Analysis of Weakly Nonlinear Effect for Varactor-Tuned Bandpass Filter

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 λ/2-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.