A new amplitude modulation strategy, suitable to solid-state RF transmitters (entirely transistorized) and operating in MW (Medium Wave: 530 kHz - 1.6 MHz), is presented. Unlike current solutions used in broadcastings, this modulation strategy provides a simple and effective answer to customer requirements in terms of cost and reliability. Amplitude modulation strategies used in working solid-state transmitters are presented first, then the principle of the new strategy is described. It is based on the phase-shifting of transmitter power amplifier control signals. However, this phase-shifting induces special switching modes which have to be studied. These switching modes are detailed as are their effects on the power amplifier's reliability and efficiency. This leads to am optimization of the power amplifiers and to the definition of a modulation strategy for high-frequency signals (over 1 MHz). A practical implementation with several power amplifiers is described. Efficiency and distortion measurements are given. The high-quality results highlight the interest in using phase-shifting control to realize a low cost, accurate and reliable amplitude modulation.
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