A double-edge detection resonant circuit for measuring dielectric constant

Abstract

The present application relates to a kind of double-edge detection resonance circuit for measuring dielectric constant, including resistance, capacitance, inductance, AC power supply, dielectric substance, digital multimeter and sample container, the dielectric substance is placed in the sample container, the capacitance, inductance, resistance is connected in series, the digital multimeter is connected at the two ends of capacitance, for the amplitude of AC voltage grade measurement capacitance two ends AC signal, wherein, the resonance circuit is two, the capacitance of two resonance circuits is in sample container, the sample container can add dielectric substance, not add dielectric substance or add different dielectric constant medium, the capacitance value is linearly changed with the dielectric constant of different medium.By the detection of double edge, not only the size of center frequency offset can be measured at one time, but also the direction of offset can be measured, and according to the calculation formula of resonance frequency, the capacitance value after adding sample can be obtained, so as to indirectly calculate the dielectric constant of sample.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of circuit analysis and measurement control of instruments, in particular to a double-edge detection resonant circuit for measuring dielectric constant. BACKGROUND

[0002] The electrical parameters of commonly used electrical components will change slightly with the use environment, and the selection of capacitive materials and dielectric materials is also a problem that must be considered for all electrical instruments and equipment. The key to the above problems is how to accurately measure the slight changes and the dielectric constant of a certain substance. The commonly used circuit resonance method is a method for measuring the dielectric constant by taking the sample as part of the resonant structure, mainly including the perturbation method, the method of filling the resonator space entirely, and the method of filling the resonator space partially.

[0003] Among them, the full filling method can be calculated by the formula , wherein ε' is the real part of the complex dielectric constant, ε'' is the imaginary part of the complex dielectric constant, Q is the quality factor, tan δ is the loss tangent, and f0 is the resonant frequency without the sample; the partial filling is mainly to reduce the size of the sample and the influence of the material on the resonator parameters, and it is difficult to accurately calculate, and is generally used for correction; the perturbation method requires a relatively small size, and the relative frequency deviation is less than 0.001, in which case the specific size and shape can be represented by a filling factor s: , wherein f0 is the resonant frequency without the sample, Q L is the quality factor, ε r is the relative dielectric constant, A(ε r ) is a function related to the relative dielectric constant and the parameters of the perturbation cavity. The key to measuring the dielectric constant using the resonance method is how to determine f, the resonant frequency of the circuit after adding the sample. The existing measurement method is to draw an amplitude-frequency characteristic curve, and the frequency value corresponding to the peak is the resonant frequency.

[0004] Since the inherent frequency of the material to be measured is not known in the actual operation process, the signal source needs to have a very wide frequency range during the sweep operation, which puts high requirements on the instrument. In addition, in order to obtain more accurate results, this method needs to measure a large amount of data, which is difficult to use in practice. Therefore, the present application provides a double-edge detection resonant circuit for measuring dielectric constant to solve the above problems. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application provides a double-edge detection resonant circuit for measuring dielectric constant, which is used to measure the slight change of capacitance, thereby measuring the dielectric constant of unknown medium, and is suitable for accurate measurement of dielectric materials in electronic instruments and equipment, and has wide application prospects in the field of electronic circuit design.

[0006] In order to realize the above-mentioned rapid and effective detection of the small change of the capacitance and reduce the requirement for the required instrument, the present application provides the following technical scheme: a double-edge detection resonance circuit for measuring the dielectric constant, the resonance circuit comprising a resistance, a capacitance, an inductance, an alternating current power supply, a dielectric substance, a digital multimeter and a sample container, the dielectric substance being placed in the sample container, the capacitance, the inductance and the resistance being connected in series, the digital multimeter being connected across the capacitance and used for measuring the amplitude of the alternating current signal across the capacitance.

[0007] Among them, the resonance circuit is two, the capacitances of the two resonance circuits are in the sample container, the sample container can be added with the dielectric substance, without the dielectric substance or with the dielectric substance of different dielectric constants, and the capacitance value changes linearly with the dielectric constant of the different dielectric substances.

[0008] Further, the capacitance is a parallel metal plate capacitor, and the digital multimeter is used for measuring the amplitude of the alternating current signal.

[0009] Further, the alternating current power supply is a signal generator with adjustable frequency, amplitude and waveform.

[0010] Compared with the prior art, the technical scheme of the present application has the following beneficial effects:

[0011] The double-edge detection resonance circuit for measuring the dielectric constant can not only measure the size of the center frequency offset at one time, but also measure the direction of the offset, and then the capacitance value after adding the sample can be obtained according to the calculation formula of the resonance frequency, so as to indirectly calculate the dielectric constant of the sample. Compared with the general measurement scheme, the present scheme does not need to obtain the frequency response characteristic through frequency sweeping, the time is shorter and the efficiency is higher when batch measurement, and it is expected to be widely applied in electronic instrument and equipment enterprises. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 It is the principle diagram of the resonance circuit of the present application;

[0013] Figure 2 It is the frequency response characteristic diagram when the sample to be measured is not added in the present application;

[0014] Figure 3 It is the frequency response characteristic diagram after the sample to be measured is added in the present application;

[0015] Figure 4 It is the frequency response curve of the traditional single-edge detection resonance circuit. DETAILED DESCRIPTION

[0016] With reference to the drawings of the embodiments of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0017] Please refer to Figure 1 The double-edge detection resonance circuit for measuring dielectric constant in the present application comprises a resistor, a capacitor, an inductor, an alternating current power supply, a dielectric substance, a digital multimeter and a sample container. The dielectric substance is placed in the sample container. The capacitor, the inductor and the resistor are connected in series. The digital multimeter is connected to the capacitor and is used to measure the amplitude of the alternating current signal at the two ends of the capacitor.

[0018] There are two resonance circuits. The capacitors of the two resonance circuits are both in the sample container. The sample container can be added with the dielectric substance, without the dielectric substance or with different dielectric substances with different dielectric constants. The capacitance value changes linearly with the dielectric constant of the different dielectric substances.

[0019] The capacitor uses a parallel metal plate capacitor, so that the formula is used to quantitatively calculate the change of the dielectric constant, where S is the effective area of the parallel plate capacitor, ε r is the relative dielectric constant, and ε0 is the dielectric constant in vacuum.

[0020] Since the parallel plate capacitor is used, as long as the sample container is added with a medium with a known dielectric constant, the capacitance value can be theoretically calculated. At this time, the medium to be measured is added, and a dielectric constant of the sample to be measured can be measured. In order to make the result more reliable, different media with known dielectric constants can be respectively added to the sample container, and a series of data can be measured by the same method. Then, the average value is taken to reduce the error.

[0021] In addition, the digital multimeter is used to measure the amplitude of the alternating current signal. Other instruments capable of measuring the effective value of the alternating current signal can also be used instead, such as an oscilloscope. The selection of the measuring instrument has great flexibility. The alternating current power supply is a signal generator with adjustable frequency, amplitude and waveform. By adjusting the input voltage amplitude, a plurality of output amplitude-frequency characteristic curves can be made, which is convenient for multiple experiments to improve the measurement accuracy.

[0022] In a specific embodiment, first, the frequency response curve when the sample to be measured is not added can be obtained according to the actual parameters of the circuit elements. The principle of the double-edge detection resonance circuit can be realized. R1, L1, C1, R2, L2 and C2 can be accurately designed according to the actual application, so that the frequency response curve meets Figure 3The state shown in the middle curve ③④ (two frequency response curves are in a partially overlapping state), under the premise that the AC power amplitude remains unchanged, if the signal frequency is constantly changed, the response of the circuit will also change accordingly, and the frequency response characteristic diagram of Figure 2 can be obtained, in which the horizontal axis represents the angular frequency, the vertical axis represents the response of the circuit, ω1 is the resonance frequency of the circuit 1, ω2 is the resonance frequency of the circuit 2, ω c is the actual frequency applied to the circuit, A c is the response when the angular frequency of the input signal is ω c , the curve ① represents Figure 1 the frequency response characteristic of the left circuit in the middle, and the curve ② represents Figure 1 the frequency response characteristic of the right circuit in the middle. Since the parameters of the two resonance circuits are different, the angular frequencies corresponding to the peak values of the responses are also different. In theory, the formula is:

[0023]

[0024] After the sample to be measured is added, it can be known from the capacitance value of the parallel plate capacitor that both capacitance values change. Let the two capacitance values after the sample to be measured is added be C'1 and C'2, then wherein ε' is the dielectric constant of the sample to be measured. Since the resonance frequency of the circuit will change, the frequency response characteristic curve will move, but the general shape does not change. The frequency response characteristic curve after the sample to be measured is added is shown in Figure 3 .

[0025] In the diagram, the horizontal axis represents the angular frequency, the vertical axis represents the response of the circuit, ω c is the angular frequency of the input signal, A1 and A2 are the responses of the two circuits after the sample to be measured is added, ω1 and ω2 are the angular frequencies corresponding to the responses A1 and A2 of the two circuits before the sample to be measured is added, the curve ③ and the curve ④ represent the frequency response characteristic curves before the sample to be measured is added, and the curve ① and the curve ② represent the frequency response characteristic curves after the sample to be measured is added. Since the sample to be measured is added, the capacitance values of C1 and C2 change, and then the resonance frequency response curves represented by ③ and ④ move towards the center frequency in the direction of increasing or decreasing relative to the curves ① and ②.

[0026] Suppose that under the condition that the input frequency of the AC power does not change, the responses of the two circuits after the sample to be measured is added are A1 and A2 respectively, and the points (ω c , A1) and (ω c , A2) are marked, wherein the point (ω c , A1) is located on the amplitude-frequency characteristic curve of the circuit 1 after the sample to be measured is added, and the point (ω cA2) is located on the amplitude-frequency response curve of circuit 2 after the sample to be tested is added. Draw a horizontal reference line through these two points, intersecting the two curves ③ and ④ at points (ω1, A1) and (ω1, A2) respectively. Figure 3 As shown, by comparing A1 and A c A2 and A c The magnitude relationship can be used to determine the direction of shift in the new frequency response curve after adding the sample. If A1 < A c If A2 < A2, the curve shifts to the left; if A1 > A c If the value is greater than A2, the curve will shift to the right.

[0027] according to Figure 3 If we take Δω1=|ω1-ω c |、Δω2=|ω2-ω c |, then the physical meaning of Δω1 and Δω2 is how much the two frequency response curves shifted after the sample was added; combined with the above analysis of A1 and A c A2 and A c The magnitude relationship between these values ​​allows for the quantitative determination of the new resonant frequency.

[0028] ω'1=ω1±Δω1, ω'2=ω2±Δω2 (1)

[0029] In the above formula, the signs of Δω1 and Δω2 are represented by A1, A2, and A... c The size relationship determines this.

[0030] After solving for ω'1 and ω'2 from the graph, the formula can be used. Find C′1 and C′2. Since R1 and R2 are both constants, therefore:

[0031]

[0032] According to the formula achievable Substituting into formulas (1) and (2), we have

[0033]

[0034] Theoretically, ε'1 and ε'2 are equal, and their average value is taken as the dielectric constant to be determined, i.e.

[0035]

[0036] The above formula is a result of the dielectric constant of the sample to be tested. Then, in order to make the measurement results more reliable, different media with known dielectric constants can be added to the sample container, and the above steps can be repeated to obtain a series of dielectric constant values. The average value is taken as the final result.

[0037] Comparative example:

[0038] The dielectric constant was measured using a traditional single-edge detection circuit; please refer to the results. Figure 4 In the figure, the horizontal axis represents angular frequency, and the vertical axis represents the circuit response (i.e., the actual output voltage value). Curve ① represents the frequency response curve of the circuit before the sample is added, and curves ② and ③ represent the frequency response curves of the circuit after the sample is added. A0 is the circuit response before the sample is added, and A1 is the circuit response after the sample is added. ω0 is the detection frequency, and ω1 and ω'1 are the angular frequencies corresponding to the circuit response of A1 before the sample is added. In the previous process of determining the resonant center frequency using the scanning frequency method, it is necessary to measure ω0-ω1 or ω0-ω'1 to determine the change in center frequency caused by the change in capacitance C. Since the change in C is very small, the change in center frequency is also very small. When the voltage measurement accuracy requirement is high, it is difficult for the signal generating instrument to effectively adjust the frequency in very small steps, thus making it difficult to obtain accurate measurement results. In the case of only one RLC series circuit, only one frequency response curve can be obtained, such as... Figure 4 As shown in curve ②, a traditional detection method involves obtaining a new frequency response curve by sweeping the frequency after adding the sample to be tested. Then, the new capacitance value is calculated based on the frequency difference corresponding to the peak values ​​of the two curves. Finally, the dielectric constant of the sample to be tested is solved using formulas (3) and (4). This method has two main drawbacks:

[0039] 1. Frequency sweeping wastes a lot of time and requires high instrument performance, which leads to increased costs;

[0040] 2. When the dielectric constant of the sample to be tested is not much different from that of the known sample, that is, when the change in capacitance is very small, the two frequency response curves almost overlap, making it difficult to distinguish the frequency difference corresponding to the two peaks, thus greatly reducing the measurement accuracy.

[0041] Another traditional detection method involves keeping the operating frequency constant and measuring the difference in circuit response. The specific steps are as follows:

[0042] exist Figure 4In the middle, take the resonant frequency ω0 as the working frequency of the circuit, after adding the sample to be measured, the response of the circuit changes from A0 to A1, according to the angular frequency corresponding to A1, the size of the curve moving can be obtained as Δω = |ω1-ω0| = |ω'1-ω0|, after determining the new resonant frequency, the dielectric constant of the medium to be measured can be obtained according to formulas (2), (3) and (4). Generally speaking, the difference ΔA = |A0-A1| of the circuit response is much larger than Δω, so this scheme can convert the measurement of Δω into the measurement of ΔA, thereby improving the detection accuracy. However, the disadvantage of this scheme is that it cannot determine the moving direction of the curve, that is, it cannot determine whether the frequency response curve after adding the sample is curve 1 or curve 2. In order to solve this problem, the working frequency of the circuit must be adjusted. Under the condition that the amplitude does not change, the frequency of the alternating current power supply is appropriately increased, and the change of the circuit response is observed. If the response decreases with the increase of the input signal frequency, the frequency response curve after adding the sample is curve 1, that is, the new resonant frequency ω'0 = ω1; if the response of the circuit increases first and then decreases with the increase of the input signal frequency, the frequency response curve after adding the sample is curve 2, that is, the new resonant frequency ω'0 = ω'1.

[0043] Compared with the two traditional detection schemes, the present scheme has the advantages of simple operation, high efficiency and high precision, mainly reflected in that only one measurement is needed to obtain the result, without frequency sweeping, greatly saving the measurement time and detection cost.

[0044] It should be noted that in this paper, relationship terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between the entities or operations. Moreover, the terms "include", "contain" or any other variant thereof are intended to cover non-exclusive inclusion, so that the process, method, article or equipment including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or equipment. Without more limitations, the element defined by the statement "including a…" does not exclude the presence of other identical elements in the process, method, article or equipment including the element.

[0045] Although embodiments of the present application have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and alterations can be made without departing from the principles and spirit of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A double-edge detection resonant circuit for measuring a dielectric constant, characterized by, The resonance circuit comprises resistance, capacitance, inductance, AC power supply, dielectric substance, digital multimeter and sample container, the dielectric substance is placed in the sample container, the capacitance, inductance and resistance are connected in series, and the digital multimeter is connected at both ends of the capacitance and used for measuring the amplitude of the AC signal at both ends of the capacitance. The resonance circuit comprises resistance, capacitance, inductance, AC power supply, dielectric substance, digital multimeter and sample container, the dielectric substance is placed in the sample container, the capacitance, inductance and resistance are connected in series, and the digital multimeter is connected at both ends of the capacitance and used for measuring the amplitude of the AC signal at both ends of the capacitance. The capacitance is a parallel metal plate capacitor, and the digital multimeter is used for measuring the amplitude of the AC signal. The AC power supply is a signal generator with adjustable frequency, amplitude and waveform. During measurement, firstly, the frequency response curve without adding the sample to be measured is obtained according to the actual parameters of the circuit elements, the principle of the double-edge detection resonance circuit is that R1, L1, C1, R2, L2 and C2 are accurately designed according to actual application, so that the frequency response curve meets the condition of partial overlap, under the premise that the amplitude of the AC power supply remains unchanged, if the signal frequency is continuously changed, the response of the circuit will also change; after adding the sample to be measured, the capacitance values of the two capacitors are changed, assuming that the two capacitance values after adding the sample to be measured are C'1 and C'2, then there is wherein ε' is the dielectric constant of the sample to be measured, since the resonance frequency response characteristic curve will move, but the general shape does not change, assuming that under the condition that the input frequency of the AC power supply does not change, the responses of the two circuits after adding the sample to be measured are A1 and A2, combining the size relationship between A1 and Ac, A2 and Ac, the new resonance frequency is quantitatively obtained, C'1 and C'2 are obtained according to the formula, and ε'1 and ε'2 are calculated according to the formula, and the average value thereof is taken as the dielectric constant to be solved, different media with known dielectric constants are added in the sample container, and then the above steps are repeated, so that a series of dielectric constant values are measured, and the average value thereof is taken as the final result.

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