Multi-bandpass filters using external resonators

The increasing demand of wireless communication applications necessitates radio frequency (RF) transceivers operating in multiple separated frequency bands such that users can access various services with a single multimode handset. In this letter, a novel compact structure is proposed that dual bandpass filter and multi-bandpass filter with adjustable second passband can be obtained by using external resonators. By properly adjusting the multi-reject bands, appropriate multi transmission bands are created. The designed filters have compact size and good stopband performance.