Circuit Descripiton; Receive Circuits - Icom ID-31A Service Manual

SECTION 4.

4-1 RECEIVE CIRCUITS

RF CIRCUITS (MAIN UNIT)
The RX signal from the antenna is passed through two LPFs
(L32, L34, C302, C303, C306, C307, C312 and L29, C281,
C285), ANT SW (D24, D25, D31, D33, L36, C313, C315,
C319) and tuned BPF (D32, L43, C353), and then applied to
the RF AMP (Q47).
The amplifi ed signal is passed through the tuned BPF (D27, D29,
D30, L28, L31, L33, C286, C290, C292, C293, C297, C299–
C301, C304, C305), and then applied to the 1st IF circuits.
• RF CIRCUITS
From the TX AMP circuits
D27,D29,D30
To the 1st IF circuits
BPF RF
1ST IF CIRCUITS (MAIN UNIT)
The RX signal from the RF circuits is applied to the 1st IF
mixer (Q46), and mixed with the 1st LO signal from the VCO
(Q27, D10–D12), resulting in the 46.35 MHz 1st IF signal.
The 1st IF signal is passed through the 1st IF fi lter (FI3) and
limiter (D23), and then applied to the 1st IF AMP (Q44). The
amplifi ed signal is passed through another limiter (D22), and
then applied to the 2nd IF circuits.
• 1ST IF CIRCUITS
From the VCO
To the 2nd IF circuits
IF
LIMIT
AMP
Q44
D22
2ND IF CIRCUITS (MAIN UNIT)
The 1st IF signal from the 1st IF circuits is applied to the IF IC
(IC13, pin 16). The IF IC contains the 2nd mixer, 2nd IF AMP,
detector, and so on, in its package.
The 1st IF signal is mixed with the 2nd LO signal from the
reference frequency oscillator (TCXO: X1), resulting in the
450 kHz 2nd IF signal. The 2nd IF signal is passed through
the 2nd IF fi lter (FI1: for DV/FM-N mode, FI2: for FM mode)
to remove sideband noise. The fi ltered signal is amplifi ed by
the 2nd IF AMP, and demodulated by the quadrature detector
with discriminator (X2).
The demodulated AF signal is applied to the RX AF circuits.
• 2ND IF CIRCUITS
Q38
X1
REF
X3
15.3MHz
45.9MHz
BPF
To the digital
demodulation circuits.
To the RX AF circuits
CIRCUIT DESCRIPTION
D24,D25
D31,D33
ANT
LPF
LPF
SW
D32
BPF
Q46
To the RF circuits
1st LO
LPF
46.35MHz
XTAL
LIMIT
BPF
D23 FI3
FM-N/DV
FM
CERAMIC CERAMIC
BPF
BPF
FI1 FI2
W/N
Q36,D15
SW
D16,D18,D19
From the 1st IF circuits
IF IC
IC13
DISCRI
X2
RX AF CIRCUITS (MAIN AND LOGIC UNITS)
• While operating in the DV mode
The demodulated AF signal is passed through the LPF (MAIN
UNIT: R118, R121, C122, C130, C135), and then applied to
the digital demodulation circuits on the LOGIC UNIT.
The AF signal from the MAIN UNIT is amplifi ed by the AF
AMP (LOGIC UNIT: IC13A, pins 1, 2) and passed through the
LPF (LOGIC UNIT: IC13B, pins 6, 7), and then applied to the
liner CODEC (LOGIC UNIT: IC15, pin 3) to be encoded in to
a digital audio signal.
The digital audio signal is demodulated by the DSP (LOGIC
UNIT: IC18), and then applied to the linear codec (LOGIC
UNIT: IC15) to be decoded into an analog audio signal.
The decoded AF signal is applied to the MAIN UNIT.
The AF signal is passed through the mode SW (MAIN UNIT:
IC8, pins 3, 4) and AF fi lters (MAIN UNIT: IC9B, pins 6, 7
and IC9C, pins 9, 10), and then applied to the D/A converter
(MAIN UNIT: IC11, pin 13) to be adjusted in level.
• DIGITAL DEMODULATION CIRCUITS
RTONE
LPF
IC13B
DDET
AMP
IC13A
• While operating in the FM/FM-N mode
The demodulated AF signal is passed through the LPF (R272,
C355), mode SW (MAIN UNIT: IC8, pins 1, 2) and AF fi lters
(MAIN UNIT: IC9B, pins 6, 7 and IC9C, pins 9, 10), and then
applied to the D/A converter (MAIN UNIT: IC11, pin 13) to be
adjusted in level.
The level-adjusted AF signal is amplifi ed by the AF power
AMP (MAIN UNIT: IC2, pin 1), and then output from the in-
ternal speaker (CHASSIS: SP1) or external speaker, through
the SP jack (MAIN UNIT: J4).
• RX AF CIRCUITS
[SP]
IC2
VO+
AF
AMP
J4
VO-
AFON
SP1
CHASSIS UNIT
4 - 1
IC15
To the RX AF circuits
LINEAR DAFO
CODEC
DSP
IC18
Q31
From the 2nd IF circuits
LPF
IC11
From the digital
IC16
demodulation circuits.
AF
MODE
D/A
SW
FILTER
IC8
IC9B,C
Q23
LPF
From the 2nd IF circuits