A model of intermodulation distortion in non-linear multicarrier systems

A heuristic model is proposed which allows calculation of the individual spectral components of the intermodulation distortion present in a non-linear system with a multicarrier input. Noting that any given intermodulation product (IMP) can only be created by a subset of the input carriers, we partition them into “signal” carriers (which create the IMP) and “noise” carriers, modeled as a Gaussian process. The relationship between an input signal and the statistical average of its output (averaged over the Gaussian noise) is considered to be an effective transfer function. By summing all possible combinations of signal carriers which create power at the IMP frequencies, the distortion power can be calculated exactly as a function of frequency. An analysis of clipping in lightwave CATV links for AM-VSB signals is used to introduce the model, and is compared to a series of experiments. Excellent agreement is obtained over the full 50 dB experimental range of distortion levels measured for CSO, CTB, and CNLD (carrier to total non-linear distortion ratio). The model is easily generalized to other types of non-linearities: limiters and the suppression of diode imperfections are briefly discussed