A study of the passive intermodulation induced by nonlinear characteristics of RF connectors

Radio frequency (RF) connectors may exhibit nonlinear characteristics in the higher levels of transmission power, which causes passive intermodulation (PIM) to seriously degrade the performance of the communication system, even when the connectors are pristine. Accordingly, it is important to investigate the PIM induced by nonlinear effects of RF connectors. In the present work, an equivalent circuit model for a contact surface of coaxial connector was analyzed and a nonlinear physical mechanism was investigated to explain the sources of the nonlinearities. Simulations were developed and evaluated to demonstrate that surface parameters such as roughness, contact pressure and film thickness affects the change of the PIM. Moreover, a traditional nonlinear current-voltage (I-V) model was developed to describe the characteristics of third-order harmonics with input power and power ratios. Based on the two-tone testing procedure, a series of experiments were also conducted to evaluate third-order IM products. The power dependence and the effect of two input power ratios were studied experimentally and the influence of connector degradation on nonlinear effects was also investigated. A prediction of third-order IM powers by the nonlinear mathematical I-V models was verified, with the testing results showing good consistency with the behavior predicted by the models.