NMR equipment with efficient current sources

Abstract

The NMR apparatus (10) comprises an NMR magnet system for generating an NMR magnetic field, the NMR magnet system including at least one NMR coil (11) for varying the NMR magnetic field, and a stable, low-noise current source including a linear current controller (12) electrically connected to a first end of the NMR coil (11) for supplying current to the NMR coil, characterized in that the current source includes a digital voltage source (13) of switching technology electrically connected to a second end of the NMR coil, whereby the NMR coil is connected as an electrical consumer between the linear current controller and the digital voltage source, the current controller being connected to the digital voltage source via a control line (14) for controlling the voltage, and the linear current controller controlling the digital voltage source so that the linear part of the configuration has a voltage within a preselected operating range, thereby minimizing the power consumed in the linear part of the configuration. Therefore, the NMR coil can provide an adjustable, stable, low-noise current with high electrical efficiency, and the homogeneity of the NMR magnetic field itself remains stable and constant even when external conditions change.

Description

[Technical Field]

[0001] The present invention relates to an NMR apparatus having a magnetic coil system for generating a uniform magnetic field. The magnetic coil system includes at least one NMR coil for varying the NMR magnetic field. The NMR apparatus includes a stable current source configured to supply current to the NMR coil, the stable current source including a linear current controller electrically connected to a first end of the NMR coil. The current source includes a digital voltage source electrically connected to a second end of the NMR coil, whereby the NMR coil is connected as an electrical consumer between the linear current controller and the digital voltage source. The linear current controller is connected to the digital voltage source via a control line for controlling the voltage. [Background technology]

[0002] An NMR device having these features is disclosed in Non-Patent Document 1.

[0003] NMR devices equipped with magnetic coil systems for generating a uniform magnetic field have been in use around the world since the 1960s, and an example is disclosed in German Patent No. 10104365 (Patent Document 1).

[0004] In such an NMR apparatus, a stable, low-noise current source configured to supply current to the NMR coil includes a linear current controller electrically connected to a first end of the NMR coil, as described in detail in Applicant's Technical Manual entitled "BSMS System for AVANCE NEO," Version 002, Chapter 10 "SCB29," Sections 10.1-10.4, pages 83-90 (= Non-Patent Document 2).

[0005] The present invention relates generally to the field of nuclear magnetic resonance ("NMR"), and in particular to techniques for further improving or varying the uniformity or strength of the NMR magnetic field, particularly in NMR magnetic systems that are cooled and typically superconducting during operation, via NMR coils.

[0006] NMR spectroscopy is a widely used and efficient method in instrumental analysis. It can be used to study the electronic environment of individual atoms and their interactions with neighboring atoms in test substances, such as hydrocarbon molecules or complex bioinorganic compounds. In this way, for example, the composition, structure, and dynamics of test substances can be elucidated, as well as the concentration of test substances can be calculated.

[0007] In an NMR measurement, a material is exposed to a strong, static, uniform magnetic field B0, which aligns the nuclear spins in the material. The material is then irradiated with a high-frequency electromagnetic pulse. The high-frequency electromagnetic field generated is detected by an NMR spectrometer, from which information about the properties of the material can be obtained.

[0008] High-resolution magnetic resonance spectroscopy and magnetic resonance imaging place very high demands on magnetic field homogeneity. To achieve the homogeneity specifications, electric cryoshims are often used. These coils generate the basic magnetic field profile. With an appropriate current supply, the coils improve the homogeneity of the NMR magnet at the sample position, thereby improving the resolution of the measurements.

[0009] A high-resolution NMR spectrometer comprising a superconducting NMR magnetic coil system cooled to extremely low temperatures by a pulse tube cooler and placed in the low temperature region of a cryostat within a vacuum vessel is described in EP 0 780 698 (=Patent Document 2).

[0010] Cryogenic ferromagnetic materials (e.g., iron or steel alloys) may also be used as shim elements to improve uniformity, as described, for example, in DE 102015225731 A1. Regardless of how a magnet is cryo-shimmed, residual inhomogeneities ultimately remain and the magnetic field must be corrected using an additional shim system that includes shim elements located outside the vacuum vessel, typically in the magnet bore. These shims may include shim coils or ferromagnetic materials, or a combination of both, that are energized by shim currents, similar to cryo-shims. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] German Patent No. 10104365 [Patent Document 2] European Patent No. 0780698 [Patent Document 3] German patent number 102015225731 [Non-patent literature]

[0012] [Non-Patent Document 1] NICK ARRANGO ET AL::Open-source,Iow-cost,flexible,current feedback-controlled driver circuit for local B0 shim coils and other applications.Veroffentlichungsdatum:7 February 2022.Quelle(URL:https: / / cds.ismrm.org / protected / 16MProceedings / PDFfiles / 1157.html) [Researched on July 27, 2023] [Non-patent document 2] Applicant's Technical Manual entitled "BSMS System for AVANCE NEO", Version 002, Chapter 10 "SCB20", Sections 10.1-10.4, Pages 83-90 Summary of the Invention [Problem to be solved by the invention]

[0013] Current shim current sources for NMR instruments are based on analog circuit technology, which is typically stable and low noise, but is highly inefficient and generates a large amount of waste heat due to the simultaneous presence of high voltages and currents in the current control circuits.

[0014] The patent document EP 0 999 563 B1, already cited at the outset, discloses an NMR device relevant to the present invention, which includes a shim system as an electrical consumer of the current source and has all the characteristic complexes defined at the outset, but this NMR device also suffers from the aforementioned efficiency problems, namely the generation of a relatively large amount of waste heat during NMR operation, due to the simultaneous presence of high voltages and high currents in the current control circuits.

[0015] This is also clear from the non-patent document 2 already cited above, which details a stable, low-noise current source that is at least so far the best that can be used in the NMR apparatus of non-patent document 1. Chapter 10 of non-patent document 2, starting on page 83, provides a more detailed explanation of the currently used current source "SCB20." As is particularly clear from the block diagram on page 86, in this case too, all power is provided by the linear part of the current source configuration.

[0016] This prior art, which is the closest to the present invention, provides the required current completely linearly from the supply voltage. When the resistance of the load, including the supply cable, is small, the power component of the unnecessary voltage is dissipated in the linear amplifier and converted into heat. This wastes current and places an excessive load on the power supply. On the other hand, this requires special heat dissipation devices, usually heat sinks, within the NMR device, which makes the current source excessively large. Furthermore, low-noise linear amplifiers that can operate in the required voltage range are typically found in the audio field. However, audio linear amplifiers are increasingly competing with class-D amplifiers optimized for audio applications, and it is expected that low-noise linear amplifiers used in audio applications will soon become unavailable commercially, or at least will not be as readily available as before. Class-D audio amplifiers are also not suited to the specific purpose of supplying current to NMR coils, which require continuous current. [Means for solving the problem]

[0017] In contrast, the present invention is based on the problem of modifying a current source for an NMR coil of an NMR device having the characteristics defined at the outset, particularly using readily available and easily accessible technical means, and as inexpensively as possible, so that the NMR coil is supplied with an adjustable, stable, low-noise current with the highest possible electrical efficiency, in particular for use as a current source for shim coils, but also for other uses in the field of NMR, so that the very high homogeneity of the NMR magnetic field remains extremely stable and constant even when external conditions change.

[0018] This relatively complex problem is solved in a surprisingly simple and effective way by the present invention: the current source is implemented with low noise, the digital voltage source is implemented with switching technology, and a linear current controller is implemented to control the digital voltage source so that the linear part of the controller architecture has a voltage within a preselected operating range, thereby consuming as little power as possible in the linear part of the controller architecture.

[0019] By using a current source modified according to the invention, the NMR coil of an NMR device can be fed with current with particularly high electrical efficiency.

[0020] The solution according to the invention comprises a highly efficient variable digital voltage source based on switching technology, an adjustable, stable, low-noise linear current controller based on mixed analog-digital technology, and an NMR coil as an electrical load supplied by the current source. The configuration according to the invention is characterized by a high inductive component of the load's electrical impedance compared to its resistive component. The NMR coil as an electrical load is intentionally arranged between the voltage source and the current controller in terms of circuit technology. Due to the high inductive component of the load due to the coil geometry, high-frequency noise, especially generated by the digital voltage source, can be kept away from the highly sensitive, low-noise linear current controller.

[0021] An important feature of the solution according to the invention is the efficient provision of a voltage to an electrical connection point of the NMR coil acting as a load. This voltage is controlled in such a way that the power loss, and therefore the waste heat, of the linear current controller connected to the other electrical connection point of the load is minimized. This, combined with the efficient voltage generation, optimizes the efficiency of the entire system consisting of the digital voltage source, the linear current controller, and the NMR coil as an electrical load, minimizing the power loss during NMR operation.

[0022] By placing an inductive load between the digital voltage source and the linear current controller, noise from the necessary switching processes in the voltage source is filtered and directed away from the current controller. This inherently increases the purity of the current set through the linear current controller, and therefore the purity of the NMR coil load. A high degree of purity in the magnetic field generated in the NMR coil is a key prerequisite for successful NMR experiments.

[0023] For a better understanding of the present invention, it should be noted that the modifications proposed by the present invention with respect to the closest prior art consist, inter alia, in improving the good commercial availability of the amplifiers, reducing the space required for the arrangement, and significantly improving the electrical efficiency.

[0024] Ideally, the current would be switched directly from the supply voltage. However, to achieve the low noise required for NMR applications, a linear controller is still required for stability. To minimize the power consumed by a linear controller, the voltage drop caused by this controller must be limited. Furthermore, amplifiers with a maximum voltage swing smaller than the supply voltage are more readily available on the market.

[0025] If a linear current controller is connected immediately after a switching voltage source, the linear controller must operate in flying mode.

[0026] When using multiple efficient current sources for an NMR spectrometer, as is required, for example, to drive multiple shim coils, the linear controllers of each current source must be implemented separately, isolated separately, and supplied with a separate power supply voltage. This often requires more than one switching power supply per current source, e.g., one for each supply terminal of the linear controller, and in some cases even more. On the one hand, this requires considerable costs, and on the other hand, the solution requires so much space that the size benefits of increased efficiency are lost.

[0027] The core idea according to the present invention is therefore to attach a linear controller to the ground side of the coil, and on the other side a digital voltage source provides a voltage potential higher or lower than the ground voltage, where ground is understood as earth or earth potential.

[0028] In particular, when multiple efficient current sources are used, the linear controllers for these can be powered from the same power supply. In the case of DACs, as well as ADCs and operational amplifiers, driving can be achieved using multi-channel, isolation-free components. Examples of such components are semiconductor components containing multiple ADCs, DACs, and / or operational amplifiers with corresponding connection contacts. In this way, the various functions of a linear current controller can be integrated into a common component, resulting in significant space and cost savings. Furthermore, the large inductance of the NMR coil serves as additional filtering before the current reaches the current-sensitive linear controller.

[0029] The voltage range over which a linear current controller operates may be limited, sometimes to a much smaller range than the range of voltages supplied by a digital voltage source, and is not allowed to exceed the voltage range over which the linear current controller operates, in contrast to the closest prior art where the current controller includes the entire range of supply voltages as its operating range.

[0030] In this case, the digital voltage source must be controlled so that this voltage range is respected at the desired current. This also applies during the transient phase when the current is adjusted to the target current through the NMR coil. This is achieved by slowly varying the current in a linear current controller and by digitally readjusting the voltage generated by the digital voltage source based on voltage measurements on the ground side of the NMR coil.

[0031] Once the target current is reached, the digital voltage source is adjusted within the digital grid. The digital voltage source is adjusted so that the voltage drop in the linear part of the current source is as small as possible, leaving a small margin defined for the control of the linear current controller. For a "positive" current, this means that the voltage is adjusted to the lower limit of the voltage range of the linear current controller, and for a "negative" current, to the upper limit. "Positive" here means that the current flows into the linear part of the current source.

[0032] To further clarify the present invention, some key terms are explained in more detail below.

[0033] Stable, low-noise current source: Actual measured noise is less than 1 μArms in the 1 Hz to 100 Hz frequency range at 1 A and a 10 ohm resistive load, and less than 20 μArms in the 1 Hz to 200 kHz frequency range. A requirement of the present invention is to keep noise as low as possible with an acceptable effort. To achieve this, the entire circuit must have sufficiently good noise suppression performance across the entire frequency range. This is explained in more detail in the pre-regulator description below.

[0034] Linear Current Controller: A current controller with a linear output stage is a linear controller. Linear current controllers can be implemented in analog and / or digital formats. In the present invention, we have a combination of both: a "fast" analog current controller controlled by a slower digital loop control circuit. For stability of the linear current controller, the following limits should be aimed for: gain drift <11 ppm / degree, and offset drift <+ / -1 μA / degree.

[0035] Digital voltage source using switching technology: A digital voltage source using switching technology is a DC voltage converter that efficiently converts a supplied DC voltage into another DC voltage by using switched elements and one or more energy stores. In the present case, a step-down converter is usually used, i.e., a reduction is achieved compared to the supply voltage.

[0036] Preferred embodiments and developments of the invention In a particularly preferred class of embodiments of the NMR apparatus according to the invention, the digital voltage source is pulseable. Its driving method is the same as that of a digital voltage source using switching technology. Pulsed voltage sources, also known as switching voltage sources, are very efficient. Therefore, less heat is generated for the same output power.

[0037] In a preferred development of this class of embodiments, the pulse frequency of the digital voltage source is in the range of 10 kHz to 1 GHz, preferably in the range of 30 kHz to 200 kHz. If the frequency is too low (<10 kHz), the filter elements placed between the digital voltage source and the NMR coil will be too large and slow. If the frequency is too high (>1 GHz), the switching losses will be too high, resulting in poor efficiency and excessive heat generation. However, the latter is constantly improving with technological advances. Higher frequencies are generally preferred because filters can be made smaller and / or improved.

[0038] In another advantageous development of this class of embodiments, the digital voltage source is driven by pulse width modulation, in which case the supply voltage of the digital voltage source is constant and the duty cycle (ratio of switch-on time to period) is increased or decreased, so that the voltage applied to the NMR coil is determined depending on the duty cycle. A defined output voltage is generated from an existing input voltage with a given duty cycle. The use of pulse width modulation allows for easy implementation and good predictability of the output voltage of the digital voltage source. Furthermore, the pulse frequency of the digital voltage source specifies a well-defined fundamental frequency that can be filtered by fixed components.

[0039] Another equally advantageous development is characterized in that the digital voltage source has a filter (usually an analog filter) including at least one inductive element, in particular one or more coils, and one or more capacitive elements, in particular capacitors, connected in parallel to ground. The filter is implemented to smooth the pulse voltage of the digital voltage source. Preferably, this filter is arranged directly between the digital voltage source and the NMR coil. The first component of the filter may, for example, include a PWM storage choke and a capacitor to remove the DC component of the rectangular voltage of the digital voltage source, and a snubber for damping the filter. The second component includes, for example, a filter choke and a capacitor for more effective filtering of the PWM frequency, where the PWM frequency is the pulse frequency of the pulsed voltage source, and another snubber for filter damping. The first component must include an inductive and / or capacitive element. The second component can be implemented in a configuration other than an LC circuit, for example, a linear configuration. However, this may result in higher losses. Alternatively, the first component may incorporate the LC circuit of the second component, thereby eliminating the second component. In this case, the size of the first component will be somewhat larger. The advantage of dividing into two parts is that by spatially separating the second component from the PWM component (e.g., supplying multiple currents simultaneously), the individual voltages can be better shielded from interference from other current sources.

[0040] Another advantageous embodiment of the NMR device according to the invention is characterized in that the digital voltage source is set to provide both negative and positive voltage values. The direction of the current flowing through the NMR coil is determined by the sign of the voltage value, which is advantageous because the magnetic field of the NMR coil can be influenced and changed in both directions.

[0041] Also particularly preferred is a class of embodiments of the invention characterized in that the linear current controller comprises a shunt resistor and an AD converter for measuring the current through the NMR coil bypassing the shunt resistor. Since the current is set digitally, it must also be possible to measure it digitally. A common type of current measurement is that using a shunt resistor. Alternative types of current measurement lack sufficient stability and accuracy.

[0042] An advantageous development of these embodiments is that the AD converter is connected to the digital control unit. By appropriately programming the digital control unit, the analog current controller can control the digital voltage source to maintain the analog side of the controller architecture at a voltage within a preselected operating range, thereby dissipating particularly little power in the analog side of the controller architecture. In the digital space, various control technologies can be optimized as programs, and post-processing adjustments to the controls are also easy. Digital voltage sources will need to be digitally controlled anyway, so digital regulators are advantageous.

[0043] In another particularly preferred embodiment of the NMR device according to the invention, the linear current controller comprises an electrical amplifier for supplying the current to the NMR coil. Electrical amplifiers are used, among other things, to achieve the required low noise values. Particularly preferred are developments of the two above-mentioned embodiments of the invention, characterized in that the digital control unit is connected to the electric amplifier via a DA converter and preferably to a digital voltage source via a control line.

[0044] The electrical amplifier functions analogically, while the digital control unit functions digitally. To drive the electrical amplifier, a digital-to-analog converter is used, which crosses the boundary between digital and analog, thereby achieving the required stability and resolution.

[0045] In another particularly preferred class of embodiments of the NMR device according to the invention, the digital voltage sources are driven according to a fixed time grid, which reduces mutual interference when multiple current sources are spatially adjacently arranged and actively switched in a time-synchronous manner. The fixed time grid operation avoids interference between multiple similarly efficient current sources. The fixed time grid results in discrete voltage levels. These discrete voltage levels of the digital voltage source can be compensated for by a linear current controller.

[0046] In a particularly advantageous class of developments of these embodiments, an integer fraction of the measurement interval of the AD converter is selected as the time grid, or the time grid is selected in such a way that the digital filter element suppresses the frequencies of the time grid as well as possible so that they do not have a disturbing effect on the current flowing through the NMR coil. Typical AD converters already have this digital filtering functionality built in, so you can tune the ADC's filter to match the grid and / or vice versa.

[0047] An advantageous variant of this development class is characterized in that at least one pre-regulator is present for the supply voltage of the digital voltage source. A pre-regulator ("power supply stabilizer"), usually integrated into a digital voltage supply, is used to further suppress noise in the mid-frequency range (10 Hz to 10 kHz) caused by the voltage supply's energy supply, while a PWM filter only filters frequencies above 10 kHz. Each current source must be low noise, generate as little interference as possible over a wide frequency range, and be very stable (= generate as little interference as possible even at very low frequencies). In the mid-frequency range, the linear controller is not effective enough, and the analog filter still does not work properly. Therefore, in order to keep the noise of the power supply of the digital voltage source away from the load in the mid-frequency range, the supply voltage of the digital voltage source is pre-adjusted.

[0048] Another advantageous embodiment of the invention is characterized in that the current source is implemented in such a way that the current through the NMR coil can be varied in the range from -20 A to +20 A, in particular in the range from -1 A to +1 A. Depending on the requirements of the NMR system, the current flowing through the NMR coil connected as an electrical load within a given range can be adjusted with high resolution and precision (e.g., up to 20 bits) and, in some cases, changed over time, thus correcting even very small changes in the NMR magnetic field of the NMR device caused by external events, together with the NMR coil.

[0049] Typical basic data for a stable, low noise current source are: - Switching frequency: >10kHz (when operating in pulse drive mode) -Current: <+ / -1A per current source - Supply voltage: + / - 24V Noise @ 1A, 10 ohm resistive load: 1Hz to 100Hz: <1μArms, 1Hz to 200kHz: <20μArms -Load: 0~20 ohms, 0~1mH -Resolution:>20bit Stability: Gain drift <11ppm / degree and offset drift <+ / -1µA / degree

[0050] In particular, embodiments of the invention that implement an NMR coil as a coil for shimming an NMR apparatus and / or as a coil for homogenizing the NMR magnetic field generated by an NMR magnet system or, in particular, for varying the NMR magnetic field as a flux pump have proven useful in practice. For example, operating a large number of shim coils may require dozens of such current sources, but the current and voltage requirements are unknown in advance: only in the final application will it become clear which sources are required to operate which shim coils at which currents. Flux pumps require relatively small voltages, but can therefore result in large maximum currents. Flux pumps in NMR instruments are used to compensate for the reduction in the main magnetic field of the NMR magnet system due to inductive coupling.

[0051] Other advantages of the present invention will become apparent from the following description and drawings. According to the present invention, the above-mentioned and further described features can be used individually or in any combination. The illustrated and described embodiments should not be understood as an exhaustive list, but rather have an exemplary character for explaining the present invention. The invention is illustrated in the drawings and will be explained in more detail on the basis of an embodiment. [Brief explanation of the drawings]

[0052] [Figure 1] 1 is a schematic block diagram of the essential parts of the invention of an NMR device modified according to the invention. [Figure 2] 1 is a schematic block diagram of a preferred embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0053] In general, the present invention is directed to a modified NMR apparatus 10 including an NMR magnet system (not specifically shown in the drawings) for generating a uniform magnetic field, with at least one NMR coil 11 for modifying the NMR magnetic field. Additionally, there is a stable, low-noise current source configured to supply current to the NMR coil 11, the current source including a linear current controller 12 electrically connected to a first end of the NMR coil 11.

[0054] As shown in FIG. 1, an NMR apparatus 10 according to the present invention differs from the prior art in that it includes a digital voltage source 13 whose current source is clocked by switching technology, and this clocked digital voltage source is electrically connected to the second end of an NMR coil 11. As a result, the NMR coil 11 is connected as an electrical consumer between a linear current controller 12 and the digital voltage source 13. The linear current controller 12 is connected to the digital voltage source 13 via a control line 14 for controlling the voltage. The linear current controller 12 according to the present invention is implemented to control the digital voltage source 13 so that the linear part of the controller configuration has a voltage within a preselected operating range (e.g., 0 to 3.3 V). This minimizes the power consumption of the linear part of the controller configuration.

[0055] Typically, the digital voltage source 13 can be pulsed. The pulse frequency is in the range of 10 kHz to 1 GHz, preferably in the range of 30 kHz to 200 kHz. In a concretely realized exemplary embodiment of the present invention, a frequency of 104.1666 kHz is used. In particular, the digital voltage source 13 can be operated with pulse width modulation. The level of the output voltage is determined by the pulse width modulation of the digital voltage source 13. The current through the NMR coil 11, in particular the finely tuned current, is determined by a linear current controller 12. The current controller 12 usually does not provide sufficient rejection of high frequency noise.

[0056] FIG. 2 shows a block diagram of a particularly preferred embodiment of an NMR apparatus 10 according to the present invention.

[0057] The digital voltage source 13 is configured to provide both positive and negative voltage values. This is evident from the designations V+ and V- for the two voltage inputs in the drawing. Typically, these voltages are in the range of +24V to -24V. This voltage range is larger than the preselected voltage range of the linear portion of the circuit.

[0058] As clearly shown in FIG. 2, in this embodiment the digital voltage source 13 comprises a filter 15 (hereinafter also referred to as "analog filter") having at least one inductive element, in particular one or more coils 15', and one or more capacitive elements, in particular capacitors 15", connected in parallel to ground. The filter 15 is implemented to smooth the voltage of the digital voltage source 13. In FIG. 2, the filter 15 is shown spatially separated from the digital voltage source 13, but it is also possible to implement the filter integrated into the digital voltage source. In either case, the filter is located between the voltage source and the NMR coil.

[0059] In the illustrated embodiment, the linear current controller 12 comprises a shunt resistor 16 and an AD converter 17 which measures the current bypassing the shunt resistor and flowing through the NMR coil 11. The AD converter 17 is connected to a digital control unit 18.

[0060] Furthermore, the linear current controller 12 comprises an electrical amplifier 19 for supplying current to the NMR coil 11 .

[0061] The digital control unit 18 is connected via a DA converter 20 to an electrical amplifier 19 and preferably via a control line 14 to a digital voltage source 13 .

[0062] The digital voltage source 13 is driven according to a fixed time grid, so that multiple current sources used simultaneously do not interfere with each other. The time grid can be selected as an integer fraction of the measurement interval of the analog-to-digital converter 17 (hereinafter also referred to as "ADC"). However, the time grid can also be selected so that the digital filter elements suppress the frequencies of the time grid as well as possible. The measurement interval of the AD converter corresponds to the ADC frequency in frequency space.

[0063] If arbitrary PWM frequencies or the voltages they generate are allowed, then all of these arbitrary frequencies, as well as mixed frequencies with other current sources, will be present in the voltage signal of the digital voltage source. These cannot be filtered out well enough with analog filters. Such filters are either space-consuming, too expensive, or consume too much power. Uncontrolled mixed frequencies can directly interfere with NMR measurements. Current stability is crucial, especially for shim applications.

[0064] A Nyquist filter must precede the ADC, eliminating frequencies above half the ADC frequency. However, due to the extremely precise measurements required for NMR operation, this filter would either introduce too much error or require permanent recalibration, which would be too costly and complex, even if possible. For this reason, Nyquist filters are not typically used in NMR. This results in mixed frequencies in the ADC, which introduce measurement errors. Typical sigma-delta ADCs incorporate a digital filter that can very well suppress the selected frequency (and its multiples).

[0065] The largest measurement errors occur where the interference is greatest: at the PWM frequency (and multiples thereof) of the digital voltage source, which cannot be completely filtered by the analog filter. The PWM frequency and the ADC filter are coordinated to prevent PWM interference from causing measurement errors in the ADC.

[0066] In an arrangement according to the invention, at least one pre-regulator should be present for the supply voltage of the digital voltage source 13. In the embodiment according to FIG. 2, two pre-regulators 21′; 21″ are shown. These are shown here spatially integrated in the digital voltage source 13, but in other embodiments can also be arranged separately from the digital voltage source 13, for example as filter 15 in FIG. 2.

[0067] The current source of the NMR device 10 modified according to the invention is typically implemented so that the current through the NMR coil 11 can be varied in the range of -20A to +20A, in particular in the range of -1A to +1A.

[0068] In a particularly important application of the present invention, the NMR coil 11 is implemented as a coil for shimming the NMR device 10 and / or as a coil for homogenizing the NMR magnetic field generated by the NMR magnet system. However, the NMR coil 11 can also be used, for example, to vary the NMR magnetic field. In this case, the device acts as a magnetic flux pump.

[0069] The level of the current is determined by the linear current controller. This is done as long as the voltage source generates a voltage that is within the tolerance of the operational amplifier, taking into account the current and the load. Otherwise, the current will not be defined, controlled, stable, and low-noise, and the current controller may be destroyed without a protection circuit. During operation, the current controller can withstand lower voltages than the digital voltage source can generate.

[0070] An electrical amplifier is typically made up of one or more interconnected operational amplifiers. Most operational amplifiers can either source or sink current. That is, current can flow into the apex of the triangle representing the operational amplifier in a circuit diagram; in this case, it is called "positive" current.

[0071] At first glance, earth appears to have no bearing on the circuit. Positive current flows from the +V supply, is converted to a lower voltage, and flows through the NMR coil to the negative supply of the operational amplifier. Negative current flows from the operational amplifier's positive supply, through the NMR coil, into the digital voltage supply, and out the -V supply. There is no earth here. However, even if the potential / voltage on each side of the NMR coil is different, as long as the voltage difference is the same magnitude, the same current can be achieved. This makes sense.

[0072] But boundary conditions still exist, and here the earth again plays a role. The voltage generated by the pulsed digital voltage source needs to be filtered / stabilized to some potential, preferably earth. In order to save one power supply and to be able to drive the operational amplifier referenced to ground, the linear final stage, i.e. the power amplifier, is not powered symmetrically, i.e. the negative supply of the power amplifier is here taken to be earth. The op amp symbol in the block diagram does not represent just a single power amplifier. Usually additional op amps that do not draw full current are required. These op amps may again require + / - power supplies, but these supplies do not need to provide as much current as the power amplifier's power supply. [Explanation of symbols]

[0073] 10 NMR equipment 11 NMR coil 12 Linear Current Controller 13 Digital voltage source for switching technology 14 Control Line 15 filters 15' inductive element 15" Capacitive Element 16 Shunt resistor 17 AD converter 18 Digital Control Unit 19 Electrical Amplifier 20 DA converter 21';21" Pre-regulator

Claims

1. an NMR magnet system for generating an NMR magnetic field, comprising at least one NMR coil (11) for modifying the NMR magnetic field; a stable current source configured to supply current to the NMR coil (11) and including a linear current controller (12) electrically connected to a first end of the NMR coil (11); An NMR apparatus (10) comprising: The current source is a digital voltage source (13) electrically connected to a second end of the NMR coil (11), whereby the NMR coil (11) is connected as an electrical consumer between the linear current controller (12) and the digital voltage source (13), and the linear current controller (12) is connected to the digital voltage source (13) via a control line (14) for controlling a voltage, The current source is implemented in a low noise manner and the digital voltage source (13) is implemented in switching technology; The linear current controller (12) The linear part of the configuration is implemented to control the digital voltage source (13) to a voltage within a preselected operating range, thereby minimizing the power consumed in the linear part of the configuration; An NMR apparatus characterized by:

2. The digital voltage source (13) is pulse-driveable. NMR apparatus according to claim 1 .

3. The pulse frequency of the digital voltage source (13) is in the range of 10 kHz to 1 GHz, preferably 30 kHz to 200 kHz.

3. The NMR apparatus according to claim 2.

4. The digital voltage source (13) can operate with pulse width modulation 4. The NMR apparatus according to claim 2 or 3.

5. The digital voltage source (13) at least one inductive element, in particular one or more coils (15'); one or more capacitive elements, in particular capacitors (15"), connected in parallel to earth; A filter (15) is provided. The filter (15) It is implemented to smooth the voltage of the digital voltage source (13) 4. The NMR apparatus according to claim 2 or 3.

6. The digital voltage source (13) Configured to provide both negative and positive voltage values NMR apparatus according to any one of claims 1 to 5, characterized in that

7. The linear current controller (12) a shunt resistor (16); an AD converter (17) for measuring the current flowing through the NMR coil (11) bypassing the shunt resistor; Equipped with NMR apparatus according to any one of claims 1 to 6, characterized in that

8. The AD converter (17) connected to a digital control unit (18) 8. The NMR apparatus according to claim 7.

9. The linear current controller (12) an electric amplifier (19) for feeding current into said NMR coil (11); 9. An NMR apparatus according to claim 1, wherein the NMR apparatus comprises:

10. The digital control unit (18) connected to the electric amplifier (19) via a DA converter (20); Preferably, it is connected to the digital voltage source (13) via the control line (14). NMR device according to claims 8 and 9, characterized in that

11. The digital voltage source (13) is driven according to a fixed time grid, so that multiple current sources used simultaneously do not interfere with each other. NMR device according to any one of claims 1 to 10, characterized in that

12. A time grid is selected as an integer fraction of the measurement interval of the AD converter (17), or said time grid being selected such that the digital filter element suppresses frequencies of said time grid as well as possible; NMR device according to any one of claims 11, 7 and 8, characterized in that

13. At least one pre-regulator (21'; 21") is provided for the supply voltage of said digital voltage source (13). NMR apparatus according to claim 12 , characterized in that

14. The current source is implemented so that the current flowing through the NMR coil (11) can be varied in the range of -20 A to +20 A, in particular in the range of -1 A to +1 A. NMR device according to any one of claims 1 to 13, characterized in that

15. The NMR coil (11) as a coil for shimming the NMR device (10); and / or Implemented as a coil for homogenizing the NMR magnetic field generated by the NMR magnet system or for varying the NMR magnetic field, in particular as a flux pump. NMR device according to any one of claims 1 to 14, characterized in that

Images