A Compact Negative-Group-Delay Microstrip Bandpass Filter

In this paper, a compact negative-group-delay (NGD) microstrip bandpass filter is proposed. The NGD characteristic is achieved by coupling a resistor-loaded microstrip line to a square open-loop resonator. To improve the selectivity, the square open-loop resonator is loaded with an open-circuited stub for realizing two transmission zeros (TZs) in the upper stopband. To verify the proposed method, an NGD microstrip bandpass filter with a size of 0.58 lambda(g) x 0.35 lambda(g) is designed and fabricated. From the measured results, the NGD time of -1.08 ns at the center frequency of 1.995 GHz is obtained with the NGD bandwidth of 34MHz (1.977-2.011 GHz), in which the insertion loss is less than 7.5 dB, and the return loss is greater than 20 dB. Furthermore, three TZs at 1.520, 2.495, and 2.735 GHz are achieved with good stopband attenuation.