Design of Miniaturized Microstrip LPF and Wideband BPF With Ultra-Wide Stopband

摘要

This letter presents a novel lowpass filter (LPF) structure which consists of a section of high-impedance microstrip line (HIML) with a pair of radial stubs (RSs) loaded at its center and a pair of stepped-impedance open stubs (SIOSs) loaded on both ends of HIML. The RSs loaded HIML exhibits a lowpass property and has a wide stopband. The loaded SIOSs not only can improve the roll-off rate of LPF significantly but also is able to extend the stopband. Based on such a LPF structure, a pair of highpass shorted HIMLs are paralleled on both sides of LPF to construct a wideband bandpass filter (BPF) with wide stopband. To validate the proposed method, two filters, i.e., a LPF with cutoff frequency $f_{c}$ at 1.07 GHz and a BPF centered at $f_{0} = 0.89$ GHz with ${-}3$ dB fractional bandwidth of 87%, are designed and fabricated. The measured results show the fabricated LPF has a ${-}20$ dB isolation bandwidth from $1.27 f_{c}$ to $13.7f_{c}$ while the fabricated BPF has a ${-}20$ dB isolation bandwidth from $1.56 f_{0}$ to $15 f_{0}$. Moreover, the fabricated LPF and BPF also have the compact size of $0.107 lambda_{gc} times 0.083 lambda_{gc}$ and $0.123 lambda_{g0} times 0.073 lambda_{g0}$, respectively. Good agreement can be observed between the - imulation and measurement.