This applet shows the Bode plot of a 2 pole and 2 pole, 1 zero system:-

For the 2 pole case the the phase goes from 0 to -p and the magnitude plot has slope 0 at low frequencies and -40dB/decade at high frequencies.

The effect of the zero can be illustrated by the eg parameters (10.0, 1.0, 1.0, 20.0, 1.0, 2.0). This shows a break in the magnitude plot and a phase advance of 45° at 20rad/s.

To have an effect on the step response the zero has to be moved to lower frequencies eg (10.0, 1.0, 1.0, 1.6, 0.5, 2.0). In this case the phase margin is ~90° for the pole zero case and ~45° for the 1 pole case.

Gif image below shows how applet should appear.

COPYRIGHT © 2006 Cuthbert Nyack.