[time-nuts] Picket fence technique

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Dec 23 05:07:43 UTC 2006


Using a GPS timing receiver to quantify the long term stability of an 
oscillator whose frequency is not a harmonic of 1Hz, then the technique 
of dividing the oscillator frequency down to 1Hz and logging the time 
delay between the GPS derived PPS pulse and the leading edge of the 
divided down reference frequency will incur several phase wraps during 
the monitoring period. Worse than this if one is using a TIC with a 
finite dead time (eg HP5370A/B) between measurements some the TIC will 
not measure some of these intervals.

These difficulties can be circumvented by using a picket fence technique 
as devised by Greenhall to measuring beat frequencies in the paper:

A Method for Using a Time Interval Counter
to Measure Frequency Stability
C. A. Greenhall
Communications Systems Research Section
This article shows how a commercial time interval counter can be used to 
measure the
relative stability of two signals that are offset in frequency and mixed 
down to a beat
note of about i Hz. To avoid the dead-time problem, the counter is set 
up to read the
time interval between each beat note upcrossing and the next pulse of a 
10 Hz reference
pulse train. The actual upcrossing times are recovered by a simple 
algorithm whose outputs
can be used for computing residuals and Allan variance. A noise-floor 
test yielded a
df/f Allan deviation of 1.3 X 10 -9/r relative to the beat frequency.

When quantifying the long term stability of an oscillator using this 
method one connects the PPS signal to the TIC START input and the picket 
fence frequency source (produced by dividing down the reference) to the 
STOP input. If the GPS pulse sawtooth correction and pulse epoch are 
recorded the sequence of time intervals measured by the TIC can be 
unwrapped using the algorithm outlined in the above paper.

With a relatively low drift oscillator one need only divide its output 
down to 1MHz or so.
Thus a simple inexpensive single chip divider such as 74HC4017 Johnson 
counter may be used for oscillator frequencies of up to 20MHz or so.

The Question is has anyone considered using this technique?

Bruce




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