Modulation Schemes

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Modulation Schemes

S. S. Sastry

The point to modulation is to take a message bearing signal and superimpose it upon a carrier signal for transmission. For ease of transmission carrier signals are generally high frequency for severable reasons:

  1. For easy (low loss, low dispersion) propagation as electromagnetic waves
  2. So that they may be simultaneously transmitted without interference from other signals
  3. So as to enable the construction of small antennas (a fraction, usually a quarter of the wavelength)
  4. So as to be able to multiplex that is to combine multiple signals for transmission at the same time.
For example AM radio is 550-1600 KHz, FM radio is 88 MHz-108 MHz, TV is 52-88 MHz (channels 1-6), 174-216 MHz (channels 7-12) and 470-900 MHz (UHF), microwave and satellite signals are of the order of several GHz and infra red fiber optic signals are of the order of 200-300 THz.

S Sastry
Sun Apr 20 16:50:27 PDT 1997