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

Didier Juges didier at cox.net
Wed Dec 20 15:13:29 EST 2006


In the matter of lifetime (outside of MTBF issues), is it correct that 
Rb has a built-in life limiting mechanism (the lamp wears out), where Qz 
does not? If so, Rb oscillators will eventually fail but one might hope 
a Qz oscillator may not.

Didier

Tom Van Baak wrote:
>> 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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