5 - 1
SECTION 5. CIRCUIT
DESCRIPTION
5-1 RECEIVER CIRCUITS
RF CIRCUITS (MAIN-A/-B UNIT)
RF signal from the antenna is passed through the LPF (as
the harmonic filter for transmitting) and antenna SW (D3,
D5, D7), and then applied to the RX BPF circuit.
The applied RX signal is passed through the 3-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 3-staged tuned BPF
(D9, D10, D19).
Total of 5 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
1ST IF CIRCUITS (MAIN-A/-B 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,
D52), 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
2ND IF AND DEMODULATOR CIRCUITS (MAIN-A/-B 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, L35,
C305–C308) 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
LPF
LP F
PWR
DET
D1,D11,D12
ANT
SW
D3,D5
BPF
D10
BPF
D19
BPF
D9
BPF
D4
BPF
D8
RF
AMP
Q2
AGC
Q1
Tuning voltage “T2”
To 1st IF circuits
Tuning voltage “T1”
450-520 MHz
400-470 MHz
RSSI
From the TX AMP
LO
SW
D14,D15
Q3
1st mixer
BPF
XTAL
FI1
IF
AMP
Q4
LPF
From RF circuits
To IF IC
46.35 MHz
1st LO signals
(353.65 MHz-423.65 MHz
403.65 MHz-473.65 MHz)
REF
X2
AMP
Q34
X1
BPF
BPF
CERAMIC
FI6
W/N
SW
Q5
BPF
CERAMIC
FI2
W/N
SW
D70,D71
W/N
SW
D68,D69
D/A
IF IC
From 1st IF circuits
IC1
IC18
NWC
15.3MHz
45.9MHz
DISC
SQIN
450kHz
To the AF circuits
NWC
