Jumpy Silver Mica Capacitors ?

[Steve Kavanagh]

A year or so ago I was working on a microwave local oscillator (at
about 2.5 GHz) multiplied up from a crystal oscillator near 40 MHz.
The output was found to jump in frequency by tens or hundreds of Hz
many times as the LO chain was warming up. I was able to reduce this
jumping by replacing all the dipped silver mica capacitors in the
crystal oscillator stage with NP0 ceramics. There is still a bit of
jumping which may come from some silver micas which remain in the
stage following the crystal oscillator.

I have just been observing the same sort of frequent jumping behaviour
(up to a kHz or so at a time) in another local oscillator (output at
about 10.5 GHz, phase locked to a crystal oscillator around 100 MHz).
I note that this one also has dipped silver mica caps in the crystal
oscillator and I wonder if it too would be improved by replacing them
with NP0 ceramics.

The capacitors used in both cases are from unknown sources and were
probably manufactured in the early 1980's.

Has anyone else experienced this behaviour ?

Steve (VE3SMA)

 

[Dave Platt]

A year or so ago I was working on a microwave local oscillator (at
about 2.5 GHz) multiplied up from a crystal oscillator near 40 MHz.
The output was found to jump in frequency by tens or hundreds of Hz
many times as the LO chain was warming up. I was able to reduce this
jumping by replacing all the dipped silver mica capacitors in the
crystal oscillator stage with NP0 ceramics.
#snip#

Has anyone else experienced this behaviour ?

Not personally, but I believe I've seen it mentioned in one of Doug
DeMaw W1FB's books on QRP transceiver design.

 

[Tom Bruhns]

Yes, a long time ago I saw just the same sort of behavior in LC
oscillators using silvered mica caps. Ever since, I haven't trusted
them, and use NPO/C0G instead for frequency control applications.

Cheers,
Tom

[email protected] (Steve Kavanagh) wrote in message
....

 

[Joe McElvenney]

Hi,

Have a look at the 'Mica' section of this -

http://my.execpc.com/~endlr/misc__dielectrics.html

It won't answer your question but should give you an idea as to
why that particular dielectric should be avoided in certain
situations.


Cheers - Joe

 

[Jan Panteltje]

Hi,

Have a look at the 'Mica' section of this -

http://my.execpc.com/~endlr/misc__dielectrics.html

It won't answer your question but should give you an idea as to
why that particular dielectric should be avoided in certain
situations.


Cheers - Joe

Nice page, I like these oldies, remind me of the past:
http://www.orenelliottproducts.com/capacito.htm
JP

 

[Steve Nosko]

A year or so ago I was working on a microwave local oscillator (at
about 2.5 GHz) multiplied up from a crystal oscillator near 40 MHz.
The output was found to jump in frequency by tens or hundreds of Hz
many times as the LO chain was warming up. I was able to reduce this
jumping by replacing all the dipped silver mica capacitors [...snip]

The capacitors used in both cases are from unknown sources and were
probably manufactured in the early 1980's.
Has anyone else experienced this behaviour ?
Steve (VE3SMA)

Steve,
Although I didn't figure it out at the time, my Knight-Kit Space Spanner
circa 1960 had terrible microphonics - very noisy. I cleaned the entire
chassis & rewired it to no avail. Today I suspect a mica cap, the old
square type, since I heard of same thing this some time ago.

 

[Ken Smith]

I have just been observing the same sort of frequent jumping behaviour
(up to a kHz or so at a time) in another local oscillator (output at
about 10.5 GHz, phase locked to a crystal oscillator around 100 MHz).
I note that this one also has dipped silver mica caps in the crystal
oscillator and I wonder if it too would be improved by replacing them
with NP0 ceramics.

Do the silver mica caps say "made in china" on them? Are they a light tan
color? If either of these are true chances are you will get a better cap
made from ear wax and tin foil. Somewhere in China there was, and perhaps
still is a factory, that made silver mica caps that change value if you
squeeze them between your fingers and go open if you heat cycle them.

I've never had much trouble with CDE caps.

 

[Jim Adney]

The capacitors used in both cases are from unknown sources and were
probably manufactured in the early 1980's.

That may be the secret here. In the past, I've used silver mica caps
in a number of applications with positive results. The only problem
I've ever seen with good quality ones was a single one in a 40 year
old HP amplifier that had become leaky.

-

 

[Rich Grise]

That may be the secret here. In the past, I've used silver mica caps
in a number of applications with positive results. The only problem
I've ever seen with good quality ones was a single one in a 40 year
old HP amplifier that had become leaky.

If silvered mica caps are so crappy, why did they ever use them in
the first place?

Thanks,
Rich

 

[Joel]

Space Spanner.. O'Man, I could only afford the Ocean Hopper.. Man, them
were the days!

Joel AG4QC

 

[Steve Nosko]

On 16 May 2004 07:06:24 -0700 [email protected] (Steve
Kavanagh) wrote:

[snip]

If silvered mica caps are so crappy, why did they ever use them in
the first place?

Thanks,
Rich

Well made, they are very stable & handle considerable current. These were
top of the line. If they make junk now, so be it.

 

[Steve Nosko]

Space Spanner.. O'Man, I could only afford the Ocean Hopper.. Man, them
were the days!

Joel AG4QC

Yea! AND, get this, I think I took plates off, or bent some, of the
bandspread cap to reduce its range and calibrated it for 40 M. Every
Saturday morning I'd listen to the same group on SSB talking about their
Collins rigs and inverted Vees.
Wish I still had it and the Knight-Kit crystal set. Had it hooked up to
a loud speaker in my room (course I was in Wood Dale, IL., not too far from
the WBBM and WGN AM transmitters...)
73

 

[Henry Kolesnik]

Real mica has to be real old and it sn't aging much in a capacitor because
it has been aging for eons and for all pracitcal purposes inert. If mica
capacitors are jumpy it must be due to the plating or encapsulation! If
there's no clue to the mfg it could be that these jumpy were not
manufacutured for your application. JMHO
73
Hank WD5JFR

 

[Jim Adney]

If silvered mica caps are so crappy, why did they ever use them in
the first place?

I was not suggesting that they were crappy. I was suggesting that of
all the high quality ones I have worked with over many years, I've
only seen 1 failure.

His experience may well be due to the fact that his were of unknown
origin and unknown quality.

-

 

[Steve Kavanagh]

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

 

[qrk]

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

 

[Steve Kavanagh]

[email protected] (Avery Fineman) wrote in message

Coming in at the end of this discussion, I'll have to question the
observation of the so-called "frequency jump." At 100 PPB we are
talking quite serious test equipment

Yes, but the equipment here is up to the task. For the 10.5 GHz case
I am using this source as a receiver LO. A second receiver is
available and both can receive a third source. Both receivers
heterodyne the signal down to audio frequencies where any frequency
steps can be easily detected by ear. The receiver with the M/ACOM LO
jumps (frequency steps) with respect to the third source while the
second receiver does not. A similar scheme was used at 2.5 GHz.

If you are locking the X-band source to a crystal reference, then
there is a great deal of this frequency-control subsystem which can
be a cause for the "jumps." That can be the sampler or prescaler,
the phase detector (assuming it is a form of PLL), the loop filter, and
even the power supply rail voltage (affecting the voltage control of the
presumed voltage-controlled frequency adjuster circuit). ANY of
those can be the culprit in small frequency "jumps."

And from qrk:

Crystals can also jump.

Yes, I have not yet narrowed down the search in the M/ACOM 10.5 GHz
source, but was wondering about the likelihood of the silver mica caps
being the culprit. It has a measured supply voltage sensitivity of
about 50kHz/V (post regulator). The plug-in (not soldered) crystal in
an oven would also seem to be a likely source, particularly as the
jumping seems to improve after warmup (but long after the oven reaches
temperature).

I'm going to question all those others' claims about "jumpy silver
mica capacitors" after about 54 years of having hands-on
experience in RF and pulse circuitry.... I've never had one either open
or shorted and never "jumpy" in value and that includes the full-on
military environment testing of temperature, altitude, shock,
vibration, etc. A very few were found not quite within the capacity
value tolerance and not a single one experienced any "jumping"
of value. I've not heard of any such stories from contemporaries
in the industry...

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. But I am
pretty sure about the source of the jumping in the 2.5 GHz source.
Each silver mica capacitor replaced made it better (unless the PC
board just needed the thermal cycling resulting from a few extra
soldering operations).

Steve

 

[ddwyer]

A year or so ago I was working on a microwave local oscillator (at
about 2.5 GHz) multiplied up from a crystal oscillator near 40 MHz.
The output was found to jump in frequency by tens or hundreds of Hz
many times as the LO chain was warming up. I was able to reduce this
jumping by replacing all the dipped silver mica capacitors in the
crystal oscillator stage with NP0 ceramics. There is still a bit of
jumping which may come from some silver micas which remain in the
stage following the crystal oscillator.

I have just been observing the same sort of frequent jumping behaviour
(up to a kHz or so at a time) in another local oscillator (output at
about 10.5 GHz, phase locked to a crystal oscillator around 100 MHz).
I note that this one also has dipped silver mica caps in the crystal
oscillator and I wonder if it too would be improved by replacing them
with NP0 ceramics.

The capacitors used in both cases are from unknown sources and were
probably manufactured in the early 1980's.

Has anyone else experienced this behaviour ?

Steve (VE3SMA)

in the crystal oscillator business silver mica capacitors were known for
scintillation . The potting compound of sm capacitors was ofteen the
cause of temperature coefficient drift. Scintilation was probably due to
delamination of the mica.
Modern NPO ceramic are probably better particularly unencapsulated
surface mount.

 

[Steve Nosko]

Recently I mentioned this old bird and just remembered I got a picture off
the net from here

http://oak.cats.ohiou.edu/~postr/bapix/SpaceSpn.html

Other links @
http://ilectric.com/glance/Recreation/Radio/Amateur/Boatanchors/Knight/

I also found a schematic, but don't know the link.

Enjoy

Found the schematic:

http://www.qsl.net/kb7rgg/allied/schematics/spacespanner.gif

 

[qrk]

On 20 May 2004 06:12:49 -0700, [email protected] (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.

Mark