JP2008064653A - Spectrometer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spectrometer for simultaneously measuring electric power component ratios of electromagnetic wave at a plurality of different frequencies without sweeping. <P>SOLUTION: A rectangular waveguide 2 where cut-off frequency gradually increases with the distance from the end surface of an opening includes a plurality of antennas 5 for measuring the intensity of the electromagnetic wave disposed in the axial direction on the inner surface of the rectangular waveguide 2, the intensities of the electromagnetic wave of the cut-off frequency corresponding to the positions of respective antennas 5 are simultaneously measured by the antennas 5. As the rectangular waveguide 2 where the cut-off frequency increases with the distance from the end surface of the opening, a waveguide of a tapered shape or a step shape can be employed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electromagnetic spectrum measurement technique, and more particularly to a technique capable of measuring an electromagnetic spectrum over a wide frequency range.

  Spectrum analyzers are widely used to measure the spectrum at microwaves and lower frequencies. A spectrum analyzer uses a high-frequency signal input from the outside to generate a difference frequency component (commonly called an intermediate frequency) by using a high-frequency signal from a local oscillator with a known internal frequency and a down-converter using a diode mixer. Is extracted, amplified and detected. Further, the spectrum of the high frequency to be measured is measured by sweeping the frequency of the local oscillator. Spectrum analyzers have a long history and have established techniques for measuring both frequency and power with high accuracy.

  Also, recently, an intermediate frequency signal is converted into a digital signal by a high-speed digital AD converter without sweeping the local oscillator of the spectrum analyzer, and this is continuously subjected to a high-speed FFT conversion to obtain a wide frequency. Technologies have been developed to measure spectra simultaneously.

On the other hand, there is no spectrum analyzer as described above in a high frequency region from millimeter waves to terahertz waves. Such high-frequency spectrum is detected by treating high-frequency power as an electromagnetic wave beam traveling in space, wavelength-decomposing using a diffraction grating, detecting power with a bolometer or semiconductor detector, and rotating the diffraction grating to change the wavelength. Is done by sweeping
Japanese Patent Application Laid-Open No. 62-201373 Japanese Patent Laid-Open No. 01-035386 JP 05-264609 A Japanese Patent Laid-Open No. 09-051307

  However, the above-described conventional techniques have the following problems.

  In the spectrum analyzer described above, sweeping takes time, and power at a plurality of frequencies cannot be measured simultaneously. In particular, measuring the spectrum of a pulsed electromagnetic wave by frequency sweeping is extremely inefficient.

  In addition, a digital spectrum analyzer can simultaneously measure a spectrum of a plurality of frequencies, but measuring a microwave frequency of 1 GHz or more has not been put into practical use.

  In addition, the method using a diffraction grating requires an incident electromagnetic wave beam to be incident on the diffraction grating at a correct angle. In actual experiments, there are many cases where the position and direction of the incident electromagnetic wave beam cannot be accurately determined, and thus measurement is often difficult. Moreover, spectrum measurement is often difficult due to scattered waves.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a spectrometer capable of simultaneously measuring a spectrum in a wide frequency range.

  In order to achieve the above object, the spectrometer according to the present invention measures the spectrum of electromagnetic waves with the following configuration. That is, the spectrometer according to the present invention has a waveguide whose cutoff frequency gradually increases according to the distance from the opening end surface, and the intensity of electromagnetic waves provided in the axial direction of the waveguide inside the waveguide. A plurality of measurement units for measurement. And the intensity | strength of the electromagnetic wave of the cutoff frequency corresponding to the position in which each measurement part was provided is measured by these some measurement parts.

  The cutoff frequency of the waveguide gradually increases according to the distance from the opening end face. This means that the cut-off frequency increases monotonously (including both true monotonic increase and non-decrease). As such a waveguide, a configuration in which the cross-sectional dimension continuously decreases in accordance with the distance from the opening end face can be employed. Further, it is possible to adopt a configuration in which the dimension of the cross section gradually decreases in accordance with the distance from the opening end surface. Specifically, a configuration in which a rectangular waveguide is employed as the waveguide and the lateral width of the cross-sectional rectangle continuously decreases, or a configuration in which the lateral width of the rectangular cross-section decreases stepwise can be employed. Here, in the rectangular cross-section of the rectangular waveguide, the direction of the polarization plane can be limited by setting the height to half or less of the lateral width. The specific shape of the waveguide may be any shape other than the above as long as the cutoff frequency is gradually increased.

  The measuring unit may be anything as long as it can measure a physical quantity corresponding to the intensity of the electromagnetic wave at the position where the measuring unit is provided. For example, the measurement unit may be configured to include an antenna and a rectification detection unit that extracts an electromagnetic wave acquired from the antenna as a low-frequency electromagnetic wave. Further, the measurement unit has an antenna, a thermistor that absorbs the electromagnetic wave acquired from the antenna, and a temperature measurement unit that measures the temperature of the thermistor, and the intensity of the electromagnetic wave due to a temperature change measured from the temperature measurement unit It is good also as a structure which measures. In addition, the measurement unit has a thermistor thin film and a resistance measurement unit that measures the resistance of the thermistor thin film, detects the temperature change of the thermistor by the resistance measured from the resistance measurement unit, It is good also as a structure which measures the intensity | strength of.

  The group velocity of the electromagnetic wave having an arbitrary frequency component is theoretically zero where the frequency of the electromagnetic wave is equal to the cutoff frequency of the waveguide. Therefore, at a position where the frequency of the input electromagnetic wave is equal to the cutoff frequency, the electric field strength of the electromagnetic wave having the frequency component takes a peak. Therefore, the measurement unit measures the intensity of the electromagnetic wave having the cutoff frequency corresponding to the position where the measurement unit is provided. Thus, since each measurement unit measures the intensity of the electromagnetic wave having the cutoff frequency corresponding to the arrangement position, it is possible to simultaneously measure the intensity of the electromagnetic wave having the cutoff frequency corresponding to each measurement unit.

  In addition, since the frequency decomposition of the electromagnetic wave is performed by the cutoff frequency of the waveguide, it is possible to measure the spectrum of the electromagnetic wave in a wide range from microwave to millimeter wave to terahertz wave.

  The intensity of the electromagnetic wave measured by each measurement unit may be the intensity of the electromagnetic wave having the cutoff frequency corresponding to the measurement unit, but this measurement value includes the influence of the electromagnetic wave of other frequency components. Therefore, it is preferable to perform correction to remove the influence of the electromagnetic wave having the cutoff frequency corresponding to the other measurement unit based on the electromagnetic wave intensity measured by the other measurement unit.

  According to the present invention, it is possible to simultaneously measure a spectrum in a wide frequency range.

  Exemplary embodiments of the present invention will be described in detail below with reference to the drawings.

(First embodiment)
FIG. 1 is a configuration diagram of a first embodiment of a spectrometer according to the present invention. In the following description, microwaves are assumed as electromagnetic waves. However, even if these are replaced with millimeter waves and terahertz waves, they operate on the same principle.

  The spectrometer in the present embodiment is generally composed of a rectangular waveguide 2 and a plurality of antennas 5 provided therein. In the present embodiment, the number of antennas 5 is described as four, but the number of antennas 5 may be any number.

  The rectangular waveguide 2 has an opening 3 only at one end. And the electromagnetic wave 1 which is a to-be-measured object which advances to az direction is introduce | transduced from this opening part 3. The other end of the rectangular waveguide 2 is provided with a non-reflection terminal 8 in order to suppress reflection of the introduced electromagnetic wave. As shown in the figure, the coordinate axis is defined by the z-axis as the axial direction of the rectangular waveguide 2, the x-axis as the major axis direction of the cross section, and the y-axis as the minor axis direction.

  As shown in FIG. 1, the cross-sectional dimension of the rectangular waveguide 2 is a function of the axial distance z from the opening 3 in both the width and height, and gradually increases according to the distance from the opening 3. Get smaller. That is, the rectangular waveguide 2 is a tapered waveguide.

  The width of the cross-sectional dimension of the rectangular waveguide 2 is a (z), and the height is b (z). Here, by limiting the height b (z) to half or less of the lateral width a (z), the wave polarization plane of the electromagnetic wave that can pass through the rectangular waveguide 2 is limited so that the electric field is in the y direction. Can do. Therefore, it is preferable that a (z) and b (z) satisfy the relationship of the following formula (1).

In the present embodiment, the lateral width a (z) of the cross-sectional dimension is a linear function and is given by the following equation (2). Here, a ₁ is the width of the entrance, a ₂ is the width at the end, l is the length of the waveguide.

  A plurality of antennas 5 are provided on the inner surface of the rectangular waveguide 2. In FIG. 1, it is provided on the upper surface of the rectangular waveguide 2, but it may be disposed on any part of the inner surface. Further, the plurality of antennas 5 are arranged at positions where the distances from the opening 3 are different. Each antenna 5 includes a pickup unit 6 that picks up electromagnetic waves and a coaxial waveguide 7 that propagates the electromagnetic waves. Then, the electromagnetic wave propagated from the coaxial waveguide 7 is rectified by a diode mixer (not shown), and the power of the rectified electromagnetic wave is measured. In this way, the powers Q1 to Q4 at the position where the antenna 5 of the rectangular waveguide 2 is provided are measured.

  Hereinafter, the principle by which the intensities of electromagnetic waves having a plurality of frequencies can be simultaneously measured in the spectrometer according to the present embodiment will be described.

First, the cut-off wavelength λ _c and cut-off frequency f _c for electromagnetic waves in the rectangular waveguide 2 are given by Expression (3) and are a function of z. Rectangular waveguide 2 are the tapered waveguide cross-sectional dimensions decreases as the distance from the opening 3, the cutoff frequency f _c as the distance from the opening 3 gradually increases. As a unit of frequency, hertz (Hz) is generally used, but in this specification, an angular frequency ω (rad / sec) often used in electrical engineering is used. The angular frequency can be easily read as Hertz by equation (3c).

  Further, the wavelength and wave number of the electromagnetic wave propagating through the rectangular waveguide are given by the following equations.

ここで、
λ_０：真空中または大気中での電磁波の波長。λ_０＝ｃ／ｆ
ｋ_０：真空中または大気中での電磁波の端数。ｋ_０＝２π／λ_０

here,
λ ₀ : Wavelength of electromagnetic wave in vacuum or air. λ ₀ = c / f
k ₀ : fraction of electromagnetic wave in vacuum or air. k ₀ = 2π / λ ₀

FIG. 2 illustrates equation (4b) and is generally called a dispersion curve. In FIG. 2, three dispersion curves at different positions in the rectangular waveguide 2 are shown, and different cutoff frequencies ω _c1 , ω _c2 , and ω _c3 are taken from the difference in cross-sectional dimensions. If the frequency is lower than the cut-off frequency, the electromagnetic wave cannot propagate through the waveguide.

  Next, the speed at which the electromagnetic wave energy propagates through the rectangular waveguide 2, that is, the group speed, is given by Expression (5) by differentiating Expression (4b).

  According to the equation (5), the group velocity of the electromagnetic wave propagating through the waveguide, that is, the propagation velocity of energy is smaller than the speed of light, and the group velocity becomes zero particularly when the cutoff frequency is equal to the frequency of the input electromagnetic wave.

Of the electromagnetic wave is propagated in the waveguide, the energy throughput per unit time of a certain frequency component, i.e. put power and P _omega. If the loss of the waveguide is ignored, P _ω is constant in the steady state regardless of the position in the waveguide from the law of energy conservation.

Further, from the definition of the group velocity, the energy amount W _ω (z) of the electromagnetic wave per unit length is given by the equation (6a). The electric field strength is given by the equation (6b) and is proportional to the square root of the energy density. FIG. 3 shows equations (6a) and (6b) as a function of z.

ここで、Ｃはエネルギー密度と電界強度の比例定数であり、導波管の形状から決定される。

Here, C is a proportional constant between the energy density and the electric field strength, and is determined from the shape of the waveguide.

  The rectangular waveguide 2 of the spectrometer according to the present embodiment has a tapered shape, and the cross-sectional dimension a (z) decreases monotonously with respect to the position z from the opening 3 of the rectangular waveguide 2 (narrow sense). Therefore, the cut-off frequency is increased according to the equation (3b). When the input electromagnetic wave enters the rectangular waveguide 2 and the cutoff frequency becomes equal to the frequency of the input electromagnetic wave, the electromagnetic wave cannot move forward due to the cutoff, and is reflected and returned to the opening 3 of the rectangular waveguide 2. go. For this reason, a standing wave is generated in the rectangular waveguide 2. For convenience, the position where the cutoff frequency is equal to the frequency of the input electromagnetic wave is called a reflection point.

  The group velocity of electromagnetic waves becomes zero at the reflection point according to the equation (5). Therefore, the energy density becomes infinite according to the equation (6a). However, this is a one-dimensional analytical solution. Actually, there is a nearby wave, the electric field spreads around the reflection point, and the energy density is not infinite, as shown in FIG. The electric field distribution has a peak but a finite value.

As shown in FIG. 4, when four electromagnetic waves having frequencies ω ₁ , ω ₂ , ω ₃ , and ω ₄ are simultaneously input, the electric field peaks at the respective reflection points z ₁ , z ₂ , z ₃ , and z _4. take. If the antennas 5a, 5b, 5c, and 5d shown in FIG. 1 are arranged at the peak positions, power corresponding to the respective frequencies is detected from the coaxial waveguide 7.

FIG. 5 shows a change in power measured at each antenna 5 when the input frequency is swept. This corresponds to a so-called filter characteristic, and it is understood that the frequency peaks at the points of ω ₁ , ω ₂ , ω ₃ , and ω ₄ and shows a band pass characteristic.

Here, the detected powers P _ω1 , P _ω2 , P _ω3 , and P _ω4 detected by each antenna 5 can be used as the power of the electromagnetic waves having the cutoff frequencies ω ₁ , ω ₂ , ω ₃ , and ω ₄ . However, the above cut-off characteristics are not perfect, and the signal detected by each antenna 5 does not become a single frequency signal but also includes other frequency components. For example, not only the frequency component of ω ₃ but also the signals of the frequency components of ω ₁ , ω ₂ , and ω ₄ are mixed and output to the antenna at the position of z ₃ . Therefore, it is preferable to perform correction to remove the influence of electromagnetic waves other than the cutoff frequency component corresponding to the position where each antenna is arranged. This correction can be performed as follows.

First, when the coupling coefficient of each antenna with respect to the frequencies ω ₁ , ω ₂ , ω ₃ , and ω ₄ of the input electromagnetic wave is described as C _ij , the total powers Q ₁ , Q ₂ , Q ₃ , and Q ₄ detected from each antenna are It is expressed by a matrix as shown in Expression (7).

Here, the coupling coefficient C _ij in the equation (7) is obtained by detecting the electric power output from each antenna by inputting a single frequency electromagnetic wave having a predetermined intensity to the rectangular waveguide 2 by the reference signal generator. Can be determined by. By storing this as correction data in a computer and calculating the inverse matrix of equation (7), the power spectrum of the electromagnetic wave is obtained as in equation (8).

  In recent years, software technology for numerically calculating propagation of electromagnetic waves has been developed, and the coefficient matrix may be determined by numerical simulation. It is also possible to select the shape of the taper, that is, the functions a (z) and b (z) so as to match the impedance viewed from the opening of the waveguide by numerical calculation, or to use a stub as appropriate. is there.

  With the spectrometer according to the present embodiment, the spectrum of electromagnetic waves over a wide frequency range from microwaves to millimeter waves and further to terahertz waves can be simultaneously measured by the plurality of antennas 5. Here, the frequency resolution can be improved by increasing the number of antennas 5 arranged.

  Further, since it is not configured to sweep the frequency of the local oscillator as in the prior art, spectrum measurement can be performed efficiently, and in particular, spectrum measurement of pulsed electromagnetic waves can be performed efficiently.

  Further, since it is not necessary to make fine adjustments for measurement unlike spectrum measurement using a diffraction grating, the spectrometer according to this embodiment can easily perform spectrum measurement.

(Second Embodiment)
FIG. 6 is a diagram showing a configuration of a spectrometer according to the second embodiment of the present invention.

  In the present embodiment, as shown in FIG. 6, a step-shaped waveguide whose cross-sectional dimensions change stepwise is employed instead of the tapered waveguide in the first embodiment.

One antenna 5 is arranged at each step, and the arrangement location is set as far as possible from the opening 3 in each step. The dimensions of the waveguide at the position where the n-th antenna provided, the width a _n, when the height b _n, the frequency bandwidth of the signal outputted from the n-th antenna, formula (9) As given. Incidentally, for the stability of the electromagnetic wave mode, the ratio of b _n breadth a _n and height of the waveguide, it is preferable to b n _{= a} n / ₂ approximately.

By equation (9), it can be seen that the frequency range of the signal taken out from the antenna can be set by selection of the width a _n steps waveguide.

Since the cutoff characteristics of the antennas are not perfect, the equation (9) is used to calculate the output powers Q ₁ , Q ₂ , Q ₃ , and Q ₄ from the antennas using the equation (8) as in the first embodiment. The power P _ω1 , P _ω2 , P _ω3 , and P _ω4 in each frequency band given by) can be calculated.

(Third embodiment)
FIG. 7 is a diagram showing a configuration of a spectrometer according to the third embodiment of the present invention.

  In this embodiment, as shown in FIG. 7, in the first and second embodiments, a rectangular waveguide 35 is used as an electromagnetic wave measuring unit for detecting electromagnetic waves instead of using an antenna and a coaxial waveguide. Also good. It is desirable that the position and size of each rectangular waveguide 35 be optimized by numerical simulation of electromagnetic waves.

(Fourth embodiment)
FIG. 8 is a diagram showing a configuration of a spectrometer according to the fourth embodiment of the present invention.

  In this embodiment, a thermistor thin film provided on the inner surface of the rectangular waveguide 2 is used instead of the electromagnetic wave detection antenna used in the first and second embodiments.

  As shown in FIG. 8, the thermistor thin film 9 is provided on both side surfaces or one side surface of the inner surface of the rectangular waveguide 2. An insulator layer is provided between the rectangular waveguide 2 and the thermistor thin film 9 so as to be electrically and thermally insulated.

  The electromagnetic wave introduced from the opening 3 of the rectangular waveguide 2 has a higher electric field strength at the position where the cutoff frequency of the waveguide and the frequency of the input electromagnetic wave are equal, as shown in FIG. The wall current density flowing through the inner surface of the rectangular waveguide 2 is proportionally increased.

  Of the wall current, the current flowing on the thermistor surface generates heat due to the electrical resistance of the thermistor thin film 9, and the temperature of the thermistor thin film 9 rises.

  The resistance of the thermistor thin film 9 can be measured through the lead wire 10 from the thermistor thin film 9, and the temperature can be measured from the temperature coefficient of the thermistor thin film 9. The wall current density of the electromagnetic wave can be measured from the temperature rise of the thermistor thin film 9.

  Thus, from the temperature rise of the plurality of thermistor thin films 9, the spectrum of the electromagnetic wave can be determined using the above-described equation (8).

(Other)
The method for measuring the electromagnetic wave in the rectangular waveguide may be realized by any method other than those described above. Examples of a method for measuring the power Q output from the antenna include a crystal detector, a heterodyne receiver, and a thermistor. Of course, various changes can be made without departing from the scope of the present invention.

  Moreover, although the cross-sectional shape of the waveguide has been described as a rectangle (rectangle), it is needless to say that various changes can be made without departing from the scope of the present invention.

It is a figure which shows the structure of the spectrometer which concerns on 1st Embodiment. It is a figure which shows the dispersion characteristic of a taper waveguide. It is a figure which shows the distribution of the axial direction of the cutoff wavelength in a taper waveguide, cutoff frequency, group velocity, standing wave electric field, and average electric field strength. It is a figure which shows distribution of the axial direction of the electric field strength with respect to four different frequencies. It is a figure which shows the change of the electric power measured in each antenna when sweeping an input frequency. It is a figure which shows the structure of the spectrometer which concerns on 2nd Embodiment (step waveguide). It is a figure which shows the structure of the spectrometer which concerns on 3rd Embodiment (rectangular waveguide antenna). It is a figure which shows the structure of the spectrometer which concerns on 4th Embodiment (thermistor use).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Input electromagnetic wave 2 Rectangular waveguide 3 Opening part 4 Waveguide upper surface 5a, 5b, 5c, 5d Antenna 6 Pickup part 7 Coaxial waveguide 8 Non-reflection termination part 9 Thermistor thin film 10 Thermistor lead wire

Claims (6)

A waveguide whose cutoff frequency gradually increases according to the distance from the opening end face;
A plurality of measuring units for measuring the intensity of electromagnetic waves provided on the inner surface of the waveguide in the axial direction of the waveguide;
Have
A spectrometer that measures the intensity of an electromagnetic wave having a cutoff frequency corresponding to a position where each measurement unit is provided by the plurality of measurement units.   The spectrometer according to claim 1, wherein the waveguide has a cross-sectional dimension that continuously decreases in accordance with a distance from the opening end face.   The measurement unit performs correction to remove an influence of an electromagnetic wave having a cutoff frequency corresponding to a position where the other measurement unit is provided based on the intensity of the electromagnetic wave measured by the other measurement unit. Item 3. The spectrometer according to Item 1 or 2. The measuring unit is
An antenna,
A frequency converter that converts the electromagnetic wave acquired from the antenna into a low frequency;
The spectrometer according to claim 1, comprising: The measuring unit is
An antenna,
A thermistor that absorbs electromagnetic waves acquired from the antenna;
A temperature measurement unit for measuring the temperature of the thermistor;
Consisting of
The spectrometer according to claim 1, wherein the intensity of the electromagnetic wave is measured by a temperature change measured from the temperature measurement unit. The measuring unit is
The thermistor thin film,
A resistance measurement unit for measuring the resistance of the thermistor thin film;
Consisting of
The spectrometer according to claim 1, wherein a temperature change of the thermistor is detected by a resistance measured from the resistance measurement unit, and an intensity of the electromagnetic wave is measured by a temperature change of the thermistor.

Images