JPH10341092A - Shield case - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a shield case which can reduce spurious radiation to be emitted externally via a through hole made in the case. SOLUTION: A recess 8 having a width of several centimeters and a depth of several millimeters is formed around a through-hole 6 made in a bottom face of a shield case 2. Embedded into the recess 8 is a material which is resistant to a high-frequency current 7 flowing through a printed circuit board 1, that is, a material 9 having a large high-frequency transmission loss for suppressing the high-frequency current 7 from flowing towards the through-hole 6. Further, the material 9 is also coated on an inner peripheral surface of the through-hole 6, to thereby make it difficult for the high-frequency current 7 to pass through the hole 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a shield case for electromagnetically shielding a high-frequency circuit, and more specifically to a shield case having a through-hole for mounting a connecting means such as a connector. About.

[0002]

2. Description of the Related Art For example, adjustment and testing of electronic equipment and electronic measuring instruments that handle high-frequency signals, such as spectrum analyzers that measure the distribution of various frequency components of input signals, or radio receivers and high-frequency amplifiers. A high-frequency circuit is used for a signal generator or the like, which is a high-frequency power supply used. Generally, this kind of high-frequency circuit is formed on a printed circuit board or mounted on a printed circuit board as a semiconductor integrated circuit (IC). I have. Normal,
A printed circuit board through which a high-frequency current exceeding 1 GHz flows
In order to prevent spurious (unnecessary) radiation from the printed circuit board, it is housed in a shield case.

FIG. 3 shows an example of a conventional shield case of this type. The shield case 2 has a planar shape substantially similar to the shape of the printed circuit board 1 housed therein (generally, substantially rectangular), and has a shield case body 2b having an open upper portion; And a shield lid 2a for closing the upper opening of the main body 2b. After storing the printed circuit board 1 in the case, the shield lid 2a is fixed to the upper opening of the shield case main body 2b by a fixing tool such as a screw, so that the inside of the case is electromagnetically completely shielded. .

In this example, a signal source 1a for generating various high-frequency signals is mounted on the printed circuit board 1. A connection between the printed circuit board 1 and an external circuit or device is made by a shield case main body 2b in the illustrated example. Is used through the bottom wall. That is, the center conductors of these through capacitors 3 are used as through terminals, respectively, and the printed circuit board 1 is connected to an external circuit or device through the through capacitors 3 to transmit and receive signals (mainly control signals). .

In recent years, high-frequency circuits (RF circuits) require a large number of control signals for switches and digital-to-analog converters (D / A converters) used in the circuits. In some cases, it may be necessary to connect several tens of control signal and other signal transmission lines (wires or cables) to the printed circuit board 1. However, the feedthrough capacitor 3 for connecting such a large number of signal transmission lines is used.
It is difficult to provide the through-hole terminal or the through-hole terminal on the bottom wall or the side wall of the shield case 2, and even if it is feasible, it becomes extremely expensive and causes a problem in terms of price.

To solve this problem, dozens of signal transmission lines are connected to a connector 5 as shown in FIG.
Further, a through hole 6 into which the connector 5 can be inserted (this type of connector has a small thickness, so the through hole is a slit or a slot) is formed in the bottom wall of the shield case 2 in this example, and the connector 5 is printed. A method of connecting to a connector socket (not shown) provided on the substrate 1 has also been proposed.

[0007]

However, if the through hole 6 for mounting a connector is formed in the shield case 2,
Since this through hole 6 is not electromagnetically shielded,
A slit 6 having a length of several cm is opened in the bottom wall of the shield case 2. For this reason, the shield case 2 operates as a slot antenna with a cavity resonator, and a specific frequency is enhanced and emitted to the outside. That is, the spurious radiation whose radiation level has increased through the slit 6 is emitted to the outside, and adversely affects peripheral devices and circuits.

Therefore, conventionally, the printed circuit board 1 provided with the high-frequency circuit of 1 GHz or more and housed in the shield case 2 through the connector 5 to which a number of signal transmission lines are connected and an external device or Connecting to the circuit was not possible because of the inability to suppress spurious emissions. SUMMARY OF THE INVENTION It is an object of the present invention to provide a shield case in which a spurious radiation level emitted to the outside through a through hole for attaching a connecting means such as a connector is significantly reduced.

[0009]

According to the present invention, in a shield case having a through hole formed therein, a high frequency transmission loss is formed around the through hole on the inner surface of the shield case and on the inner peripheral surface of the through hole. The above object is achieved by providing a shield case coated with a substance having a large size.

In a preferred embodiment, the substance having a large high-frequency transmission loss attached to the periphery of the through hole and the substance having a large high-frequency transmission loss attached to the inner peripheral surface of the through hole are continuous. It has been adhered. Further, a recess is formed around the through hole on the inner surface of the shield case, and the substance having a large high-frequency transmission loss is embedded in the recess.

In a specific embodiment, a printed circuit board having a high-frequency circuit is housed in the shield case, and a number of signal transmission lines for electrically connecting the printed circuit board to an external device or circuit are provided. The connected connector is inserted into the through hole and connected to a connector socket provided on the printed circuit board.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below in detail with reference to FIG. In FIG. 1, portions and elements corresponding to those in FIGS. 3 and 4 are denoted by the same reference numerals, and description thereof will be omitted unless necessary. As described above, in order to prevent spurious radiation from being generated from the high frequency circuit of the printed circuit board 1, when the printed circuit board 1 is housed in the shield case 2 and electromagnetically shielded, a large number of signal transmission lines are connected. If a through hole 6 into which the connected connector 5 or other connecting means can be inserted is formed in the bottom wall or side wall of the shield case 2, the portion of the through hole 6 is not electromagnetically shielded. A slit having a length of several cm is opened on the side wall, and the shield case 2 operates as a slot antenna. Therefore, the frequency having the resonance wavelength λ, λ / 2, λ / 4,... Having the same length as the length of the slit is enhanced, and spurious radiation is generated.

For example, the length of the through hole 6 and therefore the slit (length in the left-right direction in FIG. 1B) is about 3.5 cm, and its width (length in the vertical direction in FIG. 1B) is about 1.0 cm. Assuming that the wavelength that resonates at the slit length of about 3.5 cm is λ / 4, from the formula of f = C / λ (f is frequency, C is light speed, λ is wavelength), the vicinity of about 2 GHz is obtained. Large spurious radiation occurs at the frequency of FIG. 5 shows a conventional shield case 2 having the structure shown in FIG. 4 and having a length of about 3.5 cm and a width of about 1.0 cm on the bottom wall of the case.
7 shows frequency characteristics of spurious emission levels when the slit 6 is formed. From this figure, about 1.8 GHz to about 2
It can be seen that there is a large spurious emission in the GHz frequency band.

In order to greatly reduce the spurious radiation, the slot antenna effect of the shield case 2 due to the through holes 6 must be reduced. On the other hand, the high-frequency current 7 flowing through the high-frequency circuit (the signal source 1 a in this example) of the printed board 1 flows on the surfaces of the printed board 1 and the shield case 2. In one embodiment of the present invention, FIG.
As shown in FIG. 1A and FIG. 1B, a material that becomes a resistance component to the high-frequency current 7 flowing through the printed circuit board 1, that is, a material 9 having a large high-frequency transmission loss is surrounded around the through hole 6 on the inner bottom surface of the shield case 2. The high-frequency current 7, which is applied with a width of several centimeters and flows through the printed circuit board 1, is prevented from flowing into the through-hole 6. Further, this high-frequency transmission is also applied to the inner peripheral surface of the through-hole 6 (cross section of the bottom wall of the case). The high-frequency current 7 is made difficult to pass through the through-hole 6 by depositing a substance 9 having a large loss.

In this case, it is preferable that the substance 9 having a large high-frequency transmission loss around the through hole 6 on the inner bottom surface of the shield case 2 and the substance 9 having a large high-frequency transmission loss on the inner peripheral surface of the through hole 6 are continuous. This is preferable because a high-frequency current is more difficult to flow. As can be easily understood from FIG. 1A, when a recess 8 having a depth of several mm is formed around the through hole 6 and a substance 9 having a high-frequency transmission loss is embedded in the recess 8, Adheres to the substance 9 having a large high-frequency transmission loss, so that the high-frequency current flowing through the back surface of the printed circuit board 1 can be effectively suppressed, and the high-frequency current flowing through the inner bottom surface (bottom surface) of the shield case 2 can be reduced. This is preferable because it can be more effectively prevented. Furthermore, it is preferable that the exposed upper surface of the layer of the substance 9 having a large high-frequency transmission loss embedded in the recess 8 is flush with the surface of the inner bottom surface of the shield case 2.

In this embodiment, the through hole 6 is a slit having a length (length in the left-right direction in the figure) of 35 mm and a width (length in the vertical direction in the figure) of 10 mm, and is a material having a large high-frequency transmission loss. Reference numeral 9 is attached around the through hole 6 in a range of 75 mm in length and 40 mm in width. Further, the depth of the recess 8 is 1 mm, and therefore, the thickness of the substance 9 having a large high-frequency transmission loss is 1 mm. Further, the thickness of the substance 9 having a large high-frequency transmission loss attached to the inner peripheral surface of the through hole 6 is also 1 mm.

In this embodiment, the substance 9 having a large high-frequency transmission loss attached around the through-hole 6 is the same as that shown in FIG.
As is clear from the figure, the width in the left-right direction is larger than the width in the vertical direction in the figure. This is because the signal source 1a, which is a high-frequency current source, is mounted at one end (the left end in the figure) in the longitudinal direction of the rectangular printed circuit board 1, and is not limited to the illustrated example. That is, it goes without saying that the width (length) of the substance 9 having a large high-frequency transmission loss and the depth of the concave portion 9 are appropriately changed according to the printed circuit board accommodated in the shield case 2.

As the substance which becomes a resistance component to the high-frequency current, that is, the substance 9 having a large high-frequency transmission loss, a radio wave absorber or a substance having a large dielectric loss can be used. The dielectric loss is represented by the relative permittivity ε, frequency f, dielectric loss tangent tanδ, k
Where is a constant, dielectric loss = k · f · ・ ε · tanδ. Therefore, it can be understood that a substance having a large dielectric loss is a substance having a very large relative dielectric constant (ε) and a dielectric loss tangent (tanδ). .

In this embodiment, the material 9 having a large dielectric loss is used.
Nippon Paint Co., Ltd. product name "Filtech Nova EM-200" (a product which is relatively soft and is in the form of a sheet having a thickness of several mm) is used, and the recess 8 and the slit 6 are formed with an adhesive. It was stuck on the inner peripheral surface. As a result, as shown in FIG. 2, the spurious emission level was significantly reduced in the frequency band from about 1.5 GHz to about 2 GHz as compared with the conventional shield case. In particular, from about 1.8 GHz to about 2 G
In the frequency band of Hz, the frequency is reduced by 20 dB or more compared to the related art.

In the above embodiment, the shield case 2
A case where the present invention is applied to a shield case in which a through hole 6 for mounting a connector is formed on the bottom wall of
The present invention can also be applied to a case where the through hole 6 for mounting a connector is formed on the side wall of the shield case 2. In addition, the present invention can be applied to a case where a through hole (slit) for mounting other connection means is formed in the bottom wall or the side wall of the shield case, not limited to the connector, and the same operation and effect can be obtained. Needless to say.

[0021]

As is apparent from the above description, according to the present invention, a material having a large high-frequency transmission loss is coated on the periphery of the inner surface of the through hole formed in the shield case and on the inner peripheral surface of the through hole. As a result, the high-frequency current flowing through the inner surface of the shield case and the high-frequency current emitted from the through-hole are significantly reduced by the substance having a large high-frequency transmission loss. As a result, the level of spurious radiation emitted from the through-hole can be significantly reduced as compared with the related art, and there is no possibility that peripheral circuits will be adversely affected. Therefore, there is a remarkable effect that a through-hole for mounting a connector to which a number of signal transmission lines are connected and other connection means can be formed in the shield case.

[Brief description of the drawings]

FIG. 1A is a sectional view showing one embodiment of a shield case according to the present invention together with a printed circuit board and a connector, and FIG.
FIG. 3 is a plan view of A showing the inside of the case by removing the shield lid, the printed circuit board, and the connector from the shield case shown in FIG.

FIG. 2 is a diagram showing frequency characteristics of spurious radiation levels when a shield case according to the present invention is used.

FIG. 3 is a sectional view showing an example of a conventional shield case.

FIG. 4 is a sectional view showing another example of a conventional shield case.

FIG. 5 is a diagram showing frequency characteristics of spurious emission levels when a conventional shield case is used.

[Explanation of symbols]

 1: Printed circuit board 2: Shield case 5: Connector 6: Through hole 8: Concave 9: Substance with large high frequency transmission loss

Claims (4)

[Claims] 1. A shield case having a through-hole, wherein a substance having a high-frequency transmission loss is applied on the inner surface of the shield case around the through-hole and on the inner peripheral surface of the through-hole. And a shield case. 2. The high-frequency transmission-loss material adhered around the through-hole and the high-frequency transmission-loss material adhered to an inner peripheral surface of the through-hole are continuously adhered. The shield case according to claim 1, wherein: 3. The method according to claim 1, wherein a concave portion is formed around the through hole on the inner surface of the shield case, and the substance having a large high-frequency transmission loss is embedded in the concave portion. Shield case. 4. A printed circuit board having a high-frequency circuit is accommodated in the shield case, and a connector connecting a number of signal transmission lines for electrically connecting the printed circuit board to an external device is provided in the through hole. The shield case according to claim 1, wherein the shield case is inserted and connected to a connector socket provided on the printed circuit board.