Microwave inductive proximity sensors with sub-pm/Hz1/2 resolution

• Published in: Sensors and actuators. A. Physical. 2019-08, Vol.295, p.259-265
• Main Authors: Matheoud, Alessandro V., Sahin Solmaz, Nergiz, Frehner, Lénaïc, Boero, Giovanni
• Format: Article
• Language: English
• Subjects: Attenuation; Bandwidths; CMOS; Coils; Detectors; Floors; Frequency ranges; Inductive proximity sensor; LC oscillator; Microwaves; Oscillators; Picometer resolution; Proximity; Sensors; Transistors
• ISSN: 0924-4247; 1873-3069
• DOI: 10.1016/j.sna.2019.05.041

[Display omitted] •Inductive proximity sensors based on 180 nm CMOS technology.•Displacement measured as a frequency variation of LC oscillators.•Resolution of 0.1 pm/√Hz at 110 μm from the coil with 500 MHz detector.•Resolution of 0.3 pm/√Hz at 70 μm from the coil with 10 GHz detector. Inductive proximity sensors are low-cost and versatile detectors achieving resolutions in the nm and sub-nm range. Their typical working frequency ranges from tens of kHz to a few MHz. Operation at higher frequencies is considered as a possible route for the improvement of the performance. Here we report on the design of two microwave inductive proximity sensors based on LC-oscillators operating at 500 MHz and 10 GHz, respectively. Both detectors are based on a frequency-encoded architecture, leading to an intrinsic robustness against interference and signal attenuation. The 500 MHz oscillator is composed of an off-chip resonator with a planar coil having a diameter of 6.4 mm and a CMOS integrated cross-coupled transistor pair. It achieves a frequency noise floor of 0.15 Hz/Hz1/2 (above the 1/f corner frequency of 6 kHz), which leads to a distance resolution of 0.1 pm/Hz1/2 at 110 μm from the coil. The integrated noise in the 1 mHz to 1 kHz bandwidth corresponds to a distance resolution of 45 pmrms. The 10 GHz oscillator is a fully integrated CMOS differential Colpitts with a planar coil having a diameter of 270 μm. It achieves a frequency noise floor of 2 Hz/Hz1/2 (above the 1/f corner frequency of 10 kHz) which leads to a distance resolution of 0.3 pm/Hz1/2 at 70 μm from the coil. The integrated noise in the 1 mHz to 1 kHz bandwidth corresponds to a distance resolution of 100 pmrms.