Improving the stopband of microstrip parallel-coupled line bandpass filters using partially coupled resonators

An effective method is introduced to improve the stopband performance of microstrip parallel-coupled line bandpass filters. The proposed filter is constructed by adding several partially coupled resonators close to each trace in the conventional microstrip parallel-coupled line bandpass filter. The lengths of the coupled section and the stub section in each partially coupled resonator are selected such that the desired transmission zero is obtained. Proper selection of the number and the frequency of each transmission zero eventually makes it possible to suppress the unwanted spurious responses. In order to demonstrate the characteristics of the proposed filter, a third-order bandpass filter, which is centred at the operating frequency of 1 GHz with 10% fractional bandwidth, is designed, tuned, fabricated and tested. Measurement and simulation results show that the proposed prototype has a broad upper stopband performance, whereas it keeps the desired passband loss and shape almost equal to that of the conventional ones.