Section 4 Circuit Description; Receiver Circuits - Icom IC-M401E Service Manual

SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT
(MAIN-E UNIT)
The antenna switching circuit functions as a low-pass filter
while receiving and as resonator circuit while transmitting.
The circuit does not allow transmit signals to enter receiver
circuits.
Received signals enter the MAIN-E unit from the antenna
connector and pass through the low-pass filter (L21, L22,
C127, C130, C133). The signals are then applied to the RF
circuit via the antenna switching circuit (D14, D21, L31,
C141, C142).
4-1-2 RF CIRCUIT (MAIN-E UNIT)
The RF circuit amplifies signals within the range of frequen-
cy coverage and filters out-of-band signals.
The signals from the antenna switching circuit pass through
a tunable bandpass filter (D25, L35, C150, C152) where the
object signals are applied to the RF amplifier circuit (Q21).
The amplified signals at Q21are applied to the 3-stage tun-
able bandpass filter (D26-D28, L36, L38, L39, C161, C162,
C164, C173-C178) to suppress unwanted signals and
improve the selectivity. The signals are then applied to the
1st mixer circuit (Q22).
4-1-3 1ST MIXER AND 1ST IF CIRCUITS
(MAIN-E UNIT)
The 1st mixer circuit converts the received signal to a fixed
frequency of the 1st IF signal with a 1st LO (VCO output) fre-
quency. By changing the 1st LO frequency, only the desired
frequency will be passed through two crystal filters at the
next stage of the mixer.
• 2ND IF AND DEMODULATOR CIRCUITS
Squelch level
adjustment pot
(R214)
R215
R218
C218
R216
AF signal "DET"
C212
C211
R213
R212
R221
8 7
Active
C213
filter
Limiter
amp.
FM
detector
9 10
R219
C220
R205
The signals from the RF circuit are mixed with the VCO sig-
nals at the 1st mixer circuit (Q22) to produce a 21.7 MHz 1st
IF signal.
The 1st IF signal is applied to two crystal filters (FI1, FI2) to
suppress out-of-band signals and is then amplified at the IF
amplifier (Q23). The amplified signal is applied to the 2nd
mixer circuit (IC2).
4-1-4 2ND IF AND DEMODULATOR CIRCUITS
(MAIN-E UNIT)
The 2nd mixer circuit converts the 1st IF signal to a 2nd IF
signal. A double superheterodyne system (which converts
receive signals twice) improves the image rejection ratio and
obtains stable receiver gain.
The FM IF IC (IC2) contains the 2nd local oscillator, 2nd
mixer, limiter amplifier, quadrature detector, and noise
detector circuits, etc.
The 1st IF signal from Q23 is applied to the 2nd mixer sec-
tion of IC2 (pin 16), and is mixed with a 21.25 MHz 2nd LO
signal generated at the PLL circuit using the reference fre-
quency (21.25 MHz) to produce a 450 kHz 2nd IF signal.
The 2nd IF signal from IC2 (pin 3) is passed through the
ceramic filter (FI3), where unwanted signals are sup-
pressed, and is then applied to the 2nd IF (limiter) amplifier
in IC2 (pin 5). The signal is applied to the FM detector sec-
tion in IC2 for demodulation into AF signals.
The FM detector circuit employs a quadrature detection
method (linear phase detection), which uses a ceramic dis-
criminator (X2) for phase delay to obtain a non-adjusting cir-
cuit. The detected signal from IC2 (pin 9) is applied to the AF
circuit.
2nd IF filter
450 kHz
FI3
5 3
Noise
amp.
Mixer
Noise
detector
11 14
C205
X2
R5
R206
4 - 1
(21.25 MHz)
17 16
2
PLL IC
IC1
X1
21.25 MHz
IC2 TA31136F
16
1st IF (21.7 MHz)
from the IF amplifier (Q23)
"SQL" signal to the CPU
(LOGIC-E unit; IC1,pin 90)