Pll Circuit; Power Supply Circuits - Icom IC-A23 Service Manual

4-3 PLL CIRCUIT (RF UNIT)
A PLL circuit provides stable oscillation of the transmit fre-
quency and receive 1st LO frequency. The PLL output com-
pares the phase of the divided VCO frequency to the refer-
ence frequency. The PLL output frequency is controlled by
the divided ratio (N-data) of a programmable divider.
The PLL circuit contains the VCO circuit (Q31, Q32, D27).
The oscillated signal is amplified at the buffer-amplifiers (RF
unit; Q30, Q29) and then applied to the PLL IC (IC1, pin 2).
The PLL IC contains a prescaler, programmable counter,
programmable divider and phase detector, etc. The entered
signal is divided at the prescaler and programmable counter
section by the N-data ratio from the CPU. The divided signal
is detected on phase at the phase detector using the refer-
ence frequency.
If the oscillated signal drifts, its phase changes from that of
the reference frequency, causing a lock voltage change to
compensate for the drift in the oscillated frequency.
A portion of the VCO signal is amplified at the buffer-ampli-
fier (Q27, Q28) and is then applied to the receive 1st mixer
(Q12) or transmit buffer-amplifier circuit (Q6) via the T/R
switching diode (D6, D17).

• PLL CIRCUIT

Loop
filter
"2ndLO" signal to the
IF IC (IC1, pin 2)
From IC4, pin 14
C177 C198
X2
"UNLK" signal to the CPU
C175
(LOGIC unit; IC1, pin 48)
VCO
Q31, Q32, D24
R134, R135,
C168, C169
9
PLL
12
13
R158
14 OSC Tr
11

4-4 POWER SUPPLY CIRCUITS

VOLTAGE LINE
LINE
HV The voltage from the external power supply.
The same voltage as the HV line or battery volt-
VCC
age through the power switch (RF unit; Q24).
Common 5 V converted from the VCC line by the
+5 V regulator circuit (LOGIC unit; IC16 or IC3,
+5V D9). The output voltage is applied to the CPU
(LOGIC unit; IC1), reset circuit (LOGIC unit;
IC2), and etc.
5 V for transmitter circuits regulated by the TX
control circuit (RF unit; Q7, Q8, D7). The output
T+5 voltage is applied to the LO switch (RF unit; D6),
buffer amplifier (RF unit; Q6), pre-driver (RF unit;
Q4) and etc.
5 V for receiver circuits regulated by the RX con-
trol circuit (RF unit; Q20). The output voltage is
applied to the LO switch (RF unit; D17), AM
R5
detector (RF unit; Q18), IF amplifiers (RF unit;
Q17, Q16, Q15), IF IC (RF unit; IC1, pin 4) and
etc.
Buffer
Q28
Buffer
Q30
Buffer
Q29
IC3 (PLL IC)
16/17
Cont.
4 - 5
DESCRIPTION
D6
to transmitter circuit
Buffer
Q27
to 1st mixer circuit
D17
8
3
PCK
4
PDATA
5
PSTB