Icom IC-M412 Service Manual page 15

PLL (Phase Locked Loop) CIRCUIT
The PLL circuit provides stable oscillation for both of the
TX and 1st LO frequencies. By comparing fed back VCO
output and reference frequency signals, the PLL corrects
the difference of the these frequencies.
• RX VCO LOOP
A portion of RX VCO output is applied to the PLL IC (IC1,
pin 3) via buffers (Q74, 75) and LPF (L75, C624, 625).
• TX/CH70-RX VCO LOOP
A portion of TX/CH70-RX VCO output is applied to the PLL
IC (IC1, pin 17) via buffers (Q6, 3) and LPF (L2, C34, 35).
The applied VCO output signal is divided according to the
divide ratio which is included in the serial data from the
CPU (LG: IC1), at the prescaler and programmable divider.
In the same way, the reference frequency signal from the
reference frequency oscillator is applied to the PLL IC and
divided so that these two applied signals are the same
frequency.
• PLL
Q86
R6 KRC404
UNLK
1K
R431
SPS
10K
R433
10K
MPS
R1
PSTB 10K
R3
10K
PDATA
R2
10K
PCK
Control signals from the CPU (LG: IC1)
4-4 VOLTAGE LINES
Voltage from the power supply is routed to the whole of transceiver through the regulators and switches.
POWER
SW
+5
HV VCC
Regulator
IC 1 MB15F72ULPFT
DORF LD/FOUT
11 10
VPRF DOIF
12 9
PSRF VPIF
13 8
VCCRF PSIF
14 7
GNDRF VCCIF
15 6
XFINRF GNDIF
16 5
FINRF X FINIF
17 4
LE FINIF
18 3
DATA GND
19 2
CLOCK OSCI N
1
20
T5
5V T5
YGR (Q10), APC (Q14,15), ANT SW (D14, 21), LO SW (D7)
Controller
R5
R5
IF ICs (IC2,14), AF BPF (Q31, 32), etc.
Controller
3.6V
3.6V
PLL IC (IC1)
Regulator
R8
R8 RF AMP (Q21), 1st mixer (Q22), 1st IF AMP (Q23),
Regulator
TX/CH70RX VCO (Q71, 72, D513)
The frequency-matched signals (VCO output and the
reference frequency signals) are applied to the phase
comparator and phase-compared. The resulted phase
difference is detected as a pulse-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).
The capasitance of variable capacitors of each VCOs
changes corresponding to the applied lock voltage. This
causes the change of the VCO oscilating 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.
Reference Frequency
R13
Oscillator
47
CR-804
X1
Q85
KTC4080
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