[time-nuts] Re: tinyPFA feature request

[Erik Kaashoek]

I did some testing
Input A gets 10 MHz
Input B gets either 2.5 MHz or 5 MHz.
This works well if the input to B is a pulse instead of a sine wave or 
symmetrical square wave as the pulse has enough second or fourth order 
harmonics for the tinyPFA to work
As both tinyPFA inputs use the same NCO for down conversion to fit in 
the sample rate of the ADC, your idea can not be implemented.
Erik.

On 17-8-2023 21:51, Stewart Cobb via time-nuts wrote:
> I have been using a tinyPFA for several weeks now, and it does exactly what
> it's intended to do. Thanks and congratulations to Eric for creating this!
>
> I have one request for an additional feature. Many systems of interest to
> time-nuts have a 10 MHz output, but some important ones only have 5 MHz.
>
> I can't always arrange doublers and dividers to put signals from different
> gear on the same frequency.  However, for signals that are very close in
> frequency except for a factor of two, as most time-nuts signals are, it
> seems like the tinyPFA could measure them directly without much additional
> effort.
>
> I don't know exactly how the internal software works, but it seems like
> there is a software PLL which includes a software NCO. It seems that
> shifting the NCO phase output word by one bit left or right in software
> before applying it to the B channel signal would have the effect of
> changing the target frequency by a factor of two. This seems like a fairly
> simple software change to get a significant additional capability.
>
> Possible side effects of this mode might include lower resolution or
> additional noise (due to lost LSBs) and a narrower window of operation.
> Currently both signal frequencies need to be within (200 Hz)/Tau of each
> other, and that window would probably shrink by the ratio of the
> frequencies.
>
> It might be useful to allow two or three bits of shifting. There are old
> OCXOs at 2.5 MHz, and I have gear on my bench at 20 and 40 MHz that I would
> like to compare to a 10 MHz or 5 MHz reference.
>
> I would try to implement this myself as a proof-of-concept, but the
> measurement source code does not seem to be available.
>
> Cheers!
> --Stu
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