[time-nuts] Temperature controlled TCVCXO -->> Random eBay oscillators.

[Bob Camp]

HI

There are a couple of forks in this one:

If it’s a commercial (or even a kit) product, doing it with surplus parts probably isn’t a great idea. The shock
is going to be doing a low volume run at commercial prices. There really isn’t any way around that. 

If it’s for a basement project there are a couple of alternatives. The best (in my mind) is to buy the oscillators
first and make sure they do what you need to do. This fits pretty well with the eBay “here today gone tomorrow”
sort of supply process. When the “good ones” are identified, buy a reasonable inventory and then design the 
gizmo.

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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