David,
For your reference here are the FFT's of the audio signal from CH1 while connected to CH0 at various frequencies. 50 MHz, 299MHz, 1200MHz and 2100MHz, all with offset of 4kHz
Next the isolation measurements for 50MHz and 2100MHz.
This is done in my own HW so there are no changes in ADC settings, apart from the SI5351 everything else remains the same.
With strong signals the noise floor increases max 10dB but the SNR differences are as expected due to decrease in the 4kHz harmonic mixing signal.
As you can see FFT'2 are rather clean. Only the 4kHz and its harmonics.
Close observation of the 4kHz shows its starts to wobble more at higher frequencies explaining why it does not fit perfectly in the 480 samples you use.
Hope you will be able to see the same once you have done the windowing of the FFT input signal.
