Single and Double Sideband Communication

As we know that Amplitude Modulation is an inefficient and , therefore, wasteful method of communications.  Two-third of the transmitted power appears in the carrier which itself conveys no information.  The real information is contained whiting the sidebands.  Once way to overcome this problem is simply to suppress the carrier.  Since the carrier does not provide any useful information, there is no reason why it has to be transmitted.  By suppressing the carrier, the resulting signal is simply the upper and lower sidebands.  Such a signal is referred to as a double sideband suppressed carrier (DSSC or DSB) signal.  The benefit , of course, is that no power is wasted on the carrier and that the power saved can be put into the sidebands.  Double sideband suppressed carrier modulation is simply a special case of AM with no carrier.

Double and Single Sidebands

Amplitude modulation (AM) generates two sets of sidebands, each side band containing the same information.  The information is redundant in an AM or a DSB signal. Therefore, all the information can be conveyed in just one sideband.  Eliminating one sideband produces a single sideband (SSB) signal.  Eliminating the carrier and one sideband produces a more efficient AM signal.

A typical Double sideband signal is shown in figure (a).

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(a) A time domain display of a DSB AM signal

This signal is simply the algebraic sum of the two sinusoidal sidebands.  This is the signal produced when a carrier is modulated by a single tone sine wave information signal.  During the modulation process, the carrier is suppressed, but the two sideband signals remain . Even though the carrier is suppressed, the time domain double sideband signal is a sine wave at the carrier frequency varying in amplitude as shown.  Note that the envelope of this wave form is not the same as the modulating signal as it is in a pure AM signal with carrier.

A frequency domain display of a DSB (Double sideband ) signal is shown in figure (b).

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(b) A frequency domain display of a DSB signal

Note the the spectrum space occupied by a DSB signal is the same as that for a conventional AM signal

Double sideband Suppressed Carriers generation:

Double sideband suppressed carrier signals are generated by a circuit called  a balanced modulator.  The purpose of the blanced modulator is to produce the sum and difference frequencies but to cancel or balance out the carrier.

 

        Note:

  When Double Sideband AM is used, considerable power is saved by eliminating the carrier.  This power can be put into the sidebands for stronger signals over longer distances.  Although a DSB AM signal is relatively easy to generate, Double SideBand is rarely used because the signal is difficult to recover at the receiver.

 

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