Low-emission electrical module

The electrical module, in particular an optoelectronic transceiver, has a coupling region for connection of a plug connector. The coupling region projects out of a metallic structure, which surrounds a shielding housing, when the module is arranged on a printed circuit board in the shielding housing. The coupling region is composed of plastic. This prevents the coupling region from acting as an antenna and interference emissions, which are produced by radio-frequency signals in the module, from being emitted in an amplified manner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS: FIG. 1 shows, schematically, an assembly according to the invention having an optoelectronic transceiver 1 , which is inserted into a shielding housing 2 . The shielding housing 2 is mounted on a printed circuit board 3 which, for example, represents the main board of a computer. The shielding housing 2 and the printed circuit board 3 are arranged in a metallic structure 4 which is, in particular, the rear wall (backplane) of a metallic housing, for example of a computer housing. An opening 41 is provided in the metallic structure 4 , through which the optoelectronic transceiver 1 can be inserted into the shielding housing 2 . The plug-in configuration of the optoelectronic transceiver 1 means that it can be connected in a simple manner to the printed circuit board 3 of an appliance, namely by plugging it into the shielding housing 2 . The optoelectronic transceiver 1 has a coupling region, which is illustrated only schematically in FIG. 1 , and a plug holder 11 , which represents an optical port for connecting an optical plug connector to the transceiver. The coupling region is arranged outside the metallic structure 4 , and projects out of the opening 41 in the metallic structure, so that a coupling partner can be connected without any problems. According to the invention, the coupling region 11 is composed of a plastic, in particular of an electromagnetically absorbent plastic. This prevents the coupling region 11 , which projects out of the rear wall 3 of the transceiver 1 , from acting as an antenna, and emitting electromagnetic interference radiation in an amplified manner. In fact, the use of plastic for the coupling region 11 reduces the emission of the electromagnetic waves. This is desirable in order to satisfy the requirements for low radiated electromagnetic emissions from the transceiver (electromagnetic compatibility). Alternatively, the transceiver can also be permanently connected to the shielding housing, with the transceiver and the shielding housing both being mounted on the printed circuit board 3 . In this case as well, the coupling region for holding a plug connector projects out of the metallic rear wall. FIGS. 2 to 5 show one specific exemplary embodiment of the invention. According to FIG. 5 , the optoelectronic transceiver according to the invention has a coupling region 11 (corresponding to the coupling region 11 in FIG. 1 ), an upper housing shell 12 , a lower housing shell 13 which can be connected to the upper housing shell 12 by latching them together, a printed circuit board 14 located in between, an optoelectronic receiving component 16 , an optoelectronic transmitting component 15 , and an inner shielding plate 17 . The latter will be referred to in the following text as shielding goggles. The coupling region 11 is formed with two retaining apertures 111 , 112 each of which is intended to hold an optical plug connector. An unlocking element 113 is used for easily accessible unlocking of plug connectors (not illustrated) which are inserted into the retaining apertures 111 , 112 . Optical plug connectors which are inserted into the insertion apertures 111 , 112 are coupled to the receiving component 16 and to the transmitting component 15 , respectively, which each have an optoelectronic transducer and convert incoming light signals to electrical signals and, respectively, electrical signals to outgoing light signals. The receiving component 16 and the transmitting component 15 are connected to the printed circuit board 14 , and the components 15 , 16 are driven by electrical drive units, which are disposed on the printed circuit board 14 . The shielding goggles 17 are arranged behind the coupling region 11 , at right angles to the insertion direction and parallel to a metallic rear wall (cf. the rear wall 4 in FIG. 1 ), as a single metallic part of the transceiver 1 and shields the RF-contaminated signal ground, which results in the region of the components 15 , 16 and of the printed circuit board 14 , from the coupling region 11 . The shielding goggles 17 in this case have two openings 171 for holding flanges 151 , 161 of the components 15 , 16 . The flanges have ferrules, each of which are coupled to optical wave guides in a coupling partner. Furthermore, the shielding goggles 17 have contact lugs 172 for making contact with a metallic shielding housing corresponding to the shielding housing 2 in FIG. 1 , by means of which electromagnetic interference potentials are dissipated to the shielding housing. The coupling region 11 , the upper housing shell 12 , and the lower housing shell 13 are composed completely of plastic, so that they cannot receive or radiate interference potentials. However, it is also possible in alternative exemplary embodiments for the housing shells 12 , 13 to be composed of metal, or at least to be metallized, as well. The coupling region 11 which projects out of the rear wall, however, is always formed from plastic. The shielding goggles 17 together with their contact springs remain behind the plastic part 11 , which projects out of the metallic structure. The shielding goggles 17 therefore do not represent a further discontinuity for producing radiated emissions. FIGS. 2 to 4 show the transceiver 1 according to the invention with the coupling region 11 , which is composed of plastic, during the various phases of insertion of the transceiver into a shielding housing 2 , which is mounted on a printed circuit board 3 by means of pins 22 . The shielding housing 2 in the illustrated exemplary embodiment comprises a metallic shielding plate and, in its front region, has curved contact springs 21 , which are used to make contact between the metallic shielding plate 2 and a metallic rear wall 4 . The metallic rear wall in this case provides a reference ground potential for the shielding housing. Shielding plates such as these are known per se. In the completely inserted state shown in FIG. 4 , only the coupling region 11 now still projects out of the shielding housing 2 . This coupling region 11 is located in front of an opening in the metallic rear wall, through which the transceiver 1 was inserted into the shielding housing 2 , and which makes electrical contact with the shielding housing 2 by means of the contact springs 21 . Since the coupling region 11 is formed from plastic, this reduces the emission of electromagnetic waves in the area of the coupling region 11 . This has been verified experimentally, and is illustrated in the measurement curves in FIG. 6 . In this case, the left-hand measurement curve shows the field strength of the emitted electromagnetic field as a function of the frequency for a transceiver according to the invention, and the right-hand measurement curve shows the field strength as a function of the frequency for a transceiver which is known from the prior art and has a metallic coupling region. It can be seen from the measurement curves that, particularly at a frequency of around 1.25 GHz, the radiated emissions from the transceiver according to the invention are reduced considerably (a field strength of 37 dB&mgr;V/m in comparison to a field strength of 47 dB&mgr;V/m). In addition, the transceiver according to the invention has no radiated emission peak, such as that which occurs at a frequency of about 6.25 GHz with the transceiver according to the prior art. It will be understood that the embodiment of the invention is not restricted to the exemplary embodiments described above. The sole primary feature of the invention is that a coupling region of an electronic module—the coupling region projects out of a metallic structure when the module is mounted in a shielding housing—is formed from plastic.