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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: RE Descrambler
Message-ID: <16414@milton.u.washington.edu>
Date: 14 Feb 91 03:16:01 GMT
References: <1991Feb9.233031.25283@sactoh0.sac.ca.us> <17660151@hpfcdj.HP.COM>
Organization: University of Washington, Seattle
Lines: 29

In article <17660151@hpfcdj.HP.COM> myers@hpfcdj.HP.COM (Bob Myers) writes:

>"NPO" stands for "Negative-Positive-Zero" (if it stands for anything), and
>basically means a ceramic capacitor whose value changes very little with
>temperature. 
> ... [other common ceramic capacitor types are] X7R and Z5U, which
>are worse than NP0 vs. temperature. (Z5U is the worst - I think it's something
>like -20%/+80% or thereabouts over a 0-85 deg. C range.  Z5U tends to be seen
>a lot on what are basically cheap, low-quality caps - as in lots of voids and
>other defects in the dielectric.  I'd avoid them like the plague even if you
>can handle the lousy temp. coefficient.)

	I'd like to add that the high capacitance density of Z5U is 
attained by using ferroelectric materials.  These are nonlinear
in applied voltage, as well as 'drifty' in temperature, and in
fact are also liable to flex like a piezoelectric crystal would,
so have significant damping (i.e. never achieve good high Q).
If you want losses at high frequencies, like in a bypass capacitor,
they may be just what you want.
	But NEVER try 'em in a high-Q filter.  I have changed the values
by 3% on these jobbies just by laying a warm thumb alongside 'em.

	John Whitmore

	"Typically, capacitors with BX or X7R characteristic decrease
in capacitance by approximately 10% when rated DC voltage is applied.
Other formulations with very high dielectric constants may exhibit a
capacitance decrease of 50%."
			--Kemet Capacitors General Catalog