acceptable probe tolerance, or damaged compensation network? - Page 1
Forum section: electronics / beginners

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: April 22, 2025, 02:55:22 am
content: I was looking at the response of a circuit to a square wave, and the square wave itself was not too square. I don't remember if it was like this years ago, but it bothered me that the flat top and bottom of the square wave were not flat. I did the probe compensation, and the best i could get on the channel 1 probe was still a hump. I switched probes, to see if it was the channel on the scope itself, but no, it's the probe. Channel 2's probe is able to get a flat top and bottom, but this is the best i can get on channel 1:and it looks extra humpy when zooming out, as it was during the testing of my circuit:So, is this acceptable probe tolerance? is this a damaged probe? if damaged, can it be fixed, or should i just get a new one? I have a nanoVNA coming this week, can i use that to measure the probe response?Thank youedit: both probes are Keysight N2140A, which i think were the ones that a few years ago had a faulty batch or something like that? something to do with the compensation network? i'll see if i can find that post

author: RoGeorge
author_rank: Super Contributor
timestamp: April 22, 2025, 04:13:22 am
content: Quote from: Anthocyanina on April 22, 2025, 02:55:22 amLooks like a defective probe to me.Usually probes are expensive, so maybe try to open/fix it at first, and buy a new one only if unsuccessful. Not sure how much a nanoVNA can help other than maybe measuring the internal trimmer min/max values. The probe cable itself is often made of a coiled resistive wire (like a spring) along the entire length of the cable, but I never opened the kind of probe you have, so no idea how it really is inside.If you know there was a defective batch, I would definitely search for that info before any repair attempt.To search with Google, but to limit the results to EEVblog.com only, addsite:EEVblog.comafter whatever search words you may use, for example:Code:Keysight N2140A faulty batch site:EEVblog.com

author: bdunham7
author_rank: Super Contributor
timestamp: April 22, 2025, 03:54:49 pm
content: That looks like something has come undone in the probe. I'm assuming from your description of the problem that the compensation adjusment does have an effect, just not enough. Try switching back and forth from 1X to 10X a few times and see if that makes a difference. Repairing the probe may be an exercise in frustration. These are what Keysight describes as "extremely low cost probes" (for $118...) so you probably can replace them with almost anything and they'll be just as good or better. For example:https://www.ebay.com/itm/166858456945I personally would prefer 10X non-switching probes.https://www.ebay.com/itm/167394689720https://probemaster.com/4900-oscilloscope-probe-basic-kit-150-300-mhz/

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: April 23, 2025, 09:24:05 am
content: thank you both! fortunately i have another probe i can use in the meantime. I'll try to find a way to take it apart non-destructively to try to fix it, and if not, i'll keep using the other probe while i save for a pair of probemastersI've now tried the switch thing@bdunham7suggested, but it changed nothing. on 1x it behaves normally, showing the same response as the other probe that came with the scope, and yes, adjusting the trimmer cap does change things, and compensation does go above and below level, but it just won't be level.thanks again!

author: bdunham7
author_rank: Super Contributor
timestamp: April 23, 2025, 09:38:48 pm
content: Quote from: Anthocyanina on April 23, 2025, 09:24:05 amThat's actually pretty weird, usually the issue is you're maxed out one way or the other and you just can't quite get enough (or little enough) compensation.

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: April 25, 2025, 07:10:29 am
content: Quote from: bdunham7 on April 23, 2025, 09:38:48 pmi guess it can get closer, but across the entire range there's a hump and it is never as flat as the other probe. it still passes the scope's probe check thing in a significantly wide range of humpiness. the screenshot in the first post is the closest it gets to flat, a soft slope and being flat near the end of half a cycle, and anything higher than that results in a hump going higher than the top of the waveform as seen on the other channel