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LF: Re: Capacitor warning

from [James Moritz] [Permanent Link][Original]
To: <[email protected]>
Subject: LF: Re: Capacitor warning
From: "James Moritz" <[email protected]>
Date: Mon, 10 Jan 2011 15:35:08 -0000
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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
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