On 8/27/15 4:46 PM, Bob Camp wrote:
Hi
On Aug 27, 2015, at 3:58 AM, Hal Murray <hmur...@megapathdsl.net> wrote:
kb...@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.
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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