DD273129A1 - METHOD FOR CONTROLLING THE AUTODYN OSCILLATOR IN A CORE MAGNETOMETER - Google Patents

Abstract

The invention relates to the absolute value measurement of the magnetic induction of large homogeneous magnetic fields by means of nuclear magnetic resonance effect. The aim of the invention is to achieve a very good reproducibility and simultaneously high search and follow-up speeds in nuclear magnetometers. According to the invention, in a method in which the frequency of an autodyne oscillator is swept, the sweep is stopped upon first reaching a nuclear magnetic resonance signal, continued upon reaching the active edge of a clock pulse in the previous direction, and commuted after a predetermined period of time. When the resonance level is reached again, the sweep is again stopped and continued when the active edge of another clock pulse in the negative direction is reached. After a predetermined period of time, it is commutated again and the procedure described begins again. Fig. 5

Description

suitable frequency response in the LF amplifier can not be completely suppressed. It is therefore necessary to render these pseudo signals completely ineffective. This is done by a per se known electronic blanking during einos defined period of time, which begins after each detected resonance passage and ends with the expiration of a beginning in the commutation period

 Reproducibility of induction value measurement with fast magnetometers is significantly increased.

AusfOhmngsbelsplel The invention will be explained in more detail with reference to diagrams. Show it: Fig. 1: the course of the control voltage of the Aütodyn oscillator over time Fig. 2: the flow of Fig. 1 associated output voltage of the signal amplifier connected to the detector Fig. 3: derived from the measuring time quartz-stable clock 4 shows the course of the induction rate of change over the sweep time Fig. 5: the course of the control voltage over time when the induction of a magnetic field with subcritical

Speed changes

 Fig. Β: an arrangement for carrying out the method according to the invention as a block diagram.

The voltage waveforms shown in Figures 1-3 and the associated clock represent the most common Fc ¹ I, namely the use of protonon signals. In Fig. 1, the course of the control voltage of the autodyne oscillator is shown over time, wherein the actual frequency response can be derived by light filtering with low-pass character. The period of the clock frequency Ttakt is 4.7 ms. The measuring time Ttor is 50 · Ttakt = 235ms. The course of the control voltage indicates a hysteresis effect. This is because nuclear resonances are found on two different voltage and frequency, respectively, according to the sign of the sweep. This hysteresis has two causes. On the one hand, the signal of the fast passage is shifted in time and on the other hand before the capacitance diodes which vary the frequency, a time constant term is inserted which rounds the sudden changes in the control voltage curve slightly so that the resulting interference signals do not become too large. The value Bhyst resulting in the example is 0 4mT at a transit speed b = 0.3T / s. The necessary suppression tent Tdisable has been determined with 1.5ms. Calculating the limits for the possible induction rate of change a, which can follow a control according to the invention, one obtains two limiting equations, the course of which is shown in FIG. 4 above the sweep time Tsweep. The first condition follows from the consideration of Tdisable

b (Tsweep - Tdisable) + Bhyst ...

Tsweep + Tdisabla

The second condition follows from the fact that for a maximum induction rate of change a in every other period, the time for the zero sweep disappears, i. that immediately after reaching the nuclear magnetic resonance level is weiteresesept.

? Τ · Η · »... Μ. · Ρ Γ <ί.ν - .. Ι.

Both equations span an area of possible sweep times Tsweep for achievable

Induction change rates a with bhyst and tdisable as parameters. The realization of a certain value a below the maximum value is basically possible by a value interval for the sweep time Tsweep. However, the smallest value has the advantage of producing a smaller modulation stroke, which has an advantageous effect on the accuracy and reproducibility of the frequency measurement. In the example presented, the sweep time Tsweep = 0.76ms has been chosen so that a reaches the maximum value of 75 m TVs.

In Fig. 5, the curve of the control voltage over time is shown when a <a max, d. H. when the induction of a magnetic field changes with subcritical velocity. It can be seen that the shape of the modulating voltage changes depending on the timing of the induction for the presented type of control of the autodyne generator. In the block diagram of Fig. Β, the essential components are shown, which are required for the presentation of the principle. An integrator 1 supplies a control voltage U control for tuning the autodyne oscillator 2. In this case, it is decided with the aid of an analog multiplexer 3 whether it integrates a magnitude equal positive or negative input voltage Uint. This makes it possible to generate zero sweep in addition to positive and negative sweep. Uint is a voltage derived from the concatenation of an ADU 4, ROM 5 and DAU 6, which is assigned to each value of U-control. The ROM contains a value inverse of the control characteristic of the autodyne oscillator 2. The current voltage Uint is thus constantly readjusted in such a way that, in spite of the non-linear characteristic curve, the same throughput speed b always results. The drive of the analogue multiplexer 3 is effected by the memory cells 7, 8 for negative and positive sweeps. When the sweep is active, a nuclear magnetic resonance signal may occur, the result of which at the output of a chain comprising a detector 9, LF amplifier 10, signal trigger 11 and pulse shaper 12 is passed through a first port 13 which resets the sweep memory cells 7 , 8 and thus causes zero sweep. At the same time, it triggers a first monostable multivibrator 14, which opens a second gate 15 during its tilting time Tout lock. The next active edge of the clock generator signal can thus pass through the second port 1S and activates, via a third port 16, one of the memory cells 7, 8, depending on which position a memory line 17 had assumed for the sweeping direction. At the same time, the first monostable multivibrator 14

during his tipping time Tsweep blocks the first gate 13 for further impulses. With the termination of Taweep three events are triggered: a two-monostable multivibrator 18 tilts and locks / Or Tdisa'ole above the first «gate 13 further pulses. The memory cell 17 for the sweeping direction is set negated and a wide monostable multivibrator is tilted. After expiration oinur short period of time., Which is required for the safe setting of the memory cell ' Ί for the' .Sweeprichtung, passes a signal Ober the third gate 16 to one of the Sweep-Spicherzellen 7,8, so deft the sweep continues in the opposite direction becomes. If no renewed nuclear magnetic resonance passage is registered within the tipping time Tout iock given by a third monostable multivibrator 19, the second gate 15 is closed again and the clock is locked, as a result of which the instantaneous state of the sweep is maintained. From the clock signal with a suitable divider 20, the measuring time Ttor derived for a connected counter 21, the nuclear magnetic resonance frequency and thus determines the desired value dor magnetic induction.

Claims (1)

A method of controlling the autodyne oscillator in a nuclear magnetometer in which the frequency of an autodyne oscillator is swept, characterized in that the sweep is stopped upon first reaching a nuclear magnetic resonance signal to continue in the previous direction upon reaching the active edge of a clock pulse is that it is commutated after a predetermined period of time, that when the resonance level is reached, the sweep is stopped again and continued on reaching the active edge of another clock pulse in the negative direction, that after a predetermined period of time, the sweep is commutated again and that then the described procedure starts from the beginning, wherein positive and negative sweep are equal in magnitude and this amount, measured in the unit of measurement of induction per unit time, is kept constant in all points of the subregions associated control characteristics and wherein the meter width of the connected frequency counter is in an even ratio to the period of the clock frequency. For this 3 pages drawings Field of application of the invention The invention relates to the absolute value measurement of the magnetic induction of large, homogeneous magnetic fields by means of nuclear magnetic resonance effect. Nuclear magnetometers are used in HF spectroscopy, mass spectroscopy, magnetic scales, materials science and high-energy physics. Characteristic of the known state of the art The majority of known nuclear magnetometers have arrangements for sinusoidal field or frequency modulation in conjunction with phase locked loops. This achieves a reproducibility of 0.3 μT or 1 ppm. These arrangements have the disadvantage that, because of the need for safe latching to the nuclear magnetic resonance frequency, the loop filter must assume values which prevent fast seek and follow-up speeds. In addition, the phase matching conditions of traditional phase detectors, which are difficult to maintain over the entire measuring range, are a source of offset phenomena. From Polish patent 88280 a method is known which is characterized by a special mode of operation of the autodyne frequency control associated with each individual resonance pass. As a result, the resonance point is periodically traversed by a triangular frequency modulator of very low modulation frequency (2Hz). The frequency measurement is divided into two parts, with ieder time directly adjoining a Resonazdurchgang. It comes dibel for the compensation of Zählanteilen below and above the sought resonance frequency. The disadvantage of this arrangement is that the reproducibility of the frequency measurement in this method remains overrun by 2uT, since due to the signal / noise ratios of the resonance, due to the few passes within the listening time, only with greater uncertainty is detected. Object of the invention The aim of the invention is to achieve a very good reproducibility and simultaneously high search and follow-up speeds in nuclear magnetometers. Explanation of the essence of the invention The invention has for its object to control the frequency of a free-running autotivn oscillator so that measurements of the induction of a Mangetfeldes, which can be attributed to the measurement of the nuclear magnetic resonance frequency of a known core, carried out with high reproducibility. According to the invention, in a method of sweeping the frequency of an autodyne oscillator, this is accomplished by stopping the sweep the first time a nuclear magnetic resonance signal is reached, continuing it in the previous direction upon reaching the active edge of a clock pulse, and sweeping it is commuted after a predetermined period of time. When the resonance level is reached again, the sweep is again stopped and continued when the active edge of another clock pulse in the negative direction is reached. After expiry of a predetermined period of time, it is commutated again. Subsequently, the described procedure starts from the beginning. Positive and negative sweeps are equal in magnitude and this amount, measured in units of induction per unit time, is kept constant at all points in the control characteristics associated with the subregions. Furthermore, the Zähltorbreite the connected frequency counter to the period of the clock frequency is in an even ratio. The repeated sudden change in the sweep within oiner clock period deceives at the connected to the autodyne oscillator sensitive to voltage changes, useful signals, which also by a