Abstract
The transmitter whose performance is described here uses hybrid modulation to produce an AM wave directly from the digitalized audio signal. In the prototype transmitter, the efficiency of the analogue-type power amplifier was too low for transmitters with much higher output because the analogue signal from the D/A converter was amplified linearly Using a carrier PWM modulator resulted in a module efficiency of 80%, twice that of the conventional one Since this transmitter has no modulator module like that in conventional transmitter, an overall efficiency of 84-93% (RF output/DC input) is obtained in the MF band. And because transient overvoltages do not occur when the output of the transmitter is shorted, power devices can be protected from breakdowns. In addition, using the serial-type combiner could greatly suppress impulse surges from the output. When a 10-kV (peak) impulse surge was impressed at the output transmitter, only a few volts (peak) appeared at the output of the RF power modules. This means that this tramsmitter is 30-40 times more resistant to lightning surges than conventional transmitters are.
