[time-nuts] GPS Disciplined TCXO

Gregory Maxwell gmaxwell at gmail.com
Fri Oct 23 17:09:00 EDT 2015

On Tue, Oct 20, 2015 at 8:53 PM, Bryan _ <bpl521 at outlook.com> wrote:
> Saw this on the Hackaday site if anyone is interested.
> https://hackaday.io/project/6872-gps-disciplined-tcxo

Will this design that uses the output of the DAC directly not run into
problems with non-monotonicity and/or dead-zones in the DAC output?  I
would expect a PLL to behave very poorly if there is any
non-monotonicity in the least significant bit of the DAC.

My intuition would be to use TPDF dither against a higher resolution
internal value (perhaps 32 bits) with noise shaping so that the dither
has no power at DC (and not much near it), followed by a RC low-pass
filter. With this approach the fact that available DAC parts run at
speeds far faster than needed for this control helps overcome the fact
that highly monotone parts are less common (last I looked).  This
would also help give much finer control without compromising tuning

[Disclaimer: I've never built a GPSDO]

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