What is a bode plot?

Answer 1

A Bode plot, named after

Dr. Bode, is the name given to a log-log plot of the frequency response of a continuaus

time system. So then what's a frequency response?

It describes the steady-slate response of a system to a

sinusoid of a particular frequency. Recall that a sinusoidal function (complex exponential) is an eigenfunction

ofa

CT system, meaning if ei"'' goes in, Kei"'' comes out, where Kis acomplex number. A Bodeplot is simply

a graph of

K vs. ro. Since K is complex, we will have to represent the frequency response as two sub-graphs. We

could plot the real part and imaginary part separately, but it is much more intuitive to plot things as magnitude

and phase versus

ro. You can see this by rewriting K as Ael+. The outpul then becomes Aei<"'t+$J,which has

the sarne frequency as the input, but with a possible change in magnitude and phase of the sinusoid. In other

words, sin

(cot) -+ Asin ( cot+ •). Therefore, a Bodeplot is simply a graph of A vs. co and

Pieasenote that things

are plotted versus co (radians/sec) notf (Hertz or cycles/sec). If needed, the conversion

is

co = 2nf.