Nonlinear error correction for Terahertz FMCW System by a new beat frequency estimation method

Abstract

Terahertz (THz) frequency modulated continuous wave (FMCW) technology is a means of nondestructive testing. The signal’s nonlinearity is an unavoidable problem in the daily application of THz FMCW technology. The signal’s nonlinearity will lead to the spectrum broadening of the FMCW’s beat frequency (BF) signal, which degrades the range resolution and result in distance-measuring error. Traditional methods require additional hardware or require a lot of computation, which are not conducive to the miniaturization of the system and real-time measurement. A novel method for correcting the nonlinear error of THz FMCW technology has been proposed and demonstrated in this article. In the proposed method, the windowed Fourier transform (WFT) is introduced to estimate the BF corresponding to the measured target, according to the linearity distribution of voltage-controlled oscillator (VCO). In this way, the measured target's BF can be accurately estimated from the unprocessed BF signal with a poor linearity. From the estimated BF of the reference target, the non-linear compensation coefficients are calculated. With the non-linear compensation coefficients, the non-linearity of the output BF signal can be calibrated. The results of simulations and experiments show that the proposed method allows the range resolution of an FMCW system to reach the theoretical limit.