[time-nuts] A new take on the all-hardware GPSDO concept

[Charles Steinmetz]

Bryan wrote:

>  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.

Best regards,

Charles