[time-nuts] Another "atomic" clock question

[Bob Camp]

On Mar 2, 2014, at 5:29 PM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> Hi Bob,
> 
> On 02/03/14 23:16, Bob Camp wrote:
>> HI
>> 
>> 
>> 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.
> 
> Indeed. I expect it to be individual to each particular crystal.

There is a body of information that suggests that it’s related to the cut, the same way as the AT (or SC) frequency temperature performance is. It also relates to the mounting, which will vary from part to part.

> 
>>> 
>>> 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.
> 
> I already know about the fundamental and third trick, my question was if it could be done to AT-cut as well. I interpret your statement as yes, it does. I don't trust it to be perfect, but reasonable. Ideas for means to handle shift would be welcome.

It was originally proposed by a very nice guy from Ft. Monmouth for use with AT cut resonators. I believe the paper is in the FCS proceedings from the mid 1980’s. The DOD kept rights to the technique and licensed it to a couple of oscillator companies. 

Bob
 
> 
> I don't have SC-cut crystals lying around, only complete OCXOs.
> It would be fun to build a simple double-mode oscillator around an AT lying around.
> 
> Cheers,
> Magnus
> 
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