Harmonic Margin
Intermodulation distortion arises when a nonlinear transfer function processes a multi-tone input signal, generating unwanted sum and difference frequencies that degrade signal fidelity in electronic circuits. Audio power amplifiers and radio frequency transmitters encounter this metric during benchtop verification against reference test tones. Two equal-amplitude sinusoidal waves applied to a non-linear stage produce spurious products at frequencies corresponding to the sum and difference of integer multiples of the primary frequencies.
Manufacturers establish compliance limits through two-tone testing, where third-order and fifth-order products near the fundamental frequencies reveal small-signal non-linearities. Thermal drift and power supply sag exacerbate these spurious outputs during high-amplitude operation.
Spurious Product
Third-order intercept points quantify the theoretical power level where the amplitude of intermodulation distortion products equals the fundamental output power. Engineers calculate this intercept by extrapolating measured fundamental power and third-order product power on a logarithmic scale, since distortion products increase at three times the rate of fundamental signals. Signal generators supply the input tones through low-distortion attenuators while spectrum analyzers measure the resulting sideband amplitudes at room temperature.
Calibration protocols demand that the measurement equipment introduce at least twenty decibels less distortion than the device under test to prevent systematic error.
Frequency Translation
Nonlinear device characteristics force active components to multiply instantaneous input voltages, creating new spectral components absent from the original waveform. Transistors operating near saturation regions exhibit compression curves that bend the transfer function away from ideal linearity. Output filters remove harmonic components outside the band of interest, yet in-band intermodulation products remain unaltered because their frequencies fall directly within the passband.
Bandwidth Consequence
Communication systems suffer spectral regrowth when intermodulation distortion products spill into adjacent channels, violating regulatory emission masks and interfering with neighboring receivers. System designers mitigate these effects by applying feedback loops or digital predistortion algorithms to linearize the transfer function before signal amplification occurs.
