Open Hardware EMC Test Equipment - Page 1
Forum section: electronics / general

author: wilhe_jo
author_rank: Regular Contributor
timestamp: June 09, 2020, 03:45:56 pm
content: Hi all!I'm currently building test equipment for my EMC related business.Now I was wondering if anyone needs a small 9k-30MHz LISN?I opted to roll my own instead of going to get a Tekbox one because I wanted to be able to measure L/N simultaneously with a scope in addition to the traditional use of an EMC receiver.If there's demand I'll tidy up my documentation and maybe put together something like a kit to make DIY easier.BTW, I intend to do the same for all boxes filled with "some copper and a lot of air" that are used for EMC testingSo CDNs are next...vy 73

author: fcb
author_rank: Super Contributor
timestamp: June 10, 2020, 09:42:31 am
content: We built our own LISN prior to buying one. It's not that difficult - but you have to find a way to characterise it for it to be 'useful'.If I was building another one, then I would probably include a relay to kick in to discharge the line caps - I've been bitten by those more than once!Also, a LISN without a clean AC source is pretty useless.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: June 10, 2020, 12:22:06 pm
content: The nice thing is, that today you have cheapish things like the VNWA and the nanoVNA.Both seem to be pretty capable in doing this.Well, I'm just in the progress in doing all measurements with these devices and compare them...The final calibration adapter is in the works, though.Of course, I'd add the calibration information to the documentation.This is a lot of work, however.That's why I'm asking if there's demand...I quit my day job and got self-employed because I was fed up with doing work all-day long and nobody caredI'm just in a new office, so I did not investigate the "ambient" noise level, yet.However, you need to use an isolation transformer anyways (preferably a shielded one).In addition, my LISN has the 250µH inductor as well.The CISPR16-1-2 standard suggests a minimum isolation of 40dB from 50kHz.With the usual 20dB from a transformer you should not have too many problems, at least for 150k-30M measurements.I made my design pretty universal.It should be pretty straight forward to put 2 of the PCBs in the same enclosure and have the first one form an additional filter if neededThe CISPR 16 series would also allow for something like this.But you're right, an ac-voltage source is quite beneficial.73

author: prasimix
author_rank: Unknown
timestamp: June 29, 2020, 04:22:18 pm
content: Quote from: fcb on June 10, 2020, 09:42:31 amI'm in process of building one by myself since Ifailedto pass "radiated emission" test. Could you please elaborate more how your build differs from ready made one?

author: fcb
author_rank: Super Contributor
timestamp: June 30, 2020, 09:51:34 am
content: Quote from: prasimix on June 29, 2020, 04:22:18 pmHow will the LISN help you with radiated emisions?

author: prasimix
author_rank: Unknown
timestamp: June 30, 2020, 09:52:47 am
content: Sorry, I mean conducted as described in mentioned thread.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: July 11, 2020, 11:37:25 am
content: Quote from: prasimix on June 29, 2020, 04:22:18 pmWell, at this moment is offers you both signals (L and N) simultaneously.This is nice for debugging since you can use your scope to figure out if your problem is common mode or differential mode (channel1+channel2 => CM; channel1-channel2 => DM).From this you know if you need X or Y caps; increase you inductors or add a diff-mode choke...I have a post processing board already on my table which should do this for you. So you can switch to L, N, DM and CM and use your favorite spectrum analyzer.A third board will give you a USB3 digitizer. So you do not need a spectrum analyzer at all.I'll populate the post processing board soon but currently I build a radiated emission test-set in my office...Tiny turntable, antenna mast, DIY antennas,... that's currently more important as the LISN is in a usable state.The digitizer is more of a fun project which will be ready if it's readyIt should work but to get it usable a lot of software is needed...73

author: prasimix
author_rank: Unknown
timestamp: July 11, 2020, 01:05:55 pm
content: Thanks for info, that seems to work, here is how outputs from LISN look when added and subtracted:Now I have to learn how to read it.

author: coppice
author_rank: Super Contributor
timestamp: July 11, 2020, 01:08:31 pm
content: Quote from: fcb on June 10, 2020, 09:42:31 amDon't the commercial ones put a high value bleed resistor across those caps?

author: wilhe_jo
author_rank: Regular Contributor
timestamp: July 11, 2020, 02:12:53 pm
content: Well, that seems to be mostly differential mode disturbances and some nasty common mode ones.I provide this kind of debugging stuff as a service to my customers.Without knowing your'e emission spectrum and seeing some of your'e schematics, it's hard to tell which is actually the problem.However, these spikes in your' A+B trace should be around 110dBµV in your'e emission spectrum (approx 200mVpp). So my educated guess would be that these are your'e enemy (you're maybe not designing professional welding equipment >75kVA ??).As a first "brute-force-test" I'd now just take some (5-10) snap on ferrites (the bigger the better - the different the better) to see if you can reduce the noise with this simple method.I'f it does, either look for the noise source (use CH3/CH4 to find a signal which lines up with the spikes), place some more Y-caps if you can, or provide an adequate ferrit core with your product73

author: prasimix
author_rank: Unknown
timestamp: July 11, 2020, 02:26:14 pm
content: Thanks @wilhe_jo, a discussion of the problem I have can be foundhere. There were many different issues, starting with PE and GND connected together on one point on the MCU module. That issue is resolved by simply disconnecting it. The remaining one is problem with huge noise when power module (DCP405) is powered with Mean Well LRS AC/DC converter. DCP405 has two switchers (power based on LTC3864, pg.1 ofschematicsand bias based on TPS54060, pg.2) and obviously insufficient filtering at the input. In fact I've added that filter against noise that Mean Well is injecting on its DC output.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: July 11, 2020, 04:35:21 pm
content: Well, that's not really insufficient filtering.At the first glance it could be a quite obvious problem....Try following: huge caps (try 1u MLCC) from V+ of your 5V and 12V AC/DCs to PE.In addition to that it could also help to have 1u MLCC direclty at V+ and V-.If you don't see huge changes, try to drastically increase your Y caps.If this helps, your'e in the lucky position that you will need fairly big common mode chokes on the output of the module as well...The problem is that your AC/DC supply introduces noise and you have fixed one potential to PE.This sadly means, that via your stray capacitances you get that current back via the LISN.The 3.3uH inductor makes this situation even worse...at least for the common mode...You tried the modules with resistive loads and had good results.I bet you would get also very bad results if you just connect V- to PEThis actually happens with 99% of all isolated power supply.If you connect one end to PE they create a lot of noise on the mains.I don't think you are in a situation where the AC/DC modules have a nasty topology which saturates your common mode chokes (totem-pole PFCs are an example of such).The stray inductances of the transformers inside the module should be also quite small. So just caps should work...73

author: prasimix
author_rank: Unknown
timestamp: July 16, 2020, 10:30:52 am
content: Thanks for new ideas. I would rather continue the discussion about my problem on a thread that already exists. I added a few poststoday.