[time-nuts] tinyPFA feature request

[Stewart Cobb]

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