[time-nuts] Temperature controlled TCVCXO

[Bob Camp]

Hi

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


> On May 13, 2016, at 9:52 AM, Nathan Johnson <jdownj at gmail.com> wrote:
> 
> Can you elaborate just a little bit on "poor"? Is that due to the "flat points" or desired temps always being elevated above most sane ambient temps? Or just that temp control via TEC isn't as precise?
> Nathan KK4REY
> 
> Sent using CloudMagic Email [https://cloudmagic.com/k/d/mailapp?ct=pi&cv=7.4.15&pv=9.1&source=email_footer_2]
> On Fri, May 13, 2016 at 06:37, Bob Camp <kb8tq at n1k.org> wrote:
> Hi
> 
> Yes, it can be done. The performance will be poor, but it will work.
> 
> Bob
> 
> > On May 13, 2016, at 4:15 AM, David <davidwhess at gmail.com> wrote:
> >
> > Has anybody tried using a thermoelectric cooler to maintain a TCVCXO
> > at a constant temperature in lieu of using an OCXO?
> >
> > I was thinking about this after reading some application notes in
> > connection with constant temperature control using TECs with bipolar
> > drive. The power required would be pretty low so I think a linear
> > control circuit could be used avoiding the noise problems associated
> > with a switching controller. Temperature sensor placement would be a
> > challenge.
> >
> > I suppose as part of an automated calibration, the temperature could
> > be adjusted over a few degrees from ambient to find any flat spots in
> > the oscillator's ppm/C curve.
> >
> > One place I have run across this application of TECs is with laser
> > diodes where coherence length depends on very tight temperature
> > control. Typical laser diodes without temperature control have a
> > coherence length of millimeters making them useless in more demanding
> > applications like interferometry.
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