[Elecraft] More than you may want to know about crystals

[James Hammons]

      Anyone who has taken apart one of the old WWII type crystals
found a square quartz blank held between spring loaded electrodes. =20
     Modern crystals aren't like that, most common blanks are round
with deposited electrodes of gold, silver or aluminum.  In a quality
crystal the edges may be beveled like a partial lens and it may be
polished clear,  to reduce spurious response. The blanks are held in
spring clips with a dot of conductive cement, on each side.  These new
crystals will not survive the power levels that the old type did, but
they have much better aging, calibration and drift characteristics.
     After mounting, the crystal is brought to final frequency by
depositing a small additional amount of the same metals until the
frequency is correct, this is done under high vacuum produced by a
diffusion pump.=20
     The crystal is then sealed using solder or cold welded with a
punch press, while in an atmosphere of dry nitrogen and helium.  The
gas mixture must be dry enough, that it's dew point is less than the
lowest operating temperature.  -40 degrees C is common in ordinary
crystals  -60 C or lower, for military spec.  The helium is there so
that it can be inspected for leakage using a mass spectrometer.  Even
with these precautions, all crystals age a bit over time.
      If a crystal leaks it will admit air which contains moisture,
and oxygen. This will cause the crystal to change frequency due to
corrosion of the electrodes. If the crystal is operated below the dew
point, of the gas in the can, dew or ice will form on the crystal
causing it to go off frequency or go dead.
     I don't think you will harm a crystal if you stay away from the
cold weld by about 3/16 or so but you can't be sure.  You won't really
know until you use your radio in a cold environment, or time causes
the crystal to age out of tolerance.
     I can't really say if grounding the crystals better improves the
filters ultimate rejection, if you don't damage the crystals it won't
hurt.   I grounded mine on the side before I heard of this mod.  I
also used a wire and connected the cans together on the tops. I also
did this on the ssb filter cans, the two pole filter, and the PLL and
BFO crystal pairs;  because thats the way we grounded when I was
involved in the filter and crystal business.   I added a ground on the
opposite side of the 5 pole CW filter,  later.  I could not detect any
difference, however mine was not done the stock way to begin with.

James Hammons