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Chapter 12

Introduction to Software-Defined Radios (SDRs)

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Chapter 11 presented a different type of superheterodyne radio-a coil-less
receiver. Two new concepts were introduced-image signals and switching-type
radio-frequency (RF) signal mixers. It was also in Chapter 11 that image signals
were shown to be eliminated or attenuated by providing a band-pass filter prior to
RF mixer so that the desired signal is output from the mixer and not the image

signaL In the coil-less radio, a higher intermediate frequency (IF) of 600 kHz was
used with a 1.6-MHz low-pass filter to provide image signal rejection. The coil-less
radio is rather simple to implement with integrated circuits.
It should be noted that in Chapters 8 through 11, the selectivity of the
superheterodyne radio was influenced by the band-pass characteristics of the IF
filter. If more selectivity is required, then usually more IF transformers, coils,

and/or ceramic filters must be added to the design. And if the IF needs to be
changed to some frequency other than 455 kHz, then the IF transformers or
ceramic filters need to be replaced.
So one of the main motivations for a using a computer or digital processing in
superheterodyne radios is to replace the IF section, whereby the IF filters can be

generated in the digital domain. Once an analog signal is digitized by an
analog-to-digital converter, the digital domain can take over to mimic many of the
past analog functions, such as filtering, RF mixing, delaying signals, or amplifying
the IF signals. In the digital domain, the frequency and band-pass characteristic
(e.g., bandwidth) can be designed with flexibility. For example, with digital filtering,
the band-pass filter characteristics can be tailored to any specific bandwidth without
changing the hardware.

So, if one needs a 4SS-kHz filter in the digital domain to have narrower or broader
bandwidth, just setting some registers in the digital filter program changes the filter
characteristics. So there is no need to change a physical part. The filter's
characteristic is determined by software.

And as technology progresses, more and more circuits can be mimicked in a
computer or in a system on a chip via digital signal processing to provide not only
filtering but also amplification, tuning across the IF band (e.g., 44.1 kHz to 192
kHz), phase shifting, multiplication (e.g .. , RF mixing) that includes image-reject
mixing, and detecting [amplitude-modulated (AM) and frequency-modulated (FM)
demodulators] via software.

One popular implementation of a software-defined radio (SDR) on a chip is a
two-band 2-m and 70-cm transceiver made by Baofeng. This radiO sells for about
$40 and uses a dedicated 5DR integrated circuit, the RDA 1846. Because this radio