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Schematic diagram of the tuned NMR spectrometer. The two-stage SQUID sensor is depicted here as a single SQUID. The device contains an integrated input coil and spoiler. The temperature of the pickup coil and capacitor can be varied. Inductance represents the NMR transmitter coil.
Frequency dependence of the flux noise referred to the first stage SQUID measured in open loop mode, with the pickup coil at . Off-resonance noise corresponds to . Both Johnson noise in the pickup coil and circulating current noise in the SQUID contribute to the peak noise. Inset shows the noise in the vicinity of the peak for coil temperatures of 20, 55, 100, 200, 400, and .
Resonance noise spectral density normalized by the off-resonance noise for , extrapolates to zero at with .
Measured and predicted at five different working points. Predicted obtained using Eq. (7) for extracted from values.
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