Design of compact bandpass filter with enhanced stopband characteristics

A compact third-order bandpass filter (BPF) with enhanced stopband characteristics is proposed in this paper. The filter consists of two stepped impedance resonators (SIRs) and a new resonator named closed loop SIR. The closed loop SIR which is embedded into the other two resonators for compact size is not the real structure of SIR, but it is similar to the conventional λ/4 SIR in some characters. Cross coupling is introduced into the proposed filter to generate extra transmission zero (TZ) near the passband for sharp frequency selectivity. By adjusting the strength of the mixed cross coupling, the location of the extra transmission zero can be controlled. A new filter centered at 1.77GHz has been analyzed, simulated and fabricated to verify the validity of the proposed method. The measured results show that there are three finite transmission zeros near the passband, which are located at 1.4GHz with 54.2dB rejection, 1.58GHz with 35.9dB rejection, 3.72GHz with 48.5dB rejection, respectively. And, the spurious frequencies of the upper-stopband are suppressed below 30dB from 2.5 GHz to 8.0 GHz.