A suspended substrate stripline lowpass filter with a very broad stopband

Broadband filters and multiplexers are constantly required in wideband receivers which often cover the whole microwave band. Suspended substrate stripline (SSS) is a preferred medium for realising these filters or multiplexers because of its high Q (typically 600), and manufacturing repeatability through standard photolithographic procedures. Several problems exist in the design of lowpass filters with low frequency cut-offs (500 MHz-3 GHz). The technique proposed here circumvents these problems by using a new SSS lowpass filter prototype, combining high Q lumped capacitors with a distributed SSS structure to achieve low bandedge loss, high selectivity and broad stopbands. The resulting filter combines the best of both techniques: it uses the lumped capacitors to achieve low loss and small size, and the distributed network to obtain the broad stopbands