To reveal the physical mechanism and establish the prediction model of the passive intermodulation (PIM) in the microstrip circuits

we deduce a closed form PIM model based on the dielectric nonlinearity through the dependent-source equivalence

and demonstrate that the dielectric nonlinearity is one of the major causes of PIM in microstrip circuits comparing the PIM phenomenon in PolyTetraFluoroEthylene (PTFE) fiberglass dielectric and air dielectric microstip lines. The nonlinear dielectric constant was derived as well. The experiments show that the PIM levels of the PTFE fiberglass dielectric microstirp line are about 20dB and 15dB larger than that of the air dielectric microstirp line for forward PIM and reverse PIM

respectively

which indicates that the dielectric nonlinearity is the main cause of PIM in the PTFE fiberglass based microstrip line. The third order nonlinear relative dielectric constant is deduced by the analytical PIM model. The model deduced in this paper can also be extended to study the PIM phenomenon in other microstrip circuits.