[time-nuts] Yet another GPSDO project.

[Achim Gratz]

Tobias Pluess writes:
> I once tried to use the internal PLL of the microcontroller to boost
> the OCXO frequency to the max. 72 MHz which my microcontroller
> accepts. While this would greatly increase the frequency measurement
> resolution, it decreased stability because it appeared that the PLL
> had some frequency modulation at its output.

It's highly likely that the PLL on that micro is using a fractional
divider (it probably tells you somewhere in the documentation).

> I just tested different PLL multiplication factors, and according to
> my tests, the stability of the PLL improves quite a bit if a power of
> 2 multiplication factor is used, e.g. if the OCXO's 10MHz are
> multiplied to 20 MHz or to 40 MHz.

Yes, you'll have the best chances of getting the result you want with an
integer ratio and for some implementations of the divider it should be
either an even number or if you are really unlucky, a power of 2.  In
your case it might be possible to overclock the micro to 80MHz or, if
even number ratios are OK, at least use 60MHz.  It may be better to
first divide the 10MHz down by two (the input circuit in the PLL might
have a dedicated divider for doing that) and then multiply up to 60MHz
or 70MHz with a ratio of 12 or 14.


Regards,
Achim.
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