Page 1
Figure 11
All, or only one of the IF amplifier stages mayoperate as limiters. For a very weak signal,
only the fourth IF amplifier Q7 may be limiting,
and transistors Q4, Q5, and
Q6 would be
amplifying the IF signal. For a very strong
signal, all four IF amplifiers may be acting as
limiters. This limiting action removes
ampli-
tude modulation from the FM signal. Limiting
action is also provided by the self-limiting
characteristics of the ratio detector circuit.
RATIO DETECTOR CIRCUIT
From the collector of transistor Q7, the IF signal
is coupled through resistor R26 and ratio de-
tector transformer T6 to the ratio detector
cir-
cuit. This circuit, which separates the audio sig-
nal from the 10.7 mc IF signal. is shown re-
drawn for greater clarity and simplified in Fig-
ure 11. Transformer T6 is represented in this
figure by primary coil
Ll, a center tapped sec-
ondary composed of coils L2 and L3.
and a third
or tertiary winding. L4. L4 is just a few turns of
wire
tightly wrapped around the bottom of pri-
mary
Ll. NOTE: In the actual circuit, choke L5
and coil L6 are also connected in series with
coil L4, resistor R31, capacitor C39, and re-
sistor R39,
Consider a separate voltage to be induced by the
primary into each of the windings, L2, L3, and L4. L4, which is closely coupled to the primary
introduces a voltage that is in series with both
L2 and L3. This voltage across L4 is relatively
constant in amplitude as long as the voltage
across
Ll does not change. (Remember, the
voltage across
Ll will stay relatively constant
due to the limiting action of transistor Q7.) Notice that each diode has its own separate loop
through which its current flows (indicated by the
arrows). Current flowing in diode D3 is con-
trolled by the voltage induced in L2 and L4 which
charges capacitor C33. The current flowing in
diode D4 is controlled by the voltage induced
in coils L3 and L4 which charges capacitor C34.
Current flows through L4 in both directions,
since this coil is common to both current loops,
The two currents flow through capacitors C33
and C34 in the same direction. Electrolytic
capacitor C35 is connected across both of these
capacitors through resistors R27 and R30.
This large capacitor keeps the total voltage
across these two capacitors from changing, thus,
any amplitude changes on the IF signal are
damped out by this capacitor.
The audio output signal from the ratio detector
circuit is applied to the base of Q8. Note that
the two loop currents are flowing in opposite
directions through coil L4, resistor R31, capac-
itor C39, and the input resistance of Q8. At the
FM IF center frequency of 10.7 mc, the diode
currents are equal, thus they cancel each other
out and no voltage appears across the input re-
sistance of Q8.
When the IF frequency deviates from 10.7 mc
due to FM modulation (audio signal), the current in one diode loop increases while the current in
the other loop decreases. These changes are
caused by a change in phase relationship in the
signal current across coils L2 and L4. and L3
and L4. Now current flows through the input
resistance of Q8 in the direction of the larger
signal, and an output voltage is developed
Page 2
RIC>HT signal R
Figure 12
across the input resistance of Q8, The ampli-
tude of this output voltage is determined by
how far the IF frequency deviates from the
center frequency of 10.7 mc. The frequency
of this audio output voltage is determined by
how often the frequency deviates from 10.7 mc.
.-
The slug in the secondary of coil T6 is used to
balance the ratio detector circuit. Capacitor
C36 and L5 removes any remaining 10.7 mc IF
signal from the audio signal. Resistors R28 and
R29 are load resistors for diodes D3 and D4.
FM STEREO MULTIPLEX CIRCUIT
Figures 12A and 12B show two sample signals
that might appear from the left (L) and right (R) channel microphones of a radio station that
is broadcasting a stereo FM signal, The trans-
mitting circuits then combine these signals to
produce the
L+R signal shown in Figure 13Aand
the L-R subcarrier signal shown in Figure 13B.
The L-R subcarrier signal is a suppressed
carrier amplitude modulated signal on a 38 kc
subcarrier, and is called the subcarrier channel. Figure 13
These two signals,
L+R and L-R, arc then com-
bined with the 19 kc pilot signal shown in Figure 13C. This whole complex signal modulates the
FM carrier and is then radiated from the
broadcasting antenna.
Figure 14 shows the locations of
the different
components that modulate an FM stereo signal.
The main channel signal is from 50 cps to
15 kc. Monaural
FM tuners use only this part of
the signal, and the remaining parts are atten-
uated by the tuners de-emphasis network,
L+R I
L-R
AUDIO MULTIFLEX SIGNAL sca signals
Figure 14
lls
TTED
IONS 1
Page 20
5544332211DDCCBBAASTEREO
INDICATOR
PHASE
38KC
OSCILLATOR
ON PHASE
ADJUST CONTROL AUDIO AMPLIFIER
AMPLIFIER AND
EMITTER FOLLOWER19 KC6
3
4Q8STEREO LEFTSTEREO RIGHTB +17VC +8VC45
390pfR40
220C58
.002MFC48
.2MFC53
470pfQ8
2N2712C49
2700pfR33
33Q14
2N2712R50
10KR51
220KR43
10KC50
.01MFR37
2.2KR41
47KR46
6.8KR44
560R49
47KR.E.C.231Q9
2N2712+C56
10MF
+C59
10MFC55
.001MFC51
.01MFADJ
NORMALR38
47KC41
680pfR47
2.2KC42
.01MFC44
.01MFR35
120KQ11
2N2712L8R.E.C.231R42
560R48
15KC46
.01MFR39
10KL75142R34
1KQ10
2N2712T71625347
+C52
10MFQ12
2N2712R45
39KC57
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+C43
10MFC47
.01MFQ13
2N2712R36
68KREAR OF
WAFER #3R52
220KC38
200pfLAMPC54
.001MF
