Frequency mismatch

[Tom Kamphuys]

I'm trying to measure my amplifier. Yesterday I got very weird results.

As you can see, the noise steps up at around 500Hz. Harmonics start to decrease at that same point and drop down to basically the noise floor level in the 1-3kHz region.

Trying to understand this, I took a step back and measured just my DAC & ADC in loopback. My DAC is a Topping d10s and my ADC is a Behringer UMC204-HD. Both are set to 192kHz, as that is the max of the Behringer.
What seems to be the case is that the frequency at which the FFT is evaluated, does not match with the peaks in the spectrum. This is the case for the harmonics _and_ the fundamental, although the problem with the fundamental does not show up in the image above. The harmonic numbers drop of the peak and end up in the noise. Exactly what was seen in the amplifier test.

One thing that struck me is that the end frequency is clamped to 91.2kHz whereas I would expect 192kHz/2 = 96kHz. Might the frequency axis scale be a bit off? Might explain why the problem shows itself at higher frequencies...

Any idea what might be causing this? And how to solve it? I could use only the Behringer (as both DAC and DAC), but it has higher noise, its level is lower and its usable level is even lower as distortion skyrockets the last 3dB or so.

 

[John Mulcahy]

The clocks in the DAC and ADC may differ enough to cause the problem. You could test that by making a sweep measurement of a loopback connection from DAC to ADC using either the acoustic timing reference or a loopback timing reference on the other channel and enabling the Analysis preference to Adjust clock with acoustic ref or Adjust clock with loopback respectively. The Info panel for the measurement will show any clock adjustment (in ppm) that was required to make the DAC and ADC clocks match. It is generally best to use the same device for input and output when making stepped sine measurements, that way there is only one clock.