Compact bandpass filter with improved stopband characteristics using S-L coupling

A compact bandpass filter (BPF) with improved stopband performance using mixed electric/magnetic (E/M) coupling and Source-Load (S-L) coupling is presented. Due to the cancelling effect of the mixed coupling, a transmission zero (TZs) is employed near the passband. The design is implemented to a circular structure to achieve compact size. For realizing wide stopband characteristic, an extra transmission zero is achieved in the upper stopband owing to the S-L coupling introduced between the input and output port. A new filter centered at 1.08 GHz 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 0.78 GHz with 53.8 dB rejection, 1.86 GHz with 67.6 dB rejection, 3.01 GHz with 53.6 dB rejection, respectively. The out-of-band rejection better than 30 dB is up to 6 GHz. And the circuit only occupies 13.5×11.5 mm².