A SQUID is the most sensitive magnetometer available. The sensitivity is limited by the intrinsic noise which is white at high frequencies and increases inversely with frequency below a frequency called the 1/f knee. The sensitivity above the 1/f knee is typically better than 10−10GHz−1/2, but below the 1/f knee it can be orders of magnitude worse. This is a serious problem for those measurements which are intrinsically low frequency or dc; for them it is important to circumvent the 1/f noise. A well-known technique for reducing amplifier noise is applied schematically to a SQUID in Fig. 1.1 The low frequency signal is modulated at a frequency above the 1/f knee. The SQUID sees a high frequency signal and in amplification adds white noise rather than 1/f noise. The signal from the SQUID is then demodulated, recovering the original signal. This scheme requires a high speed, low noise superconducting switch. We have developed and tested such a switch.
© 1989 Optical Society of AmericaPDF Article