[time-nuts] looking for SMT oscillator SC cut, with no oven
kb8tq at n1k.org
Wed Aug 26 21:27:40 EDT 2015
> On Aug 26, 2015, at 7:28 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> dk4xp at arcor.de said:
>> SC requires high temperature, that does not go together well with SMD and
>> low power.
> 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.
To get a turn down around room, you are looking at something that is 70C away from the center
of the curve. Think of a normal AT that is operating around -40C. The analogy is not perfect, but
it is close enough. The crystal is moving a lot if it has a turn that far off center.
Now for the more complicated part. Depending on exactly what you mean by SC, the angles that
go into the blank can be fiddled a bit. You get something that has a label like “modified SC’ on it.
It no longer is stress compensated. It’s still pretty good. You can bump the center of the
curves up or down 10 or 20 degrees and still be “close” by some definition. The further
you go, the more elastic your definition needs to be. Even with some fiddling, you still have
a room temperature crystal with some major temperature slopes inside the operating range.
You do *not* want to let that crystal see a draft :) Indeed if you move the phase noise limit down
to 1 Hz and below, the thermal noise in a typical room will be your dominant noise source.
> Or is is something like SC is only used in ovens and they have to be higher
> than ambient so nobody ever makes one at the magic angle that would give a
> lower turn over temp.
> These are my opinions. I hate spam.
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