Tunable 3-pole bandpass filter with constant absolute bandwidth and >60 dB rejection at desired location of stopband

Abstract

This paper presents a novel design approach for tunable 3-pole bandpass filter (BPF) with constant bandwidth (ABW) and deep rejection at the desired stopband. First, by properly designing the coupling coefficient and Q_ext, the tunable 3-pole BPF is designed with the characteristic of constant ABW. Only two bias voltages are adopted to complete the tuning, among which one of them is used to tune the center frequency and the other one is adopted to control the Q_ext. Then, bandstop resonators are introduced to provide deep rejection at desired stopband. A deep rejection level >60 dB can be easily controlled by varying the loaded capacitor C_bs among the stopband. Furthermore, the designed deep rejection point can be constant and produces little influence on the passband. For demonstration, a 3-pole tunable BPF is designed, fabricated, and measured. The filter can be tuned from 0.98 to 1.4 GHz with constant ABW, a wide fractional tuning range up to 40% can be attained. High rejection level >60 dB is realized at the pre-designed stopband point 0.65 GHz. The measurement shows good agreement with the simulation.

Keywords:

• bandpass filter
• tunable filter
• constant absolute bandwidth
• bandstop resonator
• high rejection

Additional information

Funding

This work was supported by the National Natural Science Foundation of China (NSFC) [grant number 61271090], [grant number 61401375]; the National 863 Project of China [grant number 2012AA012305]; the Fundamental Research Funds for the Central Universities [grant number 2682014RC24]; the Cultivation Project of Sichuan Province Science and Technology Innovation Seedling Project (2014-047), “2014 Doctoral Innovation Funds of Southwest Jiaotong University” and the program of China Scholarship Council [grant number 201407000007].