[time-nuts] Xtal Oscillator Aging

Richard (Rick) Karlquist (N6RK) richard at karlquist.com
Wed Oct 26 10:41:22 EDT 2005

>    It has nothing to do with "throwing off atoms". A Xtal is actually a

This only happens in very low quality crystals that have impurities on
the surface.

>    Going in the other direction, the mechanical resonant frequency
>    changes with time because, as the xtal vibrates, microscopic cracks in
>    the structure of the quartz break apart. Running at high power makes
>    the crystal generate these microscopic faults at a faster rate; this
>    then causes the oscillator to have poorer long-term stability. When an
>    xtal is left vibrating (oscillating) in an undisturbed environment,
>    the rate of cracking of the quartz decreases, and the oscillator is
>    said to "age" to its final frequency.

The 10811 scientists agree that microscopic cracks are the main unresolved
issue regarding crystal stability (there are many other possible issues,
but they have been beat to death, at least in the top tier crystal fabs).
OTOH, at HP we never saw any drive power related aging effects.  The drive
levels we used had some effect on the frequency (as shown in my E1938 paper)
so we were limited because of that issue.  Also, the g forces were so high
that we couldn't go a lot higher without danger of losing the plating

The effect on S/N ratio is such that you need a certain amount of drive
to get a good noise floor at 10 kHz (which is only important in a minority
of applications).  Close in, the S/N ratio is determined by the quartz
not the electronics, and running higher crystal drive doesn't help.  It may
even degrade short term stability.

>    But if you subject that same crystal to a mechanical jolt will force
>    some new cracks and re-start the aging "diffusion" process. Ditto
>    turning the oscillator on & off or a thermal shock can aggravate the
>    aging.

At HP, we never saw any significant aging shift due to turning the
on and off (while maintaining the oven at the same temperature).  OTOH,
it is definitely true that any oven temperature change will have a settling
time effect on aging.

>    If the metal can or glass envelope around the xtal outgasses, some of
>    the resulting crud (a very scientific term!) from the envelope and
>    seal will deposit onto the quartz and also cause aging. For this
>    reason, only the cheapest crystals are housed in a metal can with a
>    solder seal; cold welding of the can is a much better procedure; and a
>    glass envelope is the best. Cheaper than cheap are the WW2 "FT243"

Many decades ago, glass was the "gold standard".   However, cold well metal
cans have long since superceded glass.

Rick Karlquist N6RK
(formerly HP Santa Clara Division)

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