Novel interdigital microstrip bandpass filter with improved spurious response

In this paper an interdigital bandpass filter with under-coupled quarter wavelength resonators is proposed. With this new structure, the spurious response up to the 3^rd-order harmonic passband can be suppressed significantly whilst the fundamental passband remains unchanged. Also, this design can take full advantages of the traditional interdigital filter design equations. In order to demonstrate the proposed filter performance, a 3^rd-order Butterworth interdigital bandpass filter centered at 900MHz with 15% fractional bandwidth was designed and implemented using both the proposed and the conventional structure. From the simulation and experimental results, the proposed filter obtains more than 10dB suppression in average for the entire upper stopband up to the triple of the midband-frequency value, and also a 32 dB suppression of the 3^rd harmonic spurious passband.