4.1 UWB bandpass filters using MMR

A quintuple-mode resonator is proposed to design UWB bandpass filter, and the physical layout of the presented UWB filter is sketched in Figure 7 [19]. Since the whole structure is symmetrical along the T–T' line, classical odd-even-mode method is adopted to analyze the quintuple-mode resonator. As demonstrated in Figure 8, five resonant modes can be generated by quintuple-mode resonator; besides, owing to the loaded stub, two transmission zeros are realized both at lower and upper cutoff frequencies; thus, high selectivity is approached. As shown in Figure 9, the measurement results are in good agreement which shows sharp skirt and ultra-wide stopband of the UWB bandpass filter (Figure 9).

### 4.2 UWB bandpass filters using SLMMR

As illustrated in Figure 10, dual short stub-loaded resonator is presented to construct UWB transmission characteristics [31]. Owing to symmetrical structure

Figure 7. Physical layout of the quintuple-mode resonator.

Figure 8. Transmission coefficient |S21|versus weak coupling and strong coupling.

of the presented SLMMR, classical odd-even-mode method can be introduced to analyze the resonant modes of UWB filter. With even-mode excitation and oddmode excitation, the input admittance can be, respectively, written as follows:

$$Y\_{i\pi\epsilon} = Y\_c \frac{\left(jY\_1 \tan \theta\_1 - jY\_2 \cot \theta\_2 + jY\_3 \tan \theta\_3\right) + jY\_c \tan \theta\_c}{Y\_c + (-Y\_1 \tan \theta\_1 + Y\_2 \cot \theta\_2 - Y\_3 \tan \theta\_3) \tan \theta\_c} \tag{19}$$

By using the numerical calculation method mentioned in Figure 4, the design graphs for implementing UWB bandpass filter are sketched in Figure 11. For example, by properly choosing the values of θ2, the excited four resonant frequencies can be easily adjusted to the desired UWB specifications. Therefore, the first four resonant modes are located at 2.86, 5.58, 8.56, and 10.21 GHz, and the dimension parameters are optimized by IE3D as follows: L<sup>1</sup> = 8, L<sup>2</sup> = 1.6, L<sup>3</sup> = 1.6 L<sup>4</sup> = 1.4, L<sup>5</sup> = 1.2, L<sup>6</sup> = 3, L<sup>7</sup> = 2.5, L<sup>8</sup> = 1, L<sup>9</sup> = 4, L<sup>10</sup> = 3.6, L<sup>11</sup> = 8, W<sup>1</sup> = 0.2, W<sup>2</sup> = 0.3, W<sup>3</sup> = 0.6, W<sup>4</sup> = 0.2, and W<sup>5</sup> = 0.3. It can be observed in Figure 12 that simulation results are in good agreement with measurement results, which shows UWB bandpass characteristics with small and flat group delay in the passband.

In order to design bandpass filter with UWB performance while occupied compact size, dual-layered structure is proposed in [56]. The UWB filter is constructed by substrate integrate waveguide (SIW) ridge resonator, and the bandwidth of the UWB filter can be easily tuned by properly changing the width of rod in ridge

4.3 UWB bandpass filters with multilayer structure

Simulated and measurement frequency responses of presented UWB.

Schematic diagram of proposed UWB bandpass filter with SLMMR.

Figure 9.

Review on UWB Bandpass Filters

DOI: http://dx.doi.org/10.5772/intechopen.87204

Figure 10.

87

resonator. The scheme diagram is sketched in Figure 13.

$$Y\_{ino} = Y\_c \frac{\left(-jY\_1 \cot \theta\_1 - jY\_2 \cot \theta\_2 + jY\_3 \tan \theta\_3\right) + jY\_c \tan \theta\_c}{Y\_c + \left(Y\_1 \cot \theta\_1 + Y\_2 \cot \theta\_2 - jY\_3 \tan \theta\_3\right) \tan \theta\_c} \tag{20}$$

Review on UWB Bandpass Filters DOI: http://dx.doi.org/10.5772/intechopen.87204
