An Investigation on Passive Inter-Modulation generated by antenna contact

Abstract

In modern day handheld cellular devices, low-profile is a key design target. In addition, with the introduction of 4G protocols, the number of cellular bands has increased significantly and the simultaneous transmission of multiple bands is more common than ever before. These requirements lead to challenges for designers as the close proximity of passive devices (e.g., antenna) to active devices (e.g., radio) can lead to additional distortions in the signal. Due to the overlapping nature of many frequency bands, harmonics/intermodulation distortion can seriously degrade the receiver sensitivity of coexisting radios. One way in which this can occur is that the antenna metal-to-metal contact can generate a metal-insulator/semiconductor junction due to surface roughness. If the gap between the layers is small enough, it can generate a tunnel current through the gap. This tunnel current has a non-linear I-V characteristic. For this reason, it is important to investigate the non-linearity caused by passive sources in antenna structures. Non-linearity caused by passive sources can produce significant distortion for active devices with high transmit power or low receive sensitivity requirements. The aim of this research was to devise a measurement technique to study the unknown nonlinear behavior of radio frequency antenna metal-to-metal contacts. Issues related to the measurement of low-level distortion were investigated. As part of this research, a measurement setup was developed to measure low-level distortion. Measurements made showed that intermodulation caused by metal-to-metal contact in the antenna structure can degrade receiver sensitivity significantly. This thesis also discusses an approach to modelling nonlinear relationships, based on the measurements made with the new test setup.