Multi-step impedance converter microwave sensing system based on parallel open circuit branches

Abstract

The invention discloses a multi-step impedance converter microwave sensing system based on parallel open circuit branches. By introducing the lossy microstrip line impedance formula, the optimal size of the impedance transformer under the condition of maximizing the transmission coefficient phase change rate is obtained, then the differential model of the output direct-current voltage and the dielectric constant is constructed, the optimal solution of the sensitivity is solved under the condition of considering the loss of the liquid to be detected, and therefore the detection performance is remarkably improved. In order to reduce the detection cost, a multi-step impedance converter is used as a load network, a reflection type radio frequency oscillator is designed according to a negative impedance theory, and the change characteristic of a dielectric constant is converted into oscillation frequency change. Furthermore, a frequency demodulation circuit is carried to demodulate the change of the oscillation frequency into the change of the direct-current voltage, so that the equipment cost is reduced. The microwave micro-flow sensor provided by the invention has the advantages of high sensitivity, convenience in detection and the like, and plays an important role in industrial and agricultural production.

Description

Technical Field

[0001] This invention belongs to the field of microwave technology and relates to a microwave sensing system based on a multi-stage step impedance converter with parallel open branches. It is a compact microwave sensor based on a multi-stage step impedance converter for measuring materials with different dielectric constants and different losses. Background Technology

[0002] In agricultural production, for example, the moisture content of grain needs to be tested for better storage; in industrial production, the water content of extracted crude oil needs to be tested; in drinking water testing, the pH value of the water needs to be tested; and in ensuring the quality of stealth aircraft coatings, the safety and undamaged nature of the coating materials needs to be tested. In these practical applications, the quality of grain, crude oil, water pH value, and stealth coatings can all be characterized by changes in dielectric constant. Furthermore, because the electric field of a microwave resonant detection unit is highly sensitive to changes in dielectric constant, microwave sensing technology is well-suited for detecting changes in the dielectric constant of materials. In addition, microwave sensors offer advantages such as portability, compact structure, and high system integration.

[0003] Currently, due to the wide application of microwave sensors in industrial and agricultural production, scholars both domestically and internationally have conducted in-depth research on microwave sensing and detection technology, and proposed many different types of microwave sensors. Based on whether the microwave sensor is powered during detection, it can be divided into active and passive microwave sensors. According to different detection methods, detection can be achieved using three parameters: the resonant frequency, amplitude, and phase of the S-curve. In recent years, research teams from various universities have proposed various types of resonant structures, including: split-ring resonators (SRR), interdigital capacitor-loaded split-ring resonators (ICLSRR), complementary split-ring resonators (CSRR), small stub-loaded complementary split-ring resonators (SSLCSRR), meander slot-loaded complementary split-ring resonators (MSLCSRR), electrical-LC (ELC), magnetic-LC (MLC), spiral complementary split-ring resonators (SCSRR), and substrate integrated waveguide resonators (SIWR).The split-ring resonator is a relatively simple resonant unit. When excited by a microstrip line, the excitation electric field is mainly concentrated at the opening of the split-ring resonator. However, it is insensitive to small changes in the dielectric constant of the material. To improve detection sensitivity, an interdigital capacitor structure is embedded at the opening of the split-ring resonator. This increases the area of ​​the corresponding surface at the opening, thus enhancing the equivalent capacitance. Etching the split-ring resonator to the bottom of the dielectric substrate forms a complementary split-ring resonator. The complementary split-ring resonator structure itself has more gaps, which can confine more electric field and improve detection sensitivity. In the complementary split-ring resonator, changing the straight slot to a meandering slot can also enhance the equivalent capacitance and improve the electric field confinement. The purpose of these methods is to achieve the following: Adding short-stub complementary split-ring resonators facilitates device miniaturization and enhances the compactness of microwave sensors; mirroring the split-ring resonator can form an electro-LC sensor, which can generate two resonant modes for multi-material parameter detection; mirroring the complementary split-ring resonator can form a magnetic-LC sensor, where the electric field is mainly concentrated in the middle gap; the spiral complementary split-ring resonator involves etching a spiral curve structure onto the bottom of the dielectric substrate to create a larger slot, thereby enhancing the electric field concentration; substrate-integrated waveguide resonators have the characteristics of low insertion loss and high quality factor, which is beneficial for improving the detection sensitivity of the material loss tangent. However, the above-mentioned passive microwave detection units suffer from problems such as inconvenience in portability and low system integration.

[0004] To improve the portability and integration of microwave detection sensors, radio frequency (RF) circuits are embedded into the detection system to convert changes in the dielectric constant of the analyte into changes in DC voltage. Common RF detection circuits embedded in passive microwave detection units include phase-locked loop (PLL) circuits, reflective RF oscillators, positive feedback RF oscillators, and frequency demodulation circuits. Embedding active RF circuits can effectively enhance the portability of the detection system, improve system integration, and reduce system cost.

[0005] In existing technologies, patent CN202510042683.5, "Microstrip Microwave Sensing System and Application Based on Transmission Coefficient Phase of High-Sensitivity Parallel Step Impedance Resonator," proposes a method to make the transmission coefficient phase sensitive to changes in dielectric constant by optimizing the dimensions of a multi-order step impedance transformer under ideal conditions where the sensing region is unloaded and the microstrip line is lossless. However, this scheme has significant shortcomings: its optimal dimensional solution is only applicable to lossless conditions and does not consider the influence of the measured liquid loss on the impedance transformer's sensitivity, leading to a decrease in sensitivity in practical applications and limiting the detection accuracy and applicability.

[0006] To address the aforementioned shortcomings, this invention proposes a microwave sensing system based on a multi-stage step impedance converter with parallel open stubs. By introducing a lossy microstrip line impedance formula, the optimal size of the impedance converter under the condition of maximizing the phase change rate of the transmission coefficient is obtained. Then, a differential model of the output DC voltage and dielectric constant is constructed, and the optimal solution for sensitivity is solved under the condition of considering the loss of the liquid under test, thereby significantly improving the detection performance. Summary of the Invention

[0007] The present invention addresses the shortcomings of existing technologies by proposing a microwave detection system based on a multi-order step impedance resonator with parallel open-circuit stubs. This sensor maximizes sensitivity by constructing a mathematical model relating the transmission coefficient phase to the dielectric constant and solving the model to obtain the maximum rate of change of the transmission coefficient phase with respect to the dielectric constant. A high-frequency electron mobility transistor is embedded in the microwave passive resonant unit to realize a reflective RF oscillator, reducing detection costs. A cascaded phase-locked loop circuit maps the output DC voltage to the change in dielectric constant, improving system integration.

[0008] This invention is implemented according to the following technical solution:

[0009] This invention relates to a microwave sensing system based on a multi-stage step impedance converter with parallel open-circuit stubs, comprising a reflective radio frequency oscillator and a frequency demodulation circuit. The reflective radio frequency oscillator acquires the change in dielectric constant of the material under test, which is then converted into an output DC voltage by the frequency demodulation circuit. The dielectric constant of the material under test is then estimated according to a standard model.

[0010] The reflective RF oscillator includes a multi-stage step impedance transformer and an RF amplifier;

[0011] The multi-order step impedance transformer is an N-order step impedance transformer with parallel open-circuit stubs, where N≥2. It includes a top layer, a middle layer, and a bottom layer. The top and bottom layers are metal layers, the middle layer is a dielectric substrate, and the metal layer of the top layer is an N-order step impedance transmission line, comprising an input-feed microstrip line, N series-connected different step impedance transformers, N series-connected symmetrical step impedance transformers, an output-feed microstrip line, and parallel open-circuit stub microstrip lines. The input terminals of the N series-connected different step impedance transformers are connected to one end of the input-feed microstrip line. The output terminal is connected to one end of the parallel open-circuit stub microstrip line and the input terminal of N series-connected symmetrical step impedance transformers; the output terminal of the N series-connected symmetrical step impedance transformers is connected to one end of the output-feed microstrip line; the other end of the input-feed microstrip line serves as the input port, and the other end of the output-feed microstrip line serves as the output port; the other end of the parallel open-circuit stub microstrip line is left floating; wherein, the parallel open-circuit stub microstrip line serves as the sensing area.

[0012] Preferably, the optimal dimensions of the step impedance transformer, the symmetrical step impedance transformer, and the parallel open-circuit stub microstrip line in the multi-stage step impedance transformer satisfy the following:

[0013] When considering the loss effect of the inductively connected open-circuit microstrip line, an attenuation constant is introduced. Therefore, the input admittance, taking into account the decay constant... Time is represented as:

[0014] (1)

[0015] In the formula, It is the complex propagation constant. It is the real number propagation constant. It is a plural number. It is the physical length of the parallel open-circuit stub microstrip line in a multi-stage step impedance converter. It is the characteristic impedance of the parallel open-circuit stub microstrip line in a multi-stage step impedance converter.

[0016] Assume that the electrical lengths of each step impedance transformer, the symmetrical step impedance transformer, and the parallel open-circuit stub microstrip line in the multi-stage step impedance transformer are all... When N is even, the transfer matrix of the multi-stage step impedance converter Considering the attenuation constant Time is represented as:

[0017] (2)

[0018] in It is the characteristic impedance of the i-th segment step impedance transformer and the symmetrical step impedance transformer;

[0019] Based on S-parameters and transfer matrix Conversion relationship between them, transmission coefficient Considering the attenuation constant Time is represented as:

[0020] (3)

[0021] in, Standard characteristic impedance ;

[0022] Transmission coefficient phase Further expressed as:

[0023] (4)

[0024] in This represents the characteristic impedance of a parallel open-circuit microstrip line;

[0025] Transmission coefficient phase The derivative of the electric length of the parallel open-circuit stub microstrip line is further expressed as:

[0026] (5)

[0027] It can be seen from equation (5) that The characteristic impedance of the odd-segment step impedance transformer and the corresponding symmetrical step impedance transformer increases, while the characteristic impedance of the even-segment step impedance transformer and the corresponding symmetrical step impedance transformer decreases.

[0028] When N is odd, the transfer matrix of the multi-stage step impedance converter Considering the attenuation constant Time is represented as:

[0029] (6)

[0030] Based on S-parameters and transfer matrix Conversion relationship between them, transmission coefficient Considering the attenuation constant Time is expressed as:

[0031] (7)

[0032] Transmission coefficient phase Further expressed as:

[0033] (8)

[0034] Transmission coefficient phase The derivative of the electric length of the parallel open-circuit stub microstrip line is further expressed as:

[0035] (9)

[0036] It can be seen from equation (5) that The characteristic impedance of the odd-segment step impedance transformer and the corresponding symmetrical step impedance transformer increases, while the characteristic impedance of the even-segment step impedance transformer and the corresponding symmetrical step impedance transformer decreases.

[0037] Preferably, the parallel open-circuit stub microstrip line serves as the sensing area, with a PET film laid directly above it to prevent the liquid solution to be tested from contaminating the surface of the dielectric substrate; a PTFE container is placed above the PET film, and a cavity for holding the liquid solution to be tested is opened inside the PTFE container.

[0038] Preferably, there is a one-to-one correspondence between the N series-connected symmetrical step impedance transformers and the N series-connected different step impedance transformers with respect to the parallel open-circuit stub microstrip lines. The corresponding step impedance transformers and symmetrical step impedance transformers use microstrip lines of the same width and length.

[0039] Preferably, the frequency demodulation circuit includes a downconverter, a low-pass filter, a voltage-controlled oscillator, a baseband amplifier, and an oscilloscope.

[0040] Preferably, the input port is connected to a 50-ohm load.

[0041] The beneficial effects of this invention are at least as follows:

[0042] 1. In constructing the phase model of the transmission coefficient of a multi-stage step impedance transformer, this invention innovatively introduces an attenuation constant α to characterize the loss of the liquid under test, thereby extending the model from ideal lossless conditions to actual lossy conditions. By constructing a mathematical model of the derivative of the phase of the transmission coefficient with respect to the power-saving length of the parallel open-circuit branch, and solving for the optimal solution of the characteristic impedance of each microstrip line segment when the derivative is maximized, the rate of change of the phase of the transmission coefficient with respect to the dielectric constant (i.e., sensitivity) of the sensor is systematically optimized and maximized when facing actual liquids with different loss characteristics.

[0043] 2. This invention uses a multi-stage step impedance transformer as the load network and designs a reflective radio frequency oscillator based on the negative impedance principle. This design tightly integrates the passive sensing unit with the active oscillation circuit, realizing the direct conversion of minute changes in the dielectric constant of the test material into changes in the oscillation frequency. It eliminates the need for large and expensive testing equipment such as traditional vector network analyzers, greatly enhancing the portability and integration of the system, while significantly reducing the overall hardware cost of the detection system.

[0044] 3. This invention employs a phase-locked loop (PLL) circuit as the core of its frequency demodulation circuit. Through the coordinated operation of its internal down-converter, low-pass filter, and voltage-controlled oscillator, it can stably lock and demodulate the frequency variation output by the reflective RF oscillator into a DC voltage signal. This technical feature transforms high-frequency phase or frequency information, which was originally difficult to measure directly, into a DC voltage that is easy to measure and process. This greatly simplifies subsequent signal processing circuits, reduces equipment complexity and cost, and improves the reliability and repeatability of measurements.

[0045] 4. The microwave sensor of the present invention has the advantages of low cost, portability, and easy operation. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of the proposed Nth-order step impedance converter with parallel open-circuit stubs.

[0047] Figure 2This is a schematic diagram of the proposed microwave sensing and detection system.

[0048] Figure 3 This is a planar schematic diagram of the proposed multi-stage step impedance converter with parallel open-circuit branches.

[0049] Figure 4 A three-dimensional schematic diagram of the proposed multi-stage step impedance converter with parallel open-circuit branches.

[0050] Figure 5 A sensitivity comparison between the optimal and non-optimal dimensions of the proposed multi-stage step impedance converter with parallel open-circuit stubs. Detailed Implementation

[0051] To more clearly illustrate the problems solved by the present invention, the technical solutions adopted, and the beneficial effects, the specific embodiments of the present invention are described below in conjunction with the figures. The preferred embodiments described herein are only for illustration and explanation of the present invention and are not intended to limit the present invention. All modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be within the protection scope of the present invention.

[0052] This embodiment provides a microwave sensing system based on a multi-order step impedance converter with parallel open-circuit stubs. It is a system-level detection device. See Appendix. Figure 2 It includes a reflective radio frequency oscillator and a frequency demodulation circuit. The reflective radio frequency oscillator obtains the change in the dielectric constant of the material under test, and then the frequency demodulation circuit converts it into an output DC voltage, which is then used to predict the dielectric constant of the material under test according to a standard model.

[0053] Specifically, the reflective radio frequency oscillator includes a multi-stage step impedance converter 2 and a radio frequency amplifier 3;

[0054] The multi-order step impedance transformer 2 is an N-order step impedance transformer with parallel open-circuit stubs, where N≥2, and includes a top layer 9, a middle layer 21, and a bottom layer 22. The top and bottom layers are metal layers, and the middle layer is a dielectric substrate. (See attached diagram.) Figure 1The top metal layer is an N-order step impedance transmission line, including an input-fed microstrip line 12, N series-connected step impedance transformers of different types, N series-connected symmetrical step impedance transformers, an output-fed microstrip line 19, and a parallel open-circuit stub microstrip line 20. The input terminals of the N series-connected step impedance transformers of different types are connected to one end of the input-fed microstrip line 12, and the output terminals are connected to one end of the parallel open-circuit stub microstrip line 20 and the input terminals of the N series-connected symmetrical step impedance transformers. The output terminal of the symmetrical step impedance transformer is connected to one end of the output-feed microstrip line 19; the other end of the input-feed microstrip line 12 serves as input port 10, and the other end of the output-feed microstrip line 19 serves as output port 11; the other end of the parallel open-circuit stub microstrip line 20 is left floating; wherein, the parallel open-circuit stub microstrip line 20 serves as the sensing area. The input port 10 is connected to a 50-ohm load 1.

[0055] This invention considers the influence of the loaded material on the characteristic impedance and electrical length of the parallel stubs. It transforms the relationship between the phase of the transmission coefficient of the passive detection unit and the dielectric constant into a differential relationship between the phase of the transmission coefficient and the electrical length and characteristic impedance. The main purpose is to construct a mathematical model of the derivative between the phase of the transmission coefficient and the electrical length of the parallel stubs. Based on the mathematical model, the characteristic impedance of each segment of the microstrip line of the multi-stage step impedance converter is accurately solved to maximize the sensitivity.

[0056] One implementation, a schematic diagram of an N-order step impedance converter with parallel open-circuit stubs, is shown. By constructing a mathematical model of the phase and dielectric constant of the converter's transmission coefficient, the optimal geometric dimensions that maximize the sensitivity of the multi-order step impedance converter are obtained. The optimal dimensions of the step impedance converter, the symmetrical step impedance converter, and the parallel open-circuit stub microstrip line 20 in the multi-order step impedance converter (2) with parallel open-circuit stubs are derived as follows:

[0057] When considering the loss effect of the inductively connected open-circuit microstrip line 20, an attenuation constant is introduced. Therefore, the input admittance, taking into account the decay constant... Time is represented as:

[0058] (1)

[0059] In the formula, It is the complex propagation constant. It is the real number propagation constant. It is a plural number. It is the physical length of the parallel open-circuit stub microstrip line 20 in the multi-stage step impedance converter 2. It is the characteristic impedance of the parallel open-circuit stub microstrip line 20 in the multi-stage step impedance converter 2.

[0060] Assume that the electrical lengths of each step impedance transformer, the symmetrical step impedance transformer, and the parallel open-circuit stub microstrip line 20 in the multi-stage step impedance transformer 2 are all... When N is even, the transfer matrix of the multi-stage step impedance transformer 2 Considering the attenuation constant Time is represented as:

[0061] (2)

[0062] in It is the characteristic impedance of the i-th segment step impedance transformer and the symmetrical step impedance transformer;

[0063] Based on S-parameters and transfer matrix Conversion relationship between them, transmission coefficient Considering the attenuation constant Time is represented as:

[0064] (3)

[0065] in, Standard characteristic impedance ;

[0066] Transmission coefficient phase Further expressed as:

[0067] (4)

[0068] in This represents the characteristic impedance of the parallel open-circuit stub microstrip line 20;

[0069] Transmission coefficient phase The derivative of the 20 electrical length of the parallel open-circuit stub microstrip line is further expressed as:

[0070] (5)

[0071] It can be seen from equation (5) that The characteristic impedance of the odd-segment step impedance transformer and the corresponding symmetrical step impedance transformer increases, while the characteristic impedance of the even-segment step impedance transformer and the corresponding symmetrical step impedance transformer decreases.

[0072] When N is odd, the transfer matrix of the multi-stage step impedance transformer 2 Considering the attenuation constant Time is represented as:

[0073] (6)

[0074] Based on S-parameters and transfer matrix Conversion relationship between them, transmission coefficient Considering the attenuation constant Time is expressed as:

[0075] (7)

[0076] Transmission coefficient phase Further expressed as:

[0077] (8)

[0078] Transmission coefficient phase The derivative of the parallel open-circuit stub microstrip line (20 electrical length) is further expressed as:

[0079] (9)

[0080] It can be seen from equation (5) that The characteristic impedance of the odd-segment step impedance transformer and the corresponding symmetrical step impedance transformer increases, while the characteristic impedance of the even-segment step impedance transformer and the corresponding symmetrical step impedance transformer decreases.

[0081] One implementation method is shown in the appendix. Figure 4 The parallel open-stub microstrip line 20 serves as the sensing area, with a PET film 23 laid directly above it to prevent the test liquid solution 25 from contaminating the surface of the dielectric substrate. A PTFE container 24 is placed above the PET film 23, with a cavity inside the PTFE container 24 for holding the test liquid solution 25. For liquid detection scenarios, this invention sequentially places a PET film and a PTFE container directly above the parallel open-stub microstrip line 20 (sensing area). This structural feature effectively prevents the test liquid from contaminating the dielectric substrate, ensuring the long-term stability of the sensor performance. Simultaneously, the PTFE container provides a fixed holding cavity for the test liquid, making the detection operation more convenient and standardized, and improving the practicality and lifespan of the sensor.

[0082] In one implementation, there is a one-to-one correspondence between the N series-connected symmetrical step impedance transformers and the N series-connected different step impedance transformers with respect to the parallel open-circuit stub microstrip line 20. The corresponding step impedance transformers and symmetrical step impedance transformers use microstrip lines of the same width and length.

[0083] As an example, this embodiment uses N equal to 3 as an example for illustration. See Appendix. Figure 3The top layer 9 of the multi-stage step impedance transformer 2 structure with parallel open stubs mainly includes an input port 10, an output port 11, an input-feed microstrip line 12, an output-feed microstrip line 19, a third step impedance transformer 13, a second step impedance transformer 14, a first step impedance transformer 15, a third symmetrical step impedance transformer 16, a second symmetrical step impedance transformer 17, a first symmetrical step impedance transformer 18, and a parallel open stub microstrip line 20. In the top-level 9 structure, the input port 10 is first connected to the input-feed microstrip line 12. The input-feed microstrip line 12 is connected to the third step impedance transformer 13. The third step impedance transformer 13 is connected to the second step impedance transformer 14. The second step impedance transformer 14 is connected to the first step impedance transformer 15. The first step impedance transformer 15 is connected to the third symmetrical step impedance transformer 16. The third symmetrical step impedance transformer 16 is connected to the second symmetrical step impedance transformer 17. The second symmetrical step impedance transformer 17 is connected to the first symmetrical step impedance transformer 18. The first symmetrical step impedance transformer 18 is connected to the output-feed microstrip line 19. The parallel open-circuit stub microstrip line 20 is located in the middle of the first step impedance transformer 15 and the third symmetrical step impedance transformer 16 and is closely connected to the first step impedance transformer 15 and the third symmetrical step impedance transformer 16.

[0084] Preferably, in this embodiment, the microwave microfluidic sensor has been optimized in commercial electromagnetic simulation software, and the specific values ​​can be:

[0085] The overall dimensions of the microwave sensor are Lx = 50mm and Ly = 90mm; the width of the input feed microstrip line 12 is W0 = 1.15mm, and the length is L0 = 5mm; the width of the output feed microstrip line 19 is W0 = 1.15mm, and the length is L0 = 5mm; the width of the third step impedance transformer 13 is W3 = 0.101mm, and the length is L3 = 12.53mm; the second step impedance... The width of the first step impedance transformer 14 is W2 = 3.83 mm, and the length of the second step impedance transformer 14 is L2 = 10.7 mm; the width of the first step impedance transformer 15 is W1 = 0.101 mm, and the length of the first step impedance transformer 15 is L1 = 12.53 mm; the width of the third symmetrical step impedance transformer 16 is W3 = 0.101 mm, and the length of the third symmetrical step impedance transformer 16 is L3 = 12.53 mm; the second The width of the symmetrical step impedance transformer 17 is W2 = 3.83 mm, and the length of the second symmetrical step impedance transformer 17 is L2 = 10.7 mm; the width of the first symmetrical step impedance transformer 18 is W1 = 0.101 mm, and the length of the first symmetrical step impedance transformer 18 is L1 = 12.53 mm; the width of the parallel open-circuit stub microstrip line 20 is Ws = 3.83 mm, and the length of the parallel open-circuit stub microstrip line 20 is Ls = 10.7 mm. mm; the thickness of the dielectric substrate is h1=1.27mm; the length, width and height of the PET film 23 structure are L4=24mm, W4=20mm and h2=0.0125mm respectively; the length, width and height of the PTFE container 24 structure are L5=13mm, W5=10mm and h3=5mm respectively; the length, width and height of the cavity used to hold the liquid solution 25 to be tested are L6=10.5mm, W6=6mm and h3=5mm respectively.

[0086] In one embodiment, the frequency demodulation circuit includes a downconverter 4, a low-pass filter 5, a voltage-controlled oscillator 6, a baseband amplifier 7, and an oscilloscope 8.

[0087] The working principle of the microwave sensing system with a multi-stage step impedance converter in this embodiment can be described as follows:

[0088] When the multi-stage step impedance transformer 2 is in an unloaded state, the reflective RF oscillator (composed of the multi-stage step impedance transformer 2 and the RF amplifier 3) oscillates at an initial resonant frequency of... oscillation signal The mathematical expression for this signal is written as:

[0089] (10)

[0090] in It is the amplitude of the oscillation signal. It is the initial phase. It is time;

[0091] Then the oscillation signal Entering the downconverter 4, assume that the voltage-controlled oscillator 6 is oscillating with an initial oscillation signal at this time. The mathematical expression is written as:

[0092] (11)

[0093] in It is the inherent oscillation frequency of the voltage-controlled oscillator. It is the oscillation amplitude of the voltage-controlled oscillator. It is the initial phase of the voltage-controlled oscillator;

[0094] The oscillation signal generated by the reflective radio frequency oscillator and voltage-controlled oscillator oscillation signal Simultaneously, the signal enters down-converter 4, at which point down-converter 4 outputs a signal. The mathematical expression can be written as:

[0095] (12)

[0096] Then the output signal of inverter 4. After entering low-pass filter 5, the high-frequency signal is filtered out, leaving only the low-frequency signal. Its mathematical expression is written as:

[0097] (13)

[0098] The instantaneous phase difference at this moment can be written as:

[0099] (14)

[0100] The derivative of the instantaneous phase difference with respect to time can be written as:

[0101] (15)

[0102] When equation (15) equals zero, the frequency demodulation circuit phase-locked loop achieves frequency latching, that is, the oscillation frequency of the reflective RF oscillator. With the oscillation frequency of voltage-controlled oscillator 6 equal.

[0103] When the sensing region of the multi-stage step impedance transformer 2 is loaded with substances to be tested with different dielectric constants, the frequency demodulation circuit using the phase-locked loop circuit is frequency latched, and the voltage-controlled oscillator 6 can output a DC voltage different from that under the initial conditions; this voltage is then substituted into the pre-fitted standard model to predict the dielectric constant value of the current substance to be tested.

[0104] The pre-fitted standard model is a mathematical model that establishes the relationship between the DC voltage and the sample dielectric constant by obtaining different output DC voltages from liquid samples with different known dielectric constants through a frequency demodulation circuit.

[0105] From the appendix Figure 5 It can be seen that the sensitivity of the microwave sensing system of the present invention is significantly improved when a multi-stage step impedance transformer that meets the optimal size is used. Here, Sensor 1 refers to the microwave microstrip sensor with parallel open stubs that meets the optimal size condition proposed in the present invention, and Sensor 2 refers to the microwave microstrip sensor with parallel open stubs that does not meet the optimal size condition.

[0106] The microwave sensing and detection system provided in this embodiment is a portable, convenient, and highly integrated detection device. This study explores the relationship between the phase of the transmission coefficient of a multi-stage step impedance transformer with parallel open branches and the dielectric constant of the analyte. A mathematical model is constructed primarily between the phase of the impedance transformer's transmission coefficient and the derivative of the electrical length of the microstrip line in the sensing region. The optimal solution for the length and width of each segment of the microstrip line in the multi-stage step impedance transformer is found to maximize this derivative, thereby maximizing sensitivity. To reduce detection costs, the impedance transformer is used as a load network. Based on negative impedance theory, a reflective radio frequency oscillator is designed to convert the variation in dielectric constant into a variation in oscillation frequency. Furthermore, a phase-locked loop circuit is incorporated to demodulate the variation in oscillation frequency into a variation in DC voltage, further reducing equipment costs. The microwave microfluidic sensor proposed in this invention has advantages such as high sensitivity and convenient detection, and will play an important role in industrial and agricultural production.

[0107] The above embodiments are not intended to limit the present invention, and the present invention is not limited to the above embodiments. Any embodiment that meets the requirements of the present invention is within the protection scope of the present invention.

Claims

1. A microwave sensing system based on a multi-stage step impedance converter with parallel open branches, including a reflective radio frequency oscillator and a frequency demodulation circuit, which acquires the change in dielectric constant of the material under test through the reflective radio frequency oscillator, and then converts it into an output DC voltage through the frequency demodulation circuit, thereby predicting the dielectric constant of the material under test according to a standard model; The reflective RF oscillator includes a multi-stage step impedance transformer (2) and an RF amplifier (3); Its features are, The multi-order step impedance transformer (2) is an N-order step impedance transformer with parallel open-circuit stubs, where N≥2. It includes a top layer, a middle layer, and a bottom layer, where the top and bottom layers are metal layers, the middle layer is a dielectric substrate, and the metal layer of the top layer is an N-order step impedance transmission line, including an input-side fed microstrip line (12), N series-connected different step impedance transformers, N series-connected symmetrical step impedance transformers, an output-side fed microstrip line (19), and parallel open-circuit stub microstrip lines (20); N The input terminals of the series-connected step impedance transformers are connected to one end of the input-feed microstrip line (12), and the output terminals are connected to one end of the parallel open-circuit stub microstrip line (20) and the input terminals of the N series-connected symmetrical step impedance transformers; the output terminals of the N series-connected symmetrical step impedance transformers are connected to one end of the output-feed microstrip line (19); the other end of the input-feed microstrip line (12) serves as the input port (10), and the other end of the output-feed microstrip line (19) serves as the output port (11); The other end of the input-feed microstrip line (12) serves as the input port (10), and the other end of the output-feed microstrip line (19) serves as the output port (11); the other end of the parallel open-circuit stub microstrip line (20) is suspended; wherein, the parallel open-circuit stub microstrip line (20) serves as the sensing area.

2. The microwave sensing system based on a multi-order step impedance converter with parallel open-circuit stubs according to claim 1, characterized in that, The optimal dimensions of the step impedance transformer, the symmetrical step impedance transformer, and the parallel open-circuit stub microstrip line (20) in the multi-stage step impedance transformer (2) satisfy the following: When considering the loss effect of the inductively connected parallel open-circuit microstrip line (20), an attenuation constant is introduced. Therefore, the input admittance, taking into account the decay constant... Time is represented as: (1) In the formula, It is the complex propagation constant. It is the real number propagation constant. It is a plural number. It is the physical length of the parallel open-circuit stub microstrip line (20) in the multi-stage step impedance transformer (2). It is the characteristic impedance of the parallel open-circuit stub microstrip line (20) in the multi-stage step impedance transformer (2); Assuming that the electrical lengths of each step impedance transformer, the symmetrical step impedance transformer, and the parallel open-circuit stub microstrip line (20) in the multi-stage step impedance transformer (2) are all When N is even, the transfer matrix of the multi-stage step impedance converter (2) is... Considering the attenuation constant Time is represented as: (2) in It is the characteristic impedance of the i-th segment step impedance transformer and the symmetrical step impedance transformer; Based on S-parameters and transfer matrix Conversion relationship between them, transmission coefficient Considering the attenuation constant Time is represented as: (3) in, Standard characteristic impedance ; Transmission coefficient phase Further expressed as: (4) in The characteristic impedance of the parallel open-circuit stub microstrip line (20) is represented; Transmission coefficient phase The derivative of the electrical length of the parallel open-circuit stub microstrip line (20) is further expressed as: (5) It can be seen from equation (5) that The characteristic impedance of the odd-segment step impedance transformer and the corresponding symmetrical step impedance transformer increases, while the characteristic impedance of the even-segment step impedance transformer and the corresponding symmetrical step impedance transformer decreases.

3. The microwave sensing system based on a multi-order step impedance converter with parallel open-circuit stubs according to claim 1, characterized in that, The optimal dimensions of the step impedance transformer, the symmetrical step impedance transformer, and the parallel open-circuit stub microstrip line (20) in the multi-stage step impedance transformer (2) satisfy the following: When considering the loss effect of the inductively connected parallel open-circuit microstrip line (20), an attenuation constant is introduced. Therefore, the input admittance, taking into account the decay constant... Time is represented as: (1) In the formula, It is the complex propagation constant. It is the real number propagation constant. It is a plural number. It is the physical length of the parallel open-circuit stub microstrip line (20) in the multi-stage step impedance transformer (2). It is the characteristic impedance of the parallel open-circuit stub microstrip line (20) in the multi-stage step impedance transformer (2); Assuming that the electrical lengths of each step impedance transformer, the symmetrical step impedance transformer, and the parallel open-circuit stub microstrip line (20) in the multi-stage step impedance transformer (2) are all When N is odd, the transfer matrix of the multi-stage step impedance transformer (2) Considering the attenuation constant Time is represented as: (6) Based on S-parameters and transfer matrix Conversion relationship between them, transmission coefficient Considering the attenuation constant Time is expressed as: (7) Transmission coefficient phase Further expressed as: (8) Transmission coefficient phase The derivative of the electrical length of the parallel open-circuit stub microstrip line (20) is further expressed as: (9) It can be seen from equation (5) that The characteristic impedance of the odd-segment step impedance transformer and the corresponding symmetrical step impedance transformer increases, while the characteristic impedance of the even-segment step impedance transformer and the corresponding symmetrical step impedance transformer decreases.

4. The microwave sensing system based on a multi-order step impedance converter with parallel open-circuit stubs according to claim 1, characterized in that, The parallel open-stub microstrip line (20) serves as the sensing area, and a PET film (23) is laid directly above it to prevent the liquid solution (25) to be tested from contaminating the surface of the dielectric substrate. A PTFE container (24) is placed above the PET film (23), and a cavity for holding the liquid solution (25) to be tested is opened inside the PTFE container (24).

5. The microwave sensing system based on a multi-order step impedance converter with parallel open-circuit stubs according to claim 1, characterized in that, The N series-connected symmetrical step impedance transformers correspond one-to-one with the parallel open-circuit stub microstrip line (20) and the N series-connected different step impedance transformers. The corresponding step impedance transformers and symmetrical step impedance transformers use microstrip lines of the same width and length.

6. The microwave sensing system based on a multi-order step impedance converter with parallel open-circuit stubs according to claim 1, characterized in that, The frequency demodulation circuit includes a downconverter (4), a low-pass filter (5), a voltage-controlled oscillator (6), a baseband amplifier (7), and an oscilloscope (8).

7. The microwave sensing system based on a multi-order step impedance converter with parallel open-circuit stubs according to claim 1, characterized in that, The input port (10) is connected to a 50-ohm load (1).

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