Antenna Tuner Circuits - Icom IC-756PRO Service Manual

3-4 ANTENNA TUNER CIRCUITS

3-4-1 MATCHING CIRCUIT (TUNER 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 unit; IC5) via the buffer-amplifier
(IC1, IC2) ground one of the taps of L3–L12 and add capac-
itors (C27–C43). After selecting the coils and capacitors, 2
motors (CTRL unit; MF1, MF2) adjust C44 and C45 using
the antenna tuner CPU (CTRL unit; IC5) and the motor con-
troller (CTRL unit; Q211–Q218, D211–D217) to obtain a low
SWR (Standing Wave Ratio).
3-4-2 DETECTOR CIRCUIT (CTRL UNIT)
(1) SWR detector
Forward and reflected power are picked up by a current
transformer (L1), detected by D2 and D1, 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 volt-
age. The rectified voltage signals are combined, then ampli-
fied 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 AND TUNER UNITS)
The control circuit of the internal antenna tuner consists of
the CPU, EEPROM*, tuning motors and tuning relays.
*Electronically-Erasable Programmable Read Only Memory
(1) CPU and EEPROM (CTRL unit)
The antenna tuner CPU (IC5) controls the tuning motors via
the motor controller (Q211–Q218, D211–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 and TUNER units)
A motor controller (Q211–Q218, D211–D217) rotates the
tuning motors (TUNER unit; MF1, MF2) to obtain a low
SWR.
(3) Tuning relays (TUNER unit)
According to the operating frequency band and antenna
condition, tuning relays select the capacitors and coils.
3-4-4 ANTENNA TUNER CPU PORT ALLOCATION
(CTRL unit; IC5)
Pin Port
number name
1 R
2 REF
3 FOR
4 PWRS
STDU
6
7 SETI
13 KEY
15 START
17 THRU
SEND
21
22, 23 CL1, CL2
26 DUAL
L24M, L18M,
27–32 L14M, L10M,
L7M, L3.5M
CO3, CO2,
34–40 CO1, CI3,
CI2, CI1
PZ, PY, PX,
41–48 PW, RZ, RY,
RX, RW
64 P
3 - 8
Description
Input port for the resistance com-
ponents detection voltage.
Input port for the reflected RF
power voltage.
Inpout port for the forward RF
power voltage.
Input port for the transceiver power
OFF.
Inputs low level signal when oper-
ating the antenna tuner in 50 MHz
band.
Input port for reference voltage set-
ting.
Outputs tuner data signal.
Input port for the serial signal.
Input port for the [TUNER]
ON/OFF signal.
Input port for the TX/RX switching
signal.
Input port for the antenna tunner
CPU system clock.
Outputs the coil selection signal.
High : While 46–60 MHz band is
displayed.
Output the coil selection signal.
Output the capacitor selection sig-
nal.
Output pulse-type control signals
for the tuning motors (M1, M2).
Input port for the reactance compo-
nents detection voltage.