[time-nuts] DIY GPS-DO
attila at kinali.ch
Sat Mar 10 14:59:36 UTC 2012
On Sat, 10 Mar 2012 14:07:33 +0100
Filip Ozimek <me_super at o2.pl> wrote:
> could you recommend me a good project of GPS-DO? I've got a GPS module
> with 1PPS output and a 20 MHz VCXO.
> As I understand to do a phaselock to 1 PPS signal one must build some
> kind of digital PLL. But I'm not sure
> how to calculate a correction signal to the VCXO and how to test such
> a disciplined oscillator.
Start with reading the following documents:
James Miller's GPSDO based on a 10kHz output of an Jupiter GPS unit.
It is by far the most simple system i've seen sofar. It's mostly analog
(beside the dividers) and uses a XOR based PLL for the FEC generation.
A GPSDO around a PIC16C73 from Brooks Shera. It is basically what you
want to do.
A GPSDO that is based on an FLL (frequency locked loop) by Betrand Zauhar.
Similar to the one above, but because it's using an FLL instead of an
PLL it has a slight frequency offset (instead of a slight phase offset)
You also want to read:
"Phase-Locked Loops" by Best.
The best book on PLL's i've found. Explains everything you need to know.
The PICTIC as an example how to build a time interval counter that is
good to <1ns (see  for information on PICTIC performance).
Depending on the stability and accuracy you want to accheive, it might
also be possible to use one of the modern uCs that run with >100MHz
clocks and use its internal timer as TIC. It will give you an measuremnt
accuracy better than 10ns, which is in the range of the width of a good
timing GPS receiver (see ), this would simplify your circuitry
Over the past years, there have been numerous discussions on GPSDOs
on this mailing list. I suggest you walk trough the archives and
read some of them.
Why does it take years to find the answers to
the questions one should have asked long ago?
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