The third functional block consists of a GMI fiber and a magnetic-bias coil wrapped around the GMI fiber. One end of the GMI fiber is grounded; the other end is connected via inductor L₁ to the inverting input terminal of U1. When the impedance of the GMI fiber varies in response to a change in the magnetic field applied to the fiber, the resistance between the inverting input terminal and ground changes, resulting in a change in the DC amplifier output voltage, Vout. The magnetic-bias coil is used to fix the operating point of the GMI fiber at the level of applied magnetic field that affords the greatest change in impedance per unit change in applied magnetic field, thereby maximizing sensitivity.

The fourth functional block is a crystal oscillator, U2, connected to the ungrounded end of the GMI fiber via the series combination of capacitor C₂ and resistor R₂. The oscillator U2 generates a 0-to-5-volt RF square-wave signal to be applied to the GMI fiber to excite the magnetic-fielddependent impedance. Because the crystal oscillator is many times more stable than is an oscillator of the type used in prior GMI-based magnetic sensors, frequency drift of the oscillator is small enough to be considered insignificant.

The fifth functional block is a decoupling network comprising inductor L₁ and capacitor C₁. This network keeps the RF excitation signal out of the DC paths of amplifier U1: In slightly oversimplified terms, L₁ acts as a short circuit for DC and an open circuit for the RF signal, while C₁ acts as short circuit for RF between ground and the inverting input terminal of U1. Care must be taken so that the impedance of the GMI fiber does not become capacitive enough to resonate with inductor L₁. If the impedance of the GMI fiber unavoidably becomes significantly capacitive, a damping resistor can be added in series with the GMI fiber to suppress the resonance.

Capacitor C₂ passes the RF excitation while preventing DC coupling from the oscillator to the GMI fiber. Resistor R₂ provides damping to prevent resonance of the combination of capacitor C₂ and inductor L1. R₂ can also be used to bias the RF excitation of the GMI fiber to the correct level (typically, a few milliamperes).

The sixth functional block is an analog- to-digital (A/D) converter. This block receives the analog output voltage (V_out) from amplifier U1 and converts it to a digital signal for further processing. Because the V_out signal has low frequency content, the A/D converter can be chosen to be of a low-bandwidth type to keep power consumption low.

This work was done by James D. Hagerty of the Naval Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Electronics/Computers category. NRL-0006

This Brief includes a Technical Support Package (TSP).

Improved Magnetic Sensor Based on Giant Magneto-Impedance (reference NRL-0006) is currently available for download from the TSP library.

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