The frequency response of a system is described by values of the gain and
the phase angle which occur when the sinusoidal input signal is varied
over a range of frequencies. The term Bode plot is used for the pair of
graphs of the logarithm to base 10 of the gain plotted against the
logarithm to base 10 of the frequency and the phase angle plotted against
the logarithm to base 10 of the frequency. The reason for the graphs
being in this form is that it enables plots for complex frequency response
functions to be obtained by merely adding together the plots obtained for
each constituent element.
Suppose we have a sinusoidal input to a number of systems in series
(Figure 5.7). The first produces a gain of IG~0co)l and a phase angle shift
of ~bl, the second produces a gain of IG2Oco)l and a phase angle shift of ~b2,
and the third produces a gain of IG3fjco)l and a phase angle shift of ~3.
The overall gain of the system will be the products of the gains of each of
the systems and thus be: