Question 2.7: For a tribit input of Q = 0, I = 0, and C = 0 (000), determi......
For a tribit input of Q = 0, I = 0, and C = 0 (000), determine the output phase for the 8-PSK modulator shown in Figure 23.
The inputs to the I channel 2-to-4-level converter are I = 0 and C = 0. From Figure 24 the output is -0.541 V. The inputs to the Q channel 2-to-4-level converter are Q = 0 and \overline{C} = 1.
Again from Figure 24, the output is -1.307 V.
Thus, the two inputs to the I channel product modulators are -0.541 and sin ω_{c}t. The output is
I = (-0.541)(sin ω_{c}t) = -0.541 sin ω_{c}t
The two inputs to the Q channel product modulator are -1.307 V and cos ω_{c}t. The output is
Q = (-1.307)(cos ω_{c}t) = -1.307 cos ω_{c}t
The outputs of the I and Q channel product modulators are combined in the linear summer and produce a modulated output of
summer output = -0.541 sin ω_{c}t – 1.307 cos ω_{c}t
= 1.41 sin(ω_{c}t – 112.5°)
For the remaining tribit codes (001, 010, 011, 100, 101, 110, and 111), the procedure is the same. The results are shown in Figure 25.
