High-frequency carrier type thin-film sensor using low-noise crystal oscillator

We have developed a very sensitive high-frequency carrier-type magnetic field sensor based on the giant magneto-impedance effect available at lower frequencies. Our approach is to optimize the sensor structure so as to obtain high impedance. In addition, we optimize the carrier suppressing circuit using a low-noise crystal oscillator to reduce the phase noise of the lower offset frequency (under 1 kHz) as a carrier source. When a small ac magnetic field is applied to the sensor element, a very small AM signal can be detected using a carrier suppressing circuit with a low-noise crystal oscillator. A resolution of 1.7/spl times/10/sup -8/ Oe/Hz/sup 1/2/ was obtained at 501 kHz. The detectable magnetic field is limited by the thermal noise of room temperature. Resolutions of 1.5/spl times/10/sup -7/ Oe/Hz/sup 1/2/ at 1 kHz and a resolution of 1.1/spl times/10/sup -6/ Oe/Hz/sup 1/2/ at 110 Hz were demonstrated. The resolution was basically limited by the phase noise of the crystal oscillator.