Abstract:
In an electronic circuit, comprising electrical and electronic components and their connecting lines ( 3 ) on a printed circuit board ( 2 ), in which case a shielded housing encloses the electronic circuit, attenuating radiofrequencies, in order to prevent interference which acts on the electronic circuit and interference which is caused by the electronic circuit, in particular for use in a combination display instrument in a motor vehicle. Connecting lines ( 3 ) from the electronic circuit on the printed circuit board ( 2 ) are passed out of the shielded housing ( 1 ), connecting means ( 5 ) outside the shielded housing ( 1, 15 ) are connected to the connecting lines ( 3 ), and shielding is provided which reduce or suppress the ingress of radio-frequency interference into the shielded housing ( 1 ) and the emission of radio-frequency interference from the shielded housing ( 1 ).

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The invention relates to an electronic circuit, comprising electrical and electronic components and their connecting lines on a printed circuit board, in which case a shielded housing encloses the electronic circuit, attenuating radiofrequencies, in order to prevent interference which acts on the electronic circuit and interference which is caused by the electronic circuit, in which case the circuit has connecting lines which lead out of the shielded housing to the exterior. This circuit is intended in particular for installation in a baseplate in a motor vehicle combination instrument, in which various display instruments, display areas, monitoring and warning lights as well as the electrical and electronic equipment parts required for this purpose are arranged in a common housing and are covered by a common transparent panel. 
     A shielding box is known from the prior art, in which the circuit is completely surrounded by a shielding housing, and contact means in the form of contact springs connect the circuit to a baseplate which contains all the other active and passive switching elements, conductor tracks, plug connectors and connecting points which are required for operation of the combination instrument. In this case, the circuit is accommodated on a printed circuit board and is connected to the baseplate via contact springs which have different shapes and pass through the shielding housing. A disadvantage with this type of shielding is that contact means having different shapes are required. Furthermore, the known shielding housing can be assembled only with difficulty, by hand. 
     SUMMARY OF THE INVENTION 
     The object of the invention is thus to provide an electronic circuit of the type mentioned initially which provides good shielding while being of simple design and being capable of being assembled easily. 
     With the circuit of the invention, it is advantageous that it is possible to dispense with a retaining frame for guiding the contact means and for preventing contact between the contact means and the shielded housing. Furthermore, this type of construction means that it is possible to achieve a grid size for the individual contact means with the normal value of {fraction (1/10 )}inch (corresponding to 2.54 mm) in a row. 
     It is particularly advantageous for the shielding means to be designed in such a manner that contact means are provided on the printed circuit board, which are electrically conductively connected to the shielded housing, and for the contact means to be connected to the connecting lines via capacitors inside the housing. In this case it is advantageous for the filter components, in particular capacitors, to be connected as close as possible to the housing wall, in order to keep the effective length of the unprotected connecting lines as short as possible. Furthermore, it is in this case advantageous that the filter capacitors represent an economic and effective shielding means. 
     By using pressure contacts, it is possible in an advantageous manner to dispense with a rigid connection. At the same time, this prevents the connection from becoming loose as a result of the printed circuit board and the shielded housing having different thermal expansion levels. Furthermore, the different thermal expansion levels of the printed circuit board and of the shielded housing result in friction between the components which make contact with one another, which overcomes any contamination that may be present. The pressure contact is designed, for example, in the form of a crown. 
     The configuration of the contact means as solder studs is simple to construct, while at the same time being very effective. Furthermore, it is advantageous in this context that the solder studs can be produced at the same time as the connections for the SMDs. 
     An electrically highly conductive connection between the individual contact means and a ground ring line prevents possible potential differences and enhances the radio-frequency shielding of the shielded housing overall. 
     The plated through holes for the ring lines at the same time act as a shielding grid. Where shielding must be provided for frequencies up to 1 GHz, it has been found to be advantageous for the distance between the individual contact point s to be about 12 mm. 
     Pushing the side parts of the shielded housing against the contact means results in an economic and effective pressure-contact connection between the shielded housing and the filter components. 
     The use of galvanized ferromagnetic sheeting for the shielded housing and solder studs composed of tin prevent contact oxidation of the conductor tracks, and thus ensures good electrical connection. 
     The fact that the printed circuit board has slots through which the lugs on the shielded housing are passed means that an electrical connection and mechanical joint can be produced in a simple way between the shielded housing case and the shielded housing cover by bending the lugs around, for example, the shielded housing cover. Furthermore, the bending of the lugs at the same time fixes the shielded housing on the printed circuit board in such a manner that the housing is permanently pressed against the contact means. The lugs also act as plated-through holes to the cover. 
     The use of aluminum-oxide ceramic for the printed circuit board ensures good heat dissipation for the components. 
     The multilayer technology allows complex circuits in a very small space. 
     Finally, if the capacitors are tuned to the interference frequency having the greatest amplitude on the respective lines, the interference is suppressed as well as possible. 
     If contact springs are used as the connecting means, it is possible to compensate for the different thermal expansion levels between the printed circuit board and the baseplate, without any mechanical stresses. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in the following text, for a particular, preferred exemplary embodiment, with reference to the drawings, in which 
     FIG. 1 shows a plan view of a particularly preferred printed circuit board according to the invention, 
     FIG. 2 shows the stamped shapes of a particularly preferred shielded housing according to the invention, 
     FIG. 3 shows the side view of the printed circuit board from FIG. 1 with the particularly preferred assembled shielded housing according to the invention, from FIG. 2, mounted on a baseplate, 
     FIG. 4 shows a plan view of the printed circuit board from FIG. 1, with a plan view of the particularly preferred shielded housing according to the invention, from FIG. 2, 
     FIG. 5 shows the section D-E from FIG. 4, and, 
     FIG. 6 shows the section B-C from FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, the printed circuit board  2  is composed of metal ceramic and has slots  26 . There is a ground ring line  4  with solder studs  41 ,  42  on the top surface  21  of the printed circuit board  2 . Along the longitudinal sides of the printed circuit board  2 , capacitors  6  each having a connecting pin  62  are each connected via a ground connecting line  44  to the ground ring line  4 . Connecting lines  3  connect in each case one contact spring  5  via in each case one connecting pin  61  of each capacitor  6  to the electronic circuit, comprising the microprocessor  203 , an oscillator crystal  204 , electronic components  201 ,  202  and the further other electronic parts shown, which are required for the microprocessor  203  to operate but are not provided with reference characters, in order to improve the clarity. In order to improve the clarity, only one connection between the electronic components  201 ,  202  and the respective contact springs  5  is shown in each case. The other contact springs  5  are also connected to the electronic circuit via connecting lines  3  which are not shown, in which case the connecting pins  61 ,  62  of each capacitor  6  in each case provide radio-frequency coupling from a connecting line  3 , via a ground connecting line  44 , to the ground ring line  4  and to the solder studs  41 ,  42 . In the region where it crosses over the connecting lines  3 , the ground ring line  4  is electrically insulated from these connecting lines  3 . The circuit is designed in a manner known per se using multilayer technology. In the region of the narrow side of the printed circuit board  2 , the ground ring line  4  is advantageously not only formed on the top surface  21  but also has a projection  45  beyond the edge of the upper surface  21 , on the side surface of the printed circuit board  2 . 
     The design according to the invention means that it is possible to achieve a grid size of a=2.54 mm with Size 0603 capacitors  6 . 
     FIG. 2 shows the stamped parts of a particularly preferred shielded housing composed of galvanized tin plate, comprising a shielded housing case  1  and a shielded housing cover  15 . The shielded housing case  1  has narrow sides  10  and longitudinal sides  11 . The bottom edges  111  of the longitudinal sides  11  are extended by bending lugs  12 . The bottom edges  101  of the narrow sides  10  are extended by interconnecting lugs  13 . The shielded housing cover  15  has slots  16  for the bending lugs  12  to pass through, and slots  17  for the interconnecting lugs  13  to pass through. Before assembly, the longitudinal sides  11  and the narrow sides  10  are each bent through 90°, resulting in a case having four side walls and a base. 
     FIG. 3 shows the view of the shielded housing from FIG. 2 mounted on the printed circuit board from FIG. 1, and mounted on a baseplate  9 . The bending lugs  12  on the longitudinal sides  11  are pushed through the slots  26  in the printed circuit board  2  and through the slots  16  in the shielded housing cover  15 , and are bent around so that they press against the shielded housing cover  15 . The shielded housing cover  15  furthermore rests against the bottom edge  101  of the narrow side  10 . At the same time, the interconnecting lugs  13  are pushed through the slot  17  in the shielded housing cover  15  and are interconnected. They thus also press the shielded housing cover  15  particularly permanently against the bottom edge  101  of the narrow side  10 . 
     The shielded housing is advantageously assembled as follows: the shielded housing case  1  is located in such a manner that is open upward, and the bending lugs  12  and interconnecting lugs  13  project upward. The printed circuit board  2  is now fitted, with its top surface  21  downward, onto the shielded housing case in such a manner that the bending lugs  12  project through the slots  26 . The shielded housing cover  15  is now fitted in such a manner that the bending lugs  12  project through the slots  16 , and the interconnecting lugs  13  project through the slots  17 . Finally, the bending lugs  12  are bent, and the interconnecting lugs  13  are interconnected. 
     The printed circuit board  2  is electrically connected and mechanically joined to the baseplate  9  by means of the contact springs  5 , in the present case, for example, by means of wave-soldered joints  91 . 
     In FIG. 4, the shielded housing case  1  has been fitted onto the top surface  21  of the printed circuit board  2 . The bottom edges  111  of the longitudinal sides  11  of the shielded housing case  1  are pressed against the solder studs  41 ,  42 , and the shielded housing case  1  is thus electrically connected to the ground ring line  4 . In the region between the points F and G, the narrow sides  10  of the shielded housing case  1  also cover the side edge of the printed circuit board  2  and the side of any multilayer region which may be present on the bottom surface  22  of the printed circuit board  2 . The inside of the side surface  10  of the shielded housing case  1  is thus pressed against the projection  45 , and is thus likewise electrically connected to the ground ring line  4 . 
     FIG. 5 shows the section D-E from FIG.  3 . The printed circuit board  2  has a further ground ring line  40  on its bottom surface. The bending lug  12  on the shielded housing case  1  is pushed through the slot  26  in the printed circuit board  2  and through the slot  16  in the shielded housing cover  15 , and is thus bent against the shielded housing cover  15  such that the shielded housing cover  15  is pressed against the solder stud  401  which is located on the ground ring line  40 . The ground ring lines  4 ,  40  are electrically conductively connected to one another by through-plating  46  that covers the edge of the slot  26 . There are further plated-through holes (which are not illustrated), for example under the solder studs  41  (illustrated in FIG.  1 ). The ground connecting line  44  connects the capacitor  6  to the ground ring line  4 . The contact spring  5  is electrically connected and mechanically joined to the connecting line  3  by means of the solder  50 . In this section, the rest of the multilayer board is composed of glass  7 . Other conductor tracks and multilayer boards may also be fitted on the bottom surface  22  of the printed circuit board  2 . In this case, it is necessary to ensure that there are no inadvertent short circuits between the lines and the shielded housing cover  15 . 
     In FIG. 6, the bottom edge  111  of the longitudinal side  11  is pressed against the solder stud  42 , and is thus electrically connected to the ground ring line  4 . The connecting line  3  is passed through into the interior of the shielded housing, on the printed circuit board  2  and underneath the ground ring line  4 . However, underneath the connecting pin  61  of the capacitor  6 , the connecting line  3  changes its level in the multilayer board. In consequence, the radiofrequency which is to be short-circuited by the capacitor  6  is forced to pass via the connecting pin  61  of the capacitor  6 , the capacitor  6 , the connecting pin  62  of the capacitor  6  and the ground connection  44  to the ground ring line  4  and the shielded housing  1 . If the connecting line  3  were routed with the conductor track in a straight line without bends underneath the connecting pin  61 , it would be possible for a situation to arise in which the radiofrequency would not leave the connecting line  3 . 
     In order to indicate possible orders of magnitude, it should be mentioned that, when using Size 0603 ceramic capacitors for the capacitors  6 , the dimension b (the width of the capacitor) is about 0.75 mm. It can thus be seen that the effective length of the connecting line  3  which is not short-circuited for interference frequencies inside the shielded housing by a capacitor  6  is very short so that, in the example described above, interference frequencies up to 1 GHz on the connecting line  3  can be suppressed, to be precise both those which are produced by the microprocessor  203  inside the shielded housing and should not reach the exterior, as well as those which are coupled into the connecting lines  3  outside the housing and should not pass through into the interior of the shielded housing. The effective length of the connecting line  3  between the shielded housing and the capacitor  6  is so short that it cannot act as a transmitting antenna for interference frequencies attempting to pass from the exterior to the interior on the connecting line. It likewise cannot act as a receiving antenna for interference frequencies that are present inside the shielded housing. If the unprotected connecting line  3  were too long, the interference frequencies could otherwise leave the shielded housing via the connecting line  3 . 
     The shielded housing described above and the electronic circuit protected by it have a compact design which, as a further advantage, means that the requirements for ceramic, conductor track and insulating material are small, while the baseplate  9  now virtually has to carry out only connecting functions, and can therefore be produced from simple, economic printed circuit board material.