Ultra-wideband (UWB) Bandpass Filter Using Shunt Stub with Lumped Capacitor

摘要

In this paper, we presented an ultrawideband (UWB) bandpass filter which used shunt stub with lumped capacitor. The transmission line theory and our experiments confirmed that a shunt stub with a capacitor allows large separation between the first and second resonant frequencies which then results in a wide passband and possible compactness. A length compensation technique to compensate the finite length of the via hole as a short circuit, and the finite physical size of the lumped capacitor, has been also developed in this paper. The experimental results demonstrated the usefulness and effectiveness of the developed technique. Using the shunt stub with capacitor and the compensation technique, a two-stage compact UWB bandpass filter was designed, fabricated and measured. The developed UWB filter has excellent properties: insertion loss of 0.2 dB at the center frequency; flat group delay characteristics as low as 0.13 ns; and almost meet the requirement of FCC’s UWB spectrum mask except in the low-frequency region below 2 GHz and compact size of 2 cm x 2.5 cm. The measured result had a good agreement with the designed result after the length compensation.