Practical Mitigation of Passive Intermodulation in Microstrip Circuits

This paper presents new experimental evidence and a novel practical approach for mitigation of passive intermodulation (PIM) in microstrip circuits fabricated on commercial printed circuit board laminates. The mechanisms of distributed PIM in microstrip circuits are reviewed and a phenomenology of PIM generation due to locally enhanced electromagnetic fields at the fringed etched edges of microstrip transmission lines is presented. Reducing the magnitude of the local fields is shown to be an effective means for mitigating PIM distortions. The effect is demonstrated experimentally by selectively coating the edges of a microstrip transmission line with a thin layer of high-permittivity dielectric, which resulted in more than 10 dB reduction of the PIM level without compromising the linear characteristics of the circuit, while avoiding the need for additional signal processing. Our approach has proved to be a new practical and effective means for PIM mitigation suitable for a wide range of commercial printed circuits.