On Sat, 17 Feb 2007 08:50:56 -0600, you wrote: >Angus wrote: >> On Fri, 16 Feb 2007 17:44:14 -0600, you wrote: >> >> >>> Richard W. Solomon wrote: >>> >>>> I looked at the ACE-III on Trimble and do not see mention of a 10 KHz >>>> output. >>>> Maybe I miised it, but without it, the N1JEZ board will not do unless you >>>> modify >>>> it for 1 pps. >>>> I am trying to find more info, but since it reached EOL, Trimble may have >>>> deleted >>>> it. >>>> >>>> 73, Dick, W1KSZ >>>> >>> Dick, >>> >>> You are correct, you need to patch another set of dividers in the chain. >>> Keep in mind that the only benefit of a 10 kHz output on the Jupiter is >>> to save 4 decade dividers. You should not use the fact that the Jupiter >>> outputs 10 kHz to speed up the loop in a GPSDO. The loop bandwidth is a >>> function of the crossover point between the VCXO and the GPS, which in >>> most cases will be between many minutes (cheap VCXO) and a few hours (HP >>> 10811). Whether you use 1 Hz or 10 kHz comparison frequency into the >>> phase detector will not affect the filter. >>> >> >> Hi, >> >> (As this is my first post here, I'll try not to mess up too badly :) >> >> I think that for a simple controller like the N1JEZ board, a 10KHz >> output on the GPS really is needed. >> I tried a circuit like that with 100pps once out of curiosity, but as >> I remember, the lower error sensitivity made things more difficult. >> >> BTW, with a standard GPSDO, would using the 100Hz/10KHz the pulse not >> reduce the need for a fast comparator clock, and also reduce sawtooth >> effects? >> >> Angus. >> >Angus, > >One of the problems with the 10 kHz output us that it is only updated >once per second, i.e. the period of the 10 kHz signal is the same for 1 >second, and every second, there is a small phase jump to update the >phase of the 10 kHz to match the 1 PPS.
I don't know whether the outputs on a Jupiter exhibit any granularity or not (I've not seen anything to suggest that they do), but I think that with receivers that do, changing from 1PPS to a faster PPS output would normally change the granularity effects that are seen - assuming things are not in sync. As for possibly reducing the need for a fast clock; being able to measure a number of pulses in a second and then take the average count can allow a rather better resolution than just taking one measurement - so in some circumstances this could be useful. I've not really looked into it much, but was really just interrested if anyone here had come accross the 100/1K/10K, etc pulses being used like that. >That means the 10 kHz has a one Hz component that is the correction >signal. It must be filtered out. But even filtering the 1 Hz component >is not sufficient, since the 1 Hz component has the same short term >noise that is present on the 1 PPS output of any GPS receiver. > >So while it's a lot easier to filter 10kHz down to a level where 10 kHz >ripple does not appear on the OCXO EFC input, such a filter would be >grossly insufficient to make a GPSDO. You would simply have an OCXO >phase locked to a poor reference. Although that's not what I was talking about doing above, I think that it's pretty much what some of the hardware GPSDO's actually do using various types of oscillators (and to good effect too). As in these: http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm http://www.frars.org.uk/cgi-bin/render.pl?parameter=&pageid=1285 the loop filter is basically just an RC circuit. It's not quite raw GPS, but is quite different to using a digital filter as used in a Thunderbolt, Shera, etc., which can run to hours. When the N1JEZ board was mentioned, I assumed that it was this type of controller that was meant - and I think it does need 10KHz unless it gets much more of a mod than a couple of counters. Angus. >You must filter the output signal from >the phase comparator so that the short term instabilities in the GPS >timing signal are also filtered out. > >The 10 kHz is useful for experimentation, since you could artificially >speed up the loop with one second or so time constant or even less so >that you could easily verify with an oscilloscope that the OCXO is >actually phase locked to the GPS, but you would not want to run the >system that way because it would have terrible performance. > >With a time constant of 20 minutes to an hour, it can be very >frustrating to verify that the system phase locks and it would be best >to have another hobby to attend to in the mean time :-) > >Regarding the sawtooth effect, since the 10 kHz is only updated every >second, it has no effect on the possibility of hanging bridges, and I am >not sure what you mean by reducing the need for a fast comparator clock. > >Didier >_______________________________________________ >time-nuts mailing list >time-nuts@febo.com >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
