Acom AOM 04AT User Manual page 70

During tuner operation (after tuning), in normal state of the relays KS1 and KS2, the input RF power is directly
fed from the RF Input to the input of the L matching network.
During the basic cycle of tuning, by switching the KS1 relay over to the Input switch MOSFET, the input RF
power is directed through a 27 Ohm / 100 W resistor to the transistor MOSFET INPUT SWITCH and to the
TUN input of the Sensor board. In this case, the KS2 relay on the Capacitors board is used to connect the
input sensor output to the input of the matching circuit (L-network), so that the sensor measures the antenna
impedance, transformed by the matching circuit.
In tuning mode, the input RF power is fed through the 27 Ohm / 100 W resistor and primary windings of two
current transformers, TA1 and TA2, to two branches - one to the input of the matching circuit and another
to a reference resistor 50 Ohm / 15 W, which is compared to the input impedance of the L-network, thus
forming a RF impedance bridge.
The transformers TA1 and TA2 are located on the Capacitors board, while the block
diagram shows them with their primary windings only.
The secondary windings of TA1 and TA2 feed to the input sensor RF voltages, proportional to the RF currents
in the two branches - the reference 50 Ohm / 15 W resistor and the antenna impedance transformed by the
matching circuit. The relation of amplitudes and phases of these two signals represents unambiguously the
complex value of the antenna impedance transformed by the matching circuit.
By comparing properly selected parts of these two signals and their combinations by module and by phase,
the sensor produces the following three logic output signals, characterizing the position of the antenna
impedance transformed by the matching circuit, towards the target of the tuning, i.e. towards the point
(50+j0) Ohm, on the complex impedance plane:
- "R": logic output - shows whether the real part of the transformed impedance is above or below
50 Ohm;
- "G" logic output - shows whether the real part of the transformed admittance is above or below
1/50 S (20 mS);
- "PH" logic output - shows whether the phase sign of the transformed impedance is inductive or
capacitive.
The three logic signals in question, "R", "G", "PH", and the detected forward and reflected power "fwd" and
"rfl" (from the input of the matching circuit) are the main sources of information, which are fed by the input
sensor to the Control board during the full-tuning process. They are processed by the Control board, which
provides the tuner operation algorithm (including the tuning algorithm).
As mentioned above, in tuning mode, the input RF power is mainly fed to the 27 Ohm / 100 W resistor. The
INPUT SWITCH MOSFET is normally saturated (high level on gate from *RFON signal), so the RF current is
diverted to the ground through its drain-source channel. In this state, almost all input RF power is dissipated
by the 27 Ohm / 100 W resistor and nothing is fed to the matching network. When the MOSFET is saturated,
the control system can switch over the tuner relays without a risk of relay contact arcing and deterioration,
while the tuner input impedance is kept below SWR of 2:1.
Page 70 of 88 | S e c t i o n MAINTENANCE
User's Manual | ACOM 04AT | Remote Automatic Antenna Tuner and Switch
October 2023