Amplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent. For example, changes in the signal strength can be used to specify the sounds to be reproduced by a loudspeaker, or the light intensity of television pixels. (Contrast this with frequency modulation, also commonly used for sound transmissions, in which the frequency is varied; and phase modulation, often used in remote controls, in which the phase is varied)

In the mid-1870s, a form of amplitude modulation—initially called "undulatory currents"—was the first method to successfully produce quality audio over telephone lines. Beginning with Reginald Fessenden's audio demonstrations in 1906, it was also the original method used for audio radio transmissions, and remains in use today by many forms of communication—"AM" is often used to refer to the mediumwave broadcast band (see AM radio).

Forms of amplitude modulation

In radio communication, a continuous wave radio-frequency signal (a sinusoidal carrier wave) has its amplitude modulated by an audio waveform before being transmitted.

In the frequency domain, amplitude modulation produces a signal with power concentrated at the carrier frequency and in two adjacent sidebands. Each sideband is equal in bandwidth to that of the modulating signal and is a mirror image of the other. Amplitude modulation that results in two sidebands and a carrier is often called ''double-sideband amplitude modulation'' (DSB-AM). Amplitude modulation is inefficient in terms of power usage. At least two-thirds of the power is concentrated in the carrier signal, which carries no useful information (beyond the fact that a signal is present).

To increase transmitter efficiency, the carrier can be removed (suppressed) from the AM signal. This produces a reduced-carrier transmission or ''double-sideband suppressed-carrier'' (DSBSC) signal. A suppressed-carrier amplitude modulation scheme is three times more power-efficient than traditional DSB-AM. If the carrier is only partially suppressed, a ''double-sideband reduced-carrier'' (DSBRC) signal results. DSBSC and DSBRC signals need their carrier to be regenerated (by a beat frequency oscillator, for instance) to be demodulated using conventional techniques.

Improved bandwidth efficiency is achieved—at the expense of increased transmitter and receiver complexity—by completely suppressing both the carrier and one of the sidebands. This is single-sideband modulation, widely used in amateur radio due to its efficient use of both power and bandwidth.

A simple form of AM often used for digital communications is ''on-off keying'', a type of ''amplitude-shift keying'' by which binary data is represented as the presence or absence of a carrier wave. This is commonly used at radio frequencies to transmit Morse code, referred to as continuous wave (CW) operation.

ITU designations

In 1982, the International Telecommunication Union (ITU) designated the various types of amplitude modulation as follows:

{|class="wikitable" |- !Designation!!Description |- |A3E||double-sideband full-carrier - the basic AM modulation scheme |- |R3E||single-sideband reduced-carrier |- |H3E||single-sideband full-carrier |- |J3E||single-sideband suppressed-carrier |- |B8E||independent-sideband emission |- |C3F||vestigial-sideband |- |Lincompex||linked compressor and expander |}

Example: double-sideband AM

A carrier wave is modeled as a simple sine wave, such as:

:$c(t)\; =\; C\backslash cdot\; \backslash sin(\backslash omega\_c\; t\; +\; \backslash phi\_c),\backslash ,$

where the radio frequency (in Hz) is given by:

:$\backslash omega\_c\; /\; (2\backslash pi).\backslash ,$

The constants $C\backslash ,$ and $\backslash phi\_c\backslash ,$ represent the carrier amplitude and initial phase, and are introduced for generality. For simplicity however, their respective values can be set to 1 and 0.

Let ''m''(''t'') represent an arbitrary waveform that is the message to be transmitted.  And let the constant ''M'' represent its largest magnitude. For instance:

:$m(t)\; =\; M\backslash cdot\; \backslash cos(\backslash omega\_m\; t\; +\; \backslash phi).\backslash ,$

Thus, the message might be just a simple audio tone of frequency :$\backslash omega\_m\; /\; (2\backslash pi).\backslash ,$

It is generally assumed that  $\backslash omega\_m\; \backslash ll\; \backslash omega\_c\backslash ,$  and that  $\backslash min[\; m(t)\; ]\; =\; -M.\backslash ,$

Then amplitude modulation is created by forming the product:

:{| |$y(t)\backslash ,$ |$=\; [A\; +\; m(t)]\backslash cdot\; c(t),\backslash ,$ |- | |$=\; [A\; +\; M\backslash cdot\; \backslash cos(\backslash omega\_m\; t\; +\; \backslash phi)]\backslash cdot\; \backslash sin(\backslash omega\_c\; t).$ |}

$A\backslash ,$ represents the carrier amplitude which is a constant that we would choose to demonstrate the modulation index. The values ''A''=1, and ''M''=0.5, produce a ''y''(''t'') depicted by the graph labelled "50% Modulation" in Figure 4.

For this simple example, ''y''(''t'') can be trigonometrically manipulated into the following equivalent form:

:$y(t)\; =\; A\backslash cdot\; \backslash sin(\backslash omega\_c\; t)\; +\; \backslash begin\{matrix\}\backslash frac\{M\}\{2\}\; \backslash end\{matrix\}\; \backslash left[\backslash sin((\backslash omega\_c\; +\; \backslash omega\_m)\; t\; +\; \backslash phi)\; +\; \backslash sin((\backslash omega\_c\; -\; \backslash omega\_m)\; t\; -\; \backslash phi)\backslash right].\backslash ,$

Therefore, the modulated signal has three components, a carrier wave and two sinusoidal waves (known as sidebands) whose frequencies are slightly above and below  $\backslash omega\_c.\backslash ,$

Also notice that the choice A=0 eliminates the carrier component, but leaves the sidebands. That is the DSBSC transmission mode. To generate double-sideband full carrier (A3E), we must choose:

$A\; \backslash ge\; M.\backslash ,$

Spectrum

For more general forms of ''m''(''t''), trigonometry is not sufficient. But if the top trace of Figure 2 depicts the frequency spectrum, of ''m''(''t''), then the bottom trace depicts the modulated carrier. It has two groups of components: one at positive frequencies (centered on $+\backslash omega\_c$) and one at negative frequencies (centered on $-\backslash omega\_c$). Each group contains the two sidebands and a narrow component in between that represents the energy at the carrier frequency. We need only be concerned with the positive frequencies. The negative ones are a mathematical artifact that contains no additional information. Therefore, we see that an AM signal's spectrum consists basically of its original (2-sided) spectrum shifted up to the carrier frequency.

Figure 2 is a result of computing the Fourier transform of:   $[A\; +\; m(t)]\backslash cdot\; \backslash sin(\backslash omega\_c\; t),\backslash ,$ using the following transform pairs:

:

Power and spectrum efficiency

In terms of the positive frequencies, the transmission bandwidth of AM is twice the signal's original (baseband) bandwidth—since both the positive and negative sidebands are shifted up to the carrier frequency. Thus, double-sideband AM (DSB-AM) is spectrally inefficient, meaning that fewer radio stations can be accommodated in a given broadcast band. The various suppression methods in Forms of AM can be readily understood in terms of the diagram in Figure 2. With the carrier suppressed there would be no energy at the center of a group. And with a sideband suppressed, the "group" would have the same bandwidth as the positive frequencies of $M(\backslash omega).\backslash ,$  The transmitter power efficiency of DSB-AM is relatively poor (about 33%). The benefit of this system is that receivers are cheaper to produce. The forms of AM with suppressed carriers are found to be 100% power efficient, since no power is wasted on the carrier signal which conveys no information.

Modulation index

It can be defined as the measure of extent of amplitude variation about an unmodulated maximum carrier. As with other modulation indices, in AM, this quantity, also called ''modulation depth'', indicates by how much the modulated variable varies around its 'original' level. For AM, it relates to the variations in the carrier amplitude and is defined as: :$h\; =\; \backslash frac\{\backslash mathrm\{peak\backslash \; value\backslash \; of\backslash \; \}\; m(t)\}\{A\}\; =\; \backslash frac\{M\}\{A\},$   where $M\backslash ,$ and $A\backslash ,$ were introduced above.

So if $h=0.5$, the carrier amplitude varies by 50% above and below its unmodulated level, and for $h=1.0$ it varies by 100%. To avoid distortion in the A3E transmission mode, modulation depth greater than 100% must be avoided. Practical transmitter systems will usually incorporate some kind of limiter circuit, such as a VOGAD, to ensure this. However, AM demodulators can be designed to detect the inversion (or 180 degree phase reversal) that occurs when modulation exceeds 100% and automatically correct for this effect.

Variations of modulated signal with percentage modulation are shown below. In each image, the maximum amplitude is higher than in the previous image. Note that the scale changes from one image to the next.

AM modulation methods

Modulation circuit designs can be broadly divided into low and high level.

Low-Level Generation

In many recent radio systems, modulated signals are generated via digital signal processing (DSP). With DSP, many forms of AM modulation are feasible under software control, including traditional double sideband with carrier, but also single sideband suppressed carrier, independent sideband, etc. The calculated digital samples are converted to voltages with a digital to analog converter, typically at a frequency less than the desired RF output frequency. The analog signal must then be shifted and linearly amplified to the desired frequency and power level. Linear amplification must be used to prevent modulation distortion. The low-level method for AM is used in many current Amateur Radio transceivers.

AM can also be generated at low level using various analog methods described in the next section.

High-Level Generation

Present-day high-power AM transmitters (e.g., for MF broadcasting) are based on high efficiency class-D and class-E power amplifier stages modulated by varying the supply voltage.

Older designs for broadcast and amateur AM radio also generated AM by controlling the gain of a transmitter’s final amplifier, which was generally a class-C type for good efficiency. The following types are for vacuum tube transmitters, but similar options are available with transistors.

Plate Modulation. In plate modulation, the plate voltage of the RF amplifier is modulated with the audio signal. The audio power requirement is 50% of the RF carrier power.

Heising (Constant-Current) Modulation. The RF amplifier plate voltage is fed through a “choke� (high value inductor). The AM modulation tube plate is fed through the same inductor, so that the modulator tube diverts current from the RF amplifier. The choke acts as a constant current source in the audio range. This system has low power efficiency.

Control Grid Modulation. The operating bias and gain of the final RF amplifier can be controlled by varying the voltage of the control grid. This method requires little audio power, but special care must be taken to reduce distortion.

Clamp Tube (Screen Grid) Modulation. The screen grid bias may be controlled through a “clamp tube� that reduces voltage according to the modulation signal. It is difficult to approach 100% modulation while maintaining low distortion with this system.

AM demodulation methods

The simplest form of AM demodulator consists of a diode which is configured to act as envelope detector. Another type of demodulator, the product detector, can provide better quality demodulation, at the cost of added circuit complexity.

See also

AM radio

AM stereo

Mediumwave band used for AM broadcast radio

Longwave band used for AM broadcast radio

Frequency modulation

Shortwave radio almost universally uses AM, narrow FM occurring above 25 MHz.

Modulation, for a list of other modulation techniques

Amplitude modulation signalling system (AMSS), a digital system for adding low bitrate information to an AM signal.

Sideband, for some explanation of what this is.

Types of radio emissions, for the emission types designated by the ITU

Airband

Quadrature amplitude modulation

References

Newkirk, David and Karlquist, Rick (2004). Mixers, modulators and demodulators. In D. G. Reed (ed.), ''The ARRL Handbook for Radio Communications'' (81st ed.), pp. 15.1–15.36. Newington: ARRL. ISBN 0-87259-196-4.

External links

''Amplitude Modulation'' by Jakub Serych, Wolfram Demonstrations Project.

Amplitude Modulation, by S Sastry.

Amplitude Modulation, an introduction by Federation of American Scientists.

Amplitude Modulation tutorial video with example transmitter circuit.

Category:Radio modulation modes

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