Tru Physics

Amplitude Modulation (AM)

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Introduction to Amplitude Modulation (AM)

Amplitude Modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. In AM, the strength (amplitude) of the carrier wave is varied in proportion to that of the message signal being sent.

Amplitude Modulation. AM Wave.

Basic Principle of Amplitude Modulation

Modulation Process

In an amplitude modulation process, the carrier wave’s amplitude is altered according to the properties of the modulating (message) signal. Assuming a sinusoidal carrier wave, the basic equation of the carrier signal is:

s(t) = A_c \cos(2 \pi f_c t)

where A_c is the amplitude of the carrier signal, f_c is the carrier frequency, and t is time.

During amplitude modulation, the amplitude A_c is varied according to the information signal. The equation for an AM signal is:

s(t) = (A_c + m(t)) \cos(2 \pi f_c t)

where m(t) represents the message signal.

Try the AM simulator below to get a better sense for what amplitude modulation means. Open it is Desmos for better viewing (works best in desktop mode).

Modulation Index

In AM, the modulation index, m_a, is defined as the ratio of the peak value of the message signal to the peak value of the carrier signal:

m_a = \dfrac{A_m}{A_c}

where A_m is the peak value of the message signal and A_c is the peak value of the carrier signal.

Advantages and Applications of Amplitude Modulation

Advantages

Despite its simplicity, AM signals can be easily detected and demodulated, requiring only a simple envelope detector. The equipment used to generate and receive AM signals is also less complex and less expensive than that required for frequency and phase modulation.

Applications

AM is extensively used for audio broadcasting, especially for broadcasting on the Medium Wave and Short Wave bands. Other applications include Single Side Band (SSB) modulation, Quadrature Amplitude Modulation (QAM) for digital signals, and in the design of pulse amplitude modulation (PAM) systems.

Amplitude Modulation: Advanced Topics

Bandwidth of AM Signals

The bandwidth of an AM signal is twice the bandwidth of the modulating (message) signal. If the highest frequency in the message signal is f_m, then the bandwidth of the AM signal is 2f_m.

Sidebands

In an AM signal, the frequency components that result from the modulation process appear in two “bands” on either side of the carrier frequency — these are known as the upper and lower sidebands.

Conclusion

While Amplitude Modulation might seem basic compared to newer modulation schemes, it remains fundamental in various forms of electronic communication. Understanding the principles of AM provides the groundwork for learning more advanced modulation techniques used in modern telecommunication systems.

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