Schlecker, B.; Hoffmann, A.; Chu, A.; Ortmans, M.; Lips, K.; Anders, J.: Towards low-cost, high-sensitivity point-of-care diagnostics using VCO-based ESR-on-a-chip detectors. IEEE Sensors Journal 19 (2019), p. 8995 - 9003

—In this paper, we present a new architecture for VCO-based ESR detection for a future use in portable, point-ofcare ESR spectrometers. The proposed architecture is centered around an ASIC containing a VCO-based ESR detector with two distinct tuning ports with largely different VCO gains to enable wide frequency sweeps and small-signal frequency modulations while keeping the requirements on the digital-to-analog converter driving the ports manageable. Additionally, the proposed ASIC features a second VCO for an on-chip frequency downconversion via mixing. To allow for a precise derivation of the operating frequency from an external reference as it is required for quantitative ESR experiments, the two on-chip VCOs are embedded into an offset phase-locked loop. The proposed architecture is verified with ESR experiments on commonly used ESR standard samples (DPPH and BDPA). In these experiments, a spin sensitivity of 1.7 × 109 spins/(G√ Hz) has been achieved at B0 = 450 mT, which is comparable to the state of the art, using a permanent magnet and low-cost signal processing on an FPGA. The presented proof-of-concept experiments clearly demonstrate the potential of the proposed VCO-based ESR detection system for future point-of-care applications.