Slot-Line UWB Bandpass Filters and Band-Notched UWB Filters

Slot-line ultra-wideband (UWB) bandpass filters and band-notched UWB filters are presented for UWB systems. Three types of slot-line multimode resonators are proposed and studied. Microstrip feed lines are used to realize the desired strong external coupling in a simple manner. By properly allocating the resonant modes of resonator and external coupling, UWB bandpass filters have been realized. Next, microstrip resonators, i.e., open-loop resonator, stub-loaded dual-mode resonator, and triangular dual-mode ring resonator, are loaded to the slot-line; notched bands are realized in the UWB passbands. The design methodology has been verified by the measured results.


Introduction
Ultra-wideband (UWB) technology is strongly desired in high-speed transmission systems. Since the US Federal Communications Commission (FCC) authorized the unlicensed use of UWB devices in 2002 [1], research on UWB components such as UWB bandpass filters (BPFs) becomes a hot spot. Many methods have been proposed to design various UWB bandpass filters (BPFs) including multimode resonators (MMRs) [2] and stub-loaded resonators [3,4]. To achieve the desired external coupling, extreme narrow gap between microstrip resonator and I/O feed lines is necessarily needed, which increases the difficulty of manufacturing. To keep the gap width in a moderate scale that can be realized easily by using PCB process, an aperture-backed parallel-coupled microstrip line with enhanced coupling degree is constructed to allocate the coupling peak [5,6]. For the indoor and handheld UWB systems, the FCC required that the UWB bandwidth must be strictly contained between 3.1 and 10.6 GHz. However, there is a need to avoid the interference from existing wireless communication systems such as wireless local area network (WLAN) in 5.2-GHz band. Generating a notch band in a UWB BPF is an effective and feasible method to solve this problem. As usual, an external resonator is used to create a notch band in the core of the UWB BPF at the cost of enlarged size [7]. In [8,9], a stepped-impedance resonator (SIR) is embedded to achieve a band-notched characteristic without increasing the circuit size. Band-notched filtering effect was achieved by adding a meander line slot to reject the undesired WLAN radio signals [10]. In [11], two spurline sections are employed to create a sharp band-notched filter for suppressing the signals of 5-GHz WLAN devices. In [12], a dual-mode fractal defectedground structure (DGS) bandstop filter is realized and connected with MMR; band-notched characteristics are realized. To avoid the interference of the wireless local area network (WLAN) at 5.25 and 5.775 GHz, two different quarter-wavelength lines are arranged on the ground of UWB BPF to generate dual narrow stopbands [13]. Obviously, combined bottom layer and top layer can make fully use of the circuit board, increasing the coupling between resonator and feed line, while keeping the circuit size [14].
In this chapter, slot-line multimode resonators are studied and applied in UWB BPFs. Microstrip feed lines are used to realize the desired external coupling in a simple manner. Microstrip resonators, such as open-loop resonator, stub-loaded dual-mode resonator, and square ring dual-mode resonator, are loaded to the slot-line; notch bands are realized in the UWB passbands.

Stepped-impedance slot-line UWB BPF
A diagram of the original UWB BPF using stepped-impedance slot-line resonator is shown in Figure 1. A multimode resonator is realized by using the stepped-impedance slot-line on the ground plane. By setting the impedance ratio and the length of the slot-line resonator, the first three resonant modes are equally allocated in the UWB passband. In other words, the central resonant frequency of the multimode slot-line resonator is determined by the length l 1 ; the location of first and third resonant frequency is determined by the width ratio w 2 /w 1 . l s is approximately λ g /4 of slot-line in center frequency, and l m is approximately λ g /4 of microstrip line in center frequency. To decrease the return loss in the passband of UWB, the coupling must be very strong, and microstrip feed lines are placed right on the top and orthogonal to the slot-line resonator.

Band-notched UWB filter
To effectively decrease the interference between UWB system and WLAN system, a notch band shall be produced in the UWB band. To avoid the size increment of the circuit, the open-loop resonator is placed right on the top of stepped-impedance slot-line resonator, and the circuit volume can be fully used.  The microstrip open-loop resonator provides a bypass for the signal at its resonant frequency, and a notch band is produced. The resonant frequency of the open-loop resonator is approximated by where c is the speed of light in vacuum, ε eff is the effective dielectric constant, and a and g are the side length and the gap width of the microstrip open-loop resonator, respectively.

Experimental results and discussions
A UWB BPF with notch band is designed based on the abovementioned method. To further increase the attenuation of the notch band in the UWB band, two microstrip open-loop resonators are loaded to the slot-line resonator. By proper setting the position of the two openloop resonators, a narrow notch band can be achieved in the UWB passband. The layout of the proposed notched UWB BPF is shown in Figure 3(a). Figure 3(b) illustrates a full-wave  simulated result of the location of notch band against the side length a. Increment of slotline width will increase the insertion loss in the passband. It is also observed that loaded microstrip open-loop resonator produces a notch band and decreases the insertion loss in the passband.
The UWB bandpass filter with improved performance is designed, fabricated, and measured. A substrate with relative dielectric constant of ε r = 4.5 and a thickness of h = 0.8 mm is used in the design. The parameters of the proposed filter in Figure 3 are l 1 = 12 mm, l 2 = 5.6 mm, l s = 6.2 mm, l m = 5.9 mm, w 0 = 1.5 mm, w 3 = 0.2 mm, w 4 = 1.2 mm, w r = 0.2 mm, d 1 = 1.9 mm, d 2 = 1.55 mm, a = 4.1 mm, and g = 0.2 mm. Measured frequency responses of the filter are plotted in Figure 4. The results exhibit attractive UWB bandpass behaviors in the 3.1-10.6-GHz band; the narrow notch band locates in 5-GHz band. Its insertion is greater than 29 dB and the 3-dB bandwidth is about 300 MHz. The insertion loss is about 1.2 dB at the center frequency of 6.85 GHz. The group delay is below 2 ns within the passband. Figure 5 shows the configurations of the proposed UWB BPF with three-stub-loaded slot-line MMR. Three-stub-loaded slot-line MMR is fed by microstrip feed line. The MMR and the feed lines are folded and orthogonal coupling is applied.

Three-stub-loaded slot-line UWB BPF
The slot-line MMR consists of a stepped-impedance resonator and three loading stubs, with one located at the middle of the resonator. Compared with traditional SIR and stub-loaded resonator (SLR), the proposed one has more degrees of freedom to control its resonant frequencies. Once the original parameters of the slot-line resonator are determined, EM solver is invoked to tune the structure to achieve an optimized characteristic. Figure 6 depicts the simulated transmission characteristics of the resonator with and without additional stub. The solid line and dashed line indicate the transmission characteristic of the resonator with and without additional stub, respectively. Additional stub increases the electrical length of the stub, and an additional resonant mode is shown in the UWB frequency range. Next, the influence of two parameters of the resonator on the resonant mode of the resonator is studied. Figure 7 shows the variation of resonant-mode frequencies against L 8 , length of the stub, and r, radius of the additional stub. As can be seen from Figure 7, four resonant modes (i.e., f 1 , f 2 , f 3 , and f 4 ) are created in the studied frequency range, which are applied to generate the UWB transmission characteristic. Figure 7(a) shows the variation of resonant mode against the length of the stub. As L 8 increases from 2.8 to 4.8 mm, f 3 drops from 10.5 to 8 GHz, and f 4 drops from 11.5 to 10 GHz, which are located in the UWB range, while f 1 and f 2 keep almost unchanged. Figure 7(b) depicts the variation of resonant-mode frequencies against the radius of the additional stub. As r increases from 0.4 to 1.0 mm, f 1 , f 2 , and f 3 remain stationary, while f 4 drops from 12.5 to 10.5 GHz. These resonant frequencies are basically related to the stepped-impedance resonator, and some ones also can be separately controlled by tuning the loaded stubs, which shows great convenience in relocating the required resonant modes of the resonator.

Band-notched UWB BPF
Based on the UWB filter mentioned above, a notch band is produced and a band-notched UWB BPF is designed. Figure 8(a) illustrates the circuit model of a dual-mode resonatorloaded slot-line, where the white section indicates the slot-line, the gray part is the ground, and the black one is a microstrip dual-mode resonator. Equivalent circuit of dual-mode resonator-loaded slot-line is provided in Figure 8(b). Two resonant modes of the stub-loaded resonator are coupled to the slot-line, providing a bypass for the adjacent signal of its resonance. The degree of separation of two modes determines the bandwidth of the notch band, and the coupling between resonator and slot-line influences the amplitude of the attenuation. EM solver is invoked to analyze the relationship between the parameters and the transmission characteristic of the bandstop filter. Figure 9(a) shows the transmission characteristics of the resonator versus L, the length of the loading stub. When L increases from 4.5 to 6.0 mm, the lower resonant mode keeps unchanged, and the higher mode decreases from 3.06 to 2.84 GHz.
As we all know that the bandwidth of the bandstop filter is determined by the separation of two poles and the band position of the bandstop filter can be adjusted by the length of the stub L, a transmission pole is also created, which is located near the lower transmission zero, sharping the transition band. Figure 9(b) illustrates the effect of varying S on the two modes of the dual-mode resonator. When S increases from 3.0 to 4.5 mm, the bandwidth of the filter increases from 60 to 480 MHz, and the central frequency decreases from 4.6 to 4.2 GHz. Obviously, modifying S and L can change the position and the bandwidth of the stopband.

Filter implementation and results
Based on the proposed structure, a UWB BPF with notch band from 5.15 to 5.35 GHz is designed and fabricated. The feed lines and the MMR in slot-line are folded and orthogonally coupled to acquire the desired strong coupling. A dual-mode loaded-stub open-loop   resonator is loaded to the slot-line, and a notch band for WLAN is produced, as shown in Figure 10. The dual-mode resonator is folded in order to improve the slow-wave effect for miniaturization. A substrate of 28 mm × 18 mm with a relative dielectric constant of ε r = 3.5 and a thickness of h = 0.8 mm is used in the design. Finally obtained parameters of the filter shown in Figure 1 are

Dual-stub-loaded slot-line MMR
Two stubs are symmetrically loaded to slot-line, which forms a dual-stub-loaded slot-line MMR, as shown in Figure 12(a). Because the proposed slot-line MMR is a symmetrical structure, even-odd mode theory can be applied to analyze its resonant characteristics. Figure 12(b) and (c) gives equivalent circuits of the slot-line MMR.
Under odd-mode excitation, the symmetrical plane can be seemed as short-circuited, and its resonant condition can be derived as where Z A and Z B and θ A , θ B , and θ C are the characteristic impedance and electrical length of the dual-stub-loaded MMR, respectively.
Under even-mode excitation, the symmetrical plane can be seemed as open-circuited, and its resonant condition can be summarized as Resonant modes of the resonator can be controlled and allocated according to the requirements by changing the parameters of the resonator.
To have a clear knowledge of slot-line resonator, resonant characteristics of the dual-stubloaded slot-line resonator are performed by invoking the 3D EM simulator. Resonant modes  of slot-line resonator against S, L 1 , and L 2 are plotted in Figure 13, where f 1 , f 2 , and f 3 indicate the first, second, and third resonant modes of resonator, respectively. When S increases from 0.2 to 2.2 mm, f 3 decreases from 9.6 to 8.2 GHz, while f 1 and f 2 shift slightly. When L 1 increases from 23 to 25 mm, both f 1 and f 2 , together with f 3 decrease steadily. When L 2 increases from 1 to 6 mm, f 2 drops from 8.2 to 6.5 GHz, f 3 drops from 13.5 to 8 GHz, and f 1 keeps unchanged. Obviously, three resonant modes of the resonator can be designed intuitively and well set in the UWB passband. Figure 14, which is constructed by a slot-line resonator and two microstrip feed lines. On the bottom layer, a dual-stub-loaded slot-line resonator is formed firstly, where two identical stubs are symmetrically loaded to a uniform slot-line resonator. Figure 15 illustrates the frequency responses of the slot-line UWB BPF with different lengths of feed line (L4) under all the other sizes fixed. When the slot-line resonator is fed under weak coupling case with L 4 = 4.8 mm, three resonant modes with peak S21 magnitudes are observed at about 4.08, 6.41, and 9.5 GHz, respectively. As L 4 increases to 10 mm, the S21 magnitude realizes an almost flat frequency response over a UWB passband. After its sizes are slightly adjusted, an UWB frequency response is satisfactorily realized. Under the use of this hybrid microstrip/slot-line structure, the desired strong coupling between feed lines and MMR can be easily achieved by properly selecting the relative position between them.

Realization of notch band
Considering the fact that the above-achieved UWB passband range may interfere with the existing wireless systems such as wireless local area network (WLAN), a notch band may be   highly demanded in various practical applications. For this purpose, a microstrip dual-mode triangular loop resonator is formed on the top layer of a dielectric substrate and loaded to the dual-stub-loaded slot-line MMR. Figure 16 depicts the geometry, equivalent circuit model, and simulated frequency response of a slot-line loaded with back-sided microstrip triangular loop resonator, respectively. Figure 16(a) shows a simplified circuit geometry of the structure, where the white portion indicates the slot-line and the black ones are microstrip feed lines and a dual-mode triangular loop resonator with perturbations. Its equivalent circuit model is given in Figure 16(b).
Coupling between dual-mode resonator and source/load can be intuitively neglected because its value is quite small. Next, two small patches are symmetrically added as the perturbation element to the lower angles of the triangular loop resonator. These perturbations can accomplish the further separation of the two degenerate modes, creating the dual-mode behavior of the resonator. Resonant modes of the triangular loop resonator are coupled to the slot-line, providing a bypass for the adjacent signal of its resonance, and a notch band is thus created. Figure 17(a) shows the transmission characteristics of the resonator versus L 5 , that is, the perimeter of a triangular loop resonator. As L 5 increases from 31 to 37 mm, the central frequency of notch band falls off from 6.25 to 5.39 GHz, and its absolute bandwidth decreases from 1.49 to 1.37 GHz. Obviously, the perimeter of triangular loop resonator can directly determine the position of the notch band. Figure 17(b) illustrates the influence on the frequency response from varied W 5 and width of two patches. As W 5 increases from 0.2 to 1.0 mm, the notch bandwidth goes up from 1.52 to 1.79 GHz. These exhibited characteristics can be used to determine the central frequency and bandwidth of the created notch band; thus the notch band of the UWB BPF can be fully controlled.

Filter implementation and results
Based on the filter structure and analysis approach described above, a UWB BPF with a controllable notch band is designed and fabricated on a substrate with a dielectric constant of   respectively. In general, the measured results agree well with the simulated results except the loss in the high-frequency band that may be caused by the dielectric loss and fabrication tolerance. Meanwhile, simulated and measured results indicate that the group delay within the passbands is varied between 0.3 and 0.7 ns.

Conclusions
Slot-line UWB BPFs and band-notched UWB filters are presented. UWB BPFs are implemented by using microstrip feed slot-line MMR. To acquire a notch band in the UWB passbands, microstrip resonators are loaded to the slot-line. Circuit model for microstrip resonatorloaded slot-line is given and analyzed. The design methodology has been finally verified by the measured results. Band-notched UBW BPFs are hot spots in a few years; there are some trends in this field, such as multiple notched band and tunable notched band.