Regular paperA microstrip lowpass filter with sharp roll-off using arrow-shaped resonators and high-impedance open stubs
Introduction
Microstrip technique has been of interest of many researchers for different filter structures because of suppressing the undesired harmonics, low-cost and small-size characteristics. Therefore, the low-cost efficient design can be achieved by this technology [1], [2]. This technique can be used in design of filters of wireless communication technologies such as WLAN, WIMAX and etc. Different criteria can be defined to compare various designs of microstrip filters and highlight more efficient ones in applications. In [3], using a simple structure and stepped-impedance resonators, a low pass filter (LPF) is designed. Some properties such as stop band and insertion loss of this paper must be improved. In [4], by folding of the high-low impedance resonators, cut-off frequency as 1.85 GHz has been obtained. To weaken the undesired harmonics, hexagonal suppressor cell and open stubs have been used. High insertion loss and slow roll-off are the disadvantages of this work. In [5], by using four inter-digital hairpin stepped-impedance resonators and four radial stubs, a desirable LPF has been designed. Flat pass band and good suppressing level in stop band are interesting in this paper. However, disturbing harmonics are suppressed only up to 7 times of the cut-off frequency (7fc) which is low in wireless technologies. In [6], the proposed LPF consists of three coupled stepped-impedance resonators while its roll-off rate has increased to 440 dB/GHz. Undesirable harmonics are weakened just up to 16 GHz and the level of suppressing is achieved as −15 dB which is acceptable. In [7], by using the rhombic elements and folding the transmission line, a low pass filter has been designed in which high insertion loss and slow response in the transition band are the disadvantages of the filter. A microstrip lowpass filter by T-shape and U-shape structures has been presented in [8]. To achieve the appropriate response, the defected ground structure technique has been used. The Dimensions of this filter are large compared to previous works and roll-off rate is only 78 dB/GHz indicating that the transition band is not sharp. In [9], using several triangular resonators and open-circuit stubs, a lowpass filter with cutoff frequency as 5.15 GHz is presented. One of the disadvantages of this design is high return loss in the pass band. The filter transition band has 0.36 GHz bandwidth, which can be improved in comparison with previous works. The designs with wide stop band can be mentioned as those used in [10], [11]. These structures suppress the harmonics up to 40 GHz and the relative stop-band bandwidth (RSB) parameter is very interesting. However, some defects exist in these designs such as large physical size in [10] or suppressing factor (SF) value in [11] which results in undesirable attenuation level in stop band as −16 dB. In [12], using two pentagonal resonators, a structure with a cut-off frequency of 1.27 GHz is presented. The gradual response with a roll-off rate of 92.5 dB/GHz is one of the disadvantages of this work. In [13], a low pass filter with a figure of merit (FOM) of 57,860 is provided, which has a suitable suppressing factor and bandwidth. The normalized circuit size (NCS) rate is 0.0155 , which is weak compared to other works. A defected ground structure with a cut-off frequency of 2.11 GHz is presented in [14] where the gradual response and suppressing factor of 1.8 are the main disadvantages. A low pass filter with a very sharp response is presented in [6] in which, several radial stubs have been used. Low bandwidth, low suppression factor, and inadequate NCS rate are some of the disadvantages of this design. In [15], a low pass filter with very compact dimensions is proposed, which has disadvantages such as low bandwidth and gradual response.
To explain the importance and application of the LPF, it is worth mentioning that design of passive LPFs with low cost, wide stop band, and small size is highly in demand [1]. The efficient filters play an important role in the new generation of low-cost satellite communications technology, where the cost and size of Earth receiver stations are very significant. This is especially significant for communication-on-the-move (COTM), where in addition to all the strict requirements of satellite communications, there is a serious incentive to employ affordable, passive microwave and electromagnetic components in the portable satellite receivers [16], [17], [18], [19]. In addition, the designed LPF must consist of sharp roll-off and wide stop-band which is applicable in WLAN and WiMAX systems. Moreover, the harmonics of the main frequency must be suppressed in these communication systems.
In the proposed structure of this paper, using arrow-shaped resonators, high-impedance open stubs and the folding technique of the transmission lines, an efficient LPF is suggested. The scattering parameters of the designed filter are very interesting with high figure of merit which shows the great performance of this structure in wireless communication systems such as WLAN and WIMAX. Also, wide stop band is the main characteristic of the proposed LPF in which the harmonics are significantly suppressed in this structure. Thus, the suggested structure can be applicable in satellite communications specially in portable satellite receivers, WLAN and WiMAX systems. The main novelties of the paper can be summarized as:
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Arrow-shaped resonators with very long bases leading to high selectivity property
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High-impedance open stubs resulting in a sharp response and wide bandwidth
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Technique of folding the transmission line
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Compressing of the dimensions of the filter (small size)
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Simple elements in the proposed structure
The rest of the paper is as follow. In Section 2, the design process of proposed LPF is discussed step-by-step in details and simulation results are presented. In Section 3, the measurement results of implemented LPF are stated and compared to simulation results. Finally, some conclusions are dawn in final section, Section 4.
Section snippets
Proposed resonator
In this section, the main steps to design the proposed LPF are discussed. The main resonator of the proposed filter including long and narrow base is presented in Fig. 1a. To analyze this filter, equivalent L-C circuit has been shown in Fig. 1b. With the analysis of this circuit, the transmission zero can be achieved as observable from the frequency response in Fig. 1c (the resonator has a transmission zero in 1–3 GHz shown in Fig. 1c). The higher the narrow base of resonator, the lower
Measurement and simulation results
The design of proposed LPF is performed in ADS and simulation results are obtained by this software. This filter is realized on RT/Duroid 5880 with εr = 2.2, thickness of 15 mil and tan = 0.0009 as described in Fig. 7a. The frame of total structure is 14.0211.62 mm2 which shows the low size of the filter. The simulation and measurement results are depicted in Fig. 7b. Due to this figure, the flat response in pass band is observed with low reflection loss (-20.56 dB), sharp roll-off and wide
Conclusion
In this paper, a microstrip LPF with flat pass band, low reflection loss and narrow transient band is proposed. The stop band is wide as 12 times more than the cut-off frequency in which the attenuation level of stop band is below −20 dB. The size of this filter is interestingly low and considering the cut-off frequency, low NCS has been obtained. Another significant property of this filter is the FOM parameter which indicates the overall efficiency of designed filter. The FOM of the proposed
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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