[time-nuts] Brooks Shera GPS Controller Help

Jason Rabel 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
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




More information about the time-nuts mailing list