Double Sideband Modulation

Cuthbert Nyack

Because of power considerations (too much power in the carrier which consists of a constant amplitude sine wave carrying no information) modifications were developed to regular AM. One of these is DSB double sideband. Here the carrier is removed and the spectrum consists of 2 components at w_c ± w_m.

The applet below shows that the resulting signal(in red) consists of "beats" between the 2 frequencies. Also the modulating signal shown in green does not follow the positive amplitude envelope of the DSB signal as in regular AM.

In order to demodulate the DSB signal, the carrier must be reinserted at the receiver. Orange curve in applet shows the resulting AM which is produced when the regenerated carrier has the same frequency(25.0) and phase(0.0) as that used to produce the DSB signal.
Using the scrollbars to change the frequency(f(c)) and phase (p(c)) of the regenerated carrier shows the ease with which the AM signal is destroyed. In fact if the phase changes by 90º then the resulting signal becomes more FM than AM.

Therefore in cases where DSB is used, a reference signal (can be a periodic burst) must be sent by the transmitter so that the receiver can regenerate the original carrier with the correct amplitude and phase.

Return to main page
Return to page index