Caps for CEM & SSM Filters?

Christopher_List at Sonymusic.Com Christopher_List at Sonymusic.Com
Fri Jun 13 01:01:20 CEST 1997






> Got a quick question:  Seems like somewhere I've heard
> that for CEM based (and SSM?) filters you're supposed to
> use polystyrene caps.
> I can't find any with values as low as the designs I've been building
> call for.  Here's what I have found in Mouser's catalog:

On page 216 of the current Mouser catalog (Good until July '97) you'll find
polystyrene caps down to 100pF, is that low enough for you?

When I need lower, like 22pF,  I go with ceramic or mica - the micas are
big and ugly while the radial lead ceramics are convienient and cute. I
figure noise and tolerance problems are pretty insignificant when you're
dealing with 22pF....

Getting back to the 100pF to .001uF (1nF) range, the polystyrene caps are
(like the mica caps) are not pretty and strange as far as lead spacing and
stuff goes. As a comprimise I often use the Panasonic polyester caps on
page 263 of the latest Digikey catalog - these go down to 100pF and have a
more standard (standarder?) pinout - and come in pretty colors and a nice
small package. This obviously doesn't make it sound any better, but it
makes the building more fun :). The dissipation factor, however, is 10
times worse compared with the polystyrene. I'm not sure how much that
effects filter performance.

Actually, that's a good question for the experts. The word from on high is
that you should use polystyrene for filters and VCO integration caps - and
poly-something caps are better in general for everything but power supply
buffering. In these two applications (filter and VCO), what are the key
elements of cap performance (dissipation factor, resistance, tolerance,
noise(?), etc.) that make the polystyrenes the choice of the master
builder?

- CList





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