[time-nuts] is there a "best bet" advanced hobbyist buildable GPSDOdesign?

Luis Cupido cupido at mail.ua.pt
Wed Dec 12 19:02:05 EST 2007

Ok. I understand now what you suggest.
Thanks for the explanation.

Luis Cupido.

Bruce Griffiths wrote:
> Luis Cupido wrote:
>> Bruce,
>>  > No analog filtering of the D flipflop output is required.
>> Now you got me lost.
>> We were talking about a GPSDO, that is locking
>> an VCXO on the GPS time (1pps or else)
>> So by the end of it you need an analog
>> signal to control the voltage input of the VCXO. Right ?
>> Where you get that from ?
>> If not by filtering your flip-flop output
>> what else you have in between the 1pps and the VCXO ?
>> CPU's DAC's ????
> Some software, including a sigma delta DAC, the effect of which is no
> different, in principle, than the filtering etc required by any of your
> phase detector implementations.
> The 1 bit phase error samples are processed in software (or hardware
> depending on one's inclinations, expertise, etc) in a similar way that
> samples from an N (>1) phase detector samples are, to produce a digital
> output for a DAC which drives the OCXO EFC input. The only difference is
> that a sigma delta DAC is used instead of a conventional DAC.
>> if so how does your complexity arguments still apply ?
> The interpretation of "complexity " depends on ones background and
> experience.
> The originator of the thread indicated that they had some microprocessor
> software experience.
>> Luis Cupido
> I was trying to tailor the design to the stated strengths of the
> originator of the thread.
> If one is trying to "squeeze" the ultimate in performance when using a
> GPS receiver to discipline an OCXO, then carrier phase measurements
> potentially offer much higher performance than can be achieved by using
> the PPS output of a typical GPS timing receiver.
> However only a few commercially available GPS receivers are suitable for
> this application.
> The GPS receiver oscillators all have to be phase locked to the OCXO
> being disciplined.
> This approach has been used in at least one commercially available GPSDOCXO.
> In principle a GPS receiver has all the required measurement hardware,
> so all that is required are suitable algorithms implemented in either
> software running on a DSP, microprocessor, etc, or implemented in
> hardware (CPLD etc).
> Bruce
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