SAC SA100, Manual

Page: 59 / 224

SOUTHERN AVIONICS COMPANY
Model SA100 Dual 2-29 Theory of Operation, Radiobeacon
2.3.8 Power Amplifier
The output voltage from the modulator is applied to the half bridge switching amplifier FET's Q5
and Q6 to provide high level amplitude modulation.
The RF input to the Switching Power Amplifier comes from the RF Driver on the Exciter Mother
board and consists of a TTL square wave operating at the frequency selected on the Synthesizer
PWB. This signal is buffered by U2 and applied to complementary pairs Ql, Q2, and Q3, Q4.
Each complementary pair is turned ON on alternate 1/2 cycles of the RF signal producing an AC
square wave on the primaries of T1 and T2. The signal on T2 is 180 out of phase with the signal
on T1 so that when Q5 turns ON Q6 turns OFF and vice-versa preventing the two FET's from
being ON at the same time. The square wave output is applied to T3 where it is impedance
matched to the 50 Ohm filter input.
P.A. voltage and P.A. current samples from the Power Amplifier modules are routed to a front
panel switch and meter.