[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik Kaashoek)

[Magnus Danielson]

Hi Erik,

On 7/8/22 17:12, Erik Kaashoek via time-nuts wrote:
> Not something I want to implement on short notice but maybe for the 
> future.
> The biggest limitation in this DIY PNA is the phase noise of the 
> reference OCXO and the noise of the opamp amplifying the output of the 
> mixer.
> So I was wondering if it would make sense to do the following
> 1: Split the output of the DUT into two completely separate PNA's
> 2: Feed the output of the two PNA's into the PC left/right audio 
> inputs where the noise of both ADC's gets added.
> 3: Do a cross correlation of the two inputs.
> This should (as far as I have understood the feedback) eliminate both 
> the phase noise of the two independent OCXO's used as reference and 
> eliminate the noise of the opamps in the two PNA's and the ADC's, 
> given enough time to do the correlation.

This makes perfect sense. You will not remove the noise of the two 
channels, but you will get a direct benefit and as you average the 
complex output of successive FFT-cross-correlations you will suppress 
the measurement noise even further.

Have you attempted doing a PI-loop as I've suggested?

However, you benefit greatly at optimizing the performance of a single 
channel first before going to the cross-correlation. Bob's many good 
suggestions should provide you directions enough. Cross-correlation is 
not a replacement for doing the homework well, it's to get the icing on 
the cake.

Cheers,
Magnus