Amplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. In Telecommunications, modulation is the process of varying a periodic Waveform, i In Telecommunications, modulation is the process of varying a periodic Waveform, i Single-sideband modulation ( SSB) is a refinement of Amplitude modulation that more efficiently uses electrical power and bandwidth. Phase modulation (PM is a form of Modulation that represents information as variations in the instantaneous phase of a Carrier wave. Space modulation is a radio Amplitude Modulation technique used in Instrument Landing Systems that incorporates the use of multiple antennas fed with various radio In Telecommunications, modulation is the process of varying a periodic Waveform, i On-off keying ( OOK) is a type of Modulation that represents Digital Data as the presence or absence of a Carrier wave. Frequency-shift keying (FSK is a Frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a Carrier wave Amplitude-shift keying ( ASK) is a form of Modulation that represents Digital Data as variations in the Amplitude of a Carrier Phase-shift keying (PSK is a Digital Modulation scheme that conveys data by changing or modulating the phase of a reference signal In Digital modulation, Minimum-shift keying (MSK is a type of continuous phase Frequency-shift keying that was developed in the late 1960s Continuous phase modulation ( CPM) is a method for modulation of data commonly used in Wireless modems In contrast to other coherent digital Phase Pulse-position modulation is a form of signal Modulation in which M message bits are encoded by transmitting asingle pulse in one of 2^M possible time-shifts In Telecommunication, trellis modulation (also known as trellis coded modulation, or simply TCM) is a modulation scheme which allows highly efficient Orthogonal frequency-division multiplexing ( OFDM) — essentially identical to Coded OFDM ( COFDM) and Discrete multi-tone modulation ( Spread-spectrum techniques are methods by which Energy generated in a particular bandwidth is deliberately spread in the Frequency domain, resulting Frequency-hopping spread spectrum ( FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using In Telecommunications direct-sequence spread spectrum ( DSSS) is a Modulation technique Radio is the transmission of signals by Modulation of electromagnetic waves with frequencies below those of visible Light. In Telecommunications, a carrier wave, or carrier is a Waveform (usually Sinusoidal) that is modulated (modified with an input signal 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 reflect the sounds to be reproduced by a speaker, or to specify 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. Frequency is a measure of the number of occurrences of a repeating event per unit Time. Phase modulation (PM is a form of Modulation that represents information as variations in the instantaneous phase of a Carrier wave. A remote control is an electronic device used for the remote operation of a Machine. The phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0 )

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). Reginald Aubrey Fessenden ( October 6, 1866 &ndash July 22, 1932) born in East Bolton, Quebec, Canada, was Medium Wave (MW is a part of the Medium frequency (MF radio band used mainly for AM broadcasting. A band is a small section of the spectrum of Radio communication frequencies, in which channels are usually used or set aside for the same

Fig 1: An audio signal (top) may be carried by an AM or FM radio wave.

Forms of amplitude modulation

As originally developed for the electric telephone, amplitude modulation was used to add audio information to the low-powered direct current flowing from a telephone transmitter to a receiver. As a simplified explanation, at the transmitting end, a telephone microphone was used to vary the strength of the transmitted current, according to the frequency and loudness of the sounds received. Then, at the receiving end of the telephone line, the transmitted electrical current affected an electromagnet, which strengthened and weakened in response to the strength of the current. In turn, the electromagnet produced vibrations in the receiver diaphragm, thus closely reproducing the frequency and loudness of the sounds originally heard at the transmitter.

In contrast to the telephone, in radio communication what is modulated is a continuous wave radio signal (carrier wave) produced by a radio transmitter. A continuous wave or continuous waveform ( CW) is an Electromagnetic wave of constant Amplitude and Frequency; and in Mathematical In Telecommunications, a carrier wave, or carrier is a Waveform (usually Sinusoidal) that is modulated (modified with an input signal In its basic form, amplitude modulation produces a signal with power concentrated at the carrier frequency and in two adjacent sidebands. In Radio communications a sideband is a band of Frequencies higher than or lower than the Carrier frequency, containing power as a result of Each sideband is equal in bandwidth to that of the modulating signal and is a mirror image of the other. Bandwidth is the difference between the upper and lower Cutoff frequencies of for example a filter, a Communication channel, or a Signal spectrum 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 and much of it is wasted. 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); the remaining power is split between two identical sidebands, though only one of these is needed since they contain identical information.

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. Reduced-carrier transmission is an Amplitude modulation (AM transmission in which the Carrier wave level is reduced to reduce Wasted electrical 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. A beat frequency oscillator or BFO in Radio telegraphy, is a dedicated Oscillator used to create an audio frequency signal from Carrier wave An envelope detector is an electronic circuit that takes a high-frequency signal as input and provides an output which is the "envelope" of the original signal

Even greater 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. Single-sideband modulation ( SSB) is a refinement of Amplitude modulation that more efficiently uses electrical power and bandwidth. Amateur radio, often called ham radio, is both a Hobby and a service in which participants called "hams" use various types of Radio communications

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. A digital system uses discrete (discontinuous values usually but not always Symbolized Numerically (hence called "digital" to represent information for On-off keying ( OOK) is a type of Modulation that represents Digital Data as the presence or absence of a Carrier wave. Amplitude-shift keying ( ASK) is a form of Modulation that represents Digital Data as variations in the Amplitude of a Carrier The binary numeral system, or base-2 number system, is a Numeral system that represents numeric values using two symbols usually 0 and 1. This is commonly used at radio frequencies to transmit Morse code, referred to as continuous wave (CW) operation. Morse code is a Character encoding for transmitting telegraphic information using standardized sequences of short and long elements to represent the letters numerals A continuous wave or continuous waveform ( CW) is an Electromagnetic wave of constant Amplitude and Frequency; and in Mathematical

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

Example - Double Sideband AM

Fig 2: The (2-sided) spectrum of an AM signal. In Radio communications a sideband is a band of Frequencies higher than or lower than the Carrier frequency, containing power as a result of Single-sideband modulation ( SSB) is a refinement of Amplitude modulation that more efficiently uses electrical power and bandwidth. Reduced-carrier transmission is an Amplitude modulation (AM transmission in which the Carrier wave level is reduced to reduce Wasted electrical Single-sideband modulation ( SSB) is a refinement of Amplitude modulation that more efficiently uses electrical power and bandwidth. Single-sideband suppressed-carrier ( SSB-SC) is a Telecommunication technique which belongs to the Amplitude modulation class Independent sideband (ISB is an AM Single sideband mode which is used with some AM radio transmissions Normally each Sideband In Radio communications a sideband is a band of Frequencies higher than or lower than the Carrier frequency, containing power as a result of In Telecommunication, Signal processing, and Thermodynamics, companding (occasionally called compansion) is a method of mitigating the detrimental

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

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

For generality, and are arbitrary constants that represent the carrier amplitude and initial phase. For simplicity, we set their respective values 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:

Thus, the message might be just a simple audio tone of frequency  

It is generally assumed that    and that  

Then amplitude modulation is created by forming the product:

represents another constant we may choose. 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:

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

Also notice that the choice A=0 eliminates the carrier component, but leaves the sidebands. In Radio communications a sideband is a band of Frequencies higher than or lower than the Carrier frequency, containing power as a result of That is the DSBSC transmission mode. To generate double-sideband full carrier (A3E), we must choose:  

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 + ω_c) and one at negative frequencies (centered on − ω_c). Frequency is a measure of the number of occurrences of a repeating event per unit Time. The concept of negative and positive frequency can be as simple as a wheel rotating one way or the other way 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.

For those interested in the mathematics of Figure 2, it is a result of computing the Fourier transform of:   using the following transform pairs:

Fig 3: The spectrogram of an AM broadcast shows its two sidebands (green) separated by the carrier signal (red). This article specifically discusses Fourier transformation of functions on the Real line; for other kinds of Fourier transformation see Fourier analysis and The spectrogram is the result of calculating the Frequency spectrum of Windowed frames of a compound signal.

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. In Signal processing, baseband is an adjective that describes signals and systems whose range of Frequencies is measured from zero to a maximum bandwidth 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   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

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:

  where and 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%. For 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. In Electronics systems a VOGAD or voice-operated gain-adjusting device is a type of Automatic gain control or compressor for Microphone

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.

Fig 4: Modulation depth

Amplitude modulator designs

Circuits

A wide range of different circuits have been used for AM, but one of the simplest circuits uses anode or collector modulation applied via a transformer. A transformer is a device that transfers Electrical energy from one circuit to another through inductively coupled Electrical conductors While it is perfectly possible to create good designs using solid-state electronics, valved (vacuum tube) circuits are shown here. This article is about the electronic device not an evacuated pipe used for experiments in Free-fall. In general, valves are able to more easily yield RF powers, in excess of what can be easily achieved using solid-state transistors. In Electronics, a transistor is a Semiconductor device commonly used to amplify or switch electronic signals Most high-power broadcast stations still use valves.

Anode modulation using a transformer. The tetrode is supplied with an anode supply (and screen grid supply) which is modulated via the transformer. A tetrode is an electronic device having four active Electrodes The term most commonly applies to a two-grid Vacuum tube. The resistor R1 sets the grid bias, both the input and outputs are tuned LC circuits which are tapped into by inductive coupling

Modulation circuit designs can be broadly divided into low and high level. An LC circuit is a variety of resonant circuit or tuned circuit and consists of an Inductor, represented by the letter L and a Capacitor, represented

Low level

Here a small audio stage is used to modulate a low power stage, the output of this stage is then amplified using a linear RF amplifier. Sound' is Vibration transmitted through a Solid, Liquid, or Gas; particularly sound means those vibrations composed of Frequencies In Telecommunications, modulation is the process of varying a periodic Waveform, i A linear amplifier is an electronic circuit whose output is proportional to its input but capable of delivering more power into a Load.

Advantages

The advantage of using a linear RF amplifier is that the smaller early stages can be modulated, which only requires a small audio amplifier to drive the modulator. An audio amplifier is an Electronic amplifier that amplifies low-power audio signals (signals composed primarily of frequencies between 20 Hertz to

Disadvantages

The great disadvantage of this system is that the amplifier chain is less efficient, because it has to be linear to preserve the modulation. The efficiency of an entity (a device, component, or System) in Electronics and Electrical engineering is defined as useful Hence Class C amplifiers cannot be employed. An electronic amplifier is a device for increasing the power and/or Amplitude of a signal.

An approach which marries the advantages of low-level modulation with the efficiency of a Class C power amplifier chain is to arrange a feedback system to compensate for the substantial distortion of the AM envelope. A simple detector at the transmitter output (which can be little more than a loosely coupled diode) recovers the audio signal, and this is used as negative feedback to the audio modulator stage. Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes Negative Feedback feeds part of a System 's output inverted into the system's input generally with the result that fluctuations are attenuated The overall chain then acts as a linear amplifier as far as the actual modulation is concerned, though the RF amplifier itself still retains the Class C efficiency. This approach is widely used in practical medium power transmitters, such as AM radiotelephones. A radiotelephone is a communications device that allows two or more people to talk using Radio.

High level

Advantages

One advantage of using class C amplifiers in a broadcast AM transmitter is that only the final stage needs to be modulated, and that all the earlier stages can be driven at a constant level. These class C stages will be able to generate the drive for the final stage for a smaller DC power input. Direct current ( DC) is the unidirectional flow of Electric charge. However, in many designs in order to obtain better quality AM the penultimate RF stages will need to be subject to modulation as well as the final stage.

Disadvantages

A large audio amplifier will be needed for the modulation stage, at least equal to the power of the transmitter output itself. Traditionally the modulation is applied using an audio transformer, and this can be bulky. Direct coupling from the audio amplifier is also possible (known as a cascode arrangement), though this usually requires quite a high DC supply voltage (say 30 V or more), which is not suitable for mobile units. In electronics direct coupling (also called direct wired contact and conductive charging) is a way of interconnecting two circuits such that in addition to transferring The cascode is a two-stage Amplifier composed of a Transconductance amplifier followed by a current buffer.

See also

• AM radio also referred to as Mediumwave
• Frequency Modulation
• Shortwave radio almost universally uses AM, narrow FM occurring above 25 MHz. Medium Wave (MW is a part of the Medium frequency (MF radio band used mainly for AM broadcasting. Shortwave Radio operates between the frequencies of 3000 KHz (3
• Modulation, for a list of other modulation techniques
• AMSS Amplitude Modulation Signalling System, a digital system for adding low bitrate information to an AM signal. In Telecommunications, modulation is the process of varying a periodic Waveform, i The amplitude modulation signalling system ( AMSS or the AM signalling system) is a Digital system for adding low Bit rate information to an
• Sideband, for some explanation of what this is. In Radio communications a sideband is a band of Frequencies higher than or lower than the Carrier frequency, containing power as a result of
• Types of radio emissions, for the emission types designated by the ITU
• Airband

References

• Newkirk, David and Karlquist, Rick (2004). The International Telecommunication Union uses an internationally agreed system for classifying Radio frequency signals The airband or air band is the band of frequencies used for radio communication in Aviation. 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, The Wolfram Demonstrations Project.
• Amplitude Modulation, by S Sastry.