Oscillator-Based Reactance Sensors With Injection Locking for High-Throughput Flow Cytometry Using Microwave Dielectric Spectroscopy

This paper presents the analysis and design of oscillator-based reactance sensors employing injection locking for high-throughput label-free single-cell analysis using dielectric spectroscopy at microwave frequencies. By injection-locking two sensing LC-oscillators with an I/Q excitation source, the measurement of the sample-induced frequency shift caused by the interaction with the electromagnetic fields is performed through phase detection with injection-strength-dependent transducer gain. Such inherent phase amplification offered by the injection locking not only relaxes the design requirement for the readout circuits but also maintains the highest rejection against common-mode errors associated with the drift of the supply voltage and the environmental parameters. To reduce flicker noise contribution, a chopping technique employing phase modulation is exploited. In addition, this paper presents a novel ping-pong chopping approach to alleviate chopping-induced dc offset. In this prototype, four sensing channels, covering frequencies between 6.5 and 30 GHz, are distributed along a microfluidic channel fabricated with standard photolithography. Measurements show that the proposed microwave capacitive sensors achieve a sub-aF(rms) of noise sensitivity at 100 kHz filtering bandwidth, enabling measurement throughput exceeding 1 k cells/s. The sensor prototype is implemented in 65 nm CMOS technology and consumes 65 mW at 1 V supply.