[time-nuts] looking for SMT oscillator SC cut, with no oven

[Bob Camp]

Hi


> On Aug 28, 2015, at 12:14 AM, Jim Lux <jimlux at earthlink.net> wrote:
> 
> On 8/27/15 4:46 PM, Bob Camp wrote:
>> Hi
>> 
>>> On Aug 27, 2015, at 3:58 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
>>> 
>>> 
>>> kb8tq at n1k.org said:
>>>>> Is there anything fundamental about SC that forces the turn over
>>> temperature
>>>>> to be high?
>>> 
>>>> Simple answer yes. More complicated answer : that depends.
>>> 
>>>> The crystal curve on an AT or an IT centers roughly at room temperature.
>>>> When you fiddle the  angles to get a stress compensated blank, that center
>>>> point moves up to the 90 to 100 C range.
>>> 
>>> Thanks.  I guess I thought there was an extra degree of freedom so you could
>>> pick the turn over temperature.
>> 
>> Life would be so much simpler if that was true ….
>> 
>> There are indeed a range of cuts you could make. Working out the in’s and outs
>> of any one of them is a megabuck sort of endeavor. You can predict that this or that
>> will happen. That only gets you just so far. There are a lot of fine details that
>> you can only find out by experiment.
>> 
>>> 
>>> The graph at the bottom of this URL
>>>  http://www.4timing.com/techcrystal.htm
>>> shows that there are actually 3 turn over temperatures.
>> 
>> The Beckman graph at the bottom of that page shows a number of curves that
>> have no turnover (those below 0 angle) . For the ones that do have a turnover, each
>> one has an upper turn and a lower turn. The magic point in the middle that they all
>> go through is generally called the inflection temperature.
>> 
>> Lots of make your head hurt info at:
>> 
>> http://www.ieee-uffc.org/frequency-control/learning/fc_conqtz2.html
>> 
>> I don’t see anything on a quick Google search that actually give
>> the Beckman constants.
>> 
> 
> 
> there's some C code out there that models AT (and also other cuts).. I'll see if I can find it. I found it in a PhD dissertation on designing temperature compensation neworks, as I recall.
> 

The formulas and everything *are* out there. Last time I used them I pulled them out of IEEE papers. Since I don’t have 
access to them at home, (and suspect most of us are in that boat) - citing one didn’t seem like a useful thing to do. There
are a bunch of fiddly little things about the constants used that vary a bit from paper to paper. Since those variations are 
almost all in the “past what you can measure” range for the raw quarts, it’s not real easy to work out who is right and who is wrong. 

Bob


> It's not necessarily reality, but it's a model that you could probably use to predict a range of variations.
> 
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