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Bandpass filters help sort the many different frequency bands used in modern communications systems. With the growth of portable electronic communications devices, miniaturization of these filters takes on greater importance. On that front, designers leveraged an Archimedes spiral capacitor and ground-loaded elliptical ring stubs to develop a compact dual-band bandpass filter (BPF). The filter, with passbands of 0.77 to 1.07 GHz and 1.65 to 2.02 GHz, is well-suited for GSM/DCS wireless-communications systems.
By using these novel circuit techniques, the filter can be made quite small on a printed-circuit board (PCB). The Archimedes spiral capacitor’s spiral arms can be extended for connection to the outer portion of the ground-loaded stubs to achieve a second resonant mode for dual-band operation. The size of this filter design is determined by 0.051λ g × 0.052 λ g, where λ g is the guided wavelength of the center frequency of the lower-frequency band in the dual-band filter configuration.
When fabricated and characterized with the aid of commercial RF/microwave test equipment, the filter’s two 3-dB passbands were found to be 0.77 to 1.07 GHz (the 32.6% passband at 0.920 GHz) and 1.65 to 2.02 GHz (the 20.2% passband at 1.835 GHz). As a result, the filter becomes a good component complement to GSM/DCS wireless-communications systems.
Wireless-communications systems such as GSM/DCS-based networks rely on economical and highly integrated (preferably small) dual-band BPFs to maintain separation between occupied frequency bands.1-5 For example, as ref. 1 details, half-wavelength stepped-impedance resonators were used for dual-band filter designs.
The design approaches in refs. 2 and 3 employed two different resonators to achieve the dual-mode resonances required for a dual-band filter. In ref. 4, multilayer structures and embedded resonators were used for a dual-band filter design. In ref. 5, series and shunt resonators were proposed as a practical solution to a dual-band filter.
While the performance levels of these various dual-band filter approaches were quite good, they all shared one mechanical trait—none of the design methods yielded compact circuitry. In terms of achieving filter miniaturization, a compact BPF based on an Archimedes spiral capacitor and short-loaded stubs was presented in ref. 6, but it’s a single-band filter. As a solution for a compact dual-band filter, the latter design approach was adapted by extending the spiral arms of the Archimedes spiral capacitor for connection with the outer portions of the ground-loaded stubs to excite a second resonant mode. This made it possible to fabricate an extremely compact dual-band BPF.
Figure 1 shows the configuration of the compact dual-band filter, with a three-dimensional (3D) view (left) and a top view (right) of the filter. It was fabricated on 5580 PCB material from Rogers Corp. The substrate material has relative permittivity (εr) of 2.2 and height (h) of 0.508 mm.
A two-turn Archimedes spiral capacitor in this circuit extends to the etched gap of an elliptical ring. Then, the two arms of the lengthened spiral are connected with the outer elliptical stubs. The elliptical ring connects with the ground plane by means of two viaholes with radius r. The parameters of the spiral capacitor (Fig. 1) include ri = 2.1 mm, W0 = 0.8 mm, W1 = 0.5 mm; the gap of the spiral is determined as (W0 – W1)/2 = 0.15 mm.
Additional parameters for the spiral capacitor are Wf = 1.52 mm; W2 = 0.5 mm; W3 = 0.8 mm; W4 = 0.5 mm; W5 = 0.3 mm; W6 = 0.3 mm; r1 = 6.9 mm; r2 = 6.4 mm; r3 = 1.9 mm; e0 = 0.2 mm; e1 = 1 mm; e2 = 0.2 mm; c0 = 4 mm; c1 = 6 mm; L1 = 2.7 mm; L2 = 2.1 mm; L3 = 1.5 mm; L4 = 0.8 mm; h = 0.508 mm; and θ = 45°.
The novel filter was fabricated on low-loss commercial PCB circuit material as shown in Fig. 2 (left). Good agreement exists between the measured and simulated performance results of the BPF (Fig. 2, right). The measured and simulated results reveal the two passbands, with S21 forward-transmission responses in good agreement for passband 3-dB bandwidths. The 3-dB bandwidths of the first passband are 0.75 to 1.08 GHz and 0.77 to 1.07 GHz. The 3-dB bandwidths of the second passband are 1.59 to 2.14 GHz and 1.65 to 2.02 GHz. The first passband has two simulated resonances, at 0.858 and 0.996 GHz, and the second passband has a simulated resonance at 1.919 GHz. Three measured transmission zeros occur at 1.22, 2.38, and 2.85 GHz, with respective attenuation levels of –40.99, –34.66, and –39.44 dB.
Electric energy densities at resonant frequencies of 0.858, 0.996, and 1.919 GHz are depicted for the BPF in Figs. 3a, b, and c, respectively. Electric energy densities at 0.858 and 0.996 GHz are concentrated on the spiral and the center interdigital capacitor, and the electrical energy density is very strong at the outer elliptical stubs at 1.919 GHz.
The filter provides relatively wide bandwidths of 32.6% and 20.2% in its two passbands in a relatively small circuit structure, compared to previous efforts to design and fabricate dual-band BPFs for similar commercial wireless-communications applications. Its small size is achieved through novel use of an Archimedes spiral capacitor, ground-loaded elliptical ring stubs, center-loaded interdigital capacitor, and outer elliptical stubs. Another key benefit is that the filter can be fabricated on low-cost, commercially available PCB material with excellent results.
The authors would like to acknowledge that work on the BPF was supported by the National Natural Science Foundation of China, under Grant Nos. 61401110 and 61371056, as well as by the Dean Project of Guangxi Wireless Broadband Communication & Signal Processing Key Lab, under Grant No. GXKL0614103.
Lin Peng is associate professor and Si-Min Li and Xing Jiang are professors at Guangxi Wireless Broadband Communication and Signal Processing Key Laboratory, Guilin University of Electronic Technology, Guilin, 541004, Guanbgxi, China. Ji-Yang Xie is a Master's degree candidate at Guangxi Experiment Center of Information Science, Guilin, 541004, Guangxi, China.
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