Demodulated Effect of SQUID to Magnetic Signal Outside Frequency Bandwidth

Abstract

The authors proposed a new method for image formation of magnetic particles or electric currents by detecting magnetic signal modulated by sounds like ultrasound. This method is a combination of sonography and magnetometry, thus we named it “sonomagnetometry”. The method enables the simultaneous observation of spatial distribution and time shift of magnetic source. To realize this technique, we need a SQUID magnetometer which can detect MHz-order frequency signal. Recently, some groups have realized bandwidths detection higher than MHz. In contrast to them, the authors take a different approach which is to detect MHz signal by a narrow bandwidth in basic type SQUID magnetometer system based on the idea similar to a diode demodulator. SQUID magnetometer with 8 kHz bandwidth which is normally used in MEG system was used for the examinations. When 1 MHz magnetic signal whose amplitude is modulated by 500 Hz signal applied to the SQUID, output voltage of 500 Hz was appeared. Its shape is very similar to the modulating 500 Hz signal. This result demonstrates that SQUID magnetometers have demodulation effect to magnetic signals whose frequency is outside the bandwidth. This effect can be applied to sonomagnetometry by measuring time delay from ultrasonic excitation and capturing the shape of the demodulated signals.