Abstract-Coupled-line and coupled three-line resonators are proposed to design dual-wideband bandpass filters. Compared with the shorted and open stubs shunt at the same locations of the main line, in addition to saving the circuit area, these resonators provide alternative ways to the design of dual-wideband filters, with larger possible bandwidths and different frequency ratio of the two center passbands. The geometric parameters of the coupled-line and the coupled threeline structures are determined by deriving their equivalent circuits to a shunt open stub in parallel connection with a shunt shorted stub. To extend the upper stopband, a cross-shaped admittance inverter is devised to play the role of the 90-degree transmission line section at the center frequency and to create transmission zeros at the spurious passbands, so that the upper stopband of the filter can be extended. It is a quarter-wave section with two open stubs of unequal lengths shunt at its center. For demonstration, two dual-wideband bandpass filters operating at 900/1575 MHz and 900/2000 MHz are fabricated and measured. Measured results of the experimental circuits show good agreement with simulated responses.
Planar microstrip filters are designed to have a quasielliptic function passband and extended upper stopband with a rejection level of 40 or 50 dB. The design employs stepped-impedance resonators in a compact 2 × 2 cross coupled configuration. The geometric parameters of the resonators are planned to shift the third resonance as high as possible, and then the transmission zeros created by the structure are devised to suppress the second resonance, so that a wide upper stopband up to more than 4.5 times the passband frequency can be achieved. By employing the skew-symmetric input/output feeds, three transmission zeros in the frequency band of interest can be created. The leading two zeros are allocated on the both sides of the passband, generating a quasi-elliptic function response with enhanced roll-off rate in the transition bands, and the last zero incorporating with a zero created by anti-coupled-line at higher frequencies are employed to extend the rejection bandwidth. The measurement data agree very well with the simulation responses.
This paper presents a class of reduced footprint inline microstrip bandpass filters capable of covering bandwidth up to 20% as well as good flexibility in establishing various cross-couplings for creating transmission zeros. The resonating elements are quarter-wave stepped-impedance resonators (SIRs), which have a wide upper stopband in nature. To achieve size miniaturization, the low-impedance segment of each SIR is implemented as a thick-trace ring and configured in spiral form, and the highimpedance section is deformed to accommodate the low-impedance section. An interlaced coupling structure is proposed to enhance the coupling limited by the downsized coupled segments of adjacent resonators. In addition, by properly routing the associated shorted stubs, the structure can easily establish cross-coupling between nonadjacent resonators. Thus, compact SIR filters in an inline arrangement can be achieved to have either narrow or wide passbands with plural transmission zeros. Two such circuits with sharp transition bands and upper stopband extension are synthesized, fabricated, and tested. The measured results show good agreement with simulated data. The circuit areas together with the performances of the experimental filters are compared with those in existing literature.
The electric coupling and magnetic coupling are extracted for a dual-mode resonator filter. The resonator is a half-wave section with its center being shorted to ground. The extraction relies on an equivalent circuit model. With properly tuning of the coupled-line sections, the electric and magnetic coupling coefficients of the two quarter-wave resonators can be adjusted, and hence both the filter bandwidth and zero frequency can be controlled. Two experimental filters are fabricated and measured for validation. Good agreement between measurement and simulation is obtained.
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