[time-nuts] Re: GPS Control Loop

[Bob kb8tq]

Hi

> On Apr 27, 2022, at 1:09 PM, ASSI <Stromeko at nexgo.de> wrote:
> 
> 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.

As noted in a number of papers, the actual minimum temperature sensitivity
point on an OCXO will not be exactly at the turnover temperature. That will
be the crystal’s “best” but there are a number of other items in the OCXO that
get into the mix as the crystal sensitivity goes to zero. Modern OCXO production
uses temperature test data to set the OCXO to the “best” point. 

Bob

> 
> 
> Regarts,
> Achim.
> -- 
> +<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+
> 
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> 
> 
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