Circuit Description; Receiver Circuits - Icom IC-FR4000 Service Manual

SECTION 3

3-1 RECEIVER CIRCUITS

3-1-1 RF CIRCUIT (RX UNIT)
Received signals from the RX antenna connector enter the
RX unit J1 and pass through a tuned bandpass filter (D3,
D4, L1–L3, L32, C2, C5–C8, C10–C12, C14, C205, C206,
C210) which is controlled by the D/A converter IC (LOGIC
unit; IC25). The filtered signals are applied to an RF amplifi-
er (Q1). The amplified signals are applied to a bandpass fil-
ter (D6, D7, L6, L7, C21, C22, C24, C26–C28, C30, C207,
C208, C221, C222), and are then applied to the 1st mixer
circuit.
3-1-2 1ST MIXER AND 1ST IF CIRCUITS (RX UNIT)
The 1st mixer circuit converts the received signals to a fixed
frequency of the 1st IF signal with the PLL output frequency.
By changing the PLL frequency, only the desired frequency
will pass through a crystal filter at the next stage of the 1st
mixer.
The filtered signals are applied to the 1st mixer circuit
(L8–L10, IC1), and are then mixed with the 1st LO signal
from the PLL circuit to produce a 70MHz 1st IF signal.
The 1st IF signal passes through a MCF (Monolithic Crystal
Filter; FI1) to suppress out-of-band signals. The filtered sig-
nal is applied to the 2nd mixer circuit (IC2, pin 16) via the
buffer amplifier (Q3).
3-1-3 2ND IF AND DEMODULATOR CIRCUITS
(RX UNIT)
The 2nd mixer circuit converts the 1st IF signal to a 2nd IF
signal. A double-conversion superheterodyne system
improves the image rejection ratio and obtains stable receiv-
er gain.
The amplified signal is applied to the 2nd mixer section of
the FM IF IC (IC2, pin 16), and is then mixed with the 2nd
LO signal for conversion to a 455 kHz 2nd IF signal.
• 2ND IF AND DEMODULATOR CIRCUITS (RX UNIT)
"SQL" signal to the
LOGIC unit J1
"DISC" signals to
LOGIC unit J1

CIRCUIT DESCRIPTION

8
7
Active
filter
IF
amp.
QUAD.
detector
11
9 10
X1
IC2 contains the 2nd mixer, IF amplifier, quadrature detec-
tor, active filter and noise amplifier circuits, etc. The local
oscillator section and X2 generate 69.545 MHz for the 2nd
LO signal.
The 455 kHz 2nd IF signal is applied to a ceramic bandpass
filter (narrow; FI5, wide; FI6) where unwanted signals are
suppressed, and are then applied to the IF amplifier section
in the system IC (IC2, pin 5).
The 2nd IF signal is then amplified at the IF amplifier section
(IC2, pin 5) and applied to the quadrature detector section
(IC2, pins 10, 11 and X1) to demodulate the 2nd IF signal
into AF signals.
The AF signals are output from pin 9 (IC2) and are then
applied to the AF amplifier circuit on the LOGIC unit.
3-1-5 AF AMPLIFIER CIRCUIT (LOGIC UNIT)
The AF amplifier circuit amplifies the demodulated AF sig-
nals to drive a speaker.
The AF signals from the FM IF IC (RX unit; IC2, pin 9) are
applied to the buffer amplifier (IC9, pins 8, 9). The amplified
signals pass through the high-pass filter (IC9, pins 5, 7, 12,
14) which remove CTCSS or DTCS signals.
The filtered AF signals are output from pin 7 (IC9) and are
applied to the de-emphasis circuit (IC9, pins 1, 2) with fre-
quency characteristics of –6 dB/octave, and then pass
through a low-pass filter (IC10, pins 1, 3, 5, 7). The filtered
signals are applied to the volume control (VR unit; R1) to
control the audio level.
The output AF signals from the volume control (VR unit; R1)
pass through the analog switch (IC16; pins 1, 7) and are
then applied to the AF power amplifier (IC17, pins1, 4) to
drive a speaker.
D15 D16
FI6
WIDE
FI5
NARROW
5 3
Noise
amp.
Noise
comp.
RSSI
12
13
"RSSI" signal to the CPU
3 - 1
2nd IF filter
2nd LO signal
455 kHz
(69.545 MHz)
1
2nd
IC2
Mixer
TA31136FN
16
1st IF from the IF amplifier (Q3)
"N-DET" to the CPU
X2