The Fm Radio; Fm Ratio Detector - Elenco Electronics AM/FM-108K Assembly And Instruction Manual

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
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. 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.
.01µF
SECTION 6
FM RADIO
Section 8 Section 7
1ST FM IF 2ND FM IF
AMPLIFIER AMPLIFIER
µF
Figure 26
-32-
Detector and work back to the FM Antenna. Each
stage will be tested before proceeding to the next
stage.
Section 6
FM
DETECTOR
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.
.02µF
Section 1
Speaker
AUDIO
AMPLIFIER
µF