Home made emc bench - Page 1
Forum section: electronics / projects

author: Slh
author_rank: Regular Contributor
timestamp: July 08, 2023, 08:17:42 pm
content: I thought that I'd share my home emc test bench. It's got two home made double 5uH, 350V, 20A LISNs and a couple of acrylic platforms about an inch off the ground plane. There's also a few current transducers of various sorts and a multi channel h-field probe.The ground plane is just a sheet of aluminium backed plastic that I had lying around with lots of holes drilled in it. Some plastic stand offs for isolation and brass ones for earth connections.The LISNs look pretty convincing up to 80MHz (sorry, no plots yet as I'm writing this on the phone) and I suspect that my measurement set up causes a resonance at 80MHz as banana plugs aren't great. They're built for CISPA 25 work but don't have to be perfect for what I'm up to. The one at the back has two 50 ohm loads fitted externally and the one at the front has a built in attenuator. They've got big copper tongues for a good ground connection. It's soldered directly to the ground of the SMA connectors inside the box.To go with it, there's a homemade current transformer (at the front). I think that's only good to around 30MHz.There's a non contact current probe for looking at the current in PCB tracks. Definitely works on the bigger tracks and is just about sensitive enough to look at gate drive currents. I need to build a better amplifier for it as it's rather noisy and tha bandwidth is a bit limited. It's probably good for around 10kHz to 1MHz as long as you can put up with the noise.The device under test is a dcdc I threw together in a morning. It's a bit of a mess which is great for testing test equipmentBig loops and no ground plane, it sings nicely across most of the TinySA range.There's a 16 channel h-field probe under the DUT. It's connected to the spectrum analyser (a TinySA). It's a home made knock off ofhttps://yictechnologies.com/emc-desktop-scanner/The idea is that you can stick it under a noisy DUT and identify where the noise is coming from and hopefully take repeatable measurements after you've made improvements. I've got a bash script that cycles through the channels and downloads the data to my pc. Then some octave scripts plot the magnitude versus frequency versus channel or magnitude at a frequency versus x and y positions.Total cost around £100 including the TinySA and aluminium boxes for the LISNs. It was supposed to be portable so that I can lift it on and off the desk but it's actually pretty good for normal work so it might not move much. The LISNs are a convenient place to connect the power supplies normally. Power supplies are above the desk hence the dangling cables connected to the closest LISN.So far, it's only really been used for a bit of investigation of active EMi filters. The multi-channel h-field probes only got automated this afternoon so they haven't been useful yet.All alleged measurements taken with a NanoVNA-H4 (but not saved anywhere).

author: 2N3055
author_rank: Super Contributor
timestamp: July 09, 2023, 10:40:35 am
content: Quote from: Slh on July 08, 2023, 08:17:42 pmHello!Care to share some details ?Best?

author: Slh
author_rank: Regular Contributor
timestamp: July 09, 2023, 07:52:56 pm
content: I'll share some plots when I have time at the computer.It's 16 h-field probes laid out in a grid with a 16 channel multiplexor to select which probe is connected to the spectrum analyser. The multiplexor appears to work ok for the range of the tiny SA (up to 900MHz). I need to hook it up to the nanoVNA to check the response. I've checked the S11 which is roughly the same for each of them but I haven't had a chance to check the S21 yet.

author: CosteC
author_rank: Regular Contributor
timestamp: July 10, 2023, 08:29:56 am
content: Quote from: Slh on July 09, 2023, 07:52:56 pmWow, how did you made 900 MHz, 16 channel signal multiplexer? It is not trivial thing!

author: Slh
author_rank: Regular Contributor
timestamp: July 10, 2023, 11:41:30 am
content: The important thing about this application is that it's not a calibrated tool. It's for taking relative measurements and identifying possible sources of emissions spacially on the board in a repeatable way. That's a fancy way of saying that the multiplexer doesn't have to be perfectI'm a power electronics guy really so this is just me messing around and trying to learn more about RF. I cobbled together a single channel selector using a 1n4148 dead bug style and it seemed to work so I made a 4 channel prototype on a home etched board using 1n4148 diodes. The 16 channel version was properly made with RF diodes and microstrip according to the jlc stackup calculator but it's the first time I've attempted anything like this so there are probably issues. I feel like the world may have moved on from using diodes for this level of multiplexing but it seemed to work so I'm happy. I've never done impedance controlled traces before.I've uploaded a photo of the bare PCB. It's the size of a credit card as it's a cheap size to work with.Looking at the S11, all the channels look similar to a single channel h-field probe but have a bit of a dip around 150-200MHz. It may be something to do with the multiplexer tracking as not all of the channels have the same length. At some point I'll try to upload a few plots but I tend to be lazy and only look at the screens without saving them.

author: CosteC
author_rank: Regular Contributor
timestamp: July 10, 2023, 11:47:59 am
content: ok, its antenna array with diode switches to enable only one at a time. Nice, elegant and effective. I assumed it is fully blown mux with flat RF characteristics.Still very nice design.

author: Barry Hardhouse
author_rank: Contributor
timestamp: July 10, 2023, 12:11:03 pm
content: This is fascinating, thanks for sharing your work Shl. By chance, EMC pre-compliance was a topic that came up this morning at work, and I think we might be looking at doing something similar ourselves.

author: Slh
author_rank: Regular Contributor
timestamp: July 10, 2023, 12:39:23 pm
content: Thank you for all the kind words. I'm still chuffed that it is working so well. Only one bodge and that's because I bought the wrong SMA connectorI've attached a couple of example plots. One is magnitude on Z axis vs frequency on one axis and channel on the other. The other plot is the channels laid out in a grid with magnitude on the z axis. There's a slide that can be used to select the frequency that you're interested in.Barry, if you're really after using this sort of thing for serious work then I'd say it's worth looking at the emscanner. I've only seen it at a trade show but I think the software for visualising the data is what you really want. It can be overlaid on your circuit so you can see exactly what's causing your issue. I expect that you'd spend a lot more overall making something like this work for you. They claim around 4mm resolution that's better than mine will currently do. It's probably just down to the data processing but that's not going to be trivial.