[sdiy] 8th order?

[Magnus Danielson]

From: James Dunn <james at 4thharmonic.com>
Subject: [sdiy] 8th order?
Date: Tue, 05 Jun 2007 10:40:03 +0100
Message-ID: <46652F73.3050707 at 4thharmonic.com>

> What does 8th order mean? Is it simply another way of describing the 
> number of poles in a filter or db/oct?

It relates back to the theoretical background of linear diffrential system
and describes the degree of the system. The order of the system also descibes
how many poles and how many zeros the system can have. The placement of the
zeros gives the db/Oct number, but the school-book examples with all zeroes in
a bunch over at infinity for a low-pass makes the db/Oct being -6.02 db/Oct
times the order. An n-pole system is of the n:th degree.

You require at least n reactive components (capacitors, inductors) to acheive
a n:th order system, but it is not always that you get that order, you can get
less. For example will a dual-PI notch filter with RC links have 3 capacitors
but only show up as a 2:nd degree system. The db/oct measure does not give a
good measure here.

I hope you see the difference and also apprechiate why we talk about 8th order
rather than 48 db/Oct systems.

Cheers,
Magnus