[time-nuts] Brooks Shera GPS Controller Help
jason at extremeoverclocking.com
Mon Jan 15 17:28:12 EST 2007
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
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
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
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.
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