On the design of microstrip bandpass filters/ Rafidah Ahmad

The aim of this project is to design, fabricate and characterize maximally flat (Butterworth) bandpass filter to be used in microwave communication transceiver. The Butterworth response is chosen since it has less delay distortion and offers the flattest passband compared with the other filter responses. A commercially available computer aided design (CAD) package HP Eesof LIBRA was used to simulate the filter response. The filters were designed to work from 1.9 GHz to 2.1 GHz. The specified stopband attenuation is more than 15 dB while a 1 dB loss in the passband is required The filter configuration chosen was based on microstrip planar resonators and uses a parallel-coupled filter form. The filters were fabricated on microstrip laminate, 'RT/Duroid® 5870* with a substrate thickness of 0.5 mm and relative permittivity 2.33. The circuit was fabricated using in-house facilities available at the faculty. The filter characteristics were determined using a Wiltron 562 Scalar Network Analyzer. The filters were found to exhibit excellent properties. The response obtained was very close to simulation. Typical return loss values measured were of the order of 12.56 dB, while the passband attenuation was found to be about 1 dB. This agreement with the original specification reflects the accuracy possible with CAD and indicates the capability of rf circuit fabrication facility available at the faculty.