[time-nuts] Temperature controlled TCVCXO

[Bob Camp]

Hi

If the crystal has a “most stable point” it will be a point that the slope goes
from positive to negative (or vice versa). These points are symmetrical about 
the “center” of the crystal curve. For an AT cut, your center will be just a bit above
25C. How far above depends on the geometry of the blank and a few other things.
For simplicity you see 25 used a lot.

If you look at the curves on the link posted earlier they follow a pattern. Anything 
below “zero angle” never goes through a slope change The zero angle goes flat
at the center. The “useful” part of the curves have a turn at 25 +/- X where X can 
be anything from 5 to 100C in normal crystal. 

You need to dig into the actual math to take a look at the slopes near turn. Obviously
the turn at 50C is a better bet for your OCXO than the one at 125C.

The enemy of any temperature stabilization system is waste heat. On an OCXO
with a normal heater, there is some (small) current in the circuit even when the heater
is turned off. With a TEC, there is a *lot* of waste heat when cooling. The systems
I have seen using them get into stacked devices and water cooling pretty quickly. 

Bob

> On May 13, 2016, at 11:26 PM, David <davidwhess at gmail.com> wrote:
> 
> Just to be clear here, my original question referred to using a TEC to
> maintain a constant temperature where the TCVCXO with the crystal has
> the best stability.  I assume this would be around 25C but testing
> might reveal that it is actually more stable at a point slightly
> higher or lower.
> 
> So the TCXO would be operating at or around a fixed point near 25C
> whether the ambient temperature is higher or lower.  Bidirectional
> control of the TEC with a servo amplifier is not difficult and that
> second HP link is pretty close to what I had in mind.
> 
> On Fri, 13 May 2016 16:30:20 -0700, you wrote:
> 
>> Hi Nathan,
>> 
>> Let me state what Bob is driving at in a different way.
>> 
>> Your biggest problem with a TCXO is the crystal.  It is cut so that the 
>> slope, relative to temperature, has the shallowest curve around room 
>> temperature (between 20 to 25 degrees C).  If you cool or heat it from 
>> that range the slope becomes more radical.  The more radical the slope 
>> the greater the change in frequency for a given change in temperature.  
>> The exact opposite of what is trying to be achieved.
>> 
>> This translates into having to have an extremely tightly controlled oven 
>> system in an attempt to overcome the radical slope.  In addition you 
>> will also have to consider the elevated temperature effects on the other 
>> components.  It is a never ending battle.
>> 
>> Bill....WB6BNQ
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