[sdiy] RC filter questions + SPICE RC filter simulation questions

[Scott Nordlund]

If you're expecting a filter with 4 poles at 1/(2*pi*RC), that's not what
you've built.  It's not 4 independent stages, it's all one circuit, and a 
measurement taken at any point will be affected by the entire rest of the 
circuit.  You've got 4 poles, but they're all at different frequencies, 
resulting in a rather flaccid transfer function.

If you want the poles to be at the same frequency, you'll need buffers
between the RC sections.

> Date: Fri, 16 Apr 2010 17:01:34 +0200
> From: cheater00 at gmail.com
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] RC filter questions + SPICE RC filter simulation questions
> 
> Hi guys,
> I have been playing around with pspice today, simulating and analyzing
> some very simple circuits.
> 
> I analyzed a simple RC filter: AC voltage source goes through R=1kR
> and C=1uF to ground. I analyzed it and got the familiar curve.
> 
> Then I stuck some more stages like that on it: there's a series of
> four 1kR resistors and after each one there's a 1uF cap to ground. I
> analyzed it and got the expected curve at the last stage, but the
> first stage has a distinctive bump. Additionally the cutoffs of all
> the stages are much lower. Where does the bump come from? Is there a
> specific law that defines this?
> 
> I am trying to find a way to filter a .wav through the filter and look
> at the voltage output. Is this possible to do in pspice? If not, do
> any other spice packages allow that?
> 
> Thanks
> D.
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