Receiver Circuits - Icom IC-F5220D Service Manual

SECTION 5.

5-1 RECEIVER CIRCUITS

RF CIRCUITS
The RX signal from the antenna is passed through the LPF
(L30, L33, L35, C1–C8 and C404) and antenna SW (D24,
D28, D32), then fi ltered by the 2 stage tuned BPFs (D31 and
D37) to eliminate unwanted out-of-band signals. The fi ltered
RX signal is amplifi ed by the RF AMP (Q20), and fi ltered by
another tuned BPF (D21) 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.
• RF CIRCUITS
From the TX circuits
D21
Q20
To the 1st IF circuits
RF
BPF
AMP
T2
1ST IF CIRCUITS
The RX signal from the RF circuits is applied to the 1st IF
mixer (Q19) and mixed with the 1st LO signal from the RX
VCO, resulting in the 46.35 MHz 1st IF signal. The 1st IF
signal is passed through the IF SWs (D13, D14, D17, D18)
and the crystal fi lter (FI4: analog mode, FI5: digital mode) to
be fi ltered, amplifi ed by the 1st IF AMP (Q14), then applied
to the 2nd IF circuits.
• 1ST IF CIRCUITS
Q10
RX VCO
BUFF
FI4
D13,D14
46.35MHz
XTAL
BPF
Q14
e 2nd IF circuits
IF
IF SW FI5
AMP
46.35MHz
XTAL
BPF
CIRCUIT DESCRIPTION
ANT
D24, D28, D32
ANT
LPF
SW
D31 D37
BPF BPF
T1 T1
D5,D6
Q8
LO
To the TX circuits
BUFF
SW
D17,D18
ATT
Q19
From the RF circuits
IF SW
1st IF mixer
2ND IF AND DEMODULATOR CIRCUITS
The signal from the 1st IF circuits is applied to the IF demod-
ulator IC (IC4) 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).
• WHILE OPERATING IN THE ANALOG MODE
The 2nd IF signal is filtered by the 2nd IF filter (FI2: wide
mode) or fi lters (FI1 and FI2: 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 is applied to the linear codec
(inside the AF CUSTOM IC (IC1010)). The AF signal is
encoded into a digital signal, then demodulated by DSP
(IC2007). The demodulated signal is then applied to the
linear codec (inside the AF CUSTOM IC (IC1010)) to be de-
coded into an analog audio signal.
• WHILE OPERATING IN THE DIGITAL MODE
The 2nd IF signal is fi ltered by the 2nd IF fi lters (FI1 and FI2)
to eliminate unwanted signals, and applied to the IF AMP
(IC5) through the buffer (Q5). The amplifi ed 2nd IF signal is
passed through the ceramic fi lter (FI3), and then applied to
the A/D converter (IC2002) to be encoded into a digital sig-
nal. The digital signal is demodulated by the DSP (IC2007),
and then applied to the linear codec (inside the AF CUSTOM
IC (IC1010)) to be decoded into an analog audio signal.
The AF signal is applied to the RX AF circuits.
• 2ND IF AND DEMODULATOR CIRCUITS
Q4 X2
15.3MHz
X3
TCXO
IF IC
45.9MHz
BPF
W/N
SW
FI1
450kHz
CERAMIC
BPF
5 Q
BUFF
IC1010
SQIN
BEEP
AFVO
REF
VCON
BUFF
BAL
MOD
HPF
MICI
ALC
AMP
AMP
5 - 1
IC4
From the 1st IF
circuits
D8,D9
FI2
450kHz
CERAMIC
BPF
D2,D3
W/N
SW
IC2007
De-Emphasis
F 3 I BDR0
IC5 C I 0 2 2 0
Signaling
BFSR0
CERAMIC DIF Decode
BCLKR0
AMP A/D
BPF
DSP
BCLKX1
BFSX1
BDX1
BDR1
DET
BUFF
BUFF
DAFO
DMO
LPF
LINEAR
LIMIT
AMP
CODEC
MOD
TX/RX
LINE
SW
CTRL