Patent GB-939257-A (GB)
Title: Gyromagnetic resonance apparatus
IPC: G01R33/46
Filed: 1959-10-14
Abstract: 939,257. Gyromagnetic resonance apparatus. VARIAN ASSOCIATES. Oct. 14, 1959 [Oct. 16, 1958], No. 34859/59. Class 37. [Also in Group XXXV] A gyromagnetic sample is tested by placing it in a polarizing magnetic field, applying a radio frequency magnetic field at right angles to the polarizing field, modulating one of the fields at a low frequency capable of producing a plurality of separate resonance spectra, extracting the modulation from one of the resonance spectra and mixing it with the modulating frequency so as to obtain a signal dependent on the amplitude of the modulation component of the resonance spectrum. Fig. 1 shows a sample 11 in the polarizing field of a permanent or electromagnet 12, the field being modulated at frequency # 1  by an oscillator 16. An r.f. magnetic field at frequency # is provided by a coil 13. It is shown that at the appropriate polarizing field strength, the gyromagnetic resonant frequency components induced in coil 14 are in accordance with Fig. 3. These signals are amplified at 18 and mixed with the output of the transmitter 15 so that the difference frequency # 1  may be separated in a band pass filter 21 and, after amplification, mixed with the output of oscillator 16 to provide a D.C. signal suitable for display of a cathode-ray tube 24. A frequency multiplier may alternatively be inserted between units 16 and 23 so that components such as 2# 1  may be used. Fig. 4 (not shown) relates to a similar arrangement using a single r.f. coil. In Fig. 6, the frequency # from the transmitter 15 is modulated by an audio frequency # 1  at 34. In the absence of a gyromagnetic sample, the amplitude modulation is removed by the amplifier/limiter 35, leaving only the radio frequency #. It is shown that when gyromagnetic resonance occurs at frequency #+#, quadrature components exist at frequency #+# 1 , being respectively dependent on the dispersion and absorption mode amplitudes. The frequencies are changed to # 1  by beating with # at 19. D.C. components proportional to either mode are available from unit 32 after mixing with frequency # 1 , in the required phase. Fig. 5 (not shown) relates to a similar arrangement using two r.f. coils, a phase modulated transmitter and a receiver sensitive to amplitude modulation. Reference is made to a number of combinations of the systems described. Automatic frequency or magnetic field control.- Fig. 8 shows a system in which a reference sample 46 (such as hexamethyl disiloxane) is used to stabilize frequency of the transmitting system, whilst an unknown sample 45 is being analyzed. The frequency # is amplitude modulated by frequencies # 1  # 2  respeetively in the balanced modulators 41, 42. The signals are amplified at 47 and compared with frequency # in the phase detector 48. The phase shifter 49 is first adjusted so that the reference signal is in quadrature with the amplified signals. Gyromagnetic resonance in the two samples is arranged to occur at frequencies #+# 1  and #+# 2  respectively, hence the phase detector 48 provides outputs at # 1  and # 2 . A D.C. signal dependent on the # 1  signal, provided by mixer 52 is used to control the transmitter frequency, whilst a further D.C. signal dependent on # 2  signal is displayed on the cathoderay tube 24. Phase shifters 53, 54 are provided to select the dispersion or absorption mode. In Fig. 9, the reference and test samples are contained in a measuring head having a single r.f. coil 64. A frequency # from the transmitter 61 is modulated at 62 by a frequency # 1  from audiofrequency oscillator 63 which in turn is modulated by the cathode-ray tube sweep generator 73 so that the side band is swept through the gyromagnetic resonant frequency of the test sample. The polarizing field is modulated by a frequency # 2  from audio oscillator 74 so that the side band #+# 2  is swept through the gyromagnetic resonant frequency of the reference sample. The output of the r.f. amplifier 65 is mixed with the transmitter frequency in a phase detector 66 to provide components at frequencies # 1  and # 2 . The former is mixed with a reference voltage at frequency # 1  to provide an indication of the absorption or dispersion mode depending on the phase of the reference voltage. The output at frequency # 2  is used to control the polarizing field strength.