[GPS_Standard] Peculiar problem

Fri Feb 2 08:36:57 EST 2007

Thanks,

My reference to pins 8 and 9 was a typo (or writing too late at night).
I meant 5 and 6 which are the EFC control pins.  8 & 9 are the for
applying the 12v signal to turn on the heater, which the 10544a
requires, but the 10811 doesn't.

Howard, W1WF 

-----Original Message-----
From: Bert, VE2ZAZ [mailto:ve2zaz at sympatico.ca] 
Sent: Friday, February 02, 2007 4:55 AM
To: Howard W. Ashcraft; gps_standard at mailman.qth.net
Subject: Re: [GPS_Standard] Peculiar problem

Hi Howard,

Your explaination makes sense. Connecting a generator was a good way to
sort out things.

I don't own a 10544, so don't have much more to say, but you can refer
to the 10811 service manual. Remember that the 10811 is an improved
10544, so 99% of the specsheet applies, including pinout..

http://www.hparchive.com/Manuals/HP-10811AB-Manual.pdf

I have a 10544 specsheet, but it is much slimmer than the document
above. I have attached it to this email. Not sure if all readers will
have access to it...

Cheers,

Bert, VE2ZAZ

----- Original Message -----
From: "Howard W. Ashcraft" <hashcraft at hansonbridgett.com>
To: <gps_standard at mailman.qth.net>
Cc: "Bert, VE2ZAZ" <ve2zaz at sympatico.ca>
Sent: Friday, February 02, 2007 1:47 AM
Subject: RE: [GPS_Standard] Peculiar problem

Although I haven't had extensive time to debug this problem, I think it
is not the board, but the OXCO.  My 10544a is capable of achieving the
10Mhz with the oven on, but the observed frequency on Montrol was still
way off.  I know that the system can correctly measure frequency,
because I drove it with a signal generator and adjusted the input until
the system issued a DAC correction.  Montrol showed frequencies that
were close to measured on my frequency counter and the signal generator.
But it would not work correctly with the OXCO connected.

I determined that if I disconnected the EFC connection to the OXCO, the
noise I have observed disappeared and the native 10 MHz signal from the
OXCO was evident throughout the board.  But if I connected the EFC, the
observed frequency was significantly off and erratic.  With the OXCO
powered, but the EFC (pins 8 & 9 on 10544a) disconnected, a scope shows
a slight signal on the + and ground EFC pins.  A spectrum analyzer shows
that there is erratic noise on these pins peaking at around 1.8
MHz--just like I was measuring on the board with the EFC connected.
Measured with a 500 ohm probe, the noise is peaking around -20 to -30
dbm.  (adjusted for 10x factor)

If anyone has a 10544a, I would be interested in what their OXCO's show
on these pins.  It would also be nice to see an official pinout or
schematic for the OXCO.  I am assuming these are the correct pins
because of a comment on another website.

When I next find time for this project, I will try using a low pass
filter to quench the noise and see if the problem is solved.

-----Original Message-----
From: Bert, VE2ZAZ [mailto:ve2zaz at sympatico.ca]
Sent: Wednesday, January 31, 2007 4:49 AM
To: Howard W. Ashcraft; gps_standard at mailman.qth.net
Subject: Re: [GPS_Standard] Peculiar problem

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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To ensure compliance with requirements imposed by the IRS, we inform you that any tax advice contained in this communication (including any attachments) was not intended or written to be used, and cannot be used, for the purpose of (i) avoiding penalties under the Internal Revenue Code or (ii) promoting, marketing or recommending to another party any transaction or matter addressed herein.

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