Icom AT-141 Service Manual page 5

3-4 PHASE DETECTOR CIRCUIT
The phase detector consists of L2, L3 and D81. The phase
detector's purpose is to detect reactance components and
provide a pure resistance.
The output of D81 is a reference voltage of approximately
0 V when the load of L3 is a pure resistance with no reac-
tance. An inductive load produces an output voltage from
D81 which is lower than the reference voltage, whereas, a
capacitive load produces an output voltage higher than the
reference voltage. The output voltage from the phase detec-
tor is applied to the comparator (IC19b) and then applied to
the CPU (IC15, pin 23).
• PHASE DETECTOR CIRCUIT
INPUT
(FROM RL1)
C6
R14 L2 L3
D81
D5
D6
R15
IC19
From R193
3-5 IMPEDANCE DETECTOR CIRCUIT
The tuner uses an attenuator to reduce the transmit power
to a very low level. The low power minimizes the risk of
interference to other stations while matching an antenna to
the transmitter.
The VSWR at the input terminal is usually close to 1:1 even
with a large change of impedance at the attenuator output
due to the 16 dB of isolation between the input and the
matching network.
The circuit uses the constant voltage at D7 and D8 as a ref-
erence. If the impedance of the attenuator output is higher
than 50 Ω, the detected voltage by D11 and D12 is high. If
the impedance is lower than 50 Ω, the voltage is low. Both
the reference and detected voltages are applied to the com-
parator (IC19a). The output voltage from the comparator is
applied to the CPU (IC15, pin 24)
• IMPEDANCE DETECTOR CIRCUIT
ATTENUATOR
INPUT
R16–R24
(FROM RL1)
SWR DETECTOR
C7
CIRCUIT
D8 R151 R27
C8
D7
C12
C15
C16
ATTENUATOR
OUTPUT
R16–R24
(TO RL2)
SWR DETECTOR
CIRCUIT
BUFFER
AMP
Q4, Q5
TO IC15
OUTPUT
(TO RL2)
C19
BUFFER
AMP
C17
D11
Q4
D12
C20
TO Q5
R34
TO IC15
IC19
R33
3-6 LOGIC CIRCUIT
The CPU (IC15) controls the antenna matching network.
The CPU receives +5V through IC20 when DC power is
applied to the tuner. This voltage initializes the CPU and
then the CPU sets each relay to the initial condition.
The tuning program begins only if the START line is at LOW
level. RL1 and RL2 activate when RF power is applied to
the tuner from the transceiver.
3-8 ANTENNA CURRENT DETECTOR CIRCUIT
Antenna current detected at antenna current detector (L35).
The detected voltage is converted into a DC voltage at D86
and then applied to the ANTC line after passed through the
low-pass filter (L36, C286).
• ANTENNA CURRENT DETECTOR CIRCUIT
L37
TO W501
(ANTC)
C286
L36
B - 3 - 2
ANTENNA
FROM
L302 TO L317
L35
R259 D86
C285
R258