Page 407 - Complete Wireless Design

Communications System Design

406 Chapter Nine

to properly drive the power amplifier (PA). In this FM case, the PA will run
saturated, which demands that the driver has enough gain to keep it so.
Some of the signal is tapped by the coupler for output power confirmation to
a microprocessor, with the majority of the signal sent to and filtered by the
duplexer, and broadcast out of the antenna.
The power amplifier stage can be the most difficult part of the transmitter
to design. The PA must be very tolerant of low output return losses, which are
caused by the large impedance variations created by the mobile antenna and
its rapidly changing physical environment, as well as reflections off the
duplexer’s reflective stopbands. Not only must the PA not be destroyed by
these VSWR variations, but must also not degrade total performance specifi-
cations. In fact, many FM services demand that the PA be able to vary its pow-
er output over some specified range, be highly efficient (for increased battery
life), generate minimal harmonic levels, and have enough gain for efficient sat-
urated output.

9.4.3 System design with RFICs
Most wireless system designs (Fig. 9.9) now rely heavily on the ever-increas-
ing use of radio-frequency integrated circuits (RFICs). RFICs may contain the
complete LNA/mixer stages, the entire IF stages, or even the total transceiv-
er. However, a complete transceiver on a single RFIC chip is usually available
only for low-data-rate or low-cost voice applications—at least for the foresee-
able future, and with the single exception of the upcoming one-chip solution
for Bluetooth devices (for further information on this important wireless tech-
nology, please go to Bluetooth.com). Nonetheless, higher integration levels are
slowly becoming a reality as more and more companies attempt to make a
complete wireless system on a chip.
Depending on the frequencies and specifications required, adopting RFICs
can significantly lower design and production costs, as well as the physical
size, of the complete radio system. In fact, multiple RFICs have already
replaced many discrete systems, as high levels of integration can allow one
RFIC to replace dozens, if not hundreds, of components. This not only decreas-
es the cost of designing the communications system in the first place, but also
simplifies the actual construction of the systems themselves.
RFICs are available for each part of a radio, with the level of the desired
integration depending on the design flexibility required. The design of Fig. 9.9
demonstrates low levels of RFIC integration, but still allows for a far more
rapid time to market than a completely discrete design. Most of the chips for
this, and similar radio designs, are available from MAXIM, RFMD, Mini-
Circuits, National, Motorola, etc.
Many designs must still rely on discrete, or at least individual MMIC,
amplifiers and mixers when RFICs are not compatible with a particular
design goal, while other wireless systems may have to employ an almost
entirely discrete design for price/performance reasons. Indeed, the RFIC solu-
tion is compatible with a particular wireless design only if the chip already

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