doing in house EMC pre-compliance testing - where do I start? - Page 1
Forum section: electronics / general

author: Simon
author_rank: Unknown
timestamp: August 21, 2022, 02:06:23 pm
content: We had a product at work fail EMC in CE marking. So we have bought a spectrum analyser and will be getting the required licence on it for the required filters etc. We have an aerial, and that is all I know.How do we go about setting this up for the pre-compliance the spectrum analyser claims it can do? The analyser itself has some sort of output as well as input. Is this so that it can self characterise the setup and make the required adjustments to get a roughly good metric against the standard?I am sorry to say that Telonic have been pretty useless in advising on what we need to the point that I have an aerial but no cable for it, they must be doing so well that dealing with customers in order to make a few more quid on some cable is totally not worth their while.

author: fourfathom
author_rank: Super Contributor
timestamp: August 21, 2022, 03:09:26 pm
content: What kind of antenna? What kind of product? What was the failure (frequencies, amplitudes)? I assume this is a free-space test. And what spectrum analyzer? What antenna?I used to do FCC and European (pre-EU) pre-compliance testing for our telecomms gear, and before that for wireless microphones and receivers. At the simplest, I used a spectrum analyzer, several antennas (a telescoping dipole for the lower frequencies, and a biconical for the higher ones, and sometimes a log-periodic for still-higher frequencies). Each antenna has a calibration chart. You may need a preamp -- that and the coax cable also need to be calibrated, or at least have known gain/loss at the frequencies of interest. My test range was sometimes a grassy field, or at a different company where we did a lot of this we laid down a wire "hardware cloth" screen under a layer of asphalt.So that's the setup. This was before the era of modern spectrum analyzers with pre-loaded compliance limits and calibration factors, so I had to do that part by hand: identify the emission peaks apply the correction factors, and compare that to the compliance limits.I highly doubt that your analyzer can self-calibrate the measurement system using just a measuring antenna. You're probably going to have to enter the calibration factors yourself. Again, you need antenna factor calibration data.If you have the test results for the failed run, you may be able to useful mitigation without a calibrated system. Just find the specific emissions (frequency, dB over limit) and measure the unmodified system with whatever you have. Start doing your EMI fixes, and see how much you can drop those specific emissions -- this is a relative measurement, so calibration isn't critical. If you failed by 10dB and can reduce the emissions by 20dB then you are probably ready to re-test.The test results can be greatly affected by external cabling to/from your DUT. Be realistic, don't cheat, and try to maintain the same configuration between tests.

author: Simon
author_rank: Unknown
timestamp: August 21, 2022, 03:16:47 pm
content: Yes my first aproach would be to look at the system as it failed and use that as a metric against the failure test. then change stuff and see how much better I can make it. First order of business is sniffing around with a near field probe to locate the cause.I have a tekbox biconical antenna, yes it has a chart of gains at different frequencies so that the analyser can take that into account. What I am confused about is how do I actually set up to do a test with the analyser that will apparently give me the ability to compare to standards. If I am not set up right then what I am doing is meaningless.

author: fourfathom
author_rank: Super Contributor
timestamp: August 21, 2022, 03:34:12 pm
content: Quote from: Simon on August 21, 2022, 03:16:47 pmQuick response! See my additions to my previous post.There has to be a "How to do an open-field compliance test" tutorial out there somewhere. It's really pretty simple: your correction factor is (Antenna factor @ Freq Of Interest + preamp gain @FOI + cable attenuation @FOI). See what the test range distance is for your particular requirement, and put your antenna there. Make measurements, probably with vertical and horizontal polarizations (antenna rotation) at the higher frequencies. Usually your DUT is put on a carousel, but one way or another you need to test at different orientations. Compare measurements to the standard.Your spectrum analyzer probably has compliance limit templates and the ability to enter correction factors, but if that's your question -- sorry, I've never used that stuff. The last time I did these measurements was perhaps 35 years ago. But I guarantee that antennas, coax, amplifiers, and radio propagation haven't changed all that much since then.

author: Simon
author_rank: Unknown
timestamp: August 21, 2022, 03:49:37 pm
content: So I replicate the distances of the standard and use the correction factors of the antenna plus losses/gains of stuff in between and I'm sort of doing the same as the test house.Interestingly I actually have a 9kHz to 3GHz signal generator so is there a way of testing my setup by generating a known signal and see what I get.

author: fourfathom
author_rank: Super Contributor
timestamp: August 21, 2022, 05:51:17 pm
content: Quote from: Simon on August 21, 2022, 03:49:37 pmI've never done it, but you can certainly set up two antennae on your test site, transmit into one and receive on the other. Of course you will need antenna and coax calibration data for both ends, otherwise you won't be able to separate antenna factors from site loss factors. I expect that someone has a very small "pseudo-isotropic" antenna with calibration data over a wide frequency range. It would probably be too lossy for receiving, but it could be a useful test signal source. Perhaps a design is out there, and if done right, the calibration could be simple math.But if you are doing outdoor open-space measurements the field (ground under your feet) requirements aren't too difficult. When I was doing this, the FCC's own site was essentially a grassy field, probably kept well-watered.I hope someone with more recent experience chimes in here. No doubt some techniques have changed since I was in the field.

author: srb1954
author_rank: Super Contributor
timestamp: August 21, 2022, 08:26:17 pm
content: Quote from: Simon on August 21, 2022, 03:49:37 pmYou also need to rotate the UUT to pick out the peak emissions at each frequency. Since the angle for maximum emissions may vary for different frequencies the usual approach is to set the spectrum analyser in peak hold mode and conduct multiple sweeps as the UUT is slowly rotated. Once the frequencies of the highest amplitude emissions are identified from the peak hold scan you can then zoom in for a more detailed look at each emission.

author: Simon
author_rank: Unknown
timestamp: August 21, 2022, 08:52:53 pm
content: The test house did it at every 90 degrees which makes sense.

author: martinr33
author_rank: Frequent Contributor
timestamp: August 22, 2022, 02:22:46 am
content: I've been out of this game for longer than everyone, I suspect My experience dates back to the first days of failing EMC compliance...The short of it is, don't try to replicate the test setup, but use lab tools and your SA to find the porblem emitters.1) It is tough to replicate the failure conditions unless you have a low-noise space (like a field with limited wireless noise, so away from TV and radio transmitters, cars, and houses.and the ability to rotate the device. But this is less of an issue if you know where the device is failing.2) Therefore, if you know the failing frequencies you can use near field antennae and your spectrum analyser to figure out where they are coming from.3) A technique I found effective was to use a scope to probe around the board, and feed its output into the spectrum analyzer. When you find a strong signal on the scope with harmonics in your zone of interest, you have a candidate.) If you are lucky, your system will have just a few problems. If not, you will be chasing multiple problem frequencies.

author: fourfathom
author_rank: Super Contributor
timestamp: August 22, 2022, 03:26:49 am
content: Martinr33, that is good advice. But there are often two issues: Finding the source, and then finding out how that source is escaping to the outside world.Sometimes you really can't do anything with the source -- for example a widely-distributed clock in a large system (such as the PBX I had to bring in compliance when the FCC Part 15 rules for this were introduced). It wasn't practical to filter all the clock signals without radically redesigning all the board/backplane interfaces, so instead I had to put RF gasketing on the enclosure doors and treat all the exiting cables with ferrites (in doing so I believe I invented the "ferrite clamp" core -- I had the ferrite company make me a bunch of E-I cores in an RF mix).Other times, you have a small device with a few or no external cables, and then you can often fix the source at the root.But regardless, the first step is to identify the source or sources.

author: martinr33
author_rank: Frequent Contributor
timestamp: August 22, 2022, 03:57:57 am
content: Not quite on topic, but on my problem unit we tried:- conductive paint spray on interior surfaces- thick conductive coating on interior surfaces- (my favorite) copper plating inside and outside of the plastic case with metallic copper (ands no, it did not help enough).The fix was a tiny capacitor on a long signal line.To your point, the problems stem for the sum of smaller parts, and a few single culprits. Design tools fix the worst problems, but the final assembly is another matter.

author: KE5FX
author_rank: Super Contributor
timestamp: August 22, 2022, 04:08:01 am
content: Are you able to share your failing test report? The compliance lab should have given you some paperwork showing exactly what mask was violated and the frequency(s) at which the violation(s) occurred. That data is the best starting point.

author: Simon
author_rank: Unknown
timestamp: August 22, 2022, 08:21:16 am
content: Quote from: KE5FX on August 22, 2022, 04:08:01 amWe have overshot the line by 20dB at 60MHz, this falls back to the line at 10MHz above and below. I have two candidates, 2 maxon motors and controllers that have no filtering at all and a 3m RS232 coms cable that although screened did not have the screening terminated. Either could be the source.So yes first steps are near field probes and go around it to find the issue location. But as we fix things it would be nice to have the ability to do a test similar to the one that is being done in the lab.

author: miro123
author_rank: Frequent Contributor
timestamp: August 22, 2022, 10:14:51 am
content: Hello,1. Can you perform all CE pre-complaince tests on your device?1. radiated - Wideband receive antenas has start price of 5000$. Most of the time you can avoid the farraday cage that add another 200...800K$2.Immunity- conducted is easy, it add 500...1000$ rf transformers from e-bay + hobby RF power aplifoers also from e-bay- for higher frequency get more expencive.2. De you get advice from CE lab what get wrong, what are the solutions? - my experience is this labs has good educated test engineers. Normally they guide me how to solve my EMC problems.==============Normally I perform the EMC compliance tests in 3 steps- pre compliance test on our ems lab ~100K$ in second hand uncalibrated equipment- unofficial compliance test - EMC lab near our location - they have calibrated equipment/ antennas and faraday cage.- official test - just to get paperI think that you should post in other forum section, where more EMC experts can advice you.

author: kleiner Rainer
author_rank: Regular Contributor
timestamp: August 22, 2022, 01:31:07 pm
content: Hello Simon,buy "Electromagnetic Compatibility Engineering" by Henry W. Ott. Read it - its worth the time and money. Its the EMC bible, period.Get hold of CISPR14, Part 1 and 2. The Indian version is an older edition, but free:https://law.resource.org/pub/in/bis/manifest.litd.9.htmlCISPR16, Part 1 and 2 describe measuring apparatus and methods. Another important read.Greetings,Rainer (doing EMC work for the last 25 years...)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 22, 2022, 02:21:18 pm
content: I don't feel antennas are all that helpful, outside of a lab setting; you aren't going to have the same gain calibration (due to antenna curve, environment, orientation, etc.), and ambient sources get in the way. Just a good set of probes (E and H field, current clamp), and decoupling networks (LISN, CDN), covers quite a lot.The main thing you need to know is, what source is blowing it out by 20dB, and, can you reduce it by at least as much? If the probe says, whatever say 43dBuV, in your reference orientation, then you make a change and it says 17dBuV -- that's probably a good start. You still need to know that that reduction was made in a location accessible by ambient fields (coupling to radiation). Usually this means cables and such.Note that LISNs aren't useless at 30MHz, that's just where the conducted test happens to end. You can see quite a lot carried on cables, before it gets into the air. And a current clamp probe, you can think of as an inline LISN of sorts. E/H probes help with further pinpointing sources, but mind they tend to be rather low gain, not so effective at tracking noise along cables (where a larger probe, or antenna outright, might indeed be more illuminating).About the test itself: was this sent off and done by independent techs, no one went along to see what it's doing or to troubleshoot it? If so, that's rather troubling. You can only guess what the noise is coming from, and where it's getting out. Hopefully there's only one thing making a given cluster of peaks; but multiple sources can overlap, and you won't know which. Nor what connections are likely radiating; hopefully here too the set of suspects is small, but noise can show up everywhere and a little direction goes a long way.There is at least polarization, assuming you know how the stuff was laid out for the test (got photos from the lab?). And assuming V/H are listed on the plots, not just merged together(!). Unless everything's all looped up (circular polarization?) in which case nevermind I guess, heh.Yes, unshielded cables are a likely culprit. A cable having screening/braid is meaningless; only if it's tied to local GND at both ends, is there any containment of CM noise within that path. Other things relatively easy to spot: any connections passing through a bulkhead/shield without grounding or bypassing to it; any PCBs having onboard ground loops between connectors and the main GND plane, or enclosure; etc.Also, was this just emissions? Did susceptibility pass? Is it going back for that later?I don't know what kind of motors or controllers those are (Maxon is just a brand), but definitely a possibility. Anything to do with power is an immediate suspect.RS232 for example, should be very quiet at ~60MHz (unless maybe the driver is shite -- I suppose the charge pump could be badly made or something?), and that's if it's transmitting at all, and at much baud -- it's slew-rate limited, so only the very corners of the waveform correspond to harmonics of any consequence (i.e. in the 10s of MHz), and those corners are very small indeed (i.e. the ~100s of mV during which the signal changes direction between flat and slewing). RS232 isn't shielded so much for emissions reasons, as immunity: it's a fairly high impedance, so an external electric field couples into the wire at near full scale, corrupting the signal. With a noise margin of 5V+ with typical interfaces, it should pass commercial limits (say 3V, 3V/m) well enough, but almost certainly demands filtering and/or shielding to pass more stringent, industrial / etc. levels (say 10V+).Tim

author: fourfathom
author_rank: Super Contributor
timestamp: August 22, 2022, 03:01:47 pm
content: Quote from: T3sl4co1l on August 22, 2022, 02:21:18 pmCables are often the main avenue for emissions to escape a product, and (as T3sl4co1l mentions) even shielding may make no difference. Often the radiation is common mode, carried on the shielding itself.You need to consider two things: The source of a signal, and how this signal is radiated. If you can keep the source dimensions small enough you may not even need shielding. A signal source needs an antenna to radiate, and cables are often your main antennae. So see if unplugging the external cable makes a difference. Or, use a ferrite clamp on the suspect cable, as close to the DUT as you can get. Use the proper ferrite mix, and enough turns on the core to be effective -- you aren't trying for a production-ready solution at this point, just searching for the problem. Later on you can consider cables with built-on ferrites, or common-mode chokes on the circuit board just before the cable connection, or bypassing (but bypassing is often less effective that you would hope.)I would be happy to point you to good info on suppression ferrites.

author: Simon
author_rank: Unknown
timestamp: August 22, 2022, 06:43:26 pm
content: Well on the one hand the motor controllers having no filters is quite a red flag, they are 50/8 models or 50V 8A, the person that designed the main board did not think filters were required but spent a long time navigating around ground bounce..... (yes they walk among us...). The RS232 coms happen at 12V over a 3m cable so quite the aerial.I actually suggested to my boss that we obsolete this product version and come out with a revised one as I have no wish to fix something done so badly that I'd have to start again anyway. Well I have already started again on a bigger brother to this product so once that is working it's just a case of switching the controls out for ours and just having a main board with filters and any power stuff on it being controlled by our new controller. The hard stuff would be done as part of the new project, retrofitting it could be easy.I still need to work with this product as much as I can and hopefullytellguide the person that designed it to get it right this time and we have some breathing space.

author: martinr33
author_rank: Frequent Contributor
timestamp: August 23, 2022, 07:37:03 am
content: This does not sound like a motor problem. Motors run at kHz and their inductance fights fast riding pulses. And RS232 is not that fast.Your problem will be something with a crystal oscillator on board.One possible fix is to switch to a clock generator with frequency spreading. These devices effectively blunt the peaks of a crystal oscillator by spreading the noise over a wider (but still tiny) portion of the spectrum.This modulation is key to making emissions limits easier to beat.At this point, don’t worry about near field probes. Just probe suspect signals for spikes at your failing frequencies.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 23, 2022, 12:37:14 pm
content: Quote from: martinr33 on August 23, 2022, 07:37:03 amIt does sound like a motor problem: if it's a brushed type, the brush noise can extend to 100s of MHz. Arcing is impressively wideband. (Some of the earliest microwave experiements,up to 60GHz or so, were done with spark discharge techniques -- back around 1900!) If it's PWM controlled, and unfiltered as suggested, then 60MHz is a common enough peak in inverter switching loops (corresponding to, on the order of, let's say, 1nF MOSFET capacitance, with a loop of 7nH -- a bit small for a single pair of TO-220s, but representative of a slightly lazy layout with D2PAKs say). The broad peak suggests modulation; switching noise is impulsive, and the spectrum might be a filtered response, perhaps due to the natural (under)damping of the switching loop, or perhaps due to cabling (say the cable is 2.5m long, so, 1/2-wave resonant).It can also be digital noise, again due to converters or charge pumps; or subharmonics of the clock frequency, in which case the modulation is due to whatever random data patterns are being emitted. The peaks should be visible if this is the case, but it depends: the signals might repeat at quite a low rate (MHz, kHz even), thus giving quite a dense forest, and subsequent filtering (due to poor PCB layout, cables, etc.) gives the overall figure of the spectrum. The modulation might not even be very strong; I've seen MCUs emit a highly pure F_CLK before, even at fairly high frequencies (harmonics) where you'd expect it to be dropping off instead; and selected harmonics at that (e.g., 336MHz standing out prominently, but not 168 or 504MHz, nor sidebands thereof).Given the broadening, spread-spectrum is unlikely to help the former (the data patterns are already spreading it out!), but the latter can be reduced quite effectively (since the pilot tone is F_CPU itself (and harmonics), apparently unmodulated by data).But yeh, that's all of EMC: "it depends".Tim

author: Simon
author_rank: Unknown
timestamp: August 23, 2022, 06:02:27 pm
content: The motors are brushless. I'm not sure what frequency the main micro runs at, it's an STM32 with an M3 or M4, it may have an 8MHz crystal and use the internal PLL. The cabling in the box is not the best.

author: Simon
author_rank: Unknown
timestamp: August 24, 2022, 12:11:43 pm
content: So the bit I am wondering about, well several bits.The tests are done in dBµV/m, the spectrum analyser has dBµV, does this mean that the test is measuring the V/m based on the known length of the aerial and the analyzer is just giving the voltage it picks up (referenced to 1µV)?The antenna comes with a table of antenna factors. Some dB/m, so if I put these in it adjusts the trace to take these into account. Nom the analyser only has a dB metric for these not dB/m.Really my biggest problem with all of this is that every item seems to use a different metric or that some metrics may be the same but they are called differently. I know that dBm is actually dBmW but we all call it dBm. So when it comes to dBµV is this another unit of measure that just misses the last bit off and is actually dBµV/m?Turning the attenuator on on the analyser lowers the noise floor. Turning on the preamp does too. I am guessing that this is a data acquisition thing, the attenuation is being compensated for by some amplification but the attenuation reduces the noise to a point where the ADC will not resolve it, same logic for turning the preamp on.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 24, 2022, 01:50:33 pm
content: Yes, calibrated to dBuV/m in the air; antenna factor accounts for frequency response as well as converts from uV/m to uV. And yes, dB relative to 1uV.Calibration ultimately comes from an antenna of standard geometry, like an electrically short dipole, which might not be very efficient or well-matched, but the electric field from it is well-defined.Tim

author: Simon
author_rank: Unknown
timestamp: August 24, 2022, 04:01:58 pm
content: Well I did some playing around this afternoon. I set up a signal generator with a dipole that is about 600mm and just put about 50dBµV on it. I worked out how to set the analyser up to get a readout that allowed me to see the signal against the background noise.I increased and decreased the output by 10dB and saw a corresponding change in the peak on the screen. So I now know that if I take some measurements on my failed equipment, whatever that is I need to take 20+dBµV off.So if I put 600µV into this 600mm dipole antenna, is that 1mV/m? or given the aerial characteristics I have entered does that mean that when I read off 50dBµV that is 50dBµV/m?I tried putting steel mesh around the antenna with holes that are 50mm, this did not reduce the noise floor as unplugging the aerial did not. So I assume that I am in a quiet part of the building and that the only way to further drop the noise floor if needed is to reduce the RBW, the test house runs at 100kHz, at 9kHz we can drop the noise floor, at 200Hz it's nice but takes some time. But changing the RBW does not alter the actual measurement of my sample signal so I am guessing that I can use the RBW I need and still get relatively comparable measurements.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 24, 2022, 04:24:28 pm
content: The whole story of it is:- You have two antennas of given gain factors.- You position them in some orientation.- You apply some voltage to one. It generates... whatever field, given by its antenna factor.- That field propagates (or perhaps doesn't, if in near field), with attenuation given by Friis antenna equation, plus reflections from nearby geometry, multipath, etc. (The superposition of which interferes and may reinforce or reduce the actual reading at the far end.) Geometry can be ignored in an anechoic chamber, all those modes are absorbed and direct transmission remains.- The other antenna picks up that field at whatever distance it's at.- You read some voltage at it, given by its antenna factor.Also, polarization, if oriented differently to each other, or by going through enough oddly-angled reflections.Transmission losses are quite substantial, so you may need quite some transmission power (well, ~mW say), and a preamp on the receiver, to see anything on the spec -- an instrument with fairly high noise figure, generally (but whatever it is, it's in the manual).If the antennas are identical, then you basically square (as V, or double as dB) the antenna factor (or put one into the signal generator, but if it's fixed voltage, then, this must be done at the receiver). You should then measure reasonably flat gain. But again, assuming freedom from reflections, and adequate distance between (to not be in the near field).Lower RBW gets better selectivity for narrow-band signals: random noise has a density behavior (V/rtHz), so lower RBW (effectively, the pixels on the display represent narrower bands) gives lower noise floor, while narrow signals remain full amplitude. Modulated signals are spread out by nature of the modulation, and so generally read lower, as with noise (because they don't fit within RBW).There is still a statistical bias, depending on detector type (peak / avg / etc.). At very low RBW, many sampling periods might go into one pixel, and you are simply displayed the peak or average or whatever of those readings. The spec's manual will show how these trade off.Offhand, I would expect RBW of 10 or 100kHz is fine here, unless your noise floor sucks and you need the edge from lower RBW, just to see things.Tim

author: nctnico
author_rank: Super Contributor
timestamp: August 24, 2022, 05:42:26 pm
content: Quote from: T3sl4co1l on August 22, 2022, 02:21:18 pmAgreed. Without a shielded environment, the noise from the environment will swamp any signal you measure. Any antenna is just a waste of money. Even bench measurements with E/H field probes and LISN can be affected by outside sources.A 'cheap' way to get some kind of shielded environment is a TEM cell and/or a shielded tent but you still won't get anything that is like an EMC lab grade measurement.

author: fourfathom
author_rank: Super Contributor
timestamp: August 24, 2022, 06:07:18 pm
content: Quote from: Simon on August 24, 2022, 04:01:58 pmProbably not. An E field of 1 V/m refers to a potential difference of 1 V between two points 1 m apart. There is an equation for free-space field strength given an isotropic (ideal, zero size) radiator, and the antenna factor is usually relative to that. Also, your 600mm dipole is probably not matched to your cable at the frequency of interest, so you will have reflections and frequency-dependent cable attenuation (which will be greater than the spec'd cable attenuation.)A short antenna is still a good tool. If you make it very short and put a 50 Ohm resistor at the coax connection you will have a fairly well-matched antenna with a linear antenna factor (below the antenna resonance frequency.) It's not going to be calibrated unless you do the math, but it shouldn't have any surprising behavior. Use it for relative measurements.

author: Simon
author_rank: Unknown
timestamp: August 24, 2022, 07:31:42 pm
content: Quote from: fourfathom on August 24, 2022, 06:07:18 pmThe antenna was plugged straight into the signal generator.

author: fourfathom
author_rank: Super Contributor
timestamp: August 24, 2022, 11:25:12 pm
content: Quote from: Simon on August 24, 2022, 07:31:42 pmWas it decoupled from the ground and power wiring of the generator by a choke or something? Otherwise the generator and power cable become part of the antenna. Even decoupled, the generator chassis in close proximity to the dipole will affect antenna impedance and pattern.. And how do you even connect a dipole to a signal generator without using a cable?I once made an "active dipole" signal source using a simple transistor oscillator and a 9V battery on a scrap of circuit board. The whole thing was much smaller than a credit card, with the dipole wires directly attached, but I assume that's not what you did. It was just an experiment, I was bored.

author: fourfathom
author_rank: Super Contributor
timestamp: August 24, 2022, 11:29:27 pm
content: Quote from: nctnico on August 24, 2022, 05:42:26 pmI've done lots of useful compliance work with measuring antennas in the company parking lot, in the middle of the city. Yes, there's noise, but if you already know what you're looking for you can usually work around the ambient crud.

author: Simon
author_rank: Unknown
timestamp: August 25, 2022, 11:43:58 am
content: Yes well I know I have this 70MHz peak, I need to see that on my analyser, I now know that I can keep reducing the RBW to get the noise floor down and this will eventually have my signal stand out from it hopefully. It looks like moving to an unused upstairs floor of the building has done it in terms of noise floor.

author: nctnico
author_rank: Super Contributor
timestamp: August 25, 2022, 06:04:39 pm
content: Quote from: Simon on August 25, 2022, 11:43:58 amReducing RBW is not always a good idea. If the interference signal is wideband noise, then the amplitude will drop as well. If you really want to drop the noise floor, then set the attenuation to 0 and turn on the pre-amplifier.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 25, 2022, 06:32:24 pm
content: Quote from: nctnico on August 24, 2022, 05:42:26 pmBut even with a shielded environment, reflections mean that moving anything even slightly could easily change your readings substantially. You can't win....

author: fourfathom
author_rank: Super Contributor
timestamp: August 25, 2022, 06:34:19 pm
content: Quote from: nctnico on August 25, 2022, 06:04:39 pmThat may work for the Spectrum Analyzer noise floor, but will make no difference if the noise is from the RF ambient environment (as appears to be the case). Reducing the measurement bandwidth will reduce *some* of the ambient junk, but (as mentioned) will also reduce the measured level of your own signals if they are wideband. For narrowband signals such as normal clock leakage then reducing the bandwidth shouldn't affect measurement accuracy.

author: fourfathom
author_rank: Super Contributor
timestamp: August 25, 2022, 06:44:49 pm
content: I suggest that you don't worry about not having an anechoic or shielded chamber. They are great if you can find one (and I have), but you can still make useful measurements in a noisy environment:Scan in close, to see what is radiating. If you have the test report use that to guide you.Be able to switch the DUT on and off, to identify your signals from the ambient hash.If possible test outside rather than inside next to racks of other noisy gear.Accept the fact that you will have to personally identify and measure the offending signals, and personally ignore the ambient ones. You can't just push a button and have your test equipment do a pass/fail scan for you.

author: jonpaul
author_rank: Super Contributor
timestamp: August 25, 2022, 09:18:29 pm
content: Bonjour à tousEMI and other compliance is a deep field, the equipment design at the start needs to be cognizant of requirements for conducted and radiated EMI, safety and transients susceptibility for te regions contemplated for market, and equipment class, eg consum, industrial, medical, avionicsFailing one test by a huge margin, you will not be able to put a bandage on a faulty design eg noisy SMPS.I reccomend a complete système design review by an experienced EMI, Comlliance engineer or laboratorys equipped with the cal traceable SA, LISN, Antennas.Expect $20k..150k and 6 months to finish the job.Beware of Chinese competition that will do no compliance and merely counterfeit the CE, UL, VDE mark.Bon courageJon

author: nctnico
author_rank: Super Contributor
timestamp: August 26, 2022, 12:05:29 am
content: Quote from: fourfathom on August 25, 2022, 06:44:49 pmEven then you might still end up short handed. Currently I'm working on improving a PoE power supply based on an existing design that barely passes, so I wanted to add some extra margin. I've added filters that basically seperate the PoE supply from the rest of the board and which makes the power supply look super quiet on a LISN. EMC lab result: worse than before for radiated emissions. And the biggest problem is that I can't really reproduce the radiated emissions test in my own lab so I'm basically flying blind as whether additional modifications I made based on H field probe measurements have a positive effect. I suspect there is some influence from the metal casing as well. Only going to the EMC lab for a short test will tell if I'm on the right track or not. At an EMC lab they have the proper test setups for the test (shielding, ground plating, proper probes like LISN / ISN, etc) which also affect the results.

author: tautech
author_rank: Super Contributor
timestamp: August 26, 2022, 12:26:22 am
content: Quote from: Simon on August 21, 2022, 02:06:23 pmIt seems you're just after some generic advice even after deciding to commit to a SA purchase !Did you have no idea of what you were getting into or is not revealing the brand of what you purchased didn't want to be a brand advertisement ?Most manufacturers have App notes on this stuff however without knowing which of these you selected or if it included a EMI promotion we don't know which App note to direct you to.https://telonic.co.uk/product-category/test-measurement/bench-spectrum-analysers/First steps into EMI detection are usually with near field probes, yes the sort of stuff that is often offered in promotions.

author: nctnico
author_rank: Super Contributor
timestamp: August 26, 2022, 09:14:06 am
content: IMHO having an EMI option on a spectrum analyser does not bring much to the table where it comes to hunting down causes of the excessive emissions.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 26, 2022, 10:10:35 am
content: Quote from: nctnico on August 26, 2022, 09:14:06 amIndeed, the standards use average too, and 9 isn't much less than 10kHz bandwidth. QP is less than peak, but if peak is low enough, QP certainly will also be.Doing it with a scope, would be... more of a challenge(!), but nonetheless possible. The main downsides are signals in the ~mV (preamps are a must), and working out averages vs. frequency is really hard. Can also be troublesome just to even trigger on a given signal, when there are multiple frequencies in play; when this happens, a local probe probably helps (trigger on the source you're looking for), then averaging on the trace (DSO FTW).Tim

author: nctnico
author_rank: Super Contributor
timestamp: August 26, 2022, 01:04:46 pm
content: Agreed that an oscilloscope isn't a substitute for a spectrum analyser. An oscilloscope is useful though to measure ringing in any power related circuit that uses some kind of switching / PWM topology.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 26, 2022, 01:12:53 pm
content: Indeed, the combination is powerful: correlating a spectral peak to a specific waveform and location, may take some investigation to trace out, but tells you, well, basically all there is to know about the case.Tim

author: alm
author_rank: Super Contributor
timestamp: August 26, 2022, 05:11:53 pm
content: Quote from: T3sl4co1l on August 26, 2022, 10:10:35 amWould you need to trigger? Couldn't you just acquire a few mega samples of data and run FFT on that? Of course this doesn't solve the dynamic range issues.

author: nctnico
author_rank: Super Contributor
timestamp: August 26, 2022, 05:35:06 pm
content: During radiated / conducted emissions tests average or quasy-peek measurements are used. So a pulsating / sporadic signal won't cause much trouble to pass EMC testing; the continuous tones (or frequency bands with continuous emissions) are the problem to pass EMC. There isn't much to trigger on.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 26, 2022, 08:45:51 pm
content: Quote from: alm on August 26, 2022, 05:11:53 pmBut that just tells you what the spec already tells you; which is nothing about how they're correlated in time. Usually you'll have (semi-)periodic signals in the circuit, and you can look at each, e.g. SMPS switch node, or CPU clock (if they differ), to tell which is which. But if they have similar amplitudes at the point of measurement (e.g. probing a cable or LISN), you can't trigger on one or the other, you get both. Sometimes the amplitudes differ enough to catch just the taller one, but if you're trying to track the smaller one, that's no good.Put another way: you see some peaks on the spectrum. How do you correlate those with some impulsive noise in the circuit, and ultimately to an offender?And, say, placing a finger on the offending switch node, ought to pull enough common-mode current to raise the respective peaks on the spec (assuming the switch node is safe to handle..!), so you can figure it out in that case, without having to track through a scope. But that might also be hard to do (say with everything buttoned up), but say you've got access to some related signals, that might be a use case for the scope....Oh, and about nonperiodic stuff; if you can trigger on it and see that that's the offending problem at the LISN or whatever, that helps, too. Spread spectrum, or variable-frequency (e.g. QR flyback, LLC) converters, the spectrum is just a blur, easy to hide many overlapping sources within. Scope can help you separate those too.Quote from: nctnico on August 26, 2022, 05:35:06 pmSo no one's ever had an SMPS fail radiated (>30MHz)?Trouble is, impulsive noise also tends to have high peak amplitudes. A 400V switch edge, even with considerable attenuation, can still be say 10s of mV peak on the outside, and QP won't quite save you enough to pass.Tim

author: Simon
author_rank: Unknown
timestamp: August 27, 2022, 09:18:52 pm
content: Quote from: tautech on August 26, 2022, 12:26:22 amChe'? Don't worry it's one of yours, and my next purchase will not be from telonic, they just shift boxes and can't even explain the promotion clearly or what they are selling. They are about as useless as the $5 meters they used to embarrass themselves with in sending one with an oscilloscope that cost $700 as though you did not have a meter you actually trust already before spending that much on a scope.I don't understand why you think I would intentionally not say the brand, a machine is a machine and how you use it is about the same whoever made it.If I understand correctly the EMI options are indeed a waste of time. when I use the QP filter something that I could pick up before literally vanishes, all noise including the SA's is a flat line along with a signal that stood 20dB above the noise. Could well be I do not know what I am doing yet, I have mostly read the manual.My first thing will be to run the offending machine and see what that lab test looks like in my environment. I will in fact try to get a second machine to modify so that we can always do a side by side test of the offender and the new candidate, if the new one is 25dB quieter we have some sort of chance.

author: nctnico
author_rank: Super Contributor
timestamp: August 28, 2022, 02:01:40 am
content: Having two machines is a good option but be sure they are equally bad before doing any modifications.For the project I am working on, I have 3 boards available and one of them seems to behave better than the other two. Typically I go back & forth between changes on the same board to make sure the change I'm making really has some effect. After that I use a 2nd board and also measure the difference between unmodified and modifed as a verification.

author: tautech
author_rank: Super Contributor
timestamp: August 28, 2022, 08:16:52 am
content: Quote from: Simon on August 27, 2022, 09:18:52 pmReally, so you don't think we might have some experience in this field as many of our SA sales are for exactly this and many on the forum are doing this work too however generic advice won't get you far with the different UI's, feature sets and core capability of the many different devices on the market.QuoteMine, don't you more correctly mean the same brand as we sell ?Exact model please ?Good, there's lots of guidance in US App notes:https://siglentna.com/application-note/electromagnetic-compliance-pre-compliance-test-basics/https://siglentna.com/application-note/electromagnetic-compliance-pre-compliance-conducted-emissions-testing/This content needs updating if you purchased a later SSA3000X Plus analyzer as it was written for the earlier SSA3000X that doesn't have the EMI mode or the inbuilt range of CISPR-filters the later units do.Another good guidance App note:https://siglentna.com/application-note/pre-compliance-basics-analyzer-configuration-settings-and-protection/

author: Simon
author_rank: Unknown
timestamp: August 28, 2022, 08:36:49 am
content: None of my questions have been about model specific stuff, it's literally been about, having this kit, what da hell do I do with it now? how does the, let's say, physics of it work?It's this one:https://telonic.co.uk/product/siglent-ssa3015x-plus-1-5ghz-spectrum-analyser-free-tg-licence/#reviewsHow the tests are done I would expect to be the same with any unit which is why the standard tells you what settings to use. Having realised that the spectrum analyser will have a noise floor of it's own I assume that this is what would make the most difference to measurements when you have a standard that wants signal below a certain level that the analyser may not be able to do.I'll read the app notes and see if they clear anything up for me.

author: tautech
author_rank: Super Contributor
timestamp: August 28, 2022, 09:23:10 am
content: Quote from: Simon on August 28, 2022, 08:36:49 amThe clue is in the terminologies used; EMI and RFI, namely the interference emitted and how you might detect it and measure it to prove that it's inside the standard.QuoteFor low levels you will need the Preamp engaged and especially with the HH RFI probes if emissions are somewhere down where they need be.QuoteThey'll help for sure and there's also a lot of videos on EMI/RFI work here:https://siglentna.com/videos/ssa3000x-plus/Becoming conversant with the User manual Chapter 7 EMI Measurement starting on P112 will help too:https://siglentna.com//wp-content/uploads/dlm_uploads/2022/08/UserManual_UG0703P_E02D.pdfOne of the first things with SA's and RFI/EMI probes is to leave your phone a few yards away.Do NOT be tempted to reduce the automatic input attenuation and if the overload screen warning is displayed or the overload buzzer goes off rapidly remove the DUT, detection device or kill the DUT power.

author: Simon
author_rank: Unknown
timestamp: September 09, 2022, 07:28:12 am
content: Well rather unimpressed with this spectrum analyser, the manual is a mess, the machines UI is a mess, once you get into modes like EMI menu's change and duplicate others, it's not clear what menu's apply in what mode and I can't get it to read my USB drive to do the firmware update. Why did I take the advice of those assholes at telonic?No explanation on the USB stick requirements, it's like a list of the buttons names and that is it!

author: tautech
author_rank: Super Contributor
timestamp: September 09, 2022, 08:05:31 am
content: Quote from: Simon on September 09, 2022, 07:28:12 am8GB is preferred and FAT32 is a must !The current manual covers 3 ranges all with the same basic SA capabilities and same EMI mode.Have you not followed the links I posted ?But hey if I need hold your hand I will as my SVA1032X has the exact same EMI mode as your SSA3015X Plus.First practice taking screenshots with it to USB to post here but be sure to have appropriate menus captured in them.You preference but the Normal image rather than Inverted ink saver shows up best here on the forum. Enter the System menu to find where to change it.

author: Simon
author_rank: Unknown
timestamp: September 09, 2022, 08:24:42 am
content: Yes I have 2 versions of the manual, the larger has stuff that the machine appears not to.The PC software is missing DLL's, plugging it in on USB does what? computer just blips, only driver I can find fails to install, the Ethernet requires a driver despite it being a server...... it just goes on, a shed full of promises and a broken window pane of actual delivered product.First I would like to update the firmware, that seems to have special requirements. 8GB stick? I will see if I can find one.

author: tautech
author_rank: Super Contributor
timestamp: September 09, 2022, 08:37:29 am
content: Quote from: Simon on September 09, 2022, 08:24:42 amIIRC I have used 16GB sticks in these but I don't normally.Don't use the PC SW, just don't unless you really really need to and instead just use the webserver.Connect to your LAN and enter the I/O interface and give the SA a unique IP but on your LAN subnet then Save it.Enter this unique IP into the address bar in your PC browser and hey presto we have a remote connection that you can do everything you could sitting in front of the SA.

author: Simon
author_rank: Unknown
timestamp: September 09, 2022, 08:44:25 am
content: well luckily it takes a 16GB stick, firmware now updated. At some point you may answer the question actually asked.

author: Simon
author_rank: Unknown
timestamp: September 09, 2022, 08:47:28 am
content: Quote from: tautech on September 09, 2022, 08:37:29 amYou mean a bit like my oscilloscope that silent no longer admits to the existence of, yea..... We do PC software but it's vapourware. Struggling with ethernet but I probably will on a corporate network run by security nuts.

author: tautech
author_rank: Super Contributor
timestamp: September 09, 2022, 09:04:30 am
content: Quote from: Simon on September 09, 2022, 08:47:28 amNo.TBH I haven't used EasySpectrum since the early Siglent SA's, those before the Plus series however since the addition of the webserver in Plus, X-R and SVA models it's so much simpler to use as it mimics the instruments UI on a PC plus within the Instrument control panel you have a couple of additional features too.With any of Siglents Easy**** SW care need be taken to how everything is initially set up and particularly the requirement for the bloated NIVISA SW of which a full install is north of 500 MB !We can get away with just the Runtime component for the required connectivity drivers but it's still north of 100 MB FFS !Have my SVA1032X alongside me to grab a screenshot to help with getting your connected to your LAN should it be required.

author: tautech
author_rank: Super Contributor
timestamp: September 09, 2022, 09:20:22 am
content: LAN reconfigured to home network after a recent show.System>Interface>LAN Config> IP Config>DHCP = ON allows your network to negotiate an IP with the SA after which change DHCP to Static and selecting the last digits of the IP with the touch screen use Back to delete them and enter a higher # so to prevent future IP clashes with later added devices. Then Apply.Enter the saved IP into a PC browser and you are greeted with the Welcome page.A couple of captures for guidance attached.

author: tautech
author_rank: Super Contributor
timestamp: September 09, 2022, 09:29:36 am
content: BTW you can change the screenshot to Normal from the default Inverse from within the Display menu.Far right 4th button down is Save that's used for screenshots which you can then add a filename or just punch Enter to push the screenshot to USB.While you're there have a fiddle with some of the other bits and bobs in the Display menu.....Annotation and Display Line are both features that can be useful.

author: tautech
author_rank: Super Contributor
timestamp: September 09, 2022, 10:47:20 am
content: With some poking around to get a feeling for how everything works venturing into the EMI mode shouldn't be so daunting or should it ?As you've discovered even when within a Mode UI you still need pop out of it occasionally to make/add adjustments and then re-enter the EMI mode to set up to capture the EMI measurements proper.Here things get more complex where you need know which tests to use and what detection strategies to use including those in the Amplitude menu where your inputs are to be controlled and NF probes will likely need the Preamp ON.These 2 screenshots are just an introduction as Amplitude is out of the EMI mode then we need use the Mode button to return to EMI mode.Although a BSdisplay with open input and full sweep soon after a Factory default Preset.....another little tool you need to learn about so to be able to start each project with a perfectly clean state and zero remaining settings.Anyways you need select the appropriate CISPR filter within the Scan Config menu and a Span to match and while in the EMI mode you can back out of any menu with the blue circular back arrow.Use the Stop/Start button to perform the EMI sweep then examine the Signal list results.Consulting the manual and videos linked earlier should help further once you are this far.

author: Simon
author_rank: Unknown
timestamp: September 09, 2022, 05:42:31 pm
content: yes when you are in EMI mode the trace menu is bolloks but it is accessible, the detector menu replaces it, one big mess in the UI as you don't know what is active and what is not. I should have learnt my lesson with that scope and realized that the droids at telonic who were supposed to be technical were just looking at £250 more for the siglent instead of the rigol I was looking at. Siglent and telonic, never again either!

author: fourfathom
author_rank: Super Contributor
timestamp: September 09, 2022, 06:12:40 pm
content: Have you tried just using the SA as a SA, without all the EMI software and other crap? You don't even need the fancy CISPR filter if you are just trying to do preliminary comparison and evaluation measurements. You might already be done by now. When you spend more time on the tools than the job, you're doing it wrong.

author: Simon
author_rank: Unknown
timestamp: September 09, 2022, 06:33:52 pm
content: I've not been using the EMI mode, I fail to see how it works or what it is good for. but I guess if I use the EMI filter (in normal spectrum mode) then the 25dB I want to loose is more real that guessing with the gaussian filter.i fail to see how to activate the line of the standard but that is pointless as I am comparing only anyway. Turns out it's the crappy meanwell power supply, reading between the lines on the datasheet and looking at the construction they may have just screeched under the line - or the line. When they tell you that the test was done on a 360x360x1mm metal sheet you start to think WTF meanwell, don't you know that ferrous and non ferrous are two different things but you did not specify, they fail to say if it was grounded. When they say that the unit is meant to be part of a piece of equipment it's a bit of an admission that it actually fails radiated unless you enclose it with the load on a short lead.

author: tautech
author_rank: Super Contributor
timestamp: September 09, 2022, 10:58:43 pm
content: Quote from: Simon on September 09, 2022, 05:42:31 pmBollocks !Your unfamiliarity with this instrument and unwillingness to learn how it works is screaming at us you think this is too hard to negotiate yet most of what you need is right in front of you on the display calling 'I'm here' !Take a step back and understand what every symbol and number around the display actually means when after some time using these instruments for general less complex SA work one becomes very familiar to identifying settings at a glance.Rather than dive balls deep into a real project just use a coax RF loop and start sniffing all manner of appliances, phones, tablets, laptops, SMPS walwarts to get a feel for what EMI looks like and the range of frequencies you'll likely encounter.Such exercises can be very revealing and will properly drag you into a decent learning state of mind and generate more familiarity with the tools you are working with for you to better follow the videos and App notes already linked previously.

author: Simon
author_rank: Unknown
timestamp: September 10, 2022, 07:25:58 am
content: I came here asking about the setup, you are the one that demanded to know the brand, as I said at the start, the brand is irrelevant to what I wanted to know, but as you wanted to know what I was using I've told you and I have told you what I think of it!My very first experiment was to use an RF signal generator to do as you suggest, I learned from that that what working in dB means is that if my transmission goes up or down by 10dB so does the output and what I measure, this means that if I profile the failed unit I can see in a relative measurement where I need to be using the report because everything I am doing is relative.I'm not overly sure that trying to mimic a test lab is the best use of the instrument which is what the EMI mode is for.

author: fourfathom
author_rank: Super Contributor
timestamp: September 10, 2022, 02:21:05 pm
content: Quote from: Simon on September 10, 2022, 07:25:58 amYeah, unless you have a calibrated antenna / test-site then the EMI pass/fail templates aren't much use. EMI response filters might be good to use, but as long as you can identify the original failing signal then any relative measurement should tell you what you need to know (unless your fixes actually change the nature of the signal, such as a quasi-random spreading might do.) I will admit I've never used a modern analyzer for EMI work, so there may be other advantages to those modes.

author: Simon
author_rank: Unknown
timestamp: September 10, 2022, 04:06:38 pm
content: Well having the EMI filter (quasi peak) is useful as it means that the measurements are processed in the same way as the lab, so 20dB off for my measurement should be 20dB off in the lab. Unfortunately I can't modify the failing product as it is a meanwell power supply, the specs of the EMC on the datasheet are rather dubious. I have tried filters but we just force the same energy out somewhere else. As the PSU is in an aluminium box and I think it is radiated off the supply the only way to stop it is use a larger compartmented box with firewalls so that once filtered the signal stays filtered. The alternative non meanwell PSU we have ordered has a lot of baffling on the PSU so it looks like they actually tried to pass EMC rather than fudge around it with dubious statements.

author: fourfathom
author_rank: Super Contributor
timestamp: September 10, 2022, 05:47:53 pm
content: Quote from: Simon on September 10, 2022, 04:06:38 pmI can't see the system, so this might not apply, but can you isolate all supply inputs and outputs using common-mode ferrite chokes? These should be as close to the supply as possible, to eliminate any "antenna effect" of the cabling. And I mean*effective*chokes, using multiple turns on the core, appropriate ferrite material, and perhaps several cores in series. This obviously isn't a production solution, but it will tell you if an external solution is even possible. Don't just slap a random ferrite clamp on the cables and think you're done -- that might only make a dB or two difference. You need to effectively disconnect the cables at RF frequencies.If this doesn't work and you can't modify the supply then you are left with external shielding and keeping the inside-the-shield noise from being coupled to any cables that leave your shielded enclosure. This is usually a much harder and more costly task.

author: Simon
author_rank: Unknown
timestamp: September 10, 2022, 06:33:02 pm
content: Well it's a machine that the power supply plugs into. The power supply is laptop power brick style. We put the meanwell PSU into an aluminium die cast box as it is open framed. The supply is a UHP-1000-48. With a nearfield probe plenty of just is coming out of the end of the supply as there is no barrier, it's like the final 2 transformers/inductors are just radiating. With that in an aluminium box we have no chance, the PSU needs mechanical measures that separate it from any filter we use so that as fast as we filter it does not pick up again on the cable after the filter.I am confused as there are sets of capacitors mounted to daughter PCB's when they could just be on the main PCB. There is a folded sheet metal can over one inductor / transformer with a sil pad stopping it touching the EMI grill that covers the top of the supply, it's like it failed EMC and make shifting the inductor to be a shielded one worked, they just did not need to shield the other two in their cachanded testing. I suppose that could be something I look at doing but not much space in there.

author: fourfathom
author_rank: Super Contributor
timestamp: September 10, 2022, 07:08:38 pm
content: The nearfield probe will help locate sources, but it may not tell you much about how they radiate. You can have a strong current-loop that couples into the probe, but if the loop dimensions are small it won't actually put out a significant signal at a distance. Isolate your cables / wires / connections, if only to understand the problem.

author: Simon
author_rank: Unknown
timestamp: September 10, 2022, 07:54:02 pm
content: basically if you orient the near field probe around the right way whilst holding it in front of the output terminals you see plenty of signal, turn it 90 degrees and it's gone. so stuff is radiating out of the open end and I suspect the two transformers there are the culprits.

author: fourfathom
author_rank: Super Contributor
timestamp: September 10, 2022, 09:10:15 pm
content: Quote from: Simon on September 10, 2022, 07:54:02 pmYou will see this polarity sensitivity with all sorts of radiators, including cables -- few sources are truly isotropic. I can't see your setup, but do take to heart my suggestions about choking off *all* the cable currents.

author: Simon
author_rank: Unknown
timestamp: September 10, 2022, 09:38:23 pm
content: Well I tried a pi filter but this did nothing, I tried an old RS mains common mode filter, nothing, I have a common mode choke from Wurth just arrived I will try but I think we are just barking up the wrong tree with this power supply. Tellingly we get higher peaks with no load than with a light load, there is "excess energy" in the supply that has nowhere to go.Aluminium reflects more than it absorbs so any RF energy is getting plenty of opportunity to get onto our wires after a filter.

author: fourfathom
author_rank: Super Contributor
timestamp: September 10, 2022, 10:30:02 pm
content: Quote from: Simon on September 10, 2022, 09:38:23 pmWhat common mode choke from Hurst? I'm talking about getting some big honking ferrite toroids or clamp-cores and running the wires through those, perhaps three turns, and perhaps several cores in series.Again, I can't see what you've got and perhaps you've done everything you can, but a PI filter has capacitors to ground (or at least to a common point), and where you connect that common point can make a big difference. Just *try* the simple common-mode choke, an effective one, and see what happens. And back up when doing the radiated measurements. As I've mentioned, stuff you can pick up with a near-field probe may not be the same stuff that is actually radiating. It may be the same frequency, may be the same actual signal as what is radiating, but until you know *how* it's radiating you don't know much.Quote"After a filter" has me worried. Aluminum is an excellent shield, and you're not going to fix this with a magic absorptive enclosure. If you put a filter at the supply, then run wires from that to a hole in the Al box you are going to be very disappointed.Stop looking for the production solution. First find out what you have to do, then figure out how to do it practically.I'll back off now and wish you good luck. I'm always happy to give advice.

author: Simon
author_rank: Unknown
timestamp: September 10, 2022, 10:42:54 pm
content: Well given that we are 20dB out I guessed that just a toroid core with the wires wrapped round it may not be enough, so was looking to use a full blown common mode choke. Yes all common points go back to the case but that involves separate wires. Yes aluminium is a shield but does not absorb like steel, so anything that get's out of the supply will be bouncing around in the box to be picked up. The cable is actually sort of shielded in that it's armoured and that is grounded to the case.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 10, 2022, 11:23:47 pm
content: Absorption/reflection doesn't matter at these low frequencies; most any practical thickness of aluminum is an ideal ground plane. Show your layout and wiring.Tim

author: Simon
author_rank: Unknown
timestamp: September 11, 2022, 08:06:15 am
content: i can take a picture tomorrow, essentially its a diecast box, a gland in each end, plate screwed in by the support holes you get at each end. The power supply is bolted to the plate, mains in one end, DC out the other. The lid has a ground lead for mains earthing, the armour on the DC cable is terminated to a ring terminal screwed in where the plate is fixed.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 11, 2022, 09:20:46 am
content: OK, then the glands are your source of noise (conducted). Which may end up radiated by subsequent wiring, but it's all coming through those holes, regardless.So, stop it at the source: put in a filter, grounded to that plate.Likewise, you can monitor EMI at the source: with the box placed either on a ground plane, or elevated above it, (a) LISN(s) placed nearby, to which the wiring is terminated, will capture essentially all the emissions. And it's right there to plug into, no need to probe around.This assumes the end plates have good connectivity with the rest of the box ("ends" plural, are you sure it's not an extruded type??), which can be poor with anodized aluminum, gasketing, etc. Any holes, slots or gaps in the joint are a path for EMI to escape, but even just one screw making good contact will have useful effect in the LF to SW range (say, under 10 to 30MHz). (FYI, conductive EMI gaskets are also available while meeting IP6x ratings!) The more contacts, the better, up to whatever attenuation levels @ frequencies you need. (For PSUs, usually up to a few 100 MHz; so, gaps of 10cm or so between contacts are usually where it starts to get iffy.)Armored cable, as in, flexible, spiral wound, metal conduit/cable? ("BX" in electrical, or more common in industry: "Sealtite", which is a brand, but often used generically to refer to plastic-sheathed flexible conduit.) That should be terminated with a metallic connector, not (or, not just) a ground lead. Then you will have solid shielding also between cable, panel and box, and radiated (at least from that cable) will be much less of a concern. Or, it might not matter -- if this armor is merely an option, you will need to test with unshielded cable as well (whatever the end user might use?), in which case, still providing these options (i.e. shielded connectors) is helpful for users who want to reduce noise further, but won't be required for testing purposes.If you can't mount a filter on an end plate exactly where wiring penetrates it, then, how about a filter, mounted in the main body, so that wiring (and nearby fields) cannot approach the final wiring [what goes through the plate], except through the filter?For example, you could imagine, in a longer tubular enclosure, there's a PCB in the middle, then filters on either side of it, and then the end plates through which cables pass. Fields are strongly attenuated over distance inside a metal tube*. So any fields from the PCB, or the nearby wiring (between PCB and filter(s)), stay very local, then the filters do their jobs, and then the wires go out squeaky clean.*In terms of ratio to the minimum inner dimension (i.e., min(width, height)). This is a waveguide, which does transmit well for frequencies above cutoff; below, fields drop off exponentially with distance. So, in the 100s of MHz, and for enclosures of say 10s of cm width/height, we don't have to worry much, given some distance.Or if that's not feasible, maybe the filters are all the way up on the PCB; but the PCB's edges are conductive and gasketed to a slot in the enclosure (or it's mounted with screws/standoffs, or etc.). In that case, the same principle applies, entirely within the board: PSU and stuff in the middle, filters on the sides, connections on the ends. Filters grounded to whatever shield/GND is nearest.Just throwing on a CMC without any capacitors to help it out, probably doesn't do much to a common PSU, since they'll already have a CMC inside and you're basically connecting them in series -- so, maybe shaving off 6dB or thereabouts -- not a big change. Also, throwing in capacitors (such as 'Y' type for mains to GND, or whatever suitable types for other connections to GND/shield) willy-nilly won't necessarily have effect, or indeed can make things worse. Specifically the combination of CMC inside, 'Y' cap from line to GND/shield, is the most likely to be effective. (Or, again, if the PSU has a CMC in series with the line already, then 'Y' caps on either side of the added CMC.)Tim

author: Simon
author_rank: Unknown
timestamp: September 11, 2022, 04:33:27 pm
content: The box in diecast and it is a large one so yes stuff can get out of the slot. But you are spot on with the gland radiating, it does. As for the shield this is a steel braided cable, the steel braiding is to prevent the cable being cut, usually used in industrial work environments. There does not appear to be a CMC on the supplies output, this is the catch in "designed to be installed inside the equipment. We have it in it's own box with cable and antenna to the machine.

author: Simon
author_rank: Unknown
timestamp: September 11, 2022, 04:35:14 pm
content: https://docs.rs-online.com/8e80/0900766b816e7458.pdf

author: fourfathom
author_rank: Super Contributor
timestamp: September 11, 2022, 06:20:00 pm
content: Quote from: Simon on September 11, 2022, 04:33:27 pmThe gland isn't radiating. Unless the gland is a significant fraction of a wavelength it's not going to radiate much at all. What's radiating is the cable running through the gland. Perhaps a single ferrite core on that cable will only give a few dB attenuation, so keep stacking cores until you kill the signal, if only to prove that the cable is your problem. You're going nowhere until you truly understand the problem.

author: nctnico
author_rank: Super Contributor
timestamp: September 12, 2022, 01:16:35 am
content: Just wondering: which side is radiating? The mains or the DC output side? At what frequency range is the largest spike?A good idea might be to get a LISN so you can at least do some tests to get an idea which side is behaving the worst. A LISN is typically used up to 30MHz during the official EMC tests but most are usefull to 100MHz which usually covers problems originating from switching power supplies.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 12, 2022, 01:36:34 am
content: In general, both. If it's a PSU with just the two cables, then consider each cable as a port with respect to ground. Consider the housing as floating. Obviously, the EMI must be equal on those two sides.When the PSU is grounded to an enclosure, which is in turn grounded to chassis and etc.;and when it is tested as such (enclosure aluminum-taped to EMI ground plane);the unit is a 2-port and the EMI can be different.At RF, this will again generally be true, but to an ever-lesser extent as the enclosure itself becomes a "port" to the ambient fields (i.e. antenna), and you get superpositions of all three (cable, enclosure, cable).This is why the best solution for EMI filtering, is not to return 'Y' caps to earth as such -- nothing in the circuit is actually at earth -- but to output common or whatever. Which side (input, output, or both) needs CMCs, and of what values, depends (again, mainly because of the "reaction mass" or antenna value of the enclosure, or heatsinks or ground plane or whatever else consists of the bulk of the PSU itself), but you can always filter the two lines with respect to each other, and with respect to an enclosing shield, and kill EMI that way.Whether you can violate any of those conditions, and by how much, is another matter.Examples:- Instead of filtering the lines with respect to each other,at a single point(or as point-like as possible, given the ratings of conductors/components required, with respect to the frequencies of interest), maybe it's allowable to have them distant, but still filtered in a point-like manner to the enclosure/shield/ground plane. (Can include PCB ground plane, for board-only designs.)- Or if holes can be opened in the shield, potentially letting out some E-field, but maybe not a problematic amount, and as long as the impedance between filtering points (see previous item) remains low.- Or if the shield can be opened indeed so far that it's naught but a ground plane left.- Or indeed further still, so that it's not even a PCB on top of a sheet of metal, but the PCB itself is the ground plane.Note that these describe topological transformations: ideal case is a box around everything, with connections exiting at one point only, and filtered at that point. Whether that point can be split into multiple, or whether the box can be compromised (exposing internal fields to the outside world, not just explicit connections), depends on what attenuation is required.The latter is usually acceptable for commercial purposes, but requires some optimization to achieve. Getting below say 100uV seems to be a challenge this way, i.e. without additional filtering and shielding. Especially if high voltage/power/speed is involved (compare: on-board DC-DC converter vs. AC/DC supply vs. GaN supply vs. industrial inverter).Tim

author: nctnico
author_rank: Super Contributor
timestamp: September 12, 2022, 09:11:21 am
content: I have found this document from Meanwell:https://www.meanwell.com//Upload/PDF/EMI_statement_en.pdfIn there it shows their EMC testing setup. This is a conducted emissions setup with the load closely attached to the chassis ground of the PSU.This is an interesting problem though. Perhaps it is possible to post a picture of the entire setup, not just the box. Is the AC cable armoured (shielded) as well?Another good tool to have, is an RF current probe that goes around a cable. This page has some explaination and also shows how you can DIY these quickly:http://interferencetechnology.com/the-hf-current-probe-theory-and-application/

author: Simon
author_rank: Unknown
timestamp: September 12, 2022, 09:36:33 am
content: This is the setup

author: nctnico
author_rank: Super Contributor
timestamp: September 12, 2022, 12:57:20 pm
content: First of all: I'd add a wire from the mains ground lead directly to the casing where all other earth wires are bonded together.My mental model of a switching power supply where it comes to EMC is that RF current (from the switching) is coupled from the primary side into the secondary side through capacitive coupling. Typically this is fed back into the primary side through capacitors (either directly or through capacitors connected to ground) but if there is another path for this current to take, then it will take it.In your case an alternative path is provided by the DC output wires. The mains input seems to have proper filtering already.What I would try is to add an inductor to each DC output wire in order to make the DC output high impedance for RF currents. I don't know what you have around in terms of ferrites / inductors. Using two ring cores and winding each output wire a couple of times around the core should give a bit of inductance. See if that improves things.Still I'd expect the shielding of the wiring to have more effect. Are all the shields connected to ground at both sides?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 12, 2022, 02:37:40 pm
content: Ahh, painted die cast. Yeah, not great. Lid won't seal electrically, and if the screws bear on painted surfaces, it may not do anything at all. So the enclosure may be basically useless for EMI purposes (without grinding off some paint anyway, but that may compromise corrosion resistance, say). But the PSU's own metal enclosure, and the baseplate, will still serve as acceptable shield/plane for these purposes (or, probably will).Getting those braids tied to the inside plate, ASAP, and with as wide a connection as possible, will help, as well as filtering to the leads.Probably the mains side has a CMC facing, so a Y cap to the plate should help there, and probably the output side has a low impedance (Y cap already?) so unless you can make it much lower (can you afford a few uF to GND, or does it need to be better isolated than that?), a CMC there is a good idea. And then the opposite outside that, if you need more, etc. (i.e., Y cap then CMC then Y cap then... on mains; CMC then Y cap then... on output).Quote from: nctnico on September 12, 2022, 12:57:20 pmAnywhere on the plate will do, nearer the better; for safety purposes it may still be desirable to have them all on the one node, but in that case the nearer connection can still be made (doesn't hurt to do both!).QuoteYup, this. What you think should be "ground" (or ground-ish, be it mains neutral, or output DC-, etc.), is actually some voltage on top of some more general concept of ground (such as the enclosure and base plate, or differentially between the two "grounds" of the two cables). It doesn't help to put in diff-mode CLC filters (like pictured) -- those handle diff-mode ripple, but not the common (ground-ground) mode!QuoteMmmh, not at these currents (look at those output terminals!). Well, I guess with that wire, it's not being tested anywhere near full ratings either... I hope... but, ferrite rings saturate at a couple amp-turns, even 10-20At for really big ones.Unless you meant powder rings, but those don't offer much inductance, either (but, if you can afford a few uF of DC-to-GND capacitance -- that's still enough to do a good job with).Anyway, easy enough to loop both through: keep them paired together, and wind that through, as many loops as you can fit. Then hook a capacitor to one or both of the leads, after the ferrite, to greatly increase its effect.Like the uh... oh dang, I don't have a good picture of it, hm. This thing:It's just a board (custom 100W flyback supply) in a U-channel style enclosure. Mains obviously has an extra power-entry filter, handy but also good for a bit of extra filtering. The output side, there's a brown/gray twist from the terminal block, which goes around a ferrite ring a good half dozen times or so, just out of view unfortunately. On the backside of the output screw terminals, there's extra 'Y' caps to chassis, 4.7nF each.The line-entry filter is shielded, so no matter how close it may be to the PCB, it's isolated from that noise, and handles things by itself, no worry about fields coupling directly to the line cord, bypassing the filter. The output choke and caps aren't shielded, but they are quite compact (stuffed right against the screw terminals).And the enclosure is bare aluminum, so it makes good connections, even just with regular screws, but star washers help of course. Treat the metal plate or PSU enclosure the same way.Tim

author: fourfathom
author_rank: Super Contributor
timestamp: September 12, 2022, 04:35:22 pm
content: I agree with Tim, that lid looks quite insulated from the box, at least for RF. At the very least be sure the screws bite through the paint. Use star washers and scrape away the paint.How are the cable braid shields connected to the glands? Do the glands have good electrical contact to the aluminum case? It looks like you need a star washer and the paint removed under the washer. And this assumes a good shield/gland connection. Tim's probably right though, connect the shield directly to the interior of the box.I know I keep harping on about ferrites, but once you have a good shield braid connection try putting a substantial common mode ferrite core around the cable, inside the box as close to the gland as you can put it. The ferrite adds series inductance and resistive loss, and the cable braid adds shunt capacitance. This combination can be pretty effective.Remember, at this point you should do everything you can to kill the emissions. Strip off all the paint, remove the lid gasket, stuff like that. Once you have it quieted down then you can figure out how to make it production-ready.

author: Simon
author_rank: Unknown
timestamp: September 12, 2022, 06:50:17 pm
content: We are getting glands that will make contact with the braid tomorrow, yes stripping the paint down is not a bad idea.

author: Simon
author_rank: Unknown
timestamp: September 13, 2022, 08:36:46 pm
content: Well we got the shielding glands and put one on the power out side. It reduced the noise right down but there is certainly noise being conducted so I will look at filters. At most the machine draws 10A.Now that we sort of resolved the power supply we hooked the machine back up, and yes 2 brushless motor controllers do make a pretty picture on the analyser. So the main board is housed in a machined aluminium box, it has holes with plastic glands, the negative supply is connected to the box, which is earthed, the motor phase wires are "filtered" a 22µH inductor in series and some hundreds of pF capacitors after to the negative.Now the manufacturer (maxon) of the motor controllers says filtering may help, but I don't quite see it here. these are 3 phase motors (delta), they do not have a central point that always stays at the same potential. I assume there is one and it is at 1/2 Vcc, but by putting these filters onto the phase wires to negative this is trying to force the negative supply to be the invisible neutral, the negative rail and the chassis earth, I'm not surprised it's all over the place.

author: fourfathom
author_rank: Super Contributor
timestamp: September 13, 2022, 11:48:58 pm
content: Quote from: Simon on September 13, 2022, 08:36:46 pmGood news about the shielding glands.As for filtering the 3-phase stuff, remember that you are trying to filter the EMI, not the motor power. Putting series inductors and small capacitors to ground on each wire shouldn't "force the negative supply to be the invisible neutral". I don't know what the motor drive waveform looks like, but you should be able to at least knock off the EMI-radiating edges without disturbing the motor control. And once more, a common mode choke that encompasses all three of the motor drive wires will reduce EMI without requiring a common ground.One nice thing about common-mode chokes is that current saturation isn't a problem. And a common-mode choke also acts as a balun, which lets you establish your RF ground point wherever you want.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 14, 2022, 05:12:24 am
content: The main problem with putting small filters like that (22u + 100s pF) is there's no damping. The motor will be largely inductive (an open circuit at RF), and the wiring, capacitive to inductive (depending on phase of reflections). Try an R+Cs in parallel with the C, with Cs > 3 C and R = sqrt(L/C). Also if the cable is long, consider choosing L/C such that R ~ 50-100 ohms (which will be a typical impedance for the cable).Delta means you can tie all three caps to a virtual "neutral" (one cap from each wire to a floating node), and use a relatively large cap from that "neutral" to shield/GND: the line-frequency ground-leakage currents (assuming this is a synthetic sine wave sort of inverter) cancel out, so this doesn't compromise ground leakage / touch current / etc. in mains circuits.See circuits like this example:https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=30BCF10R&DocType=Customer+Drawing&DocLang=English&DocFormat=pdf&PartCntxt=2-1609989-2The line-"neutral" caps are large X-type caps, and the "neutral"-GND caps are Y-type (which can also be relatively large).But since this thing is low voltage (and assuming an output voltage less than input, no boost inside), ground leakage doesn't matter, and it's fine to put relatively large caps on them as is. Assuming the control is okay with it of course. (Which, I guess if it's completely unfiltered, it's probably expecting a motor winding and not much else. Consequences might include reduced current limit / nuisance faulting, and increase power dissipation.)More likely though, neither case really matters: the three phases are simply half-bridges acting more or less independently, for purposes of RF filtering. The inductors should be placed at the inverter output, caps to its ground plane/enclosure, or whatever serves as ground reference for its input connection / filter. Note that, if the caps return to a ground path, that has inductance back to the unit, then there is voltage drop on that path, and if the input connects along that path as well, somewhere other than where the caps do -- exactly that noise is fed up the input cable as well. So you have not just the noise from motor output vs. GND, but extra fed up the source as well. The filter can indeed make things worse, and only works when layout is correct.The point (with respect to the above line filter datasheet example) about common mode chokes is well worth appreciating though. Note that all three windings are in phase, which is to say, take triple twist and loop it around, as the whole cable, a ferrite ring. Same as with the pair discussed earlier here. Well, usually they space out the windings to get a bit more stray inductance between them, which gives some diff-mode filtering as well, against the X caps, but that's not the most pressing matter here, and twisted cable is fine.Tim

author: Simon
author_rank: Unknown
timestamp: September 14, 2022, 07:18:56 am
content: These are the filters. It feels like the negative supply should not be connected to the enclosure that is also mains earth.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 14, 2022, 08:44:22 am
content: Depends. If Power_GND is noisy, that noise goes into the output regardless how good the filter is.Like I said, if there's impedance between PGND and other ground / earth, it can make it worse. Depends on connections.The only thing external fields care about is voltage with respect to enclosure and the cables passing through it. A little capacitance to earth makes no difference for galvanic purposes, as long as adequate ratings are used.Tim

author: Simon
author_rank: Unknown
timestamp: September 14, 2022, 06:59:37 pm
content: Well my previous experience was with battery powered stuff in military, here the golden rule was no your "earth/shield" is NOT a power conductor, you have 3 potentials, the shield/earth/chassis and the negative and positive, never should power be connected to chassis as if power flows, that is now an antenna. Obviously we were paranoid about wrapping everything in metal and went to expense to do that with compartmented boxes and metal connectors.Now I am in a commercial environment, just using metal connectors everywhere and shielding to death can be expensive, I also have the issue of fixing another's mess. There are limitations because it is a completed product already that now needs retro fixing.I have started using the free whip antenna that come with the analyser, with this I actually get my signal to be more than 20dB above the noise floor, it seems that the biconical antenna was not very sensitive in the sub 100MHz.

author: nctnico
author_rank: Super Contributor
timestamp: September 14, 2022, 07:51:13 pm
content: Quote from: Simon on September 14, 2022, 06:59:37 pmWhat you can do is buy an external amplifier (like this onehttps://www.amazon.co.uk/gp/product/B07D3KW1C6/) to pull the signal up from the analyser's noise floor. But yes, the small biconical antenna from Tekbox is pretty bad below 100MHz. They shouldn't have specced it like that. You'd need the bigger model to have better reception below 100MHz.

author: Simon
author_rank: Unknown
timestamp: September 14, 2022, 07:54:21 pm
content: Yea, just 30dB of attenuation

author: nctnico
author_rank: Super Contributor
timestamp: September 18, 2022, 01:04:22 am
content: Quote from: T3sl4co1l on September 12, 2022, 02:37:40 pmI disagree with this approach. Really try get some inductance in the individual wires (even if you need huge cores) to rule out differential mode noise. This will help finding the source. The practical implementation is for later. If you look at a typical SMSP, the negative output is connected hard to the SMPS transformer's noise while the positive output has some filters in series and no hard AC connection to the negative output. So you have quite a bit of differential mode noise here.I just did a very interesting experiment with a PoE injector causing trouble... I have a setup using my DIY -somewhat- shielded cage in which I'm trying to figure out what causes a device to fail emissions testing. There is a 4 pair unshielded ethernet cable going into my cage with a whole bunch of ferrite clamps around it. Even 2 bigger ones with the cable looped through it. That should take care of external noise entering the case...When using a clean PSU everything is fine but with the PoE injector I'm measuring lots of noise. So I created a dummy device that is a purely resistive load to make sure the device (DUT) inside the cage does not emit any RF noise. It turns out that adding or removing a ferrite clamp near the PoE injector (outside the cage) does absolutely nothing for the noise level! Only if I split the cable into pairs and put a ferrite clamp around the two active pairs and two inactive pairs seperately, I get a significant noise reduction.

author: Simon
author_rank: Unknown
timestamp: September 18, 2022, 08:15:49 am
content: Quote from: nctnico on September 18, 2022, 01:04:22 amYou are missing the um... difference... between common mode and differential. A single inductor is used on the positive to deal with differential, this is because by differential we mean that the noise is pulsing down the positive returning down the negative, whichever side you put chokes is you will stop it happening. However if you put chokes on the negative you have just created another ground potential so now you have shield, 2 negative voltages that if different from each other could emit and one positive. Sure once you put a CMC in you have a new negative but this does not rise to suck potential differences with respect to the original negative that may be connected to the shielding. Ideally you want all ground to be at the same potential, with differential you are actually achieving that by doing it on the positive rail where differences between rails is more acceptable and expected.

author: Simon
author_rank: Unknown
timestamp: September 18, 2022, 08:18:55 am
content: We have found that the metal chassis is grounded to the negative supply via the mounting holes of the PCB, as my colleague pointed out it is possible that power current is flowing around the case. Waiting for nylon fastners.

author: nctnico
author_rank: Super Contributor
timestamp: September 18, 2022, 11:31:00 am
content: Quote from: Simon on September 18, 2022, 08:15:49 amNot quite. There is shield (ground) and there is negative. Just like mains has seperate neutral and ground wires. You don't want HF current through the 'negative' wire but in your case that could be the issue. In the case I'm working on is actually looks like the common mode choke is working as a transformer which couples the HF noise into wires with a different impedance.

author: Simon
author_rank: Unknown
timestamp: September 18, 2022, 11:47:09 am
content: If there is radiated stuff that is being picked up and you put an inductor on the negative I get that, but I have seen a PCB with an inductor on the negative and positive and the designer could only say tat he though it could not hurt, So PCB now floating even higher above the negative that could have been tied to chassis so producing another emitter, in this case it was a vehicle with negative chassis.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 18, 2022, 01:10:28 pm
content: Quote from: nctnico on September 18, 2022, 01:04:22 amI don't follow; it has onboard filtering (assumed), so it's DM-noisier?Perhaps you mean that it's normal mode: whatever's left after the filter, vs. hard ground (assumed). So, with respect to an assumed ideal ground, you get Vnoise on one, 0V on the other. Which is to say, equal parts DM and CM (DM = (Vnoise - 0), CM = (Vnoise + 0) / 2). But also, if it's not hard ground, that CM noise shows stronger in, well, CM of course -- but not in DM.In any case, I agree that figuring out which is which, is helpful, for determining how to best address it. Far easier to filter a little in DM by adding a small inductor and some fat bypass caps, versus a huge CMC and 'Y' caps of limited size just to knock down what is otherwise a DM (output ripple + harmonics?) source. ('Y' caps of limited size, lest your ground leakage increase too much -- or whatever limitations you might have on these components, if any.)So, the best answer would seem to be -- both, then figure out which ones can be relaxed. You can always measure the voltage drop across these cores (CM and DM/NM) -- just loop the probe ground lead around the core and see what's there!QuoteAh, that can be annoying, yeah. Since Ethernet already has CMCs on the transceivers (give or take PoE -- surprisingly, I still haven't worked with it yet, so I'm not familiar with what all bias networks are common there), adding ferrite beads or even fairly generous CMCs, has only incremental value (some dB here or there). And you're very much in the near field, no transmission line effects / standing waves to help load down the choke. So a couple turns indeed won't do very much.For my part, even just like, a scope probe with 8 turns on a fairly large (>6uH/t^2) ferrite ring, doesn't do very much when trying to measure very noisy circuits -- typically what I get is, instead of a trashy forest of a tone burst, I get an impulse plus a slow, somewhat lower amplitude, ringdown. That is, for an equivalent circuit something like, a square wave capacitively coupled into common-mode (typical in SMPS, gate drive, etc. situations), the observed DM error at the probe looks like this. The slowed ringing at least makes it quite obvious which part is intended DM, and which part is interference superimposed upon it that I can visually ignore.So, you really need a ton of choking to get much value; for a scope probe, the probe impedances are on the order of 100 ohms, so some kohms Z_CM is needed to do much of anything. For Ethernet with some kohms already, you need, well, 10s or 100s of kohms, which just isn't going to happen.What you need is some CM shunting impedance, to make the CMCs more effective. Something like this:https://www.seventransistorlabs.com/Precompliance/index.html#isnI guess you'd need two back-to-back, to both isolate the PoE injectorandstill give the impedance stabilization needed inside the semishielded enclosure.Or you can do everything out on a ground plane, which sucks a little bit for ambient fields (as you can see from my own testing of that network), but isn't too bad for the fields in and of themselves. Of course when you're going for better measurements, getting some attenuation from those fields helps clean things up.As for choking the individual pairs -- it seems your PoE networks have a low impedance in that mode? So that would indeed have value; since you're talking 4 wires now, there's a whole mess of modes we could talk about, but suffice it to say it's some kind of hybrid mode that did it. There are other ISN designs, for whole (shielded) cable, for individual wires (each in normal mode), etc., which may be relevant for different configurations. The individual-pairs type probably being most suitable for Ethernet of course.Oh also, if you're talking injector as in an inline module, rather than a full PoE-supporting endpoint, it could be literally just CT chokes tapping onto the pair-differential mode, no CMCs per pair (which would be DM chokes with respect to the power, so, quite bulky to avoid saturation!), so it would tend to be like that I guess....Actually, I suppose that's still the issue even with a proper endpoint? Right, can't really CMC each pair that way, can you...Tim

author: nctnico
author_rank: Super Contributor
timestamp: September 18, 2022, 05:34:31 pm
content: In the setup I have used yesterday, there are no ethernet signals involved. Just the PoE injector (which is a dumb device) to put power onto two pairs. The other pairs are basically open lines (only terminated inside the PoE injector into an ethernet transformer which is open at the other end). So yes, these unused pairs can be susceptible to whatever noise is present or can be coupled in (through a common mode choke or ferrite clamp).I also modified one sample of the PoE injector with a filter consisting of two ferrite beads to add some series impedance and a capacitor as a shunt coming from the boost converter that turns the 12V input power into about 50V. This filter makes the noise go down by 15dB to 20dB where it hits the noise floor of my spectrum analyser at most of the frequency spectrum. So likely the improvement is even better but I can't measure it.I also tinkered with an ISN a little bit. Initially I wanted to build the same ISN you build but after some testing I abandoned that design. From my understanding the inductor that sits at the EUT (DUT) side, is basically a wideband transformer. I did some testing with the L1 inductor I wound (TDK RM4 core using PC200 / N59 material) but I wasn't impressed by the frequency response. I didn't want to venture too far from the beaten path and as my immediate need is for measuring on two pairs only, I decided to build the much simpler two pair ISN. This basically has a common mode choke between the EUT and external equipment. The prototype seems to work OK; I want to make a more definitive version.Later this week I'll be testing in an EMC lab again and part of the plan is to redo the measurement I did yesterday in order to verify the PoE injector is the cullprit indeed. After that more testing with the actual device. This time with the modified PoE injector and I hope to bring a more professional version of my ISN to use as an extra filter. Make sure that whatever goes into the EMC test chamber is squeeky clean. But that depends on the boards arriving in time... I want to do some verification tests on it to see what is what. Ethernet has signal content up to 125MHz so an ISN will need to have little insertion loss up to that frequency. If my own ISN is any good, I'll post how I built it.

author: Simon
author_rank: Unknown
timestamp: September 19, 2022, 12:25:10 pm
content: So lets say in theory, I purchase two identical antenna's, I put one on the spectrum analyser and one on a transmitter. I sweep the spectrum and look at the received signal versus what I transmit. The gain (loss) at any particular frequency is half the difference between the transmission and the reception (at that distance).

author: Simon
author_rank: Unknown
timestamp: September 19, 2022, 06:48:03 pm
content: I found a photo of the control box. The motors connections are on the left, power comes in on the right.

author: Simon
author_rank: Unknown
timestamp: September 28, 2022, 08:47:02 pm
content: Disconnecting the PCB from the chassis has taken 10dB off

author: fourfathom
author_rank: Super Contributor
timestamp: September 29, 2022, 05:45:09 am
content: Quote from: Simon on September 28, 2022, 08:47:02 pmSo look at the ground loop (or loops) you just disrupted. PCB / cable / chassis / PCB ? Can you reduce the loop area or reduce the loop current?

author: Simon
author_rank: Unknown
timestamp: September 29, 2022, 07:24:15 am
content: Well the PCB was mounted on the 4 corners that made electrical contact with the case. The power comes in one side of the PCB the motor controllers that plug in have their power input on the opposite side so the enclosure was an alternative conductor.I would look next at connecting up the screen of the motor cable to the case. After that the silly filtering on the PCB, I don't think it's a good idea to directly ground the filter of each phase. As suggested previously I would take them all to a central point and then ground them via a capacitor, in fact I wonder about a "capacitor divider" so that the noise can take the easiest path back to where it came from.