Design of microstrip bandpass filters with suppression of spurious harmonics

Coupled corrugated microstrip lines, suspended coupled microstrip lines, over-coupled end stages and stepped impedance resonators (SIR's) are used to construct bandpass filters that suppress spurious harmonics. These designs can greatly enhance the attenuation levels in stopband, extend the upper stopband width to at least two times, and improve the passband symmetry of a microstrip bandpass filter. The first two structures are designed to equalize the phase velocities of the eigen-modes in the longitudinal direction, so that filters of medium and wide bandwidths can be implemented with effective suppression of spurious harmonics at twice the center frequency (2f_o). Over-coupled stages are used to effectively compensate for the phase difference resulting from the un-equal even- and odd-mode phase velocities of coupled stages in a parallel-coupled line filter. SIR filters are found to have a wide stopband -up to more than eight times the passband frequency. Both simulation and measured results are presented for each structure.