Abstract:
A waveguide which is particularly suitable for transmitting electrical signals between or to and/or from electronic circuits, for example between integrated circuits disposed on a printed circuit board, has an over-voltage protection region. The over-voltage region of the waveguide region is specifically formed by the actual geometric configuration of the waveguide.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an electrical waveguide, in particular to a waveguide for transmitting radio-frequency electrical signals in the MHz and/or GHz range. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a waveguide that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which is particularly suitable for transmitting electrical signals between or to and/or from electronic circuits—for example between integrated circuits disposed on a printed circuit board. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a waveguide. The waveguide contains a waveguide body having an over-voltage protection region formed by a geometric configuration of the waveguide body. 
     Accordingly, the invention provides for the over-voltage protection region to be present in the waveguide and to be formed by the geometric configuration of the waveguide. 
     A significant advantage of the waveguide according to the invention is that electrostatic discharge (ESD) problems are avoided in a very simple manner with the waveguide according to the invention. Over-voltages, as may occur as a result of electrostatic charges, for example, are suppressed by the over-voltage protection region in the waveguide according to the invention. The over-voltage protection takes place, according to the invention, in the waveguide, with the result that there is no need for any additional, separate over-voltage protection devices. 
     A further significant advantage of the waveguide according to the invention is that it is particularly suitable for use in the case of electronic circuits to which high bit-rate data signals are applied. At radio frequencies, conventional over-voltage protection devices formed by separate electrical and electronic components bring about considerable parasitic effects which can no longer be ignored and which lead to the bandwidth of the signals being limited. Such parasitic effects are avoided by the waveguide according to the invention since separate electrical and electronic components are completely dispensed with. 
     The waveguide may be, for example, a micrometric-wave waveguide and/or a millimeter-wave waveguide. By the terms “micrometer-wave waveguide” and “millimeter-wave waveguide” are meant, in this context, waveguides that are suitable for transmitting electromagnetic waves at wavelengths in the millimeter and micrometer range, respectively, i.e. in the “submeter” range. 
     The over-voltage protection region may advantageously be formed by the geometric configuration of the individual conductors of the waveguide and/or by the nature of the dielectric between the individual conductors. 
     The over-voltage protection region in the waveguide may be formed in a particularly simple, and therefore advantageous, manner by the individual conductors being closer together in the over-voltage protection region than in the remaining waveguide region. The smaller spacing in the over-voltage protection region results in a discharge, for example a flashover, between the individual conductors when over-voltages occur. The voltage required to initiate the discharge is determined by the spacing between the individual conductors. 
     The smaller spacing between the individual conductors may advantageously be due to at least one irregularity disposed in the over-voltage protection region. Since the irregularities are deliberately disposed in the waveguide, it is possible for the over-voltage protection region to be formed in a very simple manner without major complexity and without changing the remaining waveguide geometry. 
     It is, in principle, possible for any material to be used for providing electrical isolation between the individual conductors in the over-voltage protection region. It is, however, considered to be advantageous if, in the over-voltage protection region, the at least two individual conductors are separated by an air gap, since, when air is used as an insulator, an electrostatic discharge will not cause any damage and will not impair the over-voltage protection region in the case of “triggering” or a flashover. However, a dielectric having a dielectric constant that is smaller than the dielectric constant of a dielectric in the remaining waveguide region may also be used instead of air in the over-voltage protection region. 
     Coaxial conductors, in particular, are very suitable for transmitting radio-frequency electromagnetic waves. It is therefore considered to be advantageous if the waveguide according to the invention having the over-voltage protection region is formed by a coaxial conductor. 
     In the case of a coaxial conductor, the over-voltage protection region may be formed in a very simple manner by the outer conductor of the coaxial conductor being conically tapered. The conical tapering of the outer conductor reduces the spacing between the outer conductor and the inner conductor of the coaxial conductor, with the result that a voltage flashover may occur between the outer and the inner conductor at correspondingly high electrical voltages. 
     In order to avoid significant changes in the characteristic impedance of the coaxial conductor occurring when the outer conductor is conically tapered, it is considered to be advantageous if the inner conductor of the coaxial conductor is also conically tapered in the direction of the over-voltage protection region. Due to the conical tapering of the inner conductor, the characteristic impedance remains largely unchanged when the outer conductor is conically tapered in a corresponding manner, with the result that it is not possible for any undesired electromagnetic reflection to occur in the transition region from the remaining waveguide region to the over-voltage protection region. 
     Reflections occurring on account of the transition region may be avoided completely in a particularly simple, and therefore advantageous, manner in the case of a coaxial waveguide by the outer conductor and the inner conductor being conically tapered in such a manner that the ratio of the internal diameter of the outer conductor to the external diameter of the inner conductor is the same at any point in the transition region, i.e. is not location-dependent. In the case of a coaxial conductor, the characteristic impedance Z L  may be calculated using the following formula: 
         Z   L     =           μ   0       ɛ   0         ·       ln   ⁢     D   d         2   ⁢   π   ⁢       ɛ   r                 
 
in which D is the external diameter of the coaxial conductor, d is the internal diameter of the coaxial conductor and μ 0 , ε 0  and ε r  are the material constants. It can be seen from the formula that the characteristic impedance Z L  may be kept constant if the ratio of the external diameter to the internal diameter remains unchanged. Therefore, if the diameter of the outer conductor tapers conically, the diameter of the inner conductor must also taper conically if the characteristic impedance Z L  is to remain unchanged.
 
     In addition to coaxial conductors, coplanar conductors, microstrip waveguides and stripline waveguides are also suitable for transmitting radio-frequency or high bit-rate data signals, with the result that it is considered to be advantageous if the waveguide is a coplanar conductor, a microstrip waveguide or a stripline waveguide. 
     Coplanar conductors, microstrip waveguides and stripline waveguides are advantageously disposed on substrates, in particular on printed circuit boards, in which the substrate or the printed circuit board is provided with a rear-side ground contact. 
     When using such printed circuit boards or substrates having a rear-side ground contact, in order to avoid a corresponding interference wave or a corresponding electromagnetic interference pulse being coupled into the waveguide (coplanar conductor, microstrip waveguide or stripline waveguide etc.) or into the dielectric of the substrate when an electrostatic discharge takes place in the over-voltage protection region, provision is made according to one development of the waveguide according to the invention for there to be no rear-side ground contact in the over-voltage protection region, or for the ground contact to be removed in the over-voltage protection region. This is because, if there is no corresponding rear-side ground contact in the over-voltage protection region, then it is more difficult for a corresponding interference wave to be injected. 
     The over-voltage protection region may advantageously also be formed by a recess in the substrate. The recess is preferably a through-hole that extends through the substrate. The recess should preferably have a metal-free or unmetallized surface. 
     High-bit-rate data signals are, in particular, also transmitted between electrical and electronic components on a printed circuit board, with the result that it is considered to be advantageous if the waveguide is disposed on a printed circuit board together with at least one electronic component. 
     When the waveguide is disposed on a printed circuit board together with at least one electronic component, it is considered to be advantageous if the over-voltage protection region in the waveguide is configured such that its response voltage is smaller than the maximum permissible voltage across the electronic component. 
     The maximum permissible voltage across the electronic component is governed, for example, by the protection class of the respective component. 
     In this case, the waveguide may advantageously connect at least one input and/or output of the electronic component to at least one input and/or output of a further electronic component disposed on the printed circuit board. Instead of this, it is also possible for the waveguide to connect at least one input and/or output of the electronic component to an input and/or output of the printed circuit board. In this case, the waveguide may have radio-frequency data signals, in particular at frequencies in the MHz and/or in the GHz range applied to it. 
     The invention also relates to a printed circuit board having at least one waveguide and having at least one over-voltage protection device. 
     The object of the invention as regards such a printed circuit board having a waveguide is to arrive at a solution which enables electrical signals to be transmitted particularly effectively between or to and/or from electronic circuits—for example between integrated circuits disposed on the printed circuit board. 
     The object is achieved according to the invention by the over-voltage protection device being formed by an over-voltage protection region in the waveguide. 
     As regards the advantages of the printed circuit board according to the invention, reference is made to the statements made above in connection with the waveguide according to the invention, since the advantages of the printed circuit board according to the invention correspond to the advantages of the waveguide according to the invention. Specifically, this is because the waveguide according to the invention and the printed circuit board according to the invention are based on the same inventive idea that consists of forming an over-voltage protection region in the waveguide, which ensures the operation of an over-voltage protection device. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a waveguide, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic, perspective view of a first exemplary embodiment of a waveguide having a coplanar structure according to the invention; 
         FIG. 2  is a sectional view of a second exemplary embodiment of a waveguide which is formed by a coaxial conductor; and 
         FIG. 3  is a perspective view of a third exemplary embodiment of a waveguide that is formed by a microstrip waveguide. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a substrate  10  which has a coplanar conductor  30  formed on its upper side  20 . The coplanar conductor  30  has three individual conductors  40 ,  50  and  60  that run parallel to one another. 
     It can be seen from  FIG. 1  that there are irregularities  70 ,  80  and  90  between the individual conductors  40 ,  50  and  60 , which reduce the spacing between the individual conductors  40 ,  50  and  60 . On account of the irregularities  70 ,  80  and  90 , an over-voltage protection region is formed between the individual conductors  40 ,  50 ,  60  affected in each case. If a limit voltage or triggering voltage (response voltage) defined by the spacing between the individual conductors in the region of the irregularities  70 ,  80  and  90  is exceeded, sparks will flash over at the irregularities  70 ,  80  and  90 , resulting in an electrical or electrostatic discharge. 
     In the case of the coplanar conductor  30 , the discharge will occur only in the region of the irregularities  70 ,  80  and  90 , since it is only there that the spacing between the individual conductors  40 ,  50 ,  60  is sufficiently small. In the remaining waveguide region, no flashover will occur—due to the greater spacing between the individual conductors—since the triggering voltage that is necessary for this to happen is not reached. If the voltage across the coplanar conductor  30  increases, then the triggering voltage in the region of the irregularities will, for the time being, be exceeded, which will result in a flashover at this point. The voltage is thus prevented from increasing further. It is thus no longer possible for the higher “triggering voltage” that is necessary for a flashover to occur in the remaining waveguide region to be reached. 
     This ensures that the remaining waveguide region of the coplanar conductor  30  and thus the electronic components that may be connected to the coplanar conductor  30  are effectively protected from an over-voltage that is greater than the triggering voltage in the region of the irregularities. 
     Finally, it can be seen from  FIG. 1  that a rear-side ground conductor  110  is provided on a rear side  100  of the substrate  10 . In the region of the irregularities  70 ,  80  and  90 , the rear-side ground conductor  110  has window-shaped openings  120 . The openings  120  prevent an electromagnetic interference pulse from being injected into the coplanar conductor  30  or into the dielectric of the substrate  10  when a spark discharge occurs in the region of the irregularities  70 ,  80  and  90 . 
     A second exemplary embodiment of the waveguide according to the invention is shown in  FIG. 2 . The waveguide in this instance is a coaxial conductor  200  that is formed by an inner conductor  210  and an outer conductor  220 . 
     The coaxial conductor  200  has three waveguide regions in  FIG. 2 , namely an over-voltage protection region  230 , adjoining transition regions  240  and remaining waveguide regions  250  which adjoin the transition regions. 
     In the transition regions  240  and in the remaining waveguide regions  250 , the inner conductor  210  and the outer conductor  220  are separated from one another by a plastic insulation  260 . In the region of the over-voltage protection region  230 , the outer conductor  220  and the inner conductor  210  are separated from one another by air. 
     In the transition regions  240 , the inner conductor  210  and the outer conductor  220  each taper in the direction of the over-voltage protection region  230 . An external diameter d(z) of the inner conductor  210  and an internal diameter D(z) of the outer conductor  220  are thus location-dependent, as indicated by the locational coordinate “z” in  FIG. 2 . A characteristic impedance Z L  is thus likewise location-dependent, and is dependent on the locational variable z. The characteristic impedance is given by: 
           Z   L     ⁡     (   z   )       =           μ   0       ɛ   0         ·       ln   ⁢       D   ⁡     (   z   )         d   ⁡     (   z   )             2   ⁢   π   ⁢       ɛ   r                 
 
     In the exemplary embodiment according to  FIG. 2 , the inner conductor  210  and the outer conductor  220  taper such that the ratio D(z)/d(z) remains constant independently of the point in the transition region  240 —i.e. independently of location. As a result, the characteristic impedance Z L  is independent of location in the transition region  240  and corresponds to the characteristic impedance in the remaining waveguide region  250 . 
     Since the diameters of the inner conductor  210  and the outer conductor  220  are appropriately dimensioned, it is not possible for the electromagnetic waves that are transmitted via the coaxial conductor  200  to be reflected (to any significant extent) in the transition region  240 , since the characteristic impedance Z L  is not altered. 
     It should undoubtedly be remembered that the characteristic impedance Z L  is likewise altered to a certain extent due to the “material change” between the over-voltage protection region  230 —filled with air—and the two transition regions  240 —filled, for example, with the plastic insulation  260  in this case. In order to prevent or reduce reflections occurring as a result of the sudden material change, it is possible for the ratio D/d at the transition points between the materials to be varied to a correspondingly minor extent in order to compensate for the change in refractive index. In the case of coaxial conductors in which air is used as the insulator (with reinforcing rods for holding the inner conductor) instead of plastic insulation, the problem of the change in refractive index does not, of course, exist. 
     If an over-voltage should then occur in the coaxial conductor  200 , an electrostatic flashover will occur since there is only a short distance between the inner conductor  210  and the outer conductor  220  in the over-voltage protection region  230 , and the flashover will have the effect of limiting the voltage. In the case of the coaxial conductor  200  shown in  FIG. 2 , only air is provided as the insulator instead of the plastic insulation  260  in the over-voltage protection region  230 . It is thus not possible for the material to be damaged as a result of a flashover in the over-voltage protection region  230 , as would be the case, for example, if the plastic insulation  260  were also provided in the region of the over-voltage protection region  230 . This is because a flashover or an electrostatic discharge in the region of the plastic insulation  260  would lead to the plastic material being destroyed, with the result that the coaxial conductor  200  would be damaged after the occurrence of an over-voltage. Such damage to the coaxial conductor is advantageously avoided by air in the over-voltage protection region  230 . 
       FIG. 3  shows a microstrip waveguide that is formed by a strip-typed individual conductor  310  and a metallized rear side  320  of a substrate  330 . 
     The over-voltage protection region is formed, in the case of the exemplary embodiment according to  FIG. 3 , by the geometric configuration of the dielectric—i.e. of the substrate region. Specifically, a through-hole  340 , for example a drilled-hole or an etched hole, is provided in the immediate vicinity of the individual conductor  310  in the substrate  330  and forms an air channel between the upper side of the substrate  330  and the metallized rear side  320  of the substrate  330 . The diameter of the through-hole  340  may be very small, preferably smaller than 1 mm. 
     If the voltage then rises above a limit voltage determined by the spacing between the individual conductor  310  and the through-hole  340 , then a spark discharge or a flashover results in the air channel formed by the through-hole  340 . 
     The through-hole  340  need not necessarily be filled with air. It is also feasible for a dielectric  350  to be provided in the through-hole  340 . In this case, a dielectric constant of the dielectric  350  should preferably be smaller than a dielectric constant of a dielectric in the remaining waveguide region. The outer surface or the inside of the through-hole  340  should preferably not be metallized so that it is possible for a gas discharge to form in the through-hole  340 . If the hole  340  were metallized, the path provided for gas discharge would be short-circuited, as a result of which the striking voltage would be markedly reduced since, in this case, the spark gap would be formed only indirectly between the upper end of the through-hole  340  and the individual conductor  310 . 
     Instead of the through-hole  340 , it is also possible for a recess, formed in the substrate  330 , to be provided on the surface, i.e. a hole that does not extend right through the substrate  330 . 
     Instead of a microstrip waveguide, it is also possible, for example, for a stripline waveguide to be disposed on the substrate  330 . The configuration shown in  FIG. 3  would operate in a corresponding fashion in the case of a stripline waveguide.