Harmonic-Suppressed Bandpass Filter Based on Discriminating Coupling

This letter presents a novel method to suppress harmonic responses of parallel-coupled bandpass filters. The harmonic suppression is based on discriminating coupling. The coupling coefficient at fundamental resonant frequency can be tuned to desirable values whereas it is zero at the second harmonic. Hence, the coupling region blocks the second harmonic but still allows the transmission of signals at passband frequency. Benefiting from this feature, the second harmonic can be suppressed without degrading the passband performance. Furthermore, no additional circuit is needed. For demonstration purpose, two example filters are implemented. The experimental results are presented to verify the proposed method.