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

Tom Van Baak tvb at leapsecond.com
Wed Dec 20 20:00:01 UTC 2006


> Hello folks,
>
> i like to play the bad boy again: My claim is 
>
> a) that for most of us a GPSD Rb is of little to no
> use compared to a good GPSD xtal oscillator

Ulrich,

That's a rather general statement, but I understand what
you mean. Consider, instead of a bold general assertion
which can be disproved with a single counter-example,
a simple list of advantages of quartz vs. rubidium. Then
let the user decide which is appropriate for their unique
application.

There's more to a GPSDO than its ADEV. For example,

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.

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

Physical size --
    I would guess most Qz GPSDO are smaller than Rb
    GPSDO. One could argue that the Casio GPS wrist
    watches are a type of miniature GPSDO!

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

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

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

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

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

Tuning range --
    Could it be that Rb will run for many more years without
    coarse adjust than Qz?

Lock indicator --
    Most Rb give you a signal when frequency lock has
    been achieved.

Mechanical sensitivity --
    Rb GPSDO are much less sensitive to movement and
    shock than Qz. If you don't believe me watch what
    happens if you simply take your GPSDO and bump it,
    or turn it over on your desk. This is especially important
    during hold-over when no external correction is present.

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

Surplus --
    There are so many cheap telecom Rb out there it is
    near irresistible to make a GPSDO out of them.

You get the idea; feel free to add the ones I missed.

/tvb






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