[GPS_Standard] Peculiar problem

[Zack Widup]

On Tue, 30 Jan 2007, Howard W. Ashcraft wrote:

> I have constructed the GPS standard board using the VE2ZAZ board and
> pre-programmed pic.  It is connected  to an Axiom GPS receiver and an
> HP10544a OXCO, although I am not currently supplying power to the oven
> as I am testing the unit before building a power supply and enclosure.
> 
> I can read the board via the Montrol software.  The HP oscillator starts
> below frequency (9.998....), but the "sample frequency" per Montrol is
> 60448 with a difference of -31780 and a DAC value of 14240.  The DAC has
> not changed for over 200 updates, despite the OXCO output remaining
> under 10mhz.  At time stamp 208, it finally had a change to DAC14256,
> which is a negative coarse change.  As of time stamp 279, the DAC is
> still 14256.
> 

This is just a guess; if your HP 10544a output frequency is in fact 
9.998... MHz, then you have some big problems.  I don't have a spec sheet 
for the 10544 but I am guessing it's similar to the 10811, which allows 
only 10 Hz tuning range with the 10-turn trimmer and only 1 Hz tuning 
over the -5 to +5 volt electrical tuning range. 

So you would have to use the mechanical tuning to get it within 10.000000 
plus or minus 1 Hz, then allow the electrical tuning of Bert's circuit to 
tune it more accurately.

I find that zero-beating WWV gives enough accuracy with a 10 MHz OCXO for 
the initial adjustment.

73, Zack W9SZ