最後第四種低通濾波器則為步階式阻抗低通濾波器，在通帶內的反射損耗從0 GHz至 1.77 GHz皆大於15 dB，且3-dB截止頻率為1.99 GHz，截止帶大於20 dB的範圍從 2.74 GHz 至9.3 GHz。 ;The multi-channel low-pass and band-pass filters consisting of low-pass filter and half-wavelength resonators. Each passband of BPFs is controlled by a respective pair of resonators to increase design freedom, and the four BPFs are combined by utilizing distributed coupling technique to reduce channel-to-channel loading effect. Due to the low loading effect from distributed coupling technique, the proposed circuits can obtain many channels without the need of complicated matching network. Furthermore, the low-pass filter not only provides a passband from 0 to 2 GHz, but also increases improve in-band selectivity among multiple frequency bands.
The low-pass filter in the first circuit consisting of multiple cascaded stepped-impedance resonator (SIR), This low-pass filter has a 3-dB passband from dc to 1.96 GHz. The return loss is better than 20 dB from dc to 2 GHz ,and the rejection is better than 20 dB form 2.3 GHz to 6.72 GHz.
The low-pass filter in the second circuit consisting of a transmission line and a interdigital capacitor, and make it as a unit, connected two units by transmission line. The third circuit consisting of a transmission line and a lumped capacitor for a unit, also connected two units by transmission line. The second circuit has a 3-dB passband from dc to 1.82 GHz. The return loss is better than 20 dB from dc to 2.14 GHz ,and the rejection is better than 20 dB form 2.53 GHz to 6.73 GHz. The third circuit has a 3-dB passband from dc to 1.97 GHz. The return loss is better than 15 dB from dc to 1.77 GHz ,and the rejection is better than 20 dB form 2.3 GHz to 6.68 GHz. In the end, the low-pass filter in the last circuit is a Stepped-impedance low-pass filter ,and it has a 3-dB passband from dc to 1.99 GHz. The return loss is better than 15 dB from dc to 1.77 GHz ,and the rejection is better than 20 dB form 2.74 GHz to 9.3 GHz.
