Detailed Notes Amplitude Modulation - Communication System - Electronics

Introduction

Modulation is the process of translating a message (information) signal to a different frequency band and increasing its suitability for transmission. In practical communication systems modulation increases the frequency and often the power of the message so that it can be transmitted efficiently over the chosen medium.

Amplitude Modulation (AM) is a family of modulation techniques in which the amplitude of a high-frequency carrier wave is varied in proportion to the instantaneous amplitude of the message (modulating) signal while the carrier frequency and phase remain essentially constant.

The process of AM is illustrated below.

Example context: Audio signals (speech, music) typically occupy low-frequency ranges and are difficult to radiate directly with an antenna. By modulating these audio (message) signals onto a high-frequency carrier, the resulting modulated waveform can be radiated effectively using antennas of practical size and sent over long distances.

Other common modulation types are Frequency Modulation (FM), where the carrier frequency varies with the message, and Phase Modulation (PM), where the carrier phase varies with the message.

Analog

Analog refers to quantities that vary continuously with time. In communication:

• Analog communication uses continuous-time signals to convey information and predates digital communication. It typically uses simpler, low-cost components but may be more susceptible to noise and distortion.
• Analog signal is a waveform that varies continuously with time; common examples are sinusoidal signals representing sound or light variations converted to voltage. A simple analog signal example is shown below.

What is modulation?

When a message signal is combined with a carrier signal so that the message information is carried by the resulting waveform, the process is called modulation. The message is effectively "superimposed" on the carrier.

• Message signal: the original low-frequency signal containing information to be transmitted.
• Carrier signal: a typically high-frequency sinusoidal signal of nearly constant amplitude used to carry the message.
• Baseband signal: a message signal whose frequency components lie near zero up to some maximum frequency; also called an unmodulated or low-frequency signal.
• Passband signal: a signal whose spectrum is centred at a frequency higher than the largest baseband component; this is the usual result of modulation.

Example: A speech signal has significant energy roughly in 0.3-3.4 kHz. Without modulation two such signals would interfere if transmitted on the same channel. By translating (modulating) each speech signal onto different carrier frequencies (for example around several MHz), many users can share the same physical medium without overlap.

Need for Modulation

Modulation is required in practical communication systems for several reasons:

• Frequency multiplexing: multiple messages are assigned different carrier frequencies so they can be transmitted simultaneously on the same channel and later separated by bandpass filters.
• Antennas: antenna size is related to the carrier wavelength; transmitting baseband audio frequencies directly would need impractically large antennas.
• Narrowbanding: translating a wide range of baseband frequencies to a much higher band can reduce the fractional bandwidth. For example, translating 50-10⁴ Hz to around 1 MHz + (50-10⁴) reduces the highest/lowest-frequency ratio and allows practical antenna lengths and filters.
• Common processing blocks: receivers often convert incoming signals to a fixed intermediate frequency (IF) for common amplification and filtering (superheterodyne architecture).

Types of Amplitude Modulation

The International Telecommunication Union (ITU) designates the common forms of amplitude modulation. The main types are:

• Double Sideband (DSB): AM that produces spectral sidebands symmetric about the carrier. DSB is further classified as DSB-C (with carrier) and DSB-SC (carrier suppressed).
• Single Sideband (SSB): only one of the two sidebands (upper or lower) is transmitted; the other is suppressed. SSB is bandwidth- and power-efficient compared with conventional DSB-AM.
• Vestigial Sideband (VSB): a compromise in which one sideband is transmitted fully and a vestige (partial portion) of the other sideband is transmitted; commonly used in television broadcasting to reduce bandwidth while preserving low-frequency components that are difficult to filter sharply.

The original generic name for amplitude modulation was DSB-AM because two sidebands are normally generated around the carrier.

History of Amplitude Modulation

• 1831 - Michael Faraday discovered electromagnetic induction, laying foundations for electromagnetism.
• 1873 - James Clerk Maxwell formulated the equations predicting electromagnetic wave propagation.
• 1875 - Alexander Graham Bell demonstrated practical telephony.
• 1887 - Heinrich Hertz experimentally demonstrated radio (electromagnetic) waves.
• 1901 - Reginald Fessenden (often written as R. Fessenden) transmitted one of the first amplitude-modulated signals, using spark-gap transmitters and later continuous-wave techniques.
• 1906-1910 - Practical continuous-wave AM transmitters began to be developed and used for radiotelephony between 1900 and 1920.
• 1915 - J. R. Carson initiated the mathematical analysis of amplitude modulation and demonstrated efficient single-band transmission. On 1 December 1915 he patented an approach related to SSB.
• 1920s - The invention and refinement of vacuum tubes led to widespread radio AM broadcasting.

Frequency Translation of Amplitude Modulation

Modulation may be understood in the frequency domain as multiplication of the message signal by a sinusoidal carrier.

Let the message be a single-tone sinusoid

Vm(t) = A_m cos ω_m t = A_m cos 2π f_m t

The message has a spectrum consisting of two impulses at ±f_m, each of amplitude A_m/2.

Let the carrier be

Vc(t) = A_c cos ω_c t

Multiplying the signals in time gives

Vm(t) · Vc(t) = A_m A_c cos ω_m t · cos ω_c t

Using the trigonometric identity cos a cos b = 1/2[cos(a + b) + cos(a - b)], the product contains components at frequencies ω_c ± ω_m (i.e. f_c ± f_m) in addition to components at the negatives of those frequencies. Thus the original baseband spectral lines are translated to be centred about the carrier frequency. In general, modulation produces two sideband components around the carrier for each spectral component of the message.

Modulation Index

The modulation index (often denoted by m or μ) for amplitude modulation quantifies how much the carrier amplitude varies under modulation. For a message amplitude A (or A_m) and carrier amplitude A_c it is

Modulation index = A_m / A_c (or m = μ = A_m / A_c)

For a modulating signal m(t), when m(t) is normalised so that its peak equals A_m, the instantaneous AM waveform is commonly written as

s(t) = A_c [1 + μ m(t)] cos ω_c t

where μ must normally satisfy μ ≤ 1 to avoid over-modulation (envelope distortion) when m(t) has amplitude up to unity.

Efficiency of AM

Power efficiency for AM is defined as the ratio of power present in the sidebands (which carry the information) to the total transmitted power (carrier + sidebands):

Efficiency = P_s / P_t

Total power P_t = P_c + P_s, where P_c is carrier power and P_s is total sideband power.

For an AM signal written as s(t) = A_c cos ω_c t + A_c μ m(t) cos ω_c t, the carrier power is

P_c = A_c² / 2

If P_m denotes the average power of the modulating signal m(t) (when normalised appropriately), the sideband power can be shown to be

P_s = (A_c² μ² / 2) P_m

Thus

Efficiency = (μ² P_m) / (1 + μ² P_m)

For single-tone modulation (m(t) = cos ω_m t), P_m = 1/2 and the sideband power relative to the carrier leads to the familiar expression for total power

P_t = P_c (1 + μ²/2)

and the maximum theoretical efficiency for conventional DSB-AM with carrier present and single-tone modulation is about 33.33% when μ = 1. For DSB-SC (carrier suppressed) efficiency is higher because the carrier power is not transmitted; SSBSC (single sideband suppressed carrier) can approach 100% efficiency in the ideal case because only the information-bearing sideband is sent.

Advantages

• AM transmitters and receivers are relatively simple and low cost.
• AM modulation and demodulation methods are conceptually straightforward.
• AM allows long-distance transmission using high-frequency carriers and practical antennas.
• Wide availability of legacy equipment and standards (e.g., AM broadcast band).

Disadvantages

• AM is more susceptible to amplitude noise (noise and interference that alter signal amplitude) because the information is contained in the amplitude.
• Conventional AM (DSB-C) wastes power in the carrier and in both sidebands; the carrier contains no information, and the two sidebands carry redundant information.
• AM requires greater bandwidth than single-sideband methods - DSB occupies twice the highest message frequency.

Applications of Amplitude Modulation

• Broadcasting: AM is widely used in medium-wave and short-wave radio broadcasting.
• Two-way radios and portable transceivers: simple AM/DSB implementations are used where low cost and simplicity are important.
• Legacy and specialised links: AM is used where equipment simplicity and existing infrastructures make it suitable.

Numerical Examples

Example 1: Find the total power of the amplitude modulated signal with a carrier power 400 W and modulation index of 0.8.

Sol.

Use the total power expression for single-tone AM:

P_t = P_c (1 + μ²/2)

Substitute values: P_c = 400 W, μ = 0.8

P_t = 400 × (1 + (0.8)² / 2)

P_t = 400 × (1 + 0.64 / 2)

P_t = 400 × (1 + 0.32)

P_t = 400 × 1.32

P_t = 528 W

Thus, the total power of the AM signal is 528 watts.

Example 2: What is the maximum efficiency of the single-tone modulation signal?

Sol.

For single-tone modulation the efficiency expression in terms of modulation index μ is

Efficiency = μ² / (2 + μ²)

Maximum permitted linear modulation without over-modulation is μ = 1.

At μ = 1, Efficiency = 1² / (2 + 1²)

Efficiency = 1 / 3

Efficiency = 0.3333 → 33.33%

Hence, the maximum efficiency for single-tone conventional AM (with carrier present) is 33.33%.

Summary

This chapter has described the principles of amplitude modulation: why modulation is needed, how modulation translates spectra to higher frequencies, definitions of modulation index and power efficiency, the principal types of AM (DSB, SSB, VSB), historical milestones, and simple numerical examples. Understanding these fundamentals is essential before studying AM transmitter and receiver circuits, demodulation methods (envelope detectors, coherent detection), and advanced topics such as SSB generation and demodulation and bandwidth/power trade-offs in communications engineering.