On Sun, 21 Mar 2004 20:58:07 -0500, "Tam/WB2TT"
wrote:

Paul,

I probably should have included more details. A fellow I was working with
was trying to extract a clock signal from a synchronous data stream, and was
getting nowhere. We swept the frequency back and forth to be sure he was
tuned to resonance - he was. Changed the ceramic cap to mica, and everything
worked like a charm. We never analyzed why the ceramic did not work, but I
suspect it was because of the capacitance vs. applied voltage dependence. If
the cap had 6VDC on it, and he had a few mv of RF, I expect it would have
worked. Instead, he had 0 bias, and a couple of Volts p-p signal.

Yes, ceramics are *hopeless* for tuned circuits; I wouldn't trust the
black tipped ones, either. You can't beat silver mica but they're a
bit hard to find and expensive.


BTW, somebody mentioned powdered iron toroids. Sounds like a good idea.

Indeed. I'm looking into it.
--

The BBC: Licensed at public expense to spread lies.

Paul Burridge wrote:

Yes, ceramics are *hopeless* for tuned circuits; I wouldn't trust the
black tipped ones, either. You can't beat silver mica but they're a
bit hard to find and expensive.

I strongly disagree with this. I've successfully used ceramic capacitors
many times for both high and low Q tuned circuits from HF to UHF. Years
ago, I found that NPO ceramics were decidedly superior to silver micas
for temperature stability, so I use them exclusively for VFO tank circuits.

You might have come across some bad parts. Or perhaps you don't realize
that many different types of ceramic are used for making capacitors.
Three general classes are most common. The "general purpose" class
(something of a misnomer) is a very high-k ceramic used for capacitors
of Z5U, Y5V and similar types. Those are very good for bypassing because
of their small physical size, but terrible for nearly anything else.
They're microphonic, hygroscopic, piezoelectric, and highly temperature
and voltage dependent.

Another class is used for X7R and related types, sometimes called
"stable". These are much more stable in all respects, but are physically
larger due to the lower dielectric constant (k) of the ceramic. They're
suitable for a wider variety of uses, but still not for high Q tuned
circuits. They should be used in low Q circuits only after evaluating
the potential effects of temperature and voltage dependence, at the least.

The third common class is used for making capacitors with near-zero
temperature coefficients, such as C0G (formerly and still often called
NPO). These *are* an excellent choice for tuned circuit applications,
and are often better than silver mica (whose temperature coefficient and
Q are variable and unpredictable). They're sometimes identified with a
black dot -- on "dog bone" capacitors, it's on one end.

Many years ago I published an article describing a stable, VFO
controlled QRP transceiver which used NPO ceramic capacitors for the
oscillator tank. Scattered feedback indicated that some people had
gotten "NPO" capacitors from less-than-reliable vendors and were
experiencing an objectionable amount of frequency drift. That didn't
happen with name-brand parts. So there are some flakey capacitors out
there. If you need to depend on the quality, get name brand parts from a
reputable vendor. And you'll find that NPO ceramics are a very good
choice for tuned circuits.

Roy Lewallen, W7EL

Roy,

We were talking about bypass type ceramics. See the 3/20 10:13AM posting.
BTW, I think with SM you are pretty much forced into using ceramics.

Tam/WB2TT

Tam/WB2TT wrote:
Roy,

We were talking about bypass type ceramics. See the 3/20 10:13AM posting.
BTW, I think with SM you are pretty much forced into using ceramics.

Tam/WB2TT

Perhaps you were talking about bypass type ceramics, but the posting I
responded to:


Yes, ceramics are *hopeless* for tuned circuits; I wouldn't trust the
black tipped ones, either. You can't beat silver mica but they're a
bit hard to find and expensive.

made no such qualification, and even specifically mentioned "black
tipped" (usually NPO) capacitors.

As far as I know, silver micas are getting pretty rare, except maybe for
very high power, high current RF applications, if they're being used for
even that any more. I doubt if they exist as surface mount parts. The
reason silver micas are hard to find and expensive is that they've been
made obsolete for nearly all applications by generally superior ceramic
types.

Roy Lewallen, W7EL

In article ,
Tam/WB2TT wrote:
Roy,

We were talking about bypass type ceramics. See the 3/20 10:13AM posting.
BTW, I think with SM you are pretty much forced into using ceramics.

Cornell Dubilier / Waldom makes surace mount siler mico caps. You can
get them from Digikey for under $10 US.

You can get standard PPS film capacitors from several makers. If you
don't mind the fact that they are very touchy and have a higher failure
rate they may be an option. If you want a little better, you can buy the
more costly coated ones.

I wouldn't say that you are forced into using ceramic. It is an option
you may perfer.

--
--
forging knowledge

On Tue, 23 Mar 2004 14:44:30 +0000 (UTC),
(Ken Smith) wrote:

In article ,
Tam/WB2TT wrote:
Roy,

We were talking about bypass type ceramics. See the 3/20 10:13AM posting.
BTW, I think with SM you are pretty much forced into using ceramics.

Cornell Dubilier / Waldom makes surace mount siler mico caps. You can
get them from Digikey for under $10 US.

The CDE "MC" series of cazapitors are "mica", not "silver mica". The
difference is that silver mica caps have to be sealed (dipped) or the
silver plating reacts with everything. I'm not sure what plating is
used for the "MC" series of surface mount mica. My guess(tm) is
aluminum.
http://www.cornell-dubilier.com/mica/mica.htm
http://www.cornell-dubilier.com/film/hmc.htm
http://www.cornell-dubilier.com/catalogs/MC.pdf

The big advantages of silver mica is stability, wide temp range, very
low dissipation, and tolerance to over voltage spikes. Many years
ago, I wasted a month working over an HF xmitter, trying to design out
the expensive silver mica and porcelain cazapitors and replace them
with cheaper ceramics. It was possible for the low power drivers but
a waste of time in areas that had high RF currents or required good
stability. A similar cost reduction exercise was also being done on
the automagic antenna tuner (by someone else) with similar results.
The project ended when someone suggested using high temp silver solder
to prevent the ceramic caps from reflowing their solder connections
and falling off the board.

I guess(tm) the reason that silver mica caps are difficult to find is
that there are few companies producing high power RF products as
compared to the huge number of low power RF products. It's not a big
market that probably can only support a few specialty component
vendors.


--
Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
(831)421-6491 pgr (831)336-2558 home
http://www.LearnByDestroying.com AE6KS

Thanks, Jeff and Ken, for the info about modern mica capacitors. I
haven't dealt with the high-power RF world since a previous life as a
radar technician, long ago now, so hadn't realized they're still
preferred for some applications.

Roy Lewallen, W7EL

Thanks, Jeff and Ken, for the info about modern mica capacitors. I
haven't dealt with the high-power RF world since a previous life as a
radar technician, long ago now, so hadn't realized they're still
preferred for some applications.

Roy Lewallen, W7EL

On Tue, 23 Mar 2004 14:44:30 +0000 (UTC),
(Ken Smith) wrote:

In article ,
Tam/WB2TT wrote:
Roy,

We were talking about bypass type ceramics. See the 3/20 10:13AM posting.
BTW, I think with SM you are pretty much forced into using ceramics.

Cornell Dubilier / Waldom makes surace mount siler mico caps. You can
get them from Digikey for under $10 US.

The CDE "MC" series of cazapitors are "mica", not "silver mica". The
difference is that silver mica caps have to be sealed (dipped) or the
silver plating reacts with everything. I'm not sure what plating is
used for the "MC" series of surface mount mica. My guess(tm) is
aluminum.
http://www.cornell-dubilier.com/mica/mica.htm
http://www.cornell-dubilier.com/film/hmc.htm
http://www.cornell-dubilier.com/catalogs/MC.pdf

The big advantages of silver mica is stability, wide temp range, very
low dissipation, and tolerance to over voltage spikes. Many years
ago, I wasted a month working over an HF xmitter, trying to design out
the expensive silver mica and porcelain cazapitors and replace them
with cheaper ceramics. It was possible for the low power drivers but
a waste of time in areas that had high RF currents or required good
stability. A similar cost reduction exercise was also being done on
the automagic antenna tuner (by someone else) with similar results.
The project ended when someone suggested using high temp silver solder
to prevent the ceramic caps from reflowing their solder connections
and falling off the board.

I guess(tm) the reason that silver mica caps are difficult to find is
that there are few companies producing high power RF products as
compared to the huge number of low power RF products. It's not a big
market that probably can only support a few specialty component
vendors.


--
Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
(831)421-6491 pgr (831)336-2558 home
http://www.LearnByDestroying.com AE6KS

Tam/WB2TT wrote:
Roy,

We were talking about bypass type ceramics. See the 3/20 10:13AM posting.
BTW, I think with SM you are pretty much forced into using ceramics.

Tam/WB2TT

Perhaps you were talking about bypass type ceramics, but the posting I
responded to:


Yes, ceramics are *hopeless* for tuned circuits; I wouldn't trust the
black tipped ones, either. You can't beat silver mica but they're a
bit hard to find and expensive.

made no such qualification, and even specifically mentioned "black
tipped" (usually NPO) capacitors.

As far as I know, silver micas are getting pretty rare, except maybe for
very high power, high current RF applications, if they're being used for
even that any more. I doubt if they exist as surface mount parts. The
reason silver micas are hard to find and expensive is that they've been
made obsolete for nearly all applications by generally superior ceramic
types.

Roy Lewallen, W7EL