Page 430 - Complete Wireless Design

Wireless Issues

Wireless Issues 429

adding the ultimate rejection of the SAW with the SAW’s insertion loss. For
example, if a SAW has an insertion loss of 20 dB, and its ultimate attenuation
is 50 dBc, then the PC board will be required to supply a minimum of 70 dB of
isolation to maintain the SAW’s expected performance. This can be accom-
plished by isolating the SAW filter’s input and output ports from each other by
a plated-through slot, in which a piece of metal is placed through the board’s
dielectric to its ground between the SAW’s ports. The plated-through slot will
decrease the RF leakage through the substrate material of the PCB. Other
methods that can be used in conjunction are:

Placing a metal shield around the SAW, which separates its input/output
ports above the PCB
Assuring that the SAW’s case has multiple direct connections to the ground
plane
Positioning the input/output port’s matching inductors at right angles to
each other
Using shielded inductors

10.4 Software Radio
10.4.1 Introduction
Because of advances in high-speed analog-to-digital converters (ADCs), inte-
grated mixers, and digital signal processors (DSPs), digital radio—at least up
to the first IF stage—has become a reality for some high-end communications
systems. However, for the foreseeable future, complete software radios will be
a design curiosity that will be adopted in a few expensive, and relatively low-
frequency, radios. It will take time to perfect this technology to be viable in the
lower-cost, higher-frequency radio area.

10.4.2 Software radio designs
The dream of a radio that can economically change from one type of wireless
device to another, from one modulation and band to another, and from one
bandwidth and frequency to another by simply re-programming is the promise
of software radio. A complete software radio has yet to be designed for the mass
market—but the digital back end is beginning to reach closer and closer to the
RF front end of the receiver.
There are basically three types of software radios: software-defined radio,
which changes a restricted number of distinct hardware functions by software
(this is already a reality in some dual-mode handset radios and base stations);
software radio, in which some of the analog circuitry is replaced by software,
while the rest of the analog circuits are reconfigurable by software (this is
expected to become more common by the year 2004); and ideal software radio,
in which a limited amount of front-end analog circuitry is fixed and untunable,

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