[time-nuts] Brooks Shera GPS Controller Help

[Jason Rabel]

Yep, I leave the OCXO always on since I finished that circuit.

The OCXO outputs a 6V reference, from there I go to the 10K pot, which goes
to the EFC and ground/DAC (I have it wired so I can use a jumper to ground
when not hooked to the DAC. The 5 MHz target is achieved around like
3.2-3.3V, so it's still pretty well centered, I don't remember how much pull
the DAC has going up or down, but that part is okay I know because I've
watched the EFC voltage and frequency on the scope.

I was talking about the resistors at P2 (R1 & R2). I think I can just use
the TTL input instead of the AC coupled one.

GPS is okay, it's a Motorola Oncore VP in fixed location mode. I also used a
UT+ for a while before moving that to something else.

Jason

> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Bill Hawkins
> Sent: Monday, January 15, 2007 6:58 PM
> To: 'Discussion of precise time and frequency measurement'
> Subject: Re: [time-nuts] Brooks Shera GPS Controller Help
> 
> 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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