[time-nuts] GPSDO - probably a stupid question.

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Aug 17 03:32:22 EDT 2016

The stability of the typical GPS receiver oscillator is usually inadequate to be useful as a GPSDO. An OCXO (as in the Trimble Thunderbolt for example) or equivalent is usually required. One can't usually just add a varicap to adjust the frequency of a packaged oscillator. If an external crystal is used the varicap should be placed in series with the crystal. However to do this reliably one needs to know the crystal parameters and those of the oscillator circuit. If you need to adjust the frequency both up and down (due to crystal parmeter tolerances) then an inductor in series with the crystal and varicap will also be required.

    On Wednesday, 17 August 2016 7:05 PM, Peter Reilley <preilley_454 at comcast.net> wrote:

 As a neophyte, I was wondering: rather that trying to discipline an external
oscillator to create a GPSDO and produce a precise 10 MHz why not 
discipline the oscillator
of the GPS receiver itself?  This could be done with a varactor diode 
across crystal of the
receiver's oscillator.  Of course there are the same problems with 
trying to servo this
oscillator as there are trying to servo an external oscillator but there 
are fewer parts.

Being a beginner I assume that I am missing something, but what?


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