Icom IC-211 Instruction Manual page 30

When the AGC switch is in the SLOW position, the output signal of Q27 is fed to
another peak hold detector Q26. The detected DC voltage charges C77 and switches
Q25 ON and Q24 OFF. When the input signal is removed, the charged voltage of C77 is
discharged through R101 and O25 is switched ON, and the charged voltage in C75 and
C81 is discharged through R95 and R96. Thus the AGC voltage is held during the time
constant of C77 and R101.
When the AGC switch is in the FAST position, —9 Volts is applied to the base of Q25,
turning it OFF, and turning Q24 ON. Therefore the AGC voltage does not hold, and
the AGC release time is determined by C75 and R95. When the power is turned ON, a
pulse through R92 and C74 turns ON Q22 temporarily to discharge C75 and C81 in
order to allow the AGC voltage to recover to a non-signal bias state.
Q19 superimposes the RF gain control voltage on the AGC line by way of the emitter
follower, forming a threshold-type RF gain control.
. FM IF AMPLIFIER CIRCUIT
Signals that have passed through the crystal filter and diode switch D40 are supplied,
together with 10.245MHz signals from the second local oscillator O44, to the gate of
second mixer 043 for conversion to the 455K Hz second IF.
In the second IF amplifier, which has excellent selectivity due to 2 CFU455E ceramic
filters, signals are amplified by Q40 ~ O42. After removal of noise and other АМ
components by ІСЗ functioning as а limiter, the signals are detected in terms of audio
frequency signals by a ceramic discriminator. This ceramic discriminator has outstanding
temperature characteristics, linearity and detection sensitivity, which guarantee clear,
stable reception. Audio signals from the discriminator are divided into audio signals
and noise component signals to operate the squelch circuit.
In the FM mode, the AGC circuit does not actuate, so the meter indicator needle is
caused to move by a portion of the IF signals taken from the collector of the second IF
amplifier O40 and rectified by D38. Meter indicator deflection can be adjusted by
altering gain of the second IF amplifier Q42 by R167 in series with the by-pass capacitor
of the emitter of O42.
. SQUELCH CIRCUIT
The signals from the discriminator are also supplied to the squelch circuit. The signals
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