AM-SSB Modulation

If we analyze the AM signal in frequency domain we shall realize that the  two spectral components in the AM signal that lies at equal distances above and below the carrier frequency contain identical information because they are complex conjugates (replica) of each other. The portion above the carrier frequency is called the upper sideband and the portion below the lower sideband. This means that sending an AM-DSB signal is equivalent to sending the same information twice . This is in fact wastage of bandwidth which is the most scarce resource in communication systems. that is the reason AM-SSB (Single Side Band) techniques are develop.

AM-DSB signal in frequency domain with two side bands (Src:wikipedia)

Basically this post  is a continuation of the last post. To generate AM-SSB (AM-Single Side Band) signal we first have to generate AM_DSB (Double Side Band ) signal. There is no way to directly generate an AM-SSB signal.

Bandforms of single-sideband modulation compared to amplitude modulation. (src:wikipedia)

Generating SSB Signals: The Filter Method

The filter method is the simplest and most widely used method of generating SSB signals. The modulating signal is applied to the audio amplifier/ amplifier. The amplifier’s output is fed to one input of a balanced modulator.A crystal oscillator provides the high frequency carrier signal which is also fed into to the balanced modulator. The output of the balanced modulator is a double sideband (DSB) signal.

An SSB signal is produced by passing the DSB signal through a highly selective bandpass filter. With the filter method, it is necessary to select either the upper or the lower sideband. Section of the upper or lower sideband depends upon the total bandwidth available, and the transmitter/receiver design.

Stages involved in generating AM-SSB signal