[time-nuts] Temperature controlled TCVCXO

Bob Camp kb8tq at n1k.org
Sat May 14 19:44:41 EDT 2016


Hi

Indeed, you would probably do better with a simple XO than with a TCXO. A proper 
OCXO is temperature tested and the gain and set point of the oven are adjusted to 
optimize the temperature performance. 

Bob
> On May 14, 2016, at 5:52 PM, David <davidwhess at gmail.com> wrote:
> 
> As you pointed out earlier, with a TCXO, any inflection points of the
> crystal itself will be unavailable because the temperature coefficient
> at any one point will be the product of the crystal and the
> temperature compensation circuitry.
> 
> I am left to wonder if this will work better with a TCXO or just an
> XO.  The later would allow for a better optimized voltage control but
> at that point, I will have just reinvented the ovenized crystal
> oscillator using AT cut crystals and operating at room temperature.
> 
> Do OCXOs use internal temperature compensation other than that
> provided by the oven itself?
> 
>> Bottom line: A $10 eBay OCXO is likely to beat an ovenized or cooled TCXO. 
> 
> Designing something which relies on the availability of often
> questionable Ebay items just bugs me.
> 
> On Sat, 14 May 2016 08:54:36 -0400, you wrote:
> 
>> 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
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