Strange behaviour using Automated Tests

Using automated tests to check ana audio amplifier sometimes I obtain a strange thing: a burst of distortion with a lot of spurious lines. The next test is ok. See the

enclosed pictures.
Best regards.
Pietro Cremonini

Hi @Cremo, it looks like this is test 5 of 11 in your sequence that got the strange result, and then a subsequent run showed test 5 of 11 was fine…do I understand correctly?


When you get to a place where the bad reading is shown, can you switch to time domain and inspect the waveform more closely?

For example, here is the full burst. Note defined ramp up and ramp down at beginning and end. You could, however, see gaps in here meaning something went amiss with the USB (packet loss).

And then, drag and zoom with mouse to see a few cycles. My guess is it would still look pretty good, but you might be able to detect a bit of noise.

Looking at your plot, we see the noisefloor come up, the power line hum @ 100 Hz go REALLY high, and then a bunch of intermodulation-y looking components show up around the 1 kHz (from the power line rise). These look like legit problems to me and not a momentary glitch in the analyzer. So, for that reason, I’d put the analyzer in a short FFT mode so that it is updating several cycles per second, and then stress all your cables in the setup to see if you can cause this happen again.

Thanks! Matt

Hi Matt,

thank you for your ultrafast reply.
Till now it is not clear what is happening.

The only firm thing is that the waveform in a scope connected in parallel with the output of the amplifier is correct in both cases.

In any case I dont blame to the analyzer.

I will do further tests.

One question: if the computer is too slow so the USB communication has problems similar malfunctions can occur ?

I enclose also the results of tests done with problem and without it.

Best regards.


Yes, if the computer is very slow then USB problems could be happening. But even a 6 year old laptop is plenty fast. So, as I said, I don’t think that is the issue. But we must rule it out.