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Encyclopedia > Amplitude modulation
Topics in Modulation techniques
Analog modulation

AM | SSB | FM | PM | QAM 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 the technique of amplitude modulation designed to be more efficient in its use of electrical power and bandwidth. ... In telecommunications, frequency modulation (FM) conveys information over a carrier wave by varying its frequency. ... Phase modulation (PM) is a form of modulation which represents information as variations in the instantaneous phase of a carrier wave. ... “QAM” redirects here. ...

Digital modulation

OOK | ASK | PSK | FSK | MSK | QAM | CPM | TCM | OFDM Digital modulation (also referred to as shift keying) is a modulation in which the modified parameter of the carrier signal can take only discrete values. ... 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 which represents digital data as variations in the amplitude of a carrier wave. ... Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). ... It has been suggested that this article or section be merged with audio frequency-shift keying. ... Like the OQPSK modulator (i. ... “QAM” redirects here. ... Continuous phase modulation (CPM) is a method for modulation of data commonly used in wireless modems. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... Orthogonal Frequency-Division Multiplexing (OFDM) — essentially identical to Coded OFDM (COFDM) — is a digital multi-carrier modulation scheme, which uses a large number of closely-spaced orthogonal sub-carriers. ...

Spread spectrum

FHSS | DSSS Spread-spectrum telecommunications is a technique in which a signal is transmitted in a bandwidth considerably greater than the frequency content of the original information. ... Frequency-hopping spread spectrum (FHSS) is a spread-spectrum method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. ... In telecommunications, direct-sequence spread spectrum (DSSS) is a modulation technique. ...

edit

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 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.) This article or section does not adequately cite its references or sources. ... In telecommunications, frequency modulation (FM) conveys information over a carrier wave by varying its frequency. ... FreQuency is a music video game developed by Harmonix and published by SCEI. It was released in November 2001. ... Phase modulation (PM) is a form of modulation which represents information as variations in the instantaneous phase of a carrier wave. ... A television remote control A DVD player remote control A remote control is an electronic device used for the remote operation of a machine. ... This article is about a portion of a periodic process. ...


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 the early 1900s, it was also the original method used for audio radio transmissions, and remains in use by some forms of radio communication—"AM" is often used to refer to the mediumwave broadcast band (see AM radio). Reginald Fessenden (October 6, 1866 – July 22, 1932) was a Canadian inventor, best known for his work in early radio. ... Mediumwave radio transmissions serves as the most common band for 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 purpose. ... Mediumwave radio transmissions (sometimes called Medium frequency or MF) are those between the frequencies of 300 kHz and 3000 kHz. ...

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

Contents

Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ...

Forms of amplitude modulation

what is 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 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. In its basic form, 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 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. A continuous wave (CW) is an electromagnetic wave of constant amplitude and frequency. ... This article or section does not adequately cite its references or sources. ... In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing energy as a result of the modulation process. ... -1...


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. Reduced-carrier transmission is an amplitude modulation (AM) transmission in which the carrier wave level is reduced to reduce wasted electrical power. ... A beat frequency oscillator or BFO in radio telegraphy, is a dedicated oscillator used to create an audio frequency signal for carrier wave transmissions to make them audible, as they are not broadcast as such. ... An envelope detector is a device which is used to demodulate AM signals. ...


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 the technique of amplitude modulation designed to be more efficient in its use of electrical power and bandwidth. ... Amateur radio station with modern solid-state transceiver featuring LCD display and DSP capabilities Amateur radio, often called ham radio, is a hobby that uses various types of radio broadcasting equipment to communicate with other radio amateurs for public service, recreation and self-training. ...


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. A digital system is one that uses discrete values (often electrical voltages), especially those representable as binary numbers, or non-numeric symbols such as letters or icons, for input, processing, transmission, storage, or display, rather than a continuous spectrum of values (ie, as in an analog system). ... 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 which represents digital data as variations in the amplitude of a carrier wave. ... The binary numeral system, or base-2 number system, is a numeral system that represents numeric values using two symbols, usually 0 and 1. ... 1922 Chart of the Morse Code Letters and Numerals Morse code is a method for transmitting telegraphic information, using standardized sequences of short and long elements to represent the letters, numerals, punctuation and special characters of a message. ... A continuous wave (CW) is an electromagnetic wave of constant amplitude and frequency. ...


In 1982, the International Telecommunications Union (ITU) designated the various types of amplitude modulation as follows: The International Telecommunication Union (ITU) is an international organization established to standardize and regulate international radio and telecommunications. ...

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

In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing energy as a result of the modulation process. ... Single-sideband modulation (SSB) is a refinement of the technique of amplitude modulation designed to be more efficient in its use of 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 power. ... Single-sideband modulation (SSB) is a refinement of the technique of amplitude modulation designed to be more efficient in its use of electrical power and bandwidth. ... Single-sideband suppressed-carrier is a telecommunication transmission mode, which belongs to amplitude modulation. ... Independent sideband (ISB) is an AM single sideband mode which is used with some AM radio transmissions. ... Single-sideband modulation (SSB) is a refinement of the technique of amplitude modulation designed to be more efficient in its use of electrical power and bandwidth. ... A waveform before and after the compression stage of companding In telecommunication, signal processing, and thermodynamics, companding (occasionally called compansion) is a method of reducing the effects of a channel with limited dynamic range. ...

Example

Double Sideband AM


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

c(t) = Ccdot sin(omega_c t + phi_c),,

where the radio frequency (in Hz) is given by:  omega_c / (2pi).,
Hz or hz may mean: Herero language (ISO 639 alpha-2, hz) Hertz, unit of frequency This is a disambiguation page — a list of articles associated with the same title. ...


For generality, C, and phi_c, 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:

m(t) = Mcdot cos(omega_m t + phi).,

Thus, the message might be just a simple audio tone of frequency  omega_m / (2pi).,



It is generally assumed that  omega_m ll omega_c,  and that  min[ m(t) ] = -M.,


Then amplitude modulation is created by forming the product:

y(t), = [A + m(t)]cdot c(t),,
= [A + Mcdot cos(omega_m t + phi)]cdot sin(omega_c t).

A, 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:

y(t) = Acdot sin(omega_c t) + begin{matrix}frac{M}{2} end{matrix} left[sin((omega_c + omega_m) t + phi) + sin((omega_c - omega_m) t - phi)right].,


Therefore, the modulated signal has three components, a carrier wave and two sinusoidal waves (known as sidebands) whose frequencies are slightly above and below  omega_c., In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing energy as a result of the modulation process. ...


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 ge M.,

Fig 2: The (2-sided) spectrum of an AM signal.
Fig 2: The (2-sided) spectrum of an AM signal.

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 + ωc) and one at negative frequencies (centered on − ωc). 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. Image File history File links AM_spectrum. ... Image File history File links AM_spectrum. ... FreQuency is a music video game developed by Harmonix and published by SCEI. It was released in November 2001. ... Negative frequency is the rate of clockwise rotation in phase, where phase is defined as the arctangent of the imaginary and real parts of an electrical signal or mathematical function. ...

For those interested in the mathematics of Figure 2, it is a result of computing the Fourier transform of:   [A + m(t)]cdot sin(omega_c t),, using the following transform pairs:
m(t) quad stackrel{mathcal{F}}{Longleftrightarrow}quad M(omega),
sin(omega_c t) quad stackrel{mathcal{F}}{Longleftrightarrow}quad i pi cdot [delta(omega +omega_c)-delta(omega-omega_c)],
Acdot sin(omega_c t) quad stackrel{mathcal{F}}{Longleftrightarrow}quad i pi A cdot [delta(omega +omega_c)-delta(omega-omega_c)],
m(t)cdot sin(omega_c t) quad stackrel{mathcal{F}}{Longleftrightarrow},   frac{1}{2pi}cdot {M(omega)} * {i pi cdot [delta(omega +omega_c)-delta(omega-omega_c)]},
= frac{i}{2}cdot [M(omega +omega_c) - M(omega -omega_c)],
Fig 3: The spectrogram of an AM broadcast shows its two sidebands (green) separated by the carrier signal (red).
Fig 3: The spectrogram of an AM broadcast shows its two sidebands (green) separated by the carrier signal (red).


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(omega).,  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. In mathematics, the Fourier transform is a certain linear operator that maps functions to other functions. ... Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... It has been suggested that this article or section be merged with periodogram. ... Baseband is an adjective that describes signals and systems whose range of frequencies is measured from 0 to a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies starting at 0. ...


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: The modulation index (mf) is defined as the ratio of the frequency deviation σ to the modulating signal frequency fi, i. ...

h = frac{mathrm{peak value of } m(t)}{A} = frac{M}{A},   where M, and A, 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 amplification. ...


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
Fig 4: Modulation depth

Image File history File links AM_signals. ...

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. While it is perfectly possible to create good designs using solid-state electronics, valved (tube) circuits are shown here. In general, valves are able to easily yield RF powers far in excess of what can be achieved using solid state. Most high-power broadcast stations still use valves. Figure 1:Three-phase pole-mounted step-down transformer. ... In electronics, a vacuum tube (American English) or (thermionic) valve (British English) is a device generally used to amplify a signal. ...

Anode modulation using a transformer. The tetrode is supplied with an anode supply (and screen grid supply) which is modulated via the transformer. The resistor R1 sets the grid bias, both the input and outputs are tuned LC circuits which are tapped into by inductive coupling
Anode modulation using a transformer. The tetrode is supplied with an anode supply (and screen grid supply) which is modulated via the transformer. 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. Image File history File links Ammodstage. ... Image File history File links Ammodstage. ... This article does not cite any references or sources. ... An LC circuit consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C. When connected together, an electrical current can alternate between them at an angular frequency of where L is the inductance in henries, and C is the capacitance in farads. ...


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 a disturbance of mechanical energy that propagates through matter as a wave. ... 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. Mission Cyrus 1 Hi Fi integrated audio amplifier An audio amplifier is an electronic amplifier that amplifies low-power audio signals (signals composed primarily of frequencies between 20 hertz to 20,000 hertz, the human range of hearing) to a level suitable for driving loudspeakers and is the final stage...

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. Hence Class C amplifiers cannot be employed. The efficiency of an entity (a device, component, or system) in electronics and electrical engineering is defined as useful power output divided by the total electrical power consumed (a fractional expression). ... The term amplifier as used in this article can mean either a circuit (or stage) using a single active device or a complete system such as a packaged audio hi-fi amplifier. ...


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. 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. It has been suggested that Peak Inverse Voltage be merged into this article or section. ... This article does not cite any references or sources. ... Look up radiophone, radiotelephone in Wiktionary, the free dictionary. ...


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. 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. Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. ...

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 is a way of interconnecting two circuits such that, in addition to transferring the signal (or information), the first stage also provides DC bias to the nextǂ. Thus, there is no need for a DC blocking capacitor to be used in order to interconnect the circuits... A cascode is an arrangement of electronic devices where the output of a common emitter (or cathode) circuit is coupled directly to the input of a common base (or grid) circuit (usually with no intervening components). ...


See also

Mediumwave radio transmissions (sometimes called Medium frequency or MF) are those between the frequencies of 300 kHz and 3000 kHz. ... Mediumwave radio transmissions serves as the most common band for broadcasting. ... In telecommunications, frequency modulation (FM) conveys information over a carrier wave by varying its frequency. ... This article is about an album. ... 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 analogue amplitude modulated broadcast signal in the same manner as the Radio Data System (RDS) for frequency modulated (FM) broadcast signals. ... In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing energy as a result of the modulation process. ... The International Telecommunication Union uses a special system for classifying radio frequency signals. ... This article is about the location. ... A Bendix/King KY197 Airband VHF communication radio mounted above a Cessna ARC RT-359A Transponder (the beige box) in a light airplane instrument panel. ...

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.

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