Dennis Ferguson wrote:
On 8 Feb, 2014, at 14:50 , [email protected] 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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Instead of speculating try reading the specifications.
1Hz phase modulation of the 10kHz output is present.
The receiver sawtooth error sample rate is 1Hz not 10kHz.
The 10kHz output signal phase is adjusted at a 1Hz rate by the receiver.
The only advantage of the 10KHz output is that it allows the phase error
of each 10KHz burst to be sampled up to 10,000 times a second.
This may be useful in improving estimates of this phase error due to
limited timing resolution but doesnt remove the 1Hz sawtooth phase error
that is present.
Bruce
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