SLR Helps Shrink Quad-Band Filter

Based on a novel SLR, a compact BPF with quad-band characteristics was investigated. The quad-mode SLR can achieve four passbands simultaneously centered at 1.5, 2.5, 3.5, and 5.2 GHz. To improve the selectivity of the quad-band BPF, a cross-coupling structure between the SLRs and the filter’s input/output (I/O) ports is employed. Measurements performed on a fabricated prototype agree closely with simulated results from commercial computer simulation software.

Figure 1(a) shows the structure of the novel quad-mode SLR, which consists of a common microstrip half-wavelength resonator, an open stub, and a T-type stub. One end of the T-type stub is grounded by means of a metallized via hole. The novel SLR is modified from a tri-mode SLR, as shown in Fig. 1(b). Due to its asymmetrical structure, it is difficult to obtain analytical resonant frequencies for the quad-mode SLR. But since the tri-mode SLR is symmetrical, an odd-even-mode method can be implemented, as shown in Fig. 1(b). Under conditions of Z₁ =  Z₂ = Z₃, the three resonant frequencies of the tri-mode SLR can be expressed by means of Eqs. 1-3:

f_e1 = c/4[(L₁ + L₃](ε_e)^0.5   (1)

f_e2 = c/2[(L₁ + L₂](ε_e)^0.5   (2)

f_o1 = c/4L₁(ε_e)^0.5   (3)

where:

c = the speed of light in free space; and

ε_e = the effective dielectric constant of the substrate.

After substituting the shorted stub of tri-mode SLR with the T-type stub, quad-mode performance can be obtained. Figure 1(c) shows the simulated frequency responses of the novel quad-mode SLR under weakly capacitive coupling conditions. As can be seen, the quad-mode SLR obtains four resonant poles simultaneously.

The transfer characteristics of the quad-mode SLR were studied for various dimensions with the help of the High-Frequency Structure Simulator (HFSS 11.0) full-wave electromagnetic (EM) simulation software from ANSYS, as shown in Fig. 2. It can be seen that the four resonant frequencies increase simultaneously as L₁ decreases. But only one or two resonant frequencies increase as L₂, L₄, and L₅ decrease, respectively. Therefore, by appropriately adjusting the resonator dimensions, four desired resonant frequencies can be achieved.

Figure 3 shows the structure of the novel quad-band BPF. It is composed of two SLRs and an interdigital I/O port. To realize good out-of-band performance, a cross-coupling structure between the SLRs and the I/O port was introduced, which can generate one transmission zero between two resonant frequencies.² The filter was fabricated on RT/duroid 5880 circuit substrate material from Rogers Corp. with a thickness of 1.0 mm and a relative dielectric constant (ε_r) of 2.2. The dimensions were selected as follows: W₀ =3.0 mm; W₁ = W₂ = W₃ = W₄ = W₅ = W₆ = W₇ = 1.0 mm; L₁ = 12.25 mm; L₂ = 8.5 mm; L₃ = 6.35 mm; L₄ = 2.5 mm; L₅ = 5.0 mm; L₆ = 2.5 mm; L₇ = 8.0 mm; L₈ = 2.8 mm; L₉ = 3.0 mm; S₁ = 0.75 mm; S₂ = 0.2 mm; and S₃ = 0.35 mm. The via hole radius is 0.2 mm. As for bandwidth control, the external quality factor (Q_e) and coupling coefficient (k) should be considered. The values of Q_e and k are mainly determined by the gaps S₃, as shown in Fig. 4.

The designed and fabricated quad-band BPF was measured with a model N5230A vector network analyzer (VNA) from Agilent Technologies, as shown in Fig. 5. The measured center frequencies and 3-dB fractional bandwidths (FBWs) of the quad-band filter are as follows: 1.5 GHz (FBW = 9.5%), 2.5 GHz (FBW = 19.3%), 3.5 GHz (FBW = 3.6%), and 5.2 GHz (FBW = 6.1%), as expected. The measured minimum passband insertion losses are 0.45, 0.22, 1.35, and 1.59 dB, respectively, while the return losses of each passband are better than 15 dB. In addition, the novel quad-band bandpass filter can generate transmission zeros with a better than 20-dB suppression degree on both sides of the passbands, giving much improved selectivity.

The overall size of this quad-band filter is about 30 × 33 mm, as shown in Fig. 6. A comparison of this novel quad-band BPF with other reported quad-band BPFs is shown in the table, which depicts that the proposed BPF has good characteristics with compact size. Due to its simple structure, compact size, and good performance, the novel BPF presented here should be quite suitable for multiband communications systems.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (NFSC) under Grant No. 61301071 and China Scholarship Council.

Lei Chen, Lecturer

School of Electronic and Information Engineering,

Xi’an Technological University, Xi’an, People’s Republic of China, 710032

Feng Wei

Associate Professor

National Key Laboratory of Antennas and Microwave Technology,

Xidian University, Xi’an, People’s Republic of China, 710071