Receiver Circuits - Icom iC-m34 Service Manual

SECTION 4

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCH
The received signals from the antenna connector are
passed through the antenna switch which toggles the
receive (RX) line and transmit (TX) line.
The received signals from the antenna connector are
passed through the low-pass filter (LPF; L81, L82, C80, C83
−C86, C89) and the antenna switch (D52, D90 and D353
are OFF).
While transmitting, the voltage on the T5V line is applied to
D52, D90 and D353, and these are ON. Thus the TX line is
connected to the antenna, and RX line is connected to the
GND to prevent transmit signal entering.
While receiving, no voltage is applied to D52, D90 and
D353, and these are OFF. Thus the TX line and the antenna
is disconnected to prevent received signals entering, and
RX line is disconnected from the GND and L90, C90−C92
compose an LPF which guides received signals to the RX
circuits.
The received signals are applied to the RF circuits.
4-1-2 RF CIRCUITS
The RF circuit amplifies received signals within the frequency
coverage. The received signals are filtered at the bandpass
filter (BPF) and amplified at the RF amplifier.
The received signals from the antenna switch are passed
through the two-staged tunable BPF (D92, D93, L92, L93,
C95, C97−C101) to filter out unwanted signals. The filtered
signals are applied to the RF amplifier (Q90). The amplified
signals are then applied to the 1st mixer (Q150) via another
two-staged tunable BPF (D130, D131, L96, L97, C104,
C109, C110, C112−C116).
4-1-3 1st IF CIRCUITS
The received signals are converted into the 1st IF signal,
and amplified at the 1st IF circuits.
The amplified received signals from the RF circuits are applied
to the 1st mixer (Q150), and converted into the 21.7 MHz 1st
IF signal by being mixed with the 1st local oscillator (LO)
signals from the VCO (Q21, Q22, D21−D23).
• 2ND IF AND DEMODULATOR CIRCUITS
14
D/A
(IC190) converter
13
Demodulated signals
to the AF circuits
CIRCUIT DESCRIPTION
8
Filter
amp.
Limiter
Quadrature
amp.
detector
9 10 11
X2
The converted 1st IF signal is passed through two 1st IF
filters (FI150, FI151) to filter out unwanted signals, and
applied to the 1st IF amplifier (Q151). The amplified 1st IF
signal is then applied to the FM IF IC (IC170, pin 16).
• 1st IF CIRCUITS
1st IF amp
(Q151)
1st IF signal
to the FM IF IC
(MAIN UNIT; IC170, pin 16)
4-1-4 2nd IF AND DEMODULATOR CIRCUITS
The 1st IF signal is converted into the 2nd IF signal, and
demodulated.
The 1st IF signal from the 1st IF circuits is applied to the
2nd IF mixer in the FM IF IC (IC170, pin 16) and converted
into the 2nd IF signal by being mixed with the 21.25 MHz
2nd LO signal from the reference oscillator (IC1, X1) via
LPF (L1, C8, C9, C12).
The converted 2nd IF signal is output from pin 3, and
passed through the 2nd IF filter (FI170) to suppress
sideband noise. The filtered 2nd IF signal is applied to the
limiter amplifier (IC170, pin 5). The amplified 2nd IF signal
is FM-demodulated at the quadrature detector (IC170,
pins 10, 11) and output from pin 9. The demodulated AF
signals are applied to the AF circuits.
4-1-5 AF CIRCUITS
The demodulated AF signals from the demodulator circuits are
amplifi ed and fi ltered in AF amplifi er circuits.
The demodulated AF signals from the FM IF IC (IC170, pin 9)
are passed through the AF mute switch (IC260, pins 1, 2),
LPF (IC200, pins 8, 9) and variable register (VR BOARD;
R801) for level adjustment.
The level adjusted AF signals are passed through the
de-emphasis circuit (R286, C280, C285) to obtain −6 dB
of audio characteristic. The de-emphasized AF signals are
passed through the analog switch (IC430, pins 10, 11), and
applied to the AF power amplifier (IC401, pin 7) to obtain
0.6 W of AF output power. The power-amplified AF signals are
then output from pin 1, and applied to the internal speaker via
J251.
BPF
FI170
7 5 3
Noise
detector
Mixer
12 14
R5V
"RSSIV" signal to the CPU (IC360, pin 33)
4 - 1
2 BPF's
21.7 MHz
LPF
BPF 1st IF signal
1st mixer
FI150, FI151
(Q150)
2NDLO
21.25 MHz
LPF
2
FM IF IC
(MAIN UNIT; IC170)
16
1st IF signal from the IF amplifier (Q151)
"NOISV" signal to the CPU (IC360, pin 32)
signals from the VCO
Received signals
from the RF circuits
PLL IC
17
(IC1)
X1
21.25 MHz