Communication Fundamentals: AM Signal Spectrum

Analog Modulation

• AM (Amplitude Modulation) signal spectrum can be evaluated using the formula: sin(x).sin(y) = ½[cos(x-y) - cos(x+y)]

AM Signal

• The upper sideband (USB) signal is:
• The lower sideband (LSB) signal is:

AM Signal Spectrum

• The spectrum of the AM signal can be sketched as:

Modulation Index

• Modulation index (m) is the ratio of the amplitude of the message signal to the amplitude of the carrier wave
• For 75% modulation, m = 0.75

AM Signal Power

• The power developed across a load of 100 ohms due to an AM signal can be calculated using the formula:
• The result is 16.015 watts for a 75% modulation

Double Sideband-Suppressed Carrier (DSB-SC)

• DSB-SC modulation eliminates the wastage of transmitted power in AM
• DSB-SC modulation is the product of the message signal m(t) and the carrier wave c(t)

DSB-SC Modulation Process

• The modulation process uses a product modulator to generate the DSB-SC modulated wave
• The modulated signal undergoes a phase reversal whenever the message signal crosses zero

DSB-SC Frequency Domain and Bandwidth

• The Fourier transform of the DSB-SC signal s(t) is:
• The transmission bandwidth (BT) of the modulated signal is: BT = 2W

DSB-SC Power

• The power in the DSB-SC signal is:

DSB-SC Demodulation

• Demodulation of the DSB-SC signal uses coherent detection or synchronous demodulation
• The envelope of the DSB-SC modulated wave s(t) is different from the message signal m(t)