[GPS_Standard] Peculiar problem

[Bert, VE2ZAZ]

Hi Howard,

First, the FLL will not be able to disciplin an oscillator that is 200 ppm
lower than nominal frequency. The maximum offset that the FLL can accurately
count is -166 ppm (-1664 Hz) and +243 ppm (+2432 Hz). You are currently
under-running the 16-bit counter. And besides, the HP oscillator can only
tune +/-0.5 Hz. So you must find a way to bring that oscillator closer to
frequency. Why don't you power up your oven with a temporary supply?

Are you convinced that your Axiom GPS is tracking a few satellites. In other
words, are you confident that the 1PPS is accurate?

Also, please send me your parameter settings (values in the Control Window).

Finally, I don't really understand the "dirty" behavior of your system.
Everything is quite straightforward and can be easily traced end-to-end with
a scope. You should see stable 10MHz signals everywhere. Your problem must
be caused by an assembly process error (cold solder joint, bad socket, bad
system grounding, etc...), or maybe an insufficient swing coming into U1 pin
6. From what you're saying, your problem appears to be happening at or
around U1... Simultaneously probe pin 6 and pin 1 and try to see a
correlation of behavior between the two signals. Would it be possible that
the oscillator could oscillate at its fundamental frequency of 3.33333MHz
because it is outside its operating range (oven off)? You shoud always see a
clean 10MHz sinusoidal , 1V + peak-to-peak swing at U1 pin 6.
This is really the only thing I can say about this. You are the first out of
dozens of users that is reporting such a behavior...

Let me know the outcome...

Bert, VE2ZAZ


----- Original Message -----
From: "Howard W. Ashcraft" <hashcraft at hansonbridgett.com>
To: <gps_standard at mailman.qth.net>
Sent: Wednesday, January 31, 2007 2:08 AM
Subject: [GPS_Standard] Peculiar problem


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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