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Modern Test & Measure
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TECH SERIES Filtering Signals with MATLAB Resolving Human Body Measurement ESD Challenges – Part 2 TECH REPORT Measuring Current in PCB Tracks
PRODUCT WATCH VSON MOSFET Relays from Omron COVER STORY Key Attributes of High-bandwidth Probes
EEWEB FEATURE
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Comprehensive USB 3.1 Compliance Testing from Tektronix
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Modern Test & Measure
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TECH SERIES
Filtering Signals with
MATLAB By David Maliniak, Teledyne LeCroy
Touted by its maker as “the language of technical computing,” The MathWorks’ MATLAB MATLABis a veritable Swiss Army knife for engineers, scientists, and perhaps anyone involved in technical endeavors. MATLAB serves a myriad of applications in programming, data analysis, application development, modeling and simulation, and—wait for it—instrument control!
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Modern Test & Measure
W
ith MATLAB, you can make Teledyne LeCroy’s oscilloscopes jump through all sorts of hoops. You can run it on most Windows-based instruments and livestream acquired data into MATLAB for processing and analysis, and then stream that data right back into the oscilloscope for further processing and display. It can also be used to create custom math or measurement algorithms to make non-standard measurements or filter and process signals. Let’s take a look at an example of the latter case. Often, you’ll find a need to filter a signal before analysis. It might be a matter of equalizing the frequency
Figure 1. A 2-pole, 1-MHz Butterworth low-pass filter applied to an acquired waveform.
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TECH SERIES response, or perhaps noise reduction. No matter what the reason, MATLAB provides a very handy way to embed a user-defined filter in the oscilloscope’s processing path. MATLAB comes with a number of filter types; The MathWorks offers lots of online resources resources to get you started in filter design and implementation as well. Figure 1 shows an example of a 2-pole, 1MHz Butterworth low-pass filter applied to an acquired waveform using the oscilloscope’s MATLAB math function. The MATLAB math function allows users to call the MATLAB program and execute a script file directly in the oscilloscope’s
processing path. The output from MATLAB is returned to the next processing stage and normal oscilloscope operation ensues. In Figure 1, we show the basic setup of the MATLAB math function on an HDO6000 oscilloscope. The function accepts one or two input signals and returns a single output. Selecting the MATLAB tab in the instrument’s Math dialog box allows users to load an existing MATLAB (.m) file or create a new one in the built-in editor (Figure 2). The .m file used in this example is shown in Figure 3. This code implements the 2-pole, 1MHz Butterworth low-pass filter applied to the signal in Figure 1. The filter used here is a relatively
Figure 2. A view of the built-in editing window in the HDO6000’s MATLAB Math function.
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Modern Test & Measure
slow cutoff, second-order filter. The command to create the filter coefficients is in the next-to-last line of code, where b represents the numerator coefficients of the digital filter and a represents the denominator coefficients of the digital filter.
the selected data. In this case, that is the input waveform, WformIn1). The commands in the first five lines of code queries the oscilloscope via Microsoft automation to obtain the sampling frequency.
The arguments for the Butterworth filter are order (2 in this case) and the cutoff frequency, which must be normalized to Nyquist. That’s why we have divided by half of the sampling frequency, Fs.
In this example, we’ve implemented a simple low-pass filter. But this concept can be extended for use with any of the available MATLAB functions or scripts. Try a few ideas of your own and see how the power of MATLAB can be applied to the filtering of live signal acquisitions.
The filter is implemented using the filter command in the final line of code, which applies the filter coefficients to
Figure 3. The MATLAB .m code that implements a 2-pole, 1MHz Butterworth low-pass filter applied to the signal in Figure 2.
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Modern Test & Measure
Resolving
HBM ESD Test Challenges
Using Latest Traditional Low-Parasitic Tester (Part 2)
› CLICK Resolving
To read the first part of this series, click on image
HBM ESD Test Challenges
Using Latest Traditional Low-Parasitic Tester
T
oday’s human body model (HBM) ESD tests are widely performed on automated relay-based
testers. While this is the most common test example, these testers can cause false failures due to parasitic impedance from resistance, inductance, and capacitance introduced by the relays, device under test (DUT) board, and sockets. Recent changes in the HBM
spec now allow for the use of a low-parasitic HBM tester that, because it is electrically connected at only two points, provides a highly accurate and nearly perfect
By Barry Fernelius, ESD and Latch-Up Manager Evans Analytical Group (EAG)
HBM pulse. However, it also has a drawback because it is slow and not practical to use for parts with high pin counts.
Part one of this two-part article series will provide an
overview of the HBM spec’s evolution, while part two will describe the challenges of using the associated lowparasitic HBM tester and how to resolve them with a hybrid test approach.
By Barry Fernelius, ESD and Latch-Up Manager Evans Analytical Group (EAG)
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TECH SERIES
A
s described in part one of this series, the human body model (HBM) ESD test is widely performed today on automated relay-based testers; but these testers can cause false failures due to parasitic impedance from resistance, inductance, and capacitance introduced by the relays, device under test (DUT) board, and sockets. Part one described the evolution of the HBM spec, which now allows for the use of a low-parasitic HBM tester that provides a highly accurate and nearly perfect HBM pulse. Part two will look at the challenges of using this tester, which is slow and not practical to use for parts with high pin counts. As an alternative, IC designers can now employ a hybrid test strategy with both pieces of equipment that enables them to eliminate the parasitic impedance problems and associated false failures of relay-based testers without having to slow down testing by moving completely to a low-parasitic 2-point tester.
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Modern Test & Measure
A Reduced Parasitic HBM Tester
Hybrid Test Approach
To address the problems that have been seen with relay-based systems, the last three versions of the ESDA/JEDEC HBM test allow the use of a low-parasitic HBM tester. The interface between the tester and the DUT is a probe station; the part is electrically connected at two points only. No special fixtures or sockets are required, and there is no relay array resulting in reduced parasitic impedance. The DUT can be a packaged part or a wafer. Critical structures can be characterized earlier in the design cycle. (See Figure 1.)
In a hybrid test approach, available relay testers are supplemented with best practices for minimizing tester parasitics. Then, if failures occur, 2-point testers are employed to investigate their cause. The process is as follows:
The problem with a two-point tester is that it is still too slow to do tests on large production parts. HBM testing is significantly faster and more effective with a hybrid test strategy.
Figure 1. HBM Two Point Tester and Probe Station
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1. Test the DUT on a relay tester. This is allowed by the spec, and it’s the highest speed and lowest cost solution. 2. Minimize tester parasitic impedances by using the following practices: a. When a multi-pin supply or ground plane is Terminal B (grounded terminal), tie all of the pins to ground.
TECH SERIES b. When a multi-pin supply, ground, or non-supply group is Terminal A (zapped terminal), don’t zap all pins; zap a representative pin instead. c. In the supply/ground to supply/ground tests, zap positive polarity only. 3. If there are no failures at ATE, the DUT has passed the test. 4. If failures are observed or if characterization data is required using a low parasitic 2-point tester: a. Do further stressing to identify and isolate the failing pin pairs. b. If a pin pair passes on the twopoint tester, the device passes.
HBM Current Waveform
HBM testing is significantly faster and more effective with a hybrid test strategy.
c. If a pin pair fails, use the twopoint tester to characterize the failure which will provide valuable feedback to design engineers. With the rapid changes in process technology, false failures due to relay tester parasitic impedances have become a more important HBM issue. A hybrid testing approach using both relay and 2-point testers minimizes the incidence of false failures through the most efficient process possible. This speeds HBM testing while reducing mask changes and enabling faster time to market.
HBM I-V Curve
Figure 2 – Two Point HBM Tester Waveform and I-‐V Curve
Figure 2. Two Point HBM Tester Waveform and I-V Curve The problem with a two-‐point tester is that it is still too slow to do tests on large production parts. HBM testing is significantly faster and more effective with a hybrid test strategy. Hybrid Test Approach
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Modern Test & Measure
References JEDEC JESD22-A114, “Electrostatic Discharge (ESD) Sensitivity Testing Human Body Model (HBM),” (Other revisions were A, B, C, D, E.) ANSI/ESDA/JEDEC JS-001-2014, “Electrostatic Discharge (ESD) Sensitivity Testing Human Body Model (HBM),” (Other revisions were 2010, 2011, 2012.) E.Grund, M. Hernandez, Oryx Instruments, “Methods to Remove Anomalies from Human Body Model Pulse Generators,” EOS/ESDA Symposium 2006 Evan Grund, Grund Technical Solutions LLC, “Two-Pin Human Body Model Testing,” EOS/ESDA Symposium 2009 Scott Ward, Keith Burgess, Joe Schichl, Charvaka Duvvury, Peter Koeppen, Hans Kunz, Texas Instruments; Evan Grund, Grund Technical Solutions, “Overcoming the Unselected Pin Relay Capacitance HBM Tester Artifact with Two Pin HBM Testing,” EOS/ESDA Symposium 2010 Yue Zu, Liang Wang, Rajkumar Sankaralingam, Scott Ward, Joe Schichl, Texas Instruments; “Threshold Voltage Shift due to Incidental Pulse on Nonstressed Pins during HBM Testing,” EOS/ESDA Symposium 2014
A hybrid testing approach using both relay and 2-point testers minimizes the incidence of false failures through the most efficient process possible.
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Modern Test & Measure
16
TECH REPORT
Measuring
Current
in PCB Tracks By Alan Lowne – CEO of Saelig Co. Inc. Observation and measurement of current in printed circuit board (PCB) tracks has always presented major difficulties. Previously, the only practical way has been to cut a PCB track and insert either a current shunt resistor, or a loop of wire large enough to attach a conventional closed-loop current probe. A new type of probe is now available called the I-prober 520 that can measure current by placing the insulated tip of the probe directly on to the PCB track without any cutting.
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Modern Test & Measure
The inability to observe and measure currents in a circuit under development can pose a serious problem for engineers.
M
easuring current in a PCB track presents particular difficulties because it is normally not possible either to break the track or to enclose it within a magnetic circuit. Typically, engineers have to estimate the current flowing in a track from voltage measurements made in other parts of the circuit. The inability to observe and measure currents in a circuit under development can pose a serious problem for engineers.
Conventional Current Measurement True measurement of current requires the circuit to be broken and a current measurement device inserted (e.g. a small shunt resistor that ‘converts’ current to a simple voltage measurement). Conventional DC-capable current probes do not measure current—they measure field density. Current flowing through a conductor creates a magnetic (H) field, which is directly proportional to the current. If a conductor is surrounded by a closed magnetic circuit, the whole of the field is ‘captured’ by the magnetic circuit, and the field density can be scaled to represent current. Conventional current probes concentrate the field into a gap within a loop of high magnetic permeability (Mu) material. The field is then measured by a field sensor
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inserted into the gap, usually a Hall-effect device. Alternatively, AC current can be measured by transformer action when a loop of magnetic material creates a one turn primary from the conductor that is enclosed. Hybrid devices use a field sensor for DC and low frequencies plus a transformer for higher frequencies. Normally the probe jaws provide a method of mechanically opening the magnetic circuit to enable the conductor to be inserted. The position of the conductor within the loop has relatively little effect on the measurement, and a high rejection of external fields can be achieved.
Positional Current Probes To make a quantitative measurement of current generated from the field requires that a known proportion of that field be measured. While conventional current probes achieve this by concentrating the whole of the field within a loop of high Mu material, in contrast, a positionalcurrent probe measures a known proportion of the field by positioning a sensor at a known distance from the conductor. This provides the potential to provide a calibrated measurement of current (see Figure 1). However, to be usable with a PCB track, the size of the sensor, as well as its distance from the conductor, must be very small.
TECH REPORT
The I-prober 520’s bandwidth of DC to 5MHz encompasses a wide range of applications including most power switching circuits.
Figure 1. Measuring currents up to 20A in PCB tracks.
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Modern Test & Measure
Previous positional-current probes have been physically large, capable only of measuring high currents at low bandwidth, and not usable in the high densities of modern circuit boards. But the I-prober 520 is a miniaturized positional-current probe that uses the well-established principle of a fluxgate magnetometer to measure field strength. This type of magnetometer uses a magnetically susceptible core surrounded by a coil carrying an AC excitation current, which magnetizes the core alternately in opposite directions. If there is no external magnetic field, this magnetization is symmetrical. When an external field is applied, the resulting asymmetry is detected by a feedback loop, which applies an opposing current through the coil to restore the net field to zero. The output voltage is proportional to this opposing current, and therefore to the magnitude of the field.
Figure 2. Miniature fluxgate magnetometer used in the I-prober 520
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Conventional fluxgate magnetometers are relatively large with bandwidths of a few kHz. They are typically used for precision measurement of fields within geophysics and bio-electromagnetics.
Figure 3. Compact probe tip in use
In contrast, the sensor within the I-prober 520 uses a patented miniature fluxgate magnetometer of sub-millimeter size, developed at Cambridge University in England, incorporating a highly advanced core material (see Figure 2). This enables it to use an excitation frequency of several tens of MHz resulting in a sensor with a bandwidth of DC to 5MHz combined with low noise and wide dynamic range. This miniature sensor is fitted within a double insulated probe tip (see Figure 3) which is just 1.8mm wide, with the sensor spaced 0.7mm from the surface of the tip. Because the field diminishes with the square of the distance (to a first order approximation), this spacing is critical to the operation of the probe, giving it both high sensitivity and reduced susceptibility to fields from adjacent conductors. Inconsequently, the I-prober 520 is the first and only probe that can be used to measure currents from amps (20A) down to milliamps (20mA) at frequencies from DC up to 5MHz, making practical measurement of PCB track currents a reality.
TECH REPORT
Figure 4. Clip-on toroid assembly for measuring current in a wire
Using a Positional Current Probe The magnitude of the signal from a positional-current probe (see Figure 4) is critically related to its position relative to the conductor. The size of the conductor (e.g. the width of a PCB track) also has a significant effect. This means that the sensitivity of the I-prober 520 has to be adjusted to match the track width when quantitative measurements are required. A calibrator within the control box enables sensitivity adjustment in conjunction with a calibration graph. The output from the control box is scaled to 1 volt-per-amp and is intended for connection to a conventional oscilloscope. The measurement result will also include other field effects present at the tip of the probe and not just that coming from the current through the conductor. This may include DC effects from adjacent magnetized components and from the Earth’s magnetic field, as well as AC effects from transformers and other field radiating sources. Current in adjacent tracks, or tracks on the opposite side of the PCB, will also affect the measurement.
But there are solutions to these problems. The unwanted DC can be nulled out by observing the measurement without power to the circuit, and AC interference can be attenuated using bandwidth filters. The I-prober 520 control box has been designed to include a wide-range DC offset control and switchable filters. Nevertheless, the use of the I-prober 520 requires some interpretation based on a proper understanding of circuits and systems, and is therefore really a tool for the professional engineer.
I-prober 520 is a miniaturized positional-current probe that uses the well-established principle of a fluxgate magnetometer to measure field strength.
A useful analogy might be with an ultrasound probe. Whereas the layman will just see a blur of indeterminate images, the skilled professional can use it to make valuable qualitative and quantitative measurements. Similarly, the experienced user can learn the techniques needed to distinguish the wanted from the unwanted and make observations and measurements that were previously impossible.
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Modern Test & Measure
One of the most interesting applications of the I-prober 520 is the observation of currents flowing within ground planes.
The I-prober 520’s bandwidth of DC to 5MHz encompasses a wide range of applications including most power switching circuits. The maximum current that can be measured is 20A pk-pk in a track of up to 3.5mm, or 40A pk-pk in a wider track. The minimum visible current is limited by the noise figure, which, for a 0.5mm track, is equivalent to around 6mA RMS at full bandwidth, reducing to 1.5mA at lower bandwidths. In practice, this enables AC signals as low as 10mA pk-pk to be usefully observed. It should be understood, however, that accurate measurement of very small DC currents is likely to be impracticable in most situations. The effect of the Earth’s magnetic field is equivalent to up to 180mA flowing through a 0.5mm track. While this can be nulled out within the control box, the effects of small changes in orientation can cause variations that would invalidate mA level measurements.
Figure 5. Measuring wire current.
Figure 5. Measuring wire current
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Similarly the effect of magnetized components and high Mu materials close to the probe can cause significant offsets. One of the most interesting applications of the I-prober 520 is the observation of currents flowing within ground planes. Although quantitative measurements are not possible (because the current density cannot be inferred) it is easy to see whether and where circulating currents are flowing and to find their injection points. Many circuit designs are adversely affected by power and ground connection schemes which fail to provide a true zero impedance connection, allowing unexpected interfering signals to appear. Due to the fact that the I-prober 520 is positional-current probe, there are many circumstances where current measurements can be made in the conventional way by enclosing the conductor. To increase its overall usefulness, the I-prober 520 is supplied
TECH REPORT with a clip-on toroid assembly (Figure 4), which converts it into a closed magnetic circuit probe for measuring current in a wire (Figure 5). The wide bandwidth, dynamic range and low noise of the probe are retained but higher accuracy; repeatability and unwanted field rejection are achieved.
Further Measurement Modes While the primary purpose of the I-prober 520 is as a positional-current probe, there are many circumstances where current measurements can be made in the conventional way by enclosing the conductor. To increase its overall usefulness, the I-prober 520 is supplied with a clip-on toroid assembly (Figure 4), which converts it into a closed magnetic circuit probe for measuring
current in a wire (Figure 5). The wide bandwidth, dynamic range, and low noise of the probe are retained—but higher accuracy, repeatability, and unwanted field rejection are achieved. The very small size of the field sensor within the I-prober 520 gives it some unique capabilities when used to measure magnetic fields. The variation of field with position can be accurately determined enabling the precise source of fields to be located and their variation in space measured A switch on the control box re-scales the output voltage to measure in Teslas or in amps per meter. Finally the observation and measurement of current in PCB tracks without modification is achievable.
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Modern Test & Measure
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With OMRON placing this powerful MOSFET relay technology in a VSON package, all of these benefits are now available in a miniscule form factor. VSON stands for Very Small Outline Non-Leaded, an SMT package measuring a mere 2.45 by 1.45 by 1.3 millimeters. This offers a 50-percent decrease in bottom surface area compared to USOP packaging, while the land contacts are designed to improve solderability. Despite the incredibly small size, there are models available for 20 volts, stepping all the way up to 100 volts.
MOSFET relays struggle with rounder on/off edges than reed or electromechanical relays, however, OMRON has parts designed for a lower off-state capacitance and models with lower on-state resistance to sharpen those edges.
These VSON packaged MOSFET relays are perfect for use in applications such as semiconductor test equipment, test and measurement devices, data loggers, battery powered equipment, communications equipment, and more. For great performance in a tiny package, OMRON has provided the perfect solution for any switching needs.
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Modern Test & Measure
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COVER STORY
Choosing High-bandwidth
PROBES By Jae-yong Chang, Keysight Technologies, Inc.* Modern high-speed digital applications have pushed the limit of high bandwidth and high performance probing solutions. As the bandwidth of your system increases, the edge speed of the signal gets faster, the size of the chipsets and components tends to be smaller, and the PCB layout becomes more complex. These trends pose challenges for probing solutions and there are more things to consider when choosing a high performance probe than a lower-bandwidth, general purpose probe. The higher the performance, the more care is needed. Even though a probe may have an impressive performance specification on the data sheet, the published specified performance may be under very ideal probing conditions. In a real-world probing situation, which would include using probing accessories to attach to the probe tips, the probe’s performance may be much worse than the published specified performance. The article describes key attributes to consider when choosing a high performance oscilloscope probe.
*Keysight Technologies Inc., formerly Agilent Technologies electronic measurement business
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Modern Test & Measure
PROBE LOADING An ideal oscilloscope probe would simply provide an exact replica of the signal being probed. But, in the real world, the probe becomes part of the circuit under test because the probe or probing accessories attached to the DUT introduce probe loading to the circuit. Depending on the different loading effects, the non-ideal loading effects on DUT could impose limitations on the probe’s bandwidth and frequency response flatness in the frequency domain and may cause unwanted side effects such as overshooting, ringing, and DC offset problems in the time domain. One important factor you have to keep in mind when choosing a high bandwidth probe is that the probe’s loading characteristics may be somewhat different from a conventional probe. The
conventional model of probe impedance looks like this red trace in the input impedance vs frequency chart (Fig 1). The example here is of the Keysight InfiniiMax 1169A 12 GHz differential probe. At low frequencies up to ~10 MHz, you see the input impedance is 50 kohms driven by input R of the probe. Then it intersects the 210 fF capacitance of the probe. That is what we call an RC input impedance profile. That is very traditional of any oscilloscope probes, including Keysight’s InfiniiMax I or II probes or almost any other probes you may deal with in your everyday use. But the high-bandwidth differential probes like InfiniiMax III or III+ have a different input impedance characteristic. They have a 100-kohm differential impedance at DC and at very low frequencies, then it intersects with the
Figure 1. Input impedance vs. frequency profile of common highperformance probes
Red = Keysight InfiniiMax II 1169A 12GHz probe (RC) Blue = Keysight InfiniiMax III N2803A 30GHz probe (RCRC) Pink = Another 20GHz probe in the market (RCRC)
30
COVER STORY 50nF capacitance of the probe, where you hit the mid-band impedance. And then over six decades of frequency, it maintains a 1 kohm differential input impedance until it finally intersects the 32fF capacitance. Also shown in the magenta color is the input impedance plot of the other vendor’s high bandwidth probe. Again, 100 Kohm at DC, and it falls due to its 110pF mid-capacitance down to a 450 ohm loading, and then intersects with 65fF capacitance. The crossover frequency of this probe is much higher and it levels out to the midband impedance at ~100MHz. We call this an RCRC profile for semantics here. This is very typical of the newer high frequency probe systems such as InfiniiMax III or III+.
Generally RCRC probes do an excellent job of reproducing wave-shapes with fast edge speeds, but have effects when trying to measure absolute voltage levels, especially if the source impedance of the target signal is high or if there are long time constants in the signal being probed. The RCRC type probes do not provide expected results when probing buses that transition to a “high Z” state, such as when the MIPI D-phy signal transitions from HS (high speed) mode to LP (low power) mode (Fig 2). The impedance driving the high impedance bus is typically pulled up or pulled down with a high value resistor, and this interacts with the RCRC input impedance causing very long time constant effects. Therefore, it is generally not recommended to use an RCRC type probe for this type of bus. A RC type of probe with high input impedance across the wide bandwidth
Figure 2. The RCRC type probes may not provide expected results when probing buses that transition to a “high Z” state, such as this MIPI D-phy signal. Yellow = Keysight N2832A InfiniiMax III+ 13 GHz probe (RCRC) Blue = Keysight 1169A InfiniiMax II 12 GHz probe (RC)
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Modern Test & Measure
range such as InfiniiMax I, II probe is recommended for this application. The important point here is that probe users should consider probeloading effect to ensure that the probe loading is tolerable. Most probe manufacturers provide input loading models so customers can take stab at understanding the probe loading characteristics before probe selection.
PROBE NOISE Many of you are concerned about the probe/scope’s inherent noise that contributes to the measurement. There are multiple factors contributing the noise figure of the measurement. However, one of the single most important characteristics you may want to consider is its signal-to-noise ratio. Typically lower attenuation ratio
Figure 3. Signal to Noise diagram of a scope and probe
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leads to higher signal-to-noise ratio with less noise, but at the same time, it yields a lower input resistance, lower dynamic range, and lower common mode range etc. There are some tradeoffs here. One easy way to estimate the amount of your probe noise is to check the attenuation ratio and the probe noise level of the probe from the probe’s data sheet or manual. The diagram at Figure 3 illustrates the situation with respect to signal to noise. In short, the noise floor of the probe/ amplifier system is fixed so if the signal levels being applied to summing point are not maximized, a degradation of signal to noise ratio will be seen. All scopes use an attenuator to vary the vertical scale factor. The scope’s noise arises after this attenuation occurs. So when the attenuator is set to any ratio
COVER STORY other than 1:1 that is the scope’s most sensitive vertical scale range, the noise will appear to be larger relative to the signal at the scope’s input connector. Therefore, there is an advantage in using the most sensitive vertical range of a scope possible to avoid magnifying the scope’s noise unnecessarily and resulting in degraded signal-to-noise ratio of the measurement. Also, it is advisable to use a probe with lower attenuation ratio to achieve higher signal-to-noise ratio. fg3
to use just the bandwidth that is exactly needed by using the scope’s bandwidth limit control features.
Once you settled in choosing the probe of your interest, the next step you would consider in lowing the probe noise contribution is to minimize the ground/signal loop formed by the probe head/tip. Select just enough scope/ probe bandwidth that is needed since excessive bandwidth will contribute to the system’s overall noise. Try
In fact, the oscilloscope is often “not” the weakest link in the measurement system. A measurement system also consists of probes, cables, connectors, and fixtures. Each of these elements has the potential to cause more loss of bandwidth than the oscilloscope. While cables and connectors typically have very low loss, this is not the case for probes and probe accessories.
PROBE TIP MAY BE THE WEAKEST LINK A measurement system is only as good as its weakest link. The bandwidth of an oscilloscope or a probe is always a key banner specification, but there is more to the measurement system than just the oscilloscope and the probe.
Figure 4. Probe tip leads may be the weakest link. Keep them small and the loop area as small as possible.
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Modern Test & Measure
If you see a high bandwidth probe that has obviously longer input lead wires at the tip than similar probes, suspect the frequency response variation and degradation. In general, longer the input wires or leads of a probe tip, more it may decrease the bandwidth, increase the loading, cause non-flat frequency response and result in more variation in response as span and tip wire environment changes. As the bandwidth of the system goes above a GHz, the probe tip effect may play an even more important role in the determining the system performance. If at all possible, keep the input leads of the probe tip as small as possible, and keep the loop area of connection as small as possible. And, if you’re using a single-ended probe, keep the low inductance ground connection which is short in length and fat in thickness.
CONCLUSIONS As the bandwidth of your system increases, the edge speed of the signal gets faster, the size of the chipsets and components tends to be smaller, and the PCB layout becomes more complex. These trends
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pose challenges for probing solutions and there are more things to consider when choosing a high performance probe than a lower-bandwidth, general purpose probe. The higher the performance, the more care is needed. This article discussed the key attributes to consider when choosing a high performance probing system. We looked at two different input impedance profiles of oscilloscope probes and how they are different. Probe users should consider probe loading effect to ensure that the probe loading is tolerable. Most probe manufacturers provide input loading models so customers can take stab at understanding the probe loading characteristics before probe selection. As the bandwidth of the system goes above a GHz, the probe tip effect may play an important role in the determining the system performance. If at all possible, keep the input leads of the probe tip as small as possible, and keep the loop area of connection as small as possible. Also it is advisable to use a probe with lower attenuation ratio to achieve higher signalto-noise ratio.
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Modern Test & Measure
Tektronix provides an impressively comprehensive toolset for USB 3.1 verification, characterization, debug, and compliance testing.
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EEWeb FEATURE
Comprehensive
USB 3.1
Compliance Testing From Tektronix Few people who regularly interface with personal computing devices can say that the ubiquitous Universal Serial Bus, or USB, doesn’t serve some essential function in their technological routine. The history of USB officially began in 1994, when a group of well-known companies, including Compaq, Intel, IBM, and Microsoft, led the charge to develop a more universal interface protocol for computer peripherals of all kinds. Steadily increasing in capability of both form and function as the demands of everyday data interaction have increased exponentially in the decades since, USB is still admirably keeping pace with a host of younger competitors.
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Modern Test & Measure
F
irst introduced by Intel in 1995, today’s familiar Universal Serial Bus has defined itself not only by being the groundbreaking universal interface protocol, but by continuing to grow in adept strides alongside all of the most familiar steps in the evolution of our everyday computing technology. Today, with the help of experienced companies like the 70-year-old measurement systems innovator Tektronix, the Universal Serial Bus is going strong and proving that it still has plenty of pioneering left to accomplish, especially when working in the kind of harmony that proper testing can accomplish. Companies like Tektronix, whose focus is on test and measurement, are now finding themselves most thoroughly engaged with USB’s version 3.1, which debuted in 2013 and continues the constant progress of increased speed and compatibility in interfacing. Recently, we spoke with Randy White at Tektronix to get a little bit more information about what’s happening at the crest of USB measurement and testing, and the efforts that Tektronix has put forth to ensure that USB 3.1 is functioning as smoothly as possible for everyone. Some of the first issues that come to mind when testing USB 3.1, White outlined, relate to the fact that it sports an impressive transfer rate that’s at least twice as fast—at 10 Gb/s, compared to USB 3.0’s standard 5 Gb/s rate. Not
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surprisingly, these faster speeds require additional changes to overall design, and therefore, new approaches to testing. As White explains, “changes such as the use of 128b/132b encoding and more complex equalization necessarily increase complexity. For example, this introduces new PHY validation and debug challenges on top of the usual.” There are also a handful of other recent updates and innovations in 3.1 that will similarly impact test and measurement of USB equipment, he tells us, including the release of the updated USB Power Delivery 2.0 specification and the new reversible USB Type-C connector. Most important for USB 3.1 users, however, is the fact that Tektronix and their products remain demonstrably prepared to address these and all of the other emerging demands of USB 3.1 testing. Today, Tektronix provides an impressively comprehensive toolset for USB 3.1 verification, characterization, debug, and compliance testing. In this top-ofline system, tests have been expanded to include extensive address of both the familiar generals of interface testing and the new specifics of USB 3.1. For an added dimension of ease and convenience, Tektronix test fixtures provide access to both USB transmitter and receiver signals without physical cables. Perhaps most critical for customers to consider initially, however, is the level to which Tektronix has expertly automated their testing processes for both transmitters and receivers.
EEWeb FEATURE
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Modern Test & Measure
Firstly, these include a wide range of automated USB 3.1 normative and informative transmitter tests that can all be operated with single-button execution and no further user interaction required. These systems also allow users to quickly test under different conditions with fully independent controls for de-embedding, channel embedding, and equalization, streamlining the transmitter testing process with incomparable results in speed and detail. Tektronix software separates transmitter tests into three main groups: clocking with SSC, traditional PHY measurements like jitter, voltage and eye mask, and LFPS timing and voltage swing measurements. Overall, a quarter of those tests, including a series of LFPS-specific parameters, are geared distinctly toward 3.1 and have more stringent requirements than those for earlier USB versions. “Essentially,” White expounds, “there is no longer a need to be an expert on transmitter testing procedures,” as the Tektronix USBSSP-TX solution fully automates transmitter testing. He envisions this kind of efficiency enabling engineers to simply select and run the desired tests, freeing them up for work on other tasks while the tests are being executed.
Tektronix software separates transmitter tests into three main groups: clocking with SSC, traditional PHY measurements like jitter, voltage and eye mask, and LFPS timing and voltage swing measurements.
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Of course, Tektronix’ software applies just as much care with addressing the specifics of receiver testing as well. In terms of the increased complexity of 3.1 receivers, the update adds three new sub-states in the polling state machine to allow a port to identify itself as 3.1-capable and to synchronize with another link partner. On this end, White points out, “one of the most important parts of receiver testing is link training to ensure the receiver equalizer is tuned properly for a given channel.” Here, Tektronix’ industry-leading, supersimplified “one-click” loopback process, which includes a full link handshake, not only allows for updated PHY synchronization but also for proper equalizer adaptation. Of particular value is the fact that, with this fully automated receiver compliance and margin testing system, which includes calibration and integration with a Tektronix power supply, no specialized knowledge is required to configure, calibrate, test, and document results. The company’s BERTScope USB 3.1 automated receiver test solution is specifically designed to streamline what White calls “the often tedious and laborintensive receiver test workflow.” He details how the company’s unique brand of “fast and accurate BERT-based testing provides high test throughput, intuitive and fast margin testing, and availability of a wide range of debugging tools when further investigation is required,” all on a fully automated platform.
EEWeb FEATURE Rounding out the incomparable level of automation inherent in Tektronix testing solutions, White tells us how the company’s systems “can quickly validate test status with comprehensive reporting that captures test margins, pass/fail results, and plots,” while also providing automatic processing of results without manual intervention. Test results can be easily compiled into a comprehensive report and generated in .MHT format, and can also be stored in an Excel .XLS file that can be used for an exceptional level of further analysis. Concurrently, Tektronix has partnered with Granite River Labs to offer GRL-USB-PD software power delivery test software that provides support for the latest USB PD test specification. The software for Tektronix MSO/DPO5000, DPO7000 and MSO/DPO70000 series oscilloscopes drives electrical test and decode of the Biphase Mark Coding (BMC) used in the USB Configuration Channel along with a range of other parametric measurements including rise time and reference bit rate. White elaborated on what he sees at the approaching horizons of USB 3.1 development. First, he notes the emergence of the new USB Type-C connector as a game-changing dynamic that’s only set to expand. The symmetrical, reversible Type-C connector is intended to replace both USB and Micro USB plugs with an intra-protocol standard, and even
Test results can be easily compiled into a comprehensive report and generated in .MHT format, and can also be stored in an Excel .XLS file that can be used for an exceptional level of further analysis.
now, the Tektronix website includes extensive information outlining existing test methods of implementation and test instrumentation for today’s fledgling USB Type-C connectors. “The new Power Delivery specification is one of the most exciting aspects of USB 3.1 and will definitely be opening up a broad range of interesting applications for USB,” he also suggests. “USB Power Delivery is now more complex than it has been in the past,” and as a result, he says, “solutions like GRL-USB-PD are becoming critical to efficiently debugging USB power delivery ICs and system designs.” With some deserved confidence in hand, Tektronix seems at the ready to meet the demand.
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