Receiver Circuits - Icom IC-M802 Service Manual

SECTION 3

3-1 RECEIVER CIRCUITS

3-1-1 RF FILTER CIRCUIT (FILTER AND MAIN
UNITS)
Received signals from the antenna connector are applied to
the transmit/receive switching and protection relay (FILTER
unit; RL7301) which is controlled by the CPU via the "TRXS"
line. The signals pass through the 30 MHz cut-off low-pass
filter (FILTER unit; L7321, C7321–C7323, C7325), and then
applied to the MAIN unit via the J7321.
The signals pass through the transmit/receive switch (D53)
and 1.6 MHz cut off high-pass filter (L51–L54, C54, C56,
C57, C59, C61–C64), and are then applied to one of the
bandpass filters (including one low-pass filter for below 2.0
MHz). These filters are selected by the filter control signals
(B0–B8) as described in the table below.
The filtered signals pass through the 33 MHz cut-off low-
pass filter (L202, L203, C202–C206), and are then applied
to the 1st mixer circuit (Q201, Q202).
Frequency (MHz) LPF ctrl signal
0.5–1.999
2–2.999
3–4.999
5–6.999
7–9.999
10–13.999
14–17.999
18–19.999
20–21.999
22–23.999
24–29.999
3-1-2 1ST MIXER AND IF CIRCUITS (MAIN UNIT)
The 1st mixer circuit converts the received signals into a
fixed frequency, 64.445 kHz 1st IF signal using PLL output
frequency. By changing the PLL frequency, only the desired
frequency is picked up at the pair of crystal filters (FI301a,
FI301b) via the 64.445 kHz bandpass filter (FI201) at the
next stage.
• RECEIVER CONSTRUCTION
Antenna
LPF
FILTER unit
to transmit/receiver switch
CIRCUIT DESCRIPTION
BPF ctrl signal
L0 B0
L1
B1
L2
B2
L3 B3
L4 B4
L5
B5
B6
L6
B7
L7
B8
0.5—1.9999 MHz
LPF
BPF
2—30 MHz
DSP
UNIT
IC1002
The IF amplifier (Q203) and resonator circuits are designed
between the filter pair. The PLL output signal (1LO) enters
the MAIN unit via the J601 and is amplified at the 1st LO
amplifier (Q601). The amplified signal is passed through the
100 MHz cut-off low-pass filter (L604, L605, C604,
C606–C610) to suppress harmonics components, and then
applied to the 1st mixer circuit (Q201, Q202).
3-1-3 2ND MIXER AND IF CIRCUITS (MAIN UNIT)
The 1st IF signal from the crystal filter (FI301b) is converted
again into a 455 kHz 2nd IF signal at the 2nd mixer circuit
(D302, L303, L304). The 2nd LO signal (2LO) from the PLL
unit enters the MAIN unit via the J301 to be applied to the
2nd mixer circuit.
3-1-4 3RD MIXER AND IF CIRCUITS (MAIN UNIT)
The 2nd IF signal passes through the low-pass filter (L305,
L306, C307–C311), and then applied to the IF amplifier
(Q401) via the ceramic bandpass filter (FI401). The ampli-
fied signal passes through the ceramic bandpass filter
(FI402), and then applied to the 3rd mixer circuit via the IF
amplifier (Q501). The 2nd IF signal is converted into a 12
kHz 3rd IF signal at the 3rd mixer circuit (IC501). The 3rd LO
signal (3LO) from the PLL unit enters the MAIN unit via the
J3601 to be applied to the 3rd mixer circuit.
3-1-5 DSP RECEIVER CIRCUIT (MAIN AND DSP
UNITS)
The DSP (Digital Signal Processor) circuit enables digital IF
filter, digital noise reduction, digital PSN (Pulse Shift
Network), phase demodulation, digital automatic notich, and
etc.
The 3rd IF signal is applied to the IF amplifier (MAIN unit;
IC1002, pin 5) after being passed through the low-pass filter
(MAIN unit; IC1002, pins 3, 1). The amplified 12 kHz 3rd IF
signal is amplified at the differential amplifiers (IC651a/b),
and is then applied to the A/D convertor section in the
CODEC IC (IC501) on the DSP board (EX-2432). At the
same time, the converted signal is level-shifted 5 V to 3.3 V
in the IC (IC501).
1st LO 2nd LO
64—94 MHz 64 MHz
Crystal
BPF
1st mixer
2nd mixer
FI301A
Q201
Q201
FI301B
Q202
Q202
3rd LO
443 kHz
IF
LPF
amp.
3rd mixer
IC501
3 - 1
Crystal IF
amp.
BPF
FI401 Q401
IF
Crystal
amp.
BPF
Q501
FI402