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PowerPedia:Amplitude distortion

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Amplitude distortion is distortion occurring in a system, subsystem, or device when the output amplitude is not a linear function of the input amplitude under specified conditions. Generally, output is a linear function of input only for a fixed portion of the transfer characteristics. In this region, I_c=ßI_b where Ic is collector current and Ib is base current, following linear relation $y=mx\,$ .

1 Distortion
2 Harmonic distortion
3 Intermodulation distortion
4 Related
5 References and external articles
6 See also

Distortion

A distortion is the alteration of the original shape (or other characteristic) of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted. In some fields, distortion is desirable, such as electric guitar (where distortion is often induced purposely with the amplifier to achieve the desired sound). The slight distortion of analog tapes and vacuum tubes is considered pleasing in certain situations. The addition of noise or other extraneous signals (hum, interference) is not considered to be distortion, though the effects of distortion are sometimes considered noise.

When output is not in this portion, two forms of amplitude distortion might arise

harmonic distortion
intermodulation distortion

Harmonic distortion

Harmonic distortion: The creation of harmonics of the fundamental frequency of a sine wave input to a system.

The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental.

In most cases, the transfer function of a system is linear and time-invariant. When a signal passes through a non-linear device, additional content is added at the harmonics of the original frequencies. THD is a measurement of the extent of that distortion.

The measurement is most commonly the ratio of the sum of the powers of all harmonic frequencies above the fundamental frequency to the power of the fundamental:

$\mbox{THD} = {\sum{\mbox{harmonic powers}} \over \mbox{fundamental frequency power}} = {{P_2 + P_3 + P_4 + \cdots + P_n} \over P_1}$

Other calculations for amplitudes, voltages, currents, and so forth are equivalent. For a voltage signal, for instance, the ratio of RMS voltages is equivalent to the power ratio:

$\mbox{THD} = {\sqrt{V_2^2 + V_3^2 + V_4^2 + \cdots + V_n^2} \over V_1}$

In this calculation, V_n means the RMS voltage of harmonic n.

Other definitions may be used. A measurement must specify how it was measured. Measurements for calculating the THD are made at the output of a device under specified conditions. The THD is usually expressed in percent as distortion factor or in dB as distortion attenuation. A meaningful measurement must include the number of harmonics included (and should include other information about the test conditions).

THD+N means total harmonic distortion plus noise. This measurement is much more common and more comparable between devices. This is usually measured by inputting a sine wave, notch filtering it, and measuring the ratio between the signal with and without the sine wave:

$\mbox{THD+N} = {\sum{\mbox{harmonic powers}} + \mbox{noise power} \over \mbox{total output power}}$

A meaningful measurement must include the bandwidth of measurement. This measurement includes effects from intermodulation distortion, interference, and so on, instead of just harmonic distortion.

Intermodulation distortion

Intermodulation distortion: This form of distortion occurs when two sine waves of frequencies X and Y are present at the input, resulting in the creation of several other frequency components, whose frequencies include (X+Y), (X-Y), (2X-Y), (2Y-X), and generally (mX ± nY) for integer m and n. Generally the size of the unwanted output falls rapidly as m and n increase.

Intermodulation distortion is nonlinear distortion characterized by the appearance, in the output of a device, of frequencies that are linear combinations of the fundamental frequencies and all harmonics present in the input signals.

Due to the additional outputs, this form of distortion is definitely unwanted in audio, radio and telecommunication amplifiers, and it occurs for more than two waves as well.

In a narrowband system such as a radio communication system, unwanted outputs such as X-Y and 2X+Y will be remote from the wanted band and so be ignored by the system. In contrast, 2X-Y and 2Y-X will be close to the wanted signals. These so-called third order distortion products (third order as m+n = 3) tend to dominante the non-linear distortion of narrowband systems.

Amplitude distortion is measured with the system operating under steady-state conditions with a sinusoidal input signal. When other frequencies are present, the term "amplitude" refers to that of the fundamental only.

Harmonic components themselves are not usually considered to characterize intermodulation distortion. When the harmonics are included as part of the distortion, a statement to that effect should be made. This is usually considered total harmonic distortion.

IMD in its most basic and most testable form shows up as presence of frequencies not in the input signal. If the sum of two pure tones is the input to the system, IMD shows up as the presence of new tones in the output whose frequencies are the sum and difference of the input tone frequencies.

In modern record production, it is a commonplace technique to exploit the intermodulation distortion characteristics produced by vacuum tube electronics and audio tape. For example; once a recording engineer has mixed the various tracks that make up a song into the stereo format, he may send the mix to a vacuum tube based stereo compressor and overload the vacuum tube electrical components. The resulting output will sound fuller and smoother due to the creation of second and third order harmonics.

This technique applies mostly to vacuum tube based equipment though some use electro-optical based compressors to similar effect. Solid-state or integrated-circuit based equipment is rarely used for this effect as its harmonic distortion character is not favorable.

A recording engineer may also record the mix to an audio tape format called reel to reel. In this technique, the engineer will increase the level at which the mix is recorded to audio tape far past the level recommended by the tape's manufacturer. This will result in a slight compressing of the dynamic (volume) range and the production of several second and third order harmonics.

RF technicians and audio engineers often experience problems with intermodulation distortion when setting up wireless equipment for live performances and events. Often times, wireless equipment for performer’s in-ear monitors or wireless microphones operate on similar frequencies to digital televisions signals, creating harmonic frequencies that interfere with other equipment. With security, technical crew, performance and other wireless signals in use at larger live sporting or concert events, it has become common for hundreds of individual frequencies operating in the same area. Audio engineers have to rely on complex software to calculate all of the possible overlapping and distorted frequencies when setting up such a large live event.