Sources
Help build the largest human-edited directory on the web.
Submit a site - Become an editor
Wikipedia. Licensed under the GNU Free Documentation License.
Are you an expert in this subject? Join the discussion and share your knowledge at Wikipedia.org.
Intermodulation
From 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 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 if the intent is to create specific sound effects; for instance, intermodulation is the basis of the power chord technique in rock music.
Contents |
A linear system cannot produce intermodulation. If the input of a linear time-invariant 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, non-linear systems generate harmonics, meaning that if the input of a non-linear 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 non-linear 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 non-linear 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 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 
, 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, third-order intermodulation products (IMPs) occur where 
:
In many radio and audio applications, odd-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.
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:
- '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.
As explained in a previous section, 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 galvanically unmatched metals come in contact with each other.
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.
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.