Digital Third-Order Nonlinearity Correction for Time-Interleaved A/D Converters with VCOs

Abstract

A time-interleaved analog-to-digital converter (TI-ADC) with voltage controlled oscillators (VCOs) is one promising candidate for ADCs in direct-RF sampling receivers. The VCO-based ADC, however, produces many harmonics due to the nonlinearity of the VCO gain, degrading the signal-to-noise-and-distortion ratio. We present a digital correction method for the third-order nonlinearity of TI-ADCs with VCOs in direct-RF sampling receivers dealing input signals centered at a quarter of the sampling frequency. Our method first estimates the nonlinearity with the cross-correlation function and adaptive signal processing of a third-order harmonic distortion (HD 3 ) tone for a continuous wave (CW) signal. Then, it generates HD 3 and third-order intermodulation products for modulated signals to subtract them from the output of the receiver. This approach using the CW wave makes the correction circuit much simpler; it requires no lookup tables and auxiliary ADCs (VCOs). MATLAB/Simulink simulations show that the proposed method reduces an HD 3 tone from −49.8 dBc to −78 9 dBc for a CW signal and improves an error vector magnitude from 2.48% to 0.87% for a 16-QAM signal.