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MSO-X4154A amplitude measurement

Question asked by RednBlack on Apr 29, 2020
Latest reply on Apr 30, 2020 by RednBlack

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Hi,

I have an odd problem with this scope's amplitude measurement function. The signal is a sinusoid with significant crossover distortion and the measurement occasionally picks this as the top of the waveform:

The behaviour is not consistent; the lower trace will occur maybe 1 in 20 captures (in single shot mode). I first noticed this when I had the measurement statistics running with x1000 averaging, and the results deviated from expected values by a few percent. If the affected measurement doesn't have cursors visible this could easily be missed.

I assume the algorithm is looking for a region of the waveform close to the peak with relatively low dV/dt, this is what is implied in the manual. How it misses the true top of the waveform that is about double the detected value, I'm not sure. It does seem to be worse when the peak is near the top of the screen.

I tried updating the firmware to the latest version 7.31.2020012900, this made no difference. Any suggestions?

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pat_white

Apr 29, 2020 8:02 AM

I believe vertical histograms are used to determine the top and the base (and therefore amplitude). The crossover distortion probably gives the tallest histogram bar occasionally. I don't know of a way to modify how the top is determined, so I'm thinking you need to smooth out the distortion before measuring the amplitude, by applying a low-pass filter or smoothing math function.

Best Regards,

Pat

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Tardz Apr 29, 2020 1:15 PM

Hi,

In this particular case, I suppose that the automatic measurement function "AMPLITUDE" has difficulty since there is a relatively stable level on the waveform, and probably considers that the rest of the waveform is a peak (positive side). So maybe try the "PEAK-TO-PEAK" function in this particular case, with the avg function activated, you should possibly have a better result. I know that it is undoubtedly not ideal but for you I would try the experiment.

Unfortunately I do not have MSO-X4154A (a great scope), but I did the test on my TEK DP3014 and I have the same problem, but for my part it is with the negative cycle (I used an 33220A to generate the waveform). Tested on an R&S RTB2004, it worked better but the statistics showed a less stable value for the AMPLITUDE function compared to the PEAK-TO-PEAK value, so there were probably some hiccups.

Yves

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RednBlack Apr 30, 2020 12:21 AM

OK thanks both, the histogram explanation makes sense.

I initially wanted to use amplitude rather than peak-to-peak as it is more stable with a signal with occasional transient interference present, but peak-to-peak with x1000 averaging seems to work well enough for this applciation.

My main concern was that it could be a bug in the firmware, but I think we can rule that one out now.

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pat_white Apr 29, 2020 8:02 AM
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I believe vertical histograms are used to determine the top and the base (and therefore amplitude). The crossover distortion probably gives the tallest histogram bar occasionally. I don't know of a way to modify how the top is determined, so I'm thinking you need to smooth out the distortion before measuring the amplitude, by applying a low-pass filter or smoothing math function.

Best Regards,
Pat
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Tardz Apr 29, 2020 1:15 PM
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Hi,
In this particular case, I suppose that the automatic measurement function "AMPLITUDE" has difficulty since there is a relatively stable level on the waveform, and probably considers that the rest of the waveform is a peak (positive side). So maybe try the "PEAK-TO-PEAK" function in this particular case, with the avg function activated, you should possibly have a better result. I know that it is undoubtedly not ideal but for you I would try the experiment.

Unfortunately I do not have MSO-X4154A (a great scope), but I did the test on my TEK DP3014 and I have the same problem, but for my part it is with the negative cycle (I used an 33220A to generate the waveform). Tested on an R&S RTB2004, it worked better but the statistics showed a less stable value for the AMPLITUDE function compared to the PEAK-TO-PEAK value, so there were probably some hiccups.

Yves
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RednBlack Apr 30, 2020 12:21 AM
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OK thanks both, the histogram explanation makes sense.

I initially wanted to use amplitude rather than peak-to-peak as it is more stable with a signal with occasional transient interference present, but peak-to-peak with x1000 averaging seems to work well enough for this applciation.

My main concern was that it could be a bug in the firmware, but I think we can rule that one out now.
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MSO-X4154A amplitude measurement

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