**6.3 Noise**

Orthogonal fluxgates in fundamental mode became very popular thanks to the fact that they have less noise than traditional orthogonal fluxgates. This is due to their operative mode, rather than the sensor itself. In (Paperno, 2004) it is demonstrated how the very same fluxgate (120 µm diameter Co-based amorphous wire surrounded by 400 turn pick-up coil) has 1 nT/√Hz noise at 1 Hz if operated in the second harmonic mode whereas the noise is reduced to 20 pT/√Hz when the fundamental mode is used. In this case, the fundamental mode contributes to reduce the noise by a factor of 50, obtained using the same sensor.

A similar result was obtained in (Paperno et al., 2008) for a fluxgate based on a tubular core manufactured with a 5 cm wide amorphous ribbon wrapped with 8 mm of outer diameter. In this case, both the excitation and pick-up coils are added to the core. When this sensor is operated in a fundamental mode, the noise results as being 10 pT/√Hz at 1 Hz, or 30 times lower than the value obtained in the second harmonic mode.

Therefore, noise reduction given by the fundamental mode can be generalized as it applies to all kinds of orthogonal fluxgates, based on the wire core as well as on bulk tubular core.

H

Hdc

+

M

+

H 

Z

0''

t

– –

Fig. 11. Diagram of fundamental mode orthogonal fluxgates with positive and negative dc

In order to suppress the offset we can periodically invert the dc bias and subtract the signals obtained with the positive and the negative bias. Since the sensitivity is reversed, by subtracting the characteristics we sum up the signals whereas the offset is cancelled given

The bias can be switched at a frequency much lower than the excitation frequency. For example, (Sasada, 2002b) suggests to invert the sign every 25 periods of excitation current. In this way we can reduce the effect of sudden transition from a saturation state to an opposite saturation state which could negatively affect the output noise of the sensor. To avoid the effect of bias switching on the noise we can exclude the period right before and after the transition. This can be easily done digitally (Weiss et al., 2010) or analogously using

It must be noted that all the proposed techniques require significant modification of the electronics both on the excitation side as well as on the signal conditioning circuit. While this slight complication in the electronics can be bearable for many magnetometers, it could be a

Orthogonal fluxgates in fundamental mode became very popular thanks to the fact that they have less noise than traditional orthogonal fluxgates. This is due to their operative mode, rather than the sensor itself. In (Paperno, 2004) it is demonstrated how the very same fluxgate (120 µm diameter Co-based amorphous wire surrounded by 400 turn pick-up coil) has 1 nT/√Hz noise at 1 Hz if operated in the second harmonic mode whereas the noise is reduced to 20 pT/√Hz when the fundamental mode is used. In this case, the fundamental mode contributes to reduce the noise by a factor of 50, obtained using the same sensor.

A similar result was obtained in (Paperno et al., 2008) for a fluxgate based on a tubular core manufactured with a 5 cm wide amorphous ribbon wrapped with 8 mm of outer diameter. In this case, both the excitation and pick-up coils are added to the core. When this sensor is operated in a fundamental mode, the noise results as being 10 pT/√Hz at 1 Hz, or 30 times

Therefore, noise reduction given by the fundamental mode can be generalized as it applies to all kinds of orthogonal fluxgates, based on the wire core as well as on bulk tubular core.

bias. The signal sensitivity is inverted changing the sign of dc bias but the offset is

Z

'

the fact that its sign is unchanged for both the positive and the negative bias.

a fast solid state switch before the final low pass filter (Kubik et al., 2007).

non-negligible problem for applications such as portable devices.

lower than the value obtained in the second harmonic mode.

unchanged.

Hdc

+

<sup>H</sup> <sup>M</sup>

t

– –

H

+

**6.3 Noise** 

This can be easily seen when analyzing the source of the noise. Typically, the noise of fluxgate sensors originates in the magnetic core. The reversal of magnetization from positive to negative saturation (and vice versa) involves domain wall movement, which is the origin of the Barkhausen noise. Since a pick-up coil detects time-variation of flux within the core, the Barkhausen noise will cause noise in the output voltage of the pick-up coil. Therefore, designers of fluxgates have chosen materials for the core, which are not only very easy to saturate but also present very smooth transitions between opposite saturation states.

This source of noise is dramatically reduced when a dc bias is added to the excitation current. If the bias is large enough to keep the core saturated for the whole period of the ac current Iac, then the magnetization is only rotated by Iac (Fig. 9) and no domain wall movement occurs.

Sensitivity, however, should also be considered when calculating the output noise in magnetic units. A higher dc bias Idc can significantly reduce sensitivity, because it increases the angle of magnetization M resulting in a lower projection of M on the longitudinal axis (i.e. the magnetic flux in the longitudinal direction is sensed by the pick-up coil). On the contrary, the sensitivity monotonically increases with the ac excitation current Iac (Butta et al., 2011) and therefore an increment of Iac can be useful to reduce the total noise even if a larger Iac could bring the core out of saturation.

The lowest noise of an orthogonal fluxgate in fundamental mode is then obtained selecting a pair of parameters Iac and Idc such that the sensitivity is large enough to minimize the noise but with the minimum value of the total current not too low, so as to avoid significant domain wall movement in the core. The optimum condition for noise reduction is obtained right before minor loops appear in the circumferential BH loop (Butta et al., 2011). Noise as low as 7 pT/√Hz at 1 Hz was obtained by optimizing excitation parameters, using the magnetometer structure proposed in (Sasada & Kashima, 2009).

The noise can be further reduced to 5 pT/√Hz at 1 Hz by using three-wire cores instead of a single wire, in order to increase the sensitivity.
