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3. FM Tuner
The FM Tuner section of Model 2215 is divided into three functional blocks: FM front end, IF
amplifier & Detector, Muting control] and MPX stereo decoding circuit.
FM signals induced on a FM antenna are led to FM antenna coil L103. These signals are then
applied to the FET RF amplifier which in turn applies its output to the next Transistor Mixer
H111
through
a high-Q
tuned
circuit. 'The Mixer
convert
its input signal
into
10.7MHz
intermediate frequency and amplifies it. The H110 is a local oscillator and its output is injected
into. the base of Mixer transistor, the injection voltage is about 100 to 200mV. 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
limitting characteristics, improved capture ratio and good AM suppression.
A part of IF amplifier H202 is rectified by the diodes H206 and H207 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 2215.
The DC voltage obtained by rectifing a part of IF output signal from the H204 is applied to
the base of H304 and turns on it, if the IF output is greater than predetermined level (muting
threshold
level). When
the H304
is turned on, the H305
is turned
off, thus allowing the
_ emitter-collector resistance of the H305 increasing and the collector voltage rises about 8V. The
increased
collector voltage increases the base bias voltage and makes the switching transistor
H306 turn on, thus decreasing the collector-emitter resistance to near zero ohm and allowing the
power supply path to the IC closed.
When the input signal is lower than the predetermined level, the DC output obtained is small
and can not turn on the H304, thus the H304 keeps its turn-off state and this makes H305 turn
on, decreasing the collector voltage and turning off H306. Thus no power is supplied to the IC
H205 and signals below the threshold level are muted out. The muting threshold level can be
varied by adjusting the trimming resistor R306.
The DC voltage developed at the collector of H305 is also used to make the Auto-Stereo
switching transistor H403 turn on and off.
3-2 MPX Stereo Decoding Circuit
Non-equalized audio signals are applied to the first amplifier H401 which servs as a tuned
amplifier for the pilot signal in the composite signals and as a buffer amplifier for the rest audio
signals. The amplified 19KHz pilot signal is led to the second 19KHz amplifier H402 and further
amplified
if switching transistor H403 is turned on by the controlling DC signal as descibed
above. The 19KHz
pilot signal is rectified by the doubler circuit consisting of H412 and H413 to
obtain synchronized 38KHz
signal to drive the H404. The H404 is the 38KHz tuned amplifier
and supplies its output to the switching matrix circuit consisting of four diodes. The composite
signals are applied to the center tap of switching transformer L402-2 and decoded into left and
right channel signals, then both channel signals are led to the crosstalk cancelling amplifier wh ich
utilizes complementary confuguration with NPN and PNP transistors through de-emphasiss net
works. Transistors H313 and H314 are buffer amplifiers and their outputs are led to the function
switch.
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