30-09-2013, 04:01 PM
Analogue Modulation (AM)
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Why Modulate?
All audio signals occupy the same frequency band i.e. between 0 and 20 kHz. Before being broadcast an audio signal (speech or music) must be moved, or frequency translated to a specific frequency range in order to use the available frequency spectrum. To do this the audio signal (or modulating signal) modulates a much higher radio frequency (the carrier frequency). Each audio signal is assigned a carrier - defining a channel - so that it is possible for the receiver to discriminate between all the streams of signals coming in.
Modulation Index (or Modulation Factor or Depth of Modulation)
This is defined as m =
In AM, this quantity, also called modulation depth, indicates by how much the modulated signal varies around its 'original' level. For AM, it relates to the variations in the carrier amplitude.
So if m = 0.5, the carrier amplitude varies by 50% above and below its unmodulated level, and for m = 1.0 it varies by 100%. Modulation depth greater than 100% is generally to be avoided as it creates distortion.
Suppressed Carrier Signals and Single Sideband
Single Sideband (SSB), is a form of AM
Because most of the output power from an amplitude modulator is contained in the carrier there will be major power savings if the carrier can be suppressed. The carrier amplitude and frequency do not change and so it does not contain any signal information. The suppression of the carrier will not cause any of the information in the signal to be lost.
Each sideband is the image of the other and one of them may be suppressed without the loss of any information.
All of the information is conveyed through the use of a single sideband with no carrier.