Capacitive refractive index meter based on variable capacitor

Abstract

The invention provides a capacitive refractometer based on a variable capacitor, and relates to the technical field of atmospheric waveguide parameter detection, and the capacitive refractometer comprises a capacitance detection circuit, a frequency mixing module, a filter circuit and a data processing module which are connected in sequence; wherein the capacitance sensor adopting the variable capacitance is arranged in the capacitance detection circuit; the capacitance value of the capacitance sensor is associated with the entering gas; wherein a detection electric signal influenced by the capacitance value of the variable capacitor is detected and obtained through the capacitance detection circuit, the detection electric signal is subjected to frequency mixing through the frequency mixing module, and the electric signal subjected to frequency mixing is filtered through the filter circuit to generate a stable oscillation signal; the data processing module is also connected with a temperature compensation module; detecting and acquiring a temperature electric signal through a temperature compensation module; the stable oscillation signal and the temperature electric signal are subjected to digital-to-analog conversion and data processing through the data processing module, the refractive index is obtained, and the device has the advantages of being low in cost, high in precision and high in anti-interference capacity.

Description

Technical Field

[0001] This invention relates to the field of atmospheric waveguide parameter detection technology, and more specifically to the field of atmospheric refractive index measurement design, particularly to a capacitive refractive index meter based on variable capacitance. Background Technology

[0002] Atmospheric refractive index is a key physical parameter, slightly greater than 1, describing the degree to which electromagnetic waves are slowed down relative to a vacuum in the atmosphere. It varies with temperature, air pressure, humidity, altitude, and location. This seemingly insignificant physical quantity plays a crucial role in numerous scientific and engineering fields because it directly affects the propagation path and characteristics of electromagnetic waves (light, radio waves, microwaves, radar waves, etc.) in the atmosphere, such as beyond-line-of-sight propagation. Under specific atmospheric conditions (such as the presence of a strong temperature inversion layer or humidity gradient), the gradient of refractive index with altitude is large enough that radio waves or radar waves bend downwards beyond the curvature of the Earth. This allows signals to propagate far beyond normal line-of-sight distances, i.e., "beyond-line-of-sight propagation." This is critical for radar detection range, the design of radio communication links, and interference prediction. Currently, the main devices used to measure atmospheric refractive index are microwave refractometers and Type 59 radiosondes. However, microwave refractometers are inconvenient to install when measuring the refractive index of the upper atmosphere; while the Type 59 radiosonde has advantages in stability, especially in the smoothness of humidity data, and is still cited in some studies. However, it has shortcomings in high-altitude accuracy, response speed, and data consistency. Therefore, accurate measurement, modeling, and prediction of atmospheric refractive index and its changes are crucial for scientific research, engineering applications, national defense, and daily life. It serves as a bridge parameter connecting electromagnetic wave propagation physics with atmospheric science, space information technology, communication engineering, and other fields. Summary of the Invention

[0003] In view of this, the present invention proposes a capacitive refractive index meter based on variable capacitance to solve the problems existing in the prior art.

[0004] To achieve the above objectives, this invention proposes a capacitive refractive index meter based on a variable capacitor, comprising: The circuit consists of a capacitance detection circuit, a mixing module, a filtering circuit, and a data processing module connected in sequence; a capacitance sensor with a variable capacitor is installed in the capacitance detection circuit; the capacitance value of the capacitance sensor is related to the incoming gas. The detection circuit detects the electrical signal affected by the capacitance value of the variable capacitor, the mixing module mixes the electrical signal, and the filtering circuit filters the mixed electrical signal to generate a stable oscillation signal. The data processing module is also connected to a temperature compensation module; the temperature compensation module detects and acquires the temperature electrical signal; the data processing module performs digital-to-analog conversion and data processing on the stable oscillation signal and the temperature electrical signal to obtain the corrected refractive index.

[0005] Optionally, the capacitive sensor employs a variable capacitor structure combining a cylindrical capacitor and a variable electrode spacing; the capacitive sensor has alternating positive and negative electrodes, wherein the positive electrodes are all connected by connecting wires to form the positive terminal of the capacitive sensor, and the negative electrodes are all connected by another connecting wire to form the negative terminal of the capacitive sensor. The contact area and spacing between the positive and negative electrodes are adjusted to adjust the initial capacitance value; when the gas enters the gap between the positive and negative electrodes, the capacitive sensor outputs an electrical signal corresponding to the gas.

[0006] Optionally, a copper sheet is provided on the outside of the capacitive sensor, and an opening is provided on the copper sheet at the position corresponding to the capacitive sensor, wherein a grid-like copper sheet is provided at the opening.

[0007] Optionally, the capacitance detection circuit uses a capacitor feedback three-segment oscillator.

[0008] Optionally, the filter circuit adopts a fifth-order LC resonant circuit, wherein the filter circuit includes five reactive elements connected in series, and each reactive element is connected in parallel with a corresponding loop capacitor, and the loop capacitors are all connected in parallel.

[0009] Optionally, an operational amplifier is also connected between the filtering circuit and the data processing module, wherein the operational amplifier is a decoupled single-supply operational amplifier bias circuit.

[0010] Optionally, the capacitive sensor, mixer module, and filter circuit are all mounted on the PCB board.

[0011] Optionally, the temperature compensation module employs a temperature compensation circuit based on an LM35DT sensor.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention calculates the atmospheric refractive index by converting atmospheric nodal characteristics into changes in capacitance. This method is cost-effective, has a simple circuit, and incorporates a temperature compensation circuit, improving stability and robustness. Existing instruments for measuring atmospheric refractive index include microwave refractometers and the Type 59 radiosonde. Microwave refractometers offer fast response and high accuracy, but their high cost and large size limit their application. The Type 59 radiosonde also uses sensors to directly measure corresponding atmospheric parameters and calculate the refractive index, but there are significant differences in time and space between ground decoding, calculation, and radiosonde detection in the waveguide, resulting in substantial delay errors in the equipment. Attached Figure Description

[0013] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. In the drawings: Figure 1 This is a circuit schematic diagram from an embodiment of the present invention; Figure 2 This is a front view of the variable capacitor in an embodiment of the present invention; Figure 3 This is a top view of the variable capacitor in an embodiment of the present invention; Figure 4 This is a bottom view of the PCB board in an embodiment of the present invention; Figure 5 This is a schematic diagram of the temperature compensation circuit in an embodiment of the present invention; Figure 6 This is a physical image of an embodiment of the present invention. Detailed Implementation

[0014] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the disclosure to those skilled in the art. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0015] This invention discloses a capacitive refractometer, comprising a capacitive sensor, a capacitance detection circuit, a power supply, a temperature compensation module, a filter circuit, an operational amplifier, a mixer module, and a data processing module. The capacitive sensor employs a combination of a cylindrical capacitor and a variable-gap capacitor sensor. The relative area of ​​the capacitor can be changed by rotating a knob, thereby altering the capacitance value. The capacitance can be adjusted to achieve the optimal value for different applications. When gas enters the variable capacitor, the resonant frequency of the resonator changes with the gas's dielectric constant. Since air temperature, humidity, and air pressure all affect the gas's dielectric constant, and the instrument is also susceptible to interference from environmental factors such as magnetic fields and noise, this can cause significant errors in the measurement results. To obtain an accurate atmospheric refractive index value, the instrument further processes the signal carried by the gas after it enters the measuring device. The signal affected by the environment is filtered by a filtering circuit to generate a stable oscillation signal, which is then sent to a digital-to-analog waveform conversion module. The converted data is collected and processed to finally obtain an accurate atmospheric refractive index value. Compared with microwave atmospheric refractive index measuring instruments and the "59" type radiosonde, this invention has the characteristics of simple design, low cost, high accuracy, and strong anti-interference ability.

[0016] This invention addresses the issue that atmospheric waveguides exist at high altitudes, where large diurnal temperature variations affect the dielectric constant of nearby air, thus impacting accuracy. To compensate for the temperature effect on the circuit, a temperature compensation circuit is incorporated. When air enters the capacitive sensor, the signal carrying atmospheric parameters is filtered to generate a stable oscillation signal. When the atmospheric dielectric constant changes, the capacitance C of the air capacitor also changes. A 5th-order LC oscillation circuit can measure the change in resonant frequency when the composition and structure of the gas change. Finally, the atmospheric refractive index value is obtained through mode-to-wave conversion, data acquisition, and data processing.

[0017] The capacitive refractometer of this invention is used to measure atmospheric refractive index parameters. Existing atmospheric refractive index measuring instruments mainly include microwave refractometers and the "Type 59 radiosonde". The cost of the capacitive atmospheric refractive index measurement instrument of this invention is much lower than that of microwave refractometers, and the accuracy is higher than that of the "Type 59 radiosonde". It also has the advantages of small size, light weight and high cost performance. In some engineering measurements with strict requirements for the measurement environment, the capacitive refractometer can replace microwave refractometers and the "Type 59 radiosonde".

[0018] The above content will be described in detail: The technical problem to be solved by this invention is to overcome the existing defects and to invent a capacitive refractometer based on a capacitive sensor for measuring atmospheric refractive index parameters. Existing atmospheric refractive index measuring instruments mainly include microwave refractometers and the "Type 59 radiosonde". The cost of the capacitive atmospheric refractive index measurement instrument of this invention is much lower than that of microwave refractometers, and the accuracy is higher than that of the "Type 59 radiosonde". It also has the advantages of small size, light weight and high cost performance. In some engineering measurements with strict requirements for the measurement environment, the capacitive refractometer can replace microwave refractometers and the "Type 59 radiosonde".

[0019] This invention provides a capacitive refractometer based on variable capacitance. The capacitive refractometer includes a variable capacitor mounted on a PCB board. The capacitive sensor based on the variable capacitance employs a structure combining a cylindrical capacitor and a variable-gap capacitor. Two semi-circular aluminum plates are sequentially and staggeredly fixed to copper pillars. The capacitive sensor based on the variable capacitance is fixed to a ceramic plate via the copper pillars. The entire structure is fixed to a via-hole PCB board using rotatable screws. The contact area and spacing between the variable capacitors can be changed, thereby altering the capacitance value. By rotating the rotatable screws, the contact area between the capacitor plates can be changed. Furthermore, when two capacitor plates completely overlap, the spacing between the capacitor plates is halved, changing the capacitance value.

[0020] Furthermore, a copper strip is added to the outside of the variable capacitor-based capacitive sensor. The copper strip is soldered to the back of the PCB board, and a rectangular opening is cut out on the copper strip at the height of the variable capacitor-based capacitive sensor. The opening is welded into a grid shape with copper strips, which can better prevent external interference while ensuring smooth air intake and exhaust.

[0021] Furthermore, a variable capacitor-based capacitance sensor is set in the capacitance detection circuit. The capacitance detection circuit, i.e. the first stage circuit, is a capacitor feedback three-terminal oscillator. The capacitor feedback three-terminal oscillator adopts an LC sine wave oscillation circuit. The output waveform of the capacitor feedback three-terminal oscillator is more standard because the collector and base currents can return to the emitter through the capacitor branch with low impedance to harmonics. Therefore, the feedback of higher harmonics is weakened, the harmonic components of the output are reduced, and the waveform is closer to a sine wave. Secondly, since the unstable capacitors in this circuit are all connected in parallel, appropriately increasing the loop capacitance can reduce the impact of unstable factors on the oscillation frequency, thereby improving frequency stability. Finally, when the operating frequency is high, it is even possible to use only the input and output capacitors of the device as loop capacitors; in the capacitor feedback three-terminal oscillator, the oscillation frequency can be changed by adjusting the capacitor.

[0022] When the oscillation frequency is changed by adjusting capacitor C1 or C2, the feedback coefficient of the capacitor feedback three-segment oscillator will also change. By connecting the variable capacitor in parallel across the inductor L1 and fixing the capacitance values ​​of capacitors C1 and C2, the resonant frequency can be adjusted without affecting the feedback coefficient. The capacitors C1 and C2 are 330pF and 680pF, respectively, and the inductor L1 is 4.7μH.

[0023] Furthermore, the capacitance detection circuit and the crystal oscillator are connected to the mixer. The output of the mixer is connected to an amplifier, and the output of the amplifier is connected to a filter circuit. The filter circuit adopts a 5th-order LC resonant circuit, which only requires 5 independent reactive components. At the same time, the filter circuit constructed with the LC loop can also be used as a resonant circuit. As the capacitance value changes, the output frequency of the circuit will also change accordingly. In this invention, the 5th-order LC resonant circuit is used as the core circuit of the measurement system.

[0024] Furthermore, the PCB board uses FR-4 material with a thickness of 1.6mm, lead-free tin plating, and a copper plating thickness of 1OZ.

[0025] Furthermore, an operational amplifier is connected after the filter circuit. The operational amplifier section is a decoupled single-supply operational amplifier bias circuit, which further amplifies the frequency-selected signal and increases the output amplitude.

[0026] The operational amplifier is connected to a data processing module via a digital-to-analog waveform conversion module. The converted data is collected and processed to obtain an accurate atmospheric refractive index value. Furthermore, the temperature compensation circuit employs a temperature compensation circuit based on the LM35DT sensor. The LM35DT sensor exhibits a linear relationship between the ambient temperature and the output signal, achieving an accuracy of ±0.25℃. It features precise internal process calibration, very low output impedance, and ease of use. It can be powered by a single power supply or both positive and negative power supplies, draws only 60µA of current, and has very low self-heating, with a temperature increase of only 0.1℃ in still air. The LM35 outputs 0V at 0℃, and the output voltage increases by 10mV for every 1℃ increase in temperature.

[0027] The temperature compensation circuit is placed near the variable capacitor, ensuring that the circuit operates at the same temperature and humidity as the capacitor. The circuit utilizes an LM35DT-based sensor. Before the gas to be measured enters the variable capacitor, the sensor measures its temperature and humidity. Temperature compensation is then performed in the data processing module. This module acquires the temperature data from the sensor, processes it, and adjusts the final refractive index. The adjustment method includes calculating adjustment coefficients based on temperature data and refractive index correction values ​​using linear or nonlinear fitting. The corresponding refractive index correction data is then calculated using these adjustment coefficients and temperature data, and the refractive index is corrected accordingly. This process accounts for the influence of temperature, reducing the impact of temperature changes, improving measurement accuracy, and enhancing robustness.

[0028] In the data processing module, the basic calculations related to the refractive index are as follows: First, calculate the relative permittivity ε. r :

[0029] Where f0 is the natural oscillation frequency of the oscillator resonant circuit under vacuum, f represents the oscillation frequency of the oscillator resonant circuit after real-time measurement and filtering; L represents the corresponding inductance value, and C0 represents the initial capacitance value of the corresponding variable capacitor.

[0030] Further calculate the corresponding refractive index N based on the relative permittivity:

[0031]

[0032] Where n represents the refractive index of the medium, and N represents the atmospheric refractive index.

[0033] The derivation process for the above calculation is as follows: The refractive index n of air is defined as follows:

[0034] In the formula, V0 is the speed of light in a vacuum. ε is the phase velocity of an electromagnetic wave propagating in air. r μ is the relative permittivity or relative permittivity. r This represents the relative permeability or relative magnetic flux density. Furthermore, since when μ... r When the value changes, the change in the value of n is relatively small, not exceeding 0.5 × 10⁻⁶ within the troposphere. -6 In engineering applications, μ is taken. r =1, and for the refractive index of air, it can be simplified to:

[0035] In the study of radio wave transmission, N is generally referred to as the refractive index. Therefore, the refractive index n can be represented by the refractive index N. The definition of the atmospheric refractive index N is:

[0036] The relative permittivity of air can be expressed as:

[0037] In the formula, T is the absolute temperature of the air, P is the atmospheric pressure, and e is the absolute humidity of the air.

[0038] From the formula for the refractive index of air, it can be found that when the relative permeability is 1, the refractive index of air satisfies the following formula:

[0039] In the formula, C is the capacitance of the variable capacitor in the atmosphere, and C0 is the capacitance of the variable capacitor in a vacuum. It can be seen that by measuring the capacitance value, the atmospheric parameter measurement rate of the surrounding gas environment can be indirectly obtained. This is because when the atmospheric dielectric constant ε... r When changes occur, the capacitance C of the air capacitor will also be affected and change. Using the LC oscillation circuit in the capacitance lightweight tester circuit, it is possible to measure that when the composition and structure of the gas change, the oscillation frequency of the gas will also change.

[0040] The resonant angular frequency of an LC oscillator circuit under parallel resonance is:

[0041] In the formula, Q is the quality factor, L is the inductance value in the capacitor detection circuit, i.e., the oscillation circuit, and R represents the resistance value of the corresponding oscillation circuit.

[0042] Let f0 be the natural oscillation frequency of the resonant circuit of the oscillator under vacuum, then the following conditions are met:

[0043] The dielectric constant ε is derived from the following formula. r for:

[0044] This invention provides a capacitive atmospheric refractive index measuring instrument based on a variable capacitor, the measurement of which includes the following steps: The variable capacitor, circuitry, and other components are integrated onto an FR4 substrate. Cut copper sheets are soldered to the corresponding positions. Gas is blown into the capacitive sensor, carrying different information parameters. This gas enters the variable capacitor, changing the relative permittivity of the gas inside, thus altering the capacitance and causing a frequency change. By capturing, collecting, and processing these frequency changes, the permittivity and refractive index of the gas are finally calculated using formulas. The specific method is as follows: Since temperature can affect capacitor size and electronic device performance, and thus the output frequency of the circuit and the test results, it is necessary to reduce and compensate for the errors caused by temperature. Therefore, it is essential to add a temperature compensation circuit to the circuit.

[0045] The temperature compensation circuit uses a sensor based on the LM35DT. The sensor's output signal is linearly related to the ambient temperature, with an accuracy of ±0.25℃. The chip has precise internal process calibration, very low output impedance, and is easy to use. It can be powered by a single power supply or by both positive and negative power supplies, draws only 60µA of current, and has very low self-heating; its temperature rises by only 0.1℃ in still air. The LM35 outputs 0V at 0℃, and the output voltage increases by 10mV for every 1℃ increase in temperature.

[0046] Gas is fed into a variable capacitor. When the variable capacitor detects a change in the relative permittivity of the gas in the capacitor cavity, its capacitance changes, thus generating resonance. At this time, the signal is amplified by an LC oscillation circuit and sent to a mixer along with a stable 10MHz sine wave signal generated by a 10MHz crystal oscillator. The mixer mixes the signal from the oscillation module (frequency f1) and the 10MHz signal from the crystal oscillator (frequency f2), outputting a difference frequency |f1-f2|, thus converting the signal frequency to a more easily processed frequency band.

[0047] Since the amplitude of the signal output from the mixer is usually small, the processed signal needs to be amplified by an amplifier and then sent to a frequency selection network (filtering circuit). The frequency selection network (filtering circuit) is a bandpass filter based on a 5th-order LC circuit. It filters out unwanted frequency components such as sum frequencies, harmonics, and spurious signals by selecting the target frequency signal.

[0048] The above technical solution will be described in detail with reference to the accompanying drawings: like Figure 1 , Figure 6As shown, this invention provides a technical solution: a capacitive refractometer based on a variable capacitor. Since the capacitance change caused during atmospheric refractive index measurement is extremely small, and the excitation voltage provided by the circuit is limited, the structural design of the capacitive sensor is crucial. To minimize the impact of edge effects and improve the system's anti-interference capability, stability, sensitivity, and accuracy, while ensuring uninterrupted airflow, a structure combining a cylindrical capacitor and a variable-gap capacitor is adopted. The variable capacitor is model CW1-J50P / G100P. Further details are omitted here.

[0049] To address the issue of small capacitance variations, a larger capacitance value can be obtained by adjusting the spacing and contact area between the capacitors by rotating the screws. Setting the initial capacitance value requires using a variable capacitor to operate at the corresponding frequency; the adjustments can be made according to the specific situation and will not be elaborated here.

[0050] At the same time, such as Figure 2 As shown, the capacitor employs a structure with alternating positive and negative electrodes. The electrode plates are arranged alternately, with odd-numbered layers connected by connecting wires to form the positive electrode of the capacitive sensor; even-numbered layers are connected to form the negative electrode. Both the positive and negative electrodes are mounted on corresponding screws. When the screw rotates, the corresponding electrode rotates, changing its contact area. The electrode fixed to the screw can be adjusted according to the screw's position, thus allowing for adjustment of the distance between the positive and negative electrodes.

[0051] like Figure 2 As shown, gas enters through the hollowed-out copper ring. Gases with different parameters enter the capacitor, causing a change in the relative permittivity of the gas inside the capacitor, which in turn changes the capacitance value, thus generating frequency resonance as shown in the formula:

[0052] Where f represents the resonant frequency, L represents the inductance value, C0 represents the capacitance value, and ε r This represents the relative permittivity.

[0053] Because the instrument is affected by factors such as temperature, humidity, and electromagnetic interference, it generates noise. The signal is filtered by a circuit to produce a stable oscillation signal. Finally, the atmospheric refractive index value is obtained through analog-to-digital conversion, data acquisition, and data processing.

[0054] like Figure 3As shown, the variable capacitor-based capacitive sensor adopts a structure combining a cylindrical capacitor and a variable-gap capacitor. Two semi-circular aluminum plates are staggered sequentially, with one aluminum plate fixed to a copper pillar for fixing the variable capacitor above. A ceramic plate is placed below, and the variable capacitor-based capacitive sensor is fixed to the ceramic plate by the copper pillar. The entire structure is fixed to a via PCB board using rotatable screws. The contact area and spacing between the variable capacitors can be changed, thereby changing the capacitance value. By rotating the rotatable screws, the contact area between the capacitor plates can be changed. At the same time, when the two capacitor plates completely overlap, the spacing between the capacitor plates is halved, changing the capacitance value.

[0055] like Figure 4 As shown, a copper strip is added to the outside of the variable capacitor-based capacitive sensor. The copper strip is soldered to the back of the PCB board, and a rectangular opening is cut out on the copper strip at the height of the variable capacitor-based capacitive sensor. The opening is welded into a grid shape with copper strips, which can better avoid external interference while ensuring smooth air intake and exhaust.

[0056] The capacitive refractometer uses a fixed-inductance LC sinusoidal oscillation circuit. Based on the principle that the capacitance value of the capacitor changes with the refractive index of the gas, the resonant circuit method can efficiently measure the capacitance value of the sensor. Therefore, it is widely used in the precise measurement of the dielectric properties of materials.

[0057] The present invention uses an LC sinusoidal oscillation circuit as a circuit-type capacitance detection circuit, and a capacitance sensor as part of the oscillation circuit. By measuring the LC resonant frequency containing gas refractive index information, the purpose of measuring the gas refractive index in the capacitor is achieved.

[0058] like Figure 1 As shown, the capacitance detection circuit, i.e., the LC sine wave oscillation circuit, includes a transistor and a variable capacitor. The base of the transistor is connected to the power supply VCC through a first resistor. A third capacitor and a second resistor are connected in parallel between the base and the first resistor, with the portion of the third capacitor and the second resistor furthest from the base grounded. The emitter is connected to the power supply VCC through the third resistor and grounded through a fourth resistor. A fourth capacitor is connected in parallel between the base and the third resistor, with the other end of the fourth capacitor connected to a wire as the output terminal of the capacitance detection circuit. A first capacitor is connected in parallel between the fourth capacitor and the output terminal, with the other end of the first capacitor connected to the emitter. A second capacitor is connected in parallel between the first capacitor and the emitter, with the other end of the second capacitor grounded. A variable capacitor and a first inductor are also connected in series in parallel between the fourth capacitor and the output terminal, with the other end of the first inductor grounded.

[0059] The capacitance values ​​of the first capacitor and the second capacitor are C1 and C2, respectively, which are 330pF and 680pF, and the capacitance of the first inductor is 4.7μH.

[0060] The output of the capacitor detection circuit is connected to the input of the mixer. The input of the mixer is also equipped with the output of the crystal oscillator. The pins of the crystal oscillator, arranged counterclockwise, correspond to the power supply, ground, output, and power supply in sequence. The fourth pin is connected to the power supply through a resistor, and the resistor is grounded to the third pin through a resistor. The mixer's ground pin is grounded, and its output is connected to the amplifier's input. The amplifier's output is connected to a filter circuit, which is a fifth-order LC oscillator. In this filter circuit, a second, third, fourth, and fifth inductor are connected in series from the input to the output. A first filter capacitor is connected in parallel between the second and third inductors, a second filter capacitor is connected in parallel between the third and fourth inductors, and a third filter capacitor is connected in parallel between the fourth and fifth inductors. The other ends of these three filter capacitors are connected together. These three inductors—the third and fourth inductors, the first, second, and third filter capacitors—form a fifth-order LC oscillator.

[0061] The output of the filter circuit is connected to the operational amplifier.

[0062] Among them, the second and fifth inductors have the same inductance value, the third and fourth inductors have the same inductance value, and the first and third filter capacitors have the same capacitance value.

[0063] The operational amplifier's input terminal is connected to its inverting input terminal via a series capacitor and resistor; its non-inverting input terminal is connected to its output terminal via a fifth resistor; a sixth resistor is connected in parallel between its non-inverting input terminal and the fifth resistor, with the other end of the sixth resistor grounded; a power supply is connected to the positive power supply terminal, and an inductor is connected in parallel between the positive power supply terminal and the power supply and grounded; the negative power supply terminal is grounded. The output terminal connected in parallel with the fifth resistor serves as the output terminal of the final circuit section of the capacitive refractometer.

[0064] Specifically, a 10MHz crystal oscillator is used as the reference frequency source for mixing, utilizing the piezoelectric effect of the crystal to generate a 10MHz frequency signal with extremely high frequency stability.

[0065] A mixer is a nonlinear frequency conversion device (usually composed of diodes / transistors). It mixes the "signal from the oscillation module (frequency f1)" and the "10MHz signal from the crystal oscillator (frequency f2)" to output a difference frequency |f1-f2|, thereby achieving "frequency shifting" and enabling the signal frequency to operate in a more manageable frequency band.

[0066] For the filtering circuit, a bandpass filter composed of a 5th-order LC oscillator circuit is used to screen the signal of the target frequency, allowing only the frequency required after mixing (such as the difference frequency) to pass through, and filtering out unwanted frequency components such as sum frequency, harmonics, and spurious signals to ensure the purity of the subsequent signal.

[0067] For the operational amplifier section, a decoupled single-supply operational amplifier bias circuit is used to further amplify the frequency-selected signal in order to increase the output amplitude.

[0068] Since this invention is used to measure atmospheric parameters, it is often applied in scenarios with large temperature and humidity differences. Therefore, to mitigate the influence of temperature, a temperature compensation circuit is required. For this temperature compensation circuit, a temperature compensation circuit based on the LM35DT sensor is used. The LM35DT sensor exhibits a linear relationship between ambient temperature and output signal, achieving an accuracy of ±0.25℃. Furthermore, the chip has precise internal process calibration, very low output impedance, and is quite easy to use. It can be powered by a single power supply or by both positive and negative power supplies, draws only 60µA of current, and has very low self-heating. In still air, the temperature rises by only 0.1℃. The LM35 outputs 0V at 0℃, and the output voltage increases by 10mV for every 1℃ increase in temperature.

[0069] like Figure 5 As shown, the LM35DT chip's power supply terminal is connected to a power source, and its ground terminal is grounded. The output terminal of the LM35DT chip is connected in series with a second fixed resistor and a varactor diode. The extension wire of the varactor diode serves as the output terminal of the temperature compensation circuit. A first fixed resistor is connected in parallel between the output terminal of the LM35DT chip and the second fixed resistor, with the other end of the first fixed resistor connected to the power source. A fifth fixed resistor and a third fixed capacitor are connected in parallel between the second fixed resistor and the varactor diode, with the other ends of both the fifth fixed resistor and the third fixed capacitor grounded. A fourth fixed resistor and a third variable resistor are also connected in series in parallel between the second fixed resistor and the varactor diode. The third variable resistor is an adjustable resistor, with one end of its fixed resistor portion connected to the power source and the other end grounded. A first fixed capacitor and a second fixed capacitor are connected in series in parallel between the output terminal of the temperature compensation circuit and the varactor diode and grounded. A first fixed inductor is connected in parallel across the first fixed capacitor.

[0070] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A capacitive refractive index meter based on a variable capacitor, characterized in that, include: The capacitor detection circuit, mixer module, filter circuit and data processing module are connected in sequence; A variable capacitance sensor is installed in the capacitance detection circuit; the capacitance value of the capacitance sensor is related to the incoming gas. The detection circuit detects the electrical signal affected by the capacitance value of the variable capacitor, the mixing module mixes the electrical signal, and the filtering circuit filters the mixed electrical signal to generate a stable oscillation signal. The data processing module is also connected to a temperature compensation module; the temperature compensation module detects and acquires the temperature electrical signal; the data processing module performs digital-to-analog conversion and data processing on the stable oscillation signal and the temperature electrical signal to obtain the corrected refractive index.

2. The capacitive refractometer according to claim 1, characterized in that, The capacitive sensor employs a variable capacitor structure combining a cylindrical capacitor and a variable electrode spacing. The sensor contains alternating positive and negative electrodes, with the positive electrodes connected by connecting wires to form the positive terminal and the negative electrodes connected by another connecting wire to form the negative terminal. Adjusting the contact area and spacing between the positive and negative electrodes adjusts the initial capacitance value. When gas enters the gap between the positive and negative electrodes, the capacitive sensor outputs an electrical signal corresponding to the gas.

3. The capacitive refractometer according to claim 1, characterized in that, A copper sheet is disposed on the outside of the capacitive sensor, and an opening is provided on the copper sheet at the corresponding position of the capacitive sensor, wherein a grid-like copper sheet is disposed at the opening.

4. The capacitive refractometer according to claim 1, characterized in that, The capacitance detection circuit uses a capacitor feedback three-segment oscillator.

5. The capacitive refractometer according to claim 1, characterized in that, The filter circuit adopts a fifth-order LC resonant circuit, wherein the filter circuit includes five reactive elements connected in series, and each reactive element is connected in parallel with a corresponding loop capacitor, and the loop capacitors are all connected in parallel.

6. The capacitive refractometer according to claim 1, characterized in that, An operational amplifier is also connected between the filtering circuit and the data processing module, wherein the operational amplifier is a decoupled single-supply operational amplifier bias circuit.

7. The capacitive refractometer according to claim 1, characterized in that, The capacitive sensor, mixer module, and filter circuit are all mounted on the PCB board.

8. The capacitive refractometer according to claim 1, characterized in that, The temperature compensation module uses a temperature compensation circuit based on the LM35DT sensor.

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