Patent GB-901827-A (GB)
Title: Improvements relating to nuclear magnetic resonance detectors
Inventor: WATSON EDWARD
IPC: G01R33/46
Filed: 1957-10-21
Abstract: 901,827. Nuclear resonance-testing circuits. FAIREY CO. Ltd. Oct. 14,1958 [Oct. 21, 1957], No. 32865/57. Class 37. A nuclear magnetic resonance detector comprises a tuned test coil to receive the specimen and located in a magnetic field having a field strength such as to produce a desired resonance frequency in the specimen, means for producing a radio frequency signal in the test coil having a frequency differing from the desired resonance frequency by an amount approximately in the audio range, means for periodically altering the resonance frequency or the frequency of the radio frequency signal and rapidly restoring it so as to pass rapidly through a resonance condition, and means responsive to the resulting beat signal component of the potential across the tuned circuit. In Fig. 1, the test specimen is located in a test coil 10 between the poles of an electromagnetic 12, the coil being connected in the frequency determining circuit of an oscillator 22. The normal current through the coils of the magnetic 12 is such that the nuclear resonance frequency differs from the oscillator frequency by an audio frequency and the current in the coils is periodically switched between this value and a second value so that the nuclear resonance frequency is periodically swept through the oscillator frequency and returned to its initial value. An audio frequency beat is produced in the oscillator valve and this is passed through an audio frequency amplifier 40 to a detector 42 comprising a clamping circuit 34 which is rendered effective during one extreme of the magnetic field so that beats produced at this extreme are eliminated. The amplifier is off-tuned so that the amplitude of the output signal indicates the resonance frequency. Alternatively, highly selective audio frequency circuits may be used to accurately determine the frequency. In an alternative circuit (Fig. 2), the current through the coil of the magnet 12 has a normal value which causes the nuclear resonance frequency of the specimen 11 to differ from that of a piston controlled RF generator 63 by an audio frequency, and the magnet field is periodically rapidly changed to a new value restored to the previous value. The test coil 10 is connected to a detector amplifier circuit 65 which provides positive feed-back as described in Specification 901,829 to increase the Q of the test circuit. The output of the detector is applied through an audio frequency amplifier and a detector 42, responsive only to a desired part of the magnetization cycle. The magnetizing current is derived from a thyratron pulse generator 50 which periodically fires to produce a voltage pulse across resistor 51. The frequency of operation of the pulse generator is varied by means of a time base 58 and at one point in the time base cycle the frequency is such that successive bursts of audio frequency produced in the detector circuit are in phase so that the output from the circuit consists of periodical bursts of audio frequency energy. Specifications 826,847 and 863,272 also are referred to.