Patent GB-992880-A (GB)
Title: Gyromagnetic resonance apparatus
IPC: G01R33/60
Filed: 1963-05-24
Abstract: 992, 880. Gyromagnetic resonance apparatus. VARIAN ASSOCIATES. May 24,1963 [May 31, 1962], No. 20900/63. Heading G1N. In an electron paramagnetic resonance apparatus in which two signals are derived from two samples, one of which serves as a reference, and in which a modulation frequency (100 kc/s) is imposed on the magnetic field through each sample, the amplitude of modulation of the field through the reference sample is made adjustable relative to that through the sample under test so that the difference between signals produced in a phase detector from the derived modulation frequency signals can be made equal to a reference value, the amplitude of the adjustable signal then being a measure of the amplitude of the resonance line. The gyromagnetic response to a field sweep through Ho is described with reference to Fig. 1, and the signal produced by the phase detector in response to the modulation, shown imposed on Fig. 1, is described with reference to Fig. 2. The phase of the modulation of the field through the reference sample is inserted so that the nett derived signed represents the said difference. Attenuated energy from a 9. 5 mc/s klystron is fed to a three-port circulator 11 to one port of which is coupled a test cavity resonator housing a reference sample 4 and having a quartz tube 1 through which the sample to be tested is caused to flow. The change in energy reflected, arising from changes in absorption as the magnetic field is swept, is detected by a crystal detector 12. Coils 6 and 7 are energized by the 100 kc/s source 13, and modulate the magnetic fields through the two samples, the amplitude of that through the coil 78 the reference sample being adjustable by the wipers on a potentiometer 16 in the output of a phase shifting stage 14. The difference between voltages E1 and E2 Fig. 3, in the phase detector output is maintained at a reference value in this case zero, by switching through relay K3 the field from values H A  to H B  in order to maintain coincidence of the bottom values of the peaks, E 1  being the amplitude between the two peaks at the two values. Comparison is made by the opening and closing of relay contacts K 2  and any difference in value is eliminated by the current through capacitor 23 and amplifier 22 causing a servomotor to drive the wipers of potentiometer 16 to a null position. The wiper position Vs indicates vanadium content, where the vanadium content of petrol is being measured and is calibrated by applying a direct current to terminals 24. A 60 cycle A.C. field modulation is arranged to be applied at spaced intervals to check H B  for deviation from the exact resonance value, any 60 cycle output from the phase detector whilst this test is being made being applied over contacts K1 to motor 28 to adjust the field of a bias coil 29. The adjustment circuit includes an alarm arrangement which operates if correction is not attained. Microwave energy is applied to the cavity through an iris 33 Fig. 5, the coupling of which may be adjusted, and the modulation coil 6 is carried on the outside of the quartz tube in a microwave shielding tubes 36. A helix 5 enclosing the reference sample 4 and having a coupling loop 38 is rotatable in a plastic container to adjust the degree of coupling, and the sample position within the helix is itself adjustable. The sample is packed with powdered potassium chloride to prevent movement. Adjustable iron rings 42 are provided to provide a local bias for the field through the sample to align the two resonance lines, a small D.C. may additionally be applied to the modulation coil.