Bill Janssen wrote: > WarrenS wrote: > >>> Bruce said: >>> The critical requirement is that the 2 standards being compared are >>> statistically independent. >>> Comparing a pair of Thunderbolts GPSDOs with similar time constants and >>> damping will give optimistic results for Tau comparable with or greater >>> than the loop time constant. >>> Its is even better is to use 3 or more similar standards simultaneously >>> logging phase differences between the various pairs (0.5*N(N-1) pairs for N >>> standards). >>> It is then possible to obtain estimates for ADEV, MDEV etc for each >>> standard. >>> >>> >> The optimistic results at and above the loop time constant, that results >> even when 3 or more units are used, >> is because the noise is then mostly due to the GPS signal itself and NOT the >> local oscillators in the GPSDO. >> In effect you are then using the same 1PPS signal into each unit, and any >> common noise on the >> GPS 1PPS signal will cancel and not be seen. >> So I think what Bruce is saying is that you can not (or is it should not?) >> use the GPS signal to >> measure the GPS's noise. >> But the point is, if you want to compare your GPSDO with different settings, >> or compare it to >> another OCXO, It can be done this way, if you do not have a better ref to >> use. >> You could then add the noise of the GPS nose above the control loop time to >> your >> optimistic results if you want true results at high Tau values. >> >> Also note that having the GPS noise cancle is not necessary a bad thing, It >> can be a good thing >> especially if the GPS noise is not what it is that you want to measure. >> >> >> >>> Like all digital phase detectors its best to avoid, if possible, the >>> nonlinearity inherent at the ends of the range. >>> >>> >> Using a phase detector near its end point (or at its crossover point if >> there is any deadband) >> is something that needs to be avoided. >> The two basic standard ways to insure that just the center of the phase >> detector's range is use: >> 1) Divide the signals down just enough before sending them to the phase >> detector so that >> the end points is not an issue. This works when both signals are from >> devices that are >> locked to a common signal such as the GPS. >> >> 2) When one of signals is from a non locked source such as a OCXO whose >> phase can drift >> any amount overtime, One of ways to limit phase detector issues, and use >> just the very accurate zero phase point, is to use the Phase detector's >> output to lock the OCXO in a fast control loop and then by knowing the gain >> of the EFC input, the filtered EFC voltage can be use as freq drift >> information to find the ADEV's. >> >> WarrenS >> >> *************: >> > What I am doing to ovoid the "end of range" problem is; > First I divide the signal by two to get a 50% duty cycle. Then when the > phase difference gets to > 10% or 90% of the full scale value I switch the phase detector (or > counter) to respond to > to the opposite zero crossing. I keep track of those switches in > software. I use a computer to control things and to keep a log of the > phase difference. > > Bill K7NOM > > > Bill
At this level of precision the waveform duty cycles are never precisely 50% so some correction for this also needs to be made. Bruce _______________________________________________ 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.
