A compact, two-band bandpass filter, exploiting dual-layer hexagonal shape, open-loop folded, step-impedance-resonators (SIR), for Bluetooth and WLAN applications, using low temperature co-fired ceramic (LTCC) technology, has been designed and fabricated. The filter is composed of three hexagonal shape open-loop SIRs embedded in multilayer thin ceramic to produce a compact size. The proposed topology is demonstrated with a design operating at 2.40 GHz with a bandwidth of 400 MHz and 5.20 GHz with a bandwidth of 200 MHz. More than 30 dB of spurious suppression from 5.8 to 7 GHz is demonstrated in the experimental results.
Current trends in wireless communication systems are miniaturization and high integration. Filters with compact size, low cost and good performance have attracted much research interest in recent years. Many approaches have been proposed to design wideband bandpass filters (BPF) for wireless applications.1,2 For example, dual-band BPFs have been designed using the stepped impedance resonator (SIR) method and several individual resonators in parallel conﬁgurations, which require a large size. A stack loop resonator structure with microstrip patch perturbation for a dual-mode dual-band ﬁlter has been reported.3 Embedded microstrip dual-mode resonators with microstrip patch perturbation have also been adopted for dual-band design.4 However, designs of wideband BPF with compact size remain a challenge. Among several technologies to realize highly integrated components and modules, LTCC is one of the most efficient methods owing to its 3D structure and low cost. Therefore, many RF filters with LTCC technology have been presented recently.5
In this article, a dual-layer hexagonal shape SIR is introduced, to design a compact two-band LTCC BPF. Moreover, the resonator is printed on multilayer thin ceramic, which can reduce the size of the component efficiently. Finally, this proposed filter is verified by simulation and measurements. Good agreement between the simulation and experiment results is achieved.
Dual-layer Hexagonal Open-Loop SIR
The pentagonal shape open-loop SIR, shown in Figure 1, is simply a SIR with its two low-impedance sections and high-impedance sections being printed on different layers, which can reduce the area of the circuit. The two low-impedance sections are bent to form an open-end capacitance. Tight coupling is purposely established between the two ends to provide a sufficiently large capacitance and an alternative path for the signal traveling from input to output port.6 The advantage of using a hexagonal resonator is that it has more sides than the square resonator, allowing a high degree of coupling. The front view of the resonator is also shown. The characters of this kind of resonator have been analyzed theoretically and experimentally.6
LTCC Filter Design
The configuration of the proposed ﬁlter is shown in Figure 2. The ﬁlter consists of three identical SIRs. The high-impedance sections of the first and second resonators are printed on the first layer, the thickness of which is 0.4 mm. The low-impedance sections of the first and the second resonators are printed on the second layer, the thickness of which is 0.1 mm. The high-impedance section of the third resonator is printed on the third layer and the thickness of this layer is 0.1 mm. The low-impedance section of the third resonator is printed on the fourth layer, the thickness of which is 0.1 mm also. As shown, the low-impedance section of the first resonator is overlapped with the low-impedance sections of the second and third resonators. The first and second resonators are connected with coplanar wave guide (CPW) printed on the fifth layer through metal vias and the CPWs are used as input/output ports. The thickness of the fifth layer is 0.3 mm. In order to equalize the electric potential between the upper and lower ground planes of the stripline (SL) structure, ground vias are placed around the ﬁlter and metal is printed on the front and back sides of the filter.
To validate the above design approach, the proposed ﬁlter was fabricated in multilayer ceramic, with a total component thickness of 1 mm. The relative dielectric constant of the ceramic is 27. After simulation and optimization by Ansoft HFSS, the geometric dimensions are determined as follows: L1 = 1.4 mm, L2 = 0.90 mm, W1 = 0.48 mm, W2 = 0.17 mm, S1 = 0.2 mm, S2 = 0.3 mm, L3 = 0.5 mm and the distance between the first and second resonators is L5 = 0.6 mm.
The overall size of the fabricated ﬁlter is 7 × 5 × 1 mm. The simulated and measured responses are compared in Figure 3. The passbands are centered at 2.45 and 5.25 GHz with bandwidths of 400 and 200 MHz, respectively. The measured minimum insertion losses, including the loss from the transition between the CPW and SL, are 1.20 dB, with the passband return loss better than 10 dB at 2.45 GHz and with an insertion loss of 1.78 dB and a return loss better than 10 dB at 5.25 GHz. The attenuation level is more than 30 dB from 5.8 to 7 GHz. The measured results are in close agreement with the simulated ones.
A compact, two-band LTCC ﬁlter for Bluetooth and WLAN applications has been presented. The ﬁlter employs dual-layer hexagonal shape SIRs printed on two ceramic layers, which has a volume of 7 × 5 × 1 mm. The filter operates at 2.45 and 5.25 GHz with bandwidths of 400 and 200 MHz, respectively. More than 30 dB of spurious suppression from 5.8 to 7 GHz is obtained. The simple multilayer planar structure, as well as compact size, makes it attractive for wireless communication applications.
This project is supported by the China Postdoctoral Science Foundation, People’s Republic of China [Project no.: 20100471410].
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