Amplitude Modulation


This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series teaches you a specific topic of common measurement applications by explaining the theory and giving practical examples. This tutorial covers an introduction to RF, wireless, and high-frequency signals and systems.


Amplitude Modulation

Modulation is the process of varying a higher frequency carrier wave to transmit information. Though it is theoretically possible to transmit baseband signals (or information) without modulating it, it is far more efficient to send data by modulating it onto a higher frequency "carrier wave." Higher frequency waves require smaller antennas, use the available bandwidth more efficiently, and are flexible enough to carry different types of data. AM radio stations transmit audio signals, which range from 20 Hz to 20 kHz, using carrier waves that range from 500 kHz to 1.7 MHz. If we were to transmit audio signals directly we would need an antenna that is around 10,000 km! Modulation techniques can be broadly divided into analog modulation and digital modulation. Amplitude modulation (AM) is one form of analog modulation.

Figure 1. Basic Stages of AM

Mathematical Background

The carrier signal is generally a high-frequency sine wave. There are three parameters of a sine wave that can be varied: amplitude, frequency, and phase. Any of these can be modulated, or varied, to transmit information. A sine wave can be mathematically described by a sine or cosine function with amplitude Ac, frequency fc, and phase φ.

Figure 2. Carrier Wave

The carrier signal is modulated by varying its amplitude in proportion to the message, or baseband, signal. The message signal can be represented by

and the carrier signal can be represented by

To make the equations simpler, assume that there is no phase difference between the carrier signal and the message signal and thus φ = 0.

The modulated signal can be represented by multiplying the carrier signal and the summation of 1 and the message signal, as shown below.

With some basic trigonometric manipulation, the above waveform can be written as

Types of AM Modulation

As described in the previous section, the modulated signal has waves at three frequencies: fc, fc – fb and fc + fb. Transmitting at all three frequencies wastes power and bandwidth. To avoid that problem use a filter to remove one of the sidebands (usually the lower sideband, fc – fb). Use a highpass filter to remove the lower sideband signal; this process is single sideband (SSB) modulation.

However, by removing one of the sidebands we lose some of the original power of the modulated signal. To maximize the power transmitted, transmit both the lower and the upper sideband. This process is double sideband (DSB) modulation. The following figure illustrates DSB.

Figure 3. Frequency Domain View of Double Sideband – Full Carrier

One of the components of the modulated signal is the pure carrier wave. Because the carrier wave does not have any information, we can remove the carrier wave component from the signal before we transmit it. This process is called single sideband/double sideband – suppressed carrier (SSB-SC, DSB-SC) modulation. However, we need the carrier when demodulating the signal. Special circuits can extract information about the carrier from one of the sidebands; these circuits are used when demodulating SSB-SC or DSB-SC signals.

We can also use amplitude modulation to send digital data. Quadrature amplitude modulation (QAM) uses four predetermined amplitude levels to determine digital bits.

Reality Check

Although understanding AM is helpful to understand modulation, it is not the most efficient or useful way to modulate a signal. Simple AM is slow and requires too much power. Because most communication today is digital, far more complex methods are used. Generally, phase shift keying (PSK)—a type of phase modulation—is used to transmit digital data.

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