Tektronix AWG 2021 - Manual (Page 19)

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
Figure 22. 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 –
π
to +
π
in
π
/4 steps. If the 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