Hi Everyone,
I've had some fun and learned quite a bit trying to get my Lucent RFTG rubidium module to discipline itself to a GPS module. It has an interesting way of doing this that I'm not sure is the best or the worst, but it is constantly figuring out an average phase error between the gps 1pps signal and its own 1pps signal generated by dividing the Rb clock. It can make both time corrections (to correct the phase of the 1pps generated signal) and frequency corrections by changing the control voltage on the rubidium. I've been trying to figure out exactly how it works, but can only guess at some things. It measures an average phase error over a 5 minute interval right now, and clearly it is using a 10Mhz clocked device to do this. It usually bounces between a phase error of +40nS or -60nS and it can only make time corrections in 100nS increments. My theory is that if the average phase error is large enough that it can adjust for it then it will do so, for example an average phase error of 150nS would mean that it could adjust by 100nS leaving only 50nS. It tries to adjust the 1pps (time correction) every 15 minutes. It also calculates the delta between these average phase errors and I watched them for an hour and you will see things like +6nS delta followed by a -6nS delta and adding all the delta's together for the hour I got 0. I would think it might actually use these delta. I would think looking at this delta for a long period of time might be helpful for it to determine a frequency correction, but perhaps that isn't how it works. Maybe it looks at the adjustment it made (100nS) and the time between adjustments, say 10 hours to determine the frequency adjustment. I can't think that would be as accurate because it makes time adjustments on a 15 minute schedule. You can also tweak the time correction schedule and frequency correction schedule so I'm not sure they are relatable to each other or not. I just know that my unit has been running for >300000 seconds and still hasn't made a 100ns adjustment. It has displayed no time adjustments and three frequency adjustments each for "0.0000e0". This got me to wondering - how do other GPSDO units work? Do they take days and weeks to build up enough data to discipline themselves or are there faster methods? Thanks, Alan _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
