[time-nuts] Disciplining dual oscillators using a 3-corner hat

[Richard H McCorkle]

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