Page 432 - Complete Wireless Design

Wireless Issues

Wireless Issues 431

ities, and actually supplants the LO, channel select filter, quadrature mixer,
and data decimation filter. The RSP is basically a special-purpose DSP, and
must be capable of the same speed as the A/D converter. The output of the RSP
is then sent to the DSP, which demodulates the baseband signal. The DSP,
depending on its programming, can demodulate both digital and analog sig-
nals, and can thus receive either/or FM, AM, QPSK, CDMA, etc.
A fast and quiet ADC makes software radio possible. The A/D converter
must have a high SNR rating, which embodies both quantization and thermal
noise, as well as the A/D sample clock’s wideband phase noise. High dynamic
range is also a must in order to decrease spurious responses, since in a multi-
carrier radio the spurious responses of one channel can interfere with the
weaker signal of another channel.
The transmitter of a heterodyne SDR, as shown in Fig. 10.14, functions so:
The DSP places digital data to be modulated into the transmit signal proces-
sor (TSP). The TSP then modulates the carrier with this information. The dig-
ital modulated signal is transformed to analog by the D/A converter, sent into
the analog stages and frequency up-converted to RF by the mixer/LO, ampli-
fied by the power amplifier (SSPA), filtered, and sent out of the antenna.
DC SDR. Another SDR architecture is the direct conversion (DC) design. This
technique reduces much of the heterodyne system’s disadvantages, such as a
fixed front-end filter to set the heterodyne radio’s frequency and bandwidth,
and converts the RF directly into a zero IF with no image frequency. Since
front-end RF filters that are not only center frequency tunable, but also main-
tain their return loss and have a tunable bandwidth, are extremely difficult to
design and mass-produce, each wireless standard that the heterodyne SDR
would be required to emulate would need, at a minimum, an expensive and
large switchable filter bank, along with a fine-tunable LO. The DC SDR
removes this problem and lowers system costs.

10.5 Hybrid Circuits
10.5.1 Introduction
Hybrid design and construction predates today’s monolithic ICs, but hybrids
have most assuredly not been completely supplanted by them in all elec-
tronic products and applications. But, hybrids are used only when ICs or
common surface mount discrete printed circuit board designs are inferior
and weight, space, cost, and performance are vital for specialized or high-
reliability situations. Hybrid circuits can also be an excellent choice for low-
volume production runs when custom monolithic ICs would have too high an
initial design and cost.
RF hybrids will appear to an observer as simply a large IC, or as a small
populated PCB if the hybrid is left unpackaged. In most consumer applica-
tions, the completed hybrid circuit will be placed in a special plastic package,
and silicone or resin is then poured over it, or the circuit itself can be dipped

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