[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit
Jack Hudler
jack at hudler.org
Thu Dec 14 04:52:00 UTC 2006
Bruce,
Can you describe further your idea about phase detection using an ADC.
Who produces the sinewave from the filtered counter?
(Thinking out loud) Using a 10MHz oscillator as an example:
Is this dividing the clock down to (say) 1 MHz and using a square->sine
conversion then sampling the phase angle at the 1PPS transition? The accuracy of
square->sine conversion appears as another source of error.
Perhaps the MAX9382 (http://www.maxim-ic.com/appnotes.cfm/appnote_number/1130)
could help with the problem of locking to the undesired harmonic.
Jack
-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf
Of Dr Bruce Griffiths
Sent: Wednesday, December 13, 2006 7:52 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit
The Brooks Shera circuit relies on the 24 MHz oscillator not being
locked to the OCXO or the PPS signal so that (in the absence of
injection locking) averaging the measured time interval gives an
unbiased estimate of the true value. Using a higher speed oscillator to
measure the time interval would be advantageous with the more accurate
GPS timing receivers currently available. If you have an M12+ or MTM
timing receiver and a rubidium oscillator the Brooks Shera technique is
not optimum you can easily achieve a performance that is 1-2 orders of
magnitude better with a less complex phase detector. The Brooks Shera
circuit has a single shot resolution of 41.6 nanosec which may be up to
10 times worse than the jitter in the PPS output of a good timing
receiver (after correcting for any sawtooth error - in software of
course, to avoid any additional noise and errors caused by hardware
correction).
The phase detector method employed is the digital equivalent of a
classical sampling phase detector with a linear phase detection
characteristic. A phase lock loop employing a sampling phase detector
will lock to a harmonic of the sampling frequency, which in this case is
1Hz. It is therefore necessary to divide down the OCXO (or other high
stability oscillator) frequency to a value such that the efc tuning
range of the oscillator precludes locking to the undesired harmonic.
Unfortunately a phase detector with a linear characteristic can
sometimes allow the phase lock loop to lock to a frequency which is not
a harmonic of the sampling frequency (in this case frequencies like
17/13, 12/11 Hz etc.).
A phase detector with a sinusoidal characteristic (such as an ADC
sampling a sinewave produced by filtering the output of a counter
clocked by the OCXO) avoids the problem of non harmonic locking and
eliminates most of the digital circuitry.
A loop time constant of around 120 seconds is far too short for
obtaining the optimum performance from a good rubidium standard, as the
loop will degrade the rubidium standard to an accuracy of around 35E-10
(with a 41.6ns single shot measurement resolution) when a good rubidium
standard is capable of a considerably higher stability than this.
Bruce
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