Icom IC-A5 Service Manual

4 - 5
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).
4-4 POWER SUPPLY CIRCUITS
VOLTAGE LINE
Cont.
16/17
PLL
Loop
filter
OSC Tr
VCO
Buffer
Buffer
Buffer
Q27
Buffer
Q28
Q29
Q30
R134, R135,
C168, C169
3
4
5
PCK
IC3 (PLL IC)
PDATA
PSTB
to 1st mixer circuit
D17
D6
9
12
13
14
11
8
Q31, Q32, D24
C198C177
From IC4, pin 14
C175
"2ndLO" signal to the
IF IC (IC1, pin 2)
X2
R158
to transmitter circuit
"UNLK" signal to the CPU
(LOGIC unit; IC1, pin 48)
LINE
HV
VCC
+5V
T+5
R5
DESCRIPTION
The voltage from the external power supply.
The same voltage as the HV line or battery volt-
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,
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
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
detector (RF unit; Q18), IF amplifiers (RF unit;
Q17, Q16, Q15), IF IC (RF unit; IC1, pin 4) and
etc.
• PLL CIRCUIT

Icom IC-A5 Specifications

General

Brand	Icom
Model	IC-A5
Category	Transceiver
Language	English