Micsig Diff Probe Inverting Measurements When Switching Attenuation - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: December 06, 2023, 08:07:16 pm
content: I have a Micsig x10 / x100 Diff Probe into an Agilent 54831 scope.The exact circuit I can't share, but it's basically a bridge rectifier, a TRIAC controlling the AC into the bridge rectifier, and a DC motor on the output of the rectifier. From my understanding, the motor is being used to somewhat smoothen the ripple, but the ripple isn't a concern.The main issue: I'm using the diff probe to measure across the motor (i.e. the output of the rectifier). Without changing any settings except v/div on the scope and only changing between x10 and x100 on the diff probe, I'm seeing what looks like an inverted signal.Is something wrong with the diff probe or am I overlooking a setting change (see attached scope pictures)?

author: tggzzz
author_rank: Super Contributor
timestamp: December 06, 2023, 08:13:11 pm
content: What happens when you try to measure a constant voltage, e.g. a PSU?

author: bostonman
author_rank: Super Contributor
timestamp: December 06, 2023, 08:20:42 pm
content: Good question.I actually took those measurements too, but didn't include them in my initial message.Note: the 9v battery is older and only 7.x or 8.x volts - I keep it around for general junk tests such as this.Update: just to be clear, the file name x10 / x100 is switching the diff probe, not changing the attenuation in the scope settings.

author: bdunham7
author_rank: Super Contributor
timestamp: December 06, 2023, 08:29:51 pm
content: Try changing your triggering to 'LINE' and changing your timebase to 5ms/div or even 10ms/div to get more signal on the screen. One capture is triggered, the other is not so you may be looking at different time periods of your signal. I don't see it as inverted.

author: bostonman
author_rank: Super Contributor
timestamp: December 07, 2023, 07:56:26 pm
content: I'll try (hopefully) this weekend.Just out of curiosity, should 'line' always be used for single (and maybe multiple) channel measurements?Now that I'm using a higher end scope, I'm noticing lots of noise, but always reference channel 1 (assuming that's the channel I'm using to measure) as a trigger reference. Maybe I should be using 'line' to reduce this issue.I'm not use to using these faster scopes and/or looking at the finer points of a waveform.

author: bdunham7
author_rank: Super Contributor
timestamp: December 07, 2023, 08:38:30 pm
content: Quote from: bostonman on December 07, 2023, 07:56:26 pmNo, line trigger is just triggering off the the 60Hz mains power waveform. Only non-isolated mains-powered scopes typically have it, I didn't ask what yours was. In this case, I want to see several cycles worth of waveform aligned with the 60Hz mains power.

author: bostonman
author_rank: Super Contributor
timestamp: December 08, 2023, 01:55:09 am
content: My scope is a Agilent 54831M (upgraded to 1GHz - technically a 54832B).

author: bostonman
author_rank: Super Contributor
timestamp: December 11, 2023, 01:00:37 am
content: Earlier I took more measurements (see attached).Maybe I'm wrong, but it looks inverted to me.

author: bostonman
author_rank: Super Contributor
timestamp: December 13, 2023, 04:10:32 am
content: Any feedback on these measurements?

author: bdunham7
author_rank: Super Contributor
timestamp: December 13, 2023, 04:34:48 am
content: Hmmm. Definitely appears to be inverted as you said...One thing that stands out is that although the scope settings appear to be identical for both 10X and 100X settings, the displayed amplitude is only slightly different. The main body of the signal, less the noise spikes, appears to be about 1 division at 10X and .7 divisions at 100X.I can't see any indication of probe setting on your scope--is it set for 10X, 100X, 1X, what? I wouldn't think the scope would support 20V/div at 1X attenuation. What is the expected voltage across the motor where you are measuring? If your scope is set for anything other than 1X, your signal is pretty small.Is the device grounded or ground referenced and is the scope grounded to the same potential? This looks like a common-mode issue or something like that. Do you actually need to do a differential measurement or is one side of the motor at ground or neutral potential?

author: Jeroen3
author_rank: Super Contributor
timestamp: December 13, 2023, 07:14:18 am
content: It isn't truly inverted because it stays above the channel's x axis. Only the AC coupled part inverts.I have these probes as well, not seen this before.The overrange alarm isn't triggering? buttons are not blinking?

author: bostonman
author_rank: Super Contributor
timestamp: December 14, 2023, 02:41:12 am
content: Well, I'll respectively understand if this question can't be answered due to being unable to provide the circuit, so I can imagine the confusion of trying to decipher this question without any circuit reference.The scope should be set to 10x in all conditions and the only thing I changed was the v/div along with the trigger level.As I previously mentioned, the DC motor is being driven by a bridge rectifier, which is driven by a TRIAC. The question arose regarding what the signal looks like at the motor. Prior, I didn't know if this was a DC motor or AC because there isn't any smoothing, but later learned, it's a DC motor. The motor is acting as an inductor (and/or capacitor). Although the signal looks like junk, it's basically good enough for our needs.In any case, I wanted to protect my scope and used this diff probe so I could get a good look at the signal across the motor. I began getting confused over whether my scope needed to be on 1x or 10x, so I began tinkering between x10 and x100 on the diff probe along with the scope settings. This is when I saw the wave being inverted.Eventually I began confusing myself because x10 and 10x are somewhat ambiguous. Sometimes if I'm confused enough on whether my probe is x1 or x10, or if my scope is set correctly, I'll measure a 9v battery (the test probe clock on the front doesn't seem to go 0-5, but more like 0-2.x V if I remember correctly.If I have enough guts to measure across the motor directly to the scope, then maybe I can put the diff probe on channel one and direct scope probe on channel 2, and see if they are the same or inverted; not sure I want to risk this though.

author: bdunham7
author_rank: Super Contributor
timestamp: December 14, 2023, 03:26:48 am
content: Quote from: bostonman on December 14, 2023, 02:41:12 amDon't connect directly to the motor, especially if you can't answer with certainty as to what part is ground referenced.Instead, try connecting the negative lead of the differential probe to ground and then connect the positive lead to each side of the motor, taking a screenshot each time. I can't guarantee you I'll figure out what is going on with that, but it might help.

author: bostonman
author_rank: Super Contributor
timestamp: December 14, 2023, 03:54:27 am
content: I'll have to look, but believe one side of the motor is neutral.

author: bdunham7
author_rank: Super Contributor
timestamp: December 14, 2023, 04:31:01 am
content: Quote from: bostonman on December 14, 2023, 03:54:27 amNot likely if it goes through a bridge rectifier. The test I proposed will answer that question for sure.

author: bostonman
author_rank: Super Contributor
timestamp: December 15, 2023, 11:28:37 pm
content: I took more measurements.The file names are named accordingly. GND is AC neutral (directly to the neutral wire), red and black are meaningless, however, technically positive and negative respectively.I forgot to check if the scope is on 1x or 10x, however, I'm uncertain if it matters. The x10 and x100 in the file names are, as previous, the setting on the diff probe.Update: I failed to mention, the wires to the motor are both on the output of the bridge rectifier. There is a diode across the motor (or bridge rectifier), and the neutral input side of the rectifier is fed directly back to neutral on the plug while the 'hot' is being fed by a TRIAC.

author: bdunham7
author_rank: Super Contributor
timestamp: December 16, 2023, 12:16:08 am
content: OK, after looking at those and then more closely at your previous screenshots, it looks like you are simply exceeding the 70V peak differential input of the probe. Using offset or AC coupling on theprobescopedoesn't help. If you set the scope to 10X, 20V/div (which is actually 2V/div at the scope input) and 0V offset, then if the trace exceeds 3.5 divisions from the center line you'll haveprobe amplifierclipping...and the resultant mess you are currently looking at.

author: bostonman
author_rank: Super Contributor
timestamp: December 17, 2023, 01:07:10 am
content: I hadn’t considered the maximum voltage, makes sense.Hopefully the diff probe didn’t get damaged - I’ll look on the website later to see if they state anything about exceeding the maximum input voltage.

author: bdunham7
author_rank: Super Contributor
timestamp: December 17, 2023, 02:08:14 am
content: I wouldn't expect it to be damaged. Did the range buttons blink at all?

author: bostonman
author_rank: Super Contributor
timestamp: December 17, 2023, 02:25:37 am
content: The x10 and X100 buttons? They did blink, but the little attention I gave to the blinking I associated it triggering and/or detecting a signal.I’m sure it’s not damaged as it’s probably designed for such mistakes. Worse case I’m out a diff probe and learned a lesson.

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2023, 10:20:02 am
content: Quote from: bostonman on December 17, 2023, 02:25:37 amWell, I'd say a few lessons:every piece of equipment lies; you need to learn to listen to everyting it is telling youRTFM before using any new piece of equipmentone of the two mottos I think are worth remembering: "Think" (The other is "Be Prepared")

author: bostonman
author_rank: Super Contributor
timestamp: December 17, 2023, 03:16:51 pm
content: QuoteI don't feel reading the manual would have done any good in this situation. I just looked and it only states the maximum voltage is 70V on 10x and the button light flashes when it's over range.Yes, this is sarcasm and slapping myself in the face.The whole concept of x10 and 10x needs to be sorted out for me mentally because I don't know if it's multiplying by ten in some cases or dividing by ten. Some of my scope probes are x1, some are x10 or (1x and 10x?), the scope uses ratios x : 1 (where x is manually entered), and the scope probe test connection goes somewhere between 0 and 2.x or 3.x instead of the usual 0-5V, however, it's not stamped on the front so I'm always forgetting.The only reason I began playing with the diff probe attenuation buttons was due to not being able to figure out the scope probe settings, so I just pressed buttons and saw an inverted signal, however, I wasn't paying attention and managed to not realize the mistake I was making.

author: bdunham7
author_rank: Super Contributor
timestamp: December 17, 2023, 03:30:46 pm
content: Quote from: bostonman on December 17, 2023, 03:16:51 pmGenerally speaking, scope probes divide or attentuate the voltage by the stated ratio. The setting on the scope makes no difference to its actual operation, but simply informs the scope that a certain ratio of probe is being used so that it can display a corrected voltage scale. There's no need to worry about the difference between x10 and 10x, it always means the same thing when you're dealing with passive scope probes.

author: bostonman
author_rank: Super Contributor
timestamp: December 17, 2023, 04:04:26 pm
content: This scope is the first one I've had where I tend to worry about the probe settings in the menu. It's obvious scope probes don't amplify, but most of my career I've dealt in small voltages and used the same probes, so I never had to devote time to thinking about settings (thankfully I wasn't one to just hit 'auto' and always tried to manually adjust the scope).Now I have a plethora of old and new probes, one nice Agilent, some x1, some x10, and a scope that is either sensitive to the slightest incorrect setting or causes me to wonder if something is wrong.Sometimes the triggering is awful, sometimes I can't figure out whether my voltage is 5v or 50v, etc... Then I go into the menu and say, well, my ratio is 10:1, my probe is x10, this seems correct, but wait, why is this not giving me the correct voltage. One time I set the scope to 0.1:1 when I confused myself enough. hahahhah

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2023, 08:27:03 pm
content: Quote from: bostonman on December 17, 2023, 03:16:51 pmBack in reply #2, I suggested measuring a known simple signal to see what happened. That's always a sound principle. It can clarify whether the instrument is or isn't lying to ypu.Whether a *10 factor has or hasn't been set on a scope is always a painful source of error. Even when familiar with a piece of equipment, measuring a known source clarifies what is being shown on the display.

author: bostonman
author_rank: Super Contributor
timestamp: December 18, 2023, 02:23:16 am
content: I either followed your advice and/or already did that using a 9v battery.Think I figured out the scope. Whenever I connected the (per the label) 10:1 Agilent probe (part number: 10074C) to the scope, it automatically detects it and removes a scope probe menu item which happens to be the attenuation.The menu that remains is external scaling. When I changed this setting, thinking I was changing the attenuation, I was changing the gain. At some point the attenuation must have automatically been on x10, I set the gain to x10, saw it was wrong (ten times higher), changed the gain to x1, didn't understand why the scope was on x1 and the probe being x10, so I tried 0.1.In any case, the front scope test point is 0-1V. I don't know why this is 0-1V as most scopes I've seen are usually 0-5v.Now that I understand the scope probe menu stuff, maybe I won't mess up this portion of my measurements; and I now realize I saturated the diff probe which hopefully won't happen again.