Tunnel conduction consequences in high frequency microcontacts; passive intermodulation effect

The tunneling conduction effect across the electrical contacts of coaxial microwave electromechanical switches can be a major source of non-linearity in the current-voltage characteristics. This is similar to the case of coaxial cables, where the nonlinearity is attributed to the contact resistance at the RF connectors´ interface. The current non-linearity occurs as an effect of the tunnel conduction, usually type II as classified by R. Holm, and it can be described both analytically and experimentally. The paper uses a quasi-experimental method to assess the non-linearity in the current-voltage characteristic of the switch microcontacts and, based on this, predicts the expected passive intermodulation levels for the switch. The interference phenomenon called passive intermodulation (PIM), in high power, multichannel systems, presents an important challenge in communication satellites as well as in the wireless industry. The non-linearities generate harmonics of the fundamental frequency, which ´mix´ together to generate a predictable set of new frequencies that may interfere with the receive portion of the communication system. Historically, a number of models have been developed for coaxial cables, which assume the intermodulation sources are located in the RF connectors. The paper proposes a model for the intermodulation characteristics of the switch path that is based on a previously developed contact resistance model. A 5th order source point model is used because of the potential for a significant power content in the 3rd through 5th order equations, should these frequencies fall within the receive band. The results are compared with measured data for some typical RF coaxial electro-mechanical switches used in space applications; the measurement techniques are also discussed.