Patent GB-1067621-A (GB)
Title: Improvements in or relating to gyromagnetic resonance apparatus
Inventor: BUTCHER PETER MONTAGUE
IPC: G01R33/60
Filed: 1964-09-07
Abstract: 1,067,621. Electrophysical measurements. DECCA Ltd. Sept. 7, 1965 [Sept. 7, 1964], No. 36592/64. Heading G1N. [Also in Division H3] In gyromagnetic resonance apparatus, in which a radio-frequency signal is fed from a radiofrequency source to a sample cavity, the tuning of the radio-frequency source, to adjust its frequency to compensate for variations in frequency of the cavity, is controlled by the output of phase sensitive detecting means which determines the phase relationship between a low frequency signal derived from and phase related to the radio-frequency output from the sample cavity, and a reference signal of the same low frequency. In one embodiment, for use with electron spin resonance apparatus, the sample cavity 10, Fig. 1, is located between two coils 11, 12 provision (not shown) being made for applying current to the coils to sweep a magnetic field through a range of values and also to modulate the field. A single waveguide 23 forms an input to and output from  the cavity. The input signal is taken from a klystron 13, via a cross coupler 14, isolator 16, phase shifter 17 and adjustable attenuator 18 to the cavity. The output signal, having been fed through an Isolator  25, is mixed at a directional coupler 21 with part of the input signal, fed from a directional coupler 19, via a switch 20, to the coupler 21. The  resultant signal is then fed to a straight receiver 22.  The frequency of the klystron is controlled in relation to a stable frequency of a crystal oscillator 30. The output of the oscillator 30, after being fed through a multiplier 31 and harmonic generator 15, is mixed with the output signal of the klystron at the coupler 14. The resultant beat frequency is then amplified at 32 and compared in phase with the output of an oscillator 34. Thus the frequency of the klystron is controlled by keeping this beat frequency equal to the frequency of the oscillator 34. The final adjustment to the klystron frequency is then made by adjustments to the oscillator frequency. A low frequency oscillator 39 feeds a modulation signal to the phase sensitive detector 33 to modulate the control voltage applied to the klystron and thereby to cause a small frequency modulation of the klystron output. If the tuning of the cavity differs then an amplitude modulation of its output signal results which is detected by a microwave detector at the input of the receiver 22 and amplified by the amplifier 41. It is finally detected at a phase sensitive detector 42 using part of the output of the oscillator 39 as a reference signal. The output of the detector 42 is thus a signal having an amplitude dependent on the phase relation between the low frequency modulation on the klystron and the signal from the amplifier 41. This output signal is smoothed in circuit 44 and applied as a control voltage to control the frequency of the oscillator 34, and thereby to control the frequency of the klystron. In the second embodiment, Fig. 2 (not shown) which uses a superheterodyne type of receiver the output from the cavity 10 is mixed, at a directional coupler (50) with a signal from a local oscillator (51). The resulting output is fed through amplifiers (53, 54) to two phase sensitive detectors (55, 56), operating in quadrature and using a reference signal from an amplifier (57). The reference signal is obtained by combining part of the output from the klystron with part of the output of the oscillator (51) at the cross-coupler (58). Thus the output signal from the detector (56) has a voltage directly related to the difference between the frequency of the cavity and the frequency of the input signal from the klystron, the output being zero when the two are equal. This signal is applied as a control voltage to an oscillator (60), whose output is fed to the phase sensitive detector (33) to provide a voltage control similar to that provided by the oscillator 34 in Fig. 1. In this embodiment the frequency is controlled in relation to a crystal oscillator 30 as in Fig. 1.