[time-nuts] Why do crystals go bad?

[Chris Albertson]

On Mon, Feb 14, 2011 at 7:47 PM, Shawn Tayler <shawn at xmtservices.net> wrote:
>
> My question is what is the process in the aging if the crystals that causes this drop in frequency?  I understand it may be mechanical but the level if vibration the radios are exposed to varies greatly and there does not seem to be a correlation with the frequency shift we are seeing.

All crystals change frequency as they age.  Some of this is that the
crystal absorbs mass as gas is defused into the structure or looses
mass by outgassing or the mounting metal defuses into the outer layer
or the metal oxidized.   All these are chemical changes

I've also read about microscopic cracks or defects in the crystal can
grow over time

In a radio the oscillator also has other components like caps and
resistor that can age.

Most crystal oscillators that you buy come with a spec for aging and
also an adjustment and I find the engineers have build maybe only a 20
year worst case life into the adjustment range
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Chris Albertson
Redondo Beach, California