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Networking Equipment – EMC Radiated Emissions Problem Solving

I had an email from a customer that I’m working on some design consultancy work with, saying that one of their prototype products was having some radiated emissions problems at an accredited lab. Could I take a look?

Absolutely, EMC radiated emissions problem solving is my favourite part of the job! Ironically, it is usually the customers least favourite part!

Thankfully I had a slot free the next week so they bundled their kit into the car for the long drive “Up North” from their base in the South West of the UK.

After some tinkering, the equipment was set up in the chamber for some radiated emissions work. The first scan confirmed the problem levels and frequencies that had been observed at the other laboratory.

The problem areas from their last scan were at 35MHz, 80-90MHz and a broad band between 150MHz and 220MHz.

 

System Overview

The system was housed inside a nice aluminium case that was being used for CPU heatsinking and environmental protection as well as EMC shielding. A rough diagram of the internals shows a main PCB with a large CPU / memory block in the centre and a variety of cables leaving the PCB and the casing.

The main power cable housing also had two debug connections inside the same housing that weren’t being used in the field but were available for updating software and such like.

rwns overview diagram

As is so often the case, this product was in it’s final stages of the development life cycle, meaning that no major design changes were possible. These EMC problems would have to be resolved using easy to fit additional components. Thankfully I have plenty of things in stock to try out.

 

Emissions Analysis

There are two important characteristics about these emissions that show us where to look

  1. They are predominantly broadband, an indication of analogue noise e.g. DC/DC converter / power supply. Sometimes this broadband noise is generated by digital switching but this can be less common.
  2. They are all low in frequency, where large or long structures are the most efficient antennae. This usually means cables.

So power noise and cables…. hmmm…. any good ideas?

 

OK Kids, Let’s Take a Look at the Cables.

In a very sensible move by the designer, both the DC power and Ethernet cables had some common mode filtering on the PCB.

Ethernet magnetics have common mode chokes built into the transformer stack which reduces the noise emitted and increases the susceptibility performance of Ethernet despite the often unshielded twisted pair cables used.

The caveat is that once the cables have left the magnetics that they must be protected from other interference sources. Noise coupling on to these lines is going to be heading straight out of the enclosure using these lines as the antenna. Similarly, if common mode noise gets onto the centre-tap of the output side of the magnetics then this can also cause similar issues.

I have experienced system noise coupling on internally routed Ethernet cables before and it nearly always results in lots of low frequency emissions.

The power cable had a small surface mount Murata filter in place with excellent attenuation at the frequencies of interest.

murata filter characteristics and equivalent circuit

Both the Ethernet and power cables pass through the shielded enclosure with no connection or filtering to the case. In bypassing the quite nice Faraday cage of the enclosure, any noise current on these lines will inevitably appear as radiated emissions and be picked up by the receive antenna..

Now to find out some more info.

 

Radiated Emissions Experiments

First, unplugging the Ethernet cable dropped the emissions significantly from 30MHz to 120MHz.

Secondly, some messing around with ferrite cores on the power cable reduced the 150MHz to 220MHz hump down to sensible levels.

This left a single peak at 270MHz that was traced to noise using the coaxial RF cables to the antenna to radiate.

Lets look at each of the points in a bit more detail:

 

Ethernet

The only practical method of dealing with the Ethernet emissions was to change the bulkhead connector to a metallic screened version and the external cable to a SSTP (Screened Shielded Twisted Pair) type of cable. No exciting analysis here I’m afraid.

 

Details of the Power Cable Noise Coupling

The most interesting coupling mechanism was happening inside the un-screened bulkhead power connector. Thanks to the power filter on the PCB, there was very little noise being conducted back down the cable from this line. However, the debug connections to the CPU are picking up all kinds of noise and carrying that noise to the connector.

rwns cable coupling close up

Disconnecting and bundling the debug cables near the connector cuts the radiated emissions down to next to nothing.

What’s most interesting is that the capacitive coupling region between the power cable and the internal debug cables is so small. The connector is only 20mm long and the cables run parallel with each other for barely any distance. And yet there is enough noise current being coupled onto these lines that it causes a radiated emissions problem.

 

Details of the RF Antennae Noise Coupling

By the time that all of the cables had been filtered or removed, there remained just one emission at 270MHz that was failing the Class B limit. An investigation with RF current probes showed a lack of noise on the main output cables listed above, even when they were screened or filtered appropriately.

A wander round the enclosure with an electric near field probe and spectrum analyser showed a spike in emissions near the RF antenna housing on the side of the EUT.

 

rwns antenna spurious

Checking the antenna feed cables showed them connected to the PCB pretty centrally. Disconnecting the coaxial cables from their mating halves dropped the emissions down to the noise floor.

Even though the noise isn’t in-band for the antennae themselves, they still perform well enough to radiate the noise and cause an emissions problem.

 

Summary of Fixes Applied

The below diagram shows the fixes applied to the EUT to achieve a Class B pass.

rwns applied modifications

Firstly, a fully screened metal bulkhead Ethernet connector was chosen for use with a shielded cable. This isn’t ideal from the installation point of view but is ultimately unavoidable without more significant modifications to the EUT.

Secondly, a Wurth ferrite was equipped around all three of the cables connected to the power bulkhead connector. As detailed above, it is necessary to put the ferrite around all three cables and not just the power to reduce the noise entering the capacitive coupling region around the connector.

Thirdly, a small ferrite was placed around each of the UFL cables at the point at which the antenna cables left the housing. This is a fairly common modification for radiated emissions, one I’ve employed several times before, and there are numerous suppliers of ferrites of various lengths with just the right inside diameter for the type of thin coaxial cable used with UFL connectors.

 

Results

Closing Thoughts

Any time your cable passes through a shielded enclosure with no RF termination at that point, you can pretty much guarantee its going to need some filtering.

Nothing particularly in depth in this analysis of the EUT, but I did find the coupling in and around the power connector particularly interesting.

At the end of the day, the best outcome was a happier customer with a path forward for their product.

 

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.

 

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! 😉

Busy, and a Birthday

It has been a very busy few months at Unit 3 Compliance; it feels like the chamber turntable hasn’t stopped spinning. There has been a wide range of products through the door from prosthetics to video wall controllers, from high spec IoT products to motion sensors, from lighting power supplies to RF amplifiers. I really love the variety of work!

I’ve also had some safety assessment work to carry out on which is always interesting. Disassembling mains transformers to measure the creepage distances inside is fascinating, getting out the angle grinder to hack the laminations apart just adds to the fun.

There have also been a fair amount of design reviews and general consulting work in between. To be able to work with customers right at the start of the project is invaluable as it sets them on the right path without having to find problems further down the line.

I’ve found lots of interesting nuggets of EMC information during this process that I’m looking forward to sharing with you in some future blog posts once I get time to sit down and write them up.

I managed to escape down to Lincoln to speak at the Open Source Hardware User Group oshcamp18 meetup on the subject of EMC testing. The delegates came up with lots of good questions at the end and the audience participation (see below slide) of the talk went down well. Higher! Lower!

play your compliance cards right!

Good to see reconnect with some old contacts and make some new ones. The other talks were very interesting also, lots of good work going on in the open source hardware field at the moment.

Lastly, and it snuck past without me spotting it, Unit 3 Compliance had it’s first birthday. It’s been a whole year since I got the keys to the unit. In that time, and with lots of help, it has gone from this:

the empty unit

Via this:

To this:

And finally this:

 

Here’s to the next 20 years of compliance, I hope to see you on the way.

 

 

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)
110.167
230.5
3101.67

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)
110.1670.467
230.51.4
3101.674.67

 

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.