1. Field of the Invention
The present invention relates to a layout of a cable for transmitting a high-speed digital signal for a relevant unit in a digital electronic apparatus, such as a copying machine, with respect to a metal housing of the unit. More particularly, the present invention relates to a technique for suppressing radiant noises in the electronic apparatus.
2. Description of the Related Art
Hitherto, a digital electronic apparatus, for example a digital copying machine, has been designed such that an image read section for reading an image and an image printing section for printing the image are disposed in one metallic housing. To accommodate varying specification demands of customers, however, it is desirable to be capable of changing the specifications of such an apparatus. Accordingly, an electronic apparatus of the separated-type, in combination with various functional units, has become prevalent. Specifically, in this type of apparatus, the image read section and the image printing section are constructed as separate units, i.e., a reader unit and a printer unit, which are accommodated in separate metallic housings. Also, a paper supply section for supplying paper on which an image is to be printed, a paper eject section for ejecting paper on which an image has been printed, and the like, are increasingly constructed as separate operating units. As a result of the various functional sections being split into separate units, metallic housings of the different units are commonly arranged opposite to each other.
In the separated-type digital electronic apparatus described above, at least one of the various units has a cable for transmitting a high-speed digital signal, such as a clock signal. Such a cable may be disposed externally of the housing of the unit, depending on an arrangement space and the order of unit assembly. However, when the high-speed digital signal is transmitted through a gap between opposing metallic housings, those housings become electrically coupled, whereby a parallel plate resonance occurs and radiant noises at a very high level generate at a particular frequency. Particularly, this problem is more noticeable in the case of a cable for transmitting a radio frequency signal of not lower than 100 KHz or, in the case of a cable connected to a controller board on which a controller operates, at a frequency of not lower than 10 MHz. The parallel plate resonance will be described in more detail below.
FIG. 13 is a perspective view showing a configuration of a known digital copying machine. Assuming the side where a console unit 14 manipulated by a user is arranged to be the front side, FIG. 13 is an oblique view from behind the digital copying machine. For convenience in explanation, FIG. 13 uses dashed lines to create a see-through view.
The digital copying machine comprises a reader unit 11 for reading an image, a printer unit 12 for printing the image, a paper supply unit 16 for supplying paper to the printer unit 12, an optional paper supply unit 17 that is optionally mountable, a finisher unit 18 for ejecting the paper on which the image has been printed, and a feeder unit 19 arranged above the reader unit 11 and feeding a manually set original document. An image controller unit 13 for executing image processing is mounted on the rear side (front side as viewed in FIG. 13) of the printer unit 12, and has ports for establishing network connections to external devices and for interfacing with optional units. The console unit 14 is mounted on the front side (rear side as viewed in FIG. 13) of the printer unit 12. The console unit 14 displays the operation status of the copying machine, and allows a user to operate the copying machine. The image controller unit 13 and the console unit 14 are connected to each other by a digital signal cable 15 through which a digital signal is transmitted. As illustrated, the digital signal cable 15 is laid on an upper surface of a metallic housing of the printer unit 12, and the reader unit 11 is arranged above the printer unit 12 with a predetermined gap left therebetween.
FIG. 14 is a schematic view showing a layout of the digital signal cable 15 and respective metallic housings of the reader unit 11, the printer unit 12 and the finisher unit 18 shown in FIG. 13. The digital signal cable 15 is disposed in a space defined between the upper surface of the metallic housing of the printer unit 12 and a lower surface of the metallic housing of the reader unit 11. Therefore, parallel plate resonance occurs between the upper surface of the metallic housing of the printer unit 12 and the lower surface of the metallic housing of the reader unit 11, and radiant noises at a very high level are generated at a particular frequency.
However, the mechanism of the radiant noises, which are generated due to the layout of the metallic housings and the cable and the influence of electrical coupling between the adjacent metallic housings, is very difficult and very complicated to make clear the phenomenon. Accordingly, the mechanism of the radiant noises is not yet fully clarified up to now, and it has been difficult to consider that point in the stage of apparatus design.
To suppress the radiant noises generated due to the layout of the metallic housing and the cable and the influence of electrical coupling between the metallic housings, Japanese Patent Laid-Open No. 05-159835 discloses a method of fitting a ferrite core over the cable, i.e., a noise source, so that noise components will not propagate from the cable. Also, Japanese Patent Laid-Open No. 08-106819 discloses a method of using a shielded cable prepared by covering the cable with a shield material in advance. Further, Japanese Patent Laid-Open No. 10-208791 discloses a method of interconnecting the metallic housings of the adjacent units at several points by using metallic members adapted for electrical connection, thereby stabilizing the ground potential between the adjacent units to suppress the radiant noises.
However, those known methods have problems as follows. The method of fitting the ferrite core, disclosed in the above-cited Japanese Patent Laid-Open No. 05-159835, is practiced as a countermeasure taken after manufacturing the product. This results in a serious problem from the viewpoint of productivity in that the number of manufacturing steps increases and there is no space for arranging the ferrite core. Further, because the ferrite core is expensive, it is impractical to use many ferrite cores. In addition, the ferrite core is effective for only signals in the frequency band of 30 to 500 Hz, but it is not effective for the radiant noises at frequencies higher than that band.
The method of using the shielded cable, disclosed in the above-cited Japanese Patent Laid-Open No. 08-106819, increases the total cost of the apparatus because the shielded cable is very expensive.
The method of interconnecting the metallic housings of the adjacent units at several points by using metallic members adapted for electrical connection, as disclosed in the above-cited Japanese Patent Laid-Open No. 10-208791, is not effective unless the metallic housings are interconnected at portions corresponding to the nodes in phase of a flowing current. Then, from the practical point of view, it is very difficult to interconnect the metallic housings exactly in the portions corresponding to the nodes in phase of the current. Even if such interconnection is performed, this method merely shifts the resonance frequency. In other words, the radiant noises may occur in another frequency band, and a complete solution is not obtained.