SU890184A1 - Method and device for measuring magnetic field - Google Patents

Description

(54) METHOD OF MEASURING A MAGNETIC FIELD

AND DEVICE FOR ITS IMPLEMENTATION

one

The invention relates to a measuring technique, namely, methods and devices for measuring constant magnetic fields by nuclear magnetic resonance (NMR).

A known method for measuring constant magnetic flukes by NMR, in which the measured magnetic field in the sample volume is modulated in amplitude by a periodically varying magnetic field with a modulation depth greater than the width of the NMR signal. The measurement of the field is reduced to measuring the frequency of oscillations of the radio frequency magnetic field when the phase shift between the NMR signal and the modulating voltage is zero, which is defined as the equality of the time intervals between the NMR pulses tl

The disadvantage of this method is the presence of modulation of the magnetic field, which degrades the measurement accuracy due to the effect of the inhomogeneity of the modulating field on the NMR waveform. In addition, the application of this method for measuring the magnetic field in small gaps is difficult due to the size of the sensor (probe) with the modulating coil.

The closest to the present invention is a method of measuring a magnetic field, based on the method of nuclear magnetic resonance with frequency modulation of the frequency of an oscillating radio frequency mazznitality field. According to this method, the frequency is measured together with

10 of the nuclear magnetic resonance pulse, the frequency being stabilized from the moment the pulse ends, and the sign of the stabilization factor is chosen to be the opposite sign of the change in the G2D frequency.

15

However, the accuracy of the measurement with this method ultimately reduces to measuring the resonant frequency value (FS) at the point of extreme value

20 NMR and depends largely on the frequency stabilization factor of the radio frequency magnetic field at the time of its measurement. Therefore, the measured frequency 38 CEM will differ from FO by the value, where K is the stabilization factor; slope of frequency change until stabilization; t t is the difference between the measurement period due to a change in the stabilization factor. Required high stability often imposes complexity on the implementation of the method. Even with high stability, K is finite, therefore at 0 IEM. The aim of the invention is to increase the measurement accuracy and simplify the measurement of constant magnetic fields by NMR. This goal is achieved by the fact that, according to the method of measuring a magnetic field based on nuclear magnetic resonance with frequency modulation of the frequency of the radio frequency magnetic field, when the phase of the NMR signal is equal to zero, relative to the voltage modulated phase, the frequency modulation is not switched on time measurement of the frequency of the radio frequency field The device for implementing the method of measuring a magnetic field contains a series-connected NMR sensor, autodyne and an indicator, a frequency meter connected to a resonator The autodyne circuit and the frequency modulator, as well as the phase detector, the control key, the power line, the control trigger and the AND circuit, the input of the phase detector connected to the output of the NMR signal indicator, and its output which entrance; is the start of the device, the output of the AND circuit is connected to the first input of the control trigger, the second input of which is connected to the output of the Frequency Counter, the output of the control trigger is connected to the first control input, the left key, the output of which is connected to the autodyn, and the second input is A frequency module, an input of the delay line are connected to the second input of the control unit, and its output is connected to the Start start of the frequency counter. FIG. Figure 1 shows graphs for the proposed method for measuring a magnetic field; in fig. 2 shows an apparatus for carrying out the method. In order to implement the method, the frequency of oscillations of an rf magnetic p 4 LN is modulated sinusoidally with a modulation depth greater than 1 NMR sigle by the value of t L F When an NMR signal appears, adjust the autodyne frequency until the condition of equal to zero phase shift of the NMR signal disable the modulation for the duration of the frequency measurement of the radio frequency field. Thus, the measured value of Pm is equal to (, and since at the time of the AFMc measurement, (t) 0, the MS.M.RF frequency of the radio frequency magnetic field is measured by counting the number of periods over a given time interval. This interval can be chosen in the pause between pulses NMR, which allows measuring the frequency without disturbing the operation of the stabilization system and controlling the magnetic field by means of the NMR signal (Fig. 1). Thus, the proposed method eliminates the effect of frequency instability on the measurement accuracy of the resonant frequency value and eliminates the need for In a stabilizing device, a distinctive feature of this method is that, when frequency-modulated by a radio frequency magnetic field, when the phase of the NMR signal is zero and the modulation voltage is turned off, the frequency modulation is switched off for the measurement time. the device implementing the proposed method comprises a series-connected NMR sensor, autodyne 2, an NMR-3 signal indicator, a phase detector 4, a frequency meter 5 whose input is connected to an autodyne resonant circuit, a frequency modulator 6, The output of which is connected to the autodin, AND 7, one of the inputs of which is connected to the output of the phase detector 4, the second input to the Start input, the control trigger 8, one of the installation inputs of which is connected to the output of the AND circuit 7, the second installation input is connected to the output of the frequency meter 5, the delay line 9, whose input is connected to the output of the control trigger 8, the output is connected to the start input of the frequency meter 5, the control key 10, the input of which is connected to the output of the frequency modulator b, the output is connected to the autodin 2 and the control input is connected output control trigger 8.

586

The device works as follows.

The initial search for the NMR signal is carried out by changing the frequency of autodone 2. After finding the NMR signal, autodyne 2 adjusts smoothly until the condition of zero phase shift of the NMR signal with respect to the phase of the modulating voltage is received, and the signal for passing the Run command generated external device. The start signal from the output of the AND 7 circuit goes to the control trigger 8 and switches its potential from the direct output of the control trigger 8 to the control input of the key 10, which disables the autodyne frequency modulation 2, as well as to the delay line 9, after which the signal enters the frequency counter 5 and starts it. After completion of the frequency measurement, the stop frequency meter 5 signal returns the control trigger 8 to the initial state. In this case, the frequency modulator 6 is turned on and the frequency meter 5 is blocked. To re-enable the circuit, it is necessary, if there is an enable signal from the phase detector 4, to send a Start signal.

The proposed method and device for its implementation make it possible to increase the measurement accuracy of the resonant frequency value due to the reduction of the effect of frequency instability, and also make it possible, compared to the known method, to more easily measure the magnetic field.

Claims (2)

1. A method of measuring a magnetic field based on the method of nuclear magnetic 846 Frequency modulated resonance of an rf magnetic field, characterized in that, in order to improve measurement accuracy and simplify the measurement, when the phase of the NMR signal is zero when the modulation voltage phase is zero, the frequency modulation for the frequency of the radio frequency field is turned off . 2. An apparatus for carrying out the method of p-I comprising a series-connected NMR sensor, an autodyne and an NMR signal indicator, a frequency meter connected to an NMR sensor, and a frequency modulator, characterized in that, in order to improve measurement accuracy and simplify measurement, it additionally introduces a phase detector, a control key, a delay line, a control trigger and an AND circuit, with the input of the phase detector connected to the output of the NMR signal indicator, and its output with the first input of the AND circuit, the second input of which is intended The start signal, the output of the AND circuit with the first input of the control trigger, the second input of which is connected to the output of the frequency counter, the output of the control trigger is connected to the first input of the control key, the output of which is connected to the autodyne, and the second input to the frequency modulator, input the delay line is connected to the output of the control trigger, and its output is connected to the start input of the frequency meter. Sources of information taken into account in the examination 1.Leshe A. Nuclear induction. M., Foreign Literature Publishing House, 1963, p. 215-217., 2. USSR author's certificate number 473O91, cl. UOIH 27/78, 1974 (prototype). " /