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  • 1. Koptioug, Andrei
    Application of the Time-Variable Feedback to the Input Amplifiers of Pulse Magnetic Resonance Spectrometers: Theoretical Considerations2002In: Applied Magnetic Resonance, ISSN 0937-9347, E-ISSN 1613-7507, Vol. 22, no 22/4, p. 513-537Article in journal (Refereed)
    Abstract [en]

    Analytical model for the optimization of the signal-to-noise (S/N) performance for the receiverwith input resonance circuit and variable feedback is developed. It is shown that by varyingthe feedback type and depth optimization of the receiver the best S/N performance could be achieved.This approach is based upon a resonator-receiver model with lumped elements. These assumptionsare relatively general for the model to be applicable for the design of both continuous and pulsereceivers in radio-frequency and microwave bands. The overall S/N performance of the receiver uponthe noise properties of its elements and the feedback settings in the input amplifier is studied fordifferent parameter settings. It is shown that the separate optimization of individual elements doesnot necessarily lead to the best S/N performance of the receiver, especially when bandwidth propertiesand noise contribution of the elements are substantially different. It is shown that critical couplingof the amplifier to the resonance structure could be far from optimum. In some cases the optimumS/N performance could be achieved with the coupling settings less than critical. But under the assumptionsmade the coupling over the critical value does not correspond to the best receiver S/Nperformance. Suggestions on the optimum architecture of magnetic resonance spectrometer receiverswith variable feedback are made.

  • 2.
    Koptioug, Andrei
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Antzutkin, Oleg
    Lindgren, M.
    Lund, Anders
    Optically detected ESR (OD ESR) of radical-ion pairs in coloured solutions: Observation of transient Trans-Azobenzene Radical Cation1993In: Applied Magnetic Resonance, ISSN 0937-9347, E-ISSN 1613-7507, Vol. 5, no 1, p. 77-86Article in journal (Refereed)
    Abstract [en]

    Trans-azobenzene dissolved in different liquid hydrocarbons absorbs fluorescence arising from all acceptors previously used in Fluorescence Detected Magnetic Resonance (FDMR) and Optically Detected ESR (OD ESR) spectroscopy making optical detection impossible. In this report a new acceptor,rubrene, having sufficient quantum yield of fluorescence in the red band 550–620 nm, has been proven successful. OD ESR spectra of the radical-ion pairtrans-azobenzene+/rubrene were detected in liquid squalane (2,6,10,15,19,23-hexamethyl-tetracosane) solution in the temperature range 294–243 K. The experimental isotropic hyperfine splittings of the radical cation oftrans-azobenzene (a N=1.4 mT) have been compared with those from MNDO/INDO calculations and with those of earlier work using freon matrix studies.

  • 3. Koptioug, Andrei
    et al.
    Reijerse, Edward
    Klaassen, A. A. K.
    Application of Time-Variable Feedback to the Input Amplifier of Pulse Magnetic Resonance Spectrometers: Experimental Studies2002In: Applied Magnetic Resonance, ISSN 0937-9347, E-ISSN 1613-7507, Vol. 22, no 4, p. 455-473Article in journal (Refereed)
    Abstract [en]

    Experimental research on the improvement of the sensitivity and time resolution of pulsedmagnetic resonance spectrometers is discussed. It is shown that application of a time-variable feedbackof a signal to the input of the receiver amplifier can decouple the "fixed" relationship betweenthe quality factor Q and the ringdown time of the resonance system. Experiments were performedwith low-frequency, radio-frequency and microwave pulse-type magnetic resonance receivers. Modificationsof an S/C-band electron spin echo modulation spectrometer carried out to check the “timevariablefeedback” performance is described. It is demonstrated that application of a time-variablefeedback can significantly reduce the ringdown time and improve the recovery properties of the magneticresonance receiver system. It is also demonstrated that the time-variable feedback can improvethe overall receiver sensitivity due to the fact that working bandwidth of the resonance system canbe optimized separately for the transmit and receive modes. Signal values could be increased at leastthree times and the signal-to-noise ratio about 1.5–2 times. The largest improvement is achieved withthe initially overcoupled resonator. Experimental spectra of test samples for different settings of thetime-variable feedback are shown.

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