# Poles and Zeros

#### Mazaag

Joined Oct 23, 2004
255
I know that a pole is the value of s at which the magnitude of the transfer functino goes to infinity...why is it that when we draw a bode plot, a pole is represented by a drop in magnitude with roll off = 20db/dec?

thanks

#### scubasteve_911

Joined Dec 27, 2007
1,202
I found a pretty good lecture explaining poles and zeros, with bode plots.

http://web.mit.edu/2.14/www/Handouts/PoleZero.pdf

I was under the impression that the pole occurs at the -3dB point, then it drops off to its zero at a rate determined by the order. So, a 1st order will drop off at 20dB/decade, 2nd 40dB/decade, etc. It has been a while since I took control systems, just don't ask me to draw a root locus

Steve