Phase Noise of SAW Delay Line Magnetic Field Sensors

Abstract

Surface acoustic wave (SAW) sensors for the detection of magnetic fields are currently being studied scientifically in many ways, especially since both their sensitivity as well as their detectivity could be significantly improved by the utilization of shear horizontal surface acoustic waves, i.e., Love waves, instead of Rayleigh waves. By now, low-frequency limits of detection (LOD) below 100 pT/Hz can be achieved. However, the LOD can only be further improved by gaining a deep understanding of the existing sensor-intrinsic noise sources and their impact on the sensor’s overall performance. This paper reports on a comprehensive study of the inherent noise of SAW delay line magnetic field sensors. In addition to the noise, however, the sensitivity is of importance, since both quantities are equally important for the LOD. Following the necessary explanations of the electrical and magnetic sensor properties, a further focus is on the losses within the sensor, since these are closely linked to the noise. The considered parameters are in particular the ambient magnetic bias field and the input power of the sensor. Depending on the sensor’s operating point, various noise mechanisms contribute to f0 white phase noise, f−1 flicker phase noise, and f−2 random walk of phase. Flicker phase noise due to magnetic hysteresis losses, i.e. random fluctuations of the magnetization, is usually dominant under typical operating conditions. Noise characteristics are related to the overall magnetic and magnetic domain behavior. Both calculations and measurements show that the LOD cannot be further improved by increasing the sensitivity. Instead, the losses occurring in the magnetic material need to be decreased.

Thin-Film-Based SAW Magnetic Field Sensors

“…In the 1/f noise regime, a LOD of 3.2 nT/Hz 1/2 is achieved at 10 Hz and an input power of 10 dBm. Above 1 kHz, the noise is dominated by white noise, which is additive noise and decreases with increasing signal power [27]. Here, a LOD of 246 pT/Hz 1/2 is reached for 10 kHz.…”

“…The LOD can be determined from the measured noise floor with Equation (2). A more detailed description of the measurement setup can be found in [27].…”

“…Up to a frequency of 1 kHz, a regime of flicker (1/f) noise dominates the spectra. This noise is related to defects in the substrate and the SiO 2 layer, as well as random fluctuations of the magnetization and magnetic hysteresis losses [27]. In the specific case of SiO 2 , additional surface roughness is introduced during the ion beam etching step to pattern the FeCoSiB layer.…”

“…By means of S and the power spectral density of the random phase fluctuations of the sensor, the limit of detection (LOD) of the sensor can be calculated by [27]: As the change of Young's modulus ∆E alters the phase velocity ν of the acoustic wave [25], a B-field-induced phase change ∆ϕ = 2π l f /(v(B 1 ) − v(B 0 )) can be detected at the output IDTs via the direct piezoelectric effect [26]. The sensitivity of the sensor, which is defined as the phase change per change in magnetic field S = ∂ϕ/∂H, can be written as the product of its individual contributions: magnetic layer sensitivity S mag (change in Young's modulus with magnetic field), structural sensitivity S str (change of the wave velocity with change in Young's modulus), and geometric sensitivity S geo (phase change with change in wave velocity) [1]:…”

“…By means of S and the power spectral density S ϕ of the random phase fluctuations of the sensor, the limit of detection (LOD) of the sensor can be calculated by [27]:…”