UMTS diplexer design using dual-mode ring resonators

In this letter, we propose a novel design of a UMTS diplexer using dual-mode rectangular ring-resonators. The diplexer is composed of two rectangular rings, which termed as the Tx- and Rx-ring, with each ring its input and output ports separated from each other by a quarter-wavelength to create the degenerate-mode resonance. The ratio of the long- and short-side dimension of rectangular ring can be used to fine tune the passband's bandwidth and the ratio is properly chosen to achieve sufficient bandwidth required by the UMTS diplexer applications. An enhanced parallel-coupled structure is introduced in the circuit to replace the conventional end-gap coupling for reducing the insertion losses in the passband. The effect of the coupling-line length is also simulated and the resulting data are used to precisely tune the positions of the zeros. In the zero assignment, one of the zeros possessed by each ring filter is managed to be implanted in the passband of its counterpart filter for increasing the isolation between the Rx-and Tx-port.