A compact dual-band bandpass filter with large frequency ratio based on ultra-wide-stopband bandpass filter

With the rapid development of wireless techniques, the bandpass filter (BPF) is required to cover microwave and millimeter-wave frequency bands simultaneously with good mid-band suppression. However, it is difficult to implement such BPF due to the large frequency ratio and wideband rejection. This paper presents a superior method to realize a dual-band BPF with a large frequency ratio maintaining compact size and low design complexity. This is contributed by an ultra-wide stopband BPF with inherent discriminating excited degree at spurious frequencies. By properly arranging the feeding position and electrical length ratio of stepped impedance resonator (SIR), the excited degree at specific spurious frequencies can be flexibly adjusted to achieve desired suppression level without affecting characteristics at the fundamental passband. For validation, two BPFs were simulated, fabricated and measured, exhibiting suppression levels of 20.3 dB and 35 dB up to 18fo and 10.53/f0 respectively. Based on this, a dual-band BPF with a large frequency ratio can be easily constructed. For demonstration, a dual-band BPF operating at 3.55 GHz and 43.15 GHz is implemented. A frequency ratio up to 12.15 and mid-band suppression level better than 28 dB had been achieved. Advantages of compactness, simplicity and excellent performance of the proposed work can be observed.