Feng Wei, Chang Jia Gao, Hai Wei Zhang, Bei Liu, and Xiao Wei Shi
Ultrawideband (UWB) communications makes possible a number of services within the frequency band from 3.1 to 10.6 GHz authorized in 2002 by the United States Federal Communications Commission (FCC).1 To realize a functional UWB communications system, the design and fabrication of a small but high-performance bandpass filter is critical. Many efforts in the design and development of bandpass filters (BPFs) for UWB applications have led to a variety of UWB filter types, including stepped impedance resonator and distributed highpass filter designs.2-6
Multiple-mode resonator (MMR) structures3-5 and three-line microstrip structures6 are based on the stepped impedance resonator approach and have become widely adopted as an effective form of wideband filter. These two types of BPFs have good passband frequency performance and are suitable for practical implementation. Unfortunately, both are relatively large in size and have a narrow stopband that does not provide the bandwidth or performance needed to suppress higher-order harmonics.
Based on previous works, a new UWB BPF has been developed with a wide passband, an improved stopband performance, and relatively small size. The design is based on a folded three-line microstrip structure and semicircular defected ground structure (S-DGS) on the back of the metal. The use of the S-DGS helps to improve the out-of-band performance of the filter. The 3-dB bandwidth of the fabricated filter covers a wide range from 3.1 to 10.6 GHz. The measured results agree closely with the simulated results.
Figure 1 depicts the configuration of three kinds of wideband filters. The classical multiple-mode resonator structure has the length of one wavelength, as shown in Fig. 1(a). Figure 1(b) shows a conventional three-line microstrip structure with a length of less than three-quarters wavelength. The folded three-line microstrip structure proposed in the current report makes the filter length less than one-half wavelength. On the other hand, the connecting stub of the two three-line microstrip structures can be seen as an open load that has periodic transmission zeros and can improve the passband performance, as shown in Fig. 1(c).
To achieve structural symmetry, only modes 1 and 3 can be excited; the eigenvoltage matrix for these dominant modes of the folded threeline microstrip structure can be written as shown in Eq. 16:
Assuming that the phase constants of three modes are approximately equal and comparing the Z-parameters of the folded three-line microstrip with those of the equivalent circuit in Fig. 2, the result is shown in Eqs. 2 and 3:
where Zmi = Zoi/(mi2 + 2) and Zoi = the characteristic impedance of mode i.6 The frequency characteristics of the filter design were simulated by using Version 11.0 of the commercial High-Frequency Structure Simulator (HFSS) from Ansoft (www.ansoft. com). The electromagnetic (EM) simulation software is based on a three-dimensional finite-elementmethod (FEM) to model antennas, filters, and other high-frequency circuits and structures using solutions to Maxwell's equations. Figure 3 shows the HFSS simulations for the proposed filter structure. From the simulated filter results, it is apparent that the folded three-line microstrip structure can obtain a wide passband and sharp cutoff frequency response, but its stopband is relative narrow, which limits its application.
The use of DGS design elements in microstrip lines have periodic etched defect on the ground plane and can provide good stopband characteristics.7-10Figure 4 illustrates the configurations of the proposed S-DGS fed by a 50-O microstrip line. The S-DGS is composed of two semicircular defected areas and one narrow connecting slot on the ground plane. It obtains a higher quality (Q) and better stopband characteristics compared to a microstrip filter design with a conventional DGS. The frequency characteristics of the S-DGS element can be modeled by a series-connected parallel inductive-capacitive (LC) resonance circuit. The equivalent capacitance and inductance of the circuit can be extracted by using circuit analysis theory as follows in Eqs. 4 and 58:
fc = the 3-dB cutoff frequency and
f0 = the resonant frequency of the bandstop response.
Furthermore, the effects of the proposed S-DGS dimensions on the frequency characteristics have been investigated. Figure 5 shows the transfer characteristics of the S-DGS elements for various dimensions. Different cutoff frequencies can be achieved by changing the radius of the semicircle and the width of the gap.
In order to design a compact UWB BPF using this approach, a folded three-line microstrip structure and one S-DGS were cascaded along with a microstrip line. The simple structure of the proposed UWB BPF is quite flexible in terms of design parameters, as shown in Fig. 6. The SDGS element is used to improve the out-of-band spurious performance of the miniature filter circuit. Using HFSS V.11.0, a simulation of the filter circuit was performed based on RT/duroid 5880 printed-circuitboard material from Rogers Corporation, with a dielectric constant of 2.2 and material thickness of 1 mm. The dimensions for the design were as follows: W0 = 3.0 mm, W1 = 0.4 mm, W2 = 0.2 mm, W3 = 0.15 mm, W4 = 0.3 mm, L1 = 8.0 mm, R1 = 1.1 mm, Lslot = 9.0 mm, Lopen = 11.0 mm, Wslot = 1.3 mm, and Wopen = 6.5 mm.
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The UWB BPF was fabricated and measured using a model N5230A two-port vector network analyzer (VNA) from Agilent Technologies. The analyzer covers a measurement bandwidth of 300 kHz to 20 GHz. From the measured results in Fig. 7, the fabricated UWB BPF has a passband from 3.1 to 10.6 GHz. Both the transition bands at upper and lower cutoff frequencies are very steep. The measured rejection loss is more than 20 dB at the midband frequency of the notched band and the upper-stopband with 10-dB attenuation to 20 GHz. The simulated and measured group delays are shown in Fig. 8. The deviations of the measurements from the simulations are expected mainly due to the finite substrate and the reflections from the RF connectors. Fig. 9 shows the top and bottom views of the fabricated UWB BPF. The overall size of the fabricated UWB BPF is about 16.9 x 26.8 mm.
In summary, an extremely compact UWB BPF with improved upper-stopband response has been proposed and designed based on folded microstrip lines and S-DGS transmission-line elements. The filter design is suitable for UWB communications applications from 3.1 to 10.6 GHz, in which miniature filters are important for both infrastructure equipment and embedded transceiver devices. By tuning the parameters of the elements, the proposed UWB BPF can achieve a wide passband while also providing a narrow notch band. Measurements on prototype filters compared favorably with simulated results from a commercial EM simulator. The miniature microstrip bandpass filter's simple planar geometry makes it a viable candidate for use with existing microwave-integratedcircuit (MIC) design and fabrication methods.
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