A way to tell that a device doesn't create electromagnetic interference? - Page 1
Forum section: electronics / beginners

author: rgawron
author_rank: Contributor
timestamp: January 02, 2015, 02:58:55 am
content: Is there a way to tell that a device doesn't create electromagnetic interference? I mean some sort of test suite that if performed (at home, with tools available for an amateur) is 99% enough to say "this device is OK, it won't make problem in this case"?As an example, let's assume that you build some device connected to USB.

author: marshallh
author_rank: Unknown
timestamp: January 02, 2015, 03:04:17 am
content: Search for "EMI pre-compliance testing"

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2015, 04:04:31 am
content: About the only cheap way is an RF probe, but this isn't nearly sensitive enough nor selective enough to be anywhere near sufficient.Ensuring shielding is the cheapest way to avoid EMI problems (fully shielded USB cable and connector, spring contacts from the metal box to the connector, no other ins/outs from which radiation may escape -- or if necessary, only fully shielded cables and connectors as well), though you still have to test to properly bring it to market.Tim

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: January 02, 2015, 06:43:08 am
content: It's a bit rough but holding an am or shortwave radio near the device might help.You'll hear hash or pulsing if it's emitting rf.

author: pickle9000
author_rank: Super Contributor
timestamp: January 02, 2015, 07:02:12 am
content: sdr's are fun and cheap

author: tggzzz
author_rank: Super Contributor
timestamp: January 02, 2015, 09:44:23 am
content: Quote from: rgawron on January 02, 2015, 02:58:55 amIt is very difficult to prove a negative. Doubly so with EMI/EMC since it is very easy to get wildly differing results simply by changing the tests and/or equipment slightly.First steps are to determine the relevant specifications and get some equipment that is sensitive at the relevant levels and frequencies.Next explicitly make a decision as to how much to "over-engineer" the equipment, i.e. NRE costs and the market window vs production costs. Over-engineer as much as possible within the other constraints.Finally, if it is only going to be used in one environment, test it in that environment - i.e. suck it and see.

author: Howardlong
author_rank: Super Contributor
timestamp: January 02, 2015, 11:33:07 am
content: Agreed regarding over-engineering, plenty of decoupling, use of ferrte chips, check what you should be doing with shields, ie, don't just assume connecting them straight to ground, USB devices in particular must take special care in this respect.Also you'd be surprised how much benefit there is to using multilayer (preferably 6 or more layer) boards, hiding the signal layers between well decoupled power planes.On one occasion a product I'd designed failed a CE compliance test due to a clock's harmonic radiating in certain use cases (I never did figure out why it was worse in some scenarios and not others). I popped a judicious cap on the clock buffer output after a 33ohm series termination resistor that was already there. Not the nicest, but it worked. After that expereince, I always leave room for a cap on high speed clock lines, just in case. Just my luck, they've always been DNP since.

author: G0HZU
author_rank: Super Contributor
timestamp: January 02, 2015, 04:00:50 pm
content: There isn't a simple DIY answer to this (unless the device is very obviously non compliant in a big way)Since the 1990s I've been designing products to meet MIL STD 461 and there's a whole load of test categories in there for things like radiated and conducted emissions. So I've got lots of experience when it comes to visiting EMC test houses and watching stuff get tested to MIL STD 461. I've also had stuff tested for other EMC specs besides MIL STD 461 though...When something fails, eg it fails on radiated emission levels, then it isn't always that easy or obvious howto find the leaky area in the equipment. In the test house the tests are done with calibrated antennas at acertain distance in an anechoic chamber. Often I'm allowed to tinker and play in the anechoic chamber to see if there are any obvious and easy fixes. But sometimes the radiation is caused by poor layout and ground paths inside the device itself.I often have to resort to going back to the works lab and using E and H field probes to try and sniff the device and its cabling in the near field if the device radiates in any local hotspots. You can get confusing results doing this but I would recommend making a suite of E and H field probes using semi rigid coax and having a sniff around for obvious leakage from hotspots.At work we use the classic R&S HZ-11 near field probe kit for stuff like this but I made my own for home use and they work really well and can be very useful diagnostic tools for lots of things besides EMC testing.Obviously, the signal levels and patterns you see on the probes are not the same as what will be measured in a formal test using the correct antennas in a test chamber so the aim with the probes is to try and spotleaky spots and try and reduce the emissions in a relative sense. Hopefully, the results will then be better in the real tests.So unless you have a way of calibrating your measurements then you won't know how good or bad your results are. I try and calibrate my homemade E and H field probes using a VNA and a long transmission line. This method gives good insight into the frequency response of the probe, its self resonant frequency limit and also the E rejection for the H probe and vice versa. But it only gives information in the very near field rather than at the formal test distances used at the EMC test house.For conducted emissions you can make useful measurements at home if you make your own LISNs and you can connect the test port of the LISN to a spectrum analyser or EMC receiver.

author: Codemonkey
author_rank: Regular Contributor
timestamp: January 02, 2015, 04:12:11 pm
content: Quote from: G0HZU on January 02, 2015, 04:00:50 pmAny chance you could elaborate on how you go about this ? I just made a set of probes for home use (I obtained a couple of SA's at a price too good to turn down so didn't!), an HP ESA-L1500 (9KHz to 1.5GHz), and a HP8595E (9KHz - 6.5GHz). The probes seem to work well but not being an RF guy have no idea how to calibrate them in any way. I do have access to a very nice many GHz (60 I think) Agilent VNA at work.

author: lgbeno
author_rank: Frequent Contributor
timestamp: January 02, 2015, 09:10:36 pm
content: Yeah you can use near field probes to find hotspots but you at least need to test in a chamber once to know how that relates to performance in far field tests. You will always find stuff with near field probing.If you were really dedicated you could setup a open air test site with antenna and receiver. This is a acceptable way to do emissions testing and I've done it before.Also did FCC, European, Canadian, Japanese intentional radiator certifications testing this way as well.

author: dannyf
author_rank: Super Contributor
timestamp: January 02, 2015, 09:15:22 pm
content: QuoteA sure way would be to destroy it: if it doesn't exit, it cannot create electromagnetic interference, for sure.

author: katzohki
author_rank: Frequent Contributor
timestamp: January 02, 2015, 09:20:03 pm
content: Quote from: dannyf on January 02, 2015, 09:15:22 pmMatter can neither be created nor destroyed - merely rearranged

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2015, 05:10:12 am
content: Once upon a time, I tried to destroy a yellow flashlight, but as hard as I tried, the casing was still radiating electromagnetic radiation in the 580nm band! ARGH!Tim

author: jahonen
author_rank: Super Contributor
timestamp: January 03, 2015, 09:13:13 am
content: A big problem with nearfield probes on a PCB is that they show every possible reactive nearfield as a hotspot although they don't actually radiate significantly and cause EMC problems.My experience is that almost every time, the worst source of the problem lies with common mode emission, which is usually shown only when cables are connected to the DUT. When cables are disconnected, the problem goes away or is drastically reduced. I have often put a power/IO cable through the loop probe and then scan across the cable length. If there is some point where field is picked up, there usually is an EMC problem. But proving that there isn't, is very difficult. At the end, it is usually much less hassle to bite the bullet and go to some EMC test laboratory and do couple of quick scans with peak detector. Usually even with two scans, one with horizontal and one with vertical polarization, one can get a very good feeling if the device will meet the emission requirements or not.Regards,Janne

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2015, 02:09:30 pm
content: Quote from: jahonen on January 03, 2015, 09:13:13 amYes, near fields only, of course.Whether those turn out to be radiating modes depends on distance, electrical length, and especially as you say -- if any of that crap gets onto wires, which make excellent antennas for obvious reasons. One can make reasonable hand-waving guesses about the nature of these things -- but it's a very special-case thing, not amenable to generalization. The generalization is that long wires radiate; to apply that, one must know what the system looks like.QuoteUmm.. loop probe? Like an H field probe?? There's no H parallel to a wire...You can use wideband current transformers for this purpose, though. Easy enough to make (if uncalibrated), and not too hard to improve (distributed windings, shielding; if still uncalibrated), or you can go out and buy a Pearson... for the price of a well-used car. But hey, it's clamp-on at least.Tim

author: jahonen
author_rank: Super Contributor
timestamp: January 03, 2015, 02:26:14 pm
content: Quote from: T3sl4co1l on January 03, 2015, 02:09:30 pmNot parallel, but the cable through the loop of the H-field probe, so like a current transformer. Of course, that is difficult if connectors are so big that they don't fit through the probe loop.Regards,Janne

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2015, 04:14:22 pm
content: An H-field probe senses fields going through the hole, so it will pick up almost exactly zeroarounda wire, because the fields around a wire are tangential to the wire, not parallel!A current transformer (of that design) has toroidial windings, which are parallel to the wire and therefore pick up current on the wire.Conversely, a current transformer is specifically designed to be insensitive to stray magnetic field through the opening, so that it makes a very poor H-field probe.You can use an H-field probe as a current probe (uncal.) by holding it near (but off center of) the wire, or forming the wire into a loop centered over the probe.Tim

author: lgbeno
author_rank: Frequent Contributor
timestamp: January 04, 2015, 01:46:43 pm
content: I agree with Tim current probes for me were always the most useful tool in diagnosing emissions problems.These are the de facto standard:http://fischercc.com/ViewProductGroup.aspx?productgroupid=127Common mode implies that current is not returning of the same wire as its source or vice versa. This is a formula for emissions! The larger the current loop, the more likely that you have a problem. By the way low frequency emissions are the most difficult to mitigate.Again you don't know if you have a EMC problem until pre scan so I agree, bite the bullet, there's no substitute...A note on design for emc, the resource that resonated most for me was a presentation by Silent Solutions. Henry Ott and his book are also considered the definitive resource.One thing that makes me nervous about EMC is that designers tend to go filter crazy with their designs however you have to realize that you are dealing with currents that are very determined to flow and filters can often steer them in worse directions.I like the strategy of reducing high frequency return path impedances such that return currents tend to stay close their sources reducing the loop area. This is done though strategic placement of ground wires inside the cable bundle. Ample ground planes and vias tying them together. Very robust ties of cable shields and grounds to said ground plane.Filters are expensive and dangerous, sometimes necessary though so that's why every answer in EMC is "it depends". Common mode filters like ferrites are a little more friendly.

author: jahonen
author_rank: Super Contributor
timestamp: January 04, 2015, 02:25:01 pm
content: Quote from: T3sl4co1l on January 03, 2015, 04:14:22 pmAnd it seems that you are right and I have been wrong all these years, but fortunately the result is favorable. That makes it much easier to use as there is no need to fit the cable inside the H-field probe. Holding the loop parallel to the wire gives the best signal (top case). What I suggested earlier seems to be the worst way (middle case). It is not so evident on the spectrum analyzer as the display is logarithmic but on a scope, signal vanishes completely. The setup consists of 54 MHz 3.3 volt lvttl output oscillator, connected by twisted wires to a 50 ohm termination resistor (hidden behind the tape in the picture below).Regards,Janne

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: January 04, 2015, 03:01:04 pm
content: I did not read all the way through every post , skimming through I thought I would see some setup questions - the lack of .What device and its use , along with what frequencies of concern and then what amplitude would be problematic ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 04, 2015, 03:08:59 pm
content: In the US, the FCC requires that unintentional radiators meet specific requirements of peak and quasi-peak amplitude at various frequencies, for conducted and radiated modes. If you don't mind the legalese, FCC Part 15 is available free online; or in the same search, you can find many summaries of the same.Part 15, especially the older parts of it, is geared towards TV and radio equipment, since that was pretty much all there was for consumer electronics until the 80s. But whatever's doing it, the goal remains the same: minimizing interference to any radio services that might be affected (ranging from consumer radio and TV to marine, aircraft and emergency bands as well).IEC 61000-4 is the international standard for compliance, including emissions as well as transients and susceptibility.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 04, 2015, 03:11:07 pm
content: Quote from: jahonen on January 04, 2015, 02:25:01 pmD'OH!Oh well -- thanks for the visual aid though. Some people just can't convert words to visuals, so that helps get the point across, for anyone who might be watching this thread.Tim