Design of a Compact UWB Bandpass Filter using Via-Less CRLH TL

A novel compact ultra wideband (UWB) bandpass filter (BPF) based on composite right/left handed transmission line (CRLH TL) is reported in this paper. The proposed UWB BPF is designed and developed by coupling of two unit-cells of via-less CRLH TL, excited by asymmetrical coplanar waveguide (CPW) feed-line. The unit cell of CRLH TL is designed using series interdigital capacitor (IDC) in shunt with the shorted inductive stub. Because of the CPW-fed, no via is required to get the shunt inductance for the realization of CRLH TL, which minimizes the fabrication steps. The filter is compact in size 11.9 x 6 mm 2 . The proposed filter exhibits the return-loss (|S11|) more than 11.2 dB and insertion-loss (|S21|) less than 0.5 dB and low and flat group delay response throughout the passband, 3.3 GHz to 13.0 GHz. The proposed UWB BPF also shows the good stopband rejection (|S21| > 20 dB and |S11| < 0.8 dB) from 13.6 GHz to 15 GHz and steep roll-off from passband to stopband. The fractional bandwidth (FBW) of the filter is found to be 119 %. The equivalent lumped circuit model of the filter is obtained through Ansoft Designer. All simulated results are extracted through method of moment based simulator, IE3D. Agilent vector network analyzer (VNA) is used to get the measured results. All measured results are found in close similarity with the simulated results.


I. INTRODUCTION
The left handed metamaterials can be realized mainly by two methods, firstly, using periodic arrangement of split ring resonators and thin metallic wires and, secondly by dual of conventional right handed transmission line, i.e. by periodic arrangement of series interdigital capacitors in shunt with the shorted stubs to ground.The first and second methods are respectively known as resonant and non-resonant approach of realization of left handed metamaterials.Due to large in size and lossy in nature, the resonant approach is not well suited for the realization of left handed metamaterials [1].
Since 2002 the U.S. Federal Communications Commission (FCC) [2] has regularized the unlicensed use of UWB frequency band 31.GHz to 10.6 GHz, the research in the field of UWB bandpass filter Design of a Compact UWB Bandpass Filter using Via-Less CRLH TL has gained much attention and challenges.The design of ultra wideband components need much efforts and attentions as it requires the good selectivity, low insertion-loss and low and constant group delay throughout the passband.
The UWB BPF reported in [3] is based on conventional CRLH TL designed by using via.The reported UWB BPF in [3] has good S-parameters characteristics and compact in size (16.4 x 4.8 mm 2 ).Moreover because of the use of via it requires the extra fabrication processing steps for drilling and soldering as compared to the proposed UWB BPF.The UWB bandpass filter presented in [4] is based on defected ground structure (DGS) as split ring resonator and conventional CRLH TL with via.The reported filter [4] is compact in size, 13 x 8.5 mm 2 , however because of the split ring resonator structure in ground plane and the presence on via, it requires the more extra efforts in ground plane processing and for creation of via.A single CRLH TL cell based UWB band pass filter [5] operating over the frequency band of 4 GHz to 9.5 GHz is of relatively large in size, 30 x 8.5 mm 2 and also requires the via for the realization of CRLH TL.The reported filter [5]  Many researchers and scientists worldwide have investigated, designed and developed various UWB BPFs based on different techniques such as; by use of fractal geometry, defected ground plane, coupled line structure and multimode resonator etc. References [8][9][10][11][12][13] report the various UWB BPFs based on different techniques such as a CRLH TL and fractal geometry based UWB BPF [8], the coupled structure based ultra wideband bandpass filter [9], defected ground structure [10], modified CRLH TL with cross coupling [11], cascading bandpass and bandstop filters [12] and multi mode resonator based UWB BPF [13].The UWB BPFs [8][9][10][11][12][13]   The series,   and shunt,   resonance frequencies of the CRLH TL are given as follows For balanced CRLH TL, Where,   is known as centre frequency.The lower end,   and upper end,   frequencies of CRLH TL are given as follows The design layout of proposed ultra wideband bandpass filter with the notation of all physical dimensions is depicted in Fig. 1.The filter is excited by asymmetrical CPW-fed, which consists of two ground planes.As shown in Fig. 1, UWB BPF is designed using coupling of two symmetrical series interdigital capacitors (IDC).The IDC consists of five fingers of finger length  3 , finger width  2 and finger spacing  1 .The coupling gap between the two IDCs is  6 .The two stubs each of length  9 , is connected to each IDC and ground plane 2 to get the shunt inductance.Since signal plane and ground plane both are in same planes, hence no via is required to short circuit the stubs to ground plane 2, in contrast to the conventional method of design of CRLH TL with via.The absence of via minimizes the fabrication steps, saves the fabrication time and reduces the overall bulk production cost.The filter is compact in size, 11.9 x 6 mm 2 .The effects of variation of physical parameter W 6 is shown in Fig. 2. The optimum value of W 6 is taken to be 0.9 mm.It can be seen from Fig. 2 that as the value of W 6 is increased from 0.9 mm to 1.The sensitivity for the variation of physical parameter L 9 on S-parameters response is shown in Fig.  I.    From the Fig. 13, a close similarity between the simulated and measured results can be observed.
Moreover a little deviation between the simulated and measured results can be seen, which mainly arise because of the finite ground plane, improper soldering and fabrication tolerances.The measured -3.0 dB insertion-loss frequency band of the proposed UWB BPF is found to be 3.12 GHz to 12.43 GHz, whereas the simulated -3.0 dB insertion-loss frequency band is found to be 3.3 GHz to 13.0 GHz.In throughout the passband, the measured and simulated insertion-losses are less than 0.6 dB and 0.5 dB respectively, whereas return-losses are more than 10.4 dB and 11.2 dB respectively.The mesured fractional bandwidth of the filter is found to be 120 %.
also suffers with the poor S-parameters performance in passband (|S 21 | = 1.5 dB) as well as in stopband.A compact UWB band pass filter (18.4 x 4.5 mm 2 ) reported in[6] has good S-parameters performance (|S 21 | < 0.3 dB and |S 11 | > 10 dB) is designed on via-less CRLH TL.Moreover the reported UWB BPF[6] operated over relatively low frequency band 3.1 GHz to 10.6 GHz and relatively large in size as compared to the proposed UWB BPF ( frequency band, 3.3 GHz to 13.0 GHz and size, 11.9 x 6 mm 2 ).An ultra wideband band pass filter designed using interdigitated coupled lines CRLHTL structure[7] operated over 3.9 GHz to 10.3 GHz is relatively large in size (30 x 15 mm 2 ), requires the generation of via and suffers with relatively poor insertion loss (|S 21 | = 1.5 dB).

1 2𝜋√𝐿
II. THEORY AND DESIGN OF PROPOSED UWB BANDPASS FILTERThe composite right/left handed transmission line can be realized by dual of conventional right handed transmission line (RH TL), known as non-resonant approach.The pure right handed transmission line (PRH TL) consists of series inductor in shunt with the capacitor, whereas pure left handed transmission line (PLH TL) is realized by series capacitor in shunt with the inductor.Unfortunately, either PRH TL or PLH TL cannot be physically realized because of the unavoidable parasitic effects of microstrip transmission lines.Hence because of the unavoidable parasitic effects, the CRLH TL is realized rather than the PRH TL.The CRLH TL shows the virtue of both, the nature of left handed transmission line (LH TL) at low frequencies and the nature of RH TL at high frequencies[1].The unit-cell of CRLH TL consists of per-unit series LH capacitance (  ) and per-unit right handed inductance (  ) and per-unit shunt LH inductance (  ) and per-unit RH capacitance (  ).The combination of series RH inductance (  ) and shunt RH capacitance (  ) in CRLH TL works as a lowpass filter whose cut-off frequency is given as   = 1 2√    .The combination of series LH capacitance (  ) and shunt LH inductance (  ) in CRLH TL works as a highpass filter whose cut-off frequency is given as   =    .So, when   <   , the CRLH TL works as a wideband bandpass filter.

Fig. 1 .
Fig. 1.Design layout of the proposed ultra wideband bandpass filter III.EFFECTS OF VARIATION OF VARIOUS PHYSICAL PARAMETERS ON UWB BPF All optimized physical dimensions of the proposed UWB BPF are obtained by parametric study of the geometry shown in Fig.1.The nature of variation of S-parameters characteristics of filter is investigated and analyzed for variation on one physical parameter keeping other parameters constant, to get the best optimized physical dimensions of the geometry, which can meet the low insertion-loss and return-loss better than 10 dB throughout the passband.

Fig. 2 . 6 Fig. 3 .
Fig. 2. S-parameters vs. Frequency plot for the variation of parameter W 6 5 mm, the S-parameters improve in frequency band 6.0 GHz to 8.0 GHz and deteriorate in the frequency band of 4 GHz to 4.8 GHz and 11.7 GHz to 12.4 GHz for the constant bandwidth 3.3 GHz to 13.0 GHz.As the value of W 6 is decreased from 0.9 mm to 0.5 mm, the value of S-parameters deteriorate in frequency band 6.0 GHz to 8.0 GHz and improve in 4 GHz to 4.8 GHz and 11.7 GHz to 12.4 GHz.So the value of W 6 is considered as 0.9 mm by compromising the performance in passband.The sensitivity for the variation of physical parameter, W 5 on S-parameters versus frequency plot is shown in Fig.3.As the value of W 5 increases from 1.0 mm to 1.3 mm, the Sparameters performance deteriorate in frequency band 6.0 GHz to 8.0 GHz and improve in the frequency band 4 GHz to 4.8 GHz.The vice-versa effects can be seen for the value of W 5 less than 1.0 mm.Hence the optimized value of W 5 is chosen to be 1.0 mm, by considering the performance of S-parameters in both bands.For the variation of W 5 , the overall bandwidth remains almost constant, i.e. from 3.3 GHz to 13.0 GHz.The effects of variation of physical parameters L 2 on S-parameters versus frequency plot is depicted in Fig.4.Considering the response of S-parameters in the band 4.0 GHz to 8.0 GHz and 11.7 GHz to 12.4 GHz, the optimum value of L 2 is considered as 2.7 mm.

Fig. 4 .
Fig. 4. S-parameters vs. Frequency plot for the variation of parameter L 2

Fig. 9 . 10 .Fig. 10 .
Fig. 9. S-parameters vs. Frequency plot for the variation of parameter S 6 parasitic effects of shorted stubs to ground plane 2 and the gap (W 5 -W 4 ) between topmost additional finger of IDC to the ground plane 1.The various components values of the equivalent lumped circuit model shown in Fig. 10 are listed in Table II.The comparative S-parameters versus frequency response of EM simulation and circuit model are illustrated in Fig. 11.The EM simulation results are taken corresponding to the optimized physical parameters listed in Table I.The close similarity between the EM simulation and circuit model results are observed, however a slight deviation between the two may be seen which may arise because of the ignorance of mutual coupling between the individual circuit elements.The -3.0 dB insertion-loss frequency bands for EM simulation and circuit model are found to be 3.3 GHz to 13.0 GHz and 3.2 GHz to 13.1 GHz respectively.The passband insertion-loss is less than 0.5 dB and 0.25 dB respectively and reflection-loss greater than 11.2 dB and 10.9 dB respectively for EM simulation and circuit model.The EM simulation results show the good stopband rejection (|S 21 | > 20 dB and |S 11 | < 0.8 dB) from 13.6 GHz to 15 GHz and steep roll-off from passband to stopband.The fractional bandwidth of the filter is found to be 119 %.

Fig. 11 .
Fig. 11.Comparative S-parameters vs. Frequency plot of equivalent circuit model and EM simulation V. MEASUREMENT RESULTS AND DISCUSSION Based on the optimized physical parameters listed in Table I, the proposed UWB BPF shown in Fig.1 is fabricated and its performance parameters are measured using Agilent vector network analyzer.Fig. 12 shows the photograph of fabricated prototype of proposed UWB BPF.The comparative simulated and measured S-parameters versus frequency response are shown in Fig. 13.

Fig. 14 .
Fig. 14.Comparative simulated and measured group delay vs. Frequency plot tool of Ansoft Designer.The simulation and measured results are obtained from electromagnetic (EM) simulator, IE3D and vector network analyzer respectively.All measured results are found in close similarity with the simulated results.

TABLE II .
COMPONENTS VALUES OF THE EQUIVALENT LUMPED CIRCUIT MODEL SHOWN IN FIG. 10

TABLE III .
COMPARISON OF EM SIMULATION, CIRCUIT MODEL AND MEASURED RESULTS