Filters are essential to the advancement of wireless communications networks, especially when a variety of different communications systems must be co-located or in close proximity. In such cases, compact lowpass filters with low passband insertion loss, sharp transitions, and wide rejection bandwidths can contribute a great deal to the success of multiple communications systems within the same operating area. Planar ultrawideband (UWB) microstrip filters are generally preferred for applications in high-speed or low-power transceivers due to their flexible layouts, small size, low cost, and ease of fabrication and integration with other microwave circuits. By employing a novel design approach using a folded stepped-impedance resonator (SIR), it was possible to create a compact lowpass filter with steep and broad rejection bandwidth that meets many of the requirements of modern wireless communications systems.
Lowpass filters designed for use in wireless microwave communications systems have taken on many forms and employ a variety of design techniques. Some of these filter configurations have incorporated stepped-impedance hairpin units,1 a symmetrically loaded radial patch and meandered main transmission line,2 complementary split ring resonators,3 or modified semi-elliptic and semi-circular microstrip patch resonators.4 Although high performance levels have been achieved with some of these approaches, none of these designs are suitable for UWB applications. A recently developed lowpass filter has achieved UWB coverage by using an approach based on the use of open-stub loaded and tapered compact microstrip resonator cells (CMRCs).5
Unfortunately, the lowpass filter constructed with this latter approach generates gradual cut-off frequency and has relatively complex structure with stopband that can be unstable. SIRs can yield lowpass filters with simple construction and small size, but suffering from poor spurious stopband response and harmonic performance and lacking a sharp cutoff response. But the use of a folded configuration for the SIRs has been proposed by the author as a means of designing a compact notched UWB bandpass filter.6 By building upon this approach, it was possible to develop a new configuration of folded SIR for use in a miniature lowpass filter with sharp rolloff and UWB stopband. The filter offers a deep and broad rejection bandwidth, with performance exceeding the levels shown by existing lowpass filters currently reported in the literature.
Figure 1(a) illustrates the basic configuration of a microstrip SIR. Figure 1(b) depicts its folded version, including parallel open-end high-impedance segments. The folded version offers the resonant qualities of a conventional SIR in a smaller size that leads to the size reduction of the final filter structure. In order to analyze the frequency response of a folded SIR, the structure was fabricated and simulated on commercial microwave circuit laminate material, RT/duroid 5880 from Rogers Corp. This laminate has a relative dielectric constant, er, of 2.2 in the z-direction at 10 GHz and thickness of 10 mil.
The dimensions of the folded SIR are: L1 = 2.25 mm, L2 =1.6 mm, L3 = 1.5 mm, W1 = 2 mm, and W2 = 0.1 mm. The simulated S-parameters of the folded SIR with different dimensions of W1 and L1 are shown in Figs. 1(c) and (d), respectively. One transmission zero is located at about 22.86 GHz, which its location can be adjusted by changing the width of W1 or the length of L2 or both. As can be seen from Fig. 1(c), when W1 increases from 1.6 to 2 mm (in steps of 0.2 mm), the transmission zero in 22.86 GHz will move toward higher frequency. However, in Fig. 1(d), by increasing L2 from 1.6 to 2 mm (in steps of 0.2 mm), the transmission zero in 22.86 GHz will approach a lower frequency. Although the simulated results exhibit a remarkable lowpass response, the filter design does not meet the overall performance requirements.
It is well-known that cascaded resonators can yield sharp rolloffs with extended filter stopbands because of the mutual suppression of spurious passband responses. However, the cutoff frequency of a filter may change through the use of cascaded resonators. To maintain the same cutoff frequency and provide a wide rejection bandwidth, a specific scheme was developed for combining folded SIRs. Figure 2(a) shows the configuration for a proposed lowpass filter based on combining folded SIRs. It is constructed with several cascaded folded SIRs and two microstrip open stubs at the input and output ports. Figure 2(b) shows simulation results using 1, 2, 3, and 4 folded SIRs. In all cases, the resulting cutoff frequency remains at 10.2 GHz. However, increasing the number of cascaded folded SIRs does result in a steeper descent of the transition at the cutoff frequency and increased rejection in the stopband.
Figure 2(c) shows the simulated S21 forward transmission response of the proposed lowpass filter for an UWB frequency range. To block unwanted harmonics not effectively rejected by the resonant structure and achieve an UWB stopband, open stubs are then used in combination with the folded SIRs to form the final filter structure. Figure 3(a) shows a schematic diagram of the lowpass filter based on the use of folded SIRs. The parameters of the filter were obtained and optimized as followed: L1 = 2.25 mm, L2 = 1.6 mm, L3 = 1.5 mm, L4 = 3.7 mm, L5 = 2.0 mm, L6 = 1.2 mm, W1 = 2.0 mm, W2 = 0.1 mm, W3 = 1.0 mm, W4 = 0.5 mm, and W5 = 0.2 mm.
Simulations on the performance of the lowpass filter design were performed by means of electromagnetic (EM) simulation using the Advanced Design System (ADS) suite of software simulation tools from Agilent Technologies. Figure 3(b) shows the simulated S-parameters for the novel lowpass filter. These results show that the proposed filter design exhibits insertion loss of less than 0.05-dB from DC to 8.14 GHz, which is about 80% of the passband; the return loss in this range is better than 20-dB. The filter has a 3-dB cutoff frequency located at 10.2 GHz which can be easily tuned over a wide frequency range by changing structural parameters, as shown in Figs. 3(c) and 3(d).
These simulations indicate an upper stopband with more than 30-dB rejection and bandwidth of 48.4 GHz, and relative stopband bandwidth (RSB) of 138.02%. The RSB is given by:
Compared to other reported lowpass filter designs in the literature, this high amount of rejection (30 dB) over such a wide stopband exceeds all previous results. The filter occupies a small area on its substrate as 20.9 4.8 mm2 taking into account the length of its feedlines; if the feedlines are ignored, the length of the filter is only 17.5mm. The table offers a comparison of the current design in terms of size and performance with a number of previously reported lowpass filter designs.
In summary, the proposed lowpass filter achieves outstanding electrical performance, with less than 0.05-dB passband insertion loss, better than 20-dB passband return loss, and wide stopband with better than 30-dB rejection.
MILAD MIRZAEE, Engineer, Department of Electrical and Electronics Engineering,Islamic Azad university, Eslamabad-E-Gharb Branch, Eslamabad-E-Gharb, Kermanshah, Iran; e-mail: milad.mirzaee@gmail.com
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