[sdiy] Ms20 filter clone PBC?

[Magnus Danielson]

From: "JH." <jhaible at debitel.net>
Subject: Re: [sdiy] Ms20 filter clone PBC?
Date: Wed, 27 Feb 2008 21:11:20 +0100
Message-ID: <002401c8797c$fe797f20$0400a8c0 at jhsilent>

> >I remember coming up with similar conclusions, that the HP is actually
> >more of a one pole.
> 
> No, it's still a 2-pole HPF, even though it's 6dB/Oct in HPF mode.

I agree. This is a wonderfull example that you cant use pole-count and slope-
numbers interchangeably. Slope-numbers only renders one lower limit on pole-
count. In this case, the slope requires at least 1 pole, but the resonance
proprety requires at least 2 poles, so there is at least 2 poles in that
filter.

> Must be a zero exactly compensating one of the (real) poles (as long as no 
> feedback is applied).
> But with feedback applied, the poles become complex, and you get resonance.

Yes, but the zero compensation is still there, so you keep the 6 db/oct slope.

> (You can actually measure a higher slope - like 8dB/Oct - near the resonance 
> point in that case,
> but that's an effect that happens in all resonant circuits, and isn't used 
> to characterize the "slope", so
> Ian is perfectly right with 6dB/oct.)

Agree.

Cheers,
Magnus