The Little Known Fourth Method of Generation of Single-Sideband Signals

Envelope Elimination and Restoration, from Kahn 1952 

There is a method for SSB generation which is now becoming popular with digital signal generation and digital output stages. This is the Envelope Elimination and Restoration method or Polar Modulation method. The signal is interpreted as a complex signal in the polar format, i.e., with an envelope and a phase.

This method is receiving renewed interest among radio amateurs as it is used in the SSB implemention of e.g. the interesting QRPLabs QMX transceiver. 

In the QMX a single frequency signal (the phase) is generated in the digital domain and then phase modulated (upper branch of figure). It then has a digital output stage with very high efficiency as it only switches between on and off. The output stage is amplitude modulated with the envelope signal from the lower branch.This gives a much more efficient power amplification than is possible with the three conventional methods.

The three methods

How does it compare with other methods? In 1956, Donald K. Weaver, Jr. wrote a paper with title "A Third Method of Generation and Detection of Single-Sideband Signals".