JP2004364025A - Filter - Google Patents

Abstract

An object of the present invention is to provide a filter which solves the cost increase and low reliability due to the use of the conventional feedthrough capacitor, and realizes high isolation.
A wiring board on which a circuit is mounted, a metal shield case covering a part of the surface of the wiring board, and a front surface of the back surface of the wiring board covered at least on the shield case. A metal chassis 2 mounted on or in parallel with a surface including a surface corresponding to the above, and a jumper wire 4 mounted on the back surface of the wiring board 3 and connecting the inside and outside of the shield case to each other. , A coil connected in series to the jumper wire 4, and first and second capacitors respectively connected to the outer and inner ends of the shield case 1 among the two ends of the series circuit of the jumper wire 4 and the coil. The chassis 2 has a concave portion 21 for accommodating the jumper wire 4, and at least the coil and the second capacitor constitute a low-pass filter.
[Selection diagram] Fig. 1

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a filter, and more particularly, to a filter that realizes higher isolation of relatively low-frequency noise than a conventional feedthrough capacitor.
[0002]
[Prior art]
FIG. 4 is a diagram illustrating signal isolation in a conventional shield case. The substrate 3 constitutes, for example, a high-frequency circuit for a wireless base station device, and is installed on the chassis 2 which is a metal base. Circuit blocks requiring particularly high isolation among high-frequency circuits are covered with the shield case 1 from the front surface of the substrate and sandwiched between the chassis 2 on the back surface, so that the circuit blocks are almost electromagnetically sealed and a shielding effect is obtained. ing. Further, in exchanging signals inside and outside the shield case 1 (especially at low frequency), a feedthrough capacitor 5 is used to prevent intrusion of noise transmitted through the signal line. The feedthrough capacitor 5 is of a general cylindrical shape, and is screwed through a screw hole provided in the shield case 1 made of a thick metal material.
[0003]
In addition to the above, as a technique related to the present invention, a capacitor is formed by plating electrodes on both sides, and a U-shaped lead wire is passed through a dielectric plate having holes, and the inductance of the capacitor and the lead wire is increased. Is known in which an assembly forming a π-type low-pass filter is installed in a concave portion of a metal plate such that the L portion of the π-type filter is sandwiched between a metal plate and a dielectric plate. .).
In addition, it is known that a pattern for extracting an electrode is sandwiched between a metal substrate serving as a base of the pattern and a metal plate overlaid on the pattern via an insulating layer, thereby substituting the pattern as a feedthrough capacitor. (For example, see Patent Document 2).
[0004]
[Patent Document 1] JP-A-5-322595 [Patent Document 2] JP-A-6-236571
[Problems to be solved by the invention]
However, when the feedthrough capacitor 5 is used, a through hole must be provided in the shield case 1 and the feedthrough capacitor 5 must be fixed by screwing or soldering, and it is not easy to attach and detach the shield case 1 after fixing. There has been a problem that the number of steps is increased or the order of the steps is limited. Further, since the lead of the feedthrough capacitor 5 is soldered to the terminal 31 extending vertically from the substrate 3, the dielectric portion (ceramic) of the feedthrough capacitor is damaged by cracks and the like due to the heat of soldering, and Together with the short circuit. Further, the soldered lead is mechanically weak and may be deformed or cut by an external force, and the conventional configuration is not satisfactory in both cost and reliability.
[0006]
SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide a filter that solves the cost increase and the low reliability due to the use of the conventional feedthrough capacitor and realizes high isolation.
[0007]
[Means for Solving the Problems]
In order to solve the above problems, in the present invention, a wiring board on which a circuit is mounted, a metal shield case covering a part of the surface of the wiring board, and at least the shield case of the back surface of the wiring board A metal chassis mounted on or in parallel with a surface including a surface corresponding to a surface to be mounted, and mounted on the back surface of the wiring board to connect the inside and the outside of the shield case. A conducting wire, a coil connected in series to the conducting wire, and first and second capacitors respectively connected to outer and inner ends of the shield case, of both ends of a series circuit of the jumper wire and the coil, Wherein the chassis has a recess for accommodating the conductor, and at least the coil and the second capacitor function as a low-pass filter.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the description of the present embodiment, the same parts as those of the conventional configuration are denoted by the same reference numerals, and description thereof will be omitted.
FIG. 1 is a sectional view of a filter according to the present embodiment. In this embodiment, a jumper wire 4 is used instead of the conventional feedthrough capacitor 5 to connect the inside and the outside of the shield case 1. The jumper wire 4 is a conductive wire similar to the lead wire of the component, and each of both ends is soldered to a pattern on the substrate 3 inside and outside the shield case 1 and passes through the back surface of the substrate so as to avoid the shield case. The patterns are connected to each other, and a direct-current or low-frequency signal (hereinafter, referred to as a DC signal) is introduced into or out of the shield case 1.
[0009]
The chassis 2 is in contact with the back surface of the substrate 3, and a counterbore 21 is provided at a portion where the jumper wire 4 is laid as a space for storing the jumper wire. By doing so, exposure of the jumper wire 4 can be avoided as compared with a structure in which the jumper wire 4 is laid on the surface of the substrate and a cutout is provided in the shield case, so that high isolation can be easily obtained.
[0010]
FIG. 2 is a top view of the filter in this embodiment. The jumper wire 4 has bypass capacitors C1 and C4 connected in parallel outside the shield case 1, a coil L2 connected in series inside the shield case, and bypass capacitors C2 and C3 connected in parallel. These circuits connect DC signals inside and outside the shield case.
[0011]
FIG. 3 is an equivalent circuit of the filter in the present embodiment. The bypass capacitors C1 and C4 short-circuit unnecessary AC components on DC signals and RF components generated in the shield case 1 to ground.
The coil L1 is an inductance component of the jumper wire 4. A feature of the present embodiment is that a jumper wire having a larger inductance component is used instead of the embedded wiring of the substrate, so that an effect of removing noise on a lower frequency side can be obtained. The jumper wire 4 is close to the ground pattern of the chassis 2, the shield case, or the substrate and has a capacitance between them, so that they equivalently form a feedthrough capacitor. The conventional feed-through capacitor can be considered as a feed-through capacitor, but the frequency band in which the isolation is obtained by the nominal value is fixed and narrower than necessary, but in this embodiment, the necessary frequency band is increased by increasing the inductance component. Can be used to obtain an isoleshone.
The coil L2 and the bypass capacitors C2 and C3 form a low-pass filter. The time constant of the low-pass filter is determined so that the necessary cutoff characteristics can be obtained.
With the above configuration, this circuit realizes high isolation in total. As an example of the specifications of this circuit, when importance is placed on isolation at a frequency of 300 MHz or less, C1 to C4 = 0.1 μF, L2 = 100 nH, and the jumper wire length of L1 = 20 mm.
[0012]
In this embodiment, the substrate 3 does not necessarily need to be attached so as to be directly bonded to the chassis 2, and an insulator may be interposed therebetween. Further, the jumper wire may be a covered wire such as a wrapping cable.
[0013]
【The invention's effect】
According to the present invention, a highly reliable filter can be realized at low cost, and the character constant of the filter can be arbitrarily set, so that necessary frequency isolation can be obtained.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a filter according to an embodiment of the present invention.
FIG. 2 is a top view of the filter according to the embodiment of the present invention.
FIG. 3 is an equivalent circuit of a filter according to the embodiment of the present invention.
FIG. 4 is a diagram illustrating signal isolation in a conventional shield case.
[Explanation of symbols]
1 shield case, 2 chassis, 3 substrate, 4 jumper wire, 5 feedthrough capacitor

Claims (1)

A wiring board on which the circuit is mounted;
A metal shield case covering a part of the surface of the wiring board,
A metal chassis mounted on or in close proximity to a surface including a surface corresponding to at least the surface mounted on the shield case of the back surface of the wiring board,
A lead wire mounted on the back surface of the wiring board and connecting the inside and the outside of the shield case,
A coil connected in series to the wire;
First and second capacitors respectively connected to the outer and inner ends of the shield case, of both ends of the series circuit of the conductor and the coil,
The chassis has a recess for accommodating the conductor,
A filter, wherein at least the coil and the second capacitor function as a low-pass filter.

Images