Many thanks to Eric for clarifying how the spectrum of the processed signal can be obtained.
So, NanoVNA outputs signal, and mixes it with a signal with an offset of XXX Hz (IF freq), all this goes to the SA612 mixer. From where the intermediate frequency goes to the ADC.
There the signal is digitized, and at the output we get I/Q samples
Further, there is the mathematics how to extract the amplitude and phase of the IF signal, compare it with the reference.
Well, according to this information, take everything further ... (apply calibration, made calculation)
I needed to understand how the ADC works qualitatively, which intermediate frequencies are better or worse. What is the noise level.
Now, thanks to Eric's help, you can see it visually. And make changes to the settings on the fly and see the result.
So, in the end:
Attached screenshots for ADC 384k (full screen data)
1 screenshot offset 50k
2 screensot offset 98k
3 screenshot offset 196k
You can see clean data, some offset harmonics, in futuire i try made more screenhots for show how it work on higher frequency, noise level and so
