6 - Polar transmitter principles

Summary

A polar transmitter by definition operates in accordance with one or more of the principles shown in Figure 2-3. By far the most common implementation is direct control of output signal magnitude from (2.8) and (2.9) and illustrated in Figure 2-3(a). The implementation shown in Figure 2-3(b) is actually a phase modulator, and is usually also implemented ahead of the stage controlling output magnitude.

History of the technique

Polar modulation, defined as a signal process implementing Figure 2-3(a), has been in use for nearly a century. Early in the implementation of AM broadcasting, it was discovered that the energy efficiency of class A transmitter power amplifiers was unacceptably low. Many alternatives were attempted [6-1], including constant current modulation, variable gain (controlled grid) modulation, and plate modulation.

The technique that won this competition is plate modulation.

6.1.1 Plate modulation (predates 1920)

The controlled port of a vacuum tube (valve) is called the “plate.” It was found before 1920 that the output power from an amplifiers can be made to vary if the power supply applied to the amplifiers varied. This output signal variation was roughly proportional to the power supply value, as long as the amplifiers operated in compression [1-1]. The proportionality was not perfect, but for the modulation requirements of AM broadcasting it was good enough. Later it was learned that when class C bias was used, along with very large drive signals to achieve sufficient output compression, the operation of a plate modulated transmitter could achieve efficiency values greater than 90% at AM band frequencies [4-3]. Plate modulation of class C biased power amplifiers dominated AM broadcast transmitter designs for more than 60 years, making it a very successful technology.

Plate modulation, as used for AM broadcasting, is a single coordinate polar signal process. Only signal magnitude is varied according to the information signal.