In article , Ian White, G3SEK
writes
qrk wrote:
On 20 May 2004 06:12:49 -0700, (Steve
Kavanagh) wrote:

[snippage]
Well, Tom Bruhns is the first one I have run across who has also noted
this, so it can't be a very commonly experienced effect.
[snippage]

Steve

I doubt that many people would notice ppb changes and jumps. This
takes a bit of patience and ruling out bad test equipment/setup to
observe this phenomena.

It has certainly been noticed by many more people than Tom.

The word "scintillation" rang a very faint bell, and Google found a
reference at:
http://www.seas.gwu.edu/~ecelabs/app...data/page2.pdf

These scanned pages from an unknown reference book define:
"Scintillation: minute and rapid fluctuations of capacitance, formerly
exhibited by silvered mica and silvered ceramic types [of capacitors]
but overcome by modern manufacturing techniques."

Well, maybe not *totally* overcome...

This explains why we only tend to hear about the problem in very old
capacitors (probably WW2 era) or in critical applications such as
precision oscillators.

The reference to silvered-ceramic capacitors is interesting. Evidently
that scintillation problem was "overcome" more completely than for
silvered-mica, which is why NP0 ceramic are now the capacitors of choice
for oscillator applications.


in the crystal oscillator business silver mica capacitors were known for
scintillation . The potting compound of silvered mica capacitors was
often the cause of temperature coefficient drift. Scintilation was
probably due to delamination of the mica.
There are 2 types of mica caps cleaved mica and compressed? which
powdered the mica and then re-formed. Im not sure about their relative
scintillation .
Modern NPO ceramic are probably better particularly un-encapsulated
surface mount.
Note that I would design an overtone crystal oscillator with only enough
reactance to remove the manufacturing tolerance. This reactance does not
have to be capacitative could be inductive capacitative reactance could
alternatively be a varicap then the problem would be a clean varicap
supply.
Note crystals can do strange things, the jumps described are too small
for unwanted modes but there is the well known (to TCXO designers) band
breaks these are small frequency jumps that occur at exact temperatures
and are due to minor modes passing through the major mode frequency at a
particular temperature. This is what limits TCXO performance. OCXO
makers make sure that the set temperature is not on a bandbreak.

--
ddwyer