A 25-35 GHz 5-bit digital attenuator with low RMS amplitude error and low phase variation in 65 nm CMOS

A mm-wave 5-bit digital attenuator with low RMS (root mean square) amplitude error and low phase variation is presented in 65 nm CMOS. The attenuator combines the PI/T-type topology with embedded switches and PI-type topology with the SPDT (single-pole-double-throw) switches to alleviate the insertion loss issue of the conventional PI/T-type topology with embedded switches in mm-wave frequency band, and achieves high attenuation range while maintaining compact chip size. The amplitude/phase calibration technique is proposed to reduce the RMS amplitude error/phase variation and improve the circuit robustness. The presented attenuator has been integrated in a Ka-band phased-array transmit front-end module and achieves 15.5 dB attenuation coverage with the step of 0.5 dB. The RMS amplitude error and RMS phase variation are 0.13-0.48 dB and 1.24-2.08° across 25-35 GHz, respectively. Especially, the proposed attenuator could achieve the RMS amplitude error of 0.13-0.25 dB if the operation frequency is limited to 26.9-31.4 GHz. The core chip size is 434 µm × 360 µm.