[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit
phk at phk.freebsd.dk
Wed Dec 20 15:32:18 EST 2006
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
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.
> 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".
> 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.
> 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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