IJMOT-2014-1-541 © 2014 IAMOT INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY, VOL.9, NO.1, JANUARY 2014 78 Design Procedure for Compact Asymmetric SPDT Switches and Full X-Band Demonstrator Mirko Palomba*, Riccardo Cleriti, Sergio Colangeli and Ernesto Limiti Università degli Studi di Roma “Tor Vergata”, Via del Politecnico, 1, 00133, Rome (IT) Tel: +39-0672597343; E-mail: mirko.palomba@uniroma2.it Abstract – A design procedure for asymmetric SPDT switches is proposed and validated by a MMIC demonstrator covering the full X-Band, well suited to integration in T/R modules. The demonstrator exhibits 0.7 dB and 1.6 dB insertion loss along its TX and RX paths, respectively, and 21.5 dB isolation in TX mode. Chip size is as compact as 1  1 mm 2 . Index Terms – asymmetric SPDT, compact MMIC, SPDT switch design, X-band. I. INTRODUCTION In the past, the best way to separate transceivers’ TX and RX chains consisted in using ferrite circulators, thanks to their low insertion loss and high isolation. Unfortunately these bulky and heavy devices could not be integrated. Therefore, an increasing interest in replacing ferrite circulators with FET-based circulators or switches resulted, thus leading to small, lightweight systems. However, although active circulators represent a valid improvement, they are typically larger than simple switches; furthermore, in particular cases (e.g., T/R modules) signal circulation along TX or RX path is mutually exclusive, and therefore an SPDT switch solution is sufficient. Several satellite systems for Earth Observation operate around 9.6 GHz, and it is therefore interesting to investigate switches operating in the X-Band. Contributions in open literature use particular techniques to null parasitic effects. Resonating two shunt FETs [1] results in 0.6 dB insertion loss and 17 dB isolation at 10 GHz. The resulting circuit is 0.675  1.15 mm 2 but the performance results to be narrowband since isolation rapidly degrades with frequency. Another available technique is presented in [2, 3] by resonating FET in series connection instead of shunt devices. This approach leads again to narrowband behavior, but multi-octave performance on a single series resonated FET are achievable by introducing a resistor on the resonating arm [4]. A study about single gate FET SPDTs can be found in [5], highlighting the possibilities underlying FET resonant techniques. In this case, 0.7 dB insertion loss and 28 dB isolation was reached by a 0.76  1.78 mm 2 chip. Also in this case, the circuit exhibits a narrowband behavior. In fact, at 0.9 GHz from central frequency, isolation drops down to 20 dB. A deep study about the relationship between FET’s physical parameters and its quality factor is presented in [6]: according to this study, the best isolation for a single-throw-single-pole (SPST) switch, with 1 dB insertion loss, is 50.7 dB. A power switch with 10 W capability is reported in [7]: this circuit exhibits 1 dB insertion loss and 26 dB isolation for a 4.5  3.7 mm 2 chip. The bandwidth is 2 GHz around 9.5 GHz. A very small circuit is described in [8], with only 0.3  0.5 mm 2 chip size, by developing the dielectric overhang gate process to reduce the distance between source and drain electrodes of the multiple-gate HEMTs used for the switch. Good performance is registered at 900 MHz but they rapidly degrade to 1 dB insertion loss and 16 dB isolation at 2.7 GHz. A high-power SPDT switch with selectively anodized aluminum substrate is reported in [9], exhibiting 1.3 dB insertion loss and 20.3 dB isolation in a 1 GHz bandwidth around 9.5 GHz; chip size is in a 4.4  3.1 mm 2 . A quarter-wave PIN switch architecture used for T/R modules is presented in [10], providing better bandwidth performance