EP0472698A1

EP0472698A1 - Shielded pc

Info

Publication number: EP0472698A1
Application number: EP19910906080
Authority: EP
Inventors: Thomas Vielhaben
Original assignee: PANAVIA AIRCRAFT GmbH
Current assignee: PANAVIA AIRCRAFT GmbH
Priority date: 1990-03-15
Filing date: 1991-03-15
Publication date: 1992-03-04
Also published as: JPH04506283A; DE4008335A1; WO1991014354A1

Abstract

Un ordinateur personnel blindé portatif est blindé de manière à rendre impossibles pratiqement toutes lectures ou écoutes non autorisées des données en-dehors de l'espace de travail habituel. A cet effet, le boîtier extérieur de l'ordinateur est composé d'un matériau ferromagnétique à haute perméabilité et tous les composants sont enduits de cuivre, puis d'étain. Une deuxième cage intérieure de Faraday est emboîtée dans ce boîtier extérieur de l'ordinateur et tous les passages forment des conduits amortisseurs de HF.A portable screened personal computer is screened to make it virtually impossible for unauthorized reading or listening to data outside of the normal workspace. For this purpose, the outer casing of the computer is made of a ferromagnetic material with high permeability and all the components are coated with copper, then tin. A second inner Faraday cage is fitted into this outer case of the computer and all the passages form HF damping conduits.

Description

Shielded PC

The invention relates to a shielded personal computer, i.e. a PC that is shielded against (high-frequency) electromagnetic (radio) radiation, or "eavesdropping", and thus the possibility of reading the data by unauthorized third parties, but is still locally mobile, i.e. remains portable.

Such computers are of great importance when used in areas requiring confidentiality, which include not only military areas of application, but often also development departments in industry, because they prevent unauthorized data interception (via the

Computer-induced electromagnetic fields), or at least make it considerably more difficult.

The easiest way to shield a computer so that it is bug-proof would be to install the computer in a Faraday's cage. This could shield the room in which the computer is installed. However, this means that the computer can only be used in this room and is therefore no longer mobile. It has also been shown that the working conditions for the

Operating personnel in such a, necessarily quite small and windowless, shielded room, are absolutely unreasonable in the long run.

There is therefore a need for tap-proof shielding of PCs in ordinary work rooms in such a way that the shield is integrated with the computer itself, which has the further advantage that the computer remains mobile and the operating personnel can work in familiar work rooms.

There are shielded PC's in the trade where the damping

-2- takes place electronically via electrical modules. Such shielded computers, which are also called radiation-preventing or suppressing devices, are divided into four classes, class 1 being the least shielded and class 4 the best, ie the safest class.

But even these commercially available PCs, shielded according to the TÜV regulations, still have so much radiation that the information implemented during work can be read without authorization with the help of special listening devices and sometimes even at distances of 100 m to several hundred meters. A mobile device that can no longer be read outside the work area is not yet known.

The aim of the invention is therefore a PC that is so strongly electromagnetically shielded that practically no unauthorized reading or listening to the unprotected data is possible practically outside the usual workspace, in particular a PC that at least meets the requirements of class 4 of the current definition.

This is achieved by the outer computer housing being made of a ferromagnetic material with high permeability and all of its components being copper-plated and additionally tinned, and this outer computer housing being nested with a second, inner Faraday * cage and by all bushings being designed as damping HF chimneys are. Additional constructive measures in the design and system assembly / assembly of the individual electronic components of the computer hardware reinforce the shielding effect.

Due to the type of construction, the emergence of unwanted signals is reduced both through inevitable openings in the housing and via the outgoing signal lines (= data cable). With a suitable design, the nesting of two Faraday cages results in adequate overall damping.

Attached absorber materials convert the field energies into heat. The outer Faraday's cage is made of a ferro-magnetic material with high permeability especially for damping the magnetic field components. Since the use of Permalloy or Dynamoblech IV is not always given in terms of production technology, components made from deep-drawn sheet metal must be heat-treated accordingly, if necessary. All components are copper-plated for better surface conduction. In order to ensure good electrical contact with the inserted contact spring strips, the housing components are additionally tinned, which has an additional absorbing effect due to the electromagnetic penetration.

The copper plating and tinning is essential for the shielding of all transitions that are not sealed by contact springs. As mentioned above, not only is the entire outer housing made of annealed thermoformed sheet, but so is the printer housing. The inner housing, that is to say the actual housing which contains the computer, preferably consists of 1 mm thick electrolytic copper.

All outgoing signal lines are double shielded, with the inner shield also connected to the innermost cage and the outer to the U housing of the computer.

Although this allows the components connected via cables (signal lines) to be included in the concept of nested shielding, these cable connections form weak points. For this reason, additional measures are required, namely correct cable termination, bleeder resistances between the individual shields and suitable cable lengths in order to avoid jacket waves (i.e. standing waves) in the cable.

Since the usual PC is a system consisting of a computer unit (chip microelectronics on printed circuit boards plus mass storage and drives in one housing), keyboard, monitor (screen) and printer, the power lines and data cables form the individual, external inputs - / output devices the critical,

Radiating cable connections for which special measures are necessary.

Where there are supply voltage feeds

{Power cable), can use a suitable filter

Separation from the information-carrying signals.

Positive openings for ventilation, cable inlets and openings for floppy disks as well as exchangeable disks are realized by hollow-conductor openings that are operated below the critical wavelength due to their physical dimensions.

The shielding is partly in the special application a known components for this special purpose of the shielded PC; partly these are new components that have been adapted for this special purpose, the suitable combination having the desired effect.

The individual components and measures are now to be explained in the following for the critical areas:

1. the radiation via the mains supply is shielded by special mains filters known to si,

2. the radiation from the supply line to the printer, monitor and keyboard is shielded by multiply shielded data lines with specially screwed connections to the housings,

3. the radiation through ventilation holes is shielded by the use of honeycomb filters,

4. The radiation of diskette and removable disk slots is a new construction of the diskette and removable disk slots as HF chimneys shielded,

5. the radiation from the main housing cover is shielded by the use of beryllium contact spring strips and a low-torsion cover construction,

6. the radiation from the system board is shielded by using a copper inner housing, preferably made from Irrm electrolytic copper, 7. the radiation from the cathode ray tube of the

The monitor is shielded by using a special screen with HF sealing gauze

8. radiation from the control opening of the printer is prevented by decommissioning the printer when the cover is opened.

Thus, the suppression of radiation is primarily achieved through the implementation of a double Faraday cage. The outer cage encloses all system components in the form of individual housings for computers, printers, monitors and keyboards. The outer shielding lies on a common shielding potential.

All electronic components are insulated and isolated inside the outer housing and enclosed with a second Faraday's cage. This results in an inner and an outer shield, the shielding potential of which is different from one another.

The cable connections between the individual outer housings are double-shielded lines, the inner cable shield lying on the inner shielding potential and the outer cable shielding on the outer shielding potential. So the double Farada 'see cage is not canceled by the cable connections. System overview

The computer system normally consists of four individual devices that are connected to one another by data cables (signal lines). These 4 devices are:

I. the computer case,

II. The monitor housing,

III. the keyboard and

IV. The printer housing.

The computer, monitor and printer have their own power supply with appropriate HF filtering.

The network is fed into the computer, monitor and printer via a normal IEC socket with a downstream EMI filter to block RF signals via the mains supply.

The cable bushings are protected against escaping radiation by special covers with HF chimneys. The ventilation openings are sealed with a honeycomb filter.

The radiation protection for the individual components is described in more detail below:

I. calculator

The computer case is made of annealed deep-drawn sheet steel. The steel sheet is copper-plated and tinned for better radiation reduction.

The housing has two large side openings that are closed with lids made of the same material. The covers are screwed together in one fold and for better contact with beryllium-copper contact spring strips Mistake. The connecting cables to the printer, monitor and keyboard are routed out of the housing via screw connections with an attached HF chimney made of electrolytic copper. Penetrations that are not used are sealed with electrolytic copper plates of approx. 1.5 mm thickness. The cable entries are made using a known R + S HF connector. The calculation of the HF chimneys, which are intended for all feedthroughs that emit, is given at the end of the description using an example for Hi ~ ^Wel l ^en '.

The following sub-housings are located in the computer housing (outer F'-cage) with line filter and cable glands:

a) Mains filter housing (made of copper) b) CPU housing (made of copper) c) Mass storage housing with front plate (made of ordinary sheet metal, but copper and tin-plated) d) Power supply housing (made of tinplate galvanized on both sides) e) Ventilation fan f) Signal outputs (plug) for connecting the data cable

a) Line filter housing

The line filter housing is made of soldered copper sheet (approx. 1 mm, preferably electrolytic copper). It is screwed to the outer computer housing in a conductive and HF-tight manner. The line filter housing houses a special EMI filter. The EMI filter shuts off the RF signals that are directed towards the outside via the mains cable. The EMI filter principle is known per se, but here a special design takes place, a common-mode suppression takes place via a suitably wound yoke. The filter must therefore be calculated for the respective size. The position of the components is important, as is the physical dimensions. To avoid parasitic capacitances, the capacitors must be at a certain point with the metal wall be glued, which must be determined experimentally for different types of line filter housings in order to achieve damping optimization.

In the back of the mains filter housing there is an IEC socket with an integrated fuse known per se.

b) CPU case = inner Faraday cage

The CPU case is made of soldered copper sheet (approx. Lirai, preferably electrolytic copper). The CPU case has a screwed metal sealing lid that can be removed for service purposes. The multiple screw connection is one calculated

Screw distance created connection from the inner housing with the inner housing cover. The screw spacing is less than 1/2 of the critical wavelength. The CPU housing accommodates the CPU main board and the extensions (plug-in cards) plugged into the main board. The

The CPU housing is insulated into the outer computer housing by means of six plastic spacers. Signal lines are led out of the CPU housing from openings in the housing wall, which are secured against HF leakage by HF chimneys with inserted HF sealing foam. These lines lead to the connectors on the outer wall of the computer case. (See f)). As mentioned earlier, the example given at the end can be used to calculate the HF chimneys

c) Mass storage enclosure with front panel

The mass storage case with front panel is built into the computer case from the front. The mass storage case contains: hard disk, floppy disk drive and removable disk drive. The

The housing is made of sheet steel, which is copper-plated and tinned, and the front plate is milled from full aluminum (preferably approx. 3 mm) The hard disk is built into a sheet steel insert - the hard disk can be removed from the computer after finishing work. The front plate of the insert has a circumferential fold which, when inserted, engages in the front plate of the mass storage housing with a projecting shoulder which dampens the radiation.

The control openings for the removable disk and the floppy disk drive are designed as HF chimneys. As in all the following cases, the example given at the end can be used to calculate the HF chimneys.

The key switch for keyboard lock and power switch as well as the operating status LEDs and the reset and load switches are built into the front panel.

All bushings and passageways, e.g. B. the light-emitting diode holes are designed as HF chimneys.

The mass storage housing with front panel is sealed with a circumferential metal cord for better contact. This seal is placed under high mechanical pressure by means of a screw or clamping attachment. This metal cord is a commercial product consisting of conductive rubber and a copper / iron mixture. Since a pressure of several kg / cm ^{2 is to be} achieved, a screw fastening, e.g. B. with M4 or M5 screws, preferred.

d) power supply housing

The power supply housing is a largely closed sheet metal housing made of tinplate galvanized on both sides. It contains the power supply boards and a fan for heat dissipation. The power supply housing is built on top of the CPU housing, so it has an inner potential. The Ohmic resistance at the contact points between the power supply housing and the CPU housing should be as low as possible and care should also be taken to avoid possible corrosion. The direct assembly of the power supply housing on the CPU housing suppresses the radiation at the source, i.e. where the highest currents flow. The direct mounting of the power supply housing on the CPU housing also creates the shortest possible cable routes.

e) ventilation of the housing by the ventilation fan

The computer outer housing is provided with openings for ventilation of the interior. The breakthroughs are secured with known honeycomb filters against HF radiation leakage. Several built-in fans ensure the circulation of the indoor air. The heat of the CPU inner casing is dissipated to the outside in the exchanger principle.

f) Signal outputs (sockets or plugs) for connecting the data cable

Normal, known shielded sockets or plugs can be used here, filters between sockets or plugs for the cables to the screen and printer being used.

II. Monitor housing with line filter

The monitor housing completely encloses the commercially available monitor. To view the screen image, a plastic disc with a fine-meshed metal gauze is used. The metal gauze is completely conductively connected to the monitor housing. The metal gauze consists of an iron / copper alloy, with a good conductive Connection of the crossing points of the individual wires must be observed. The mesh size results from the critical wavelength and is preferably less than 1 mm. The

Mesh size is chosen according to the critical wavelength. So that a color rendering is possible, that is

Network by 20 ° to 25 ° and in particular about 23 ° to

Horizontal inclined. Such gauze nets made of nylon threads are known as contrasting disks and gauze nets were also used in X-ray technology. Until now, such networks were only useful for black and white images. It is only through the inclination that you get a color-compatible net.

A line filter is used to filter out the image information signals of the monitor on the voltage supply line. The inner housing of the line filter results from its embodiment. The line filter solution is generally known. The inner housing, ie the monitor housing, is not connected to the outer housing, so that there are two different ground potentials. The mains filter is attached to the outside of the monitor housing so that the largest possible HF separation of the interior is achieved. The individual housing parts are made of specially heat-treated and therefore magnetically significantly improved thin sheet. The sheet is copper-plated and tinned for optimal attenuation of the HF when passing through the housing wall. All housing parts _. are screwed together using the tinning as a sealing medium. The sheet metal of the housing parts is about 4 to 5 times better than normal sheets due to the soft annealing. However, it does not become as soft as transformer sheet or permalloy, which are too soft. The choice of the sheet metal can prevent the occurrence of further magnetic or electrical fields which could radiate due to the occurrence of magnetic fields. _? ^ _Pr _? S] 5_rI9 l} ys_ _iB __: ^ _- ^ ^^ i ^ _? R

The printer housing, which is also made of steamed deep-drawn sheet, preferably copper-plated and tinned, consists of an upper and a lower shell. To operate the printer, the top cover must be opened up.

The EMI line filter is flanged from the inside to the rear of the printer housing in a housing. The individual housing parts are preferably copper-plated and tinned. A shielded plastic pane, like the one used for the monitor housing, allows a view of the display elements of the printer. Also, as explained above, a fine metal gauze is inserted into this plastic disc, which is connected to the housing with a low resistance all around by a press frame. To ensure that the housing is radiation-proof, an all-round seal is made with beryllium-copper contact spring strips. The two locking levers ensure a high contact pressure on the sealing surfaces. The contact spring strips, which represent a mechanical and electrical connection, replace the multiple screw connection, e.g. is used to connect the mass storage enclosure to the front panel.

The printer only works when the cover is closed. When the cover is open, the mains voltage supply to the printer is interrupted, so that no compromising radiation can occur;

IV. Keyboard

The keyboard is built into an aluminum housing. The. Keycaps of the keyboard can for technical reasons, ie. because of the need for user operation, no further shielding. The HF shielding takes place here through a special layout of the keyboard logic. The keyboard switches are actuated via a plastic bushing that leads through a metal shield plate mounted over the circuit board. This screen plate is preferably a 3-4 mm thick aluminum plate. The cable bushings therefore form HF chimneys themselves.

V. Individual technical measures

IC

Signal line and data cable bushings

All feedthroughs of the data lines through the housing walls perform several functions. she

15 connect the outer cable shield to the outer shielding potential of the computer. Furthermore, they take over the strain relief of the cable. The tube of the cable duct is designed as an HF chimney, which further increases the HF tightness of the connection.

All individual parts of the cable duct are made of high quality, i.e. sufficiently hard, electrolytic copper. The contact to the outer housing is again made by several screws that generate high contact pressure 5.

Ventilation of the housing

To ventilate the interior, the computer outer casing 0 is provided with openings. The breakthroughs are secured with honeycomb filters to prevent HF radiation from escaping. Such honeycomb filters are generally known. In the present case, filters made of silver-plated copper are used, which are inserted into the slots in the outer housing. The honeycomb size 5 results from the critical wavelength. The thickness of these filters is approximately 2 cm and the honeycombs are preferably used in a frame, for example 2.5 cm thick. The damping on the device is fundamentally dependent on the housing, since the outer surfaces of the housing are not homogeneous. It can therefore vary from place to place on the housing. With just a few trials, you can determine what is best for a particular device type.

For the calculation of the HF chimneys, only the outward waves are of interest, which when considering the critical wavelength result in the most unfavorable attenuation value (the Hi _Q limit wavelength depends on the width of the waveguide). The following always applies to cable inlets and ventilation grilles: >> 2 GHz.

The view for the breakthroughs is more critical

Removable disk and floppy disk. For the removable disk, the assumed waveguide is approx. 140/10/100 mm (W / H / L).

Claims

Patent claims

1. Shielded, portable personal computer, which is shielded in such a way that practically no unauthorized reading or listening to the data is possible outside the usual work area, characterized in that the outer computer housing consists of a ferromagnetic material with high permeability and all components are copper-plated and additionally tinned are and this outer computer housing is nested with a second, inner Faraday's cage and that all bushings are designed as damping HF chimneys.

2. Personal computer according to claim 1, characterized in that the second, inner Faraday cage consists of sheet copper.

3. Personal computer according to claim 1, characterized in that all outgoing signal lines are double-shielded, the inner shield being connected to the inner cage and the outer to the computer outer housing, and that all supply lines are shielded by multiply

Data lines with screwed connections on the housings are shielded.

4. Personal computer according to claim 1 or 2, characterized in that the cooling by ventilation of the

Interior by means of at least one used

Fan blower takes place and the computer outer case with

Breakthroughs are provided with honeycomb filters

HF radiation leakage are secured.

5. Personal computer according to one or more of the preceding claims, characterized in that the front panel of the insert for the mass storage has a circumferential fold, which in the inserted state in

⁵ engages the front plate of the mass storage housing with a projecting shoulder and that the mass storage housing is sealed with a front plate with a circumferential metal cord, this seal being set with a screw fastening under a sufficiently high mechanical pressure.

6. Personal computer according to one or more of the preceding claims, characterized in that the floppy disk and removable disk shafts are designed as ⁵ HF chimneys.

7. Personal computer according to one or more of the preceding claims, characterized in that the radiation through the main housing cover is shielded by the ° use of beryllium / contact spring strips and a low-torsion cover construction.

8. Personal computer according to one or more of the preceding claims, characterized in that the CPU housing has a much screwed metal sealing cover, the screw spacing being less than 1/2 of the critical wavelength.

9. Personal computer according to one or more of the preceding ⁰ claims, characterized in that the radiation of the system board is shielded by covering with an inner housing.

5

10. Personal computer according to one or more of the preceding claims, characterized in that all electronic components are electrically isolated, installed inside the outer housing and isolated from

⁵ a second Faraday ¹ see cage are enclosed.

11. Personal computer according to one or more of the preceding claims, characterized in that the radiation through the cathode steel tube of the ° monitor is shielded by the use of a plastic disc with an inserted fine-meshed metal gauze which is completely conductively connected to the monitor housing.

12. Personal computer according to claim 10, characterized in that the metal gauze consists of parallel, perpendicular rows of metal wires which are inclined 20 ° to 25 ° to the horizontal or vertical.

13. Personal computer according to one or more of the preceding claims, characterized in that the printer is deactivated when the printer cover is opened.