ABSTRACTIn this paper, a DC-20GHz 5-bit step T-attenuator including five cascaded single-bit switched T-attenuators is presented. The attenuation can be digitally controlled in steps of about 1 dB. To improve the flatness and insertion loss of the attenuator, nMOS switches with optimized switching performance are used as shunt switches. The circuit is implemented in a standard 0.18 µm CMOS (complementary metal-oxide semiconductor) technology. Over DC-20GHz, the proposed circuit shows the attenuation flatness of <2.6 dB (average of 2.1 dB) and attenuation range of 27–29.5 dB. Its insertion loss is 1.4–2.4 dB in the entire frequency range. Furthermore, the simulation results confirm good impedance matching and reasonable phase imbalance. The worst-case return loss is better than 11 dB with the rms (root mean square) phase imbalance of <16o (average of 13.8o).