[time-nuts] Brooks Shera GPS Controller Help

Richard H McCorkle mccorkle at ptialaska.net
Tue Jan 16 08:55:27 UTC 2007


Jason,
What divider output are you using? The typical input to the phase detector
should be between 312.5KHz and 625KHz and an appropriate jumper has to be
installed on the divider header on the A at A board to tie the selected divider
output to the phase detector. Sounds like you aren't getting a good clock
into the phase counter reset.
Hope this helps,
Richard

----- Original Message ----- 
From: "Jason Rabel" <jason at extremeoverclocking.com>
To: "'Discussion of precise time and frequency measurement'"
<time-nuts at febo.com>
Sent: Monday, January 15, 2007 1:28 PM
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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