The basic idea I had for the GPSDO was to use a micro clocked by the 10MHz OCXO and count clock cycles between the 1pps signal, then do some fancy math over a period of time to slowly drive the control voltage with a ~12 bit dac. I was hoping this would accommodate for any jitter.
I'm sure there are designs already out there, I see one on the instructables site and I briefly caught another using an atTiny.
Any ideas on this or suggestions of where to go looking for more info? I'd need some help probably with the math programming and maybe some of the circuit design and component selection. I'd probably use an Atmega 328p.
I have been thinking of doing it the same way.
I do not think you need to worry about the maths, the instructables way is simple enough - it just averages the count over a very long period (1, 2, 4, 12, and 24 hours). This is enough to average out the jitter in the PPS. Since you sound like you are using a comercial OCXO, their drift is usually less than 20ppm/year so this technic should be fine.
Basically, the count should be dead on 10MHz (+/- 1 count because of the PPS jitter - you cannot over come this unless you sychronise the 10MHz to the PPS but then you will end up adding jitter to the 10MHz, so in my opinion it is better to live with the 1 count error and have the stability of the OCXO) if the count is off, then you adjust the 10MHz. Whether you do this on the 1, 2, 4, 12 or 24hour is up to you.
Do not forget to have some non volatile storage to save the current DAC output, for cold starts and when GPS lock is lost.
Obviously, do not average when GPS lock is lost.
As Gyro pointed out the M variant of the Ublox devices are the general purpose positioning devices. The T variant is the timing variant but just like you, I found they are more difficult to get hold of.
The main difference from your point of view is the M variant does not have a TCXO so the temperature of the GPS module will have an effect on the stability of the PPS.
Other differences are that it has less facility for timing eg does not provide 10KHz signal. (other GPSDO seem to be beased on PLL the 10MHz to the 10KHz because the 10KHz is derived off the timing from the satellite signals).
I am curious, you say you already have a Trimble GPS. There are other project blogs on the web which use the Trimble. The Trimble has the 10KHz signal.
I will follow your progress with interrest. Good luck and have fun
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