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