On Amplitude Modulation in CW Doppler Radars for Remote Displacement Sensing Using ENVSIL Radar

This article introduces an innovative Doppler radar system designed for remote displacement sensing: the envelope detection and self-injection-locked (ENVSIL) radar system. First, we conduct the theoretical analysis of the amplitude modulation (AM) mechanism in continuous-wave (CW) Doppler radar sensors. Our study reveals that in the presence of transmit (TX)–receive (RX) leakage, target motion can produce an effective AM of the carrier whose relative magnitude can be as large as that of the phase modulation (PM). Theoretical derivation, numerical simulation, and experimental validation are provided with excellent agreement between each other. This theoretical advancement presents new possibilities for radar receiver design employing envelope detectors. Moreover, guided by the AM theory, we develop the ENVSIL system to leverage the complementary relationship between AM and PM for IQ plot construction. The ENVSIL system exhibits notable performance, achieving a root mean square error (RMSE) of 0.66 mm for a 5-cm linear motion and an RMSE of 2.26% when measuring a 4-mm motion displacement at a distance of 2.7 m. The absence of a mixer in this architecture reduces the signal power requirement to 0 dBm, contributing to high power efficiency.