US Patent 8027110 Apparatus for measuring magnetic field of microwave-assisted head

A measurement circuit system of a magnetic field measurement apparatus of the present invention includes an amplifier, a mixer circuit and a band-pass filter that are connected in order on an output end side of a microstrip line or a coplanar wave guide, which is an in-plane high frequency magnetic field intensity measurement element, a frequency immediately before being inputted in the band-pass filter is down-converted by the mixer circuit to a frequency so that a band width of the band-pass filter can be used, the band-pass filter uses a narrow band of ±0.5-±10 KHz (1 KHz-20 KHz as a bandwidth) centering a fundamental frequency selected from 5-20 MHz that is down-converted by the mixer circuit as a center peak passing frequency, and the measurement circuit system is configured to obtain 3 dB or greater of a signal-to-noise ratio (SNR) that is a ratio of S with N; where S represents the reproduction voltage (reproduction output) of the high frequency reproduction signal induced by the in-plane high frequency magnetic field intensity measurement element, and N represents a total noise voltage of a circuit noise including a noise generated by the in-plane high frequency magnetic field intensity measurement element. Therefore, the in-plane high frequency magnetic field that a microwave-assisted magnetic head generates can be measured with high reliability and precision.