X-Band dielectric resonator bandpass filter

2010 International Conference on Computer Applications and Industrial Electronics (ICCAIE 2010), December 5-7, 2010, Kuala Lumpur, Malaysia X-Band Dielectric Resonator Bandpass Filter M. F. Ain, Z. A. Ahmad, M.A. Othman, I. A. Zubir, S. D. Hutagalung. A. A. Sulaiman, A. Othman Faculty of Electrical Engineering, UiTM, 40450 Shah Alam, Malaysia e-mail: [email protected] School of Electrical and Electronics / Material and Mineral Resources Engineering, USM, 14300 Penang, Malaysia. e-mail: [email protected] The most famous filter in microwave applications is the bandpass filter [3]. The filter can be narrow- or wide-bands depend on the applications. A narrow-band bandpass device is designed for stringent specifications of passband insertion loss and stopband rejection. While a wide-band bandpass filter is normally used for high data transmission involving a lot of video and data communications [6]. Their practical realization are varies depend on the applications. For a solution to be practically useful, it needs to be easily manufactured, reliable, add little or no extra cost to the system in a mass production. The solutions also add only a minimum insertion loss to the filter since this attenuation will directly limits the performance of the wireless system. Abstract— This paper presents a new approach of designing a bandpass filter by applying a combination of microstrip and cylindrical shape of dielectric resonators for X-Band application. Three dielectric resonators with a same permittivity and diameter of 60 and 5 mm respectively are applied in the circuit in order to obtain a wideband of more than 1.0 GHz. The interaction between the microstrip transmission line and dielectric resonators increases the coupling effect as well as minimizing the insertion loss in the filter passband. An analysis on the effect of the height of the dielectric resonators has been investigated in order to prove that the new approach contributes more advantages and viable at the desired application band. Keywords- Bandpass filter; Dielectric resonator; wideband. I. The DR filters are good for mobile and satellite communications. A typical DR filter consists of a number of dielectric resonators that are mounted in a planar configuration to obtain a good resonant frequency [7]. The relative dielectric constant of the DR in microwave filters generally was chosen from a higher value compared to the base substrate. The primary advantage in using a high dielectric constant is to miniaturize the filter size. The size of DR filter is considerably smaller than the dimension of waveguide filters operate at the same frequency. Furthermore, these DR filters are employed to replace waveguide filters in applications such as satellite communication systems where the planar filters cannot be used due to the inherent of high loss. INTRODUCTION Communication systems demand a large number of basestation filters that not only excellent in performance of low loss, but also a good out-of-band spurious performance. A high performance resonator is an important element in many microwave circuits such as filters, amplifiers, couplers, and antennas. There are variety of geometrical resonators have been reported by Virdee [1]. The Dielectric resonator (DR) offers a lot of advantages in increasing the performance of RF and microwave devices which make it an ideal candidate for wireless application; low design profile and wide bandwidth application [2]. However, the performance of the most distributed resonators is limited due to the use of effective dielectric constant and discontinuity of the transmission line. Strip line structures have grasped substantial research interests due to the advantages such as ease of realization both in series or shunt stubs and without require any via holes [3-5]. The design procedure of bandpass filter microstrip lines is well documented in literature [6]. In this paper, a novel bandpass filter that consists of three dielectric resonators excited with a microstrip transmission line that used to increase the bandwidth of the design. The idea of using the three dielectric resonators is to generate few additional frequencies which can be merged together to produce a wideband devices, increase the transmitting power and reduce the insertion loss in the passband. The optimum coupling effect in the filter was obtained from the matching position of the resonators on the microstrip line. An analysis on the height of all dielectric resonators also has been done on simulation with the help of CST Microwave Studio while measurement on the S-parameters was done by E8364B Network Analyzer. Dielectric resonators are mainly designed to replace resonant cavities in microwave circuits such as filters and oscillators. Dielectric resonator filters are preferable for wireless base stations due to their superior characteristics of a high quality Q-factor and miniaturization. The advantages of dielectric resonators are high temperature stability and ease to be applied. Moreover they can be amenable in multitechnology such as printed circuit and surface mounts technology. Dielectric resonators are also usually shielded to prevent radiation as well as maintain a high-Q that required by filter and oscillator circuits [7]. 978-1-4244-9055-4/10/$26.00 ©2010 IEEE II. DESIGN METHODOLOGY The dielectric resonators can increase the Q-factor in a microwave circuit. The size, location and shape of the dielectric resonators including the height and size area will influence the matching of the circuit. In this project, three 406 is also inversely proportional to the square root of the dielectric The resonant frequency and radiation Q-factor can be varied even dielectric constant of the materials are fixed due to the dielectric resonators able to offer flexible dimensions. It is amenable in integrating to the existing technologies by exciting using probes, slots, microstrip lines, dielectric image guides or coplanar waveguide. dielectric resonators were excited on a microstrip transmission line in order to obtain the optimum coupling effect. The dielectric resonators offer advantages in increasing the performance of RF and microwave devices. The match combination of dielectric resonators and microwave circuit capable to generate additional coupling effect that can be merged together to produce a wideband device as well as increasing the transmitting power and reduce the insertion loss. This combination proficiently produces a low design profile. Fig. 1 shows the simulated and fabricated circuit layouts of the bandpass filter. The microstrip transmission line is made up of a copper metal with electrical conductor of 5.8 e+7 S/m, while dielectric resonator is a ceramic type made up from ZnSnTiO with dielectric constant, εr = 60 and tangent loss of 0.002. The base substrate is a Duriod type with εr = 2.5 and tangent loss of about 0.002. Vacuum box Input and output ports Dielectric resonator The detail dimensions of the circuit layout including the transmission line and dielectric resonators in millimeters are shown in Fig. 2. The radius, r, of the dielectric resonators are equal to 2.55 mm, while the 50  transmission line is 2.18 mm. Input and output of the circuit are connected to the end of the transmission line from both sides. The overall circuit length is 49 mm, while the location of dielectrics are DR1 = 14.5 mm, DR2 = 24.5 mm and DR3 = 41.55 from the input port. Substrate DR1 DR2 DR1 Strip line (a) Simulated layout r = 2.55 2.18 14.5 10 40 17 (b) Fabricated layout Fig. 1. Geometry of the simulated and fabricated bandpass filter. 49 Fig. 2. Circuit layout. The dielectric constant of a material is a parameter that reflects the capability of a material to confine a microwave. The higher this parameter means better in term of microwave signals confinement in the substrate. There is an inversely proportional between size and dielectric constant. A high dielectric constant is required to reduce circuit size of a device. Since cylindrical shape of dielectric resonators have a flexible radius, height, h and dielectric constant due to various sizes can be obtained from the market. The applications of these resonators have been widely used in filters and oscillators [7]. Such shape offers a wide degree of freedom in microwave designs since the ratio of r/h could determine the Q-factor for a given dielectric. Thus a height of the slender cylindrical DR can be made to resonate at the same frequency as a wide and thin DR. However, the Q-factors for these two resonators will be different. This characteristic offers a flexible degree for choosing the most suitable ratio to be the best frequency and bandwidth. The high Q-factor and compact size make it an ideal couple especially in microstrip technology. The main difference lies in the fact that the wavelength in dielectric materials is divided by the square root of the dielectric constant, ε r in a function of λ g = λ o , where εr λ o is the free space wavelength at the resonant frequency. Moreover, unlike resonant cavities, the reactive power stored during resonance is not strictly confined inside the resonator. The leakage fields from the resonator can be used for coupling or adjusting the frequency. The wavelength inside the DR, λ g 407 III. are almost having a same pattern of responses. However insertion loss from measurement is higher than the simulation. The simulated result shows a very good flat insertion loss in the passband frequencies. The return loss from the simulated result is higher than the measured value. These mean that the simulated results are better than the measured values due to the fact that the simulation is an approximate method to predict the result based on theoretical approach and practically it is quite difficult to locate all dielectric resonators exactly on the coordinates such as in simulation especially when the process of measurement in progress. In term of transition bands, the results from the simulation are steeper than the measurement. It is also clearly shows that the bandwidth of the simulated result is wider than the measurement. RESULTS AND DISCUSSION Dielectric resonator will interact with the microwave transmission line. Wideband devices can be designed using two or more DRs. All DRs are operating in a same principle. Each DR will resonate for a same mode but with different frequency such that the combination response is an additional result from the single response which able to increase the overall bandwidth. TABLE I. For example if DR1 has a normalized resonant frequency of f1 and bandwidth of BW1, while DR2 has a normalized resonant frequency of f2 and bandwidth of BW2, then the combination response could has a bandwidth BW that is larger than BW1 and BW2, if f1 and f2 are properly chosen. Let the Qfactors of the two resonators are approximately the same ( Q 1 ≈ Q 2 = Q o ) and if the return loss of the combined response is equal to or better than 10 dB over the bandwidth BW, then the required values for the resonant frequencies of the individual DRs can be approximately equal to [2]: 5 , 6Q o f2 ≈1+ 5 6Q o 71 · ¸ fo = 72 ¹ fo , 5 · 73 § fh = ¨1 + ¸ fo = 360 ¹ 72 © (1) fo Simulation Measurement Insertion loss -0.86 dB -3.53 dB Return loss -15.54 dB -19.42 dB Bandwidth 1.28 GHz 1.03 GHz The combination of dielectric resonators and microstrip line in designing a bandpass filter with such structure is a novel. The idea of designing this bandpass filter was due to the dielectric resonator can increase Q-factor in a circuit response and able to maximize power transfer in dielectric resonator antennas. Since antenna is a single port device and filter is a two ports device, the same advantages and design techniques have been used to achieve the objectives. Ignoring any mutual interaction as well as any loading effects of the feed, that could either increase or decrease the bandwidth response. For example, if all DRs having a Q-factor of 60, the cutoff frequencies can be simplified as equation below [2]: 5 § fl = ¨1 − 360 © Items Table 1 shows the summary of few parameters from both simulated and measured responses for apparent proven in some of critical points as a comparison. The best insertion loss of 0.86 dB was obtained from the simulated result, while only 3.53 dB from the measured response. This was due to the high dissipation effect of the material loss in microwave frequencies. However, the maximum return loss of measurement value in the pass band of the filter is about 4 dB better than the simulated result. The bandwidth of the measured circuit is only 1.03 GHz compared to 1.28 GHz from the simulation result. The wideband was obtained from both result were due to the implementation of few dielectric resonators on the design. Fig. 3. Measurement and simulation results. f1 ≈ 1 − COMPARISON VALUES OF SIMULATION AND MEASUREMENT A dominant parameter affecting the degree of coupling is the dielectric constant of the DR. For the higher values of dielectric constant, the stronger coupling will be. Nevertheless, the maximum amount of coupling is significantly reduced if the dielectric constant of the DR is low. This can become a problematic if low dielectric constant values are applied to obtain a wideband operation. (2) where fl and fh are the lower and upper cutoff frequencies, respectively. In order to obtain a compact size of a design, a DR that contain of a high dielectric constant must be chosen. However, the range of dielectric constants that can be used is limited, Fig. 3 shows the wideband results from simulation and measurement of the filter for a comparison. Both of the graphs 408 proven that the results of the filter are agreed well to the microwave theory. However the magnitude values of both insertion and return losses almost have no change. since there is a tradeoff between the compact circuit and the dielectric constant due to the high percentage of power being trapped in the surface waves of the microstrip substrate. Since surface waves are not generated in DRs, the radiation efficiency is not affected by the highest dielectric constant on the top. At the same time, the Q-factor is increases proportionally to the dielectric constant. This will reduce the bandwidth of the filter. By properly choosing the dielectric constant, the Q-factor can be reduced. The volume of the DR and Q-factor can be traded off depending on the particular design application. For a low profile design, a combination of high dielectric constant and large DR area can be used to obtain a reasonable bandwidth. Feed line is a very important element in microwave devices. The design and implementation of this network involves a significant part of the overall design effort. A proper design of the feed networks is able to minimize losses due to a match combination circuit reduces the reflected signal. However, the bandwidth specifications must be achieved within the limited circuit area. The choice and design of a feed network involves a tradeoff between bandwidth and circuit efficiency. If the circuit does not match, no maximum power will be transferred due to the signal was reflected back to the source. Fig. 4. The effect of dielectric height on return loss Alternatively, series feeding technique results in a more compact size together with a lower loss network compared to parallel method for the combination of DR devices. Power is transferred from the line to the DRs by electromagnetic coupling that can be controlled by adjusting the spacing between the DRs and the line. In the resonant approach, the microstrip line is terminated in an open circuit by the DRs. This approach will create a standing wave on the line where the voltage maxima/minima of each wave are located at multiples of λg/2 such as in slotted line. Normally this approach is used to achieve a systematic approach for controlling the amplitude weighting of the elements [8]. Input signal can be coupled into or out of the DR through one or more ports. The port types and position can determine which mode will be excited and how much power will be coupled between the port and the device. The amounts of coupling and resonant frequency are important in order to determine the performance of the device that applies DRs. These quantities can be predicted by approximately calculate the field distributions of modes from the isolated DRs and using the Lorentz Reciprocity Theorem and coupling theories such as in resonant circuits that explained by [9]. Frequency (GHz) Fig. 5. The effect of dielectric height on insertion loss IV. CONCLUSION A bandpass filter was designed to operate at center frequency of 10 GHz for X-band application. The filter has advantages of very small ripple at the passband insertion loss and able to operate with a wide bandwidth. The structures of the filter are simple and easy for fabrication process. The measurement values are closely agreed to the simulation results. From the analysis, the height of the dielectric is inversely proportional to the operating frequency. The analysis on the DR height, h, shows a major influence to the response of the design. The height/thickness of the dielectric affects on the coupling performance inherently has a relationship on the insertion, S21 and return, S11 losses. Fig. 4 and Fig. 5 show three samples size of the dielectric height of the cylindrical dielectric resonators. The response of S11 and S21 will be shifted to the higher frequency when the height was decreased. The size of the dielectric will become thinner. This experiment only varies the height without change the surface area of the dielectric resonators. Since, the relationship between wavelength and frequency has known as inversely proportional according to the equation, Ȝ= c/f. This analysis has ACKNOWLEDGMENT Authors would like to thank Universiti Teknologi Mara, and Universiti Sains Malaysia for supporting the project. 409 [5] REFERENCES [1] [2] [3] [4] Bal S. Virdee, Christos Grassopoulos, “Folded Microstrip resonator,” IEEE MTT-S Int. 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