Jumpy Silver Mica Capacitors ?

qrk wrote:
On 20 May 2004 06:12:49 -0700, skavanagh72nospam@yahoo.ca (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/appnotes/PDF/refdata/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
 
In article <3A4Bt$C8VarAFAV1@ifwtech.co.uk>, Ian White, G3SEK
<G3SEK@ifwtech.co.uk> writes
qrk wrote:
On 20 May 2004 06:12:49 -0700, skavanagh72nospam@yahoo.ca (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/appnotes/PDF/refdata/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
 
Mark
Any more info on the jumpy HPs appreciated.
--
73
Hank WD5JFR
"qrk" <SpamTrap@reson.com> wrote in message
news:kd4oa0ldgi0j07dsiodvjgd524lpqkdl0m@4ax.com...
On 19 May 2004 06:08:37 -0700, skavanagh72nospam@yahoo.ca (Steve
Kavanagh) wrote:

Thanks for all your comments. Since speculation has started here is
what I know about the capacitors.

Those used in the 2.5 GHz source are surplus from a company that makes
high quality stuff. They were probably procured to a military or
space specification but I am not sure. The 10.5 GHz source was
manufactured by MA/COM about 20 years ago. All of them are the usual
deep maroon (is that the right word ?) to brown colour.

Keep in mind I am being pretty picky. I consider short term frequency
jumps of much over 100 Hz to be unsatisfactory - that is 10-40 parts
per billion depending on which source is considered. The largest
observed jumps are about ten times this. Of course, since these are
crystal oscillators, the corresponding capacitance jumps must be much
larger, since the crystal should dominate the oscillator stability. I
would not consider them "crappy", just not as good as one might be led
to expect. I have used capacitors from the same provenance as those
in the 2.5 GHz source in LC oscillators at a few MHz with no observed
problems. The smooth portion of the warm-up drift is reasonably
normal in both cases...only the jumpiness is unusual.

Steve

Crystals can also jump. Just look at HP oven oscillators used in the
GPS time/frequency references. Very jumpy. A collegue tried 5
oscillators in the GPS time/freq reference and all were jumpy to
various degrees. He was noting sub-ppb jumps.

Mark
 
In article <2Y%tc.1241$sJ2.646@newssvr23.news.prodigy.com>, Henry
Kolesnik <kolesnik@sbcglobal.net> writes
Mark
Any more info on the jumpy HPs appreciated.
--
73
This thread seems to be cloned from last week ; anyway as I said then
silver mica are known to be subject to scintillation which may be due to
de-lamination , later mica used powdered mica which may or maynot be
better. Encapsulated low value caps are often degraded by the
encapsulation material so sm would be better use NPO ceramic, microwave
types should be lower loss.
Crystals jump in a changing temperature due to unwanted modes passing
through the main mode as they have different temperature coeggicient.
These are fairly gross effects of .01 to 10ppm jumps.

--
ddwyer
 

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