Frequency Synthesizer Circuits - Icom IC-F5220D Service Manual

MODULATION CIRCUIT
The modulation signal from the TX AF circuits is applied to
D10 of the TX VCO (Q7, D6, D7, D10, D12) to modulate it
(FM for the analog mode, 4FSK for the digital mode). The
modulated signal from the TX VCO is buffer-amplified by
two buffers (Q12, Q15), and applied to the TX AMP circuits
through the LO SW (D15).
• MODULATION CIRCUITS
Q12 Q15
TX VCO
Q7,D6,D7 BUFF BUFF
D10
MODULATION FM:Analog
4FSK:Digital
MOD
From the TX AF circuits
TX AMPLIFIERS
The buffer amplified signal from the LO SW (D15) is se-
quentially amplifi ed by the drive AMP (Q17) and power AMP
(IC3), to obtain TX power. The amplifi ed TX signal is passed
through the antenna SW (D24, D28 and D32) and the LPFs,
which eliminates harmonics, and then fed to the antenna.
APC CIRCUITS
D27, D29 and D35 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 CIRCUITS
Q17
From the TX VCO
DRIVE
AMP
D15,D16
To the TX AMP circuits
LO
SW
D24, D28, D32
IC3
ANT
PWR
LPF
LPF
AM P
SW
IC2
APC PWR
D27,D29,D35
AMP
T1 DET
Q23
MUTE TMUT
To the RX circuits
SW

5-3 FREQUENCY SYNTHESIZER CIRCUITS

The RX VCO is composed of Q6, and D4, D5, D11. The
VCO output signal is buffer-amplified by two buffers (Q12
and Q15), and then applied to the 1st IF mixer, through the
LO SW (D16) and the attenuator.
The TX VCO is composed of Q7, D6, D7, D10 and D12. The
VCO output signal is buffer-amplified by two buffers (Q12
and Q15), and then applied to the drive AMP (Q17), through
the LO SW (D15) and the LPF.
A portion of signal generated by each VCO is fed back to the
PLL IC (IC8, pin 17) through the buffer (Q11) 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 (IC8), 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 CIRCUITS
X2
15.3MHz
VCON
TCXO
Q4
IC8
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
D11
RX VCO
Q6,D4,D5
LV
ADJ
LOOP
LV
FILTER
Q12 Q15
BUFF BUFF
D12
TX VCO
LVA
TLV
ADJ
Q7,D6,D7
Q11
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
D15,D16
LO
To the TX AMPs
SW
ATT
Q19
1st IF mixer
VCC
W2
HV
PWON