Test & Measurement Handbook June 2017

How to reduce oscilloscope noise during measurements Page 6

Simplifying signal generation using arbitrary waveform generators Page 10

June 2017

Test & t n e m e r u s a Me H A N D B OO K

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TEST & MEASUREMENT HANDBOOK

SEEN A GHOST? BLAME ELECTROMAGNETIC FIELDS WE’D

like to think that the average viewer who tunes into a TV show on the SciFy channel called Ghost Hunters might get a laugh out watching investigators prowl around places that are reported to be haunted. But the show participants seem to be completely serious and unconcerned about the fact they find essentially nothing week after week.

Our own impression of the show is that the producers are quite lucky that average TV viewers don’t understand the workings of either gauss meters or RF power meters.

Unsurprisingly, not much happens on Ghost Hunters.

Investigators set up electronic equipment in what are supposedly paranormal hotspots. They then spend several hours taking electromagnetic field and temperature readings, recording audio, and filming with digital video cameras.

Perhaps to make up for the lack of action, show investigators have tended toward instrumentation that provides more audiovisual interest than just numbers on a display. They have used an EMF meter on which LEDs, rather than a number on an LCD, give a measure of field strength. They’ve also employed a custommade geophone (normally used for detecting seismic disturbances) which flashes LEDs in proportion to the intensity of vibrations. Another EMF detector they use buzzes when it detects an electromagnetic field.

The fact that Ghost Hunters uses EMF detectors in any capacity might lead the average viewer to think that “ghosts” are some how expected to generate electromagnetic energy. But a better grasp of the origin of this idea could pour even more cold water on the proceedings in the show.

The connection between ghostly appearances and EMFs was theorized by Michael Persinger, a neuroscientist at Laurentian University in Canada. In one of his studies he describes the experiences of a teenager who in 1996 2

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LEE TESCHLER EXECUTIVE EDITOR

claimed to get nocturnal visits from the Holy Spirit.

Persinger’s group found that an electric clock near the 17-year-old’s bed generated electromagnetic pulses with waveforms resembling those found to trigger epileptic seizures. Removing the clock stopped the girl’s visions.

Persinger theorized that the clock, in combination with mild brain damage that the girl had sustained at birth, were likely contributing to the perceived ghostly experiences.

Persinger has done a lot of research on how electromagnetic stimulation of the frontal lobes of the brain can induce feelings of a “sensed presence.”

However, the field levels involved must be pretty high and are usually generated by having a subject wear special headgear sometimes dubbed a God helmet. Experiments with lower levels of EM stimulation, as might arise when someone wanders around a room, have been somewhat controversial. Research groups have mostly been unable to see any “ghostly” hallucinations under such circumstances though Persinger claims some success in this area.

Getting back to Ghost Hunters, investigators on the show seem to act as though fluctuations on their gauss meters may indicate a ghostly presence. If Persinger is correct, a noteworthy reading on a gauss meter is more likely an indication of hallucinations rather than any spiritual activity.

And there’s no reason spirit hunters on the show need special instruments to see magnetic fields. There are some pretty nifty apps for the iPhone that will use the phone’s Hall sensors to read out not only the magnitude but also the direction of magnetic fields on the order of 25 to 65 mT range as created by the earth.

Of course, waving around an iPhone rather than a gauss meter probably doesn’t look particularly convincing if you are playing to an audience hoping to see evidence of ghosts.

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CHARGE UP

YOUR SUMMER

b

a t

t e r y h o l d e r s . c o m

MPD m e m o ry p r o t e c t i o n d e v i c e s

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INSIDE THE TEST & MEASUREMENT HANDBOOK

23

10

16

02 Seen a ghost? Blame electromagnetic fields 06 How to reduce oscilloscope noise during measurements

Several tricks can help reveal small signals that are obscured by measurement system noise.

10 Simplifying signal generation using arbitrary waveform generators

New instruments excel at constructing super-complicated waveforms that once took a lot of time to create.

16 Better testing with well-controlled ac power

Modern ac power supplies are adept at cleaning up line conditions that can lead to garbled electrical test results.

23 The case for dc power quality measurements

New instruments excel at constructing super-complicated waveforms that once took a lot of time to create.

27 Safer testing with dynamic power supplies

4

It can be easier to use programmable auto-ranging power supplies as alternatives to fuel-cell power sources that would otherwise force the installation of safety and compliance equipment just to run a few characterization tests.

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FIND IT ONLINE:

OSCILLOSCOPE BUYERS GUIDE Our oscilloscope buyers’ guide gives instrument users a handy way to compare scope offering and make side-by-side comparisons. This online guide will help you become familiar with different types of scopes, common accessories and features.

PRODUCTS.DESIGNWORLDONLINE.COM /PRODUCTS/OSCILLOSCOPES

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TEST & MEASUREMENT

HOW TO REDUCE OSCILLOSCOPE NOISE DURING MEASUREMENTS Several tricks can help reveal small signals that

H A N D B O O K

are obscured by measurement system noise.

6

KENNY JOHNSON KEYSIGHT TECHNOLOGIES, INC.

THERE

is a need to observe small signal details in many modern applications.

Transducers, biomedical sensors, high energy physics, power integrity, and high-speed digital designs are examples of situations where details can be obscured by measurement system noise. Measurement system noise is the noise of the oscilloscope, probes and connection method that is superimposed on the signal of interest.

When the signal being observed is small, like the ac ripple and noise on a power supply, the signal presented on the screen of the oscilloscope may only vaguely represent what is real if care is not taken to reduce measurement system noise. The complete elimination of measurement system noise is not a realistic goal though there are some practical steps that can be taken to substantially reduce it.

One obvious suggestion is to choose the low-noise path. Unfortunately, many users stumble here, not knowing they may have even better options available to them. The oscilloscope measurement path consists of the oscilloscope being used and the scope input termination—50 Ω or 1 MΩ. For many oscilloscopes, the 50-Ω input is a lower-noise path than the 1-MΩ path.

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The baseline noise of the 50-Ω

A measurement input (top) and 1-MΩ input of of the baseline noise a Keysight DSOS054A Highof the oscilloscope Definition Oscilloscope (500MHz, four channels). measurement system is a sanity check similar to shorting the leads together on a DMM before making a continuity or resistance measurement. It is a good practice to perform what’s called a null measurement on the complete oscilloscope measurement system— including probe and connection accessories—to be confident that the oscilloscope, probe and connection method are appropriate for the measurement about to be undertaken.

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HOW TO REDUCE OSCILLOSCOPE NOISE

To make a null measurement simply configure the oscilloscope and probes as they will be used during the measurement, including sensitivity (volts/division) and time base (time/division), then short the probe input to ground (or short the inputs together on a differential probe) and measure the noise. If the results from the null measurement are not acceptable, it may mean that a different oscilloscope, probe, or connection method needs to be used.

TABLE 1. NULL MEASUREMENT

Bandwidth limits More bandwidth is better, right? Not always. The noise voltage of an oscilloscope, probe, and connection accessory are a function of frequency. Limiting the used bandwidth to only the amount necessary for the given measurement will reduce the amount of oscilloscope, probe and connection noise that shows up in the measurement.

Oscilloscope manufacturers recognize the need to adjust bandwidth to make different measurements and have provided a variety of bandwidth limit presets. Some manufacturers also provide the ability to set any bandwidth limit to further tailor the limits to the measurement. If a desired preset or adjustment is not available a math function can be implemented to filter the signal, though this can reduce throughput because there are calculations performed on each acquisition.

To use this filtering technique, one must know the amount of bandwidth necessary for the signal of interest.

There are resources available on oscilloscope manufacturer websites that explain, in depth, how to determine the needed bandwidth. In summary, for digital signals, the necessary oscilloscope bandwidth is 2X the signal

Table 1. Null measurements taken with a Keysight MSO804A oscilloscope (8 GHz, 10-bit ADC, 20 GSa/sec) with an N7020A Power Rail probe (2 GHz, 1:1 attenuation).

BANDWIDTH

V pp

Vr m s

2 GHz

1,040 µV

110 µV

1 GHz

860 µV

90 µV

500 MHz

800 µV

80 µV

20 MHz

460 µV

60 µV

bandwidth. The signal bandwidth can be calculated from the rise time of the signal: signal bandwidth = 0.5/rise time.

If the signal of interest is not digital or is of an unknown source, like noise on a power rail, there’s an alternative approach. In this case, the signal is probed at the full bandwidth of the measurement system and observed in the frequency domain via an FFT (Fast Fourier transform) to see the frequency at which the signal content diminishes.

A bandwidth limit can then be set at or near the frequency observed with the FFT. Oscilloscope probes can have an impact on noise.

Probes come in a variety of attenuation ratios. Probably most familiar is the 10:1 passive probe. One benefit of using a 10:1 probe is that it allows the measurement of signals that otherwise would exceed that maximum input to the oscilloscope. The down side of attenuation is that the scope noise relative to the size of the signal being measured increases proportionally to the attenuation ratio.

Fill the screen Oscilloscope noise, resolution, and accuracy are a function of the full-screen voltage or volts-perdivision. It is a good practice, then, to always expand the signal being measured to fill the whole screen of the oscilloscope. This not only minimizes the amount of oscilloscope noise but also improves accuracy and resolution.

Noise comparison of a 1:1 and 10:1 probe measuring a 50 mVpp sine wave. Both a 10:1 probe and a 1:1 probe measure the same signal, simultaneously—a 20-MHz 50 mVpp sine wave. The only difference between the two measurements is the attenuation ratio. The 1:1 measurement is 52 mVpp while the 10:1 measurement is 65 mVpp. The higher attenuation ratio overstates the measurement by at least 25% due to the reduced signal-to-noise ratio resulting from the higher attenuation. This illustrates that with small signals where oscilloscope and probe noise can problematic, it is best to use as small an attenuation ratio as possible to minimize noise.

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TEST & MEASUREMENT

TABLE 2. SCOPE NOISE VERUS SENSITIVITY

H A N D B O O K

Accuracy is commonly 1-2% of the full screen voltage.

RMS NOISE VERTICAL Taking an eight-bit A/D as an example, the resolution is the FLOOR (V rms AC) SENSITIVITY full-screen voltage divided by 256 (28). A “bad habit” that many ON 50-Ω INPUTS users have is when viewing more than one signal on-screen, they will scale each signal so the signals do not overlap and 1 mV/div 260 µV are easier to see rather than creating individual windows or 2 mV/div 260 µV graticules for each waveform.

5 mV/div 320 µV Noise can impact probe use in another way: Oscilloscope probes usually come with a wide variety of connection 10 mV/div 390 µV accessories to accommodate various targets and improve ease20 mV/div 620 µV of-use. For example, probes come with a variety of ground leads and ground clips. The long ground leads are included 50 mV/div 1.4 mV as a way of making convenient qualitative measurements 100 mV/div 3.1 mV like checking that data lines are toggling or that a supply is up. When making important, quantitative measurements like 200 mV/div 6.4 mV rise time, overshoot, ripple, et cetera, the shortest ground 500 mV/div 13.3 mV connection possible should be used. External noise sources can 1 V/div 24.1 mV couple in the loop of the ground connection. The smaller the loop area of the ground connection the less susceptible it is to Table 2. The baseline noise of a Keysight DSO-S804, 8-GHz oscilloscope at different picking-up external noise. vertical sensitivities (volts per division).

If the signal of interest is repetitive and there is no desire to capture transient events, then averaging is an effective method of reducing noise. Averaging is an acquisition mode where the oscilloscope overlays or averages a predetermined number of acquisitions into one waveform. Over multiple acquisitions, random noise will be averaged-out resulting in a crisp view of the signal.

Another noise-reduction option available on some oscilloscopes is “highresolution” mode. This mode is like averaging in the way it reduces noise, yet it can be used on non-repetitive signals. High-resolution mode averages together multiple

Consider the waveforms of an FPGA core supply (top trace) and FPGA data line (bottom trace) measured on a Keysight MSO-S804 (8-GHz Mixed Signal Oscilloscope). Visually, the information presented to the user’s eye looks the same yet any measurements made on the waveforms on the left side screen will have half the oscilloscope noise, twice the vertical accuracy and twice the resolution as measurements made on the waveforms on the right-side screen. This is because the signals on the left are expanded to fill the screen (there are two screens or graticules placed side-by-side) while the signals on the right only fill half the screen. Considering the top trace, FPGA core voltage, the screen on the left is 100 mV/div versus 200 mV/div on the right. From Table 1, we see the oscilloscope noise is 3.1 mVrms vs 6.4 mVrms (left/right respectively).

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HOW TO REDUCE OSCILLOSCOPE NOISE

SIMPLE SCOPE BLOCK DIAGRAM

A BRIEF LES S ON IN S C OPE N OI S E :

There are two primary sources of noise in an oscilloscope-and-probe system. The input amplifier and buffer circuits in the scope contribute some noise, and the probe amplifier of an active probe has its own noise. Scopes use an attenuator to vary the vertical scale factor. The scope’s noise arises after this attenuation. When the attenuator is set to something greater than 1:1 (the scope’s most sensitive hardware range) the noise will appear to be larger relative to the signal at the input connector of the scope.

Consider a scope that has a basic sensitivity of 5 mV/div with no attenuation inserted (1:1). For this example, we will say this scope has a noise floor of 500 µVrms at 5 mV/div. If we change the sensitivity to 50 mV/div the scope inserts a 10:1 attenuation in series with the input. The noise then appears as if it were 5 mVrms relative to the input (500 µV×10). The same thing happens when a probe with attenuation is attached to the scope. The scope noise appears larger relative to the signal at the input to the probe by the amount of the attenuation.

adjacent sample points of a single acquisition to create a single sample point. In this way, it averages out the effects of random noise on the signal captured by the oscilloscope. The limitation to high-resolution mode is that it reduces the bandwidth of the measurement.

For example, if an oscilloscope sampled data at 8 GSa/sec its Nyquist bandwidth would be 4 GHz. If the same oscilloscope averaged together four adjacent points to create one new point in high-resolution mode, its effective sample rate would be 2 GSa/sec and its Nyquist bandwidth would be 500 MHz. In a nutshell, there are multiple, easy-to-implement techniques available to help measure small signals or signals where measurement system noise can be troublesome. Sometimes only one or two techniques -- like filling the screen or limiting bandwidth -- are enough. However, it is good to be aware of all these techniques for occasions when more challenging conditions present themselves.

REFERENCES HOW TO DETERMINE HOW MUCH BANDWIDTH YOUR SCOPE NEEDS, www.youtube.com/watch?v=ZuhLDAPH7FE www.keysight.com/main/redirector.jspx?action=ref&cname= EDITORIAL&ckey=509008&lc=eng&cc=US&nfr=-11143.0.00 EVALUATING OSCILLOSCOPE BANDWIDTHS FOR YOUR APPLICATION http://literature.cdn.keysight.com/litweb/pdf/5989-5733EN. pdf?id=885255

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H A N D B O O K

TEST & MEASUREMENT

SIMPLIFYING SIGNAL GENERATION USING ARBITRARY WAVEFORM GENERATORS New instruments excel at constructing super-complicated waveforms that once took a lot of time to create.

CHRISTOPHER SKACH SAHAND NOORIZADEH TEKTRONIX

INSIDE AN AWG Int. or Ext. Noise Source

Waveform Memory

Shift Register

Memory Address Control

Clock Oscillator

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DAC

IT’S

hard to beat arbitrary waveform generators (AWG) for convenience and flexibility when it comes to generating signals for test applications. If a waveform can be defined or captured, chances are good that an AWG can generate it. What’s more, the last few years have seen significant advances in digital-toanalog converter (DAC) technology, making it possible to directly generate high-bandwidth, high-frequency signals with high fidelity.

As DAC technology and AWG instruments have become more capable, so too have the techniques for designing the various waveform types necessary for radar and electronic warfare (EW) or for a variety of applications in verification, characterization, and stress/margin testing. The traditional approach has involved defining waveforms in software programs such as Excel or Matlab or even in simple .txt text files and importing them into the AWG. This approach offers flexibility but also requires in-depth knowledge about the waveforms in question, and it can be tedious and time consuming. To improve efficiency and productivity, AWG vendors are now adopting a modular software-based plug-in architecture that allows for point-and-click waveform design.

Analog Output Circuit

Out

From Ext. Trigger

From Ext. Clock

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A simplified block diagram of an AWG architecture.

AWG trends Fundamentally, an AWG is a sophisticated playback system that delivers waveforms based on stored digital data. The digital data describes the constantly changing voltage levels of an arbitrary signal. An AWG starts with a digital representation of waveforms that are either defined mathematically or derived from measurements made on actual signals and then stored as waveform files.

These files are then played back from memory. As you might expect, this practice makes memory a valuable commodity for AWGs. The more

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TEST & MEASUREMENT

memory available, the longer the run times for generated waveforms.

Sample rate, usually specified in terms of megasamples (MS/sec) or gigasamples per second (GS/sec), denotes the maximum clock or sample rate at which the instrument can operate. The sample rate affects the frequency and fidelity of the main output signal. The Nyquist criterion states that the sampling frequency, or clock rate, must be more than twice that of the highest spectral frequency component of the generated signal to ensure accurate signal reproduction. To generate a 1-MHz sine wave signal, for instance, it is necessary to produce sample points at a frequency of more than 2 MS/sec. Although Nyquist is usually cited as a guideline for acquisition, as with an oscilloscope, its pertinence to signal generators is clear: Stored waveforms must have enough points to faithfully retrace the details of the desired signal.

Modern, high-performance AWGs offer deep memory depth and high sample rates. These instruments can store and reproduce complex waveforms such as pseudo-random bit streams.

Similarly, these fast sources with deep memory can generate extremely brief digital pulses and transients.

Memory depth plays an important role in signal fidelity at many frequencies because it determines how many points of data can be stored to define a waveform. Particularly in the case of complex waveforms, memory depth is critical to reproducing

signal details accurately. Expressed by the number of binary word samples, memory can range from 2 Gsamples for mid-range AWGs to 16 Gsamples for highend instruments. For reference, 2 Gsamples of waveform memory can play 400 msec of data at 5 GS/sec, which is sufficient for most applications.

An instrument’s bandwidth is independent of the sample rate. The analog bandwidth of a signal generator’s output circuitry must be sufficient to handle the maximum frequency that its sample rate will support.

In other words, there must be enough bandwidth to pass the highest frequencies and transition times that can be clocked out of the memory, without degrading the signal qualities.

Another important AWG consideration is vertical resolution. It pertains to the binary word size, in bits, of the instrument’s DAC. The vertical resolution of the DAC defines the amplitude accuracy and distortion of the reproduced waveform. A DAC with inadequate resolution contributes to quantization errors, causing imperfect waveform generation. While more is better, in the case of AWGs, higher-frequency instruments usually have lower resolution. Taking advantage of the latest advances in DAC technology, AWGs are now appearing on the market with an impressive combination of 16-bit resolution and a 10 GS/sec sample rate. AWGs with higher sample rates in the 50 GS/sec range or so typically have 8- or 10-bit vertical resolution.

A growing number of applications require multiple output channels that must be precisely synchronized.

THE COMPLEX PATH FROM WAVEFORM MEMORY TO ANALOG OUTPUT

As this simplified block diagram of the DAC in a Tektronix AWG5200 illustrates, modern DACs incorporate digital signal processing and conditioning.

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SIMPLFYING SIGNAL GENERATION

For obvious reasons, anti-lock braking systems in cars require four stimulus signals. Biophysical research applications call for multiple generators to simulate various electrical signals the human body produces. Quantum computing applications push multi-channel output requirements to new levels. These applications involve sending dozens or even hundreds of synchronized signals to manipulate q-bits. Another extreme example is that of occupied spectrum measurements where designers must try to simulate an environment filled with RF/microwave signals from military and commercial radar and radios as well as countless consumer devices. For these measurements, the more test sources the better.

To meet multi-channel signal generation requirements, the latest AWGs offer up to eight analog output channels per instrument, and multiple instruments can be synchronized to create large systems with 32+ channels. In the case of the Tektronix AWG5200 introduced earlier this year, each of the instrument’s eight independent channels provide less than 10 psec channel-to-channel skew. Each of the channels have independent paths out, individual amplification, separate sequencing, up-conversion, dedicated memory, and can be controlled independently with minimized cross talk or limitations on performance. The only common factor is that all channels share a common clock or, if the user chooses, can share an externally supplied reference clock.

Advancing DACs The push to reduce the size and cost of telecommunication and military systems is driving manufacturers to integrate more functions into a single DAC chip. Some advanced high-speed DACs also incorporate digital signal processing and conditioning functions such as digital interpolation, complex modulation, and numerically controlled oscillators (NCO). These

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The ability of AWGs to work in one of several DAC modes allows instrument users to output signals at the cleanest portion of the DAC bandwidth and frequency roll-off positions.

features enable direct generation of complex RF and other signals in an efficient and compact way.

AWGs that incorporate these highly-integrated DACs offer such features as a digital complex modulator and multi-rate interpolation. With internal quadrature (also called IQ) modulation, IQ mismatches often attributed to external modulators and mixers are eliminated. (IQ imbalances arise from mismatches between the parallel sections of the receiver chain dealing with the in-phase (I) and quadrature (Q) signal paths.) Also, there is no inband carrier feed-through, and there are no images.

Built-in interpolators also support the ability to create waveforms more efficiently. This feature reduces waveform size and compilation times and extends playback time as well.

DACs offer a variety of familiar modes including non-return-to-zero (NRZ) mode, return-to-zero (RZ) and mix-mode. AWGs provide access to these modes to ensure signals are output at the cleanest portion of the DAC BW and frequency roll-off positions.

Creating Waveforms It has always been a significant design challenge

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TEST & MEASUREMENT

H A N D B O O K

to create stimulus signals that can fully exercise a prototype. In the case of RF applications, complex high-frequency modulated signals with jitter, spread-spectrum clocking, and other time-variant effects have traditionally required a benchtop of pulse, function, modulation and RF generators.

Compared to these alternatives, an AWG provides finer control, granularity and repeatability for the signals and stresses being generated.

The wide variety of different waveforms that AWGs create can ultimately lead to thousands of different test scenarios. Additionally, the AWG’s sequencing abilities can be helpful. Sequencing means iterating over several segments with each segment comprised of several samples. AWGs may support several stored sequences and may also allow programmatic control of the sequences using external triggers. Sequencing allows users to sweep through a wide range of voltage values and stresses. The AWG generates multiple waveforms and sequences them so they emulate the effect of turning the knob to dial-in jitter.

Another advantage of AWGs is their ability to save waveforms, edit them offline, then share them with teams globally. This can help to isolate bugs early in the development cycle for globally dispersed teams, for example.

For many users, the default approach for generating signals is to use MathWorks’ Matlab and the Signal Processing Toolbox, which provides a broad set of functions and apps to generate, measure, transform, filter and visualize signals. The Instrument Control Toolbox lets users then configure and control AWGs in Matlab.

For users who fall short of Matlab guru status, however, it can be a challenge to create the signals needed for complex applications, particularly if there are many subtleties and changes that must be made quickly for what-if analysis. It can be time consuming, for example, to use Matlab in applications such as the testing of receivers for compliance to complex standards such as MIPI D-PHY or PCIe Express. The process will involve painstakingly walking through the standards’ test documentation to

add appropriate stresses and impairments needed to simulate real-world conditions.

To speed the process of creating waveforms for specific applications or to simulate complex real-world environments, AWGs are now available with a special plug-in architecture.

It integrates into the AWG’s GUI, or can be run on a PC and be moved to the AWG over an Ethernet LAN. The library of available plug-ins continues to grow. If plug-ins are available for a particular application area of interest, the plug-in should be considered the go-to starting point. The plug-in can always be complemented with waveforms created externally or imported from a test instrument such as an oscilloscope.

Here’s a rundown on some of the more common plug-ins and their capabilities: Generic precompensation – Users today need the cleanest signals and the lowest EVMs possible. The precompensation plug-in simplifies the process of generating correction factors and applying them to get the best signals and cleanest performance from an AWG. For instance, when creating waveforms that test wideband receivers, it is important that the AWG signals have

0

R

The importance of precompensation for correcting AWG signals is evident in this correction example using a 16 QAM 32-Gbaud waveform.

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SIMPLFYING SIGNAL GENERATION

a flat frequency and linear phase response. Users can compensate for the first and second Nyquist zones of the AWG. Users can define the LO frequency and choose to get correction coefficients for either the lower or upper side bands as well as define the carrier frequency. Users can also define the bandwidth of compensation either by specifying start and end frequencies (RF & IF) or bandwidth (in IQ/IQ with modulator).

High-speed serial – High-speed serial data signals continue to grow more complex. The plug-in for high-speed serial simplifies signal creation and jitter simulations to reduce overall development and test time. It can be used to create the waveforms required for thorough and repeatable design validation, margin/ characterization and conformance testing.

Specific capabilities include generation of jitter (random, periodic (sinusoidal), inter-symbol interference (ISI), and duty cycle distortion (DCD) as well as spreadspectrum clocking (SSC), pre-emphasis, and noise addition. A combination of various impairments can be created simultaneously to stress the receiver. The input data pattern can be scrambled by defining a polynomial. The pattern duty cycle can also be defined using pulse-width modulation (PWM), which allows for alternatively encoding the bit stream at up to 16-PAM. Multi-tone and chirp – This plug-in is useful for military, aerospace, and RF applications where creating and generating tones are part of a successful mission.

Tones can be created for various applications, including noise power ratio (NPR), with a set of desired start and end frequencies, spacing or the number of tones.

Frequencies can be notched out by setting the start and end frequency of choice.

When generating chirps, the user can decide between high-to-low or low-to-high frequency sweeps and define chirp qualities by sweep time or by sweep rate.

While creating waveforms for testing wideband receivers, it is important that the test equipment generate signals with flat frequency and linear phase response.

In this regard, correction files can be directly applied to tones or chirp waveforms while they are being compiled.

Optical – High-speed telecommunications networks are moving toward faster and more complex modulated signals. The optical plug-in simplifies waveform creation to reduce design iteration intervals. Single or dual-polarization modulation

schemes can be defined with a variety of preloaded modulation formats including BPSK, QPSK, OQPSK, OOK, NRZ, up to 8 PAM and QAM8. Baseband data can be selected from several predefined patterns, user-defined patterns, or from a pseudorandom binary sequence (PRBS) 31 generator, and each N-bit word can be defined with symbols from N unique data streams.

Whether you use one of the above plug-ins standalone or design waveforms using other software tools like Matlab or Excel, it has never been easier to generate high-fidelity signals for test and measurement applications in conjunction with a modern AWG.

REFERENCES TEKTRONIX INC., AWG5200, www.tek.com/datasheet/arbitrarywaveform-generators-1

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TEST & MEASUREMENT

BETTER TESTING WITH WELL-CONTROLLED AC POWER Modern ac power supplies are adept at cleaning up line

H A N D B O O K

conditions that can lead to garbled electrical test results.

STEVE BOEGLE BEHLMAN ELECTRONICS, INC.

IF

you plug your product’s power cord into any unregulated standard factory ac main, there is a possibility that the power you are using could corrupt any testing you do. The quality of ac power can degrade the performance of equipment being tested. Corrupted ac power can make equipment appear to be on-spec when it is off-spec, or off-spec when it is on-spec. Either way, bad test results can be costly. They can bring unnecessary engineering time spent fixing non-problems, or worse, result in costly fixes of faulty units in the field, with a simultaneous loss of customer confidence.

Simply put, plugging sensitive instruments to be tested into your factory ac mains is a gamble not worth the risk.

Commercially generated electricity is distributed at high voltages over long distances by power utilities. At local substations, that electricity is stepped down onto lower-voltage power lines. (Note that some large power users such as iron smelting facilities may have what amounts to their own substation inside the walls of the plant.) Power from the substation routes to distribution transformers at each residence and commercial

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One example of an ac supply and frequency converter is the Behlman Model P1351, a 1.2-kVA single-phase bench-top (or rackmount) instrument that delivers clean regulated ac power in a (2U) 3.5-in.-high and (3U) 5.25-in.high form factor. The P Series can simulate power from any utility as well as aircraft and shipboard power.

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TEST & MEASUREMENT

H A N D B O O K

building. These transformers step the voltage down to 120 Vac. But outside factors can cause sags or spikes in the electricity available from the ac socket. For example, consider the effects of a brownout, a drop in voltage on an electrical power distribution system. Brownouts can last for seconds, minutes or hours.

There are also short-term voltage sags (dips). Even under normal conditions, variations of ±5% at the point of service are within typical power utility specifications.

A problem faced by equipment manufacturers and testing facilities is that they can never be 100% certain that the voltage from their ac mains is at exactly the right level. In fact, their ac mains power may not be sufficiently predictable for accurate, repeatable testing. In addition, their inability to control the voltage and frequency may also cause other problems.

Fortunately, a regulated electronic ac supply can solve power quality issues as well as provide additional testing flexibility.

Even occasional, self-inflicted power quality issues can disrupt operations. A typical production facility may experience voltage fluctuations caused by heavy loads switching on during a typical day.

In addition to voltage fluctuations and distortion, there is also the possibility of high-voltage, highfrequency transients from nearby load switching, lightning, and electromagnetic interference from faulty or improperly installed devices connected to the same line.

The layout of the facility distribution system wiring will also affect the voltage delivered to any individual outlet.

Three-phase systems may additionally suffer from phase imbalance caused by poorly distributed single-phase loads.

Any voltage fluctuations at the point of testing become more apparent as the power level of the product being tested rises. For these reasons, testing facilities may use electronic supplies like the Behlman P1351 series which can reduce voltage variations to below 1% over specified line and load conditions.

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TOP: Small shop vacuum started with a Behlman Model P1350 power supply.

This illustrates the use of a supply able to provide a temporary high output for starting. Note the maximum RMS current value exceeds the unit’s 10-A rating. (Top trace = 100 V/div; bottom trace = 20 A/div) BOTTOM: Small shop vacuum softstarted with a Behlman model P1350 power supply. The voltage was set to zero, and then adjusted to 115 V using the front panel control.

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BETTER TESTING

Harmonic distortion and deviations in voltage can have a large negative impact on test results. During efficiency measurements, for example, variations in the applied voltage over time can degrade results. Power factor and in-rush current measurements will be affected by line impedance. In a typical factory, this impedance varies over time and with location within the facility (distance to service entrance). Varying line impedance can also be a problem when performing certain commercial product tests, where the impedance must be known or measured. Use of a regulated ac supply can allow for standardization of tests results.

Product design and development Product development often requires testing beyond operating limits. The inability to accurately measure incremental differences in design changes or verify product

specifications can cause problems for manufacturers.

In addition to adding engineering costs and delaying products, inaccurate measurements could bring reliability and safety ramifications. Ultimately, customer satisfaction suffers.

Electronic ac power supplies can help evaluate components and sub-assemblies. They can also help verify the performance of items like transformers, fans, relays, actuators, and other ac-operated components. These power supplies can additionally play a role in what-if scenarios for design changes. Many include meters to monitor voltage, current, power, and frequency. Just like a dc bench supply, a bench-top or rack-mounted ac power supply can be an asset to any engineering or test department.

Many commercial safety test standards require the application of operating voltages and frequencies above or below normal limits. It’s possible to adjust voltages using

BEHLMAN P SERIES AC POWER SUPPLIES / FREQUENCY CONVERTERS

MODEL

POWER (VA)

OUTPUT VOLTS VAC (RMS)

OUTPUT (AMPS)

OUTPUT FREQUENCY (HZ)

INPUT VOLTS 10 VAC

DIMENSIONS (19-IN. RACH MOUNT)

STANDARD FEATURES

P1350

1,350

0-135, 0-270

10, 5

Fixed at 50, 60 and 400

120

3.5 in. H x 17.5 in. D

Three fixed frequencies and variable voltage

P1351

1,350

0-135, 0-270

10, 5

45-500

120

3.5 in. H x 17.5 in. D

Variable voltage and frequency

P1352*

1.350

0-135, 0-270

10, 5

45-500

120

3.5 in. H x 17.5 in. D

Programmable voltage and frequency, plus RS-232

P2001

2,000

0-135, 0-270

45-500

45-500

120

5.25 in. H x 17.5 in. D

Variable voltage and frequency

P2002*

2,000

0-135, 0-270

15, 7.5

45-500

120

5.25 in. H x 17.5 in. D

Programmable voltage and frequency, plus RS-232

PF1350

1,350

0-135, 0-270

10, 5

Fixed at 50

95-270

3.5 in. H x 21 in. D

Three fixed frequencies and variable voltage with CE mark

PF1351

1, 350

0-135, 0-270

10, 5

45-500

95-270

3.5 in. H x 21 in. D

Variable voltage and frequency with CE mark

PF1352*

1,350

0-135, 0-270

10, 5

45-500

95-270

3.5 in. H x 21 in. D

Variable voltage and frequency with CE mark, plus RS-232

*P1352, P2002, and PF1352 also offer Option U, which includes USB, Ethernet, and RS-232 Interface using SCPI protocol. (This option enables faster communication speed, power supply programming from greater distances, and compatibility with newer computer systems.) Optional IEEE488 is also available on the Behlman P1351, P1352, P2002, PF1351, and PF1352.

Examples of ac power supplies sporting variable frequencies and output parameters optimized for specialized testing as often arises in motors.

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H A N D B O O K

TEST & MEASUREMENT

Three-phase ac power supply models from 1 – 20 kVA.

simple tapped transformers and variable auto-transformers. The problem is such components suffer from poor voltage regulation with load and line changes. This makes the adjustment of the test voltage like trying to shoot a moving target.

In addition, the cost of high-quality adjustable auto-transformers, also known as Variacs, has risen dramatically in recent years. (Instrument Service Equipment now owns the Variac trademark but the word has become generic for hand-variable autotransformers.) Another disadvantage of autotransformers is that they cannot vary the ac line frequency. Products intended for international sales generally must operate over a frequency range of 47 to 63 Hz per IEC (International Electrotechnical Commission) test specifications. For the aviation industry, frequencies in the range of 360 to 880 Hz are common.

Electronic ac supplies are well suited for production line testing. They can be used to provide bulk-regulated ac to test stands and fixtures.

Automated control features like computer or analog control via PLCs are available to suit most needs. Control via RS-232, IEEE-488, USB and Ethernet interfaces are common. Single-phase systems in the range of 500 VA to 40 kVA and three-phase systems in the 1 kVA to 120 kVA range are available from various suppliers. These supplies range from reference quality instruments to modified UPS units.

Selecting the right ac supply When it comes to purchasing an ac power supply, obvious factors like output voltage, current, and frequency range are determined by user needs and/or third-party test specifications. Additional considerations include surge currents and possible non-linear currents associated with the tested products. Products that incorporate pumps, compressors or other motor-driven loads can have high starting currents. These currents can present issues that can cause test failures. Products with non-linear input currents can also distort the ac output.

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Other considerations involve the type of test performed. Simple functional tests generally need simpler supplies than tests aimed at qualifying a product for efficiency or making power factor measurements.

For example, certain IEC test specifications spell out how to verify the short circuit current available from the ac power source. The specification attempts to provide some sort of standardization so test results can be compared. One such test would be the quantification of in-rush current or motor-locked rotor current. If the ac supply used does not have sufficient transient capability, the test can be invalid.

A high source impedance during testing can mask the true in-rush current experienced when the product serves in its intended application.

The implication here is that certain tests are better served by over-sizing the supply to provide a low source impedance. Consultation with the manufacturers’ engineering staff can help with sizing the power source for a particular test.

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BETTER TESTING

Tests of ac motors can pose special challenges for ac supplies. The most common type of single-phase ac motor is the induction motor with capacitive starting. Induction motors that operate from three-phase power need no capacitors for starting. Here locked rotor current (or LRA for locked rotor amps) is the worst-case current the induction motor draws. LRA is generally measured with the rotor anchored so it can’t spin. As the motor starts turning, the rotor current drops and continues dropping until the rotor hits full or rated speed. At this point the current is at minimum. Rated speed is slightly less than the speed of the rotating magnetic field and depends on how much torque the motor must produce to turn the load.

The duration of the LRA current depends on the motor construction and the mechanical load at start-up. Air conditioning compressors and liquid pumps are some worst-case examples of start-current duration. Their LRA can range from several cycles of the ac waveform to several seconds. Motor manufacturers normally rate their product to either IEC or NEMA (National Electrical Manufacturers Association) standards. IEC standards provide values for typical start currents depending on induction motor size. NEMA tables provide this information in the form of volt-amps during startup. This information, along with the type of test to be conducted, should be known before selecting the ac power source.

It is common practice to specify ac supplies around LRA demands. But for a typical functional or burn-in test, the power supply need only be rated for the continuous current. And several methods could be used for motor starting that would reduce the overall continuous power requirement.

Some power supplies offer a motor-test option. These units feature oversized output devices that allow much higher transient currents than a standard model while maintaining the size and pricing of a unit rated to only supply the run current. Some power supplies also provide a constant-current mode. This mode will automatically reduce the output voltage to limit current while maintaining a sinusoidal current waveform. This action will allow the motor to soft start.

Soft starting is often used for induction motors. A soft starter initially applies a reduced voltage to get the rotor spinning. Once the rotor is up to speed, applied voltage is allowed to reach the rated running value. This method works well for both single- and three-phase motors that do not have a substantial mechanical load at startup (low starting torque).

Commercial motor starters can be as simple as a fixed resistor in series with the motor winding that is switched out once the motor has started. More sophisticated versions allow for adjustable starting voltages as well as adjustable timing. Timing and voltage levels are determined from motor specifications or empirical testing.

The constant-current mode that some power supplies provide creates soft starting automatically. Soft starting can also make testing safer as the motor housing will not tend to move when the motor starts. In one case, a manufacturer of vacuum cleaner motors experienced this problem: Its test stands started motors directly from the ac line. High torque during starting could make a motor leap or roll off the test Three-phase ac power supply m odels from 1 – stand. The addition of a power supply having a constant20 kVA. current mode eliminated the need for restraining the motor.

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H A N D B O O K

TEST & MEASUREMENT 22

This change reduced the test set-up time as well as nuisance fuse tripping in the test stand. The start current exceeded 20 A when connected directly to the ac line, But the 10-A-rated power supply softstarted the motor and brought the nominal run current to about 5 A. There is a type of soft start called ramp-up soft start. Here, the test voltage is applied by connecting the motor to a supply that ramps the voltage up to the run value. This technique works well for applications requiring a bit more control of starting torque. Power supplies can be modified to apply ramp-up voltage for starting compressors and pumps using smaller power supplies than would be necessary otherwise. For really tough products like high-pressure pumps, the voltage and frequency can both be ramped. This method resembles what takes place in VFD (variable frequency drive) circuits and allows for controlled torque and acceleration. To perform this type of test, a power supply with either analog remote control or computer remote control would be useful.

However, some products are designed with control circuits that do not respond well to reduced operating voltages. In addition, some motor tests do not allow for limited current or voltage during starting.

This might be the case, for example, in a test that attempts to quantify a product’s maximum start current. In these cases, the ac power supply must have enough transient power capacity to get the load started.

Power supplies having a Motor Test option provide this kind of temporary high output. These models are designed to allow high short-term output currents for periods typically on the order of 500 msec. Supplies with a motor-test option have over-sized output amplifiers (using IGBTs in these models) and tailor the current-limit response to allow a particular starting transient. In some cases, the ac supplies are tailored to a particular motor or product. Again, consultation with the ac power supply manufacturer is a good place to begin.

Once the requirements are understood, a discussion with a power supply manufacturer should be able to pinpoint the correct model to achieve consistent, reliable results.

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Three-phase ac power supply models from 1 – 20 kVA.

REFERENCES BEHLMAN ELECTRONICS, INC., www.behlman.com

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6/12/17 10:17 AM

THE CASE FOR DC POWER INTEGRITY MEASUREMENTS

THE CASE FOR DC POWER INTEGRITY MEASUREMENTS KENNY JOHNSON KEYSIGHT TECHNOLOGIES, INC.

New instruments excel at constructing super-complicated waveforms that once took a lot of time to create.

WITH

the continual rise in circuit density and performance of modern electronics there has been a proportional increase in the scrutiny applied to the dc power supplies within these devices. The term applied to the study of this dc power is power integrity (PI). PI is the analysis of how effectively power is converted and delivered from the source to the load within a system. The power is delivered through a power distribution network (PDN) that consists of passive components and interconnects from the source to the load. The PDN includes device packaging as well as the semiconductor. And it typically spans measurements made from dc to the multi-gigahertz range.

Product functional reliability is proportional to the quality of the dc powering the product. Because modern products have many more capabilities than previous generations, more can go wrong with them if they lack quality dc power. Thus, there is more emphasis on PI than ever.

Common PI measurements include: • PARD—Periodic and random deviation is the deviation of the dc output from its average value with all other parameters constant.

It is a measure of the undesirable ac and noise components that remain in the dc output after the regulation and filtering circuitry.

It is measured in rms or peak-to-peak, the latter being more common; over a bandwidth range of 20 Hz to 20 MHz. PARD-like variations that arise below 20 Hz are usually called drift.

AN ILLUSTRATION OF PARD

•

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Keysight T&M 6-17 V3.indd 23

Load Response—This can refer to a static or transient load and is a measure of a supply’s ability to remain within specified output limits for a predetermined load. Load response usually includes

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TEST & MEASUREMENT

H A N D B O O K

a measurement of the transient recovery time of the supply.

This is the time needed to settle within a predefined band in response to a load change.

• Noise—This is a deviation of the dc supply from its nominal value. Noise can include random noise, like thermal noise, and spurious signals such as switching waveforms coupling in from adjacent circuits or PARD and load response.

• PDN Impedance— This is the impedance-versus-frequency of the PDN, with or without the dc/dc converter. Often a target impedance is determined during the design phase based on acceptable voltage variation, and then it is measured on the actual target.

The idea of analyzing the power distribution path is not novel.

Engineers have been working with the concept of measuring voltages and currents on power lines since the 1920s. What, then, has led to the current urgency to make PI measurements?

As IC gate density rises, so does the power density of ICs. More gates in a smaller space result in more current consumed in that same small space. Even if the current draw of each gate is reduced, the higher number of gates in that small area offsets the savings. Power, being the product of voltage and current, will therefore rise. This higher power consumption can lead to thermal failures, reduced battery life, a larger environmental footprint, and higher product costs from bigger/more power supplies, more cooling fans, more heat sinks, and larger enclosures.

To reduce power demands, designers have dropped operating voltages. As silicon geometries have shrunk, IC operating voltages

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A simple example showing the effects of power supply jitter (PSIJ) on an FPGA data line. This data was captured using a Keysight S-Series oscilloscope and N7020A Power Rail probe. The target initially supplied the FPGA core with a 5% variation in core voltage. An eye diagram was constructed on one of the FPGA data lines and shows an eye width of ~73 psec. Engineers cleaned up the supply and reduced the variation to <1%.

A new eye diagram on the same data line reveals that the eye width is nearly 55% wider (~114 psec). The only difference between these two measurements is the amount of noise on the dc supply.

Such improvements are not limited to the core voltages of microprocessors, microcontrollers and FPGAs. Sensors, radios and displays will also provide poor service if there is excessive dc supply variation.

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THE CASE FOR DC POWER INTEGRITY MEASUREMENTS

have dropped as well to prevent their damage. Core voltages for microcontrollers, FPGAs, and memory ranged from 3.3 to 5 V twenty years ago and have steadily declined.

Modern microcontrollers and FPGAs have a Vcore of 0.9 to 1.3 V while some LPDDR4 memories have a 0.6-V supply.

Simply reducing dc voltages is not enough. Previousgeneration products with 5-V supplies commonly had a tolerance of ±250 to 500 mV. That same leeway on a modern microcontroller or FPGA would equate to ±25 to 50% tolerance on a 1-V supply leaving such a device inoperable. Supply tolerances have shrunk along with operating voltages and today they are typically 1 to 3% (±10 to 30 mV for a 1-V Vcore ). These tolerances include the dc level and static and dynamic transient current response of the supply. As the amount of acceptable dc supply variation has shrunk, it has become increasingly important to verify the PDN performs as needed to prevent product failures.

Transient current loads challenge the dc supply to maintain voltages within the specified limits, from dc up to the bandwidth of the switching current, typically above 1 GHz. The classic example of a transient current load is

that of a digital line and its associate switching load on the supply. As the digital line goes from low to high it creates a sudden current load that may result in a temporary dip in supply voltage. When the same line goes from high to low it releases the load on the supply that potentially results in a momentary spike on the supply.

What happens if dc is not in spec? A loss or compromise of functions is at stake if the dc supply variation is not controlled. Excessive noise on the power bus may degrade operation and cause data corruption. Device delay is affected by variations in the dc supplying that device. As the supply voltage drops there’s more delay through the gates of that device and vice versa. Thus, variations in supply voltage translate into timing jitter, referred to as power supply induced jitter (PSIJ). Power supply noise is one of the major sources of timing jitter. For example, it is possible to see up to a 50% wider eye diagram on signals from FPGAs if core voltage variations are reduced from 5% to below 1%.

The need for PI is compounded by the growing number of dc supplies inside products today that must be measured

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New Fused Disconnect Switch UL98 Rated for CC fuses up to 30A & 600V The new Fused Disconnect Switch (FDS) series incorporates the switch functionality of a circuit breaker with the high protection level of a fuse. The FDS allows end-users to shut off and isolate branch circuits in electrical control systems in order to safely perform maintenance on the downstream circuit components.

To view the product data sheet and learn more about the FDS, please visit: www.marathonsp.com/New Products/Fused Disconnect Switch Regal and Marathon are trademarks of Regal Beloit Corporation or one of its affiliated companies. ©2016 Regal Beloit Corporation, All Rights Reserved. MCAD16061E • SB0045E

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TEST & MEASUREMENT

H A N D B O O K

Schematic illustration of digital switching loads on dc supplies (L) and actual switching loads of an IoT development board captured using a Keysight S-Series oscilloscope and N7020A power rail probe.

The adjacent screen shot depicts switching loads of an IoT development board. As the digital line goes from low to high it creates a sudden current load resulting in a temporary dip in supply voltage. When the same line goes from high to low it releases the load on the supply resulting in a momentary spike on the supply. The scope screen shot shows the IoT device in three different modes, idle, simple three-bit counter counting up and four lines simultaneously switching. The digital lines of the microcontroller are connected to the MSO channels of the oscilloscope and 3.3-V supply is being probed by the Keysight N7020A power rail probe. The baseline supply ripple, while the device is idle, is a small 6 mVp-p. During the counting up operation, the switching currents increase the supply noise nearly 3X, and simultaneously switching four lines boosts the noise nearly 7X. The IoT device being used has 16 digital data lines, and the switching noise will increase proportionally with more lines active. This illustrates the burden that switching currents place on dc supplies.

and verified. Solid-state drives typically have 12 or more dc supplies while entry level tablets have about 24 dc supplies.

Modern test equipment like oscilloscopes have about 75 dc supplies, and next-generation mobile products are approaching 200 supplies.

There are usually only a handful of unique dc voltages inside a product. The supply count rises because many products carry multiple copies of common dc supply voltages. There may be several different functional blocks inside a product that operate from the same voltages, but each block is given its own supply to prevent interaction or supply contamination from the dynamic loads constituted by the other functional blocks.

For example, when a Bluetooth module begins to transmit data it will present a sudden load on the supply because of inrush current. If another resource shares that supply, the compounded supply load may degrade its operation. To avoid the problem, it is common to arrange things so subcircuits have their own dc/dc converters acting as a buffer to prevent the loading of other subcircuits.

In all, modern life is filled with electronic products which all depend on the prosaic dc supply to function properly. Engineers and technicians responsible for bringing these products to market must ensure these products have dc supplies that meet specifications under all operating conditions.

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REFERENCES KEYSIGHT TECHNOLOGIES www.keysight.com DIBENE, J. T. II FUNDAMENTALS OF POWER INTEGRITY FOR COMPUTER PLATFORMS AND SYSTEMS. JOHN WILEY & SONS, 2014 www.wiley.com/WileyCDA/WileyTitle/ productCd-1118091434.html BOGATIN, E. SIGNAL AND POWER INTEGRITY—SIMPLIFIED, 2ND ED., PRENTICE HALL, 2010. www.amazon.com/Signal-PowerIntegrity-Simplified-2nd/dp/0132349795 WANG, XINJIE - REDUCTION OF POWER SUPPLY INDUCED JITTER WITH APPLICATIONS TO DDR CONTROLLER. A THESIS SUBMITTED TO THE FACULTY OF GRADUATE AND POSTDOCTORAL AFFAIRS, CARLETON UNIVERSITY. 2016. www.curve.carleton.ca/system/ files/etd/b281b419-abb5-4179-8b36c14963ad4d00/etd_pdf/4e191bf57f073a 569fc9913eab2bfc03/wang-reductiono fpowersupplyinducedjitterwithapplica tions.pdf

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SAFER TESTING

SAFER TESTING WITH DYNAMIC POWER SUPPLIES It can be easier to use programmable auto-ranging power supplies as alternatives to fuel-cell power sources that would

ERIC TURNER INTEPRO SYSTEMS

otherwise force the installation of safety and compliance equipment just to run a few characterization tests.

ONE

of the most challenging aspects of configuring a test system is that the test rig must be “better” than the situation being simulated. If a device is rated to a certain temperature level or vibration standard or moisture resistance, the test systems involved must exceed the test parameters or the test may not effectively represent realworld performance.

This relationship between test systems and the things they test is extremely important when it comes to characterizing power systems. Improper characterization of power system performance can result in a product that fails to perform at the edges of its envelope of operational tolerances, often with catastrophic results. This failure to perform is especially an issue with power systems driven by alternate energy sources like solar panels and fuel cells.

In particular, the simulation of a fuel-cell-based power system is more complex than just attaching a dc source to the circuit. Polymer electrolytic membrane fuel cells and related chemical energy harvesting systems, like reflow batteries and such, don’t generate power with a perfect consistency.

The loads that the fuel cell powers must be able to handle variations efficiently and effectively. Individual cells in a stack can receive fuel inconsistently, and thermal issues also impact fuel-cell performance.

Several issues arise when running a real fuel cell in a real test environment. One of the biggest challenges is instrumenting a lab that meets all the regulations and safety concerns that arise when working with devices that use hazardous materials. Fuel cells commonly used in transportation systems, for example, normally have pressurized hydrogen as a fuel source. Labs working with pressurized hydrogen must adhere to safety standards associated with the handling of these pressurized tanks.

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Intepro T&M 6-17 V3.indd 27

ANATOMY OF A HYDROGEN FUEL CELL

Fuel cells come in many varieties but all work in the same general way. Chemical reactions take place at the interfaces of the anode, cathode, and electrolyte. The reactions consume fuel (often hydrogen in the case of land or air vehicles) and create water and carbon-dioxide as an exhaust as well as electricity.

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DESIGN WORLD — EE Network

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TEST & MEASUREMENT

POWER SUPPLY FUEL CELL SETUP

VALUE

RANGE

DESCRIPTION

Point 1: Uoc

0 V... UN om

Open circuit voltage at no load

Point 2+3: U

0 V... UN om

Point 2+3: I

0 A... IN om

Voltage and current define the position of these two points in the U-I coordinate system, which represent two supporting points on the curve to be calculated

Point 4: Isc

0 A... IN om

DC output current at the MPP

H A N D B O O K

Data points entered into a table to describe the V-1 inflection points on a fuel cell performance curve.

U

TYPICAL FUEL CELL POLARIZATION CURVE

A typical polarization curve for a hydrogen–oxygen polymer electrolyte membrane fuel cell. Most fuel cells have the three operating ranges visible here: The activation polarization region (left-most part of the curve), the ohmic polarization region (linear middle portion), and the mass transfer limited region (far right). The polarization curve is obtained by varying the external load resistance. This curve is what simulators set up to mimic fuel cells must reproduce.

P2

Uoc P3

I Ioc

Consequently, it is often advantageous to emulate the fuel cell with a programmable power source for testing purposes, rather than use real cells. In addition, a testing setup should enable devices and components to be easily tested under a wide range of parameters. And tests can proceed more quickly than when using a live fuel cell.

Another advantage to having a dynamic testing setup is being able to test the system beyond the limits of the fuel cell, to create simulation profiles for circumstances when the fuel cell is not working properly, and to test the system’s end-of-life qualities. Emulation greatly reduces time and cost of research and development testing, production testing, and certification testing. Highly advanced, programmable power sources should have the programming capability to perform this type of emulation for testing dc/dc convertors or ac inverters.

For example, the Intepro Systems PSI 9000 series of fast-response dc sources includes just such a fuel-cell emulator — creating a non-linear voltage output that

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Intepro T&M 6-17 V3.indd 28

6 • 2017

simulates a fuel cell or fuel cell stack output voltage.

The fuel cell table function is used to prescribe the voltage and current qualities of a fuel cell. The procedure consists of first setting up the parameters that define points on a typical fuel cell curve.

This information is used to calculate a voltagecurrent table that is passed to the internal function generator. The emulator function includes a set-up feature that walks the user through the process of entering four V-I support points. When finished, these points will be used to calculate the curve.

The fuel cell emulator is an application-specific implementation of an FPGA-based function generator. The function generator uses a tablebased regulation circuit for the simulation of nonlinear internal resistances. Complex progressions can be created by linking together several differently configured sequences. Smart configuration of the arbitrary generator can be used to match triangular, sine, rectangular or trapezoidal wave functions to create, for example, a sequence of rectangular waves with differing amplitudes or duty cycles.

Programmable Power Sources Fuel cell simulation is just one of myriad scenarios needed in both design and production testing of power conversion devices. Tools like the Intepro PSI

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COMPARING OUTPUT CURVES FOR AUTORANGING AND CONVENTIONAL SUPPLIES

H A N D B O O K

TEST & MEASUREMENT

Series specifically target difficult test situations resembling those involving fuel cells. They also are important in similar applications demanding fast response times, advanced power simulation modes, and precision control. Advanced programmable supplies should always include an integrated function generator that allows creation of arbitrary disturbances for complex testing. They should also include a galvanicallyisolated analog interface for voltage, current, and power programming and monitoring.

Another useful feature in some programmable supplies (such as the Intepro PSI 9000) is auto-ranging, which allows for more voltage and current combinations in the output.

This feature allows you to put out maximum power in more ways than a traditional power supply. A traditional supply generally puts out its rated wattage only when it simultaneously generates both its rated maximum voltage and rated maximum current.

Auto-ranging dc power systems are typically a bit more costly than conventional supplies with the same power rating. The price difference is mostly because the output stages of auto-ranging systems must be designed to operate reliably over a wider range of output voltages and currents. But the real cost is lower because one auto-ranging unit can be used to replace multiple conventional units. Each chassis features a controller which brings the flexibility of separating the instrument into individual sources or sources that are paralleled for high-power applications.

There are several features that are increasingly considered ‘must haves’ for advanced power sources. For example, they should be equipped with common communications protocols enabling remote programming and operation. In nearly all testing applications, users should also look for remote sensing to compensate for voltage drops along the load cables. The power supply automatically detects whether the sensing input is connected and will stabilize the voltage directly at the load.

A PSI9000 auto-ranging power supply and the

Auto-ranging output provides wider operating range than avaliable in conventional supplies. Ordinary power supplies provide max power only at one voltage and current. Auto-ranging supplies provide max power over a range of voltages and currents, yielding a power curve like this one.

Using a regenerative load The programmable source used to emulate the output of fuel cells, in our example, draws its power directly from the ac line. The power source output is applied as the input to the device under test. By using a programmable electronic regenerative load, a high percentage of the power output from the DUT can be regenerated and returned to the ac line. This has the exceptional benefit of dramatically reducing the direct energy costs plus reducing the need to expensive, noisy and energy-consuming cooling systems.

As a quick review, an electronic regenerative load redirects the power it receives back to the utility by using an internal micro-inverter stage that is synchronized to the power line input. Use of a regenerative load can reduce the amount of energy that would otherwise be dissipated by up to 93%. And it doesn’t take much to cool a regenerative load. A 10-kW regenerative load dissipates just 700 W of heat, about as much as a typical hair dryer.

power output curve it typically produces.

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SAFER TESTING

THE REGENERATION LOOP Internally, the dc energy that goes to the regenerative load flows into a dc-dc converter which is tied into a dc-ac inverter. The output of the inverter synchronizes with the utility grid to recycle the energy. The regenerative load must also include an automated grid monitoring system that detects the phase voltage and frequency that is used for grid synchronization. If the grid drops out, so does the regenerative load. The unit simply shuts down and waits for the operator to turn it back on. In general, savings through reduced energy costs will pay for a regenerative load in about three years.

All in all, proper system setup when testing power systems is more critical than ever, as energy densities rise and power requirements rise. The performance of the power system is directly linked to effective thermal management as well as safety and reliability, so it is not an area to indulge in half measures. The right emulation system will give the best test results, and your products will reflect the effort.

The ultimate energy-saving test setup includes a regenerative electronic load which recycles energy normally dissipated in a load back to the ac mains through use of a dc/ac converter synchronized to the ac line frequency.

REFERENCES INTEPRO SYSTEMS, www.inteproate.com

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10 10 16 24 30 50 50 8 8 12 16 38 40 25 25 25

3.8 3.8 3.8 4 3.7 3.7 3.7 4 4 4.5 4 4 4 4 4 4

Single Copacked Copacked Single Copacked Copacked Single Single Copacked Copacked Single Copacked Single Single Copacked Copacked

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