Circuit Description; Receiver Circuits - Icom IC-H6 Service Manual

SECTION 3 CIRCUIT DESCRIPTION
3 - 1 RECEIVER CIRCUITS
3 - 1 - 1 ANTENNA SWITCHING CIRCUIT
Signals from the antenna connector are fed to the antenna switching circuit through Chebyshev low-pass
filter consisting of L229, L230, C295, C297 and C298 in the PLL board.
The antenna switching circuit employs a quater wave switching circuit.
In the receive mode, switching diodes, 0216 and 0217 are turned OFF, and they make isolation against
the transmitter circuit and matching circuit, and the incoming signals are fed to the RF amplifier.
3 - 1 • 2 RF AMPLIFIER AND FIRST MIXER
The signals from the switching circuit are fed to the cascode amplifier 0101 and 0102.
The amplified signals are fed to the gate of the first mixer 0103 through the band-pass filter L102 -
L 104, which reduces interference and intermodulation from out of the band signals.
To the source of 0103, a 140MHz signal* is supplied from the PLL circuit to convert the RF signals into
16.9MHz first IF signals. (*This frequency differs depending on the version, and it can be calculated by
formula; "Receive frequency" - 16.9MHz.)
The first IF signals are taken from the drain of 0103 and fed to the IF circuit.
3 - 1·3 IF CIRCUIT
The first IF signals from 0103 are fed to the matched pair crystal filter F 1101, then IF amplifiers 0104
and 0105.
The amplified signals are fed to IC101. IC101 is composed of the second localoscillator, second mixer,
limiter amplifier, quadrature detector and active filter circuits.
The second local oscillator oscillates 16.445MHz with X101, 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 F 1102 which has excellent selectivity, then fed to IC101 (Pin 5) again to
amplify and detect.
The detected AF signals are put out from Pin 9.
3 - 1 - 4 AF AND SQUELCH CIRCUITS
The detected AF signals are put 6dB/Octave de-emphasis by integral circuit consisting of R 117 and Cl 26,
and fed to AF power amplifier IC102 through the VOLUME control R1, to obtain enough power to
drive the speaker.
Noise components put out from Pin 9 of IC101 are fed to IC101 (Pin 10) again through the SQUELCH
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 0113, integrated by R135, R136 and C136, and turns 0114 ON and
turns OFF the regulator for AF power amplifier consisting of 0103, 0115 and 0116.
This reduces the current drain of the set, in the standby condition. When a signal is received, noise is
suppressed by the signal and turns 0114 OFF and the regulator is turned ON and supplies regulated
voltage to the AF power amplifier, and incoming signal can be heard from the speaker.
In the transmit mode, a voltage is applied to 0114 and turns it ON, and turns the regulator OFF the
same as in the standby condition.
3-1