This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS 1 A Switchable Iris Bandpass Filter Using RF MEMS Switchable Planar Resonators K. Y. Chan, Member, IEEE, R. Ramer, Senior Member, IEEE , and R. R. Mansour, Fellow, IEEE Abstract—A concept of using planar circuit resonance to 1 disable 3D cavity resonance inside a rectangular waveguide filter 2 is demonstrated. Switchable RF MEMS planar resonators are 3 introduced inside the resonant cavities of a high Q-factor iris 4 bandpass filter to turn the filter ON and OFF. The measurement 5 confirms that this high Q-factor filter with insertion loss better 6 than 0.1 dB can be converted to a bandstop filter with an isolation 7 better than 30 dB for the same frequency and bandwidth. 8 Index Terms— MEMS switches, microelectromechanical 9 systems, microwave filters, microwave switch, radio frequency 10 MEMS (RF MEMS), waveguide switch. 11 I. I NTRODUCTION 12 M ULTI-band and multi-standard operations are now 13 present in all communication systems. The use of recon- 14 figurable RF front-end offers size, weight and cost reductions. 15 Microwave filters are some of the most important devices 16 and the reconfigurable RF front-end relies highly on the 17 reconfigurability of microwave filters. 18 Most reconfigurable filters were designed in planar technol- 19 ogy employing PIN diodes, FET, RF MEMS and ferroelectric 20 switches and varactors. These filters offer versatile tunability 21 and RF performance in frequency of operation and bandwidth 22 but provide limited Q-factor. 23 Three-dimensional reconfigurable filters deliver much 24 higher Q-factors, and micromachining technology has been 25 used for fabrication with embedding tuning elements [1]–[7]. 26 Small to moderate tuning range and relatively high Q-factor 27 were achieved. Most research has been focusing on high 28 Q-factor tunable filters and limited research effort has been 29 allocated into designing switchable high Q-factor filters. 30 Switchable filters are important in many applications including 31 time-division multiplexing (TDM) systems where traditionally, 32 switching is performed by using RF switches [8] to switch 33 between different filters. 34 Another way to switch between different filter responses 35 is to physically switch ON and OFF each filter. Switching 36 3D filters can be achieved by detuning the resonators in 37 the filters. This is usually achieved by applying mechanical 38 adjustment using mechanical tuning screws. Yet, mechanical 39 screw tuning is inherently bulky and slow when compared with 40 electrical tuning. 41 In this letter, a novel method to switch ON and OFF high 42 Q-factor bandpass filters, using reconfigurable planar res- 43 onators with RF MEMS switches, is introduced. It should 44 be noted that our proposed concept is not limited to the use 45 Manuscript received March 31, 2016; revised June 7, 2016 and July 22, 2016; accepted September 5, 2016. K. Y. Chan and R. Ramer are with The University of New South Wales. R. R. Mansour is with the University of Waterloo. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LMWC.2016.2629960 TABLE I LIST OF DIMENSIONS IN FIG. 5. of RF MEMS switches; PIN diodes and FET switches can 46 be used and can operate well within the required switching 47 speed for time-division multiplexing systems. There are sev- 48 eral advantages in our approach compared with the traditional 49 approach in inserting the switches at the input/output ports: 50 In our approach, the switches are in the OFF-state with a very 51 little impact on the filter insertion loss as demonstrated in 52 later section, when we have transmission between the input 53 and the output. On the other hand, in the traditional approach 54 the switches have to be in the ON-state when we have 55 transmission between the input and output. The insertion loss 56 of the switches degrades the overall insertion loss performance 57 of the switchable filter. This feature becomes more pronounced 58 at millimeter-wave frequencies where switches exhibit high 59 insertion loss at that frequency range. 60 II. SWITCHABLE BANDPASS FILTER DESIGN 61 The concept behind the proposed design is to utilize low 62 Q-factor planar resonators to disable the high Q-factor 3D res- 63 onant mode inside a 3D waveguide filter. Conventionally, 64 tunable or switching elements are employed to adjust the 65 resonant frequency or the coupling factor within a filter. 66 In this work, switching elements are used to enable or disable 67 transmission zeros that counter the transmission poles within 68 a 3D bandpass filter. To demonstrate this, a three-pole iris 69 bandpass filter is designed, fabricated and tested. The filter has 70 the following specifications: centre frequency f 0 = 14.25GHz, 71 bandwidth = 500 MHz (or 3.51% fractional bandwidth). 72 Fig. 1 shows the proposed iris waveguide filter with three 73 identical switchable planar resonators adhered onto the side- 74 wall of each resonator cavity. Table I details the dimensions 75 of the parameters given in Fig. 1. 76 These switchable resonators are electrically switched using 77 monolithically integrated RF MEMS ohmic switches that 78 are placed along the centre as shown in Fig. 2. When the 79 RF MEMS switches are turned ON, they form a resonator that 80 resonates at a specific frequency providing a notch. When the 81 switches are OFF, the whole structure only slightly loads the 82 TE 10 mode and with negligible influence. 83 Fig. 3 illustrates the equivalent circuit of a single waveguide 84 resonant cavity loaded with a switchable planar resonator. 85 In this equivalent circuit, a waveguide resonator, λ/4 + λ/4 86 long, has adjacent inverters and the switchable planar resonator 87 connected in the middle. When the MEMS switch is ON, shunt 88 resonance occurs that shorts the RF signal to ground causing 89 the RF signal to reflect back. 90 1531-1309 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.