Subject: Re: Ceramic Capacitors
Date: Wed, 25 Sep 2002 15:50:52 -0400
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Thanks a lot for this detailed explanation ( and to other people who
contributed earlier ).
Do you know what the situation is regarding other dielectrics, such as X5R,
I'm kind of looking for some kind of quantification of the piezoelectric
versus voltage, versus size ( 0603, 0805, 1206... ), versus dielectrics...
I assume that a smaller size like 0603 would be less likely to convert
vibrations present on the PCB into spikes than a larger size like 1206...
would you agree ?
Thanks in advance.
"Phil Hobbs" wrote in message
> Matthew Kendall wrote:
> > C0G good, X7R and others bad. In particular, use only C0G in PLL loop
> > filters and anywhere else near VCOs unless you want lots of microphony.
> > VHF transceiver I saw was so bad it didn't need a microphone, you could
> > shout at the loop filter.
> In order to get a lot of capacitance in a small space, X7R and Z5U caps
> use dielectrics containing barium and strontium titanate, which have
> huge dielectric constants. Unfortunately, they're also piezoelectric.
> Using sintered powder with (we hope) random grain orientation helps a
> great deal, but external effects such as mechanical stress due to CTE
> mismatch, sustained high-temperature operation under high bias, and that
> sort of thing, causes a net piezoelectric response to reappear.
> And, of course, since the cancellation is only statistical anyway,
> piezoelectricity will always appear at some level.
> NPO ceramics are not piezoelectric, and neither are most kinds of
> plastic films. Microphonics from actual capacitance variations happen
> only with air-spaced capacitors, e.g. panel mount variables or jiggling
> Phil Hobbs