[time-nuts] DIY GPS-DO

Attila Kinali 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 [5] 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 [6]), this would simplify your circuitry
quite considerably.

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. 


				Attila Kinali

[1] http://www.jrmiller.demon.co.uk/projects/ministd/manual.pdf
[2] http://www.rt66.com/~shera/QST_GPS.pdf
[3] http://ve2zaz.net/GPS_Std/Downloads/VE2ZAZ_GPS_Derived_Std_QEX_09_10_2006.pdf
[4] http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:pictic
[5] http://www.ke5fx.com/pictic.htm
[6] http://www.u-blox.com/images/downloads/Product_Docs/Timing_AppNote_(GPS.G6-X-11007).pdf

Why does it take years to find the answers to
the questions one should have asked long ago?

More information about the time-nuts mailing list