Suppression of Spurious Resonance by Using Multistep Transmission Lines

Miniaturized bandpass filters with high performance have become more and more important in today´s wireless communication industry. Bandpass filters implemented with half-wavelength resonators have inherently harmonic frequencies at the multiples of frequency. A novel multistep transmission filter based on the strip-width modulation reformulation is proposed to suppress the spurious passband at the second harmonics of the operating frequency. A three- stage Chebyshev hairpin type bandpass filter was designed and implemented on FR-4 and alumina ceramic substrates, respectively. The center frequency of the bandpass filter is at 0.9 GHz and the fractional bandwidth is 5%. With the multistep structure, the spurious passband at the second harmonics is suppressed.