Icom IC-F4261DS - 4-3 FREQUENCY SYNTHESIZER CIRCUITS

4 - 3
• MODULATION CIRCUITS
• TX AMP AND APC CIRCUITS
• FREQUENCY SYNTHESIZER CIRCUITS
4-4 VOLTAGE BLOCK DIAGRAMS
Voltage from the power supply is routed throughout the
transponder, through regulators and switches.
SW
LO
BUFF
LO
AMP
Q32,D9
MODA
From the TX AF circuit
To the TX AMP circuit
MOD
D13
Q40Q34 D19,D20,D39
MAIN-A/B/C UNIT
ANT
SW
APC
AMP
PWR
AMP
LPF
LPF
LPF
PWR
DET
DRIVE
PRE
DRIVE
PRE
BUFF
Q603
RF-A/B/C UNIT
Q602Q601
D605-D607,
D609
To the RX circuit
From the VCO
D604,D608
Q42
IC601
BPF
LO
TX AMPs
1st IF mixer
1st LO signal
SW
BUFF
PLL
IC FILTER
LOOP
LO
AMP
BUFF
Q32,D9
Q38
Q33
Q40Q34
IC14
2ND LO signal
D5,D7
D19,D20,D39
TX:700-1040 MHz
RX:607.3-947.3 MHz
ATT
Q21
X3
RF-A/B/C UNIT
MAIN-A/B/C UNIT
BATT + BATT -
CPU5
PWR
T5V
From Battery
Q6
R5V
Q7
VCC
+5V
+5V
Q16,Q17
CPU5V
CPU
CPU
CPU
IC3 and IC18
HV
R5V
RX circuit
TX circuit
COMMON circuit
FRONT and VR UNITs
FRONT and JACK UNITs
GPS
S5V
S5V
Q9
T5V
IC11
Q4,Q5
VREF
IC3
VREF
Q11
Q8
SW
VCC
SW
Q14,Q15
PWON
BATV
IC2
+3.3V
REG
DC/DC
+3.3V
1.35V
IC32
MODULATION CIRCUIT (MAIN UNIT)
The MIC signal from the TX AF circuit is amplified by the
AMP (IC3), and then applied to the D/A converter (IC9,
pin 4) to be adjusted in level.
The level-adjusted signal is applied to the TX VCO (Q32, D9,
D12, D13) as a modulation signal.
The modulation signal is applied to D13 to obtain GMSK (For
digital mode) or frequency modulation (For analog mode).
The modulated VCO output signal is passed through the buf-
fer (Q34) and amplifi ed by the LO AMP (Q40), and then ap-
plied to the TX AMP circuit on the RF UNIT as the TX signal,
through the LO SW (D20 and D39).
TX AMP CIRCUIT (RF UNIT)
The TX signal is sequentially amplified by the pre-drive
AMPs (Q601 and Q602) and power AMP (Q603), and then
passed through the LPF (L607, L608, C629, C631, C634,
C636, C639). The filtered TX signal is passed through the
ANT SW (D605) and another LPF (L612–L614, C653–C656,
C658, C659), before being applied to the antenna.
APC CIRCUITS (RF UNIT)
The voltage produced at the LPFs is rectified by D604 and
D608, and it is used as the TX power sensing voltage.
The voltage is applied to the APC AMP (IC601, pin 3), and
the output voltage controls the bias voltages of pre-drive
AMP (Q602) and power AMP (Q603) to keep the TX output
power constant.
4-3 FREQUENCY SYNTHESIZER CIRCUITS
(MAIN UNIT)
VCO
The transceiver has 3 VCOs; RX VCO-A (Q30, D10, D14),
RX VCO-B (Q31, D11, D15) and TX VCO (Q32, D9, D12,
D13).
The RX VCO-A (Q30, D10, D14) and RX VCO-B (Q31, D11,
D15) generate the 1st LO signal, and the output signal is
amplifi ed by the buffer (Q34) and LO AMP (Q40), and then
applied to the 1st IF mixer (Q33), through the LO SW (D19),
LPF (L37 and C281) and ATT (R222, R223 and R226).
The TX VCO generates the TX LO signal. The output of buf-
fer (Q34) is applied to the TX AMP circuit, through the LO
SW (D20 and D39).
PLL
A portion of VCO output signal is passed through two buffers
(Q34 and Q38) and tuned BPF (D5, D7, L6, C178, C185,
C190), and then fed back to the PLL IC (IC14, pin 6).
The PLL IC (IC14) phase-compares the output of reference
frequency oscillator (TCXO; X3) and VCO, and the phase
difference is output as the charge pump current. The current
is passed though the loop filter (R165, R173, R179, C202,
C482, C483) to be converted into the lock voltage, which
controls the oscillating frequency of VCO.
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.