A Multi-Stub Lowpass Filter

The stepped-impedance lowpass filter is modified by replacing the low-impedance lines with lowpass transmission lines. The modified stepped-impedance lowpass filter is called a multi-stub lowpass filter because it is largely composed of open-circuited stubs. For experimental verification of the multi-stub filters, we design a 1 GHz seventh-order Butterworth LPF. The proof-of-concept prototype is a compact design that fits into a circuit size of 0.09λ_g × 0.136λ_g on an RO4003 substrate. The measurement results show that the multi-stub lowpass filter retains the characteristics of a Butterworth filter in the passband, while substantially improves the performance in the transition and stopband regions. It achieves state-of-the-art performance compared to lowpass filters in the literature.