If two or more signals of high levels are present together with the useful signal at the antenna input, their interaction always causes additional synthetic signals that were not present in the signal spectrum before. This is called intermodulation (IM). According to the origin, one distinguishes between signals resulting from addition or subtraction of the interfering carrier frequencies and signals grouped immediately above or below the interfering carriers.
All of these signals are called intermodulation products. They occur in non-ideal receivers and can have a negative effect on the SIR of signals at the receive frequency, depending on the frequency constellation .
Limitations or non-linear amplification cause such effects. This is the case if signals present in the existing system exceed the linear range of the transfer characteristic for the system (Fig. III.56). If the gain characteristic of an active component is known, its properties with single-tone and multi-tone control and the resulting distortion products cannot only be measured, but calculated as well. This includes the compression point (Section III.8.1), intermodulation products, and the origin of cross-modulation (Section III.10).
Based on the process of its formation, the relation between these interferences and their interaction can be determined mathematically. This will be rigorously explained with mathematical formulas in the Section V.3, while ...