Response Diversity of Stub-Loaded Ring Bandpass Filter Based on Commensurate Line Element: Single- and Dual-Band Applications

Response diversity of a commensurate line bandpass filter is described and discussed. It is found that a stub-loaded ring topology can generate different bandpass responses in a single band or two symmetric bands, depending on the line admittances in the topology. For a single band, the response can be designed to be an equiripple delay, Chebyshev in magnitude, or elliptic in magnitude. For dual bands, the out-band attenuation curve can be adjusted with almost unchanged in-band matching. A generalized synthesis algorithm, supporting both the single- and dual- band applications, is given to calculate the admittance of each line element, according to the prescribed in-band specifications, e.g., ripple factor and cut-off frequencies. Further study reveals that the admittance of the line segment between the stubs is the key element dominating the response type in the single-band case. When the admittance is positive, zero, and negative, the response is an equiripple delay, Chebyshev in magnitude, and elliptic in magnitude, respectively. To validate the synthesis algorithm, four filter sets with different responses are given and simulated through a transmission line model. For circuit verification, a microstrip filter with a single-band elliptic response is designed, fabricated, and measured, of which the negative admittance line is realized by a coupled line section with shorted ends. The experimental results show that the filter achieves elliptic response successfully. The measured 3-dB fractional bandwidth is 86.6% centered at 0.97 GHz, within which the minimal insertion loss is 0.88 dB at 0.81 GHz. Furthermore, two transmission zeroes beside the passband are observed clearly at 0.31 and 1.59 GHz, offering a sharp rejection in the stopband.