Thread: Homebrew GPSDO
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Old 6th Mar 2021, 10:31 am   #16
Vintage_RC's Avatar
Join Date: Sep 2018
Location: Twickenham, London, UK.
Posts: 316
Default Re: Homebrew GPSDO

As I mentioned in my post #8 regarding my Quartzlock 2A modified with a Jupiter GPS to give a GPSDO function but still allowing switching back to off air (198KHz) operation, I have never seen any difference between the two methods of obtaining the frequency reference. One thing I noticed during the development of this dual mode standard was that the Jupiter provided the 10KHz output even without having acquired any satellites. It is close enough to the precise frequency to allow the 2A phase locked loop to lock onto it thus giving the illusion that it is providing a GPS accuracy output. To get around this a wrote a PIC program to monitor the NMEA output of the Jupiter to show (via an LED) whether there was a true GPS source providing the 10KHz signal. After the Jupiter has acquired satellite lock and the 10KHz is GPS referenced I noticed that if you remove the GPS antenna the internally generated 10KHz initially remains at the locked correct frequency and only slowly drifts back to the internal reference frequency. There must be some sort of phase locked loop running in the Jupiter Rx. This means that for short interruptions in the GPS signal the 10KHz output remains steady.
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