Amplitude modulation
Encyclopedia : A : AM : AMP : Amplitude modulation
Amplitude modulation (AM) is a form of modulation in which the amplitude of a carrier wave is varied in direct proportion to that of a modulating signal. (Contrast this with frequency modulation, in which the frequency of the carrier is varied; and phase modulation, in which the phase is varied.)
AM is commonly used at radio frequencies and was the first method used to broadcast commercial radio. The term "AM" is sometimes used generically to refer to the AM broadcast (mediumwave) band (see AM radio).
Forms of AM
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. Thus, most of the power output by an AM transmitter is effectively wasted: half the power is concentrated at the carrier frequency, which carries no useful information (beyond the fact that a signal is present); the remaining power is split between two identical sidebands, only one of which is needed.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. 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.
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.
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.
In 1982, the International Telecommunications Union (ITU) designated the various types of amplitude modulation as follows:
| 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
Suppose we wish to modulate a simple sine wave on a carrier wave. The equation for the carrier wave of frequency [\omega_c], taking its phase to be a reference phase of zero, is
- [c(t) = C \sin(\omega_c t)].
- [m(t) = M \sin(\omega_m t + \phi)],
Amplitude modulation is performed simply by adding [m(t)] to [C]. The amplitude-modulated signal is then
- [y(t) = (C + M \sin(\omega_m t + \phi)) \sin(\omega_c t)]
- [y(t) = C \sin(\omega_c t) + M \frac - M \frac]
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:- [h = \frac m(t)}].
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.
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.
Modulation circuit designs can be broadly divided into low and high level.
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.
- Advantages
- Disadvantages
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.
High level
- Advantages
- Disadvantages
See also
- AM radio also referred to as Mediumwave
- shortwave radio almost universally uses AM modulation, narrow FM occurring above 25 MHz.
- 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.
- Sideband, for some explanation of what this is.
- Types of radio emissions, for the emission types designated by the ITU
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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