The total power that is developed and radiated at the transmitter has no direct bearing on bandwidth requirements, the saving in power that can be affected by suppressing the carrier and one of the sidebands cannot be totally ignored.
Overview of transmission efficiency:
- Considering the power relations in the modulated wave. Based on eqn. (1) the total power Pt, in the modulated wave is the sum of the carrier power Pc, and the power in the two sidebands.
- This can be expressed as
Where is the r.m.s. value of the sinusoidal carrier wave, and R is the resistance in which the power is dissipated. Equation (4.3) can be simplified to read as
- From the above expression that Pc remains constant but Pt depends on the value of the modulation index m.
- When several frequency components of different amplitudes modulate the carrier wave, which in fact is the rule rather than an exception, the carrier power Pt is unaffected but the total sideband power gets distributed in the individual sideband component powers.
- This is so because the total modulating voltage is equal to the square root of the sum of the squares of individual modulating voltages.
- It can be seen from eqn. (2) that at 100% modulation (m = 1) the transmitted power attains its maximum possible value. Pt(max) = 1.5 Pc, where the power contained in the two sidebands has a maximum value of 50% of the carrier power.
- It is clear then, that the carrier component that is redundent, so far as the transmission of intelligence is concerned, constitutes about 72% of the total power that is radiated in the double sideband, full carrier (better known as A3 modulation) AM system.
- Therefore, a lot of economy can be affected if the carrier power is suppressed and not transmitted.