Intermodulation Distortion

Intermodulation - Wikipedia, the free encyclopedia
Intermodulation should not be confused with general harmonic distortion (which ... Software that calculates possible intermodulation distortion at large live sound ...
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intermodulation: Definition from Answers.com
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Intermodulation Distortion (IMD) Measurements Application Note
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Intermodulation or intermodulation distortion (IMD), or intermod for short, is the result of two or more Signal (electrical engineering) of different frequencies being Frequency mixer together, forming additional signals at frequencies that are not, in general, at harmonic frequencies (integer multiples) of either.

Intermodulation is caused by non-linear behaviour of the signal processing being used. The theoretical outcome of these non-linearities can be calculated by conducting a Volterra series of the characteristic, while the usual approximation of those non-linearities is obtained by conducting a Taylor series.

Intermodulation is rarely desirable in radio, as it essentially creates spurious emissions, which can create minor to severe interference to other operations on the resulting frequency. Intermodulation may be desirable in Audio frequency if the intent is to create specific sound effects; for instance, intermodulation is the basis of the power chord technique in rock music.

Causes of intermodulation A linear system cannot produce intermodulation. If the input of a LTI system theory system is a signal of a single frequency, then the output is a signal of the same frequency; only the amplitude and Phase (waves) can differ from the input signal. However, non-linear systems generate harmonics, meaning that if the input of a non-linear system is a signal of a single frequency, ~f_a, then the output is a signal which includes a number of integer multiples of the input frequency; (i.e some of ~ f_a, 2f_a, 3f_a, 4f_a, \ldots).

Intermodulation occurs when the input to a non-linear system is composed of two or more frequencies. Consider, an input signal that contains three frequency components at~f_a, ~ f_b, and ~f_c; which may be expressed as

\ x(t) = M_a \sin(2 \pi f_a t + \phi_a) + M_b \sin(2 \pi f_b t + \phi_b) + M_c \sin(2 \pi f_c t + \phi_c)

where the \ M and \ \phi are the amplitudes and phases of the three components, respectively.

We obtain our output signal, \ y(t), by passing our input through a non-linear function: \ y(t) = G\left(x(t)\right)\,

\ y(t) will contain the three frequencies of the input signal, ~f_a, ~ f_b, and ~f_c (which are known as the fundamental frequencies), as well as a number of linear combination of the fundamental frequencies, each of the form

\ k_af_a + k_bf_b + k_cf_c

where ~k_a, ~ k_b, and ~k_c are arbitrary integers which can assume positive or negative values. These are the intermodulation products (or IMPs).

In general, each of these frequency components will have a different amplitude and phase, which depends on the specific non-linear function being used, and also on the amplitudes and phases of the original input components.

More generally, given an input signal containing an arbitrary number N of frequency components f_a, f_b, \ldots, f_N, the output signal will contain a number of frequency components, each of which may be described by

k_a f_a + k_b f_b + \cdots + k_N f_N,\,

where the coefficients k_a, k_b, \ldots, k_N are arbitrary integer values.

Intermodulation order

The order \ O of a given intermodulation product is the sum of the absolute values of the coefficients,

\ O = \left|k_a\right| + \left|k_b\right| + \cdots + \left|k_N\right|,

For example, in our original example above, third-order intermodulation products (IMPs) occur where \ |k_a|+|k_b|+|k_c| = 3:

\ (f_a + f_b - f_c), (f_a + f_c - f_b), (f_b + f_c - f_a) \ (2f_a - f_b), (2f_a - f_c), (2f_b - f_a), (2f_b - f_c), (2f_c - f_a), (2f_c - f_b)

In many radio and audio applications, low-order IMPs of are most interest, as they fall within the vicinity of the original frequency components, and may therefore interfere with the desired behaviour.

Intermodulation noise In a transmission path or device, intermodulation noise is noise, generated during modulation and demodulation, that results from nonlinear characteristics in the path or device. Intermodulation noise occurs when the frequency sum or difference of a particular signal, S1, interferes with the component frequency sum or difference of another signal, S2.

Someone listening to a car radio while driving close by an amplitude modulation or FM radio transmission tower may hear two types of 'interference' / distortion:

'break-through', where the transmission from the near station overwhelms the car radio; and
intermodulation, where another station entirely is heard.

On musical instruments, it is the beat frequency produced when two other notes are produced.

==Passive intermodulation==As explained in intermodulation#Intermodulation and non-linear systems, intermodulation can only occur in non-linear systems. Non-linear systems are generally composed of active components, meaning that the components must be biased with an external power source which is not the input signal (i.e. the active components must be "turned on").

Passive intermodulation (PIM) occurs in passive systems (i.e. the input signal is the only source of energy to the system) when the input signal is very high power, and the system consists of junctions of dis-similar metals or junctions of metals and oxides. These junctions effectively form diodes, which are non-linear. The higher the signal amplitude, the more pronounced the effect of the non-linearities, and the more prominent the intermodulation may occur, even though upon initial inspection, the system would appear to be linear and unable to generate intermodulations.

PIMs can also occur in connectors, or when conductors made of two Galvanization unmatched metals come in contact with each other.

Intermodulation in audio applications All electronic audio recording and production systems use some form of signal processing. The human ear can often discern certain forms of intermodulation distortion, so consideration of these issues is particularly relevant in the design of such systems.

Use in music production 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.

See also

Rusty bolt effect
Beat (acoustics)
Audio system measurements

External links

A program for the calculation of intermodulation products and for the search for free frequencies with wireless radio microphones.
Software that calculates possible intermodulation distortion at large live sound events.
Good explanation of Intermodulation

intermodulation distortion from FOLDOC
intermodulation distortion < electronics, communications > (IMD) Nonlinear distortion in a system or transducer, characterised by the appearance in the output of frequencies equal ...

Intermodulation - Wikipedia, the free encyclopedia
Intermodulation or intermodulation distortion (IMD), or intermod for short, is the result of two or more signals of different frequencies being mixed together, forming additional ...

Intermodulation distortion in in UHF transistors - ECS EPrints ...
Stewart, R. D. and Kasim, T. A. (1979) Intermodulation distortion in in UHF transistors. Circuit Theory and Applications, 27 . Full text not available from this repository.

intermodulation distortion definition of intermodulation distortion in ...
electronics, communications) intermodulation distortion - (IMD) Nonlinear distortion in a system or transducer, characterised by the appearance in the output of frequencies equal ...

intermodulation distortion
The Free Online Dictionary of Computing (http://foldoc.doc.ic.ac.uk/) is edited by Denis Howe < dbh@doc.ic.ac.uk >. Previous: Intermetrics, Inc. Next: internal field separators

intermodulation distortion definition |Dictionary.com
Free On-line Dictionary of Computing - Cite This Source - Share This. intermodulation distortion electronics, communications (IMD) Nonlinear distortion in a system or transducer ...

Third-Order Intermodulation Distortion - What does IMD3 stand for ...
Acronym Definition; IMD3: Third-Order Intermodulation Distortion

Intermodulation Distortion (IMD) Measurements Application Note
2 OVERVIEW Intermodulation distortion (IMD) has become increasingly important in microwave and RF amplifier design. As modulation techniques become more sophisticated, greater ...

Intermodulation Distortion (IMD)
2 Introduction In this application note methods to successfully measure two-tone intermodulation distortion products using the IMD option on the MS462xx Vector Network Measurement ...

Definition: intermodulation distortion
intermodulation distortion: Nonlinear distortion characterized by the appearance, in the output of a device, of frequencies that are linear combinations of the fundamental ...