Application of the time-variable feedback to the input amplifiers of pulse magnetic resonance spectrometers: Theoretical considerations

Abstract

An analytical model for the optimization of the signal-to-noise (S/N) performance for the receiver with input resonance circuit and variable feedback is developed. It is shown that by varying the 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 assumptions are retatively general for the model to be applicable for the design of both continuous and pulse receivers in radio-frequency and microwave bands. The overall S/N performance of the receiver upon the noise properties of its elements and ~he feedback settings in the input amplifier is studied for different parameter settings. It is shown that the separate optimization of individual elements does not necessarily lead to the best S/N performance of the receiver, especially when bando width properties and noise cont¡ of the elements are substantially different. It is shown that critical coupling of the amplifier to the resonance structure could be lar from optimum. In some cases the optimum S/N performance could be achieved with coupling settings below the c¡ value. But under the assumptions made the coupling above the critical value does not correspond to the best reeeiver S/N performance. Suggestions on the optimum architecture of magnetic resonance spectrometer receivers with variable feedback are made.