Compact Microstrip Lowpass Filter with Ultra-Wide Stopband using Stepped-Impedance Trapezoid Resonators

A new microstrip lowpass filter with compact size and ultra-wide stopband is presented. The resonance properties of a microstrip main transmission line parallel loaded with the stepped-impedance trapezoid resonator are studied. Analysis results reveal that a compact size and ultra-wide stopband lowpass filter can be realized by properly introducing multiple stepped-impedance trapezoid resonators in the design. A demonstration filter with 3 dB cutoff frequency at 0.8 GHz has been designed, fabricated, and measured. Results indicate that the proposed filter is able to suppress the 17th harmonic response by 15 dB, together with a small size of 0.057 lambda g (X)0.077 lambda g, where lambda g is the guided wavelength at 0.8 GHz.