Parallel-Coupled Bandpass Filter With Higher-Order Harmonic Suppression up to 36.7f0 Using Resonant Frequency Separation Technique

초록

This work presents a microstrip bandpass filter (BPF) featuring suppression of higher-order harmonics up to 36.7f0 while maintaining a well-defined fundamental passband. To suppress the higher order harmonic other than the fundamental mode, the harmonic-suppressed filter was designed based on the resonant-mode separation technique, in which the resonant- mode frequencies of the fundamental mode were placed close to each other, while those of the remaining harmonics were intentionally separated as far as possible. Using the second- harmonic-suppressed periodic slits resonator as the fundamental structure, an additional resonator was developed by strategically placing it at the averaged antinode positions of the first, middle, and end regions of the third- and fifth-order harmonic modes. This modification induced frequency shifting of the higher- order harmonic modes except for the fundamental. By coupling these resonators, the transmission coefficient of the higher- order harmonics other than the fundamental mode could be maximally suppressed. As a result, the fabricated proposed filter exhibits harmonic suppression exceeding 28 dB for all measured harmonics up to 90 GHz (36.7 f0 ), with return loss (RL) of under 20 dB and insertion loss (IL) of 1.73 dB at the fundamental frequency.

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