Inductance-Loaded Y-Shaped Resonators and Their Applications to Filters

Inductance-loaded Y-shaped resonator and its applications to ultra-wideband (UWB) bandpass filters have been presented in this paper. Based on the odd- and even-mode equivalent circuits, the resonant characteristics of the proposed microstrip resonator with various electrical length and admittance ratio values are investigated. The relationship between the external quality factor and the parameters of the coupling structure has been analyzed to give a design approach for the UWB filter. Two types of UWB bandpass filters are designed to demonstrate the practicality of the proposed resonator. To get tight coupling, the high-impedance parallel-coupled line has been used in the input/output of the first UWB bandpass filter. Both simulated and measured results show that the first UWB filter has a 10-dB return loss bandwidth of 5.6 GHz, a group delay variation of less than 0.3 ns, and a wide upper stopband with more than 20-dB attenuation up to 17.5 GHz. The multilayer technology is applied to the second UWB bandpass filter to provide tight coupling by using the broadside-coupled structures. The measured results show that the multilayer UWB filter exhibits excellent performance, including 1-dB insertion loss and 10-dB return loss bandwidths of 3.3-10.4 GHz with a minimum insertion loss of 0.45 dB at 6.85 GHz, as well as 3-dB insertion loss bandwidth and group delay variation < 0.22 ns over the entire UWB band.