Alinco DR-135 Service Manual page 7

3. Demodulation Circuit
4. Audio Circuit
5. Squelch Circuit
6. WIDE/NARROW Switching circuit
After the signal is amplified by the first IF amplifier (Q105), it is input to pin16
of the demodulator IC (IC108). The second local signal of 30.395MHz (Crystal
oscillator) is input through pin 1 of IC108. Then, these two signals are mixed
by the internal mixer in IC108 and the result is converted into the second IF
signal with a frequency of 455kHz. The second IF signal is output from pin 3 of
IC108 to the ceramic filter (FL101 or FL102), where the unwanted frequency
band of that signal is eliminated, and the resulting signal is sent back to the
IC108 through pin 5.
The second IF signal input via pin 5 is demodulated by the internal limiter
amplifier and quadrature detection circuit in IC 108, and output as an audio
signal through pin 9.
The audio signal from pin 9 of IC 108 is amplified by the audio amplifier
(IC104:A), and switched by the signal switch IC (IC111) and then input it to the
de-emphasis circuit.
And is compensated to the audio frequency characteristics in the de-emphasis
circuit (R203, R207, R213, R209, C191, C218, C217) and amplified by the
AF amplifier (IC104:D). The signal is then input to volume (VR1). The adjusted
signal is sent to the audio power amplifier (IC117) through the pin 1 to drive
the speaker.
The detected output which is outputted from pin 9 of IC108 is inputted to pin 8
of IC108 after it was been amplified IC104:A and it is outputted from pin 7 after
the noise component was been eliminated from the composed band pass
filter in the built in amplifier of the IC, then the signal is rectified by the internal
diode in IC108 to convert into DC component. The adjusted voltage level at
VR101 is delivered to the comparator of the CPU.The voltage is led to pin 2 of
CPU and compared with the setting voltage. The squelch will open if the input
voltage is lower than the setting voltage.
During open squelch, pin 30 (SQC) of the CPU becomes "L" level, AF control
signal is begin controlled and sounds is outputted from speaker.
The second IF 455kHz signal which passes through filter FL101 (wide) and
FL102 (narrow) during narrow, changes its width using the width control
switching IC103 and IC102.
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