Compact quasi‐elliptic bandpass filter with high selectivity using short‐circuited coupled line

Abstract

A design strategy of a bandpass filter (BPF) with sharp roll-off and improved stopband performances is presented in this paper. The proposed filter prototype consists of a couple of parallel-coupled stepped-impedance resonators (SIRs) and two short-circuited coupled lines (SCCLs). The theoretical analysis and numerical simulations on the parallel-coupled SIRs and coupling structure are carried out. Furthermore, a Butterworth BPF is studied and designed first. By loading the SCCL at the I/O ports of the Chebyshev BPF, four transmission zeros can be introduced beside the passband, which improves the selectivity and stopband performances significantly. For demonstration, a BPF prototype with the center frequency of 2.02 GHz and 3-dB fractional bandwidth of 5.2% is designed, fabricated, and measured. Moreover, the measured insertion loss is less than 1.5 dB and the rejection levels are larger than 40 dB in both lower and upper stopbands. Thus, the measurement results verify the feasibility of the design. K E Y W O R D S bandpass filter, high selectivity, short-circuited coupled line (SCCL), transmission zeros 1 | INTRODUCTIONWith the rapid development of wireless communication systems, filters with sharp roll-off, wide stopband, and deep suppression are highly desired. 1,2 Many different filter design methods have been proposed, such as using multimode resonators, 3 substrate-integrated waveguide, 4 and filter synthesis. 5 Because the parallel coupled line filter has the advantages of simple structure, easy design and implementation, reliable performance, and easy integration, it has been widely used in microwave system. 6,7 However, the selectivity and stopband performance are not so good, because there are no transmission zeros (TZs) besides the passband. Filters with high selectivity are in great demand because of the scarcity of the electromagnetic spectrum resources. Common methods for improving the filter frequency selectivity mainly include increasing the filter order and introducing finite frequency TZs. 8,9 Increasing the order of the filter results in increased insertion loss and circuit size. Therefore, introducing TZs as a better way to enhance filter selectivity has been extensively studied. [10][11][12][13][14][15][16][17] In Reference 10, multiple TZs can be obtained by a compact parallel-coupled transmission line sections which are connected at their both ends. Four TZs are introduced by multiple capacitive cross-couplings in Reference 11, and the locations of the TZs can be adjusted by changing the values of cross-coupled capacitances. In Reference 12, by properly selecting the length of the stubs and their positions, extra TZs are created in the stopband before the spurious passband. Moreover, source-load coupling structure has been employed to introduce TZs. 13,14 In Reference 13, two TZs generated by source-load coupling structure were applied to suppress the first and second spurious harmonics. In addition, two TZs close to the cut-off frequencies which were introduced by the source-load coupling...

Spoof Surface Plasmon Polaritons Developed from SIW Using Ring Slots and Vias

“…Compared with the traditional microwave filters, such as coupled line filters [11][12][13][14], waveguide filters [15][16][17], the SSPP based filters have the advantages of easily tunable bandwidth and fast design period through observing the dispersion characteristics of the SSPP unit cell, which are also a good candidate for the 5G or internet of thing applications [18]. However, the conventional ultra-thin SSPPs are generally constructed by corrugated strip lines, which are all open structures.…”

Bandpass filter using coupled‐line‐stub cascaded structure with high stopband rejection

“…In recent years, some methods have been introduced to design the high-performance filters. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] A novel BPF, composed by three pairs of parallel-coupled lines, is designed with high stopband suppression and steep roll-off skirt in Reference 1, but its bandwidth was limited. In Reference 2, the concept of transmission zero (TZ) resonator pairs was proposed to design a BPF, in which the TZ pairs are symmetrically located at the out-of-band.…”

“…As one of the most important microwave components, bandpass filters (BPFs) with low in‐band insertion losses and high out‐of‐band rejection levels are extremely desirable in the modern wireless communication systems. In recent years, some methods have been introduced to design the high‐performance filters 1‐15 . A novel BPF, composed by three pairs of parallel‐coupled lines, is designed with high stopband suppression and steep roll‐off skirt in Reference 1, but its bandwidth was limited.…”

“…However, the rejection level at the stopband is not very high. By loading short‐circuited coupled line at input/output ports of the proposed structure, an enhanced quasi‐elliptic BPF with good selectivity and stopband performance is designed in Reference 3. The source‐load coupling method is utilized to design high‐selectivity BPF in Reference 4, and the cross coupling concept is used to enhance the stopband rejection level by introducing an additional TZ at high stopband without size increased in Reference 5.…”