[time-nuts] GPSDO Alternatives

David davidwhess at gmail.com
Fri Dec 7 01:45:54 UTC 2012

Sorry.  The Shera counter is 16 bits and not 12 bits but that does not
change what I posted.

On Thu, 06 Dec 2012 19:17:48 -0600, David <davidwhess at gmail.com>

>It is not a 16 bit capture and it does not run at 1/4 the clock rate.
>The Shera uses a 12 bit counter to capture the phase difference
>between the OCXO frequency divided by 16 and GPS pulse per second
>output to a resolution of about 42ns.
>What I suggested effectively captures the same phase difference but
>divides the OCXO by 65536 and only has a resolution of 100ns.  The
>OCXO division is just conceptual though as it comes from the 16 bit
>timer/counter overflow internal to the microcontroller so in practice,
>the timer/counter is arbitrary length with an interrupt about every
>6.5ms doing the housekeeping for the extra bits.  It directly measures
>the pulse per second period to a resolution of 100ns using the OCXO as
>a clock.  If a 20 MHz OCXO was used (limited by the maximum clock
>frequency of the microcontroller), then the resolution would be 50ns.
>That is not how I plan on designing my own GPSDO which has taken a
>significant turn from what I posted about here some time ago but I
>have stopped discussing that until I have some results to share.

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