Frequency Synthesizer Circuits - Icom IC-F6220D Service Manual

MODULATION CIRCUITS
The modulation signal from the TX AF circuits is applied to
D26 of the TX VCO (Q23, D16, D26, D64) to modulate it (FM
for the analog mode, 4FSK for the digital mode). The modu-
lated signal from the TX VCO is buffer-amplifi ed by two buf-
fers (Q11, Q10), and applied to the TX AMP circuits through
the LO SW (D25).
• MODULATION CIRCUIT
Q11 Q10
TX VCO
BUFF BUFF
Q13,D6
D26
MODULATION
MOD
From the TX AF circuits
TX AMPLIFIER
The buffer amplified signal from the LO SW (D25) is se-
quentially amplifi ed by the pre-drive AMP (Q25), drive AMP
(Q8) and power AMP (IC3), to obtain TX power. The ampli-
fi ed TX signal is passed through the antenna SW (D5, D24)
and the LPFs, which eliminates harmonics, and then applied
to the antenna.
APC CIRCUIT
D11, D12 and D29 rectify a portion of the TX signal to direct
current, and the APC AMP (IC2) compares the voltage and
the TX power control reference voltage, "T1." The resulting
voltage controls the gain of the power AMP (IC3) to keep the
TX power constant.
• TX AMPLIFIERS AND APC CIRCUIT
Q8
From the TX VCO
DRIVE
AMP
D25
To the TX AMP circuits
LO
SW
D3,D5,D6,D68
IC3
ANT
PWR
LPF
LPF
AM P
SW
IC2
APC PWR
D1,D11,D12
AMP
DET
Q6
MUTE TMUT
To the RX circuits
SW
5-3 FREQUENCY SYNTHESIZER CIRCUIT
The RX VCO is composed of Q24, D33 and D62. The VCO
output signal is buffer-amplified by two buffers (Q11 and
Q10), and then applied to the 1st IF mixer, through the LO
SW (D15) and the attenuator.
The TX VCO is composed of Q23, D16, D26 and D64. The
VCO output signal is buffer-amplified by two buffers (Q11
and Q10), and then applied to the pre-drive AMP (Q25),
through the LO SW (D25) and the LPF.
A portion of signal generated by each VCO is fed back to the
PLL IC (IC8, pin 17), through the buffer (Q12) and the LPF
(L13, C298–C300).
The applied VCO output signal is divided and phase-com-
pared with a 15.3 MHz reference frequency signal from the
TCXO (X2), which is also divided. The resulting signal is
output from the PLL IC (IC3), and DC-converted by the loop
fi lter, and then applied to the VCO as the lock voltage.
When the oscillation frequency drifts, its phase changes
from that of the reference frequency, causing a lock voltage
change to compensate for the drift in the VCO oscillating fre-
quency.
• FREQUENCY SYNTHESIZER CIRCUIT
X2
15.3MHz
VCON
TCXO
Q4
IC3
PLL
X3
IC
ANT
SO,SCL,PLST
45.9MHz
IF IC
BPF
IC4
5-4 VOLTAGE DIAGRAMS
AF power AMP
VCOs and LO buffers
Transmit circuits
Receive circuits
TX/RX common circuits
Logic circuits
5 - 3
D62
RX VCO
Q24,D33
LV
ADJ
LOOP
LV
FILTER
Q11 Q10
BUFF BUFF
D64
TX VCO
LVA
LV
ADJ
Q23,D16
Q12
LPF
BUFF
VCC
8V
D1008,Q1011,
Q1018,Q1021 I C1002
VCC
T8 +8
T8V TXC
REG 8V REG
Q1001
Q1013,Q1016
Q 1003
8V
R8
R8V VCC POWE R
REG
RXC SW
Q1014,Q1015
I C1001
8V
+5
+5V
REG +3
REG
Q1017,Q1019
8V
+3
+3.3V
REG
CPU3.3V
CPU
D14,D15
LO
To the TX AMPs
SW
ATT
Q3
1st IF mixer
VCC
W2
HV
PWON