SU1545176A1 - Device for measuring permanent magnetic field - Google Patents

Abstract

The invention relates to a technique for measuring permanent magnetic fields by nuclear magnetic resonance (NMR) using frequency modulation of a radio frequency magnetic field containing a nuclear sample. The goal is to simplify the device and improve the measurement accuracy in units of the magnetic field. The device includes blocks that form a harmonic signal for modulating the frequency of the autodyne with a period multiple of the inverse value of the gyromagnetic ratio of the sample nuclei. The summation of autodyne frequency periods during NMR is performed by a frequency meter, the measurement time of which is a multiple of the modulation signal period. NMR signals from autodyne are fed to the indicator device through a high-pass filter with a cut-off frequency in the interval between the first and third harmonics of the modulation frequency, which significantly increases the signal-to-noise ratio during NMR recording and the resulting accuracy of magnetic field measurements.

Description

The invention relates to a measurement technique, in particular to a technique for measuring constant magnetic fields by nuclear magnetic resonance (NMR).

The purpose of the invention is to simplify the device and improve the measurement accuracy in units of the magnetic field.

Fig „1 shows the block diagram of the device; Fig. 2 illustrates a form of autodyne frequency modulation; fig.Z - NMR signals; figure 4 - counting pulses in the frequency counter.

The device contains a 1 NMR sensor, autodyne 2, a frequency meter 3, an NMR signal indicator 4, a crystal oscillator 5, the first frequency divider 6, the second

frequency divider 7, symmetrical integrator 8, low-frequency filter 9 (n.p.), high-frequency filter 10 (v.ch.), controlled amplifier 11.

The device works as follows, i

The NMG sensor 1 consists of a coil with a nuclear sample inside it and is placed in a measured constant magnetic field Brt3 (vv. A radio frequency (r.h.) magnetic field is excited in the coil, since the coil is connected to an autodin 2, part of its rc contour. The autodyne frequency F is modulated by an external harmonic signal with a period Tm () D (Fig.2) and a modulation depth of ± dF (greater or meHI me

JV JV

about

ten

15

signal width NMR) to discriminate NMR. In order to determine In Vnazchasto ™, the F of autodyne 2 is adjusted in such a way that by giving a NMR signal from the autodyne in v, h filter 10 on the indicator 4 of the NMR signal, with its help fix the condition of the NMR (FIG. 3),

In the device, using a series-connected blocks (crystal oscillator 5, first frequency divider 6, symmetrical integrator 8, 9 nch filter and controlled amplifier 11), harmonic modulation F of autoditch with period T mod is multiplied by the inverse gyromagnetic ratio for example, pteothon. Perhaps, for example, the numerical value of Tmod 23.48736 ms (fMtvf42ruJ,

To measure F (average value), it is necessary to sum the number of periods within the period Tmo.

With a linear frequency modulation (CflF / F Ј0.01), the sum of the number of periods is equal to the average frequency for the period Tmod. It does not matter when to start counting periods, the counting time is exactly T, the data point and the sum of the periods remains constant.

Numerically the sum of the periods for T d., "

MOV zi corresponds to the value of the measured

the magnetic field is VIZLAV in units of Tesla, if it is preliminarily agreed with the VIZM the position of the bit comma in the sum.

Exact correspondence of T., and T opre-35

LLO | TS AND ZM

made by a rectangular signal from the output of the first divider

25

frequency having a period t

Tod

through

the second frequency divider at the input of the measurement time of the frequency meter 3. By setting different conversion factors for the second divider (for example, 2 or 20)

it is possible to increase T multiple

Maud

to increase the accuracy of frequency measurement (magnetic field).

Cutoff frequency of filters n.h. and hf is selected in the frequency range of the first and the third harmonics of the modulation frequency for the following reasons. The input of the filter is a signal in the form of a symmetrical saw consisting of 1,3,5 ... harmonic components, of which only the first harmonic must be passed by the filter n.h. The filter should suppress in the measuring channel basically the first harmonic, the koterach with frequency modulation in the autodyne

0

five

R

„

and

five

five

0

five

0

five

due to a corresponding change in the quality factor of the circuit and the occurrence of amplitude modulation in the output signal in connection with this. Due to possible nonlinearities in some elements of the circuit, the parasitic second harmonic may appear in the output signal, although it is small in amplitude,

Claims (1)

The proposed device can operate at amplitudes of frequency modulation larger and smaller than the width of the NMR signal (integral and differential approach), the device requires additional equipment to control the modulation system in measurement k, which leads to a decrease in error — measurements. The device may have significant variations in the values of Tmodi Ti $ m relative to each other. Introduction of filters n.ch. Hz, h, in the measurement and modulation channels of the device provides a significant increase in the signal-to-noise ratio and, accordingly, reduces the error in measuring permanent magnesium — i th lo in the range (0.05-2.5) 1 at least to levels below 0.001%. Invention Formula A device for measuring a constant magnetic field based on nuclear magnetic resonance (NMR), comprising a series-connected NMR sensor with a sample and tunable frequency autodynes, with an input for modulating frequency and with NMR and autodyne frequency outputs, an NMR signal indicator, a frequency meter, and also a series-connected crystal oscillator and a frequency divider, the frequency output of the autodyn is connected to the first frequency input, characterized in that, in order to simplify and improve the measurement accuracy in units magnetic field, a second frequency divider, a minimal integrator low-pass filter and a high-pass filter controlled by an amplifier are additionally introduced, the output of the first divider is connected to the second frequency counter through a second frequency divider, and a low-pass filter through series-connected symmetric integrator and a controlled amplifier with an auto-modulation frequency input. 5 1545176 " the output of the symmetric integrator is connected to the output of the NMR signals to the second input of the autodyne indicator and the first input of the NMR signal indicator, and the high frequency filter of the NMR signals is “ Fiyo.1 Fu & 4