Receiver Circuits - Icom IC-F3210D Service Manual

SECTION 4.

4-1 RECEIVER CIRCUITS

RF CIRCUIT
The RX signal from the antenna is passed through the LPF
(L1–L3, C2–C7, C24, and C37) and antenna SW (D1, D22,
D23), then filtered by the 2-staged tuned BPF (D24 and
D26) to eliminate unwanted out-of-band signals. The fi ltered
RX signal is amplifi ed by the RF AMP (Q22), and fi ltered by
another 2-staged tuned BPF (D28) to obtain a good image
response, then applied to the 1st IF circuit.
The BPFs are tuned to the RX frequency by applying ad-
equate tuning voltages: "T1" and "T2" to the variable capaci-
tors.
• RF CIRCUIT
From the TX circuits
D28
Q22
To the 1st IF circuits
RF
BPF
AMP
T2
1ST IF CIRCUIT
The RX signal from the RF circuit 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
circuit.
• 1ST IF CIRCUIT
Q10
RX VCO
BUFF
FI1
D33,D34
46.35MHz
XTAL
BPF
Q24
To the 2nd
IF circuit
IF
IF SW FI2
AMP
46.35MHz
XTAL
BPF
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).
CIRCUIT DESCRIPTION
ANT
D1,D22,D23
ANT
LPF
SW
D24,D26
BPF
T1
D5,D6
Q8
LO
To the TX circuit
BUFF
SW
D31,D32
ATT
Q23
From the RF circuit
IF SW
1st IF mixer
• WHILE OPERATING IN THE ANALOG MODE
The 2nd IF signal is filtered by the 2nd IF filter (FI3: wide
mode) or fi lters (FI3 and FI4: narrow) to eliminate unwanted
signals. It is amplifi ed by the 2nd IF AMP, and then demodu-
lated by the detector circuit, which employs the discriminator
(X1) as the phase shifter.
The demodulated AF signal, output from pin 9, is applied to
the linear codec inside IC2 (a custom AF IC). The AF signal
is encoded into a digital signal, then demodulated by DSP
(IC903). The demodulated signal is then applied back to the
linear codec to be 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 applied to
the A/D converter (IC901) to be encoded into a digital signal.
The digital signal is demodulated by the DSP (IC903), and
then applied to the linear codec (inside IC2) to be decoded
into an analog audio signal.
The AF signal is applied to the RX AF circuit.
• 2ND IF AND DEMODULATOR CIRCUITS
X2
TCXO
15.3MHz
Q26
X3 BPF
45.9MHz
IF IC
X1
SQIN RSSI NOIS
DET
(Digitalpath)
BUFF
DET
IC2
SQIN
BUFF
VOL1
BEEP
AFVO
BUFF
VOL1
REF
VCON
BUFF
BAL
VOL1
MOD
LPF
VOL1
LIMIT
HPF
AMP
MOD
MICI
ALC
AMP LINE
AMP
CTRL
D53
AMP DET
4 - 1
IC3
From the 1st IF
D36,D37
FI3
450kHz
W/N CERAMIC
BPF
SW
FI4 D38,D39
450kHz
W/N
CERAMIC
BPF
SW
FI5
IC901
Q28 IC5
CERAMIC DIF
A/D
AMP
BPF
Q901
W/N SW
NWC
T1
R89 T2
LVA
VOL1
SSO,SSK,DAST
DISC
DAFO
DMO
LINEAR
CODEC
TX/RX
SW
W/N SW
VOXV
circuit
IC903
DSP