[time-nuts] Brooks Shera GPS Controller Help

Bill Hawkins bill at iaxs.net
Mon Jan 15 19:57:40 EST 2007


First, take your OXCO, heat it up, and use just a pot for the 0 to 6
volt control range. Find out what control voltage is needed for the
proper frequency. The DAC can't achieve lock if it can't reach that
voltage.

If that's all OK - the DAC output is within range - then find out what
AC input voltage is required from the OXCO to the board. 0.4 volts
sounds really low. What's all this about resistors? You can't achieve
lock if the board has no input from the OXCO. Are you sure the GPS input
is OK?

Did that help?

Bill Hawkins
 

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Jason Rabel
Sent: Monday, January 15, 2007 4:28 PM
To: 'Discussion of precise time and frequency measurement'
Subject: [time-nuts] Brooks Shera GPS Controller Help

You guys might have noticed a little bit I posted in another topic, but
I figured it's time I started my own now that I have the setup all
hooked back up and running again.

I'm having trouble getting the controller to lock the OCXO. The phase
reading only goes between about 10 - 155... Nowhere near the 800 that
means it is locked on. :(

I have a Brandywine GPSDO that I used to coarse adjust the 5 MHz (both
hooked into my oscope), and I got it spot-on, probably way more than
necessary. I flip the Shera board to Mode 1, and the DAC voltage jumps
to about 1V and goes up and down between that and about 1.1V, slowly
back and forth.

If I switch to Mode 2 or higher, then the DAC voltage will go all the
way through its -3V to +3V range, when it hits the top of the +3V it
jumps down to -3V and works its way up again.

I'm using the A&A board, all my solder joints are good, even the couple
that were the least bit questionable I fixed just to be 100% sure about
that.

I'm using a 5 MHz OCXO, with a tuning range between 0V to 6V. I have S4
closed, as my voltage increases the frequency decreases. On the A&A
board I have pins 1 & 2 jumpered together on P3. The GPS is a Motorola
Oncore VP, going through the TTL in. The OCXO is going through the AC
coupled input, after the resistors its still showing about .4V p-p. Just
the plain output is about 2.5-3V p-p, I *think* I could just use the TTL
in then? The notes says about 2V or less so that's what I have it going
through the resistors.

The first LED always stays lit signifying that the lock is questionable
(obviously).

My coarse adjust is a 10K pot, I did just add a 2K pot inline as a fine
adjust, but have not re-calculated the R5 & R6 values. It is acting
exactly as before so I don't think that is the cause. But tomorrow I'll
probably re-do the calculations for S, then also R5 & R6...

My only guess is that something is wrong with the 5 MHz signal
somewhere...
The way the DAC voltage cycles makes me think it is just completely
ignoring something. 

If anyone is interested in the raw phase data, email me directly and I
can send them to you, but I don't know how helpful they will be.

Jason





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