Nonlinear Characterisation for Microstrip Circuits with Low Passive Intermodulation

This paper presents a new methodology for experimental charaterisation of distributed nonlinearity in microstrip circuits fabricated on commercial grade microwave laminate materials. The circuits exhibit excessive intermodulation response when subject to relatively high power transmit multi-carrier waveforms. Weak intermodulation products generated in passive circuits may cause strong interference in the conventional FDD and emerging broadband and multi-radio communication systems. The proposed phenomenological model of distributed passive intermodulation in printed transmission lines extends the previous work by incorporating concurrent circuit nonlinearities and providing closed-form expressions for the intermodulation products of arbitrary order. An efficient characterization approach is devised with the aid of the commercial RF-CAD software. For the first time, it has been observed that the nonlinear sources in typical microstrip circuits may exhibit extremely uneven distribution which require a different means of characterisation and modelling. The proposed approach is verified by accurately predicting the intermodulation response in complex circuit layouts. The results of this work contribute to the development of a holistic approach to the design of planar microwave circuits under the linearity constraints.