Dual band Stepped Impedance Interdigital Microstrip Filter for LTE Bands

In this paper, a dual band microstrip bandpass filter has been proposed utilizing three edge coupled resonators, interdigital stubs and DGS technique. To enhance the coupling degree, two interdigital coupled feed lines are employed in this filter. The suppressing cell consists of stepped impedance ladder type resonators, which provides a wide stopband. The proposed suppressing cell has clear advantages like low insertion loss in the passband and suitable roll off. The frequency response of the filter looks like a standard dual band band-pass filter. The filter exhibits a dual passband with resonant frequencies at 2.2GHz and 3.45GHz covers LTE1 and LTE22 bands.


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Abstract-In this paper, a dual band microstrip bandpass filter has been proposed utilizing three edge coupled resonators, interdigital stubs and DGS technique.To enhance the coupling degree, two interdigital coupled feed lines are employed in this filter.The suppressing cell consists of stepped impedance ladder type resonators, which provides a wide stopband.The proposed suppressing cell has clear advantages like low insertion loss in the passband and suitable roll off.The frequency response of the filter looks like a standard dual band band-pass filter.The filter exhibits a dual passband with resonant frequencies at 2.2GHz and 3.45GHz covers LTE1 and LTE22 bands.Index Terms-Microstrip, Stepped Impedance, Interdigital stubs, DGS, CSRR.

I. INTRODUCTION
Communication systems have developed tremendously since there was prompt development in the ultra-wideband systems, wireless internet like Wi-Fi and WiMAX, broadband personal communication systems and 3G, 4G technologies.Microstrip bandpass filters combine the RF/microwave signals within certain spectral boundaries within the electromagnetic spectrum.Dual mode resonator usage allows to realize the compact high quality of bandpass filter (BPF) [1][2][3].Due to this rapid development, there was a need for more rigid and precise microwave components.These microwave components such as microwave filters, dielectric resonant antenna arrays, duplexers play an important role in the satellite systems.
Because of the rapid development, it created more challenging requirements such as the small size of the filter, selectivity, phase linearity and bandwidth.Nowadays satellite systems are focusing more on mobile, remote sensing and navigation applications.In [4] Physical realization of LH materials is reviewed based on a general transmission line (TL) approach.DGS techniques have been implemented on a BPF structure to examine how the techniques impacts the result.Baena et al [5] investigated coupling effect of transmission line with split-ring resonators (SRRs) and complementary split-ring resonators (CSRRs).The performance of the Metamaterial based Composite Right-Left Handed transmission line structure with microstrip gap capacitors and open stub inductors discussed in [6].Zhang Q et al [7] achieved desired passband by periodically loaded combination of split-ring resonators (SRRs) and complementary split-ring resonators (CSRRs).They made use of CSRR which introduced Published on July 17, 2019.Authors are with the Department of ECE, Agnel Institute of Technology and Design, Goa, India.
(e-mail: jw@aitdgoa.edu.in,15ec11@aitdgoa.edu.in,15ec17@aitdgoa.edu.in,15ec14@aitdgoa.edu.in,15ec30@aitdgoa.edu.in)compactness, low profile and flexibility in the choice of the substrate permittivity, which made their proposed design valuable for the future microwave devices like filters and couplers.With recent development in wireless communication systems, dual-band filters have been needed for many dual-band operation systems.
Traditionally any dual-band bandpass filter is implemented by a cascade connection of two single-band filters.However, this approach not only increases the overall size of the filter, but also requires extra impedance matching networks.So a new concept of Defected ground structure is used to achieve dual-band filter.In [8], a novel low-pass filter is designed using defected ground structure (DGS) slot with a protruded T-shaped strip inside the slot.The resonant frequency of the slot can be easily controlled by changing the protruded T shaped strips dimensions, without changing the area taken by the structure.In [9] a dumbbell-shaped defected ground resonator and its application in the design of differential filters are studied.Christopher James [10] developed a filter using single unit CSRR and concluded that the band-pass frequency response is dependent on the physical dimensions of both the microstrip line and the CSRR structure.Hayati M et al [11] introduced a simple stepped impedance structures to design a very compact size lowpass filter (LPF) as a suppressing cell.The proposed suppressing cell consists of stepped impedance ladder-type resonators, which provides a wide stopband by creating the transmission zeros to its frequency response.
Wei et al [12] presented a new design of multi-band bandpass filters employing stub loaded stepped-impedance resonator with defected microstrip structure (SL-SIR-DMS).In [13] an ultra-wideband (UWB) bandpass filter (BPF) using radial-stub-loaded resonator is proposed, to enhance the coupling degree, two interdigital coupled feed lines are utilized in this UWB filter.In [14] a compact broadband bandpass filter using triple-mode stepped-impedance stubloaded resonator is proposed.Filter is composed of an impedance-transforming unit and two asymmetrical openended stepped-impedance stubs at the center.By simply adjusting the impedances of the SIS, the resonant frequencies of the filter can be flexibly changed.The interdigital coupled-lines are adopted to feed the proposed filter.
This paper is organised as, Design of proposed filter is discussed in the section II, section III describes the structure of the filter, followed by discussion on results and analysis in section IV, and section V concludes the paper.

II. DESIGN OF PROPOSED FILTER
The proposed filter is composed of two techniques where in feed line is designed using interdigital technique and resonator is a type of stepped impedance micros trip stubs Defected ground structure is an important technique for designing filters where in a definite pattern is etched in the ground plane of micro strip which causes disturbance in the current distribution in the ground plane.CSSR consists of two slot rings where the inner and outer slots are separated be metal strip.Presence of CSSR in the ground plane creates negative permittivity instead of negative permeability near the resonance frequency.Figure 2 shows the CSRR inserted back view of the proposed filter structure.

III. STRUCTURAL DESCRIPTION
The substrate used is FR4 with relative dielectric constant Ɛr=4.3 and thickness of 1.6mm.The thickness of the copper strip is 0.035mm.Resonators determine the size of the filter since they are the fundamental elements of a filter.Figure 3 shows the structure of stepped impedance resonator.It consists of low impedance and high impedance lines.For this of low impedance stubs and L1 represents the overall inductance of high impedance.L represents the inductance of high impedance line.By increasing the lengths dL1, the first transmission zeros move towards left and decreasing dL1, the capacitance decreases and transmission zero moves towards right.Inter digital feed lines are used to enhance the coupling between the resonator and the feed lines.The parameters l2, w1, s1 in Figure 4 affects the coupling strength between the resonator and feed lines.By increasing the length l2, inductance increases.The space s1 between the two feed lines varies the capacitance.In this proposed filter the optimized values of strip and slot width are chosen as 0.5 mm and 1.25 mm respectively.The coupling length L2 is chosen as 6 mm.Table 1 shows the designed parameter values of the proposed Dualband BPF.IV. RESULTS AND DISCUSSION Based on the above analysis, dual band BPF is designed and fabricated on FR4 substrate with a relative permittivity of 4.3, thickness of 1.6mm and loss tangent of 0.002.The overall size of the filter amounts to 40 mm x 24mm.The filter was designed and simulated using CST (Computer Studio Technology) software.The Fabricated filter structure is shown in figure 5.The measurements are done using vector network analyzer ZVB 14 which operates at a range of 20MHz-14GHz.The simulated and measured S parameters of the fabricated dual band BPF is shown in Figure 6.The frequency range of measurement is set from 1 to 5 GHz.It is observed from Figure 6 that the designed and fabricated filter has two passbands.An ideal filter has -3dB cut off frequency and steeper roll off.Ideally a steeper roll off is required to get a narrow pass band.The two passbands that are observed from the graph have a range from 1.9GHz-2.3GHzand from 3.2GHz-3.6GHz.The group delay is a measure of the time delay of various frequency components going through a filter.A constant group delay results in less signal distortion when passing a signal through a filter.The group delay for any filter is found by taking the negative derivative of phase with respect to frequency.Figure 7 shows the group delay of the filter having less than 4 nano seconds difference.Slight deviation is observed between the simulated and measured results which are due to unexpected tolerances in fabrication, material properties and soldering.

Fig. 1 .
Fig. 1.Front view of the proposed filter

Fig. 2 .
Fig. 2. Back view of the proposed filter

Fig. 4 .
Fig. 4. Schematic of the Interdigital section of the filter

Fig. 6 .
Fig. 6.Comparison of the Measured and Simulated Results

Fig. 7 .
Fig. 7. Group delay of the proposed filter

TABLE I :
DESIGN PARAMETER VALUES