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THE FM RADIO
Section 6 begins the construction of the FM radio. The
stages that we will build are shown in the block diagram
below. We will begin with the FM Ratio Detector and
Section 9
FM RF
FM MIXER
AMPLIFIER
FM
AFC
OSCILLATOR
FM RATIO DETECTOR
In the AM DETECTOR section we observed that the
audio was detected from changes in the amplitude of
the incoming signal.
detected from changes in frequency of the incoming
signal. The RATIO DETECTOR has built-in limiting
action which limits the signal so that any noise riding on
the FM carrier will be minimized.
DETECTOR is redrawn below for ease of explanation.
When an incoming signal is present at T4 and T5, a
current flows through D2, R26, R28, R27 and D3. At no
modulation, the current through the diodes D2 and D3
are equal because T5 is center tapped.
SECTION 6
Section 8
1ST FM IF
AMPLIFIER
In FM detection, the audio is
The RATIO
Thus, no
µF
µF
work back to the FM Antenna. Each stage will be
tested before proceeding to the next stage.
FM RADIO
Section 7
Section 6
2ND FM IF
AMPLIFIER
DETECTOR
current is drawn through C23 resulting in zero audio
output voltage. When the incoming signal is modulated,
the current through one diode will be greater than the
other. This causes a current to flow in C23 which will
produce an audio voltage across C23. If the modulation
is of opposite direction than before, more current will
flow in the other diode, which will again cause current
to flow in C23 in the opposite direction resulting in an
audio voltage being produced across C23. The large
current drawn from the audio which causes the battery
voltage to vary. The ratio detector is decoupled further
by the resistor R23 and capacitor C21.
Figure 27
-33-
Section 1
FM
AUDIO
AMPLIFIER
µF
µF
Speaker
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