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.

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! 😉

EMC Immunity Issues with RS-232 to USB Converters

These little converters are super handy to interface between your modern PC or laptop and the simpler, lower technology RS-232 serial port used by many pieces of equipment for control or debug purposes. However, like any commodity item there are design compromises, including EMC ones, that you need to be aware of.

I was recently performing some Electrical Fast Transient (EFT) testing on a customers product and was surprised to observe it failing at quite a low level of injected transient of 200V. It appeared that the whole system crashed when the bursts were applied to any of the digital I/O ports.

Even more confusing was that I’d looked over the schematic and the port protection measures that they had implemented were very sensible with ferrite beads and diode clamps.

A pointer came from observing the front panel of the device with all of it’s indicator LEDs blinking away as if it was working properly. Yet the equipment under test (EUT) wasn’t responding to serial communications and the TeraTerm serial port software was still showing a connection.

Checking through the test setup, I theorised that the RS-232 to USB converter that I was using might be crashing or responding to the EFT pulse as a start bit to a frame. Despite being isolated with a Coupling/Decoupling Network (CDN), when a scope probe was added to the RXD line on the decoupled side of the CDN, a transient with 30V of pk-pk amplitude was visible when the EFT burst was applied.

I tried two other converters that I had in the lab and none of them were happy with this pulse and also refused to work correctly.

a selection of usb to serial converters

So I knocked up a small filter PCB with a pi filter on each line (RXD, TXD and 0V) consisting of 2 x 100pF capacitors and a ferrite bead. The non-line side of the caps was taken to the HF ground plane using some adhesive copper tape (the EMC scoundrel’s last resort!) to return the currents back to the generator and not into the converter.

EFT test setup showing flow of HF current and position of small filter

Success! No more interference and the converter works perfectly.

As an experiment (OK, I got slightly distracted by something interesting) I played around optimising the filter and managed to get it down to just two components – a 100pF capacitor on the TXD and RXD lines of the converter.

Now I know that these devices will be designed to the lowest price point but two 0402 capacitors is hardly breaking the bank! It does make you wonder how they managed to get through their own EMC testing, if at all.

Incidentally when this was later tested in the chamber it had some fairly strong 12MHz harmonics from the USB 1.2 data lines that only just squeaked under the limit line lending further weight to my suspicions of corner cutting and poor design!

So today’s lessons are:

  • Beware of cheap generic test adaptors and EMC issues caused by them – both immunity and radiated.
  • Consider your port filtering carefully. Many I/O interfaces can stand a small capacitor or filter adding to it and the benefits for EMC are significant. It gives a path for interfering signals to the local ground and will also improve your emissions too. The customer who’s product I was testing had such parts fitted; it passed the testing at 1kV EFT without issue (the spec is 0.5kV).
  • Using a fibre optic serial port adaptor would probably have helped here by increasing the common mode impedance of the connection (assuming of course it had been designed properly!)