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Bode Plot Torture Test

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mawyatt:

--- Quote from: Someone on November 11, 2022, 01:13:51 am ---In which case adding noise to the stimulus should have minimal effect as a) noise has shown to be supressed by the frequency selective detector b) the detector for both sides of the measurement should be equally affected
So the original post may be mostly about the amplitude of the measurement on each side, which could be reproduced by adjusting the vertical gains.

--- End quote ---

The phase has shown more sensitivity to Noise or Spurious Signals in the measurements than the amplitude, which from past experience has generally been the case for corrupted signals.

Best,

2N3055:
Averaging will suppress only random signals and frequency selectivity can suppress only out of band signals. If you overlap tone agresor tone on top of stimulus, it will vector add to it and will either suppress or amplify the amplitude... No way to discriminate it.
But a single tone of significant amplitude will stand out on a plot as a discrete peak, obviously out of place..

In case of testing switcher PSU stability, for instance, Bode sweep would be performed in frequency range of regulation loop, well below switching frequency. In which case interference will be outside the swept range and suppressed well enough.

Good practice is to run FFT on input and output before FRA to check for both interference and spurs in system. With Bode plot most important part is gain/phase margin. A bit of error because of interference won't kill you because you need to have healthy margin anyways.. I would like a Nyquist polar plot for stability check, they are what I'm used to, and they make it easy to see problems with a quick look.

Very clean sweep results would be more important if used for impedance analysis. In which case de embedding and calibration would need to be added to scope so it can perform like a propper VNA/impedance analyser.

I took a closer look at how Keysight is doing FRA: they use averaging acquisition mode and grab a screen of data, several triggers, waiting for averaging to converge and use that data for calculations.. I also know they use Chirp-Z transform for it's internal spectral analysis (they say FFT on a scope, but it is not classic Cooley-Tukey FFT, but a Chirp-Z), so maybe they use it for calculations.. code is there in the scope... But they might be just using literal interchannel phase/amplitude measurements, because they get very little data, exactly 4 periods..

precaud:

--- Quote from: nctnico on November 10, 2022, 04:10:54 pm ---IMHO noise is not the main problem for doing network analysis as most of the suitable algorithms get rid of noise anyway....

It would be interesting to come up with a DUT that really tests the limits of FRA / network analysis.

--- End quote ---

Try using it to measure the output impedance of voltage regulators and power supplies. That will test the limits.

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