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V2 pocket EMC debug probe PCB - near field probe set - board front

Pocket EMC Debug Probe V2

This is a work in progress guide for the assembly and use of the Version 2 “Pocket EMC Debug Probe” from Unit 3 Compliance.

Whilst this page is being written, you can also refer to this page for the previous version which has much of the information that you need.

 

V2 pocket EMC debug probe PCB - near field probe set - board front V2 pocket EMC debug probe PCB - near field probe set - board rear

 

Introduction

A short introduction to near field probes and their uses.

 

Assembly Guide

Components required (0805 resistors, R1 = 470R, R2 = 10k, gives 450 ohm parallel combination, required for the 10:1 into a 50 ohm input, capacitor 1nF, C0G/NPO dielectric, 50V)

Sourcing SMA edge mount female connectors (RS, eBay)

Recommendations for the probing pin (socket strip or a bit of wire)

Suitable ferrite cores for the current transformer

90 degree options for the B-field loop probe and E-field capacitive probe (on E-field probe snap off – scrape copper on each side of slot before you snap off the end to enable soldering)

 

Usage

Narrow down emissions from a PCB by scanning over the surface and looking for noisy traces

Don’t just use the B-field probe, the E-field probe is just as important. Both will respond differently.

Use the current probe attachment to check for noisy cables

Narrow down to individual traces using the high bandwidth probe

 

 

 

Thoughts on In House Pre-Compliance Test Equipment

From an email sent to a customer who asked for some feedback on their list of proposed EMC pre-compliance test equipment.

 

“On the subject of equipment, sounds like you’ve identified a nice little pre-compliance setup there! I agree that investing in equipment is a much better long term view than just hiring some in (for the common tests at least).

Com-Power stuff is very good, I use some of their kit myself. The TekBox stuff is very reasonably priced for pre-compliance and again, I use some of their equipment in my test setups. If you are making conducted emissions measurements with a LISN you’d be wise to put a limiter in series with your spectrum analyser input to prevent costly damage. Armoured cable isn’t strictly necessary as it can be difficult to handle, just regular good quality coax is fine.

As a substitute for radiated measurements you can use a current probe around cables as these tend to be excellent emitters of radiated noise. It helps if you already know what the problematic frequencies are. Again, Tekbox make some very reasonably priced probes.

I would also consider the Signal Hound SA44B or BB60C (I have one and I really like it) spectrum analysers as an alternative to the Tektronix one. There’s quite an in depth review of the BB60C here. Their software is easy to use and crucially is free with an EMC pre-compliance option.

If you need test equipment support, I usually talk to Joy Torres at Instruments 4 Engineers in Stockport. She is very helpful and can often get you access to good prices. joyt@instruments4engineers.com. I know she represents Tek, Com-Power and Signal Hound.

The main downsides of making your own emissions measurements is the amount of ambient noise from other electronics, radio sources, reflections off nearby surfaces, etc. It is good for “is A better than B” testing, but it doesn’t really get you to a “but does it pass?” kind of answer. This takes some practice to get right and to get to know your equipment.

The question you could ask yourself is “do I have both the capital and the time to invest in this solution?”. I’ve talked to other companies about their setting up of pre-compliance facilities in house and their struggles tend to be:

  • Engineers lack time to work on the EMC project aspects as well as their regular project work
  • A lot of up front time required to learn the variables of the test setups and standards
  • How to make useful measurements and interpret the results
  • How to match the results from this testing to predicted test lab pass/fail results (spoiler: it is very tricky)
  • EMC knowledge is not well shared amongst employees or the engineer who has the knowledge is on holiday / off sick / unavailable / working on something else

None of these obstacles are insurmountable with good planning and management  🙂  but it is worth going in with eyes open.

In my experience, the best weapon in the EMC armoury is not a spectrum analyser, nor an antenna, not even a full test lab. It is a design review. When we design something we define its EMC characteristics. Getting this up-front bit right is the key to shorter design cycles, fewer prototype runs, reduced time to market and much less stress. Working together to catch the problems before they become problems gives experience in how to design for EMC and the lessons learned can be carried forward from project to project. We’ve helped many customers this way and we’d be happy to help out on your future product ideas.

Similarly, if you want to run quick checks on equipment that needs equipment that you don’t have then we are happy for you to send us the kit via courier and to run the tests on your behalf. I appreciate that our office isn’t exactly nextdoor so anything we can do to help minimise disruption to you and your team would be our pleasure.”

 

Hope some of this is useful

All the best

James

 

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.

 

 

Specifications

  • 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.

Schaffner/Teseq NSG 5500 test system

New Automotive Test Capabilities ISO 7637-2

The best day is new equipment day 🙂

We are continuing to invest in our test capabilities. As such, the Unit 3 Compliance EMC test laboratory has just acquired a Schaffner (Teseq) NSG 5500 automotive surge/EFT test generator.

Schaffner NSG 5500 test systemWith this, we now have the capability to test your equipment to the ISO 7637-2 standard for automotive conducted transients.

The NSG 5500 will generate the ISO pulses 1, 2a, 3a and 3b, along with the Load Dump and Clamped Load Dump pulses 5a and 5b.

This gives us the capability to support your automotive product development to these standards:

  • EN 50498:2010 – Aftermarket electronics for vehicles – full testing for CE marking
  • CISPR 25 for non Immunity Related Function EUTs
  • UNECE R10.06 (pre-compliance)
  • ISO 13766-1:2018 Earth Moving Machinery (pre-compliance)
  • ISO 7637-2:2011 automotive conducted transients
  • ISO 16750-2:2012 automotive electrical loads (part)

 

Footnote:

Timing is a curious thing. Like two buses arriving simultaneously after a long wait I find things tend to cluster up. This acquisition occurred not long after publishing this blog post on how to test to the automotive standards without an automotive surge generator.

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.

James

 

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

 

Simple RF Current Transformer for EMC / EMI Investigation

This post contains some background info related to the video I posted on YouTube on how to make a simple RF current transformer, a great tool for debugging EMC / EMI issues such as radiated emissions from cables, or tracing conducted RF immunity noise paths.

RF current transformers (or probes) are commercially available products from places like Fischer CC or Solar Electronics and they work really well, have specified bandwidth and power handling characteristics, built in shielding, robust case, etc.

They also cost a few hundred £$€ each which, if you are on a budget like most people, represents a significant investment for a individual or small laboratory. However, this one can be built very cheaply; most labs will have a development kit with some clip on ferrite cores, if not the core I used only costs £5 from RS.

DIY Current Probe

I’m a big fan of making my own test adaptors and equipment as its a great way to really understand how things work and the compromises in any design. As such I decided to share how I go about making this kind of really useful tool.

It’s primary use is for A-B comparison work; measuring the current, performing a modification and then measuring the current to see the improvement.

It is to be stressed that my version is a crude but effective piece of equipment and does not replace a well designed commercial product. There’s a time and a place to invest in quality equipment and one should use engineering judgement on when that is. For instance, measuring the RF current accurately is definitely a job for a properly designed and characterised device.

If you want to explore RF current transformers in more detail then there is plenty of info on Google, but these links are useful places to start.

Some of the design compromises involved in this low cost approach include:

Core Losses / Insertion Loss

The ferrite material in these cores is specifically designed to be lossy at the frequencies of interest, which will result in a lower reading than a higher bandwidth core and a reduction in the amount of noise on the cable downstream from the noise source. This can in some cases mask the effect you are trying to measure. The commercially available products use low loss, high bandwidth ferrite cores.

A high insertion loss also makes these parts more unsuitable for injecting noise into circuits for immunity testing. they can be calibrated for this task using a simple test setup (to be covered later)

Secondary Turns

Number of secondary turns controls sensitivity but the more you add, the inter-winding capacitance increases, decreasing the bandwidth of the tool. I generally use 5 or 6 turns to start with but I do have a 20 turn part made with micro coax on a solid core which also helps to deal with…

Capacitive pickup

From the cable under test to the secondary winding. Normally a split shield (so that it doesn’t appear as a shorted turn) is built in to commercial products. Guess what, that’s easy to do on this with a spot of copper tape or foil.

Not as Robust

Although a well designed product, the plastic hinges and clips on the cores are not designed for repeated opening and closing. The Wurth Elektronik system of a special key to open and close the core is much more robust at the expense of having to keep a few keys to hand for when they inevitably go missing. However these parts are so cheap and quick to make that a broken clip on core is no real obstacle.

Future Videos

I’ll be following this video with some hints and tips on how to use these devices effectively for finding radiated emissions problems and for looking at conducted RF immunity issues. Stay tuned.

Video and Construction Errata

The sharp eyed of you will have spotted that I originally assembled the BNC connector on the core so that it covered the key-way to open the clamp. I rectified this but didn’t film the change.

Also, you can wrap the wire round the core without removing it from the housing but that means you don’t have a nice flat surface to affix the BNC connector to. It does make it easier to close the clamp however so make your choice.