Richard H McCorkle wrote: > Hello Time-Nuts, > I am currently disciplining two MTI260 oscillators in a > dual standard to a common GPS timing receiver 1PPS with > two highly modified Shera style controllers that use a > 100 MHz TIC with sawtooth correction and a 23-bit DAC. > Phase samples are accumulated over identical 30-second > periods between updates and the updates are logged over > identical sample intervals from both controllers using > a common receiver. When the phase data from the two > controllers are compared there is a striking similarity > in the short-term phase variations in both data sets > when both oscillators are locked. > Extreme care was taken to minimize coupling between > the oscillators by using separate power supplies, > physical separation, and shielding of the two systems > and their associated wiring. Intentionally varying the > frequency in either of the oscillators has no visible > effect in the phase data from the other oscillator so > I don’t believe injection locking is occurring between > the oscillators. > The MTI260 has very good short-term stability so I am > assuming the short-term phase variations of nanoseconds > per update seen in both data sets are predominantly the > result of changes in the GPS 1PPS timing. I am wondering > if anyone on the list has explored the concept of using > the common phase variations from multiple disciplined > high-stability oscillators driven from a common GPS > receiver to determine the actual GPS variation (using a > 3-corner hat analysis) and apply that information in the > disciplining routines to improve oscillator short-term > stability. > I am considering a methodology of doing comparisons of > A to GPS in controller A, B to GPS in controller B, and > then having the two controllers share their phase data > and do a comparison in each controller to determine the > common GPS variation and correct the raw phase data before > calculating the EFC. Each controller outputs the combined > phase effects of the GPS and its oscillator and by sharing > the phase data between two controllers fed by a common > receiver I believe the GPS variations in the raw phase > data could be eliminated using simple PIC math as shown > in the following equations using Gp as the GPS phase, Ap > as the A oscillator phase, and Bp as the B oscillator phase. > > Controller A raw phase data = (Gp + Ap) > Controller B raw phase data = (Gp + Bp) > Difference in readings = (Gp + Ap) – (Gp + Bp) = (Ap – Bp) > A reading – difference = (Gp + Ap) – (Ap – Bp) = (Gp – Bp) > B GPS difference = (Gp + Bp) + (Gp – Bp) = (Gp * 2) > GPS phase data = (Gp * 2) / 2 = Gp > Controller A corrected phase data = (Gp + Ap) – Gp = Ap > Controller B corrected phase data = (Gp + Bp) – Gp = Bp > > One concern I have is a 3-corner hat is generally > performed on three sources of similar stability. In > this case the short-term stability of the two MTI260 > oscillators will be much better than the GPS short-term > stability and I am questioning how valid the data will be. > I would appreciate any comments on the concept, flaws in > the methodology, or pitfalls that might result during > implementation before I attempt this in a working system. > > Thanks for your input, > > Richard > Richard
1) The above is not a 3 cornered hat comparison. You have to compare oscillators A and B directly with each other as well. The 3 cornered hat technique assumes that the phases of all 3 oscillator are statistically independent. This assumption almost inevitably fails for sufficiently long Tau. (In your case when Tau is a significant fraction of the averaging time.) If the oscillators/frequency sources are statistically independent then one can determine the individual instabilities from measurements of the 3 phase differences, however one cannot determine the individual phase errors for each oscillator. If all one source has significantly greater instability than the other 2 then the accuracy in determining the instability of the other 2 suffers. 2) Your maths is incorrect, The 4th line should be: A reading – difference = (Gp + Ap) – (Ap – Bp) = (Gp + Bp) Thus as one should expect you technique doesnt work, with only 2 measurements you cannot determine 3 quantities. The only way you are going to significantly improve the performance of your GPSDOs is to use GPS carrier phase measurements, the Rockwell/Connexant/Navman Jupiter receivers have the GPS carrier phase data available. However to be useful the receiver local oscillator has to be phase locked to the OCXOs being disciplined. 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.
