Tri-band isolation circuits using both stop-band and pass-band of double-Lorentz transmission lines for quadruplexers

In this paper, novel method to use stop-band of a double-Lorentz transmission line (DL TL) for an isolation circuit is proposed. Combining this concept with conventional usage (operating only in the pass-band) of a DL TL, design flexibility of a DL TL for an isolation circuit can be increased and an engineer can have more choices for designing a DL TL. Also, a designer could get a robust DL TL for an isolation circuit avoiding use of vicinity of stop-band but actively entering into the stop-band, because phase change of a DL TL is fast in the vicinity of stop-band and it could bring difficulty of designing and fabricating a DL TL. The quadruplexer using the proposed method is designed and fabricated. It has still the advantages, i.e. no modifying filters and straightforward design process.