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 dont 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 _______________________________________________ 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.
