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.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek