intermodulation

thumb|upright=1.4|A frequency spectrum plot showing intermodulation between two injected signals at 270 and 275 MHz (the large spikes). Visible intermodulation products are seen as small spurs at 280 MHz and 265 MHz. right|3rd order intermodulation products (D3 and D4) are the result of nonlinear behavior of an amplifier. The input power level into the amplifier is increased by 1 dB in each successive frame. The output power of the two carriers (M1 and M2) increases by about 1 dB in each frame, while the 3rd order intermodulation products (D3 and D4) grow by 3 dB in each frame. Higher-order intermodulation products (5th order, 7th order, 9th order) are visible at very high input power levels as the amplifier is driven past saturation. Near saturation, each additional dB of input power results in proportionally less output power going into the amplified carriers and proportionally more output power going into the unwanted intermodulation products. At and above saturation, additional input power results in a decrease in output power, with most of that additional input power getting dissipated as heat and increasing the level of the non-linear intermodulation products with respect to the two carriers.

Intermodulation (IM) or intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies, caused by nonlinearities or time variance in a system. The intermodulation between frequency components will form additional components at frequencies that are not just at harmonic frequencies (integer multiples) of either, like harmonic distortion, but also at the sum and difference frequencies of the original frequencies and at sums and differences of multiples of those frequencies.