[time-nuts] DIY GPS-DO

[Chris Albertson]

On Sat, Mar 10, 2012 at 5:07 AM, 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.


THe references you got were good.  But if you are looking for the big
picture, it's simple.   There are many methods but they are all much
alike if you step back and look at what's really going on.   What you
need is a digital signal line that goes high on the leading edge ofht
e PPS and then goes low on the next leading edge of the VCXO.  Then
you build a controller to keep the time signal is high at a constant.

You can use a time to voltage converter and be 100% analog or you can
go digital.

The first thing you need is a divider.  20MHz is kind of fast.   you
might even go for a divide by 20.   then your controller sees a more
manageable 1Mhz.   If the vxco is not ultra stable it is best to
divide it down quite a bit.

I assume this project is just for education and entertainment.  If you
really need a GPSDO in your llab it is cheaper and simpler to buy a
Thunder both from eBay.  You can buy one for many $120 and they are
hard to build at that price.
Chris Albertson
Redondo Beach, California