Sub-THz Ruler: Spectral Bistability in a 235GHz Self-Injection-Locked Oscillator for Agile and Unambiguous Ranging

Abstract

Sub-THz/THz radars are paramount technologies in scientific and industrial metrology, offering high-precision and fast-acquisition target ranging. Among sub-THz/THz radars, the FMCW and pulse radars are well-studied approaches, where they use interferometry and time-of-flight techniques, respectively, to measure the target range. Also, both utilize the Doppler effect to calculate the target velocity by estimating the frequency shift of the TX signal (fd=(2Vr)⁄lambda, where fd is the Doppler frequency, Vr is the radial velocity, and lambda is the wavelength). However, the fundamental challenge of these radars is the ambiguities that arise in the simultaneous measurement of the range and velocity, represented in the ambiguity diagram, which significantly constrains the accuracy of range and velocity measurements. For applications that require a wide range of target velocities, sub-THz/THz radars need a bank of Doppler filters to cover the expected range of Doppler frequencies, enormously increasing the complexity of the radar hardware. Furthermore, the processing time required to resolve the ambiguities slows down the radar operation, limiting the radar capability in determining the range and velocity of agile targets. To overcome these challenges, we propose the idea of a sub-THz ruler sensor that uses self-injection locking (SIL) nonlinearity in a sub-THz oscillator to discretize the free space with lambda⁄2 steps. The sub-THz ruler sensor can promptly and unambiguously determine the target’s relative range and velocity by counting the number and measuring the time intervals of a series of sharp polarized pulses at the sensor’s output. The proposed sensor is also equipped with an FMCW radar to find the initial range (R0) of the target needed for absolute ranging (R=R0+nlambda⁄2). Using these approaches, this paper demonstrates a 235GHz ruler sensor with lambda⁄2=638μm range accuracy and the capability to measure velocities up to 638m/s and 840m/s for receding and approaching targets, respectively. Also, the backup FMCW radar for initial ranging has 16GHz bandwidth from 230-to-246GHz. Across state-of-the-art, this is the first demonstration of the SIL technique in the sub-THz band for agile and unambiguous ranging and velocimetry.

Morteza Tavakoli Taba

mm-Wave Design Engineer

My research interests include mm-wave and terahertz circuit design.