Intermodulation
Unwanted frequency components from nonlinear mixing of multiple signals.
Intermodulation, also called intermodulation distortion (IMD), occurs when a system’s nonlinearities or time-varying behavior amplitude-modulate a signal that contains two or more distinct frequencies. Unlike harmonic distortion, which only produces new frequencies at integer multiples of a single input frequency, intermodulation creates additional components at the sum and difference of the original frequencies, as well as at sums and differences of multiples of those frequencies. This nonlinear behavior can arise in physical equipment or in algorithms, and its theoretical effects can be described using a Volterra series or, more approximately, a Taylor series.
Nearly all audio equipment has some degree of nonlinearity, so some IMD is always present, though it may be too low for humans to notice. Due to how the human auditory system works, the same percentage of IMD is generally perceived as more annoying than an equivalent amount of harmonic distortion. In radio, intermodulation is usually undesirable because it generates spurious emissions—often as sidebands—that widen the occupied bandwidth. This can cause adjacent channel interference, reducing audio clarity or increasing spectrum congestion.
IMD and harmonic distortion are not fundamentally different; they are produced by the same nonlinear system. Harmonic distortion results from a single sine wave input, while IMD arises from more complex tones. In music, however, IMD is intentionally applied to electric guitars using overdriven amplifiers or effects pedals to create new tones at subharmonics of the notes being played. IMD also differs from intentional modulation, such as in a superheterodyne receiver’s frequency mixer, where signals are deliberately fed into a nonlinear element (like a mixer diode or single-transistor oscillator-mixer circuit) to multiply them. Even so, superheterodyne mixers can simultaneously produce unwanted intermodulation from strong nearby signals that fall within the receiver’s passband.
A linear time-invariant system cannot produce intermodulation: if its input is a single frequency, the output is the same frequency, differing only in amplitude and phase. Nonlinear systems generate harmonics from a sinusoidal input, meaning a single-frequency input yields output at integer multiples of that frequency.
- Order definition
- The order of a given intermodulation product is the sum of the absolute values of the coefficients.
- Third order example
- Third-order intermodulation products (IMPs) occur where the sum of the absolute values of the coefficients equals 3.
- Imd3 frequencies
- For two sine waves at frequencies f1 and f2, third-order IMD produces frequencies at 2f1 – f2 and 2f2 – f1.
Lore & Background
Intermodulation is distinct from harmonic distortion only in that the stimulus signal is different; the same nonlinear system will produce both total harmonic distortion (with a solitary sine wave input) and IMD (with more complex tones). In radio, intermodulation is usually undesirable as it creates unwanted spurious emissions, often in the form of sidebands, increasing occupied bandwidth and leading to adjacent channel interference. In audio, practically all equipment has some non-linearity, so it will exhibit some amount of IMD, which may be low enough to be imperceptible by humans; due to the characteristics of the human auditory system, the same percentage of IMD is perceived as more bothersome compared to the same amount of harmonic distortion. In music, IMD is intentionally applied to electric guitars using overdriven amplifiers or effects pedals to produce new tones at subharmonics of the tones being played.
Reader's Guide
Intermodulation is a fundamental consequence of nonlinearity in electronic systems, with significant implications for both radio and audio. In radio communications, intermodulation products from strong nearby signals can fall within the passband of a receiver, causing interference that cannot be filtered out. The third-order products are of particular concern because they lie close to the original frequencies. Passive intermodulation (PIM) occurs in passive devices such as cables, antennas, and connectors when subjected to two or more high power tones, arising from nonlinearities like junctions of dissimilar metals or metal-oxide junctions. PIM is a major concern in modern communication systems where a single antenna is used for both high-power transmission and low-power reception, as the PIM signal can be on the same order of magnitude as the receive signal and cannot be separated from it. Ferromagnetic materials, including ferrites, nickel, and steels, are common sources of PIM due to hysteresis. The legacy of intermodulation understanding is that it drives design requirements for linearity in amplifiers, mixers, and passive components, and informs the specification of third-order intercept points and other linearity metrics.
Did You Know?
- Third-order intermodulation products grow by 3 dB for every 1 dB increase in input power, while the carriers grow by about 1 dB.
- Passive intermodulation can occur between different frequencies within a single broadband carrier, showing up as sidebands.
Frequently Asked Questions
What is Intermodulation in radio electronics?
Intermodulation, often shortened to IMD, is a phenomenon in which a system's nonlinear or time-varying behavior mixes two or more input frequencies to generate unwanted new components at their sums and differences. It is fundamentally different from simple harmonic distortion, which only yields integer multiples of a single input tone.
How do you calculate the order of an intermodulation product?
The order is simply the sum of the absolute values of the integer coefficients that describe how the original frequencies are combined. For example, a product described by 2f1 – f2 has an order of 2 + 1 = 3, making it a third-order intermodulation product.
What frequencies does third-order IMD produce for two input tones?
Given two sine waves at f1 and f2, third-order intermodulation generates spurious components at 2f1 – f2 and 2f2 – f1. These fall close to the original signals, which is what makes them especially troublesome for receiver selectivity.
Where does Intermodulation originate—hardware or software?
IMD can arise in physical components such as amplifiers and mixers whose transfer functions are inherently nonlinear, and it can also appear in purely algorithmic signal-processing chains. In both cases the root cause is any deviation from a strictly linear, time-invariant response.
Why is Intermodulation a bigger concern than harmonic distortion in multi-signal environments?
Because IMD products land at sums and differences of the original tones rather than at integer multiples, they can fall directly on or near other active channels. This means a receiver handling several simultaneous signals will see IMD products masquerading as legitimate in-band signals, whereas harmonic distortion is easier to filter out.
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