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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.
The f.m. band covers
88-108 MHz.
There are signals from
many radio transmitters
in this band inducing
signal voltages in the
aerial.
The rf amplifier selects
and amplifies the
desired station from the
many.
It is adjustable so that
the selection frequency
can be altered.
This is called TUNING.
In cheaper receivers the
tuning is fixed and the
tuning filter is wide
enough to pass all
signals in the f.m.
band.
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 10.7 MHz.
No matter what the
frequency of the
selected radio station
is, the i.f. is always
10.7 MHz.
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.
Some of the audio is fed
back to the oscillator
as an AUTOMATIC
FREQUENCY CONTROL
voltage.
This ensures that the
oscillator frequency is
stable in spite of
temperature changes.
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. |
