Signal Processing for Wireless Transceivers - Signal Processing Systems - page 202

the desired channel center-frequency. In any case, the frequency separation between
the desired and image signals is always 2 f LO . Thus in practice the image band is
located at the distance 2 f IF either below or above the desired channel, depending on
the side of LO injection. The basic superheterodyne principle can also be extended
to double-IF or triple-IF scenario where the signal is brought to baseband through
many consecutive IFs, and selectivity is implemented step by step.
From the receiver design point of view, a proper compromise is required in
selecting or specifying the intermediate frequency. On one hand, a high enough
IF should be used since the desired and image bands are separated by 2 f IF and the
image rejection filtering is performed at RF. On the other hand, a low enough IF is
needed to make the implementation of the IF channel selectivity filtering as feasible
as possible. As an example, intermediate frequencies around 71 MHz (first) and
13 MHz (second) are traditionally used in superheterodyne based GSM receivers,
whereas IFs around 10 MHz are typical in broadcast FM receivers.

4.4.2 Direct-Conversion Receiver

Due to the high number of discrete components and high power consumption, the
above superheterodyne architecture is, however, not the most appropriate choice for
highly integrated transceiver implementations in mass-market devices. Furthermore,
the use of fixed discrete components in the RF front-end limits the receiver
flexibility. Thus, architectures with more simplified analog front-ends with less RF
processing are in general desirable.
A simple way to reduce the number of components in the receiver and alleviate
the problem of receiver complexity is to avoid the use of intermediate frequency
stage and use complex or quadrature downconversion of the desired channel signal
from RF directly to baseband. Complete elimination of the IF stage results in highly
simplified structure where most of the channel selectivity and amplification are
implemented at baseband. In practice, depending on the performance of the A/D
interface, the overall selectivity can be split properly between analog and digital
filters. On one hand, since most of the signal processing tasks take place at low
frequencies, the power consumption of the radio is minimized. On the other hand,
very low noise operation is called for in all the remaining analog components since
the amplification provided by the RF stage is only moderate. The basic block-
diagram for RF I/Q downconversion based receivers is illustrated in Fig. 21 .
In theory, the complex mixing approach corresponds to pure frequency trans-
lation and the image signal related problems present in real mixer are basically
avoided. In practice, however, complex-valued processing always calls for two
parallel signal branches (I and Q, e.g. two mixers and LO signals in case of real-
valued input and complex mixer) whose characteristics are (unintentionally) likely
to differ to some extent. This so-called I/Q imbalance problem has the net effect of
reducing the image rejection capability to only 20 40 dB in practical analog I/Q

front-ends, at least without digital calibration. In the pure direct-conversion radio,

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