Receiver Circuits - Icom IC-F5012 Service Manual

SECTION 5.

5-1 RECEIVER CIRCUITS

RF CIRCUITS (MAIN-A UNIT)
RF signal from the antenna is passed through the LPF (as
the harmonic filter for transmitting) and antenna SW, and
then applied to the RX BPF circuit.
The applied RX signal is passed through the 2-staged tuned
BPF (D4, D8) to remove unwanted out-of-band signals, and
amplified by the low-noise RF AMP (Q2), and then applied
to the 1st mixer (Q3) through another 2-staged tuned BPF
(D9, 10).
Total of 4 stage BPFs are tuned to the RX frequency by the
tuning voltage "T1" and "T2" from the D/A converter (IC20),
to obtain required IMD characteristic.
• RF CIRCUITS
D2,D3,D5,
D6,D47,D48
ANT
From the TX AMP
SW
RSSI
D9_A
D9_B
To 1st IF circuits
BPF BPF
Tuning voltage "T2"
1ST IF CIRCUITS (MAIN-A UNIT)
The RX signal from the BPF is mixed with the 1st LO signal
to be converted into the 46.35 MHz 1st IF signal.
The 1st LO signal is generated by the RX VCO (Q14, D33,
D34, L26, L39, etc.), and passed through the LO SW (D15)
and attenuator, and then applied to the 1st mixer (Q3)
The converted 1st IF signal is filtered by the crystal filter (FI1)
and amplified by the 1st IF AMP (Q4), then applied to the IF
demodulator IC (IC1).
• 1ST IF CIRCUITS
1st LO signals
(182.35 MHz-220.35 MHz)
FI1
Q4
46.35 MHz
To IF IC IF XTAL
AMP BPF
CIRCUIT DESCRIPTION
136-174 MHz
D1,D11,D12
PWR
LP F LPF
DET
Q1
AGC
D4
D8
Q2
RF
BPF BPF
AMP
Tuning voltage "T1"
D15
LO
SW
LPF
Q3
From RF circuits
1st mixer
2ND IF AND DEMODULATOR CIRCUITS (MAIN-A UNIT)
The amplified 1st IF signal is mixed with the 2nd LO signal
at the internal 2nd IF mixer of the IF demodulator IC (IC1),
to obtain the 450 kHz 2nd IF signal.
The 15.3 MHz signal generated by the reference oscillator
(X2) is passed through the filter AMP (Q34, L33, 35, C305–
308) to extract the 45.9 MHz 3rd harmonic component.
The 45.9 MHz signal is then applied to the pin 2 of IF
demodulator IC (IC1) as the 2nd LO signal.
The converted 2nd IF signal is output from the pin 3 of IF
demodulator IC (IC1), and filtered by the ceramic filters
(FI2 and FI6 for Narrow mode; FI2 only for Wide mode) to
remove unwanted out-of-band signals, and then applied to
the internal frequency-demodulator of IF demodulator IC
(IC1) from pin 5.
The demodulator is a quadrature type which uses X1 as the
phase shifter.
The frequency-demodulated AF signal is output from pin 9 to
AF circuits.
• 2ND IF AND DEMODULATORCIRCUITS
X2
REF
15.3MHz
X3 Q34
IC1
45.9MHz
BPF
Q5
SQIN
NWC
D/A
W/N
DISC
IC18 SW
AK2330
To the AF circuits
5 - 1
From 1st IF circuits
IF IC
FI2
D66,D67
CERAMIC
W/N
X1
BPF
SW
FI6
450kHz
CERAMIC
W/N
BPF
SW
NWC Q40 D64,D65