How to use a Network Analyzer With Current Probes? - Page 1
Forum section: electronics / projects

author: SniperFox
author_rank: Contributor
timestamp: February 14, 2020, 07:36:54 pm
content: Hi everyone,I'm wondering how to set up current probes to use it with a network analyzer. I don't know how to calibrate them properly for short, open, and 50ohm terminations (oh and through, too. SOLT). It's straight forward with BNC or SMA connections but what about current probes?What I tried:Short -> I made a small wire loop.Open -> left it open.Load -> I made a small wire loop with a 50ohm resistor.Through -> I Put a small wire loop between both probes.This didn't seem to work, as the trace has lots of spikes and is bouncing up and down all over the place... so I'm seeing nothing but noise when I try to take measurements.I'm trying to measure S11 and S21 btw.What I'm trying to do is use a network analyzer with current probes to measure the impedance of a LISN and SMPS power supply. This is step towards systematically designing a conducted EMI filter.I found this neat scholarly article explaining how to measure those impedances under operating conditions with clamp-on current probes. See link below.https://www.researchgate.net/publication/260589392_EMI_filter_design_Part_II_Measurement_of_noise_source_impedancesIf you scroll down a bit, you can view the article without downloading it.They explain everything very well and derive the equations well too. The problem is that they don't explain how to calibrate and use the VNA with the current probes.They also don't say what probes or VNA they used. Although I am 90% sure that they used CT1 or CT2 probes because a few of their references use those probes.If anyone wants details on the instruments I'm using: I'm using E5061B VNA and two P6021 current probes.(I tried searching the forums for this topic but could not find anything)

author: graybeard
author_rank: Frequent Contributor
timestamp: February 15, 2020, 12:11:18 am
content: I made a video on how to do it with the bode plot function with a Siglent o-scopehttps://youtu.be/T6mbTblw4QE

author: graybeard
author_rank: Frequent Contributor
timestamp: February 15, 2020, 12:59:44 pm
content: I read the paper. It was very interesting. However as they demonstrate with their measurements the frequency range is limited to about 2 decades with a high end in the 10s of MHz.The calibration method is implicitly described in the paper.You first need to establish your S parameter calibration planes at the current probes as shown in Figure 2. If you are using the typical 50 Ohm Tek CT1 or CT2 probes you will need to make a set of female SMB calibrations; short, open, load, and through to place the s parameter calibration planes as close as possible to the current probes.You then need to make two calibration measurements with the current probes; one with a resistance standard whose value is near the impedance you expect to measure, and one with a short. You first get the ratio of the probe voltages at every frequency using equation 9 for both measurements; RSTDand short. Substitute that result into two equations 10 & 11 and solve for the complex quantities K and Zsetupat each frequency point.Since you now have K and Zsetupyou can measure your unknown impedance and using equation 8 calculate the unknown impedance at each frequency point.This method will not work with the P6021 current probes you are using.I hope that helps.. . . Chris

author: SniperFox
author_rank: Contributor
timestamp: February 15, 2020, 06:24:38 pm
content: Thanks! I'll check out the video on Monday when I have time.Quote from: graybeard on February 15, 2020, 12:59:44 pmThis shouldn't be a problem as I only need to measure for the conducted EMI range of frequencies (150kHz-30MHz).QuoteWhat is "SMB"? Is that the type of connection on the CT1 & CT2 probes?Also, if I use a CT1 or CT2 probe: Where do I do the short, open, load, and through? At the green arrow or at the red arrow? (see the attached picture)QuoteYeah, I understand this part and it's pretty cool how they came up with this.I just don't know how to get the setup ready and prepared to do these Rstd and the short measurements.QuoteHow come it won't work with the P6021? This is what I suspect, but why? Is it because the P6021 requires a 1Mohm termination?

author: graybeard
author_rank: Frequent Contributor
timestamp: February 15, 2020, 08:59:04 pm
content: Quote from: SniperFox on February 15, 2020, 06:24:38 pmThe SMB is the connector on the current probes.Quote from: SniperFox on February 15, 2020, 06:24:38 pmThe red arrow, the green area point to a mounting screw, not a connector.Quote from: SniperFox on February 15, 2020, 06:24:38 pmIt is both high impedance (1MOhm) which is way to far away from 50Ohms and non-reciprocal (it has a built in amplifier) and thus S11 will be independent of the load and the current probe will not transmit any energy into the load for the S21 probe to detect.

author: SniperFox
author_rank: Contributor
timestamp: February 15, 2020, 11:01:40 pm
content: Thanks for the responses! Very helpful.How did you know the CT1 & CT2 use a SMB connection? I tried to search for it just now but I can't find this information online or in the manuals.So if I understand this correctly, it looks like it's just the cable we calibrate for the s-parameter measurements..And the current transformer is not calibrated and is a part of the measurement?Which would you suggest buying for this type of measurement? The CT1? Or the CT2?Also, is there a different kind of current probe you'd suggest buying instead of the CT1 or CT2?Maybe a clamp on version so I won't have to de-solder my DUT (device under test) every time I want to put the probes on for measurements.

author: graybeard
author_rank: Frequent Contributor
timestamp: February 16, 2020, 01:11:24 am
content: Quote from: SniperFox on February 15, 2020, 11:01:40 pmQuote from: SniperFox on February 15, 2020, 11:01:40 pmI have used both of them.Quote from: SniperFox on February 15, 2020, 11:01:40 pmthe current probes are calibrated as part of the secondary short RSTDcalibration.Quote from: SniperFox on February 15, 2020, 11:01:40 pmwhich ever one does the best job of covering your frequency range on the low endQuote from: SniperFox on February 15, 2020, 11:01:40 pmnot off the top of my headQuote from: SniperFox on February 15, 2020, 11:01:40 pmI don't know of any passive clamp on ones designed to work into 50 Ohms.

author: SniperFox
author_rank: Contributor
timestamp: February 17, 2020, 06:50:20 pm
content: I just realized that the CT1 and CT2 probes have a really tiny hole.The AC mains wires won't fit into it. Datasheet says that the hole size is ~1.4mm, and the AC mains wires are 2mm. Not to mention I'm going to have to put 2 of them through the hole sometimes to make measurements, so it needs to be at least 4mm...If I solder a thinner piece of wire onto the DUT in series to be able to fit the CT2 probe on, that would mess with the results, right?I'm assuming so, because of the slightly higher resistance in the thinner wire causing slightly less current...Would this be a reasonable compensation for the small hole or would it mess up my results too much?

author: chris_leyson
author_rank: Super Contributor
timestamp: February 17, 2020, 07:10:33 pm
content: I've seen clamp on probes used for EMC measurements, something like this.https://www.emchire.co.uk/category/current-probes-clamps-lisns-cdns/current-probes--clampsHate to think of the cost though.Maybe 10 or 20 turns terminated with 50 Ohm on an RF toroid would do the trick but I suspect it would only be useful above 1 or 2 MHz.

author: SniperFox
author_rank: Contributor
timestamp: February 17, 2020, 09:47:54 pm
content: Quote from: chris_leyson on February 17, 2020, 07:10:33 pmThanks for the suggestions, Chris. I'll look for probes rather than doing it DIY because I want to make sure I'm doing this properly and with the least amount of error.I'm browsing the internet (and the link you provided) to look for those clamp on passive probes, but unfortunately they are few. And a lot of them are missing information on how many mV/mA is generated..I had no idea it's this hard to find a correct current transformer/probe...I'm tempted to get the CT2 and just solder a thinner piece of wire on for the measurements.

author: graybeard
author_rank: Frequent Contributor
timestamp: February 18, 2020, 12:15:32 am
content: Quote from: SniperFox on February 17, 2020, 06:50:20 pmIt should work so long as you incorporate it into your cal.

author: SniperFox
author_rank: Contributor
timestamp: February 24, 2020, 03:40:28 pm
content: Quote from: graybeard on February 15, 2020, 12:11:18 amDidn't say which Monday I would watch, haha.I finally watched your video today and it was great. Thanks for making it!

author: Wolfgang
author_rank: Super Contributor
timestamp: February 25, 2020, 11:49:25 am
content: Quote from: SniperFox on February 14, 2020, 07:36:54 pmSeen this ?https://electronicprojectsforfun.wordpress.com/power-supply-impedance-measurements-for-various-power-supplies/power-supply-impedance-measurements-using-the-bode100-lf-vna/powwer-supply-impedance-measurements-using-a-current-injector-and-a-current-probe/