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

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.