[time-nuts] GPSDO Alternatives

Bob Camp lists at rtty.us
Fri Dec 7 00:34:08 UTC 2012


Again, the math is pretty simple. 

A 16 bit capture running at a 1/4 clock is not going to get you very near a Shera. It's even further from the more modern "enhanced Shera" designs. 


On Dec 6, 2012, at 6:59 PM, David <davidwhess at gmail.com> wrote:

> You can use the ATmega328 16 bit timer/counter in input capture mode
> to count the number of 10 MHz OCXO cycles per pulse per second period
> to a resolution of 100ns but there are some problems:
> The ATmega328 16 bit timer/counter external clock is limited to 1/4 of
> the CPU frequency with an asynchronous source so the 10 MHz OCXO would
> need to be divided down which would further limit performance and
> require an external divider.  Modifying the Aruino board to use the 10
> MHz OCXO in place of the CPU clock solves that problem.
> Then operating the counter/timer in input capture mode with the GPS
> pulse per second signal connected to the input capture pin would allow
> almost Shera like performance.  The timing resolution would be 2.4
> times lower (and not asynchronous) limiting performance over short
> time spans.
> On Thu, 6 Dec 2012 14:57:19 -0800, Chris Albertson
> <albertson.chris at gmail.com> wrote:
>> You'd have to seriously divide down the output from the 10MHz OCXO if you
>> were going to use it as an interrupt.  Maybe to divide by 10,000? and even
>> at the higher clock rate you'd still have poor resolution.
>> I image each interrupt handler would sample some internal counter and the
>> background task would look at the delta between the two and adjust the DAC
>> to drive the OCXO to close the difference.   The resolution would be
>> (maybe?) a "handful" of clock cycles.   Given enough time, say a 1000
>> second period it might wrk well enough.  I can't know without doing a more
>> detailed design
>> On Thu, Dec 6, 2012 at 1:59 AM, Don Latham <djl at montana.com> wrote:
>>> Chris:
>>>> The question I have again is about a simple phase detector.
>>> I did ask if the arduino interrupt ports could be used as a phase
>>> detector; one on the GPS and one on the OXCO. Too much jitter? If the 12
>>> MHz clock is too slow, would an 80 MHz clock ARM arduino style processor
>>> work? I'm simply too new at this to decide.
>>> Don
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