Miniaturized wideband bandpass filter with enhanced selectivity and stopband suppression

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Abstract
In this article, a wideband microstrip bandpass filter with cross‐coupling is presented. The filter adopts a short‐stub‐loaded dual‐mode resonator (SSL‐DMR) and two stepped‐impedance spurline resonators (SISLRs) to achieve size miniaturization. Meanwhile, the trisection topology is utilized to achieve two transmission zeros (TZs) on both sides of the passband, with which good sideband selectivity can be exhibited. Furthermore, the SISLRs at the input and output ports can also contribute to another TZ at higher frequency of the upper stopband, which cannot be acquired by utilizing the conventional uniform‐impedance resonator (UIR) or stepped‐impedance resonator (SIR). To verify the aforementioned concept, a filter with a miniaturized size of 0.16 λg × 0.75 λg is designed, fabricated, and measured (λg denotes to the guided wavelength at central frequency). Measured results of the fabricated prototype are in good agreement with the simulated ones. Meanwhile, the fabricated filter achieves a fractional bandwidth about 44.5%, a minimum in‐band insertion loss of 0.6 dB, and a stopband rejection over 20 dB up to the fourth harmonic of the operation center frequency.

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