A filter and an assembling method thereof

Abstract

A frequency selection device including a tubular conductive housing having a polygonal cross-section. The present invention provides a filter, comprising: a branched strip conductor disposed within the conductive housing, forming a series portion of a transmission line and an open stub connected in parallel with the series portion of the transmission line, and forming attenuation poles within a stop band of the filter; wherein the branched strip conductor is disposed within an elongated conductive housing having a wide wall and a narrow wall; wherein at least one narrow wall includes an opening, and an end of the branched strip conductor forming the filter is disposed at the opening; wherein the conductive housing includes at least two regions, wherein a distance between two wide walls in a first region is at least 20% greater than a distance between two wide walls in a second region; and the open stub is disposed in the first region. The filter structure of the present invention is simple, forming a pass band and a stop band separated by a narrow boundary band. The filter provides a low passive intermodulation value, low insertion loss, and low production cost.

Description

Technical Field

[0001] The present invention relates to frequency selective devices, such as filters and duplexers, which include strip conductors disposed within a conductive housing.

[0002] Background Introduction

[0003] Many different designs of filters and other frequency selective devices have been developed to improve their frequency characteristics and reduce their size. Stripline filters manufactured using conventional printed circuit board technology have a low production cost, but when the passband and stopband are separated by a narrow frequency band, the insertion loss of the device is excessive. Therefore, it is necessary to dispose a metallic strip conductor within a conductive housing to reduce the insertion loss. A passband filter including a main transmission line and an open stub shunted to the main line is most suitable for manufacture because it can be manufactured by stamping. Several improvements of such filters are described in patents US 5015976, US 5192297, and US 5291161, but when the passband and stopband are separated by a narrow frequency band, the known designs cannot be used because the impedance of some of the strip lines forming the open stub is a very high and very narrow metallic strip line and cannot be made by stamping. Therefore, providing a passband and stopband filter separated by a narrow frequency band includes cavity resonators described in many patent applications, such as US 3448412, US 6735766, EP2928011 A1, EP 3104452 A1, EP 3179552 A1. The production cost of such filters is much higher than the production cost of filters including strip lines because the manufacture of cavity resonators is more complex than the manufacture of strip lines.

[0004] Since the modern wireless communication industry uses a large number of frequency selective devices, it is necessary to develop a simple structure that can provide low insertion loss and low production cost when the passband and stopband are separated by a narrow boundary band. Summary of the Invention

[0005] An object of the present invention is to develop a filter having a simple structure that forms a passband and a stopband separated by a narrow boundary band. The filter provides a low passive intermodulation value, low insertion loss, and low production cost.

[0006] An object of the present invention is to overcome the deficiencies of known passband filters and provide a filter having a simple structure, the insertion loss of which increases sharply on the boundary band from the passband to the stopband. For example, reduce the production cost and insertion loss of filters operating in the 600 - 6000 MHz frequency band used in modern mobile communications, and provide a 1 - 7% boundary band between the passband and the stopband.

[0007] A filter for achieving the object of the present invention includes a branched strip conductor forming a series portion of a transmission line, and an open stub connected in parallel with the series portion, disposed within a tubular conductive housing having wide walls and narrow walls.

[0008] The narrow wall includes an opening, and the ends of the branched strip conductor of the filter are disposed at the opening. The conductive housing includes at least two regions, where the distance between the wide walls differs by at least 20%. For example, the distance between the two wide walls in the first region is at least 20% greater than the distance between the two wide walls in the second region. The portion of the branched strip conductor forming the open stub is disposed in the region where the distance between the wide walls is the largest, such as the first region.

[0009] The open stub forms attenuation poles at frequencies above or below the passband. The branched strip conductor is made of a single piece of metal isolated from the conductive housing and is connected to the inner conductor of the coaxial cable only through its ends, and the coaxial cable is connected to the input and output ports of the filter. The branched strip conductor is supported by a foam dielectric substrate and fixed thereto by dielectric pins.

[0010] Compared with the impedance of the series connection portion disposed at the portion of the housing where the distance between the wide walls is smaller, the open stub disposed at the portion where the distance between the wide walls is the largest has a significantly higher impedance. This structure produces a narrow attenuation band near the attenuation poles, and thus can provide a narrowband boundary between the passband and the stopband. Using an open stub in a flat spiral shape can additionally increase its impedance. Therefore, the filter according to the present invention sharply increases the insertion loss from the passband to the stopband and can provide a boundary band of up to 1 - 2%.

[0011] The provided filter does not include separate metal parts in contact with each other because the branched strip conductor made of a single piece of metal is isolated from the conductive housing, and the conductive housing is tubular and also made of a single piece of metal. As a result, compared with known filters including a conductive housing, the filter with this simple structure provides a lower level of passive intermodulation and a lower production cost, and the conductive housing has a cover pressed together by screws and a short - circuit stub pressed or welded to the conductive housing.

[0012] The branched strip conductor is made of a single piece of metal by stamping and is supported by a foam dielectric substrate, which can provide very small production tolerances. Therefore, the provided filter is manufactured without a tuning process and does not contain tuning screws. Thus, its production cost is lower than that of known filters and provides low insertion loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Some embodiments of the present invention are described by the following drawings, wherein:

[0014] Figure 1a and 1bShows a top view of a stripline of a first type of bandpass filter, the stripline including a main line and an open stub described by US 5291161 (prior art), and the frequency characteristic of its insertion loss increasing sharply in a frequency band higher than the passband.

[0015] Figure 2a and 2b Shows a top view of a stripline of a second type of bandpass filter, the stripline including a main line and an open stub described by US 5291161 (prior art), and the frequency characteristic of its insertion loss increasing sharply in a frequency band lower than the passband.

[0016] Figure 3 Is a schematic diagram of a filter with two stubs according to the present invention.

[0017] Figure 4 Are the frequency characteristics S11 and S21 of the filter, the schematic diagram of which is as shown in Figure 3 Shown, the transmission line lengths Ln and impedances Zn are shown in Table 1.

[0018] Figure 5 Are the simulated frequency characteristics S11 and S21 of the filter, the schematic diagram of which is as shown in Figure 3 Shown, the transmission line lengths Ln and impedances Zn are shown in Table 2.

[0019] Figure 6a and 6b Show a straight open stub having a thickness of 0.01 mm and an open stub in a flat spiral shape having a width of 1.4 mm and a pitch of 1.4 mm.

[0020] Figure 7 Show the frequency characteristics S11 and S21 of the filter, the schematic diagram of which is as shown in Figure 3 Shown, the transmission line lengths Ln and impedances Zn are shown in Table 2.

[0021] Figure 8 Shows the metal housing of the filter according to the present invention.

[0022] Figure 9 Is according to Figure 3 The top view of the branch strip conductor of the filter formed according to the schematic diagram shown.

[0023] Figure 10 Is a perspective view of an assembly including a branch strip conductor, the assembly being disposed between four foam dielectric substrates.

[0024] Figure 11 Shows the side view of the filter composed of the components shown in Figures 9 - 10 Shown.

[0025] Figure 12is a perspective view showing a first preferred embodiment of a filter according to the present invention, with coaxial cables connected to the input and output ports of the filter.

[0026] Figure 13 For Figures 8 - 12 the frequency characteristics S11 and S21 of the filter shown. Detailed Description

[0027] It should be understood that the present invention is not limited to the specific forms disclosed in the above drawings. The present invention will cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

[0028] The object of the present invention is to reduce the production cost and insertion loss of a bandpass filter, provide a separation of 1 - 7% between the passband and the stopband, and be used for modern mobile communications.

[0029] As Figure 1a and 2a shown, the stripline of the known filter consists of a main stripline conductor connected to the input and output ports of the filter, and an open stub electrically connected in parallel with the main stripline conductor. The filter with this structure forms a frequency characteristic of a sharp increase in insertion loss at a frequency band higher than the passband, as shown in Figure 1; and forms a frequency characteristic of a sharp increase in insertion loss at a frequency band lower than the passband, as shown in Figure 2.

[0030] Calculations and optimizations were performed on different schematic diagrams to find the optimal structure of the filter, which provides low insertion loss at the 824 - 880 MHz passband and provides an attenuation of about 20 to 21 dB at the 900 - 960 MHz stopband. Therefore, the Figure 3 shown schematic diagram was selected to provide ideal matching at the passband and attenuation at the stopband. The filter includes two open stubs connected in parallel with the series part of the stripline.

[0031] Figure 4 are the calculated frequency characteristics S11 and S21 of the filter, and its schematic diagram is as Figure 3 shown. The filter provides S11 = -21 dB at the 824 - 880 MHz passband and S21 = -20 dB at the 900 - 960 MHz stopband. The passband and the stopband are separated by a 20 MHz frequency band, that is, 2.2% of the intermediate frequency between the passband and the stopband.

[0032] Table 1 includes the lengths Ln and impedances Zn of the transmission lines forming the filter, and the filter provides the Figure 4 shown frequency characteristics. The third row of Table 1 includes the formation of Figure 3The width of the strip conductor of the components in the schematic diagram shown. For a strip conductor with a thickness of 0.5 mm, calculate the width Wn of the strip conductor, which is arranged inside the conductive housing, with a distance of 7 mm between the wide walls of the conductive housing. The strip conductor is arranged between foam dielectric substrates with a dielectric permeability of 1.06.

[0033] Table 1

[0034] N1 N2 N3 N4 N5 L, mm 99.95 79.74 7.31 83.06 94.83 Z, Ohm 30 87.5 100 156.5 21.5 W, mm 16.19 3.26 2.43 0.52 24.05 <![CDATA[W n ,mm]]> 13.62 8.57 1.98 1.95 20.27

[0035] The impedances of the transmission lines differ from each other by more than 7 times, and the widths of the branched strip conductor parts differ from each other by more than 38 times. The narrowest part N4, W = 0.52 mm, cannot be made by stamping. The widest part N5, W = 24 mm, so the calculated size of the filter is too large. The strip conductors of the calculated filter cannot be manufactured because the narrowest part is not suitable for mass production. It is necessary to increase the distance between the wide walls of the conductive housing to make the strip line N6 wider, but all other parts of the branched strip conductor will also be wider, and the size of the filter will be larger than the specified size. Therefore, it is necessary to find another method to develop a filter with a simple structure, specified size and low insertion loss.

[0036] The conductive housing is made in an elongated shape, with wide walls and narrow walls, including two regions. The distances between the wide walls of the two regions are different. The parts of the branched strip conductor forming the open stub are arranged in the region with a larger distance between the wide walls, while the other parts are arranged in the region with a smaller distance between the wide walls.

[0037] The last row of Table 1 includes the calculated width Wn of the strip conductor, which constitutes Figure 3 the components in the schematic diagram shown. The open stubs N2 and N4 are arranged in the region with a distance of 16 mm between the wide walls, and the series parts N1, N3 and N5 are arranged in the region with a distance of 6 mm between the wide walls. The width of the open stub N4 is increased to 1.95 mm, and the width of N5 is reduced to 20.3 mm. This shape of the conductive housing and the configuration of the conductor parts increase the width of the open stub and reduce the width of the series part. The insertion loss of the filter is small because the insertion loss mainly depends on the width of the open stub. The length of the filter depending on the width of the series part is also smaller. All parts N1 - N5 are wider than 1.9 mm, so the branched strip conductor can be made by stamping. Therefore, the provided filter structure is simple, with low manufacturing cost and suitable for mass production.

[0038] Calculate the second embodiment of the filter with Figure 3 the schematic diagram shown to provide a stop band below the pass band. Table 2 includes the length Ln and impedance Zn of the transmission lines, providing as Figure 5The frequency characteristics shown. The filter provides S11 = -20 dB in the 1885 - 2025 MHz passband and S21 = -19.5 dB in the 1710 - 1830 MHz stopband. For a strip conductor with a thickness of 0.5 mm, the width Wn of the strip conductor is calculated. The strip conductor is arranged between foam dielectric substrates with a relative permittivity of 1.06. The foam dielectric substrates are located inside a conductive housing, and the distance between the wide walls of the conductive housing is 7 mm.

[0039] Table 2

[0040] N1 N2 N3 N4 N5 L, mm 27.36 41.1 67.24 43.65 20.28 Z, Ohm 27.2 249.8 60.5 122.2 32.99 W, mm 18.23 - 6.26 1.42 14.4 <![CDATA[W n ,mm]]> 15.35 0.01 5.21 2.31 12.1

[0041] The calculation results shown in the third row of Table 2 indicate that the N2 part of the branch strip conductor with Z = 249.8 Ohm is too narrow and thus cannot be manufactured. The last row of Table 2 includes the calculated width Wn of the branch strip conductor. The open stubs N2 and N4 are arranged in the area where the distance between the wide walls is 10 mm, and the series parts N1, N3, and N5 are arranged in the area where the distance between the wide walls is 6 mm. The width of the open stub N2 is increased to 0.01 mm, and the width of N1 is decreased to 15.35 mm. The calculated filter cannot be manufactured by stamping because the open stub N2 is still too narrow.

[0042] After further research, it is found that the second method can increase the width of the strip conductor, thus forming an open stub with high impedance. The study of the frequency characteristics of the open stub in the shape of a flat spiral shows that, compared with the straight open stub, the width of the strip conductor of the flat spiral is significantly wider and can provide approximately the same frequency characteristics near the attenuation pole. In Figure 6a and 6b a straight open stub 26 with a width of 0.01 mm and an open stub 27 in the shape of a flat spiral with a width of 1.4 mm and an inter-turn distance of 1.4 mm are correspondingly shown.

[0043] The open stubs 26 and 27 are arranged in the area with a wide wall spacing of 10 mm and are correspondingly connected to the strip conductors 28 and 29, which are arranged in the area with a wide wall spacing of 6 mm, forming a transmission line with an impedance of 50 Ohm. As Figure 7 shown, the simulated frequency characteristics S11 and S21 of these open stubs have attenuation poles at the same frequency of 1832 MHz. Frequency characteristics 1 and 3 correspondingly describe the S11 of the straight open stub and the S11 of the open stub in the shape of a flat spiral.

[0044] The frequency characteristics 2 and 4 respectively describe S21 of a straight open stub and S21 of an open stub in a flat spiral shape. The open stub with a width of 0.04 mm provides 10 dB of attenuation in the 78 MHz band, and the flat spiral stub with a width of 1.4 mm provides 10 dB of attenuation in the 80 MHz band. Therefore, the narrow straight stub can be replaced by a flat spiral stub with a significantly wider width. The open stub in the flat spiral shape with a width of 1.4 mm can be manufactured by stamping and manufactured together with other parts of the branched strip conductor, so the manufacturing cost of this filter is low.

[0045] Figure 8 Shows the metal housing 7 of the filter according to the present invention. The metal housing 7 includes a cavity 8 formed by a wide wall and two narrow walls 11 and 12, and the wide wall is composed of parts 9, 10 and 15, 16. The distance between parts 9 and 10 is 6 mm, and the distance between parts 15 and 16 is 10 mm. The narrow wall 11 includes a longitudinal cavity 20 having a longitudinal slit 19. The first openings 13a and 13b penetrate the inside of the cavity 8. The second openings 14a and 14b on the narrow wall 11 cut the longitudinal cavity 20 near the first openings 13a and 13b to form parts 30a and 30b. The third opening 14c cuts the middle part of the longitudinal cavity 20.

[0046] Figure 9 is according to Figure 3 Shown in the schematic diagram is a top view of the branched strip conductor 5 forming the filter, where the transmission line has the impedance shown in the last row of Table 2. Two open stubs are arranged in the area where the distance between the wide walls is 10 mm. The open stub N2 has a flat spiral shape to additionally increase its impedance. The open stub N4 is bent 90 degrees to reduce the size of the filter. Parts N1, N3 and N5 are arranged in the area where the distance between the wide walls is 6 mm. The hole 6 is for the dielectric pin to fix the branched strip conductor 5 to the dielectric foam substrate. The dimensions of the branched strip conductor are optimized to compensate for the influence of discontinuities at the positions where the parts of the branched strip conductor are connected to each other.

[0047] Figure 10 Is a perspective view of a component composed of the branched strip conductor 5, which is arranged between four foam dielectric substrates 18a - 18d and fixed thereto through dielectric pins 17 through holes 6. The foam dielectric substrates 18a - 18d include holes 25a and 25b, and the holes 25a and 25b are arranged opposite to the open stubs N2 and N4. The holes 25a and 25b reduce the dependence of the resonance frequency characteristics of the open stubs N2 and N4 on the thickness and dielectric constant of the foam dielectric substrate. Therefore, the frequency characteristics of the filter are less dependent on production tolerances.

[0048] Figure 11 is a side view of a filter composed of the components shown Figures 8 - 10 as shown. The assembly shown is installed in the cavity 8 of the metal housing 7 shown Figure 10 and moves towards the first openings 13a and 13b, thereby inserting the ends 5a and 5b of the branched strip conductors 5 into the first openings 13a and 13b respectively Figure 9 as shown.

[0049] Figure 12 is a perspective view showing a second embodiment of a filter according to the present invention, in which coaxial cables 21a and 21b are connected to the ports of the filter. As shown Figure 10 the assembly shown is provided at the cavity 8 inside the housing 7, and the ends 5a and 5b of the branched strip conductors 5 are provided in the first openings 13a and 13b respectively. The coaxial cables 21a, 21b are installed in the longitudinal cavity 20 through the third opening 14c, and their inner conductors 22a, 22b are welded to the ends 5a and 5b of the branched strip conductors 5 respectively. The outer conductors 23a, 23b of the coaxial cables 21a, 21b are provided inside the longitudinal cavity 20. A part of the coaxial cable 21a provided between the first opening 13a and the second opening 14a is welded to the part 30a

[0050] Solder penetrates into the longitudinal cavity 20 through the longitudinal slit 19. The first opening 13a and the second opening 14a partially separate the part 30a from the other parts of the narrow wall 11 and prevent heat from spreading from the part 30a during the welding of the outer conductor 23a to the part 30a. Therefore, less heating is required for the welding process. The protective tubes 24a and 24b cover the outer conductors 23a and 23b and provide support near the welded ends, preventing the outer conductors 23a and 23b from cracking during vibration. In addition, the protective tubes 24a and 24b isolate the unwelded parts of the outer conductors 23a and 23b from the housing 7 and prevent them from contacting. Therefore, the filter according to the present invention provides a low level of passive intermodulation

[0051] Figure 13 is Figures 8 - 12 the simulated frequency characteristics S11 and S21 of the filter shown. The filter provides S11 = 19 dB in the 1885 - 2025 MHz passband, and the insertion loss S21 is less than 0.2 dB. In the 1710 - 1830 MHz stopband, the attenuation S21 = -19.5 dB

[0052] The spiral shape of the strip conductor forming the open stub allows an additional increase in the width of the strip conductor. Therefore, the filter according to the present invention provides a smaller insertion loss than known filters. In addition, compared with the matching of known filters having very narrow open stubs, the matching of the provided filter is less dependent on production tolerances

[0053] The branched strip conductors can also be manufactured using conventional printed circuit board technology, providing a very narrow strip conductor width and forming an open-ended stub with a high impedance. For example, the substrate is disposed between two metal plates spaced 10 mm apart, having a thickness of 0.27 mm and a dielectric constant of 2.5. The impedance of a strip conductor with W = 0.1 mm formed on this substrate is approximately 250 Ohm. Thus, the provided filter can be used in many applications where the passband and stopband must be separated by a narrow boundary band. A notch filter that suppresses a very narrow frequency band from a wide frequency band can also be created.

Claims

1. A filter, comprising: a branched strip conductor disposed within a conductive housing, forming a series portion of a transmission line and an open stub connected in parallel with the series portion of the transmission line, and forming an attenuation pole within a stop band of the filter; wherein the branched strip conductor is disposed within an elongated conductive housing having a wide wall and a narrow wall; wherein at least one narrow wall includes a first opening, and an end of the branched strip conductor forming the filter is disposed at the first opening; wherein the conductive housing includes at least two regions, wherein a distance between two wide walls in a first region is at least 20% greater than a distance between two wide walls in a second region; the open stub is disposed in the first region.

2. The filter according to claim 1, wherein, the branched strip conductor is a single piece of metal, isolated from the conductive housing, and connected to an inner conductor of a coaxial cable only at a port of the filter.

3. The filter according to claim 1, wherein, at least one open stub has a flat spiral shape.

4. The filter according to claim 1, wherein, at least one open stub has a curved shape.

5. The filter according to claim 1, wherein, the branched strip conductor is disposed between foam dielectric substrates within the conductive housing.

6. The filter according to claim 5, wherein, the foam dielectric substrate includes holes, and the holes are disposed opposite to the open stub.

7. The filter according to claim 5 or 6, wherein, the branched strip conductor is fixed to the foam dielectric substrate by dielectric pins.

8. The filter according to claim 7, wherein, the conductive housing includes a second opening on the narrow wall and a longitudinal cavity, and the second opening cuts the longitudinal cavity near the first opening, forming a portion for welding an outer conductor of a coaxial cable.

9. The filter according to claim 8, wherein, the longitudinal cavity includes a longitudinal slit.

10. A method of assembling the filter according to claim 9, comprising the following steps: installing a coaxial cable in the longitudinal cavity and welding an outer conductor of the coaxial cable to the narrow wall near the second opening of the conductive housing; assembling the branched strip conductor and the foam dielectric substrate into a component by dielectric pins, and when ends of the branched strip conductor are oppositely disposed at the first opening of the conductive housing, installing the component at a position inside the conductive housing; moving the assembled branched strip conductor towards the first opening such that an end of the branched strip conductor and an inner conductor of the coaxial cable are in direct contact; welding an end of the branched strip conductor to the inner conductor of the coaxial cable.

11. The filter according to claim 1, wherein, the branched strip conductor is formed on a surface of a dielectric substrate disposed inside the conductive housing by printed circuit board technology.

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