[time-nuts] Brooks Shera's GPS locking circuit faults and fixes

[Dr Bruce Griffiths]

The phase detector itself in Brooks Shera's GPS locking circuit has 
several problems:

1) There are no synchronisers so that the partial width pulse response 
of the counters biases the output.

2) Possible synchronism between the (24MHz) phase measurement clock and 
the time interval being measured will bias the result.

The cures are
1) use synchronisers to ensure that only full width pulses are counted.

2) Randomly phase modulate the (24MHz) clock to destroy any possible 
coherence.

The necessary phase modulation amplitude can be found in

/Time Interval Averaging: Theory, Problems, and Solutions/, David Chu, 
HP Journal June 1974 pp12-15.

Of course one still has to cure the hanging bridges and slow ramp timing 
errors produced by the GPS receiver.
Of course these could be cured by random phase modulation of the GPS 
receiver PPS output pulse timing clock.
Phase modulating the GPS receiver timing clock may also be sufficient to 
eliminate the synchronous bias in the Brooks Shera phase detector, 
however synchronisers are still required. Random phase modulation of the 
receiver local oscillator frequencies may be somewhat counter-productive.

Bruce