Ah, OK. So you keep the antenna connected and you keep the GPS receiver in position hold mode, still receiving fixes.
All you're doing is disabling the software disciplining algorithm. That sounds like a good test. Let me try this too and see if I agree with your conclusions. I'll try to simultaneously measure the 1PPS or 10 MHz outputs as you suggest. Has anyone hacked a TBolt yet to find which internal pin has a raw 1PPS from the GPS engine (as opposed to the 1PPS divided down from the OCXO)? /tvb ----- Original Message ----- From: "Bruce Griffiths" <[email protected]> To: "Tom Van Baak" <[email protected]>; "Discussion of precise time and frequency measurement" <[email protected]> Sent: Tuesday, January 20, 2009 9:04 PM Subject: Re: [time-nuts] Setting loop TC using thunderbolt internal data. > Tom > > I turned of the OCXO discipling using Thunderbolt monitor menu: > > Control:Disable Discipling > > > Then I used the Window: Logging Dialog box to enable logging of > > 1) Time of week (sec) > 2) PPS offset (ns) > 3) 10MHz offset (ppb) > 4) DAC voltage > 5) Temperature > > to a data file for about 40K sec. > > > The Thunderbolt keeps the DAC voltage constant and continues to measure > the oscillator offset and PPS offset even whilst the OCXO disciplining > is disabled. > > The measurement noise on the LHS is that of the Thunderbolt receiver etc > itself. > The RHS is essentially OCXO drift whilst discipling is turned off. > > The thunderbolt has its own internal TIC or equivalent thereto. > > To increase the certainty that the receiver is behaving as I believe it > is when configured like this, it would be nice to simultaneously measure > the ADEV of the undisciplined OCXO output by other means. > > Unfortunately I am not at present able to do this even though I do have > another GPSDO availble. > > Bruce > > Tom Van Baak wrote: >> Hi Bruce, >> >> Couple of questions. To me the plot looks like it is dominated >> with measurement noise (negative slope on the left) and OCXO >> frequency drift (positive slope on the right). >> >> You mention "GPS receiver noise" but I don't see that in the >> plot. Is this a free-running TBolt? If so, what 1pps source are >> you comparing it to? >> >> The title of the plot says, "Undisciplined Thunderbolt OCXO vs. >> Thunderbolt GPS receiver"; so is this maybe a plot between >> two different Thunderbolts; one locked, the other unlocked? >> What TIC was used to collect the data? >> >> /tvb >> >> ----- Original Message ----- >> From: "Bruce Griffiths" <[email protected]> >> To: "Tom Van Baak" <[email protected]>; "Discussion of precise time and >> frequency measurement" <[email protected]> >> Sent: Tuesday, January 20, 2009 6:04 PM >> Subject: Setting loop TC using thunderbolt internal data. >> >> >> >>> If one turns of the disciplining of the Thunderbolt's internal OCXO, >>> then one can log various parameters such as the 1 pps error. >>> A plot of the resulting OADEV (all tau) using such data is attached. >>> >>> For short tau the GPS receiver noise dominates. >>> For large tau the OCXO noise dominates. >>> >>> If the OADEV plot is symmetric about the minimum then the value of tau >>> at the Allan intercept coincides with the value of tau at the minimum in >>> the OADEV plot. >>> In this case the plot has greater slope after the minimum than before so >>> the value of Tau at the Allan intercept is about 500 sec rather than the >>> 400sec corresponding to the minimum in the OADEV plot. >>> The optimum loop time constant for this particular Thunderbolt is thus >>> around 500sec. >>> >>> In the absence of any other means of measuring the performance of the >>> Thunderbolt as a function of the loop time constant, this technique >>> allows the loop time constant to be set near the optimum for each >>> particular Thunderbolt. >>> >>> 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. >> >> > > _______________________________________________ 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.
