EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: Faringdon on October 24, 2023, 05:58:57 pm
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Hi,
How common is it in a scope, where you sample and hold 2 channels together....and they are not positioned properly with ref to each other in time...eg one is incorrectly time shifted? Is this common?
Say you have two signals, where they definitely pulse up at the same time.....but in the scope shot, they are not sync'd....because the scope is in error. Is this common?
Please also consider when say one channel is a big signal on 200x and 100V/sec, and the other channel is say 1V/div and on 20x magnification....is this error more likely then?
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What? No, this is not common. This is not even a thing at all.
There may be some propagation delay differences between the channels, but they are not going to be significant compared to the sample rates.
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Connect both probes to the same node and post a picture of the waveform and probes connected together. Only then can the bad workman stop blaming his tools and work out why the signals he thinks "definitely pulse up at the same time" in fact don't!
EDIT: This may be related to https://www.eevblog.com/forum/beginners/high-voltage-seen-when-scoping-with-diff-probe/ (https://www.eevblog.com/forum/beginners/high-voltage-seen-when-scoping-with-diff-probe/) who knows.
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It can happen. See around time 10:23 in https://www.youtube.com/watch?v=9cbK5o_7D6U&ab_channel=TechCornerTVElectronics%2CProjects%26Tutorials. (https://www.youtube.com/watch?v=9cbK5o_7D6U&ab_channel=TechCornerTVElectronics%2CProjects%26Tutorials.) Review of a small oscilloscope.
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Ok, I assume we are talking about actual tools, not random junk for $20 from Aliexpress. Obviously absolutely anything can happen there.
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If you are using different probes (especially if one of them is active DIFF probe and other one is passive one) then both of these probes will have different propagation delay. Also if cables are not same length. Or using combination of voltage and current probes...
Proper scopes have Channel Deskew function in channel setup to compensate for that.
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@Faringdon: What's the scope model? Could you share a screenshot which shows the settings and the problem?
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Ok, I assume we are talking about actual tools, not random junk for $20 from Aliexpress. Obviously absolutely anything can happen there.
The Hantek2000, for example, has such a problem (1/2 sample offset between the two channels). In single-channel mode, the two halves of the AD9288 are interleaved (in order to double the sample rate), so of course the two clocks must be interleaved as well. However, it seems that the clocks are still interleaved in dual-channel mode :(
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From the other thread, I suspect that the OP is working at low frequency, so ADC interleaving shouldn't be an issue.
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1/2 sample offset between the two channels
This is what I understood under "insignificant". If 1/2 sample starts to matter, you need to get a better scope.
But as usual with OP, we are not likely to see any answers. He will just make another pointless thread.
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If you are using different probes (especially if one of them is active DIFF probe and other one is passive one) then both of these probes will have different propagation delay. Also if cables are not same length. Or using combination of voltage and current probes...
Proper scopes have Channel Deskew function in channel setup to compensate for that.
I was just about to say the same thing :) Deskew fixtures and compensation exist -- even in mid to low range scopes -- for a reason.
If you're probing the same point with two identical voltage probes and see offset/skewed waveforms, I would definitely find that odd.