The contributions of amplitude-and phase-sensitive detection to the synchronous demodulation in a RF SQUID magnetometer

Abstract

It is known that the tank-circuit voltage of the rf SQUID carries information about the magnetic flux to be detected in amplitude and phase. The coherent demodulationtechnique utilizes mixing circuits to shift the rf tank voltage spectrum to dc without loss of any signal-information both in amplitude and phase (AM & Pm-detection). Vice versa such a combined detection can be devided into two several detection channels, amplitude sensitive detection (AM) and phase sensitive detection (PM). The contribution of each single mode to the combined detection AM & PM is analysed theoretically in the limit of small magnetic signals and measured in special rf circuits of the magnetometer, which apply PLL-techniques. The influence of feedback is also calculated. For refined studies of the interesting parameters the reference signal of the rf mixer allows to be phase-shifted by an arbitrary amount. All three modes of detections are sketched in diagrams as magnetometeroutput versus phase-shift of that reference signal. In addition, noise analysis is included and measured for vanishing signal. Basic formulas are derived for signal-to-noise ratios. A correlation coefficient is defined for recording correlated noise sources between AM and PM, whereby the possibility of S/N-improvement is pointed out. By computer support the graphs of equations are fitted to measured data with least mean-square deviation. More logical than functional circuit diagrams complete the insight into the treated basic ideas.