Design of Compact Uwb Bandpass Filter with Sharp Rejection Characteristics

This study presents an Ultra-Wideband (UWB) filter with sharp rejection characteristics at the upper Wireless Local Area Network (WLAN) band. The filter adopts the modified Composite Right/Left-Handed (CRLH) Transmission Lines (TLs), the unit cell of which is theoretically analyzed to derive the design formulas. Subsequently, a UWB bandpass filter is designed using these formulas. The measurement results show that the fabricated UWB bandpass filter has an insertion loss of less than 1.1 dB, bandwidth of 3.1~4.9 GHz and a rejection of more than 50 dB at 5.8 GHz.


INTRODUCTION
Since the Federal Communications Commission (FCC) released the frequency band of 3.1~10.6GHz for Ultra-Wideband (UWB) communication in 2002, high data rate, short-range and short-pulse UWB communications have developed rapidly.A Bandpass Filter (BPF) is an essential component in an UWB system and UWB BPFs have thus been studied widely.Many filters have been designed using microstrip lines (Gao et al., 2008;Duong and Kim, 2009;Watanabe et al., 2009;Wu et al., 2013) and a combination of microstrip lines and coplanar structures (Kuo et al., 2007;Chen et al., 2009), as well as ones with a combination of microstrip lines and defected ground structures (Huang et al., 2010;Tian and Chu, 2011;Gao et al., 2012;Kumar and Upadhayay, 2012).Some have been implemented using Composite Right/Left-Handed (CRLH) Transmission Lines (TLs) (Lin et al., 2007;Gong and Chu, 2008;Shen and Law, 2012;Su et al., 2012) Size reduction and bandwidth enhancement of bandpass filters can be achieved with the use of CRLH TLs (Lai et al., 2004).
In this study, we present the design of a compact UWB BPF with sharp rejection characteristics.This is achieved using modified CRLH TLs, which help avoid interference between the UWB and WLAN systems.The simulated and measured results of the proposed UWB BPF are in good agreement.and the left-handed shunt inductor, L L and series capacitor, C L , are similar to the ones founded in the conventional CRLH structure (Caloz and Itoh, 2006).Otherwise than the conventional CRLH structure, another right-handed series inductor, L Rp , is in parallel with the series of L Rs and C L.

METHODOLOGY
Since the circuit model is symmetrical, the even/odd-mode analysis can be used to derive the transfer function S 21 .The even-and odd-mode admittances of the modified CRLH unit cell are as follows: (1) (2) where, (3) where, Y 0 is the admittance of the input/output port.A transmission zero is created when the transfer function S 21 is equal to zero, i.e.: (7) As a result, the transmission zero frequency is ω rej , which is the resonant frequency of the series branch of the modified CRLH unit cell.The resonant frequency of the shunt branch is ω sh and the resonant frequency of the series elements of the series branch is ω se .
According to the Bloch-Floquet theory, the dispersion relation and characteristic impedance can be expressed as (Pozar, 2012): where, Z B is the characteristic impedance normalized to the port impedance, Z 0 and: Figure 2 shows the dispersions and attenuations of the modified CRLH unit cell.It operates as a CRLH TL when ω rej <ω sh and as a Right-Handed (RH) TL when ω sh <ω rej .Unlike conventional CRLH TLs, only the characteristics of RH TLs appear in the unbalanced case of ω sh <ω rej .In the balanced case, ω rej = ω sh , it operates in the non-propagating mode.Figure 2 shows the sharp  3), ( 4), ( 6) and ( 9), we can calculate the element values of the modified CRLH unit cell.We have one degree of freedom among 5 element values of the unit cell.C R determines the slope parameter of the shunt resonator that creates the passband of the UWB BPF.Therefore, C R is regarded as a free variable element.Design specifications are as follows: the passband of the UWB BPF is 3.1~4.9GHz, the attenuation pole frequency is 5.8 GHz and the characteristic impedance of the modified CRLH unit cell is 50 Ω at the center frequency.When the value of C R is selected as 1.5 pF, the calculated element values are L Rp = 1.19 nH, L Rs = 0.99 nH, L L = 2.0 nH and C L = 0.34 pF, respectively.The design results are shown in Fig. 3. To satisfy the design specification, the number of unit cells is increased to three.Figure 3a shows 1, 2 and 3 unit cell characteristics with the

RESULTS AND DISCUSSION
The UWB bandpass filter with sharp rejection characteristics was implemented on a 0.787-mm-thick Taconic TLX-9 substrate with a dielectric constant of 2.5 and a loss tangent of less than 0.002.Figure 4 shows the fabricated UWB bandpass filter, in which L Rs and C L are realized by the upper gap coupling structure, L L is realized by the short-circuited stub and L Rp and C R are implemented by the main transmission line.With the aid of an Electromagnetic (EM) simulator, the dimensions of the structure can be optimized to w 1 = 2.2 mm, w 2 = 1 mm, l 1 = 7 mm, l 2 = 5 mm, l 3 = 7 mm, l 4 = 6.5 mm, l 5 = 4.7 mm, l 6 = 4 mm, l 7 = 6.5 mm, l 8 = 1 mm and l 9 = 1.5 mm.The size of the fabricated UWB BPF is 24.5×13.9mm 2 .The measurements are carried out using an Anritsu 37347C vector network analyzer.Figure 5a shows the simulated and measured It is more compact than the one in Watanabe et al. (2009) and has a simpler single-layer structure compared to Shen and Law (2012), in which the UWB BPF is implemented using a three-layer structure.

CONCLUSION
This study proposes a UWB BPF with sharp rejection characteristics at the WLAN band that adopts the modified CRLH TLs.The fabricated filter has a compact size, low insertion loss and flat group delay; it also possesses a sharp skirt property near the passband.The measured results appear to agree with the EM simulation results.
Fig. 1: Circuit model of the modified CRLH unit cell

Fig. 2 :
Fig. 2: Dispersion and attenuation diagram of the modified CRLH unit cell, (a) ω rej <ω sh , (b) ω sh <ω rejattenuation characteristics in the lower stopband when ω rej <ω sh and in the upper stopband when ω sh <ω rej .Thus, we can use the modified CRLH structure to obtain the sharp rejection skirts of a UWB BPF for avoiding interference between the UWB and WLAN systems.According to Eq. (3), (4), (6) and (9), we can calculate the element values of the modified CRLH unit cell.We have one degree of freedom among 5 element values of the unit cell.C R determines the slope parameter of the shunt resonator that creates the passband of the UWB BPF.Therefore, C R is regarded as a free variable element.Design specifications are as follows: the passband of the UWB BPF is 3.1~4.9GHz, the attenuation pole frequency is 5.8 GHz and the characteristic impedance of the modified CRLH unit cell is 50 Ω at the center frequency.When the value of C R is selected as 1.5 pF, the calculated element values are L Rp = 1.19 nH, L Rs = 0.99 nH, L L = 2.0 nH and C L = 0.34 pF, respectively.The design results are shown in Fig.3.To satisfy the design specification, the number of unit cells is increased to three.Figure3ashows 1, 2 and 3 unit cell characteristics with the

Fig. 3 :
Fig. 3: Design results of the UWB BPF, (a) 1, 2 and 3 unit cell characteristics, (b) before and after optimization results calculated element values.Following parameter optimization, we obtain the design values: C R = 2.58 pF, L Rp = 1.04 nH, L Rs = 1.02 nH, L L = 1.08 nH and C L = 0.32 pF.The design characteristics before and after optimization are depicted in Fig. 3b.