A new compact CMOS distributed digital attenuator

This article presents a new millimeter-wave (mm-wave) distributed digital attenuator with a compact size and high linearity. To overcome the large area consumption of conventional distributed attenuators, multiple unit attenuation cells are combined at a single node, forming a multistate cell. By distributing the multistate cells along transmission lines (T-lines), the number of T-lines is reduced, leading to a compact chip size at a given attenuation range and step. The linearity is improved by stacking multiple FET varistors in each unit attenuation cell. An analytical analysis confirms that the proposed distributed attenuator topology maintains a low phase error comparable to that of the conventional counterpart. To experimentally verify the proposed topology, two different mm-wave digital attenuators are designed and implemented using a 65-nm CMOS technology. The first attenuator (Att1) uses a regular nFET as varistor of the attenuation cell, whereas the other attenuator (Att2_TW) uses a triple-well nFET to reduce the insertion loss. The maximum attenuation range of both attenuators is 14 dB with a step of 1 dB. The measured insertion losses of Att1 and Att2_TW are 4.8 and 4.1 dB at 35 GHz, respectively. The insertion losses are no more than 6.2 dB over 10-50 GHz and 4.3 dB over 15-43 GHz, respectively. The input 1-dB compression powers are 15 and 14 dBm, respectively, at 35 GHz. The chip sizes, excluding probing pads, are as small as 0.19 and 0.29 mm2.