[time-nuts] Why do crystals go bad?

jimlux jimlux at earthlink.net
Tue Feb 15 04:31:01 UTC 2011


On 2/14/11 7:47 PM, Shawn Tayler wrote:
> Very interesting Bob Thanks.
>
> It brings to mind an annoying issue I run into from time to time.
>
> VCXO 12.8 Mhz used as a reference in communications gear.  Most of the gear is roughly 10 years old and of similar make, both mobile and portable styles.  They all are dropping in operating frequency. Many as far as 3000 hz at 850 MHz but it does vary with most in the 1300 hz area.  The trim adjustment is made through a programming interface and in some cases we are running out of adjustment.
>
> 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.
>
> Just thought I start a discussion.  I thinks still soy of on topic?
>

I think aging of crystals is sufficiently time-nutty..

I had heard that one theory on aging is that the (microscopic) 
vibrations are throwing off mass one molecule at a time (which would 
raise the frequency).  And on the other hand, it's microfractures which 
reduce the stiffness (lowering the frequency)...

I think the short answer is "nobody knows" except in pathological cases 
(cheap crystal oscillators with aluminized plates that are oxidizing, or 
something like that)




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