Basic Principles of Digital Modulation - Digital Video and Audio Broadcasting Technology - page 214

quency but also two sidebands spaced from the carrier at an offset of the
modulation frequency. For example, if a 1 MHz carrier is amplitude-
modulated with a sinusoidal 1 kHz signal, a modulation spectrum with the
carrier signal at 1 MHz and two sideband signals at 1 kHz above and be-
low the carrier will be obtained. The bandwidth is 2 kHz in this case.
Fig. 13.5. Amplitude modulation with suppressed carrier
As mentioned above, the carrier is suppressed by the mixer. If a mixer is
used for amplitude modulation and the modulation signal itself has no DC
component, no spectral line at the carrier frequency will be found in the
modulation spectrum. There are only the two sidebands. Fig. 13.5. shows
amplitude modulation effected by means of a double-balanced mixer. In
the modulation spectrum, we find not only the two sidebands but also har-
monic sidebands about multiples of the carrier frequency. The latter have
to be suppressed by lowpass filters. Fig. 13.5. also shows a typical ampli-
tude-modulated signal in the time domain with suppressed carrier. The
bandwidth is the same as with "normal" amplitude modulation, i.e. with
unsuppressed carrier.