Circuit Descripiton; Receiver Circuits - Icom IC-M35 Service Manual

SECTION 4.

4-1 RECEIVER CIRCUITS

RF CIRCUITS
RF circuits consist of RF fi lters, antenna switch (ANT SW),
RF amplifi er (RF AMP), etc., and extracts and amplifi es the
signal of frequency which is desired to receive.
• ANTENNA SWITCHING CIRCUIT
The RF signals from the antenna are passed through the
LPF (L81, 82, C83–86, 89) and antenna SW (D52, 90, L90,
C90, 91), then applied to the RF AMP (Q90) via the 2-staged
tuned BPF (D92, 93, L92, 93, C95, 97–100, C117).
• RF AMPLIFIER
The filtered RX signals are amplified by the RF AMP (Q90),
and passed through another 2-staged BPF (D130, 131, L96,
97, 110, 112–115) to remove unwanted signals. The filtered
signals are applied to the 1st mixer (Q150).
• RF CIRCUITS
D131 D130
To 1st mixer
BPF BPF
1ST IF, 2ND IF AND DEMODULATOR CIRCUITS
• 1ST IF CIRCUITS
The RX signals are mixed with 1st LO signals to be
converted into the 21.7 MHz 1st IF signal at the 1st mixer
(Q150). The converted 1st IF signal is filtered by the crystal
filter (FI150, 151) to remove unwanted out-of-band signals.
The filtered 1st IF signal is amplified by the 1st IF AMP
(Q151), then applied to the IF IC (IC170, pin 16).
• 2ND IF AND DEMODULATOR CIRCUITS
IC170 is an IF IC contains 2nd local oscillator, 2nd mixer,
limiter and quadrature detector in its package.
The 21.25 MHz reference frequency signal from the PLL
IC (IC1, pin 2) is applied to the IF IC (IC170, pin 2) as the
2nd LO signal, and mixed with 21.7 MHz 1st IF signal from
the 1st AMP (Q151). The resulting signal of 450 kHz 2nd IF
signal is output from pin 3 of IC170.
The 2nd IF signal is filtered by external ceramic filter (FI170)
to extract 450 kHz signal only, then applied to internal
quadrature detector and frequency-demodulated. The
demodulated AF signals are output from pin 9.
• 1ST IF CIRCUITS
FI150 FI151
Q151
To IF IC
IF
BPF
AMP
1st IF signal
CIRCUIT DESCRIPTION
D52 D90
ANT
TX CIRCUITS
LPF
SW
D 93 D 92
Q90
RF
BPF BPF
AMP
T1CON
1st LO signals
from VCO
21.7MHz From RF AMP
Q150
The quadrature detector is a frequency demodulator which
uses a discriminator (X170) as a phase delayer, and
provides demodulation without any adjustment.
SQUELCH CIRCUIT
A portion of AF signals from the IF IC is level-adjusted by
the D/A converter (IC190), and applied to the noise amplifer
(IC170, R174–176, C179, 180).
Only noise components (approx. 30 kHz signal) are
amplified by the noise amplifier, and rectified by internal noise
detector to be converted into DC voltage corresponding
to noise level; the squelch voltage. The squelch voltage is
applied to the A/D port of the CPU (IC360). The CPU (IC360)
compares input voltage and preset squelch level to control
audio signals ON/OFF (emit/mute).
RX AF CIRCUITS
The demodulated AF signals from the IF IC (IC170, pin 9) are
ANTENNA
passed through the AF mute SW (IC260) and LPF (IC200C,
R271–273, C264, 265). The filtered AF signals are passed
through another LPF (R279, C266) to remove 3 kHz and
higher signals, and level-adjusted by the D/A converter (IC190),
and de-emphasized by R286, C285 to obtain –6 dB/oct. of
frequency response, then applied to AF power amplifier (IC362).
IC362 is an AF power amplifier IC which can provide 0.35 W
of output power with 8 ohms load, or 0.7 W of output power
with 16 ohms load.
• 2ND IF, DEMODULATOR AND RX AF CIRCUITS
IC200C IC260D
LPF
BEEP DETMS
SP1
Int. SP
J250
SPO
Ext. SP
AF OUT
4 - 1
IC1
PLL IC
21.25 MHz
2ND LO signal
X1
I C170
21.25MHz
DET
IF IC
AF
MUT E
450KHz
X170
VOLIN D/A IC
IC190
IC362
NJM2073M
AF
DE -
AM P ENPHA
FI170
450KHz
BPF
From 1st IF AMP
RSSI
NOISE LEVEL