Hi The first hit on Google for AT cut curves is:
http://www.aextal.com/tutorial-frequency-stability.htm One common way to make a TCXO is to start with a minimum error cut over the intended temperature range. If that is -30 to +70C, you do get turns in the curve. There are several other ways to do it. The gotcha is that you have no way of knowing *which* way your TCXO was done. Bob > On May 13, 2016, at 7:30 PM, wb6bnq <wb6...@cox.net> 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 > > > Bob Camp wrote: > >> 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 <jdo...@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 <kb...@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 <davidwh...@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. >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@febo.com >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
