Broadband external suppression stub-loaded band-pass filter

Abstract

A stub-loaded band-pass filter with broadband external suppression belongs to the field of radio frequency passive devices in the radio frequency technology, is of a single-layer structure, and comprises a square substrate, a microstrip stub-loaded resonator consisting of two short-circuit microstrip lines, a U-shaped microstrip line and an open-circuit microstrip line, and two differential coupling ports, a metal floor and two metal columns. The metal floor is printed on the lower surface of the square substrate, and other structures are printed on the upper surface of the square substrate. Two ports of the filter adopt a differential coupling structure, and an open-circuit line is added to one port. The differential coupling structure can generate four controllable transmission zeros outside the band. The position of each zero point can be controlled by adjusting the length of the coupling feeder line. And the open-circuit line at the first port can generate a sixth zero point at high frequency, so that the out-of-band suppression width of the filter is further increased. The band-pass filter has a wide pass band and a wide stop band, the out-of-band rejection capability reaches 25dB, and the band-pass filter can be widely applied to the 5G radio frequency field.

Description

Technical Field

[0001] The invention belongs to the field of radio frequency passive devices in radio frequency technology, and relates to a microstrip branch-loaded bandpass filter, which is a short-branch-loaded broadband external suppression bandpass filter. Background Art

[0002] With the advent of the 5G era, wireless communication technology has developed rapidly. At the same time, in the base station system, the bandpass filter plays a vital role in the RF front-end system. Its signal selection and interference suppression performance can effectively alleviate the spectrum congestion problem of the communication system and improve the stability of the RF front-end system. As an important member of RF passive devices, the bandpass filter's main function is to select signals within a specific frequency range within the passband, and attenuate or block signals in other frequency ranges, and filter the signals to meet the needs of various electronic communication systems. It is of certain importance in different applications such as wireless communication, RF technology, audio processing, radar and sensors. Therefore, the performance of the filter directly affects the communication quality of the entire electronic communication system.

[0003] It is a very common method to design a microstrip bandpass filter using a microstrip branch-loaded resonator structure in modern communication systems. However, loading the microstrip branch line will cause the filter to produce some harmful resonance points at high frequencies outside the band, and these high-order resonance points will produce some unnecessary passbands. In the present invention, no additional circuit is required. By using differential coupling and open microstrip lines, the harmonics of the bandpass filter outside the high-frequency band are well suppressed. Through simulation verification, the bandpass filter in this article can generate six controllable transmission zeros outside the band, and can achieve a very wide out-of-band suppression width (up to 5.1 times the passband center frequency). Summary of the invention

[0004] In view of the problems existing in the prior art, the present invention provides a wideband external suppression bandpass filter operating in the 1.72-2.84 GHz frequency band.

[0005] The technical solution adopted by the present invention is:

[0006] A branch-loaded bandpass filter with broadband out-of-band suppression is a bandpass filter with multiple transmission zeros out-of-band that operates in the 1.72-2.84 GHz frequency band. The bandpass filter includes a square substrate 13, a microstrip branch-loaded resonator composed of two short-circuited microstrip lines and an open-circuited microstrip line 6, two differential coupling ports, a metal floor 14 and two short-circuited metal pillars (including a first metal pillar 4 and a second metal pillar 5). The bandpass filter is a single-layer structure, and each metal microstrip line is printed on the upper surface of the square substrate 13, and the metal floor 14 is printed on the lower surface of the square substrate 13. Specifically:

[0007] The microstrip branch-loaded resonator comprises a U-shaped microstrip line 1, a first microstrip line 2 and a second microstrip line 3 of the same size with short-circuited terminals, and an open microstrip line 6 with an open terminal. The first microstrip line 2, the second microstrip line 3 and the open microstrip line 6 are parallel to each other and are vertically connected to the middle horizontal part of the U-shaped microstrip line 1 respectively. One end of the open microstrip line 6 is connected to the middle part of the U-shaped microstrip line 1, and the other end is in an open state. One end of the first microstrip line 2 and the second microstrip line 3 are connected to the U-shaped microstrip line 1, and the other end is connected to the metal floor 14 through the first metal column 4 and the second metal column 5 respectively, so as to achieve terminal short circuit.

[0008] Furthermore, the open microstrip line 6 is located at the center of the middle horizontal part of the U-shaped microstrip line 1, forming an E-type structure with the U-shaped microstrip line 1, and the first microstrip line 2 and the second microstrip line 3 are respectively located on both sides of the open microstrip line 6, that is, the entire microstrip branch loaded resonator is symmetrical about the open microstrip line 6 in the middle.

[0009] The two differential coupling ports are two ports arranged at the left and right positions of the microstrip branch-loaded resonator, which are the first port 15 and the second port 16. The first port 15 includes a first main microstrip line 7 and two L-shaped microstrip lines, which are the first L-shaped open microstrip line 8 and the second L-shaped open microstrip line 9. One end of the first main microstrip line 7 is connected to the feeding port, and the other end is divided into two paths, which are respectively connected to the first L-shaped open microstrip line 8 and the second L-shaped open microstrip line 9. The second L-shaped open microstrip line 9 is formed by a metal patch arranged in the vertical direction and folded 90 degrees to the middle. The second L-shaped open microstrip line 9 is parallel to the right vertical part of the U-shaped microstrip line 1, so as to realize coupled transmission. The first L-shaped open microstrip line 8 is formed by a metal patch arranged in the horizontal direction and folded 90 degrees to the middle. The second port 16 is composed of a second main microstrip line 10, an open microstrip line 11 and a third L-shaped open microstrip line 12, and the center of the second main microstrip line 10 is a feeding point. The open microstrip line 11 and the third L-shaped open microstrip line 12 are respectively connected to the two sides of the second main microstrip line 10, and the open microstrip line 11 is located on the inner side of the U-shaped microstrip line 1, while the third L-shaped open microstrip line 12 is located on the outer side thereof. The long side of the third L-shaped open microstrip line 12 and the microstrip line 11 are parallel to the left vertical part of the U-shaped microstrip line 1, thereby forming a differential coupling region.

[0010] Furthermore, in order to optimize the impedance matching within the band and increase the transmission zero point outside the band, the width of the first L-shaped open microstrip line 8 of the bandpass filter is not uniform. Among them, the horizontal part where the first L-shaped open microstrip line 8 is connected to the main microstrip line 7 is wider and has a smaller impedance, while the end thereof is narrower and has a larger impedance. The end of the first L-shaped open microstrip line 8 is far away from the end of the U-shaped microstrip line 1 to reduce the coupling between them.

[0011] Furthermore, without considering the two ports (the first port 15 and the second port 16), since the entire microstrip branch-loaded resonator is symmetrical about the open microstrip line 6 in the middle, the resonance point of the bandpass filter can be divided into odd-mode resonance points and even-mode resonance points using the odd-even mode analysis method. By changing the length of each microstrip line in the microstrip branch-loaded resonator (U-shaped microstrip line 1, first microstrip line 2, second microstrip line 3, open microstrip line 6), the bandpass filter can generate three resonance points within the passband 1.7-2.8GHz, including two even-mode resonance points and one odd-mode resonance point. The U-shaped microstrip line 1, the first microstrip line 2, the second microstrip line 3 and the open microstrip line 6 all have an impact on the three resonance points, and the positions of the three resonance points will move downward as their lengths increase. The open microstrip line 6 can control the even-mode resonance point separately. The microstrip branch-loaded resonator itself has a transmission zero point TZ 1, and its position is determined by the lengths of the U-shaped microstrip line 1, the first microstrip line 2, the second microstrip line 3, and the open microstrip line 6. That is, the microstrip branch-loaded resonator can generate three resonance points within the band and a transmission zero point at a high frequency outside the band.

[0012] Furthermore, the bandpass filter includes two differential coupling regions. At the first port 15, the right vertical portion of the U-shaped microstrip line 1 and the long side of the second L-shaped open microstrip line 9 form a differential coupling region A. At the second port 16, the left vertical portion of the U-shaped microstrip line 1 and the long side of the third L-shaped open microstrip line 12 and the open microstrip line 11 form a differential coupling region B. By adjusting the length of each microstrip line in the two differential coupling regions so that its coupling coefficient is equal to zero, the bandpass filter can generate four controllable transmission zeros outside the high frequency band, and the order from low frequency to high frequency is recorded as TZ 2 to TZ 5. Among them, the transmission zeros TZ 3 and TZ 5 are generated by the differential coupling region A of the first port 15, and the positions of these two zeros can be controlled by adjusting the length of the second L-shaped open microstrip line 9. The differential coupling region B of the second port 16 can generate transmission zeros TZ 2 and TZ 4, and the positions of these two zeros are jointly determined by the lengths of the open microstrip line 11 and the third L-type open microstrip line 12. In order to further increase the number of transmission zeros to improve the suppression width outside the filter band, a first L-type open microstrip line 8 is added at the connection point between the first main microstrip line 7 and the second L-type open microstrip line 9. The long side of the first L-type open microstrip line 8 is parallel to the microstrip line 7, and its end is far away from the U-shaped microstrip line 1 to ensure that no coupling is generated. Based on the transmission line impedance transformation principle, the first L-type open microstrip line 8 can generate the sixth transmission zero TZ 6. By adjusting the five controllable transmission zeros TZ 2 to TZ 6, the high-frequency resonance points generated by the microstrip branch loading resonator itself can be well suppressed.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] The short branch loaded bandpass filter provided by the present invention can achieve wide passband, multiple transmission zeros, wideband out-of-band suppression, and the transmission zeros TZ 2 to TZ 6 outside the high frequency band are all controllable. The final simulation shows that the passband of the bandpass filter is 1.72-2.84 GHz, and its relative bandwidth is 49.12%. From 3.01 GHz to 11.61 GHz, the S21 of the bandpass filter is lower than -25 dB, and the out-of-band suppression width reaches 5.1 times the center frequency. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic diagram of the planar structure of the microstrip branch-loaded bandpass filter proposed in the present invention.

[0016] Figure 2 It is a schematic diagram of the three-dimensional structure of the microstrip branch-loaded bandpass filter proposed in the present invention.

[0017] Figure 3It is the reflection coefficient and isolation obtained by simulation of this filter. Among them, S11 is the reflection coefficient of the first port when the second port is matched, which reflects the impedance matching of the filter; S21 is the forward transmission coefficient from the first port to the second port when the second port is matched, which reflects the transmission characteristics of the filter.

[0018] In the figure: 1 U-shaped microstrip line; 2 first microstrip line; 3 second microstrip line; 4 first metal column; 5 second metal column; 6 open microstrip line; 7 first main microstrip line; 8 first L-shaped open microstrip line; 9 second L-shaped open microstrip line; 10 second main microstrip line; 11 open microstrip line; 12 third L-shaped open microstrip line; 13 square substrate; 14 metal floor; 15 first port; 16 second port. DETAILED DESCRIPTION

[0019] The specific implementation scheme of the present invention is described in detail below in conjunction with the accompanying drawings and technical solutions of the specification.

[0020] refer to Figure 1 and Figure 2 The bandpass filter of the present invention is a single-layer structure, each metal microstrip line is printed on the upper surface of the square substrate 13, and the metal floor 14 is printed on the lower surface of the square substrate 13. The thickness of all metal microstrip lines and the metal floor 14 is 0.035mm. The material of the square substrate 13 is Rogers RO4350B, which has a dielectric constant of 3.48 and a thickness of 0.762mm.

[0021] Furthermore, the U-shaped microstrip line 1, the short-circuited microstrip line 2, the short-circuited microstrip line 3 and the open-circuited microstrip line 6 form a microstrip branch-loaded resonator. The length of the horizontal middle part of the U-shaped microstrip line 1 is 15.2 mm, and the length of the vertical parts on the left and right sides is 14.8 mm. The first metal column 4 and the second metal column 5 are connected to the first microstrip line 2 and the second microstrip line 3 on the front side, and connected to the metal floor 14 on the back side, so that the first microstrip line 2 and the second microstrip line 3 are short-circuited at the terminal. The open-circuited microstrip line 6 is a terminal open-circuited microstrip line. The first microstrip line 2, the second microstrip line 3 and the open-circuited microstrip line 6 are all located in the middle horizontal part of the U-shaped microstrip line 1, wherein the open-circuited microstrip line 6 is located in the center and forms an E-type structure with the U-shaped microstrip line 1. The other two short-circuit lines are located on both sides of the open microstrip line 6. The spacing between the first microstrip line 2, the second microstrip line 3 and the open microstrip line 6 is 2.2 mm. The spacing between the first microstrip line 2, the second microstrip line 3 and the two vertical parts corresponding to the U-shaped microstrip line 1 is 2.2 mm. The entire microstrip branch-loaded resonator is bilaterally symmetrical. The microstrip branch-loaded resonator can generate three resonance points and a high-frequency transmission zero point.

[0022] The first main microstrip line 7 in the first port 15 is connected to the first port 15 of the bandpass filter, and the other side of the first main microstrip line 7 is divided into two paths: the first path is the second L-shaped open microstrip line 9, which is formed by a metal patch arranged in the vertical direction and folded 90 degrees to the middle. The long side of the second L-shaped open microstrip line 9 is parallel to the vertical part of the U-shaped microstrip line 1, and the part forms a differential coupling area A, with a spacing of 0.2mm and a length of 18.8mm on the long side. The short side of the second L-shaped open microstrip line 9 is parallel to the horizontal part of the microstrip line 1, with a spacing of 1.5mm and a length of 5.9mm on the short side. The differential coupling area can realize two transmission zeros TZ 3 and TZ 5 outside the band. Since the distribution of voltage in the coupling area A is controlled by the length of the second L-shaped open microstrip line 9, the positions of the transmission zeros TZ 3 and TZ 5 can be adjusted by changing the length of the microstrip line. The other is the first L-shaped open microstrip line 8, which is formed by a metal patch arranged in the horizontal direction and folded 90 degrees to the middle. In order to adjust the impedance matching, the impedance values ​​of the horizontal and vertical parts of the first L-shaped open microstrip line 8 are not the same. The length of its horizontal part is 3.8mm and the width is 1.5mm. The length of its vertical part is 1mm and the width is 0.4mm. Based on the transmission line quarter impedance transformation principle, the length of the first L-shaped open microstrip line 8 is equal to one quarter of the 10GHz operating wavelength. Therefore, at this frequency point, the current is short-circuited by the first L-shaped open microstrip line 8 to generate a transmission zero point TZ6.

[0023] At the second port 16, the L-shaped microstrip line 12, the open microstrip line 11 and the U-shaped microstrip line 1 form a differential coupling region B, and the center of the second main microstrip line 10 is the feeding point. When the filter is working, the inner core of the coaxial line is connected to the middle of the second main microstrip line 10, and its outer conductor is connected to the metal floor 14. The third L-shaped open microstrip line 12 is formed by a metal patch arranged in the vertical direction and folded 90 degrees to the middle. The length of its long side is 18.8mm, and the distance between the long side and the vertical part on the left side of the U-shaped microstrip line 1 is 0.2mm. The length of its short side is 2.6mm, and the distance between the short side and the horizontal part in the middle of the U-shaped microstrip line 1 is 1.5mm. The open microstrip line 11 is formed by a metal patch arranged in the vertical direction, and its length is 5.3mm. The distance between the U-shaped microstrip line 1 and the second main microstrip line 10 is 1.3mm to avoid contact with each other.

[0024] Figure 3 The reflection coefficient curve and isolation curve obtained by bandpass filter simulation. It can be seen that the filter passband bandwidth is 1.72-2.84GHz, (1.12GHz, 49.12%). From 3.01GHz to 11.61GHz, the S21 of the bandpass filter is lower than -25dB, and the out-of-band suppression width reaches 5.1 times the center frequency.

[0025] The above examples are only for illustrating the technical concept and features of the present invention, and are only used for a specific description of the present invention so that people familiar with the technology can understand the content of the present invention and implement it accordingly, and cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the content of the present invention should be included in the protection scope of the present invention.

Claims

1. A branch-loaded bandpass filter with broadband external suppression, characterized in that: The branch-loaded bandpass filter is a single-layer structure, comprising a square substrate (13), a microstrip branch-loaded resonator composed of two short-circuited microstrip lines, a U-shaped microstrip line (1) and an open-circuited microstrip line (6), two differential coupling ports, a metal floor (14) and two metal columns; the metal floor (14) is printed on the lower surface of the square substrate (13), and other structures are printed on the upper surface of the square substrate (13).

2. A branch-loaded bandpass filter with wideband external suppression according to claim 1, characterized in that: The branch-loaded bandpass filter is specifically: The microstrip branch-loaded resonator comprises a U-shaped microstrip line (1), a first microstrip line (2) and a second microstrip line (3) of the same size and short-circuited at the terminal, and an open-circuited microstrip line (6) with an open terminal; the first microstrip line (2), the second microstrip line (3) and the open-circuited microstrip line (6) are parallel to each other and are respectively vertically connected to the middle horizontal part of the U-shaped microstrip line (1); one end of the open-circuited microstrip line (6) is connected to the middle part of the U-shaped microstrip line (1), and the other end thereof is in an open-circuited state; one end of the first microstrip line (2) and the second microstrip line (3) are connected to the U-shaped microstrip line (1), and the other end thereof is respectively connected to a metal floor (14) through a first metal column (4) and a second metal column (5) to achieve a terminal short circuit; The two differential coupling ports are two ports arranged at the left and right positions of the microstrip branch loading resonator, which are respectively a first port (15) and a second port (16); the first port (15) comprises a first main microstrip line (7) and two L-shaped microstrip lines, which are respectively a first L-shaped open-circuit microstrip line (8) and a second L-shaped open-circuit microstrip line (9); one end of the first main microstrip line (7) is connected to the feeding port, and the other end is divided into two paths, which are respectively connected to the first L-shaped open-circuit microstrip line (8) and the second L-shaped open-circuit microstrip line (9); the long side of the second L-shaped open-circuit microstrip line (9) is parallel to the right vertical part of the U-shaped microstrip line (1), so as to achieve coupling. transmission; the second port (16) comprises a second main microstrip line (10), an open microstrip line (11) and a third L-shaped open microstrip line (12), the center of the second main microstrip line (10) being a feeding point; the open microstrip line (11) and the third L-shaped open microstrip line (12) are respectively connected to two sides of the second main microstrip line (10), the open microstrip line (11) is located on the inner side of the left vertical part of the U-shaped microstrip line (1), and the third L-shaped open microstrip line (12) is located on the outer side thereof; the long side of the third L-shaped open microstrip line (12) and the open microstrip line (11) are both parallel to the left vertical part of the U-shaped microstrip line (1), forming a differential coupling region.

3. A branch-loaded bandpass filter with wideband external suppression according to claim 2, characterized in that: The open microstrip line (6) is located at the center of the middle horizontal part of the U-shaped microstrip line (1), and forms an E-shaped structure with the U-shaped microstrip line (1); the first microstrip line (2) and the second microstrip line (3) are respectively located on both sides of the open microstrip line (6); and the entire microstrip branch-loaded resonator is left-right symmetrical about the middle open microstrip line (6).

4. A branch-loaded bandpass filter with wideband external suppression according to claim 2, characterized in that: The second L-shaped open microstrip line (9) is formed by folding a metal patch arranged in a vertical direction toward the middle by 90 degrees; the first L-shaped open microstrip line (8) is formed by folding a metal patch arranged in a horizontal direction toward the middle by 90 degrees.

5. The branch-loaded bandpass filter with wideband external suppression according to claim 2, characterized in that: The width of the first L-shaped open microstrip line (8) is not uniform: the horizontal portion of the first L-shaped open microstrip line (8) connected to the main microstrip line 7 is wide, and the vertical portion at the end thereof is narrow.

6. A branch-loaded bandpass filter with wideband external suppression according to claim 2, characterized in that: The branch-loaded bandpass filter is a bandpass filter with multiple transmission zero points outside the band and operating in the 1.7-2.8 GHz frequency band.

7. A branch-loaded bandpass filter with wideband external suppression according to claim 6, characterized in that: The branch-loaded bandpass filter can generate six controllable transmission zeros outside the band, including: The microstrip stub-loaded resonator itself has a transmission zero point; By adjusting the length of each microstrip line in the two differential coupling regions of the branch-loaded bandpass filter, four controllable transmission zeros are generated; The sixth transmission zero point generated by the first L-shaped open microstrip line (8).

8. A branch-loaded bandpass filter with wideband external suppression according to claim 7, characterized in that: The two differential coupling regions in the branch-loaded bandpass filter are specifically: at the first port (15), the right vertical portion of the U-shaped microstrip line (1) and the long side of the second L-shaped open microstrip line (9) form a differential coupling region A; at the second port (16), the left vertical portion of the U-shaped microstrip line (1) and the long side of the third L-shaped open microstrip line (12) and the open microstrip line (11) form a differential coupling region B.

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