A3 - ORIGINAL CONTRIBUTIONS Nuclear resonance absorption...

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ORIGINAL CONTRIBUTIONS Nuclear resonance absorption circuit By F. N. H. ROBINSON,* M.A., D.Phil., Clarendon Laboratory, Oxford [Paper first received 13 April, and in,finalform 14 May, 19591 The new circuit described in Part 1 has the sensitivity and flexibility of conventional circuits using a separate oscillator, combined with the convenience and freedom from microphonics of marginal oscillator circuits. In particular, it permits the use of very low levels of oscillation. Because the factors governing the sensitivity of conventional circuits are not widely known, this first paper is largely devoted to their delineation. Only the principles and theory of the new circuit are given in Part 1. Practical design details are relegated to the second paper. Part 1. Sensitivity considerations Nuclear resonance absorption in a specimen surrounded by a coil forming part of a tuned circuit changes the quality factor Q by where 77 is a filling factor and x” is the imaginary part of the nuclear susceptibility. The excellence of any circuit for detecting nuclear resonance (n.r.) absorption can therefore be specified by giving that change 6(l/Q) which yields a signal equal to noise. The simplest n.r. circuit (Rollin(’)) is shown in Fig. 1, in which a constant current generator I drives the resonant circuit (L, C with loss G) at its natural frequency. N.R. absorption changes the shunt impedance of the circuit and thus the voltage across it. This change is detected by a receiver of bandwidth B and noise figure F. 6(l/Q) = 4~77~” (1) ffl Fig. 1. Simple nuclear resonance circuit using a separate oscillator A change 6(l/Q) results in a fractional change Q6(l/Q) in the shunt impedance and so, if the r.m.s, voltage in the absence of signal is VI, it is changed by: SVi = - V:Q6(l/Q) (2) The effective rms. noise voltage across the coil is V,, = (4kTFB/G)’12 = (4kTFBQjwc)lI‘ (3) Thus unity signal-to-noise ratio is achieved when (4) Identical results are obtained for bridge circuits. the only purpose of which is to give the experimenter freedom to minimize the receiver noise figure by working with the optimum input. The voltage Vi cannot be indefinitely increased because of nuclear saturation. If the maximum permissible r.f. magnetic * English Eiectric Research Fellow. field is Hl then energy considerations yield the following relation where U is the specimen volume. Y:Cg H?U/4q (5) At the optimum level a value of satisfying gives a signal equal to noise. Both H1 and x’’ are beyond the experimenter’s control. To optimize the signal-to-noise ratio one can only increase Q, U, 7 and decrease F and B. Of these only Q, F, B pertain to the circuit. Factor Q is limited partly by the experimental arrangement and partly by the physical properties of materials. Bandwidth B is limited by the stability of the equipment and by the tinir available for measurement. Thus, all that the circuit designer can do is to attempt to ensure that F does not appreciably exceed unity.
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A3 - ORIGINAL CONTRIBUTIONS Nuclear resonance absorption...

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