Troubleshooting EMC results from a lab - Page 1
Forum section: electronics / general

author: trevwhite
author_rank: Frequent Contributor
timestamp: November 28, 2017, 11:01:43 am
content: Hi allI have some EMC results back from a lab concerning a current project. It has a couple of places where it has failed radiated emissions. I was hoping I could use a LISN and CDN on cables to track down sources of noise. But I do not really feel confident I can track down all the noise and the EMC lab is asking for a lot of money to retest so I want to be confident before I go back.One source of noise at 190Mhz I do not actually see on my CDN unit so am thinking the noise might not be on that cable. If I use my near field probes around the circuit I can not find a place where noise at that frequency is much greater than other frequencies. I can find places where I get noise in 100Mhz - 200Mhz but not somewhere where there is a large peak of 190Mhz only I do not currently see.It seems very tricky.Anyone got any suggestions, help guides or even know of cheap pre-approval labs in Midlands area of UK that might be able to help?ThanksTrev

author: tszaboo
author_rank: Super Contributor
timestamp: November 28, 2017, 12:47:59 pm
content: Get in touch with Würth. They will send an application engineer, who will propose (their) part numbers, and solve these issues. I'm very happy with them. The parts might be more expensive, but failing the test again is probably more expensive. And I'm no EMC specialist.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 28, 2017, 01:13:20 pm
content: Sending stuff off and hoping really isn't the ideal way to do it.It's better to go there yourself with a box full of tools and parts, so if any issues are identified in a preliminary scan, you can try fixes there and then. if it doesn't fully fix it, at least you can do experimentation to see what things make a difference and eliminate things.The only difference between a "pre approval" and a full approval is their level of liability ( and the cost).It's usually exactly the same kit, facilities and staff. You can typically book a day or half-day and do whatever you can within that time - if you get it all sorted, the test results are just as valid.This is especially useful if you have multiple variants or products, as you can test as much as you can in the time, and also be fixing one thing while another is being tested.The only place I've used is RN Electronics in Essexhttps://www.rnelectronics.com/but I'm sure there are plenty of others

author: Tomorokoshi
author_rank: Super Contributor
timestamp: November 28, 2017, 01:14:29 pm
content: What clock rates do you have in your system? Look for an integer sub-multiple of 190 MHz. Can you post a view of that scan spectrum?

author: trevwhite
author_rank: Frequent Contributor
timestamp: November 28, 2017, 01:19:48 pm
content: Thanks for the repliesI believe I am now booked in for tomorrow at a lab to do all my tests. I was really hoping to be able to do some work before going there. I want to be confident I can solve the problems when I get there.

author: fcb
author_rank: Super Contributor
timestamp: November 28, 2017, 01:25:48 pm
content: If you get really stuck trevwhite, and it's not too big - we have an small anechoic chamber, 8591EM, 11940A/11941A probes etc.. in our lab. Your welcome to noddle about for a day (near Ipswich).

author: nctnico
author_rank: Super Contributor
timestamp: November 28, 2017, 01:49:50 pm
content: The trick is to use a near field probe and identify where the frequency band is generated. If I read your text so far think a switching PSU is the cullpritt. If that is the problem area you found with the H-field probes then you can try to put a ferrite bead (with low DC resistance and a peak impedance at around 190MHz) in series with the rectifier diode or add a series resistor (around 5 to 15 Ohm) in series with the gate of the MOSFET. If the MOSFET is internal then you can also put a resistor in series with the boost capacitor. You'd have to check if the PSU still behaves well (doesn't get hot) under full power.

author: trevwhite
author_rank: Frequent Contributor
timestamp: November 28, 2017, 02:10:28 pm
content: fcb: That is really good of you thanks. Did you make your own chamber? Do you have any information on its build?nctnico/All: The system consists of two units. The lab results I got were the two units together which did not help. I should have really gone with the units but that is another story really...Anyway the one unit is an STM32f429 running a full colour screen powered from an off the shelf 5V mains input power supply. The uP is runnning up at the 190Mhz frequency but its an internal PLL so not sure about that. It does interface some external memory and this is possibly the route of some noise as I think this might be interface at 90Mhz. Not sure if its down to tracking or not because I have not been able to isolate any peak noise at that level. I have seen a spread of noise from 100Mhz to 200Mhz but the 190Mhz was within that range so I would have thought the other noise would have been picked up as well on the original test results. That's why I say I cant find any 190Mhz noise that is stronger than other noise. There is a switch mode power supply providing 20mA or so to the display backlight but the switching on that is lovely. No ringing and a slope on the switching so I think it should be quiet there.The second board is powered from an off the shelf, open frame 48V. The board is an LED driver based on a constant current step down buck converter. This is all boxed within an aluminum extrusion. There is mains coming in and leads going out to the LED board it drives. That is why I thought it was noise emitting from the cables. I believe this is the source of the 60Mhz noise. Now I did actually add series resistance to the FET gate and it did make a significant difference and I was quite happy. I also put a common mode choke from the 48V to my board but that seemed to make no difference.My measurements are all done with a very old CDN unit and also a bit with a 5uH LISN.Any other suggestions on how to compare emitted noise with that shown on my CDN? Anyone got information on how the two should correlate if at all?Trev

author: fcb
author_rank: Super Contributor
timestamp: November 28, 2017, 03:37:34 pm
content: Nope - we bought it 2nd hand.I think it's an M&A Instruments "micro anechoic chamber" - it weighs half a tonne and is about 9ft x 4ft x 5ft or thereabouts, it sits on a six wheeled base. You can get in it... I'd hate to think what they cost new.

author: Andreas
author_rank: Super Contributor
timestamp: November 28, 2017, 03:49:44 pm
content: Quote from: trevwhite on November 28, 2017, 11:01:43 amIt usually is.There are "antenna resonances" which can amplify a harmonic of a fundamental frequencyin far field (so your near field probes only detect the fundamental).So I would look for a fundamental frequency fitting to the 190 MHz and any structure(housing or cable length) that has its lambda/4 or lambda/2 near 190 MHz.so 0.8 m or 0.4 m * shortening factor (NVP).with best regardsAndreas

author: dmills
author_rank: Super Contributor
timestamp: November 28, 2017, 04:44:40 pm
content: Any slots or painted metal to metal in the case about 35cm long?If the CPU is clocking at 190MHz, and that is where you are seeing the fail, I would be paying close attention to power and ground around that part, also anything being clocked at around 64MHz would be suspicious (Third harmonic).A 90Mhz DDR memory will tend to draw current pulses on both clock edges, look to your layout and decoupling, usually obvious when things just dont work, but....If it is in fact radiated then just making up a dipole or small yagi for 190Mhz and doing the open field site thing can get you hints. Copper tape and posterboard are your friends for making the aerial, and copper tape is your friend for closing up possible leakage paths.Regards, Dan.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 28, 2017, 04:57:02 pm
content: Quote from: dmills on November 28, 2017, 04:44:40 pmAnd decoupling - at 190MHz you may want some small caps as well as the usual 100n decouplers.Isolating the power to a noisy MCU with ferrite beads can also helpif you can, tweaking clock frequencies and seeing if the peaks move can be a useful way to narrow down the cause

author: trevwhite
author_rank: Frequent Contributor
timestamp: November 28, 2017, 05:27:36 pm
content: This is all really great advice. ThanksI have done some more testing w.r.t. to the gate resistor of the FET on the LED driver. I used a value of 10R initially and it generated noise. This is all crude stuff by me but I have included a screenshot. Blue trace is the 10R resistor.I changed the resistor to 47R and you can see the difference. This is from a 5uH LISN between the 48V power supply and the LED driver board. It is not a quiet environment, it's just on my bench, etc but as a comparison of the effect on a conducted cable, I believe it shows something real.

author: trevwhite
author_rank: Frequent Contributor
timestamp: November 28, 2017, 05:30:39 pm
content: This is one of the measurements the EMC lab took

author: AndyC_772
author_rank: Super Contributor
timestamp: November 28, 2017, 05:35:57 pm
content: Your peak around 190 MHz isn't a narrow spike, so it's unlikely to be directly caused by your microcontroller or its memory. Those devices are driven off a crystal and will generate noise at very specific frequencies which are exact multiples (or fractions) of the crystal frequency.My first suspect would be your LCD display; they're notoriously noisy. Does the radiated spectrum change according to the displayed image? (Try an all black or all white image and see what happens).

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 28, 2017, 05:36:24 pm
content: But make sure the gate resistor doesn't kill efficiency or cause heating/melting of the FET. It's a balance between EMC and still doing anything useful!

author: fcb
author_rank: Super Contributor
timestamp: November 28, 2017, 05:54:00 pm
content: That's quite a smudge. 190MHz seems to high for SMPSU noise. LCD screen drive cable (ribbon)? - copper foil, ferrites and a near field probe might get you somewhere tonight.Your EMC lab should have an EM investigation kit (usually an older spec/an with a near field probe on it), or perhaps a flat-bed EM scanner (surprisingly useful).Just remember that EMI is like a squeezing a balloon, you squeeze it out of one area, and it'll pop up somewhere else - so just when you think you've solved it...

author: coppice
author_rank: Super Contributor
timestamp: November 28, 2017, 05:54:22 pm
content: Quote from: mikeselectricstuff on November 28, 2017, 04:57:02 pmIts not just the value of the decoupling caps that is important. If you really want to see if decoupling is the issue, get a small value SMD cap soldered right across the supply pins of the MCU. Even small lead lengths add enough inductance at 190MHz to spoil the effect of the caps.

author: nctnico
author_rank: Super Contributor
timestamp: November 28, 2017, 05:56:35 pm
content: 190MHz can be SMPS noise! I've seen SMPS noise in this frequency range before several times. If it is noise from a clock then it will be a single spike.

author: dmills
author_rank: Super Contributor
timestamp: November 28, 2017, 06:35:32 pm
content: I have seen insufficiently snubbered flybacks ring like a bell in the tens of MHz, not sure I have seen one go quite that fast, but it is possible.That broad mess looks like something data dependent to me combined with an unfortunate but fairly low Q resonance, a graphic LCD is entirely possible as a source, but if you have screwed up badly enough it could be memory data or even an unfortunate power plane resonance.Make up an H field probe out of some small hardline or handiform and use a pawn shop scanner tuned to 195MHz or so to have a quick sniff around.I had not considered that such a thing as an E/H field flatbed scanner existed, I **MUST** build one... Pass that old flatbed plotter and some small coax, I think I have a 'project'.Regards, Dan.

author: Andreas
author_rank: Super Contributor
timestamp: November 28, 2017, 06:39:25 pm
content: Quote from: nctnico on November 28, 2017, 05:56:35 pmHello,Modern ECUs partly have spread spectrum oscillators to distribute the peaks over a wider frequency area.Dont know if it is already active here.Perhaps the problem could be solved by software.If its the power supply a small capacitor over the Schottky-Diode could reduce the radiation.with best regardsAndreas

author: nctnico
author_rank: Super Contributor
timestamp: November 28, 2017, 06:41:03 pm
content: Quote from: dmills on November 28, 2017, 06:35:32 pmActually there are three potential sources for generating noise by an SMPS: MOSFET turn on edge, ringing due to the inductor and the rectifier diode's reverse snap off. The latter can easely get in the 100Mhz to 200MHz range especially if it is a fast diode.

author: dmills
author_rank: Super Contributor
timestamp: November 28, 2017, 06:46:07 pm
content: And here was me thinking that nobody used snap diodes in that sort of power supply.... But yea, I could see reverse recovery doing that if somebody picked a fast diode without soft recovery.The fact that all that UHF grass goes down with a little added gate resistance is pointing to something like this for that part of the problem.Regards, Dan.

author: Pitrsek
author_rank: Regular Contributor
timestamp: November 28, 2017, 06:46:28 pm
content: Off the shelf power supplies.... have you tested them separately? Claiming compliance, and being compliant are two way different things. It happens more often than you think. At 190MHz, your regular x5r/x7r capacitors are not capacitors anymore. Would not bother with adding more capacitors, adding multiple values only adds more resonant peaks.What's your board stackup?Is everything going to and from the board filtered?Are your connectors close to each other?Do you have a solid ground plane?+1 for the LCD team, this would be first thing I'd test.How is the LCD connected to rest of the device - especially return/ground?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 28, 2017, 07:04:37 pm
content: If you're not supplying the psu with the kit, it makes sense to usr the quietest one you can find. For 48v, telecom rectifiers are awesome.Most SMPSUs are characterised with a resistive load, so anything else can affect their EMC performance significantly.If possible, use a linear PSU or batteries!

author: trevwhite
author_rank: Frequent Contributor
timestamp: November 28, 2017, 07:33:00 pm
content: Well this is presenting me with great aspects of the setup to test. I can code a simple main loop where no data is being sent to the display and the external memory is not being used. I think the LCD display is spitting out rubbish - I have seen that with my near field probe but it wasnt specific to 190Mhz but maybe that frequency resonated with the antenna?Maybe this is why ribbon cables from LCD displays are tucked underneath the display so that they are sandwiched between the board and the screen. My ribbon cable is running away from the display so it could be one potential source of noise. I can tape over that to screen it out and see if that affects the tests.The screenshot today was with the LISN between the 48V supply and the LED driver board. So that noise is all from the conducted power lines coming back from the led driver board. Interesting about the diode possibly causing noise. I am using a PMEG6020 which has a reverse recovery time of 8.5nS typical.I may be able to change the processor timing. I can also just unplug the display and see what happens.Sure there is some bits I missed but I have noted most of what people have said. It has been a great help. Also it seems the lab tests have been moved to Friday so might have a play a bit more before then getting things ready. I would like to get some data saved to see how well I mange to measure noise sources here.

author: nctnico
author_rank: Super Contributor
timestamp: November 28, 2017, 07:36:07 pm
content: Quote from: mikeselectricstuff on November 28, 2017, 07:04:37 pmIn addition: keep whatever is not part of the system to test outside the room and put a massive amount of ferrite clamps (test housing usually have a large ferrite clamp for this purpose) on any cable going into the test chamber. Also emission is usually tested with the cable going vertical or horizontal for a certain distance. After that distance you can put as many ferrite clamps onto the cable as you like so the rest of the cable isn't acting as an antenna.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 28, 2017, 08:12:27 pm
content: If the display is on a cable you should try a ferrite round the cable. I see this a lot in teardown kit. Remembe the whole glass face is an antenna. Also make sure the lcd frame is well bonded.As regards software for testing, try to build as much in ahead of time, and make it easily accessible. The test place is an expensive and stressful environment, and you don't want to be wasting time dicking about on a laptop loading different firmware etc.

author: Berni
author_rank: Super Contributor
timestamp: November 28, 2017, 08:48:05 pm
content: Oh yes EMC trouble can be quite the chase.Sometimes something seemingly insignificant can turn your product in to quite a radio transmitter, here is a list of common things that cause it from my experience:1) Fast switchmode regulators:As suggested before putting resistors in some places to slow down switching regulators can quiet them down a lot but every time you add a component to slow it down it has to be tested over the entire designed voltage and load range as sometimes you can get weird behavior. This is not only limited to adding gate resistors. Changing the capacitors and inductors can change things a lot, adding SMD ferites on the input and/or output can help a lot etc.2) Ground return paths:Large ground loops are very good antennas. This is particularly important around switchmode supplies again. The ground fill on the PCB for the switching regulator has to be carefuly designed to keep all the high frequency currents contained, grounding topologies also have to be designed to not cause ground currents to flow around a alternate route. For example you have a grounded screw mounting hole near your switching regulators ground fill and somewhere else you have a connector such as SMA or D-Sub or something that is also grounded, then you add a metal case and connect the two. This creates a path to ground for the high speed current that takes it around the entire case and radiates it all out.3) Conections:Wires or ribbon cables are offten used to connect multiple parts together. If they are carrying any high speed signals (Especialy non differential) then you need lots of distributed ground lines in your ribbon cable to offer a solid and close by return path. Sometimes this means having 10 ground lines on a 20 pin ribbon cable.4) Go only as fast as you need:A signal doesn't need to be high freqency to radiate, it just needs fast rising edges to make a 10MHz signal show up up to 1GHz. Test the rise times around a design, if you don't need a rise time to be as fast as it is then slow it down by adding a series resistor at the driving side or even a ceramic EMC filter network.5) Combinations:A LCD screen is covered under point 3 since it is a subsystem connected by a cable, but they already have a given pin out that maybe only has 2 ground pins. Well since you can't do anything about that you can use point 4. These screens can have a very wide 24bit bus and that can hammer your ground return very hard, since you can't add more ground you just slow the signal down as much as possible while still having it look mostly like a square wave. This can make a big diference6) Unlucky clock speedsSometimes you simply happened to pick the worst possible frequency to run something at because you caught a resonance or something or got your harmonic in a particularly sensitive spot(Like having a GPS receiver onboard and having your EMI tone smack on the satellites carrier frequency and so making for bad reception). Just tweak your CPU or memory or PLL clock settings a tiny bit because running at 98.333MHz is basically the same as 100MHz for a CPU, but it might move your pesky spike enugh to not hit that resonance or not interfere with what it used to.7) Put shields on stuffPutting a metal can over stuff or wrapping conductive EMI tape on cables or between assemblies can help. But don't expect it to do miracles. In most cases it will make a tiny difference in EMI, but if you tried everything and are trying to squeeze just a few extra dB under that test limit sure. Tho a PCB well designed around shielding boxes can have a significant improvement in some cases (Correct grounding and routing etc).In the end its a whole lot of trial and error

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: November 29, 2017, 01:06:57 am
content: Testing with near field probes can distort the picture considerably - did not believe it myself until I experienced it. There is a company selling nice&cheap PCB-constructed LogPer-Antennas, get one of those and a Preamp (some standard Minicircuits or whatever), get out from the near field and you will be much closer to what the lab sees.Also this is really some signal, so the search for the radiating structure might be a good idea. Did they make measurements in more than one position/orientation? Any clue from those?Also, going through the modes and activities of the DuT might show something.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2017, 07:15:33 am
content: Quote from: trevwhite on November 28, 2017, 02:10:28 pmFYI, I've seen a similar symptom before. Situation was a base board with MCU, RAM, various devices, and a header. Header was a board stacker, for a 16 bit parallel LCD, running on the memory bus.I tried to introduce as many grounds as possible, and covered the boards with ground. The header pinout was mostly set by the customer, though.There was a spike somewhere around there, 3rd harmonic of clock or something like that, 168MHz I think it was.Would've preferred:1. More grounds in the header. Preferably every other pin.2. Grounding the boards together via mounting hardware. (Customer wasn't fond of metallic spacers, either, unfortunately.)3. Ferrite beads -- BUT, this is rather impossible as it would have to be a huge wraparound shape to slide over the header. I don't think that exists. FBs on individual lines is not possible -- the data would turn to mush at either end.4. SPI or I2C, if possible -- slower, fewer lines and easier to deal with. Also not an option, because of the graphical bandwidth involved IIRC. (Which they probably didn't use anyway, but who knows -- this was part of a development program, that sort of thing is normal.)5. LVDS, even if just transceiver pairs over this short distance. Balanced current flow means less to radiate. It may not have a similar gain in immunity -- LVDS V_ICM does not extend beyond the rails -- but it also shouldn't be worse than the original CMOS levels.6. To further elaborate on that -- pack the bus into a serdes, or use an FPGA (hah, right), or use MCU and display that support LVDS or HDMI or MIPI or what have you. Downsides: much fancier, more expensive, and may be hard to use (MIPI is license only IIRC?).Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2017, 07:21:52 am
content: Quote from: mikeselectricstuff on November 28, 2017, 08:12:27 pmThe aforementioned development project moved away from the discrete approach; the customer eventually settled on an industrial display unit (with orders of magnitude more compute power inside, than on any of the support boards -- go figure).Which, wouldn't you know it -- failed radiated immunity, at just the frequency where the screen opening is a resonant aperture.The module's rated for the same test we were doing, too (V/m field).The module was conveniently painted all around with a nice heavy epoxy(?) paint, making any ground bonding impossible.(Although we tried that anyway, scraping off paint -- it didn't actually help. Probably it was the panel itself getting screwed up, perhaps driven by the cutout it was positioned over. Who knows?)Tim

author: trevwhite
author_rank: Frequent Contributor
timestamp: December 01, 2017, 07:05:05 pm
content: Thought I would say big thanks and to let everyone know I managed to get a pass at the emc lab today.Been a learning curve but thankfully a good learning curve.TrevThanksTrev

author: nctnico
author_rank: Super Contributor
timestamp: December 01, 2017, 07:13:16 pm
content: What did you fix on your boards?

author: trevwhite
author_rank: Frequent Contributor
timestamp: December 01, 2017, 07:29:14 pm
content: Common mode choke from power supplies to my processor board and led driver board. Ferrite bead on led driver to led board. Choke on mains input to both power supplies.Gate drive resistor of fet in led driver adjusted.Lab said it did not look like processor or display noise. Thankful of that.The lab did test both units at the same time though. Not sure if they might have been better done one at a time?Even wrapped whole unit it tin foil at the end to see what difference it made. It had no affect on measurements so all the noise seemed to be radiating from the cables.Was a little stressful but really enjoyed going to the lab. Was a great place.ThanksTrev