A Ka-Band 3-Bit GaN Distributed Step Attenuator and the Limits of Tuning-Resistor Control

초록

This paper presents a 3-bit distributed step attenuator for Ka-band applications in a 100 nm GaN HEMT process. Designed for a 0 to -7 dB range across 28-38 GHz, the fabricated device provides 0 to -10.9 dB with a measured insertion loss of 0.94-1.11 dB, return losses better than 10 dB, and an RMS phase error of 4.15 degrees at 38 GHz. Every bit was realized 40 to 58 percent above its design target at both design frequencies, a deviation that simulation did not reproduce. To identify its origin, the sensitivity of the attenuation to the tuning resistor is quantified for each cell. A sweep across 5-20 Omega gives 6.5 mdB/Omega for the 1 dB cell, rising to 42.6 mdB/Omega for the 4 dB cell. In the 1 dB cell the measured step cannot be reproduced at any resistor value, which excludes resistor variation and locates the discrepancy in the switch branch. The branch on-resistance is estimated at about 81 Omega per device, placing the tuning resistors at 0.11 to 0.20 of the branch impedance. The analysis yields a design guideline: step-size margin must be secured through device sizing rather than resistor selection, with the largest margin allocated to the cells of smallest attenuation.

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