Test & Measurement Handbook June 2018

Why 5G is going to over-the-air testing Page 8

Probing small signals on a high-voltage bus Page 12

Tips on buying a new oscilloscope Page 18

JUNE 2018

Test & Measurement

HANDBOOK

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TEST & MEASUREMENT HANDBOOK

Will high test instrument costs keep the next Steve Wozniak out of the tech business?

Back

in the heyday of personal computing, it was possible to do state-of-the-art work without making a big investment in test equipment.

To see what I mean, consider the development of the first 5.25-in. floppy disk drive for the Apple II PC. Introduced at the Consumer Electronics Show in early 1978, the hardware design was largely by Apple cofounder Steve Wozniak. The timing signals on the Apple Bus included 1 and 7-MHz clocks. The controller board only carried eight chips which Woz used to fashion a state machine.

I recently had a chance to ask Woz about what kind of test instruments went into developing the floppy controller for the Apple II. He explained, “I ran the state machine at 7 MHz, as I recall, so I only needed an oscilloscope that could work at that speed. But I really didn’t use the scope for this. … I looked at the results and fixed bugs to get what looked like the right 1’s and 0’s coming back. I could write a track and see that I got the same 1’s and 0’s back without any scope. I was almost totally in the digital world on this project…. I had no logic analyzer, but really didn’t have to deal with parallel data outside of simple read and write commands from the Apple II processor….I only had to see that my 7-MHz timing wound up generating 4-µsec. timing, and that was all I needed in a scope. When you make a project small and it’s all digital, your equipment needs are less.”

For Woz to see a 7 MHz signal in 1977, he would have needed a scope with a bandwidth of at least 14 MHz. Instruments of that caliber were easily within reach of small companies back then. A good candidate would have been the

Tektronix 423, a widely used analog scope of the day with a 25 MHz bandwidth and a list price of about $1,600.

Online calculators compute that $1,600 in 1978 had roughly the same buying power as $6,400 today.

The floppy drive for the Apple II was a first-of-its-kind, never-been-done-before item. It is noteworthy that the only test gear necessary to conceive it was a scope that was reasonably priced.

And it is a little scary to think about what kind of instrumentation is necessary for state-of-the-art work now. Consider, for example, what it might take to field something in the 5G telecom area which is expected to dominate IoT applications. Developers probably would need instruments able to generate the IQ waveforms and envelope tracking involved, as well as an oscilloscope with a 1-GHz bandwidth to see the complex baseband IQ signals.

The $1,600 that got Woz up and running in 1977, or even the equivalent $6,400 of today, wouldn’t go far for work in 5G. A look at PXIe modular instruments widely used for 5G illustrates the cost chasm over which developers must leap. To cite a few examples, a 1-GHz PXIe scope from Keysight goes for $12,000. An IQ arbitrary waveform function generator in the same format able to work at 1 GHz runs $22,000. A 500 MS/sec PXIe digitizer, also handy for 5G work, runs $7,800.

It’s tough to look at the cost of this test equipment and not wonder about the fate of two-guys-in-agarage tech start-ups. There will certainly always be entrepreneurs, but the daunting cost of test equipment may prevent them from unleashing the kind of breakthrough hardware technology that came out of Apple in the 1970s.

LEE TESCHLER EXECUTIVE EDITOR

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INSIDE T E S TA N D M E A S U R E M E N T T I P S . C O M

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THE TEST & MEASUREMENT HANDBOOK JUNE 2018

OSCILLOSCOPE B U Y E R ' S G UID E

18

PAG E 7

02 Will high test instrument costs keep the next Steve Wozniak out of the tech business?

08 Why 5G is going to over-the-air testing Cables and connectors may become artifacts of the past for communication electronics as the world moves to 5G standards.

12

Probing small signals on a high-voltage bus Subtleties of scope probes can lead to garbled measurements when examining power circuits.

18

Tips on buying a new oscilloscope Answers to a few basic questions can help clarify what kind of scope makes sense for your workbench.

20 You passed: Getting products through EMC/EMI compliance tests

Pre-compliance tests run in your own lab can avoid bad news when products go into formal conformity checks.

24 Better instrumentation for insulation testing

Megohmmeter measurements can now be more accurate, rapid and safer than ever before.

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30

30 Infrared cameras for PCB development and diagnosis

IR cameras can help size up the surface temperatures of electronic components, PCBs, and other objects to reveal unseen problems.

34 Functions and features of advanced hipot testers

Modern instruments help check whether manufactured equipment meets the proliferating variety of international safety standards that govern factors such as grounding and the ability to withstand overvoltages.

39 Interactive debugging on a budget

The cost of undertaking multi-domain debugging has come down. New instrumentation can timecorrelate RF and embedded signals to let designers quickly uncover the root causes of knotty problems.

44 Interoperability testing for the Internet of Things

The Internet of Things might also be thought of as the internet of interoperable things. A systematic testing regime can help uncover problems when devices work in concert.

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ONLINE RESOURCE

Oscilliscope Buyer’s Guide

WWW.PRODUCTS.DESIGNWORLDONLINE.COM/PRODUCTS/OSCILLOSCOPES.HTML

It can be frustrating wading through tons of product information when selecting an oscilloscope. Our oscilloscope buyers’ guide gives instrument users a handy way to compare scope offering and make side-byside comparisons. This online guide will help you become familiar with different types of scopes, common accessories and features.

We polled major scope manufacturers to get the latest prices, specifications, and features that let scope users make intelligent evaluations of instrument brands. Our online guide can be found at https://products.designworldonline.com/ products/oscilloscopes.html. It is interactive and includes data about triggering and

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Buyer's Guide T&M 6-18 V1.indd 7

automated measurement capabilities And it is updated continually to make it an even more useful scope selection tool.

Scope qualities covered in the online guide include instrument form factor, bandwidth, sampling rate, number of channels, memory depth, waveform update rate, display size, vertical and horizontal resolution, vertical sensitivity, time base range, footprint size, operating system, and instrument price.

Scope manufacturers included in the online guide include B&K Precision, Cleverscope, Fluke, Keysight Technologies, National Instruments, Picoscope, Rigol, Rohde & Schwarz, Siglent, Tektronix, Teledyne LeCroy, and Yokogawa.

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TEST & MEASUREMENT HANDBOOK

Why 5G is going to over-the-air testing SHERI DETOMASI KEYSIGHT TECHNOLOGIES, INC.

Cables and connectors may become artifacts of the past for communication electronics as the world moves to 5G standards.

5G

PHASED ARRAY PRINCIPLE

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emphasis on OTA testing in 5G? First a little background. The 5G vision set forth by IMT-2020 is amazing. Conceived by the International Telecommunication Union Radiocommunication Sector, this term stands for International Mobile Telecommunication system with a target date set for 2020 (for standardization).

It opens many possibilities for consumers, the environment, humanity, health and safety. Virtually every industry will be transformed, and new ones will emerge. Three defined use cases represent the foundation for the 5G specifications: enhanced mobile broadband (eMBB) to support extreme data rates, ultra-reliable lowlatency communications (URLLC) for near instant communications, and massive machine-type OF OPERATION communications (mMTC) for massive IoT interconnects.

It will take new technologies to enable the 5G vision. The 5G New Radio (NR) specification adds new operating bands in both sub-6-GHz and millimeter-wave (mmWave) frequencies to extend the available spectrum. 5G NR identifies almost 10 GHz of new spectrum in sub-6 GHz and up, to mmWave frequencies that will be implemented on a country-by-country basis.

FR1 (Frequency Range 1): 400 MHz to 6 GHz adds 1.5 GHz of new spectrum in frequency bands: 3.3 – 4.2 GHz, 3.3 – 3.8 GHz, 4.4 – 5 GHz FR2 (Frequency Range 2): 24.25 to 52.6 GHz adds 8.25 GHz of new spectrum in frequency bands: 26.5 -- 29.5 GHz, 24.25 -- 27.5 GHz, 37 – 40 GHz. Initial target frequency bands are 28 GHz and 39 GHz. A phased array antenna is formed by an FR2 mmWave frequencies array of smaller antenna elements. up to 52.6 GHz can have channel bandwidths up to 800 MHz when

is rapidly approaching and there is a lot of talk about over-the-air (OTA) testing – that is, testing without the DUT physically connected to the test equipment.

I’ve seen test vendors show off their equipment with multi-element antennas attached, or even fully operational anechoic test chambers featuring automated OTA testing of wireless devices. Equipment vendors and operators now promote OTA test solutions as well. This scenario differs dramatically from 4G where most tests used cables. So, what is driving the

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WHY 5G

HOW PHASED-ARRAY FIELDS CHANGE WITH DISTANCE carrier aggregation is used to combine multiple component carriers. This additional spectrum is essential to enabling 5G’s promise of extreme data rates of 20 Gbps in the downlink (DL) and 10 Gbps in the uplink (UL).

While mmWave frequencies enable more bandwidth, they also expose signal propagation issues that are not a problem at sub6-GHz frequencies. Signal propagation issues such as increased path loss, delay spread, and blockage make it more difficult to establish and maintain a wireless communications link between a mobile device and base station.

At mmWave frequencies, the placement of a user’s hand on a mobile device or the orientation of their body can significantly degrade the radio link performance. Signals also experience more attenuation going through different types of materials, which can limit outdoor-to-indoor coverage. Non-lineof-sight (NLOS) scenarios can also incur delay spread.

To overcome these signal propagation issues, 5G radio systems will utilize multiple antennas on base stations and mobile devices to implement spatial diversity and beam steering techniques to reliably direct narrow beams in a specific direction.

In sub-6-GHz frequency bands, massive MIMO (multiple-input, multiple output) technologies will be deployed to boost cell capacity and realize the

Beam properties at different distances from the antenna array.

throughput envisioned by eMBB use cases. Massive MIMO will use a much greater number of antenna elements on the base station directed to multiple user devices simultaneously in the same frequency and time resource, also known as multi-user MIMO (MU-MIMO). With the large number of antenna elements, massive MIMO antenna designers will be challenged with the ability to visualize, characterize, and validate the antenna array beam patterns.

WHY OTA TESTS? In that 5G will utilize multiple antennas for massive MIMO and beam steering, consider what’s driving the need for OTA tests: Increasing density of multi-element antenna arrays – Phased-array antennas are becoming a preferred technique for mobile communications.

These antennas can use changes in

HOW RADIATING APERTURES AFFECT PATH LOSS AND FF-DISTANCE D (cm)

Frequency (GHz)

Far-field Distance (m)

Path Loss (dB)

5

28

0.5

54

10

28

1.9

66

15

28

4.2

73

20

28

7.5

78

25

28

11.7

82

30

28

16.8

85

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relative phase and amplitudes of a signal applied to each antenna element to create narrow beams and dynamically steer the beams in a desired direction. This is also known as beam steering.

Phased-array antennas will be used in sub-6-GHz frequency bands to boost capacity and in mmWave frequency bands to overcome path loss issues. 5G NR (New Radio) Release 15 specifies up to eight layers, or streams of data, in the downlink, and up to four layers in the uplink.

While this is not an improvement over 4G LTE-Advanced, expectations are that 5G base stations may employ hundreds of antennas for massive MIMO, and devices will implement many more antennas than are in production today.

Due to the number of antenna elements configured as narrow beams, it will be difficult to fully characterize and validate beam performance. Designers will need to measure beam patterns both in 2D and 3D and understand beam width, side lobe levels, null depths, and symmetry. These types of tests must use OTA test methods.

Shrinking size of mmWave components -- Higher frequencies imply shorter wavelengths. Multi-antenna arrays at mmWave frequencies will help overcome signal propagation issues and deliver directional antennas with higher gain. With shorter wavelengths, antenna elements can be spaced more tightly, resulting in

Estimated far-field distance and path loss for different radiating apertures.

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TEST & MEASUREMENT HANDBOOK INDIRECT FAR-FIELD COMPACT ANTENNA TEST RANGE extremely compact arrays. Many vendors even opt to develop arrays that integrate into ICs. Of course, highly integrated ICs have no place to probe and no place to put connectors.

A consequence of this integration is it has become impractical to use traditional RF connectors between the radio circuit and the antenna, bringing the need for OTA tests.

HOW ARE OTA TESTS PERFORMED? OTA tests typically take place in either the near-field or far-field regions of the antenna array. The characteristics of the transmitted electromagnetic (EM) wave change depending on the distance from the transmitter. As the signal propagates from the antenna array, the signal becomes more developed. The amplitude of the peaks, sidelobes, and nulls in the radiation pattern evolve towards the farfield pattern.

Close to the antenna, typically on the order of a few wavelengths or less, resides the reactive near-field. The far-field, or Fraunhofer distance, begins at 2D2/λ, where D is the maximum diameter of the radiating elements and λ is the wavelength. Some communication systems, such as near-field communications (NFC) or radio-frequency identification (RFID), use the near-field region for communication.

However, 5G cellular communication links must be evaluated using far-field assumptions.

Measurements such as radiated power can take place in the near-field. However, the far-field beam pattern is not fully formed in the near-field. Some near-field scanning techniques, using a Fourier transform of the near-field pattern, can be used to predict the

far-field pattern. However, measurements in the reactive near-field are not as accurate because the receiving antenna can interact with the transmitting antenna and degrade measurement results.

We can calculate the far-field distance for a typical 5G mmWave device. Assume a 15-cm radiating antenna element (D) operating at 28 GHz; from the above equation, it would have a far-field distance of 4.2 m and a path loss of approximately 73 dB. This kind of distance introduces new challenges in design and test. RF parameters such as transmitted power, transmit signal quality, and spurious emissions are measured in radiated transmitter tests. Such tests are more difficult as the far-field distance lengthens and the path loss grows. To make matters worse, path loss deteriorates as the radiating element dimensions (D) get larger or the frequency rises.

It takes different instrumentation setups to gauge RF parameters as designs progress from early R&D through conformance and manufacturing tests. During the prototyping phase, designers must characterize the performance of the chipsets, antennas, and devices

TYPICAL NFTF MEASUREMENT SETUP OF EIRP/TRP

DIRECT FAR-FIELD TEST SETUP

As defined by the 3GPP, the test setup for the traditional DFF test method (left), the IFF test method based on a CATR (top), and the near-field with transform (NFTF) test method (directly above) for EIRP/TRP measurements (used with permission).

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WHY 5G

TYPICAL 5G CHANNEL SIMULATION SETUP in a controlled over-the-air environment. Engineers will need to characterize and evaluate their designs to meet the minimum requirements specified by the 3GPP (3rd Generation Partnership Project, the group developing and maintaining 5G standards) before releasing their device to the market.

In 4G, radiated tests are required for safety (Specific Absorption Rate - SAR), electromagnetic compatibility (EMC), and more recently to validate MIMO throughput. Most other tests take place in a cabled or conducted environment.

Moving to 5G, the low-frequency tests will be like 4G tests. But for mmWave, the following tests now must take place using OTA methods:

RF performance – Minimum level of signal quality Demodulation – Data throughput performance RRM – Radio resource management – initial access, handover and mobility Signaling – Upper layer signaling procedures Manufacturing test – Calibration and validation of performance

Standards committees have yet to define many of these tests. RF radio transmission and reception requirements for NR user equipment (UE) and base stations, and conformance tests, for example, will be specified in the 3GPP 38 Series.

COMPARING OTA TEST METHODS OTA tests will be critical to developing, validating, and commercializing 5G NR devices. A typical OTA test will involve an anechoic chamber, different probing techniques, and test equipment to generate and analyze the radiated signals in a spatial setting. The anechoic chamber provides a non-reflective environment with shielding from outside interferences so radiated signals of known power and direction can be generated and measured in a controlled environment.

To date, the 3GPP has defined three permitted test methods: direct far-field method (DFF), indirect far-field method (IFF), and near-field to far-field transform (NFTF). In the DFF method, the DUT mounts on a positioner that rotates in azimuth and elevation to enable measurement of the DUT at any angle on the full 3D sphere. The range length of the chamber is determined by the Fraunhofer far-field distance mentioned earlier. The IFF test method is based on a compact antenna test range (CATR) and uses a parabolic reflector to collimate the signals transmitted by the probe antenna to create a far-field test environment in a much shorter distance than the DFF method. The NFTF method samples

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the phase and amplitude of the electrical field in

An example setup to reconstruct realistic channel conditions for the signals transmitted/ received by a base station or device emulator.

the near-region and uses a Fourier transform to predict the far-field pattern.

The best test method depends on the radiating DUT antenna size and configuration.

While the IFF CATR method can be used for the three current DUT categories identified by 3GPP, the excessive path loss with larger radiating elements limit the DFF method to DUTs having radiating antenna elements smaller than 5 cm.

Both DFF and IFF methods can be used for RF parametric tests to characterize beam patterns and validate beam steering.

The main differences are in the required size of the chamber and the associated path loss. The CATR method is most flexible with DUT size and frequency requirements, and the small chamber involved can be used in lab environments.

To understand device performance in real-world conditions, developers test a device’s end-to-end system performance while including impaired signals. This can take place using a PROPSIM 5G channel emulator. A channel emulator simulates real-world signal impairments including path loss, multi-path fading, delay spread, and Doppler shift.

To ensure accurate and repeatable measurements through a design cycle, new OTA test methods are being studied and approved by 3GPP. Direct far-field, indirect far-field, and near-field to far-field transformation has been approved by 3GPP to date. As 5G goes mainstream, OTA test methods will be critical for moving from R&D to design verification, through conformance and into manufacturing.

REFERENCES Keysight Technologies Inc., 5G solutions www.keysight.com/us/en/solutions/5g.html More information about OTA challenges and test methods can be found in the OTA Test for millimeter-wave 5G NR Devices and Systems White Paper www.literature.cdn.keysight.com/litweb/pdf/5992-2600EN.pdf 3GPP TR 38.310 technical report defining DFF, IFF, and NFTF test methods www.3gpp.org/ftp/Specs/archive/38_series/38.810/

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TEST & MEASUREMENT HANDBOOK

Probing Small Signals on a High-Voltage Bus Subtleties of scope probes can lead to garbled measurements when examining power circuits.

TYPICAL HIGH-VOLTAGE PROBE EQUIVALENT SCHEMATIC

KEN JOHNSON, DAVID MALINIAK TELEDYNE LECROY CORP.

When

an oscilloscope is used for debugging, validation, or device characterization, measurements generally take place with the help of a scope probe. There are several types of scope probes because manufacturers optimize different types for specific applications.

In some cases, though, a given probe may have an Achilles’ heel. For example, high-voltage, active differential probes are excellent for all-purpose uses. But they may not be the best option for a specialized task such as measuring conduction loss in a power-semiconductor device.

One such specialized task is the measurement of small signals riding on a high-voltage bus. Examples of such signals include upper-side gate-drive signals, floating control signals, and sensor signals.

Two key probe specifications determine why a probe is, or isn’t, useful in power-electronics applications: high-voltage isolation and common-mode rejection ratio (CMRR).

High-voltage isolation is the maximum common-mode voltage an attenuating probe can safely handle. In the powerelectronics realm, the maximum common-mode voltage is the dc bus voltage. One must use an isolated probe to properly measure signals floating on the dc bus. That small 3-to-24-V gate-drive signal might be riding atop a 500-V bus voltage, so the probe must be able to safely handle the sum of the two.

Common dc bus voltages include: 500-V dc for 120/240-V ac line inputs; 1,000-V dc for 600-V ac-class line inputs; 1,500-V dc for grid-tied solar photovoltaic inverters and UPS systems; and 6,000-V dc for 4,160-V ac inputs.

Conventional high-voltage-rated probes carry a safety agency rating from bodies such as Underwriters’ Laboratory (UL). The rating indicates a maximum safe common-mode voltage to ensure that neither the oscilloscope, device under test (DUT), or operator can sustain harm.

CMRR is the ability of the differential amplifier to ignore the component of a signal that is common to both inputs. For starters, the perfect differential amplifier does not exist: In the real world, a diff amp cannot remove all the common-mode

A simplified schematic of a high-voltage differential probe or amplifier.

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TEST & MEASUREMENT HANDBOOK HVFO EQUIVALENT CIRCUIT

A schematic rendering of the HVFO high-voltage, fiber optically-isolated probe.

signal. Then, differential probe/lead pairs must be perfectly matched for frequency response, which is difficult to realize with attenuating probes.

The measure of effectiveness of common-mode rejection is CMRR expressed either in decibels or as a ratio of rejected voltage. You might see CMRR reported as 100,000:1 or as 100 dB, but they mean the same thing:

The HVFO probe’s three-lead configuration contributes to its high CMRR performance.

20log10 (VSIGNAL/VMEASURED) = CMRRdB A conventional high-voltage (high attenuation) probe topology has a tough time realizing a high CMRR at high frequencies, but oscilloscope manufacturers do the best they can by binning, sorting, testing, and calibration.

It’s not difficult to get a handle on a given probe’s CMRR in a field measurement.

A DIFFERENT PROBE TOPOLOGY One probe technology -- high-voltage, fiber optically-isolated (HVFO) probes -- optimizes both CMRR performance and high-voltage isolation. To understand why, it is useful to compare the topology of a conventional high-voltage differential probe or amplifier with that of the HVFO probe.

Often, users of conventional probes aren’t aware that they employ high levels of attenuation, or that the probe is grounded. Because these probes are grounded, they use two leads, and current flows in both. On the negative side, current flows because of the influx of commonmode voltage into the probe. On the positive side, it flows due to the commonmode voltage plus signal swing into the high side of the differential amplifier. Thus, the probe is measuring small signals plus the common-mode voltage across the lead capacitance, which results in more probe

X

R • •

Plot of typical CMRR vs. frequency for HVD3106A high-voltage differential probe.

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PROBING SMALL SIGNALS

loading on the device under test, especially at high common-mode voltages.

In addition, the probe pair must be precisely matched both in impedance and in frequency response to maintain CMRR across the probe’s rated frequency range. As noted earlier, this matching is difficult to realize in practice.

In contrast, the HVFO probe topology possesses inherent advantages for maintaining a high CMRR and for minimizing probe loading. The single-ended HVFO topology acts differentially in a way because the whole battery-powered amplifier is floating. As a result, it measures only the small-signal swing. This, in turn, results in only a small load current drawn from the circuit.

The signal lead, being coaxial, does not require matching to realize high CMRR, while the fiberoptic isolation between transmitter and receiver also contributes to the CMRR specification.

One significant difference between the HVFO and conventional high-voltage differential probes is its three-lead configuration: The blue lead is coaxial; its center conductor carries the signal current. The return path for signal current is through the coax cable outer conductor. The green lead connects to measurement reference and to the blue coaxial cable’s outer signal conductor. The black lead also connects to the measurement reference.

Connecting the green and black leads to the same point ensures that the signal and return

Plot of CMRR vs. frequency for HVFO highvoltage, fiber optically-isolated probe.

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TEST & MEASUREMENT HANDBOOK

A simple test will give a reasonable estimation of a probe’s CMRR. Though not highly accurate, it will correlate well enough with the manufacturer’s specifications to serve as a reality check.

The yellow trace is an upper-side gate-drive (VG-E) signal acquired with a Teledyne LeCroy high-voltage fiber-optic probe (HVFO). The signal has an amplitude of about 15 V dc as it swings from low to high. It floats on a dc bus voltage of about 465 V. The blue trace is the same signal acquired with a Teledyne LeCroy HVD3106A high-voltage differential probe, which delivers good CMRR for its class. But here the positive and negative leads connect at the measurement reference location, which, in this case, is the emitter or source location of the upper-side gate-drive device.

its leads (signal, ground, and shield) connected at the emitter.

Thus, the HVD3016A is giving us a CMRR of about 15:1. From the equation 20log10 (VSIGNAL/VMEASURED) = CMRRdB, the CMRR is about 24 dB. An eyeball estimate of the rise time for the upper-side gate-drive signal is about 40 nsec. Using the 0.35/ TRISE rule of thumb gives a bandwidth of about 9 MHz. Compare these estimates to the data published in the HVD3106 data sheet: The data sheet CMRR plot shows about 30 dB at 10 MHz (using the 500x attenuation path required for the high common-mode voltage). Note that characterization of probes for data-sheet plots takes place in highly controlled and optimized conditions. These optimized conditions probably account for the difference between the 24 dB and the data sheet’s 30 dB.

With both the positive and negative leads connected at the emitter, the HVD3106A probe should not pick up any signal.

The fact that it does demonstrates the probe’s inability to separate interference from the true signal of interest (VG-E). A probe with near-perfect CMRR would give a flat line; instead, we get a transient spike with an amplitude of about 1 V. That spike represents measured common-mode interference.

Meanwhile, the magenta trace is acquired using a Teledyne LeCroy high-voltage fiber-optic (HVFO) probe with all three of

currents are equal and opposite at the tip of the commonmode choke located near the amplifier jack. As a result, the current in the black reference wire (shield current) drives the reference voltage for the single-ended amplifier, and it accounts for and minimizes any parasitic capacitance effects.

Note there is no connection to earth ground whatsoever.

Its topology gives the HVFO optically-isolated probe a significantly higher CMRR than that of a conventional highvoltage differential probe/amplifier. An example of the latter, Teledyne LeCroy’s HVD3106A, is an exceptionally good probe for its class; its CMRR plot-vs.-frequency shows CMRR topping out at 85 dB at 60 Hz and dropping to 65 dB at 1 MHz and to 40 dB at 5 MHz. The nearby CMRR plot for the HVFO reflects use of the probe’s 1X tip and shows a CMRR of 140 dB at 60 Hz, 120 dB at 1 MHz, and 85 dB at 10 MHz. This level of performance is what makes the HVFO a candidate for upper-side gate-drive measurements and sensor-voltage measurements, floating incircuit in either case. Similarly, the probe works well for EMI/RFI measurements in situations where a sensor signal from a board gets bombarded with EMI within a test chamber as it travels to the oscilloscope outside of the chamber.

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REFERENCES Teledyne LeCroy Corp. www.teledynelecroy.com

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TEST & MEASUREMENT HANDBOOK

Tips on buying a new oscilloscope JAMES MCGREGOR NEWARK ELEMENT14

Answers to a few basic questions can help clarify what kind of scope makes sense for your workbench.

For

many engineers, the oscilloscope is the most useful and versatile tool on the bench. So when it’s time to buy a new one, it’s important to make sure the scope meets your test and measurement needs. But before you go shopping, it might be helpful to bone up on the latest trends in test and measurement equipment based on what we’re seeing in our T&M unit at Newark element14. Some recent developments may affect the choices you make. And a quick checklist of tips and questions may help you make the right decision.

TRENDS IN THE OSCILLOSCOPE MARKET The oscilloscope market has traditionally divided into two streams: Mainstream scopes that are completely adequate for most applications, and higher-end scopes that offer more sophisticated and precise measurements for research and specialized industrial applications. Within this range of capabilities, most manufacturers have kept prices in a sweet spot between $3,000 and $7,000 per scope.

But more recently, we’re seeing an expanded range of options at both the lower and the upper edge of the market. In addition to oscilloscopes that offer the basics, now priced at $2,000, there are also units priced upwards of $30,000 that allow engineers to take extremely precise measurements.

Generally speaking, this trend shows the T&M industry is reacting to movements in the larger consumer market where customers want and expect more choices.

In turn, this increased range of options in the oscilloscope market amplifies the importance of T&M distributors for equipment manufacturers.

Previously, engineers mostly had a short list of multi-use workhorse devices to choose from. But now, with a much broader range of choices, distributors offer equipment from a variety of manufacturers all in one place, so buyers get a fuller view of the options now available.

In terms of widely used functions, new touchscreen user interfaces on some of the higher-end oscilloscopes make the product more intuitive to use, and they can save users time when learning a new piece of equipment. Touch-screens also speak to a new generation of users—millennials— who expect a more interactive interface than traditional knobs and dials.

The Picoscope 2204 (left) the Tektronix MDO3024 and the Tektronix TBS2104 (at right on facing page) scopes were all in the “best seller” category recently on the Newark element14 site.

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TIPS ON BUYING A NEW OSCILLOSCOPE

Additionally, there is a growing market for handheld oscilloscopes, which suggests that scopes increasingly find use in the field, not just in the lab. There is also a move from fixed-function T&M units to more modular, software-defined instrumentation, in which a standard testing device can change its functions by running software. Some higher priced equipment currently offers software options, but right now the softwaredefined approach to T&M equipment is mostly aspirational, and at the high end of the market. It will be a while until generalpurpose oscilloscopes offer a modular software option.

One final trend observation: From a unit perspective, sales of T&M equipment is growing in North America, from an already strong position. This is consistent with reports that some high-volume manufacturing of consumer products, like smartphones, is returning to the U.S. Reestablishing these factories would require appropriate T&M instruments, which would account for an increase in demand.

TIPS FOR PURCHASING YOUR NEXT OSCILLOSCOPE By answering four questions, you’ll be able to focus on what to look for as you go shopping for a scope.

What is the highest signal frequency you are likely to measure? It’s not enough that your oscilloscope is fast enough to capture the fastest signal you’ll need to measure – it needs to capture signals five times faster. That’s because your scope will need to measure the signal’s fundamental, third and fifth harmonics to accurately display the full digital signal. If you are testing for 100-MHz bandwidth, you’ll need five times that capacity in your equipment, or an oscilloscope rated for 500 MHz. Don’t forget that your scope’s measurement is only as good as the probe you use. It should have the same bandwidth as your scope, or better.

What is the fastest rise time you are likely to measure? Signal rise time is critical information when measuring digital signals; it’s also required for accurate timing measurements. The rise time for an oscilloscope is calculated as (k/bandwidth):

for scopes under 1 GHz, k=0.35, for scopes over 1 GHz, k=0.40 or 0.45. The 5x rule applies here, too. An oscilloscope’s rise time should be one-fifth the fastest rise time of your signals. For example, to measure a 4-nsec rise time accurately, you need a scope with a rise time of 800 psec.

How many channels do you really need? The more channels on your oscilloscope, the more flexibility you’ll have for viewing and debugging circuitry, which can save time. On the other hand, the more channels you buy, the more your unit will probably cost. So plan accordingly.

Two or four analog channels enable you to view and compare signal timings of multiple waveforms. Debugging a digital system with parallel data requires at least an additional eight or 16 channels. A mixed signal oscilloscope (MSO) adds digital timing channels, used to indicate high or low states and display them as a bus waveform. While having more channels than you need may cost a bit more, having fewer than you need will definitely cost you time.

How do you choose where you buy your equipment? When you’ve narrowed your choices and it’s time to make a purchase, here are three top considerations. First, select a distributor with a broad range of devices, with trained sales people who deeply understand customer needs and can support your decision-making processes. Second, make sure the distributor you’re buying from actually has your product in inventory and has it available for next day delivery in case you need it quickly. If your distributor must order the instrument from the manufacturer before delivery to you, it will be a while before you get your scope.

Third, ask if your distributor offers inside technical support free to customers. Your distribution partner should make itself relevant to you by offering a deep level of support and product expertise, all free.

With so many options available, online forums like the element14 community site also can provide reviews and evaluations, as well as provide opportunities to post questions and get advice from peers in the field.

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REFERENCES element14 community site www.element14.com/community/welcome Newark element14 www.newark.com

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TEST & MEASUREMENT HANDBOOK

You passed: Getting products through EMC/EMI compliance tests DYLAN STINSON TEKTRONIX INC.

Pre-compliance tests run in your own lab can avoid bad news when products go into formal conformity checks.

EMI

regulations are in place to ensure reliability and safety for users of electrical and electronic equipment. With few exceptions, any product emitting or susceptible to electromagnetic interference must be certified for EMI compliance. Those who’ve taken a product through EMI certification know that a great deal of time and effort goes into minimizing both EMI signatures and the possibility of failing EMC certification.

To ensure compliance with regulations, many companies have historically employed a specialized test facility, or test house, to perform EMC pre-compliance and compliance testing.

The downside to this approach is that third-party test houses can add significant

development cost and time to a project.

As an alternative, a growing number of design teams now perform affordable pre-compliance testing themselves, in their workspace, throughout the design process.

Pre-compliance testing in-house offers an efficient way to identify and address potential EMI issues before full compliance testing. It also lets developers methodically isolate problem areas and apply appropriate corrections, saving time and money.

RADIATED AND EMISSIONS TESTING The term radiated emissions refers to both the intentional and unintentional release of

electromagnetic energy from an electronic device. A radiated test ensures emissions emanating from the device under test (DUT) or equipment under test (EUT) comply with the applicable limits.

The term conducted emission refers to electromagnetic energy created in an electronic device and coupled to its power cord. As with radiated emissions, regulatory agencies dictate the allowable conducted emissions from electronic devices.

A full compliance test in a certificated lab can be expensive. Costs range from $1,000 to greater than $20,000 per submission depending on the device and the number of countries to be covered.

Full compliance testing can also be time

Good EMC design gets progressively tougher to implement as the development process proceeds.

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YOU PASSED

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Affordable pre-compliance testing can easily be set up to uncover potential problems and eliminate the need for test time in expensive compliance test facilities.

consuming. Emissions and immunity testing generally takes two to six days and another three-to-ten days for generating the final test report. And, of course, this doesn’t include the time spent waiting in the test queue.

To prepare for testing, many companies have historically rented time in full-compliance test facilities.

This, too, can be expensive with costs ranging from $700 for a half day to $10,000 per day depending on the services needed. As an alternative, some firms set up fully compliant test facilities in-house. Certainly, this is a luxurious option – and one few companies can afford with costs ranging from hundreds of thousands of dollars on up.

Fortunately, there is a happy middle ground that significantly reduces the risk of failing EMC compliance testing. For a fraction of the cost of a full test facility, design teams can acquire everything necessary for pre-compliance checks. Modern software tools greatly simplify test set-up and analysis so even non-RF experts can gain meaningful insights.

During development, periodic diagnostics help produce designs that emit little electromagnetic energy and aren’t susceptible to interference. In later development stages, pre-compliance testing can catch compliance problems and improve the chances of passing EMC compliance testing on the first try.

The goal of pre-compliance testing is to mimic the compliance-test set-up well enough to uncover potential problems. Pre-compliance testing does not require test equipment identical to that for compliance testing. It typically involves a spectrum analyzer with a quasi-peak detector, a preamplifier (optional), an antenna with a nonmetallic stand for radiated emissions, a line impedance stabilization network (LISN) for conducted tests, a power limiter for conducted tests, near-field probes for diagnostics (optional), and an oscilloscope with frequency

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TEST & MEASUREMENT HANDBOOK

An ambient trace and a first peak scan compared to the limit line of FCC part 15 Class A up to 960 MHz. The ambient noise is the teal trace and the peak scan is the yellow trace. The table shows the results of peak scan versus the limit line. The results in this example show numerous spurs in the A region as well as a failure. The next scan uses quasi-peak detection across region A (zoomed up to 59 MHz) for more precise results. This technique is more in line with the official testing done at an accredited test house. The figure at right shows the results for peak (green trace) and quasi peak (yellow trace) versus the limit line. In this example, the device now passes with a margin of 1.5 dB. However, this is probably not enough to ensure the device will pass real compliance tests at the test house. and time correlation capabilities to assist in debugging (optional). The latest USB-based spectrum analyzers offer general purpose filters and detectors to support precompliance testing and for troubleshooting any EMI issues.

Pre-compliance testing can employ fast measurement techniques intended to give a quick look at problem areas.

The test site is usually temporary. When selecting a site, it is best to pick a location with an electromagnetic environment as free as possible from external signal sources. Rural areas, conference rooms or basements are good because they tend to see a minimum of signals that might mask the DUT/EUT emission levels you are trying to measure. Other considerations that improve measurement accuracy include having a good ground plane and minimizing the number of metal objects near the test area.

Standards documents spell out the setups for radiated compliance tests. Developers running their own tests try to mimic these setups as closely as possible. In setting up radiated testing, there are several challenges. For one, electromagnetic waves don't extend out from your product in a nice spherical pattern. The emissions tend to be directional.

To address this, a test lab will typically vary the height of the receiving antenna between one and four meters, and it will rotate the EUT on a turntable. The receiving antenna picks up both the signal direct from the EUT, as well as reflections

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from the ground. Additionally, the floor should be covered with an electromagnetically reflective surface (aluminum, steel, wire mesh, etc.) to create a ground plane for measurement accuracy.

And the ground-plane area should be relatively flat. RF receivers then scan EUT in the frequency band of interest while technicians look for emissions close to the limits.

A radiated emission pre-compliance test setup includes your EUT, ground plane, an antenna on a tripod, a pre-amplifier, and a spectrum analyzer. The pre-amp amplifies the output for better sensitivity. The output of the pre-amplifier goes to the input of the spectrum analyzer for analysis, generally with the help of EMC test software.

CONDUCTED TESTS Conducted emission tests consider electromagnetic energy the DUT conducts onto the power supply cord. The goal is to minimize this conducted interference. These emissions (usually from 150 kHz to 30 MHz) must stay within specified limits.

For this testing, it helps to ensure the local power source is relatively clean, and your DUT won’t affect nearby devices.

Conducted emissions testing usually takes place on devices that connect to an ac power supply. For some standards, there are also limits placed on devices that operate from dc supplies.

Conducted measurements use a LISN (line impedance stabilization network), a low-pass filter placed between an ac or dc power source and the EUT. The LISN creates a known impedance and provides an RF noise measurement port. It also isolates unwanted RF signals from the power source. Adding a pre-amplifier is a good way to boost the relative EUT signal levels.

In some cases, the interference conducted on a 60 or 50-Hz power supply can be an issue. While most of conducted

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YOU PASSED

EMI tests specify a measured frequency range of 9 kHz to 1 GHz, it can be useful to measure the signals at lower frequencies when the need arises.

MEASUREMENT PARAMETERS The bandwidth of an EMC measurement is defined by a receiver bandwidth shape or a resolution bandwidth (RBW) filter in the case of a spectrum analyzer. The bandwidths used are representative of the perceived threats within the spectrum, and the bandwidths vary with frequency. The reading of any non-continuous signal by a receiver or spectrum analyzer will depend on the measurement bandwidth used. For consistent results, regulatory agencies define the bandwidth and shape of the filters used in compliance measurements.

The standards also specify how EMI noise must be detected. Most commercial standards use three major detection methods defined by CISPR16-1: positive or negative peak, the RMS or average value of voltage, or the quasi-peak (QP) value.

Compliance labs use QP detection for full compliance tests. The tests usually begin with a scan using simple peak detection to find problem areas that exceed or are near specified limits. Signals that approach or exceed the limits are then quantified with QP measurements. QP detection serves to detect the weighted peak value

(quasi-peak) of the envelope of a signal.

It weights signals depending on their duration and repetition rate. Signals that arise more frequently or last longer will result in a higher QP measurement than infrequent, short impulses.

Thus, the first scan for conducted emission (after determining the ambient electrical noise in the environment) should be via peak detection. It is fast and gives a quick overall perspective of both the noise amplitude margin from the limit and which frequencies are concerning.

To account for inaccuracies in the pre-compliance test setup, it is best to have a sizeable signal margin -- ideally, at least 6 dB. For EMC/EMI pre-compliance testing, it is important to select a spectrum analyzer with software supporting quasipeak, peak and average detection. During product design and testing, there are situations where all three of these tests are necessary for an accurate picture of EMC/EMI compliance status.

While most modern spectrum analyzers are up to the task, one of the more economical and efficient options may be a real-time USB-based spectrum analyzer. In addition, comparisons with swept-spectrum analyzers have shown pre-compliance tests such as quasi-peak can process faster on a USB real-time

spectrum analyzer with a laptop, tablet or desktop computer.

A real-time spectrum analyzer can reveal failures hidden under broadband noise that swept spectrum analyzers are not be able to see.

All in all, tests run with a real-time spectrum analyzer and EMC analysis software help ensure that EMI emissions don’t leave your products short of the finish line.

REFERENCES Tektronix Inc. www.tek.com

The right side shows what a swept spectrum analyzer sees, once it is done sweeping a 40 MHz span starting at 2.4262 GHz. At left is what a real-time spectrum analyzer sees.

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TEST & MEASUREMENT HANDBOOK

Better instrumentation for insulation testing JEFF JOWETT MEGGER

Megohmmeter measurements can now be more accurate, rapid and safer than ever before.

A

considerable number of electronics personnel working throughout the electrical industry are military trained.

And many of them learned on a simple hand-cranked analog meter, properly termed a megohmmeter, or insulation tester. These testers were typically 500-V units that measured to a few hundred megohms and could run a continuity test to perhaps 100 Ω. Test voltages came from an on-board generator powered by the operator cranking a handle, with a rectifier converting the output to dc. A switch provided the option of a high-resistance insulation test or a low-resistance continuity test. Many of these testers are still operable and, provided they are in good repair and calibrated, there’s no reason they shouldn’t be.

Megohmmeters for decades remained quite similar in design and function. Differences came mainly in quality of manufacture. But the revolution in microelectronic circuitry created an explosion in rapid upgrading to newer and better designs. Measurements can now be more accurate, rapid and safer than ever before.

First the basics: A megohmmeter measures the quality of electrical insulation by applying a voltage across the insulation and measuring the amount of current that “leaks” through (hence the term leakage current). Voltages are typically applied at rated operating voltage for routine maintenance or twice-rated and higher for troubleshooting. Currents are minuscule… typically nanoamps…and therefore the tester must have

A technician uses a hand-cranked megohmmeter to check the insulation resistance of motor windings.

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BETTER INSTRUMENTATION

extreme sensitivity. A current of just 5 mA is enough to shock a human. Test voltage and measured current are converted to resistance, in units of millions of ohms (megohms, MΩ). Anything less than a megohm is generally considered unfit for service (exceptions are equipment operating at extremely low voltages and sub-assemblies that will be encased in additional insulation inside of larger equipment).

The original testers did all this, but not much more. Electrical insulation tests determined what needed to be cleaned,

repaired, or scrapped and what could remain reliably in service. Insulation testing is a vital link in fire protection, elimination of costly in-service failures, and assurance of safe operation. Simple instruments can perform these functions quite well, and a certain amount of lore has grown up around them over the decades they have been in service.

The original testers all had analog movements. They had to; there were no microelectronics. The pointers rested on the high end, pegged low at the start of the test due to capacitive charging

currents, then drifted steadily (it was hoped) back to the high end or stopped on a measurement. Many operators became adept at watching the travel and paid less attention to actual numbers.

This skill was hard to teach; it had to be learned and is still practiced by veteran technicians. But analog movements were sensitive and withstood little banging about. They also could suffer from parallax and the operator’s interpretation as to just where the pointer stopped.

LCDs introduced digital measurements. These units generally

Modern megohmmeters: The MIT4002 family from Megger. Note the use of both digital and analog log-scale displays of resistance.

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TEST & MEASUREMENT HANDBOOK

It is still possible to obtain insulation testers containing a hand crank. An example is the MJ159 which also features multiple test voltages for spot and step voltage testing, a guard terminal to eliminate surface leakage current, and a read-out without scale multipliers to avoid possible operator reading errors.

could be dropped and put right back into service, provided they didn’t land directly on the display; a huge bonus in savings of time and cost. Digital measurement also could be extremely precise, down to a percent or two in quality instruments, and required no interpretation. But the pointer travel cherished by the veteran techs was lost.

Then technology came to the rescue once more! Combination displays are available in quality instruments, with an electronic pointer and digital result when it rests. Remember: Look for a logarithmic arc that is expanded for better resolution on the all-important low end of the scale. A mere curved bar graph doesn’t behave like a genuine analog.

Analog technicians got accustomed to good insulation measuring off the high end of the scale, marked with an infinity symbol. This is always desirable, but not always understood.

Infinity is not a measurement; it simply means the insulation is better than that particular tester can measure within its stated parameters. Old original testers might have gone to only 200 MΩ, or more likely 1,000 MΩ (1 Gigaohm). This was enough to weed out bad or faulty equipment. But it didn’t provide much more information.

Quality testers now measure into the Gigaohm or Teraohm (1,000 GΩ) ranges. There are two prime advantages to this expanded range. Insulation resistance drifts slowly and relentlessly down during operation and can act like a car odometer in reverse; the lower the number, the less remaining life.

This behavior can be trended to provide a timeline for maintenance and replacement. Higher numbers enable early warning if resistance is falling rapidly, as from moisture ingress or nearby sources of contamination. Second, manufacturers of insulating materials constantly develop larger cross-linked macromolecules that boost quality and raise early measurement values. The measurement capabilities of testers have had to keep up with such developments.

Finally, you must record the result if your test does go to infinity (over-range) and know how high your tester can measure. Range limit typically rises with test voltage, so be aware of the voltage used and

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the limit of that range. Then record it as greater than that limit (>100 GΩ, for instance). Nothing is a “fail” at range limits.

HAND-CRANK VS BATTERY Line power was not amenable to many of the environments in which testing took place, such as construction sites and remote circuits, so the hand-cranks developed a considerable mystique over the years. When batteries came into use, they reinforced, rather than superseded, the handcrank mystique. Early battery operation was spotty and earned a bad reputation, even to the extent of banishment in some quarters. Batteries could die before the end of a shift, leaving a technician without a tool. Worse, as they lost charge, readings could become regressively less accurate.

By the late 1970s, battery technology had improved enormously, and these problems could be circumvented. A quality insulation tester now can run 2,000 tests with a single set. Furthermore, full specified capabilities are available right up until the LO BAT warning appears. Nonetheless, handcranks have become so entrenched that they continue to enjoy wide use. Veteran operators may insist they can tell something about the quality of a test item by the turn of the generator. But, like the handling feel of a car, this claim is scientifically unquantifiable.

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BETTER INSTRUMENTATION

Insulation testers provide a lot of voltage but have little power. Initially, this may seem counterintuitive, but a little reflection makes it clear. An item under test that passes more than a few milliamps is no longer fit as insulation. Hence, megohmmeters are commonly limited in output to about 5 mA or less. That low level makes the tester essentially safe, but not the test item. Highly capacitive test items (long runs of cable, large windings in motors and transformers) could store enough energy to be lethal. When the test ends and the voltage gradient provided by the megohmmeter ceases, all this stored energy will discharge.

In the past, protection against such difficulties was largely left to good working practices. Some testers had discharge switches, but they could be accidentally overlooked. A rule of thumb was to discharge five times the length of the test; i.e., a ten-minute test was left grounded for fifty minutes before disconnecting, presuming that would be more than enough.

The order of the day now is redundant safety.

Safe working practice is augmented by a discharge circuit in the instrument, complete with audible and visual warnings. The operator need only watch while the progress of the discharge is shown on the display.

Protective circuitry is also in place at the beginning and during a test. If a circuit is energized or becomes energized during the test, modern testers warn the operator and disable testing.

In bygone days, insulation testers were routinely returned for “warranty” repair with burn tracks across the board. Live hookup; operator error; no warranty.

Now, a quality tester senses live voltage and disables testing. It doesn’t stop there. Continuity testing -- the corollary to insulation testing, to assure that circuits are properly connected -- requires a low-impedance test circuit. But the high resistance discharge circuit remains engaged until the tester senses that both leads are connected across a safe, low-resistance circuit.

Old testers once came with a stack of test record cards. The technician would write in the data and sometimes connect the dots to develop a graph. These were often hung on machinery in waterproof jackets. The practice was time consuming and prone to human error.

Modern testers store data with the push of a button; even all the data of a prolonged procedure. Aside from easy storage, this practice also eliminates a lot of dispute with third parties and authorities. Test reports and certificates are printed just as easily. And mathematical calculations, such as temperature correction, take place automatically and without error.

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TEST & MEASUREMENT HANDBOOK

TEST VOLTAGE Insulation tests once took place at a single voltage to which were added a few critical selector choices. But only one read directly. Other measurements had to be corrected by a multiplier or division, which was printed on the selector. Variacs provided infinite voltage adjustment, but only one or two positions read directly. All others had to be corrected by a factor on the scale plate. Eventually, there were multiple selector positions that could all be read directly. They were an enormous help and dominated for about half a century. Now, advanced testers afford direct reading in 1-V increments across the entire range of the tester.

Additionally, testers can measure multiple parameters besides insulation resistance, directly displaying leakage current (the inverse of resistance), frequency, actual test voltage, capacitance, and other parameters. Pass/fail audible indicators can be set up and multiple measurements displayed simultaneously. Standard procedures can run automatically while the operator attends to another task.

International standards provide operating procedures and results analysis. Two of the most important are IEC 61010, which specifies general safety requirements for several types of electrical equipment, and IP (ingress protection) rating. The IEC CAT, or category, rating indicates the level of safety against arc flash/arc blast. Always know the CAT rating of an instrument and apply it accordingly.

Ingress of foreign materials…dust, moisture…won’t be lethal to the operator but can be to the instrument. Enclosures have vastly improved over the old Bakelite and phenolic materials. The IP rating quantifies enclosure performance, indicating objectively and reliably under what environmental conditions the instrument will remain in service. There is even a rating for immersion, although insulation testing generally doesn’t take place underwater.

All in all, the evolution of instrumentation over a century has greatly reduced the possibility for error. But there are still some best practices to keep in mind when running insulation tests: Testing should follow Safe Working Practices of the employer, union, or standards source. Isolate the test item and keep it inaccessible to onlookers or passersby. Run a performance test on the test equipment and include the leads. Damaged leads are often an overlooked source of confusing or inaccurate results.

Know the basic electrical configuration of the test item; you will test whatever insulation is between the leads. Motors and transformers will have open windings so you’re not running a continuity test. Cap open ends or separate them to eliminate the possibility of arcing. Know the unit of measurement to prevent confusing MΩ with GΩ or TΩ. A good tester will show units on the display, but operators sometimes overlook it.

Above all, be sure to account for time and temperature. Both profoundly affect readings. Adjust to a common temperature using the coefficient for the insulating material (part of the specs). Take readings at the same test time, once digits settle (e.g., 30 sec, 1 min).

Finally, single-conductor cable cannot be tested conventionally because there is no place to attach the second lead. Special accommodations can be made to test single conductors, but don’t expect the standard procedures for multi-core to apply.

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REFERENCES Megger www.megger.com

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TEST & MEASUREMENT HANDBOOK

Infrared cameras for PCB development and diagnosis IR cameras can help size up the surface temperatures of electronic components, PCBs, and other objects to reveal unseen problems.

ALAN J. LOWNE SAELIG CO. INC.

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Thermal

imagers, also called infrared cameras, capture a 2-D representation of objects in front of their germanium optics (ordinary optical glass impedes IR wavelengths). They present a visible version of thermal energy (heat). These images can be used for immediate diagnosis or processed via software for further evaluation and reporting.

The surface temperatures of electronic components, PCBs, and other objects can be measured with sufficient accuracy to reveal problems unseen to the human eye. 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.

It can also accurately measure an IC’s steady state temperature to verify operation within specified limits. In soak-testing imagers can ensure temperature limits are not exceeded on mission-critical boards. Desktop thermal imagers can provide a powerful set of diagnostic tools for testing, debug, and development. A non-contact thermal measurement system which pairs a highsensitivity infrared camera with an integrated stand lets technicians make quick thermal behavior checks of a PCB or Ty p ical other electronic assembly. th Professional-grade thermal the erma Fo t l i m cameras with interchangeable lenses ric 220 agers accurately measure temperature and provide Ser inclu ies. de high-resolution imaging for a wide range of scientific applications. Some can connect with smartphones, tablets, and computers across Android and Windows systems.

Modern thermal cameras are easy to use and can quickly identify problems with PCBs via lenses that can focus down to 20 µm lines.

Software provided with some cameras allow picture-in-picture overlays of the heat profile on top of a visible-wavelength image, which helps to visually locate problem areas. Comparing a good board’s thermal image with that of a faulty one can quickly pinpoint where a problem lies, saving valuable debug time.

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INFRARED CAMERAS

An example of thermal imaging today, resolving dimensions as small as 20 µm.

Some cameras can record up to 1,000 frames of full radiometric video, capturing temperature changes in real time with a user-defined sampling rate of up to 5 frames/sec. Up to 640x480pixel resolution is commonly available and each pixel represents a temperature measurement point in the field-of-view.

Data collection can be automatic with no need for an attached PC, allowing an instant analysis of thermal images and videos on the instrument itself.

Individual emissivities can be entered for up to 12 box areas or spots to improve the temperature accuracy of different scenes and materials. Designers can use the radiometric video stream plotting and 3D visualization of temperature images to note hot-spots and subtle temperature changes during product development.

Data can be automatically gathered, triggered by time, temperature, or an external signal, with threshold alarms available as well as the capturing of high, low, and average or user-defined temperatures.

Thermal imagers can be valuable for noncontact testing of circuit boards during the design and prototyping phases, but are equally useful in production, soak testing, process monitoring, and quality assurance. Finding and fixing hot spots in electronics designs is somewhat of a guessing game without the assistance of a thermal camera to quickly identify

heat dissipation issues and find effective solutions.

Unlike thermocouples or spot pyrometers, IR cameras can take as many as 300,000 individual accurate temperature readings (each pixel!) on large boards or even micro traces. Heat signatures in electronics while charging or in battery use can all be accurately documented in a thermal video during actual charge and discharge. Modern IR cameras can record fast-moving data, with the sensitivity and spatial resolution needed to characterize critical thermal transients.

IN THE DESIGN AND MANUFACTURING PHASE IR cameras can play a useful role when evaluating circuit design and heat dissipation choices.

In some cases, even counterfeit components can be detected because their thermal signature differs from that of authentic parts.

Applying power to the board and letting it run in various conditions and with

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different loads will insure a reliable finished product.

Imaging the thermal signature of poweredup prototype boards can reveal possible electrical or mechanical design improvements. It also helps to avoid unobvious design faults as well as expensive potential product recalls.

Discovering that a board or component is overheating can warn a design engineer to investigate active cooling or indicate the need for improved heat-sinking. A circuit design that does not run hot is especially important for prolonging battery life in a final product. The alternative of using thermistors with their associated messy thermal compound on a single point is much less attractive for obtaining accurate and comprehensive temperature measurements.

For quality assurance, a thermal imager viewing powered-up boards can be extremely useful. During automated board inspection, the camera can be controlled

Picture-in-Picture Super-impositioning the hot spot on a visible image of the scene helps to better identify the exact location of a problem.

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TEST & MEASUREMENT HANDBOOK

Real-time thermal data. Real-time data can be captured to show individual component temperature changes as well as final steadystate conditions.

by a remote PC to identify, for instance, insufficient solder which raises both circuit resistance and the local temperature enough to be detectable by an IR camera. A faulty circuit will produce a different temperature profile from a good one.

Model board comparison involves storing a thermal signature of one or more known good boards by powering the board and recording the temperature changes.

Production boards are then tested against the model to identify thermal differences that can indicate problems, saving costly product recalls. Entire boards can be inspected at once without contact, regardless of component density. Board comparison tests can find and locate subtle temperature anomalies that are almost impossible to find with other methods.

Conventional optical inspection systems often cannot be used to detect possible shorts, VIA (vertical interconnect access) abnormalities, or incorrect component placement. The cost justification for thermography is growing as electronic components and products shrink.

The world of portable electronics has a strong trend toward miniaturization, forcing packaging density to rise significantly. Flexible PCBs using conductive adhesives are designed in, using flip-chip components with fine pitches.

In these PCBs, the quality of contact resistance can be efficiently assessed with thermal imaging cameras.

PCB DIAGNOSTICS It’s often tough to know where to start troubleshooting when presented with a dead board. Thermal cameras can quickly speed up the repair of PCBs and components.

Comparing good (gold standard) and faulty boards can give

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a quick indicator of nonworking parts or unusual turn-on heat qualities, indicating where to start using traditional tools like oscilloscopes and meters. Customer service departments and same-day repair centers for electronic devices can speed their repair work this way using thermal cameras. Poor thermal management can be quickly discovered with an IR camera – perhaps a lack of thermal compound or thermal gasket, which lets components overheat and fail.

PCB defects such as defective ball-grid arrays and overstressed components cannot easily be identified with the conventional methods of in-circuit test equipment, optical inspection, or X-ray inspection, but reveal themselves readily with thermal imaging.

Short circuits also can be difficult to troubleshoot and isolate. Engineers can spend hours locating a single short, especially if it happens on an interlayer. But a quick way to find such problems is to briefly power the board and use the IR camera to locate temperature rises from the heat of the short. Operating the camera in a differential temperature mode displays thermal images that show temperature changes from the moment power is applied. A temperature change as small as 0.1°C is enough to identify a short.

Differential mode is also extremely useful when comparing a good and questionable board; temperature differences of the subtracted images will show problems immediately.

One camera maker reports that pre-production screening of PCBs using thermography saved a printer manufacturer from shipping faulty product. One of their engineers discovered during routine testing of new printer PCBs that some – but not all - of the boards overheated and tripped protective devices. He found that the motor

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Elec

INFRARED CAMERAS

Specs to compare

Super-resolution function which can expand 640x480 to 1,280x960 resolution.

Battery lasts 10+ hours, or powered by ac for non-stop testing Cost of camera and Ge lenses Cost of analysis software for PC Functions and ease-of-use Functions of PC software for report writing, etc.

Temperature range and accuracy Size of built-in LCD screen Number of available image spot/box analysis areas Recording of thermal videos Minimum focal distance Picture-in-picture capability Minimum target resolution Image triggering and alarm choices 3D graphing and software capabilities

driver chips on the failed boards were hot to the touch.

They used a thermal imager to accurately characterize the thermal picture of all the boards. The investigator found that the malfunctioning units were all from the same supplier who had selected a motor driver chip from an alternative manufacturer.

SELECTING AN IMAGER When engineers must determine the temperature of objects as small as individual IC pins, camera spatial resolution and thermal sensitivity are important selection factors. And with regard to thermal sensitivity, resolution down to 0.05°C at 30°C may be needed.

In a nutshell, thermal infrared cameras will likely become standard tools on every test bench, alongside Saelig Co. Inc. DMMs, oscilloscopes, and www.saelig.com spectrum analyzers.

REFERENCES

Proven integrity AND industry know-how Electrocube is one of the most respected design manufacturers of passive electrical component products for a wide range of standard and custom applications – from aerospace and audio to elevators and heavy equipment – as a capacitor supplier, resistor-capacitor distributor, and more.

Bishop Electronics, Seacor, Southern Electronics, F-Dyne

ELECTROCUBE.COM | 800.515.1112 | INFO@ELECTROCUBE.COM

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TEST & MEASUREMENT HANDBOOK

Functions and features of advanced hipot testers Modern instruments help check whether manufactured equipment meets the proliferating variety of international safety standards that govern factors such as grounding and the ability to withstand overvoltages.

KEVIN CLARK VITREK, INC.

Electrical

others. The agency certification safety TYPICAL DIELECTRIC-WITHSTAND TEST process confirms compliance testers – with the relevant standard(s). often referred to as hipot This compliance evaluation testers – are an integral part investigates two key areas: of electrical and electronic Construction - mechanical equipment manufacturing. construction, spacing, Hipot testers get their name clearances, etc.; and safety – to from the high-potential (high assure safe operation (even voltage) they produce to under high-stress conditions). perform dielectric withstand Work is progressing to and insulation resistance harmonize standards from global tests. In addition to these agencies. For example, the IEC tests, many hipot testers 61800-5-1 is a safety standard provide accurate lowspecified by the International resistance measurements and Electrotechnical Commission low-resistance/high-current for adjustable-speed electrical outputs to test ground power drive systems. It covers resistance and ground bond the safety aspects related to integrity.

During dielectric-withstand tests, hipot is applied to electrical, thermal and energy.

Electrical safety both conductors and leakage is measured in return The former UL standard testing and certification is circuit through the ground connection.

(UL508C) has now been a requirement for virtually supplanted by new standard, every electronic device and harmonized with the IEC requirements. electrical apparatus. The details of what constitutes a certified The UL document announcing this change put it this way: product depends upon a daunting number (hundreds) of safety “This harmonization work was undertaken with the intent of standards and in what region of the world the device will be sold creating a standard that, while being based upon and adopting and used. Safety standards-setting organizations include: EN / IEC requirements, would incorporate national differences IEC (European), UL (US), CSA (Canada), CCC (China), and JEIDA / that would address U.S. installation requirements (NFPA 70, MITI (Japan).

U.S. National Electrical Code). This goal has largely been Manufacturers must submit samples of their products accomplished in all cases.” to recognized certification agencies. Nationally Recognized To further help manufacturers address this often-bewildering Certification Laboratories (NRTLs) include UL, VDE, FM, ETL and

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newark.com | 1 800 463 9725 Your Trusted Source for Engineering Solutions

Industry’s Best Website Voted #1 by United Business Media

Industry’s First Catalog 88 Years of Publication

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TEST & MEASUREMENT HANDBOOK The Vitrek V7X Hipot Tester is well-suited to the requirements of electrical safety production testing.

array of international (and sometimes conflicting) standards, the Power Sources Manufacturers Association (PSMA) has established a standing committee and forum on its website.

PRODUCTION ELECTRIC SAFETY TESTING Electrical safety testing is an important final step in the production process for most electrical and electronic equipment. It aims to assure compliance safety agency labeling requirements, detect defective components or assembly flaws, and reduce the incidence of latent field failures and the attendant warranty costs.

Once in production, products must be 100% tested to confirm compliance with the related agency certifications and safety standards. Production tests are less stringent than initial certification but will generally include basic dielectric-withstand and shock-hazard (leakage) tests. Plug-connected devices will also be subjected to ground-resistance and (if the standard requires) ground-bond tests. Electrical motors, transformers, and other such devices will likely include insulation resistance tests.

Periodic inspection and calibration of test equipment is a standard requirement to maintain NRTL certification. Agency inspection will include a check of hipot instrument calibration certification. This “cal cert” is typically required annually.

(UL and other NRTLs require compliance certification with ISO17025.) Another common requirement prescribed by most NRTLs is a daily functional test of the hipot equipment.

The basic hipot test applies a high voltage from the conductors to the chassis of the device-under-test (DUT).

This test is often referred to as dielectric or voltage withstand.

Its purpose is to confirm that there’s sufficient insulation and isolation of the non-conducting surfaces from the operating voltage to avoid a shock hazard. The typical specification for this test is: 1,000 V + 2x normal operating voltage.

Both ac and dc hipot tests are possible and, in general, the test should use the same type of voltage as would be present during normal operation. However, if a dc hipot test is used on an ac circuit, the hipot voltage should be two times the peak (2 x 1.4 x RMS) + 1,000 V. Depending on the applicable standard, units pass this test if either the leakage current measured is less than the maximum allowable current, or no breakdown occurs, i.e., no sudden and uncontrolled flow of current.

For double-insulated products, test standards will often specify higher voltages. In addition, this class of device typically requires special fixturing to connect the nonconductive outer shell to a conductive element.

Several features may come in handy on instrumentation designed to run dielectric-withstand tests. The maximum output voltage is adjustable – a 5-kV maximum is adequate

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for many applications, but higher voltages (up to 30 kV) may occasionally be necessary. Both ac and dc outputs are advisable, with excellent regulation on both the line and load. Ramp rates, dwell times and discharge features should all be controllable, and the instrumentation should be capable of measuring the phase angle of leakage current via capacitive coupling detection.

Some standards allow for in-phase and quadrature current to be measured separately. Leakage current due to capacitive coupling may not be a safety concern. Instrumentation also should also include a way to specify current limits for min/ max pass/fail, as well as separate limits during ramps, and programmable multichannel testing.

Defects that are often detected with the hipot test include contamination (dirt, debris) and lack of proper spacing (creepage and clearance) of components. Creepage is measured across surfaces, clearance is the air gap between components.

Contamination would likely cause an unacceptable level of leakage current. Clearance problems could result in breakdown.

Insulation resistance testing is likely to be required for motor windings, transformer windings, and other applications

TYPICAL INSULATION INSULATION RESISTANCE RESISTANCE TEST TEST TYPICAL

During insulation resistance tests, voltage is applied to one conductor at a time while adjacent conductors are bundled.

Resistance is calculated based on leakage current.

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ADVANCED HIPOT TESTERS

involving cabling or insulated wire. Insulation resistance testing typically involves confirming that the resistance exceeds a defined high resistance value.

In many instances, insulation resistance must be measured between several conductors. Examples include cable/connector assemblies, multiconductor cables and relays. To make this measurement, all conductors except one are shorted together and the test voltage is applied from the remaining conductor across the shorted bundle. The test is then repeated for each wire in the bundle.

Instrumentation for insulation resistance tests have several features that simplify the measurement process. Among them are the inclusion of a wide range of selectable test voltages, an ability to run accurate and repeatable high-resistance measurements, a programmable high-voltage switching accessory, and a multichannel programmable testing facility.

Ground continuity testing confirms that the conductive chassis of a device safely connects to the earth-ground pin on the power plug.

This connection assures protection against shock hazards even if the equipment suffers an internal short to the chassis. The current would be shunted via the ground wire and would likely trip the breaker or blow the fuse.

Checks of ground continuity apply a low current (e.g., 50 mA) and calculate the resistance from the ground pin on the power plug to selected locations on the exposed surfaces of the DUT. Desirable features of ground continuity instrumentation include an accurate, repeatable low-resistance meter, and a plug adaptor accessory to speed testing.

GROUND-BOND TESTS Where ground continuity measures the resistance of the safety ground connection, the ground-bond test assures the integrity of the connection.

Using the same test setup, a high current is passed through the circuit.

If the ground bond is solid, the current passes without a change in resistance. A weak ground bond will cause resistive heating that would induce a failure of the bond.

Instrumentation for ground bond tests generally should include an accurate high current source, a means of programming test currents and test times, a plug adaptor accessory to speed testing, and a four-wire milliohm meter -- one providing a Kelvin connection for highly accurate low resistance measurements.

Electrical safety testing is a universal requirement for electrical and electronic equipment. Testing to specific regional requirements can be a daunting task, but programmable features and functions of advanced hipot testers can help simplify the task.

NRTLs in every region of the world provide services to certify compliance with applicable standards. They will also regularly inspect the equipment Vitrek and testing facilities used to perform www.vitrek.com production testing.

REFERENCES

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The Vitrek 964i offers automated multiconductor, multi-point hipot testing.

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.

The Vitrek 952i is representative of the highest performance hipot testers featuring up to 40-A ground bond capability and four-wire teraohm insulation resistance measurement capability.

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INTERACTIVE DEBUGGING

Interactive debugging on a budget The cost of undertaking multidomain debugging has come down. New instrumentation can time-correlate RF and embedded signals to let designers quickly uncover the root causes of knotty problems.

CHRIS ARMSTRONG RIGOL TECHNOLOGIES INC.

Examine

a typical device that is part of the Internet of Things and you’ll likely find a product that combines RF, digital, and analog design elements. Ultimately, developers of such products must interactively debug systems that include both RF and embedded subsystems. Design issues can be seen in embedded signals, RF signals, or radiated emissions while the root cause of the issues may reside in any of those sections or in the mechanical design.

To address such complex debugging issues, it is useful to employ both real-time spectrum analysis and high-performance oscilloscopes. Recently developed instruments combine these facilities to simplify the task of multidomain analysis: sizing up digital, analog, and RF signals that characterize IoT devices.

A case in point is the Rigol RSA (real-time spectrum analyzer) series.

Unlike many basic real-time spectrum analyzers, these instruments combine hardware triggering and IF output in a way calculated to help in advanced multi-domain analysis.

The identification of most issues in troubleshooting starts with the capture and verification of signals either in the time domain or RF domain.

One of the advantages of instruments that combine real-time spectrum analysis with scope displays is that the operator can view signals over time or in the form of a frequency spectrum. When symptoms appear in the RF transmissions, operators can monitor the frequency band of interest to investigate transient events.

Operators can extend this analysis into the time domain by either looking at a power-vs-time view or by monitoring the IF signal on the scope. Deep memory and waveform recording verify signals as they change on longer time scales.

It is useful to examine the new RF domain views available in realtime analysis. As a debugging tool, Rigol’s RSA enables viewing time in three distinct modes. One of the most important is the density view. The density view color-codes the probability of occurrence to highlight transient signals that are difficult to capture using other

The Rigol Real-Time Spectrum Analyzer (RSA5000)

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Density view of a hidden signal artifact

techniques. Density view makes it possible to see signals obscured by the spectrum of some other signal.

With frequency on the X axis and power on the Y axis, color coding shows how often a frequency of interest reaches a particular power level.

Yellow and red signify more frequent visits, and bluer hues for signal levels that are more rare.

The second mode displays a spectrogram history of power across the spectrum. The spectrogram shows the sequencing of signals over longer time periods. A typical task where spectrogram history comes in handy is in the characterization of a hopping FSK-modulated signal. The spectrogram helps identify the hopping algorithm and channel spacing over time. Markers can be used on the spectrogram display to calculate the timing between transmissions.

Transient events like a hopping signal can be further characterized through use of the third mode, the power vs. time display. It shows the timing of power changes from microseconds to seconds.

CORRELATION It is often necessary to correlate embedded signals or serial data packets with RF signals to find the root cause of a bug. There are three ways the RSA and an oscilloscope can be used together to correlate these signals.

All three methods use the same connections. The RSA trigger-out connects to the scope external input or one of the standard channels.

The oscilloscope trigger output connects to the RSA trigger input. Finally,

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an RSA output called the IF output connects to a 500-MHz scope channel in 50-Ω mode. (The IF output reproduces the RSA’s real-time bandwidth on a 430-MHz carrier, making the data accessible on a 500 MHz scope.)

With the connections in place, the instruments can be set to trigger together in three different ways. The first method involves triggering on the oscilloscope itself. With the RSA in real-time mode, you can select a view and trigger on the scope channel connected to the RSA IF output. The scope can be set to trigger on RF power changes. Operators can correlate these changes with the other signals the scope captures. This triggering method can select a trigger signal from any embedded signals on the mixed-signal oscilloscope or from the IF output channel.

The second method facilitates a detailed analysis by ensuring both instruments pause together. Here we trigger the RSA from the scope’s trigger output. Whenever the scope identifies a trigger event, the RSA will also trigger, and displays on both instruments will correlate. The FFT math function on the oscilloscope can display the spectrum in this

RSA can display both signal density and a spectrogram view simultaneously. Here the signal depicted is a hopping FSK modulated signal. The screen shot directly below shows the same signal with the addition of the power vs. time display in the top right. The signal’s 1-msec repetition rate becomes clear in this view.

The spectrogram (on the left) shows the same hopping sequence and the spectrum in the bottom panel shows the latest capture of the FSK pulse. This capture triggers when it detects a power level of at least -60 dBm. Individual pulses in this example are too short to measure.

To zoom in on this pulse width, the operator would connect the scope to the IF output of the RSA. This tactic makes it possible to view the precise timing of the RF pulse and see it in context of other signals.

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INTERACTIVE DEBUGGING

The power trigger being used to capture an FSK pulse on an RSA (top). The resulting pulse and trigger appears on the scope (bottom). In this mode, both instruments will trigger together only when power is detected in the 2.4-GHz band at or above -70 dBm. mode. This method works well for viewing signals containing complex time-correlated events as well as RF signal patterns that are best viewed on the analyzer as well.

For more complex RF signals, the third triggering method comes into play. It takes advantage of the RSA real-time capabilities to trigger on the power level or specific values within the spectrum. Here we set the RSA trigger mode to Power or Frequency Mask Trigger. The RSA’s trigger-out signal then triggers the scope. This setup lets the operator view the status of embedded, power, and serial signals at the time of a RF event or EMI emission.

Having investigated potential bugs and correlated signals across the relevant sections of our device we can now analyze changes and improvements to solve the problem.

ANALYSIS With a deep memory scope like the 4000 Series, the long record length can help view the time before and after an RF event to find the root cause of any errors. This time-based analysis is critical because many causes of problems are not

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TEST & MEASUREMENT HANDBOOK INSTRUMENT CONNECTIONS FOR MULTI-DOMAIN DEBUGGING Three different triggering and display connection schemes can help implement multidomain debugging on an RSA and mixed-signal scope: The RSA trigger-out connects to either the scope external input or a standard channel.

Alternatively, the scope trigger output connects to the RSA trigger input. Finally, the RSA IF output connects to a 500-MHz scope channel in 50-Ω mode.

instantaneous but result from a previous event. Programmable components like FPGAs hide many of these errors. One way to debug and verify their performance is to monitor changes over time continuously to locate the logic or state error.

Rigol’s waveform-record mode is another tool for multidomain analysis. Record mode makes it possible to capture a sequence of thousands of trigger events, then play back captured data using pass/fail masks or a point-by-point RMS difference analysis. The resulting data helps compare error occurrences and find a common cause.

All in all, an RSA combined with a 500 MHz mixed-signal oscilloscope bridges the gap between RF and embedded signals making true multi-domain analysis possible. This multi-domain analysis comes at significant savings to a single, new instrument with multi-domain capabilities that would typically cost $20,000 – but Rigol Technologies, Inc., RSA details provide a more www. rigolna.com/product-tour/ limited ability to Multi-Domain Debugging and Analysis visualize complex www.rigolna.com/multi-domain-debug RF signals in realtime.

REFERENCES

A screen caption from a 4000-Series scope (top) shows the IF pulse (in purple) as well as the RSA trigger (in yellow) near the bottom of the main display. Here we have detected errant pulses in the two FSK series and have located the related embedded signal that is the root cause. In record mode, correlation between the blue trace (named D4) and RF power (in purple) becomes evident. Once these issues are rectified, the RSA helps compare the realized modulation scheme to the design specifications. The RSA includes a 2FSK signal analysis package that aids in this evaluation (left). The 2FSK mode in the analyzer helps characterize each channel level and frequency to ensure it is within the error budget. The screen grab shows the results of the analysis on the improved hopping FSK signal.

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TEST & MEASUREMENT HANDBOOK

Interoperability testing for the Internet of Things

DELMAR HOWARD INTERTEK GROUP PLC

The Internet of Things might also be thought of as the internet of interoperable things.

A systematic testing regime can help uncover problems when devices work in concert.

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The

Internet of Things (IoT) is growing at an exponential rate. In three years, the number of IoT-enabled products is expected to nearly triple from 8.4 billion to 20 billion. Additionally, there are already more than 450 IoT platforms currently on the market.

Many IoT manufacturers have proprietary protocols, making it difficult to ensure IoT end-products work together in their intended environments. Additionally, IoT manufacturers must account for future updates and security patches, device upgrades, and user experience and expectations. Perhaps one of the biggest challenges for IoT-enabled products is an absence of standard guidelines to help ensure interoperability, leaving risk assessment, testing, and action to individual manufacturers.

There are numerous risks that any IoT-enabled device may encounter. They include vulnerabilities in the software running on other devices connected to the networks, access control through the network and other devices, potential ecosystem disruptions, vulnerabilities in default and/or hard coded credentials, no clear path to update legacy firmware, sending data in unencrypted text, open ports vulnerable to data breaches, interference from other products, signals or electronics; and cybersecurity concerns of other devices and networks.

Testing devices for interoperability ensures that products work together securely, without sacrificing performance.

In the world of cyber security analysis, information security management systems

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utilize the four-stage “Plan, Do, Check, Act” system. This methodology can also be employed to test interoperability.

Plan: This phase involves identifying improvement opportunities within a product and its systems. Evaluating the current process and pinpointing causes of failures will allow you to mitigate risk and address issues. Start the process by identifying the test type. Typical test types might include performance, security, compatibility or an ad-hoc approach. Each IoT device may require its own special considerations, so particularities of the product itself, as well as its intended environment and use, can factor into test plans.

It’s important to keep in mind that testing should include a mix of both automated and manual testing; it should also include negative testing as a complement to positive evaluations.

Interoperability test conditions should not be limited to the individual app or DUT (device under test); they should factor in how the DUT interacts with all the applications in the system.

And as plans develop, it can be helpful to create a data repository that can serve as a reference for future products and for updates or upgrades of the current device. Some developers create an RTM (Requirements Traceability Matrix) to help keep track of test cases and test conditions and acceptance test conditions/requirements for individual test cases.

The RTM basically helps ensure 100% test coverage. Here 100% coverage simply means the tests laid out in the plan cover

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INTEROPERABILITY TESTING

IOT UNITS BY INDUSTRY

Here’s a sobering thought: The market research firm Gartner (www.gartner.com/ newsroom/id/3598917) predicts some 20 billion IoT devices will be operation by 2020 – any many of them will have to interoperate with each other.

every device function. The RTM helps in this endeavor by creating a snapshot to identify coverage gaps. Testers usually start the RTM by writing their test scenarios and objectives, then the test cases. For each test scenario, there will be one or more test cases. As tests proceed, the RTM expands to include test-case execution status and defects.

Do: At this step in the process, it is time to conduct evaluations and assessments; collect analytics and data; and document any issues and failures. The Do step includes executing test cases, logging defects, getting them resolved, retesting and regression testing (described later) of the system as a whole. It is important to keep all the information on hand for use in redesign, as well as for future product development initiatives.

The “Plan” and “Do” phases can require a good deal of time and effort,

especially because the steps involved will vary depending on factors such as the operating environment. A medical device intended for use in an operating room, for example, requires different considerations than a consumer electronic device in the home. A server or automobile would undergo a different process than that for wireless speakers or a tablet.

When it is time to put the test plan into action, several evaluations can help gauge interoperability issues.

Simulation lets you to test an IoT-enabled product or app without using real boards or servers. The method is particularly effective for large environments. It requires basic programming knowledge, but templates can help replicate specific tests. The simulated server allows you to evaluate scale, security, and reliability when accounting for other devices, traffic, interference, data loads or other concerns.

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Usability evaluations account for the end-user, considering human factors as opposed to machine interactions. In many IoT products, human factors can often be overlooked during the prototyping phase.

Yet, human factors can be at odds with how a product operates. Assessments for usability help ensure a product provides an enjoyable, seamless user experience.

Testing for a device’s overall performance can be a straightfoward assessment. There’s usually no preset standard protocol for performance testing, but it generally involves assigning a certain number of user interactions over a specified time period. The difficulty comes in identifying bottlenecks and deciding how to fix them.

It is important to ensure that previously developed (and tested) software performs correctly after it has been altered or connects with other software. This is especially important with

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TEST & MEASUREMENT HANDBOOK

IoT products because they interact with each other. It is also critical when new features are added during development.

Common methods of regression testing include re-running previously completed tests, checking to see whether programs behave differently, and verifying previously fixed faults have not re-emerged. Regression testing can often be automated and can complement other tests. It is a structured assessment but is essential in continuous quality models like “Plan, Do, Check, Act”.

Data breaches are a constant concern with connected products. IoT devices can be vulnerable to cyber security issues, including those arising from software defects, open ports, and unencrypted communications, among other things. In the U.S., ANSI/ UL 2900 was adopted in 2017 for software security. It highlights testing for vulnerabilities, software weakness and malware in networked components.

It applies to products that include ATMs, fire alarm controls, network-connected locking devices, smoke and gas detectors, burglar alarms, and numerous others.

Check: During the “check” phase, any results from the previous stages must be reviewed and analyzed. With the analysis completed, it is time to identify whether improvements can be made or whether corrections from pervious tests were successful. If there are still issues to be addressed, you will need to return to the “Plan” and “Do” phases. Specifically, developers may refer to the RTM and check whether the design has met all the expected requirements and that tests exercised all the vulnerable functions.

Act: Based on the observations and failures of previous stages, change anything that did not work and continue any practices that were effective. It is important to iterate the PDCA process until a product meets interoperability requirements satisfactorily.

It is important to remember that products on the market still require vigilance for security and interoperability. To that end, you should plan to issue regular updates, upgrades and patches with a well-defined testing and release process. Such practices will help protect against zero-day style exploits and keep the product relevant as new software and hardware capabilities emerge.

With no specific interoperability standards in place to ensure products work seamlessly together, manufacturers can still take steps to help protect their devices, their reputation and their brand.

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Simulated servers are utilities used to help test and troubleshoot server/client applications and connections. Testing applications on "live" servers may result in loss of actual production data. Simulated servers create a simulated environment so in the event of a problem, no real process data gets lost.

REFERENCES Intertek Group plc www.intertek.com Gartner Says 8.4 Billion Connected “Things” Will Be in Use in 2017, Up 31% From 2016. www.gartner.com/newsroom/id/3598917 Security Sales and Integration. Toll Brothers Builds Smart Home Appeal in New Construction Market. www.securitysales.com/automation/smart-home/toll_brothers_ builds_smart_home_appeal_in_new_construction_market/.

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It’s not a web page, it’s an industry information site Stay current with the latest electronic tips, resources, and news, visit eeworldonline.com and stay on Twitter, Google plus, Facebook and Linkedin. It’s updated regularly with relevant technical information and other significant news to the electrical design engineering community.

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