Dielectric spectroscopic sensor and method for measuring reflection coefficient

Abstract

A dielectric spectroscopic sensor comprises: a printed circuit board (2); a first integrated probe (3); a second integrated probe (4); and a high-frequency switch (5). The first integrated probe (3), the second integrated probe (4), and the high-frequency switch (5) are provided on one main surface (2a) of the printed circuit board (2). The first integrated probe (3) includes a first transmission line (6) and a first opening end portion (7) that is in communication with the first transmission line (6) and in contact with a sample serving as a calibration standard. An end of the first transmission line (6) on the side opposite to the first opening end portion (7) is connected to the high-frequency switch (5). The second integrated probe (4) includes a second transmission line (8) and a second opening end portion (9) that is in communication with the second transmission line (8) and that is an open end. An end of the second transmission line (8) on the side opposite to the second opening end portion (9) is connected to the high-frequency switch (5).

Description

Dielectric spectroscopy sensor, method for measuring reflection coefficient

[0001] The present invention relates to a dielectric spectroscopy sensor and a method for measuring a reflection coefficient.

[0002] As the aging population advances, responding to adult diseases has become a major issue. Tests for blood glucose levels, etc., require blood sampling, which places a heavy burden on patients. For this reason, non-invasive element concentration measuring devices that do not require blood sampling are attracting attention.

[0003] Known non-invasive constituent concentration measurement devices use electromagnetic waves in the microwave to millimeter wave band because they scatter less in vivo than optical methods such as near-infrared light, and because the energy of one photon is low. Examples of non-invasive constituent concentration measurement devices include those using a resonant structure, as described in Non-Patent Document 1. In this device, a measurement sample is brought into contact with a device with a high Q factor, such as an antenna or resonator, and frequency characteristics around the resonant frequency are measured. Because the resonant frequency is determined by the complex dielectric constant of the device's surroundings, the constituent concentration can be estimated from the resonant frequency shift by predicting the correlation between the resonant frequency shift and the constituent concentration.

[0004] Another known method using microwave-millimeter wave electromagnetic waves is dielectric spectroscopy, as described in Patent Document 1. In dielectric spectroscopy, electromagnetic waves are irradiated into the skin, absorbed by the blood component being measured, e.g., glucose molecules, in accordance with the interaction between the molecules and water, and the amplitude and phase of the electromagnetic waves are observed. The dielectric relaxation spectrum is calculated from the amplitude and phase of the observed electromagnetic waves at different frequencies. The dielectric relaxation spectrum is generally expressed as a linear combination of relaxation curves based on the Cole-Cole equation, and the complex dielectric constant is calculated. In measuring biological components, for example, the amount of blood components, such as glucose and cholesterol, contained in the blood is correlated with the complex dielectric constant, and the complex dielectric constant is measured as an electrical signal (amplitude, phase) corresponding to this change. A calibration model is constructed by measuring the correlation between changes in complex dielectric constant and component concentration in advance, and the component concentration is calibrated from changes in the measured dielectric relaxation spectrum. Regardless of which method is used, improved measurement sensitivity can be expected by selecting a frequency band that is strongly correlated with the target component. Therefore, it is important to measure the change in dielectric constant in advance using broadband dielectric spectroscopy.

[0005] Among dielectric spectroscopy methods, methods using a coaxial probe (open-ended coaxial probe or open-ended coaxial line) as shown in Non-Patent Documents 2 and 3 and Patent Document 2 allow for the use of readily available samples such as water to calibrate the measuring instrument, and also allow the dielectric constant of a sample to be measured by contacting the sample to the probe end without requiring special processing of the material. Therefore, this method is suitable for measuring the dielectric constant of samples whose electrical properties need to be evaluated without processing, such as living organisms, fruits, soil, and hydrogels. To measure these dielectric constants, a reflected signal measuring instrument as shown in Non-Patent Document 1 or a reflected signal measuring method using a general-purpose measuring instrument such as a portable vector network analyzer is used.

[0006] JP 2013-32933 A Japanese Patent No. 6771372 A

[0007] M. Hofmann, G. Fischer, R. Weigel, and D. Kissinger, “Microwave-Based Noninvasive Concentration Measurements for Biomedical Applications”, IEEE Trans. Microwave Theory and Techniques, Vol.61, No.5, pp. 2195-2203,2013J P. Grant, R N. Clarke, G T. SYymm and N M. Spyrou, “A critical study of the open-ended coaxial line sensor technique for RF and microwave complex permittivity measurements”, J. Phys. E: Sci. Instrum, Vol. 22, pp. 757-770, 1989T.P. Marsland, and S. Evans “Dielectric measurements with an open-ended coaxial probe”, IEE Proceedings, Vol. 134, No. 4, 1987

[0008] However, because conventional dielectric spectroscopy sensors have three calibration standards, it is assumed that errors in individual sensing data will propagate to other sensing data, which poses the problem of reduced measurement stability in applications where errors in individual sensor characteristics are likely to occur due to surrounding conditions, such as wearable sensors and environmental sensors.

[0009] The present invention has been made in consideration of the above circumstances, and aims to provide a dielectric spectroscopy sensor that can be applied to applications where errors in individual sensor characteristics are likely to occur due to surrounding conditions, such as wearable sensors and environmental sensors, and that has excellent measurement stability, as well as a method for measuring a reflection coefficient using a dielectric spectroscopy sensor.

[0010] One aspect of the present invention is a dielectric spectroscopy sensor comprising a printed circuit board, a first integrated probe, a second integrated probe, and a high-frequency switch, the first integrated probe, the second integrated probe, and the high-frequency switch being provided on one main surface of the printed circuit board, the first integrated probe having a first transmission line and a first open end communicating with the first transmission line and in contact with a sample that serves as a calibration standard, the end of the first transmission line opposite the first open end being connected to the high-frequency switch, and the second integrated probe having a second transmission line and a second open end communicating with the second transmission line that is an open end, the end of the second transmission line opposite the second open end being connected to the high-frequency switch.

[0011] One aspect of the present invention is a method for measuring a reflection coefficient using a dielectric spectroscopy sensor of one aspect of the present invention, the method comprising the steps of: measuring, at the first open end of the first integrated probe, the dielectric constant of a sample with a known dielectric constant that is in contact with the first open end of the first integrated probe; bringing the second open end of the second integrated probe into contact with the sample with a known dielectric constant and measuring the dielectric constant; calculating a difference (characteristic difference Δtl) between the characteristics due to the length of the first transmission line of the first integrated probe and the characteristics due to the length of the second transmission line of the second integrated probe according to the following equation (a): measuring, using the first integrated probe, the reflection coefficient of the sample with a known dielectric constant that is in contact with the first open end; measuring, using the second integrated probe, the reflection coefficient of the sample to be measured that is in contact with the second open end; and correcting the reflection coefficient of the sample to be measured based on the characteristic difference Δtl:

[0012]

[0013] According to the present invention, it is possible to provide a dielectric spectroscopy sensor with excellent measurement stability that can be applied to applications such as wearable sensors and environmental sensors where errors in individual sensor characteristics are likely to occur depending on the surrounding conditions, and a method for measuring a reflection coefficient using a dielectric spectroscopy sensor.

[0014] It is a schematic diagram showing the basic configuration of a dielectric spectroscopy sensor according to one embodiment of the present invention. It is a flowchart of a method for measuring a reflection coefficient according to one embodiment of the present invention. It is a schematic diagram showing a dielectric spectroscopy sensor used in a conventional method for measuring a reflection coefficient. It is a diagram showing the results of measuring the reflection coefficient of air continuously for two hours using the dielectric spectroscopy sensor of the example or the dielectric spectroscopy sensor of the comparative example.

[0015] [Dielectric Spectroscopy Sensor] Fig. 1 is a schematic diagram showing the basic configuration of a dielectric spectroscopic sensor according to one embodiment of the present invention. As shown in Fig. 1, the dielectric spectroscopic sensor 1 of this embodiment includes a printed circuit board 2, a first integrated probe 3, a second integrated probe 4, and an RF (radio frequency) switch 5.

[0016] The first integrated probe 3 and the second integrated probe 4 are formed on one main surface 2a of the printed circuit board 2. The RF switch 5 is mounted on one main surface 2a of the printed circuit board 2.

[0017] The first integrated probe 3 has a first transmission line 6 and a first open end 7 communicating with the first transmission line 6. The end of the first transmission line 6 opposite to the first open end 7 is connected to the RF switch 5.

[0018] The first open end 7 is in contact with a sample that serves as a calibration standard. The first integrated probe 3 is used as a standard for sequential calibration. Examples of the sample that serves as a calibration standard that comes into contact with the first open end 7 include a dielectric material that is a substrate material, air, and a metal.

[0019] The first transmission line 6 may be a microstrip line, a coplanar line, a coplanar strip, or the like.

[0020] The first open end 7 is made of a material that mimics the dielectric constant of the object to be measured, such as a hydrogel material such as agar or a polymer, or a metal material such as gold, copper, or aluminum that shorts the opening to achieve an ideal reflection state.

[0021] The second integrated probe 4 has a second transmission line 8 and a second open end 9 communicating with the second transmission line 8. The end of the second transmission line 8 opposite to the second open end 9 is connected to the RF switch 5.

[0022] The second open end 9 is an open end having a central portion 9A disposed so as to penetrate the printed circuit board 2 in the thickness direction, and an annular portion 9B disposed concentrically about the central portion 9A and also penetrating the printed circuit board 2 in the thickness direction. The reflection characteristics of the end face of the second open end 9 change when it comes into contact with a sample under test (DUT).

[0023] The second transmission line 8 may be a microstrip line, a coplanar line, a coplanar strip, or the like.

[0024] The second open end 9 is composed of a central portion 9A, an annular portion 9B, and a dielectric filled between the conductors. The conductors are made of metals such as copper or gold, and the dielectrics are made of glass epoxy, Teflon (registered trademark), alumina, silicon, etc.

[0025] The inner diameter of the first open end 7 is preferably equal to the inner diameter of the second open end 9. The outer diameter of the first open end 7 is preferably equal to the outer diameter of the second open end 9.

[0026] Examples of the printed circuit board 2 include a printed circuit board using glass epoxy or paper phenol as a substrate, a printed circuit board using polytetrafluoroethylene as a substrate material, and a semiconductor substrate.

[0027] The RF switch 5 may be a single-pole double-throw or single-pole four-throw switch using a diode or a CMOS transistor.

[0028] The dielectric spectroscopy sensor 1 of this embodiment can be applied to applications where errors in individual sensor characteristics are likely to occur depending on the surrounding conditions, such as wearable sensors and environmental sensors, and can provide a dielectric spectroscopy sensor with excellent measurement stability.

[0029] [Method for measuring reflection coefficient] A method for measuring a reflection coefficient according to one embodiment of the present invention is a method for measuring a reflection coefficient using the dielectric spectroscopy sensor of the above-described embodiment, and includes the steps of: measuring, at the first opening end of the first integrated probe, the dielectric constant of a sample with a known dielectric constant that is in contact with the first opening end; bringing the second opening end of the second integrated probe into contact with the sample with a known dielectric constant and measuring the dielectric constant; calculating, according to the following formula (a), a difference (characteristic difference Δtl) between a characteristic due to the length of the first transmission line of the first integrated probe and a characteristic due to the length of the second transmission line of the second integrated probe; measuring, using the first integrated probe, the reflection coefficient of the sample with a known dielectric constant that is in contact with the first opening end; measuring, using the second integrated probe, the reflection coefficient of the sample to be measured that is in contact with the second opening end; and correcting, based on the characteristic difference Δtl, the reflection coefficient of the sample to be measured.

[0030] 2 is a flowchart of a method for measuring a reflection coefficient according to an embodiment of the present invention. Here, a method for measuring a reflection coefficient using the dielectric spectroscopy sensor 1 according to the embodiment described above will be described.

[0031] In the dielectric spectroscopy sensor 1, the reflection coefficient Γ returning to the input of the RF switch 5 ｍ can be expressed by the following formula (1).

[0032]

[0033] In the above formula (1), T is the pass characteristic of the RF switch 5, tl ｎ is the characteristic impedance of a finite length transmission line, Γ ａ is the reflection coefficient at the end face of the second open end 9 of the coaxial probe (second integrated probe 4). ｎ and Γ ａtakes into account the round-trip propagation of the electromagnetic wave due to incidence and reflection on the RF switch 5. In this case, if there is a difference in characteristics between the lengths of the first transmission line 6 of the first integrated probe 3 and the second transmission line 8 of the second integrated probe 4, the characteristics can be expressed by the following equations (2) and (3).

[0034]

[0035]

[0036] In the above formula (2), tl １ is the impedance of the first transmission line 6, and A １ is an arbitrary coefficient for expressing the transmission characteristics of the first transmission line 6, and jβl1 is the propagation constant of the first transmission line 6. In the above equation (3), tl ２ is the impedance of the second transmission line 8, and A ２ is an arbitrary coefficient for expressing the transmission characteristics of the second transmission line 8, and jβI2 is the propagation constant of the second transmission line 8.

[0037] Prior to actual measurement, calibration is performed to determine the difference (characteristic difference) between the characteristics due to the length of the first transmission line 6 of the first integrated probe 3 and the characteristics due to the length of the second transmission line 8 of the second integrated probe 4. First, the permittivity of a sample with a known permittivity that is in contact with the first open end 7 of the first integrated probe 3 is measured (S1). Next, the second open end 9 of the second integrated probe 4 is brought into contact with the sample with a known permittivity, and the permittivity is measured (S2). Next, the difference (characteristic difference Δtl) between the characteristics due to the length of the first transmission line 6 of the first integrated probe 3 and the characteristics due to the length of the second transmission line 8 of the second integrated probe 4 is calculated (S3) according to the following equation (4):

[0038]

[0039] In the above equation (4), Γ ｍ１ is the actual measurement value of the reflection coefficient by the first integrated probe 3, Γ ｍ２ is the actual measurement value of the reflection coefficient by the second integrated probe 4, Γ ｓｔｄ is the reflection coefficient in the standard configuration, Γｃａｌ２ indicates the reflection coefficient of a known sample when measured.

[0040] By using Δtl, the reflection coefficient taking into account the above characteristic difference Δtl can be expressed by the following equation (5).

[0041]

[0042] This makes it possible to calculate a reflection coefficient that reduces the influence of the difference (characteristic difference Δtl) between the characteristics due to the length of the first transmission line 6 of the first integrated probe 3 and the characteristics due to the length of the second transmission line 8 of the second integrated probe 4.

[0043] In this embodiment, the first integrated probe 3 measures the reflection coefficient of a sample with a known dielectric constant that is in contact with the first open end 7 (S4). Furthermore, the second integrated probe 4 measures the reflection coefficient of a sample to be measured that is in contact with the second open end 9 (S5).

[0044] Next, the reflection coefficient of the sample under test is corrected based on the characteristic difference Δtl calculated in S3 (S6).

[0045] The reflection coefficient measurement method of this embodiment can be applied to applications where errors in individual sensor characteristics are likely to occur depending on the surrounding conditions, such as wearable sensors and environmental sensors, and has excellent measurement stability.

[0046] The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0047] Example A dielectric spectroscopic sensor 1 shown in FIG. 1 was fabricated.

[0048] Comparative Example A dielectric spectroscopic sensor was produced as shown in Fig. 3. Fig. 3 is a schematic diagram showing a dielectric spectroscopic sensor used in a conventional method for measuring a reflection coefficient.

[0049] As shown in Figure 3, the dielectric spectroscopy sensor 100 has a dielectric substrate 101, a switch 102, a first antenna conversion section 103, a second antenna conversion section 104, an open-end conversion section 105, a short-circuit conversion section 106, an RF input terminal 107, and a load resistor 108.

[0050] The switch 102 is mounted on one main surface 101a of the dielectric substrate 101. The first antenna conversion section 103 is formed on the one main surface 101a of the dielectric substrate 101 and connected to the switch 102 via wiring 111. The second antenna conversion section 104 is formed on the one main surface 101a of the dielectric substrate 101 and connected to the switch 102 via a switch output terminal 109 and wiring 112. The open-end conversion section 105 is formed on the one main surface 101a of the dielectric substrate 101 and connected to the switch 102 via wiring 113. The short-circuit conversion section 106 is formed on the one main surface 101a of the dielectric substrate 101 and connected to the switch 102 via wiring 114. The RF input terminal 107 is connected to the switch 102 via a switch input terminal 110 and wiring 115. The load resistor 108 is connected to the switch 102 via wiring 116.

[0051] [Evaluation] Comparing the dielectric spectroscopy sensor of the Example with the dielectric spectroscopy sensor of the Comparative Example, the dielectric spectroscopy sensor of the Example uses fewer measurement values ​​for sequential calibration than the dielectric spectroscopy sensor of the Comparative Example. That is, the dielectric spectroscopy sensor of the Example uses two measurement values ​​for calibration: the reflection coefficient by the first opening end 7 and the reflection coefficient by the second opening end 9. In contrast, the dielectric spectroscopy sensor of the Comparative Example uses four measurement values: the measurement value by the first antenna conversion unit 103, the measurement value by the second antenna conversion unit 104, the measurement value by the open-end conversion unit 105, and the measurement value by the short-circuit conversion unit 106. Therefore, the dielectric spectroscopy sensor of the Example can be expected to perform measurements with less error propagation.

[0052] Furthermore, when measuring time-series data using the dielectric spectroscopy sensor of the embodiment, it is conceivable that the difference in the characteristics of the transmission line may change over time due to changes in the ambient temperature, etc. In such a case, the change in the characteristics of the first transmission line 6 of the first integrated probe 3 may first be calculated using the following equation (6), and the reflection coefficient of the sample to be measured may be calculated using the amount of change as a correction term using the following equation (7).

[0053]

[0054]

[0055] This suppresses the effects of signal fluctuations originating from the transmission line that occur over time, and is expected to improve stability after a certain period of time has passed.

[0056] The results of measuring the reflection coefficient of air continuously for two hours using the dielectric spectroscopic sensor of the example and the dielectric spectroscopic sensor of the comparative example are shown in Figure 4. It was found that when the dielectric spectroscopic sensor of the example was used, the signal stability was improved after 60 minutes had passed compared to when the dielectric spectroscopic sensor of the comparative example was used.

[0057] The dielectric spectroscopic sensor and the method for measuring the reflection coefficient of the present invention can be applied to a dielectric spectroscopic sensor for a solution present in a human or an animal, and to a dielectric spectroscopic sensor for a solution or semi-solid material collected from a human or an animal.

[0058] REFERENCE SIGNS LIST 1 Dielectric spectroscopy sensor 2 Printed circuit board 3 First integrated probe 4 Second integrated probe 5 Radio frequency (RF) switch 6 First transmission line 7 First open end 8 Second transmission line 9 Second open end

Claims

1. A dielectric spectroscopy sensor comprising a printed circuit board, a first integrated probe, a second integrated probe, and a high-frequency switch, the first integrated probe, the second integrated probe, and the high-frequency switch being provided on one main surface of the printed circuit board, the first integrated probe having a first transmission line and a first open end communicating with the first transmission line and in contact with a sample that serves as a calibration standard, the end of the first transmission line opposite the first open end being connected to the high-frequency switch, and the second integrated probe having a second transmission line and a second open end communicating with the second transmission line that is an open end, the end of the second transmission line opposite the second open end being connected to the high-frequency switch.

2. The dielectric spectroscopy sensor according to claim 1, wherein the calibration standard sample is a dielectric, air, or metal.

3. A method for measuring a reflection coefficient using a dielectric spectroscopy sensor as defined in claim 1 or 2, comprising the steps of: measuring, at the first opening end of the first integrated probe, the dielectric constant of a sample with a known dielectric constant that is in contact with the first opening end; bringing the second opening end of the second integrated probe into contact with the sample with a known dielectric constant and measuring the dielectric constant; calculating the difference (characteristic difference Δtl) between the characteristics due to the length of the first transmission line of the first integrated probe and the characteristics due to the length of the second transmission line of the second integrated probe according to the following formula (a): measuring, using the first integrated probe, the reflection coefficient of the sample with a known dielectric constant that is in contact with the first opening end; measuring, using the second integrated probe, the reflection coefficient of the sample to be measured that is in contact with the second opening end; and correcting the reflection coefficient of the sample to be measured based on the characteristic difference Δtl.

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