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Bandpass Signaling Principles and Circuits Chap. 4
Basebands circuits
x(t)
I channel
RF circuits
m(t)
Modulation
in
Baseband signalprocessing
(Type II)
circuit may
be nonlinear
y(t)
cos(v c t)
Q channel
+
–
v(t)=x(t) cos(v c t)-y(t) sin(v c t)
Carrier
–90°
oscillator f
phase
c
shift
sin(v c t )
cos(w c t)
Figure 4–28
Generalized transmitter using the quadrature generation technique.
Example 4–5 GENERATION OF A QM SIGNAL BYIQ PROCESSING
Rework Example 4–1, but generate the QM signal by using IQ processing as shown in
Fig. 4–28. See Example4_05.m for the solution. Note that the QM signal generated by IQ
processing is identical to that obtained by complex envelope processing used for Example 4–1.
Once again, it is stressed that any type of signal modulation (AM, FM, SSB, QPSK,
etc.) may be generated by using either of these two canonical forms. Both of these forms
conveniently separate baseband processing from RF processing. Digital techniques are especially
useful to realize the baseband-processing portion. Furthermore, if digital computing
circuits are used, any desired type of modulation can be obtained by selecting the appropriate
software algorithm.
Most of the practical transmitters in use today are special variations on these canonical
forms. Practical transmitters may perform the RF operations at some convenient lower RF
frequency and then up-convert to the desired operating frequency. In the case of RF signals
that contain no AM, frequency multipliers may be used to arrive at the operating frequency.
Of course, power amplifiers are usually required to bring the output power level up to the
specified value. If the RF signal contains no amplitude variations, Class C amplifiers (which
have relatively high efficiency) may be used; otherwise, Class B amplifiers are used.
Generalized Receiver: The Superheterodyne Receiver
The receiver has the job of extracting the source information from the received modulated
signal that may be corrupted by noise. Often, it is desired that the receiver output be a replica
of the modulating signal that was present at the transmitter input. There are two main classes
of receivers: the tuned radio-frequency (TRF) receiver and the superheterodyne receiver.