**2. Open-loop resonator-loaded slot UWB filter**

#### **2.1. 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

**Figure 1.** The schematic of original UWB filter using stepped-impedance slot-line resonator.

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*<sup>2</sup> /*w*<sup>1</sup> . *l* s is approximately λg /4 of slot-line in center frequency, and *l* <sup>m</sup> is approximately λg /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.

#### **2.2. Band-notched UWB filter**

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

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

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

resonator and feed line, while keeping the circuit size [14].

**2. Open-loop resonator-loaded slot UWB filter**

**Figure 1.** The schematic of original UWB filter using stepped-impedance slot-line resonator.

**2.1. Stepped-impedance slot-line UWB BPF**

UWB passbands.

44 UWB Technology and its Applications

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. **Figure 2(a)** shows the physical layout of an open-loop resonator-loaded slot-line. Frequently used structures for creating notch band include conventional open stub, spurline, embedded open stub (EOS), and open-loop resonator (OLR). **Figure 3** provides a comparison of transmission characteristic at 5.25 GHz between these methods. Open-loop resonator can produce the sharpest notch band, and conventional open stubs produce the widest notch band. The spurline and embedded stub do not increase the size of the circuit, while open-loop resonator and conventional open stub may increase the circuit size. With respect to that, the WLAN passband is quite narrow, the transition band shall be very sharp, and the open-loop resonator is preferred.

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

$$f\_1 = \frac{c}{2\sqrt{\overline{\varepsilon\_{gf}}}(4a - g)}\tag{1}$$

**Figure 2.** (a) Open-loop resonator-loaded slot-line resonator and (b) simulated frequency responses of the four frequently used structures.

where *c* is the speed of light in vacuum, εeff is the effective dielectric constant, and *a* and *g* are the side length and the gap width of the microstrip open-loop resonator, respectively.

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

The UWB bandpass filter with improved performance is designed, fabricated, and measured. A substrate with relative dielectric constant of ε<sup>r</sup> = 4.5 and a thickness of h = 0.8 mm is used

*d*<sup>2</sup> = 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

**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

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

**Figure 5.** Configurations of the proposed UWB filter with three-stub-loaded slot-line MMR. (a) Top view and (b) bottom

<sup>m</sup> = 5.9 mm, *w*<sup>0</sup> = 1.5 mm, *w*<sup>3</sup> = 0.2 mm, *w*<sup>4</sup> = 1.2 mm, *w*<sup>r</sup> = 0.2 mm, *d*<sup>1</sup> = 1.9 mm,

<sup>1</sup> = 12 mm, *l*

http://dx.doi.org/10.5772/intechopen.80004

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

<sup>2</sup> = 5.6 mm,

47

in the design. The parameters of the proposed filter in **Figure 3** are *l*

of 6.85 GHz. The group delay is below 2 ns within the passband.

**3. UWB BPF with three-stub-loaded slot-line multiple mode** 

stub, and an additional resonant mode is shown in the UWB frequency range.

passband.

<sup>s</sup> = 6.2 mm, *l*

**resonator (MMR)**

**3.1. Three-stub-loaded slot-line UWB BPF**

lines are folded and orthogonal coupling is applied.

*l*

view.

#### **2.3. 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

**Figure 3.** (a) Layout of the proposed band-notched UWB BPF using open-loop resonator-loaded stepped-impedance slot-line resonator and (b) simulated result of the band-notched UWB BPF with varying *a*.

**Figure 4.** Measured frequency responses of proposed band-notched UWB BPF using open-loop resonator-loaded stepped-impedance slot-line resonator.

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<sup>r</sup> = 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* <sup>1</sup> = 12 mm, *l* <sup>2</sup> = 5.6 mm, *l* <sup>s</sup> = 6.2 mm, *l* <sup>m</sup> = 5.9 mm, *w*<sup>0</sup> = 1.5 mm, *w*<sup>3</sup> = 0.2 mm, *w*<sup>4</sup> = 1.2 mm, *w*<sup>r</sup> = 0.2 mm, *d*<sup>1</sup> = 1.9 mm, *d*<sup>2</sup> = 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.
