JPH031828Y2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a nuclear magnetic resonance (NMR) device, and is a method for generating a radio frequency (RF) signal suitable for generating radio frequency pulses for exciting magnetic resonance. Regarding circuit improvement.

[Conventional technology]

In a pulsed NMR apparatus, a sample is placed in a static magnetic field, and a high-frequency magnetic field having a resonance frequency of an observation nucleus is applied to the sample in a pulsed manner from a coil wound around the sample. Therefore, a means for supplying a pulse-modulated radio frequency (RF) signal to the coil is required. FIG. 4 schematically shows a conventional RF signal generation circuit used for this purpose. In the figure, reference numeral 20 denotes a first RF oscillator that generates a first high-frequency output, and the high-frequency output f ₁ from the first RF oscillator 20 is sent to a mixer 22 via a gate circuit 21 . 23 is a second RF oscillator that generates a second high frequency output, and the second RF oscillator 2
The high frequency output f ₂ from No. 3 is sent to mixer 22 via gate circuit 24 . In the mixer 22, the high frequency output _f1 of the first RF oscillator 20 and the second
The high frequency output f ₂ of the RF oscillator 23 is mixed, and an RF signal of f ₁ ±f ₂ is obtained from the mixer 22. Of these RF signals, those having a desired frequency (for example, f ₁ +f ₂ ) are extracted through the filter 25 , amplified by the amplifier 26 , and supplied to the transmitting/receiving coil 27 . 28 is a magnet that generates a static magnetic field, 29 is an amplifier for amplifying the detection signal from the transmitting/receiving coil 27, and 30 is a gate circuit that turns on/off the detection signal, and the detection signal is transmitted to a demodulation circuit, etc. is sent to the CPU via

[Problem that the invention attempts to solve]

In the conventional circuit configured in this way, the gate circuits 21 and 24 are kept on for a predetermined period to control the frequency.
An RF pulse of f ₁ + f ₂ is taken out and irradiated to the sample as an RF pulse magnetic field via the coil 27. Immediately after the RF pulse magnetic field is irradiated, the gate circuit 30 is turned on, and the detection signal induced in the transmitting/receiving coil 27 is Detected. If there is a so-called RF carrier leak in which an RF signal leaks from the transmission system including the oscillators 20 and 23 to the detection circuit system during this detection period,
Since this adversely affects the obtained NMR spectrum, gate circuits 21 and 24 are made of gate circuits with extremely low leakage. However, it is impossible to completely eliminate leakage in the gate circuit, and even when the gate circuit is off, it is inevitable that the f ₁ +f ₂ component will leak slightly to the detection circuit system.
Moreover, the detection circuit system is highly sensitive in order to detect weak NMR signals, and even the slightest leak can be detected. Therefore, in the past, it was unavoidable that the negative effects of RF carrier leak remained. The purpose of the present invention is to solve the above problems, eliminate carrier leakage of RF signals during detection, and prevent abnormal signals from being generated in the detection circuit system.

[Means for solving problems]

The configuration of the present invention to achieve this object includes a waveform storage circuit that stores a large number of digital values constituting an arbitrary waveform, and a readout means that repeatedly generates a readout signal for reading out a waveform from the waveform storage circuit. and a D/A converter that converts the digital value read out from the waveform storage circuit into an analog signal based on the signal from the reading means, and between the D/A converter and the waveform storage circuit or the D/A converter. The device is characterized by comprising a gate circuit inserted between the D/A converter and the reading means, and a mixer that mixes the signal from the D/A converter and a signal having a predetermined frequency.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a block diagram showing a device according to an embodiment of the present invention, and FIG. 4 shows the same parts of the conventional device denoted by the same reference numerals and the explanation thereof will be omitted. In Figure 1,
1 is a clock generation circuit, 2 is a counter circuit, 3
4 is a waveform storage circuit, 4 is a gate circuit, 5 is a gate timing generation circuit for controlling on/off of each gate circuit, 6 is a D/A converter, and 7 is a filter circuit.

In the device configured as described above, the waveform storage circuit 3 divides one cycle of the sine waveform into a large number of points, as shown in FIG. 2, and stores digital values indicating the intensity of each point. There is. Using the output of the counter circuit 2 as an address signal, data is sequentially and repeatedly read out from the waveform storage circuit 3, converted into an analog signal (staircase waveform) by the D/A converter 6, and further waveform-shaped by the filter 7. For example, a sine waveform
An RF signal f ₁ is obtained. Also, if the data for one cycle is appropriately thinned out and read in a short time, the RF
The frequency of signal _f1 can be set arbitrarily.

FIG. 3 is a waveform diagram for explaining the operation of the circuit of FIG. 1. For example, the pulses shown in FIG. 3B generated by the clock generation circuit 1 are counted by the counter circuit 2. The count value is sent to the waveform storage circuit 3 as an address designation signal. Based on the addressing signal, digital data (for example, 8 bits) is sequentially read out from the waveform storage circuit 3 as shown in FIG. The data supplied to the gate circuit 4 is
It is turned on/off by the pulse shown in FIG. 3C generated by the gate timing circuit 5. When the gate circuit 4 is turned on by this pulse, the data shown in FIG _.
is converted to The RF signal converted into an analog signal by the D/A converter 6 is sent to the third filter circuit 7.
The waveform is shaped into the signal shown in the figure and sent to the mixer 22. On the other hand, an RF signal f ₂ generated by a second RF oscillator 23 is supplied to the mixer 22 via a gate circuit 24 that is turned on and off at the same time as the gate circuit 21, as shown in FIG. As a result, new frequency components such as f ₁ ±f ₂ are generated as the output of the mixer 22. Then, from the filter circuit 8, only the f ₁ +f ₂ component is obtained as shown in FIG. Now, considering the RF carrier leak, the waveform data is not sent to the D/A converter while the gate circuit 4 is off, as shown in FIG.
There is no RF signal f ₁ at all. Therefore, mixer 2
2 and later as well, since there are no composite components such as f ₁ ±f ₂ at all, an ideal RF carrier leak suppression effect can be obtained.

By the way, the mixer 22 via the gate circuit 24
It is inevitable that the RF signal _f2 leaks slightly to the RF signal _f1 , but the RF signal f1 is completely blocked,
Only the RF signal _f2 leaks to the detection circuit system. Since this f ₂ is significantly different from the resonant frequency f ₁ +f ₂ , even if it leaks, it does not cause any problem.

It should be noted that the present invention is not limited to the above embodiments and can be modified. In this embodiment, the gate circuit is provided before the D/A converter, but the counter circuit 2
and the waveform storage circuit 3. Also, in this example, the present invention was applied to the RF signal _f1 side, but the RF
It may be applied to the signal f ₂ side, or f ₁ , f ₂
It is also possible to apply the present invention to both. Furthermore, although the present invention was applied to an RF signal generation circuit for transmission in this embodiment, it is also possible to apply it to an RF signal generation circuit for detection in a reception system, or to generate an RF signal for decoupling. It is also possible to apply it to an RF signal generation circuit for.

〔effect〕

As detailed above, according to the present invention, a part of the RF signal generation circuit is configured with a digital circuit, and the signal is gated at the time of the digital signal.
RF carrier leak when the gate circuit is off can be eliminated. Therefore, when detecting an FID signal, high frequencies at or near the resonant frequency do not leak into the detection circuit system, and there is no problem in observation due to the generation of abnormal signals.

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram showing one embodiment of the present invention;
The figure is a waveform diagram for explaining the storage state of digital values in the waveform storage circuit, FIG. 3 is a diagram showing signal waveforms of the circuit, and FIG. 4 is a diagram for explaining a conventional device. 1: Clock generation circuit, 2: Counter circuit,
3: Waveform memory circuit, 4, 24: Gate circuit, 5:
Gate timing circuit, 6: D/A converter, 7,
25: filter circuit, 22: mixer, 23: second RF oscillator, 26, 29: amplifier, 27: transmitting/receiving coil, 28: magnet.

Claims (1)

[Scope of utility model registration request] a waveform storage circuit that stores a large number of digital values constituting an arbitrary waveform; a readout means that repeatedly generates a readout signal for reading out a waveform from the waveform storage circuit; A D/A converter that converts the digital value read from the storage circuit into an analog signal, and inserted between the D/A converter and the waveform storage circuit or between the D/A converter and the reading means. Gate circuit and the D/A
A high-frequency signal generation circuit for a nuclear magnetic resonance apparatus, comprising a mixer that mixes a signal from a transducer and a signal having a predetermined frequency.