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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] 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 _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
