[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit

[Poul-Henning Kamp]

In message <003501c72471$6fe791e0$3b8c8843 at computer>, "Tom Van Baak" writes:

>Warm-up time --
>    Many Rb will lock in 5 minutes, typically. Some Qz
>    take much longer to get on-frequency from cold start.
>    This can simplify the initial loop locking algorithm.

Initial capture is best done with a looser timeconstant in any
circumstances.

Note that the integrator terms initial condition is undefined in a
PLL, this can be used to achieve lock without the initial overshot:

Clamp the integrator term to zero and let the proportional term
drag the offset into range (use a high rate for this, there is
no stability issues).

Once the second derivative of the offset approaches zero, unclamp
the integrator and switch to a normal but loose set of constants
for the PLL.

With properly chosen values, you can drag any frequency source
into submission of a PPS that way in a fraction of a minute.

After this the PLL can adapt its constants based on the statistics
(remember what I said earlier about looking at the ADEV shape).

>Power consumption --
>    Probably Qz-based GPSDO have much lower power
>    consumption than Rb.

Single oven: Maybe, double oven: certainly.

>Hold-over performance --
>    For mid- to long-term, Rb is vastly superior to Qz;
>    most Rb have daily drift rates 100x better than Qz.

I is likely to be the difference between replacing the GPS antenna
now or after the snowstorm is over.  Given the price difference,
this may be a no-brainer advantage to the Rb.

>Stand-along performance --
>    Without GPS lock, a free-running Rb can be trusted
>    to be orders of magnitude more accurate than Qz.

Also, if the qz in the rb jumps, the Rb is very likely
to tell you it lost lock.  A Qz unit will jump and you
will not know it, unless the resulting phase jitter
kills your microprocessor or similar.

>Environmental --
>    Is it the case that Rb is less sensitive than Qz to
>    extreme environments?

No significant difference with proper design.  The Rb's
cooling requirements are tricker to design for than an
Qz units "bolt down and forget".

>Cost --
>    As a rule, Qz-based GPSDO are cheaper than Rb.

That's actually not a given.  For a decent Qz performance
new price approaches $1k and a PRS10 is only $1.5k.

>Phase noise --
>    I'd guess that Qz-based GPSDO could have better
>    short-term stability and phase noise than Rb.

Depends on your PLL more than anything else.

>Lifetime --
>    Is the MTBF of Qz much longer than Rb due to fewer
>    parts and simpler design?

Yes, no chemical stress and with proper drive levels,
no mechanical stress either.

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