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Bandpass Signaling Principles and Circuits Chap. 4
signal with digital signal-processing (DSP) hardware. Software would be used to compute the
receiver output. The difficulty with this approach is that it is almost impossible to build
ADCDSP hardware that operates fast enough to directly process wideband modulated signals
with gigahertz carrier frequencies [Baines, 1995]. However, the complex envelope of these
signals may be obtained by using a superheterodyne receiver with quadrature detectors
(Fig. 4–31). For sufficiently modest bandpass bandwidth (say, 25 MHz), the I and Q components,
x(t) and y(t), of the complex envelope can be sampled and processed with practical DSP
hardware so that software programming can be used.
In another approach, a high-speed ADC can be used to provide samples of the IF signal
that are passed to a digital down-converter (DDC) integrated circuit (e.g., Intersil, HSP50016)
[Chester, 1999]. The DDC multiplies the IF samples with samples of cosine and sine LO
signals. This down-converts the IF samples to baseband I and Q samples. The DDC uses
ROM lookup tables to obtain the LO cosine and sine samples, a method similar to the direct
digital synthesis (DDS) technique discussed in Sec. 4–15. To simultaneously receive multiple
adjacent channel signals, multiple DDC ICs can be used in parallel with the LO of each DDC
tuned to the appropriate frequency to down convert the signal to baseband I and Q samples for
that signal. (For more details, see the Intersil Web site at http: www.intersil.com.)
The I and Q samples of the complex envelope, g(t) = x(t) + jy(t), can be filtered to
provide equivalent bandpass IF filtering (as described in Sec. 4–5). The filtering can provide
excellent equivalent IF filter characteristics with tight skirts for superb adjacent channel interference
rejection. The filter characteristic may be changed easily by changing the software.
Raised cosine-rolloff filtering is often used to reduce the transmission bandwidth of digital
signals without introducing ISI. For minimization of bit errors due to channel noise, as well as
elimination of ISI, a square-root raised-cosine filter is used at both the transmitter and the
receiver [as shown by Eq. (3–78) of Sec. 3–6].
AM and PM detection is accomplished by using the filtered I and Q components to
compute the magnitude and phase of the complex envelope, as shown by Eqs. (4–4a) and
(4–4b), respectively. FM detection is obtained by computing the derivative of the phase, as
shown by Eq. (4–8).
The Fourier transform can also be used in software radios, since the FFT can be computed
efficiently with DSP ICs. For example, the FFT spectrum can be used to determine the
presence or absence of adjacent channel signals. Then, appropriate software processing can
either enhance or reject a particular signal (as desired for a particular application). The FFT
can also be used to simultaneously detect the data on a large number of modulated carriers
that are closely spaced together. (For details, see Sec. 5–12 on OFDM.)
The software radio concept has many advantages. Two of these are that the same hardware
may be used for many different types of radios, since the software distinguishes one type
from another, and that, after software radios are sold, they can be updated in the field to include
the latest protocols and features by downloading revised software. The software radio concept
is becoming more economical and practical each day. It is the “way of the future.”
For additional reading about practical software radio design and designed circuits, see
the 2011 ARRL Handbook [ARRL, 2010]. To explore hands-on design of a software radio, go
to http:gnuradio.org. GNU Radio is a free software toolkit for learning about, building, and
deploying Software Defined Radio systems. A description of GNU Radio is also available on
Wikipedia.