Proceedings of the International Conference on Computer and Communication Engineering 28 May 13-15, 28 Kuala Lumpur, Malaysia RF Bandpass Tunable Filter using RF MEMS A.H.M. Zahirul Alam, Md. Rafiqul Islam, Sheroz Khan, Soheli Farhana, Nik Noor Atikah Bt. Nik Mohd. Salleh, Noraini Aziz Faculty of Engineering, International Islamic University Malaysia (IIUM) Email (zahirulalam@iium.edu.my) Abstract A bandpass tunable RF filter is proposed by using Radio Frequency (RF) Microelectro Mechanical Systems (MEMS). The tenability is obtained by using capacitive MEMS switches. The performance of the filter depends on geometry and location and types of the MEMS switches. The paper described the optimization procedure to design a bandpass filter that can be tuned within the bandwidth of 3.6GHz to 4.4GHz by using high frequency electromagnetic simulator (HFSS). I. INTRODUCTI Micro-electromechanical systems (MEMS) devices for radio frequency (RF) and microwave applications, which are already acclaimed in the past decade as one of the most promising emerging technologies, have recently received further attention for their ability to implement reconfigurable passive networks for futuregeneration multiple standards and multiple frequency wireless terminals [1]. These devices have potential performance that can surpass the limits of their current equivalent implementations using more traditional solid-state technologies. In fact, all RF-MEMS devices (not only RF switches) maintain good miniaturization, and they can be integrated with solid-state circuits either above the IC or in the same package. These kinds of devices exhibit almost zero power consumption, extremely good linearity both in terms of IIP2 and IIP3 (both > 7 m), and very low losses (high Q), making them very suitable for tuning [2]. Concerning RF MEMS switch peculiarities, they achieve very low insertion loss (<.1 up to 1 GHz) while maintaining high isolation (> 2 ). From the technological standpoint, they tend to require low-cost fabrication processes compared to RF and microwave solid-state ICs. Reconfigurable bandpass filters (BPFs) draw much attenuation in modern systems because of their diversity. Reconfigurable BPFs with tunable bandwidth [3], [4], tunable center frequency [5] [8], and switch ability [6] are reported. Among these researches, PIN diodes, piezoelectric transducers, and MEMS cantilever/bridge structures are used as the switch components. In this paper, RF MEMS switches have been proposed to design a reconfigurable interdigital bandpass filter. II. DESIGN AND ANALYSIS The schematic diagram of the interdigital filter is shown in Fig. 1. The center frequency f = 4GHz, and bandwidth =.8GHz have been chosen for designing the filter. There are five resonators are assembled on the alumina substrate in the design. The alumina substrate is placed on the ground plane made of copper plate with.5mm thickness. The filter requires use of grounding of the resonators. The grounding is placed at the side of one end of the resonator and connected to the ground plane via hole through the substrate. The microstrip line was used as the feeding technique. Figure 1. The layout of inter digital bandpass filter. 978-1-4244-1692-9/8/$25. 28 IEEE 69
Figure 2 shows the inter-digital bandpass filter response without the MEMS bridge/switches. The frequency response is obtained by using electromagnetic simulator HFSS. Attenuation and Return Loss, -1-3 -4-5 -6-7 -8 S21 S11 3. 3.5 4. 4.5 5. Figure 2. Simulated results of interdigital filter without MEMS switches. A. One MEMS Bridge Parallel with the Resonator The effect of one MEMS bridge placed above the middle resonator has been studied. The MEMS bridge consists of thin strip of metal (membrane) and insulator that is fixed at both ends and suspended over is suspended above an electrode with spacing, forming a capacitor between these two conductors. The bridge is considered silicon nitride of dielectric constant εr = 6.8 with copper coating. The dimension of the bridge is 1.689mm x 14mm x.1mm. The structure is shown in Fig. 3. The insertion loss and return loss is shown in Fig. 4. The result shows that the bandwidth is decreasing as the distance between the bridge and the resonator becomes closer. The upper frequency is nearly maintained while the lower frequency is changing. In the up-state the capacitance is small, since air is separating the electrode and the switch. By applying a voltage the upper membrane is deflected by the electrostatic force, the membrane will snaps down to the opposite electrode, the spacing is reduced and the capacitance is increased. Electrostatic force between the top and bottom electrodes actuates the switch. The variation in capacitance will disturb the frequency response of the filter. Furthermore, the fringing fields capacitance of MEMS switches depending on the bridge dimensions and height. As a result the lower cutoff frequency decreases to lower frequency. The results are summarized in the table 1. Figure 3. The layout of the filter with RF MEMS bridge above the middle resonator. -1-3 -4 S11 S21 1mm 2mm 3mm 4mm 5mm 3. 3.5 4. 4.5 5. -1-3 -4-5 -6-7 -8-9 Figure 4. Simulated results of interdigital filter with MEMS switch placed parallel to the centre resonator. TABLE I. THE RESULTS FOR E MEMS BRIDGE PARALLEL WITH THE RESATOR Distance d(mm) f L f U f C Bandwidth 1 3.64 4.444 4.42.84 2 3.57 4.447 4.85.877 3 3.544 4.455 3.9995.911 4 3.52 4.454 3.987.934 5 3.55 4.467 3.986.962 B. Two MEMS Bridge Parallel with the Resonator The performance of the filter is analyzed by turning the switch on and off by placing two bridges parallel to the resonator places beside the centre resonator. It is observed from the Fig. 5 that there is not much in difference in bandwidth variation between the filter 61
with placing only single MEMS bridge in parallel with the resonator. However, for two bridges, the bandwidth for the down-state is much wider than the single bridge. Return Loss, Insertion Loss, -1-3 -4-5 -6-7 -1-3 -4 3. 3.5 4. 4.5 5. Figure 5. Simulated results of interdigital filter with MEMS switch placed parallel to the centre resonator. C. One MEMS Bridge Perpendicular to the Resonator The effect of MEMS bridge placed perpendicularly above the resonator has been studied. The bridge is made of silicon nitride with.889mm x 24.885mm x.1mm with copper coating as shown in Fig. 6. therefore the capacitance was increased and the bandwidth tends to degrade since the capacitive part of the resonators was changed. In this design, the lower frequency remains, while the upper frequency decreased when the switch is in the down-state. It is also observed that the lower frequency shifted to lower frequency region for the case of parallel bridge whereas the higher frequency shifted to high frequency for the case of perpendicular bridge when the MEMS switch is at down state. Return Loss, Insertion Loss, -1-3 -4-5 -6-7 -1-55 -15-25 -3-35 3. 3.5 4. 4.5 5. Figure 7. Simulated results of interdigital filter with MEMS switch placed perpendicular to the centre resonator. D. MEMS bridges parallel covering all the resonators The filter was designed with MEMS bridges placed on the top of the each resonator, and all the MEMS bridges had the same width as the resonator. The layout of the filter is depicted in Fig. 8. The performance of the filter is analyzed by varying the height of the resonators. Figure 6. The layout of the filter with RF MEMS bridge perpendicularly above the resonator The performance of the filter is analyzed by turning the switch on and off. Fig. 7 shows the frequency response of the filter when the MEMS switch is downstate and up-state. From Figure 6, it can be observed that the bandwidth varies within the frequency range of 3.6GHz to 4.4GHz. In the off-sate position, the bandwidth and center frequency has not changed. On the other hand, when the MEMS bridge is moved down (on-state position), the gap was reduced and Figure 8. The layout of the filter with RF MEMS bridges above all the resonators. 611
The frequency response of the filter is shown in Fig. 9 for different MEMS height. It is observed that the variation in bandwidth within the same frequency when the height of the bridge varies. In this configuration, the bandwidth is dependent on the height of the bridge, because reducing the gap of the bridge and the resonators will result in increasing the capacitance of the bridge. Therefore, increasing or decreasing the bridge capacitance enables the bandwidth to be tuned within the same pass band. Table II depicted the center frequency and bandwidth for the filter. -1-3 -4 S11 S21 4mm 5mm 6mm 7mm 3. 3.5 4. 4.5 5. Figure 9. Simulated results of interdigital filter with MEMS bridges covering all the resonators for various heights. TABLE II. THE RESULT FOR THE FILTER WITH FIVE MEMS BRIDGES PARALLEL WITH THE RESATOR Height of the bridge, (mm) fl fu fc -4-6 -8 Bandw idth 4. 3.76 4.36 4.6.6 5. 3.72 4.38 4.5.66 6. 3.68 4.4 4.4.72 7. 3.62 4.4 4.1.78 III. CCLUSIS The criteria of RF filters with a higher performance, smaller size, lighter weight, lower cost, low loss, and high selectivity can be achieved by using RF MEMS technology. The designing of the interdigital bandpass filter have been presented with the analysis of the frequency responses of the filter. The use of capacitive RF MEMS switch in the application of interdigital bandpass filter is investigated in this paper. The designs indicated that the application of capacitive RF MEMS switches in the tunable filter enables the filter to tune the bandwidth in the frequency range of 3.6GHz to 4.4GHz, which was the range for the filter design without MEMS switch. The tunability has made the filter suitable to be used in different types of application. REFERENCES [1] G. Rebeiz, RF MEMS: Theory, Design, and Technology. New York:Wiley-Interscience, 23. [2] B. Razavi, RF Microelectronics. Englewood Cliffs, NJ: Prentice-Hall, 1997. [3] C. Rauscher, Reconfigurable bandpass filter with a three-toone switchable passband width, IEEE Trans. Microw. Theory Tech., vol. 52, no. 3, pp. 573 577, 23. [4] W.H. Tu, K. Chang, Piezoelectric Transducer-Controlled Dual-Mode Switchable Bandpass Filter IEEE Microw. And Wireless Components Lett, vol. 17, no. 3, pp. 199 21, 27. [5] E. Fourn, A. Pothier, C. Champeaux, P. Tristant, A. Catherinot, P. Blondy,G. Tanne, E. Rius, C. Person, and F. Huret, MEMSswitchable interdigital coplanar filter, IEEE Trans. Microw. Theory Tech., vol. 51, no. 1, pp. 32 324, 23. [6] Y.-H. Shu, J. A. Navarro, and K. Chang, Electronically switchable and tunable coplanar waveguide-slotline band-pass filters, IEEE Trans. Microw. Theory Tech., vol. 39, no. 3, pp. 548 554, 1991. [7] K. Entesari, K. Obeidat, A.R. Brown, G. Rebeiz, A 25-75- MHz RF MEMS Tunable Filter, IEEE Trans. Microw. Theory Tech., vol. 55, no. 11, pp. 2399 245, 27. [8] A. Abbaspour-Tamijani, L. Dussopt, and G. M. Rebeiz, Miniature and tunable filters using MEMS capacitors, IEEE Trans. Microw. Theory Tech., vol. 51, no. 7, pp. 1878 1885, 23. 612
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