Nonlinear Response of a Continuous Wave NMR Spectrometer for Samples with a Large Magnetization |
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Authors: | Agnès Roni Pierre Thibault Gerard Vermeulen |
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Affiliation: | (1) Centre de Recherches sur les Très Basses Températures, laboratoire associé à l'Université, Joseph Fourier, CNRS, BP 166, Grenoble, France |
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Abstract: | We present a new insight in the response of a CW NMR spectrometer for highly magnetized samples above 100 MHz. The spectrometer is a bridge made of a magic T . The output of the bridge is proportional to the reflection coefficient, , of a resonant circuit, which is built with a coil containing the sample. The sensitivity of the reflection coefficient to the complex susceptibility of the sample, ( ) = ![chi](/content/w20q255pn45633h2/xxlarge967.gif) ( ) – j![chi](/content/w20q255pn45633h2/xxlarge967.gif) ( ), depends on the quality factor, Q, of the circuit and filling factor, . When the condition Q![eegr](/content/w20q255pn45633h2/xxlarge951.gif) ![chi](/content/w20q255pn45633h2/xxlarge967.gif) 1 is not fulfilled, we show indeed that the use of a simple crystal detector, which is only sensitive to | | gives rise to a strong nonlinear response of the spectrometer. Measurements of the complex value of by means of phase sensitive detection allow to recover a linear behavior. We discuss and illustrate those issues with a few circuits we designed for our measurements on liquid
3
He with spin polarizations up to 15 %. A method is described to build in a reproducible and predictable way resonant circuits matched to 50 in the frequency range 100 – 400 MHz with a quality factor as high as 1000 at 4 K. |
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