EMC chamber without absorber material - Page 1
Forum section: electronics / rf-microwave

author: chiplukes
author_rank: Newbie
timestamp: October 23, 2018, 02:59:37 am
content: I have a few questions for those of you who have a solid understanding of RF (which is not me, I’m an FPGA guy).The company I work for is constructing a new building and has plans for a large dedicated room for debugging EMC issues. The room will be built such that it could become an anechoic chamber someday, but that will not happen initially due to cost. The plan is to attempt to shield this room (solid metal floor, walls, etc), but not add any RF absorbent material.Given that initial state, what useful EMC testing can be done in such a room?I am thinking that it might be useful for identifying EUT emissions that would normally be masked by ambient RF outside this chamber.I am assuming that with reflections that any sort of useful field strength measurement will not be possible, correct? I realize EUT, antenna placement, and room geometries all factor in to this.Also, with all the potential reflected RF energy from the EUT bouncing around in there, are we creating some sort of self-immunity test? I’m envisioning the inside of a microwave, but at whatever frequencies are coming from the EUT 😊Anyway, any thoughts or comments about this would be great!Thanks,Chip

author: mjs
author_rank: Regular Contributor
timestamp: October 23, 2018, 12:10:07 pm
content: About right, quantitative measurements are very difficult. Close field measurements and finding sources is easier, especially if the room is enough and there's at least something absorbing.At AT&T lab I saw a rather small mesh room with a small booth made of absorber foam structure - it was quite OK for doing antenna tuning measurements with Emscan RFX2 - we got within 1-2dB at 800-2400MHz from an accredited measurements.

author: Berni
author_rank: Super Contributor
timestamp: October 23, 2018, 03:05:23 pm
content: Yep it helps a lot because it keeps away RF crap from tge envioement. So any peak you see on the screen is you.But you have to shield it really well. Those cell towers and FM broadcast is powerful stuff.One bad joint in the wall and it all gets inside.

author: chiplukes
author_rank: Newbie
timestamp: October 23, 2018, 04:31:35 pm
content: Thanks for the responses!We have read that sealing the room will be difficult in these forums and elsewhere, but lack that "gut feeling" that a true RF engineer would have regarding these things.Assuming we have a very thin seam not making contact (lets say < 1mm) and a 10cm or so in length. At the frequencies that can get through that, is the field strength inside the room pretty much the same as outside, or will it still be reduced substantially?This building will have a huge cell phone tower next door...Chip

author: izsurk
author_rank: Newbie
timestamp: October 23, 2018, 04:50:47 pm
content: Only for radiated emissions: Non-RF absorbing walls will reflect the waves thus produce standing waves in your chamber (depending on the frequency of interest + size of your chamber). This will add peaks (where there should be no peak) in the spectrum, but also extinct peaks where there should be some.The metal shielding of the chamber is to prevent signal content from outside to distort your measurements (and prevent the DUT from disturbing).Measurements without a chamber a basically useless. Every radio / TV / WLAN transmitter near your DUT is much much more powerful than the "EMC-signals" you are looking for. This will prevent you from "seeing" problems. Missing absorbers will make you search for disturbances at frequencies that are not existent. You can do like pre-compliance and some relative measurements (if you know where your problematic frequencies are). But you won't be able to tell about pass / fail.I'd suggest: Buy a GTEM cell or TEM cell. Much more useful and reliable.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 23, 2018, 06:15:56 pm
content: I only know of two applications for such a chamber:1. Shielding external noise. A Faraday cage. The equipment inside is made very quiet, to listen to sensitive experiments.2. Partial discharge testing. High voltage AC is applied to the DUT, and a nearby antenna listens for broadband (impulsive) noise. The magnitude of the impulse corresponds to the amount of partial discharge. This test is sensitive to pC of charge.A local major-brand power electronics company has a room paneled with solid copper sheet for this purpose. Sure looks fancy.Doing conventional EMC (susceptibility / emissions) testing inside such an environment, is unlikely to be productive. It's a blown-up microwave oven cavity, it's going to have very distinct modes, down to fairly modest frequencies (~50MHz say?), with absorption of each mode depending on the location and consistency of lossy bodies within the cavity (technicians).Tim

author: chiplukes
author_rank: Newbie
timestamp: October 23, 2018, 08:25:43 pm
content: Very helpful everyone. Thanks again!I'm not sure I can wrap my mind around the statement about seeing frequencies in the chamber that are "non-existent".For a given measurement arrangement, I would expect to either miss a frequency that is present but reflections have cancelled out, or measure a frequency with greater amplitude (reflections adding together).How would I actually get frequencies other than those coming from the EUT?Chip

author: Berni
author_rank: Super Contributor
timestamp: October 23, 2018, 08:35:00 pm
content: Getting quantitative measurements gets expensive really quickly tho. Not only do you need a very big well shielded room but you also need great quality absorbing material on all walls and special calibrated antennas along with a spectrum analyzer with a quazi peak detector. And even if you do have all that you need accreditation to do EMI certification of products.Just being able to see interference and test emi counter measures you can get by a lot cheaper. What we use for that is a rectangular box made out of a conductive steel fabric and some RF and power feed trough ports on one end. Put some absorbing foam in one side of it and stick a basic antenna in there, hook up whatever spectrum analyzer you got to it and you are off to the races. Are there reflections and standing waves inside the chamber? Yes there are. Is my antenna flat for the entire frequency range? Not even close. Does it get the job done? Hell yes it does. With the thing properly shielded so well and with a LNA in the chain the noise floors you can get are well below -130dBm with a pretty open RBW. If you see a -60dBm spike in there when you switch on the DUT you know you fucked up. Yes the level will change as you move the antenna around, but that's normal since the DUT radiates around itself in all sorts of strange patterns and sure the levels go up and down by as much as 10dB when pushing and bending the chamber walls if you happen to be riding on a resonance mode inside there. But if something is radiating even a little bit somewhere it will show up clear as day every time. Then once you determine some peaks being unacceptable you can fix the DUT in a jig and fix the antennas in place while adding modifications to the device like extra ferites, changing components, changing sheilding cans etc. And with each modification you can clearly see exactly how much it affected the result so you can see if your EMI fixes are going in the right direction. Yes its not ideal, but having a big shielded box still is much better than waving an antenna around your DUT on a office desk. It can be really difficult to tell apart your devices radiation from the RF noise in the environment and to make it worse the noise in the environment keeps changing. Additionally the powerful RF sources like towers and broadcast can overwhelm your LNA or your spectrum analyzers mixer even if its not listening to that frequency and that forces you to put attenuators in and making your systems noise floor worse.As for shielding. Its not the width of the gap that's the problem, its instead the fact that the gap is not conductive. When you introduce a long cut in your wall this essentially creates a single turn inductor around it and the gap itself forms two plates of a capacitor. Some outside RF could excite these parasitics into oscillation and since they are part of your wall they radiate some of the energy inside. This is why it is critical to make sure the walls have a good connection to each other at all points so that they look like one solid piece of metal. If you just stick a small piece of insulated wire trough the door of the chamber suddenly you will see quite a bit of RF making its way in. A lot of it has to also do with how dBm works. if the chamber has 60dB of attenuation to outside noise that means only one millionth of the power makes it inside and a good chamber will have more than 60dB. So it only takes nanowatts of power to leak in and your attenuation is ruined. It just means you have to construct it very carefully so that everything is electrically connected and any door at MUST have EMI finger contacts all the way around it.

author: izsurk
author_rank: Newbie
timestamp: October 24, 2018, 12:04:06 pm
content: Quote from: chiplukes on October 23, 2018, 08:25:43 pmYeah, my bad. Of course, I am talking about frequencies that are existent in your EUT. They might be far below EMC limits in real, but as a standing wave, you'll see them as a peak. Most digital DUTs have like a wideband EMC-noise floor in addition to the peaks (which are mostly paid attention). The problem is that frequencies from this additional noise floor can be "used" the standing waves and thus create peaks. You'll spend a lot of time hunting for those frequencies although they are no problem for the EMC test. Of course, reflections won't synthesize frequencies ...

author: chiplukes
author_rank: Newbie
timestamp: October 24, 2018, 04:44:08 pm
content: Lots of great information here! Thanks everyone!It is sounding like we should be able to at least make some measurements that will be useful to us.Berni, the next time you are near your testing chamber could you snap a picture or two and post them here? I realize sometimes company policies may not allow that. If not, no big dealAlso, thanks for the better way to think about what a hole in the shield is.We are still about a year out on the building timeline, but I will try to update this thread regarding our success or failure with the processChip

author: cdev
author_rank: Super Contributor
timestamp: October 24, 2018, 10:42:16 pm
content: Outdoors- do you have access to a yard or similar, with no metal anything in it, and earth as the ground surface?In terms of flatness (as in response) that may be preferable to a conductive box with cheaper and presumably less effective RF absorptive materials lining its interior.The downside of course is that there may be strong signals from elsewhere that obscure the signals you want to detect. That may defeat the entire purpose.Depending on where you are located, it may be possible to find some location inside a natural valley without any transmitters nearby that acts as a natural RF absorptive environment.Were it not for military bases nearby someplace like Death Valley would be ideal.

author: Berni
author_rank: Super Contributor
timestamp: October 25, 2018, 06:11:12 am
content: We use something like this:https://www.select-fabricators.com/rf-emi-shielding/rf-emi-test-shielding/portable-enclosures/Its just a box made out of very fine steel wire fabric. We use ours laying on the side so that there is lots of room to work in and the aluminum plate holding the pass trough connectors is located on the short side of the box. The passtrough plate provides three N connectors and BNC along with two DC power ports and mains input, all of those having big chunky RF filters on them to make sure no crap gets in. The 'door' is simply made out of velcro but also made out of steel wire so that it makes good contact all the way around.You can even buy room sized tents that you can actually walk inside or even park a car inside:https://hollandshielding.com/EMI-RFI-shielded-Faraday-tentAs for doing your measurements in a open field, i wouldn't recommend it for EMI emission testing. Its hard to find an area that is quiet over the entire band and its likely going to be in a pretty remote area. Caves also keep away radio waves well but again its hard to find a easy to access one. A shielded box however does an even better job, the line on the spectrum analyzer simply flat lines below -130dBm once you close the door on of these chambers. Using an open field however is a great place to do antenna testing since for that reflections are a major problem and will significantly affect the test results. Its much easier to simply let the RF power dissipate into the environment rather than building a wall that perfectly absorbs it. The noise in the environment is not a problem as you can simply use enough transmit power to overpower it. And by enough transmit power its still under 0dBm since your detector antenna is probably going to be only 5 to 20 meters away, so you wouldn't be jamming anyone far away.Some of the devices we make end up installed next to radio equipment like airband radios, transponders, flarm, gps etc. If you emit just a tiny bit of EMI on the frequencies they operate you can really degrade there performance. So we absolutely need a shielded box as giving out as little as -110dBm on the wrong frequency can cause problems.

author: wolfp
author_rank: Contributor
timestamp: October 25, 2018, 06:58:34 am
content: @chipTo reply to your initial question:You should do immunity tests like ESD (61000-4-2), Burst (61000-4-4), Surge (61000-4-5), conducted RF (61000-4-6) in a shielded room.You can not do an immunity test against radiated RF (61000-4-3) in a shielded room (without absorbers).You can make the measurement of conducted emission in a shielded room.You can find radiated emissions of your product but you can not measure them in a shielded room.I attached a few pictures of an (fully) anechoic chamber which is used up to 6 GHz for small devices.Wolfgang

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 25, 2018, 02:34:33 pm
content: Oh weird, bent log periodic. This seems to be the respective press release:https://www.arworld.us/pr_html/080108_log_periodic_antenna_6GHz.aspI wonder how flat it actually is...Tim

author: chiplukes
author_rank: Newbie
timestamp: October 25, 2018, 06:43:00 pm
content: Those are great pictures. I'm sure there are more than a few people around here that are jealous of that spaceWe've actually spent a fair amount of time outside performing "pre-compliance" type measurements as well as post-lab failure testing. We do have lots of outside space here in Montana, although in the winters it can get coldHowever, we are located near some powerful cellular, tv, and radio antennas and that has been very problematic for us at times.I suspect there are plenty of somewhat radio dead areas tucked away in mountain valleys around, but we are looking for something more convenient and less time consuming. Definitely if we were planning on doing full certification testing finding a good spot and setting up an OATS would be the way to go.I suspect that a combination of shielded room (to find frequencies from eut and other debugging) and then occasional outside testing (when possible given frequencies from EUT, weather, etc.) will be what we end up with.Chip

author: wolfp
author_rank: Contributor
timestamp: October 26, 2018, 09:52:06 am
content: In the meantime you do not need an OATS anymore. The standards for measuring radiated emissions allow the use of a fully anechoic chamber if it is verified (validated) according to CISPR 16-1-4 .Wolfgang

author: joeqsmith
author_rank: Super Contributor
timestamp: October 26, 2018, 10:20:00 am
content: Most of my early exposure to screen rooms was just that, rooms made of copper screen. I was looking for an article to show a rotating scatterer and came across this presentation that may be of interest:https://ewh.ieee.org/r6/scv/emc/archive/2016/112016Kate.pdf

author: SimonST
author_rank: Newbie
timestamp: October 27, 2018, 11:20:15 pm
content: Hi, I have worked very little with EMC but I have done some work on antenna design and testing. Using a reverberation chamber is getting more and more common to use for both antenna parameters and OTA testing. I think the reverberation chamber was invented originally for EMC testing.Depending on what you want to test and the time you have for testing and playing with the chamber, the metal for mode stirrers is cheaper than absorbers. Once you got a good shielded chamber that you trust you can continue to do many fun things.