Heathkit AR-14 - Page 80

Page 70
A 19 kc pilot signal is transmitted to give the
proper phasing for the demodulated subcarrier
channel. The 38 kc subcarrier channel is AM
modulated from 23 kc to 53 kc.
A second subcarrier signal is transmitted by
some stations at 67 kc. This is usually a com-
mercial music signal. This signal is called the
SCA (Subsidiary Communications Authorization)
channel.
The signal that is used for stereo multiplex
operation is coupled from the ratio detector,
through capacitor C39 and the SCA filter, to
audio amplifier transistor Q8. The SCA filter,
which consists of coil L6 and capacitors C40
and C41, removes the 67 kc SCA signal. These
signals are not used for stereo reception.
The complete stereo multiplex signal consists of
the (L+R) main channel, the (L-R) subchannel,
and the 19 kc pilot signal. The complete stereo
signal is amplified by transistor Q8 and coupled
through capacitor C43 to 19 kc amplifier tran-
sistor Q9. The collector circuit of transistor Q9
is tuned to 19 kc by coil L7 and capacitor C44.
Phase control R43 and capacitor C45 are con-
nected across a portion of coil L7 so the phase
of the 19 kc signal can be adjusted. The 19 kc
signal is then coupled to the base of the 38 kc
oscillator transistor, QlO, where it locks the
38 kc oscillator in phase and frequency with the
transmitted 38 kc subcarrier signal.
The 38 kc oscillator signal from transistor QlO
is applied through transformer T7 to the base
circuits of switching detector transistors Qll
and Q12. At the same time, the main channel
(L+R) and subchannel (L-R) signals are coupled
from the emitter of transistor Q9 to the emitters
of transistors Qll and Q12.
When
the main channel and subchannel signals
are combined with the 38 kc oscillator signal in
the switching detector circuit, the 38 kc carrier
that was removed at the transmitter (suppressed
carrier transmission) is reinserted into the
stereo signal (waveform 3 on Block Diagram).
Figure 15 shows the various waveforms that
are present in the switching detector circuit.
Waveform 1 is the suppressed carrier stereo
I
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and main channel signal that comes from tran-
sistor Q9. Waveform 2 is the 38 kc oscillator
signal that is reinserted in the stereo signal at
the
same phase and frequency as the original 38
kc carrier. Remember, this 38 kc oscillation
was locked at the correct frequency and phase by
the 19 kc pilot signal from Q9.
The actual detection process takes place in the
following
manner: When waveform 3 is applied
to the switching detector transistors, Q12 only
conducts on that part of the waveform that car-
ries the L waveform. Thus, only the L wave-
form 4 appears at its output. Transistor Qll
only conducts on the R portion of the 38 kc
waveform, thus only the R waveform 5 appears
at its output. These are the left and right sig-
nals originating at the broadcasting station. ,
In Figure 16, the 38 kc signal is shown super-
imposed on the stereo signal. At each 38 kc
peak on the L waveform, Q12 conducts and Qll
is cut off. At each 38 kc peak on the R wave-
form, Q12 is cut off and Qll conducts. The L
signal from transistor Q12, charges capacitor
C54; the R signal from transistor Qll, charges
capacitor C55.
L WAVEFORM
1 012 CONDUCTS )
1
011 CUT - OFF )
R
WAVEFORM
(012 CUT-OFF)
( 011 CONDUCTS)
Figure 16