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

Bob Camp kb8tq at n1k.org
Thu Aug 27 19:46:25 EDT 2015


> 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:


I don’t see anything on a quick Google search that actually give 
the Beckman constants. 

>  Do AT crystals used 
> in ovens take advantage of the UTP?

Yes. On a precision AT based oven, the oven temperature is matched to the 
turn over of the crystal by a process known as “turn hunting”. Once this is done
the unit is run over temperature and the offset from this point is adjusted to 
optimize the temperature performance. In a modern approach, the oven
gain is also optimized for best temperature performance during the same
set of temperature runs. 

> ----------
> So we are just lucky that an AT cut works well at a convenient temperature 
> and that an SC cut works well with an oven.  A life form on some other star 
> might not be so lucky.

The AT is far from luck. A *lot* of people spent a few decades running experiments 
to come up with what we call the AT. I often wonder if we have run out of two letter 
combinations for naming cuts and soon will have to go to three letter combos. 

Likewise the SC was not so much a search for temperature performance as for stress 
compensation in a single plane. An enormous amount of effort went into both the theoretical
and the experimental sides of that discovery. 

In both cases, the parts we have are as much a function of equipment as anything else.
The SC required double axis cutting and x-ray gear to be perfected. Earlier, the AT and 
it’s many cousins required the whole single axis X-ray and cutting prices to be worked out. 
That doesn’t even get into mounting structures or enclosures …


> -- 
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