Compact wide stopband microstrip lowpass filter using polygon patches and meandered lines

Received Dec 12, 2019 Revised Jan 2, 2020 Accepted Jan 21, 2020 In this paper, a low pass filter based on T-Shaped resonator is presented. The T-Shaped resonator consists of meandered lines and rectangular patches. Also, the LC model and transfer function of the proposed resonator is presented. For suppression of spurious harmonics, a bandstop structure consists of hexangular patches and open stubs has been utilized. Finally, the wide stopband microstrip lowpass filter with cutoff frequency 2.72 GHz has been simulated, fabricated and measured. The LPF has good characteristics such as wide stopband and insertion loss lower than 0.18 dB in the passband region. The rejection level is less than -20 dB from 2.98 up to 21.3 GHz. The filter size is 10.5 mm×12.7 mm, or 0.131 λg× 0.158 λg, where λg is the guided wavelength. The measured and simulated results of the filter is in good agreement with each other, which show the merits of low insertion loss and wide stopband.


INTRODUCTION
The lowpass filters (LPFs) are used in the communication systems for elimination of undesired frequencies. There are several requirements for a LPF such as small size, low loss in passband region, and wide stopband. Planar filters have attracted considerable attention due to low cost, simple structures and capability of integration with other microwave circuits [1]. To suppress the spurious frequencies, the stepped impedance filters were introduced in [2][3]. To have high order harmonic suppression, stepped impedance hairpin resonator was proposed in [2]. In [3], a compact low pass filter with widestop band by using transformed stepped impedance resonator, was proposed. However, the reported work in [3] has poor selectivity from passband to stopband region. Other methods such as defected ground structure (DGS) and slotted structure to design of filters have been proposed, but the main problem is complexity of fabrication process [4][5][6][7]. In [4], the bandwidth of filter has been increased by a DGS structure. The filter based on DGS in [5] has wide stopband. Yet, it provides high insertion loss and gradual cutoff frequency. Also, multistopband filter based on tree fractal slotted structure in [6] and LPF using DGS in [7] have been proposed. The microstrip lowpass filters with compact size and ultrawide stopband using triangular and polygonal patch resonator are presented in [8] which have poor selectivity. In [9], multimode resonators to design of wide stopband LPF with high selectivity have been used. However, the multimode LPF has the stopband region with low attenuation level. In [10], a novel design of a LPF based on metamaterial was introduced. The proposed filter is investigated based on square split ring resonators (SRRs) with good attenuation level in the stopband but suffers from low stop bandwidth and large size. In [11], the presented filter used meandered lines, and polygonal patches with low passband insertion loss which suffer from low selectivity and relatively low rejection level in the stopband region.
To deal with problems such as low selectivity, narrow stopband, high insertion loss, and large size, a new microstrip lowpass filter with good performance has been presented in this work. This paper, addresses all those problems by obtaining wide stopband by bandstop structure instead of cascaded resonators since caseded resonators result in size increasing. From the proposed design, it is found that there is only optimum number of elements to maximize filter stopband and minimize passband losses.
The rejection level better than −20 dB is obtained in the range of 2.98 up to 21.3 GHz. The filter size is 10.5 mm×12.7 mm, or 0.131 λg× 0.158 λg, where λg is guided wavelength. Measured results of the developed lowpass filter are in good agreement with the simulation ones, thereby proving the validity of the proposed filter. The paper is organizaed as the following. In the first section, we provide an overview of the proposed resonator with lumped and distributed elements. In the second section, we argue the bandstop structure based on lumped and distributed elements. The LPF design and the obtained results are presented in third and fourth sections, respectivey. Finally, the conclusion is presented in the fifth section.

DESIGN OF RESONATOR
A new T-shaped resonator using meandered line is presented. Meandered lines are employed and loaded with rectangular patches to achieve the high selectivity and low insertion loss. In Figure 1  (1) and = 1 1 and the capacitance of the open stub is where Zsi is the characteristic impedance of the line, li is its length, c is the velocity of light in free space and λg is the guided wavelength at the cut-off frequency. The values of these parameters after optimization obtained as L1=3.1 nH, L2=3.05 nH, C1=0.07 pF, and C2=0.8 pF. The dimensions are l1=5.25 mm, l2=1 mm, l3=0.6 mm, w1=3.9 mm, w2=4.1 mm, and w3=0.4 mm. Figure 1(c) shows the LC and EM simulation results of the basic resonator. As it is illustrated, the agreement between LC and EM simulation results has been met perfectly. The S21 parameter of the basic resonator shows that the resonator has low rejection level in the stopband. Also, the transmission zero (TZ) of the basic resonator is 3.15 GHz, which is obtained as Hence, the position of transmission zero, fz, can be determined by setting the T1(s) = 0, which results in fz=3.3 GHz. Also, the variation of cut-off frequency of the basic resonator versus w2 and l2 parameters is shown in Figure 2(a) and 2(b), respectively. As it can be seen, the cutoff frequency has been decreased by increasing w2 and l2. By varying the dimensions of the patch structure, the cutoff frequency of the proposed resonator can be tuned.

DESIGN OF BANDSTOP STRUCTURE
The bandstop structure (BSF) are designed based on two open stubs and high impedance line loaded by four hexangular patches (resonator-II). The main section is a high impedance line having an impedance of 134 Ω connected in series with a hexangular patch. The resonator-II has been designed and simulated with the Advanced Design System (ADS). When the layout of the resonator has been generated, an electromagnetic momentum (EM) simulation has been carried out to analyze its performance. Its layout and LC equivalent circuit is shown in Figure 4. The dimensions are l4 =3.13 mm, and l5 =1.7 mm. This unit has one transmission zero at 8 GHz with attenuation level -43 dB. From (6), fz1=7.9 GHz and fz2=8.5 GHz is obtained. By connecting two units symmetrically connected at the centre of main high impedance transmission line, the structure in Figure 6 (a) is obtained. Figure 6(b) exhibits two transmission zeros, which are located at about 7.67 GHz and 9 GHz with attenuation level near -62 dB and -60 dB. These transmission zeros are caused by the resonance of the high impedance single stepped stub loaded by triangular patches and its resonant frequency depends on the structural parameters.
In practical applications, a filter with wide stopband and high suppression level is necessary. The bandstop unit in Figure 8(a) has been added to the resonator. The S21 parameters of bandstop structure are depicted in Figure 8

FILTER DESIGN AND LAYOUT
The schematic showing the generation process of the proposed filter on substrate with the permittivity of 2.2, loss tangent of 0.0009 and thickness of 0.508 mm is given in Figure 9(a). The LPF occupies 10.5 mm×12.7 mm . Fig 9(b)  resonator was embedded between spaces of the bandstop structure to avoid size increasing of the filter. Conventional microstrip filters are usually implemented using cascaded resonator and bandstop structure which results in size increasing and losses. The filter was simulated by ADS 2014 and measured by HP8510B Network Analyzer. The measured and simulated results will be presented in the next section. Two microstrip lines connected at both sides of the LPF are in order to match the impedance at the input and output ports to 50 Ω with the Wf = 0.7 mm, and lf=2.4 mm.  Figure 10(b) shows the measured and simulated S-parameters of the proposed LPF. One transmission zero is located at 2.97 GHz to have high selectivity. In addition, extra transmission zeroes in the stopband region can be observed. In general, these transmission zeros help us in out-of-band rejection enhancement. The measured insertion loss is less than 0.18 dB, while the return loss is greater than 12.5 dB from DC up to 3.39 GHz. The simulated group delay is shown in Figure 10(c). It can be seen that the group delay is less than 0.32 ns in the passband. Furthermore, over 20 dB attenuation level in the stopband region is achieved from 2.98 up to 21.3 GHz, so that the unwanted signal is eliminated. For comparisons, Table 1 illustrates the measured results for some previuos works and proposed filter, where roll off rate ζ is defined as

RESULTS AND DISCUSSION
where αmax and αmin represent the −20 dB and −3 dB attenuation level, respectively, fc is the −3 dB cutoff frequency, and fs is the −20 dB stopband frequency. In this case, the bandwidth divided by center frequency of stopband is the relative stopband width (RSB) as  Comparing with the other filters [2][3], [5], [7][8][9][10][11], clearly a transmission zero close to the passband is realized with T-shaped resonator and the filter has sharp roll-off rate in comparison to [2][3], [5], [7][8][9][10][11]. Moreover, the stop bandwidth of the proposed filter can be further extended by using Split Ring Resonators as [7]. Also, the proposed filter has lower insertion loss in comparison to other works. However, the filter is symmetrical and miniaturized in dimension which is less costly to implement in communication applications. The measured and simulated results of the filter have good agreement at lower frequency. The difference at high frequencies is due to the imperfect soldering of the ports and inaccurate implementation of the proposed filter.

CONCLUSION
An LPF with cutoff frequency 2.72 GHz by modified T-shaped resonator has been designed, fabricated, and measured. To improve the stopband bandwidth, the bandstop structure consists of using open stubs and hexangular patches has been added at the two sides of the proposed resonator. An approximate LC equivalent circuit of each structure has been derived and its response has been compared with simulated result. A high attenuation over the stopband up to 21.3 GHz has been obtained so that the unwanted out-ofband signal is eliminated. The obtained results show that the proposed filter has good performance such as low insertion loss, wide stopband and high selectivity in comparison to other filters. Due to compact size, low insertion loss and wide stopband, the proposed LPF is expected to be applied in modern wireless communication systems.