Patent GB-807841-A (GB)
Title: Gyromagnetic resonance apparatus
IPC: G01R33/24
Filed: 1955-03-29
Abstract: 807,841. Gyromagnetic resonance apparatus. VARIAN ASSOCIATES. March 29, 1955 [March 29, 1954], No. 9159/55. Class 37. The invention refers to gyromagnetic resonance apparatus and discloses the use of a reference radio frequency signal phase-adjusted to select either the in-phase or quadrature component of the nuclear induced resonance signal. The apparatus comprises a polarizing field perpendicular to mutually perpendicular transmitter and receiver coils or perpendicular to a single coil which serves as transmitter and receiver. Also the apparatus may utilize (i) several transmitter/receiver coils arranged to balance out errors due to off-perpendicular changes in the direction of the polarizing field; and (ii) square wave modulation of the polarizing field in equipment designed to maintain constant resonance. General description and use of phase-adjusted reference signal; separate receiver and transmitter coils, Figs. 1 to 3.-In Fig. 1 nuclei in gyromagnetic matter 1 precess under the joint effect of the driving R.F. magnetic field of transmitter coil 4 and the polarizing field of D.C. coils 2 and induce a voltage in receiver coil 5 and this voltage is a maximum when the nuclei precess at a frequency in resonance with the angular rate of the R.F. applied to coil 4 from R.F. transmitter 3. An L.F. (e.g. 60 c/s.) sweep voltage varies or modulates the polarizing field of coils 2 so that the point of resonance is periodically swept through. A leakage flux voltage from coil 4 is also induced in receiver coil 5 and this voltage although small is much higher in value than the nuclear induced voltage. Both signals have components in phase or in quadrature with respect to the R.F. voltage applied to coil 4 and one or the other of these components is selected by the phaseshifter 11, Fig. 2, as follows. The total signal from receiver coil 5 amplified at 6 is mixed in phase-sensitive detector 12 with a reference R.F. voltage taken from transmitter 3 and phaseadjusted at 11 so that it is in phase with the in-phase or quadrature component of the signal in receiver coil 5 (as desired by the operator) and the modulation envelope emerging from detector 12 is amplified and displayed on C.R.O. 8 so as to exhibit the resonant voltage peaks identifying the nuclear species in substance 1. In the circuit details of Fig. 3 translated from Fig. 2 the reference R.F. voltage is phase-adjusted by condenser-resistor 13-14 at 11 and applied through limiter 15 as one input to discriminator 16 whose other input is the R.F. signal from pick-up coil 5 as amplified by R.F. amplifier or receiver 6. The modulation envelope signal as the output of discriminator 16 is delivered into audio-amplifier 19 for display on C.R.O. 8. Bridge designed to avoid errors due to tilt or movement of the polarizing magnetic field, Fig. 4.-The precession-frequency variable bridge arm consists of three series-connected and mutually perpendicular R.F. fed transmitter/ receiver coils 25, 26, 27, each having its own sample of gyromagnetic matter 29. The polarizing field affects each sample of matter 29 and is shown as being produced by a magnet 7 (one pole only shown) but could be that of the earth. When resonance occurs the bridge is unbalanced, thus producing an output signal. It is explained mathematically that any change in the direction of the polarizing field in relation to the coils 25, 26, 27 produces balancing changes in the signals induced in the coils. This Fig. 4 apparatus has particular application in geophysical prospecting where the earth's magnetic field is the polarizing field and the apparatus is carried in an aircraft. Maintaining resonance constant, Fig. 6.-A square-wave generator 31 modulates the polarizing field of D.C. coils 2 and the output from the receiver coil 5 is composed of the leakage signal plus the nuclear induced signal during one half of the square wave and only the leakage signal during the other half. The phaseadjuster 11 selects the in-phase component of the receiver coil signal as stated above and the output from phase-sensitive detector 12 is a square-wave voltage representing the envelope of the induced resonance signal. This square wave is fed to a second phase-sensitive detector 32 as is the square wave from generator 31. The output from the second detector 32 is a D.C. voltage or error signal, the magnitude and polarity of which is proportional to the magnitude and phase-sense of the square wave output of detector 12; the phase-sense of the output of detector 12 is with reference to the square wave signal of generator 31. This error signal is transmitted to a servo system 35 to control the strength of the polarizing field through coil 36 or control the frequency of R.F. transmitter 3 so as to maintain constant resonance. Specifications 753,495, 807,842 and 807,843 are referred to.