[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
> 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).
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
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
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.
More information about the time-nuts