Three-Tone Intermodulation Distortion Generated by Superconducting   Bandpass Filters
  Microwave bandpass filters constructed from materials exhibiting some nonlinearity, such as superconductors, will generate intermodulation distortion (IMD) when subjected to signals at more than one frequency. In commercial applications of superconductive receive filters, it is possible for IMD to be generated when a weak receive signal mixes with very strong out-of-band signals, such as those coming from the transmitter. A measurement procedure was developed and data were taken on several different types of superconducting bandpass filters, all developed for commercial application. It was found that in certain interference situations, the 3-tone mixing can produce a spur that is noticeable by the receiver, but that there are simple preventative design solutions. 
Stephen K. Remillard, H.R. Yi, and Amr Abdelmonem