# Poles of filter

Discussion in 'General Electronics Chat' started by cooded, Jan 18, 2011.

1. ### cooded Thread Starter Member

Jul 20, 2007
28
0
Hello everybody,

I am a newbie at analog electronics. I am studying filters now, i wanted to know how do you figure out from the poles of the transfer function of the filter(active and passive) as to what kind of filter is it?( i.e LPF,HPF,BPF,Band stop etc.)

Regards

2. ### Audioguru Expert

Dec 20, 2007
10,607
1,182
The slope of the transfer function of a filter shows how many poles it has. Each pole is 6dB (half or double the amplitude) per octave (halving or doubling frequency).

After learning about the basics of electronics then simply looking at the schematic of a filter will tell you which type it is.

3. ### hgmjr Retired Moderator

Jan 28, 2005
9,030
218
For low order filters like those of the first and second order it is possible with study to determine the category in which the filter fits without graphing it. For filters of order three or greater, it is safest to plot the filter before leaping to any conclusions. As has already been mentioned, the more your study filter theory the easier it is to correctly recognize filters by type.

hgmjr

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,440
704
You are looking at the z-plane.

Low-pass IIR Filter
Poles are inside the unit circle.
Poles are to the right of the imaginary axis.
Zeros are, usually, to the left of imaginary axis, they can even be way way to the left, outside of the unit circle. In some cases zeros are at the origin.

High-pass IIR Filter
Poles are inside the unit circle.
Poles are to the left of the imaginary axis.
Zeros are, usually, to the right of the imaginary axis, they can even be way way to the right, outside of the unit circle. In some cases zeros are at the origin.