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 wc ± wm.

When Fn = 1, the applet below shows that the resulting signal(in yellow) 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. Red curve in applet shows the resulting AM which is produced when the receiver generated carrier has the same frequency(wc) and phase(0.0) as that used to produce the DSB signal.
Using the scrollbars to change the frequency(wcr) and phase (fcr) of the regenerated carrier shows the ease with which the AM signal is destroyed. This is illustrated by Fn = 2, 3 and 4.

Fn = 5, 6 and 7 shows that the information is also lost if the regenerated carrier frequency is different from the carrier frequency used by the transmitter.

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 generate the original carrier with the correct amplitude, frequency and phase.

Fn = 8 shows a phasor representation of DSB. Fn = 9, 10, 11 show the phasor as Pt is changed.

Fn = 12 shows a phasor representation of AM. Fn = 13, 14, 15 show the phasor as Pt is changed.

Fn = 16, 17 shows the phasor representation when the phase is incorrect.

Instead of adding the carrier and using an envelope detector, the DSB can be mutiplied by the carrier and the modulating signal can then be recovered by low pass filtering( the resulting signal has components at wm and 2wc. ± wm.
Fn = 18 shows this case.

Fn = 19, 20 shows what happens when the phase is incorrect.
Fn = 21, 22 and 23 shows what happens when the frequency is incorrect.

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COPYRIGHT © 1996, 2011 Cuthbert Nyack.