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

[Magnus Danielson]

Hi,

On 7/11/22 18:05, Mike Monett via time-nuts wrote:
> To Bob kb8tq
>
>    Now for my new method.
>
>    The schematic   for   a   phase-frequency   detector   is   shown in
>    DBAND2S.PNG. In  operation, a pulse arrives at the DATA pin  and pin
>    U1Q goes high. Then a pulse arrives at the VCO pin and pin  U2Q goes
>    high.
>
>    This allows  the  NAND  gate  to bring  the  CLR  signal  low, which
>    immediately resets both d-flops.
>
>    The result  is shown in ZERODB.PNG. It is a very  narrow  pulse with
>    both d-flops superimposed.
>
>    This is the basis for my new approach. Simply tie both inputs of the
>    PFD together  and  measure  the noise spectrum  of  the  output. (Of
>    course, you have to ensure that both outputs match at zero error.)
>
>    Once you have the PFD noise, you can enable the loop and measure the
>    total noise  spectrum. Then simply subtract the PFD spectrum  to get
>    the OCXO  noise.  If  you   have   two  identical  VCXO's,  each one
>    contributes half the noise.
>
>    I don't know if this method would work with a double-balanced mixer.
>    The problem is a DBM requires quadrature signals, so the noise  is a
>    function of the OCXO noise as well as the mixer diodes. But the OCXO
>    noise is what you are trying to measure.
>
>    This method  works with the PFD since only a single pulse  is needed
>    to activate both d-flops, so you are measuring only the PFD noise.

A few comments.

First of all, there is no problem with quadrature signal on DBM, since a 
standard PI-loop lock drives it into quadrature, so no extra quadrature 
hardware needed and all very simple.

Secondly, noise differs very distinctly from systematic signals in how 
convergence work. You need to average longer for good confidence bounds 
on noise level compared to a systematic. The uncertainty of the noise 
vs. frequency will make suggested delta-processing much less rewarding 
unless you average over a long time.

Also, as working close-in, flicker noise is likely to be a major issue, 
and I doubt the DFF and PFD is optimized for that, rather the opposite.

Using a delta-approach have proven hard to make useful, because 
averaging time is what is used to cram the last dB of noise out only 
after as good as possible detection has been used.

Cheers,
Magnus