Dual-band bandpass filters using SIRs with open-stub line and zero-degree feed structure

A novel compact dual-band bandpass filter (DBPF) using SIRs with open-stub line and 0-degree feed structure is presented in this paper. The low-band is obtained by using hairpin type Stepped Impedance Resonators (SIR) split ring through electrical coupling, And by tuning a pair of open-stub lines the high-band is realized too. In order to improve the stop band attenuation, zero-degree feed structure is implemented to introduce three transmission zeros. So the proposed DBPF has high selectivity, and is also simple in design and compact in structure. Finally the proposed filter is designed and fabricated, and the simulation and test results are in good agreement.

shiftable even-mode resonance and almost unchanged fundamental odd-mode resonant when tuning the open stub [9,10].
Compared with the traditional filters, 0-degree feed structure filters can improve the decay rate of the transition from pass band to stop band, without increase of order number or the introduction of new resonance unit, which ensures a compact design of the filter [11,12]. The proposed DBPF in this paper is mainly based on λ/2 SIRs structure and stub-loaded resonators and 0-degree feed structure. In comparison with other filters, the proposed filter distinguishes itself with its simplicity in design, which is realized with compact SIRs and an easily accessible second passband by using the open-stub lines. Both the dual-band filter in [11] and the DBPF in this paper are based on 0-degree feed structure. However, the DBPF in this paper has smaller size due to one pair (rather than two pairs) of SIRs structure and has an easily controllable second resonant band with open-stub lines. So the filter is simple in design and small in size. Finally, the 2.4/3.5GHz microstrip DBPF for short range wireless communication and FWA application is fabricated to verify the proposed design. The measurement of the filter is in good agreement with the simulation.

II.THE DESIGN OF DBPF
The dual-band bandpass filter design process is divided into three steps. The first step is to get basic resonator unit using SIR structure which is detailed in section A. Then, section B demonstrates how hairpin SIR filter with 0-degree feed structure is designed to obtain the lowband passband. The third step is to produce the high-band passband through adding a pair of open-stub lines in section C. Finally, the design parameters of microstrip DBPF using SIRs with open-stub line and 0-degree feed structure are obtained.

A.The basic resonator unit using SIR structure
In this section, SIR split ring is used as the basic resonator unit of the filter. Instead of uniform impedance transmission line resonators, SIRs [13] are used. They play an even more significant role in miniaturization and harmonics suppression. Half wavelength structure of SIR is shown in Fig. 1.
It can be seen in Fig. 1 that the λ g /2 SIR is symmetric and has two different characteristic impedences of Z 1 and Z 2 . And the corresponding two electrical lengths are θ1 and θ2, respectively. For simplicity, let θ 1 =θ 2 =θ in practice. The admittance of the resonator at the open end, Y in is given as follows: [12]   Where the impedance ratio R Z =Z 2 /Z 1 . The resonance condition can be obtained from the following equation: It is now readily to calculate the resonant frequency corresponding to the length value. The resonant frequency is set as f 0 ccorresponding to an electrical length of θ 0 . Letting first spurious resonant frequency of λ g /2 SIR be fs1 corresponding to θ s1 , we can obtain Even the fundamental resonance frequency is settled, the impedance ratio can be adjusted to control other resonant response positions, which explains why SIR can be used for harmonic suppression.

B.Hairpin SIR Filter with zero-degree feed structure
Hairpin resonator is a kind of improved filter structure of half wavelength coupled structure, while SIR hairpin resonator structure is more compact than the traditional hairpin resonator, so the latter is preferred. Fig.2 shows two feed structures of hairpin type microstrip SIR split ring, one ( Fig.2 (a)) is non 0-degree feed structure; the other ( Fig. 2(b)) is 0-degree feed structure.
The center frequencies of the filters are both at 2.4GHz, the return loss are both below 25dB. However, the zero degree structure has produced two transmission zeros at 2.12GHz and 2.67 GHz, respectively, so the stopband rejection is greatly increased. Fig .1 The general structure of λg/2 SIR  On the basis of the design of section B, by adding a pair of open-stub lines, the second passband is produced, so the DBPF using SIRs can be realized. The structure of DBPF is shown in Fig. 4.

C．Hairpin Dual-Band Pass Filter Using SIRs with Open-Stub Line Based on 0 Degree Feed Structure
The properties with varying stub length are verified by the simulated curves shown in Fig. 5.
The odd-mode resonant frequencies are not affected by the open stubs; the low-band center frequency is mainly determined by the hairpin type SIRs. While the high-band center frequency, corresponding to the resonant frequency of the even-mode, is easily varied by changing the stub length Ls.
When the length of the open stubs is 13.2mm, the highband center frequency is found to be 3.5GHz. The main parameters of the DBPF are the same as that in Fig .2 (b) in section B. The width of the open stub is 1.1mm, and the open stub is bent to minimize circuit board size. Finally, the proposed DBPF is fabricated, and the photo is shown in Fig.6.
As can be observed from Fig.7, although the measured S parameters are slightly worse than the simulated one, and the high-band passband is a little offsetting, the agreement between simulated and measurement results is still good. The measured dual passbands are at 2.42 and 3.6GHz with insertion losses of 1.1dB and 3.16dB and the fractional frequency bandwidths of about 5.8% and 3.6%, respectively. And both the simulated and the measured return losses within the low-band passband are better than 15dB, while the measured return losses within the high-band passband is about 10dB, which is less than the simulated one. Three transmission zeros are realized with more than 35 dB attenuations. The measured insertion loss is mainly due to the conductor loss, and frequency deviation can be attributed to the possible tolerance of fabrication. Thus, the proposed novel DBPF can meet the 2.4 GHz and 3.5GHz wireless communication requirements.

III. CONCLUSION
A novel compact dual-band bandpass filter (DBPF) with high selectivity is proposed. The basic resonator unit is SIR split ring, the low-band passband is obtained through the SIRs electric coupling. By adding open-stub lines, the high-band passband is easy to generate and adjust almost without influence to the low-band passband.In order to further improve the selectivity, 0degree feed structure is applied to introduce three transmission zeros. Finally, it is observed that there is a good agreement between the simulated and measured results. So the proposed novel DBPF using SIRs with open-stub lines and 0-degree feed structure not only has high selectivity, but also is compact in size, and easy to realize. It can meet the 2.4 GHz and 3.5 GHz wireless communication requirements such as short range wireless communication in WLAN and FWA application.