As discussed in the previous section, we regard the signal

as a positive-frequency sinusoid when
. In a
manner analogous to spectral magnitude plots (discussed in
§4.1.6), we can plot this complex sinusoid over a frequency
axis as a vertical line of length at the point
, as shown in Fig.4.10. Such a plot of
amplitude versus frequency may be called a spectral plot, or
spectral representation [43] of the (zero-phase)
complex sinusoid.

Figure 4.10:
Spectral plot of a complex sinusoid
.

More generally, however, a complex sinusoid has both an amplitude and
a phase (or, equivalently, a complex amplitude):

To accommodate the phase angle in spectral plots, the
plotted vector may be rotated by the angle in the plane
orthogonal to the frequency axis passing through , as done
in Fig.4.16b below (p. )
for phase angles
.