A Fourth Method of Generation and Detection 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. It 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 is generated in the digital domain and then phase modulated (upper branch of figure). It then has a class D 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".

In the first method, shown below, the balanced modulators suppress the carrier frequency, and the undesired sideband is removed in the filters. This is the Filter method dating back to the 1930's, and the most common one to use. 

Filter method, from Weaver 1956

In the second method, a 90° phase shift network, implements a Hilbert transform. This was difficult to do accurately with analog electronics, but with digital signal processing it is quite feasible.

Phasing method, from Weaver 1956

The third method, also called Weaver's method, is based on four balanced modulators and two carefully matched filters. Like the phasing method, this method has not been so popular, but it is also easier to implement in the digital than the analog domain.

Third method, from Weaver 1956

All three methods generate the SSB signal at a low level and require linear amplification to bring the signal to the desired transmit level. This consumes power and easily generates unwanted signal components due to nonlinearity.

The Envelope Elimination and Restoration Method for SSB Generation

Weaver's paper refers to Leonard Kahn's paper from 1952 "Single-Sideband Transmission by Envelope Elimination and Restoration", but doesn't discuss it. Curiously, it predates the third method by some four years, but seems to have been neglected almost until now.

The advantage of the Envelope Elimination and Restoration method is that a nonlinear amplifier can be used as a last stage - in the figure from 1952 (on top) it is shown as a class C stage. The disadvantage is that the separation of the SSB signal into a phase component (upper branch), and the envelope part (lower branch) has not been feasible in practice until the advent of digital signal generation. In the upper branch there is "envelope elimination" and in the lower branch there is "envelope restoration".

This method is really the third method as it predates Weaver's method. For historical reasons it would be better now to call it the Fourth Method of Generation and Detection of Single-Sideband Signals as in the title of this blog post. 

But its original name seems to be the most common one: The Envelope Elimination and Restoration Method for SSB Generation.

(First published on the LA3ZA Radio and Electronics blog)