Compact Microstrip Bandpass Filter With Multispurious Suppression Using Quarter-Wavelength and Half-Wavelength Uniform Impedance Resonators

In this paper, an effective and simple method for suppressing the spurious frequency in bandpass filter is proposed. Quarter- and half-wavelength uniform impedance resonators are used to generate the filtering function. The transmission zero caused by the structure of coupled line terminated with short-circuited stub is discussed in different mediums. By adjusting the coupled line and stub electrical lengths, controllable transmission zeros can be generated that can suppress the spurious passband, thereby extending the stopband. The concept has been experimentally tested in a fourth-order microstrip bandpass filter operating in 1 GHz, which provides an attenuation of 26.4 dB up to 15.9 times the center frequency and exhibited a size reduction of 50% compared with the half-wavelength resonators structure at the same frequency.