Ultra-Wideband mmW Digital Step Attenuator

In this paper, we describe the design, analysis and evaluation of an ultra-wideband distributed-type digital step attenuator (DSA) covering most of the mmW frequency bands (between 40 and 220 GHz). Based on Fraunhofer IAF’s 35-nm mHEMT technology, we compare various approaches to realizing the unit cell (UC) of a distributed DSA. By using a tapered-stack topology, we minimize the circuit’s insertion loss and improve manufacturability. The fabricated 20-UC DSA features an attenuation depth of between 12.5 dB at 40 GHz and 23.5 dB at 220 GHz with a total of 11 steps. At the same time, its return loss above 40 GHz is better than 10 dB for all states. With these figures, this is the most broadband DSA published to date.