| Products > Test Equipment |
| EMC pre compliance test equipment recommendations |
| (1/3) > >> |
| TwinScroll:
Just curious on people’s thoughts / suggestions or advice on EMC pre compliance test equipment. The company I work for has given approval to spend up to around £8K GBP. We have a large test area (that is a long distance from other working areas) and we can improve on the shielding of the room / area if found needed, the cost for that would be separate. We already have a Tektronix MDO3000 scope with a 3Ghz spectrum Analyser option, but if a separate analyser will be easier to use and possible within budget that is an option. We mainly do DC powered equipment 12v / 24v typically less than 10Amps (sometimes marginally higher but that is rare). We want to concentrate on emissions over susceptibility to start with. Any suggestions on must haves, caution on items we are hardly ever likely to use etc would be great. I am thinking we need :: -Near field probe set -RF current monitor -DC LISN (ideally ability to measure Differential and Common Mode) -Spectrum Analyser (Big Ticket) -Far field Emissions Antenna or Antennas -Various high quality interconnection cables Best Regards TwinScroll |
| nctnico:
I'd forget about trying to measure far-field. You need to go to a lab for those measurements. Shielding a room isn't enough; the walls need to be non-reflective to avoid standing waves. Near field probes, LISN and some cables are good investments. Don't overdo the cables. RG319 cables are cheap and OK up to 1GHz. You don't have to break the bank for a spectrum analyser either. The 3GHz one from Siglent will do just fine. Also think about getting an ESD gun. ESD can be nasty to pass as well c.q. it is good to test how your devices responds to ESD discharges. Again this will require some room to setup because you don't want to damage other equipment. IMHO the biggest mistake is to think you can make accurate measurements with an error less than 3dB. Many tests require very specific test setups which take a lot of space. The best way is to do a pre-compliance run at an accredited lab and then use near field probes to see if you can lower emissions at the offending frequencies by modifying the circuit with a healthy margin. |
| twospoons:
Thats a pretty tight budget, depending on what you want to achieve. For that money I doubt you'll get good data relative to the EMC limits (that takes calibration and $$$), but at least you'll see the obvious issues and will be able to evaluate mitigations. The NF probes and the LISN should get you most of the way there, at least as far as finding potential issues goes. You might consider an absorption clamp instead of far field antennas - takes less space, will get you to 1GHz, without needing an anechoic chamber Shielding a room without adding absorbers might make things worse rather than better ( EMI gets bounced around inside the shield), unless you have a powerful source of interference you know you need to block. If the devices you are testing are small enough, another potential route is the GTEM cell. They can be used for both emissions and susceptibility. |
| T3sl4co1l:
Conducted is easy enough, given that you aren't too picky about the precision of results, especially at high frequencies. Traditionally, it ends at 30MHz on the dot. There's no reason you can't look further -- just the sanity of making that measurement at all, versus who knows what's evaporating off the cables or enclosure... So, any time you can make a port-like measurement, it helps. For example, instead of measuring conducted emissions at a distance (after standard e.g. power cords and such), measure it at the enclosure -- assuming the enclosure is metallic so a good ground reference can be made there, and every connection exiting through that reference can be considered an RF port. (And if not grounded, then the nearest thing that can -- perhaps just floating the board over another ground plane and measuring all wires coming off the board.) An open air test, boards laid out over a ground plane, is still susceptible to ambient fields, which you'll have to subtract from the measurement somehow -- hopefully they're below threshold so your measurement is still meaningful, and if not, good luck with the shielding budget -- but given that hurdle is passed, it's a good way to get some idea of the wideband emissions from individual boards, or equipment. An adequate LISN can even be hand made (we have some excellent threads on that here), or they are easy to find for sale. Use the kind appropriate for your work of course; if your 12/24V DC equipment is like, automotive, look up the whatever standard is typically used for that; if it's more like uh, I suppose industrial kit would have to pass some relative of FCC Part 18, or the IEC/BS equivalent? Then, give that a look. (Although that might not be all that helpful as I think Part 18 is mostly OAT anyway, who needs LISNs?) Also add CDNs for stuff like telecom pairs, etc. YMMV, may not be the easiest thing for like CAN bus, since it's DC coupled -- Ethernet and some RS-485 can go through transformer coupled CDNs which is nice, when it can't you have to use an attenuating (resistor coupled) CDN so keep that in mind. I don't know what the MDO3000 spec option is like; if it's basically like any other spec, useful dynamic range, all the RBW settings and stuff, that's fine. If it has quasi-peak (QP) detection that's even better, but you can do precomp just fine with peak, too. I'd just be weary of anything that's just a regular scope with FT, those tend to have poor dynamic range, no spectrum averaging, variously no frequency offset or detector or RBW options, etc... Yeah, current probes aren't a bad idea, but be mindful of where those currents are going -- or not. Some stuff laid out over ground plane, will just have whatever resonances between the stuff, and not much resonance towards the LISN(s) since those are resistive terminated. Resonances on cables and equipment are pretty easy to spot: keep an eye out for peaks/valleys, and what physical lengths those frequencies correspond to. A couple 10-20dB wideband preamps, good idea. Attenuators too. Sometimes you want to use both simultaneously, just to have the safety padding, or freedom from reflections, and then claw back whatever SNR you have left since the spec probably doesn't have the greatest SNR (or rather, noise factor). Near field probes, seconded. Not really meaningful in terms of relation to radiated emissions -- they're more for tracking down culprits, and then you can decide what to do with it (improved routing, bypassing, ferrite beads, shielding, etc.). Yeah, ESD is a good tool, both for causing the upsets it's famous for, as well as a source of wideband RF. The low duty cycle makes it not a great substitute for the modulated CW used in immunity testing, but if you can deal with that (perhaps with an impulse or error detector on suspect circuits?) you can get some idea of general susceptibility too. Tim |
| tautech:
--- Quote from: nctnico on May 05, 2021, 11:21:58 pm ---I'd forget about trying to measure far-field. You need to go to a lab for those measurements. Shielding a room isn't enough; the walls need to be non-reflective to avoid standing waves. Near field probes, LISN and some cables are good investments. Don't overdo the cables. RG319 cables are cheap and OK up to 1GHz. You don't have to break the bank for a spectrum analyser either. The 3GHz one from Siglent will do just fine. --- End quote --- You can peel back more cost when the 1.5 GHz SSA3015X Plus can easily cover EMI precompliance needs however many select the 2.1 GHz model for the bit high BW and better TG spec it offers for GP SA work. All Siglent SSA and SVA models have an EMI promotion running where you get a set of characterised EMI probes and the EMI SW license for a much reduced cost when buying the 2 together. https://www.siglenteu.com/news-article/a-new-bundle-for-emi-pre-compliance-test/ Such is the cost of EMI compliance testing just one fail will have funded one of these SA's and not needing to rework designs when you can do EMI tests as you go and have a product properly ready for testing by an EMI test house. |
| Navigation |
| Message Index |
| Next page |