Attenuators are extensively employed as an amplitude control circuit in communication and radar systems. Flatness, attenuation range, and bandwidth are typical specifications for attenuators. Most attenuators in previous studies rely on the basic topologies of the Pi-, T-, and distributed attenuators. The performance of the Pi- and T-attenuators, however, is affected substantially by the switch performance of transistors, and it is hard to obtain optimum flatness, attenuation range, and bandwidth with these attenuators. The conventional distributed attenuator also demands a large chip area for large attenuation ranges. We report the design of a new microwave/millimeter-wave CMOS attenuator. A design method is proposed and implemented in the attenuator to improve its flatness, attenuation range, and bandwidth. It is recognized that the Pi- and T-attenuators at a certain attenuation state inherently has the insertion-loss slope increasing as the frequency is increased. This response is owing to the off-capacitance of the series transistors. On the other hand, the distributed attenuators can be designed to have the opposite insertion-loss slope by shortening the transmission lines. The reason is that short transmission lines causes the center frequency be shifted higher. The proposed design method utilizes the two opposite insertion-loss slopes and is implemented for the Pi-, T-and distributed attenuators in a cascade connection. Over 10-67 GHz, the measured results exhibit attenuation flatness of 6.8 dB and attenuation range of 32-42 dB.


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