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Power Efficiency: AM vs DSB-SC vs SSB-SC


AM Power Calculation, DSB-SC, SSB-SC Efficiency and Total Transmitted Power Explained

AM Power Calculation, DSB-SC and SSB-SC Efficiency Explained

Understanding power distribution in modulation systems is one of the most important topics in communication engineering. This article explains:

  • Carrier Power in AM
  • Sideband Power Calculation
  • Total Transmitted Power
  • AM Transmission Efficiency
  • DSB-SC Efficiency
  • SSB-SC Efficiency
  • Numerical Examples
  • Comparison of AM, DSB-SC and SSB-SC

What is an AM Signal?

A standard Amplitude Modulated (AM) signal is represented as:

s(t) = Ac [1 + m cos(ωm t)] cos(ωc t)

Where:

  • Ac = Carrier Amplitude
  • m = Modulation Index
  • ωc = Carrier Angular Frequency
  • ωm = Modulating Angular Frequency

Carrier Power in AM

The carrier component is:

Carrier = Ac cos(ωc t)

Carrier power is:

Pc = Ac² / 2R

where R is the load resistance.

Power in Upper and Lower Sidebands

Expanding the AM equation: 

s(t) = Ac cos(ωc t)
      + (mAc/2) cos(ωc + ωm)t
      + (mAc/2) cos(ωc - ωm)t

Power in each sideband:

PUSB = PLSB = (m²/4) Pc

Total Sideband Power

PSB = PUSB + PLSB
PSB = (m²/2) Pc

Total Transmitted Power in AM

Total transmitted power equals carrier power plus sideband power.

PT = Pc + PSB
PT = Pc (1 + m²/2)
This is the most important formula for total transmitted power in conventional AM.

Power Distribution Diagram in AM

Carrier Power Pc USB m²Pc/4 LSB m²Pc/4

AM Transmission Efficiency

Efficiency is defined as:

η = Useful Power / Total Power

Since only sidebands contain information:

η = PSB / PT
η = (m²/2) / (1 + m²/2)

Maximum Efficiency at 100% Modulation

For: 

m = 1
η = 0.5 / 1.5
η = 33.33%

Thus, conventional AM wastes most of its power in the carrier.

Numerical Example

Given: 

Carrier Power = 1000 W
Modulation Index = 1
Parameter Value
Carrier Power 1000 W
Upper Sideband Power 250 W
Lower Sideband Power 250 W
Total Sideband Power 500 W
Total Transmitted Power 1500 W
Efficiency 33.33%

DSB-SC (Double Sideband Suppressed Carrier)

In DSB-SC, the carrier is completely removed. Only the upper and lower sidebands are transmitted.

Carrier Power = 0
Total Power = Sideband Power

Therefore:

ηDSB-SC = 100%
All transmitted power carries information.

Advantages of DSB-SC

  • No carrier power wastage
  • Improved power efficiency
  • Better utilization of transmitter power

SSB-SC (Single Sideband Suppressed Carrier)

SSB-SC transmits:

  • Only one sideband
  • No carrier

Therefore:

Carrier Power = 0
Unused Sideband Power = 0
Total Power = Information Power
ηSSB-SC = 100%

SSB-SC is both power-efficient and bandwidth-efficient.

Bandwidth Comparison

Modulation Type Bandwidth
AM (DSB-FC) 2fm
DSB-SC 2fm
SSB-SC fm

Efficiency Comparison

Modulation Scheme Carrier Present Sidebands Efficiency
AM (DSB-FC) Yes Two Maximum 33.33%
DSB-SC No Two 100%
SSB-SC No One 100%

AM vs DSB-SC vs SSB-SC

Feature AM DSB-SC SSB-SC
Carrier Transmission Yes No No
USB Yes Yes One Only
LSB Yes Yes One Only
Power Efficiency Low High Highest Practical
Bandwidth Efficiency Low Medium High
Receiver Complexity Simple Moderate High

Key Formulas for Exams

Formula Expression
Carrier Power Pc = Ac² / 2R
One Sideband Power (m²/4)Pc
Total Sideband Power (m²/2)Pc
Total AM Power Pc(1 + m²/2)
AM Efficiency (m²/2)/(1 + m²/2)
Maximum AM Efficiency 33.33%
DSB-SC Efficiency 100%
SSB-SC Efficiency 100%

Conclusion

Conventional AM is simple to generate and detect but suffers from poor power efficiency because most transmitted power is consumed by the carrier. At 100% modulation, the maximum AM efficiency is only 33.33%.

DSB-SC improves power utilization by suppressing the carrier, achieving 100% power efficiency. SSB-SC goes one step further by transmitting only one sideband, providing both 100% power efficiency and a 50% reduction in bandwidth.

For this reason, SSB-SC is widely used in long-distance radio communication, military communication systems, marine communication, and amateur radio applications.

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