EUT Monitoring Hardware

For Equipment Under Test (EUT) monitoring during EMC tests we’ve adapted a Digilent Analog Discovery 2, added an input filter board, and enclosed in a nice case from Lincoln Binns.

This connects to the (rapidly evolving) Monitor-o-Matic 8000 software mentioned in a previous post.

This adaptor and software is going to be used this week during testing of a piece of industrial equipment. This test adaptor will be monitoring both 4-20mA and relay outputs (using the in built power supplies to generate the voltage on one pin of the relay contacts and the digital inputs to monitor the other pin).


Analog Discovery 2

The AD2 is a very versatile piece of kit with a good balance of analogue and digital input and output for a reasonable price. It includes

  • 2 x 14-bit, 30MHz differential scope channels
  • 2 x 14-bit, 10MHz waveform generators
  • 16 x digital logic I/O with pattern generation and logic analyser
  • 2 x programmable power supplies


Future Plans

We’re going to be monitoring how well this device performs for monitoring during testing. We may need to add extra filtering beyond that already fitted such as optical isolation for the digital inputs.

We may also look at fitting a battery and a USB to fibre optic converter for fully isolated measurements in a variety of EMC environments.



EMC Immunity Testing EUT Monitoring Software

One of the hardest parts of EMC immunity testing is monitoring EUT (Equipment Under Test) performance. Not that it is hard-as-in-complicated but it is hard-as-in-difficult.

Concentrating on a display of figures scrolling past looking for small deviations in one or two characters sounds easy, but try doing it for a couple of hours straight whilst doing Radiated RF Immunity testing and you will be fighting an itch to defocus, stare off into the distance or check the news on your phone.

Go on, ask me how I know  😉

Not ideal when you only have a short (think a few seconds) window to catch potential problems or if you have multiple screens to monitor.


Introducing the Monitor-o-Matic 8000

To remedy this and improve the quality of our testing we’ve written a simple application in LabView to handle logging and display of data captured from the EUT during testing.




  • COM Serial input to monitoring PC from EUT. all standard serial port baud rates and configurations supported
  • Use USB to RS-232 or RS-485 adaptors to connect serial port to EUT
  • Extract values / parameters from data stream
  • Plot numeric values on graph
  • Record min and max values seen during test to determine if EUT meets appropriate performance categories
  • Logging of all data during test (all data will be made available as part of any immunity testing carried out at U3C for post testing analysis)
  • Alerts/alarms for data that exceeds defined performance limits. These can be set to latch on in case of problems to prevent missed alarms


Use Requirements

1) EUT has the ability to output serial debug ASCII text data for all key parameters like

  • analogue sensors (e.g. temperature, pressure, humidity, light, voltage, current, etc)
  • digital I/O values (e.g. High/Low, True/False)or system status
  • raw digital values read from other parts of EUT
  • checksums from memory
  • whatever other parameters that you need to monitor to ensure the EUT is working as intended during the tests

2) Format could be human readable text, comma delimited, JSON, XML… whatever gets the job done for you. So long as the values are extractable from the text using regular expressions we can log and plot the data.

3) These can either be output as a continuous stream of data that the MoM8000 software will parse, or the EUT could require separate commands to read each parameter. If you can send us an example serial output ahead of time we can get the software setup before your arrival so that no testing time is wasted during setup.

4) We also need to know what performance limits you might have (e.g. temperature deviation of +/- 0.5C) so that we can enter the appropriate limits. This notification is key as it lets us quickly evaluate EUT performance to the Immunity Criteria (A/B/C) in the appropriate standard.


Future Additions

We’ll be adding extra functionality to this software over time when we develop new requirements. This includes:

  • Subscribe to MQTT topics on local or remote server
  • Read HTTP data
  • Read text data file on local network
  • Tighter integration of test equipment and software to speed up EMC tests

Discuss with us in advance if you have a special requirement for testing and we will do our best to accommodate you.

ESD Latch Up Behaviour in Diodes Inc. Power Switch Parts

A new customer came to me with their product that was having problems during testing at another laboratory. There were radiated emissions problems (mostly solved with improvements to the ground plane scheme on the PCB) and a very interesting (and challenging) ESD problem which I’ll cover in this blog.

Here was the device exhibiting the problem, a Diodes Inc AP22802AW5-7 “power distribution load switch”. Input VBAT from a stick of AA batteries, SW_PWR from a rotary switch, and output to the rest of the circuit.

Problem outline

The ESD problem was described by the customer:

The EUT stopped working when 4kV contact discharges were applied on discharge point shown. I removed the batteries and I put them [in] again and there was not any response from the sample (no otuput and the green LED remained OFF).

[A second sample] was then tested with the same result, although this time not on the first discharge

Upon inspection both devices had failed due to the load switch (AP22802AW5-7Diodes), with one failing open and one failing short and both becoming very warm.

ESD diode placed on input and output of load switch (with no effect)

ESD diodes placed on all [discharge points] (with no effect)

ESD diode places on VCC close to pullup resistors for [discharge points] with no effect

First thing first was to get the product set up on the ESD table (with a bit of added blur to protect the innocent).

It was very easy to re-create the problem observed at the original test lab with the second contact discharge to the EUT exposed contact point causing the unit to shut down.

In each case, the power switch was failing low resistance from IN to GND. The initial theory was that the device was being damaged by the high voltage punching through the silicon layers leaving a conductive path.


Eliminate the possible

I made a series of experiments to determine the coupling path into the problematic device. Working on the principle that, because of the 15cm distance between discharge point and problem device, that conduction might have been the problem.

  • Capacitors on Vin and EN
  • plus disconnect EN line
  • plus ferrite beads and capacitors on Vin, Vout and EN
  • plus local TVS diodes on pins of device
  • plus ferrite beads in series with [EUT input] lines

Whilst none of these experiments were successful they certainly helped eliminate conduction as the coupling path.

Because of the very high frequency content of the ESD pulse, capacitive coupling is likely going to be the dominant coupling method. Whilst it could couple into the device directly, there was more opportunity for the pulse to couple into the traces connected to the device first. Filtering the inputs eliminates two coupling possibilities


Change of sample

The PCB was starting to get a bit tired from the repeated hot air SMT de-soldering and re-soldering so I swapped to another supplied sample. To be able to operate the unit out of the casing I swapped to a linear DC bench supply instead of the AA batteries.

This proved to be an interesting mode as it allowed me to kill the power quickly. The next set of experiments were in an attempt to reduce the effect of capacitive coupling to the problem device.

  • Improved ground stitching / connection
  • Changing supply voltage
  • Indirect HCP discharge – not to EUT but to the Horizontal Coupling Plane albeit with a vertical ESD gun to increase capacitive coupling to EUT.
  • Reduction of coupling into Vin terminal by removing components and copper
  • Addition of copper foil shield over the top of the device


Failure mode discovery

Setting the current limit on the DC supply to a fairly low value (about 20% higher than nominal current draw) was a good idea.

When applying the ESD strikes the supply went into foldback as the EUT power input went low resistance. I discovered that quickly turning off the power and then turning it back on effectively reset the failure mode of the device. This proved to be repeatable over several discharges: zap – foldback – power cycle – EUT OK.

What silicon component behaves like this? A thyristor.

This is a phenomena known as “latch up” where the parasitic thyristor structure present in the CMOS process fires due to over voltage… such as an ESD strike for instance!

Because the device is only small the power dissipation caused by the battery short circuit current is enough to “pop” the device through overheating.


Out of circuit testing

Whilst it doesn’t get used very often, my Sony Tektronix 370 curve tracer is perfect for testing components like this.

(not mine, picture From CAE Online)

Here’s the VI curve of an undamaged device. It’s a bipolar voltage between VIN and GND. On the left of centre is the standard forward biased body diode. On the right is the reverse biased breakdown of around 8V.

Now for a damaged device. In this case the current changes quickly for a small applied voltage and there is no non-linear characteristic. Essentially, a short circuit.

Turning up the maximum voltage that the curve tracer can apply and dialling down the series impedance allowed me to simulate the over voltage fault condition and create a latch up condition. This latch up wasn’t permanent due to the bipolar sine wave nature of the curve tracer applied voltage.

However turning up the voltage enough to cause excess power dissipation inside the device did result in the same failure mode using the curve tracer.



I have never encountered a device that is this unusually sensitive to ESD events before. A nearby 2kV discharge on the PCB top layer ground plane was enough to cause the latch up condition.

I noted in the report to the customer that this device had been changed to “not recommend for new designs” by Diodes Inc. I wonder if they identified this condition in the device and withdrew it for that reason.

The customer resolved the issue by replacing the device with a different part and we all lived happily ever after.

The end.




10g 16ms half sine shock test profile

EN 60068-2-27 Shock Testing of Anti-Shock Rubber Mounts

We’ve been vibration and shock testing of some heavy equipment designed for the construction environment. This is one of the toughest environments for product environmental testing. It’s wet, it’s dusty, it gets hot and cold… sometimes all at the same time! Not only that but it’s a very physical environment where rough treatment is the norm.

This customer is well versed in the art of protecting their equipment from such conditions using a robust frame with the key part of the product mounted on beefy rubber shock mounts.

This slow motion footage of captured of the unit undergoing shock testing really shows you just how useful these parts are.

Test was being performed to EN 60068-2-27, 10g shocks with a 16ms half sine profile. There is significant pulse pre- and post-loading as the piezolectronic accelerometer I use has a pretty poor low frequency response and this seems to help.

10g 16ms half sine shock test profile

The use of these anti shock mounts isn’t without issue. In this case, the springiness/stiffness of the anti shock mount combined with the mass of the equipment leads to a resonance at around 25Hz with quite large displacement of the main equipment mass.

The losses in the anti shock mounts causes a damping effect leading to a softer, wider resonance. The equivalent of resistance in an LC resonator causing a reduction in the Q of the circuit.

Compared to a much sharper resonance (caused by a different physical structure) the overall gain is much lower. The tradeoff is selecting a stiffer mount to damp the resonances but at the expense of transmitting more force through to the unit under protection.

25Hz soft resonance vs other sharper resonance



Compromise EFT Test Setup

When the customer supplied cable isn’t long enough to fit inside the standard EFT/B capacitive clamp what do you do?

One answer, for pre-compliance testing, is to make your own clamp from aluminium foil cut to length and separated from the GRP by expanded foam blocks.

The capacitive clamp is not a sophisticated piece of test equipment and a close compromise can be achieved quickly with commonly available lab materials.

Details of a compromise EFT test setup using aluminium foil and foam blocks.

Making sure there is good contact to the GRP from the generator is important which is partly achieved by taping the cable down with some conductive adhesive aluminium tape.

Overall area of the injection plate is reduced by 25% from the standard capacitive clamp plate area. Therefore the injection voltage was increased by 25% to compensate for the reduced capacitance.

Safety warning: don’t touch the foil when the generator is running!

Obviously not good enough for exact testing to the standard but it is within the spirit of the test and will give some useful information.

Useful Test Adapters for EMC Testing and Electronics Development

Working in an EMC test lab means I get to see all kinds of equipment. No two devices are ever the same so I have to make up / adapt cables to interface various devices. If you work with a wide range of products or just want a bit more versatility in your lab then read on.

I have no affiliation to any of these products, I just use them a lot.

Clever Little Boxes

These versatile little test adapters from Clever Little Boxes are great for being able to quickly hook up one thing to another. As you can see they come in all shapes and sizes. I’ve got a box full of various ones, including the ones shown in the above photo.

Go Bananas

The ubiquitous 4mm “banana” plug and socket is super common on power supplies and other kinds of test equipment. They give a surprisingly low resistance connection for their size which, along with their simplicity, goes a long way to explaining their popularity.

If you’ve ever made up a cable assembly with standard connectors then you know they can be a pain. That’s why I really like these connectors that have a spring loaded gate that accepts a bare wire up to 2.5mm^2.

I’ve just got the standard red and black colours to keep things simple. These work well when paired with a set of crocodile clips

Get Me a Crocodile Sandwich…

I really like to pair these crocodile clips with the 4mm connectors above for super versatile connections to anything big like metal frames or enclosures of equipment.

Hook and Spring

Big numb adapters get a bit crowded when trying to connect onto individual connector pins or component legs. That’s where these teeny spring clips come in. I’ve often ended up with one of my development boards looking like an electronic porcupine with these stuck all over them!


Something More Permanent for Sir?

If I’m wiring up anything using mains voltages that I want to be a bit more permanent and safe then my go to are these spring terminal blocks from Wago. They are like choc block terminal strips with the main exception that these are not rubbish. Rated at 32A they can accept much larger wires that you would think and the spring clips retain the wires with a remorseless grip.

They come in multiple ways although I tend to use 2, 3 and 5 by default. Best of all they are ridiculously cheap. Just don’t get your thumbnail caught underneath the orange lever when it clicks down otherwise you’ll be using some language that is distinctly NSFW.

So “be prepared” (Scout motto) and happy testing.



p.s. don’t get me started on the adaptor vs adapter debate.


radiated emissions plot

RS-232 to USB Converters – EMC Problems Part Two

A while ago, I wrote about EMC immunity problems with USB to serial converters and how it was easy to fix with a small 100pF capacitor to ground on the TXD and RXD lines for a bit of filtering. Well, now I’ve found the opposite problem of EMC radiated emissions failures caused by these periodically problematic products.

In this case it appears to be harmonics of the 48MHz internal clock of a SiLabs CP2102 being conducted out of the converter on the TXD and RXD pins.

These little boards are generally used as development tools in a laboratory setting but there’s nothing to stop this IC or module being integrated into a product where these problems would manifest themselves.

The below plot shows the radiated emissionsbefore (light blue) and after (red). This module was connected to it’s host by 10cm unshielded wires, not an unreasonable application by any means.

radiated emissions plot

And what was the fix? Yep, you guessed it, some 0603 100pF capacitors on the output pins to ground. I bet that would help with immunity too! 😉

crude differential mode surge spice circuit

Surge Testing, MOV Position and Fuse Current

I’ve been working on a power supply product for a customer with a very tight limit on the AC mains fuse rating. One of the problems this causes is during differential mode surge testing.

When the metal oxide varistor (MOV) connected line-to-line fired, the resulting current was enough to blow the fuse after a couple of surges at the specified 1kV surge (1.2/50us, 2 ohm). Clearly there wasn’t enough headroom for the product to pass the test. A different MOV with a higher clamping voltage would have reduced the peak current but at the cost of higher voltage stress elsewhere in the circuit.

I decided to look at if the position of the varistor within the circuit made a difference to the surge current in the fuse. It started off in the middle of the mains filter (PCB routing convenience I suspect) but perhaps mounting it before the filters would help? What about at the end of the filter chain, then the X2 capacitors can go to work on the surge pulse first.

The easiest way to try these scenarios was to stick it into SPICE (I like SiMetrix) and have a look at the variables. I crudely modelled the input stage of the power supply as shown below. I guessed at many of the series impedances for the fuse and the capacitor. However the leakage inductances and DCR for the inductors I measured using my excellent Peak Electronics LCR45 component meter. The MOV was simply a 1N4004 diode with a 400V reverse breakdown and the surge was only applied in the +ve direction.

crude differential mode surge spice circuit

I varied the position of the “MOV” between positions A, B and C to see if there was a difference in the surge current through the fuse (R15). Interestingly enough, there was.

surge test spice output

Red = A, Green = B, Blue = C

So the further down the filter chain that the MOV is placed, the less the peak surge current (56% lower) and the RMS current (23% lower) through the fuse.

The results were positive too. The power supply went from failing on the 5th strike at 1kV to passing 10 strikes at 1.75kV. A marked improvement resulting in a more robust product.


conducted rf immunity calibration impedance and measurement voltages

When is a Test Level Not a Test Level?

Answer: When you don’t read the standard properly!

I was verifying my EN 61000-4-6 conducted RF immunity test setup after the construction of some new test adaptors and acquisition of some new equipment when came across something that left me scratching my head. I figured it out eventually and updated my calibration procedure with a note but it did have me puzzled for an hour!

Like most conducted immunity signal generators, the one I use combines a signal generator and modulator with a power amplifier and some front panel controls/readouts for performing the basic functions. It also has an RF Input for calibrating Coupling/Decoupling Networks (CDNs) which measures the voltage at the 150/50 ohm calibration adaptor and sets the output voltage of the generator to the correct level. My generator has a LED bargraph display showing the level which provides a reassuring visual confirmation that everything is OK.


Confused by Conducted, Stumped by the Scope

Having calibrated my new CDN at 3V, since I had a scope within reach, I decided to run the test but monitor the output of the calibration adaptor with the scope to make sure it was all working OK.

I did not see the expected 3V level, instead the RMS measurement on the scope was 0.5V and the pk-pk was just over 1.5V. I checked my 50 ohm thru termination on the scope input and even swapped it for a different one. My second scope also read the same voltage so it clearly wasn’t the scope. Puzzling.

I swapped the CDN for one that had been previously calibrated CDN and the lower than expected output voltage persists. Try turning up the generator voltage to 10V and I can’t even achieve 3V on the scope. Yet when I swap the connection from the scope to the RF generator it proclaims that yes, that is indeed the level that the generator says it is outputting.

Putting a BNC T-piece in series and monitoring the voltage with the RF input terminating the signal still achieves the same result. Is the generator RF input broken and reading the wrong voltage?

I checked the operating manual of the generator – the cal setup I’ve been using for years is correct. Then I carefully read the standard, focusing on the section that deals with calibration of the test adaptors. All became clear…


Open Circuit Voltage vs Loaded Voltage

EN 61000-4-6 specifies the test levels in terms of Uo, open circuit voltage. However the generator level setting part of the calibration is based on a measurement of Umr, the measured output voltage. This is a slightly simplified version of Figure 9 from the 2014 version of the standard showing the impedances of each part of the system.

conducted rf immunity cdn calibration impedances

Tucked away at the bottom of the calibration section is the formula that links the two together.

Uo = Umr / 6

Which yields the following values that the input of the generator or the scope should be looking to measure:

Test LevelUo (Vrms)Umr (Vrms)

For the measurement, the impedance of the decoupling part of the CDN is big enough that the termination of the AE port is not significant to the measurement, making most of the current flow through the EUT port network. You should be able to open or short the 150 ohm AE port termination and not see the measured output voltage change significantly.

By simplifying the above image and a bit of Ohms law you can clearly see that Umr is 1/6 of Uo.

conducted rf immunity calibration impedance and measurement voltages

Of course these are RMS voltages. If your scope that you are measuring with doesn’t have an RMS function then you’ll probably be measuring the peak to peak voltages. The conversion factor is:

Vpk-pk = Vrms x 2 x sqrt(2)

Which when added to the above table makes life a bit easier.

Test LevelUo (Vrms)Umr (Vrms)Umr (Vpk-pk)


Panic Over

Armed with this new knowledge I revisited my calibrations to find that yes, everything was measuring correctly. The RF generator, being designed specifically for conducted RF immunity testing, takes care of the divide by 6 in it’s calculations.

As an ex-colleague was often heard to remark “every day is a school day” and today’s lesson was a good one. I hope this article saves you a bit of head scratching next time you are verifying your conducted RF immunity test setup.