JPH06133947A - High-frequency signal generator - Google Patents

Abstract

PURPOSE:To perform simultaneous frequency conversion through digital frequency synthesizing so as to directly obtain a signal for exciting nuclear magnetic resonance by tuning an output value from a data table and a certain value to an oscillator, and DA-converting both of the values alternately. CONSTITUTION:A reference frequency oscillator 1 oscillates rectangular waves whose duty cycle can be varied. Either of two outputs is added to an address generator 2 to determine the cycle. The address generator 2 originates an address so that data for a data table ROM 3 are repeated at a predetermined frequency. Sine wave data of one cycle for the raw address are written in said data, and a multiplier 4 calculates the product of the output of the data table ROM 3 and the other output of the reference frequency oscillator 1 and adds it to a DA oscillator 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital frequency synthesizer which is a high-frequency signal generator for exciting nuclear magnetic resonance, and a high-frequency signal generator for directly generating a nuclear magnetic resonance frequency by multiplying another frequency during DA conversion. Regarding vessels.

[0002]

2. Description of the Related Art In a conventional digital frequency synthesizer, as described in Japanese Patent Publication No. 3-35934, only the output of a sine wave data table is DA-converted. It has been difficult to directly obtain a high-purity signal of megahertz. Therefore, in order to obtain a signal for nuclear magnetic resonance excitation, a digital frequency synthesizer of several megahertz and several stages of frequency conversion were required.

[0003]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a digital frequency synthesizer for performing frequency conversion simultaneously,
The purpose is to obtain the signal for nuclear magnetic resonance excitation directly.

Also, the duty cycle at the time of DA conversion is changed to amplitude-modulate the output signal.

[0005]

In order to achieve the above object, as a DA conversion data of a digital frequency synthesizer, a sine wave data table value and a fixed value are alternately used, and Frequency conversion of wave data can be performed. After this, the necessary frequency is taken out by a bandpass filter.

Further, the hold time of the sine wave data in the DA converter is related to the amplitude value of the output and causes a spectrum deterioration called a zero-order hold characteristic (aperture effect). Therefore, the output is amplitude-modulated by limiting the hold time of the sine data in the DA converter.

[0007]

The reference frequency oscillator generates the read frequency of the sine wave data table and the carrier frequency that generates the sideband, and changes the duty cycle thereof to amplitude-modulate the output by the filter effect by the aperture effect. .

The multiplier multiplies the output of the sine wave data table and the carrier.

After passing through a DA converter, an output is obtained through a bandpass filter to obtain the desired frequency.

[0010]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 3 shows a block diagram of the device of the present invention. The frequency required for nuclear magnetic resonance excitation depends on the nuclide and the magnetic field strength, but here we will describe the case where the magnetic field strength is 0.5 Tesla symmetrically with respect to hydrogen nuclei. The nuclear magnetic resonance frequency in this case is about 21.1 MHz.
The reference frequency oscillator 1 has a duty cycle of 0 to 10
It oscillates a 20 MHz square wave that can be varied up to 0 percent. One of the two outputs is added to the address generator 2 for synchronization. The address generator generates the address so that the data in the data table ROM3 is repeated at a frequency of 1.1 MHz obtained by subtracting the oscillation frequency of 20 MHz of the reference frequency oscillator 1 from the desired 21.1 MHz. As the data of the data table ROM3, one cycle of sine wave data is written for the raw address. This data table ROM3
And the other output of the reference frequency oscillator 1 are multiplied by the multiplier 4 and added to the DA converter 5. If the digital data values up to this point are 8 bits or more, the object of the present invention can be achieved.

The output of the DA converter 5 has a waveform as shown in FIG. 1, which is apparent from the above description. Here, Ts is the oscillation cycle of the reference frequency oscillator 1, ts is the pulse width determined by the duty cycle of the reference frequency oscillator 1, and Ts is
₀ is the repetition cycle of the address generator 2. Since ts in this case is variable, it is shown as constant in FIG. 1, but in reality, it varies between 0 <ts <Ts. Next, a part of the spectrum waveform of FIG. 1 is shown in FIG. The output is
Sidebands are generated by folding back at an integral multiple of the oscillation frequency of the reference frequency oscillator 1 based on the repetition frequency f ₀ of the address generator 2. That is, ts
When ≈0, f _OUT = f ₀ + (fs ± f ₀ ) + (2fs ± f ₀ ) + …………………… (nfs ± f ₀ ) n = 0,1,2 ... .

When ts is finite, the spectrum is deteriorated according to the zero-order hold characteristic due to the aperture effect. In FIG. 2, the envelope of maximum amplitude is shown by a one-dot chain line for ts = 0 and ts = Ts. Figure 2
As is apparent from the above, the desired frequency fs + f ₀ is 20 + 1.1 = 21.1 MHz, but the amplitude of this spectrum can also be changed from zero to the maximum value by changing ts. Therefore, amplitude modulation at a frequency sufficiently lower than the repetition frequency fs of the address generator 2 (for example, several kilohertz) is sufficiently possible,
It can be used for amplitude modulation of an excitation waveform required in an MR imaging device.

The signal thus generated does not require components other than the desired frequency and must be attenuated to a sufficiently low value. Therefore, the band pass filter 6 shown in FIG. Unnecessary frequency component by -30
Must be below decibels.

Although the nuclear magnetic resonance frequency of 21.1 MHz has been described above, it is obvious that the same applies to other frequencies.

In this embodiment, the desired frequency is
Although it is set to fs + f ₀ as described in FIG. 2, the present invention is not limited to this frequency and nfs ± f _0.
It can be easily understood that the same operation can be performed for any frequency (n = 1, 2, 3, ...).

[0017]

EFFECTS OF THE INVENTION In a high-frequency generator for nuclear magnetic resonance excitation, a high-frequency modulation waveform similar to the conventional one can be obtained with a simple digital signal without the need for an analog frequency converter or modulator as in the conventional one. There is an effect that can be realized by the configuration of the circuit, the AD converter, and the bandpass filter.

[Brief description of drawings]

FIG. 1 is a diagram showing a typical example of an output waveform after AD conversion.

FIG. 2 is a diagram showing a part of the spectrum waveform of FIG.

FIG. 3 is a block diagram of a device showing an embodiment of the present invention.

[Explanation of symbols]

1 ... Reference frequency oscillator, 2 ... Address generator, 3
... data table ROM, 4 ... multiplier, 5 ... DA converter,
6 ... Bandpass filter.

Claims (2)

[Claims] 1. An oscillator that oscillates a standard frequency, an adder that is synchronized with this oscillator, a data table that generates a sine wave from the output of this adder, a DA converter that converts a digital signal into an analog signal, and a necessary frequency. In a high-frequency signal generator for nuclear magnetic resonance excitation, which comprises a filter to be selected, high-frequency signal generation characterized in that an output value from the data table and a fixed value are synchronized with the oscillator and DA conversion is alternately performed. vessel. 2. The filter according to claim 1, wherein the hold time of the output value from the data table in the DA converter and the hold time of the DA converter having a certain fixed value are mutually controlled to pass the filter. A high frequency signal generator characterized by controlling the output amplitude afterwards.