Compact microstrip bandpass filters with quarter-wavelength stepped-impedance resonators

Compact microstrip bandpass filters utilizing quarter-wavelength (lambda/4) stepped-impedance resonators (SIRs) are proposed. The lambda/4 SIR has the abilities to reduce the filter size and to push up the filter spurious frequency. By arranging lambda/4 SIRs properly in the filter layout, two compact 4th-order microstrip bandpass filters with extended rejection band and good selectivity are designed and implemented. Specifically, a miniature lambda/4-SIR 6th-order cross-coupled microstrip bandpass filter with a size of only 0.3 lambda x 0.17 lambda is realized to possess a better rejection up to 3.6f(0), where f(0) denotes the passband center frequency.