On Mon, Jan 30, 2012 at 5:16 AM, Bob Camp <[email protected]> wrote: > Hi > > The GPS ops signal (or 10 KHz signal) you will be trying to track will be > moving by 10's or 100's of ns per second. 10 ns per second is 10 ppb. 10 ppb > at 10 GHz is 100 Hz. You need to smooth this out or your LO will be moving > all over the place.
Up at the top of this thread I said you need to have a long loop constant for exactly this reason. It does not matter of the GPS has a 1PPS 100PPS or 1000PPS the problem is GPS itself. GPS simply does not have good short term stability. In the short term the OCXO will be better. The reason we build GPSDOs is to combine the best of both, the short term characteristics of an OCXO and the long term characteristics of GPS. If you try and build a GPSDO using a very short loop filter time constant then you have a GPSDO with same characteristics of GPS averaged over your short time constant. In other words you select the time constant based of your frequency stability requirements. Look at the first blue line in the top graph. This is very much what a Jupiter GPS has. If you need 10E-10 you need a 100 second constant . The line is straight to for 10x better accuracy you need 10x longer time constant. I think using an analog PLL you can get to about the 1E-10 level. I think a non-expert could build a good 100 second RC filter. Doing better is going to require a longer time constant then is reasonable. a uP based PLL can do much better and likely with effort equal the performance of a Trimble Thuderbolt. But then you can simply buy a Thunderbolt for $150. I'm just starting work on a PLL that (I hope) can be built using just an Aduino as a controller and an 4046 chip (or the 74hct9046) as the phase detector. The Arduino costs more than a bare uP chip but I don't want to have to build a PCB. -- Chris Albertson Redondo Beach, California _______________________________________________ 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.
