Compact Tri-Band Bandpass Filter with Multiple Transmission Zeros

In this article, a compact tri-band bandpass filter with high selectivity using novel tri-mode resonators is proposed. The novel tri-mode resonator consists of a modified /4 stepped-impedance resonator with a closed rectangular loop and two loaded open stubs. A pair of transmission zeros are generated at the lower and upper stopband of each passband for high selectivity and excellent band-to-band isolation. A 1.90/3.50/5.75-GHz tri-band experimental bandpass filter has been designed and fabricated to demonstrate the proposed method. The measured results show six finite transmission zeros near the passbands located at 1.65 GHz with 30.14-dB rejection (TZ(1)), 2.64 GHz with 51.35-dB rejection (TZ(2)), 3.06 GHz with 49.36-dB rejection (TZ(3)), 3.95 GHz with 43.76-dB rejection (TZ(4)), 5.40 GHz with 30.67-dB rejection (TZ(5)), and 6.16 GHz with 35.15-dB rejection (TZ(6)), respectively. The circuit only occupies 0.04 (g) x 0.07(g).