Heathkit AR-14 - Page 79

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LEFT
MICROPHONE
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LEFT SIGNAL (LI
I
Figure 12
across the input resistance of QB. The ampli-
tude of this output voltage is determined by
how far the IF frequency deviates from the
center frequency of 10. 7 me. The frequency
of this audio output voltage is determined by
how often the frequency deviates from 10.7 me.
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 me 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 13A and
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.
I
Page 69
L+R ( MAIN CHANNEL )
(A)
L-R ( SUBCARRIER I
(B)
19 KC
(C)
Figure 13
These two signals, L+R and L-R , are 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 thispart of
the signal, and the remaining parts are atten-
uated by the tuners de-emphasis network.
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Figure 14