Hi,
I posed this question to Jack Kusters, now retired
from HP/Agilent. He and Charles Adams commercialized
the SC-cut crystal for HP in the 10811A oscillator. He
gave me permission to post his response on the reflector.
Jim Johnson
[EMAIL PROTECTED]
===
Hi Jim,
In addition to everything Tom Clark said (I agree in general with his
explanation), there is another aging mechanism associated with stress. When
the crystal blank is manufactured, it is sawn, lapped, ground, etched, and
otherwise abused. All of this produces stress in the blank. In addition,
there are mounting stresses that arise because of the way the blank is mounted
on its header, surface stresses that develop because the electrode material
when evaporated and then condensed on the surface shrinks as it cools.
All of these result in long-term aging as these stresses need to equilibrate
out. In addition, there are other stress related mechanisms that may result in
either long- or short- term aging. The quartz material is anisotropic, the
mounts, electrode material are isotropic.
So, lets assume that we've had the crystal at an elevated, constant
temperature. Over a period of time, all stresses, material, mount, electrode,
cracks, etc. equilibrate to their lowest energy level and it appears that aging
has stopped.
Now, take it down in temperature. The anisotropic quartz and the isotropic
mount and electrode, have different contraction rates, so the overall system
now has a new set of stresses.
Let the unit come to full equilibrium at the new, lower temperature. Now take
it up in temperature to where it was before. Now we see a whole new set of
aging and stress relief. The only virtue is that aging due to cracks and
material stress from manufacturing processes should be mostly gone, so the unit
should come to equilibrium much faster.
One further comment, glass sealed crystals are not necessarily the best way to
seal a crystal. It takes heat from a source sufficiently elevated in
temperature to melt the glass. This tends to cause contaminents to migrate
from the area being sealed to a cooler spot in the package, usually the
crystal. Contaminents come from gasses from the torch or from junk trapped in
the glass.
The cleanest mount one can do is a cold-weld seal under proper conditions.
For example, the HP crystals were put into a vacuum furnace, heated to 300+
deg-C overnite at 10E-7 torr with the can stored next to the crystal. After
reducing the temperature to about 80-84 deg-C, the crystal was frequency plated
to within several parts in 10E7. After that, the mount was placed in the can,
the temperature raised up to about 150 deg-C, stabilised in temperature and
vacuum, then cold-welded.
Done properly, there is essentially no contamination inside the crystal
assembly, most of the other stresses are gone, and the typical HP SC-cut would
reach an aging rate of better than 1E-7 per day, within the first 5 days.
Best regards,
Jack Kusters
==
-Original Message-
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Tom Clark, W3IWI
Sent: Sunday, October 23, 2005 9:21 PM
To: time-nuts@febo.com
Subject: [time-nuts] Xtal Oscillator Aging
Brooke (no relation) commented
Hi Richard:
>
> 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 lef
