[time-nuts] Another "atomic" clock question

Bob Camp lists at rtty.us
Sun Mar 2 17:16:27 EST 2014


On Mar 2, 2014, at 4:00 PM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> On 02/03/14 21:45, Bob Camp wrote:
>> Hi
>> The gotcha is that there are second order temperature effects. If you are going to run the crystal very far off turn, you need to keep it more stable than you might think.
> "hysteresis", "memory effect", "restart of frequency drift”

In addition to those there is actually a “temperature rate of change” dependent frequency change coefficient. It varies with the angle of cut and the temperature.

> Yeah, it puts a limit on how good a TCXO can track-and-compensate.
> I was also considering the use of XOs for temperature sensing, it has the benefit that it is relatively easy to sense with resolution, but after that frequency/phase measures is in, getting a good temperature reading isn't as easy.
> Is there a good temperature-sensing set of modes in AT-cut crystals, as I know being used in SC-cut crystals?

As Jim mentions in another post, you can run on the fundamental and the third (or 5th or 7th) and get a thermometer out of the delta between the two modes. The gotcha is that a change in load impedance will shift the frequencies unequally. That will give you an apparent temperature change. 


> Cheers,
> Magnus
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