Adjustable low-pass filter circuit

Abstract

The invention discloses an adjustable low-pass filter circuit which comprises two first defected ground structures, two second defected ground structures, two first additional defected ground structures and two second additional defected ground structures which are symmetrically distributed relative to an X axis and are sequentially etched along the Y axis at the position, under a micro-strip transmission line, on a ground plane, and the two first defected ground structures, the two second defected ground structures, the two first additional defected ground structures and the two second additional defected ground structures are symmetrically distributed relative to the X axis. The first defected ground structure is connected with the first direct current bias circuit, and the second defected ground structure is connected with the second direct current bias circuit; variable capacitance diodes are inserted into the first defected ground structure and the second defected ground structure, the sizes of introduced equivalent capacitance and equivalent inductance are changed by changing the sizes of the defected ground structures and the additional defected ground structures, the transmission characteristics of the microstrip transmission line are changed, and the transmission characteristics of the microstrip transmission line are changed by changing the bias voltage of the direct-current bias circuit. The equivalent capacitance of the variable capacitance diode is changed, and the cut-off frequency of the low-pass filter circuit is tuned; the filter has the advantages that high selectivity of the filter is guaranteed, and meanwhile high stopband suppression is taken into consideration.

Description

Technical Field

[0001] The present invention relates to the technical field of filters, and particularly relates to an adjustable low-pass filter circuit. Background Art

[0002] In recent years, with the rapid development of 5G communication and multifunctional radar systems, the demand for dynamic control of spectrum resources in modern radio frequency (RF) front-ends has become increasingly urgent. Various wireless communication products and RF microwave signal processing modules have developed rapidly, and device units are moving towards high density, high integration, and high sensitivity, while the functions of the system are becoming increasingly complex. For example, the number of transceiver components in next-generation multifunctional phased array radars shows a significant growth trend, and more components need to be integrated in the same or smaller space. For example, shipborne early warning radars use up to thousands of transceiver modules, which poses higher requirements for the multifunctionality, high performance, miniaturization, and stability of device units in the system.

[0003] The RF front-end is a key component of a wireless communication system, mainly including devices such as filters, mixers, low-noise amplifiers, and power amplifiers. The function of the mixer in downconversion is to mix the RF signal with the local oscillator signal to generate an intermediate frequency signal. However, in addition to generating the intermediate frequency signal, many harmonic and spurious components will also be output, which will interfere with the useful signal and affect the overall performance of the system. The low-pass filter placed at its rear end is used to screen and filter the output signal. In the face of the frequency-domain migration of parasitic harmonic components during the switching of the intermediate frequency output channel of the mixer, in the traditional RF front-end receiving link, a method combining switches and filter banks is adopted. This simple solution results in a non-compact circuit size, low system integration, and in addition, the discrete adjustment method leads to a slow switching speed, and the scheme of multiple parallel filter banks will increase the system complexity.

[0004] An adjustable low-pass filter is an electronic circuit that can dynamically adjust its cut-off frequency. Its core function is to allow low-frequency signals to pass through while suppressing high-frequency signals above the cut-off frequency. The working principle of an adjustable low-pass filter using a varactor diode as a tuning element is as follows: Utilize the non-linear characteristic that the junction capacitance of the varactor diode is inversely proportional to its reverse bias voltage. By adjusting the reverse bias voltage, the capacitance value of the diode is continuously changed, thereby changing the cut-off frequency of the filter. The cut-off frequency is inversely proportional to the capacitance. When the voltage increases, the junction capacitance decreases, and the cut-off frequency increases, thereby dynamically controlling the highest frequency allowed to pass through. It can flexibly adapt to different frequency requirements without replacing hardware.

[0005] So far, numerous studies on varactor diode-tuned adjustable filters have been carried out. For example, the multifunctional filter with adjustable center frequency and bandwidth based on a microstrip resonator disclosed in Chinese Patent Publication No. CN109599648A. In the proposed adjustable low-pass filters, most are based on loading silicon varactor diode elements equipped with a bias network on a microstrip resonator to obtain the frequency tuning function. However, due to the skin effect and the frequency-dependent characteristics of the series resistance caused by the low carrier mobility of silicon, as well as the parasitic inductance and capacitance introduced by the package leads forming a resonant circuit with the junction capacitance, resulting in impedance mismatch and additional losses, the quality factor (Q value) of the silicon varactor diode deteriorates at high frequencies, making it impossible to achieve both high selectivity and high stopband rejection level of the adjustable low-pass filter simultaneously; and the quality factor of the microstrip transmission line resonator also deteriorates rapidly at high frequencies. These reasons have restricted the operating frequency of the research on varactor diode-based adjustable low-pass filters to below 10 GHz, limiting the development of high-performance adjustable low-pass filters at high frequencies.

[0006] Therefore, in the co-design of varactor diodes and microwave passive circuits for adjustable low-pass filters, how to apply the varactor diode-based tuning scheme to the high-frequency band above 10 GHz, while ensuring high selectivity of the filter and taking into account high stopband rejection, is an urgent problem to be solved currently. Summary of the Invention

[0007] The technical problem to be solved by the present invention is how to ensure high selectivity of the filter while taking into account high stopband rejection.

[0008] The present invention solves the above technical problems through the following technical means: An adjustable low-pass filter circuit includes a ground plane (10), a dielectric substrate (15) located above the ground plane (10). A first DC bias circuit (16), a second DC bias circuit (17) and a microstrip transmission line are attached to the upper surface of the dielectric substrate (15). The direction of the microstrip transmission line is the Y-axis, the direction perpendicular to the direction of the microstrip transmission line is the X-axis, and the origin is the center of the microstrip transmission line. On the ground plane (10), at the position directly below the microstrip transmission line, two first defected ground structures (11), two second defected ground structures (12), two first additional defected ground structures (13) and two second additional defected ground structures (14) which are symmetrically distributed with respect to the X-axis are etched out in sequence along the Y-axis. Each first defected ground structure (11) is connected to the first DC bias circuit (16), and each second defected ground structure (12) is connected to the second DC bias circuit (17). A varactor diode is inserted into both the first defected ground structure (11) and the second defected ground structure (12). By changing the sizes of the first defected ground structure (11), the second defected ground structure (12), the first additional defected ground structure (13) and the second additional defected ground structure (14), the magnitudes of the introduced equivalent capacitance and equivalent inductance are changed, and the transmission characteristics of the microstrip transmission line are changed. By changing the bias voltages of the first DC bias circuit (16) and the second DC bias circuit (17), the equivalent capacitance of the varactor diode is changed, and the cut-off frequency of the low-pass filter circuit is tuned.

[0009] Further, the first defected ground structure (11) and the second defected ground structure (12) have the same structure, only with different sizes. The first defected ground structure (11) includes 2 first rectangular slots (111) that are symmetric about the Y-axis and 4 first rectangular branch slots (112) distributed at the ends of the first rectangular slots (111) and extending towards the middle. The two first rectangular slots (111) are connected by 2 first equal-width slot gaps (113) that are symmetric about the X-axis. The 2 first equal-width slot gaps (113) and the 2 first rectangular slots (111) enclose a non-etched rectangular area as the metal electrode (115). A varactor diode (114) is loaded into one of the first slot gaps (113). The negative electrode of the varactor diode (114) is connected to the metal electrode (115), and the positive electrode of the varactor diode (114) is grounded.

[0010] Furthermore, the magnitude of the equivalent inductance L1 of the first defected ground structure (11) is related to the width W s1 and length L s1 of the first rectangular slot (111). The equivalent capacitance C1 of the first defected ground structure (11) is related to the length L a and width g1 of the first equal-width slot gap (113), the width W r and length L rand the equivalent capacitance value C of the varactor diode (114) v1 Regarding this, the transmission response of the first defected ground structure (11) is equivalent to a series-parallel network of an inductor and a capacitor.

[0011] Furthermore, when the first defected ground structure (11) and the second defected ground structure (12) are loaded under the microstrip transmission line, the slow-wave factor of the microstrip transmission line increases with the introduction of the defected ground structure, and at the same time, it also disturbs the shielding current distribution on the ground plane (10), thereby introducing additional effective capacitance and inductance, enabling the structure to exhibit stopband characteristics without increasing the circuit area, and the entire structure behaves as an LC resonator.

[0012] Further, the first additional defected ground structure (13) and the second additional defected ground structure (14) have the same structure but only different sizes; the first additional defected ground structure (13) includes two first additional rectangular slots (131) symmetric about the Y-axis, and the two first additional rectangular slots (131) are connected by three second slot gaps (132) of equal size, and the three second slot gaps (132) form a rectangular open-loop stepped impedance form.

[0013] Furthermore, the first additional defected ground structure (13) and the second additional defected ground structure (14) behave as DGS resonators. For the first additional defected ground structure (13), when radio frequency energy is transmitted, at the resonant frequency position of the resonator, the magnetic flux energy is blocked at the input end of the first additional defected ground structure (13), and its magnetic field energy shows a circumferential distribution around the first additional defected ground structure (13), while the electric field energy is highly concentrated in the second slot gap (132) of the first additional defected ground structure (13), and the structure exhibits stopband characteristics. This energy distribution characteristic increases the magnitude of the equivalent capacitance of the first additional defected ground structure (13), shifting the frequency towards the low frequency. The equivalent inductance of the first additional defected ground structure (13) is related to the width W s3 and the length L s3 of the first additional rectangular slot (131), and the equivalent capacitance of the first additional defected ground structure (13) is related to the width g2 of the second slot gap (132).

[0014] Furthermore, the first DC bias circuit (16) includes a bias microstrip line (161), a first bias resistor (162), and a first metal via hole (163) connected in sequence; the first defected ground structure (11) is connected to the first bias resistor (162) through the first metal via hole (163); the second DC bias circuit (17) includes a bias microstrip line (171), a second bias resistor (172), and a second metal via hole (173), and the second defected ground structure (12) is connected to the second bias resistor (172) through the second metal via hole (173).

[0015] Furthermore, the resistance values of the first bias resistor (162) and the second bias resistor (172) are equal. The first DC bias circuit (16) and the second DC bias circuit (17) form a DC bias network. The bias microstrip line (161) and the bias microstrip line (171) receive a bias voltage and vertically feed the bias voltage from the upper metal plane to the metal electrode (115) isolated from the ground on the lower layer.

[0016] Furthermore, the bias voltages of the first DC bias circuit (16) and the second DC bias circuit (17) are V1 and V2 respectively. By changing the bias voltages V1 and V2, the equivalent capacitance of the varactor diodes loaded on the first defected ground structure (11) and the second defected ground structure (12) is changed, and the resonant frequency of the LC resonator composed of the first defected ground structure (11) and the second defected ground structure (12) is changed, thereby tuning the cut-off frequency of the low-pass filter circuit.

[0017] Further, a second additional defected ground structure (14), a first additional defected ground structure (13), a second defected ground structure (12), a first defected ground structure (11), another first defected ground structure (11), another second defected ground structure (12), another first additional defected ground structure (13), and another second additional defected ground structure (14) are etched at equal intervals in sequence along the direction of the microstrip transmission line.

[0018] The advantages of the present invention are as follows:

[0019] (1) In the technical solution of the present invention, the first defected ground structure (11) and the second defected ground structure (12) of the tunable low-pass filter circuit introduce additional effective capacitance and inductance, so that the structure exhibits stopband characteristics without increasing the circuit area. By changing the bias voltages of the first DC bias circuit (16) and the second DC bias circuit (17), the equivalent capacitance of the varactor diodes is changed, and the cut-off frequency of the low-pass filter circuit is tuned, thereby enhancing the frequency selection characteristics of the center filter. The overall solution ensures high selectivity of the filter while taking into account high stopband suppression.

[0020] (2) After the first defected ground structure (11) and the second defected ground structure (12) are loaded under the microstrip transmission line, the slow-wave factor of the microstrip transmission line increases with the introduction of the defected ground, and at the same time, it will disturb the shielding current distribution on the plane of the microstrip line, thereby introducing additional effective capacitance and inductance, making the structure exhibit a stopband characteristic without increasing the circuit area, and the whole structure behaves as an LC resonator; both two-stage resonators can be equivalently modeled as an LC parallel network topology. The symmetric first slot introduces a capacitive series network, and cooperates with the capacitive compensation structure composed of four rectangular stub slots distributed diagonally to increase the equivalent capacitance value of the resonator. Based on the synergistic effect of the slow-wave effect, this composite structure successfully shifts the resonance frequency 3 dB towards the low-frequency band with a relatively small change in the cut-off frequency. This frequency regulation mechanism not only realizes the miniaturization improvement of the device size, but also significantly enhances the frequency selection characteristic of the center filter through Q-value optimization.

[0021] The first additional defected ground structure (13) and the two second additional defected ground structures (14) behave as a two-stage loaded rectangular split-ring stepped impedance DGS resonator. When radio frequency energy is transmitted, at the resonance frequency position of the resonator, the magnetic flux energy is blocked at the input end of the structure, and its magnetic field energy shows a circumferential distribution around the defected ground structure, while the electric field energy is highly concentrated in the second slot of the additional defected ground structure. This energy distribution characteristic increases the size of the equivalent capacitance and shifts the frequency towards the low frequency. Based on this, the studied resonator requires a smaller size than the traditional dumbbell-shaped defected ground structure at the same resonance frequency, and has the advantages of miniaturization and high Q value at the same time. This composite characteristic can not only effectively suppress the parasitic passband near the cut-off frequency, but also improve the stopband rejection depth of the filter.

[0022] (3) The DC bias network of the adjustable low-pass filter circuit in the technical solution of the present invention realizes the bias voltage transmission through the vertical path from the upper metal plane to the lower ground isolation metal electrode, effectively removing the DC-blocking capacitor in the traditional bias voltage circuit and reducing the circuit design complexity. Description of the Drawings

[0023] Figure 1 It is a three-dimensional structure diagram of an adjustable low-pass filter circuit disclosed in an embodiment of the present invention;

[0024] Figure 2 It is a planar structure diagram of an adjustable low-pass filter circuit disclosed in an embodiment of the present invention;

[0025] Figure 3 It is an equivalent circuit schematic diagram of an adjustable low-pass filter circuit disclosed in an embodiment of the present invention;

[0026] Figure 4The equivalent schematic diagram of a single DGS in the first defected ground structure and the second defected ground structure in an adjustable low-pass filter circuit disclosed in an embodiment of the present invention;

[0027] Figure 5 The S parameter comparison diagram of a single DGS in the first defected ground structure and the second defected ground structure in an adjustable low-pass filter circuit disclosed in an embodiment of the present invention and a dumbbell-shaped DGS of the same size; 21 Parameter comparison diagram;

[0028] Figure 6 The quality factor comparison diagram of a single DGS in the first defected ground structure and the second defected ground structure in an adjustable low-pass filter circuit disclosed in an embodiment of the present invention and a dumbbell-shaped DGS under the same size change condition;

[0029] Figure 7 The S parameter comparison diagram of a single DGS in the first additional defected ground structure and the second additional defected ground structure in an adjustable low-pass filter circuit disclosed in an embodiment of the present invention and a dumbbell-shaped DGS of the same size; 21 Parameter comparison diagram;

[0030] Figure 8 The quality factor comparison diagram of a single DGS in the first additional defected ground structure and the second additional defected ground structure in an adjustable low-pass filter circuit disclosed in an embodiment of the present invention and a dumbbell-shaped DGS under the same size change condition;

[0031] Figure 9 The result comparison diagram of an adjustable low-pass filter circuit with or without an additional unit disclosed in an embodiment of the present invention;

[0032] Figure 10 The S parameter curve diagram of an adjustable low-pass filter circuit disclosed in an embodiment of the present invention with voltage tuning; 21 Parameter curve diagram;

[0033] Figure 11 The S parameter curve diagram of an adjustable low-pass filter circuit disclosed in an embodiment of the present invention with voltage tuning; 11 Parameter curve diagram. Detailed implementation manners

[0034] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Apparently, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0035] As Figure 1As shown, the present invention provides an adjustable low-pass filter circuit, which can achieve continuous adjustment of the cut-off frequency from 12 GHz to 13.5 GHz. It includes a ground plane 10 and a dielectric substrate 15 located above the ground plane 10. The dielectric material substrate 15 is Rogers RO4003(tm), with a relative dielectric constant of 3.55, a dielectric loss tangent of 0.0027, and a thickness of 0.508 mm. The upper surface of the dielectric substrate 15 is attached with a first DC bias circuit 16, a second DC bias circuit 17, and a microstrip transmission line. The direction of the microstrip transmission line is the Y-axis, the direction perpendicular to the microstrip transmission line direction is the X-axis, and the origin is the center of the microstrip transmission line. On the ground plane 10, at the position directly below the microstrip transmission line, two first defected ground structures 11, two second defected ground structures 12, two first additional defected ground structures 13, and two second additional defected ground structures 14 that are symmetrically distributed with respect to the X-axis are etched out in sequence along the Y-axis, that is, a second additional defected ground structure 14, a first additional defected ground structure 13, a second defected ground structure 12, a first defected ground structure 11, another first defected ground structure 11, another second defected ground structure 12, another first additional defected ground structure 13, and another second additional defected ground structure 14 are etched out at equal intervals along the microstrip transmission line direction. Each first defected ground structure 11 is connected to the first DC bias circuit 16 through a first metal via 163, and each second defected ground structure 12 is connected to the second DC bias circuit 17 through a second metal via 173.

[0036] The first defected ground structures 11 and the second defected ground structures 12 are symmetrically loaded to form a center filter for realizing the cut-off frequency response of the low-pass filter. The first additional defected ground structures 13 and the second additional defected ground structures 14 form an out-of-band rejection structure for introducing transmission zeros at the right stopband of the filter to realize the suppression of the parasitic passband to improve the stopband rejection and improve the return loss of the passband. The first DC bias circuit 16 and the second DC bias circuit 17 are respectively connected to the first defected ground structures 11 and the second defected ground structures 12 to form a DC bias network, providing a bias voltage for adjusting the cut-off frequency response of the filter. The input / output 50Ω transmission line 18 and the upper-layer microstrip line 19 form a microstrip transmission line, providing a transmission channel for the input / output signals.

[0037] The first defected ground structure 11 has the same structure as the second defected ground structure 12, only with different sizes; the first defected ground structure 11 includes two first rectangular slots 111 symmetrical about the Y-axis and four first rectangular branch slots 112 distributed at the ends of the first rectangular slots 111 and extending towards the middle. The two first rectangular slots 111 are connected by two first slit slots 113 with the same width symmetrical about the X-axis. The two first slit slots 113 with the same width and the two first rectangular slots 111 enclose a non-etched rectangular area as the metal electrode 115. A varactor diode 114 is loaded in one of the first slit slots 113. The negative electrode of the varactor diode 114 is connected to the metal electrode 115, and the positive electrode of the varactor diode 114 is grounded. Similarly, the second defected ground structure 12 is respectively loaded on the left and right sides of the first defected ground structure 11 and is composed of the following structural relationship: two second rectangular slots symmetrical about the Y-axis and four second rectangular branch slots, connected by two first slit slots with the same width symmetrical about the X-axis. The negative electrode of another GaAs varactor diode loaded in the right first slit slot is connected to another metal electrode, and the positive electrode of this metal electrode is grounded.

[0038] The DGS (defected ground structure) in the first defected ground structure 11 effectively disturbs the shielding current distribution of the ground plane, changes the characteristics of the upper-layer transmission line at this position, and due to the slow-wave effect, increases the effective capacitance and effective inductance. The magnitude of the equivalent inductance L1 of the first defected ground structure 11 is related to the width W s1 and length L s1 of the first rectangular slot 111. The equivalent capacitance C1 of the first defected ground structure 11 is related to the length L a and width g1 of the two symmetrical first slit slots 113 with the same width, the width W r and length L r of the first rectangular branch slot 112, and the equivalent capacitance value C v1 of the varactor diode 114. The transmission response of the first defected ground structure 11 is equivalent to a series-parallel network of inductance and capacitance. The same is true for the second defected ground structure 12. The DGS in the second defected ground structure 12 changes the characteristics of the upper-layer transmission line at this position due to the disturbance of the shielding current distribution of the ground plane, shows the slow-wave effect, and increases the effective capacitance and effective inductance. Among them, the magnitude of the equivalent inductance L2 of the second defected ground structure 12 is mainly related to the width W s2 and length L s2It is related to the length, width of the two symmetrically equal-width first slot grooves of the second defected ground structure 12, the width and length of the second rectangular stub groove, and the equivalent capacitance value of the varactor diode. Its transmission response is equivalent to a series-parallel network of inductance and capacitance. Therefore, when the first defected ground structure 11 and the second defected ground structure 12 are loaded under the microstrip transmission line, the slow-wave factor of the microstrip transmission line increases with the introduction of the defected ground structure, and at the same time, it disturbs the shielding current distribution on the ground plane 10, thereby introducing additional effective capacitance and inductance, making the structure exhibit a stopband characteristic without increasing the circuit area, and the whole structure behaves as an LC resonator.

[0039] The first additional defected ground structure 13 and the second additional defected ground structure 14 have the same structure, only different in size; the first additional defected ground structure 13 includes two first additional rectangular slots 131 symmetrical about the Y-axis, and the two first additional rectangular slots 131 are connected by three second slot grooves 132 of the same size. The three second slot grooves 132 form a rectangular open-loop stepped impedance form, that is Figure 1 and Figure 2 as shown, the starting end of one second slot groove 132 is connected to one of the first additional rectangular slots 131, and the left and right sides of the end of this second slot groove 132 are respectively connected to the other first additional rectangular slot 131 through a second slot groove 132. Based on this, the first additional defected ground structure 13 and the second additional defected ground structure 14 can be collectively referred to as a DGS resonator loaded with a rectangular open-loop stepped impedance. The first additional defected ground structure 13 is loaded on both sides of the center filter, and the second additional defected ground structure 14 is loaded on both sides of the first additional defected ground structure 13 in the same form.

[0040] For the first additional defected ground structure 13, when the radio frequency energy is transmitted, at the resonant frequency position of the resonator, the magnetic flux energy is blocked at the input end of the first additional defected ground structure 13, and its magnetic field energy shows a surrounding distribution around the first additional defected ground structure 13, while the electric field energy is highly concentrated in the second slot groove 132 of the first additional defected ground structure 13. The structure exhibits a stopband characteristic, and this energy distribution characteristic increases the magnitude of the equivalent capacitance of the first additional defected ground structure 13, shifting the frequency to the low frequency. The equivalent inductance of the first additional defected ground structure 13 is related to the width W s3 and the length L s3 of the first additional rectangular slot 131, and the equivalent capacitance of the first additional defected ground structure 13 is related to the width g2 of the second slot groove 132.

[0041] The first DC bias circuit 16 includes a bias microstrip line 161, a first bias resistor 162, and a first metal via 163 connected in sequence; the first defected ground structure 11 is connected to the first bias resistor 162 through the first metal via 163; the second DC bias circuit 17 includes a bias microstrip line 171, a second bias resistor 172, and a second metal via 173, and the second defected ground structure 12 is connected to the second bias resistor 172 through the second metal via 173.

[0042] The resistance values of the first bias resistor 162 and the second bias resistor 172 are equal. The first DC bias circuit 16 and the second DC bias circuit 17 form a DC bias network. The bias microstrip line 161 and the bias microstrip line 171 receive bias voltages, and vertically feed the bias voltages from the upper metal plane to the metal electrode 115 isolated from the ground on the lower layer, effectively eliminating the DC-blocking capacitor in the traditional biased voltage circuit and reducing the circuit design complexity. The bias voltages of the first DC bias circuit 16 and the second DC bias circuit 17 are V1 and V2 respectively. By changing the bias voltages V1 and V2, the equivalent capacitances of the varactor diodes loaded on the first defected ground structure 11 and the second defected ground structure 12 are changed, and the resonant frequencies of the LC resonators formed by the first defected ground structure 11 and the second defected ground structure 12 are changed, thereby tuning the cut-off frequency of the low-pass filter circuit.

[0043] As Figure 2 shown, the widths and lengths of the first rectangular slot 111 of the first defected ground structure 11 and the second rectangular slot of the second defected ground structure 12 are W s1 、W s2 、L s1 、L s2 , the widths of the first slit slots of the first defected ground structure 11 and the second defected ground structure 12 are both g1, and the lengths are the same as the width of the upper microstrip line, denoted as L a , the widths and lengths of the four rectangular blocks, that is, the first rectangular stub slot 112, are W r 、L r， The widths and lengths of the second rectangular stub slot of the second defected ground structure 12 are also W r 、L r , the varactor diodes 114 of the first defected ground structure 11 and the varactor diodes of the second defected ground structure 12 are respectively described as C v1 and C v2 , the width of the center of the first defected ground structure 11 from the metal ground electrode isolated from the ground is W d , and the width of the center of the second defected ground structure 12 from the metal ground electrode isolated from the ground is also W d。The width of the second slot of the first additional defect ground structure 13 and the second additional defect ground structure 14 is g2, and the width and length of the two end rectangular regions, namely the first additional rectangular slot 131 and the second additional rectangular slot of the second additional defect ground structure 14, are described as W s3 、W s4 and L s3 、L s4 , the resistance values of the first bias resistor 162 and the second bias resistor 172 in the DC bias network are R b , the two-end 50Ω transmission line 18 is designed to be suitable for soldering SMA-RF (Subminiature A-Radio Frequency) connectors, and the width and length are W0 and L0 respectively.

[0044] After the first defect ground structure 11 and the second defect ground structure 12 are loaded under the microstrip line, the slow-wave factor of the microstrip line increases with the introduction of DGS. At the same time, it will also disturb the shielding current distribution on the plane of the microstrip line, thereby introducing additional effective capacitance and inductance, making the structure exhibit stopband characteristics without increasing the circuit area. The whole structure behaves as an LC resonator; both two-stage resonators can be modeled as an LC parallel network. The symmetric first slot introduces a series network of capacitors, and the four diagonal rectangular stub slots are introduced as capacitive compensation structures, increasing the equivalent capacitance of the resonator. Combining with the slow-wave effect of the resonator, the resonance frequency is shifted down to a lower frequency range while the 3dB cut-off frequency changes less, enhancing the structural compactness while achieving a high Q value of the resonator, which can effectively enhance the frequency selectivity of the center filter; the first additional defect ground structure 13 and the second additional defect ground structure 14 behave as a two-stage loaded rectangular open-loop stepped impedance DGS resonator. When radio frequency energy is transmitted, at the resonance frequency position of the resonator, the magnetic flux energy is blocked at the input end of the structure, the magnetic field energy is distributed around the DGS, and the electric field energy is concentrated in the second slot of the DGS, increasing the size of the equivalent capacitance and shifting the frequency down to a lower value. Therefore, the studied resonator requires a smaller area than the dumbbell-shaped DGS at the same resonance frequency, realizing the miniaturization and high Q value of the resonator, and is suitable for eliminating and suppressing the parasitic passband near the cut-off frequency and improving the stopband suppression depth of the filter.

[0045] As Figure 3 shown, the equivalent circuit of the adjustable low-pass filter circuit is shown. C1 represents the total capacitance of a single resonator in the first defect ground structure 11, which is composed of the slot capacitance, the rectangular stub slot capacitance, and the varactor capacitance. C2 ( Figure 3 not shown in the figure) represents the total capacitance of a single resonator in the second defect ground structure 12, which is composed of the slot capacitance, the rectangular stub slot capacitance, and the varactor capacitance. Figure 3Since the equivalent circuits of the second defective ground structure 12 and the first defective ground structure 11 are the same, the equivalent circuit of the second defective ground structure 12 is not shown. Among them, the transmission zeros formed by L1, L2, C1, and C2 jointly control the cut-off frequency of the filter. The transmission responses generated by the first additional defective ground structure 13 and the second additional defective ground structure 14 are equivalent to the parallel resonators composed of L3, C3 and L4, C4. The zeros they generate are used as an out-of-band rejection circuit and help improve the impedance matching of the filter passband. The equivalent circuit structure of the first additional defective ground structure 13 is the same as that of the second additional defective ground structure 14. Figure 3 Not shown in the figure. Parallel capacitors such as C5 - C9 are used to help construct the passband. The parameter values of each component are directly affected by the size parameters of the first defective ground structure 11, the second defective ground structure 12, the first additional defective ground structure 13, and the second additional defective ground structure 14.

[0046] As Figure 4 shown, the equivalent circuits of single DGSs in the first defective ground structure 11 and the second defective ground structure 12 are presented. Among them, L and C are the equivalent inductance and capacitance values of the resonator. The resistor R is used to simulate the equivalent circuit of a single resonator and is ignored in the overall equivalent circuit of the filter. C g1 and C g2 are the capacitances of the first rectangular stub slot 112 and the first slot 113 in the DGS. C v is the equivalent capacitance of the varactor diode, which overall shows a series - parallel equivalent circuit here.

[0047] As Figure 5 , Figure 6 shown, compared with the dumbbell - shaped DGS resonator, a single resonator in the first defective ground structure 11 and the second defective ground structure 12 exhibits a lower resonance frequency and a better quality factor under the same size conditions. This characteristic can effectively improve the frequency - selection characteristic of the filter designed based on such resonators, so that while reducing the order of the resonator, it can still maintain a frequency - selection performance comparable to that of a low - pass filter using a dumbbell - shaped defective ground structure.

[0048] As Figure 7 , Figure 8 shown, compared with the dumbbell - shaped defective ground structure, a single resonator in the first additional defective ground structure 13 and the second additional defective ground structure 14 shows a better quality factor and a lower resonance frequency for its resonance unit. This structural characteristic not only enables the resonator to achieve a more compact physical layout under the same size conditions while reducing the influence of ground - plane discontinuity, but also has a deeper stop - band suppression level.

[0049] As Figure 9As shown, the simulation results of the center filter show good frequency selection characteristics. However, without an additional defected structure, a parasitic passband will occur at a high frequency around 30 GHz, which limits the bandwidth of the stopband. By introducing additional defected ground cells as out-of-band rejection structures, the frequency selectivity of the tunable filter remains consistent, the parasitic passband is well suppressed, and at the same time, the return loss in the passband is improved.

[0050] As Figure 10 and Figure 11 shown, a result display diagram of the variation of the S parameters of the tunable low-pass filter circuit based on series-parallel capacitive compensation and loaded rectangular split-ring stepped impedance DGS with the bias voltage is provided, showing that the proposed tunable low-pass filter has good frequency selectivity and stopband rejection function in the high-frequency band above 10 GHz.

[0051] Through the above technical solutions, the first defected ground structure 11 and the second defected ground structure 12 of the tunable low-pass filter circuit in the technical solution of the present invention utilize the slow-wave effect of DGS and the perturbation of the shielding current distribution on the microstrip line ground plane to introduce additional effective capacitance and inductance, so that the structure exhibits stopband characteristics without increasing the circuit area. The symmetric first slot introduces a series network of a capacitor, and the four diagonal rectangular stub slots are introduced as capacitive compensation structures, increasing the equivalent capacitance of the resonator, thereby shifting the resonance frequency down to a lower frequency range with a relatively small change in the 3 dB cut-off frequency, realizing the miniaturization and high Q value of the DGS resonator, and effectively enhancing the frequency selectivity of the center filter.

[0052] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An adjustable low-pass filter circuit, characterized in that, It includes a ground plane (10), a dielectric substrate (15) located above the ground plane (10). A first DC bias circuit (16), a second DC bias circuit (17) and a microstrip transmission line are attached to the upper surface of the dielectric substrate (15). The direction of the microstrip transmission line is the Y-axis, the direction perpendicular to the microstrip transmission line direction is the X-axis, and the origin is the center of the microstrip transmission line. At positions directly below the microstrip transmission line on the ground plane (10), two first defected ground structures (11), two second defected ground structures (12), two first additional defected ground structures (13) and two second additional defected ground structures (14) that are symmetrically distributed with respect to the X-axis are etched in sequence along the Y-axis. Each first defected ground structure (11) is connected to the first DC bias circuit (16), and each second defected ground structure (12) is connected to the second DC bias circuit (17) through. Varactor diodes are inserted into both the first defected ground structure (11) and the second defected ground structure (12). By changing the sizes of the first defected ground structure (11), the second defected ground structure (12), the first additional defected ground structure (13) and the second additional defected ground structure (14), the magnitudes of the introduced equivalent capacitance and equivalent inductance are changed, and the transmission characteristics of the microstrip transmission line are changed. By changing the bias voltages of the first DC bias circuit (16) and the second DC bias circuit (17), the equivalent capacitance of the varactor diode is changed, and the cut-off frequency of the low-pass filter circuit is tuned.

2. The adjustable low-pass filter circuit according to claim 1, characterized in that, The structures of the first defected ground structure (11) and the second defected ground structure (12) are the same, only the sizes are different. The first defected ground structure (11) includes two first rectangular slots (111) that are symmetric about the Y-axis and four first rectangular branch slots (112) distributed at the ends of the first rectangular slot (111) and extending towards the middle. The two first rectangular slots (111) are connected by two first slit slots (113) of equal width that are symmetric about the X-axis. The two first slit slots (113) and the two first rectangular slots (111) enclose a non-etched rectangular area as the metal electrode (115). A varactor diode (114) is loaded into one of the first slit slots (113). The negative electrode of the varactor diode (114) is connected to the metal electrode (115), and the positive electrode of the varactor diode (114) is grounded.

3. The adjustable low-pass filter circuit according to claim 2, characterized in that, The magnitude of the equivalent inductance L1 of the first defected ground structure (11) is related to the width W of the first rectangular slot (111) s1 and the length L s1 The equivalent capacitance C1 of the first defected ground structure (11) is related to the length L of the first slit slot (113) with equal width a and the width g1, the width W of the first rectangular stub slot (112) r and the length L r as well as the equivalent capacitance value C of the varactor diode (114) v1 The transmission response of the first defected ground structure (11) is equivalent to a series-parallel network of an inductor and a capacitor.

4. The adjustable low-pass filter circuit according to claim 3, wherein, When the first defected ground structure (11) and the second defected ground structure (12) are loaded below the microstrip transmission line, the slow-wave factor of the microstrip transmission line increases with the introduction of the defected ground structure, and at the same time, it disturbs the shielding current distribution on the ground plane (10), thereby introducing additional effective capacitance and inductance, making the structure exhibit a stopband characteristic without increasing the circuit area, and the whole structure behaves as an LC resonator.

5. An adjustable low-pass filter circuit according to claim 1, characterized in that, The first additional defected ground structure (13) and the second additional defected ground structure (14) have the same structure, only different in size; the first additional defected ground structure (13) includes two first additional rectangular slots (131) symmetrical about the Y-axis, and the two first additional rectangular slots (131) are connected by three second slot gaps (132) of equal size, and the three second slot gaps (132) form a rectangular open-loop stepped impedance form.

6. The adjustable low-pass filter circuit according to claim 5, characterized in that The first additional defected ground structure (13) and the second additional defected ground structure (14) behave as DGS resonators. For the first additional defected ground structure (13), when radio frequency energy is transmitted, at the resonant frequency position of the resonator, the magnetic flux energy is blocked at the input end of the first additional defected ground structure (13). Its magnetic field energy shows a surrounding distribution around the first additional defected ground structure (13), while the electric field energy is highly concentrated in the second slot (132) of the first additional defected ground structure (13). The structure exhibits a stopband characteristic. This energy distribution characteristic increases the magnitude of the equivalent capacitance of the first additional defected ground structure (13), causing the frequency to shift towards the low frequency. The equivalent inductance of the first additional defected ground structure (13) is related to the width W s3 and the length L s3 of the first additional rectangular slot (131). The equivalent capacitance of the first additional defected ground structure (13) is related to the width g2 of the second slot (132).

7. An adjustable low-pass filter circuit according to claim 2, characterized in that, The first DC bias circuit (16) includes a bias microstrip line (161), a first bias resistor (162), and a first metal via hole (163) connected in sequence; the first defected ground structure (11) is connected to the first bias resistor (162) through the first metal via hole (163); the second DC bias circuit (17) includes a bias microstrip line (171), a second bias resistor (172), and a second metal via hole (173), and the second defected ground structure (12) is connected to the second bias resistor (172) through the second metal via hole (173).

8. An adjustable low-pass filter circuit according to claim 7, characterized in that, The resistance values of the first bias resistor (162) and the second bias resistor (172) are equal. The first DC bias circuit (16) and the second DC bias circuit (17) form a DC bias network. The bias microstrip line (161) and the bias microstrip line (171) receive a bias voltage and vertically feed the bias voltage from the upper metal plane to the metal electrode (115) isolated from the ground in the lower layer.

9. An adjustable low-pass filter circuit according to claim 7, wherein, The bias voltages of the first DC bias circuit (16) and the second DC bias circuit (17) are V1 and V2 respectively. By changing the bias voltages V1 and V2, the equivalent capacitance of the varactor diodes loaded on the first defected ground structure (11) and the second defected ground structure (12) is changed, and the resonant frequency of the LC resonator composed of the first defected ground structure (11) and the second defected ground structure (12) is changed, thereby tuning the cut-off frequency of the low-pass filter circuit.

10. The adjustable low-pass filter circuit according to claim 1, wherein A second additional defected ground structure (14), a first additional defected ground structure (13), a second defected ground structure (12), a first defected ground structure (11), another first defected ground structure (11), another second defected ground structure (12), another first additional defected ground structure (13), and another second additional defected ground structure (14) are etched out at equal intervals in sequence along the direction of the microstrip transmission line.

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