A phase coherent 7-bit digital step attenuator on 0.18μm SOI

Abstract

We present a novel digital step attenuator (DSA) with low phase variation under attenuation state and frequency changes. This is achieved while keeping all other specifications comparable with the state-of-the-art. To compensate the phase shift, a number of switchable phase compensating blocks are employed. Unlike previous studies, this work achieves very low phase variation in a commercial, 4×4 quad-flat no-leads (QFN) package, where wirebond effects are significant. The proposed attenuator has 7-bit control with 0 to 31.75 dB attenuation range with 0.25 dB step sizes and achieves accurate attenuation settings in a wide frequency range. The attenuator is fabricated in a commercial 0.18 pm RF silicon-on-insulator (SOI) process. The measurement results show that the attenuator has an amplitude error of less than 1 dB, while introducing a maximum of ±3° phase shift up to 2.2 GHz and less than ±6° between 2.2–3.5 GHz. This approach provides at least 2.5-fold improvement in the phase shift when compared to commercial attenuators. The input 1 dB compression point and IIP3 are measured typically higher than 35 dBm and 45 dBm, respectively. Total chip size, including pads, is 1.95mm × 0.95mm.