The Design of 5 dB Attenuator in Coplanar Waveguide for DC to 67 GHz Amir Effendy Microwave Modules and Accessories Agilent Technologies (M) Sdn. Bhd. Bayan Lepas, Malaysia amir_effendy@agilent.com Ashraff Ali Microwave Modules and Accessories Agilent Technologies (M) Sdn. Bhd. Bayan Lepas, Malaysia ashraff-ali_thajuddin@agilent.com Abstract— Resistive attenuator is used to reduce incident RF power and for impedance matching due to its absorptive and dissipative nature. A resistive 5 dB attenuator has been designed for an electromechanically selectable attenuators module. The electromechanical switch structure necessitates the attenuator to be designed to approximate a coplanar waveguide. As the attenuator has to work up to 67 GHz, parasitics of the attenuator circuit and the peripheral mechanical components had to be dealt with. Three dimensional EM field simulations were performed to understand the effect of the parasitics by monitoring the field patterns. Modification to the coplanar waveguide structure was introduced to overcome the parasitic degradation. A  topology 5 dB attenuator has been successfully developed with attenuation flatness of 0.25 dB and return loss less than -10dB up to 67 GHz. Keywords- attenuator, EM field, parasitic, coplanar waveguide I. INTRODUCTION Resistive attenuators find their use in RF and microwave applications [1-5] because of their precise attenuation level and good impedance matching as the attenuations are provided by resistor networks which are absorptive instead of capacitive element like PIN diode or FET which are reflective and by themselves are lossy, as evident in [6, 7]. In [8], Yiqin et al describe attenuator design using thin film resistor deposited on FR4 substrate, in microstrip configuration. They observed that as the length of the thin film resistor approximates the higher frequencies wavelength, the resistors behave like transmission lines resulting in VSWR close to 1.5 at 20 GHz. Resistive attenuators built from thin film resistors on alumina substrates enable the attenuators to handle a relatively high average power i.e. 30 dBm [9] because of the alumina high thermal conductivity. In this paper, a 5 dB attenuator circuit design to meet attenuation accuracy within 0.25 dB and for application up to 67 GHz is discussed. The initial design was simulated to predict its performance as well as to detect any potential problems. The circuit was then optimized based on the simulation results – upon which, we introduced a unique capacitive patch which enable the attenuator to function accurately up to 67 GHz. All simulations used Agilent EEsof EMPro three dimensional electromagnetic (3D-EM) simulator. Figure 1: Dimensions of a multilayered CPW II. 5 DB ATTENUATOR DESIGN A. Coplanar Waveguide Dimensions To achieve this precision, the resistive attenuators are configured in a multilayered coplanar waveguide (CPW) structure with air as the dielectric layers above and below the circuit, thus minimizing shunt parasitic capacitance; this is shown in Figure 1. When the parasitic elements are reduced, the resistive attenuator operation can be extended into the millimeter wave range. The alumina substrate has a dielectric constant, r = 9.6, its thickness is 127 m. For a system characteristic impedance, Z0 = 50 , the initial CPW dimensions is derived from the formula for CPW characteristic impedance given in [10] with the effective dielectric constant, eff for multilayered CPW calculated based on equation 7.44 in [11]. This yields a width, WC = 960 m for the CPW center conductor and a gap, g = 210 m between the center conductor and the ground planes; these are depicted in Figure 1. Given the area of the substrate is 1600 m long by 1800 m wide, the width of the ground planes on each side of the center conductor is 210 m.