[time-nuts] GPS W/10KHz

Dennis Ferguson dennis.c.ferguson at gmail.com
Sun Feb 9 19:59:16 EST 2014


On 8 Feb, 2014, at 14:50 , EWKehren at aol.com wrote:
> The problem with the PLL analog version is the same as with any digital  
> GPSDO. The saw tooth is present at 10 KHz just like 1 Hz. To the best of my  
> knowledge there is no GPS receivers out there for less than $ 1000 with out 
> saw  tooth. Timing receivers output the correction value and you can either 
> with  software or a variable delay do correction.

This is very true, though the sawtooth at a 10 kpps sample rate is going
to a little different than the sawtooth at a 1 pps sample rate.  The frequency
of the sawtooth noise will lie somewhere in the Nyquist bandwidth.  At a 1 pps
sample rate the frequency of the sawtooth noise will hence be somewhere between
0 Hz and 0.5 Hz, while at 10 kpps the sawtooth frequency will range from 0 Hz
to 5 kHz.

Noise at less than 0.5 Hz is not easy to filter, so you are going to require
the correction from the receiver and/or an integrator with a time constant
that can only be realized digitally.  Sawtooth noise over most of a 0 Hz to
5 kHz range, on the other hand, should be eliminated by the analog low pass
filter after the phase detector in the PLL, giving you something nice and clean
coming out.  It is only if you get unlucky and the beat frequency between GPS
time and the receiver's oscillator ends up very close to an integer multiple of
10 kHz that you'll see noise at a low enough frequency to leak through into the
control response.

This is interesting because it suggests that very simple GPSDOs using 10 kHz
from the receiver might at times work worse than you are likely to observe in
a single bench measurement as aging (or something) moves the receiver's oscillator
frequency through one of the "bad" frequency errors.  Or is there a way to avoid
that altogether (maybe if the receiver does dithering)?

Dennis Ferguson


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