A Ka-Band 3-Bit GaN Digital Step Attenuator Using Phase Compensation Method

In this study, a 3-bit digital step attenuator (DSA) based on the GaN HEMT process was designed. A distributed structure was adopted to secure stable RF characteristics within a given operating frequency band. To suppress the phase variation according to the attenuation level in the distributed structure, a technique that utilizes a tail capacitor connected in series with a switch transistor has been proposed. The theoretical functionality of the proposed technique has been verified with a numerical analysis. For experimental feasibility verification of the proposed structure, a 3-bit DSA was designed using a 150-nm GaN HEMT process providing two metal layers. The chip size of the designed attenuator was 0.95 mm2. The measured total attenuation range was 7 dB with 1 dB step. It was confirmed that the measured insertion loss was suppressed to less than 1.7 dB in the range of 26.5 to 40.0 GHz. The RMS amplitude and phase errors were measured to be less than 0.16 dB and 4.87°, respectively.