Hi guys,
Based on my very limited understanding on electronics, osclilators and other such things, I would expect oscillator effects to cancel out when looking at the relative phase of two channels. I would expect the phase jitter on the relative phase to be dominated by what happens in the ADC and the digital down conversion. That is why I am running the two dongles off the same clock.
I have some measurements of the relative IQ signal (z_1/z_2) at 100 kHz, 10 Hz, and 1 Hz sample rates. I did a power spectrum estimate using a Hanning window to look at relative phase noise. I didn't do any incoherent averaging on any of these, so there is some statistical noise. I only see some "imperfections" in the 10 Hz and 1 Hz spectrum. However, I see these kinds of effects in 100 times more expensive receivers too. For my purposes, the relative phase behaviour of the two channels is good enough.
I have attached the spectra. I have also attached the 1 Hz IQ signal, which shows a small systematic wiggle, but very little phase difference between the two channels.
The samples also are aligned over two hours of sampling, which means that there cannot really be a very large amount of clock drift between the two systems.
Sure, the thing has it's faults. But with $16, you really can't beat the price.
juha