[time-nuts] Thunderbolt? (re simple gpsdo.)

Chris Albertson albertson.chris at gmail.com
Fri Dec 30 12:26:52 EST 2011

On Thu, Dec 29, 2011 at 8:24 PM, Mark Spencer <mspencer12345 at yahoo.ca> wrote:
> One of the designs using the 10 kHz output from a Jupiter gps engine and a simple PLL to discipline an ocxo might be good starting point if suitable gps engines are still available.  There won't be much to tweak but the performance could be quite good.

I just finished reading about that one.  It requires no longer
available GPS reciever.   Maybe I should r-phrase the question:

"What is the simplest design for a GPSDO that uses only the PPS signal
from a modern GPS?"

Half of the reason for the question is academic.  Then if a simple
enough design presented itself it would be fun to try it.

The simplest design I can think of now is based on a flip-flop.  The
PPS "sets' the FF and the next raising edge of the local oscillator
"resets" it.   (The local oscillator might need to be divided down or
a slower 1Mhz oscillator used so the FF remains "on" for a reasonable

Next the FF gates a current source to a capacitor.   The voltage in
the cap is amplified and controls the local oscillator frequency via a
low pas filter.  Likely the "low pass filter" would be an active
device that we call an "integrator"

You need to discharge the cap for the next cycle.  One could rig a
one-shoot timer to discharge the cap.  Actually there are about four
states that need to be cycled every second.

Chris Albertson
Redondo Beach, California

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