Autotune - SAC SS-800AVS Manual

3.2.3 Autotune
The autotune system.is an electronic antenna tuning system located
in the antenna coupler and comprises the Autotune Motor Drive PWB,
the Limit Switch PWB, and the Tuning Motor and Tuning Ring Assembly.
The relative phase of the current and voltage signals to the 50 ohm
RF input of the coupler are compared by electronic circuits to
determine if the antenna is tuned inductive, resistive, or capac-
itive. A properly tuned antenna system is considered to have a
zero relative phase difference between voltage and current. As
the system changes so that the load exhibits reactive components,
whether inductive or capacitive, no retuning action will be taken
until the phase magnitude exceeds a level determined by the tolerance
adjust control (RI6) on the Autotune Motor Drive PWB. When this
level of phase error is exceeded, the logic circuits will activate
the tuning motor in the proper direction to return the system to
a resistive load state° Motor drive continues until a relative
phase change is detected, indicating passage of tuning through a
resistive state. Circuits on the Autotune Motor Drive PWB prevent
any tuning action during periods of low or no input signals, during
transmitter ident, and when the Setup/Run Switch (Sl) is in the
Setup position.
RF current from the transmitter passes through T1 of the Autotune
Motor Drive PWB. Limiting amplifiers Ul and U2 amplify and square
the current and voltage signals and feed them into U3 for deter-
mination of the magnitude of the phase difference between the current
and voltage signals. The output of U3, pin ii, is filtered by RI2
and CI0 to produce a DC voltage at TPI (Brown) proportional to the
phase difference. The DC voltage is compared by voltage comparator
U5 with a reference voltage at TP2 (Red) set by the tolerance adjust
control (RI6). As the TPI voltage becomes more positive than the
TP2 voltage, the output of U5, pin 7, changes from 0 V to 14 V to
indicate an out of tolerance antenna tuning condition. The tolerance
limit set by the tolerance adjust control (RI6) is adjustable to
accommodate different antenna and frequency conditions. U3 outputs
at pins 3 and 4 are compared by type D flip-flop U4 to determine
whether the current signal is leading or lagging the voltage signal.
If the voltage signal is leading the current signal as with an
antenna tuned to the inductive side of resonance, the data input,
pin 2 of U4, will become positive just before the positive transition
of the clock input, pin 3. On the positive clock transition, the
Q output, pin 5, of U4 is latched into the same state as the U4
data input, pin 2, with a logic 1 (14 V) indicating an inductive
antenna condition. This condition is also indicated by LED DSl.
As the U5 output, pin 7, changes from 0 V to 14 V, to signal an out
of tolerance tuning condition the positive transition will cause
the antenna condition to be latched at U4, pin 9, and through the
logic of U7 will determine the motor drive direction. The U5 output,
SS-800AVS
Southern Avionics
73
Company