A comprehensive guide for undergraduate students covering fundamentals, mathematical representation, modulation index, waveforms, and applications of AM
Amplitude Modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting information via a radio carrier wave.
• Carrier Wave: High frequency wave that carries the information
• Modulating Signal: The information signal (voice, music, data)
• AM Wave: The modulated carrier wave with varying amplitude
In AM, the amplitude of the carrier wave is varied in proportion to the instantaneous amplitude of the modulating signal while the frequency remains constant.
c(t) = Ac cos(2πfct)
Where Ac is carrier amplitude and fc is carrier frequency.
m(t) = Am cos(2πfmt)
Where Am is modulating amplitude and fm is modulating frequency.
Where μ is the modulation index (0 ≤ μ ≤ 1)
The modulation index (μ) is a measure of the extent of modulation and is defined as:
• μ = 0: No modulation (carrier only)
• μ = 0.5: 50% modulation
• μ = 1: 100% modulation (ideal)
• μ > 1: Overmodulation (causes distortion)
Percentage modulation = μ × 100%
Note: The envelope of the AM wave corresponds exactly to the shape of the modulating signal when 0 ≤ μ ≤ 1.
The AM signal consists of three frequency components:
Bandwidth = (fc + fm) - (fc - fm) = 2fm
Where fm is the highest frequency in the modulating signal.
Total power in AM wave:
Where Pc is the carrier power
| Advantages | Disadvantages |
|---|---|
| Simple to implement and demodulate | Inefficient power usage (carrier power) |
| Requires simple and inexpensive receivers | Inefficient spectrum utilization (2× bandwidth) |
| Compatible with many existing systems | Susceptible to noise and interference |
| Long distance propagation possible | Poor signal-to-noise ratio compared to FM |
Standard broadcast band: 535-1705 kHz
Video signal in analog television
Pulse amplitude modulation
Aviation and marine communication