[time-nuts] Xtal Oscillator Aging
Tom Clark, W3IWI
w3iwi at toad.net
Mon Oct 24 00:21:11 EDT 2005
Brooke (no relation) commented
> It's my understanding that this optimization can be done by changing the
> oscillator power level at the crystal.
> In the case of the 32768 Hz watch crystal, it must be run a very low power
> and it has a very low aging rate when compared to higher frequency
> crystals that are typically run at higher power levels. I think this is
> related to the crystal throwing off atoms, so more power means more
> acceleration and more atoms thrown off.
> Have Fun,
> Brooke Clarke, N6GCE
It has nothing to do with "throwing off atoms". A Xtal is actually a
mechanical oscillator, with the quartz slab vibrating (in either its
fundamental mode, or on an odd overtone); quartz is a piezo-electric
material so the voltage across the pins of the xtal has a direct
connection to the mechanical vibration. When an xtal oscillator starts
up, the associated amplifier generates noise, which then starts the
xtal vibrating, which generates signal at the right frequency and a
feedback loop is set up. When you crank up the power to the mechanical
resonator, the signal increases with respect to the background noise
(i.e. S/N gets better) which improves the short-term stability.
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.
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
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"
xtals where the seal is just a rubber gasket or the epoxy seals used
in some consumer-grade surface mount oscillators.
The main reason that the 32768 Hz oscillators operate at low power is
so that watches can run for years on small batteries. But even at
that, the mechanical xtal resonator (which is built as a tuning fork
for these low frequencies) is much better than any watch escapement
73, Tom Clark
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