Section 4 Circuit Description; Receiver Circuits - Icom IC-M422 Service Manual

SECTION 4

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT
The antenna switching circuit toggles the receive line and
the transmit line. This circuit does not allow transmit signals
to enter the receiver circuits.
The received signals from the antenna connector (CHASSIS
UNIT; J1) are passed through a two-stage low-pass filter
(LPF; L21, L22, C127–C130, C132) and then applied to the
type antenna switching circuit (D14, D21).
While receiving, no voltage is applied to D14 and D21. Thus,
the receive line and the ground are disconnected and L31
and C142 function as an LPF which leads received signals
to the RF circuits. The received signals are applied to the RF
circuits via the attenuator (D22).
4-1-2 RF CIRCUITS
The RF circuits amplify received signals within the range of
frequency coverage and fi lters out-of-band signals.
The received signals from the antenna switch are passed
through a tunable bandpass filter (BPF; D25, L35, C149,
C151, C152, C154) to suppress unwanted signals. The fil-
tered signals are amplified at the RF amplifier (Q21). The
amplified signals are passed through another three-stage
tunable BPF (D26–D28, L36, L38, L39, C161, C162, C164,
C173–C178) to suppress unwanted signals again.
The fi ltered signals are then applied to the 1st IF circuits.
4-1-3 1st IF CIRCUITS
The 1st IF circuits contain the 1st mixer, IF amplifier and
the 1st IF fi lter circuits, and the 1st IF mixer converts the re-
ceived signals into a fi xed frequency of the 1st intermediate
frequency (IF) signal. The converted 1st IF signal is fi ltered
at the 1st IF fi lters, then amplifi ed at the 1st IF amplifi er.
The signals from the three-stage tunable BPF are converted
into the 21.7 MHz 1st IF signal at the 1st mixer (Q22) by be-
ing mixed with the 1st LO signal generated at the VCO (Q4,
Q5, D3, D4).
• 2ND IF AND DEMODULATOR CIRCUITS
R214
demodulated signals
to the AF circuits
CIRCUIT DESCRIPTION
8
Filter
amp.
Limiter
Quadrature
amp.
detector
10 11
9
X2
The converted 1st IF signal from the 1st mixer is passed
through the monolithic fi lters (FI1, FI2) to suppress unwant-
ed signals, and then amplifi ed at the 1st IF amplifi er (Q23).
The amplifi ed 1st IF signal is applied to the FM IF IC (IC2, pin 16).
4-1-4 2nd IF AND DEMODULATOR CIRCUITS
λ The 1st IF signal is converted into the 2nd IF signal and de-
/
4
modulated at the detector section in the FM IF IC. The FM IF
IC contains 2nd mixer, limiter amplifi er, quadrature detector,
etc. in its package.
The 1st IF signal from the 1st IF amplifi er (Q23) is applied to
the mixer section in the FM IF IC (IC2, pin 16). The applied
1st IF signal is mixed with the 21.25 MHz 2nd LO signal from
the PLL IC (IC1, pin 17) to be converted into the 450 kHz 2nd
IF signal.
The 2nd IF signal from the mixer section is output from pin 3
and passed through the ceramic fi lter (FI3) to suppress the
heterodyne noise. The fi ltered signal is applied to the FM IF
IC (IC2, pin 5) again, and amplified at the limiter amplifier
section and demodulated by the quadrature detector.
The demodulated AF signals are output from pin 9, and ap-
plied to the AF cricuits.
4-1-5 AF CIRCUITS
The demodulated AF signals from the FM IF IC are amplifi ed
and fi ltered in AF circuits.
The AF signals from FM IF IC (IC3, pin 9) are passed
through the BPF (Q31, Q32) and AF mute switch (IC4, pins 8, 9),
and then applied to the volume control pot (VR BOARD; R1)
to be adjusted its level. The level adjusted AF signals are
applied to the AF power amplifi er (IC9, pin1) to obtain 4.5 W
(typ.) of AF output power.
The power amplifi ed AF signals are output from pin 4, and
applied to the internal speaker (FRONT UNIT; SP1) via J4 or
connected external speaker/hailer.
FI3
7 5 3
Noise
detector
Mixer
14
+
IC11
–
Squelch amp.
R5V
4 - 1
21.25 MHz
PLL IC
17
(IC1)
2
X1
21.25 MHz
IC2
TA31136FN
16
1st IF signal from the IF amplifier (Q23)
"SQL" signal to the CPU (LOGIC BOARD; IC1, pin 5)