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? Thanks, Pete.

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

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. Bruce On Wednesday, 17 August 2016 7:05 PM, Peter Reilley <preilley_454@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? Thanks, Pete. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.

 Björn

 On Wednesday, 17 August 2016 7:05 PM, Peter Reilley

Some (all?) Novatel receivers have an option to sync their internal TCXO or let it freewheel. -- Björn > 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. > Bruce > > > On Wednesday, 17 August 2016 7:05 PM, Peter Reilley > <preilley_454@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? > > Thanks, > Pete. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >

On Tue, Aug 16, 2016 at 6:02 PM, Peter Reilley <preilley_454@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 is actually done. But you need to design the GPS receiver from the ground up to use a very high quality oscillator. The Thunderbolt is an example of this. It has a 10MHz OCXO used as the internal clock. I've seen the same thing done with NTP servers. Rather then adjust the system time using software, why not discipline the mother board's XO? Yes but it is a large amount of work to modify an existing product. -- Chris Albertson Redondo Beach, California