Frequency Synthesizer Circuits - Icom IC-F50V Service Manual

4-3 FREQUENCY SYNTHESIZER CIRCUITS

VCOs
A VCO is an oscillator which its oscillation frequency is
determined by the applied voltage. This transceiver has
two VCOs; RX VCO and TX VCO. The RX VCO generates
the 1st LO signals for the 1st IF produce, and TX VCO
generates TX signal. The VCO SW toggles these VCOs.
• RX VCO
The RX VCO oscillates 89.65 to 127.65 MHz LO signals.
The generated 1st LO signals are applied to the 1st mixer
(Q3) via two buffers (Q12 and Q10) and LPF.
RIPPLE
Q17
R9 8
FILTER
RX VCO
L28
Freq.
adjust
LV
TX VCO
L27
Freq.
adjust
MOD
PLL (Phase Locked Loop) CIRCUIT
The PLL circuit provides stable oscillation for both of the
transmit and 1st LO frequencies (for receive). By comparing
the feed backed VCO output and the reference frequency
signal, the oscillating frequency is stabilized.
The PLL output frequency is controlled by the serial data
including divide ratio from the CPU.
A portion of VCO output from the buffer (Q11) is applied
to the PLL IC via LPF. The applied VCO output is divided
according to the serial data including divide ratio from
the CPU, at the prescaler and programmable divider. In
the same way, the reference frequency signal from the
reference frequency signal oscillator is applied to the PLL
IC and divide so that these are the same frequency.
• PLL CIRCUITS
R103
C211
BAL
IC16
2
X2
1 1
VCON
R104
3
REF
2
GND
CR-783
Ref. Freq.
OSCILLATOR
LVIN
A CZ1005Y
EP6
S5V
R9 3
D24
UNLK
PLL IC
R117
SC K
MB15A02PFV1
R118
CLOCK
SO 9
DATA
10
PLST
LE
11
FC
12
NC
13
FOUT
14
P OSCOUT
15
B OSCIN
16
IC4
R8 5 R7 9
L2 6
C127 C126
Q14
R81
L3 9
R8 4
R8 3
Modulation
L25
C124
C123
R82
L43
L3 8
4
VCC
3
Q18
OUT
R97
PLL
LOOP
FILTER
FIN
8
LD
7
GND
C207
6
DO
5
VCC
L3 2
4
VP
3
2
1
LPF
• TX VCO
The TX VCO oscillates 136 to 174 MHz TX signal. The
generated TX signal is applied to the pre-driver (one of TX
AMPs) via the TX/RX SW.
A portion of the VCO output is applied to the PLL IC via the
buffer (Q11).
• VCOs AND BUFFERS
C 102
Q12
Q11
BUFFER BUFFER
Q13
Q15
5 1
2
Q16
R88
4 3
VCO
SW
from VCO
The frequency-matched signals are applied to the phase
comparator and phase-compared. The resulted phase
difference is detected as a phase-type signal, and level-
adjusted at the charge pump then output. The output pulse
type signal is passed through the loop filter to be converted
into the DC voltage (=Lock Voltage).
Applying the lock voltage to the variable capacitors
(VD) which composes a part of the resonator of VCO,
the capasitance of VDs changes corresponding to the
applied lock voltage. This causes the change of resonation
frequency that determines the VCO oscillating frequency to
keep the VCO frequency constant.
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
frequency.
L28
Freq.
adjust
LV
R9 5
TX VCO
C150
L27
Freq.
adjust
MOD
4 - 6
T5V
Pre-
TX/RX
DRIVE
SW
D14
C100 C93
TX
R72
D15
Q10
BUFFER
LPF
L14
C49
RX VCO
R8 5
C127 L2 6 C126
R81
L3 9
R8 4
Modulation
L25
C124
C123
L43
L3 8
C92
Q9
R7 9
Q14
R8 3
Q13
R82
R
from VCO