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Thread: RC oscillator
www.piclist.com/techref/microchip/time.htm?key=oscillator
picon face BY : Russell McMahon email (remove spam text)



>        I have an application which requires an RC oscillator.  Since the
>design will be subjected to high vibration in the field, it is felt that a
>crystal or resonator would be too fragile.


Don,

RC oscillators can be made stable enough for controlling serial async data
transfer but in your circumstances I would first consider whether a
resonator or crystal is really as fragile as you think it is. How strong is
a capacitor compared to a resonator?

What sort of environment are you in?
Properly mounted a resonator or crystal will survive massive accelerations.
I don't know details on what they use but the military fire GPS units in
artillery shells (!)
Unless you are using this equipment to tamp in your sticks of gelignite you
are unlikely to get higher accelerations than that.
Equipment with survivable accelerations of in the order of 1000 g is quite
standard.
Close attention to mounting detail and massive common sense seems to cover
most of it.
I don't know about the internal mounting structures in crystals but
resonators "seem" to be rather more monolithic.

You could try an easy test.
Mount a resonator on a steel block - hold in place with epoxy and press
firmly into place against the steel as the epoxy sets.
Drop steel block from various heights onto paper sheet on steel plate on
concrete floor (resonator uppermost :-)) from various heights.
If one assumes about zero compression of the steel-steel interface any
deceleration is carries out over the thickness of the paper (or less as it
will have some compression thickness).

Acceleration in g's = Fall distance/Stopping-distance
eg for a 0.010" = 0.25mm = 0.00025m thick stopper
A drop from 250mm results in a deceleration of 0.250/0.00025
= 1000g

IF this kills your resonator try embedding it in a very small amount of
stiffish padding and try again.
The g forces will be MUCH reduced (as seen above) by allowing it even a
small amount of "compliance".

regards


     Russell McMahon
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