Elenco SEE AMFM108CK - Manual (Page 33)

SECTION 6
THE FM RADIO
Figure 26
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In the AM DETECTOR section we observed that the
audio was detected from changes in the amplitude of
the incoming signal. In FM detection, the audio is
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. The RATIO
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. Thus, no 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.
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 work back to the FM Antenna. Each
stage will be tested before proceeding to the next
stage.
FM RATIO DETECTOR
Section 9
FM RF
AMPLIFIER
FM
OSCILLATOR
1ST FM IF
AMPLIFIER
AFC
Section 8 Section 7 Section 6
FM MIXER
2ND FM IF
AMPLIFIER
FM
DETECTOR
AUDIO
AMPLIFIER
Speaker
FM RADIO
Section 1
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