A humidity sensor based on substrate-integrated waveguide reentrant resonant cavity and six-port interferometer network

Abstract

A humidity sensor based on substrate integrated waveguide and six-port interference network includes a passive resonant cavity and a six-port interference circuit. The passive resonant cavity is composed of three layers of dielectric substrates stacked vertically. The compact structure formed by the non-metallized through-holes on the top layer, the air-filled dielectric in the middle layer and the metallized through-holes on the capacitor column on the bottom layer greatly enhances the fluidity of the humid air in the resonant cavity, so that the water molecules in the humid air can be more fully attached to the resonant cavity and fully react with the hygroscopic material in the groove. The bottom metal of the dielectric substrate is designed with a coplanar waveguide feeding structure, which is connected to an external six-port interference circuit. The six-port interference circuit is composed of a power divider, a coupler, a phase shifter and a resonant cavity. The signal is fed from the power divider, and the equal power and phase are distributed to the reference and sensing branches. By loading the different phase information brought by the resonant cavity and adjusting the phase shifter, the two lines are interfered, thereby realizing high-sensitivity humidity sensing measurement.

Description

Technical Field

[0001] The invention belongs to the field of sensors, and in particular relates to a microwave active sensor suitable for detecting relative humidity of air. Background Art

[0002] As an important detection technology, humidity measurement is widely used in environmental monitoring, agricultural production, food safety, clinical medicine, etc. Real-time and high-precision humidity measurement depends on high-performance sensors. Therefore, the development of high-performance humidity sensors is of great significance to modern humidity measurement.

[0003] As the most common humidity sensors, resistive humidity sensors and capacitive humidity sensors are widely used for their low power consumption, low cost and fast response. However, they also have some disadvantages, such as large size, difficult to integrate with the system, inability to work in harsh environments, limited sensing distance, etc. Other types of sensors, such as microwave-based humidity sensors, have attracted widespread attention for their low cost, stable response, easy integration and higher frequency. In 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. To make a significant improvement on their basis, it is necessary to improve the structure. Among them, the design of the resonator has a great influence on the performance of microwave humidity sensors. Among the many resonators, the substrate-integrated waveguide reentrant resonant cavity is widely used in high-sensitivity sensing due to its high quality factor and high density of electric field in the bandgap capacitor region. Interferometry relies on a process of phase change detection and wave superposition. Based on the principle of interference, the sensor is designed as two branches with a phase difference of 180 degrees but approximately equal attenuation, one for reference and the other for sensing. By detecting the S21 signal, a significant interference effect can be detected. This technology has been proven to have high sensitivity. However, existing interference systems still have problems such as large size and poor adjustability. Summary of the invention

[0004] The present invention mainly aims at the problems of large size and low sensitivity of previous interference sensors, and proposes a humidity sensor based on a substrate integrated waveguide reentrant resonant cavity and a six-port interference network. The humidity sensor adopts a combination of a sensing cavity, a reference cavity and a six-port interference circuit, and utilizes the phase loading characteristics of the through end and the coupled end of the microstrip branch line orthogonal coupler and the phase shift characteristics, broadband balance characteristics and adjustability of the left-handed composite transmission line to improve the system adjustability and reduce the system size on the basis of previous interference sensors.

[0005] The design concept of the present invention is: in the sensing cavity and the reference cavity, metallized through holes and metal grids are used to form equivalent metal walls to effectively confine electromagnetic waves in the resonant cavity while greatly improving the fluidity of humid air in the resonant cavity; in order to balance the maximization of the reduction of the resonant cavity size and the existing milling groove process, the feed line is designed on the bottom metal of the bottom dielectric substrate, and is connected to the straight-through end and the coupling end of the microstrip branch line orthogonal coupler through a gradient microstrip line, and then the signal is output from the isolation end of the coupler to realize the loading of phase information; the input port and the output port of the entire system are connected through a Wilkinson power divider to ensure that the phases of the reference and sensing lines are equal; in order to ensure that the two branches have a phase difference of 180 degrees, a phase shifter composed of left-handed composite transmission lines is placed on the reference line, and the phase shift of the entire branch can be adjusted in real time by adjusting the bias voltage. At the same time, the phase shifter is in a balanced structure so that the S21 difference loss is extremely small, which greatly improves the adjustability and interference effect of the interference system.

[0006] The technical solution of the present invention is as follows:

[0007] A humidity sensor based on substrate integrated waveguide and six-port interference network includes a passive resonant cavity and a six-port interference circuit. Two passive resonant cavities are provided, one as a sensing cavity and the other as a reference cavity, and the other structures of the two cavities are exactly the same except whether humidity sensitive materials are placed therein.

[0008] The passive resonant cavity has three layers of dielectric substrates, each layer of the dielectric substrate includes a top metal layer, a dielectric layer and a bottom metal layer, and a substrate integrated waveguide reentrant resonant cavity is formed in the three layers of dielectric substrates. A circle of metallized through holes is distributed in each layer of the dielectric substrate, and the metallized through holes of each layer are connected to the top metal layer and the bottom metal layer of the layer. The radius of the circle formed by the metallized through holes of each layer is the same and the center of the circle is aligned, thereby forming an equivalent metal wall of the resonant cavity, and the area within the equivalent metal wall is the resonant cavity area.

[0009] In the resonant cavity region of the passive resonant cavity, a metallized through hole array arranged in a circular pattern is provided at the center of the bottom dielectric substrate, the bottom and top of the metallized through hole array are respectively connected to the bottom metal and top metal of the bottom dielectric substrate, and the top metal is partially etched away in the resonant cavity region, leaving only a circular metal layer in the center that is slightly larger than the radius of the metallized through hole array, thereby forming a capacitor column of the reference cavity.

[0010] In the resonant cavity region of the passive resonant cavity, the intermediate dielectric substrate is completely hollowed out to serve as an air-filled region.

[0011] In the resonant cavity area of ​​the passive resonant cavity, the bottom metal of the top dielectric substrate is completely etched away. A circular groove is dug from bottom to top in the central area of ​​the top dielectric substrate to form a sensing area; an array of unmetallized through holes arranged in a circular pattern is provided in the dielectric layer of the top dielectric substrate and around the circular groove, and the array of unmetallized through holes and the top metal of the substrate together form a metal grid.

[0012] The passive resonant cavity allows moist air to freely enter and exit the resonant cavity area through the metal grid of the top dielectric substrate, the air filling area of ​​the middle dielectric substrate, and the capacitor column of the bottom dielectric substrate, thereby enhancing its fluidity.

[0013] The passive resonant cavity has a coplanar waveguide feed line etched on the bottom metal of the bottom dielectric substrate, and the microstrip line connected to the feed line is in a gradient form for connecting an external six-port interference circuit.

[0014] The six-port interference circuit includes two power dividers, two couplers and a phase shifter, wherein one path of the two power dividers is connected to a reference cavity through a coupler to form a reference circuit, and a phase shifter is arranged in the reference circuit; the other path of the two power dividers is connected to a sensing cavity through another coupler to form a sensing circuit.

[0015] The six-port interference circuit feeds a signal into one end of the power divider through a coplanar waveguide feeding line.

[0016] The humidity sensor of the above structure, by providing an array of unmetallized through holes in the top dielectric substrate, not only greatly increases the air flowability, but also forms a metal grid connected with the top metal of the dielectric substrate, which can be approximately equivalent to a metal wall.

[0017] In the humidity sensor of the above structure, the area surrounded by the metal wall, the metal grid, the capacitor column and the bottom metal of the bottom dielectric substrate shields the electromagnetic wave well, so that the electromagnetic field is confined in the resonant cavity.

[0018] The interference circuit of the humidity sensor inputs a signal from one end of a power divider, outputs half-power equal-phase signals at equal power from the other two ends, connects a reference cavity and a sensing cavity through symmetrically placed microstrip branch line couplers, and outputs signals at the same-side ports of the microstrip branch line orthogonal couplers, and finally inputs the signals at the two ends on the same side of another power divider at the end, and outputs the signals at one side of the single port.

[0019] In the humidity sensor, the through and coupling ends of the microstrip branch line orthogonal coupler are directly connected to the feeding ports at both ends of the resonant cavity through a gradient microstrip line, so as to load the phase change brought by the resonant cavity.

[0020] In the humidity sensor, in the reference circuit, a separate phase shifter is placed between the microstrip branch line orthogonal coupler and the terminal power divider. The phase shifter is essentially a left-handed composite transmission line, and the principle is to adjust the phase of the phase shifter by adjusting the voltage of its DC bias circuit.

[0021] The power divider of the humidity sensor is a Wilkinson power divider, and a 100-ohm resistor is connected between the two ends on the same side to isolate the two ends and output signals of equal power and equal phase at the same time; the phase shifter is a composite left-handed transmission line, and since a DC bias circuit is connected, a DC-isolating capacitor is connected to the input and output ends of the phase shifter, and an AC-isolating inductor is connected at the bias interface. The phase shifter can be divided into two identical sections, each of which is composed of a microstrip line, a capacitor, and an inductor. The capacitor and the inductor are used to form the left-hand part of the composite left-handed transmission line, and the microstrip line is used to form the right-hand part of the composite left-handed transmission line.

[0022] In the humidity sensor, the coplanar waveguide feed line connected to the resonant cavity has a width of 2 mm, a length of 5.5 mm, and a gap width of 0.16 mm. The connected gradient microstrip line consists of two sections, the first section is directly connected to the coplanar waveguide feed line, has a width of 2 mm, and a length of 0.91 mm, the first section of the second section is connected to the first section, and the end is connected to the 50 ohm microstrip line, the width gradually changes from 2 mm to 1.08 mm, and the length is 3 mm; the coplanar waveguide feed line connected to the power divider has a width of 1.08 mm, which is the same as the width of the 50 ohm microstrip line in the interference circuit, the length of the feed line is 5.6 mm, and the gap width is 0.118 mm.

[0023] In the humidity sensor, the dielectric layer material of all dielectric substrates of the resonant cavity is Rogers 4350, which has a relative dielectric constant of 3.66, a relative magnetic permeability of 1, and a loss tangent of 0.004.

[0024] The beneficial effects of the present invention are as follows:

[0025] 1. The humidity sensor based on substrate integrated waveguide and six-port interference network proposed by the present invention has two resonant cavities, one for sensing and one for reference, and loads phase information in the six-port interference network, thereby achieving a significant interference effect, having high sensitivity and meeting the requirements of miniaturization. When the resonant cavity works at the resonant frequency, it can be equivalent to an LC circuit, and its reflection coefficient is approximately 1. Since the through end and the coupling end of the coupler are connected to the resonant cavity, the signal will be reflected back from the resonant cavity and the synthesized signal reflected back has a lower attenuation.

[0026] 2. The resonant cavity proposed by the present invention, based on the existing process, designs the feed line on the bottom metal of the bottom dielectric substrate, and places the groove where the sensing area is located on the top dielectric substrate, thereby realizing the ultra-miniaturization of the resonant cavity and the minimum frequency design under the corresponding size. The design of the metal grid, capacitor column, and air-filled dielectric area allows humid air to freely enter and exit the resonant cavity, and can react more fully with the humidity-sensitive material.

[0027] 3. The resonant cavity proposed in the present invention is designed with a six-port interference network, which 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 and sensor circuits with equal power and phase. Then the signal is input to the input end of the coupler, and the reflected signals of the through end and the coupled end are output from its isolation end. At this time, due to the difference in the presence or absence of moisture-sensitive materials, the phase information of the reference and sensor circuits is already different. A phase shifter is placed after the coupler of the reference circuit to adjust the phase information of the reference circuit. The phase shifter has an adjustment range of 90°, which can make the phases of the two branches differ by 180°, forming interference, and at different humidity, that is, corresponding to different phase information, a significant interference effect can be formed by adjusting the phase shifter, so that the device has high sensitivity.

[0028] 4. The resonant cavity and phase shifter proposed in the present invention are composed of composite left-handed and right-handed transmission lines, and are designed in a balanced structure. The entire phase shifter has a wide-band characteristic, and the operating frequency can be adjusted within a larger range, so that the entire system has better adjustability and lower loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a cross-sectional schematic diagram of the resonant cavity of the humidity sensor proposed by the present invention;

[0030] Figure 2 It is a three-dimensional exploded schematic diagram of the resonant cavity of the humidity sensor proposed by the present invention;

[0031] FIG3( a ) is a front view of a top dielectric substrate of a resonant cavity of a humidity sensor according to the present invention;

[0032] FIG3( b ) is a schematic diagram of the back side of the top dielectric substrate of the resonant cavity of the humidity sensor proposed by the present invention;

[0033] FIG. 4( a ) is a front view of a dielectric substrate in an intermediate layer of a resonant cavity of a humidity sensor according to the present invention;

[0034] FIG4( b ) is a schematic diagram of the back side of the intermediate dielectric substrate of the resonant cavity of the humidity sensor proposed by the present invention;

[0035] FIG5(a) is a front view schematic diagram of the bottom dielectric substrate of the resonant cavity of the humidity sensor proposed by the present invention;

[0036] FIG5( b ) is a schematic diagram of the back side of the bottom dielectric substrate of the resonant cavity of the humidity sensor proposed by the present invention;

[0037] FIG6( a ) is a perspective view of a humidity sensor according to the present invention;

[0038] FIG6( b ) is a bottom view of the humidity sensor provided by the present invention;

[0039] Figure 7 is a circuit diagram of the humidity sensor proposed by the present invention (excluding the resonant cavity);

[0040] Figure 8 It is a microstrip branch line coupler of the humidity sensor proposed by the present invention;

[0041] Fig. 9 It is a Wilkinson power divider of the humidity sensor proposed by the present invention;

[0042] Fig.10 It is the left-handed and right-handed composite transmission line of the humidity sensor proposed by the present invention;

[0043] Fig.11 This is a relationship diagram between the transmission coefficient and frequency of the humidity sensor proposed by the present invention under humidity-sensitive materials with different dielectric constants. DETAILED DESCRIPTION

[0044] In order to better explain the design process and purpose, the present invention is further described below in conjunction with embodiments and drawings:

[0045] The humidity sensor proposed by the present invention comprises a passive resonant cavity and a six-port interference circuit.

[0046] The passive resonant cavity is a substrate-integrated waveguide reentrant resonant cavity, and the resonant cavity is composed of three layers of dielectric substrates stacked vertically. The compact structure formed by the non-metallized through-holes on the top layer, the air-filled dielectric in the middle layer, and the metallized through-holes on the capacitor columns on the bottom layer greatly enhances the fluidity of the humid air in the resonant cavity, so that the water molecules in the humid air can more fully adhere to the resonant cavity and fully react with the moisture-sensitive material in the groove. The bottom metal of the dielectric substrate is designed with a coplanar waveguide feeding structure for connecting an external six-port interference circuit.

[0047] The six-port interference circuit is composed of a Wilkinson power divider, an orthogonal coupler, a phase shifter, and a resonant cavity. The signal is fed from the power divider and distributed to two branches with equal power and phase, one for reference and the other for sensing. By loading the different phase information brought by the resonant cavity, the two lines will obtain different phases, where the reference resonant cavity does not have a hygroscopic material, while the sensing resonant cavity has a hygroscopic material. By adjusting the phase shifter, the two lines can be made to interfere, thereby achieving high-sensitivity humidity sensing measurement.

[0048] Specifically, the passive resonant cavity of the present invention is provided with two identical structures, which serve as the sensing cavity 3 and the reference cavity 1 respectively.

[0049] In one embodiment of the present invention, Figure 1 to Figure 5(b) As shown, the resonant cavity structure serving as the reference cavity (1) and the sensing cavity (3) is formed by longitudinally stacking an upper dielectric substrate (1-1), an intermediate dielectric substrate (1-2), and a bottom dielectric substrate (1-3), and each dielectric substrate layer includes a top metal layer, a dielectric layer, and a bottom metal layer. In each dielectric substrate layer, a circle of metallized through holes (1-1-1, 1-2-1, 1-3-1) is distributed, and the metallized through holes of each layer are connected to the upper and lower metal layers of the layer. The circles formed by the metallized through holes (1-1-1, 1-2-1, 1-3-1) of each layer have the same radius and are aligned in center, thereby forming an equivalent metal wall of the resonant cavity and enclosing a resonant cavity region.

[0050] In the resonant cavity region of the resonant cavity, a metallized through hole array (1-3-4) distributed in a circular pattern is provided at the center of the bottom dielectric substrate (1-3), the bottom of the metallized through hole array (1-3-4) is connected to the bottom metal, and the top is connected to a circular metal layer (1-3-3) slightly larger than the radius thereof, thereby forming a capacitor column (1-3-2) of the resonant cavity (1). The space between the circular metal layer (1-3-3) and the equivalent metal wall is completely etched away.

[0051] In the resonant cavity region of the resonant cavity, the intermediate layer dielectric substrate (1-2) is completely hollowed out in the formed resonant cavity region and is directly filled with air to form an air-filled region.

[0052] In the resonant cavity region of the resonant cavity, the lower metal of the top dielectric substrate (1-1) only exists in the region outside the equivalent metal wall. In the central region of the top dielectric substrate (1-1), a circular groove (1-1-3) is dug from bottom to top to form a sensing region. The center of the circular groove is aligned with the center of the circular metal layer and has the same diameter. The dielectric layer of the top dielectric substrate (1-1) is provided with an unmetallized through hole array (1-1-4) distributed in a circular pattern. The unmetallized through hole array (1-1-4) is located between the equivalent metal wall and the circular groove (1-1-3) and is arranged in an array around the circular groove (1-1-3). Thus, the unmetallized through hole array (1-1-4) and the top metal of the substrate together form a metal grid (1-1-2).

[0053] The area surrounded by the above metal wall, metal grid (1-1-2), capacitor column (1-3-2) and the bottom metal of the bottom dielectric substrate (1-3) can well shield electromagnetic waves, so that the electromagnetic field is confined in the resonant cavity. At the same time, the unmetallized through-hole array (1-1-4) in the top dielectric substrate (1-1) is not only connected to 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, but also connected to 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), which greatly enhances the fluidity of the air, so that the humid air can be well attached to the resonant cavity and fully react with the humidity sensitive material.

[0054] The bottom metal of the bottom dielectric substrate (1-3) of the resonant cavity is etched with a coplanar waveguide feed line (1-3-5). In order to achieve impedance matching with an external circuit and reduce signal loss, a microstrip line connected to the feed line is in a gradient form and is used to connect an external six-port interference circuit (2).

[0055] After optimization, the parameters of the above resonant cavity structure are as follows: the three dielectric substrates are all Rogers4350, with a relative dielectric constant of 3.66, a relative magnetic permeability of 1, and a loss tangent of 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 bottom dielectric substrate (1-3) are both 0.508mm, and the thickness of the metal layer is 0.035mm. The length and width of the three dielectric substrates are both 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 the circular array metallized through hole (1-1-1) used to form an equivalent metal wall is designed to be 0.8 mm, and the hole spacing is 1.13 mm; the diameter of the metallized through hole (1-3-4) used to form a capacitor column (1-3-2) is designed to be 1.6 mm, and the hole spacing is 2.4 mm; the diameter of the unmetallized through hole (1-1-4) used to form a metal grid (1-1-2) is designed to be 0.8 mm; the center of the groove (1-1-3) is aligned with the center of the circular metal layer (1-3-3), the diameter is 8 mm, and the depth is 0.5 mm. The coplanar waveguide feed line (1-3-5) connected to the resonant cavity (1) has a width of 2 mm, a length of 5.5 mm, and a slot width of 0.16 mm. The connected gradient microstrip line consists of two sections, the first section is directly connected to the coplanar waveguide feed line (1-3-5), has a width of 2 mm, and a length of 0.91 mm, and the first section of the second section is connected to the first section, and the end is connected to a 50 ohm microstrip line, has a width that gradually changes from 2 mm to 1.08 mm, and has a length of 3 mm.

[0056] In one embodiment of the present invention, Figure 6(a) , 6(b) As shown, the complete resonant cavity structure also has 8 screw holes, which have a diameter of 1.6 mm and have no effect on the resonant cavity, and only play a fixing role. Secondly, the entire system has two resonant cavities, one as a reference cavity 1, in which no humidity sensitive material is added to the circular groove (1-1-3), and one as a sensing cavity 3, in which humidity sensitive material is added to the circular groove (1-1-3). Since the dielectric constant of air can be approximately regarded as unchanged as the humidity changes, the phase information of the reference resonant cavity is almost unchanged, while the phase information of the sensing resonant cavity will change with the change of the dielectric constant of the humidity sensitive material.

[0057] The complete sensor also includes a six-port interference circuit (2), and two resonant cavities, namely a reference cavity (1) and a sensing cavity (3), are placed on the reference circuit and the sensing circuit respectively.

[0058] In one embodiment of the present invention, the principle of the six-port interference network circuit (2) is as follows Figure 7As shown, it is composed of two power dividers (2-1, 2-5), two couplers (2-2, 2-6) and a phase shifter (2-3). The signal is input by the first power divider (2-1), and is output to the reference and sensing circuits with equal power and phase. After that, the signal is input to the input end (2-2-1) of the coupler (2-2), and the reflected signals of the through end (2-2-2) and the coupled end (2-2-3) are output from its isolation end (2-2-4) (taking the reference circuit as an example). At this time, due to the difference in the presence or absence of moisture-sensitive materials, the phase information of the reference and sensing circuits is already different. A phase shifter (2-3) is placed behind the coupler (2-2) of the reference circuit to adjust the phase information of the reference circuit. The phase shifter (2-3) has an adjustment range of 90°, which can make the phases of the two branches differ by 180° to form interference. In different humidity, i.e., corresponding to different phase information, a significant interference effect can be formed by adjusting the phase shifter (2-3), so that the device has high sensitivity. The coplanar waveguide feeder (2-4) is connected to the head and tail power dividers (2-1, 2-5). In order to achieve impedance matching, the width of the coplanar waveguide feeder here is 1.08 mm, which is the same as the width of the 50 ohm microstrip line in the interference circuit. The length of the feeder is 5.6 mm, and the gap width is 0.118 mm. There are two rectangular metal sheets on both sides of the coplanar waveguide feed line, with a length of 8.33mm and a width of 5.56mm. The large through hole on it has a diameter of 2mm and is used to connect to the solder-free joint. No metallization is required. The remaining small through holes have a diameter of 0.8mm and are used to indicate grounding and require metallization.

[0059] In one embodiment of the present invention, the coupler is Figure 8 As shown, the coupler (2-2, 2-6) is a microstrip branch line orthogonal coupler, wherein the length of the through arm is 4.8mm and the width is 1.84mm, and the length of the coupled arm is 4.9mm and the width is 1.08mm. When it works at 7.9Ghz, S21 and S31 are both -3dB, that is, equal power is output at the through end (2-2-2) and the coupled end (2-2-3), meeting the design requirements. When the impedance of the external circuit connected to the through end and the coupled end is not matched, reflection will occur. Since the resonant cavity can be equivalent to an LC circuit when resonating, the reflection coefficient is approximately 1, and the phase information is also loaded to the isolation end (2-2-4) output through the reflected signal. At the same time, the system has low attenuation.

[0060] In one embodiment of the present invention, Fig. 9As shown, the Wilkinson power divider (2-1, 2-5) proposed by the present invention has the most important function of outputting signals with equal power and equal phase. At 7.9Ghz, its S21 and S31 are both -3dB. A 100 ohm resistor is also connected between the two branches of the power divider to achieve isolation between the output ports. Similarly, it is used as a power combiner (2-5) at the end, but the principle is the same.

[0061] In one embodiment of the present invention, the phase shifter (2-3) provided by the present invention is as follows: Fig.10 As shown in the figure, it is composed of left-hand and right-hand composite transmission lines. This structure can be regarded as consisting of two sections with a 90° adjustment range. The variable capacitance diode SMV1430 and the inductor L constitute the left-hand part, and the transmission line constitutes the right-hand part. The phase shift of the right-hand part is approximately constant, while the phase shift of the left-hand part will change with the change of the bias voltage because the capacitor here uses the SMV1430 variable capacitance diode. Its junction capacitance formula is Where C 0 The junction capacitance is 1.11pF when unbiased, V 0 is 0.86V, and γ is 0.5. Due to the DC bias part, an AC isolation inductor Lac is connected between the DC bias circuit (2-7) and the left-hand composite transmission line, and a DC isolation capacitor Cdc is connected between the left-hand composite transmission line and the main circuit. When the DC voltage increases, the capacitance value of the varactor diode decreases, and the phase shift brought by the left-hand part increases. This is the working principle of the phase shifter. Since the left-hand composite transmission line is always in a balanced state within a certain voltage adjustment range, the operating frequency has a better adjustment range. After optimization, the selected AC isolation inductor is MHQ0603P4N1BT000, whose self-resonant frequency is 8.3GHz; the DC isolation capacitor is GJM1555C1H1R1CB01, whose self-resonant frequency is 8GHz; the left-hand inductor is MLG0603S0N3BT000; the left-hand capacitor is smv1430, and the DC bias voltage adjustment range is set to 0V to 30V.

[0062] Fig.11 It is a graph used to characterize the relationship between the transmission coefficient and frequency of the entire system under humidity-sensitive materials with different dielectric constants. Among them, the resonant cavity of the reference circuit does not have humidity-sensitive materials but is filled with air, that is, the dielectric constant is 1, while the resonant cavity in the sensing circuit has humidity-sensitive materials whose dielectric constant changes with humidity. Fig.11 As shown in the figure, the dielectric constants of the humidity sensitive material in the resonant cavity of the sensor circuit are 2, 3, and 10, respectively. At this time, the phase shifter is adjusted to make the phase difference of the two branches 180°, thereby forming interference. Fig.11 It can be seen that the whole system has significant interference effect and high sensitivity.

[0063] It can be seen from the above embodiments that the humidity sensor based on substrate integrated waveguide and six-port interference network proposed by the present invention adopts bottom layer feeding and ultra-thin dielectric substrate in the resonant cavity to achieve an ultra-compact structure. The reference and sensing resonant cavities are designed by the presence or absence of humidity-sensitive materials. The metal grid, air filling area and capacitor column also make the change of humidity-sensitive materials extremely significant, and finally the phase information brought by the two resonant cavities has significant difference. In the six-port network adopted by the interference circuit, due to the phase loading function of the coupler, the two branches have approximately equal loss, and it is no longer necessary to design an attenuator as in previous studies. The phase shifter is designed with left-handed composite transmission lines so that it has a wider phase adjustment range. At the same time, the balanced structure of the left-handed composite transmission lines is relied on to make the phase shifter have broadband characteristics, which can meet the wider frequency design. The whole system has a very strong interference effect and ultra-high sensitivity to meet the urgent needs of various disciplines for high-performance air humidity sensors.

[0064] The present invention is not limited to the above-mentioned embodiments. If various changes or modifications to the invention do not depart from the spirit and scope of the present invention, and if these changes and modifications fall within the scope of the claims and equivalent technologies of the present invention, the present invention is also intended to include these changes and modifications.

Claims

1. A humidity sensor based on substrate integrated waveguide and six-port interferometer network, characterized in that: It includes a passive resonant cavity and a six-port interference circuit; the passive resonant cavity is provided with two identical structures, which are used as a sensing cavity and a reference cavity respectively; The passive resonant cavity has three layers of dielectric substrates, each layer of the dielectric substrate includes a top metal layer, a dielectric layer and a bottom metal layer, and a substrate integrated waveguide reentrant resonant cavity is formed in the three layers of dielectric substrates; A circle of metallized through holes is distributed in each layer of the dielectric substrate, and the metallized through holes of each layer are connected to the top metal layer and the bottom metal layer of the layer, and the radius of the circle formed by the metallized through holes of each layer is the same and the center of the circle is aligned, thereby forming an equivalent metal wall of the resonant cavity, and the area within the equivalent metal wall is the resonant cavity area; In the resonant cavity region of the passive resonant cavity, a metallized through hole array arranged in a circular pattern is provided at the center of the bottom dielectric substrate, the bottom and top of the metallized through hole array are respectively connected to the bottom metal and top metal of the bottom dielectric substrate, and the top metal is partially etched away in the resonant cavity region, leaving only a circular metal layer in the center that is slightly larger than the radius of the metallized through hole array, thereby forming a capacitor column of the reference cavity; In the resonant cavity region of the passive resonant cavity, the intermediate dielectric substrate is completely hollowed out to serve as an air-filled region; In the resonant cavity region of the passive resonant cavity, the bottom metal of the top dielectric substrate is completely etched away; a circular groove is dug from bottom to top in the center region of the top dielectric substrate to form a sensing region; An array of unmetallized through holes arranged in a circular pattern is provided in the dielectric layer of the top dielectric substrate and around the circular groove, and the array of unmetallized through holes and the top metal layer of the substrate together form a metal grid; The passive resonant cavity allows moist air to freely enter and exit the resonant cavity area through the metal grid of the top dielectric substrate, the air filling area of ​​the middle dielectric substrate, and the capacitor column of the bottom dielectric substrate, thereby enhancing its fluidity; The passive resonant cavity has a coplanar waveguide feed line etched on the bottom metal of the bottom dielectric substrate, and the microstrip line connected to the feed line is in a gradient form for connecting to an external six-port interference circuit; The six-port interference circuit includes two power dividers, two couplers and a phase shifter, wherein one path of the two power dividers is connected to a reference cavity through a coupler to form a reference circuit, and a phase shifter is arranged in the reference circuit; the other path of the two power dividers is connected to a sensing cavity through another coupler to form a sensing circuit; The six-port interference circuit feeds a signal into one end of the power divider through a coplanar waveguide feeding line.

2. The humidity sensor according to claim 1, characterized in that: The six-port interference circuit inputs a signal from one side of a single port of a power divider, outputs a half-power equal-phase signal with equal power from the other two ends, connects two symmetrically placed couplers, outputs signals from the ports on the same side of the two couplers, and finally inputs signals from the two ends on the same side of another power divider and outputs signals from one side of the single port, forming a reference circuit and a sensor circuit respectively; In the reference circuit, a separate phase shifter is provided between the coupler and the power divider at the end. The phase shifter is essentially a left-handed composite transmission line, and the phase of the phase shifter is adjusted by adjusting the voltage of its DC bias circuit.

3. The humidity sensor according to claim 1 or 2, characterized in that: The coupler is a microstrip branch line orthogonal coupler, and the through and coupling ends of the coupler are directly connected to the feeding ports at both ends of the passive resonant cavity through a gradient microstrip line, which is used to load the phase change brought by the resonant cavity.

4. The humidity sensor according to claim 1 or 2, characterized in that: The power divider is a Wilkinson power divider, and a 100-ohm resistor is connected between the two ends on the same side to isolate the two ends and output signals of equal power and equal phase at the same time.

5. The humidity sensor according to claim 1 or 2, characterized in that: The phase shifter is a composite left-handed transmission line, connected to a DC bias circuit, connected to a DC blocking capacitor at the input and output ends, and connected to an AC blocking inductor at the 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 to form the left-hand part of the composite left-handed transmission line, and the microstrip line is used to form the right-hand part of the composite left-handed transmission line.

6. The humidity sensor according to claim 1 or 2, characterized in that: The circular groove and the circular metal layer are aligned in the horizontal direction and have the same radius.

7. The humidity sensor according to claim 2, characterized in that: The coplanar waveguide feed line connected to the passive resonant cavity has a width of 2 mm, a length of 5.5 mm, and a gap width of 0.16 mm. The connected gradient microstrip line consists of two sections, the first section is directly connected to the coplanar waveguide feed line, has a width of 2 mm, and a length of 0.91 mm, the first section of the second section is connected to the first section, and the end is connected to the 50 ohm microstrip line, the width gradually changes from 2 mm to 1.08 mm, and the length is 3 mm; the coplanar waveguide feed line connected to the power divider has a width of 1.08 mm, which is the same as the width of the 50 ohm microstrip line in the interference circuit, the length of the feed line is 5.6 mm, and the gap width is 0.118 mm.

8. The humidity sensor according to claim 1 or 2, characterized in that: The dielectric layer material of all dielectric substrates of the passive resonant cavity is Rogers 4350, which has a relative dielectric constant of 3.66, a relative magnetic permeability of 1, and a loss tangent of 0.004.

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