Design and Analysis of a Sub-THz Resonator-Based High-Resolution Permittivity Sensor

In this work, we propose a high-resolution CMOS permittivity sensor integrated with a high-<inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> whispering gallery mode (WGM) resonator at sub-THz frequencies. The system compares the transmission coefficients of a pair of WGM resonator sensors to detect small complex permittivity difference of the sensing and the reference material under test (MUT). A sub-THz voltage-controlled oscillator (VCO) in the transmitter (TX) generates the excitation signal for sensing. The TX noise is suppressed by injection locking the receiver (RX) through the sub-THz local oscillator (LO) feedforward path. The low-frequency noises are suppressed by a BPSK chopping scheme, and the high-frequency noises are reduced by windowed integration. Detailed analog signal processing analysis provides design insights into noise reduction. The WGM sensor is fabricated in house with lithography, and the CMOS transceiver (TRX) is implemented in a 28-nm CMOS technology. The boosted sensitivity by the WGM sensor and the multifold noise reduction enable the sensing system to achieve a record permittivity sensing resolution of 0.05% with an integration time of <inline-formula> <tex-math notation="LaTeX">$14~\mu \text{s}$ </tex-math></inline-formula> and a power consumption of 54 mW.