Microstrip filters provide vital frequency control functions in a wide range of RF/microwave systems. To be effective, they must be compact, with low passband insertion loss and high stopband rejection. In particular, lowpass filters can help limit spurious and harmonic signal content in higher-frequency systems, including at millimeter-wave frequencies, by passing desired frequency bands with minimal loss while attenuating unwanted higher frequencies. Among different emerging techniques for improving high-frequency circuit performance, the use of defected ground structures (DGSs) has shown great promise in the synthesis of microwave filters. A DGS is fabricated by etching a geometric shape from the ground plane of a printed circuit board (PCB); this results in an increase in the effective inductance and capacitance of a microstrip transmission line, supporting the design of ultrawideband (UWB) filters in relatively small circuit sizes. The use of UWB frequencies from 3.1 through 10.6 GHz in the United States, for example, is expected to increase rapidly in the next few years for short-range, high-data-rate communications.

DGS circuit elements offer great promise for the design and fabrication of not only filters and other passive planar circuits, but also in active circuits, such as amplifiers and oscillators. Essentially, a DGS is an intended defect etched into the ground plane of a planar transmission line, such as a microstrip or coplanar-waveguide (CPW) transmission line. The defect in the ground plane affects the current distribution of the transmission line, resulting in changes in the capacitance and inductance characteristics. In fact, any defect that is etched into the ground plane of a microstrip circuit can increase the effective capacitance and inductance of the transmission line in proximity to the defect or DGS circuit element.

A DGS can be formed in a periodic or nonperiodic cascaded configuration, with the resulting circuit becoming much smaller than a conventional circuit without DGS elements. The key to successful implementation of DGS-based planar circuits is the use of DGS circuit elements that are relatively simple to design and fabricate, and which have also been well characterized. Although there is still much to know about the effective use of DGS circuit elements, many researchers have reported on different DGS configurations for which S-parameters have been measured and equivalent circuits extracted, for the purpose of analyzing DGS-based circuits with commercial computer-aided-engineer (CAE) software tools.

A number of circuit approaches have been employed to achieve good out-of-band performance and sharp rolloff in UWB lowpass filters, including cross-shaped DGS forms,1 dumbbell and spiral-shaped slots,2 semicircular DGS forms,3 and quasi-p slots in the ground plane.4 However, the radiating fields from the defected structures can also cause problems in the measurement and integration of other components to the circuit. The use of tapered compact microstrip resonator cells (TCMRCs) has been proposed for creating lowpass filters with wide stopbands.5 Even though the performance of such a filter is good, the periodic arrangement of the CMRCs can result in a large physical size. Lowpass filters recently developed with slit-loaded tapered CMRC (SLTCMRC) structures have achieved sharp cutoff frequency. But they yield inadequate stopband rejection at typically 20 dB and can also suffer poor impedance matching in the passband that makes the design unsuitable for UWB use.6

One study revealed that open complementary split ring resonators (OCSRRs) can contribute to lowpass filters with very narrow transition bands.7 Unfortunately, the design is still unreasonably large. In attempts to improve filter performance and reduce size, some of these filter configurations employ symmetrically loaded triangular or radial patches and a meandered main transmission line.8,9 These filter designs do not appear suitable for applications requiring an extended passband, however. Different filter topologies using folded stepped impedance resonators (FSIRs) with parallel high-impedance segments have been proposed by this author for a compact notched UWB bandpass filter10 and an UWB lowpass filter offering a wide stopband.11

The use of a T-shaped stub loaded (TSSL) folded-stepped-impedance-resonator (TSSL-FSIR) approach, using a folded compact microstrip resonator cell, has shown great promise for designing compact, UWB lowpass filters with sharp rolloff response. This approach yields a deeper and broader rejection bandwidth than previously reported for such UWB lowpass filters. It achieves smaller size than lowpass filters that have been realized with other cascaded microstrip structures, including tapered compact microstrip resonator cells (TCMRCs), slit-loaded tapered compact microstrip resonator cells (SLTCMRCs), and OCSRRs.

Figure 1 depicts the evolution of the proposed TSSL-FSIR lowpass filter from a basic SIR circuit element. In the beginning, the SIR of Fig. 1(a) is converted into two parallel segments to create the loop shown in Fig. 1(b). Then, the folded SIR is symmetrically loaded by T-shaped open stubs in the middle of parallel low-impedance segments at the top and bottom sections of the loop as shown in Fig. 1(c). The use of T-shaped open stubs (compared to other structures) yields a steeper descent from the passband to the stopband, with a lower cutoff frequency and higher return loss in the passband. Figure 2 shows the parameters for the proposed TSSL-FSIR lowpass filter.

To study the frequency response of the TSSL-FSIR circuit, its behavior was simulated by means of a commercial computer-aided-engineering (CAE) software tool, the Advanced Design System (ADS) suite of simulators from Agilent Technologies. Assumptions for the simulation included a printed-circuit-board (PCB) material with relative dielectric constant (er) of 2.2 and a thickness of 20 mil and loss tangent equal to 0.0009. The parameters for the folded SIR are L1 = 4 mm, L2 = 1.5 mm, L3 = 1.8 mm, L4 = 1 mm, L5 = 9 mm, W1 = 0.2 mm, W2 = 0.5 mm , W3 = 1 mm, and W4 = 0.5 mm. To better understand the impact of the T-shaped open stubs, the proposed resonator structure was simulated with and without the T-shaped open stubs, as shown in Figs. 3(a) and 3(b), respectively. As these responses indicate, the T-shaped open stubs support a sharpness in transition from the filter passband to the stopband, while also improving the return loss and lowering the cutoff frequency compared to the resonator structure without the T-shaped stubs.

To better understand the influence of different T-shape open stub parameters on the frequency response of the resonator, simulated S-parameters were calculated for different dimensions of L4, L5, and W3, as shown in Figs. 4(a), 4(b), and 4(c), respectively. One transmission zero is located at about 11.23 GHz, but its location can be adjusted by changing the values of W3, L4, and L5. As can be seen from Fig. 4(a), when W3 increases from 0.5 to 1 mm in 0.25-mm steps, while the other parameters remain fixed, the transmission zero at 11.23 GHz will move higher in frequency. As shown in Figs. 4(b) and 4(c), however, by increasing L4 from 1 to 2 mm and L5 from 8 to 9 mm in 0.5-mm steps, the transmission zero at 11.23 GHz will approach the lower frequency.

Although the simulation results indicate impressive lowpass filter response, the filter design falls short of some requirements, notably in the transition region. By utilizing three TSSL-FSIRs, it was possible to develop a lowpass filter with wide stopband, as shown in Fig. 5(a). It is constructed by using three cascaded TSSL-FSIRs with same physical dimensions and microstrip open stubs at the input and output ports in order to obtain low stopband radiation loss. The simulated response of this lowpass filter is shown in Fig. 5(b). To eliminate unwanted harmonics that are not completely suppressed by the other circuit structures, so as to achieve a wide stopband, multiple open stubs are used in combination with the TSSL-FSIRs to form the final filter structure.

A schematic diagram of this final filter configuration is shown in Fig. 6(a), along with its simulated S-parameter responses in Fig. 6(b). The parameters of the filter were obtained and optimized as followed: L1 = 4 mm; L2 = 1.5 mm; L3 = 5.6 mm; L4 = 1 mm; L5 = 9 mm; L6 = 3 mm; L7 = 1.2 mm; L8 = 8.9 mm; L9 = 4.1 mm; W1 = 0.2 mm; W2 = 0.5 mm; W3 = 1.0 mm; W4 = 0.5 mm; W5 = 1 mm; and W6 = 1 mm. The results show that the proposed filter has a 3-dB cutoff frequency of 4.17 GHz and insertion loss of less than 0.1 dB from DC to 3.23 GHz, or around 77% of the bandwidth. The return loss is better than 20 dB in this region.

The stopband return loss indicates that this filter structure exhibits low radiation loss. A transmission zero can be observed near the passband edge at 8.17 GHz, with an attenuation level of 62.92 dB. The transition band is a relatively narrow 13% of the bandwidth from 4.17 to 4.72 GHz, with attenuation levels of 3 and 20 dB, respectively, at those two frequency points. The filter design promises a stopband extending from 5.3 to 60 GHz with better than 30-dB stopband suppression and approximately 168% relative stopband bandwidth. The design has demonstrated a 123% increase in passband and 40% increase in relative stopband with -20 dB rejection, while achieving a 55% reduction in size compared with its classical counterpart in ref. 6 using the same PCB substrate.

In summary, a novel T-shaped stub loaded, folded SIR cell (TSSL-FSIRC) has been proposed as an effective design element for a lowpass filter with deep and wide stopband. A proposed filter consists of three TSSL-CMRCs connected in series. To cancel the spurious response resulting from the expanded rejection bandwidth, multiple open-circuited stubs were used in combination with the FCMRCs in the filter structure. The simulation results confirm that at a 3-dB cutoff frequency of 4.17 GHz the lowpass filter exhibits less than 0.1 dB passband insertion loss, better than 20-dB passband return loss, sharp skirt performance, wide stopband with high suppression to 60 GHz, and compact physical size. This filter offers great potential for UWB communication systems where wide stopband, compact size, and easy integration with other microwave circuits are essential requirements.

MILAD MIRZAEE
Engineer
Department of Electrical and Electronics Engineering, Eslamabad-E-Gharb Branch, Islamic Azad University, Eslamabad-E-Gharb, Kermanshah, Iran; e-mail: milad.mirzaee@gmail.com.

References

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