A transmission-line bandpass filter based on bandstop filter is proposed in this paper. A key merit of the filter configuration is that the position of the transmission zeros can be conveniently controlled whereas the bandwidth is fixed. The bandpass filter consists of two half-wavelength resonators, two open circuited uniform impedance resonators and one stepped impedance resonator.
With five reflection zeros generated in the passband, six controllable transmission zeros are introduced to sharpen the passband skirts. Without coupling gaps between resonators, the new filter show low insertion loss and is easy for fabrication. To verify the design concept, one filter example has been designed, fabricated and measured. Good insertion/return losses, flat group delay, sharp transition and simple topology are achieved as demonstrated in both simulation and experiment.