Radar-Based Measurement of Image Rejection Ratio in Sub-THz Hartley Receivers with its Impact on Doppler Detection Accuracy

As communication and radar systems advance into the sub-THz frequency range, the Hartley receiver plays an increasingly critical role in enabling heterodyne front-ends. This paper introduces an innovative method for evaluating the image rejection capability of sub-THz Hartley receivers. The proposed approach adapts the Hartley receiver into a heterodyne Doppler radar system designed to detect moving metallic objects, generating the baseband I/Q signal trajectory to estimate the image rejection ratio (IRR) and to extract Doppler phase-shift signals. The baseband signal model developed in this study shows strong agreement with experimental measurements. It not only provides an accurate prediction of the relationship between IF phase shift adjustments and the IRR but also reveals that insufficient image rejection distorts Doppler phase-shift signals, generating broadband odd-order harmonic components. This distortion degrades the detection performance of the heterodyne Doppler radar.