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Bandpass Signaling Principles and Circuits Chap. 4
IF signal
v IF (t)=Re[g(t)e v IF t ] 2 cos(v IF t)
I channel
LPF
x(t)
Q channel
LPF
y(t)
–2 cos(v IF t)
Oscillator
f=f IF
Figure 4–31
±90°
phase shift
IQ (in-phase and quadrature-phase) detector.
product detectors, as illustrated in Fig. 4–31. x(t) and y(t) could be fed into a signal processor
to extract the modulation information. Disregarding the effects of noise, the signal processor
could recover m(t) from x(t) and y(t) (and, consequently, demodulate the IF signal) by using the
inverse of the complex envelope generation functions given in Table 4–1.
The superheterodyne receiver has many advantages and some disadvantages. The main
advantage is that extraordinarily high gain can be obtained without instability (self-oscillation).
The stray coupling between the output of the receiver and the input does not cause oscillation
because the gain is obtained in disjoint frequency bands—RF, IF, and baseband. The receiver is
easily tunable to another frequency by changing the frequency of the LO signal (which may be
supplied by a frequency synthesizer) and by tuning the bandpass of the RF amplifier to the
desired frequency. Furthermore, high-Q elements—which are needed (to produce steep filter
skirts) for adjacent channel rejection—are needed only in the fixed tuned IF amplifier. The
main disadvantage of the superheterodyne receiver is the response to spurious signals that will
occur if one is not careful with the design.
Zero-IF Receivers
When the LO frequency of a superheterodyne receiver is selected to be the carrier
frequency (f LO = f c ) then f IF = 0, and the superheterodyne receiver becomes a zero-IF
or direct conversion receiver. † In this case, the IF filter becomes a low-pass filter (LPF).
This mixer–LPF combination functions as a product detector (and the detector stage of
Figure 4–29 is not needed). A quadrature down converter can also be added so that the x(t)
and y(t) components of the complex envelope can be recovered. In this case the zero-IF
receiver has a block diagram as shown in Fig. 4–31, where the input signal is at f c and v c
replaces v IF in the figure. The components x(t) and y(t) may be sampled and digitized with
ADC so that the complex envelope, g(t) = x(t) + jy(t), may be processed digitally with DSP
hardware, which is discussed in Sec. 4–17. The analog LPF acts as an antialiasing filter for
the sampler and the DSP hardware. The zero-IF receiver is also similar to a TRF receiver
with product detection.
† A direct-conversion receiver is also called a homodyne or synchrodyne receiver.