Communication System Notes - Part 1

Communication System

GATE Scoring:

GATE 2013: 12 Marks
GATE 2012: 9 Marks

Modulation:

Modulation may be defined as a process by which some characteristics of a signal known as carrier is varied according to the instantaneous value of another signal known as modulating signal.

• Continuous Wave Modulation
• Pulse Modulation

Amplitude Modulation:

The process by which some characteristic of a carrier wave is varied in accordance with an information-bearing signal.

                                              (or)

AM is formally defined as a process in which the amplitude of the carrier wave c(t) is varied about a mean value linearly with a message signal m(t).

                                               (or)

In amplitude modulation, the amplitude or "strength" of the carrier oscillations is what is varied. For example, in AM radio communication, a continuous wave radio-frequency signal (a sinusoidal carrier wave) has its amplitude modulated by an audio waveform before transmission

1.Amplitude modulation (AM)

2.Double sideband-suppressed carrier (DSB-SC)

3.Single sideband (SSB)

4.Vestigial sideband (VSB)

Double sideband-suppressed carrier (DSB-SC)

Double-sideband suppressed-carrier transmission (DSB-SC) is transmission in which frequencies produced by amplitude modulation (AM) are symmetrically spaced above and below the carrier frequency and the carrier level is reduced to the lowest practical level, ideally being completely suppressed.

Spectrum:

DSB-SC is basically an amplitude modulation wave without the carrier, therefore reducing power waste, giving it a 50% efficiency. This is an increase compared to normal AM transmission (DSB), which has a maximum efficiency of 33.333%, since 2/3 of the Power is in the Carrier which carries NO intelligence, and each Sideband carries the SAME Information. Single_Side_Band (SSB) Suppressed_Carrier is 100% efficient.

                                                 fig. Spectrum Plot

• An amplitude modulation wave without the carrier
• Therefore reducing power wastage, giving it a 100% efficiency rate. 
• The increasing compared to normal AM transmission, which has a maximum efficiency of 50%
• Efficiency
  , E = (Sideband Power)/(Sideband Power + Carrier Power).

Modulation:

DSBSC is generated by a mixer. This consists of an audio source combined with the frequency carrier.

Demodulation:

For demodulation the audio frequency and the carrier frequency must be exact otherwise we get distortion.