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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
