Also known as: IMD, intermod, IM products
Intermodulation distortion (IMD) is the generation of unwanted new frequencies when two or more signals pass together through a device that is not perfectly linear.1 Unlike harmonics, which are integer multiples of a single tone, intermod products are sums and differences of multiples of two or more input tones. The most troublesome are the third-order products at 2f₁ − f₂ and 2f₂ − f₁, because they land right next to the original signals and so cannot be filtered away. IMD is the fundamental reason a receiver can only handle a limited range of signal strengths at once.
How it works
A perfectly linear device only scales and delays its input, so its output contains no new frequencies. Real amplifiers and mixers have a slightly curved transfer characteristic that can be expanded as a polynomial: Vout = a₁V + a₂V² + a₃V³ + …. Feed in two tones at f₁ and f₂ and each nonlinear term multiplies them together, producing sums and differences of their harmonics.
The order of a product is the sum of the multiplier coefficients. Second-order products (f₁ + f₂, f₁ − f₂) usually fall far from the originals and are easy to filter. The dangerous ones are the third-order products from the V³ term at 2f₁ − f₂ and 2f₂ − f₁: if f₁ and f₂ are close, these spurs sit just outside the pair, squarely inside the receiver’s passband, masquerading as real signals. A defining behaviour follows from the cubic term: a third-order product grows three decibels for every one decibel that the input tones rise. This 3:1 slope is what makes intermod explode as signal levels climb, and it is the basis of the third-order intercept point (IP3), the extrapolated level where the third-order product would notionally equal the wanted signal — a single figure of merit for a stage’s linearity.
In practice
Intermodulation is a symptom of driving a stage too hard, so it is tightly linked to the 1 dB compression point: both describe the onset of nonlinearity, and IP3 typically sits about 10–15 dB above the 1 dB compression point. Higher IP3 means a device tolerates stronger signals before intermod becomes objectionable. The usable window between the noise floor and the level where third-order products emerge is the spurious-free dynamic range.
IMD is also a real-world nuisance beyond a single receiver. Nearby transmitters can mix in any shared nonlinearity — a corroded connector, a rusty tower joint, even a diode-like oxide layer (the “rusty bolt” effect) — producing intermod that radiates and interferes on frequencies where nothing is actually transmitting. Passive intermodulation at antenna sites is a recurring source of hard-to-trace interference.
Relevance to SDR
An SDR’s front end is a chain of nonlinear stages — low-noise amplifier, mixer, and ADC — each with finite linearity. In a busy RF environment, strong out-of-band signals (pagers, broadcast FM, cellular) can drive these into producing third-order products that appear as phantom carriers across the tuned band, or that desensitize the receiver by raising its effective noise. Wideband direct-sampling SDRs are especially exposed because they present the whole spectrum to the ADC at once, so front-end filtering and attenuation are the usual defences. IP3 and the 1 dB compression point are the datasheet numbers that predict how a given SDR will cope.
GopherTrunk operates on the IQ samples after this front end, so intermod that has already occurred in the analog chain is baked into the samples it decodes — the software cannot remove a spur that looks like a legitimate signal. The project’s own DSP notes reinforce this: symptoms that appear only at higher capture rates and reproduce in offline replay point at front-end overload or intermod in the captured data, not at GopherTrunk’s steady-state processing. Recognising intermod is therefore a diagnostic skill for GopherTrunk operators: phantom control channels or elevated noise that vanish when an attenuator is added are the classic signature.
Sources
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Intermodulation — Wikipedia, the nonlinear-mixing definition, product orders, and the 2f1−f2 / 2f2−f1 third-order products. ↩