4 - 1
SECTION 4. CIRCUIT
DESCRIPTION
• RF CIRCUIT
• 1ST IF CIRCUIT
• 2ND IF AND DEMODULATOR CIRCUITS
4-1 RECEIVER CIRCUITS
RF CIRCUIT
The RX signal from the antenna is passed through the LPF
(L1–L3, C2–C7, C24, and C37) and antenna SW (D3, D22),
then filtered by the 2-staged tuned BPF (D24 and D26) to
eliminate unwanted out-of-band signals. The filtered RX
signal is amplified by the RF AMP (Q22), and filtered by
another 2-staged tuned BPF (D28) to obtain a good image
response, then applied to the 1st IF circuit.
The BPFs are tuned to the RX frequency by applying ad-
equate tuning voltages: “T1” and “T2” to the variable capaci-
tors.
1ST IF CIRCUIT
The RX signal from the RF circuit is applied to the 1st IF
mixer (Q23) and mixed with the 1st LO signal from the RX
VCO, resulting in the 46.35 MHz 1st IF signal. The 1st IF sig-
nal is passed through the IF SWs (D31–D34) and the crystal
fi lter (FI1: analog mode, FI2: digital mode) to be fi ltered, am-
plified by the 1st IF AMP (Q24), then applied to the 2nd IF
circuit.
• WHILE OPERATING IN THE ANALOG MODE
The 2nd IF signal is filtered by the 2nd IF filter (FI3: wide
mode) or fi lters (FI3 and FI4: 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, output from pin 9, is applied to
the linear codec inside IC2 (a custom AF IC). The AF signal
is encoded into a digital signal, then demodulated by DSP
(IC903). The demodulated signal is then applied back to the
linear codec to be decoded into an analog audio signal.
• WHILE OPERATING IN THE DIGITAL MODE
The 2nd IF signal is fi ltered by the 2nd IF fi lters (FI3 and FI4)
to eliminate unwanted signals, and applied to the IF AMP
(IC5) through the buffer (Q28). The amplifi ed 2nd IF signal is
passed through the ceramic fi lter (FI5), and then applied to
the A/D converter (IC901) to be encoded into a digital signal.
The digital signal is demodulated by the DSP (IC903), and
then applied to the linear codec (inside IC2) to be decoded
into an analog audio signal.
The AF signal is applied to the RX AF circuit.
LPF
ANT
SW
D3,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
D31,D32
D33,D34
Q23
BPF
XTAL
FI1
IF
AMP
Q24
BUFF
Q8
BUFF
Q10
ATT
IF SW
IF SW
RX VCO
To the TX circuit
From the RF circuit
To the 2nd
IF circuit
46.35MHz
BPF
XTAL
FI2
46.35MHz
1st IF mixer
W/N
SW
D38,D39
BPF
CERAMIC
FI3
BPF
CERAMIC
FI4
W/N
SW
D36,D37
IF IC
From the 1st IF
circuit
(Digital path)
IC3
450kHz
450kHz
VOL1
AMP
ALC
AMP
VOL1
AMP
LPF
BUFF
VOL1
W/N SW
Q901
BUFF
VOL1
LIMIT
AMP
DET
D53
W/N SW
BUFF
HPF
MOD
LINE
CTRL
VOL1
R89
LINEAR
REF
DET
AFVO
BEEP
MOD
MICI
TX/RX
SW
DMO
IC2
BAL
CODEC
DAFO
SQIN
VCON
DISC
VOXV
T1
LVA
T2
NWC
X1
RSSI NOIS
SQIN
DET
SSO,SSK,DAST
X3
Q26
BPF
X2
TCXO
15.3MHz
45.9MHz
CERAMIC
BPF
FI5
DIF
AMP
IC5
BUFF
Q28
A/D
IC901
DSP
IC903
2ND IF AND DEMODULATOR CIRCUITS
The signal from the 1st IF circuits is applied to the IF demod-
ulator IC (IC3) 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).
