Circuit Description; Receive Circuit; Antenna Switch; Rf Circuits - Icom IC-M73 Service Manual

SECTION 4.

4-1 RECEIVE CIRCUIT

• ANTENNA SWITCH (RF UNIT)
The received signal from the antenna connector are passed
through the low-pass filter (LPF; L81, L82, C83−C86 and
C89) and the antenna switch (D90), which toggles the re-
ceive (RX) line and transmit (TX) line.
While transmitting, the voltage on the T5V line is applied to
D52 and D90, and these are ON. Thus the TX line is con-
nected to the antenna, and RX line is connected to the
ground to prevent transmit signal entering.
While receiving, no voltage is applied to D52 and D90, and
these are OFF. Thus the TX line is disconnected from the
antenna to prevent received signal entering, and RX line is
disconnected from the ground and L90, C90 and C91 com-
pose a two-staged LPF, which guides received signal to the
RX circuits.
The received signal is applied to the RF circuits.
• RF CIRCUITS (RF UNIT)
The RF circuit amplifi es received signal within the frequency
coverage. The received signal is fi ltered at the bandpass fi l-
ter (BPF) and amplifi ed at the RF amplifi er.
The received signal from the antenna switch is passed
through the two-staged tunable BPFs (D92, L92, C95, C97
and D93, L93, C99, C100) to fi lter out unwanted signal. The
fi ltered signal is applied to the RF amplifi er (Q90). The am-
plifi ed signal is then applied to the 1st mixer (Q150), through
another two-staged tunable BPF (D130, L96, C110, C112
and D131, L97, C114, C115).
• 2ND IF AND DEMODULATOR CIRCUITS
19
D/A
(IC190) converter
18
Demodulated signal
to the AF circuits

CIRCUIT DESCRIPTION

8
Filter
amp.
Limiter
Quadrature
amp.
detector
9 10 11
X170
• 1st IF CIRCUITS (RF UNIT)
The received signal is converted into the 1st IF signal, and
amplifi ed at the 1st IF circuits.
The amplified received signal from the RF circuits are ap-
plied to the 1st mixer (Q150), and mixed with the 1st local
oscillator (LO) signal from the VCO (Q21, Q22, D20−D22),
resulting in the 21.7 MHz 1st IF signal.
The converted 1st IF signal is passed through two 1st IF
filters (FI150 and FI151) to filter out unwanted signal, and
then applied to the 1st IF amplifi er (Q151). The amplifi ed 1st
IF signal is applied to the IF IC (MAIN UNIT: IC170, pin 16).
• 1st IF CIRCUITS
1st IF amp
(Q151)
1st IF signal
to the FM IF IC
(MAIN UNIT; IC170, pin 16)
• 2nd IF AND DEMODULATOR CIRCUITS
(MAIN UNIT)
The 1st IF signal is converted into the 2nd IF signal, and de-
modulated.
The 1st IF signal from the 1st IF circuits is applied to the 2nd
IF mixer in the IF IC (IC170) and mixed with the 21.25 MHz
2nd LO signal from the reference oscillator (RF UNIT: IC1,
X1), resulting in the 450 kHz 2nd IF signal.
The converted 2nd IF signal is output from pin 3, and passed
through the 2nd IF fi lter (FI170) to suppress sideband noise.
The fi ltered signal is amplifi ed by the limiter amplifi er (IC170,
pin 5) and FM-demodulated by the quadrature detector with
the discriminator (X170), and then output from pin 9. The de-
modulated AF signal is applied to the AF circuits.
BPF
21.25 MHz
FI170
7 5 3
Noise
detector
Mixer
12 14
R5V
"RSSIV" signal to the CPU (IC360, pin 33)
4 - 1
signals from the VCO
2 BPF's
21.7 MHz
LPF
1st IF signal
BPF
1st mixer
FI150, FI151
(Q150)
2NDLO
LPF
2
RF UNIT
IF IC
(IC170)
16
1st IF signal from the IF amplifier (RF UNIT: Q151)
"NOISV" signal to the CPU (IC360, pin 32)
Received signals
from the RF circuits
PLL IC
17
(IC1)
X1
21.25 MHz