Bandpass Filter with Capacitively Loaded Coupled Lines for Wideband Spurious Rejection

A stepped-impedance resonator (SIR) bandpass filter with capacitively loaded coupled lines for wideband spurious rejection is proposed in this paper. The SIR unit has the ability to realize a wideband spurious rejection while the capacitively loaded coupled lines can provide two additional transmission zeros at the upper stopband, which are essential to improve the rejection performance. Simulated and measured results show that the center frequency of the proposed second-order filter is at 2.184 GHz with -2.6 dB insertion loss. It can provide -19 dB rejection levels in the stopband up to 6 times of the passband center frequency.