| Noise figure characterization of preamplifiers at NMR frequencies. | |
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MedLine Citation:
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PMID: 21439871 Owner: NLM Status: Publisher |
Abstract/OtherAbstract:
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A method for characterizing the noise figure of preamplifiers at NMR frequencies is presented. The noise figure of preamplifiers as used for NMR and MRI detection varies with source impedance and with the operating frequency. Therefore, to characterize a preamplifier's noise behavior, it is necessary to perform noise measurements at the targeted frequency while varying the source impedance with high accuracy. At high radiofrequencies, such impedance variation is typically achieved with transmission-line tuners, which however are not available for the relatively low range of typical NMR frequencies. To solve this issue, this work describes an alternative approach that relies on lumped-element circuits for impedance manipulation. It is shown that, using a fixed-impedance noise source and suitable ENR correction, this approach permits noise figure characterization for NMR and MRI purposes. The method is demonstrated for two preamplifiers, a generic BF998 MOSFET module and an MRI-dedicated, integrated preamplifier, which were both studied at 128MHz, i.e., at the Larmor frequency of protons at 3 Tesla. Variations in noise figure of 0.01dB or less over repeated measurements reflect high precision even for small noise figures in the order of 0.4dB. For validation, large sets of measured noise figure values are shown to be consistent with the general noise-parameter model of linear two-ports. Finally, the measured noise characteristics of the superior preamplifier are illustrated by SNR measurements in MRI data. |
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Authors:
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J A Nordmeyer-Massner; N De Zanche; K P Pruessmann |
Publication Detail:
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Type: JOURNAL ARTICLE Date: 2011-2-3 |
Journal Detail:
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Title: Journal of magnetic resonance (San Diego, Calif. : 1997) Volume: - ISSN: 1096-0856 ISO Abbreviation: - Publication Date: 2011 Feb |
Date Detail:
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Created Date: 2011-3-28 Completed Date: - Revised Date: - |
Medline Journal Info:
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Nlm Unique ID: 9707935 Medline TA: J Magn Reson Country: - |
Other Details:
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Languages: ENG Pagination: - Citation Subset: - |
Copyright Information:
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Copyright © 2011 Elsevier Inc. All rights reserved. |
Affiliation:
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Institute for Biomedical Engineering, ETH Zurich and University of Zurich, Zurich, Switzerland. |
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From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine
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