Miniaturization and Harmonic Suppression Open-loop Resonator Bandpass Filter with Capacitive Terminations

A miniaturized open-loop resonator bandpass filter with suppression of second and third harmonics is demonstrated in this paper. This novel filter is based on the common open-loop resonator bandpass filter with capacitive terminations to achieve size minimization, while the spurious response are suppressed by low characteristic impedance microstrip open stub lines as capacitive terminations. The simulations and measurements of a 0.9GHz prototype bandpass filter are presented. The measured results agree well with the simulation. Compared with the common open-loop resonator bandpass filter, the measured upper stopband from 1GHz to 3GHz including second and third spurious response are suppressed below -28.1dB. In addition, the circuit size of the prototype filter can be reduced up to 60%