Bandpass filters with single transmission zeros at finite frequencies for adjusting slopes of frequency response

The purpose of this work is to present new possibilities of designing known microstrip band-pass filters with one or two independent transmission zeros located in the stop-band far or close to the passband. A method of independently adjusting the slopes of the filter frequency response by proper placing transmission zeros is proposed. Each zero can he tuned to reject unwanted signals in the frequency range out of the passband. The transfer function of this type of filter is analytically expressed. The behaviour of a simple third-order lossless open-loop resonator filter is presented. The experimental results obtained for a third-order filter with a fractional bandwidth of 6 % at 2.4 GHz and six-order filter with a fractional bandwidth of 7% at 1.8 GHz are in a good agreement with the theoretical ones. The excellent performance and advantages of this class of filter have been presented.