Acom 03AT Operating Manual page 23

Two small-signal RF voltages U(i'-i") and U(u'-u") are produced on the tow current-transformer secondary windings
(low-resistive loaded). These voltages are proportional to the momentary voltage and current values fed to the
matching-circuit input. The phase and amplitude relations between both voltages define positively the complex
antenna impedance as seen through (or transformed by) the matching circuit.
Combining appropriate portions of the mentioned signals U(i'-i") and U(u'-u") and using several detector and DC
comparators, the following logic signals are derived that are characteristic of the transformed impedance Zt position
on the complex-impedance plane, in respect to the "matching target" (50+j0)
"Rsensor" – a high level represents a high real part of the impedance Re(Zt) > 50
-
"Gsensor" – a high level represents a high real part of the admittance Re(1/Zt) > 20mS
-
"Ssensor" – a high level represents a high SWR > 1.5:1 (bad SWR of Zt).
-
Part of the RF Sensor as well as other AATU modules are not included here in order
not to overcrowd the schematic diagram. Full schematic diagrams for each AATU
module as well as PCB layouts are included in the Technical Supplement. Please
contact your dealer for details.
Two fast CMOS limiters and a D-type trigger in the RF Sensor (not shown in this figure) are involved to detect the
sign of the phase between both voltages mentioned above. These voltages are fractions from the input voltage and
current of the matching circuit. This scheme produces the "PHsensor" signal. A high level represents that the input
impedance is inductive (i.e. the input current lags behind the input voltage), respectively represents a capacitance
(the voltage lags behind the current).
The four logic signals R, G, S, and PH mentioned above are the main information sources that the AATU uses during
the automatic impedance-matching process. These are fed to the micro-controller in the "ATU Control" PCB, where
also the operating algorithm is programmed.
The MOS-FET Q1, that is connected between the C8-C9 joint and ground, (fig.5-1) is normally saturated with
positive voltage on its gate (signal *RFON), thus branching the RF current through its drain-source channel to the
ground. In this situation nearly all the input RF power is being dissipated by the swamping resistor R4 (30
and the input impedance is nearly good (input SWR is about 50/30=1,67:1). In this way, via placing the *RFON
control signal to a high level, the micro-controller can safely switch-over the matching-circuit relays with no danger for
their contacts.
The tuning process runs in a discrete stepping way. For each step, the micro-controller has to alternatively enable
and disable the RF power fed to the sensors and to the matching-circuit input. Thus, it can control the RF power as
needed to read the sensors as well as to safely switch the relays.
On each relay switching, the micro-controller has to first let the RF power to the sensors for a while and then to read
their refreshed output status. This is accomplished by setting a low level on the Q1 gate (*RFON = 0) thus the
transistor is cut off and the RF power reaches the voltage transformer TV1. Respectively, the detectors on the
Sensor PCB are being activated. This takes only several milliseconds until the sensor readings settle, and the micro-
controller disables the RF again (*RFON = 1). Then it analyses the information read from the sensors R, G, S and
PH in order to asses the current impedance position on the complex impedance plane, in respect to the target
Ω.
No relays are switched during this process since RF voltage or current is present on their contacts.
(50+j0)
After an impedance analysis based on the information read, the micro-controller provides for which relays to switch
ON and which ones – OFF at the next execution step. The decision is determined according to a certain algorithm
that is pre-programmed in the micro-controller program memory. The algorithm is composed so that it is convergent
to the "SWR<1.5:1" circle (in respect to the 50
expected in the whole frequency range, while using the inductance- and capacitor- resource of the remote unit at a
possible maximum.
The matching process continues making a series of tries as for each switching the RF power is disabled, the new
status is given to the relays and a certain pause is made until the given status is executed. A new sensor-reading
pulse follows, and so on the process is continued until the impedance will reach the "SWR<1.5:1" circle.
Alternatively, the inductors/capacitors resource might be run out (if the antenna SWR is too high), and the AATU
could not manage to improve the SWR below 1,5:1. When the procedure ends up, the micro-controller reports the
tuning result to the RCU and a "Stop RF ..." prompt appears on the LCD. After the operator will stop the tuning
power, both relays KAT1 and KAT2 are de-energized and the AATU is ready for transmission.
N O T E
Ω−
target). This is done irrespective of a variety of antenna impedance
Ω
:
Ω;
;
Ω
/100W)
22