CN115541669A

CN115541669A - Humidity sensor based on substrate integrated waveguide re-entry resonant cavity and six-port interference network

Info

Publication number: CN115541669A
Application number: CN202211211019.1A
Authority: CN
Inventors: 黄杰; 刘兴川
Original assignee: Southwest University
Current assignee: Southwest University
Priority date: 2022-09-30
Filing date: 2022-09-30
Publication date: 2022-12-30

Abstract

A humidity sensor based on a substrate integrated waveguide and a six-port interference network comprises a passive resonant cavity and a six-port interference circuit. The passive resonant cavity is composed of three layers of medium substrates which are vertically overlapped, and the compact structure formed by the non-metalized through hole at the top layer, the air filling type medium at the middle layer and the capacitor column metalized through hole at the bottom layer greatly enhances the circulation of the wet air in the resonant cavity, so that water molecules in the wet air can be more fully attached to the resonant cavity and fully react with the humidity sensitive material in the groove. The bottom metal of the dielectric substrate is designed with a coplanar waveguide feed structure which is connected with an external six-port interference circuit. The six-port interference circuit consists of a power divider, a coupler, a phase shifter and a resonant cavity. Signals are fed in from the power divider, equal power equal phase is distributed to the reference branch and the sensing branch, the two branches form interference by loading different phase information brought by the resonant cavity and adjusting the phase shifter, and therefore high-sensitivity humidity sensing measurement is achieved.

Description

Humidity sensor based on substrate integrated waveguide re-entry resonant cavity and six-port interference network

Technical Field

The invention belongs to the field of sensors, and particularly relates to a microwave active sensor suitable for detecting air relative humidity.

Background

Humidity measurement is an important detection technology and is widely applied to the aspects of environmental monitoring, agricultural production, food safety, clinical medicine and the like. The instant and high-precision humidity measurement depends on a high-performance sensor, so that the development of the high-performance humidity sensor has extremely important significance for modern humidity measurement.

As the most common humidity sensors, resistive humidity sensors and capacitive humidity sensors are widely used due to their characteristics of low power consumption, low cost, and fast response. It also has some disadvantages such as a large size that is difficult to integrate with the system, inability to operate in harsh environments, limited sensing distance, etc. Other types of sensors, such as microwave-based humidity sensors, are receiving considerable attention due to their low cost, stable response, ease of integration, and higher frequency. In the current research, humidity sensors based on interferometry have been proposed, but most of them have the problems of large size, difficulty in integration or low sensitivity, and structural improvement is necessary to make a large improvement on the basis of the problems. The design of the resonator has great influence on the performance of the microwave humidity sensor, and in numerous resonators, the substrate integrated waveguide based reentrant resonant cavity is widely applied to high-sensitivity sensing due to high quality factors and high electric field density in a band gap capacitance region. Interferometry relies on a process of phase change detection and superposition of waves. Based on the principle of interference, the sensor is designed as two branches with 180 degrees phase difference but approximately equal attenuation, one as a reference and the other as a sensing, and the obvious interference effect can be detected by detecting the S21 signal, which has proved to have high sensitivity. However, the existing interference system still has the problems of larger size, poor adjustability and the like.

Disclosure of Invention

The invention mainly aims at the problems of larger volume, low sensitivity and the like of the traditional interference sensor, and provides a humidity sensor based on a substrate integrated waveguide re-entry resonant cavity and a six-port interference network.

The design idea of the invention is as follows: in the sensing cavity and the reference cavity, metallized through holes and metal grids are adopted to form equivalent metal walls to effectively restrain electromagnetic waves in the resonant cavity and greatly improve the circulation of wet air in the resonant cavity; in order to balance the maximum reduction of the size of the resonant cavity and the conventional slot milling process, a feeder line is designed on the bottom metal of a bottom medium substrate, is connected with a straight-through end and a coupling end of a microstrip branch line orthogonal coupler through a gradient microstrip line, and then outputs a signal through an isolation end of the coupler to realize the loading of phase information; the input port and the output port of the whole system are connected through a Wilkinson power divider to ensure that the phases of the reference line and the sensing line are equal; in order to ensure that the two branches have a phase difference of 180 degrees, a phase shifter formed by a left-hand and right-hand composite transmission line is arranged on a reference line, the phase shift of the whole branch can be adjusted in real time by adjusting the bias voltage, and meanwhile, the phase shifter is in a balanced structure, so that the S21 difference loss is extremely small, and the adjustability and the interference effect of an interference system are greatly improved.

The technical scheme of the invention is as follows:

a humidity sensor based on a substrate integrated waveguide and a six-port interference network comprises a passive resonant cavity and a six-port interference circuit. The two passive resonant cavities are respectively used as a sensing cavity and a reference cavity, and the other structures of the two passive resonant cavities are completely the same except that whether a humidity sensitive material is placed.

The passive resonant cavity is provided with three layers of dielectric substrates, each layer of dielectric substrate comprises a top layer metal, a dielectric layer and a bottom layer metal, and a substrate integrated waveguide reentrant resonant cavity is formed in the three layers of dielectric substrates. A circle of metalized through holes are distributed in each layer of medium substrate, the metalized through holes of each layer are connected with the top layer metal and the bottom layer metal of the layer, the radiuses of circles surrounded by the metalized through holes of each layer are the same, the circle centers are aligned, so that an equivalent metal wall of the resonant cavity is formed, and a resonant cavity area is arranged inside the equivalent metal wall.

In the resonant cavity region of the passive resonant cavity, a metalized through hole array which is circularly arranged is arranged at the center of the bottom dielectric substrate, the bottom and the top of the metalized through hole array are respectively connected with the bottom metal and the top metal of the bottom dielectric substrate, the top metal is partially etched in the resonant cavity region, and only a circular metal layer with the center slightly larger than the radius of the metalized through hole array is left, so that the capacitor column of the reference cavity is formed.

And in the resonant cavity region of the passive resonant cavity, the middle layer dielectric substrate is completely hollowed out to serve as an air filling region.

And in the resonant cavity region of the passive resonant cavity, the bottom metal of the top dielectric substrate is completely etched. A circular groove is dug from bottom to top in the central area of the top dielectric substrate and is used for forming a sensing area; and an unmetallized through hole array which is circularly arranged is arranged in the dielectric layer of the top dielectric substrate and surrounds the circular groove, and the unmetallized through hole array and the top metal of the substrate form a metal grid together.

The passive resonant cavity enables wet air to freely enter and exit the resonant cavity region through the metal grating of the top dielectric substrate, the air filling region of the middle dielectric substrate and the capacitor column of the bottom dielectric substrate, and the circulation of the passive resonant cavity is enhanced.

The passive resonant cavity is provided with a coplanar waveguide feeder line etched on the bottom metal of the bottom dielectric substrate, and the microstrip line connected with the feeder line is in a gradual change form and is used for connecting an external six-port interference circuit.

The six-port interference circuit comprises two power dividers, two couplers and a phase shifter, wherein one path of the two power dividers is connected with a reference cavity through one coupler to form a reference circuit, and the phase shifter is arranged in the reference circuit; the other path of the two power dividers is connected with the sensing cavity through the other coupler to form a sensing circuit.

And the six-port interference circuit feeds signals into one end of the power divider through a coplanar waveguide feeder line.

According to the humidity sensor with the structure, the unmetallized through hole array is arranged in the top layer medium substrate, so that the air circulation is greatly increased, and the metal grating is connected with the top layer metal of the medium substrate to form a metal grating which can be approximately equivalent to a metal wall.

According to the humidity sensor with the structure, the electromagnetic wave is well shielded in the area surrounded by the metal wall, the metal grating, the capacitance column and the bottom metal of the bottom dielectric substrate, so that the electromagnetic field is restrained in the resonant cavity.

The interference circuit of the humidity sensor inputs signals from one end of one power divider, outputs half-power equal-phase signals from the other two ends in an equal-power mode, is respectively connected with the reference cavity and the sensing cavity through the symmetrically arranged microstrip branch line couplers, outputs signals at ports on the same side of the microstrip branch line orthogonal coupler, and finally inputs the signals at two ends of the other power divider at the tail end on the same side, and outputs the signals at one side of a single port.

The straight-through end and the coupling end of the microstrip branch line orthogonal coupler of the humidity sensor are directly connected with the feed ports at the two ends of the resonant cavity through the gradient microstrip line, and the humidity sensor is used for loading phase change brought by the resonant cavity.

In the humidity sensor, in the reference circuit, a separate phase shifter is arranged between the microstrip branch line orthogonal coupler and the tail end power divider. The phase shifter is essentially a left-right hand composite transmission line, and the principle is to adjust the phase of the phase shifter by adjusting the voltage of a direct current bias circuit of the phase shifter.

The power divider of the humidity sensor is a Wilkinson power divider, and a 100-ohm resistor is connected between two ends on the same side and used for isolating the two ends and simultaneously outputting signals with equal power and equal phase; the phase shifter is a composite right-left hand transmission line, and is connected with a direct current bias circuit, so that the input and output ends of the phase shifter are connected with a blocking capacitor, and a blocking alternating current inductor is connected at a bias interface. The phase shifter can be divided into two identical sections, each section is composed of a microstrip line, a capacitor and an inductor, the capacitor and the inductor are used for forming the left-hand part of the composite left-hand and right-hand transmission line, and the microstrip line is used for forming the right-hand part of the composite left-hand and right-hand transmission line.

The width of a coplanar waveguide feeder line connected with the resonant cavity of the humidity sensor is 2mm, the length of the coplanar waveguide feeder line is 5.5mm, the width of a gap is 0.16mm, the connected gradient microstrip line consists of two sections, the first section is directly connected with the coplanar waveguide feeder line, the width of the first section is 2mm, the length of the first section is 0.91mm, the second section is connected with the first section at the first end, the tail end of the second section is connected with a 50 ohm microstrip line, the width of the second section is 1.08mm in gradient from 2mm, and the length of the second section is 3mm; the width of a coplanar waveguide feeder line connected with the power divider is 1.08mm, the width of the coplanar waveguide feeder line is the same as that of a 50-ohm microstrip line in the interference circuit, the length of the feeder line is 5.6mm, and the width of a gap is 0.118mm.

According to the humidity sensor, the dielectric layers of all dielectric substrates of the resonant cavity are made of Rogers4350, the relative dielectric constant of the humidity sensor is 3.66, the relative magnetic permeability of the humidity sensor is 1, and the loss tangent angle of the humidity sensor is 0.004.

The beneficial effects of the invention are as follows:

1. according to the humidity sensor based on the substrate integrated waveguide and the six-port interference network, one resonant cavity is used as a sensor and the other resonant cavity is used as a reference, phase information is loaded in the six-port interference network, the obvious interference effect is realized, and the humidity sensor has high sensitivity and can meet the miniaturization requirement. When the resonant cavity works at a resonant frequency point, the resonant cavity can be equivalent to an LC circuit, the reflection coefficient of the LC circuit is approximately 1, and because the straight-through end and the coupling end of the coupler are connected with the resonant cavity, signals can be reflected back from the resonant cavity, and the reflected back synthesized signals have lower attenuation.

2. According to the resonant cavity provided by the invention, on the basis of the existing process, the feeder line is designed on the bottom metal of the bottom dielectric substrate, and the groove where the sensing area is located is arranged on the top dielectric substrate, so that the microminiaturization of the resonant cavity and the minimum frequency point design under the corresponding size are realized. The design of the metal grating, the capacitance column and the air filling medium area enables the wet air to freely enter and exit the resonant cavity and to fully react with the humidity sensitive material.

3. The resonant cavity provided by the invention is designed with a six-port interference network, and consists of two power dividers, two couplers and a phase shifter. The signal is input by the first power divider, and is output to the reference circuit and the sensing circuit in equal power and equal phase, and then the signal is input to the input end of the coupler, and the reflected signals of the straight-through end and the coupling end are output from the isolation end of the coupler. At this time, the phase information of the reference and sensing circuits is already different due to the difference of the presence or absence of the humidity sensitive material. A phase shifter is arranged behind a coupler of the reference circuit and used for adjusting the phase information of the reference circuit, the phase shifter has an adjusting range of 90 degrees, the phase difference of two branches can be enabled to be 180 degrees, interference is formed, and in different humidity, namely corresponding to different phase information, a remarkable interference effect can be formed by adjusting the phase shifter, so that the device has high sensitivity.

4. According to the resonant cavity provided by the invention, the phase shifter is composed of the composite left-hand and right-hand transmission lines, and is designed under a balanced structure, so that the whole phase shifter has a broadband characteristic, and the working frequency point can be adjusted in a larger range, so that the whole system has better adjustability and lower loss.

Drawings

FIG. 1 is a schematic cross-sectional view of a resonant cavity of a humidity sensor in accordance with the present invention;

FIG. 2 is a schematic perspective exploded view of a resonant cavity of the humidity sensor of the present invention;

FIG. 3 (a) is a schematic front view of a top dielectric substrate of a resonant cavity of the humidity sensor proposed by the present invention;

FIG. 3 (b) is a schematic diagram of the back side of the top dielectric substrate of the resonant cavity of the humidity sensor proposed in the present invention;

FIG. 4 (a) is a schematic front view of a middle dielectric substrate of a resonant cavity of a humidity sensor proposed by the present invention;

FIG. 4 (b) is a schematic backside view of the middle dielectric substrate of the resonant cavity of the humidity sensor proposed in the present invention;

FIG. 5 (a) is a schematic front view of the bottom dielectric substrate of the resonant cavity of the humidity sensor proposed by the present invention;

FIG. 5 (b) is a schematic backside view of the bottom dielectric substrate of the resonant cavity of the humidity sensor proposed by the present invention;

fig. 6 (a) is a perspective view of a humidity sensor according to the present invention;

fig. 6 (b) is a bottom view of the humidity sensor proposed by the present invention;

FIG. 7 is a circuit diagram of a humidity sensor (without a resonant cavity) according to the present invention;

fig. 8 is a microstrip branch line coupler of the humidity sensor proposed by the present invention;

FIG. 9 is a Wilkinson power divider of the humidity sensor proposed by the present invention;

FIG. 10 is a left and right hand composite transmission line for a humidity sensor in accordance with the present invention;

FIG. 11 is a graph of transmission coefficient versus frequency for humidity sensors of the present invention with humidity sensitive materials of different dielectric constants.

Detailed Description

For better illustration of the design process and purposes, the present invention is further described below with reference to the following examples and the accompanying drawings:

the humidity sensor provided by the invention comprises a passive resonant cavity and a six-port interference circuit.

The passive resonant cavity is a substrate integrated waveguide reentrant resonant cavity which is formed by three layers of dielectric substrates which are longitudinally superposed. The compact structure formed by the non-metallized through holes in the top layer, the air filling type medium in the middle layer and the metallized through holes in the capacitor column in the bottom layer greatly enhances the circulation of the wet air in the resonant cavity, so that water molecules in the wet air can be more fully attached to the resonant cavity and fully react with the humidity sensitive material in the groove. The bottom metal of the dielectric substrate is designed with a coplanar waveguide feed structure for connecting an external six-port interference circuit.

The six-port interference circuit consists of a Wilkinson power divider, a quadrature coupler, a phase shifter and a resonant cavity. Signals are fed in from the power divider, equal power equal phase is distributed to the two branches, one branch is used as a reference, and the other branch is used as a sensing branch. By loading different phase information brought by the resonant cavity, the two lines can obtain different phases, wherein the reference resonant cavity does not have a humidity sensitive material, and the humidity sensitive material is placed in the sensing resonant cavity. By adjusting the phase shifter, two lines can form interference, thereby realizing high-sensitivity humidity sensing measurement.

Specifically, the two passive resonant cavities of the invention are provided with the same structure and respectively used as the sensing cavity 3 and the reference cavity 1.

In an embodiment of the present invention, as shown in fig. 1 to 5 (b), a resonant cavity structure serving as a reference cavity (1) and a sensing cavity (3) is formed by longitudinally stacking an upper dielectric substrate (1-1), a middle dielectric substrate (1-2), and a bottom dielectric substrate (1-3), and each dielectric substrate includes a top metal layer, a dielectric layer, and a bottom metal layer. A circle of metalized through holes (1-1-1, 1-2-1 and 1-3-1) are distributed in each layer of dielectric substrate, and the metalized through holes of each layer are connected with the upper layer and the lower layer of the layer. The circle formed by the metalized through holes (1-1-1, 1-2-1 and 1-3-1) of each layer has the same radius and the centers of the circles are aligned, so that equivalent metal walls of the resonant cavity are formed and a resonant cavity area is surrounded.

In a resonant cavity region of the resonant cavity, a metalized through hole array (1-3-4) distributed in a circular shape is arranged at the center of a bottom layer dielectric substrate (1-3), the bottom of the metalized through hole array (1-3-4) is connected with bottom layer metal, and the top of the metalized through hole array is connected with a circular metal layer (1-3-3) with a radius slightly larger than that of the metalized through hole array, so that a capacitor column (1-3-2) of the resonant cavity (1) is formed. The round metal layer (1-3-3) and the equivalent metal wall are all etched away.

In the resonant cavity area of the resonant cavity, the middle layer dielectric substrate (1-2) is completely hollowed in the formed resonant cavity area and is directly filled with air to form an air filling area.

In the resonant cavity area of the resonant cavity, the lower metal of the top dielectric substrate (1-1) only exists in the area outside the equivalent metal wall. A circular groove (1-1-3) is dug from bottom to top in the central area of the top dielectric substrate (1-1) and used for forming a sensing area, the circle center of the circular groove is aligned with the circle center of the circular metal layer, and the diameters of the circular groove and the circle center of the circular metal layer are equal. The dielectric layer of the top dielectric substrate (1-1) is provided with unmetallized through hole arrays (1-1-4) distributed in a circle, the positions of the unmetallized through hole arrays (1-1-4) are equivalent to the positions between the metal walls and the circular grooves (1-1-3), and the unmetallized through hole arrays (1-1-4) and the circular grooves (1-1-3) are arranged in an array mode, and therefore the unmetallized through hole arrays (1-1-4) and the top metal of the substrate jointly form a metal grid (1-1-2).

The electromagnetic wave can be well shielded in the area surrounded by the metal walls, the metal grids (1-1-2), the capacitance columns (1-3-2) and the bottom metal of the bottom dielectric substrate (1-3), so that the electromagnetic field is restricted in the resonant cavity. Meanwhile, the unmetallized through hole array (1-1-4) in the top dielectric substrate (1-1) is connected with the top metal of the top dielectric substrate (1-1) to form a metal grid (1-1-2) which is approximately equivalent to a metal wall, and is connected with the air dielectric filling area of the middle dielectric substrate (1-2) and the capacitor column (1-3-2) of the bottom dielectric substrate (1-3) to greatly enhance the air fluidity, so that the wet air can be well attached in the resonant cavity to fully react with the humidity sensitive material.

The bottom layer metal of the bottom layer dielectric substrate (1-3) of the resonant cavity is etched with a coplanar waveguide feeder line (1-3-5), and in order to realize impedance matching with an external circuit and reduce signal loss, a microstrip line connected with the feeder line is in a gradual change form and is used for being connected with an external six-port interference circuit (2).

The resonant cavity structure is optimized, and the determined parameters are as follows: the three dielectric substrates are all Rogers4350, the relative dielectric constant of the three dielectric substrates is 3.66, the relative magnetic permeability of the three dielectric substrates is 1, and the loss tangent angle of the three dielectric substrates is 0.004. The thickness of the top dielectric substrate (1-1) is 1.016mm, the thickness of the middle dielectric substrate (1-2) and the thickness of the bottom dielectric substrate (1-3) are both 0.508mm, and the thickness of the metal layer is both 0.035mm. The length and width of the three dielectric substrates are all 27mm, the diameter of the middle coaxial resonant cavity is 20mm, and the diameter of the circular metal layer (1-3-3) is 8mm. The diameter of a circular array metalized through hole (1-1-1) for forming an equivalent metal wall is designed to be 0.8mm, and the hole spacing is 1.13mm; the diameter of a metalized through hole (1-3-4) for forming the capacitor column (1-3-2) is designed to be 1.6mm, and the hole spacing is 2.4mm; the diameter of the unmetallized through holes (1-1-4) for forming the metal grid (1-1-2) is designed to be 0.8mm; the circle center of the groove (1-1-3) is aligned with the circle center of the circular metal layer (1-3-3), the diameter is 8mm, and the depth is 0.5mm. The width of a coplanar waveguide feeder line (1-3-5) connected with the resonant cavity (1) is 2mm, the length of the coplanar waveguide feeder line is 5.5mm, the width of a gap is 0.16mm, the connected gradient microstrip line is composed of two sections, the first section is directly connected with the coplanar waveguide feeder line (1-3-5), the width of the first section is 2mm, the length of the first section is 0.91mm, the first section of the second section is connected with the first section, the tail end of the second section is connected with a 50 ohm microstrip line, the width of the second section is 1.08mm in gradient from 2mm, and the length of the second section is 3mm.

In an embodiment of the present invention, as shown in fig. 6 (a) and 6 (b), the complete resonant cavity structure further has 8 screw holes, which have a diameter of 1.6mm and have no influence on the resonant cavity, and only play a role of fixing. Secondly, the whole system is provided with two resonant cavities, wherein one resonant cavity is used as a reference cavity 1, no humidity sensitive material is added in a circular groove (1-1-3) of the reference cavity, the other resonant cavity is used as a sensing cavity 3, and the humidity sensitive material is added in the circular groove (1-1-3) of the sensing cavity. Since the dielectric constant of air can be approximately regarded as constant with the change of humidity, the phase information of the resonant cavity as a reference is almost constant, while the phase information of the resonant cavity as a sensing changes with the change of the dielectric constant of the humidity sensitive material.

The complete sensor also comprises a six-port interference circuit (2), and two resonant cavities, namely a reference cavity (1) and a sensing cavity (3), are respectively arranged on the reference circuit and the sensing circuit.

In an embodiment of the present invention, the principle of the six-port interference network circuit (2) is as shown in fig. 7, and comprises two power dividers (2-1, 2-5), two couplers (2-2, 2-6) and a phase shifter (2-3). The signal is input by a first power divider (2-1), and is output to a reference circuit and a sensing circuit in equal power and equal phase. Then the signal is input into the input end (2-2-1) of the coupler (2-2), and the reflected signals of the straight-through end (2-2-2) and the coupling end (2-2-3) are output from the isolation end (2-2-4) thereof (taking a reference circuit as an example). At this time, the phase information of the reference and sensing circuits is already different due to the difference of the presence or absence of the humidity sensitive material. A phase shifter (2-3) is arranged behind a coupler (2-2) of the reference circuit and used for adjusting the phase information of the reference circuit, the phase shifter (2-3) has an adjusting range of 90 degrees, the phase of two branches can be enabled to be different by 180 degrees, interference is formed, and in different humidity, namely corresponding to different phase information, a remarkable interference effect can be formed by adjusting the phase shifter (2-3), so that the device has high sensitivity. The head-tail power dividers (2-1, 2-5) are connected with coplanar waveguide feeder lines (2-4), in order to realize impedance matching, the width of the coplanar waveguide feeder line is 1.08mm, the width of the coplanar waveguide feeder line is the same as that of a 50-ohm microstrip line in an interference circuit, the length of the feeder line is 5.6mm, and the gap width is 0.118mm. Two rectangular metal sheets with the length of 8.33mm and the width of 5.56mm are arranged on two sides of the coplanar waveguide feeder line, the diameter of a large through hole on the rectangular metal sheets is 2mm and is used for being connected with a welding-free head without being metalized, and the diameters of the other small through holes are 0.8mm and are used for representing grounding and needing metallization.

In one embodiment of the invention, the couplers are as shown in fig. 8, the couplers (2-2, 2-6) are microstrip branch line orthogonal couplers, wherein the length of the through arm is 4.8mm and the width is 1.84mm, and the length of the coupling arm is 4.9mm and the width is 1.08mm. When the power amplifier works at 7.9Ghz, both S21 and S31 are-3 dB, namely, equal power is output at a straight-through end (2-2-2) and a coupling end (2-2-3), and the design requirement is met. When the impedance of an external circuit connected with the straight-through end and the coupling end is not matched, reflection occurs, the resonant cavity can be equivalent to an LC circuit during resonance, so that the reflection coefficient is approximately 1, phase information is loaded to the isolation end (2-2-4) to be output through a reflected signal, and meanwhile, the system has low attenuation.

In one embodiment of the present invention, as shown in fig. 9, the wilkinson power dividers (2-1, 2-5) provided by the present invention have the most important function of outputting signals with equal power and equal phase, and at 7.9Ghz, both S21 and S31 are-3 dB. And a 100 ohm resistor is also connected between the two branches of the power divider and used for realizing the isolation between the output ports. Likewise, at the end is used as a power combiner (2-5) but the principle is the same.

In an embodiment of the present invention, the phase shifter (2-3) proposed by the present invention is composed of a left-right hand composite transmission line as shown in fig. 10, and the structure can be regarded as being composed of two sections with an adjustment range of 90 °. Varactor smv1430 and inductor L form the left hand section and the transmission line forms the right hand section, where the phase shift in the right hand section is approximately constant while the phase shift in the left hand section varies with the bias voltage, since the capacitance here is a smv1430 varactor. The junction capacitance formula is

In the formula C ₀ The junction capacitance is 1.11pF in the unbiased state ₀ 0.86V, γ is 0.5. Because of the DC offset part, the DC offset is realizedAn AC isolating inductor Lac is connected between the circuits (2-7) and the left-right hand composite transmission line, and a DC isolating capacitor Cdc is connected between the left-right hand composite transmission line and the main circuit. When the direct current voltage is increased, the capacitance value of the variable capacitance diode is reduced, and the phase shift brought by the left-hand part is increased, which is the working principle of the phase shifter. Because the left-hand and right-hand composite transmission lines are always in a balanced state within a certain voltage regulation range, the working frequency point has a better regulation range. Through optimization, the alternating current isolation inductor is selected to be MHQ0603P4N1BT000, and the self-resonance frequency of the alternating current isolation inductor is 8.3GHz; the blocking capacitor is GJM1555C1H1R1CB01, and the self-resonant frequency of the blocking capacitor is 8GHz; the left-hand inductance is MLG0603S0N3BT000; the left-hand capacitance is smv1430, and the DC bias voltage adjustment range is set to 0V to 30V.

FIG. 11 is a graph of transmission coefficient versus frequency for a humidity sensitive material of different dielectric constant for the entire system, where the resonant cavity of the reference circuit has no humidity sensitive material but is air filled, i.e., has a dielectric constant of 1, and the resonant cavity in the sensing circuit has a humidity sensitive material with a dielectric constant that varies with humidity. Fig. 11 shows the case of the humidity sensitive material with dielectric constants of 2, 3 and 10 in the resonant cavity of the sensing circuit, respectively, when the two branches are out of phase by 180 ° by adjusting the phase shifter to form interference, as can be seen from fig. 11, the whole system has significant interference effect and high sensitivity.

According to the humidity sensor based on the substrate integrated waveguide and the six-port interference network, the resonant cavity adopts the bottom feed and the ultrathin medium substrate, and the ultra-compact structure is realized. The reference resonant cavity and the sensing resonant cavity are designed by the existence of the humidity sensitive material, the change of the humidity sensitive material is very obvious by the metal grating, the air filling area and the capacitance column, and finally the phase information brought by the two resonant cavities has obvious difference. In a six-port network adopted by an interference circuit, due to the phase loading function of the coupler, two branches have approximately equal difference loss, and an attenuator is not required to be designed in the prior research. The phase shifter is designed by the left-hand and right-hand composite transmission lines, so that the phase shifter has a wide phase adjusting range and has broadband characteristics by depending on a balance structure of the left-hand and right-hand composite transmission lines, and the wide frequency point design can be met. The whole system has extremely strong interference effect and ultrahigh sensitivity so as to meet the urgent requirements of various subject fields on high-performance air humidity sensors.

The present invention is not limited to the above-described embodiments, and various modifications and variations of the present invention are intended to be included within the scope of the claims and the equivalent technical scope of the present invention if they do not depart from the spirit and scope of the present invention.

Claims

1. A humidity sensor based on a substrate integrated waveguide and a six-port interference network is characterized by comprising a passive resonant cavity and a six-port interference circuit; the passive resonant cavities are provided with two same structures and respectively used as a sensing cavity and a reference cavity;

the passive resonant cavity is provided with three layers of dielectric substrates, each layer of dielectric substrate comprises a top layer metal, a dielectric layer and a bottom layer metal, and a substrate integrated waveguide reentrant resonant cavity is formed in the three layers of dielectric substrates; a circle of metalized through holes are distributed in each layer of medium substrate, the metalized through holes of each layer are connected with the top layer metal and the bottom layer metal of the layer, the radiuses of circles surrounded by the metalized through holes of each layer are the same, and the circle centers are aligned, so that an equivalent metal wall of the resonant cavity is formed, and a resonant cavity area is arranged inside the equivalent metal wall;

in a resonant cavity region of the passive resonant cavity, a metalized through hole array which is circularly arranged is arranged at the center of a bottom dielectric substrate, the bottom and the top of the metalized through hole array are respectively connected with bottom metal and top metal of the bottom dielectric substrate, the top metal is partially etched in the resonant cavity region, and only a circular metal layer with the center being slightly larger than the radius of the metalized through hole array is left, so that a capacitor column of the reference cavity is formed;

in the resonant cavity region of the passive resonant cavity, the middle layer dielectric substrate is completely hollowed out to be used as an air filling region;

in the resonant cavity region of the passive resonant cavity, the bottom metal of the top dielectric substrate is completely etched; a circular groove is dug from bottom to top in the central area of the top dielectric substrate and is used for forming a sensing area; an unmetallized through hole array which is circularly arranged is arranged in the dielectric layer of the top dielectric substrate and surrounds the circular groove, and the unmetallized through hole array and the top metal of the substrate form a metal grid together;

the passive resonant cavity enables wet air to freely enter and exit the resonant cavity region through the metal grating of the top-layer dielectric substrate, the air filling region of the middle-layer dielectric substrate and the capacitor column of the bottom-layer dielectric substrate, so that the liquidity of the passive resonant cavity is enhanced;

the passive resonant cavity is provided with a coplanar waveguide feeder line etched in the bottom metal of the bottom dielectric substrate, and a microstrip line connected with the feeder line is in a gradual change form and is used for connecting an external six-port interference circuit;

the six-port interference circuit comprises two power dividers, two couplers and a phase shifter, wherein one path of the two power dividers is connected with a reference cavity through one coupler to form a reference circuit, and the phase shifter is arranged in the reference circuit; the other path of the two power dividers is connected with the sensing cavity through the other coupler to form a sensing circuit;

2. The humidity sensor of claim 1, wherein: the six-port interference circuit inputs signals from one side of a single port of one power divider, outputs half-power equal-phase signals from the other two ends in an equal-power mode, is respectively connected with two symmetrically arranged couplers, outputs signals from ports on the same side of the two couplers, and finally inputs the signals from the two ends on the same side of the other power divider, and outputs signals from one side of the single port to respectively form a reference circuit and a sensing circuit; in the reference circuit, a separate phase shifter is arranged between a coupler and a power divider at the tail end, the phase shifter is essentially a left-right-hand composite transmission line, and the phase of the phase shifter is adjusted by adjusting the voltage of a direct current bias circuit of the phase shifter.

3. A humidity sensor according to claim 1 or 2, wherein: the coupler is a microstrip branch line orthogonal coupler, and a straight-through end and a coupling end of the coupler are directly connected with feed ports at two ends of the passive resonant cavity through gradient microstrip lines and are used for loading phase change brought by the resonant cavity.

4. A humidity sensor according to claim 1 or 2, wherein: the power divider is a Wilkinson power divider, and a 100-ohm resistor is connected between two ends on the same side and used for isolating the two ends and outputting signals with equal power and equal phase.

5. A humidity sensor according to claim 1 or 2, wherein: the phase shifter is a composite left-right-hand transmission line and is connected with a direct-current bias circuit, a blocking capacitor is connected at the input end and the output end, and an alternating-current blocking inductor is connected at a bias interface; the phase shifter can be divided into two identical sections, each section is composed of a microstrip line, a capacitor and an inductor, the capacitor and the inductor are used for forming the left-hand part of the composite left-hand and right-hand transmission line, and the microstrip line is used for forming the right-hand part of the composite left-hand and right-hand transmission line.

6. A humidity sensor according to claim 1 or 2, wherein: the circle centers of the circular groove and the circular metal layer are aligned in the horizontal direction, and the radiuses of the circular groove and the circular metal layer are equal.

7. A humidity sensor according to claim 2, wherein: the width of a coplanar waveguide feeder line connected with the passive resonant cavity is 2mm, the length of the coplanar waveguide feeder line is 5.5mm, the width of a gap is 0.16mm, the connected gradient microstrip line is composed of two sections, the first section is directly connected with the coplanar waveguide feeder line, the width of the first section is 2mm, the length of the first section is 0.91mm, the first section of the second section is connected with the first section, the tail end of the second section is connected with a 50-ohm microstrip line, the width of the second section is 1.08mm and the length of the second section is 3mm, the width of the coplanar waveguide feeder line connected with the power divider is 1.08mm, the width of the coplanar waveguide feeder line is the same as the width of the 50-ohm microstrip line in the interference circuit, the length of the feeder line is 5.6mm, and the width of the gap is 0.118mm.

8. A humidity sensor according to claim 1 or 2, wherein: the dielectric layers of all dielectric substrates of the passive resonant cavity are made of Rogers4350, the relative dielectric constant of the passive resonant cavity is 3.66, the relative magnetic permeability of the passive resonant cavity is 1, and the loss tangent angle of the passive resonant cavity is 0.004.