Noise in single-core gradiometers based on orthogonal fluxgate

In the design of orthogonal fluxgate sensors, a common approach involves employing a pick-up coil of maximum length to enhance signal strength. Traditionally, gradiometers are implemented by utilizing two distinct sensors and calculating the gradient as the difference of their outputs. Alternatively, the use of two shorter coils on the same core is often avoided due to concerns that a shorter coil may yield a lower signal-to-noise ratio. Contrary to this conventional belief, this paper demonstrates that a two-coil gradiometer can be a viable and effective solution. Specifically, we establish that opting for a 15 mm long coil over a 50 mm long coil results in only a marginal increment from 0.9 to 1.3 pT/√Hz, a level deemed acceptable for numerous applications with proper design considerations. Additionally, we elucidate the distribution of magnetic flux density along the magnetic core, highlighting its role in detecting rapidly decaying magnetic gradients even at considerable distances. Furthermore, we delve into the correlation of noise in multiple coils on the same core and provide insights into achieving independent measurements for each coil.