[time-nuts] Disciplining dual oscillators using a 3-corner hat
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sat Apr 5 00:47:45 EDT 2008
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
More information about the time-nuts
mailing list