Diego Stutzer wrote in message
m...
Hi,
Every one knows, that e.g. a simple RC-parallel circuit has a
frequency-dependent impedance-characteristic (Absolute Value) - the
impedance (Abs) raises as the Frequency approaches zero. As a formula:
Zin
= 1/(1/R + i w C) , where i ist the imaginary number and w the frequency.

Now the hard part. How does one create an Impedance, which decreases
"slower", for frequencies close to zero but then decreases "faster" for
higher frequencies, than the simple parallel RC-Circuit?
Is there some kind of procedure like the one for syntesizeing LC-Filters
(Butterworth, Chebychev,..)?

Simply increasing C does not really help, because this equals a factoring
of
the frequency.
Increasing R does not help as well, as it seems.


I hope one of you cracks can help me out.
So far, thanks for reading.
Diego Stutzer

You need to graph out the required frequency-impedance slope then
approximate the required roll off rates using a segmented breakpoint scheme
consisting of a number of CR series sections in parallel. Essentially it's a
straight line approximation to the required Z-F curve. The CR's adding
zeroes as the frequency goes up.

Estimating the individual time constants can be irksome as each has effect
outside it's area of interest. Use a 'least-squares approximation' to obtain
a best curve fit for the number of sections involved.

It's an interesting subject but I've come across nothing out there that's of
use.

regards
john