What you should know about Wi-Fi in the 6Hz Band Page 14
Understanding total harmonic distortion measurements Page 33
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Test & Measurement Handbook
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TE S T & ME ASUREMENT HANDBOOK
Will 5G be lethal? Peruse a certain kind of website these days and you’ll find warnings about the lethality of RF transmissions in the frequency ranges slated for 5G networks. One site quotes Devra Davis, director of the Board on Environmental Studies and Toxicology of the U.S. National Research Council, who warns that our sweat glands act as antennas for 5G wavelengths and 5G wavelengths have never been tested for health effects. Another points to an IEEE paper by Om Gandhi, chair of the EE Dept. at the University of Utah, who found that a large number of cell phones can emit 11x the US FCC radiation limit and three times the European/ICNIRP limits. Yet another webpage shrieks that 5G uses nearly the same frequency as weaponized crowd control systems (96 GHz). Of course, it’s easy to find fringe websites warning about all sorts of health issues, and it appears that health issues concerning 5G are no exception. But some of the claims these sites make has questionable scientific underpinnings. For example, when sciencebasedmedicine.org examined claims about harm from cell phone radiation, it found that virtually none of the harmful “effects” had been reproduced in follow up studies. It also found evidence that proponents of theories about harmful cell phone radiation misstate the findings of key studies that have found cell phones cause no harm. That brings us to Gandhi’s IEEE paper. His group examined SAR (specific absorption rate) test data for 450 cell phones that were held in contact with the human body. SAR tests quantify the magnitude and distribution of electromagnetic energy that biological objects absorb when exposed to RF fields. Gandhi thinks these close-contact tests approximate the way most people use cell phones though manufacturers now recommend phones be held 5, 10, or 15 up to 25 millimeters (about an inch) from the body. Gandhi has a point. I can’t ever recall being told when purchasing a cell phone that I should keep it an inch away from my skin. One thing is clear: SAR tests are likely to become increasingly important and potentially controversial as 5G gear proliferates. So it’s useful to take a closer look at the tests themselves. For that, we spoke with Bryan Taylor and Nicholas Abbondante, both engineers at Intertek Group plc, an independent test lab that, among other things, runs SAR tests.
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The tests use a “phantom” filled with a liquid that simulates the electrical qualities of the human head or body. Hall-effect magnetic probes measure SAR in watts-per-kilogram of tissue. The Intertek engineers say the FCC SAR levels can be tough to meet. They estimate that only about half the equipment they test passes the first time. When there’s a problem, it is usually because a portion of the device other than the antenna radiates energy. Additionally, manufacturers that have been through SAR testing develop a bag of tricks to help mitigate difficulties, the engineers say. One example: Designing the chassis to keep the RF transmitter farther away from the human user. The Intertek engineers also point out that manufacturers increasingly design-in proximity sensors to detect when the RF antenna is too close to a human. Tablets, for example, generally employ these sensors to either reduce the RF output from specific antennas or switch it off completely when a user puts the tablet on their lap. The jury is still out on what levels of 5G RF energy can be considered safe for humans and at what distances. But you can probably expect to see a lot more measures aimed at squelching RF output when humans are close by.
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JUNE 2019
C ON T EN T S
Test & Measurement Handbook
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02 06 10 15 18 22
WILL 5G BE LETHAL? SAFETY GUIDELINES FOR HIPOT TESTING AND SETTING UP A SAFE TESTING AREA A few simple steps ensure testing takes place safely when potentially lethal voltages and currents are involved in the testing process.
TEST INSTRUMENTS TACKLE 5G
The high frequencies involved in 5G will force a re-think of how to go about characterizing circuits and systems.
WHAT YOU SHOULD KNOW ABOUT WI-FI IN THE 6GHz BAND
While there are still interference issues to be resolved, new specifications for Wi-Fi in the 6-GHz band will bring instantly better wireless performance.
HOW STANDARD AND CONFORMANCE TESTS SHAPE THE FUTURE OF 5G NEW RADIO Specs for 5G NR release-15 have been finalized and plans call for product introductions this year.
TEST INSTRUMENTS AND PCB REPAIR
28 33 36
40 44
Basic tools such as DMMs and oscilloscopes may be the only test gear needed for small printed circuit board repairs, but it pays to know when automated systems would be a better choice.
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WHAT'S NEW IN USB POWER DELIVERY 3.0
A new version of the USB spec puts additional demands on testing regimes that wring out power-handling options.
UNDERSTANDING TOTAL HARMONIC DISTORTION MEASUREMENTS
Most electronics engineers and technicians have a good understanding of total harmonic distortion (THD). But there are a few elusive details that come into play during THD measurements.
TESTING LVDS DEVICES AT THE MARGINS WITH AN AWG
Advanced arbitrary waveform generators have simplified the testing associated with communications over low-voltage differential signaling lines.
REMOTE COMMUNICATION WITH USBTMC
Simple connections employing USB can replace more expensive GPIB setups in test instrumentation.
HOW TO MEASURE CURRENT AND ENERGY USE ACCURATELY
General-purpose test equipment can have trouble measuring small currents, particularly if they only happen briefly. New specialized instruments go a long way toward solving this problem.
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TE S T & ME ASUREMENT HANDBOOK
Safety guidelines for hipot testing and setting up a safe testing area A few simple steps ensure testing takes place safely when potentially lethal voltages and currents are involved in the testing process. KEVIN CLARK | VITREK INC. Hipot testing has long been a standard procedure for assuring the electrical safety compliance of electronic equipment. Early commercial hipot testers were actually not much more than a variac-driven step-up transformer used to ramp up the test voltage and then dwell at the specified voltage for the prescribed time period – all the while monitoring for excessive leakage current or device breakdown. This method could easily lead to incorrect results when leakage current caused the voltage output from a high impedance transformer source to droop. Modern hipot testers utilize electronic source technology to assure compliance with IEC-61010. This standard explicitly requires that “the voltage withstand test equipment shall be able to maintain the required voltage for the specified period of time.” There is no substitute for operator competence, so the first step to a safe testing environment is to simply provide training. The operator should be in good health; operators with special medical conditions should not work with high voltage. All operators should understand that high voltage is dangerous, and care must to taken to avoid touching energized circuits. They should have knowledge of the effects of electrical currents on the human body and how best to avoid shock hazards. It is a good idea for all employees to learn how to perform compression-only CPR.
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The Vitrek TL-UP1 Test Adaptor accepts widely used North American, European and Asian style power cords. The Vitrek TL-UP1 accessory visible here is an example of an accessory device the simplifies ground continuity test setup. With its four-foot leads, the accessory offers easy hipot and continuity test connections of corded products.
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HIP OT T E S T ING Operators must understand the workings and importance of safety interlocks and why the interlocks should never be disabled. They must also understand the hazards of wearing metallic jewelry around electrical equipment and show how to interrupt power quickly in emergency situations. Other operator requirements include programming the necessary tests and storing them in memory. There should be a procedure available showing which memory location should be used for each individual device being tested. The procedure should also outline the test being performed (ac or dc, voltage, test time and limits). The operator should use the key lock feature on the tester. This will avoid programs being changed to unknown values. Those who train the operators should explain the object of each test, show how it should be executed, and show how to handle both normal and abnormal situations that may arise. Make sure each operator understands how much he or she can handle alone and when supervisory personnel should be called in for help. They should hold regular meetings to review and update safety procedures and regulations. The next step is determining where the test station will reside. The test area should be isolated from the factory assembly area. It should be away from foot traffic to assure the safety of passersby and, of course, the safety of the station operator. Operator distractions should be kept to a minimum, and the area should be conspicuously marked with internationally approved signage, such as “DANGER – HIGH VOLTAGE.” During testing, the hipot tester itself should have indicator lights to denote when high voltage is present. There should be ample and reliable power supplied to the test station. Verify that the power wiring meets electrical code requirements for polarization and grounding. Always use an outlet that has a properly connected protection ground and makes sure this ground has been tested to ensure a low impedance path to the panel ground and earth bonded ground. Operator injury may result if the hipot tester is not connected to earth ground properly. The work area and bench surface should consist of non-metallic materials; which means that metal work surfaces should be avoided, and metal objects should not be placed between the operator and DUT. All other metal objects should be grounded or be out of the test area altogether. An ESD mat is not a recommended platform for your test station, as it may cause erroneous readings for leakage and is unnecessary in this application. In addition, the test equipment should provide for immediate and safe removal of the output voltage using internal discharge circuity at the conclusion of the test or if the test is interrupted. Never remove power for the hipot tester. If there is a power interruption, use extreme care in any contact with the DUT. The safest approach is to leave the DUT connected to the hipot tester until power is restored and the tester can conduct its discharge function. eeworldonline.com | designworldonline.com
Vitrek — Test and Measurement HB 06-19.indd 7
DUT with palm switches and foot switch
Enclosure with interlocks
Safety features such as guards or enclosures can be added to the test station to prevent the operator from encountering high voltage. In addition, it is easy to implement circuit palm switches that prevent the operator from encountering high voltage during testing.
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OPERATOR SAFETY CONSIDERATIONS
The test station should have sufficient space for the tester and the DUT without the operator having to reach over the DUT to access the tester. The tester should be at least three inches away from the wall to provide proper airflow for the unit. Ideally the DUT should be isolated from the operator and tester. For larger DUTs, which are wheeled to the test station, the cart should be non-conductive and have locking wheels. (This also applies if the tester needs to be wheeled to the DUT.) Keep the area clean and neat and arrange the equipment so that it is easy and safe for the operator to use. There are many safety features such as guards or enclosures that can be added to the test station to prevent the operator from encountering high voltage. When placed around a DUT they should be non-conducting and carry safety interlocks that interrupt all high voltages when open. Interlocks should be arranged so operators are never exposed to high voltages under any conditions. In addition, it is easy to implement circuit palm switches that prevent the operator from encountering high voltage during testing. The basic operation of a palm switch requires the operator to use both hands to initiate a test with, potentially, a foot switch to activate the test. The test stops immediately if the operator removes one or both hands from the switches.
The switches are placed directly in front of the operator and spaced shoulders-width apart. Spacing the switches prevents an operator from trying to press down both buttons with one hand or object. No high voltage can be applied to the output terminals and DUT until both switches are pressed simultaneously. The operator cannot touch the DUT or test leads if both hands are on the palm switches. The palm switches are connected to the digital I/O on the hipot tester. Putting the switches in the down position enables the start. When one switch goes up the safety interlock is enabled, terminating the output voltage of the hipot test. This method is safe, quick and effective. There are two alternative approaches to the setup of a benchtop hipot test. In one case, the DUT is placed on the test bench and a combination of palm switches and a foot switch ensure that the operator cannot touch the DUT while the test is underway. The operator is wearing safety glasses. As a practical matter, the use of palm switches is typically restricted to shortduration tests done repetitively with a series of DUTs. If this test set up is used for longer tests, operators will find a way to defeat the palm switches. In the second case, the DUT sits under a protective cover with an interlock to isolate the operator during the test. The use of an enclosure is a more reliable means to assuring operator safety, particularly when tests require longer time periods.
The Vitrek V7X Hipot Tester is well-suited to the requirements of electrical safety production testing.
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HIP OT T E S T ING
More elaborate test stations can include a hipot tester interlock. One safety method that utilizes the interlock employs a light curtain, which is an infrared light beam that will open the interlock if anyone interrupts any part of the beam. The output of the light curtain connects to the interlock terminal on the hipot tester. If the interlock opens, high voltage is immediately terminated. The light curtain is placed between the hipot tester or the DUT and the operator. An operator touching the high voltage would have to pass through the light curtain, hence opening the interlock, which terminates the high voltage. Of course, if the hipot sits behind the light curtain there must be a way to start the test. A foot switch is an easy solution. Keep in mind the setup must ensure nobody can reach the high voltage by going around the light curtain. TEST SETUP
Regularly, typically at the start of every shift, the tester itself should be checked by connecting it to both PASS and FAIL samples. These samples should be designed to confirm the tester works properly for the type(s) of tests to be conducted (hipot, insulation resistance, ground resistance and ground bond). The operator should also confirm that all test parameters, according to the test documentation, display on the tester screen. Then the operator can run the test, keeping in mind the safety considerations this article describes. Operator Checklist for HiPot Testing: Basic Safety Guidelines and Procedures
• Verify the tester is functioning properly by use of a performance verification device. This will also confirm the condition of the test leads. Keep the equipment on a regular calibration cycle. • Have a “hot stick” handy when performing a dc test and use it to discharge any connection or device that may become disconnected during a test. This is necessary because unexpected, dangerous charges can build up during a test if a connection comes loose. • At the completion of a test, observe that the HV light is out. If the test was dc, the discharge may take some time. • Ensure the tester and test station uses all the built-in safety features and functions of the hipot tester. • Periodically test the memory to ensure consistent testing and that the parameters are not altered. • Make sure the ac mains to the tester is properly installed with low impedance ground connections. Also make sure that the emergency switch disconnects all power from the tester and the DUT and all electrical equipment and feeds in the testing area. • Operator and nearby co-workers should be trained in compression-only CPR in the event of a heart attack event or contact with high voltage.
Vitrek Inc. http://vitrek.com/
• Only properly trained operators should be allowed to use the equipment and have access to the test area. • Do not make any connections to a DUT unless you have verified the high voltage warning light is OFF. • Never touch a DUT, the tester or the test leads. • When connecting leads to the DUT, always connect the ground clip first. • Never touch the metal of a high voltage probe or HV test lead directly. Only touch the insulated parts and only when no high voltage is present. • When possible, use interlocked test fixtures only. • Verify all DUT connections before starting a test. Make sure no other objects are near the DUT or the tester. • Keep the area neat and uncluttered and avoid crossing test leads. • Suspend the test leads to minimize capacitive coupling. • Follow the prescribed procedure for each test exactly as written. • Verify all setup conditions before starting a test and examine all leads for signs of wear.
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Test instruments tackle 5G The high frequencies involved in 5G will force a re-think of how to go about characterizing circuits and systems.
Engineers responsible for designing and manufacturing 5G chipsets, devices, and systems have more than enough challenges. From a technical standpoint, they must deliver high bandwidth, low latency, and other performance benchmarks. They also must meet the corporate business case for
DEVELOPING TEST PROCESSES
5G – cost and profitability. So they must
Compared to 4G LTE, 5G technology brings greater capacity, lower latency, and a more uniform user experience. No universally approved 5G test standards have been adopted -- test specifications are evolving. But 5G testing parameters
efficiently verify products according to the 3GPP Release 15 specifications to meet key performance indicators (KPIs) in the most timely and economical manner possible.
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Test instruments will be important in reaching these goals. Test solutions must address the requirements of measuring present-day 5G signals yet have the flexibility to meet future versions of the 3GPP specifications as they continue to advance.
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ADNAN KHAN | ANRITSU CO.
Signal analyzers such as the MS2850A target applications in next-generation broadband and multicarrier communication systems found in 5G mobile and broadcast satellites. Features include built-in 5G measurement software, 5G NR/V5G (sub-6 GHz/ mmWave) coverage, a wide dynamic range to bring high EVM performance (EVM: <1%), one-button dynamic-range optimization at EVM measurements, amplitude/phase/timing difference measurements for each carrier, and a flat amplitude and phase characteristic.
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5G T E S T ING
have a much stronger foundation than was the case with LTE, thankfully. Test companies have worked with customers to determine likely 5G test processes and procedures. Additionally, leading test equipment manufacturers have made accelerated contributions in the development of 5G 3GPP specifications, not the case in early stages of 4G. The result is a consensus in the industry about the likely specifications for the first phase of 5G New Radio (5G NR), also known as 3GPP Rel-15. Based on all this data, guidelines have been established to help test vendors introduce solutions that help engineers develop accurate models, thereby expediting the design process and controlling the cost-of-test. The key aspects engineers must address to successfully launch 5G include: Antenna and device design Beamforming/millimeter wave (mmWave) Price and time Present-day LTE mobile terminals have several built-in antennas which can be physically connected to test equipment; 5G terminals could have a significantly higher variety of antennas. The 5G device has LTE antennas as well as 5G antennas, and 5G antennas may include an array of antenna elements. In 5G mmWave applications, high-gain, steerable phased array antennas will be used to overcome high propagation loss. Adding to the complexity is that mmWave antenna arrays are embedded in the chip of a 5G mobile device which makes it impossible to test with a physical connection. Implementing a measurement connector for each antenna would boost the mobile/ terminal size and contradict cost-reduction trends. And use of cable would also increase signal loss. All these factors make it significantly more challenging for engineers to verify antenna performance. The testing of 5G antennas employs Over-the-Air (OTA) measurements, including power and sensitivity. OTA and interoperability testing ensure the mobile device performs according to 3GPP 5G NR specifications by simulating 5G NR technologies in a real-world environment across a broad set of use cases and deployment scenarios. These tests take place in low-band spectrum below 1 GHz, mid-band spectrum such as 2.5 GHz, 3.7 GHz to 4.2 GHz, as well as the 28 GHz and 39 GHz mmWave bands. To control test costs, engineers must decide which tests must take place in an OTA chamber. Chipset/device manufacturers and carriers all must agree on an acceptable margin of error for certain performance parameters to eliminate the need for some OTA tests. For example, there are a considerable number of protocol tests. Because the verification of the protocol stack does not require RF measurements, protocol testing may take place without a chamber. The measurement of RF parameters in the mmWave range requires simultaneous OTA antenna measurements. Signal analyzers eeworldonline.com | designworldonline.com
Anritsu — Test and Measurement HB 06-19.indd 11
must be able to accurately test parameters that include Effective Isotropic Radiated Power (EIRP), Total Radiated Power (TRP), and Effective Isotropic Sensitivity (EIS). Most OTA antenna measurements should take place either in the far field or in a computationally simulated far field. The farfield distance rises with frequency and drops with antenna size. For example, at 28 GHz, the far field distance begins at 34.2 cm whereas at 39 GHz it begins at 24.5 cm. Antennas must also be calibrated at mmWave frequencies to ensure accurate directivity and beam width, as well phase and gain. Conventional OTA tests on mmWave designs can be costly. Traditionally, they require two test chambers – the reverberation chamber and the more-expensive anechoic chamber. Studies indicate that better results on mmWave designs come from conducting farfield measurements (FFM). That implies measurements taken 1.5 – 2 m away from devices supporting 28 GHz. Such testing will require a significant additional investment, considering RF measurements at LTE frequencies could take place in conducted mode. A second testing method is to develop FFM conversions or reflection parameters. Using this approach, engineers can conduct near-field measurements (NFM) and use industry-accepted algorithms to transform them to FFM. An NFM application can employ compact mmWave measuring instruments and the radiation pattern can be measured using a simple radio anechoic box in a room. This approach eliminates the high cost and long configuration time associated with measurements in a large radio anechoic chamber. OTA measurements are necessary for other reasons as well. With 4G UE, the transceiver and antenna are separately evaluated. In 5G mmWave, the introduction of high frequency and massive MIMO force the transceiver and antenna to be tightly integrated, making it difficult to evaluate each separately.
Downlink peak data rate
20 Gbps
Uplink peak data rate
10 Gbps
Average data rates anywhere in the cell
100 Gbps
Devices per km2
1 million
Area capacity
10 Mbps/m2
Over-the-air latency in user plane
1 ms roundtrip
Over-the-air latency in control plane
1 ms
A summary of the KPIs associated with 5G.
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Comparison of NFM and FFM Parameter
NFM
FFM
Measurement location
Simple radio anechoic box
Radio anechoic chamber
Measurement range
Near field about 3 λ (ex. 15 to 25 mm @ 60 GHz)
Far field (ex. 3 m or 10m)
Radiation pattern measurement
3D
2D (3D radiation pattern measurement requires time and facilities)
Antenna diagnostics and analysis
Yes
Difficult A comparison of FFM and NFM.
Verifying that 5G devices conform to specific feature sets is another consideration. The focus today is on relatively simple capabilities — two-component-carrier (2 CC), carrier aggregation (CA), for example — but 5G equipment will quickly ramp up to higher bandwidths with more carriers. This scenario is one reason why engineers need test solutions that support up to eight component carriers (8 CCs) in single instrument. Again, a signal analyzer will be the instrument of choice to test 5G device performance. Among the RF tests that must take place with a high degree of accuracy are frequency power, EVM, and spurious emissions. These tests require a signal analyzer with a high dynamic range as well as extremely flat amplitude and phase. Though the emphasis today is on 5G NR, beamforming and other mmWave applications are not far behind. These higherfrequency 5G designs create special test factors, as engineers must conduct static tests on devices and antennas in active beamforming environments. Engineers need to determine how many points are necessary to obtain measurements, trading off measurement accuracy for an acceptable verification time/cost of test. One of the most important tests on mmWave devices is propagation loss. Signal power at mmWave frequencies can drop significantly from path loss caused by environmental conditions, much more so than the path loss experienced at traditional frequencies (i.e., ≤ = 6 GHz). At 28 GHz, the path loss is approximately 40 dB higher than at traditional LTE frequencies. This is considerable because the received power at the far end of the link is halved for every 3 dB of path loss. Engineers need test solutions that can conduct this critical measurement with a high degree of accuracy.
environments that minimize costs. Test times are important at sub-6 GHz, but their importance will be compounded at mmWave frequencies where the tests inherently can take significantly longer. In response, test solutions will need to be built on flexible platforms that facilitate upgrades of current solutions. This approach allows engineers to protect their investments in existing test solutions and economically upgrade to 5G. It also saves time because engineering staff needn’t learn new instrument controls, operations, and test cases. Software will also play a key role in 5G testing. With appropriate software, a single instrument can verify 5G NR, as well as LTE, LTEAdvanced, and other legacy technologies, saving time and money. Future standards can be addressed with software rather than via purchasing more expensive hardware. In a nutshell, 5G presents several test challenges for engineers, ranging from higher frequencies and performance expectations to more complex designs, time and cost pressures. Test instruments having flexibility and reliability will bring the efficiencies needed to meet current requirements and evolve as 5G advances.
Anritsu Co. 5G https://www.anritsu.com/en-US/test-measurement/technologies/5geverything-connected/5g-everything-connected-detail
COST-OF-TEST AND TIME-TO-MARKET
Engineers designing and manufacturing their chipsets and devices have more to consider than the complexities associated with 5G. Test solution companies must work with customers to develop
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TE S T & ME ASUREMENT HANDBOOK
What you should know about Wi-Fi 6 in the 6 GHz band EVE DANEL | LITEPOINT
While there are still interference issues to be resolved, new specifications for Wi-Fi in the 6-GHz band will bring instantly better wireless performance.
Channel 1
Channel 1
Wi-Fi technology turns 20 this year, and it has proven to be successful beyond the wildest expectations. The recent announcement by the FCC to consider allowing 1,200 MHz of spectrum in the 6 GHz band for unlicensed use, as well as similar work in the E.U., promises to free up enough spectrum to move
Local AP
Neighbor AP
Channel 38 (40 MHz)
Channel 36 (20 MHz)
Local AP
Neighbor AP
Wi-Fi forward into a new era of high performance. In late 2018, the FCC released a notice of proposed rulemaking to promote new opportunities for unlicensed use in portions of the 1,200 MHz of spectrum in the 5.925-7.125 (6 GHz) band. WiFi users should rejoice, as Wi-Fi in the 6-GHz band could become a reality in 2020. To understand why this event is cause for celebration, we examine the reasons that make the 6 GHz band compelling for unlicensed use and why additional spectrum is so badly needed to sustain Wi-Fi growth. Cisco’s annual Mobile Visual Networking Index (VNI) forecasts that by 2022, Wi-Fi will carry 51% of global IP traffic, more than any other wired or wireless access technology, while the total number of Wi-Fi hotspots (including home spots) is expected to reach 549 million by 2022. Wi-Fi carries more internet traffic than any other wireless technology. And this took place despite an available frequency range of less than 600 MHz (70 MHz in the 2.4-GHz band, 500 MHz in the 5-GHz band). But a study commissioned by the Wi-Fi Alliance predicts an 800-MHz spectrum shortfall to handle traffic by 2020, with that shortfall growing to 1.12 GHz by 2025. The study stresses the importance of making available continuous spectrum to enable 160-MHz-wide channels (or future 320 MHz). The 6-GHz band and its 1,200 MHz of contiguous spectrum can fulfill the growth requirements, and this is why the FCC is currently considering it.
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Chan 38 Chan 36 5,170 MHz
5,190 MHz
5,210 MHz
Devices on the same channel and on adjacent or overlapping channels can be part of the same contention domain.
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WiF i 6 IP traffic by access technology 26% CAGR 2017-2022
Mobile (46% CAGR) Fixed/Wi-Fi from mobile devices (53% CAGR)
450
Fixed/Wi-Fi from Wi-Fi-only devices (18% CAGR)
400
Fixed/wired (15% CAGR)
350 Exabytes per month
48% from mobile devices
300 250 200
51% is Wi-FI
150 100 IP traffic predictions from the Cisco annual Mobile Visual Networking Index.
50 0
2017
2018
2019
2020
2021
2022
Note: Fixed/Wi-Fi from mobile devices may include a small amount of fixed/wired from mobile devices Source: Cisco VNI Mobile, 2019
WHY IS MORE SPEC TRUM NEEDED?
The availability of unlicensed spectrum in the 2.4-GHz and 5-GHz bands has driven Wi-Fi growth. It has lowered the barrier of entry for new-comers and enabled lowcost deployments that have made Wi-Fi a ubiquitous local network connectivity technology. But it has also contributed to its woes, as unlicensed spectrum is being shared by a multitude of devices (including non-Wi-Fi equipment using Bluetooth, Zigbee and others) and forced the technology to make compromises as it coexists with many users. Additional unlicensed spectrum will address the two main problems that users face: Congestion – The low number of Wi-Fi channels available today forces many users to share available bandwidth and creates congestion. Restricted 80-MHz and 160-MHz channel availability – Today the limited amount of contiguous spectrum makes it difficult to enable 80-MHz or 160-MHz channels, yet high data throughput can only come when wide channels are available. CHANNEL CONGESTION
In Wi-Fi networks using Wi-Fi 5 (802.11ac) or older Wi-Fi versions using OFDM eeworldonline.com | designworldonline.com
Litepoint — Test and Measurement HB 06-19.indd 15
(orthogonal frequency-division multiplexing) access technology, all devices connected on the same channel split its capacity. When connected on a channel with a high number of devices, users will experience low data throughput, as each one of them waits their turn to transmit (or receive) data. Congestion is caused by devices connected on the same access point and sharing the same channel. Though it may be less intuitive, congestion is also caused by devices in neighboring networks, on the same channel, or on an overlapping channel. All these devices compete for access to the same RF channel spectrum. Channel congestion is particularly problematic in dense urban areas, airports, or stadiums where there are Wi-Fi signals from hundreds of access points and client devices. The 2.4-GHz and 5-GHz frequency bands support a limited number of nonoverlapping 20-MHz and 40-MHz channels. Wi-Fi 5 and Wi-Fi 6 (802.11ax) standards added support for an 80-MHz and 160MHz channel width to allow for higher data speed. But in practice, the wider channels are rarely deployed. Only six 80-MHz and two 160-MHz channels are currently available in the 5-GHz band. In dense environments or large enterprises, it’s hard to find an 80-MHz or 160-MHz channel free from interference 6 • 2019
caused by devices on overlapping channels. In response, network administrators often choose to disable these capabilities, thereby restricting the data speed available on their network. When considering the case of a twoantenna (MIMO streams) client station typically used with phones, laptops, and tablets, the max achievable speed on a 40-MHz Wi-Fi 5 deployment is only 400 Mbps, while a Wi-Fi 6 deployment can only hit a maximum of 574 Mbps at the physical layer. At the TCP layer -- i.e. usable data throughput -- the results are in the range of 60% to 70% of the above numbers. It is obvious that spectrum availability limits will quickly hamper Wi-Fi technology evolution. Meanwhile, broadband access speeds (DOCSIS, passive optical network, fiber to the home) keep rising at a rapid pace, and applications like video streaming, VR/AR and gaming require more and more bandwidth. Additionally, Wi-Fi is a critical enabler of 5G technology; it likely will be widely used for mobile-traffic offload and providing 5G indoor coverage via FWA (Fixed Wireless Access). In the near future, with no improvements to the spectrum availability, the Wi-Fi network may well become a bottleneck as all services are ultimately accessed via Wi-Fi.
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TE S T & ME ASUREMENT HANDBOOK Mid-band spectrum roadmap 5925 MHz
6425 MHz
6525 MHz
6875 MHz
7125 MHz
United States European Union 59 x 20 MHz channel 29 x 40 MHz channel 14 x 80 MHz channel 7 x 160 MHz channel
The Unlicensed National Information Infrastructure (U-NII) radio band is part of the radio frequency spectrum used by IEEE 802.11a devices and by many wireless ISPs. U-NII is an FCC regulatory domain for 5 GHz wireless devices. The European HiperLAN standard operates in same frequency band as the U-NII. WI-FI 6 AND THE 6-GHZ BAND
It is important not to confuse Wi-Fi 6 and Wi-Fi in the 6-GHz band, as these are two separate, but interconnected, topics. Wi-Fi 6 is the Wi-Fi Alliance’s new consumer-friendly name for the IEEE 802.11ax standard. The 802.11ac standard has also been renamed Wi-Fi 5; 802.11n is now Wi-Fi 4. The purpose of this new naming scheme is to make it easier for the general public to identify device generations without remembering the complex 802.11 alphabet soup. IEEE 802.11ax has been defined to operate in the 2.4GHz and 5-GHz bands, and many device manufacturers are already shipping Wi-Fi 6 equipment operating in these bands. Now the stage is set for operation from 5.925-7.125 GHz. Wi-Fi 6-compliant devices operating in the 6-GHz band could become available as early as 2020. Wi-Fi 6 is already a revolutionary standard as it introduces orthogonal frequency-division multiple access (OFDMA). OFDMA is a real disruptor in the way Wi-Fi operates because it allows multiple users to transmit simultaneously. Unlike with OFDM, the technology used for older Wi-Fi generations, OFDMA Wi-Fi channel bandwidth (20, 40, 80 or 160 MHz) is divided amongst multiple users who simultaneously transmit on smaller subchannels called resource units (RU). 802.11ax is big step toward addressing network congestion. Its impact often gets compared to upgrading from a single-lane road (OFDM) to a multi-lane freeway (OFDMA). But OFDMA only becomes effective with wide adoption of Wi-Fi 6 technology in client devices and access points. Indeed, each time a legacy device (11ac, 11n or older) transmits in the network, the transmission reverts back to standard OFDM mode with a single transmission occupying the entire spectrum. Only Wi-Fi 6 devices are capable of participation in an OFDMA transmission. The truth is that until most consumer devices employ Wi-Fi 6, users may not notice much performance improvement over older technologies. On the other hand, the introduction of the 6-GHz band for
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Wi-Fi use provides instant gratification for all Wi-Fi 6 users. With enough spectrum to safely deploy 80-MHz or 160-MHz-wide channels and green-field deployments alleviating concerns of backward compatibility, the 6-GHz frequency band could become the VIP Lounge for Wi-Fi 6 users, where high throughput rates and congestion-free network access can be instantly achieved. A DEEPER LOOK AT THE 6-GHZ FREQUENCY BAND
The 6-GHz frequency band, also called mid-band spectrum, spans 5.925 GHz to 7.125 GHz. It is divided into four bands: UNII-5: 5925-6425 MHz UNII-6: 6425-6525 MHz UNII-7: 6525-6875 MHz UNII-8: 6875-7125 MHz
(500 MHz) (100 MHz) (350 MHz) (250 MHz)
The mid-band spectrum is currently for licensed users who have deployed services in these frequencies. For example, users include point-to-point microwave links and mobile TV pickups at sporting events relaying signals back to a studio. There are currently around 100,000 microwave links in the U.S. The operators of these links are common carriers (AT&T, Verizon, etc.), industrial and business entities (utilities, railroad, oil and gas) and public safety agencies (safety, emergency services, transportation). These incumbents have concerns about coexistence and possible signal interference with unlicensed services. FCC rulings will likely include mechanisms to mitigate interference from Wi-Fi operation in the 6-GHz band, either by having devices operate indoors only at low power levels or by having AFC (Automated Frequency Coordination) mechanisms in place. While these coexistence processes are still being finalized, Wi-Fi chip and device makers are already planning to offer products soon after the regulatory bodies grant their final approval. The IEEE incorporated definitions for the new 6 GHz frequency band channels in the latest draft of the 802.11ax standard. eeworldonline.com | designworldonline.com
6/10/19 1:48 PM
WiF i 6 How WiFi speeds have risen Number of antennas (MIMO streams)
Wi-Fi standard
20 MHz
40 MHz
80 MHz
160 MHz
2
Wi-Fi 4 (11n)
144 Mbps
300 Mbps
Not supported
Not supported
2
Wi-Fi 5 (11ac)
173 Mbps
400 Mbps
867 Mbps
1733 Mbps
2
Wi-Fi 6 (11ax)
286 Mbps
574 Mbps
1200 Mbps
2400 Mbps
Maximum achievable physical layer speed on a 2x2 station.
In the U.S. a plethora of new channels could soon be added to Wi-Fi devices, immediately alleviating congestion and enabling the highest data speed in Wi-Fi 6 devices operating in the 6-GHz band.
move to ensure this widely used wireless technology can deliver the performance needed for future applications and networks.
59 x 20 MHz channels 29 x 40 MHz channels 14 x 80 MHz channels 7 x 160 MHz channels
Cisco annual Mobile Visual Networking Index (VNI) https://www.cisco.com/c/en/us/solutions/collateral/service-provider/ visual-networking-index-vni/white-paper-c11-738429.html#_Toc953328
All in all, Wi-Fi in the 6-GHz band means instant performance increases for Wi-Fi 6 users. While there are still interference issues to be resolved, opening up 6-GHz frequency bands for Wi-Fi is the right
Wi-Fi Alliance study https://www.wi-fi.org/download.php?file=/sites/default/files/private/ Wi-Fi%20Spectrum%20Needs%20Study_0.pdf
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TE S T & ME ASUREMENT HANDBOOK
How standards and conformance tests shape the future of 5G New Radio Specs for 5G NR release-15 have been finalized and plans call for product introductions this year.
The development of 5G standards and their commercial rollout appears to be on a fast track. Mobile operators and network equipment makers are conducting field trials and the first smart phones are expected out this year. Predictions are that nearly half of global businesses are either already deploying 5G networks or plan to deploy them within the next 12 months. Is 5G at the tipping point? The MerriamWebster dictionary defines a tipping point as a point of no return, after which significant transformation occurs. 5G New Radio (NR) has come a long way with Release-15 standard frozen in June 2018. You may hear some industry experts say the standards are
SHERI DETOMASI | KEYSIGHT TECHNOLOGIES, INC.
complete, but are they really? Let’s explore where we are in the 5G NR rollout with respect to required tests, challenges, and risks. 5G products and components require testing through the ecosystem, including modems, antennas, sub-systems, and fully assembled end-user devices. Devices and base stations will follow with a similar testing workflow: research and development (R&D), design validation, conformance, and acceptance testing for devices. Manufacturers must follow many rules and regulations when bringing mobile wireless communications products to market. Testing takes place throughout the product lifecycle to ensure the end product meets Third Generation Partnership Project (3GPP) specifications and provides the quality of
service mobile operators aim to deliver. Conformance tests are required by most vendors before release of a device into the market. Conformance tests are a key requirement and involve connecting a device to a wireless test system and performing the required 3GPP tests: Radio frequency (RF) transmission and reception performance – minimum level of signal quality Demodulation – data throughput performance Radio resource management (RRM) – initial access, handover, and mobility Signaling – upper layer signaling procedures.
3G PP specs Minimum requirements
Conformance tests TS 38.141-1: Part 1: Conducted conformance testing for FR1
Base station
TS 38.104
TS 38.141-2: Part 2: Radiated conformance testing for specific base station configurations in FR1 and FR2 TS 38.521-1: Range 1 Standalone – FR1 Conducted tests
User equipment
18
TS 38.101
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TS 38.521-2: Range 2 Standalone – FR2 Radiated tests TS 38.521-3: Range 1 & 2 Interworking operation with other ratios – FR1 Conducted and FR2 Radiated
6 • 2019
The minimum requirements and the conformance test specifications and test methods for all frequency ranges can be found in several 3GPP documents.
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5G NE W R ADIO How OTA tests take place Shielded box or environmental noise camber OTA probe antennae DUT
Channel emulator
Base station network emulator
3GPP Standards identify a minimum level of performance for user equipment (UE) terminals and base stations (gNB). These requirements are defined in the RAN4 and RAN5 technical specifications (TS). RAN4 defines minimum requirements for base stations and UEs. It also specifies test cases and test methods for base station conformance tests. RAN5 defines the UE conformance test specifications and test methods. 5G NR products can operate in two frequency ranges – frequency range 1 (FR1: 410 MHz to 7.125 GHz) or frequency range 2 (FR2: 24.25 to 52.6 GHz), or both FR1 and FR2. Testing in FR1 below 3.5 GHz is firmly established with well understood test methods and associated uncertainties. Given that FR1 below 7 GHz operates under the same characteristics, tests in FR1 will resemble that of LTE with most conformance tests employing a cable connected to the device; antenna characteristics and multiple input multiple output (MIMO) performance will be completed over-the-air (OTA). FR2 at millimeter-wave (mmWave) frequencies adds a completely new twist on testing because all measurements in FR2 will be tested OTA. Third-party test labs perform conformance tests to determine if a UE is compliant. These tests are expensive, so most companies conduct their own pre-conformance tests before engaging an independent lab. Currently, these tests are still in development and will be defined over the next year for different use cases. 5G NR introduces many new features that make testing more complex. Higher-frequency operation, wider channel bandwidths, flexible waveform structure, and the rising number of test cases that must be validated all complicate test design. For the most part, the testing of designs under 6 GHz in FR1 resembles that of LTE. In FR1, the key challenges lie in testing designs operating between 3.5 GHz and 7.125 GHz that use wider bandwidths and massive MIMO, and designs operating in FR2 because they require OTA test methods. OTA introduces many challenges that UE and base station designers haven’t seen and has big implications for the test environment. User Equipment Test Challenges: 5G NR must accommodate many different usage scenarios from high throughput to low packet eeworldonline.com | designworldonline.com
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A typical (simplified) OTA test setup with a network emulator and channel emulator.
size, to low latencies with high reliability. The 5G NR physical layer was given the flexibility to support such a wide variety of use cases, changing the way signals are created and operate. In addition, there are seven different system architecture options and dual connectivity with 4G LTE. Testing must cover the many different use cases. Furthermore, testing involves signaling and full end-to-end performance with real-world impairments like excessive path loss, multi-path fading, and delay spread. Wringing out these functions requires an OTA test solution that can emulate base station protocols and channel conditions. The drastically higher number of test cases that must be validated for conformance and device acceptance testing dramatically lengthens test times. To minimize the risk of parallel development, ensure you are testing to the latest specifications and getting regular 5G NR test software updates to help meet evolving 5G NR requirements. Base Station Test Challenges: The active nature of 5G beam steering and beamforming requires validation in an OTA setting. Key aspects such as antenna gain, side lobe, and null depth for the full range of 5G frequencies and bandwidths can severely impact system performance. In particular, 3D antenna beam measurements introduce many complexities into the test. While path loss and signal impairments were not a big issue at 3.5 GHz and below, such phenomena are problematic at mmWave frequencies. Therefore, test solutions for mmWave frequencies must accommodate higher frequencies with wider channel bandwidths and also address higher path loss at mmWave frequencies. To that end, a test solution must have adequate signal-to-noise ratio (SNR) to accurately detect and demodulate 5G signals. When testing transmitters, high SNR in the test analyzer is critical for accurate error vector magnitude (EVM) and adjacent channel leakage ratio (ACLR) measurements, and achieving higher SNR becomes even more of a challenge for those testing at mmWave frequencies. The use of signal generators with higher output power per EVM and ACLR becomes important for testing receivers. In addition, it is also critical that a system-level calibration correct for system-level phase and magnitude shifts over the bandwidth of the measurement. 6 • 2019
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TE S T & ME ASUREMENT HANDBOOK Correcting waveforms for good channel response Dual-channel signal generator
DUT plane Test fixture Adapters, amplifiers, filters, cables, switches, mixers, etc
DUT
Channel response Corrected waveform
Channel response Preferred practice today is to correct for channel response by modifying the output waveform of the signal generator.
How well will the 5G NR RAN perform? Will devices connect flawlessly from one base station to another and provide dual connectivity with 4G LTE? Will devices, base stations, and the complete ecosystem deliver on key performance indicators (KPI) such as 20 GBps in the downlink (DL) for UHD video streaming? Will they provide the expected low latency for driverless automobiles? And will they deliver the high reliability required for no-fail applications? These are just a few of the questions keeping design engineers up at night. Meanwhile, 5G conformance tests are key milestones in the product lifecycle but remain incomplete. Many performance and RMM tests are still being defined, and test cases specifications for OTA test have yet to be specified. However, the most relevant test cases for RAN5 are expected by mid-2019. This development will help phase in new capabilities in new products and limit risks. In addition to updating 5G NR release-15, 3GPP has already started work on release-16. This covers new types of services, devices, deployment models, and spectrum bands. The emphasis is on ultra- reliable low-latency communications (URLLC) enhancements for industrial internet of things (IIoT), utilization of unlicensed bands, vehicle-to-everything communications (V2X), and UE positioning and power efficiency. However, incomplete and evolving targets are pressuring developers who need to ensure their designs and tests are flexible enough to handle future requirements. Designs and test systems must scale to new higher frequency bands, potentially with higher channel bandwidths, and support lower latency and co-existence with unlicensed bands. Given that 5G NR Release-15 is frozen, are we at the 5G tipping point? It is fair to say we’re on the precipice: We are at a point where specifications are finalized for 5G NR release-15 and designs are planned for introduction in 2019.
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But is it a point of no return? No. Most vendors working on 5G NR are planning to phase-in their 5G capabilities. Initial releases such as dual-connectivity mobile phones will rely heavily on 4G for critical applications and use 5G for supplemental high-speed downloads. 5G NR Release-15 focuses on specifying the underlying foundation for the flexible air interface and enables the enhanced mobile broadband (eMBB) use case for high data throughput. These capabilities are highly focused, giving the first 5G products greater leeway to withstand a rocky start. The real test for 5G products is farther down the road when critical applications that have higher consequences are introduced. It will be important for companies developing 5G products to engage with a test partner that has been working with leaders across the wireless ecosystem and has already faced most of the difficult problems involved in verifying compliance and capability.
Keysight Technologies www.keysight.com
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TE S T & ME ASUREMENT HANDBOOK
Test instruments and PCB repair Basic tools such as DMMs and oscilloscopes may be the only test gear needed for small printed circuit board repairs, but it pays to know when automated systems would be a better choice. PCBs are more complicated to repair today than even a few years ago. Manufacturing mistakes and in-service component failures have become a fact of life. Circuit boards will be manufactured with errors, parts will be soldered incorrectly, and components will fail. While soldering and component changes may be enough for simple fixes, some repairs may require a more sophisticated approach to find fault causes. The repair of PCB assemblies can seem intimidating, but a methodical approach can help find and fix problems quickly. It is wise to refrain from to powering up a damaged PCB at first. If, for instance, a simple blown fuse is the problem, the reason for the issue must be determined rather than just replacing the fuse (with a bigger one!) Short circuits or overloads usually leave tell-tale signs. If the PCB has been conformally coated to keep out moisture and dust, the coating must be removed (at least at a few critical test points) before for fault diagnosis can begin. Conformal coatings can be removed by solvents, peeling, or blasting, but a new technique is being developed whereby the coating can be pierced with sharp test pins.
ALAN LOWNE | SAELIG CO. INC.
Before commencing the repair, assemble any circuit-relevant diagrams and the appropriate test equipment, such as a DMM, solder/desolder hand tools, an oscilloscope, etc. – and preferably on a static-free bench. Another helpful “tool” is the user report of how the failure happened or what fault was observed. The most versatile tool is the multimeter, but depending on the complexity of the PCB, an LCR meter, oscilloscope, power supply, and logic analyzer may also be needed to investigate the operation of the circuit. RF circuits may need more sophisticated tools such as a spectrum analyzer to check frequencies and signal levels. Troubleshooting is also much easier if a known-good board is available so that visual and signal comparisons can be made. Lack of a comparative board or documentation makes the challenge more daunting.
An example of an ATE PCB repair system is the ABI Electronics System 8 - a board-test system that uses a selection of CD-drive-size modules to create a customized PC-driven PCB test station. Built in a PC case or 19-in rackmount, System 8 is a mix-and-match set of test instruments that target testing and fault-finding.
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P CB T E S T ING PCB Repair Flowchart Visual inspection (check for damaged components, broken tracks and signs of damage)
No
Use V-I tests and traditional instruments • • • • • •
Can the PCB be powered up?
Yes
• Check current consumption • Use current limiter • Check PCB for heat
Check power rails for shorts Check fuses for open circuits Check caps/inductors for shorts Check resistors for open circuit Measure resistors valves Check relays for shorts
Check all voltages • • • • •
No
Is a reference PCB available?
Replace components as required
Check for design similarities
Check all human/ machine interfaces • Check switches, LEDs etc. • Check displays
Test transistors (V-I) Test transistors (V-T + pulse) Test diodes (V-I) Check thyristors, SCRs etc. (V-I)
Run functional tests for digital devices • • • • •
Check all ICs with clips and V-I • Use standard V-I • Use 3D V-I • Use matrix V-I
VISUAL CHECKS
Check for loose connectors or components in sockets, which can often get dislodged in shipping. Look for burned or damaged parts, or solder bridges causing a shortcircuit of signal or power lines. This is where a high-power digital microscope is extremely useful! Visual inspection is an essential first step in troubleshooting. Components or parts such as transformers, power output transistors, resistors, and capacitors that show a burn mark can be detected easily by observation. Apparent burns and brown eeworldonline.com | designworldonline.com
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Measure all test points Measure regulators, converters Measure transformers Measure Vcc on familiar ICs Use DMM and DSO
Check clock signals
Check all discrete components with V-I testing • • • •
Power up PCB
Truth table Connections Voltage V-I Thermal
Run functional tests for analog devices stains (and a terrible smell) can identify the overheated components. But why did they overheat? A poor solder joint or bridge is another common item found during a visual inspection. Good solder joints always look smooth, bright, and evenly flowed. A dull surface can suggest a defective joint. Are there any solder bridges between tracks? Reversed or incorrect components? Short circuits also can be difficult to troubleshoot. A board test may indicate that a short exists, but often the location of the short is elusive. Technicians can 6 • 2019
• Functional • Connections • Voltage Run custom tests • Check relay activations • Check ADC/DAC with MIS Run JTAG tests
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TE S T & ME ASUREMENT HANDBOOK spend a long, frustrating time trying to locate a single short, particularly an interlayer short. Briefly powering up the board when observed under a thermal (IR) camera can show a location that heats up more than the surrounding components. Power the rail voltage with something less than the 3.3V or 5.0V required and limit the power supply current too. Stimulus Start with low volts/amps and bring both up waveform slowly. PCBs may have limited life through the poor design of excessive component heating. A quick way to find a short fault is to compare the thermal images of a known “good board” with the device under test. Significant temperature differences highlight the fault location. Using this approach, entire complex boards can be inspected in a non-contact manner. Common defects such as power-toground shorts and bad components, can quickly be found with this method. A changing or different color representation of the image may indicate overheating in a solder joint, circuit trace, or show a portion of the board that is malfunctioning Visually inspect capacitors. If leaks, cracks, bulges or other signs of deterioration are evident, replace it. Capacitors have a limited life and are often the cause of a malfunction. Look for broken leads on the components. Some devices have tiny leads that can easily break off at the circuit board. IC legs can become bent during assembly. Look for cracks on the circuit board, leading to broken circuit traces or broken components. You can laboriously test every resistor, capacitor, diode, transistor, inductor, MOSFET, LED, and discrete active component with a multimeter or LCR meter, but this is not an efficient way to do debugging. If the board can be powered up, a digital multimeter can check rail voltages at ICs, outputs of voltage regulators, and obvious signals such as clocks and I/O communications. An oscilloscope can be used to verify voltage waveforms of a powered board. To check for the presence of a WiFi signal output, even a cellphone can come in handy.
Simple V/I test setup
IL
ZL = VL/ IL
Current limiting resistor
VL
Device under test
Leaky capacitors can be found using the resistance setting of the DMM. Set the meter to read in the high ohms range and touch the meter leads to the corresponding leads on the capacitor, red to positive and black to negative. The meter should start at zero and then move slowly toward infinity. The ramp will be slow with large capacitance values. Note: A good capacitor stores an electrical charge and may remain energized after power is removed. Before taking a measurement of electrolytics, disconnect the power and carefully discharge the capacitor by connecting a resistor across the leads. With the meter in the ohms setting, there will be some constant current sent out from positive to negative leads. An open cap will show open, a shorted one will show close to zero ohms. A check of HMI interface items such as touch panels and switches may reveal functional issues caused by connection or component problems. It takes some understanding of the circuit to interpret the results of signal probing with a DMM or oscilloscope. DC voltage tests start with probes referenced to ground. When checking an IC, start by testing the voltage supply pin. Touching low-voltage parts of the circuit can change the impedance of the circuit which can alter the behavior of the system. Used in conjunction with a scope, this technique can help identify locations that need additional capacitance to remove unwanted oscillations, for instance. Most ICs can be identified by their markings and many can be operationally tested against their published specifications using scopes and logic analyzers. Comparing
A quick way to find a short fault is to compare the thermal images of a known “good board” with the device under test. Significant temperature differences highlight the fault location. Using this approach, entire complex boards can be inspected visually.
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6/7/19 1:26 PM
P CB T E S T ING Open circuit:
Short circuit:
Diagram 1:
Diagram 2: Master Actual
V/I testing generates different signatures depending on the current flow through the device as the applied voltage changes. Short circuits produce a vertical line because the current flow for any applied voltage would be theoretically infinite; open circuits generate a horizontal line because the current is always zero irrespective of the applied voltage. Pure resistors would produce a diagonal line with a slope proportional to the resistance. The higher the resistance value, the closer the line gets to the horizontal (open circuit). A difference in the slope of the curve when comparing a good and suspect board would indicate a difference in the resistor values on the two boards. Low-value capacitors produce flattened, horizontal, elliptical signatures; capacitors with relatively high values produce flattened, vertical, elliptical signatures. The optimal signature is a nearly perfect circle, obtained by selecting the appropriate test frequency and source impedance. Typically, the higher the capacitance, the lower the test impedance and frequency. A leaky capacitor would give a sloping curve due to the effective resistance in parallel with the capacitor.
the behavior of an IC to a known-good one is a quick way to identify anomalous behavior. Intermittent failures are the most challenging and time-consuming aspect of the troubleshooting process. Common irregular faults can be caused by component overheating or degradation, poor soldering, and loose connections. Long memory in a scope can be helpful for zooming in to a signal record for finding rare events. Applying freezer spray in the right location can sometimes aggravate and identify intermittent issues. If the board can’t be powered up safely, then power-off testing such as V/I and Signature testing can take place. POWER-OFF V/I TESTING
V/I testing (also known as analog signature eeworldonline.com | designworldonline.com
Saeling — Test and Measurement HB 06-19.indd 25
820 Ohm resistor Settings Frequency: Source impedance: Curve: Voltage:
Leaky
1.2Hz 1k Ohms sine wave 10V peak to peak
Diagram 3:
0.47uF capacitor Settings Frequency: Source impedance: Curve: Voltage:
4.8Hz 100 Ohms sine wave 2V peak to peak
Diagram 4:
Leakage
10mH inductor Settings Frequency: Source impedance: Curve: Voltage:
1.2Hz 100 Ohms triangle wave 4V peak to peak
Diagram 5:
1N4148 Diode Settings Frequency: Source impedance: Curve: Voltage:
60Hz 1k Ohms sine 6V peak to peak
Diagram 6: V-I
Bad
Pulse
Good
BZX55C5V1 zener diode Settings Frequency: 60Hz Source impedance: 1k Ohms Curve: sine Voltage: 20V peak to peak
analysis) is a technique which is excellent for fault finding on PCBs and is ideal when diagrams and documentation are minimal. Analog signature analysis was brought into wide use by the Huntron Tracker series of instruments. It can be used to perform powered-off troubleshooting of electronic components in PCB assemblies. It could be considered a vital diagnostic tool for PCB repair tasks because it is suitable for ‘dead’ boards which cannot safely be powered up. Applying a current-limited ac signal across two points on a circuit causes vertical deflection of the scope trace, while the applied voltage produces a horizontal deflection. This forms a characteristic V/I signature that can show if a component is good, bad or marginal. It is important to focus on differences between curves
6 • 2019
PNP transistor Settings Frequency: Source impedance: Voltage: Pulse type: Postive start: Negative start:
120Hz 1k Ohms 4V peak to peak bipolar: (V+0.12) (V- -0.7V) 0us stop: 4.18ms 4.18ms stop: 8.33ms
for good and suspect boards rather than analyzing the meaning of the curves in great detail. The majority of nodes on a PCB will contain parallel and series combinations of components, making exact analysis difficult. The majority of faults on failed boards are major failures such as short or open circuits, which are easy to detect with the V-I technique without complex analysis. The voltage across the DUT is plotted on the horizontal axis against the current through it on the vertical axis. The stimulus waveform is usually a sine wave. From Ohm’s law, (Z = V/I) the resulting characteristic represents the impedance of the DUT. The impedance of components such as capacitors and inductors varies with frequency, so they require a variablefrequency stimulus.
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A new technique is being developed whereby conformal coatings on PCBs can be pierced with sharp test pins.
Most applications use comparative analog V-I testing, so it’s unnecessary to understand the displayed characteristic. Comparisons of the curves for a known good board and a suspect board can often identify faults with a minimum of knowledge. Different devices in different configurations produce different signatures, depending on the current flow through the device as the applied voltage changes. A short circuit, for example, would generate a vertical line because the current flow for any applied voltage would be theoretically infinite, whereas an open circuit would generate a horizontal line because the current is always zero irrespective of the applied voltage. A pure resistor would give a diagonal line with a slope proportional to the resistance, because the current is proportional to the applied voltage. The higher the resistance value, the closer the line gets to the horizontal (open circuit). The source impedance of the V-I tester should be selected so the slope of the line, for a good resistor, is as close as possible to 45°. A difference in the slope of the curve when comparing a good and suspect board would indicate a difference in the resistor values on the two boards. More complex curves describe frequency dependent components such as capacitors and inductors. Ditto for nonlinear devices such as diode and
transistor junctions. Capacitors with relatively low values have flattened, horizontal, elliptical signatures. Capacitors with relatively high values have flattened, vertical, elliptical signatures. The optimal signature is a nearly perfect circle which can be obtained by selecting the appropriate test frequency and source impedance. Typically, the higher the capacitance, the lower the test impedance and frequency. A leaky capacitor would give a sloping curve due to the effective resistance in parallel with the capacitor. AUTOMATED TESTING EQUIPMENT (ATE)
In situations where faulty PCBs come in a steady stream, universal automated test systems have replaced individual test instruments. PC-based in-circuit testers perform both a powered, in-circuit logical test of digital and many analog ICs, as well as V-I signature analysis of the chips, using a variety of test clips. The Diagnosys PinPoint System is one such system that contains libraries of digital chip pinouts to assist technicians in troubleshooting and can determine wiring patterns of the circuits. ATEs can check for digital functionality of ICs and also provide a signature analysis of both active and passive components. Unknown chips can be identified by their Boolean output. Some ATEs can be extremely expensive and may come with a steep
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P CB T E S T ING learning curve, meaning that after purchase they sit idle in a storeroom. ATEs can perform automated or computerized test procedures on a device under test, including functional testing of ICs, analog and digital components, complete boards, etc. These products vary in complexity depending on the different levels of test capabilities needed for various board needs. Computer-based automated test procedures can run reliably and consistently with test results captured automatically, with high accuracy, at high test speeds, and with extreme flexibility. Typical ATEs include: In-circuit testers, performing device level tests on components mounted circuit boards; Functional testers, used to test full functionality of boards and modules via edge connectors; Boundary scan testers for products that are JTAG-compliant such as BGA, FPGA, CPLDs, or even complete boards with a JTAG connector. Another example of an ATE PCB repair system is the ABI Electronics System 8 - a board-test system that uses a selection of CDdrive-size modules to create a customized PC-driven PCB test station. Built in a PC case or 19-in rackmount, System 8 is a mix-and-match set of test instruments to suit most testing and fault-finding needs. Comparing results from a known good board with automatedsequence fault-finding procedures, fault diagnosis becomes possible by minimally trained staff. The System 8 software can be configured to guide less well-trained users step-by-step through a test procedure, with custom-annotated picture images, instructions, and attached datasheets to give quick Pass/Fail results. This is much faster and more economical than using traditional oscilloscopes, meters and other bench test methods. System 8 modules include: Board Fault Locator: 64 test channels for multiple test methods for fault diagnosis and functional testing of digital ICs (in-circuit/outof-circuit), IC connections status, and voltage acquisition, V-I curve testing of components on unpowered boards. Analog IC Tester: for in-circuit functional testing of analog ICs and discrete components (no programming or circuit diagrams needed). Fully configurable V-I Tester for detection of faults on unpowered boards. Multiple Instrument Station: includes eight high specification test eeworldonline.com | designworldonline.com
Saeling — Test and Measurement HB 06-19.indd 27
Microscopes can help find poor solder joints or bridges. Good solder joints always look smooth, bright, and evenly flowed. A dull surface can suggest a defective joint.
and measurement instruments in one module (frequency counter, digital storage oscilloscope, function generator, digital floating multimeter, auxiliary PSU, and universal I/O). Advanced Test Module: offers powerful test combinations for flexible, comprehensive fault diagnosis, including functional, connections, voltage, thermal and V-I signature tests. Advanced Matrix Scanner: 64 channels for fast data acquisition to test highpin-count devices as well as complete PCBs; sweep signal frequency to observe device under test response over a frequency range. Triple Output Variable Power Supply: provides required supply voltages to the unit under test. ATE applications include: PCB testing and troubleshooting, digital/ analog IC test, digital/analog V-I test, visual short identification with audible/visual indication of probe distance to short, live board comparison, manufacturing defects analysis, power-on/power-off testing, QA reporting, embedded real-time control, calculation and logging, component and board testing, digital and analog functional tests, automated test sequences, etc. The choice of the troubleshooting method depends on the complexity of the circuit and the knowledge and experience of the person who performs the troubleshooting. The methodical use of relevant test tools will help the engineers and technicians to identify the cause of failure quickly and accurately and subsequently increase the productivity of PCB repair. When it comes to circuit boards, it is often more cost-effective to repair than replace. Businesses have begun to realize this and have started incorporating ATEs into their support and development infrastructure.
Saelig Co. Inc. www.saelig.com
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What’s new in USB Power Delivery 3.0? A new version of the USB spec puts additional demands on testing regimes that wring out power-handling options.
DAVID MALINIAK | TELEDYNE LECROY
Source Sink
Since its official debut in 1996, the Universal Serial Bus (USB) protocol has seen numerous revisions spanning three generations of the standard, with USB 4.0 waiting in the wings for later in 2019. Meanwhile, in parallel, the USB Implementer’s Forum (USB-IF), which oversees the USB standard, has maintained parallel development tracks for USB interconnects and USB Power Delivery (USB PD) technology.
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The USB Type-C interconnect (2014) brought a smaller, slimmer plug that’s reversible, eliminating the fumbling around to properly align the plug with the jack. USB Type-C also delivered higher data transfer rates and higher charging voltages/currents. When USB 3.1 arrived, so did USB PD 2.0. Now, USB 3.2 has been released as has PD 3.0. It is useful to review some of the changes from PD 2.0 to the current 3.0 revision of the standard as well as elements of PD compliance testing.
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A representative USB PD 3.0 compliance test setup featuring Teledyne LeCroy’s Voyager M310P test platform. This setup handles the test suites for both PD 2.0 and PD 3.0. The platform, which has native Type-C ports, can test both sources and sinks.
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P OWER DELIVERY 3.0 Typical lithium-ion charging sequence
Voltage
Amps
Voltage on the battery
Current into the battery
Time
USB PD is a specification for handling higher power on USB and allows a range of devices to charge quickly over a USB connection. It facilitates negotiation between two devices for a power contract, so they can determine how much power can be pulled from the charger. Power Delivery starts at the 5-V setting and is configurable up to 20 V. Using a standard USB-C cable, it can handle up to 60 W and will go up to 100 W using a fullfeatured USB Type-C (ECMA) cable. Another point of interest regarding Power Delivery is that it allows for power to flow both ways, with no set direction based on circuit or connection. For example, if you were to connect two phones that support Power Delivery with a USB-C charging cable, one phone could charge the other and vice versa. A newly introduced PPS (Programmable Power Supply) protocol requires that a PPS-capable sink be able to request adjustments of source output voltage in increments as small as 20 mV. Likewise, the sink can request source current limits in 50-mA steps. The sink should check in with the source at least every
A simplified model of a preferred charging regime for a lithium-ion battery, as created by the USB Implementers Forum.
10 seconds with a voltage/current request. Sources operate in constant-voltage or current-limit modes depending on the load; current-limit mode happens only when the sink attempts to draw more than the negotiated current. POWER DELIVERY 3.0 VS 2.0
There are a number of significant differences between the PD 2.0 specification and the latest 3.0 specification: • Power Delivery Profiles (PDPs): Since the PD 1.0 specification, USB PD’s Power Profiles were a way to communicate power-supply capabilities to a USB end user. PDPs are a revamp of Power Profiles that now tell the user what the power supply can deliver in Watts; a higher value means more power. A PDP is the highest nominal voltage times the nominal current reported in fixed Power Delivery Objects (PDOs).
Capacity labels From the USB Implementors Forum. At left, an example of an Assured-Capacity Charger with a total capacity of 60 W and a USB Charger certification of 30 W. At right, an example of Shared-Capacity Charger with a total capability of 60 W and a USB Charger certification of 27 W.
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TE S T & ME ASUREMENT HANDBOOK
• Standards-Based Charging: For years, the USB Implementer’ Forum (USB-IF), which oversees the USB standard, has been working to globally standardize around USB for charging. The International Electro-Technical Commission (IEC) has long since formally adopted USB, and more recently, USB Type-C and PD. • USB PD Multi-Port Chargers: PD 3.0 needed a way to communicate PDPs to multi-port chargers. The standard settled on two categories of charger ports—AssuredCapacity Ports and Shared-Capacity Ports. In the former case, each port is able to deliver its rated/labeled power capacity independent of all other ports. In the latter, each port is able to deliver its rated/labeled capacity depending on the remaining available capacity that’s shared among multiple ports. The total available power capacity of the multiple ports is indicated to the user, and all ports on the charger are capable of delivering the same power rating. So, for example, as long as one port on the charger can maintain 30 W, charger packaging can claim a PDP of 30 W.
• PPS: Another outcome of the USB-IF’s efforts to create an international standard that uses the USB Type-C interconnect is the concept of PPSs. There are two usage models: Powering devices in use with a fixed source and charging of battery-powered devices. The fixed-voltage features were in place with PD 2.0; PD 3.0 introduces PPS, which attempts to make battery charging more efficient. For power delivery to devices, the key characteristics include a fixed-voltage supply, stable current, and the ability to handle dynamic loads. For battery charging, the source must deliver variable voltage and current and account for charging rate and thermal constraints. Most battery manufacturers recommend beginning with a constant current and gradually increasing voltage followed by constant voltage and gradual reduction in current. With fixed PDOs, the charger advertises its maximum output and the sink side chooses the closest fit. Fixed-voltage chargers are typically unable to charge quickly without generating excessive heat. In the PPS methodology, however, the sink “micro-manages” its own charging regime by requesting that the
USB-Type testing matrix Testing required Product type
USB-C CabCon
USB-C EPC
USB PD
Cable
X
X
X
Charger & battery pack
X
Host & hub
USB-C IOP
USB-C source power
USB 3.1 and 2.0
X
X
X
X
X
X
X
X
X
X
PD host & PD hub
X
X
X
Host alt mode only
X
X
X
Device
X
X
X
X
PD device
X
X
X
X
OTG
30
USB-C functional
The USB Type-C compliance test matrix, created by the USB Implementors Forum, indicates a large amount of testing for products to be certified as compliant with the standard. Essentially, everything but cables must be subjected to the Type-C test suite for compliance purposes. If a device is “PD enabled,” it must also pass the PD compliance suite. There is also an interoperability test suite as well as several tests specifically for power sources.
Not compatible with USB Type-C
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P OWER DELIVERY 3.0 Fast role swapping sequence Power DRP
USB PD capable host
Power DRP
DFP
USB PD capable hub UFP
Bus powered accessory
Power source
As described by the USB Implementors Forum, Fast Role Swapping is a new PD 3.0 feature for USB hubs and docking stations.
Power
source make voltage changes in small steps. As a result, charging completes in less time while keeping heat under control, which can also help extend the battery’s lifespan. Once the source reaches predetermined voltage level, the sink re-negotiates by requesting a constant voltage. • New PD 3.0 terminology for PPS mode: Many different devices might be plugged into a given PD charger, but PD aspires to universality. Thus, the PPS approach demands that the sink side be smarter, given that the sink knows how much power it needs. This gives rise to the new term Augmented PDO (APDO) to define chargers that can deliver a range of voltages. Yet, a maximum current still must be defined. PPS sinks will now use what are termed Request Data Objects (RDOs) to make these real-time dynamic adjustments to power. • Fast Role Swap (FRS) Hub: This new PD 3.0 feature specifically targets USB hubs and docking stations. Typically, users plug their docks into a wall outlet; thus, it is the dock that is powering a laptop and any other USBpowered accessories plugged into the dock. If power is lost by, say, the cord being unplugged, users run the risk of losing data being saved to a USB device attached to the dock at that moment.
After a power loss, the sink must be monitoring VCC. When VCC goes low for 30-60 µsec, that constitutes a Fast Role Swap request, and the sink then becomes the source by supplying 5 V to the bus. The key here is that this FRS capability is relevant only for Dual-Role Power (DRP) ports that can alternate between sink and source roles. • Extended Messages: PD 3.0 offers an extended payload size for larger data transfers with packets of up to 260 bytes. Initially, these larger packets were added to allow firmware updates delivered over the PD channel, but their usage was extended into arenas such as security (authentication), battery information, and other data.
Teledyne LeCroy Voyager M310P test platform https://teledynelecroy.com/protocolanalyzer/usb/voyager-m310p
• FRS hubs detect the loss of ac power and quickly perform a “power role swap.” Upon sensing loss of power, the dock speculatively signals FRS by pulling VCC to ground for 60-120 µsec. The dock must have hold capacitors that can maintain Vbus at 5 V for at least 150 µsec after Vbus falls below 5 V. In this manner, the unpowered dock continues to source Vbus on its downstream ports while the Fast Role Swap takes place.
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T HD
Understanding total harmonic distortion measurements THD is the ratio of the sum of the powers of all harmonic
Most electronics engineers and technicians have a good understanding of total harmonic distortion (THD). But there are a few elusive details that come into play during THD measurements.
components to the power of the fundamental frequency. Properly
DAVID HERRES | CONTRIBUTING EDITOR
not by the random frequency, broad-spectrum distortion that is
speaking, the fundamental frequency is the first harmonic, but THD discussions frequently don’t acknowledge this fact. THD considers distortion contributed by second-order and higher harmonics but known as noise. THD + noise is a separate though important metric. The familiar sine wave is comprised of a single frequency, while nonsinusoidal waveforms are made up of two or more sine waves that can be added together on a point-by-point basis moving along the time-domain X-axis. Breaking down a complex non-sinusoidal waveform’s sine wave components is a mathematically difficult process but became practical with the advent of the Fast Fourier Transform in the 1960s. Today, one simply imports the nonsinusoidal signal into a spectrum analyzer or, using Math Mode in an oscilloscope, presses FFT. Then, displayed on the screen in real time, is the signal at the channel input in the frequency domain.
Scopes and THD meters aren’t the only instruments capable of gauging harmonic content. Power analyzers, such as the PA3000 from Tektronix, are optimized for characterizing power sources, including their harmonic content.
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TE S T & ME ASUREMENT HANDBOOK Typical block diagram, THD meter Input
Input attenuator
Impedance matching
Sensitivity control
Bridge amp
Post attenuator
Frequency range select
Wien bridge
Pre-amp
Meter amp
Output
Amplitude, in units of power (dB) rather than volts, displays on the Y-axis and frequency, rather than time, displays along the Y-axis. These are the harmonics that, added together and divided by the fundamental, make up THD. A high THD level in power systems is harmful for the system as well as for connected equipment. Lower THD equates to lower peak currents, higher efficiency and higher power factor. Power factor is generally thought of as determined by the phase relationship between voltage and current, in accordance with: Power Factor PF = cos θv - cos θi, where θv is the phase angle of the voltage and θi is the phase angle of the current. While this equation, known as the displacement factor, is valid when voltage and current are sinusoidal, it does not account for THD in non-sinusoidal circuits, which are prevalent today thanks to the rise of nonlinear loads with abundant harmonics. Loads that include power conversion equipment -- such as ac-dc, dc-ac and dcdc, or nonlinear loads such as fluorescent ballasts -- create a heavy nonlinear
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Before scopes began to double as spectrum analyzers, the typical means of gauging THD was with a fundamental suppression THD analyzer. The instrument input is typically impedance-matched with the rejection circuit via an attenuator and an impedance matcher. This signal is then pre-amplified and sent to a Wien bridge notch filter tuned to reject the fundamental frequency and balanced for minimum output by adjusting the bridge controls. The output is the remaining signal after the fundamental has been suppressed. A feedback loop from the bridge amp output to the pre-amp input helps eliminate any remaining contribution from the fundamental frequency. The output from these blocks is measured, typically using an instrumentation amp driving an analog or digital meter. The voltage at the meter is caused by the harmonic distortion products plus noise.
environment in which harmonics and THD abound. Switching power supplies, now common in office and home, contribute to this mix. This loading modifies the higher-quality sinusoidal power at the utility generator terminals. (Generators do contribute some fifth-order harmonics because of magnetic flux that takes place at the stator slots in addition to non-sinusoidal flux across the air gap.) Scopes and THD meters aren’t the only instruments capable of gauging harmonic content. Power analyzers, such as the PA3000 from Tektronix, are optimized for characterizing power sources, including their harmonic content. VFDs, welders and arc furnaces also generate prodigious amounts of THD. Because harmonic currents are at higher frequencies that the power system fundamental, they see greater impedances. The cause of this strange phenomenon is that greater amounts of higher-frequency current flow near the surface of a conductor. With less usable cross-sectional area, the effective resistance of the conductor rises, resulting in more heat. This is seen in three6 • 2019
phase neutral conductors and transformer windings. When an ac motor is powered by a VFD, it gets a powerful direct dose of harmonics. This is a consequence of the high-speed switching in the VFD inverter section. Most of the ambient harmonics caused by other nonlinear loads in the same building or neighborhood are not much of a problem because they generally get suppressed when the power goes through the dc bus midway through the VFD. These outside harmonics do, however, assault the many autonomous motors that are found in the workplace. For one thing, harmonics create flux distribution in motor air gaps, causing poor start-ups and abnormally high slip in induction motors. A serious problem in motors and generators is pulsating torque, causing losses and mechanical oscillations with harmful heat. Here’s the greatest problem in motors when there is high THD riding on the good power at the input:
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6/11/19 2:03 PM
T HD
Because of the alternating magnetic field, there is a normal temperature rise in the iron core due to eddy current and hysteresis loss. That is a given, and the iron core is by design sufficiently massive to dissipate this heat. But as it happens, the amount of eddy current loss varies with the square of the frequency. When highfrequency harmonics come along, the heat rises dramatically and, as it dissipates, a significant portion migrates into the windings, adding to the excess heat generated there by the unwanted harmonics and further stressing the winding insulation. Hysteresis varies directly with the frequency, not with its square, but still, it adds to the total. Another factor, even more harmful, is a loss within the windings. This source of heat varies with the square of the current (I2R) and the harmonics have a significant negative impact. Additionally, these highfrequency components exhibit harmful skin effect, reducing effective conductor size. If a generator is to supply nonlinear loads, it should be derated because it has higher reactance and impedance than a similar size motor. Combined with high-frequency magnetic flux resulting from the presence of powerful harmonics, they boost the stator temperature. Rotor heating also results from these high-frequency currents. Additionally, harmonics set the stage for often catastrophic transformer failure. Generally trouble-free, transformers without warning may explode as big nonlinear loads abruptly switch on. The problem is compounded in older transformers containing toxic PCBladen cooling oil. Copper and iron losses combine to create a hazardous situation. Eddy current rises when harmonics enter a transformer from line or load. Because eddy current is proportional to the square of the applied current and the square of its frequency, a transformer catastrophe can happen suddenly and without warning. Harmonic current in transformers is a source of electromagnetic interference that can degrade nearby communication circuits. Shielding, increased spatial separation and suppression of the harmonics are used to mitigate these effects. To summarize, Fourier analysis (as opposed to Fourier synthesis), of a periodic signal reveals the harmonic frequencies that are components and integer multiples of the signal. This is where THD appears. The reason that a voltage and its associated current are purely sinusoidal is that they consist of a single frequency. Multiple higher frequency components contribute to the observed THD. A square wave has a great amount of this distortion while a sine wave that is in the real, non-ideal world has a small amount of it. In most cases, that component is not visible in the time domain, but it can usually be observed just above the noise floor in the frequency domain. THD is a constant concern in power systems. Low power factor, higher peak currents and low efficiency accompany high THD. In audio reproduction, a low THD equates to better fidelity. In communications systems, high THD means a potential for interference with nearby equipment and greater power consumption at the transmitter. A THD analyzer can be used to measure the distortion of a eeworldonline.com | designworldonline.com
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waveform in comparison to a distortion-free sine wave. The instrument breaks the wave under investigation into its harmonics and compares each harmonic to the fundamental. An alternate procedure is to remove the fundamental by means of a notch filter, then measuring the remaining signal which will be the THD plus noise. In audio equipment development, a low-distortion arbitrary function generator is used to insert an input into the unit being evaluated. Distortion at constituent frequencies is then measured for comparison of prototypes. In such procedures, crossover distortion for any given THD level is more audible and thus tends to outweigh clipping distortion, which produces higher-order harmonics. Generally, harmonics are beneficial only to the musician, who uses them in a flute or guitar to produce sounds that would otherwise be beyond the capability of the instrument. The best way to mitigate harmonics is to suppress them at the source. An alternative is to create shielding or filters at the equipment that is affected by the harmonics. Then, measuring the amount of THD, the success of these measures can be evaluated.
Tektronix PA3000 power analyzer https://www.tek.com/power-analyzer/pa3000
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Testing LVDS devices at the margins with an AWG Advanced arbitrary waveform generators have simplified the testing associated with communications over low-voltage differential signaling lines.
First introduced in 1994, low-voltage differential signaling (LVDS) has gone on to become widely used in products such as LCD panels, automotive infotainment systems, industrial cameras and machine vision, notebook and tablet computers, communications systems, and more. Unlike other 90s-era standards that have come and gone, LVDS continues to thrive in its traditional applications as well as in newer applications that require high-data-rate transmissions including automotive radar and lidar, industrial, Internet of Things (IoT), and high-energy physics applications.
CHRIS LOBERG |
TEKTRONIX
The need for robust signal margin performance rises as a generalpurpose interface like LVDS is applied in these more demanding settings. This testing requires robust signal generation to test and stress LVDS receivers from R&D through to production. In the past, these signals were generated in number of ways, ranging from use of a data-pattern generator to development of custom FPGAs. Those methods have become outdated with the introduction of arbitrary waveform generators (AWG) which have the channel counts and data rates needed to thoroughly test LVDS protocol and receiver margins. LVDS applications often require a large number of outputs, with over five differential pairs being the norm.
Typical LVDS setup Coupled fields
Driver Current source
3.5 mA
Fringing fields 350 mV 100Ω
Cross section of differential pair
Receiver
LVDS is a differential signaling system defined under ANSI/TIA/EIA-644.
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T E S T ING LVDS
To meet these requirements, the latest AWGs feature highresolution DACs and up to eight analog channels and 32 digital channels in the same box. Multiple units can be synchronized for even higher channel counts. Given the growth in application areas for LVDS, the data signals involved are progressively becoming more complex, just as the use-cases become more varied. The availability of pattern generation software for AWGs simplifies signal creation for a large number of channels. Using this software, or by importing CSV or text files generated from an external tool, designers can quickly and flexibly create the signals they need to determine how well a device is working.
Both analog and digital channels are being used to stimulate an LVDS video display device.
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LVDS BASICS
LVDS is a differential signaling system defined under ANSI/TIA/ EIA-644 that transmits information as the difference between the voltages on a twisted pair of wires. The two wire voltages are compared at the receiver. In a typical implementation, the transmitter injects a constant current of 3.5 mA into the wires, with the direction of current determining the digital logic level. As long as there is tight electric- and magnetic-field coupling between the two wires, LVDS reduces EMI output and susceptibility, making it suitable for challenging automotive and industrial applications. This noise reduction arises because of the current flow in the two wires creating equal and opposite electromagnetic fields that tend to cancel each other. The tightly coupled transmission wires also reduce susceptibility to electromagnetic noise interference because the noise is common mode. An LVDS receiver is unaffected by commonmode noise because it senses the differential voltage – changes in common mode voltage doesn’t affect reception. LVDS transmitters also consume a constant current, placing less demand on supply decoupling and reducing or eliminating phenomena such as ground bounce. The low common-mode voltage of about 1.2 V permits the use of LVDS in a wide range of integrated circuits with power supply voltages down to 2.5 V or lower. The low differential voltage, about 350 mV, enables LVDS to consume little power compared to other signaling technologies. At a 2.5 V supply voltage, the power to drive 3.5 mA becomes 8.75 mW, compared to the 90 mW dissipated by the load resistor for an RS-422 signal. For a thorough evaluation of an LVDS receiver, there are a number of jobs that must take place before a design can move into production. Here is a rundown on what typically takes place testing an LVDS receiver, such as a flatpanel display system:
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TE S T & ME ASUREMENT HANDBOOK
By assigning bit sequences to both analog and digital outputs, both analog and digital outputs are available, important when high channel counts are required.
Margin testing take place through selecting low and high data rates.
Protocol-level testing: Validate functionality with a known-good signal Test how protocol defects affect the receiver performance Digital receiver margin testing: Inter-channel skew margin performance Jitter and SSC margin performance Data-rate margin performance Amplitude margin performance Offset margin performance Duty cycle margin performance Analog receiver margin testing Characterize receiver performance across a wide range of analog pulses
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In addition, an AWG-based test set up is useful for performing environmental margin testing on an ASIC or other device that incorporates LVDS. In this case, the AWG generates a known-clean signal that is sent to the device under test (DUT) sitting in a temperature chamber. The impact of temperature variations is then evaluated using an oscilloscope. For testing applications that involve large numbers of channels, pattern generation software makes it possible to use both analog and digital outputs for as many as 40 channels on a single AWG. This is accomplished by assigning bit sequences to both analog and digital outputs. 6 • 2019
For complex test requirements, pattern generation tools make it possible to create digital pattern waveforms with a variety of custom-defined impairments and distortions. In the case of imported text files with predefined bit patterns, these patterns can be edited and modified within the tool after importing. Periodic and sinusoidal jitter with different amplitudes, frequencies and phases can be added to a base pattern, and skew can be applied between both analog and digital channels. And for LVDS applications, both inter-channel and intra-channel skew can be applied, and LVDS pairs can be created from a single bit pattern by copying and inverting bits. eeworldonline.com | designworldonline.com
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T E S T ING LVDS Typical LVDS test setup Generate known, clean signal Temperature chamber
8-channel AWG
For LVDS environmental testing, an AWG such as the Tektronix AWG5200 generates a known clean signal that is sent to DUT and evaluated by an oscilloscope under varying conditions.
DUT
8-channel oscilloscope
After importing a data sequence, the pattern generation tool allows for receiver margin testing. Before compiling the waveform, users first select the desired data-rates for margin testing. By selecting a low and high data rate, and by specifying the step increment, the software creates a sequence of waveforms that run the digital pattern through a stepped range of data rates. Alternatively, margin testing with skew, jitter, amplitude, and offset adjustments can be realized with individual signal parameter selections. It’s also possible to adjust the amplitude, as well as add skew and periodic jitter. Offset can be adjusted in the AWG after signals are assigned to a given output. PUSH THE LIMITS
Despite the emergence of wireless and fiber optic transmissions, plenty of data transmissions still go through good old-fashioned copper using LVDS signaling. With its combination of high performance, low power consumption and immunity to EMI, LVDS continues to serve in wide range of high-performance applications. True margin and stress testing of LVDS devices involves the use of complex waveforms such as bio signals for medical applications, automotive test bus signal simulations and telecommunications network testing as well as the need to add a range of impairments. The recent availability of high-channel count AWGs coupled with pattern generation software significantly simplifies the task of thoroughly testing LVDS devices.
Tektronix AWG5200 https://www.tek.com/arbitrary-waveform-generator/awg5200
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TE S T & ME ASUREMENT HANDBOOK
Remote Communication with USBTMC Simple connections employing USB can replace more expensive GPIB setups in test instrumentation.
Engineers from a variety of industries
AARON FERNANDO B&K PRECISION CORP.
test and measurement instruments from
use USB and USBTMC (USB Test & Measurement Class) to remotely control a host computer. It is useful to compare USBTMC to the widely used GPIB interface. The process of setting up an instrument for remote communication using USBTMC is relatively simple.
Host PC with USBTMC driver (VISA software)
USB cable
Typically, end-users are looking for a simple way to connect test instruments to a computer for automated control and measurement recording. For many years, these connections typically followed the IEEE 488 format. This is a short-range digital communications eight-bit parallel multi-master interface bus specification first created as HP-IB and now commonly called GPIB. In recent years, it has also become attractive to make simple test-instrument
USBTMC compliant device
VISA write VISA read
The USBTMC communication setup using a PC with supporting VISA software and a standard A-to-B USB cable as it would take place with a B&K 9115 Multi-range DC Power Supply.
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U SBT MC Getting started with USBTMC communication
A view of the B&K 8600 DC Load Rear Panel shows the USBTMC marking.
connections via USB. Although USB provides the hardware, it is the TMC (Test and Measurement Class) driver that standardizes the process of initializing and programming test instruments remotely. The USBTMC driver provides simple plug-and-play operation and GPIB-like communication at a relatively low cost. Remote communication is made possible by a standard USB cable where data transfers in a binary and/or ASCII format. The TMC driver on the PC and the instrument TMC firmware provide two-way communication for reading and writing instrument commands. Unlike USB raw connections, the TMC driver does not require any complicated setup or configuration procedures. The only requirement for plug-and-play operation with a USBTMCsupported device is installation of a VISA (Virtual Instrument Software Architecture) I/O library which contains all the necessary drivers for remote communication. Input and output functions are handled with the VISA write command strings such as “*IDN?” and the VISA read response string from the instrument. WHY USB/USBTMC?
The application determines whether or not a USBTMC scheme makes sense. For simple benchtop or laboratory applications, USBTMC is often the preferred interface for remote communication. In addition to USB and LAN interfaces, GPIB also remains widely used, especially in larger ATE systems. Although GPIB has been the staple interface for over 40 years, its cost has remained rather high. Compared to GPIB, the ubiquitous use of USB by system and computer manufacturers make it commonly available and easily accessible. Most modern PCs have at a minimum of four to six standard USB interfaces. In contrast, GPIB takes place via controller cards added to a PC and can cost an additional $200 or more. Alternatively, USB-to-GPIB controller Integration adapters range from $500 to $1,000. The GPIB cables alone (depending on length), cost anywhere from $80 to over $300. All in all, GPIB can be an expensive proposition for simple benchtop applications. Most instrument manufacturers, including B&K Precision, now provide
A comparison of GPIB and USBTMC shows why the latter standard is becoming more widely used.
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B&K — Test and Measurement HB 06-19.indd 41
USB connections and support for their equipment, making USBTMC a more attractive option. Nevertheless, both interfaces have their own advantages and disadvantages. Specifically, consider the approximate cost for a five-meter cable – a mere $6 for USB compared to $170 for GPIB. USBTMC is built upon the USB 2.0 standard which supports a theoretical bandwidth of up to 480 Mbps (60 MB/sec). However, with isolation it is de-rated to 12 MB/sec. This speed is still much higher than GPIB at 1 MB/sec. USBTMC-compliant instruments are also backwards-compatible with GPIB protocols that comply with IEEE 488.1 and IEEE 488.2 standards. These standards are important because they define a set of instrument-specific commands known as Standard Commands for Programmable Instrumentation (SCPI). In a nutshell, USBTMC requires little effort to setup thanks to its true plug-and-play operation, especially useful in cases where instruments are often interchanged. Rated for a longer cable length, GPIB does still have some advantages over USB. The higher noise immunity and ruggedness of GPIB connectors appeals to many engineers working around large ATE systems and numerous arrays of test racks. And sometimes engineers are more comfortable with GPIB and cannot justify spending the time or money to replace existing GPIB systems. Many manufacturers, including B&K Precision, label some USBcompliant instruments ‘USBTMC’ directly. Other instruments labeled ‘USB’ or ‘Device’ might still be USBTMC compliant, so it’s best to check the instrument specifications or documentation to verify compatibility.
USBTMC USBTMC (USB (USB 2.0) 2.0)
GPIB
Plug-and-play auto-detection
Requires setup procedure
Typical latency
125 μsec
300 μsec
Bandwidth (transfer speed)
60 MB/sec
1 MB/sec
Max cable length
5m
20 m
Approximate cost for 5m cable
$6
$170
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TE S T & ME ASUREMENT HANDBOOK General comm model for USBTMC interface device
Control endpoint (required)
Bulk-OUT endpoint (required) Host
USBTMC interface
Bulk-IN endpoint (required)
Interrupt-IN endpoint (optional, subclass may require)
The initial setup of a USBTMC instrument only requires VISA software installation. NI-VISA is widely used software provided by National Instruments Corp. that implements this architecture. This package also includes the Measurement & Automation Explorer (NI-MAX) which allows for auto-detection and remote communication with test instruments from a PC. Once NI-VISA is installed on the computer, the connection and powering-up of any USBTMC device would trigger the automatic installation of the correct instrument drivers. Check the device manager on a Windows system to verify a resource called “USB Test and Measurement Device” is available. A common problem faced by Windows users is the automatic download of incorrect drivers as a result of not first downloading and installing VISA software before connecting the instrument to the computer; be sure that the instrument communication setting is set to USBTMC. Moving forward, test and measurement manufacturers will continue to incorporate USB and USBTMC as a standard interface. GPIB connectivity is now commonly offered as a more expensive option and is reserved for larger test systems. For simple benchtop applications, the combination of plug-and-play connectivity, low cost, and GPIB emulation combine to make USBTMC a more widely used option for test and measurement equipment. In that regard, B&K Precision moderates a github repository resource where users can find code samples in a variety of languages, LabVIEW drivers, and USBTMC utilities for Linux environments. Users can also find a full step-by-step tutorial video on installing the required software and setting up an instrument for remote communication using USBTMC on the B&K Precision YouTube page.
The general communication model for a USBTMC interface. USBTMC client software must be able to support the endpoints shown. A USBTMC subclass specification may make some of the endpoints optional. The control endpoint is required by the USB 2.0 spec. The Bulk-OUT endpoint is required and is used to provide a high-performance, guaranteed delivery data path from the Host to the device. The Host must use the Bulk-OUT endpoint to send USBTMC command messages to the device, and the device must process the USBTMC command messages in the order they are received. The Host must also use the Bulk-OUT endpoint to set up all transfers on the Bulk-IN endpoint. The Bulk-IN endpoint is required and is used to provide a high-performance, guaranteed delivery data path from the device to the Host. The Host must use the Bulk-IN endpoint to receive USBTMC response messages from the device. The Interrupt-IN endpoint is used by the device to send notifications to the Host. A USBTMC subclass specification may require an Interrupt-IN endpoint. If the interface descriptor has bInterfaceProtocol = 0, then no subclass specification applies and the USBTMC interface is not required to have an Interrupt-IN endpoint. The Host USBTMC driver may optionally support additional endpoints if the endpoints are required by a USBTMC subclass specification.
B&K Precision Corp. www.bkprecision.com
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TE S T & ME ASUREMENT HANDBOOK
How to measure current and energy use accurately General-purpose test equipment can have trouble measuring small currents, particularly if they only happen briefly. New specialized instruments go a long way toward solving this problem.
The direct measurement of current is difficult. It’s usually impractical to count individual electrons, so the two primary techniques for measuring current actually measure “side-effects” of current. The first technique uses the fact that moving charged particles create a magnetic field (Ampère’s Law). The second technique uses the fact that charged particles moving through resistance create a voltage (Ohm’s Law). Both these techniques can be derived from Maxwell’s equations.
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MATT LIBERTY | JETPERCH LLC
The fact that current creates a magnetic field was first discovered by Ørsted in 1820 using a compass. This technique was later refined into the modern galvanometer. Most modern galvanometers have a permanent magnet and a pivoting coil of wire. In a typical D’Arsonval/Weston type galvanometer, current flowing through the coil pushes towards or away from the permanent magnet. The coil magnetic field is counteracted by the permanent magnet which forces the coil to twist, moving the pointer. If you have ever seen analog multimeters or vintage stereo equipment, you have likely seen a galvanometer.
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Selected Fluke 87 specs Range
Resolution
Accuracy
Burden voltage
10 A
10 mA
±(0.2% + 2)
30 mV/A
6A
1 mA
±(0.2% + 4)
30 mV/A
400 mA
0.1 mA
±(0.2% + 2)
1.8 mV/mA
60 mA
0.01 mA
±(0.2% + 4)
1.8 mV/mA
6 mA
1 μA
±(0.2% + 2)
100 μV/μA
0.6 mA
0.1 μA
±(0.2% + 4)
100 μV/μA Table 1
The magnetic field caused by current flow can also be measured using Hall-effect sensors. Changing magnetic fields caused by ac currents can use a sense coil inductance that will measure the rate of change of coil current, which can then be processed to yield a figure for ac current flow. A point to note is that with all these techniques, it becomes more difficult to measure the magnetic field with sufficient precision as the current becomes smaller (under 1 mA). That brings us to shunt resistors. A resistor placed in the path of current produces a voltage according to Ohm’s Law: V = I × R, or I = V /R when solved for current. If the resistance is known and we measure voltage across the resistor, we can compute current. Resistors used for current measurements are called shunt resistors. Most modern ammeters and DVMs measuring current use shunt resistors. The best part about this approach is that we can select a shunt resistor value that gives us a suitable voltage range! A shunt resistor is also called a “current-sense resistor,” or simply “sense resistor.” By design, shunt resistors cause a voltage drop, also called burden voltage or insertion loss. If this voltage is too large, it affects the load. The additional resistance also changes the source impedance as seen by the load, which can cause some load circuits to behave differently. Ideally, the shunt resistance would be so small that it would not affect the target circuit. Practically, the shunt resistance has to create a measurable voltage. It’s difficult to measure a large current range with a single shunt resistor. The voltmeter has a fixed range. To expand the range, most ammeters use multiple shunt resistors, each with different resistances. However, if the current changes over time, a shunt resistor that is too large can cause an excessive voltage drop that affects the behavior of the target circuit. If the shunt resistor is too small, it cannot accurately measure the current. Multimeters are well-suited for measuring currents that are constant, either as direct current or “constant” RMS alternating The recently released Joulescope is designed to automatically handle wide current ranges and rapid changes in energy consumption, while allowing the target device to run normally. This instrument displays data via a connection to a PC and accurately measures electrical current over nine orders of magnitude from amps
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CURREN T ME A SUREMEN T S current. Multimeters cannot easily measure currents that vary rapidly or that change dramatically over time. Most ammeters, including those in multimeters, have significant limitations including: Burden voltage: The voltage drop (also called insertion loss) across the ammeter which results in a lower voltage being delivered to the device under test. Leakage current: The amount of current diverted through the ammeter and not delivered to the device under test. Bandwidth: The response of the measurement in the presence of a time-varying signal. For target devices that use a positive dc supply, the bandwidth relates to the change in load presented by the target device. Dynamic range: The variation between the minimum current and the maximum current used by the device under test. Consider the specifications for a well-known, quality hand-held multimeter, the Fluke 87 (See Table 1). The specification is silent on leakage current. The dc bandwidth is on the order of 1 Hz. The ac bandwidth has much worse performance (±1%) and the bandwidth is 45 Hz to 2 kHz. Now suppose we connect the multimeter to estimate the energy consumed by a target device. Further suppose the target device periodically takes sensor measurements and reports them over RF. The target device must take the measurement from the sensor, send the measurement over RF and then go back to sleep, a typical sequence for IoT devices. In our simple example, the target device has three states: radio, active and sleep. To estimate the total energy consumption, recall energy is the integral of power over time (P = I × V, E = ∫ P dt). For constant power, the integral can be simplified to just the term for power multiplied by the time duration, E = P × t. The classic way to estimate energy is to first measure the duration of each state, often either via an oscilloscope inspecting the voltage across a fixed shunt resistor or with a logic analyzer inspecting bits set by the microcontroller. You can then force the system into each state and directly measure the current using the multimeter. Suppose the device uses a 3.3-V supply and we found the device drew 200 mA during its 50 msec radio state, 50 mA during its 100 msec active state, and 1 µA during its sleep state where it spends the
down to nanoamps. This wide range allows accurate and precise current measurement for modern devices where sleep modes are often just nanoamps (nA) or microamps (μA). The Joulescope also has a total voltage drop of 25 mV at 1 A, allowing the target device to run correctly. Joulescope’s extremely fast current range switching
6 • 2019
maintains a low voltage drop even under rapidly varying current demands. Via a connection to a PC, Joulescope reports cumulative energy consumption along with real-time current, voltage, and power. The multimeter view shows the most recent value while an oscilloscope view allows you to explore changes over time.
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TE S T & ME ASUREMENT HANDBOOK
Typical ammeter test setup
Ammeter Multimeters generally measure current via an ammeter included in their functions. The most common ammeter implementation places a current shunt resistor in series with the load.
rest of its time. We can estimate the energy consumed by the target device per sensor measurement as: Energy = (3.3 V × 200 mA × 0.050 sec) + (3.3 V × 50 mA × 0.100 sec) + (3.3 V × 1 µA × (measurement interval - 0.15) sec) If the device takes one sensor measurement hourly, the total energy per sensor measurement is then E = 0.033 J + 0.165 J + 0.0119 J So, how accurate is this estimate? Start with the first radio state energy estimate. The Fluke 87 states that the burden voltage is 1.8 mV/mA in the 400-mA range. For our measurement, we have 1.8 mV/mA × 200 mA = 360 mV drop Due to the voltage drop across the multimeter, the target only receives 2.94 V rather than the supplied 3.3 V. This reduced voltage may have caused unexpected behavior, such as brown outs, on the target during the measurement. If the target contains a dc-dc converter, the measured current will be higher than in the final product. We would reduce the burden voltage by setting the multimeter to a larger range at the expense of resolution. Fortunately, the active state uses a current range with the same burden voltage specification as the radio state. If the active state was only 6 mA and the multimeter was on the 6-mA range, the burden voltage would be 0.6 V! Because the multimeter switches current-shunt resistors with the current range setting, reducing current does NOT necessarily reduce the burden voltage.
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Device under test
+
-
±(0.2% × 1 µA + 4 × 0.1 µA) = 0.402 µA This amounts to 40% error on 19.4% of the total budget, or 7.7% total error! Bench multimeters usually provide additional resolution and accuracy. However, the burden voltage specifications are similar and often in the range of 0.7 V for fullscale measurements. You can sacrifice the additional resolution to provide a reasonable burden voltage. A multimeter is a vital tool, but the operator must be constantly aware of the burden voltage and resolution. Developers must be diligent in performing this tedious task regularly. In practice, developers using multimeters to measure power rarely perform testing frequently. Infrequent testing allows the product to collect “surprises” that are not discovered until late in the design cycle. OSCILLOSCOPES
Oscilloscopes sample voltages at regular intervals, often over a million times per second, to construct a voltage waveform. Oscilloscopes then display a graph showing changes in voltage over time. By measuring
the voltage over an external shunt resistor, oscilloscopes can effectively display changes in current over time. However, current measurements via scope have two primary challenges. First, the shunt resistor measurement technique has the dynamic range issues associated with shunt resistors. Oscilloscopes usually trade-off speed for limited dynamic range and typically have just 10 or 12 bits of dynamic range. Second, oscilloscopes are usually earth-ground referenced. The oscilloscope measures the voltage difference between earth ground and the signal. However, we want the differential measurement across the shunt resistor. Introducing shunt resistance into the ground path often causes signal integrity issues. We often want “high-side” shunt resistors on the positive power supply. However, if the test circuit is also earth-ground referenced, we cannot use the oscilloscope’s standard probe to measure the voltage difference across the shunt resistors. We can either use two oscilloscope probes and use a mathematical subtract feature, which introduces additional measurement error, or we can use differential oscilloscope probes, which are often quite expensive. Either way, we are still left with the dynamic range issue.
Joulescope typical connection setup PC running Joulescope app USB connection Target embedded system
DC power supply
This multimeter is also not able to accurately measure the sleep state current, which is 19.4% of the total energy! In the 0.6 mA setting, the accuracy is:
46
Power supply
Joulescope
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CURREN T ME A SUREMEN T S An expanded display of an Arduino current waveform at turn-on, top, revealing details of the turn-on sequence. The bottom display depicts a view of the Arduino current draw that includes the max and min measured current (red lines) and the average current (yellow line). Both displays were generated by a Joulescope which uses switched current-sense resistors to accurately gauge current.
Oscilloscope manufacturers also provide current probes, which are usually just a combined shunt resistor and differential probe. These probes also allow the oscilloscope to get the units right, so you needn’t do Ohm’s law calculations every time you measure current. However, dynamic range is still limited. Oscilloscopes also have current-clamp probes, which are limited to around 1 mA resolution.
change through the resistor. For example, suppose the target system takes 3.3 V and can tolerate a temporary 3% voltage glitch on a 1-A change. If the system has 10 μF of capacitance, the required shunt resistor switching time is:
SPECIALIZED EQUIPMENT
The second drawback is that this approach presents a variable impedance to the target circuit. Some circuits may exhibit unusual behavior to a changing supply impedance. However, we can mitigate this susceptibility by adding decoupling capacitors, which effectively lower the input impedance at the higher frequencies of interest. Most modern electronics already require bypass capacitors, so this drawback is often not a concern when measuring current to target devices. This sort of specialized current-measurement equipment must also account for Johnson-Nyquist noise, the noise any resistor generates. This noise, the voltage measurement accuracy, and bandwidth are the critical design factors. An example of specialized current-measurement equipment that uses this approach is the Joulescope. It switches shunt resistors in approximately 1 μsec on over-range to keep the target device running correctly. It maintains a maximum burden voltage of 20 mV across the shunt resistor for any current up to 2 A. The Joulescope is electrically isolated to avoid any grounding and ground loop concerns. In addition to being an ammeter, a Joulescope simultaneously measures voltage so it can compute power (P = I × V) and energy (E = ∫ P dt).
A variety of other equipment can measure current, sometimes while sourcing or sinking current. Equipment of this type includes electrometers, picoammeters, and Source Measurement Units (SMUs). These products are specially designed to overcome some of the standard multimeter drawbacks, and many do have lower burden voltages and lower input-bias currents. However, the primary drawback is cost. These devices often employ multiple, more complicated, active-feedback ammeter sensing methods. Some current-measurement equipment automatically and instantaneously selects the shunt resistor to keep the voltage in range. This approach maintains a maximum burden voltage while also accurately measuring current. Until recently, this type of dynamically switching equipment was either too slow (introduced too large of a dynamic burden voltage) or expensive. This approach does have two drawbacks, but these can be mitigated. The first drawback is the shunt resistor switching time, especially when the current exceeds the range for the current-shunt resistor value. If the resistor value does not switch quickly enough, the burden voltage becomes excessive and affects the target device. The required switching time can be calculated. A simplified equation, suitable for many practical applications, is:
10 μF × 3.3 V × 0.03 / 1 A = 1 μsec
t = C × ΔV / ΔI where t = resistor switching time, sec.; C = system capacitance, F; ΔV = amount of tolerable voltage change, V; and ΔI = current eeworldonline.com | designworldonline.com
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Jetperch LLC www.joulescope.com
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