Meaning
Nonlinear interference occurring when two or more signals interact in a system to produce new frequencies that fall within the desired bandwidth. Measuring third order intermodulation distortion identifies the linearity of amplifiers and mixers.
Signal Interaction
Sums and differences of the fundamental frequencies and their harmonics create the distortion products. The third order intermodulation distortion is particularly problematic because the resulting frequencies are located close to the original signals. Filtering cannot easily remove these products without also affecting the primary data stream.
System Linearity
Power levels of the interference products increase at a rate of three decibels for every one decibel increase in the input signal. The third order intermodulation distortion is often characterized by the third-order intercept point, a theoretical value where the distortion power would equal the fundamental power. High intercept points indicate a more linear system capable of handling multiple signals without crosstalk.
Measurement Technique
Two-tone testing is the standard method for quantifying this effect in the laboratory. The third order intermodulation distortion is observed on a spectrum analyzer as sidebands appearing around the test frequencies. Reducing the input power or using more linear components are the primary ways to mitigate the impact of these distortion products on system performance.
