JP2017184178A - Low pass filter - Google Patents

Abstract

Provided is a low-pass filter which can realize a steep filter characteristic with low loss and which is easy to miniaturize.
The low-pass filter (1) includes a metal housing (2) having a hollow portion (2a) inside, and a dielectric substrate (3) held by the hollow portion (2a), on one side of the dielectric substrate (3). On the surface 3a, a main line 31 made of a suspended line, branch lines 32A to 32F made of a suspended line connected to branch from the main line 31, and one electrode of a parallel plate capacitor are formed to form branch lines 32A to 32F. And the other electrode 3 of the parallel plate capacitor, which is connected to the metal housing 2 on the other surface 3b of the dielectric substrate 3. The second electrode patterns 34A to 34F are formed.
[Selection] Figure 1

Description

  The present invention relates to a low-pass filter in a microwave band used for a radio communication device of a mobile phone base station.

  In a wireless communication device of a mobile phone base station, a band-pass filter in the microwave band is used. Since the band-pass filter generally uses a resonator, a harmonic spurious band having a higher frequency than the band that the band-pass filter originally wants to pass is allowed to pass. Therefore, a low-pass filter is often used in combination so as to suppress the harmonic spurious band. Conventionally, a coaxial low-pass filter has been frequently used as a low-loss low-pass filter. A coaxial low-pass filter includes a metal casing and a central conductor held in a hollow portion inside the metal casing, and the central conductor includes large-diameter low-impedance lines and small-diameter lines arranged alternately and continuously. It consists of a high impedance line.

  In recent years, due to the need for miniaturization of wireless communication devices of mobile phone base stations, the size of bandpass filters has been steadily reduced by various technological developments. However, miniaturization of the low-pass filter used in combination has not progressed so much, so the restrictions due to the size of the low-pass filter, the mounting position, the degree of freedom of layout, etc., prevent the miniaturization of the entire wireless communication device. This is the current situation.

  The low-pass filter disclosed in Patent Document 1 is one of coaxial low-pass filters. In this low-pass filter, a bent portion is formed that is bent so that the region of the tip portion in the radial direction of the low-impedance line becomes an LC circuit and is parallel to the high-impedance line. With this LC circuit, the filter characteristics can be made steep so as to have an attenuation pole, and the number of stages of the low-pass filter can be reduced.

  The low-pass filter disclosed in Patent Document 2 is one of flat-type low-pass filters, and includes a metal casing and a dielectric substrate held in a hollow portion therein, and the dielectric substrate Is a configuration having a high-impedance line composed of suspended lines and a low-impedance line composed of microstrip lines arranged alternately and continuously. A suspended line is a metal layer line provided on one surface of a dielectric substrate, and has a structure in which a metal layer for grounding is not provided on the back surface, and the metal housing is grounded. Functions as an electrode. The microstrip line is a metal layer line provided on one surface of a dielectric substrate, and has a structure in which a ground metal layer is provided on the back surface.

JP 2011-142394 A JP-A-10-276006

  However, the coaxial low-pass filter disclosed in Patent Document 1 and the like has a limit for further miniaturization from the current state. In addition, the planar low-pass filter disclosed in Patent Document 2 is considered to be smaller than the coaxial low-pass filter, but in order to use it in a wireless communication device of a mobile phone base station, It is necessary to realize further low loss and steep filter characteristics.

  The present invention has been made in view of such a reason, and an object of the present invention is to provide a low-pass filter that can realize a steep filter characteristic with low loss and that can be easily downsized.

  In order to achieve the above object, the low-pass filter according to claim 1 comprises a metal housing having a hollow portion therein, and a dielectric substrate held in the hollow portion, wherein the dielectric On one side of the substrate, a main line consisting of a suspended line, a branch line consisting of a suspended line connected to branch from the main line, and one electrode of a parallel plate capacitor are formed on the branch line. A second electrode which is connected to the metal casing on the other surface of the dielectric substrate and forms the other electrode of the parallel plate capacitor. It has an electrode pattern.

  The low-pass filter according to claim 2 is the low-pass filter according to claim 1, wherein the branch line, the first electrode pattern, and the second electrode pattern are provided on both left and right sides of the main line. It is characterized by being.

  The low-pass filter according to claim 3 is the low-pass filter according to claim 1 or 2, wherein the branch line and the parallel plate capacitor are made to resonate in series to have a filter characteristic having an attenuation pole. Features.

  The low-pass filter according to claim 4, further comprising a metal plate disposed on at least a part of an outer edge of the dielectric substrate in the low-pass filter according to claim 1, wherein the second electrode pattern is provided. Is connected to the metal casing via the metal plate.

  The low-pass filter according to claim 5 further comprises a metal plate disposed on at least two sides of the outer edge of the dielectric substrate in the low-pass filter according to claim 2, and the second electrode pattern. Is connected to the metal casing via the metal plate.

  The low-pass filter according to claim 6 is the low-pass filter according to claim 4 or 5, wherein the metal plate is a lead frame, and is connected to each of the second electrode patterns. A dielectric substrate is mounted.

  The low-pass filter according to claim 7 is the low-pass filter according to claim 6, wherein the metal plate is bent in a thickness direction of the dielectric substrate at a position of an input / output end portion of the main line. It is characterized by being.

  According to the present invention, it is possible to provide a low-pass filter which can realize a steep filter characteristic with low loss and which can be easily miniaturized.

It is a top view of the low-pass filter concerning the embodiment of the present invention. It is a bottom view of a low-pass filter same as the above. It is sectional drawing cut | disconnected by the line shown by AA of a low-pass filter same as the above. It is a top view which shows the concept (schematic) of the layout change of a low-pass filter same as the above. It is a bottom view which shows the concept (outline) of the change of the layout of a low-pass filter same as the above. In the same low-pass filter, the metal plate is a lead frame, where (a) is a plan view and (b) is a bottom view. It is a schematic diagram which shows the state of the bending part in case a metal plate is a lead frame in a low-pass filter same as the above. It is an equivalent circuit diagram of a low-pass filter same as the above. It is a top view of the coaxial low-pass filter of a prior art. FIG. 10 is an equivalent circuit diagram of the conventional low-pass filter shown in FIG. 9. FIG. 10 is a filter characteristic diagram of the low-pass filter according to the embodiment of the present invention and the conventional low-pass filter shown in FIG. 9.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. A low-pass filter 1 according to an embodiment of the present invention includes a metal housing 2 and a dielectric substrate 3 as shown in FIGS. 1 and 2. The low-pass filter 1 is one of planar low-pass filters. 1 and 2 (and FIG. 9 to be described later) show the metal housing 2 in a transparent manner.

  The metal housing 2 has a hollow portion 2a inside, and usually has a rectangular parallelepiped shape, and input / output terminals 2A and 2A 'are attached to two surfaces facing each other. The metal housing 2 functions as a grounding electrode. As the input / output terminals 2A and 2A ', a coaxial connector having a center conductor 2Aa and a substantially cylindrical outer conductor 2Ab surrounding the center conductor 2Aa can be used. The metal housing 2 is connected to the external conductor 2Ab of the input / output terminals 2A and 2A ′ (more specifically, connected via the fixture 2B).

  The dielectric substrate 3 is held in the hollow portion 2 a inside the metal housing 2. Specifically, it is attached to the metal casing 2 via a metal plate 4 to be described later, or is supported by attaching legs to four corners, or a part of the end surface of the dielectric substrate 3 is bonded to the metal casing 2. And so on.

  The first surface 3a, which is one surface (front surface) of the dielectric substrate 3, has a main line 31 composed of a suspended line. The main line 31 extends in a straight line, and input / output end portions 31t and 31t 'at both ends thereof are connected to the center conductor 2Aa of the input / output terminals 2A and 2A' by soldering or the like. The main line 31 is an element having an inductance component. Hereinafter, the direction in which the main line 31 extends is the X-axis direction, the direction parallel to the first surface 3a of the dielectric substrate 3 and orthogonal to the X-axis direction is the Y-axis direction, and the X-axis direction and the Y-axis direction are orthogonal to each other. The direction (thickness direction of the dielectric substrate 3) is taken as the Z-axis direction.

  The first surface 3 a of the dielectric substrate 3 also has branch lines 32 </ b> A, 32 </ b> B, 32 </ b> C, 32 </ b> D, 32 </ b> E, 32 </ b> F composed of suspended lines connected so as to branch from the main line 31. The number of branch lines may vary depending on the filter characteristics of the low-pass filter 1, but is usually a plurality (six in FIG. 1). Each of the branch lines 32A to 32F is an element having an inductance component.

  Branch lines (for example, branch lines 32A and 32C) connected to different positions of the main line 31 are usually different in length, width, or shape from each other. Thereby, the inductance value is different. A branch line 32A shown in FIG. 1 has a small width portion and a medium width portion connected in series, and the branch line 32C is composed of only a small width portion.

  The first surface 3a of the dielectric substrate 3 also forms one electrode of the parallel plate capacitor and is connected to each of the branch lines 32A, 32B, 32C, 32D, 32E, and 32F. 33A, 33B, 33C, 33D, 33E, 33F.

  As shown in FIG. 2, the second surface 3b which is the other surface (back surface) of the dielectric substrate 3 is a flat plate-like second electrode pattern 34A, 34B, 34C that forms the other electrode of the parallel plate capacitor. 34D, 34E, 34F. That is, the first electrode pattern 33A, 33B, 33C, 33D, 33E, 33F on the first surface 3a, the second electrode pattern 34A, 34B, 34C, 34D, 34E, 34F on the second surface 3b and the dielectric substrate 3 , Respectively, to form parallel plate capacitors. For example, as shown in FIG. 3, the first electrode pattern 33C provided on the first surface 3a of the dielectric substrate 3 and the second electrode pattern 34C provided on the second surface 3b of the dielectric substrate 3 are: Opposing to each other with the dielectric substrate 3 in between. Similarly, the first electrode pattern 33D and the second electrode pattern 34D are opposed to each other with the dielectric substrate 3 interposed therebetween. The second electrode patterns 34C and 34D (and 34A, 34B, 34E, and 34F) are continuously formed on the outer edge side of the dielectric substrate 3 so as to overlap with the metal plate 4 to be described later and to be connected by soldering or the like. (See FIG. 3).

  Further, the branch lines 32A to 32F, the first electrode patterns 33A to 33F, and the second electrode patterns 34A to 34F are arranged on the left and right sides of the main line 31 (relative to the extending direction of the main line 31), as shown in FIGS. They can be provided in both orthogonal directions (in the figure, both Y-axis positive and negative directions). Then, for example, the two branch lines 32A and 32B, the first electrode patterns 33A and 33B, and the second electrode patterns 34A and 34B are provided on the left and right sides of the main line 31 to have a symmetrical pattern shape. In order to lay out a single element in a circuit, it can be divided into two and arranged in parallel. Thereby, it is possible to easily realize a desired inductance value and capacitance value. Further, as will be described later, since each of the second electrode patterns 34A to 34F is connected to the metal casing 2 (more specifically, the side wall thereof), the density of the current flowing through the metal casing 2 (particularly, the side wall). Therefore, the loss can be reduced. Of course, as shown in FIGS. 4 and 5, all the branch lines 32A ′, 32C ′, 32E ′, the first electrode patterns 33A ′, 33C ′, 33E ′, and the second electrode patterns are provided only on one side of the main line 31. It is also possible to provide 34A ′, 34C ′, 34E ′. 4 and 5 simply show the concept (outline) of layout change.

  Each of the second electrode patterns 34 </ b> A to 34 </ b> F is electrically connected to the metal housing 2. It is very important to reliably connect each of the second electrode patterns 34 </ b> A to 34 </ b> F to the metal housing 2 in order to suppress the characteristic variation of the low-pass filter 1. As a method for improving the mass productivity while reliably performing this, a thin metal plate 4 prepared separately is disposed on at least a part of the outer edge of the dielectric substrate 3, and the second electrode patterns 34A to 34F are attached to the metal plate 4. It is preferable to connect to the metal housing 2 via In this case, the metal plate 4 is connected to each of the second electrode patterns 34A to 34F in advance with solder or the like, and then screwed to the metal housing 2 or sandwiched (divided metal housing 2). And put it between them using a). A portion of the metal plate 4 to be attached to the metal housing 2 may be bent so as to be easily attached. When the branch lines 32A to 32F, the first electrode patterns 33A to 33F, and the second electrode patterns 34A to 34F are provided on the left and right sides of the main line 31, as described above, the metal plate 4 is attached to the outer edge of the dielectric substrate 3. The second electrode patterns 34 </ b> A to 34 </ b> F are preferably connected to the metal housing 2 via the metal plate 4.

  Furthermore, the metal plate 4 is a lead frame (thin metal frame) (see FIG. 6), and is structured to be cut out from a belt-like multiple (two in the figure) lead frame by cutting. It is preferable for increasing mass productivity and realizing cost reduction. In this case, as shown in FIG. 6, the metal plate 4 is connected to each of the second electrode plates 34A to 34F by soldering or the like, and mounted on the dielectric substrate 3 in a connected state where all the metal plates 4 are connected. Thereafter, the metal casing 2 is cut and separated at the broken line portion LA shown in the drawing, bent in the thickness direction (Z-axis positive direction or Z-axis negative direction) of the dielectric substrate 3 at the one-dot chain line portions LB and LB ′ shown in the drawing. Attach to. The alternate long and short dash lines LB and LB ′ are the positions of the input / output ends 31t and 31t ′ of the main line 31, and the metal plate 4 is provided with a notch 4a. It has been. A notch 4a is formed in the metal plate 4 and is bent in the thickness direction (Z direction) of the dielectric substrate 3 at the alternate long and short dash line portions LB and LB ′, so that the metal plate 4 and the metal plate 4 are inserted as shown in FIG. A short circuit with the center conductor 2Aa of the output terminals 2A, 2A ′ is prevented, and the main line 31 can be connected to the center conductor 2Aa of the input / output terminals 2A, 2A ′. The metal plate 4 shown in FIG. 6 is attached to the metal housing 2 by pinching (the pinching of the upper and lower portions in the figure), and the bent portion is metalized by soldering or screwing. Connected to the housing 2. Moreover, FIG. 7 is a schematic diagram when bent in the negative direction of the Z-axis.

FIG. 8 shows an equivalent circuit of the low-pass filter 1. L ₁₁ is an inductance component of the portion from the input / output end 31 t of the main line 31 to the connection portion of the branch line 32 A (and 32 B), and L ₁₂ is the connection portion of the branch line 32 A (and 32 B) in the main line 31. inductance component part before the connection portion of the 32C (and 32D) of the inductance component of the portion to the connecting _{portion, L 13} is branched lines 32E from the connecting portion of the branch line 32C (and 32D) in the main line 31 (and 32F), L ₁₄ consists of an inductance component of the portion up to input and output ends 31 t 'from the connecting portion of the branch line 32E (and 32F) in the main line 31. L ₂₁ is a combined inductance component of the branch line 32A and the branch line 32B, L ₂₂ is a combined inductance component of the branch line 32C and the branch line 32D, and L ₂₃ is a combined inductance component of the branch line 32E and the branch line 32F. _{Moreover, C 11} is the combined capacitance component parallel plate capacitor first electrode pattern 33A and the parallel plate capacitor and the first electrode pattern 33B and the second electrode pattern 34B where the second electrode pattern 34A is formed to _{form, C 12} is first electrode pattern 33C and the combined capacitance component parallel plate capacitor parallel-plate capacitor and the first electrode pattern 33D and the second electrode pattern 34D is formed to the second electrode pattern 34C is _{formed, C 13} is the first electrode pattern 33E and the second electrode It consists of a combined capacitance component of a parallel plate capacitor formed by the pattern 34E and a parallel plate capacitor formed by the first electrode pattern 33F and the second electrode pattern 34F.

In the equivalent circuit, the first electrode patterns 33A to 33F and the second electrode patterns 34A to 34F have a finite size, and thus have an inductance component although they are small. This inductance component is included in L _{21 to} L ₂₃ . The inductance component values of the first electrode patterns 33A to 33F and the second electrode patterns 34A to 34F can be controlled by changing the aspect ratio of the rectangular pattern.

Such a low-pass filter 1 can resonate the branch line and the parallel plate capacitor, that is, L ₂₁ and C ₁₁ , L ₂₂ and C ₁₂ , and L ₂₃ and C ₁₃ in series at a predetermined frequency. Thereby, the low-pass filter 1 can be made to have a filter characteristic having an attenuation pole, that is, an elliptic function characteristic. The illustrated low-pass filter 1 constitutes a low-pass filter having a total of 10 elements having seven stages of elliptic function characteristics.

  Next, a specific design of the low-pass filter 1 will be described below in comparison with the low-pass filter 101 of the prior art. The low-pass filter 101 is a coaxial type that has been conventionally used in a wireless communication device of a mobile phone base station.

The dielectric substrate 3 of the low-pass filter 1 has a size ((size in the X-axis direction) × (size in the Y-axis direction)) 15.3 mm × 7.6 mm, made by Arlon Diclad 880 (relative dielectric constant 2. 2, dielectric loss tangent 0.0009, thickness 0.25 mm). The low-pass filter 1 has an L ₁₁ of 0.982 nH, an L ₁₂ of 2.50 nH, an L ₁₃ of 2.35 nH _{, an} L ₁₄ of 0.581 nH, an L ₂₁ of 0.183 nH, and an L ₂₂ of 0.806 nH. , _{L 23} is _{0.713nH, C 11} is _{0.914pF, C 12} was _{0.865pF, C 13} is a 0.684PF.

  As shown in FIG. 9, the low-pass filter 101 of the prior art includes a central conductor 103 held in a metal casing 102 and a hollow portion 102a therein, and the central conductors 103 are alternately arranged continuously. The large-diameter low-impedance lines 103a, 103c, 103e, 103g, and 103i and the small-diameter high-impedance lines 103b, 103d, 103f, and 103h are configured. A cylindrical insulator made of Teflon (registered trademark) is disposed on the inner surface of the metal casing 102. The metal casing 102 is connected to the external conductor 102Ab of the input / output terminals 102A and 102A ′ (more specifically, connected via the fixture 102B), and the center conductor 103 has both ends 103t and 103t ′ at the input / output terminals. 102A and 102A ′ are connected to the central conductor 102Aa.

FIG. 10 shows an equivalent circuit of the low-pass filter 101 of the prior art. C ₁₁ 'is the capacitance component of the low impedance line 103a, C ₁₂ ' is the capacitance component of the low impedance line 103c, C ₁₃ 'is the capacitance component of the low impedance line 103e, C ₁₄ ' is the capacitance component of the low impedance line 103g, and C ₁₅ 'Consists of the capacitance component of the low impedance line 103i. L ₁₁ ′ is mainly an inductance component of the high impedance line 103 b, L ₁₂ ′ is mainly an inductance component of the high impedance line 103 d, L ₁₃ ′ is mainly an inductance component of the high impedance line 103 f, and L ₁₄ ′ is mainly high impedance. It consists of an inductance component of the line 103h.

This prior art low-pass filter 101 has C ₁₁ 'of 0.682 pF, C ₁₂ ' of 1.51 pF, C ₁₃ 'of 1.60 pF, C ₁₄ ' of 1.51 pF, and C ₁₅ 'of 0.682 pF. , L ₁₁ ′ is 2.99 nH, L ₁₂ ′ is 3.59 nH, L ₁₃ ′ is 3.59 nH _{, and} L ₁₄ ′ is 2.99 nH. The low-pass filter 101 constitutes a nine-stage low-pass filter with a total of nine elements.

  FIG. 11 shows filter characteristics by simulation of the low-pass filter 1 and the low-pass filter 101 of the prior art. Curves a and b are the pass characteristic S (2,1) and reflection characteristic S (1,1) of the low-pass filter 1, respectively. The low-pass filter 1 has a filter characteristic in which the pass characteristic S (2, 1) is about −50 dB at a frequency of about 6 GHz by having an attenuation pole at a frequency of about 6 GHz. Curves c and d are the pass characteristic S (2,1) and reflection characteristic S (1,1) of the low-pass filter 101, respectively. The low-pass filter 101 is a Chebyshev type, and has a filter characteristic in which the pass characteristic S (2, 1) is about −50 dB at a frequency of about 6 GHz. Therefore, it can be seen that the low-pass filter 1 can realize the steep filter characteristics required for the wireless communication device of the mobile phone base station, similar to the low-pass filter 101 of the prior art. The simulation was performed by connecting a 50 ohm resistor between the input / output terminals 2A and 2A '(and the input / output terminals 102A and 102A') between the ground potentials.

The size ((size in the X-axis direction) × (size in the Y-axis direction) × (size in the Z-axis direction)) of the hollow portion 2a of the low-pass filter 1 is 15.3 mm × 7.6 mm × 6.25 mm. There, the volume is 727mm ^3. In contrast, the size of the hollow portion 102a of the low-pass filter 101 ((size in the X-axis direction) × (area perpendicular to the X-axis direction)) is 32.4 mm × (π × 3.5 × 3.5). ) Mm ² and the volume is 1246 mm ³ . When the thickness of the metal casing 2 is 3 mm, the size of the low-pass filter 1 ((size in the X-axis direction) × (size in the Y-axis direction) × (size in the Z-axis direction)) is 21.3 mm × It is 13.6 mm × 12.25 mm and the volume is 3549 mm ³ . On the other hand, when the thickness of the metal casing 102 is 3 mm, the size of the low-pass filter 101 ((size in the X-axis direction) × (area perpendicular to the X-axis direction)) is 38.4 mm × (π × 6.5 mm × 6.5) mm and the volume is 5094 mm ³ . As a result, the low-pass filter 1 has a hollow portion 2a size of about 58% and the overall size when the thickness of the metal housing 2 is 3 mm, compared to the low-pass filter 101, about 70%. This means that it can be downsized. The scale is different between the plan view of the low-pass filter 1 shown in FIG. 1 and the plan view of the low-pass filter 101 shown in FIG.

  Thus, the low-pass filter 1 is easier to miniaturize than the low-pass filter 101 of the prior art. This is because the low-pass filter 101 is a nine-stage low-pass filter, whereas the low-pass filter 1 may be a seven-stage low-pass filter, and thus the main line 31 is shortened. Contributes greatly. Note that the fact that the main line 31 is shortened also contributes to the realization of the low loss described later.

  The dielectric substrate 3 shown in the figure is a single flat plate. However, if a flexible dielectric substrate is used, it can be bent and its shape can be freely changed. Further, the dielectric substrate 3 may be divided into two or more. The metal housing 2 and the like can be shaped according to the structure of the dielectric substrate 3. Therefore, the structure of the dielectric substrate 3, and thus the structure of the low-pass filter 1, has a high degree of freedom in layout, and can be mounted in a gap space when combined with the above-described band-pass filter in the microwave band, The whole of the above-described wireless communication device or the like can be further reduced in size.

  In addition, since the main line 31 and the branch lines 32A to 32F are each composed of a suspended line, the loss as a transmission line in the passband of the microwave band can be greatly reduced. The main loss of the transmission line on the dielectric substrate 3 occurs between the transmission line and the grounding electrode, but the metal casing 2 as the grounding electrode is separated from the main line 31 and the branch lines 32A to 32F. This is because the density of the current flowing through the air layer is very small because it is far away. Therefore, the low-pass filter 1 can have a low loss in the microwave pass band.

  As described above, the low-pass filter 1 can realize a steep filter characteristic with a low loss, and can be easily downsized.

  Although the low-pass filter according to the embodiment of the present invention has been described above, the present invention is not limited to that described in the above-described embodiment, and various modifications within the scope of the matters described in the claims. Design changes are possible.

DESCRIPTION OF SYMBOLS 1 Low pass filter 2a Hollow part 2 Metal housing | casing 2A, 2A 'Input / output terminal 3 Dielectric substrate 3a One side surface (1st surface) of a dielectric substrate
3b The other side of the dielectric substrate (second side)
31 Main line 32A-32F Branch line 33A-33F 1st electrode pattern 34A-34F 2nd electrode pattern 4 Metal plate

Claims (7)

A metal housing having a hollow inside,
A dielectric substrate held in the hollow portion,
On one surface of the dielectric substrate,
A main line consisting of suspended lines,
A branch line composed of suspended lines connected to branch from the main line;
A first electrode pattern that forms one electrode of a parallel plate capacitor and is connected to the branch line;
On the other surface of the dielectric substrate,
A low-pass filter that is connected to the metal casing and has a second electrode pattern that forms the other electrode of the parallel plate capacitor.   The low-pass filter according to claim 1, wherein the branch line, the first electrode pattern, and the second electrode pattern are provided on both left and right sides of the main line.   3. The low-pass filter according to claim 1, wherein a filter characteristic having an attenuation pole is obtained by causing the branch line and the parallel plate capacitor to resonate in series. A metal plate disposed on at least a part of the outer edge of the dielectric substrate;
The low-pass filter according to claim 1, wherein the second electrode pattern is connected to the metal casing through the metal plate. A metal plate disposed on at least two sides of the outer edge of the dielectric substrate;
The low-pass filter according to claim 2, wherein the second electrode pattern is connected to the metal casing through the metal plate.   The low-pass filter according to claim 4 or 5, wherein the metal plate is a lead frame, and the dielectric substrate is mounted so as to be connected to each of the second electrode patterns.   The low-pass filter according to claim 6, wherein the metal plate is bent in the thickness direction of the dielectric substrate at a position of an input / output end of the main line.

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