[time-nuts] Improving the stability of crystal oscillators

[Bill Powell]

I doug up and included below a time-nuts email from a couple of years 
ago on improving oscillator stability, with some thoughts from Jack 
Kusters, Tom Clark, and Brooke Clarke.

Also, there's a good history of high precision oscillators at:

http://www.ieee-uffc.org/fc_history/norton.html

Regards,
Bill Powell
af4jg



FROM:     
    <jim_johnson at agilent.com>
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DATE:           Wed, 26 Oct 2005 13:20:14 -0700
TO:           <w3iwi at toad.net>, <time-nuts at febo.com> SUBJECT:           
RE: [time-nuts] Xtal Oscillator Aging

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
jim_johnson at agilent.com

=======================================================================

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: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Tom Clark, W3IWI
Sent: Sunday, October 23, 2005 9:21 PM
To: time-nuts at 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 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
  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"
  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
  ever was!
  73, Tom Clark