Re: Phase noise/drift and the accuracy of measurements
Daniel Marks
I was thinking about what I said, and I think it may be even simpler to do what I suggested if you use Plancherel's theorem (https://en.wikipedia.org/wiki/Plancherel_theorem). In LaTeX code \int\limits_{t=\infty}^{\infty}{f(t) g(t)^* dt} = \frac{1}{2\pi} \int\limits_{\omega=\infty}^{\infty}{F(\omega) G(\omega)^* d\omega} Therefore you can do this calculation either in the frequency or time domain. If you do it in the time domain and average, you cancel the common noise phase. However, you should get the exact same result in the frequency domain because this is what Plancherel's theorem says. Therefore to implement this, perhaps you should just instead of using only one bin, use the two or three adjacent bins, and multiply together F(omega) (the frequency domain sample of the first signal bin) with G(omega)^* (the complex conjugate of the frequency domain sample of the second signal bin) and just add the results from the bins together. You need only do this for a narrow band around the center frequency enough to encompass the spreading of frequency due to phase noise. Because it will only involve multiplying and adding a few more samples, this should add minimal overhead. The reason I think this may be very helpful is I noticed when looking at the IF signal with a scope, there is definitely some jitter because of the frequency synthesis method of the SI5351A. If I tried to demodulate the signal directly using a carrier (like using the FFT) there was definitely some error there. When I trigger the sampling using the zero crossings of the IF signal, this noise pretty much disappeared. I think you can probably effectively do the same thing in software, and the addition in computational cost will be minimal, especially if you use the Plancherel theorem trick. Anyways I would be happy to explain more, but using the Plancherel theorem the FFT is already performing the crosscorrelation that is needed to demodulate the signal. 73, Dan KW4TI
On Mon, Aug 12, 2019 at 12:15 PM Yury Kuchura EU1KY <kuchura@...> wrote:

