From: Larry Miller ([email protected])
Date: Thu Jun 08 2000 - 06:57:52 PDT
I think you are right.
For garden-variety boards we see buried capacitance and ground planes start
to give out (i.e., impedance starts to increase) at about 600-ish MHz when
you measure them on a network analyzer. The measurement referred to is
driving one point on a board, say near where the DC supplies connect, and
measuring on another point some distance away. This is not to say the board
does not work above this range. At somewhere in the 2 to 4 GHz range the
dielectric losses for FR-4 start to get very bad as well.
Somehwere in this range you have to start treating things like what they
are-- microwave structures.
Larry Miller
> -----Original Message-----
> From: Barry Ma [SMTP:[email protected]]
> Sent: Wednesday, June 07, 2000 4:10 PM
> To: [email protected]
> Subject: [SI-LIST] : Upper limit of interplane capacitance
>
> Hi,
>
> The interplane capacitance between pwr and gnd planes in multilayer PCB
> has super performance at higher frequencies, as opposed to ordinary
> decoupling caps. But there must be an upper limit of operating frequencies
> for the planecap. Please allow me to list two considerations on the upper
> limit:
>
> (A) The real value of planecap can only be considered within 1/8
> wavelength.
> (B) It can only be calculated within Tr distance.
>
> As frequency gets higher and higher, the real available planecap would
> become less and less. I think that's right. What do you think?
>
>
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