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 signals at frequencies that are not, in general, at harmonic frequencies (integer multiples) of either.
Intermodulation is caused by nonlinear behaviour of the signal processing being used. The theoretical outcome of these nonlinearities can be calculated by conducting a Volterra series of the characteristic, while the usual approximation of those nonlinearities is obtained by conducting a Taylor series.
Intermodulation is rarely desirable in radio or audio processing, as it essentially creates spurious emissions, which can create minor to severe interference to other operations on the signal. Intermodulation should not be confused with general harmonic distortion (which does have widespread use in audio effects processing). Intermodulation specifically creates nonharmonic tones ("offkey" notes, in the audio case) due to unwanted mixing of closely spaced frequencies.
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A linear system cannot produce intermodulation. If the input of a linear timeinvariant system is a signal of a single frequency, then the output is a signal of the same frequency; only the amplitude and phase can differ from the input signal. However, nonlinear systems generate harmonics, meaning that if the input of a nonlinear system is a signal of a single frequency, then the output is a signal which includes a number of integer multiples of the input frequency; (i.e some of ).
Intermodulation occurs when the input to a nonlinear system is composed of two or more frequencies. Consider an input signal that contains three frequency components at, , and ; which may be expressed as
where the and are the amplitudes and phases of the three components, respectively.
We obtain our output signal, , by passing our input through a nonlinear function:
will contain the three frequencies of the input signal, , , and (which are known as the fundamental frequencies), as well as a number of linear combinations of the fundamental frequencies, each of the form
where , , and 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 nonlinear 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 , the output signal will contain a number of frequency components, each of which may be described by
where the coefficients are arbitrary integer values.
The order of a given intermodulation product is the sum of the absolute values of the coefficients,
For example, in our original example above, thirdorder intermodulation products (IMPs) occur where :
In many radio and audio applications, oddorder IMPs are of most interest, as they fall within the vicinity of the original frequency components, and may therefore interfere with the desired behaviour.
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 AM or FM radio transmission tower may hear two types of 'interference' / distortion:
As explained in a previous section, intermodulation can only occur in nonlinear systems. Nonlinear 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"). However, even passive components can perform in a nonlinear manner and cause intermodulation. Diodes are widely known for their passive nonlinear effects, but parasitic nonlinearity can arise in other components as well. For example, audio transformers exhibit nonlinear behavior near their saturation point, electrolytic capacitors can start to behave as rectifiers under largesignal conditions, and RF connectors and antennas can exhibit nonlinear characteristics. Even the air itself can behave in a nonlinear fashion, which can be exploited to produce audible sound from intermodulation of ultrasonic frequencies.
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 dissimilar metals or junctions of metals and oxides. These junctions effectively form diodes, which are nonlinear. The higher the signal amplitude, the more pronounced the effect of the nonlinearities, 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.
PIM can also occur in connectors, or when conductors made of two galvanically unmatched metals come in contact with each other. However, the most common source of passive intermodulation in connectors comes from the conduction of signal current through ferromagnetic metals such as nickel, which has a nonlinear magnetizationinductance hysteresis. This effect has been exploited to make reliable sources of PIM^{[1]}, which can be used to cancel unwanted PIM from a system^{[2]}.
Intermodulation is a very specific type of distortion, and should not be confused with harmonic distortion in general. The simplest way to recognize this is that a singlefrequency tone can be harmonically distorted (by clipping, for example); intermodulation requires at least two tonal frequencies, and only occurs when they mix according to a nonlinear relationship.
Audio engineers and producers will often intentionally add harmonic distortion to a recorded track to create a desired sound. This may be intended to replicate the spectral character of a particular amplifier (such as a vacuum tube amplifier). However, these techniques almost exclusively rely on harmonic distortion and gain compression. It is extremely unlikely that any audio application uses intermodulation distortion to improve the acoustic effects of recorded music, because intermodulating the high spectral content of a complex audio signal would result in a very garbled output. The product frequencies that occur specifically in intermodulation are sums or differences of desired frequencies (and not harmonic multiples). The result would mix atonal (offkey) notes into the music. Furthermore, unlike a fuzztone synthesizer or pedal, the spectrum is not flat white noise; instead, the resulting sound would be unpleasant.
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MILSTD188).
