The design of a compact, wide spurious-free bandwidth bandpass filter using stepped impedance resonators

We propose an analytical design for a microstrip broadband spurious-suppression filter. The proposed design uses each section of the transmission line as both a coupling and a spurious suppression element. Every two sections of the transmission line are combined to create a quarter-wavelength stepped impedance resonator (λ/4 SIR). The optimal resonator length is proven to be the shortest, while delivering the broadest spurious-free bandwidth. The experimental result shows that the size of the 4th order bandpass filter with a stepped impedance ratio (R) of 0.2 is 0.33λ, long and 0.18λ wide, which is less than half of the size of the filter designed using the in-line coupling approach. The out-of-band attenuation of the developed filter is greater than 50 dB; and the first spurious mode is shifted to approximately 5.4 times the fundamental frequency.