Compact Cross-Coupled Bandpass Filter With Second and Third Harmonic Suppression

A compact cross-coupled bandpass filter with second and third harmonic suppression is proposed. Quarter-wavelength resonators and 0° feed structure are introduced to suppress second and third spurious responses. The measured return loss and insertion loss of the proposed filter with four quarter-wavelength resonators are better than 17.2 dB and less than 1.85 dB in the range of 3.42 GHz to 3.6 GHz, respectively, and the upper stopband is up to $3.7f_{0}$ with an insertion loss larger than 20.1 dB.