amplify the small signal received.
Some cheap receivers omitted this
stage. The signal then goes to the
mixer stage that does the down
conversion to IF. It does this by
heterodyning the incoming signal
with a higher frequency local
oscillator (LO) signal.
The mixer produces the sum and
difference signals. The difference
signal is usually selected as the IF.
For example, assume the signal is
on 98.7 MHz. The mixer combines
this with a 109.4 MHz LO signal to
generate the sum of 208.1 MHz and
the difference of 10. 7 MHz. The sum
signal is filtered out and the
difference is the IF. The 10. 7 MHz
signal still has all the original
modulation on it. That IF signal is
then amplified, and selective filters
make sure that signals outside the
band are rejected.
After some IF amplification and
filtering, the signal goes to a detector
or demodulator where the
modulation is removed. Therefore,
the originally transmitted voice or
music is recovered. It is then
amplified in an audio amplifier and
applied to a speaker or some
THE DIGITAL RADIO
Many units today are what we
call software-defined radios (SDR).
These radios use a mix of analog and
While the rest of the
has gone digital,
broadcast radio is
still mostly analog.
digital techniques to allow almost full
integration of most radio functions —
including tuning — inside a few ICs.
The SDR still uses an RF amplifier to
boost the signal level and a mixer to
down-convert the signal to a lower IF.
Then, however, an analog-to-digital
converter (ADC) digitizes the signal
into binary words that are then sent
Heterodyning is a radio signal processing technique invented
in 1901 by Canadian inventor-engineer Reginald Fessenden, in
which new frequencies are created
by combining or mixing two
frequencies. Heterodyning is useful
for frequency shifting signals into a
new frequency range, and is also
involved in the processes of
modulation and demodulation. The
two frequencies are combined in a
nonlinear signal-processing device
such as a vacuum tube, transistor,
or diode, usually called a mixer. In
the most common application, two
signals at frequencies f1 and f2 are
mixed, creating two new signals,
one at the sum f1 + f2 of the two
frequencies, and the other at the
difference f1 − f2. These new
frequencies are called heterodynes.
Typically only one of the new
frequencies is desired, and the
other signal is filtered out of the
output of the mixer. Heterodynes
are closely related to the
phenomenon of "beats" in music.
June 2013 63