External active shielding

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.External active shielding (art. no. NM23040N) is a noise cancel-lation technology designed for protecting magnetometer systems from environmental interferences. Combined with a magnetically shielded room, external active shielding provides additional protection against magnetic field distortions without additional weight or size.

External active shielding reduces slow disturbances caused by moving objects, including subways, trams, elevators, or vehicles, as well as by power lines by establishing a compensation field that cancels out the interfer-ence. With compensation coils mounted on the outside surface of the magnetically shielded room, external active shielding reduces the need for expensive passive shielding.

Operating principleThe external active shielding sys-tem attenuates ambient magnetic distortions by measuring fluctua-tions of the magnetic field using a triaxial fluxgate magnetometer placed outside of the magnetically shielded room and feeding a cor-responding compensation signal independently to three compensa-tion coil pairs mounted on the outside surface of the magneti-cally shielded room.

Magnetic shieldingThe attenuation provided by the external active shielding system depends on the presence of an identifiable interference source as well as on the tuning of the com-pensation against the identified source. Therefore, the magnetic shielding performance of the active shielding system may not be guaranteed. However, under typical conditions in which the system is tuned against a remote, identifiable interference source, attenuations up to 20 dB may be reached.

ComponentsSix feedback compensation ■

coils mounted on the external surfaces of the magnetically shielded roomA three-channel magnetic field ■

compensation unitA triaxial fluxgate magnetom- ■

eterA mounting kit for the fluxgate ■

magnetometerCabling ■

AmplifierOutput current: max. 2.5 A ■

(single amplifier)Total output current: max. ■

3.5 ALoad to single amplifier: ■

≥3.0 Ω

Compliance at 2.5 A: typi- ■

cally ±7 V

MagnetometerMeasurement range: ■

±19.99 μT and ±199.9 μTResolution: 10 nT ■

Noise: <0.7 nT RMS ■

Accuracy: 0.5 % ± 25 nT ■

±1 digitOffset adjustment range: ■

±500 μT (coarse and fine)Analog output: 0.03 V/ ■ μT (BNC connector)Output voltage: ±6 V ■

Bandwidth: 0–1 kHz (–3 dB) ■

FilterBand-pass filter: 2nd order ■

biquad active filterBandwidth: ~2.6 Hz ■

Phase adjustment range: –120° ■

to +0° and –30° to +90°Channel separation: >20 dB ■

Operating environmentThe proper functioning of the ■

external active shielding system depends on the presence of an identifiable, far-field interfer-ence.Operating temperature: ■

0–40 °C

Technical specifications