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Its 235GHz antennas and the core active components are integrated on a single IC, and across a desktop it measures the range to a target with sub-mm accuracy, even if the object is moving at 600m/s – twice the speed of sound.
These core components implement a ‘self-injection-locking’ oscillator, which exploits the sometimes undesirable effect of oscillator ‘pulling’ where varying the load on an oscillator alters its frequency.
The oscillator in this case is a differential second-harmonic Colpitts (‘3’ in the diagram) and it is attached to a slot antenna (‘1’ in the diagram).
Its power is transmitted, reflected of the object, and returns into the same antenna to get back into the oscillator.
Over a certain range of reflected amplitudes, this reflection pulls the frequency of the oscillator, swinging it between 234.9 and 235.1GHz depending on the phase of the return.
Frequency varies steadily with phase until, once every full turn around the phase circle, frequency suddenly snaps back: from 235.1 to 234.9GHz or vice versa depending on which direction the target was travelling.
These snaps occur every 638μm (λ/2) of target displacement – and fairly simple signal processing produces a voltage pulse from this snap, whose polarity depends on target movement direction.
Simply adding or subtracting from a counter depending on pulse polarity keeps track of the target distance.
Physically, the chip is bonded to the flat side of a hyper-hemispherical silicon lens as the primary focusing optic, then a secondary PTFE lens produces the parallel radiation beam needed by the radar. The return path is a reverse trip through the same optics.
The second antenna on the die (‘2’, a folded dipole) is the receive antenna for an FMCW radar that has been built on the same chip.
Its transmitter uses the same hardware and antenna as the self-injection-locking oscillator radar, reconfigured to produce a frequency chirp – its bandwidth is 230 to 246GHz.
The IC has an area of 0.42mm2 and consumes 186mW. It was fabricated by STMicroelectronics in 55nm BiCMOS – and the university also worked with King Abdulaziz City for Science and Technology (KACST) on this project.
ISSCC 2024 paper 24.4: Sub-THz ruler: Spectral bistability in a 235GHz self-injection-locked oscillator for agile and unambiguous ranging
Image credit: ISSCC 2024, University of Michigan, King Abdulaziz City for Science and Technology (KACST), STMicroelectronics. The other objects on the die are: a sub-harmonic mixer (4) and an IF low-noise amplifier (5).
