TRABEM wrote:
On Tue, 25 Oct 2005 21:08:34 -0400, Bill wrote:


wrote:


Someone tell Trabem about polystyrene and/or NPO disc please!!!!!!!!!! N5TDE

Where can you buy .2uf NP0 discs?



And, even if we could buy them, why would we use a disc ceramic cap in
an environment that was not controlled due to stability issues?

I'm totally lost by the suggestion of a disc ceramic.

You shouldn't lump "ceramic cap" into a single group and assume it has
one set of characteristics. There are many types of ceramic capacitors,
with widely varying characteristics. The four most common groups a

1. Hi-K. These are made with a class of ceramic having a very high
dielectric constant. This results in physically small capacitors which
are excellent for many RF applications such as bypassing. Their small
physical size results in a high self resonant frequency for a given
capacitance. The tradeoff is instability in nearly every other respect.
They have a very high temperature coefficient, are sensitive to humidity
and physical stress, and the capacitance varies with frequency and
applied voltage. A common type is Z5U, which has a specified maximum
capacitance change of +22 - 56% over the temperature range of +10 to +85 C.

2. "Temperature stable". These are made with ceramic which has
considerably lower dielectric constant than used for the hi-k types.
Consequently, they're larger for the same capacitance. Their temperature
coefficient is much lower, and they're much less sensitive to other
factors. They're good for many general purpose uses such as low- to
moderate-Q filters and resonant circuits, coupling, and so forth. Q can
be quite good. A common type is X7R, which has a specified maximum
capacitance change of +/- 15% over the temperature range of -55 to +125 C.

3. Temperature compensating. These are much larger yet, being made from
ceramics with a low and controlled temperature coefficient. They're
typically very stable with respect to voltage, frequency, mechanical
stress, and humidity. A common type is P2G (N150), which has a
temperature coefficient of -150 +/-30 ppm/C.

4. NPO. (Common type is C0G.) These are made from a sandwich of two
temperature compensating types with equal and opposite coefficients.
They have excellent Q and are very stable. They're good for use in
resonant circuits including oscillator tanks, high-Q filters, and other
demanding applications. Q can be very good. A common type is C0G, which
has a temperature coefficient of 0 +/-30 ppm/C.

NPO capacitors are substantially larger than "temperture stable" types,
and much larger than hi-k types. You won't typically find them in large
values.

Roy Lewallen, W7EL