Compact tri-band bandpass filter with two closely frequency spaced passbands

Abstract

A compact tri-band bandpass filter based on a novel six-mode resonator is proposed. The filter has three passbands centered at 2.45, 5.2, and 5.8 GHz for WLAN applications. Among them, two closely frequency-spaced passbands with a small center frequency ratio (1/1.11) are realized. The 3 dB bandwidth of the two passbands are 5.8 and 5.2%, which means they have equal absolute bandwidth. Source-load-coupling structure is adopted to enhance the high-isolation performance. And the filter has six transmission zeros, resulting in high-frequency selectivity. The passband frequencies and bandwidths of the filter can be controlled. The measured results are in good agreement with electromagnetic simulation. Its circuit only occupies 0.14 λg × 0.09 λg (11.2 mm × 7.1 mm), where λg is the guided wavelength at the lowest passband center frequency.

Keywords:

• Tri-band
• closely frequency spaced
• compact
• high selectivity