[time-nuts] GPSDO Alternatives

Bob Camp lists at rtty.us
Fri Dec 7 01:50:37 UTC 2012


The Shera counter is not running in the same fashion you would be running an input capture pin. 


On Dec 6, 2012, at 8:45 PM, David <davidwhess at gmail.com> wrote:

> 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>
> wrote:
>> 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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