Nonlinear Distortion Measurements of Discrete-Time Radio Receivers

Modern deep-submicron CMOS processes enable the use of switched capacitor techniques to convert the RF continuous-time input of an RF radio into a discrete-time signal using discrete-time signal processing techniques. The discrete-time output of this type of receiver front-end requires a different measurement methodology than typically used in purely continuous-time systems. A mixed discrete-time/continuous-time measurement setup is necessary to analyze the system´s linear and nonlinear behavior. This paper presents measurement methodologies to characterize both the linear and the nonlinear behavior of multi-rate discrete-time sampling mixers. It presents single and multi-tone measurements of a discrete-time RF receiver front-end designed in a 90 nm digital CMOS process. Reduction of the quantization noise floor in the digital oscilloscope is crucial to resolve low-power intermodulation tones. Ways to improve the measurement setup´s SNR are shown. The measurement results were validated with transistor-level simulations of the device under test.