[time-nuts] GPS: ADEV or MDEV?

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Apr 21 21:59:18 EDT 2007


Tom Van Baak wrote:
> Bruce, I'm getting the idea you don't like _any_ of the
> hobbyists GPSDO's that have come out in the last ten
> years...
>
> So I'm curious what then would qualify as a well designed
> GPSDO in your opinion? (and please don't bring up the
> Quartzlock thing; it's a hundred times more expensive than
> the GPSDO's we're talking about here).
>
> /tvb
>   

Tom

If one insists on using a GPS timing receiver rather than using carrier 
phase disciplining techniques, then anything that:

1) Has a one shot phase of sufficient resolution (and accuracy) (1ns or 
preferably a little better for an M12M ) so that performance is limited 
by that of the GPS receiver rather than the phase detector.

2) Corrects for sawtooth error (when correction data is made available 
by the GPS receiver) in software (hardware correction also acceptable 
but probably unnecessarily expensive).

3) Uses statistical filtering to eliminate phase error outliers.

4) Has adjustable PLL parameters so that the disciplining algorithm can 
be tuned to obtain the best performance from a particular receiver and 
the OCXO or other source being disciplined. A third order loop or 
equivalent  is perhaps desirable to correct for linear frequency drift.

5) Uses a sufficiently high resolution monotonic low noise DAC (or 
equivalent DDS system) that the OCXO short term stability limits the 
performance.

6) Doesn't rely on the relative phase of an independent oscillator being 
random with respect to the PPS signal or the OCXO signal. This is rarely 
satisfactory unless one makes heroic efforts to isolate the oscillator 
from the PPS and the OCXO. Injection locking of the "independent" 
oscillator is almost inevitable unless there is sufficient isolation.

7) Allows the GPS receiver data, PLL loop parameters and phase error 
statistics etc to be monitored/logged.

8) Indicates when the GPS receiver data is too noisy/unavailable for 
disciplining so that the OCXO is operating in holdover mode.

9) Uses synchronisers where needed to effectively eliminate 
metastability as a significant concern.

None of these requirements is particularly difficult or expensive to 
implement.

The most difficult being the high resolution phase detector, 
particularly if one has stringent holdover requirements.
Constructing a phase detector with a range of say 1us and a  sub 
nanosecond resolution is relatively easy and inexpensive , extending the 
range to greater than 1us is somewhat more difficult/expensive unless 
one uses a 1GHz clock.

The high resolution DAC is also a bit of a challenge in that high 
resolution audio DACs have relatively poor tempcos which means that 
their temperature needs to constant to within a fraction of a degree for 
a a time interval of around the loop response time to avoid degrading 
the short term stability. It may be better to use a lower tempco higher 
resolution monotonic (e.g. string DAC) DAC and just dither its output. 
If a suitable low pass analog filter is used between the DAC and the 
OCXO frequency control input then the residual dither amplitude at the 
filter output can be extremely small.

Optional but desirable characteristics (particularly when disciplining 
"well aged" OCXOs):

10) Allow disciplining of an OCXO whose frequency can no longer be 
adjusted to nominal.

11) Include an offset DDS system so that a output at the nominal 
frequency is available.


Bruce




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