[time-nuts] Re: GPS Control Loop

[ASSI]

On Dienstag, 26. April 2022 23:20:34 CEST Lux, Jim wrote:
> That's the MCXO  - uses third harmonic and fundamental to measure the
> temperature.  Q-Tech sells them. Or, more properly, has them in their
> catalog and may be happy to quote a price and delivery. The datasheet
> revisions are >5 years ago, except for the Space version which was
> updated a couple years ago.  They're fairly good temp stability (a few
> ppb over  -40 to +90) and lower power than a OCXO of comparable
> performance.

Neat.  Well, they're certainly not on their stock list (nor the TCXO ultra-
miniature that would drop into a rasPi).

> The fancy temperature compensation doesn't say anything
> about aging, of course.

It seems that aging is a thing with this particular implementation, seeing 
that they can use a reference to tune it out.  But they also seem to have 
implemented a scheme that determines aging by taking the crystal out of the 
synthesis loop for a little while.  The data-sheet dooesn't really say if it's 
only active at start-up nor whether the continuous correction requires a 
reference.

> It's unclear whether cycling the temperature up
> and down at room temp makes an oscillator age faster than one held at a
> constant higher oven temp.

The control loop I implemented for my self-ovenized rasPi can do linear sweeps 
of the temperature (usually across the turnover point to determine or refine 
the control parameters, you can set both the slope and the span).  I have no 
hard data, but my impression is that both the sweeping itself and especially 
the ramp speed contribute to an accelerated drift that takes a few days to 
subside when returning to constant temperature.


Regarts,
Achim.
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