LF: Re: Capacitor warning

["James Moritz" <james.moritz@btopenworld.com>]

Dear Andy, LF Group,

If you want low tempco (and low RF loss), you really need ceramic Cs with 
low-K dielectric like NP0,C0G. These may be specialised RF components, but 
some are also available in "disc ceramic" format. The ones found in SMPSUs 
are most often the much less stable and more lossy medium- and high-K types 
like X7R or (worse) Z5U. The high K types hve other nasty habits too, like 
voltage-dependent capacitance. In many noise filtering and "snubber" 
appliations, the capacitance and loss isn't that critical, so the smaller, 
cheaper, higher dielectric constant typs work OK.
The other favourable type are metalised polypropylene - if you go to 
"capacitors - film" in the online Farnell catalogue, select polypropylene as 
the dielectric, it is easy to narrow things down to find what you want. Some 
other "poly" capacitors, particularly polyester, have much higher loss and 
are unsuitable. Polystyrene caps have low loss, but aren't generally 
available in useful TX ratings. Silver mica are good, but rather expensive 
if you need many picofarads...
Cheers, Jim Moritz
73 de M0BMU