Dual-Band Substrate Integrated Waveguide Balun Bandpass Filter With High Selectivity

A new method of designing a dual-band balun bandpass filter (BPF) with high selectivity is proposed and implemented by taking advantage of the existence of multiple substrate integrated waveguide (SIW) cavity modes. The major design concept applied in this letter is based on two mechanisms: one is by properly placing the output ports to achieve the balanced output for both passbands, and the other is by taking cross coupling of higher/lower order modes to achieve transmission zeros (TZs) at each side of the passband. Two internal coupling slots are involved to separately control the coupling level between cascaded cavities for each passband. A prototype operating at 9.0 GHz and 9.8 GHz has been realized and shows the desired filtering performance. The differences between the two outputs are within 180 degrees +/- 2 degrees in phase and 0.6 dB in magnitude across the two passbands. Measured frequency responses agree well with the simulated ones in this work.