[time-nuts] Brooks Shera GPS Controller Help

Tue Jan 16 01:49:09 UTC 2007

Hi Jason:

1)    The oscillator needs to go into the VCXO In jack and then if you 
look at U1-2 (same as U2-1) you should see a zero to +5 square wave at 
the frequency of you oscillator.

2)    The GPS 1 PPS goes into GPS In and of  you look at U3-15  you 
should see a zero to +5 pulse once per second.

If (1) and (2) above are correct then the inputs are OK.

Do you know what the control voltage right at the input pin of the VCXO 
should be when you are locked?
(3) Measure the voltage at the input pin with a scope while the 
Controller is ramping to see if it goes past the required voltage.

Have Fun,

Brooke Clarke

w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
http://www.precisionclock.com

Jason Rabel wrote:

>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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