Patent Publication Number: US-2023135564-A1

Title: NMR Measurement Apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-176250 filed Oct. 28, 2021, the disclosure of which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE DISCLOSURE 
     Field of the Disclosure 
     The present disclosure relates to an NMR measurement apparatus, and in particular to a technique for generating a plurality of transmission signals. 
     Description of Related Art 
     An NMR measurement apparatus is an apparatus which measures an NMR (Nuclear Magnetic Resonance) phenomenon which occurs in a sample. The NMR apparatus includes a spectrometer and a probe. The probe corresponds to a detector. The spectrometer includes a transmission section and a reception section. 
     Document 1 (JP 2016-24118 A) discloses an NMR measurement apparatus. A transmission section in the NMR measurement apparatus has a frequency converter. In a mixer in the frequency converter, a transmission signal is multiplied by a local signal from a local oscillator. The transmission signal after the frequency conversion, which is output from the frequency converter, is sent to the probe. Document 2 (JP 2002-31673 A) also discloses an NMR measurement apparatus. The NMR measurement apparatus has a demultiplexing circuit (multi-channel selection and routing unit) which generates a plurality of transmission signals from a single transmission signal. 
     Documents 1 and 2 do not disclose a combination of the frequency conversion technique and the demultiplexing technique. In particular, Documents 1 and 2 do not disclose a technique for generating a plurality of transmission signals while using a single local signal for frequency conversion. 
     In the NMR measurement, in some cases, simultaneous irradiation (or sequential irradiation) of a plurality of electromagnetic waves onto a sample becomes necessary. For example, when simultaneous performance of excitation of an observation nucleus and excitation of a peripheral nucleus for decoupling between the observation nucleus and the peripheral nucleus (operation for weakening an interaction) is desired, the sample is simultaneously irradiated with a plurality of electromagnetic waves having a plurality of different irradiation frequencies respectively suited to the objectives. If a plurality of transmission sections (for example, a plurality of power amplifiers and a plurality of frequency converters) are to be provided in parallel with each other for such irradiation of a plurality of electromagnetic waves, that is, for generation of a plurality of transmission signals having a plurality of different frequencies, the apparatus structure becomes complex, and the manufacturing cost is subsequently increased. In addition, if a plurality of reception sections (for example, a plurality of preamplifiers and a plurality of frequency converters) are to be provided in parallel with each other in correspondence to the plurality of transmission sections, the complication of the apparatus structure and the increase in the manufacturing cost are worsened. 
     An advantage of the present disclosure lies in enabling, in an NMR measurement apparatus, generation of a plurality of transmission signals having a plurality of different frequencies with a simple structure. Alternatively, an advantage of the present disclosure lies in setting, in an NMR measurement apparatus, of a suitable frequency conversion condition when a plurality of transmission signals are generated via frequency conversion. 
     SUMMARY OF THE DISCLOSURE 
     According to one aspect of the present disclosure, there is provided an NMR measurement apparatus comprising: a mixer that multiplies a transmission signal having a plurality of different frequency components by a local signal, to thereby generate a frequency-converted transmission signal; a power amplifier that amplifies the frequency-converted transmission signal, and outputs an amplified transmission signal; a demultiplexing circuit that has a plurality of filters having a plurality of different frequency characteristics, that allows the amplified transmission signal to pass through the plurality of filters so as to generate, in parallel with each other, a plurality of transmission signals having a plurality of different irradiation frequencies, and that outputs the plurality of transmission signals to an NMR detection circuit; and a controller configured to set a condition for generating the plurality of different irradiation frequencies as a condition for frequency conversion by the mixer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiment(s) of the present disclosure will be described based on the following figures, wherein: 
         FIG.  1    is a block diagram showing an NMR measurement apparatus according to an embodiment of the present disclosure; 
         FIG.  2    is a diagram showing a plurality of frequency sets; 
         FIG.  3    is a diagram showing an example calculation of a frequency set in a first combination; 
         FIG.  4    is a diagram showing an example calculation of a frequency set in a second combination; 
         FIG.  5    is a diagram showing an example calculation of a frequency set in a third combination; 
         FIG.  6    is a diagram showing an example calculation of a frequency set in a fourth combination; 
         FIG.  7    is a diagram showing a method of determining a frequency set when a difference method is employed; 
         FIG.  8    is a diagram showing a method of determining a frequency set when a summation method is employed; 
         FIG.  9    is a diagram showing an alternative configuration of a transmission unit; 
         FIG.  10    is a diagram showing an alternative configuration of a frequency converter; 
         FIG.  11    is a diagram showing some specific examples of a demultiplexing circuit (combining circuit); and 
         FIG.  12    is a diagram showing an alternative configuration of a demultiplexing circuit (combining circuit). 
     
    
    
     DESCRIPTION OF NON-LIMITING EMBODIMENTS OF THE DISCLOSURE 
     Overview of Embodiment 
     An NMR measurement apparatus according to an embodiment of the present disclosure comprises a mixer, a power amplifier, a demultiplexing circuit, and a controller. In the mixer, a transmission signal having a plurality of different frequency components is multiplied by a local signal. In this manner, a frequency-converted transmission signal is generated. In the power amplifier, the frequency-converted transmission signal is amplified, and an amplified transmission signal is output from the power amplifier. The demultiplexing circuit has a plurality of filters having a plurality of different frequency characteristics. In the demultiplexing circuit, the amplified transmission signal is allowed to pass through the plurality of filters, to generate, in parallel with each other, a plurality of transmission signals having a plurality of different irradiation frequencies. The plurality of transmission signals are output to an NMR detection circuit. The controller sets a condition for generating the plurality of different irradiation frequencies as a condition for frequency conversion by the mixer. 
     In the above-described configuration, the condition for the frequency conversion is set such that a plurality of transmission signals having a plurality of different irradiation frequencies are appropriately generated. With the condition set, the mixer frequency-converts the plurality of frequency components, and then, the power amplifier amplifies the plurality of frequency components after the frequency conversion. Then, the demultiplexing circuit extracts the plurality of frequency components after the amplification as a plurality of transmission signals, in parallel with each other. Thus, according to the above-described configuration, the plurality of transmission signals necessary for the NMR measurement can be generated by operating a simple structure under an appropriate condition. In particular, because a common constituent element (for example, a single mixer, or a single power amplifier) may be employed for the plurality of frequency components for generating the plurality of transmission signals, the device structure can be simplified. 
     The condition for the frequency conversion performed using the mixer may include a frequency of a local signal, a plurality of frequencies of the plurality of frequency components included in the transmission signal, a frequency conversion method, and the like. Alternatively, a condition which is determined in advance may be set in the transmission section by the controller, or a condition calculated by the controller may be set in the transmission section by the controller. When the plurality of transmission signals are sent to the NMR detection circuit, a sample is irradiated with a plurality of electromagnetic waves simultaneously or sequentially. Basically, the plurality of different irradiation frequencies of the plurality of transmission signals are determined based on an observation nucleus (an atomic nucleus to be observed), measurement objective, measurement conditions, and the like. The NMR detection circuit described above is a circuit having an electromagnetic wave irradiating function, and in this sense is an irradiation circuit and a detection circuit for NMR detection. 
     In an embodiment of the present disclosure, the plurality of transmission signals include a first transmission signal having a first irradiation frequency and a second transmission signal having a second irradiation frequency. The condition for frequency conversion includes a frequency of the local signal. The frequency of the local signal is a frequency calculated based on at least one of the first irradiation frequency or the second irradiation frequency. 
     The local signal is a common signal which is multiplied to the plurality of frequency components, and the frequency of the local signal directly defines the plurality of frequencies generated by the frequency conversion. Thus, the frequency of the local signal is one of important parameters in the frequency conversion. Such a parameter is calculated based on one or both of the first irradiation frequency and the second irradiation frequency. 
     In an embodiment of the present disclosure, one of the first irradiation frequency and the second irradiation frequency is a primary reference frequency. As a frequency conversion method, a difference method or a summation method to be described below is employed. The frequency of the local signal is a frequency calculated based on the primary reference frequency and the frequency conversion method. 
     As is well known, a difference frequency component (difference component) and a sum frequency component (sum component) appear at an output side of the mixer as results of multiplication of two frequencies which are input to the mixer. The difference method is a method in which the difference component is extracted and the sum component is removed, and the summation method is a method in which the sum component is extracted and the difference component is removed. The difference method and the summation method may be selectively employed, or one of the difference method and the summation method may be fixedly employed. In either case, in the calculation of the frequency of the local signal, the frequency conversion method is taken into consideration. 
     In the determination of the frequency of the local signal, if one of the first irradiation frequency and the second irradiation frequency is set as a basis, that is, if the primary reference frequency is set as the basis, the frequency of the local signal can be uniquely determined. 
     In an embodiment of the present disclosure, the plurality of different frequency components include a first frequency component having a first frequency and a second frequency component having a second frequency. The other of the first irradiation frequency and the second irradiation frequency (that is, the frequency which is not the primary reference frequency) is a sub reference frequency. The condition for the frequency conversion further includes the first frequency and the second frequency. At least one of the first frequency or the second frequency is a frequency calculated based on the sub reference frequency, the frequency of the local signal, and the frequency conversion method. 
     In an embodiment of the present disclosure, a frequency, among the first frequency and the second frequency, which corresponds to the primary reference frequency, is a designated frequency within a range of frequencies which can be generated. A frequency, among the first frequency and the second frequency, which corresponds to the sub reference frequency, is a calculated frequency within the range of frequencies which can be generated. The frequency of the local signal is a frequency calculated based on the primary reference frequency, the designated frequency, and the frequency conversion method. The calculated frequency is a frequency calculated based on the sub reference frequency, the frequency of the local signal, and the frequency conversion method. 
     When one of the first frequency and the second frequency is designated as the designated frequency, the frequency of the local signal can be calculated based on the designation. Then, the other of the first frequency and the second frequency is calculated as the calculated frequency. With such a stepwise frequency determination, the process for determining the condition for the frequency conversion can be simplified. In an embodiment of the present disclosure, the above-described frequency range coincides with a first range of frequencies which can be generated by a first frequency generator which generates the first frequency, and a second range of frequencies which can be generated by a second frequency generator which generates the second frequency. The designated frequency is a frequency which is determined first, and the calculated frequency is a frequency which is determined later. 
     When the frequency conversion method is the difference method and the designated frequency is a frequency exceeding 1/2 of the range of frequencies which can be generated, a lower frequency among the first irradiation frequency and the second irradiation frequency is desirably set as the primary reference frequency (lower frequency criterion). On the other hand, when the frequency conversion method is the difference method and the designated frequency is a frequency below 1/2 of the range of frequencies which can be generated, a higher frequency among the first irradiation frequency and the second irradiation frequency is desirably set as the primary reference frequency (higher frequency criterion). By employing the lower frequency criterion or the higher frequency criterion according to a combination of the difference method and highness/lowness of the designated frequency, it is possible to enlarge an acceptable frequency difference (difference between the first irradiation frequency and the second irradiation frequency). In other words, by determining a high designated frequency or a low designated frequency in the range of frequencies which can be generated, according to the combination of the difference method and a selected criterion (lower frequency criterion or higher frequency criterion), it is possible to enlarge the acceptable frequency difference. 
     When the frequency conversion method is the summation method and the designated frequency is a frequency below 1/2 of the range of frequencies which can be generated, a lower frequency among the first irradiation frequency and the second irradiation frequency is desirably set as the primary reference frequency (lower frequency criterion). On the other hand, when the frequency conversion method is the summation method and the designated frequency is a frequency exceeding 1/2 of the range of frequencies which can be generated, a higher frequency among the first irradiation frequency and the second irradiation frequency is desirably set as the primary reference frequency (higher frequency criterion). An acceptable frequency difference can be enlarged by employing the lower frequency criterion or the higher frequency criterion according to a combination of the summation method and highness/lowness of the designated frequency. In other words, by determining a high designated frequency or a low designated frequency in the range of frequencies which can be generated, according to the combination of the summation method and the selected criterion (lower frequency criterion or higher frequency criterion), it is possible to enlarge the acceptable frequency difference. 
     When the first irradiation frequency and the second irradiation frequency belong to a low frequency band, generally, the difference method is employed as the frequency conversion method. When the first irradiation frequency and the second irradiation frequency belong to a high frequency band, generally, the summation method is employed as the frequency conversion method. By keeping the frequency of the local signal low, advantages may be obtained such as that a degree of difficulty on the device design can be reduced, or that degradation of signal purity can be avoided. The low frequency band and the high frequency band are relative concepts, and the high frequency band is a frequency band higher than the low frequency band. 
     In an embodiment of the present disclosure, the difference method or the summation method is selected as the frequency conversion method, and the designated frequency is designated in such a manner that the calculated frequency is not a negative frequency and the calculated frequency does not exceed an upper limit of the frequency. The controller may select the difference method or the summation method. In this case, a frequency band to which the first frequency and the second frequency belong may be taken into consideration. The above-described frequency upper limit is an upper limit of the range of frequencies which can be generated. 
     In an embodiment of the present disclosure, the plurality of filters included in the demultiplexing circuit includes a first filter having a first frequency characteristic for generating the first transmission signal, and a second filter having a second frequency characteristic for generating the second transmission signal. The first frequency characteristic and the second frequency characteristic differ from each other. In general, the frequency characteristic may also be called a transmission band characteristic or a frequency band characteristic. 
     The NMR measurement apparatus according to an embodiment of the present disclosure further comprises a probe having the NMR detection circuit, a filter bank provided between the mixer and the power amplifier, and a bypassing path provided between the mixer and the power amplifier, and that bypasses the filter bank. When the demultiplexing circuit is to function between the power amplifier and the probe, the bypassing path functions. When the demultiplexing circuit is not to function between the power amplifier and the probe, the filter bank functions. 
     Each filter in the demultiplexing circuit may function as a difference component extractor or a sum component extractor. That is, each filter in the demultiplexing circuit may realize a function identical to each filter forming the filter bank. Thus, the difference component or the sum component can be appropriately extracted even when the transmission signal which is output from the mixer is sent to the demultiplexing circuit without passing through the filter bank. With this configuration, reduction of purity of the transmission signal can be prevented or reduced. Alternatively, a plurality of demultiplexing circuits may be prepared corresponding to a plurality of groups of irradiation frequencies, and a demultiplexing circuit suited to a selected group of irradiation frequencies may be selectively used. On the other hand, when suppression of unnecessary components to be removed is insufficient, or it is desired to suppress unnecessary components included in the signal which is input to the power amplifier, desirably, the filter bank and the demultiplexing circuit are caused to function simultaneously. 
     In an embodiment of the present disclosure, during reception, the demultiplexing circuit functions as a combining circuit. A combined reception signal is extracted from between the power amplifier and the demultiplexing circuit. For example, the combined reception signal is multiplied by a signal identical to the local signal used for the frequency conversion of the transmission signal, so that the combined reception signal is frequency-converted. Then, a signal processing for the frequency-converted combined reception signal is performed. When this configuration is employed, because it is not necessary to provide a plurality of preamplifiers and a plurality of reception frequency converters, the structure of the reception system can be simplified. 
     Details of Embodiment 
       FIG.  1    shows an NMR measurement apparatus according to an embodiment of the present disclosure. The NMR measurement apparatus is one type of a sample analysis apparatus. Specifically, the NMR measurement apparatus is an apparatus which causes an NMR phenomenon to occur in a sample, and analyzes the sample by observing and analyzing an NMR signal caused by the NMR phenomenon. The NMR measurement apparatus according to the embodiment of the present disclosure has a function to simultaneously irradiate a plurality of electromagnetic waves having a plurality of different frequencies onto a sample. 
     In  FIG.  1   , the NMR measurement apparatus comprises a spectrometer  10 , a static magnetic field generator  12 , a probe  14 , and the like, and further comprises a coupler  16  and a demultiplexing circuit  18 . The demultiplexing circuit  18  is formed from, for example, a diplexer (which is also called a duplexer). The static magnetic field generator  12  has a superconductive magnet for generating a static magnetic field, a correction coil for correcting the static magnetic field, and the like. The probe  14  is formed from an insertion portion  14 A and a base portion  14 B. The insertion portion  14 B is inserted into a bore of the correction coil, or the like. An NMR detection circuit  20  is provided in the insertion portion  14 A. 
     The NMR detection circuit  20  is an electronic circuit for detecting the NMR signal, and has a function to irradiate an electromagnetic wave, in order to detect the NMR signal. The NMR detection circuit  20  has, for example, a transmission and reception coil and a transmission coil. The transmission and reception coil is, for example, a coil for irradiating an observation nucleus in the sample with an electromagnetic wave, and detecting the NMR signal generated in the observation nucleus. The transmission coil is, for example, a coil for irradiating an electromagnetic wave for decoupling between the observation nucleus and a peripheral nucleus. A plurality of transmission signals may be supplied to the plurality of coils, or a plurality of transmission signals may be supplied to one coil. The NMR detection circuit  20  also has an adjustment circuit (for example, a tuning circuit and a matching circuit). The NMR detection circuit  20  may be operated at room temperature or at a very low temperature. 
     The base portion  14 B is a portion placed at an outer side of the bore. A plurality of ports A, B, and C are provided on the base portion  14 B. In the illustrated example configuration, a first transmission signal is supplied to the port A, and a second transmission signal is supplied to the port B. The port A and the port B are connected to the NMR detection circuit  20  via two internal signal lines. Further, in the illustrated example configuration, the port C is also provided on the base portion  14 B. Another transmission signal is supplied to the port C. In  FIG.  1   , illustration of a transmission section which generates the other transmission signal is omitted. In the embodiment of the present disclosure, the ports A and B are ports for a low frequency band, and the port C is a port for a high frequency band. Such an assignment of frequency bands is merely exemplary. 
     In  FIG.  1   , a frequency of a first electromagnetic wave (first irradiation frequency) is represented by RF 1 , and a frequency of a second electromagnetic wave (second irradiation frequency) is represented by RF 2 . For example, a combination of the irradiation frequencies, that is, RF 1  and RF 2 , is determined according to the observation nucleus, the peripheral nucleus, the static magnetic field intensity, the measurement objective, the measurement conditions, and the like. A frequency of the first transmission signal is identical to the first irradiation frequency and is RF 1 , and a frequency of the second transmission signal is identical to the second irradiation frequency and is RF 2 . In the following, the frequency of the first transmission signal will be simply referred to as RF 1  and the frequency of the second transmission signal will be simply referred to as RF 2 . 
     The spectrometer  10  comprises a controller  25 , a transmission section  22 , a reception section  24 , an oscillator  48 , and the like. The controller  25  is formed from, for example, a computer having a processor. The controller functions as a controlling unit or a controlling means. The controller  25  controls operations of various elements in the NMR measurement apparatus. The controller  25  of the embodiment of the present disclosure has a function to set a frequency conversion condition (more specifically, a set of frequencies (a frequency set) to be described below) for the transmission section  22 . The controller  25  of the embodiment of the present disclosure also has a filter selection function, a routing control function, and the like. The controller  25  has a memory, or a memory is provided external to the controller  25 . The memory stores conditions necessary for transmission and reception. The conditions include the frequency set. The controller  25  controls transmission and reception based on the information stored in the memory. 
     The transmission section  22  has a signal generation unit  26 , a combiner  36 , a frequency converter  28 , and a power amplifier  30 . In the illustrated example configuration, the signal generation unit  26  has a first signal generator  32  and a second signal generator  34 . A frequency of a first original transmission signal (first frequency) generated by the first signal generator  32  is represented by F 1 . A frequency of a second original transmission signal (second frequency) generated by the second transmission generator  34  is represented by F 2 . In the following, the first frequency will simply be referred to as F 1  and the second frequency will simply be referred to as F 2 . From the viewpoint of frequency conversion to be described below, each of F 1  and F 2  may also be called an intermediate frequency (IF). F 1  and F 2  are set by the controller  25 . 
     When a range of frequencies which can be generated by the first signal generator  32  is expressed as a first frequency range, and a range of frequencies which can be generated by the second signal generator  34  is expressed as a second frequency range, in the embodiment of the present disclosure, the first frequency range and the second frequency range are identical to each other. In the following, the first frequency range and the second frequency range will simply be referred to as a frequency range. As will be described later in detail, in the embodiment of the present disclosure, one of F 1  and F 2  is a designated frequency designated within the frequency range, and the other of F 1  and F 2  is a calculated frequency which is calculated within the frequency range. In the process of determining an apparatus operation condition, the designated frequency is a frequency which is determined first, and the calculated frequency is a frequency which is determined later. 
     The combiner  36  combines the first original transmission signal and the second original transmission signal, to generate a combined original transmission signal. The combined original transmission signal is a single transmission signal including a first frequency component having F 1  and a second frequency component having F 2 . 
     The frequency converter  28  has a mixer  38 , a filter bank (FB)  40 , a switch  42 , a switch  43 , and the like. With the switches  42  and  43 , one of a mixing path  44  and a bypassing path  46  is selected. The mixer  38  is a multiplier for frequency conversion. That is, the combined original transmission signal and a local signal are multiplied in the mixer  38 . With this process, a difference component and a sum component are generated at an output side of the mixer  38 . 
     In the embodiment of the present disclosure, a difference method is employed as the frequency conversion method. In the difference method, the difference component is used among the difference component and the sum component generated as a result of the multiplication. Specifically, a filter in the filter bank  40  is selected such that the difference component is extracted and the sum component is removed. A transmission signal after frequency conversion, which is output from the mixer, is sent to the selected filter. Each filter in the filter bank  40  is formed from, for example, an LPF (Low Pass Filter) or a BPF (Band Pass Filter). 
     Alternatively, a summation method may be employed as the frequency conversion method. In this case, a filter in the filter bank  40  is selected such that the sum component is extracted and the difference component is removed. In this case, each filter in the filter bank  40  is formed from, for example, an HPF (High Pass Filter) or the BPF. Alternatively, the difference method or the summation method may be selected by the controller  25  according to RF 1  and RF 2 . 
     The transmission signal after the frequency conversion is sent to the power amplifier  30 . The power amplifier  30  is an electronic circuit which amplifies the transmission signal. In the embodiment of the present disclosure, a single frequency converter  28  and a single power amplifier  30  are provided in the transmission section  22 , and the structure of the transmission section  22  is thus simplified. Various power amplification devices may be used as the power amplifier  30 . Alternatively, the power amplifier  30  may be formed from a plurality of amplifier circuits. 
     The frequency converter  28  in the embodiment of the present disclosure includes the bypassing path  46 . When the frequency conversion is not necessary, the bypassing path  46  is selected by the switches  42  and  43 . In this case, the transmission signal is sent to the power amplifier  30  via the bypassing path  46 . 
     The oscillator  48  generates a local signal having a frequency F 0 . That is, the oscillator  48  is a local oscillator. The local signal is a signal for the frequency conversion. The frequency F 0  of the local signal is set by the controller  25 . The local signal is supplied to both the transmission section  22  and the reception section  24 . Alternatively, a phase adjuster may be provided on a supply path of the local signal as necessary. In the following, the frequency of the local signal will simply be referred to as F 0 . 
     The coupler  16  is also called a head amplifier chassis. In the coupler  16 , a T/R switch (transmission/reception path selection switch)  50  and a preamplifier  52  are provided. The preamplifier  52  is a circuit which amplifies a weak NMR signal which is output from the NMR detection circuit  20 . The T/R switch  50  is a circuit for supplying the transmission signal which is output from the power amplifier  30  to the demultiplexing circuit  18 , and for sending the reception signal which is output from the demultiplexing circuit  18  to the preamplifier  52 . That is, the T/R switch  50  is an electronic circuit for switching the transmission/reception path. The T/R switch  50  may be formed from a directional coupler or the like. The demultiplexing circuit  18  functions as a combining circuit (multiplexing circuit) during reception, as will be described below. 
     The demultiplexing circuit  18  has a first filter  54  and a second filter  56  which are provided in parallel with each other. The first filter  54  has a first frequency characteristic, and the second filter  56  has a second frequency characteristic. The first frequency characteristic and the second frequency characteristic differ from each other. The frequency characteristic may also be called a frequency band characteristic. A frequency component having RF 1  (that is, the first transmission signal) is extracted by the first filter  54 , and other frequency components are suppressed. A frequency component having RF 2  (that is, the second transmission signal) is extracted by the second filter  56 , and the other frequency components are suppressed. RF 1  and RF 2  differ from each other. In this manner, the demultiplexing circuit  18  generates the first transmission signal and the second transmission signal in parallel with each other, and outputs the first transmission signal and the second transmission signal in parallel with each other. The demultiplexing circuit  18  is specifically a diplexer. 
     During transmission, the first transmission signal which is output from the demultiplexing circuit  18  is sent to the NMR detection circuit  20  via the port A, and the second transmission signal which is output from the demultiplexing circuit  18  is sent to the NMR detection circuit  20  via the port B. The first electromagnetic wave and the second electromagnetic waves are irradiated onto the sample by the NMR detection circuit  20 . 
     During reception, the demultiplexing circuit  18  functions as a combining circuit. More specifically, a first reception signal appears at an output side (a side of the T/R switch) of the demultiplexing circuit  18  via the port A and the first filter  54 . In addition, a second reception signal appears at the output side of the demultiplexing circuit  18  via the port B and the second filter  56 . At the output side of the demultiplexing circuit  18 , a combined reception signal is generated, which is formed from the first reception signal and the second reception signal. The combined reception signal is sent to the reception section  24  via the T/R switch  50  and the preamplifier  52 . Normally, one reception signal among the first reception signal and the second reception signal forming the combined reception signal is set as a processing target. Alternatively, both of the first and second reception signals may be set as the processing target. 
     In the embodiment of the present disclosure, the demultiplexing circuit  18  can be inserted or removed. Each of the first filter  54  and the second filter  56  may be formed from a plurality of filter elements. As will be described below, three or more filters may be provided in the demultiplexing circuit  18 . Alternatively, the demultiplexing circuit  18  may be provided in the coupler  16 , or in the probe  14 . In a state in which the demultiplexing circuit  18  is removed, a typical NMR measurement apparatus is formed in which a single frequency signal is transmitted and received at each transmission and reception channel. In a state where the demultiplexing circuit  18  is inserted, a special NMR measurement apparatus is formed in which a plurality of transmission/reception signals are transmitted and received at a single transmission and reception channel. The use/non-use of the demultiplexing circuit  18 , and a type of the NMR probe, are selected according to a nucleus species of the nucleus to be measured, or the like. 
     The reception section  24  has a frequency converter  58  and a reception signal processing unit  70 . The frequency converter  58  has a mixer  60 , a bandpass filter  62 , a switch  64 , a switch  65 , and the like. In the mixer  60 , the combined reception signal is multiplied by a signal which is identical to the local signal supplied to the mixer  38 . Of a difference component and a sum component generated by the multiplication, the difference component is extracted by the bandpass filter  62 . The bandpass filter  62  has a role of passing an observation frequency component after the frequency conversion and removing other frequency components. Alternatively, as the bandpass filter  62 , a filter in which the pass band can be varied may be provided. 
     The switches  64  and  65  are switches for selecting one of a mixing path  66  and a bypassing path  68 . When the frequency conversion is to be performed, the mixing path  66  is selected, and, when the frequency conversion is not to be performed, the bypassing path  68  is selected. The reception signal processing unit  70  has an A/D converter, a quadrature detection circuit, and the like. Alternatively, frequency analysis of the reception signal may be performed in the reception signal processing unit  70 , to generate the NMR spectrum. 
     With the embodiment shown in  FIG.  1   , it is not necessary to provide a plurality of frequency converters and a plurality of power amplifiers for the generation of the first transmission signal and the second transmission signal. That is, the first transmission signal and the second transmission signal can be generated using a single frequency converter and a single power amplifier. Therefore, the structure of the transmission section can be simplified. In addition, it is not necessary to provide a plurality of frequency converters and a plurality of preamplifiers for detection of the first reception signal and the second reception signal. That is, the first reception signal and the second reception signal can be detected using a single frequency converter and a single preamplifier. Thus, the structure of the reception section can be simplified. 
     Because the demultiplexing circuit  18  is a relatively inexpensive component, in the embodiment of the present disclosure, by the simplification of the structures of the transmission section and the reception section, significant cost reduction can be achieved. In the processing of the reception signal, a state in which the phase is synchronized between the transmission signal and the reception signal must be maintained. When a plurality of reception sections are provided in parallel with each other, desirably, the performances of the plurality of reception sections are matched to the maximum possible degree. However, in reality, it is not easy to completely satisfy this need. When the plurality of reception sections are provided in parallel with each other, a problem of reduction in processing precision due to differences in performance tends to easily occur. On the contrary, in the structure of the embodiment of the present disclosure, such a problem does not occur. 
     In the structure of the embodiment of the present disclosure, F 0 , F 1 , and F 2 , that is, the “frequency set”, must be appropriately determined according to the employed frequency conversion method so that RF 1  and RF 2  are appropriately generated. Conditions for the frequency conversion will now be described in detail. 
     When the difference method is employed as the frequency conversion method, in the frequency conversion, conditions that F0-F1=RF1 and F0-F2=RF2 must be satisfied. On the other hand, when the summation method is employed as the frequency conversion method, in the frequency conversion, conditions that F0+F1=RF1 and F0+F2=RF2 must be satisfied. 
     As already described, F 1  and F 2  are frequencies that are designated or calculated within the frequency range. One of F 1  and F 2  is the designated frequency, and the other of F 1  and F 2  is the calculated frequency. The frequency range is, for example, 200 MHz. A lower limit of the frequency range is, for example, 0 MHz or 1 MHz, and an upper limit of the frequency range is 200 MHz. Based on these values, a relatively high frequency such as 125 MHz, 150 MHz, 175 MHz, or the like is set as the designated frequency, or a relatively low frequency such as 25 MHz, 50 MHz, 75 MHz, or the like is set as the designated frequency. 
     When the method is the difference method, and the designated frequency exceeds 1/2 of the frequency range (an average between an upper limit and a lower limit of the frequency range), that is, in the case of a high frequency designation, a lower RF among RF 1  and RF 2  is set as the primary reference frequency, and a higher RF among RF 1  and RF 2  is set as the sub reference frequency. In short, in the case of the combination of the difference method and the high frequency designation (hereinafter referred to as a “first combination”), the lower frequency criterion is employed. On the other hand, when the method is the difference method, and the designated frequency is below 1/2 of the frequency range, that is, in the case of a low frequency designation, a higher RF among RF 1  and RF 2  is set as the primary reference frequency, and a lower RF among RF 1  and RF 2  is set as the sub reference frequency. In short, in the case of the combination of the difference method and the low frequency designation (hereinafter referred to as a “second combination”), the higher frequency criterion is employed. 
     When the method is the summation method, and the designated frequency is below 1/2 of the frequency range, that is, in the case of the low frequency designation, a lower RF among RF 1  and RF 2  is set as the primary reference frequency, and a higher RF among RF 1  and RF 2  is set as the sub reference frequency. In short, in the case of the combination of the summation method and the low frequency designation (hereinafter referred to as a “third combination”), the lower frequency criterion is employed. On the other hand, when the method is the summation method, and the designated frequency exceeds 1/2 of the frequency range, that is, in the case of the high frequency designation, a higher RF among RF 1  and RF 2  is set as the primary reference frequency, and a lower RF among RF 1  and RF 2  is set as the sub reference frequency. In short, in the case of the combination of the summation method and the low frequency designation (hereinafter referred to as a “fourth combination”), the higher frequency criterion is employed. 
     By determining the frequency set according to the requirements as described above, the acceptable frequency difference (difference between RF 1  and RF 2 ) can be enlarged, as will be described below with reference to  FIGS.  7  and  8   . In other words, frequency generation corresponding to a larger number of nucleus species can be enabled. In the embodiment of the present disclosure, the first combination is employed, but other combinations may be selectively employed according to the situations. 
     In any of the combinations, first, the frequency F 0  of the local signal is calculated based on the primary reference frequency (RF 1  or RF 2 ) and the designated frequency FS (═F1 or F2). Then, the calculated frequency (F 2  or F 1 ) is calculated based on the sub reference frequency (RF 2  or RF 1 ) and F 0 . Through the stepwise process as described, the frequency set (F 0 , F 1 , F 2 ) is specified. 
     In the above description, RF 1  and F 1  are fixedly correlated to each other, and RF 2  and F 2  are fixedly correlated to each other. Alternatively, other correlation relationships may be employed as the relationship between RF 1  and RF 2 , and F 1  and F 2 . For example, a lower RF among RF 1  and RF 2  may be assigned to F 1 , and a higher RF among RF 1  and RF 2  may be assigned to F 2 . 
       FIG.  2    exemplifies four frequency sets, that is, four frequency sets corresponding to the first combination (#1), the second combination (#2), the third combination (#3), and the fourth combination (#4). In  FIG.  2   , among RF 1  and RF 2 , a frequency in a cell which is shown in gray is the primary reference frequency. Among F 1  and F 2 , a frequency in a cell which is shown in gray is the designated frequency. The frequency range FR is 0~200MHz. The numerical values described in the present disclosure, however, are merely exemplary. 
     With reference to the frequency set corresponding to the first combination (#1), RF 1  is 60 MHz, and RF 2  is 150 MHz. The designated frequency is 125 MHz. With the difference method, RF 1  which is the lower RF is set as the primary reference frequency, and 125 MHz which is the designated frequency is assigned to F 1  corresponding to RF 1 . As a result, F 0  is calculated as 185 MHz, and F 2  is calculated as 35 MHz. 
     With reference to the frequency set corresponding to the second combination (#2), similar to the above, RF 1  is 60 MHz, and RF 2  is 150 MHz. The designated frequency is 25 MHz. With the difference method, RF 2  which is the higher RF is set as the primary reference frequency, and 25 MHz which is the designated frequency is assigned to F 2  corresponding to RF 2 . As a result, F 0  is calculated as 175 MHz, and F 1  is calculated as 115 MHz. 
     With reference to the frequency set corresponding to the third combination (#3), RF 1  is 560 MHz, and RF 2  is 600 MHz. The designated frequency is 25 MHz. With the summation method, RF 1  which is the lower RF is set as the primary reference frequency, and 25 MHz which is the designated frequency is assigned to F 1  corresponding to RF 1 . As a result, F 0  is calculated as 535 MHz, and F 2  is calculated as 65 MHz. 
     With reference to the frequency set corresponding to the fourth combination (#4), similar to the above, RF 1  is 560 MHz, and RF 2  is 600 MHz. The designated frequency is 125 MHz. With the summation method, RF 2  which is the higher RF is set as the primary reference frequency, and 125 MHz which is the designated frequency is assigned to F 2  corresponding to RF 2 . As a result, F 0  is calculated as 475 MHz, and F 1  is calculated as 85 MHz. 
     In the embodiment of the present disclosure, the first combination is employed, but alternatively, the combination to be used may be selected from among the four combinations. In the embodiment of the present disclosure, the designated frequency is fixed regardless of the combination of RF 1  and RF 2 . Alternatively, the designated frequency may be varied according to the combination of RF 1  and RF 2 . 
       FIG.  3    schematically shows an example method of calculating the frequency set in the first combination. In  FIG.  3   , reference numeral  74  shows a combination of RF 1  and RF 2 . RF 1  is 60 MHz as shown by reference numeral  75 , and RF 2  is 150 MHz as shown by reference numeral  76 . A frequency which is relatively low, that is, RF 1 , is set as the primary reference frequency, and a frequency which is relatively high, that is, RF 2 , is set as the sub reference frequency. As shown by reference numeral  77 , F 1  is the designated frequency FS. That is, F 1  is 125 MHz. 
     First, as shown by reference numeral  78 , F 0  is calculated based on F 1  and RF 1 . Specifically, F 0  is calculated by F0=F1+RF1, as 185 MHz. Then, as shown by reference numeral  80 , F 2  is calculated based on RF 2  and F 0 . Specifically, F 2  is calculated by F2=F0-RF2, as 35 MHz. 
       FIG.  4    schematically shows an example method of calculating the frequency set in the second combination. In  FIG.  4   , reference numeral  81  shows a combination of RF 1  and RF 2 . RF 1  is 60 MHz as shown by reference numeral  82 , and RF 2  is 150 MHz as shown by reference numeral  83 . A frequency which is relatively high, that is, RF 2 , is set as the primary reference frequency, and a frequency which is relatively low, that is, RF 1 , is set as the sub reference frequency. As shown by reference numeral  84 , F 2  is the designated frequency FS. That is, F 2  is 25 MHz. 
     First, as shown by reference numeral  85 , F 0  is calculated based on F 2  and RF 2 . Specifically, F 0  is calculated by F0=F2+RF2, as 175 MHz. Then, as shown by reference numeral  86 , F 1  is calculated based on RF 1  and F 0 . Specifically, F 1  is calculated by F1=F0-RF1, as 115 MHz. 
     As described, with the difference method, the designated frequency which is fixedly determined is prepared, and the stepwise calculation is performed. With this process, it is possible to quickly and easily specify or calculate the frequency set suited to the combination of RF 1  and RF 2 . 
       FIG.  5    schematically shows an example method of calculating the frequency set in the third combination. In  FIG.  5   , reference numeral  87  shows a combination of RF 1  and RF 2 . RF 1  is 560 MHz as shown by reference numeral  88 , and RF 2  is 600 MHz as shown by reference numeral  89 . A frequency which is relatively low, that is, RF 1 , is set as the primary reference frequency, and a frequency which is relatively high, that is, RF 2 , is set as the sub reference frequency. As shown by reference numeral  90 , F 1  is the designated frequency FS. That is, F 1  is 25 MHz. 
     First, as shown by reference numeral  91 , F 0  is calculated based on F 1  and RF 1 . Specifically, F 0  is calculated by F0=RF1-F1, as 535 MHz. Then, as shown by reference numeral  92 , F 2  is calculated based on RF 2  and F 0 . Specifically, F 2  is calculated by F2=RF2-F0, as 65 MHz. 
       FIG.  6    schematically shows an example method of calculating the frequency set in the fourth combination. In  FIG.  6   , reference numeral  93  shows a combination of RF 1  and RF 2 . RF 1  is 560 MHz as shown by reference numeral  94 , and RF 2  is 600 MHz as shown by reference numeral  95 . A frequency which is relatively high, that is, RF 2 , is set as the primary reference frequency, and a frequency which is relatively low, that is, RF 1 , is set as the sub reference frequency. As shown by reference numeral  96 , F 2  is the designated frequency FS. That is, F 2  is 125 MHz. 
     First, as shown by reference numeral  97 , F 0  is calculated based on F 2  and RF 2 . Specifically, F 0  is calculated by F0=RF2-F2, as 475 MHz. Then, as shown by reference numeral  98 , F 1  is calculated based on RF 1  and F 0 . Specifically, F 1  is calculated by F1=RF1-F0, as 85 MHz. 
     As described, even when the summation method is employed, the designated frequency which is fixedly determined is used, and the stepwise calculation is performed. With this process, it is possible to quickly and easily specify or calculate the frequency set suited to the combination of RF 1  and RF 2 . 
     In the first combination, as a case in which an inappropriate setting value is calculated, for example, the following case may be exemplified. A case which is opposite to the above description will be considered. Namely, a frequency which is relatively high, that is, RF 2 , is set as the primary reference frequency, and a frequency which is relatively low, that is, RF 1 , is set as the sub reference frequency. F 2  is the designated frequency FS, and thus is 125 MHz. F 0  is calculated based on F 2  and RF 2 . Specifically, F 0  is calculated by F0=F2+RF2, as 275 MHz. Then, F 1  is calculated based on RF 1  and F 0 . Specifically, F 1  is calculated by F1=F0-RF1, as 205 MHz. This frequency exceeds FR, and is inappropriate. 
       FIG.  7    diagrammatically shows the calculation of the frequency set with the difference method, more specifically, the calculation of the frequency sets in the first combination and the second combination. The difference method is generally employed when RF 1  and RF 2  belong to the low frequency band. Reference numeral  114  shows the first combination, and reference numeral  116  shows the second combination. The vertical axis shows the frequency. 
     In both the first combination and the second combination which are illustrated, the condition of RF1&lt;RF2 is satisfied. In the first combination, the designated frequency FS is a high frequency exceeding 1/2 of the frequency range FR. In the second combination, the designated frequency FS is a low frequency below 1/2 of the frequency range FR. Needless to say, the designated frequency FS is a frequency which is lower than or equal to the upper limit value of the frequency range FR. 
     In the first combination, the lower RF, RF 1 , is set as the primary reference frequency, that is, the criterion frequency. F 0  is calculated by adding F 1  (=FS) to RF 1 . Then, F 2  is calculated by subtracting RF 2  from F 0 . In the illustrated example configuration, F 2  is a positive frequency. A difference between RF 1  and RF 2  is shown by ΔRF. 
     For example, when RF 2  becomes RF 2 ′, ΔRF’ becomes large. When ΔRF’ exceeds F 1  (=FS), F 2 ′ to be calculated becomes a negative frequency in the numerical value calculation. In order to avoid such a situation, desirably, a largest possible value is given to FS. For example, the upper limit value of the frequency range FR may be given to FS. When a large value is given to FS, the difference between the summation component and the difference component which are output from the mixer becomes large, that is, the frequency difference between the necessary signal and the unnecessary signal becomes large. Thus, it becomes easier to remove the unnecessary signal. That is, by giving a large value for FS, the filter design can be facilitated. 
     In the second combination, a higher RF, that is, RF 2 , is set as the primary reference frequency, that is, the criterion frequency. F 0  is calculated by adding F 2  (=FS) to RF 2 . Then, F 1  is calculated by subtracting RF 1  from F 0 . A difference between RF 1  and RF 2  is shown by ΔRF. In order to enlarge ΔRF, desirably, FS is as small as possible. A minimum value (lower limit value) in the frequency range FR may be given to FS. When a small value is given to FS, advantages may be obtained in that it becomes not necessary to use a high-performance device, in that the signal purity is not significantly reduced, and the like. 
       FIG.  8    diagrammatically shows the calculation of the frequency set with the summation method, more specifically, the calculation of the frequency sets in the third combination and in the fourth combination. The summation method is generally employed when RF 1  and RF 2  belong to a high frequency band. Reference numeral  120  shows the third combination, and reference numeral  122  shows the fourth combination. The vertical axis shows the frequency. 
     In both the third combination and the fourth combination which are illustrated, the condition of RF1&lt;RF2 is satisfied. In the third combination, the designated frequency FS is a low frequency below 1/2 of the frequency range FR. In the fourth combination, the designated frequency FS is a high frequency exceeding 1/2 of the frequency range FR. 
     In the third combination, RF 1  which is the lower RF among RF 1  and RF 2  is set as the primary reference frequency. F 0  is calculated by subtracting F 1  (=FS) from RF 1 . F 2  is calculate by subtracting F 0  from RF 2 . Needless to say, F 2  must be within the frequency range FR. In the third combination, in order to enlarge the acceptable ΔRF, FS is desirably small, and FS may be matched with the lower limit value of FR. When FS is small, it is possible to obtain advantages similar to the advantages which can be obtained when FS is set to a small value in the second combination. 
     In the fourth combination, RF 2  which is the higher RF among RF 1  and RF 2  is set as the primary reference frequency. F 0  is calculated by subtracting F 2  (=FS) from RF 2 . Then, F 1  is calculated by subtracting F 0  from RF 1 . In the fourth combination, in order to enlarge the acceptable ΔRF, FS is desirably large, and FS may be matched with the upper limit value of FR. When FS is large, it is possible to obtain advantages similar to the advantages which can be obtained when FS is set to a large value in the first combination. 
     The illustrations in  FIGS.  7  and  8    are merely exemplary. The frequency conversion method, the criterion RF, the designated frequency, and the like may be selected according to the specific frequency relationship. Alternatively, the designated frequency may be variable. For example, the designated frequency may be adaptively determined based on ΔRF. 
       FIG.  9    shows an alternative configuration of the transmission section.  FIG.  9    shows a part of a transmission section  22 A. The transmission section  22 A has a signal generation unit  26 A and a combiner  36 A. The signal generation unit  26 A is formed from a plurality of signal generators  32 A. Each signal generator  32 A is formed from a sequencer  130  and a DDS (Direct Digital Synthesizer)  132 . The sequencer  130  generates a desired pulse sequence or a corresponding command sequence, and the DDS  132  generates the original transmission signal according to the pulse sequence or command sequence. The DDS  132  has a function to generate various waveforms. In the combiner  36 A, a plurality of original transmission signals are combined. The combined original transmission signal after being combined is sent to the frequency converter (not shown). 
       FIG.  10    shows an alternative configuration of the frequency converter. In  FIG.  10   , elements similar to the elements shown in  FIG.  1    are assigned the same reference numerals, and descriptions thereof will be omitted. 
     A plurality of filters in the demultiplexing circuit may realize a function similar to the function of the filter bank  40 . That is, the plurality of filters in the demultiplexing circuit may also function as a difference component extractor or a sum component extractor. Presuming this configuration, a bypassing path  138  may be selected by switches  134  and  135 , so that the purity of the transmission signal can be maintained. When it is desired to send the transmission signal to the filter bank  40 , a filtering path  136  may be selected through switches  134  and  135 . 
       FIG.  11    shows specific examples of the structure of the demultiplexing circuit. A combination of two BPFs identified by TYPE 1 has a frequency characteristic shown by reference numeral  140 - 1 . A combination of an HPF and an LPF identified by TYPE 2 has a frequency characteristic shown by reference numeral  140 - 2 . A combination of a BPF and an LPF identified by TYPE 3 has a frequency characteristic shown by reference numeral  140 - 3 . A combination of an HPF and a BPF identified by TYPE 4 has a frequency characteristic shown by reference numeral  140 - 4 . A combination of 2 BRFs (Band Rejection Filters) identified by TYPE k has a frequency characteristic shown by reference numeral  140 - k . A BRF is also called a BEF (Band Elimination Filter). 
       FIG.  12    shows an alternative configuration of the demultiplexing circuit. A demultiplexing circuit  18 A has a first filter  142 , a second filter  144 , and a third filter  146 , which are placed in parallel with each other. The demultiplexing circuit  18 A may also be called a triplexer. A transmission signal having a frequency of RF 1  is generated by the first filter  142 , a transmission signal having a frequency of RF 2  is generated by the second filter  144 , and a transmission signal having a frequency of RF 3  is generated by the third filter  146 . 
     A frequency set is calculated based on the method described above, according to RF 1 , RF 2 , and RF 3 . Specifically, when the difference method and the high frequency designation are employed, the lowest frequency among RF 1 , RF 2 , and RF 3  is set as the primary reference frequency, and the two remaining frequencies are set as the sub reference frequencies. A frequency, among three frequencies corresponding to RF 1 , RF 2 , and RF 2 , corresponding to the primary reference frequency, is set as the designated frequency, and the two remaining frequencies are set as the calculated frequencies. The above configuration is merely exemplary, and alternatively, in the present alternative configuration, the summation method or the low frequency designation may be employed. 
     In the embodiment of the present disclosure described above, a lower frequency or a higher frequency among two RFs is set as the criterion, but alternatively, an average frequency based on the two RFs may be set as the criterion. Further, in the embodiment of the present disclosure described above, the designated frequency for determining F 0  may be varied according to the measurement objective and the measurement situation. Alternatively, F 0  may be designated by the user. Another alternative configuration may be considered in which F 1  and F 2  are adaptively determined without the use of the designated frequency. Alternatively, a plurality of demultiplexing circuits corresponding to a plurality of groups of irradiation frequencies may be prepared, and a demultiplexing circuit suitable for a selected group of irradiation frequencies may be selectively used. Alternatively, for each of the transmission section and the reception section, a plurality of frequency converters connected in series may be provided. In this case, a stepwise frequency conversion is performed. Alternatively, a structure may be employed in which a plurality of demultiplexing circuits are provided in parallel with each other. In this case, the structure shown in  FIG.  1    is provided for each of the demultiplexing circuits.