Analysis of Weakly Nonlinear Effect for Varactor-Tuned Bandpass Filter

摘要

This paper presents a general analytical approach to accurately predict the nonlinear distortion of varactor-tuned bandpass filters (BPFs) with arbitrary topology. The linearized circuit and its admittance matrix are derived to study the nonlinear behavior by means of the Volterra-series representation. The effects on third-order intermodulation distortion are characterized for the principal filter parameters such as tunable resonator configuration, susceptance slope, fractional bandwidth, unloaded quality factor, and filter topology. The theoretical analysis indicates that -type varactor-tuned BPFs possess a better linearity property due to its large susceptance slope and the fractional bandwidth also plays a dominant role in the generation of nonlinearities. This analysis also reveals that the nonlinear distortion can be further reduced by source–load coupling for the first time. The analytical results are fully coincident with the harmonic-balance simulations using computer-aided design varactor model. A two-pole 1.0–1.3-GHz tunable BPF with a constant bandwidth of 105 MHz is fabricated and measured to validate the theory.