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Most of these blocks are
discussed individually,
and in more detail, on
other pages.
See filters, mixers,
frequency changers, am
modulation and
amplifiers.
There are signals from
thousands of radio
transmitters on many
different frequencies
inducing signal voltages
in the aerial.
The rf filter selects
the desired station from
the many.
It is adjustable so that
the selection frequency
can be altered.
This is called TUNING.
The selected frequency
is applied to the mixer.
The output of an
oscillator is also
applied to the mixer.
The mixer and oscillator
form a FREQUENCY CHANGER
circuit.
The output from the
mixer is the
intermediate frequency (i.f.)
The i.f. is a fixed
frequency of about 455
kHz.
No matter what the
frequency of the
selected radio station
is, the i.f. is always
455 kHz.
The i.f. signal is fed
into the i.f. amplifier.
The advantage of the i.f.
amplifier is that its
frequency and bandwidth
are fixed, no matter
what the frequency of
the incoming signal is.
This makes the design
and operation of the
amplifier much simpler.
The amplified i.f.
signal is fed to the
demodulator.
This circuit recovers
the audio signal and
discards the r.f.
carrier.
It usually incorporates
a diode in the circuit.
Some of the audio is fed
back to the i.f.
amplifier as an
AUTOMATIC GAIN CONTROL
voltage.
This ensures that when
tuning from a weak
station to a strong one,
the loudness from the
loudspeaker stays the
same.
The audio signal voltage
is increased in
amplitude by a voltage
amplifier.
The power level is
increased sufficiently
to drive the loudspeaker
by the power amplifier. |
