Icom IC-4A Instruction Manual page 23

ΙΕ CIRCUIT
The first IF signals from O3 are fed to the matched pair crystal filter F11, then IF amplifiers Q4 ~ 06.
The amplified signals are fed to IC1. IC1 is composed of the second local oscillator, second mixer,
limiter amplifier, quadrature detector and active filter circuits.
The second local oscillator oscillates 21.345MHz with X1, and is fed to the second mixer with the
first IF signals to convert into 455KHz second IF signals. The second IF signals are put out from Pin
3, and fed to external ceramic filter F12 which has excellent selectivity, then fed to ІСІ (Pin 5) again
to amplify and detect.
The detected AF signals are put out from Pin 9.
AF AND SQUELCH CIRCUITS
Noise components put out from Pin 9 of IC1 are fed to IC1 (Pin 10) again through the SOUELCH
control
R2, which controls the squelch threshold level, filtered about 20KHz signal (noise) and put
out from Pin 11.
This signal (noise) is rectified by O14, integrated by R39, R40 and 051, and turns O15 ON and turns
OFF the regulator for AF power amplifier consisting of D4, Q16 and O17.
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