The mixing of different frequency components of a signal in a nonlinear component or active device in a circuit, producing unwanted frequency components. If two frequencies f₁ and f₂ are applied to a nonlinear element, then a number of new frequency tones will be generated, as shown in Fig. a. These new frequency components are called intermodulation products, and are distortions of the original signals. The IM products are described in terms of their order: for example, 3rd order means all 3-fold combinations of the two mixing frequencies, i.e. 3f_x and 2f_x ± f_y. An nth-order nonlinearity will produce nth and lower-order products.

(a) Spectrum of intermodulation products up to 3rd order

(b) Determination of the nth-order intercept point

The nth-order intercept point represents a fictitious signal amplitude where the extrapolated values of the magnitudes of the desired input signal and the unwanted nth-order distortion components are equal. The nth-order intercept point is written IP_n and measured in terms of power. The definition of intercept point is illustrated in Fig. b. The distortion products increase by n dB in amplitude for every 1 dB increase in the linear signal. Both input and output intercept points are often quoted: the output intercept point equals the input intercept point plus the stage gain. Knowing the value of the nth-order intercept point and the input power level P_in, the (nth-order) intermodulation level can be found:

See also IP2; IP₃.