Page 184 - Fundamentals of Communications Systems

6.6 Chapter Six

The demodulator is quite simple once the phase induced in the propagation
from transmitter to receiver is identified.

6.1.2 Coherent Demodulation
An important function of a DSB-AM demodulator is producing the appropriate
value of φ p for good message reconstruction. Demodulators that require an ac-
curate phase reference like DSB-AM requires are often called phase coherent
demodulators. Often in practice this phase reference is obtained manually with
a tunable phase shifter. This is unsatisfactory if one or both ends of the link are
moving (hence a drifting phase) or if automatic operation is desired.
Automatic phase tracking can be accomplished in a variety of ways. The tech-
niques available for automatic phase tracking are easily divided into two sets
of techniques: a phase reference derived from a transmitted reference and a
phase reference derived from the received modulated signal. Note, a transmit-
ted reference technique will reduce the efficiency of the transmission since part
of the transmitted power is used in the reference signal and is not available at
the output of the demodulator. Though a transmitted reference signal is waste-
ful of transmitted power it is often necessary for more complex modulation
schemes (e.g., see Section 6.3.3). For each of these above mentioned techniques
two methodologies are typically followed in deriving a coherent phase reference;
open loop estimation and closed loop or phase-locked estimation. Consequently,
four possible architectures are available for coherent demodulation in analog
communications.
An additional advantage of DSB-AM is that the coherent reference can easily
be derived from the received modulated signal. Consequently, in the remainder
of this section the focus of the discussion will be on architectures that enable
automatic phase tracking from the received modulated signal for DSB-AM. The
block diagram of a typical open loop phase estimator for DSB-AM is shown in
Figure 6.7. The essential idea in open loop phase estimation for DSB-AM is
that any channel induced phase rotation can easily be detected since DSB-AM
only uses the real part of the complex envelope. Note that the received DSB-AM
signal has the form

y z (t) = x z (t) exp[ j φ p ] = A c m(t) exp[ j φ p ] (6.6)

Vt () = A m t ()exp [ j2φ p] + N t ()
2
2
z
c
V
() • 2 Hf () arg • {} ÷2 φ ˆ p
z
Yt () = () [ jφ p] + N t () 2φ ˆ p
x t exp
z
z
z
Figure 6.7 An open loop phase estimator for DSB-AM.