Icom ID-31A

4 - 3
• FREQUENCY SYNTHESIZER CIRCUITS
4-4 VOLTAGE DIAGRAM
Voltage from the power supply is routed throughout the transceiver, through regulators and switches.
4-3 FREQUENCY SYNTHESIZER (MAIN UNIT)
• VCO
The ID-31A/E has a VCO (Q27, D10–D12) which generates
both the 1st LO signal and the TX signal. The output of buffer
(Q30) is amplifi ed by the LO AMP (Q32), and then used as
the TX/RX LO signal.
While receiving, the LO signal is applied to the 1st IF mix-
er (Q46), through the LO SW (D13) and LPF (L30, C280,
C289).
While transmitting, the LO signal is applied to the TX AMP cir-
cuits, through the LO SW (D14), buffer (Q48) and attenuator
(D20, D21).
LPF
FILTER
LOOP
VCO
LPF
BUFF
PLL
IC
BUFF
LO
AMP
BPF
Q38
FIN
D13,D14
X1
Q29
IC13
DET
SW
FM
Q27,D10,D11,D12
LO
15.3MHz
Q46
IC6
Q32
Q30
1st LO
1st IF mixer
TX AMPs
X3
REF
45.9MHz
J2
CHG
IC4
IC5,D9
VCC
TXI-V
HV
CURRENT
S
-
SW
+
R3C
CTRL
T3C_INV
DC
CHGC
PCON
REG
Q15,Q16
VCC
Q24
RX circuits
Common circuits
Common circuits
TX circuits AF power AMP
VCO, reference
frequency oscillator,etc.
CHG
BTVIN
Q6,Q7,Q9,Q10
VCC-I
R3
DETECT
T3C
R3
SW
REG
REG
Q37,D17
T5
T3C_INV
YGR
REG
IC10
3VS
REG
SP
BTL5V
3VS
3VS
BTDET
IC1,
Q1,Q2,Q3,Q8,
D1,D3,D4
D2,D5,D6
CHGH
PS3
+5V
+5V
REG
5VS
IC15
Q17,Q19
• PLL
A portion of VCO output signal is passed through two buffers
(Q29 and Q30) and LPF (L7, C137–C139, C144), and then
fed back to the PLL IC (IC6, pin 8).
The PLL IC (IC6) phase-compares the output of reference
frequency oscillator (TCXO; X1) and VCO, and the phase-
difference is output as the charge pump current. The current
is passed though the loop fi lter (R67, R71, R90, R93, C74,
C91, C92, C96, C102) 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 frequency.