Graphical Phase Response Calculation
If is real, then is either 0 or . Terms of the form can be interpreted as a vector drawn from the point to the point in the complex plane. The angle of is the angle of the constructed vector (where a vector pointing horizontally to the right has an angle of 0). Therefore, the phase response at frequency Hz is again obtained by drawing lines from all the poles and zeros to the point , as shown in Fig.8.4. The angles of the lines from the zeros are added, and the angles of the lines from the poles are subtracted. Thus, at the frequency the phase response of the two-pole two-zero filter in the figure is .
Note that an additional phase of radians appears when the number of poles is not equal to the number of zeros. This factor comes from writing the transfer function as
Graphical Computation of Amplitude Response from Poles and Zeros