Design of Wide-Stopband Bandpass Filter and Diplexer Using Uniform Impedance Resonators

This paper proposes a simple and effective method to designing wide-stopband bandpass filters and diplexers using uniform impedance resonators. Microstrip terminated parallel coupled lines are utilized to generate the filtering functions. By adjustment of the electrical lengths of the parallel coupled lines and open-circuited stubs, multiple transmission zeros can be obtained. Appropriate positioning of these zeros enables the suppression of spurious frequencies. A complete analytic analysis is proposed to fully control the positions of transmission zeros, and the design equations and design rules for both homogeneous and inhomogeneous mediums are given in detail. Different combinations of the frequency position of the transmission zeros can provide various characteristics. To extend the stopband bandwidth, one or more transmission zeros can be distributed on different harmonic frequencies. To demonstrate the proposed method, two bandpass filters and a diplexer are designed and fabricated. The measured results are in good agreement with the simulated ones. The first filter provides a high attenuation of 40 dB up to 6.8 times the fundamental frequency (f) and the second filter provides a lower attenuation of 28 dB, but this time extending up to 14.9 f.