UWB Bandpass Filter with Dual Notched Bands Using T ... - MDPI

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UWB Bandpass Filter with Dual Notched Bands Using T-Shaped Resonator and L-Shaped Defected Microstrip Structure Xuemei Zheng 1,2 , Yuwen Pan 3 and Tao Jiang 1, * 1 2 3

*

College of Information and Communication Engineering, Harbin Engineering University, Harbin 150001, China; [email protected] College of Information Engineering, Northeast Electric Power University, Jilin 132012, China Research and Development Department, Sainty-Tech Communications Limited, Nanjing 211100, China; [email protected] Correspondence: [email protected]; Tel.: +86-451-8251-9808

Received: 12 April 2018; Accepted: 29 May 2018; Published: 1 June 2018







Abstract: In this paper, an ultra-wideband (UWB) bandpass filter (BPF) with dual notched bands using a T-shaped resonator and L-shaped defected microstrip structure (DMS) is proposed and fabricated. First, the principle of generating notched bands by using a T-shaped resonator and L-shaped defected microstrip structure to determine the size parameters of the structure is analyzed. High frequency structure simulator (HFSS) software is used to analyze the performance of the filter, and advanced design system (ADS) is used to extract the equivalent circuit model parameters. The two simulation results are consistent, which further verifies the correctness of the circuit model. Finally, the filter is fabricated and measured. The measured results are in good agreement with simulated results, demonstrating good insertion loss and return loss. The proposed filter has dual independently controllable notched bands which are implemented by coupling the T-shaped resonator to the transmission line and by etching the L-shaped defected microstrip structure respectively. The proposed filter can suppress dispensable bands at 3.5 GHz and 7.5 GHz in WiMAX band and X-band to improve the performance of the ultra-wideband communication system. By adjusting the parameters of the T-shaped resonator and L-shaped defected microstrip structure, the UWB bandpass filter with dual notched bands working at WiMAX band and X-band can be designed and applied to the wireless communication system. Keywords: microstrip filters; defected microstrip structure; microstrip circuits; bandpass filter

1. Introduction Ultra-wideband (UWB) communication systems are widely used in wireless communication because of low cost and high data rate. The ultra-wideband (3.1–10.6 GHz) frequency spectrum was announced by the Federal Communications Commission in 2002 [1] for unlicensed indoor and hand-held commercial applications. ultra-wideband filters, as one of the key components of the radio frequency front-end, directly affect the quality of communication systems. Some other existing narrowband services already occupy frequencies in the ultra-wideband band, such as WiMAX in some Asian and European countries (3.4–3.6 GHz), and X-band satellite communication services frequency band (7.25–8.395 GHz). Therefore, it is necessary to design a ultra-wideband bandpass filter with notched bands. In order to realize the miniaturization and narrow-band notch of microwave integrated circuits, ultra-wideband filters require better frequency selection performance, a more compact structure, and seamless integration.

Micromachines 2018, 9, 280; doi:10.3390/mi9060280

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A number of techniques to design ultra-wideband bandpass filters with notched bands have been studied [2–16]. The notch bands are generated by introducing a resonance unit, such as a stepped impedance resonator (SIR) [2,3] and multi-mode resonator (MMR) [4–9]. The disadvantage of adopting stepped impedance resonator or multi-mode resonator is that the size of the notch filter is too large and the transmission zero cannot be flexibly adjusted. The stop-band function is achieved by introducing defect ground structure (DGS) [10,11] and defect microstrip structure (DMS) [12,13]. Defect ground structure can effectively reduce the size of the filter, but it is difficult to obtain good broadband and sharp skirt selectivity. In addition, there is leakage of the floor. The microstrip filter based on defect microstrip structure can solve the floor leakage problem. The coplanar waveguide (CPW) [14–16] is used to achieve notch characteristics. The coupling between the microstrip line and the coplanar waveguide is strong and the filter structure is compact, but the stop band characteristic is generally not good. Novel dual-band bandpass filters are implemented for 3.5 GHz and 5.8 GHz in [17], and the pass band of the proposed filter covers 2.4 GHz, 2.5 GHz, and 3.5 GHz in [18]. In order to remove interference from WiMAX and satellite communication bands, a dual notched ultra-wideband bandpass filter is proposed in this paper. A dual notched ultra-wideband bandpass filter using a T-shaped resonator and L-shaped defect microstrip structure is proposed and fabricated. The first and second notched band functions are respectively implemented by coupling the T-Shaped resonator to the transmission line and etching the L-shaped defect microstrip structure on the bandpass filter. In the second section, we will discuss the design of the ultra-wideband filter with dual notched bands and study the frequency rejection function based on a T-shaped resonator and L-shaped defect microstrip structure. In the third section, a dual bandpass filter with notched bands is fabricated and measured, with the measured results in good agreement with simulated results, demonstrating good insertion loss and return loss. By adjusting the parameters of the T-shaped resonator and L-shaped defect microstrip structure, the ultra-wideband bandpass filter with dual notched bands working at WiMAX band and X-band can be designed and applied to the wireless communication system. 2. Filter Design The proposed dual notched ultra-wideband bandpass filter using a T-shaped resonator and L-shaped defect microstrip structure is designed on a bandpass filter in the literature [5]. The structure of a ultra-wideband bandpass filter without a T-shaped resonator and L-shaped defect microstrip structure is shown in Figure 1a. Figure 1b lays out the simulation results of a basic ultra-wideband bandpass filter by using high frequency structure simulator (HFSS) software (ANSYS, Inc., Canonsburg, PA, USA). To satisfy the requirements for ultra-wideband performance and improve edge steepness of the passband, four short-circuited stubs (length of λ/4) are used. Figure 1b indicates that the bandwidth of the bandpass filter is from 2.632 to 10.623 GHz and the fractional bandwidths (FBW) is 123.9%. The insertion loss of the ultra-wideband bandpass filter is less than 0.8 dB, while the simulated return loss is greater than 16 dB within the passband. Figure 2 shows the design process of the dual notched bands filter. The basic filter can form the first notched band by using the T-shaped resonator and the second notched band by using the L-shaped defect microstrip structure. Figure 3a,b shows the circuit configuration of the proposed filter and its S parameters, which are calculated by HFSS. The proposed filter achieves dual notched bands, whose center frequencies are at the 3.5 and 7.5 GHz, respectively. On notched frequencies, the corresponding reflection coefficient is −28.6 dB and −25.1 dB with a the fractional bandwidths (FBW) of 15.76% and 4.96%, respectively.

ultra-wideband bandpass filter by using high frequency structure simulator (HFSS) software (ANSYS, Inc., Canonsburg, PA, USA). To satisfy the requirements for ultra-wideband performance and improve edge steepness of the passband, four short-circuited stubs (length of λ/4) are used. Figure 1b indicates that the bandwidth of the bandpass filter is from 2.632 to 10.623 GHz and the fractional bandwidths (FBW) is 123.9%. The insertion loss of the ultra-wideband bandpass filter is Micromachines 2018, 9, 280 3 of 11 less than 0.8 dB, while the simulated return loss is greater than 16 dB within the passband.

Via hole W3

L3

L1

L2 W1

0 -3 dB -10

2.632 GHz

S11and S21（dB）

L4

R1

10.623 GHz

-20 -30

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W2

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S11

Figure 2 shows the design process of the dual notched bandsS21filter. The basic filter can form the L3 Figure 2 shows the design process of the dual notched -60 bands filter. The basic filter can form the 3 4 notched 5 6 7 8 by9 using 10 11 first notched band by using the T-shaped resonator and1 the2 second band the first notched band by using the T-shaped resonator and the secondFrequency(GHz) notched band by using the W3 L4 L-shaped defect microstrip structure. Figure 3a,b shows the circuit configuration of the proposed L-shaped defect microstrip of the proposed (a) structure. Figure 3a,b shows the circuit configuration (b) filter and its S parameters, which are calculated by HFSS. The proposed filter achieves dual notched filter and its S parameters, which are calculated by HFSS. The proposed filter achieves dual notched Figure (a)Structure Structure ofultra-wideband ultra-wideband bandpass filter without T-shapedresonator resonator andL-shaped L-shaped bands, whose center frequencies are at thebandpass 3.5 and 7.5 GHz, respectively. On notched frequencies, Figure 1.1.(a) of without T-shaped and bands, whose center frequencies are at the 3.5 andfilter 7.5 GHz, respectively. On notched frequencies, defect microstrip structure; (b) simulation results of the ultra-wideband bandpass filter without the corresponding reflection coefficient is −28.6 dB and −25.1 dB with a the fractional bandwidths defect microstrip structure; (b) simulation results of the ultra-wideband bandpass filter without the corresponding reflection coefficient is −28.6 dB and −25.1 dB with a the fractional bandwidths T-shaped resonator and L-shaped defect microstrip structure. (FBW) of 15.76% and 4.96%, respectively. T-shaped resonator and L-shaped defect microstrip structure. (FBW) of 15.76% and 4.96%, respectively. Filter with single notched band using Filter with single notched band using L-shaped defect microstrip structure L-shaped defect microstrip structure

Filter with single notched band Filter with single notched using T-shaped resonatorband using T-shaped resonator Basic ultra-wideband Basic ultra-wideband bandpass filter bandpass filter

Filter with dual notched bands using Filter withresonator dual notched bands using T-shaped and L-shaped T-shaped resonator structure and L-shaped defect microstrip defect microstrip structure

Figure 2. Design process of the dual notched bands filter. Figure Figure 2. 2. Design Design process process of of the the dual dual notched notched bands bands filter. filter.

W2 W2Via hole Via hole R1 R1 W 3 W3

L1 L1

Via hole Via hole R1 R1

L3 L3

W1 W1

g1 g1 W5 W6 L 3 L3 W 5 R2 W 6 L5 L3 L3 R2 L5 Via hole Via hole L6 L6

0

L4 L4

0

S and S21（dB） S11 11 and S21（dB）

R1 R1 Via hole W3 Via hole W 3 L2 L2 L9 L9

-10 -10 -20 -20

L4 L4 hole R1 Via R1 Via hole

S11 S11 S21 S21

-30 -30 -40 -40 1 1

2

2

3

3

4

4

5 6 7 8 9 10 11 12 5 6 7 8 9 10 11 12 Frequency(GHz) Frequency(GHz)

(a) (b) (a) (b) Figure 3. (a) The circuit of the proposed filter; (b) simulation results of the proposed filter. Figure 3. 3. (a) (a) The The circuit circuit of of the the proposed proposed filter; filter; (b) (b) simulation simulation results resultsof ofthe theproposed proposedfilter. filter. Figure

Figure 4 shows the structure of the T-shaped resonator and further analyzes its even-mode and Figure 4 shows the structure of the T-shaped resonator and further analyzes its even-mode and odd-mode circuit. By using odd-moderesonator and even-mode analysis method [6], the input Figureequivalent 4 shows the structure of the T-shaped and further analyzes its even-mode and odd-mode equivalent circuit. By using odd-mode and even-mode analysis method [6], the input odd-mode of equivalent circuit. By using odd-mode and even-mode analysis (1) method [6],where the input  impedance even-mode and odd-mode can be expressed as the Equations and (2), impedance of even-mode and odd-mode can be expressed as the Equations (1) and (2), where 1 1 impedance of even-mode and odd-mode can be expressed as the Equations (1) and (2), where θ and 1 and  2 are the electrical length of the open-circuited stub and the short-circuited stub, respectively. the electrical of the open-circuited stub the short-circuited stub, respectively.  2 are θand electrical length length of the open-circuited stub and theand short-circuited stub, respectively. 2 are the Z tan 6  2Z5 tan 5 tan 6  2Z5 tan 5 Zine  jZ 6 ZZ6 6tan (1) θ6 + 2Z tan Z  jZ 6 (1) 6 5tan 5θ5 Zineine= jZ6 6ZZ6 22ZZ5 tan (1) tan  tan  6 5 6 5 Z6 − 2Z5 tan θ6 tan θ5  jZ6 tan  ZZino Zinoino= jZ jZ66 tan tan 6θ66

(2)(2) (2)  6 is defined as the electrical length of the open-circuited stub,  5 is defined as the electrical  6 is defined as the electrical length of the open-circuited stub,  5 is defined as the electrical length of the short-circuited stub, which can be expressed as: length of the short-circuited stub, which can be expressed as: 5   L6 , 6   L6 / 2 (3) 5   L6 , 6   L6 / 2 (3)

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θ6 is defined as the electrical length of the open-circuited stub, θ5 is defined as the electrical length of the short-circuited stub, which can be expressed as: θ5 = βL6 , θ6 = βL6 /2

(3)

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Z5 ,L5

2Z5 ,L5 2Z5 ,L5

Z5 ,L5 Zine

Z6 ,L6/2 Z6 ,L6/2 Z6 ,L6/2Zino Z6 ,L6/2 （a） （a）

（b）（b）

Zine Z6 ,L6/2 Z6 ,L6/2

Zino Z6 ,L6/2 Z6 ,L6/2

（c）（c）

Figure 4. (a) The structure of T-shaped resonator; (b) even-mode equivalent circuit; (c) odd-mode Figure 4. (a)4.The structure of T-shaped resonator; (b) even-mode equivalent circuit; (c) odd-mode Figure (a) The of T-shaped resonator; (b) even-mode equivalent circuit; (c) odd-mode equivalent circuit (Zstructure 5 and Z6 are the characteristic impedance. L5 and L6 /2 are length of the equivalent circuit (Z 5 and Z6 are the characteristic impedance. L5 and L6/2 are length of the equivalent circuit (Z5 and Z6 are the characteristic impedance. L5 and L6/2 are length of the open-circuited stuband andthe the short-circuited stub). open-circuited stub stub stub).stub). open-circuited andshort-circuited the short-circuited

even-mode resonant frequency feven and odd-mode resonant frequency fodd fcan be expressed The The even-mode resonant frequency f even and odd-mode resonant frequency can beexpressed expressed even-mode resonant frequency feven and odd-mode resonant frequency fodd odd can be as shown Equations (4) and (5). If the two signals are in the same phase, they are stacked together as in Equations (4) and (5). If the two signals are in the same phase, they are stacked together as shown in Equations (4) and (5). If the two signals are in the same phase, they are stacked togetherto to form a pass band. However, this frequency range, thetwo two signals’ phase opposite, and and the stop formtoa form pass However, inin this range, the phase isisopposite, and the aband. pass band. However, infrequency this frequency range, thesignals’ two signals’ phase is opposite, the stop band is formed. In order to further reduce the difficulty of design, the parameter is made L bandstop is formed. In orderIntoorder further reducereduce the difficulty of design, the parameter is made L66 ==L2L band is formed. to further the difficulty of design, the parameter is made 6 =5 . 2L 5. In this way, the resonant frequency of odd mode and even mode are only related to L6, and the 2L 5 . In this way, the resonant frequency of odd mode and even mode are only related to L 6 , and the In this way, the resonant frequency of odd mode and even mode are only related to L6 , and the first firstfirst resonance frequency is adjusted by adjusting resonance frequency is adjusted by adjusting resonance frequency is adjusted by adjusting L6 .L6. L6. c c c c c c f even      c c c f  even = f even = 4( L6 4( √ /2  L ) √ 4( L /2 L  L/26 /2) 4L6 4= eff √  LL6 /2) Leff6 4L  6 eff eff 4( L6 /2 +L6L/255 ) L5εeff)eff eff 46(4( L6 /2 + /2 ) ε 6 6 ε eff eff

(4) (4) (4)

c cc ffodd =  √ oddf odd 2L62L26Leff εeff 6 eff

(5) (5)(5)

Figure 5a illustrates illustrates thestructure structure the L-shaped defect microstrip structure, whose width Figure 5a the ofofthe L-shaped defect microstrip structure, whose width is isis Figure 5a illustrates the structure of the L-shaped defect microstrip structure, whose width W length the sumsum and .L8The notch band characteristics by using using defect W77 and and length isis the ofofLL7of7and L8L. 8The notch band characteristics are are formed by using defect W7 and length is sum the L7 and . The notch band characteristics are formed formed by defect microstrip structure similar to using defect ground structure [19], and the resonant unit cell are modeled microstrip structure similar to using defect ground structure [19], and the resonant unit cell are microstrip structure similar to using defect ground structure [19], and the resonant unit cell are modeled by L-shaped defect microstrip structure and and the T-shaped resonator, i.e., Figure 4b, only by L-shaped defect microstrip structure andstructure the T-shaped resonator, i.e.,resonator, Figure 4b,i.e., only considers the modeled by L-shaped defect microstrip the T-shaped Figure 4b, only considers the first and and second resonant modes, which shows the equivalent circuit ofelements the first and second modes, which shows the equivalent lumped circuitlumped of lumped the resonant considers theresonant first second resonant modes, which shows the equivalent circuit of the resonant elements composed of the resonator and and L-shaped defect microstrip structure. composed of elements the T-shaped resonator L-shaped defect microstrip structure. resonant composed of T-shaped theand T-shaped resonator L-shaped defect microstrip structure. L9

L7

L9 L8

L7

Lp1

Lp1 Ls1

Cp1

Cp1

L8 W1 W7

W1

Ls2

Ls1

Cp

Cp

Ls2

Lp2

Lp2

Cp2

Cp2

W7

L1

L1

(a) (a)

(b) (b)

Figure 5. (a) (a)5.The The element structure L-shaped defect microstrip structure; Figure 5. structure ofofL-shaped defect microstrip structure; (b)(b) the the equivalent lumped Figure (a)element The element structure of L-shaped defect microstrip structure; (b) equivalent the equivalent lumped circuit of T-shaped resonator and L-shaped defect microstrip structure. circuit of T-shaped resonator and L-shaped microstrip structure. structure. lumped circuit of T-shaped resonator anddefect L-shaped defect microstrip

According to the theory of transmission line,line, eacheach resonant element can can be equivalent to a to a According to theory the theory of transmission resonant element be equivalent According to the of transmission line, each resonant element can be equivalent to a simple simple LC parallel resonant circuit withwith a characteristic that that is similar to the order Butterworth simple LC parallel resonant circuit a characteristic is similar to first the first order Butterworth LC parallel resonant circuitthe withequivalent a characteristic that is similar to the(6)–(10) first order Butterworth low-pass low-pass filter. Therefore, circuit model Equations of the two two resonant low-pass filter. Therefore, the equivalent circuit model Equations (6)–(10) of the resonant filter. Therefore, the equivalent circuit model Equations (6)–(10) of the two resonant elements composed elements composed of the resonator and and the L-shaped defect microstrip structure can can be be elements composed of T-shaped the T-shaped resonator the L-shaped defect microstrip structure of the T-shaped resonator andlow-pass the L-shaped defectmodel, microstrip structure can be deduced from the deduced from the Butterworth filter circuit where means a transit frequency, f T deduced from the Butterworth low-pass filter circuit model, where fT means a transit frequency, Butterworth low-pass filterthe circuit model, fresonant a transit frequency, while f 01 and f 02 T means frequencies. while lower and and thewhere higher The The 3-dB3-dB bandwidth at at f 01 and f 02 describe while the lower the higher resonant frequencies. bandwidth f and f 02 describe describe the01 lower and the higher resonant frequencies. The 3-dB bandwidth at f 01 and f 02 are 3dB_f02 . The. The by  imaginary partsparts of three Z parameters at at f 01 and f 02 are f 01 and and are denoted by3dB_ imaginary of three Z parameters 3dB_ f 01 and f 02 denoted 3dB_ f02 f 01 11, X22 and X21. The following equivalent formulas regarding resonant elements composed fT are are X11, X22 and X21. The following equivalent formulas regarding resonant elements composed fT X of the T-shaped resonator and and L-shaped defect microstrip structure can can be derived fromfrom Reference of the T-shaped resonator L-shaped defect microstrip structure be derived Reference [3]. [3]. The The equivalent equations of resonant elements composed of the T-shaped resonator and and equivalent equations of resonant elements composed of the T-shaped resonator L-shaped defect microstrip structure are shown in Reference [19]. L-shaped defect microstrip structure are shown in Reference [19].

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denoted by ∆3dB_ f01 and ∆3dB_ f02 . The imaginary parts of three Z parameters at f T are X11 , X22 and X21 . The following equivalent formulas regarding resonant elements composed of the T-shaped resonator and L-shaped defect microstrip structure can be derived from Reference [3]. The equivalent equations Micromachines 2018, 18, x 5 of 11 are of resonant elements composed of the T-shaped resonator and L-shaped defect microstrip structure shown in Reference [19]. 11 C pi = forii= C pi  for 1, 21, 2 (6) (6) 4πZ ∆ 40Z 0  3dB_ 3dB_ ff0i 0i

1

L pi =

for i = 1, 2 1 Lpi(2π  f )2 C for i  1, 2 (20if 0i )2 Cpsi psi Cp =

(7)

1

1 C p 2π f T X21 2 fT X 21

Lsi =

(8)

L pi Xii − X21 + for i = 1, 2 L X  X pi 2 f Tii 21 ( f T / f 0i ) −for1 i  1, 2 L2π si  2 fT

(7) (8) (9)

(9)

( fT / f 0 i )  1 2

Defect microstrip structure has the same slow wave characteristics as DGS, so the resonant Defect microstrip structure has the same slow wave characteristics as DGS, so the resonant frequency can be derived from the method of DGS [20]. The L-shaped defect microstrip structure can frequency can be derived from the method of DGS [20]. The L-shaped defect microstrip structure be regarded as a half guided wavelength crooked slot line resonator. Therefore, the resonant frequency can be regarded as a half guided wavelength crooked slot line resonator. Therefore, the resonant f mayfrequency be achieved as:be achieved as: f may c (10) f = √ c f 2L ε slot (10) 2L  where c is the free-space speed of light, L = L7 + L8 theslottotal length of the L-shaped defect microstrip structure, and is the effective of the acquired by the closed-form equations where c isεthe speeddielectric of light, L constant = L7 + L8 the totalslot length of the L-shaped defect microstrip slot free-space in Reference [21]. parameters extractedconstant from the is shown in Table 1. closed-form Figure 6 shows  slotcircuit structure, andThe is the effective dielectric ofADS the slot acquired by the the comparison HFSS and ADS results regarding the equivalent equation of resonant equations inofReference [21]. Thesimulation circuit parameters extracted from the ADS is shown in Table 1. elements composed the T-shaped microstrip It can be Figure 6 shows with the comparison of resonator HFSS and and ADSL-shaped simulationdefect results regardingstructure. the equivalent equation of 6 resonant composedresults with and the T-shaped and L-shaped seen from Figure that theelements ADS simulation the HFSSresonator simulation results havedefect the same microstrip structure. It can be seen from Figure 6 that the ADS simulation results and the HFSS resonant frequencies at 3.5 GHz and 7.5 GHz with the same insertion loss 28.6 dB, which proves the simulation results have same resonant at 3.5 the GHz and 7.5 GHz withfabricated the samefilter validity and correctness of thethe equivalent circuit.frequencies Figure 7 shows photograph of the insertion loss 28.6 dB, which proves the validity and correctness of the equivalent circuit. Figure 7 which selects RT/Duorid5880 (Rogers Corporation, Chandler, AZ, USA) as a substrate with the size of shows the photograph of the fabricated filter which selects RT/Duorid5880 (Rogers Corporation, 32 mm × 10 mm × 1 mm. Chandler, AZ, USA) as a substrate with the size of 32 mm × 10 mm × 1 mm.

Table 1. The circuit parameters in Figure 5b extracted from the advanced design system (ADS). Table 1. The circuit parameters in Figure 5b extracted from the advanced design system (ADS).

C (pF) C (pF) = 2.415 Cp1 C=p12.415 = 2.489 Cp =Cp2.489 Cp2 C=p20.1216 = 0.1216 – --

LL (nH) (nH) LpL1p1 = 0.85 = 0.85 Ls1Ls1 = 0.18 = 0.18 −0.105 LL s2s2 == −0.105 Lp2 = 0.47 Lp2 = 0.47

S11 and S21（dB）

0 S21 -10 -20

S11

-30 -40 3

HFSS simulation ADS simulation

4

5 6 7 Frequency(GHz)

8

9

6. comparison The comparison of simulationresults resultsby by using using high structure simulator (HFSS) FigureFigure 6. The of simulation highfrequency frequency structure simulator (HFSS) and advanced design system (ADS). and advanced design system (ADS).

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Figure The photograph fabricated filter. Figure 7. 7. The photograph of of thethe fabricated filter.

Results and Discussion 3.3. Results and Discussion thispaper, paper, the is coupled to the ultra-wideband filter and theand L-shaped InInthis theT-shaped T-shapedresonator resonator is coupled to basic the basic ultra-wideband filter the defect microstrip structure is realized by etching an asymmetric L slot line line whose width is W L-shaped defect microstrip structure is realized by etching an asymmetric L slot whose width is 7 . The T-shaped resonator is comprised of short-stub (L , W ) and open-stub (L , W ) with equal length W7. The T-shaped resonator is comprised of short-stub 5 (L55, W5) and open-stub 6 (L66, W6) with equal and width (L5 = L(L the impedance ratio (Kratio = Z2 /Z on theiron electrical length and width = 5L=6, W W65)=soWthat 6) so that the impedance (K 1=) only Z2/Z1depends ) only depends their 6 ,5W length. The final size of the is optimized by usingby HFSS with 0.1with GHz0.1 sweep electrical length. The final sizefilter of the filter is optimized using HFSS GHzstep. sweep step. Optimizeddesign design parameters of the proposed ultra-wideband is given in 2. Table Optimized parameters of the proposed ultra-wideband filter filter is given in Table The 2. The simulated and fabricated results proved the first notched band is adjustable parameters simulated and fabricated results proved that that the first notched band is adjustable byby thethe parameters , 6Land ) of T-shaped resonator,while whilethe thesecond secondnotched notchedband bandis iscontrolled controlledbybythe the (g(g 1, 1L WW 6) 6of thethe T-shaped resonator, 6 and parameters(L(L L-shaped defect microstrip structure. parameters 7, 7L,8Land W7W ) of thethe L-shaped defect microstrip structure. 8 and 7 ) of Table 2. Optimized design parameters of proposed the proposed ultra-wideband filter units mm). Table 2. Optimized design parameters of the ultra-wideband filter (The(The units are are mm).

Length L1 = 15 L2 = 5 L3 = 4 L3 = 4

Length Length Width Length Width L5 = 5.3 W1 = 1.95 L1 = 15 L5 = 5.3 W 1 = 1.95 L65= 10.6L = 10.6 W2 = W 3 =3 L2 = 6 2 L 7 = 9 W 3 = 0.3 L3 = 4 L7 = 9 W 3 = 0.3 L3 =L48 = 0.1 L8 = 0.1 W4 = W 1.44 = 1.4

Width Width W5 = 0.5 W 5 = 0.5 WW 6 = 0.3 6 = 0.3 WW 7 = 0.1 7 = 0.1 = 0.3 WW 8 =80.3

Radius Radius R1 = 0.15 R1 = 0.15 0.15 RR22 == 0.15 -– –--

Length Gap Gap Length g1 = 0.1 g1 = 0.1 -– -– – --

The the T-shaped T-shapedresonator resonatorand and L-shaped defect microstrip structure is Thesimulation simulationresults results of of the L-shaped defect microstrip structure is given given in Tables 3 and 4. It can be concluded that the lower frequency notched band with a 3-dB in Tables 3 and 4. It can be concluded that the lower frequency notched band with a 3-dB bandwidth bandwidth of 1.07 was generated by the T-shaped and the higher frequency notched of 1.07 GHz was GHz generated by the T-shaped resonator,resonator, and the higher frequency notched band with band with a 3-dB bandwidth of 0.61 GHz was realized by etching the L-shaped defect microstrip a 3-dB bandwidth of 0.61 GHz was realized by etching the L-shaped defect microstrip structure. It can structure. It can seen that the has firsta notch band has a fractional bandwidths (FBW) of about be seen that thebe first notch band fractional bandwidths (FBW) of about 15.47% and the second 15.47% and the second notch band has a FBW of about 8.76%. Moreover, the measured results show notch band has a FBW of about 8.76%. Moreover, the measured results show that the dual notched that the are dualobserved notchedatbands are7.5 observed at the 3.5 and 7.5 GHz thedB rejection level 25.2 dB and bands 3.5 and GHz with rejection levelwith of 25.2 and 17.3 dB, of with a fractional 17.3 dB, with a(FBW) fractional bandwidths (FBW) of 15.91% and 9.63%. bandwidths of 15.91% and 9.63%. Table 3. 3. Simulation results ofof the dual notched bands. Table Simulation results the dual notched bands. 3 dB Bandwidth (GHz) 1.07 1.07 0.61 0.61

NotchFrequency Frequency (GHz)3 dB Bandwidth (GHz) Notch (GHz) 3.53.5 7.5

7.5

Fractional Bandwidths (FBW) % 15.62 15.62 8.91 8.91

Level (dB)(dB) Fractional Bandwidths (FBW) %Rejection Rejection Level 28.6 28.6 25.1 25.1

Table Measurement results the dual notched bands. Table 4. 4. Measurement results ofof the dual notched bands. Notch Frequency (GHz)

3 dB Bandwidth 3 dB Bandwidth (GHz)

Notch Frequency (GHz) 3.5 7.53.5

7.5

(GHz) 1.09 0.661.09 0.66

Fractional Bandwidths Fractional Bandwidths (FBW) % (FBW) 15.91% 9.63 15.91 9.63

Rejection Level (dB)

Rejection Level (dB) 25.2

25.2 17.3 17.3

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Figure Figure8a 8ashows showsthe theexamination examinationphoto photoof ofthe thefabricated fabricatedfilter filterby byVector VectorNetwork NetworkAnalyzer Analyzer Figure 8a shows the examination photo of the fabricated filter by Vector Network Analyzer Plannar 804/1 (Copper Mountain Technologies, Indianapolis, IN, USA) with the frequency range Plannar 804/1 (Copper Mountain Technologies, Indianapolis, IN, USA) with the frequency range of88 Plannar 804/1 (Copper Mountain Technologies, Indianapolis, IN, USA) with the frequency range of ofGHz. 8 GHz. The simulation data and the measured data are imported into the Excel form, and the The simulation simulation data data and and the the measured measured data data are are imported imported into into the the Excel Excel form, form, and and the the GHz. The comparison between the simulation results and the measured results isisdrawn by Origin software comparison between the simulation results and the measured results drawn by Origin software comparison between the simulation results and the measured results is drawn by Origin software (OriginLab (OriginLabCorporation, Corporation,Northampton, Northampton,MA, MA,USA) USA)as asshown shownin inFigure Figure8b. 8b.According Accordingto toFigure Figure8b, 8b, (OriginLab Corporation, Northampton, MA, USA) as shown in Figure 8b. According to Figure 8b, the measured insertion loss is lower than 1.2 dB and the reflection coefficient is higher than 15 dB the measured measured insertion insertion loss loss isis lower lower than than 1.2 1.2 dB dB and and the the reflection reflection coefficient coefficient is is higher higher than than 15 15 dB dB the over the ultra-wideband passband. The measured and simulation results have the same resonant over the ultra-wideband passband. The measured and simulation results have the same resonant over the ultra-wideband passband. The measured and simulation results have the same resonant frequencies. frequencies. The Theequivalent equivalent model model of ofresonant resonantelements elementscomposed composed of ofthe theT-shaped T-shapedresonator resonatorand and frequencies. The equivalent model of resonant elements composed of the T-shaped resonator and L-shaped defect microstrip structure is proven to be correct by the consistent resonant frequencies. L-shaped defect defect microstrip microstrip structure structure is is proven proven to to be becorrect correct by by the theconsistent consistent resonant resonant frequencies. frequencies. L-shaped 0 0 S

dB ）） S11S and dB andS21S（（

11 -10 S11 -10

S S2121

11

21

-20 -20 -30 -30

Simulation Simulation Measurement Measurement

-40 -40 1 1

2 2

3 3

(a) (a)

4 4

5 5

6 6

Frequency(GHz) Frequency(GHz)

7 7

8 8

9 9

(b) (b)

Figure8.8. 8.(a) (a)The Theexamination examinationphoto photoby byVector Vector Network Analyzer Plannar 804/1(Copper (CopperMountain Mountain Figure Network Analyzer Plannar 804/1 Figure (a) The examination photo by Vector Network Analyzer Plannar 804/1 (Copper Mountain Technologies, Indianapolis, IN, USA); (b) comparison of the the simulation and measurement results of Technologies, Indianapolis, IN, USA); (b)(b) comparison of the simulation andand measurement results of the Technologies, Indianapolis, IN, USA); comparison of simulation measurement results of the proposed filter. proposed filter.filter. the proposed

Figure 99 illustrates illustrates the proposed proposed filter performance performance with different different parameters (g (g11,, LL66,, W W66)) of of the the Figure Figure 9 illustrates the the proposed filter filter performancewith with differentparameters parameters (g 1 , L6 , W 6 ) of T-shaped resonator. resonator. Figure Figure 9a 9a depicts depicts that that the proposed proposed filter filter performance performance is is adjusted adjusted by by changing changing T-shaped the T-shaped resonator. Figure 9a depictsthe that the proposed filter performance is adjusted by L6 of the T-shaped resonator under the condition of L 6 = 2L5. As L6 increases from 9.4 to 10.6 mm, f01 L6 of the T-shaped the condition L6 = 2L5. As increases 9.4 to 10.6 mm,9.4 f01 changing L6 of the resonator T-shaped under resonator under theof condition of LL66 = 2L5 . Asfrom L6 increases from decreases from from 44 to to 3.5 3.5 GHz, GHz, while while ff0202 is is still still 7.5 7.5 GHz. GHz. ItIt can can be be derived derived from from the the Equations Equations (4) (4) and and decreases to 10.6 mm, f 01 decreases from 4 to 3.5 GHz, while f 02 is still 7.5 GHz. It can be derived from the (5) that that this this leads leads to the the decreasing decreasing of of ff0101.. Figure Figure 9b describes describes the proposed proposed filter filter performance performance (5) Equations (4) and (5)tothat this leads to the decreasing9b of f 01 . Figurethe 9b describes the proposed filter changing with different W 6 of the T-shaped resonator when L7 = 11 mm. It can be clearly seen that changing with different W6different of the T-shaped L7 = when 11 mm. canmm. be clearly seen that performance changing with W6 of theresonator T-shapedwhen resonator L7 It = 11 It can be clearly when W 6 increases from 0.4 to 0.6 mm, f01 increases from 3.4 to 3.6 GHz, while f02 is 7.5 GHz. The when W6when increases from 0.4from to 0.6 increases from 3.4 to 3.4 3.6 to GHz, whilewhile f02 is f7.5isGHz. The seen that W 6 increases 0.4mm, to 0.6f01mm, f 01 increases from 3.6 GHz, 7.5 GHz. 02 FBW of of the the first first notched notched band band decreases decreases from from 16 16 to 14.8% 14.8% and and the the FBW FBW of of the the second second notched notched band FBW to band The FBW of the first notched band decreases from 16 to 14.8% and the FBW of the second notched increases from from 11.24% 11.24% to to 10.9%. 10.9%. increases band increases from 11.24% to 10.9%.

-10 -10

-10 -10

S21 dB ）） S（（ dB

0 0

S21 dB ）） S（（ dB

0 0

-20 -20

21

21

-20 -20 L6=9.4mm L6=9.4mm L6=10.2mm L6=10.2mm L6=10.6mm L6=10.6mm

-30 -30 -40 -40 1 1

2 2

3 3

4 4

5 6 7 8 9 5 6 7 8 9 Frequency(GHz) Frequency(GHz)

(a) (a)

10 11 12 10 11 12

W6=0.4mm W6=0.4mm W6=0.5mm W6=0.5mm W6=0.6mm W6=0.6mm

-30 -30 -40 -40 1 1

2 2

3 3

4 4

5 6 7 8 9 5 6 7 8 9 Frequency(GHz) Frequency(GHz)

10 11 12 10 11 12

(b) (b)

Figure 9. 9. (a) Simulation Simulationresults results with withdifferent different LL66 (L (L66 == 9.4 9.4 mm, ff0101 ==44GHz, GHz, LL66 =1 =10.2 0.2mm, mm, ff0101 ==3.6 3.6GHz, GHz, Figure Figure 9. (a)(a) Simulation results with different L6 (Lmm, 6 = 9.4 mm, f 01 = 4 GHz, L6 =1 0.2 mm, L 6 = 10.6 mm, f01 = 3.5 GHz); (b) Simulation results with different W6 (W6 = 0.4 mm, f01 = 3.4 GHz, W6 = mm, fL01 ==3.5 GHz); with different W6with (W6 =different 0.4 mm,W f01 =(W 3.4 GHz, W6 = fL016 == 10.6 3.6 GHz, 10.6 mm,(b) f Simulation = 3.5 GHz);results (b) Simulation results 6 6 = 0.4 mm, 0.5mm, mm, ff0101 ==3.5 3.56 GHz, GHz, W W66 ==0.6 0.601mm, mm, ff0101 ==3.6 3.6GHz). GHz). 0.5 f 01 = 3.4 GHz, W 6 = 0.5 mm, f 01 = 3.5 GHz, W 6 = 0.6 mm, f 01 = 3.6 GHz).

Figures 10 10 respectively respectively illustrates illustrates that that the the performance performance of of the the second second notched notched band band is is adjusted adjusted Figures Figure 10 the respectively illustrates that the the second notched band is adjusted The by by changing relevant parameters (L 7, L8performance , W 7) of the of L-shaped defect microstrip structure. by changing the relevant parameters (L7, L8, W7) of the L-shaped defect microstrip structure. The changing the of relevant parameters (L7 , L , W ) of the L-shaped defect microstrip structure. The total total length the defect microstrip structure is equal to L 7 + L 8 and the width is W 7 . Figure 10a 8 7 total length of the defect microstrip structure is equal to L7 + L8 and the width is W7. Figure 10a describes the the proposed proposed filter filter performance performance with with changing changing LL77 of of the the L-shaped L-shaped defect defect microstrip microstrip describes structure. When L 7 increases from 7.5 to 9.3 mm, f02 decreases from 7.7 to 6.2 GHz, while f01 is 3.5 structure. When L7 increases from 7.5 to 9.3 mm, f02 decreases from 7.7 to 6.2 GHz, while f01 is 3.5

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length of the2018, defect Micromachines 18, xmicrostrip structure is equal to L7 + L8 and the width is W 7 . Figure 10a describes 8 of 11 the proposed filter performance with changing L7 of the L-shaped defect microstrip structure. When L7 GHz. Figure simulated insertion with different 8. When L8 increased increases from10b 7.5shows to 9.3 the mm, f 02 decreases fromloss 7.7only to 6.2 GHz, while fL01 is 3.5 GHz. Figure from 10b 0.1 to the 0.3simulated mm, f01 isinsertion 3.5 GHz f02 with decreases from 7.2 to L78 increased GHz. It can from shows lossbut only different L8 . When frombe0.1derived to 0.3 mm, f 01the is equivalent of resonant elements C2 of the equivalent circuitequation increases with the 3.5 GHz but equation f 02 decreases from 7.2 to 7 GHz. Itthat canthe be derived from the equivalent of resonant increasing of the theCtotal length of thecircuit L-shaped defectwith microstrip structure (L7 total + L8length ). Figure 10c elements that equivalent increases the increasing of the of the 2 of the illustratesdefect the proposed performance different W7 of the theproposed L-shapedfilter defect microstrip L-shaped microstripfilter structure (L7 + L8 ).with Figure 10c illustrates performance structure. When W 7 increases from 0.2 to 0.4 mm, f 01 and f 02 are maintained at 3.5 GHz and with different W 7 of the L-shaped defect microstrip structure. When W 7 increases from 0.2 to7.2 0.4 GHz. mm, seen thatatwhen W7 and increases from 0.2 be to 0.4 mm,seen thethat FBW of the notched fIt andbe f 02clearly are maintained 3.5 GHz 7.2 GHz. It can clearly when W 7second increases from 01 can band increases from 15.71% 22.68%. The resonant frequency f of15.71% the L-shaped defect 0.2 to 0.4 mm, the FBW of theto second notched band increases from to 22.68%. Themicrostrip resonant structure decreases with the increasing of the total length of the L-shaped defect microstrip frequency f of the L-shaped defect microstrip structure decreases with the increasing of the total length structure (L7 + Ldefect 8) as shown in Equation (10). of the L-shaped microstrip structure (L7 + L8 ) as shown in Equation (10). 0

0 -10

S21（dB）

S21（dB）

-10

-20

-20

-30 L7=7.5mm

-40 -50 1

L8=0.1mm

-30

L8=0.2mm

L7=8.0mm L7=9.3mm

2

3

4

5 6 7 8 Frequency(GHz)

9

10

-40 1

11

L8=0.3mm

2

3

4

5

6

7

8

9

10

11

Frequency(GHz)

(a)

(b) 0

S21（dB）

-10 -20 W7=0.25mm W7=0.30mm W7=0.40mm

-30 -40 1

2

3

4

5

6 7 8 Frequency(GHz)

9

10

11

12

(c) Figure10. 10.(a) (a)Simulation Simulationresults resultswith with different 7 (L7 = 7.5 mm, f02 = 7.7 GHz, L7 = 8 mm, f02 = 7.2 GHz, Figure different L7L(L 7 = 7.5 mm, f 02 = 7.7 GHz, L7 = 8 mm, f 02 = 7.2 GHz, GHz); (b)(b) simulation results with different L8 (LL8 =(L0.1=mm, f02 = 7.2 8 = 0.2 LL77 == 9.3 9.3 mm, mm, ff0202= =6.26.2 GHz); simulation results with different 0.1 mm, f 02GHz, = 7.2LGHz, 8 8 mm, f 02 = 7.1 GHz, L8 = 0.3 mm, f02 = 7 GHz); (c) simulation results with different W7 (W7 = 0.2 mm, W7 L8 = 0.2 mm, f 02 = 7.1 GHz, L8 = 0.3 mm, f 02 = 7 GHz); (c) simulation results with different W 7 = 0.3 mm, W7 = 0.4 mm, f01 = 3.5 GHz, f02 = 7.2 GHz). (W 7 = 0.2 mm, W 7 = 0.3 mm, W 7 = 0.4 mm, f 01 = 3.5 GHz, f 02 = 7.2 GHz).

According to the above analysis, the frequency of the second notched band decreases by According to the above analysis, the frequency of the second notched band decreases by increasing increasing the total length of the L-Shaped defect microstrip structure (L7 + L8) and the fractional the total length of the L-Shaped defect microstrip structure (L7 + L8 ) and the fractional bandwidths of bandwidths of the second notched band increases with the increasing of W7, while the frequency of the second notched band increases with the increasing of W 7 , while the frequency of the first notched the first notched band is still at 3.5 GHz. band is still at 3.5 GHz. Finally, a comparison of characteristics between the proposed filter and some other filters is Finally, a comparison of characteristics between the proposed filter and some other filters is presented in Table 5, in which insertion loss and return loss are over the whole pass-bands, while fL presented in Table 5, in which insertion loss and return loss are over the whole pass-bands, while f L and fH means the central frequency of lower and higher frequency notched bands. It can be seen and f H means the central frequency of lower and higher frequency notched bands. It can be seen that that the proposed filter has dual notched bands at 3.5 and 7.5 GHz with the rejection level of 25.2 dB the proposed filter has dual notched bands at 3.5 and 7.5 GHz with the rejection level of 25.2 dB and and 17.3 dB and an FBW 123.9% higher than the other 10 works [22–31]. The measured results 17.3 dB and an FBW 123.9% higher than the other 10 works [22–31]. The measured results illustrate illustrate that the dual notched bands are centered at 3.5 and 7.5 GHz with an FBW of 15.91% and that the dual notched bands are centered at 3.5 and 7.5 GHz with an FBW of 15.91% and 9.63%. 9.63%.

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Table 5. Comparison of the proposed filter with recently proposed ultra-wideband bandpass filters with notched bands. Ref.

Insertion Loss (dB)

Return Loss (dB)

f L (GHz)

f H (GHz)

Fractional Bandwidths (FBW) %

[22] [23] [24] [25] [26] [27] [28] [29] [30] [31] This work

0.5 0.6 0.65 1.2 1.1 1.5 0.7 1.1 2 1.12 0.8

13 12 10 11 10 15 15 15 15 11 15.5

5.5 6.55 4.9 4.3 2.4 2.4 1.16 2.62 5.8 2.4 3.5

13.6 8.66 8.5 9.1 5.5 5.2 3.5 5.32 8 5.2 7.5

112 130 110 82.4 110 105 95.5 105 92 89.6 123.9

4. Conclusions In this paper, a UWB bandpass filter with dual notched bands using a T-shaped resonator and L-shaped defect microstrip structure is designed and fabricated. The proposed filter has dual adjustable notched bands which are implemented by changing the parameters of the L-shaped defect microstrip structure and T-shaped resonator respectively. As the two simulation results obtained by using ADS and HFSS are consistent, the correctness of the equivalent lumped circuit composed of a T-shaped resonator and L-shaped defect microstrip structure is further verified. The proposed filter can suppress dispensable bands at 3.5 GHz and 7.5 GHz, which can be designed and applied to the wireless communication system at WiMAX band and X-band by adjusting the parameters of the T-shaped resonator and L-shaped defect microstrip structure. Author Contributions: X.Z. and T.J. conceived and designed the experiments; Y.P. tested the performance of the antenna and collected the measured data; X.Z. and T.J. analyzed the data; X.Z. wrote the paper. Acknowledgments: This paper is funded by the International Exchange Program of Harbin Engineering University for Innovation-oriented Talents Cultivation. This work was partially supported by the National Key Research and Development Program of China (2016YFE0111100), Key Research and Development Program of Heilongjiang (GX17A016), the Science and Technology innovative Talents Foundation of Harbin (2016RAXXJ044), the Natural Science Foundation of Beijing (4182077) and China Postdoctoral Science Foundation (2017M620918). Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

References 1.

2. 3.

4.

5.

Federal Communications Commission. Revision of Part 15 of the Commission’s Rules Regarding Ultra-Wideband Transmission Systems; First Note and Order Federal Communications Commission, ET-Docket 98-153; Federal Communications Commission: Washington, DC, USA, 2002. Wei, F.; Li, W.T.; Shi, X.W.; Huang, Q.L. Compact UWB bandpass filter with triple-notched bands using triple-mode stepped impedance resonator. IEEE Microw. Wirel. Compon. Lett. 2012, 22, 512–514. [CrossRef] Wei, F.; Qin, P.-Y.; Guo, Y.J.; Shi, X.-W. Design of multi-band bandpass filters based on stub loaded stepped-impedance resonator with defected microstrip structure. IET Microw. Antennas Propag. 2016, 10, 230–236. [CrossRef] Lu, X.; Wei, B.; Xu, Z.; Cao, B.; Zhang, X.; Wang, R.; Song, F. Superconducting ultra-wideband (UWB) bandpass filter design based on quintuple/quadruple/triple-mode resonator. IEEE Trans. Microw. Theory Tech. 2015, 63, 1281–1293. [CrossRef] Wei, F.; Wu, Q.Y.; Shi, X.W.; Chen, L. Compact UWB bandpass filter with dual notched bands based on SCRLH resonator. IEEE Microw. Wirel. Compon. Lett. 2011, 21, 28–30. [CrossRef]

Micromachines 2018, 9, 280

6.

7.

8. 9.

10. 11.

12.

13. 14.

15.

16. 17.

18.

19. 20. 21. 22. 23. 24. 25.

10 of 11

Zheng, X.; Liu, W.; Zhang, X.; Jiang, T. Design of dual band-notch UWB bandpass filter based on T-shaped resonator. In Proceedings of the 2016 Progress in Electromagnetic Research Symposium (PIERS), Shanghai, China, 8–11 August 2016; pp. 4482–4486. Zheng, X.; Jiang, T. Realization of Dual Notch Bands in UWB Bandpass Filter Using Two T-shaped Resonators. In Proceedings of the 2017 International Applied Computational Electromagnetics Society Symposium, Florence, Italy, 26–30 March 2017. [CrossRef] Zhao, J.; Wang, J.; Zhang, G.; Li, J.-L. Compact microstrip UWB bandpass filter with dual notched bands using E-shaped resonator. IEEE Microw. Wirel. Compon. Lett. 2013, 23, 638–640. [CrossRef] Zheng, X.; Jiang, T. Design of UWB bandpass filter with dual notched bands using E-shaped resonator. In Proceedings of the IEEE/ACES International Conference on Wireless Information Technology and Systems, Honolulu, HI, USA, 13–18 March 2016; pp. 1–2. [CrossRef] Zhang, C.; Zhang, J.; Li, L. Triple band-notched UWB antenna based on SIR-DGS and fork-shaped stubs. Electron. Lett. 2014, 50, 67–69. [CrossRef] Zhou, L.-H.; Ma, Y.; Shi, J.; Chen, J.; Che, W. Differential dual-band bandpass filter with tunable lower band using embedded DGS unit for common-mode suppression. IEEE Trans. Microw. Theory Tech. 2016, 64, 4183–4191. [CrossRef] Zakaria, Z.; Mutalib, M.A.; Ismail, A.; Isa, M.S.M.; Ismail, M.M.; Latiff, A.A.; Zainuddin, N.A.; Sam, W.Y. Compact structure of band-pass filter integrated with Defected Microstrip Structure (DMS) for wideband applications. In Proceedings of the European Conference on Antennas and Propagation, The Hague, The Netherlands, 6–11 April 2014; Volume 21, pp. 2158–2162. Wang, J.; Zhao, J.; Li, J.L. Compact UWB bandpass filter with triple notched bands using parallel U-shaped defected microstrip structure. Electron. Lett. 2014, 50, 89–91. [CrossRef] Yao, B.; Zhou, Y.; Cao, Q. A UWB bandpass filter with multi notched bands using microstrip/coplanar waveguide. In Proceedings of the International Symposium on Antennas, Propagation and EM Theory, Kunming, China, 2–5 November 2008; pp. 637–640. Mao, S.G.; Chueh, Y.Z.; Chen, C.H.; Hsieh, M.C. Compact ultra-wideband conductor-backed coplanar waveguide bandpass filter with a dual band-notched response. IEEE Microw. Wirel. Compon. Lett. 2009, 19, 149–151. [CrossRef] Honarvar, M.A.; Sadeghzadeh, R.A. Design of coplanar waveguide ultrawideband bandpass filter using stub-loaded resonator with notched band. Microw. Opt. Technol. Lett. 2012, 54, 2056–2061. [CrossRef] Senior, D.E.; Cheng, X.; Yong, K.Y. Dual-band filters using complementary split-ring resonator and capacitive loaded half-mode substrate-integrated-waveguide. In Proceedings of the Antennas and Propagation Society International Symposium, Chicago, IL, USA, 8–14 July 2012; pp. 1–2. Pandey, A.K.; Chauhan, R.K. Miniaturized dual-band BPF for WiMax/WLAN applications. In Proceedings of the IEEE International Conference on Power Electronics, Intelligent Control and Energy Systems, Delhi, India, 4–6 July 2016; pp. 1–4. Hong, J.S.; Karyamapudi, B.M. A general circuit model for defected ground structures in planar transmission lines. Frequenz 2015, 15, 706–708. [CrossRef] Wang, J.; Ning, H.; Xiong, Q.; Li, M.; Mao, L.F. A novel miniaturized dual-band bandstop filter using dual-plane defected structures. Prog. Electromagn. Res. 2013, 134, 397–417. [CrossRef] Gupta, K.C.; Garg, R.; Bahl, I.; Bhartia, P. Microstrip Lines and Slotlines, 2nd ed.; Artech House: Norwood, MA, USA, 1996. Wang, K.; Wong, S.W.; Chu, Q.X. A compact UWB CPW bandpass filter with short-ended H-shaped resonator and controllable notched band. Microw. Opt. Technol. Lett. 2013, 55, 1577–1581. [CrossRef] Nosrati, M.; Daneshmand, M. Compact microstrip ultra-wideband double/single notch-band band-pass filter based on wave’s cancellation theory. Microw. Antennas Propag. Lett. 2012, 6, 862–868. [CrossRef] Song, K.; Xue, Q. Asymmetric dual-line coupling structure for multiple-notch implementation in UWB bandpass filters. Electron. Lett. 2010, 46, 1388–1390. [CrossRef] Wang, H.; Tam, K.W.; Ho, S.K.; Kang, W.; Wu, W. Design of ultra-wideband bandpass filters with fixed and reconfigurable notch bands using terminated cross-shaped resonators. IEEE Trans. Microw. Theory Tech. 2014, 62, 252–265. [CrossRef]

Micromachines 2018, 9, 280

26.

27. 28.

29. 30. 31.

11 of 11

Lung, C.K.; Chin, K.S.; Fu, J.S. Tri-section stepped-impedance resonators for design of dual-band bandstop filter. In Proceedings of the European Microwave Conference, Rome, Italy, 29 September–1 October 2009; pp. 771–774. Chen, F.C.; Qiu, J.M.; Chu, Q.-X. Dual-band bandstop filter using stub-loaded resonators with sharp rejection characteristic. Electron. Lett. 2013, 49, 351–352. [CrossRef] Gao, L.; Cai, S.W.; Zhang, X.Y.; Xue, Q. Dual-band bandstop filter using open and short stub-loaded resonators. In Proceedings of the International Conference on Microwave and Millimeter Wave Technology, Shenzhen, China, 5–8 May 2012; Volume 4, pp. 1–3. Feng, W.J.; Che, W.Q.; Shi, S.Y.; Xue, Q. Compact dual-wideband bandstop filters based on open-coupled lines and transversal signal-interaction concepts. IET Microw. Antennas Propag. 2013, 7, 92–97. [CrossRef] Peng, H.; Luo, Y.; Zhao, J. Compact microstrip UWB bandpass filter with two band-notches for UWB applications. Prog. Electromagn. Res. Lett. 2014, 45, 25–30. [CrossRef] Tilanthe, P.; Sharma, P.C.; Bandopadhyay, T.K. A monopole microstrip antenna with enhanced dual band rejection for UWB applications. Prog. Electromagn. Res. B 2012, 38, 315–331. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).