Receiver Circuits - Icom IC-F6021 Service Manual

SECTION 5.

5-1 RECEIVER CIRCUITS

RF CIRCUITS
RF signals from the antenna are passed through the LPF (as
the harmonic filter for transmitting) and antenna SW, then
applied to the RX BPF circuit.
The applied RX signals are passed through the 2-staged
tuned BPF (D8, 20 and D4, 21) to remove unwanted out-of-
band signals, and amplified by the low-noise RF AMP (Q2),
then applied to the 1st mixer (Q3) via three BPFs (D9, 10
and D19).
Each BPF is 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
D3,5,6
ANT
From the TX AMP
SW
D10
D19
To 1st IF circuits
BPF BPF
1ST IF CIRCUITS
The RX signals from the BPF are mixed with the 1st LO
signals to be converted into the 46.35 MHz 1st IF signal.
The 1st LO signals are generated by the RX VCO (Q14,
D33, 62, 63), and passed through the LO SW (D15) and
attenuator, 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
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
D1,11,12
PWR
LP F LPF
DET
Q1
RSSI
AGC
D4,21 D8,20
D9
Q2
RF
BPF BPF BPF
AMP
Tuning voltage "T2" Tuning voltage "T1"
D14,15
TX/RX
SW
LPF
Q3
From RF circuit
1st mixer
To 1st IF circuits
2ND IF AND DEMODULATOR CIRCUITS
The amplified 1st IF signal is mixed with the 2nd LO signal
at the internal 2nd IF mixer of the IF 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) to extract the
45.9 MHz 3rd harmonic component. The 45.9 MHz signal is
then applied to pin 2 of IF IC (IC1) as the 2nd LO signal.
The converted 2nd IF signal is output from pin 3 of IF 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, then applied to the internal frequency-
demodulator of IF IC (IC1) from pin 5.
The demodulator is a quadrature type which uses X1 as the
phase shifter.
The frequency-demodulated AF signals are output from pin 9
to AF circuits.
• 2ND IF AND DEMODULATORCIRCUITS
REF X2
15.3MHz
X3
Q34
IC1:TA31136FN
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
D57
CERAMIC
W/N
X1
BPF
SW
FI6 D56
450kHz
CERAMIC
W/N
BPF
SW
NWC