A New and Simple Design Approach for Harmonic Suppression in Bandpass Filter

This letter introduces a new and simple approach for designing a harmonic-suppressed bandpass filter (BPF), which is effective for both single-ended and balanced BPFs. According to the planar transmission line theory, there must be a voltagenull point on the quarter-wavelength resonator at the third harmonic. It is chosen as the exciting location of the feeding line so that the harmonic of the designed single-end BPF cannot be excited and then is suppressed successfully, while the passband of the BPF operating at the fundamental resonant frequency (f
₀
) of the resonator can be optimized and obtained by tuning other parameters. This technique can be easily transplanted to balanced design, and then the differential-mode (DM) third harmonic of the balanced BPF using the half-wavelength resonator can also be rejected effectively. For demonstration, two design examples, i.e. single-ended and balanced BPFs, are designed and measured. Their simulated and experiment results are presented.