CN115377632A

CN115377632A - Frequency selection device comprising a tubular conductive housing having a polygonal cross-section

Info

Publication number: CN115377632A
Application number: CN202210818580.XA
Authority: CN
Inventors: 李梓萌; 鲁达科夫·维亚切斯拉夫
Original assignee: Guangzhou Sinan Technology Co ltd
Current assignee: Guangzhou Sinan Technology Co ltd
Priority date: 2022-07-13
Filing date: 2022-07-13
Publication date: 2022-11-22

Abstract

A frequency selective device comprising 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 an attenuation pole within a stop band of the filter; wherein the branch strip conductor is disposed within an elongated conductive housing having wide walls and narrow walls; wherein at least one of the narrow walls comprises an opening and the end of the branch strip conductor forming the filter is arranged at the opening; wherein the conductive housing comprises 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. The filter of the invention has simple structure and forms a pass band and a stop band which are separated by a narrow boundary band. The filter provides low passive intermodulation values, low insertion loss and low production cost.

Description

Frequency selection device comprising a tubular conductive housing having a polygonal cross-section

Technical Field

The present invention relates to frequency selective devices, such as filters and duplexers, comprising a strip conductor disposed within a conductive housing.

Background introduction

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 are inexpensive to produce, but the insertion loss of the device is excessive when the passband and stopband are separated by a narrow band. Therefore, it is desirable to dispose the metal ribbon wire within a conductive housing to reduce insertion loss. The pass-band filter including the main transmission line and the open stub connected in parallel with the main line is most suitable to be manufactured because it can be manufactured by stamping. Patents US 5015976, US 5192297 and US5291161 describe several improvements to such filters, but when the pass-band and stop-band are separated by narrow bands, the known designs cannot be used because the impedance of some of the striplines forming the open stub lines are very high and very narrow metal striplines, which cannot be made by stamping. Thus, providing passband and stopband filters separated by narrow frequency bands includes the cavity resonators described in many patent applications, such as US 3448412, US 6735766, EP 2928011 A1, EP 3104452 A1, EP 3179552 A1. The production cost of such a filter is much higher than the production cost of a filter comprising striplines, since the manufacture of cavity resonators is more complicated than the manufacture of striplines.

Since the modern wireless communication industry uses a large number of frequency selective devices, it is desirable to develop a simple structure that provides low insertion loss and low production cost when the pass band and stop band are separated by a narrow boundary band.

Disclosure of Invention

The object of the invention is to develop a simple-structured filter which forms a pass band and a stop band separated by a narrow boundary band. The filter provides low passive intermodulation values, low insertion loss and low production cost.

It is an object of the present invention to overcome the disadvantages of the known pass band filters and to provide a filter of simple construction, the insertion loss of which increases sharply over the boundary band from the pass band to the stop band. For example, the production cost and insertion loss of filters operating in the 600-6000MHz band used in modern mobile communications are reduced, and a boundary band of 1-7% is provided between the pass band and the stop band.

A filter intended to achieve the object of the invention comprises a branched strip conductor forming a series part of a transmission line, and an open stub connected in parallel with the series part, arranged in a tubular conductive housing having wide walls and narrow walls.

The narrow wall comprises an opening at which the ends of the branched strip conductors of the filter are arranged. The conductive housing comprises at least two regions wherein the distance between the broad walls differs by at least 20%. For example, the distance between two broad walls in the first region is at least 20% greater than the distance between two broad walls in the second region. The portion of the branched strip conductor forming the open stub is disposed in a region where the distance between the wide walls is the largest, for example, a first region.

The open stub forms an attenuation pole at frequencies above or below the passband. The branch strip conductors are made of a monolithic piece of metal that is isolated from the conductive housing and are connected only by the ends to the inner conductors of the coaxial cables that are connected to the input and output ports of the filter. The branch ribbon conductors are supported by a foamed dielectric substrate and are secured thereto by dielectric pins.

The open stub disposed at the largest distance between the wide walls has a significantly higher impedance than that of the series-connected portion at the housing portion where the distance between the wide walls is smaller. This structure creates a narrow attenuation band near the attenuation pole and therefore can provide a narrow band boundary between the pass band and the stop band. The impedance of an open stub having a flat spiral shape can be additionally increased by using it. Therefore, the filter according to the present invention sharply increases the insertion loss from the pass band to the stop band and can provide a boundary band of up to 1-2%.

The filter does not include the separation metal part of mutual contact, because the branch strip conductor that forms with monoblock metal preparation keeps apart with conductive housing, and conductive housing is the tubulose, also makes with monoblock metal. As a result, this simple structure of the filter provides a lower level of passive intermodulation and lower production costs than known filters that include a conductive housing having covers that are pressed together by screws and short-circuited stubs that are pressed or soldered to the conductive housing.

The branch strip conductors are made in one piece of metal by stamping and are supported by a foamed dielectric substrate, providing very small production tolerances, and therefore the filter provided is manufactured without a tuning process and does not contain tuning screws. Therefore, the production cost thereof is lower than that of the known filter, providing a low insertion loss.

Drawings

Some embodiments of the invention are described by the following drawings, in which:

fig. 1a and 1b show top views of striplines of a first-class passband filter, which includes a main line and open stubs described by US5291161 (prior art), and whose frequency characteristics of insertion loss sharply increase over a frequency band higher than the passband.

Fig. 2a and 2b show top views of striplines of a second type of passband filter, which includes a main line and open stubs described by US5291161 (prior art), and whose frequency characteristics of insertion loss sharply increase at a frequency band lower than the passband.

Fig. 3 is a schematic diagram of a filter having two stubs according to the present invention.

Fig. 4 shows the frequency characteristics S11 and S21 of the filter, which are schematically shown in fig. 3, and the transmission line length Ln and the impedance Zn are shown in table 1.

Fig. 5 shows the analog frequency characteristics S11 and S21 of the filter, which are schematically shown in fig. 3, and the transmission line length Ln and the impedance Zn are shown in table 2.

Fig. 6a and 6b show a straight open stub having a thickness of 0.01mm, and an open stub of a flattened helical shape having a width of 1.4mm and a pitch of 1.4mm.

Fig. 7 shows the frequency characteristics S11 and S21 of the filter, whose schematic diagram is shown in fig. 3, and the transmission line length Ln and the impedance Zn are shown in table 2.

Fig. 8 shows a metal housing of a filter according to the invention.

Fig. 9 is a top view of a branched strip conductor of a filter formed according to the schematic diagram shown in fig. 3.

Fig. 10 is a perspective view of an assembly including a branch ribbon conductor, the assembly being disposed between four dielectric foam substrates.

Fig. 11 shows a side view of a filter made up of the elements shown in fig. 9-10.

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

Fig. 13 is a graph of the frequency characteristics S11 and S21 of the filters shown in fig. 8-12.

Detailed Description

It should be understood that the invention is not intended to be limited to the particular forms disclosed in the above drawings. The intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

The object of the invention is to reduce the production cost and insertion loss of a pass-band filter, providing a separation of 1-7% between pass-band and stop-band for modern mobile communications.

The stripline of the known filter as shown in fig. 1a and 2a consists of a strip-shaped main conductor connected to the input and output ports of the filter, and an open stub electrically connected in parallel with the strip-shaped main conductor. The filter of this structure forms a frequency characteristic of a sharply increased insertion loss in a frequency band higher than the pass band, as shown in fig. 1; a frequency characteristic of insertion loss sharply increasing in a frequency band lower than the pass band is formed as shown in fig. 2.

Calculations and optimizations were performed on the different schematics to find the best configuration for a filter that provides low insertion loss at the 824-880MHz passband and about 20-21 dB attenuation at the 900-960MHz stopband. The schematic shown in fig. 3 is therefore chosen to provide a perfect match at the pass band and attenuation at the stop band. The filter includes two open stubs connected in parallel with the series portion of the stripline.

Fig. 4 is calculated frequency characteristics S11 and S21 of the filter, and a schematic diagram thereof is shown in fig. 3. The filter provides S11= -21dB at the 824-880MHz pass band and S21= -20dB at the 900-960MHz stop band. The pass band and the stop band are separated by a 20MHz band, i.e. 2.2% of the intermediate frequency between the pass band and the stop band.

Table 1 includes the length Ln and the impedance Zn of the transmission line forming a filter that provides the frequency characteristics shown in fig. 4. The third row of table 1 includes the widths of the ribbon conductors that form the elements of the schematic diagram shown in fig. 3. The width Wn of the strip conductor was calculated for a strip conductor having a thickness of 0.5mm, which was disposed inside a conductive housing having a distance of 7mm between the wide walls thereof, and which was disposed between foamed dielectric substrates having a dielectric permeability of 1.06.

TABLE 1

	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
W _n ,mm	13.62	8.57	1.98	1.95	20.27

The impedances of the transmission lines differ from each other by more than 7 times, and the widths of the branched strip conductor portions differ from each other by more than 38 times. The narrowest part N4, W =0.52mm, cannot be made by stamping. The widest part N5, W =24mm, so the size of the calculated 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 broad walls of the conductive housing so that the strip line N6 is wider, but all other parts of the branched strip conductor will also be wider and the size of the filter will be larger than specified. Therefore, there is a need to find another way to develop a filter with a simple structure of a specified size and low insertion loss.

The conductive housing is formed in an elongated shape having a wide wall and a narrow wall, and includes two regions. The distance between the broad walls of the two zones is different. The portion of the branched strip conductor forming the open stub is disposed in a region where the distance between the wide walls is large, and the other portion is disposed in a region where the distance between the wide walls is small.

The last row of table 1 includes the calculated strip conductor width Wn which constitutes the element of the schematic diagram shown in fig. 3, the open stubs N2 and N4 are provided in the region of a distance of 16mm between the broad walls, and the series portions N1, N3 and N5 are provided in the region of a distance of 6mm between the broad walls. The width of the open stub N4 was increased to 1.95mm and the width of N5 was reduced to 20.3mm. This shape of the conductive housing and the configuration of the conductor portions increases the width of the open stub and reduces the width of the series portion. The insertion loss of the filter is small because the insertion loss is mainly dependent on the width of the open stub. The filter length, which depends on the width of the series part, is also smaller. All the N1-N5 sections are wider than 1.9mm, so that the branch strip conductors can be manufactured by stamping. Therefore, the filter has simple structure and low manufacturing cost and is suitable for mass production.

A second embodiment of a filter having the schematic diagram of fig. 3 is calculated to provide a stop band disposed below the pass band. Table 2 includes the length Ln of the transmission line and the impedance Zn, providing the frequency characteristics as shown in fig. 5. The filter provides S11= -20dB in the passband of 1885-2025MHz and S21= -19.5dB in the stopband of 1710-1830 MHz. The width Wn of the strip conductor was calculated for a strip conductor of 0.5mm thickness, which was disposed between foamed dielectric substrates having a dielectric permeability of 1.06, the foamed conductive substrates being located inside a conductive housing having a distance of 7mm between the wide walls of the housing.

TABLE 2

	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
W _n ,mm	15.35	0.01	5.21	2.31	12.1

The calculation results shown in the third row of table 2 show that the N2 portion of the branched ribbon conductor with Z =249.8Ohm is too narrow to be manufactured. The last row of table 2 includes the calculated width Wn of the branched strip conductor, the open stubs N2 and N4 are disposed in the region where the distance between the wide walls is 10mm, and the series portions N1, N3, and N5 are disposed in the region where the distance between the wide walls is 6 mm. The width of the open stub N2 was increased to 0.01mm and the width of N1 was decreased to 15.35mm. The calculated filter cannot be manufactured by stamping because the open stub N2 is still too narrow.

Through further research, it was found that the second method can increase the width of the strip conductor, thereby forming an open stub having high impedance. Studies on the frequency characteristics of open stubs in the form of flat spirals have shown that the width of the ribbon conductor of the flat spiral is significantly wider than that of a straight open stub, providing approximately the same frequency characteristics in the vicinity of the attenuation pole. In fig. 6a and 6b, respectively, a straight open stub 26 is shown, the width of which is 0.01mm, and an open stub 27 in the form of a flat spiral, the width of which is 1.4mm and the inter-turn distance of which is 1.4mm.

Open stubs

26 and 27 are provided in the region of a wide wall spacing of 10mm and are connected to strip

conductors

28 and 29, respectively, and

strip conductors

28 and 29 are provided in the region of a wide wall spacing of 6mm, forming a transmission line with an impedance of 50Ohm. As shown in fig. 7, the analog frequency characteristics S11 and S21 of these open stubs have attenuation poles at the same frequency of 1832 MHz. The

frequency characteristics

1 and 3 describe S11 of the straight open stub, and S11 of the open stub in the shape of a flat spiral, respectively.

The frequency characteristics 2 and 4 describe S21 of the straight open stub, and S21 of the open stub in the shape of a flat spiral, respectively. An open stub having a width of 0.04mm provides 10dB of attenuation in the 78MHz band, and a flat spiral stub having a width of 1.4mm provides 10dB of attenuation in the 80MHz band. Thus, the narrow straight stub can be replaced by a flat spiral stub having a significantly wider width. The open stub having a flat spiral shape with a width of 1.4mm can be manufactured by pressing together with the other parts of the branched strip conductor, and therefore, the manufacturing cost of such a filter is low.

Fig. 8 shows the metal housing 7 of the filter according to the invention. The metal housing 7 comprises a cavity 8, the cavity 8 being formed by a wide wall and two

narrow walls

11 and 12, the wide wall being composed of

parts

9, 10 and 15, 16. The distance between the

portions

9 and 10 is 6mm and the distance between the

portions

15 and 16 is 10mm. The narrow wall 11 comprises a longitudinal cavity 20 with a longitudinal slit 19.

Openings

13a and 13b extend through the interior of cavity 8. The

openings

14a and 14b in the narrow wall 11 cut the longitudinal cavity 20 near the

openings

13a and 13b, forming

portions

30a and 30b. The opening 14c cuts the middle of the longitudinal cavity 20.

Fig. 9 is a top view of a branched strip conductor 5 forming a filter according to the principle diagram shown in fig. 3, where the transmission line has the impedance shown in the last row of table 2. Two open stubs are arranged in the region of a distance of 10mm between the broad walls. The open stub N2 has a flat spiral shape to additionally increase its impedance. The open stub N4 is bent by 90 degrees to reduce the size of the filter. The portions N1, N3 and N5 are provided in the region having a distance of 6mm between the broad walls. The holes 6 are used as dielectric pins to secure the branch strip conductors 5 to the dielectric foam substrate. The dimensions of the branch strip conductors are optimized to compensate for the effect of discontinuities at the locations where the portions of the branch strip conductors are connected to each other.

Fig. 10 is a perspective view of an assembly consisting of a branch strip conductor 5, which branch strip conductor 5 is arranged between four dielectric foam substrates 18a-18d and is fixed thereto by means of dielectric pins 17 through holes 6. The dielectric foam substrates 18a to 18d include

holes

25a and 25b, and the

holes

25a and 25b are disposed opposite to each other in 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 dielectric foam substrate. Therefore, the frequency characteristics of the filter are less dependent on production tolerances.

Fig. 11 is a side view of a filter made up of the elements shown in fig. 8-10. The assembly shown in fig. 10 is mounted in the cavity 8 of the metal shell 7 shown in fig. 9 and moved towards the

openings

13a and 13b so as to place the

ends

5a and 5b of the branch strip conductors 5 in the

openings

13a and 13b, respectively.

Fig. 12 is a perspective view showing a second embodiment of the filter according to the present invention, and

coaxial cables

21a and 21b are connected to ports of the filter. The assembly shown in fig. 10 is arranged at the cavity 8 inside the housing 7, the

ends

5a and 5b of the branch strip conductors 5 being arranged in the

openings

13a and 13b, respectively.

Coaxial cables

21a, 21b are mounted in the longitudinal cavity 20 through the opening 14c, the inner conductors 22a, 22b of which are welded to the

ends

5a and 5b, respectively, of the branch strip conductor 5. The outer conductors 23a, 23b of the

coaxial cables

21a, 21b are arranged inside the longitudinal cavity 20. A portion of the coaxial cable 21a disposed between the

openings

13a and 14a is soldered to the portion 30a.

The solder penetrates into the longitudinal cavity 20 through the longitudinal slit 19. The

openings

13a and 14a partially separate the portion 30a from the other portions of the narrow wall 11 and prevent heat from diffusing from the portion 30a during soldering of the outer conductor 23a to the portion 30a. Thus, 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 weld ends to prevent the outer conductors 23a and 23b from cracking when vibrated. Further, the protective tubes 24a and 24b isolate the non-welded portions of the outer conductors 23a and 23b from the outer shell 7 and prevent them from coming into contact. The filter according to the invention thus provides a low level of passive intermodulation.

Fig. 13 is a graph of the analog frequency characteristics S11 and S21 of the filters shown in fig. 8-12. The filter provides S11=19dB in the 1885-2025MHz passband, with an insertion loss S21 of less than 0.2dB. At a stopband of 1710-1830MHz, attenuation S21= -19.5dB.

The spiral shape of the strip conductor forming the open stub allows the width of the strip conductor to be additionally increased. The filter according to the invention thus provides a smaller insertion loss than the known filter. Furthermore, the matching of the provided filter is less dependent on production tolerances than the matching of known filters with very narrow open stubs.

The branch strip conductors can also be manufactured using conventional printed circuit board technology, providing very narrow strip conductor widths, and forming open-ended stubs with high impedance. For example, a substrate is disposed between two metal plates, the two metal plates are separated by 10mm, the thickness of the substrate is 0.27mm, the dielectric constant of the substrate is 2.5, and the impedance of a strip conductor formed on the substrate and having a W =0.1mm is about 250Ohm. Thus, the provided filter can be used in many applications where the pass band and the stop band have to be separated by a narrow boundary band. A notch filter that rejects very narrow bands from the wide band can also be created.

Claims

1. 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 an attenuation pole within a stop band of the filter;

wherein the branch strip conductor is disposed within an elongated conductive housing having wide walls and narrow walls;

wherein at least one of the narrow walls comprises an opening and the end of the branch strip conductor forming the filter is arranged at the opening;

wherein the conductive housing comprises 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 of claim 1, wherein the branch strip conductor is a monolithic piece of metal, is isolated from the conductive housing, and is connected to the inner conductor of a coaxial cable only at a port of the filter.

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

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

5. The filter of claim 1, wherein the branch strip conductors are disposed between foam dielectric substrates within the conductive housing.

6. The filter of claim 5, wherein the foam dielectric substrate includes holes for forming portions of the branched strip conductors of the open stubs.

7. A filter according to claim 5 or 6, wherein the branch strip conductors are fixed to the foam dielectric substrate by dielectric pins.

8. The filter of claim 1, wherein the conductive housing comprises an opening in a narrow wall.

9. The filter of claim 8, wherein the conductive housing comprises a circular longitudinal channel along a narrow wall, the narrow wall comprising an opening.

10. The filter of claim 9, wherein the circular longitudinal channel comprises a longitudinal slit.

11. Method of assembling a filter according to claim 9 or 10, comprising the steps of:

mounting a coaxial cable in the circular longitudinal passage and soldering an outer conductor of the coaxial cable to the narrow wall adjacent the opening of the conductive shell;

assembling the branch strip conductor and the foam dielectric substrate into an assembly through a dielectric pin, and mounting the assembly at a position inside the conductive shell when the tail end of the branch strip conductor is oppositely arranged at the opening of the conductive shell;

moving the assembled branch ribbon conductor toward the opening such that the end of the branch ribbon conductor is in direct contact with the inner conductor of the coaxial cable;

the ends of the branch strip conductors are soldered to the inner conductor of the coaxial cable.

12. The filter of claim 1, wherein the branch strip conductors are formed by printed circuit board technology on a surface of a dielectric substrate disposed inside the conductive housing.