Antenna Tuner Circuits - Icom IC-756PRO III Service Manual

3-4 ANTENNA TUNER CIRCUITS

3-4-1 MATCHING CIRCUIT (TUNER-A UNIT)
The matching circuit is a T-network. Using 2 tuning motors,
the matching circuit obtains rapid overall tuning speed.
Using relays (RL1–RL15), the relay control signals from
the antenna tuner CPU (CTRL-A unit; IC5) via the buffer
amplifier (IC1, IC2) ground one of the taps of L3–L12 and
add capacitors (C34–C43). After selecting the coils and
capacitors, 2 motors (CTRL-A unit; MF1, MF2) adjust C44
and C45 using the antenna tuner CPU (CTRL-A unit; IC5)
and the motor controller (CTRL-A unit; Q211–Q218, D211,
D213, D215, D217) to obtain a low SWR (Standing Wave
Ratio).
3-4-2 DETECTOR CIRCUIT (CTRL-A UNIT)
(1) SWR detector
Forward and reflected power are picked up by a current
transformer (L1), detected by D2 (FOR) and D1 (REF), and
then amplified at IC1a and IC1b, respectively. The amplified
voltages are applied to the antenna tuner CPU (IC5, pins 2,
3). The tuner CPU detects the SWR.
(2) Reactance components detector
Reactance components are picked up by comparing the
phases of the RF current and RF voltage. The RF current is
detected by L4 and R16 and buffer-amplified at IC14e and
IC2a and then applied to the phase comparator (IC3a). RF
voltages are detected by C12–C14 and then applied to the
phase comparator (IC3b) after being amplified at the buffer
amplifiers (IC14c, IC2b). The output signal from the phase
comparator (IC3a, pin 6 for RF current, IC3b pin 7 for RF
voltage) is rectified at D7 and D6 for conversion into DC
voltage. The rectified voltage signals are combined, then
amplified at the inverter amplifier (IC4b), then applied to the
antenna tuner CPU (IC5, pin 64).
A C-MOS IC is used for the buffer amplifier (IC14) to
improve functionable sensitivity; the inverter amplifier
(IC4) is very responsive even with a low signal level input.
Together, these ensure quick and stable signal detection
even at low RF signal level input.
(3) Resistance components detector
Resistance components are picked up by L8, and detected
by D8, D9 and Q5. The detected resistance components
are amplified at the inverter amplifier (IC4a), and then
applied to the antenna tuner CPU (IC5, pin 1).
3-4-3 MOTOR CONTROL CIRCUIT
(CTRL-A AND TUNER-A UNITS)
The control circuit of the internal antenna tuner consists of
the CPU, EEPROM (Electronically-Erasable Programmable
Read Only Memory), tuning motors and tuning relays.
(1) CPU and EEPROM (CTRL-A unit)
The antenna tuner CPU (IC5) controls the tuning motors
via the motor controller (Q211–Q218, D211, D213, D215,
D217) and tuning relays, and memorizes the best preset
position in 100 kHz steps. The memory contents are stored
in the EEPROM (IC6) without a backup battery.
(2) Tuning motors (CTRL-A and TUNER-A units)
A motor controller (Q211–Q218, D211, D213, D215, D217)
rotates the tuning motors (TUNER-A unit; MF1, MF2) to
obtain a low SWR.
(3) Tuning relays (TUNER-A unit)
According to the operating frequency band and antenna
condition, tuning relays select the capacitors and coils.
4-4 ANTENNA TUNER CPU PORT ALLOCATION
3-
(CTRL-A unit; IC5)
Pin Port
number name
1 AN2
2 AN1
3 AN0
4 PERS
13 IKEY
15 ISTA
17 THRU
21 TRC
26 VHF
24M, 18M,
27–32 14M, 10M,
7M, 4M
CO1–CO3,
34–40
CI1–CI3
AZ, AY, AX,
41–48 AW, PZ, PY,
PX, PW
24M, 18M,
27–32 14M, 10M,
7M, 4M
64 AN3
3 - 8
Description
Input port for the resistance compo-
nents detection voltage.
Input port for the refl ected RF pow-
er voltage.
Input port for the forward RF power
voltage.
Input port for the transceiver power
OFF.
Outputs transmit control signal to
the main CPU (IC 3501).
Input port for the antenna tuner
start signal.
Input port for the [TUNER] ON/OFF
signal from main CPU (IC3501)..
Input port for the TX/RX switching
signal.
Output the coil select signals for the
tuner unit.
Output the coil select signals to the
tuner unit.
Output the capacitor select signal
to the tuner unit.
Output pulse-type control signals
for the tuning motors. (MF1, MF2)
Output the coil select signal to the
tuner unit.
Input port for the reactance compo-
nents detection signal.