[time-nuts] Re: tinyPFA feature request

[Stewart Cobb]

After finding and analyzing the schematic for the NanoVNA-H4, which is the
underlying hardware for the tinyPFA, I now realize that my requested
feature is impossible. The hardware is extremely narrowband. It only
digitizes signals within a few kHz of the selected frequency. Therefore, it
can't simultaneously receive (say) 5 and 10 MHz. To do that, it would need
wideband digitizers, such as are found in the (much more expensive) TimePod
and PhaseStation designed by John Miles. The tinyPFA is a very clever reuse
of existing inexpensive hardware, but it does have limitations.

Cheers!
--Stu

On Thu, Aug 17, 2023, 14:51 Stewart Cobb <stewart.cobb at gmail.com> 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
>