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FM signals induced on a FM antenna are led to FM antenna coil L101 through a balun coil.
These signals are then applied to the FET
RF amplifier which in turn applies its output to the
next Transistor Mixer H102 through the double tuned high selective circuits. The Mixer convert
its input signal into 10.7MHz
intermediate
frequency
and amplifies it at the same
time. The
H103
is a local oscillator
and
its output
is injected
into the base of Mixer transistor, the
injection voltage is about 50mV.
The
10.7MHz
front end IF output is led to the next IF
amplifier unit through a coaxial cable.
The IF amplifier unit consists of five stages of IF amplifiers. Two pieces of ceramic filters
are used to obtain high selectivity a pair of symmetrical diode limiter is also employed for the
best limiting characteristics, improved capture ratio and good AM suppression.
A part of IF amplifier H202 is rectified by the diodes H211 and H212 and its DC output is
fed back to the gate of FET RF amplifier to decrease the gain of it with increased input signal
strength.
3-1. Muting and Auto-Stereo Switching Circuits
The muting circuit consisting of all solid-state electrical switching has been incorporated in
the Model 2220B.
The DC voltage obtained by rectifing the sub |F output signal from the H206 is applied to
the base of H207
and turns on
it, if the sub IF output
is greater than predetermined
level
(muting threshold level).
When
the H207 turns on, the muting switch transistor H208 is turned on, thus decreasing
the emitter collector resistance to near zero ohm and allowing emitter current path to the Final
1F amplifier H205.
When the input signal is lower than the predetermined level, the DC output obtained is small
and can not turn on the H207 thus the H207 keeps its turn off state and this makes the switch
transistor keep H208 turn off, then no emitter current is supplied to the H205 and signals below
the threshold level are muted out.
The muting threshold level can be varied by adjusting the trimming resistor R253.
The DC voltage obtained is also used to.make the Auto-Stereo switching transistor H209
turn on and off.
3-2. MPX Stereo Decoding Circuit
The
stereo
composite
signal
from the FM detector undergoes
a phase compensation
by
R303 and C304, is applied to the input terminal pin 2 of the MPX stereo decoding IC H301 on
a PLL (Phase Locked
Loop) basis, and decoded into the left and right stereo signals, which
become available at pins 4 and 5 respectively. These decoded left and right stereo audio signals
are introduced
through
a !ow pass filter composed
of L301
to L304 and C309 to C318 for
elimination
of
undesirable
residual
switching
signal
and
through
a de-emphasis
network
consisting of R314,
R315, C319
and C320, into the npn-pnp direct coupled audio amplifier,
where the signals are amplified to a required level for the output from J307 and J308. From
these terminal
the audio
signals are
led to the TAPE
OUTPUT
jacks through
the function
switch. Figure 1 presents an internal block diagram showing the functions of the PLL basis MPX
stereo
decoding
IC HA1156.
The
input stereo
composite
signal, amplified
by the audio
amplifier, is delivered to the phase detectors PD-1 and PD-2. A part of the stereo composite
signal is also applied to the stereo decoder
section. The VCO
(Voltage Control Oscillator)
produces
a free
run
oscillation
in the
neighborhood
of 76KHz
with
the
time
constant
determined by a capacitor C303 and resistors R304 and R305 set on the outside of pin 14.
The VCO output has its freqeuncy divided into 19KHz through the two stages of the frequency
divider (DIV-1
& DIV-2), and is reverted to the phase detector PD-1, which contains two input
terminals designed to produce an output in proportion to the product of the two input signals.
The signal applied to.one of the inputs of PD-1
is the 19KHz square wave formed through
frequency division of the 76KHz VCO output signal by the two stages of the frequency divider
'
NOON
Citm iene
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