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

Magnus Danielson magnus at rubidium.dyndns.org
Sat Apr 5 09:07:31 EDT 2008

From: "Richard H McCorkle" <mccorkle at ptialaska.net>
Subject: [time-nuts] Disciplining dual oscillators using a 3-corner hat
Date: Fri, 4 Apr 2008 20:07:23 -0800 (AKDT)
Message-ID: <24158. at mymail.acsalaska.net>

> 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)

I am going to be very picky now, but I think you want to express it as a
difference. Each phase comparator/TIC is registrating the difference in phase.
The sum of the GPS phase and oscillator phase would result in a phase ramp
having the sum frequency of the two sources. You are looking for the difference
in phase such that you can steer the oscillator phase to roughfly be aligned to
the GPS phase.

For a control-loop analysis, remember that the reference signal has positive
sign and the controlled oscillator is being controlled using negative feedback,
so it has negative sign, i.e. Ref - Osc.


Controller A raw phase data = Gp - Ap
Controller B raw phase data = Gp - Bp

> Difference in readings = (Gp + Ap) � (Gp + Bp) = (Ap � Bp)

Here a sign-reversal is needed according to above change.

You could say that you do a common-view phase-comparision. However, this
assumes that you consider the high and low frequency phase deviation on either
A or B comparision arms to be of no concern. However, for propper three-corner
hat you need to separately measure the A-B relation in which case the you can
separate these things out a bit more. With a good steering, the A-B measure
will contain mostly noise and the difference in residual control error.

However, it is the noise aspects which do interest you.

I think you are using a measure too little for it to properly cancel things

>    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.

My main concern is that you are not doing a real three-cornered hat to begin
with. Also, the three corner hat methodology fails to properly analyze near and
below the PLL bandwidth controling one or two of the oscillators from the
reference. You need to include the full PLL loop for proper analysis.

Another flaw is that I think more concern should be taken to the fact that the
GPS noise sources includes a two local noise sources, the GPS TCXO and the
phase restamping of that TCXO. If you where probing the TCXO and using that,
you could gain advance time information of PPS moves, if you have a more stable
oscillator to measure against. The MTI260 should fit that spot well. Naturally,
the gain may be insignificant when using sawtooth corrections anyway.


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