**2. UWB-bandpass filters with slot resonator**

### **2.1. Interdigital coupled line characteristics**

Fig. 1(a) shows a conventional interdigital coupled line which has been widely used as a capacitive coupling element in multi-stage bandpass filters. The optimized interdigital coupled lines must be performed to achieve design-specified coupling factor between two adjacent line resonators. The usual procedure is to reduce both strip and slot widths in order to achieve a tight coupling and lower insertion. However, it may introduce some difficulties into the design procedure and fabrication process as the coupling response is sensitive to the strip/slot widths configuration. In this work, we redesign and optimize the interdigital coupled line, as a result shown in Fig. 1(b). An RT/Duroid3003 substrate, which has a given dielectric constant of 3.0, a thickness of 1.524 mm, and a loss tangent of 0.0013 is used for designing the new interdigital coupled lines at a central frequency of 6.85 GHz and a fractional bandwidth of 100%. The conventional and new coupled lines are evaluated by using an electromagnetic simulation program, IE3D, which is based on the method of moments and proven to be quite accurate in its prediction. The response curves of both coupled lines are demonstrated in Fig. 2. It can be noticed that both interdigital coupled lines have almost the same resonant frequency of 6.85 GHz. Nevertheless, the new coupled line has superior performances with better *S11* and *S21* in the passband. This means that it is more suited for use as a bandpass filter element than the conventional one.

**Figure 1.** Interdigital coupled lines: (a) conventional one with *w*1 = 4*.*0mm, *l1* = 6*.*0mm, l*2* = 6*.*45mm, *g1* = 0*.*2mm, and *w2*=1*.*2mm and (b) optimized one with *w1*=4*.*0mm, *l1*=6*.*0mm, *l2* = 6*.*45mm, *g1* = 0*.*2mm, and *w2* = 0*.*5mm.

**Figure 2.** Compared responses of interdigital coupled lines with *w2* = 1*.*2mm for the conventional one and *w2* = 0*.*5mm for the optimized one.

#### **2.2. SLTR and SSIR characteristics**

296 Ultra Wideband – Current Status and Future Trends

obatained.

fabricated to prove that they improve the passband and upper stopband performances with

Then, UWB-bandpass filters based on SLTR and SSIR with embedded slot feed structure for notched band are presented. The embedded slot feed at the end of resonators will be comprehensively described. The proposed filters have narrow notches in the passband, resulting from the embedded slot feed. The center frequencies and bandwidths of the notched band can be easily adjusted by tuning the length and width of the embedded slot parameters. The wider upper stopbands caused by resonator characteristics have been also

After that, UWB-bandpass filters with single-notched and dual-notched bands and improved stopband performance are proposed using SLTR and SSIR as multi-mode resonator (MMR) and embedded slotted feed. To avoid the existing interferences in the UWB passband, two different embedded slotted feed are employed to obtain two narrownotched bands. The center frequency and bandwidth of the notched bands can be controlled by adjusting the dimensions of the embedded slotted feed. To further suppress the upper stopband, the defected slot in λ/2 stepped impedance resonator fed by interdigital coupled line is introduced. Very good agreements between the measured and simulated filter

Finally, UWB-bandpass filters based on SLTR and SSIR with embedded fold-slot are presented. The proposed filters have narrow notches in the passband and size reduction, resulting from the embedded fold-slot. The length and width of the embedded fold-slot

Fig. 1(a) shows a conventional interdigital coupled line which has been widely used as a capacitive coupling element in multi-stage bandpass filters. The optimized interdigital coupled lines must be performed to achieve design-specified coupling factor between two adjacent line resonators. The usual procedure is to reduce both strip and slot widths in order to achieve a tight coupling and lower insertion. However, it may introduce some difficulties into the design procedure and fabrication process as the coupling response is sensitive to the strip/slot widths configuration. In this work, we redesign and optimize the interdigital coupled line, as a result shown in Fig. 1(b). An RT/Duroid3003 substrate, which has a given dielectric constant of 3.0, a thickness of 1.524 mm, and a loss tangent of 0.0013 is used for designing the new interdigital coupled lines at a central frequency of 6.85 GHz and a fractional bandwidth of 100%. The conventional and new coupled lines are evaluated by using an electromagnetic simulation program, IE3D, which is based on the method of moments and proven to be quite accurate in its prediction. The response curves of both coupled lines are demonstrated in Fig. 2. It can be noticed that both interdigital coupled lines have almost the same resonant frequency of 6.85 GHz. Nevertheless, the new coupled

sharpened rejection skirts outside the passband and widened upper stopband.

characteristics have been obtained validating the proposed filter prototypes.

parameters resulting in their performances have been also studied.

**2. UWB-bandpass filters with slot resonator**

**2.1. Interdigital coupled line characteristics** 

Fig. 3(a) shows a conventional λ*/*2 microstrip linear tapered-line resonator (LTLR). This resonator has inherently spurious resonant frequencies at 2*fo* and 3*fo*, where *fo* is the fundamental frequency, which may be too close to the desired wide passband. A microstrip SIR has been proposed as shown in Fig. 3(b) for higher stopband performances. In this chapter, microstrip SLTRs as shown in Fig. 3(c) and (d), composed of a microstrip taperedline with slots are proposed. A microstrip SSIR consisting of a microstrip stepped impedance

line with a slot has been also investigated, as shown in Fig. 3(e). Fig. 4 shows the current densities of resonators at 3*fo* about 21 GHz (stopband frequency). We can notice that in Fig. 4(a) and (b), the current densities pass through the resonators. For the proposed resonators in Fig. 4(c) and (d), the current densities cannot pass through the resonators but stop at the slot. It means that the stopband frequency occurs at 3*fo*, which is about 21 GHz. We then perform the parameter study for the proposed SLTR and SSIR. The same substrate with the interdigital coupled lines in previous section has been employed. The IE3D program had been used to determine the frequency response of *S*21. The input and output ports have been defined at both ends of the proposed resonator. Fig. 5(a) shows the bandstop characteristics of *S21* when varying *w*3 of the SLTR from 4.5 to 7.5 mm. It can be found that a stopband center can move from the frequency of 24 GHz down to 15.5 GHz. As we can see that the slot is longer, lower stopband center can be obtained due to the slot length affects the distance of current distribution in resonator. When varying *g*2 of the SLTR, a stopband center is slightly shifted as shown in Fig. 5(b). Fig. 6(a) and (b) shows bandstop characteristics of the SSIR when varying *w*3 and *g*2. The results are same with responses of the SLTR but better *S21* magnitudes.

UWB-Bandpass Filters with Improved Stopband Performance 299

**Figure 4.** Current densities of resonators: (a) a conventional linear tapered-line resonator, (b) a conventional SIR, (c) the proposed SLTR, (d) the proposed SLTR with three slots and

**Figure 5.** Bandstop characteristics at 3*fo*: (a) SLTR with varied *w3* and (b) SLTR with varied *g2*

(a) (b)

(e) the proposed SSIR

In order to obtain good stopband characteristics without passband perturbations of the desired UWB-bandpass filters, slot length *w*3 =5*.*5mm and slot width *g*2 =0*.*2mm have been chosen as optimized parameters. It can be clearly noticed that the conventional microstrip resonators and the SIR have not obtained stopband characteristics while the proposed slotted resonators have stopband responses with various resonant frequencies. With these stopband characteristics, superior suppression of the spurious responses in the upper band could be obtained when they have been applied to the proposed bandpass filters.

**Figure 3.** A conventional linear tapered-line resonator, (b) a conventional SIR, (c) the proposed SLTR, (d) the proposed SLTR with three slots, and (e) the proposed SSIR. The dimensions are as follows: *l*1 =14*.*0mm, *l*2 =11*.*0mm, *l*3 =5*.*4mm, *l*4 =3*.*47mm, *l*5 = 4*.*5mm, *l*6 = 2mm, *w*1 = 4mm, *w*2 = 6mm, *w*3 = 5*.*5mm, *w*4 = 4*.*5mm, and *g*2 = 0*.*2mm

*w*4 = 4*.*5mm, and *g*2 = 0*.*2mm

line with a slot has been also investigated, as shown in Fig. 3(e). Fig. 4 shows the current densities of resonators at 3*fo* about 21 GHz (stopband frequency). We can notice that in Fig. 4(a) and (b), the current densities pass through the resonators. For the proposed resonators in Fig. 4(c) and (d), the current densities cannot pass through the resonators but stop at the slot. It means that the stopband frequency occurs at 3*fo*, which is about 21 GHz. We then perform the parameter study for the proposed SLTR and SSIR. The same substrate with the interdigital coupled lines in previous section has been employed. The IE3D program had been used to determine the frequency response of *S*21. The input and output ports have been defined at both ends of the proposed resonator. Fig. 5(a) shows the bandstop characteristics of *S21* when varying *w*3 of the SLTR from 4.5 to 7.5 mm. It can be found that a stopband center can move from the frequency of 24 GHz down to 15.5 GHz. As we can see that the slot is longer, lower stopband center can be obtained due to the slot length affects the distance of current distribution in resonator. When varying *g*2 of the SLTR, a stopband center is slightly shifted as shown in Fig. 5(b). Fig. 6(a) and (b) shows bandstop characteristics of the SSIR when varying

*w*3 and *g*2. The results are same with responses of the SLTR but better *S21* magnitudes.

could be obtained when they have been applied to the proposed bandpass filters.

**Figure 3.** A conventional linear tapered-line resonator, (b) a conventional SIR, (c) the proposed SLTR, (d) the proposed SLTR with three slots, and (e) the proposed SSIR. The dimensions are as follows: *l*1 =14*.*0mm, *l*2 =11*.*0mm, *l*3 =5*.*4mm, *l*4 =3*.*47mm, *l*5 = 4*.*5mm, *l*6 = 2mm, *w*1 = 4mm, *w*2 = 6mm, *w*3 = 5*.*5mm,

In order to obtain good stopband characteristics without passband perturbations of the desired UWB-bandpass filters, slot length *w*3 =5*.*5mm and slot width *g*2 =0*.*2mm have been chosen as optimized parameters. It can be clearly noticed that the conventional microstrip resonators and the SIR have not obtained stopband characteristics while the proposed slotted resonators have stopband responses with various resonant frequencies. With these stopband characteristics, superior suppression of the spurious responses in the upper band

**Figure 4.** Current densities of resonators: (a) a conventional linear tapered-line resonator, (b) a conventional SIR, (c) the proposed SLTR, (d) the proposed SLTR with three slots and (e) the proposed SSIR

**Figure 5.** Bandstop characteristics at 3*fo*: (a) SLTR with varied *w3* and (b) SLTR with varied *g2*

UWB-Bandpass Filters with Improved Stopband Performance 301

*g1*

*w1*

*g1 l3 l2 l1*

**2.3. Filter designs and measured results** 

In the following, the two UWB-bandpass filters have been built using the MMR conventional λ/2 resonators and the proposed SLTR and SLTR with three slots fed by λ/4 interdigital coupled lines at both ends with a central frequency of 6.85 GHz and a fractional bandwidth of 100% as shown in Fig. 7(a) and (b). The RT/Duroid 3003 substrate with a dielectric constant of 3.0, a thickness of 1.524mm and a loss tangent of 0.0013 has been used. The optimized dimensions of the resonators and the interdigital coupled lines have been obtained in the previous section. Their electrical performances are then simulated by using IE3D program.

Fig. 7(c) shows schematics of the the proposed single-microstrip SSIR bandpass filters. The optimized dimensions of the resonators and the interdigital coupled lines are the same as

(a)

*l4 l4 l4 g1 l4*

*w2 w2*

*w3*

*l3 g1 l3 l2 l1*

*w2 w3*

*l3 g1 l3 l2 l1*

*w3*

*g1*

*g1*

*l6g1 l6 l6g1 l6*

*w4 w4*

*w1 w2 w2*

*l1 l2 g1 l5 g1*

(b)

*w2 w2*

*g1 l7 l7 l7 l7* (c)

*w1 w1*

*w2 w2*

**Figure 8.** Two-resonator bandpass filters: (a) the SLTR with one slot, (b) the SLTR with three slots, and (c) the SSIR. The dimensions are as follows: *l1*=6*.*0mm, *l2*=6*.*45mm, *l3*=11*.*0mm, *l4*=5*.*4mm, *l5*=6*.*5mm,

*l6*=3*.*47mm, *l7*=4*.*5mm, *w1* = 4*.*0mm, *w2* = 0*.*2mm, *w3* = 5*.*5mm, *w4* = 4*.*5mm, and *g1* = 0*.*2mm

*2.3.1. Single-SLTR filter* 

*2.3.2. Single-SSIR filter* 

*w1*

resulting from the previous section.

*l1 l2 g1 l5*

*l3*

*g1*

*w3*

*w4 w4*

*g1*

*l6g1l6 l6g1 l6*

*l1 l2 g1 l5*

*w3*

*g1*

*w3*

**Figure 6.** *S21* responses: (a) SSIR with varied *w3* and (b) SSIR with varied *g2*

**Figure 7.** Bandpass filters with a single resonator using: (a) the SLTR (b) the SLTR with three slots, and (c) the SSIR. The dimensions as follows: *l1*=6*.*0mm, *l2*=6*.*45mm, *l3*=11*.*0mm, *l4*=5*.*4mm, *l5*=3*.*47mm, *l6* = 4*.*5mm, *w1* = 4*.*0mm, *w2* = 0*.*2mm, *w3* = 5*.*5mm, *w4* = 4*.*5mm, and *g1* = 0*.*2mm

#### **2.3. Filter designs and measured results**

#### *2.3.1. Single-SLTR filter*

300 Ultra Wideband – Current Status and Future Trends

**Figure 6.** *S21* responses: (a) SSIR with varied *w3* and (b) SSIR with varied *g2*

(a) (b)

**Figure 7.** Bandpass filters with a single resonator using: (a) the SLTR (b) the SLTR with three slots, and (c) the SSIR. The dimensions as follows: *l1*=6*.*0mm, *l2*=6*.*45mm, *l3*=11*.*0mm, *l4*=5*.*4mm, *l5*=3*.*47mm,

*l6* = 4*.*5mm, *w1* = 4*.*0mm, *w2* = 0*.*2mm, *w3* = 5*.*5mm, *w4* = 4*.*5mm, and *g1* = 0*.*2mm

In the following, the two UWB-bandpass filters have been built using the MMR conventional λ/2 resonators and the proposed SLTR and SLTR with three slots fed by λ/4 interdigital coupled lines at both ends with a central frequency of 6.85 GHz and a fractional bandwidth of 100% as shown in Fig. 7(a) and (b). The RT/Duroid 3003 substrate with a dielectric constant of 3.0, a thickness of 1.524mm and a loss tangent of 0.0013 has been used. The optimized dimensions of the resonators and the interdigital coupled lines have been obtained in the previous section. Their electrical performances are then simulated by using IE3D program.

#### *2.3.2. Single-SSIR filter*

Fig. 7(c) shows schematics of the the proposed single-microstrip SSIR bandpass filters. The optimized dimensions of the resonators and the interdigital coupled lines are the same as resulting from the previous section.

**Figure 8.** Two-resonator bandpass filters: (a) the SLTR with one slot, (b) the SLTR with three slots, and (c) the SSIR. The dimensions are as follows: *l1*=6*.*0mm, *l2*=6*.*45mm, *l3*=11*.*0mm, *l4*=5*.*4mm, *l5*=6*.*5mm, *l6*=3*.*47mm, *l7*=4*.*5mm, *w1* = 4*.*0mm, *w2* = 0*.*2mm, *w3* = 5*.*5mm, *w4* = 4*.*5mm, and *g1* = 0*.*2mm

## *2.3.3. Two-SLTR filter*

Fig. 8 depicts schematics of the bandpass filters with two linear tapered-line resonators connected in cascade. Fig. 8 (a) and (b) was the proposed SLTR filters with single- and threeslotted structures, respectively. All dimensions of the resonators and interdigital coupled lines have been shown.

UWB-Bandpass Filters with Improved Stopband Performance 303

**Figure 10.** Photographs of the fabricated two-filters: (a) two-SLTR, (b) two-SLTR with three slots, and

(c)

(a)

(b)

insertion losses above 25 dB in a range of 19–25 GHz and above 47 dB at 24 GHz as shown in Fig. 11 (f). However, the measured passband insertion loss is higher than the simulation result due to the dimension of the conductor and further the conductivity slightly deviated from the design. The group delay of both filters slightly varies between 0.2 and 0.3 ns in the

This section proposes new UWB-bandpass filters using slotted linear tapered-line resonators (SLTR) and slotted step-impedance resonator (SSIR) structures driven by interdigital coupled lines at both ends of the resonators for improving the stopband performances. Also, using embedded slot structure in the input and output feed line can create a notched band.

**3. UWB-bandpass filters with embedded slot**

(c) two-SSIR

passband.
