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OPENCOMMUNICATION
; BY LOU FRENZEL W5LEF
THE LATEST IN NETWORKING AND WIRELESS TECHNOLOGIES
Making a Modern Radio
A look at how digital techniques have
changed and improved analog radios.
esign and construction
of a basic analog radio has
changed drastically over the
years. Radio architecture
has evolved from multi-tube
designs, to transistors, to
integrated circuits, and now,
today, to a single chip.
While the rest of the
electronics world has gone
digital, broadcast radio is
still mostly analog. AM, FM,
and shortwave (SW)
broadcasts are analog, and
there are few amongst us
that do not listen to one or
more of them. That is why
there are still hundreds of
millions of analog radios
still made each year for
automobiles, bedside clock
radios, portables, high-end
stereo systems, and others.
With its enormous
population, China is now
the largest market for
analog radios, but there are
still millions sold here in the
US.
D
RADIO BASICS
Any radio needs two main
things: sensitivity and selectivity.
Sensitivity means the radio’s ability
to receive very small signals and
extract the voice, music, or data.
Receivers are sensitive because of
the gain they provide. Usually,
multiple amplifiers are used to allow
microvolt level signals to produce full
output. The signals are usually
amplified before demodulation and
after demodulation.
As for selectivity, a radio must be
able to be tuned to the desired
station, and it must be able to
distinguish one signal from another. A
radio’s tuner is just a variable filter
that will let the desired signal
through, and reject others nearby in
frequency. In crowded urban areas
with many stations, this is especially
critical when most broadcast stations
are running many kilowatts of power
and can easily encroach on one
another. Older inductor-capacitor
(LC) filters did a fair job, but more
recent crystal or ceramic resonator
filters are superior. Today, modern
digital signal processing (DSP) filters
do an even better job.
RADIO CIRCUITS
Most radios evolved from a basic
design referred to as a
superheterodyne. This type of radio
converts the incoming signal down in
frequency, usually to a lower
frequency called the intermediate
frequency (or IF). This is done to
simplify tuning and provide good
fixed frequency selectivity; refer to
Figure 1. The basic design includes
an RF amplifier after the antenna to
; FIGURE 1.
Block diagram of a basic superheterodyne radio
showing the mixer that down-converts the signal to a
low intermediate frequency before demodulation.
62
June 2013
