第二個電路中的低通濾波器是基於第一個電路的低通濾波器增加一段並聯的傳輸線來增加零點；第三種低通濾波器則為以串聯兩組由一對耦合線連接一開路殘段結構當成一單位所組成的低通濾波器。第二個電路在通帶內的反射損耗從0 GHz至1.85 GHz皆大於10 dB，且3-dB截止頻率為1.93 GHz，截止帶大於20 dB的範圍從 2.68 GHz 至6.86 GHz。第三種低通濾波器的3-dB通帶從0 GHz 至 1.9 GHz，通帶內的反射損耗從0 GHz 至 1.85 GHz 皆大於10 dB，截止帶大於20 dB之範圍從2.86 GHz 至 9.85 GHz;The passband of the multi-band filter of this paper consists of a low-pass filter and two pairs of half-wavelength stepped impedance resonators. Distributed coupling technique is used to reduce the loading effect between the passbands, and every two passbands of the band-pass filter are controlled by a set of half-wavelength stepped impedance resonators. The use of half-wavelength stepped impedance resonators can reduce the circuit size compared to traditional half-wavelength resonators. The frequency of the passbands is designed in 3,4,5 and 6 GHz. In the paper, three different low-pass filters are fabricated on Rogers RO4003 substrate with a relative dielectric constant of 3.55, a thickness of 0.8 mm, and a loss tangent of 0.0027 to verify the possibility of the design. The low-pass filter in the first circuit is a stepped impedance low-pass filter. The return loss in the passband is greater than 20 dB from 0 GHz to 2 GHz, and there is a 3-dB cutoff frequency close to 2 GHz at 1.93 GHz. The cutoff band is greater than 20 dB from 2.66 GHz to 9.32 GHz. The low-pass filter in the second circuit is based on the band-pass filter of the first circuit by adding a shunt transmission line to increase the transmission zeros. The third type of the low-pass filter is a low-pass filter composed of two series circuit of a pair of coupled lines connected to an open stub structure as a unit. The return loss of the second circuit in the passband is greater than 10 dB from 0 GHz to 1.85 GHz, and the 3-dB cutoff frequency is 1.93 GHz. The cutoff band is greater than 20 dB from 2.68 GHz to 6.86 GHz. The third low-pass filter has a 3-dB passband from 0 GHz to 1.9 GHz, and the return loss in the passband is greater than 10 dB from 0 GHz to 1.85 GHz. The cutoff band is greater than 20 dB from 2.86 GHz to 9.85 GHz.
