Frequency Synthesizer Circuits; Voltage Diagram - Icom ID-31A Service Manual

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).

• FREQUENCY SYNTHESIZER CIRCUITS

IC6
PLL
15.3MHz REF IC
X1
FIN
X3
Q38
45.9MHz
BPF
FM
DET
IC13

4-4 VOLTAGE DIAGRAM

Voltage from the power supply is routed throughout the transceiver, through regulators and switches.
RX circuits Common circuits TX circuits
3VS
R3
R3
REG
Q24
R3C T3C_INV
3VS
REG
IC10
LOOP
VCO
FILTER
Q27,D10,D11,D12
LPF BUFF
Q29
VCO, reference
frequency oscillator,etc.
AF power AMP
T5 BTL5V
YGR SP
REG REG
Q37,D17 Q15,Q16
3VS
SW
REG
IC4
• 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.
LO
BUFF
AMP
Q32
Q30
Common circuits
+5V
T3C_INV
+5V
REG
PS3 IC15
5VS
PCON
4 - 3
LO
TX AMPs
SW
D13,D14
Q46
1st LO
LPF
1st IF mixer
BTVIN
HV
VCC DC
SW
CHGC CHGH
IC1,
Q1,Q2,Q3,Q8,
D1,D3,D4
CHG
CTRL CHG
Q6,Q7,Q9,Q10
BTDET
D2,D5,D6
TXI-V VCC-I
CURRENT IC5,D9
Q17,Q19
DETECT
T3C
VCC
J2
+
S
-