A 1050-GHz CMOS distributed step attenuator with low loss and low phase imbalance

This paper presents the design and measurement of a 10-50-GHz low-loss distributed CMOS step attenuator with low phase imbalance. The attenuation is controlled by 12 nMOS varistors, and the nMOS parasitics are absorbed in a synthetic transmission line to result in a wide bandwidth. The electrical distance between the varistors is explored to minimize the size of the distributed attenuator, and a method to balance the insertion phase is presented. At 33-37 GHz, the minimum attenuation state loss is 2.1 dB, and the maximum attenuation state loss is 13.0 dB. The attenuator has a maximum attenuation range of 11 dB with 0.9-dB steps (13 states). The rms phase imbalance is less than 3° at DC-50 GHz for all attenuation states. The attenuator does not consume any static power and the input 1-dB compression point is 5 dBm (defined as the 1-dB drop in the maximum attenuation range) at 20 GHz. The total chip size excluding pads is 200 × 750 μm² (0.15 mm²).