Circuit Descripiton; Receiver Circuits - Icom IC-F4101D Service Manual

SECTION 4.

4-1 RECEIVER CIRCUITS

RF CIRCUITS
The RX signal from the antenna is passed through the LPF
and antenna SW (D3, D22), then filtered by the 2-staged
tuned BPF (D24 and D26) to eliminate unwanted out-of-
band signals. The filtered RX signal is amplified by the RF
AMP (Q22), and filtered by another 2-staged tuned BPF
(D28) to obtain a good image response, then applied to the
1st IF circuits.
The BPFs are tuned to the RX frequency by applying ad-
equate tuning voltages: "T1" and "T2" to the variable capaci-
tors.
1ST IF CIRCUITS
The RX signal from the RF circuits is applied to the 1st IF
mixer (Q23) and mixed with the 1st LO signal from the RX
VCO, resulting in the 46.35 MHz 1st IF signal. The 1st IF sig-
nal is passed through the IF SWs (D31–D34) and the crystal
fi lter (FI1: analog mode, FI2: digital mode) to be fi ltered, am-
plified by the 1st IF AMP (Q24), then applied to the 2nd IF
circuits.
• RF CIRCUITS
From the TX circuits
D28
Q22
To the 1st IF circuits
RF
BPF
AMP
T2
2ND IF AND DEMODULATOR CIRCUITS
The signal from the 1st IF circuits is applied to the IF demod-
ulator IC (IC3) which contains the 2nd IF mixer, 2nd IF AMP,
FM detector, squelch circuit and AF AMP in its package.
The 1st IF signal is applied to the 2nd IF mixer and mixed
with the 2nd LO signal resulting in the 450 kHz 2nd IF signal.
The 2nd LO signal is generated by tripling the 15.3 MHz
reference frequency signal generated by the reference fre-
quency oscillator (TCXO; X2).
• 1ST IF CIRCUITS
Q10
RX VCO
BUFF
FI1
D33,D34
46.35MHz
XTAL
BPF
Q24
e 2nd IF circuits
IF
IF SW FI2
AMP
46.35MHz
XTAL
BPF
CIRCUIT DESCRIPTION
ANT
D1,D22
ANT
LPF
SW
D24,D26
BPF
T1
D5,D6
Q8
LO
To the TX circuits
BUFF
SW
D31,D32
ATT
Q23
From the RF circuits
IF SW
1st IF mixer
• WHILE OPERATING IN THE ANALOG MODE
The 2nd IF signal is filtered by the 2nd IF filter (FI3: wide/
middle mode) or fi lters (FI3 and FI4: narrow) to eliminate un-
wanted signals. It is amplifi ed by the 2nd IF AMP, and then
demodulated by the detector circuit, which employs the dis-
criminator (X1) as the phase shifter.
The demodulated AF signal is amplified by the AF AMP
(IC21), and then applied to the linear codec (IC902) through
the fi lter SW (Q901). The fi lter SW toggles the frequency re-
sponse of the AF fi lter (R904, R908, C924), according to the
type of reception; Wide, Mid or Narrow.
The AF signal is converted into a digital audio signal by the
linear codec (IC902), processed by the DSP (IC903), and
then decoded into an analog audio signal.
• WHILE OPERATING IN THE DIGITAL MODE
The 2nd IF signal is fi ltered by the 2nd IF fi lters (FI3 and FI4)
to eliminate unwanted signals and applied to the IF AMP
(IC5) through the buffer (Q28). The amplifi ed 2nd IF signal is
passed through the ceramic fi lter (FI5), and then directly ap-
plied to the DSP (IC903).
The 2nd IF signal is demodulated by the DSP (IC903), and
then applied to the linear codec (IC902) to be decoded into
an analog audio signal.
The AF signal is applied to the RX AF circuits.
• 2ND IF AND DEMODULATOR CIRCUITS
Q26 IC2
PLL 15.3MHz
X3
IC
TCXO
45.9MHz
BPF
D/A
converter
IC57
SQIN
DISC
(Digitalpath)
(Analog path)
Q901
DISC
FILTER
To the RX AF circuit
4 - 1
X2
X1
Discriminator
IC3
IF IC
D36,D37 FI3
450kHz
CERAMIC
W/N
BPF
SW
FI4 D38,D39
NOIS
450kHz
W/N
CERAMIC
BPF SW
FI5
IC901
Q28 IC5
CERAMIC DIF
BUFF BUFF A/D
BPF
R904,R908,C924 IC902
AF
FILTER
SW
LINEAR
CODEC
DAFO
From the 1st IF
circuits
IC903
DSP