DIY EMC Testing of simple devices - Page 1
Forum section: electronics / projects

author: Mangozac
author_rank: Frequent Contributor
timestamp: September 21, 2015, 12:02:31 pm
content: Hi all, I was hoping to get some comments from those familiar with EMC testing.Here in Australia it's not a legal requirement that a certified test lab conduct your EMC testing - it just needs to be done by a competent body (i.e. a business who has the equipment and technical skills) and a report generated. While at my work we have had the odd design EMC tested by a certified lab, it's just not feasible to outlay that cost for many very low volume designs that we produce for small businesses. We assess the risk and if a design is very simple we simply ignore the EMC testing.Call it "professional improvement" but I would like to begin performing a degree of in house EMC testing for such designs. I've read a lot of material on the subject (both reference books and online) and feel that I've got a pretty good understanding of it.The biggest hurdle to DIY EMC testing is an anechoic chamber for radiated emissions testing. Conducted emissions can be measured relatively easily using a LISN and current clamps.My question is: for a simple electronic design can it be argued that extensive radiated emissions testing is unnecessary?By simple design I mean something aong the lines of:- Transformer and linear regulator power supply <10W- PIC microcontroller with internal oscillator running at 4MHz- Relays for switching 240VAC loads- Nothing else in the circuit operating above tens of HzThe highest frequency of 4MHz is at such a low power level that it would require a wire of tens of metres to be an efficient radiator, so even a few harmonics of that frequency are just unable to generate strong enough emissions. The fact that it's constrained to inside of the microcontroller makes it even less likely to generate emissions. Bypass capacitors should take care of any 4MHz ripple in the 5V rails.Conducted emissions on the other hand are a lot more likely. Wires that connect to the product have a much higher chance of being at wavelength integer fractions and nonlinear currents in such cables can cause EMI to be generated.If conducted emissions levels are far below the limits and probing the device with an antenna shows no detectable radiated emissions at harmonics of the MCU clock frequency then surely full testing at an OATS or in an anechoic chamber is next to worthless?Thanks for reading - I'd really appreciate any feedback on this.

author: AndyC_772
author_rank: Super Contributor
timestamp: September 21, 2015, 02:08:55 pm
content: I'm sure you won't be at all surprised to hear that this question comes up a lot.Unfortunately, almost any electronic device "can" radiate noise in excess of the limits, if it's either badly designed, or you're just plain unlucky.I'd be very surprised if your PIC is a problem, unless you've got its I/O pins connected to long, unshielded wires. Even then, its edge rate isn't likely to generate the sort of high frequency harmonics that tend to be a problem.A mains power supply is another can of worms entirely, though. One of the noisiest I ever saw was a linear design. Each time a bridge rectifier starts conducting, there's a large dI/dt.Regardless of the technical aspects, though, I think you'd have a hard time demonstrating that whoever conducted your testing was "competent" unless they were able to make quantitatively accurate measurements. If you need to legally demonstrate compliance, I see no real substitute for an OATS or chamber which has been properly calibrated.

author: MikeW
author_rank: Regular Contributor
timestamp: September 21, 2015, 02:26:19 pm
content: If you have the equipment to test your conducted emissions (analyser, LISN and CDN) then adding the antenna to measure radiated emissions can't be that much more of an expense?

author: IconicPCB
author_rank: Super Contributor
timestamp: September 21, 2015, 10:24:02 pm
content: You dont really need an anechoic chamber..Do a survey of local environment and subtract from test readings.Yes this calls for some specialised software but then..While this might not be in compliance with rules and regs.. it will give you an insight into equipment performance.You will also need a reference antenna and a rotating table to rotate the product on and check its signature from various directions

author: Mangozac
author_rank: Frequent Contributor
timestamp: September 21, 2015, 10:55:56 pm
content: Quote from: MikeW on September 21, 2015, 02:26:19 pmNo, the antenna isn't a big expense; it's the anechoic chamber or open area test site that isQuote from: AndyC_772 on September 21, 2015, 02:08:55 pmAnd I certainly understand that, however as a professional engineer who is aware of designing for EMC I feel that I am able to make educated conclusions (and perform basic tests to back it up) as to the likelihood of a design to potentially generate emissions.Quote from: AndyC_772 on September 21, 2015, 02:08:55 pmExactly (put more elegantly than I tried to describe).Quote from: AndyC_772 on September 21, 2015, 02:08:55 pmAbsolutely, but that will show up as conducted (and some near field) emissions more than radiated. With the impedance of a small <10VA transformer I still struggle to imagine a large enough dI/dt being generated to cause significant conducted emissions, let alone radiated.Quote from: AndyC_772 on September 21, 2015, 02:08:55 pmI do agree, however in practice if we have done all of the conducted emissions tests accurately along with some "indicative" emissions tests, then the likelihood of the device generating any EMI is very low and the chance of actually being queried by the governing body is even lower. Surely making most of the measurements correctly, along with technical reasoning, is enough to show due diligence for a simple, low risk design? It may not be 100% to the legal requirements, but in practice they're also not going to be issuing big fines or jail sentences (as they apparently have the power to do).Quote from: IconicPCB on September 21, 2015, 10:24:02 pmYes, that is true, but it's still a lot of work for a test that is ultimately not absolutely accurate. Is there much point if you conclude from the conducted emissions tests and a technical design perspective that the chance of failing the emissions is next to nil?

author: skipjackrc4
author_rank: Regular Contributor
timestamp: September 22, 2015, 03:07:43 am
content: Quote from: IconicPCB on September 21, 2015, 10:24:02 pmThis might get you somewhat close, but the anechoic chamber does far more than just shield your test setup from the outside world. Reflections off your surroundings will interfere constructively and destructively with the signal you're trying to measure, resulting in potentially inaccurate measurements. How inaccurate? It depends very heavily.This is why open air sites were so common--if you're testing in the middle of the desert, the only reflections you get are those from the ground, and the standards address this.It is possible to take near-field measurements very close to the DUT and then extrapolate the far-field for comparison to the test standards, but this is a rather intensive process.

author: coppice
author_rank: Super Contributor
timestamp: September 22, 2015, 03:35:27 am
content: Quote from: skipjackrc4 on September 22, 2015, 03:07:43 amYou can do pretty good testing in any reasonably large field, like a local football pitch. You probably need a small portable generator, or a UPS, to power your equipment so far from a mains socket, but that's a small cost compared to an RF chamber. You might need more formal testing later on, but testing in a field will let you flush out most problems.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: September 22, 2015, 03:38:00 am
content: Quote from: coppice on September 22, 2015, 03:35:27 amSure. I was more referring to working inside a traditionally small lab space. Any type of sporting field would be large enough to avoid major reflection errors for most products.

author: Mangozac
author_rank: Frequent Contributor
timestamp: September 22, 2015, 03:53:21 am
content: Testing on a nearby sports field is certainly feasible, but still a big headache supplying power and setting everything up.

author: Bud
author_rank: Super Contributor
timestamp: September 22, 2015, 04:10:42 am
content: Lots of fun when a concerned citizen will call the police

author: skipjackrc4
author_rank: Regular Contributor
timestamp: September 22, 2015, 04:14:10 am
content: Quote from: Bud on September 22, 2015, 04:10:42 amJust don't try to test your suitcase-clock-thing while a game is going on

author: Mangozac
author_rank: Frequent Contributor
timestamp: September 22, 2015, 04:36:33 am
content: Quote from: Bud on September 22, 2015, 04:10:42 amI know that's a tongue-in-cheek comment, but I'm sure if an officious, self important council worker noticed it they would get their knickers in a knot and we'd probably get booted off!

author: coppice
author_rank: Super Contributor
timestamp: September 22, 2015, 05:49:28 am
content: Quote from: Mangozac on September 22, 2015, 04:36:33 amWell, if you are going to use a sports facility it might be a good idea to actually rent your time there.

author: IconicPCB
author_rank: Super Contributor
timestamp: September 22, 2015, 09:37:18 pm
content: I have had to get a piece of equipment tested and certified.It would have been awkward to move it due to its bulky nature to a laboratory facility some hundred or so kilometers away.We are located o an acreage allotment so the test eng came to us, we set up the equipment on a timber dolly so it could be rotated.The fellow spent an hour or so operating his spec an and sampling the background.He then tested the equipment ( both radiated and conducted tests ) and issued a test certificate in compliance with Australian requirements.Same equipment was then tested in an anechoic chamber in Europe for compliance with CE requirements and similar results were obtained.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 22, 2015, 10:11:17 pm
content: Quote from: IconicPCB on September 21, 2015, 10:24:02 pmSounds like a nice idea but much less useful in practice, as many ambient signals will be varying all the time, either due to their source or reflections etc.Where possible, cycling the DUT on & off and looking for deltas is probably a bit more effective.For smaller items, a GTEM cell may be worth a look.

author: Mangozac
author_rank: Frequent Contributor
timestamp: September 23, 2015, 12:43:00 am
content: Quote from: mikeselectricstuff on September 22, 2015, 10:11:17 pmIndeed - that's why I'm reluctant to invest much time and effort into such testing with a significant degree of uncertainty.Quote from: mikeselectricstuff on September 22, 2015, 10:11:17 pmCycling power to the DUT will certainly help with identifying peaks being generated by the DUT. Unfortunately, according to CISPR 14.1, a GTEM doesn't seem to be an option for devices that have attached cables.One interesting thing I did note however is that clause 4.1.2.3.2 and figure 10 of CISPR 14.1:2013 seems to indicate that for appliances with clock frequencies <30MHz you may alternatively choose to test for radiated emissions using an RF absorbing clamp on the cables. I did read the reasoning behind that somewhere but I can't find it right now. That is certainly a more feasible test method...

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 23, 2015, 06:31:08 am
content: Quote from: Mangozac on September 23, 2015, 12:43:00 amI believe the rationale for that is that for low frequencies, the only significant way for RF to radiate is from long cables. However I can't see how a clock frequency limit can be that useful as you don't know what edge rates are involved - I'm sure 30MHz feeding into some 74AC gates with a few inches of wire on the output would produce plenty of crap well into several hundred MHz

author: coppice
author_rank: Super Contributor
timestamp: September 23, 2015, 06:54:19 am
content: Quote from: Mangozac on September 23, 2015, 12:43:00 amGTEM cells are widely used for testing things which do have cables attached. Often power devices with really thick high current cables. Maybe this is not considered good enough for formal testing, but I see GTEM cells used in house for development testing in a number of companies located far from the madding crowd.While a GTEM cell costs nothing like as as much as a ferrite lined room, it is still not exactly cheap.

author: Mangozac
author_rank: Frequent Contributor
timestamp: September 23, 2015, 07:05:02 am
content: Quote from: mikeselectricstuff on September 23, 2015, 06:31:08 amYes, it's something along those lines and I think makes sense.I agree that the 30MHz threshold seems uncharacteristically (for a standard) loose. Perhaps that few inches of wire in your example would cause coupling to the external connection cables (which would then be detected by the absorbing clamp)?Quote from: coppice on September 23, 2015, 06:54:19 amIt's true that a GTEM is not cheap either! My understanding is that they're more commonly used for testing smaller, battery powered devices. I'm sure that some people test other devices with it where appropriate, but as best I can tell CISPR 14.1:2003 doesn't allow it for devices with cables.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 23, 2015, 05:01:25 pm
content: Quote from: coppice on September 23, 2015, 06:54:19 amBut unlike ferrites, not inherently expensive - it's just a metal box, so could be DIY'd

author: Mangozac
author_rank: Frequent Contributor
timestamp: September 23, 2015, 10:10:52 pm
content: Quote from: mikeselectricstuff on September 23, 2015, 05:01:25 pmBut for anything other than very small devices the size of the GTEM grows quickly, so even DIY can get expensive. It is however rare that we do anything particularly large, so it's certainly worth doing some more research into exactly what size is required and which standards will allow the GTEM to be used for devices with cables. There seems to be sufficient information online to enable design and construction of a DIY GTEM. By far the most difficult part is doing characterisation of the field strength inside the GTEM.One advantage of the GTEM is that it can also be used to do immunity testing.