James Waldby wrote:

On Thu, 28 Jan 2010 23:47:03 +0000, DoN. Nichols wrote:
On 2010-01-27, Jim Wilkins kb1dal... wrote:
On Jan 27, 5:00Â pm, "DoN. Nichols" ... wrote:
...I wonder whether using low TC mica capacitors and hand-drawn
resistors would produce something more stable. Â ...

I was curious and checked the temperature change of several types of
caps with a Boonton meter. Baked-bean micas changed perhaps 5% from ice
water to a heat gun blast. Multilayer ceramics dropped about in half
when heated, tantalums much more. The 1% metal film resistors that
company used for everything didn't change much at all.

O.K. So the micas would probably have been an improvement. The
trimmer caps were ceramic ones in the low pF range. However, the
resistors which I had available were the old carbon composition
resistors (RC20GF???J IIRC). And I don't know how the CMOS thresholds
might have changed with temperature.

I *am* amazed that the multilayer ceramics had that much change.

For several kinds of high-dielectric MLCC's[*], capacitance
decreases as higher voltage is applied, which for many apps is a
worse problem than temperature dependence. For example, see
chart near middle of http://my.execpc.com/~endlr/ceramic.html
where it shows that Z5U-dielectric MLCC's have 120% of rated
capacitance at 10% of rated voltage; 50% at 50%; and 30% at 100%.

As the article says later: "Why use them if their electrical
properties are inferior to C0G? Size and cost. You will rarely
see C0G larger than 0.047 uF (at least as a standard product),
but Z5U can be found as large as 22 uF, even in SMD. Poor as it
is by C0G standards, a 22 uF Z5U can compete with the equivalent
electrolytic in many high-frequency applications with a much lower
ESR."

Also see www.murata.com/cap/measure.pdf and
www.niccomp.com/catalog/nmc2.pdf.

* MLCC = one or more of "multi-layer ceramic capacitor",
"multi-layer ceramic chip", or "multi-layer chip capacitor"
SMD = surface mount device.
--
jiw


Z5U are crap grade capacitors intended for bypass applications. They
are not suited for tuned circuits.



--
Greed is the root of all eBay.