Tri-band isolation circuits based on double-Lorentz transmission lines for quadruplexers

This paper presents a novel tri-band isolation circuit based on a double-Lorentz (DL) transmission line (TL). Since a DL TL has three arbitrary phase constants at three target frequencies, the combination of a DL TL and a band-pass filter can have tri-band isolation characteristic. The quadruplexer, discussed in this paper, consists of four tri-band isolation circuits, and each circuit is isolated from an input at the center frequencies of filters of other channels. As a result, the combined four circuits can satisfy the required characteristics for a quadruplexer. This quadruplexer has two significant advantages. Since modifying filters is not required and there is no limitation to choose filters, commercial filters can be used for a quadruplexer. Also, the quadruplexer can be designed easily with a simple and straightforward design concept. Thus a complex numerical optimization is not necessary. For verifying the design concept, a quadruplexer, which has 942.5 MHz, 1575 MHz, 2140 MHz, and 2450 MHz center frequencies, is designed and fabricated. Filters are commercial Surface Mounted (SMT) type. The measured results show good agreement with the simulation results.