Circuit Descripiton; Receiver Circuits - Icom IC-F4029TDR Service Manual Addendum

SECTION 4

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT (PA-C UNIT)
The antenna switching circuit toggles the receive (RX) line and
transmit (TX) line.
The received signals from the antenna are passed through
the low-pass filter (LPF; L709, L710, C742, C744−C748)
and antenna switch (D701, D704).
While transmitting, the voltage on the T5V line is applied
to D701 and D704, and these are ON. Thus the TX line is
connected to the antenna. Simultaneously, the RX line is
connected to the GND to prevent transmit signal entering.
While receiving, no voltage is applied to the D701 and
D704, and these are OFF. Thus the TX line and the antenna
is disconnected to prevent received signals entering.
Simultaneously, the RX line is disconnected from the GND
and the received signals are passed through the LPF (L712,
C750−C752). The filtered signals are applied to the RF
circuits.
4-1-2 RF CIRCUITS (MAIN-C UNIT)
RF circuits fi lter and amplify the received signals within the
frequency coverage.
The received signals from the PA-C UNIT are passed though
the two-staged bandpass fi lter (BPF; D19, D25, L7, L8, C21
−C23, C25, C27−C29) to filter-out unwanted signals, and
the fi ltered signals are applied to the RF amplifi er (Q5). The
amplifi ed received signals are then applied to the 1st mixer
(Q6) via another two-staged BPF (D14, D15, L9, L11, C19,
C36, C39−C41, C44, C45).
4-1-3 1st IF CIRCUITS (MAIN-C UNIT)
The received signals are converted into the 1st IF signal,
fi ltered and amplifi ed at the 1st IF circuits.
The received signals from the two-staged BPF (D14, D15,
L9, L11, C19, C36, C39−C41, C44, C45) are applied to
the 1st mixer (Q6) and converted into the 46.35 MHz 1st IF
signal by being mixed with the local oscillator (LO) signal
from the RX VCO (Q17, D9, D11).
• 2nd IF AND DEMODULATOR CIRCUITS
23
D/A converter
(IC12)
24
Demodulated signals
to the AF circuits
2nd IF signal to the DSP UNIT
CIRCUIT DESCRIPTION
8
Filter
amp.
Limiter
Quadrature
amp.
detector
9 10 11
Q8
Buffer
X1
The converted 1st IF signal is passed through the 1st IF fi lter
(FI1) to fi lter-out adjacent signals, then applied to the 1st IF
amplifi er (Q7). The amplifi ed 1st IF signal is then applied to
the FM IF IC (IC9, pin 16).
4-1-4 2nd IF AND DEMODULATOR CIRCUITS
(MAIN-C UNIT)
The 1st IF signal is converted into the 2nd IF signal, and
demodulated.
The 1st IF signal from the 1st IF amplifi er is applied to the
2nd mixer in the FM IF IC (IC9, pin 16), and converted into
the 450 kHz 2nd IF signal by being mixed with the 45.9 MHz
2nd LO signal from the reference frequency oscillator (X2)
via the tripler (Q22) and BPF (L33, C163, C164, C166). The
converted 2nd IF signal is output from pin 3, and passed
through the 2nd IF fi lters to remove sideband noise.
In analog mode, the 2nd IF signal is passed through the 2nd
IF fi lter (FI2) and applied to the FM IF IC (IC9, pin 5) again
via A/D switches (D2, D3).
In digital mode, the 2nd IF signal is passed through two 2nd
IF fi lters (FI2 and FI3) via A/D switches (D2, D3) and applied
to the FM IF IC (IC9, pin 5) again.
The fi ltered 2nd IF signal is amplifi ed at the limiter amplifi er,
and FM-demodulated by the quadrature detector (IC9, pins
10, 11, X1). The demodulated AF signals are output from pin
9, then applied to the AF amplifi er circuits.
4-1-5 AF AMPLIFIER CIRCUITS (MAIN-C UNIT)
The demodulated AF signals from the FM IF IC are amplifi ed
and fi ltered at AF circuits.
The demodulated AF signals from the FM IF IC (pin 9) are
passed through HPF (IC5, pins 1, 2) to remove tone signals.
The fi ltered AF signals are passed through the de-emphasis
circuit (R142, C249) to obtain the −6 dB/oct of frequency
characteristic. The de-emphasized AF signals are passed
through the RX mute switch (Q32, Q33), AF switch (Q36,
Q37), HPF (IC5, pins 13, 14), analog switch (IC3, pins 1, 2),
AF mixer (IC5, pins 6, 7) and analog switch (IC3, pins 10,
11) in sequence.
D2
D3
A/D A/D
SW SW
FI3
FI2
7 5
Noise
Noise
amp.
detector
RSSI
12 13
R5V
4 - 1
45.9 MHz
BPF
2
3
TCXO
X2
Mixer
FM IF IC (IC9)
16
1st IF signal from the 1st IF amplifier (Q7)
"NOIS" signal to the CPU (IC22: pin 75)
"RSSI" signal to the CPU (IC22: pin 50)
Q22
3
15.3 MHz