4 - 1
SECTION 4. CIRCUIT
DESCRIPTION
• RF CIRCUITS
• 1ST IF CIRCUITS
• 2ND IF CIRCUITS
4-1 RECEIVER CIRCUITS
RF CIRCUITS
The RX signal from the antenna is passed through the LPF
and antenna SW, then filtered by the 2-staged tuned BPF
to eliminate unwanted out-of-band signal. The filtered RX
signal is amplified by the RF AMP, and filtered by another
2-staged tuned BPF to obtain a good image response, then
applied to the 1st IF circuits.
The tuned BPFs are tuned to the RX frequency by applying
adequate tuning voltages; “T1” and “T2” to the variable
capacitors.
1ST IF CIRCUITS
The RX signal from the RF circuits is applied to the 1st IF
mixer 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
filtered by the crystal filter, amplified by the 1st IF AMP, then
applied to the 2nd IF circuits.
2ND IF CIRCUITS AND DEMODULATOR
The signal from the 1st IF circuits is applied to the IF
demodulator IC which contains the 2nd IF mixer, 2nd IF
AMP, FM detector, etc. 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 oscillated by the reference
frequency oscillator (TCXO).
The converted 2nd IF signal is filtered by the crystal filter/
filters to eliminate unwanted signal, and amplified by the
2nd IF AMP, then demodulated by the detector circuit which
employs a discriminator as the phase shifter.
The demodulated AF signal is applied to the RX AF circuits.
LPF
ANT
SW
D1,D22
RF
AMP
Q22
BPF
D28
D24,D26
BPF
T2
T1
From the TX circuits
To the 1st IF circuits
ANT
LO
SW
D5,D6
Q23
BPF
XTAL
FI1
IF
AMP
Q24
BUFF
Q8
BUFF
Q10
ATT
RX VCO
To the TX circuits
From the RF circuits
To the 2nd IF&demodulator circuits
46.35MHz
1st IF mixer
PLL
IC
IC2
MB15A02
X3
Q26
BPF
X2
TCXO
W/N
SW
Q27,
D33,D34
X1
Discriminator
BPF
CERAMIC
FI2
BPF
CERAMIC
FI3
W/N
SW
D31,D32
D/A
converter
IF IC
From the 1st IF
circuits
To the RX AF circuits
IC3:NJM2591
15.3MHz
45.9MHz
DISC
SQIN
IC57
R2A20178
450kHz
450kHz
NWC
NOIS
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