An Energy-Efficient Microwave Magnetic Field Generator for NV Center Quantum Magnetometers Based on an Array of Four Injection-Locked VCOs

This paper proposes the use of chip-integrated LC voltage-controlled oscillators (VCOs) as the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$B_{1}$</tex> field source in NV diamond-based magnetometers. VCOs provide a compact, energy-efficient solution by directly converting DC power into the microwave <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$B_{1}$</tex> field required for NV center control, bypassing conventional RF amplification stages. Additionally, the VCO's intrinsic frequency agility, enabled by the varactor inside the LC tank, which simultaneously changes the resonant frequency of the LC resonator and the oscillation frequency, ensures a near-constant <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$B_{1}$</tex> field across the tuning range. Current state-of-the-art chip-integrated diamond magnetometers, while showing promising results, typically make use of classical transmitter architectures, in general, providing a lower energy efficiency compared to the presented VCO-based approach. With its very good energy efficiency, the proposed VCO-based approach offers a path to scalable, low-power NV-based magnetometers suitable for portable and embedded applications. To provide a sufficiently large active area for typical diamond samples, the presented VCO-based <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$B_{1}$</tex> generator uses an array of four injection-locked VCOs. An on-chip phase-locked loop (PLL) precisely defines the frequency of the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$B_{1}$</tex> field from an external reference. The design is validated using electrical measurements that reflect the key metrics of the target quantum magnetometry application.