Page 202 - Fundamentals of Communications Systems

6.24 Chapter Six

0.8

0.6

0.4

0.2
x Q (t) 0

−0.2

−0.4

−0.6

−0.8
−1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8
x (t)
l
Figure 6.26 Vector diagram for the SSB-AM transmission in Example 6.10.

the SSB-AM transmission of Example 6.10. Clearly if one compares the vector
diagram shown in Figure 6.8(a) to the vector diagram in Figure 6.26, it is easy
to see why automatic phase recovery from the received signal is tough. So, while
VSB-AM provides improved spectral efficiency it does it at the cost of increased
demodulator complexity.
If automatic phase tracking is desired with VSB-AM, a transmitted reference
signal is often used. We will explore this idea in a SSB-AM application. The block
diagram of an example SSB-AM transmitter system that uses a transmitted
reference is given in Figure 6.27. The notch filter removes the message signal
components from around DC. The in-phase signal consists of this notched-out
message and a DC term. This DC term when upconverted will result in a carrier

A r

Notch x (t)
I
m(t) Σ x (t)
c
Filter I/Q
x (t) Upconverter
Q
h (t)
Q
f c
Figure 6.27 A modulator implementation for SSB-AM using a transmitted reference signal.