[time-nuts] A new take on the all-hardware GPSDO concept
csteinmetz at yandex.com
Mon Sep 12 19:57:28 EDT 2016
> From the Jupiter-T TU60-D120 datasheet
> Figure 1-3 (next page) shows the typical 1PPS performance of the Jupiter-T GPS receiver. The 10 kHz output is also available from the receiver and is phase coherent with the 1PPS signal. This output is made available for functions such as phase locking of crystal oscillators, frequency synthesisers, and similar applications.
Yes, but.... The devil is in the details.
The fact that one thing is phase-locked to another does not necessarily
mean it puts out a good, clean signal. At short time scales (tau less
than ~100 seconds), the PPS signal from any GPS receiver is noisy. At
tau = 1second, it is shockingly noisy (~5e-9), and it decreases by a
nominal factor of 10 per decade as the averaging time (tau) is made
longer. By tau = 1000 seconds, it is pretty respectable (~2e-12 if the
GPS rx designers did their job well).
So, the trick is to use the noisy source (GPS) to discipline the clean
source (OCXO) very gently and very slowly. That way, the OCXO remains
in control of the output at short tau (< 100 to 1000 seconds), while it
is kept on-frequency over the long term by the GPS. This requires a PLL
control loop with a very long time constant (equivalently, a very low
cutoff frequency, in the microHertz to milliHertz region). It is not
practical to build analog filters with time constants that long, so one
must design a digital filter (far from impossible, but not the sort of
thing most hobbyist GPSDO designers are willing to undertake).
If you use a control loop with a short time constant, then the quiet
OXCO just follows the noisy reference source and doesn't improve anything.
If a GPS rx puts out a phase-locked audio or RF frequency (10kHz or
10MHz in the examples we've been discussing), the question becomes
whether that output has better stability (lower jitter) over short
averaging times than the PPS. The usual way to do this would be to use
a clean local oscillator disciplined in a very slow loop -- the same
thing discussed above, only at 10kHz rather than 10MHz. To my
knowledge, the Jupiter receivers don't have internal OCXOs devoted to
this, so if the 10kHz output really does have better stability at short
tau than the PPS, it isn't exactly obvious how the designers did it.
Presumably, they would need a very high-Q resonator of *some* sort.
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