19

Digital Phase Modulation — PSK

8
The modulating signals in the
foregoing examples have been
sinusoidal or continuous wave-
forms. A simple step to digital
modulation is made with a slight
variation to sinusoidal modula-
tion. Figure 21 shows one cycle
of a sinewave that has been
quantized into steps between
–0.5 and +0.5. The equation
defining these steps is shown in
Figure22. The second line
simply quantizes a cosine wave
by rounding and scaling the
continuous waveform to the
nearest eighth. This quantized
modulating pattern is then
directly inserted in the phase
argument of a cosine carrier.
Thus, the phase argument takes
on values between –πto +πin
π/4 steps. Ifthe polar graphical
representation of the signal is
used, a family of eight points of
equal magnitude is defined,
spaced around the circle in π/4 or 45°phase increments.
Figure 21. The sinusoidal modulating pattern is
quantized into discrete steps. The steps are
equally spaced in amplitude and will shift the
phase of the carrier in π/4 or 45°increments.
Digital Modulation