[time-nuts] Temperature controlled TCVCXO

[Bob Camp]

Hi

Different compensation approaches call for different crystal angles. Most (but not all)
TCXO’s use AT cut crystals. What varies is the angle of cut and thus the turnover temperatures. 

If you are buying in sufficient volume, the price of a simple OCXO is well below $10. You 
need to be in the “thousands per week” range, but they are out there.

Bob

> On May 13, 2016, at 8:46 PM, David <davidwhess at gmail.com> wrote:
> 
> Thanks for this.  It is exactly the kind of response I was looking
> for.  Based on what you say, it seems like a $10 eBay OCXO will always
> beat an temperature controlled TCXO. 
> 
> The varactor tuning range is going to be a problem for which the only
> solution is a discrete crystal oscillator.  I wonder if there are any
> non-custom sources for suitable crystals in a design like this.
> 
> I assumed that they used AT cut crystals with a temperature curve of
> the same form if not the same uncompensated temperature range.  Of
> course the temperature compensation circuit should completely dominate
> that anyway.
> 
> I was hoping for something with a better risk for cost even if not as
> good as the typical Ebay used OCXO of questionable heritage.
> 
> On Fri, 13 May 2016 17:57:58 -0400, you wrote:
> 
>> Ok, with a TCXO you have a temperature sensor that tries to servo the crystal on to frequency. 
>> You also have a crystal with a temperature dependance. As you try to heat / cool the TCXO
>> your thermal variation hits one before it hits the other. The net effect is that the ADEV is actually
>> worse with a TCXO than with an un-compensated crystal. Consider that a good oven has variation 
>> in temperature on the order of micro C over a few seconds to tens of seconds. That’s what you 
>> are trying to “emulate”. 
>> 
>> The next issue is that the TCXO has a loop design oriented towards wide tune range. Things like 
>> varicap diodes have a much bigger impact on a TCXO loop than they do on a narrow tune range
>> OCXO loop. That (and possibly a coil or two) are yet another source of thermally induced variation 
>> in the oscillator. Just like the temperature sensor, there is a delay in the temperature hitting them 
>> compared to other parts of the oscillator. Again, you get a degradation in ADEV over the bare
>> TCXO crystal. 
>> 
>> Next up is the crystal it’s self. A TCXO crystal likely has a turn temperature that is optimized to 
>> match the compensation approach being used. In some cases there will not actually be a turn
>> at all. Thus there is no temperature “zero slope” point to hit with your temperature controller. This
>> means that rather than you now need much better temperature variation control on your system. 
>> 
>> Bottom line: A $10 eBay OCXO is likely to beat an ovenized or cooled TCXO. 
>> 
>> Bob
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