Microstrip bandpass filters based on coupling matrix with second harmonic suppression

An approach for suppressing the second order harmonic responses in open loop resonator bandpass filters is presented. A known technique for the effective suppression of even-mode harmonic responses is based on loading the open loop resonator at its center with a resistor. A basic analysis of this structure leads to the conclusion that the even-mode resonance is controlled without affecting the responses at the odd-mode resonant frequencies. More detailed simulations, however, that take into account the size of the resistor via hole and the substrate losses, reveal that the value of the resistor has significant impact on the overall response, including fundamental odd-mode. In particular, the insertion loss in this mode becomes worse. By replacing via holes by open stubs with quarter-wavelength length at the even frequency, we have managed to achieve better rejection level in the even-mode with affecting the low insertion loss in the fundamental mode. Experiments verify the simulated predictions.