[GPS_Standard] Peculiar problem

[Howard W. Ashcraft]

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.

I am receiving the 1pps pulse and I can steer the final op amps.  (If I
ground the input, the output swings to near -5v, as it should.) However,
I have never seen a negative steering voltage.  The voltage is generally
around 3.4 v or above. In several runs on several days I have seen a
handful of  DAC changes, but generally in the wrong direction.  (The
tuning slope is correctly set to negative for the HP OXCO.)

Sometimes, the 10mhz output of the board drops to around 3-4Mhz, when
measured with a Racal 1992 frequency counter, and does so from pin 1 of
U1 downstream.  If you place a frequency counter probe on the board
ground, you pick up a very erratic signal running from 9.5 to 14 mhz.  I
took a look with a spectrum analyzer and found a -40 to -30dbm signal
floating on the board ground that was jumping around and centered on
about 1.8 Mhz.

What I think is happening is that the noise signal is hetrodyning with
the OXCO output causing the controller to become confused.  If the
trigger level is right, the frequency counter starts measuring the 3-4
Mhz signal.  But if you look at the signal with a scope, you get a
somewhat dirty looking signal at around 10 Mhz.

I'm at somewhat of a loss because, everything looks right on an
individual basis, but the pic is not providing any steering.

Any ideas?



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