Abstract:
The invention relates to a protective circuit ( 10 ) for the input-side protection of an electronic device ( 30 ) operating in the maximum frequency range from high-power interfering impulses in the working frequency range of the device ( 30 ), said protective circuit ( 10 ) between an input ( 11 ) and an output ( 18 ) comprising a first limiting circuit ( 12 ) having at least one gas discharge tube (GDT 1 , GDT 2 ) for limiting high interference powers and a second limiting circuit ( 14, 16 ) disposed behind the first limiting circuit ( 12 ), said second limiting circuit having semi-conductor components (D 1 , . . . , D 4 ) for limiting smaller interference powers. Protection from particularly high interference powers is achieved in that the first limiting circuit ( 12 ) comprises two gas discharge tubes (GDT 1 , GDT 2 ) connected in parallel and preferably identical.

Description:
BACKGROUND 
     In wireless communication, in the case of radar installations or other receiving devices which operate at microwave frequencies, and whose input is normally coupled to an antenna, high-power interference pulses or frequency bursts, with steep rising flanks can deliberately or inadvertently be applied to the input of the appliance, whose frequencies fall in the operating frequency range of the appliance, where they temporarily or permanently affect the sensitive input stages or, furthermore, cause damage. 
     In the past, it has therefore variously been proposed that appliance inputs such as these be protected by means of protective circuits upstream of the input, which reliably limit or attenuate such fast high-power interference pulses without noticeably impeding the wanted signals being fed into the appliance input. In this case, a plurality of limiter circuits, which cover a coarse and a fine range of the interference power, are frequently connected in series, for example with gas discharge paths being used for the coarse range and limiting semiconductor components being used for the fine range. 
     By way of example, CH-A5-654 149 discloses a protective circuit against NEMPs, in which a gas cell as coarse protection means and a varistor as fine protection means are connected in series via a frequency-selective delay element. 
     DE-A1-34 25 296 discloses a protective apparatus for electronic appliances, in which a coarse protective device with a gas discharge path and a fine protective device which operates with semiconductor elements are accommodated in separate housings and are connected to one another via a relatively long connecting cable. DE A1-36 26 800 also discloses a comparable solution. 
     DE A1-39 07 199 discloses an overvoltage protective device for coaxial cables, in which a three-electrode suppressor is used for coarse protection, and a combination of switching diodes and limiter diodes is used for fine protection. The three-electrode suppressor is inserted into a printed circuit board on which the other circuit elements are also arranged. The end-face electrodes of the three-electrode suppressor are connected to the inner conductor and outer conductor of the coaxial cable, while the center electrode is connected to a separate ground line. 
     EP-A1-0 855 758 discloses a circuit arrangement for protection of RF input circuits of telecommunication appliances, in which a fine protective circuit having a varistor and a limiter diode is provided behind a gas capsule suppressor or a lambda/4 short-circuit line. 
     Finally, it has been known for a long time for PIN diodes to be used for limiting interference pulses at the input of appliances which operate in the microwave range, and these are distinguished by very small internal capacitances and short response times (see for example U.S. Pat. No. 5,345,199 or WO-A1-2006/124104). 
     Other protective methods relate to quick active regulation of a limiting element in the input of the appliance (US-A1-2006/0293020). 
     The known protective circuits and protective methods are not sufficient for reliably limiting or attenuating very high power interference pulses, for example those transmitted by pulsed radars or specific interference pulse generators. 
     SUMMARY 
     The object of the invention is to specify a protective circuit which avoids the disadvantages of known protective circuits and is distinguished by a simple, compact and robust design, as well as reliable limiting of fast and, in particular, high-power interference pulses in the microwave band. 
     The object is achieved by the totality of the features of claim  1 . One essential feature for the invention is a limiter circuit, arranged at the input of the protective circuit, for the coarse power range, which comprises two parallel-connected gas discharge tubes which are connected in parallel in a specific manner. The two gas discharge tubes, which are preferably of the same type and are arranged one behind the other connected in parallel allow the isolation with respect to interference powers at the input to be improved by an order of magnitude (a factor of approximately 10) by very simple means and with a very compact design. 
     For optimum limiting, it is advantageous for the two gas discharge tubes to be coupled to one another as closely as possible, with regard to their ignition. According to one refinement of the invention, this is achieved in that the two gas discharge tubes have a common gas space. 
     It is particularly advantageous for the two gas discharge tubes to be physically combined in one gas capsule, which comprises a cylindrical gas space with two mutually opposite end-face electrodes and a center electrode which is arranged in the center between the two end-face electrodes, wherein the end-face electrodes each form one electrode of the two gas discharge tubes, and the center electrode forms the other electrode of the two gas discharge tubes. This makes it possible for one gas discharge tube to likewise be ignited comparatively quickly when the (adjacent) other gas discharge tube ignites, because of the plasma that is created in the gas space. 
     A bridging circuit which is in the form of a T-element with two inductances and one capacitance is preferably arranged between the end-face electrodes of the gas capsule, in order to pass on the wanted signals. 
     The coupling between two gas discharge tubes is improved further if the end-face electrodes extend so far in the axial direction into the gas space that their ends are covered by the center electrode, when seen in the radial direction. This results in the discharge paths of the two tube or electrode systems being directly adjacent, and influencing one another particularly intensively. 
     In order to ensure that the proximity of the two gas discharge tubes does not lead to undesirable electromagnetic coupling between the input and the output of the limiter circuit, means are preferably provided for electromagnetically decoupling the two gas discharge tubes. 
     In particular, for electromagnetic decoupling of the two gas discharge tubes, the gas capsule is installed with its cylinder axis essentially at right angles to the plane into an electrically conductive flat plate, such that the plate is located between the end-face electrodes of the gas capsule. The plate then has a shielding effect. 
     Another refinement of the invention is characterized in that PIN diodes are used as semiconductor components for limiting relatively low interference power levels, with the PIN diodes preferably being connected back-to-back in parallel, in pairs. 
     A further refinement of the invention is distinguished in that a third limiter circuit is arranged behind the second limiter circuit and likewise comprises semiconductor components, in that PIN diodes are in each case connected back-to-back in parallel, in pairs as semiconductor components in the second and the third limiter circuit, and in that the PIN diodes in the third limiter circuit have a lower power and respond more quickly than the PIN diodes in the second limiter circuit. The third limiter circuit reduces the response delay of the protective circuit even further. At the same time, the limiting range is extended toward lower power levels. 
     In order to achieve a compact design and good heat dissipation from the limiter elements, it is advantageous if the PIN diodes are arranged as unpackaged semiconductor chips directly on a thin ceramic substrate which is mounted flat on a cold plate for heat dissipation. 
     Another refinement of the invention is distinguished in that in order to ensure the response of the limiter circuits, a decoupling circuit is in each case arranged between the limiter circuits and between the last limiter circuit and the output, wherein, in particular, the decoupling circuit which is arranged between the first and the second limiter circuit comprises a series circuit formed from a capacitance and a transmission line, and the decoupling circuits in front of and behind the third limiter circuit each comprise a transmission line. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail in the following text with reference to exemplary embodiments and in conjunction with the drawings, in which: 
         FIG. 1  uses two figure elements to show a block diagram of a protective circuit according to one exemplary embodiment of the invention ( FIG. 1   a ), and the arrangement of the protective circuit from  FIG. 1   a  in front of an appliance to be protected ( FIG. 1   b ); 
         FIG. 2  shows the internal design of the first limiter circuit shown in  FIG. 1   a , with a gas capsule with a center electrode as a limiting element; 
         FIG. 3  shows the circuit of the two other limiter circuits from  FIG. 1   a , in each case with a pair of PIN diodes, connected back-to-back in parallel, as limiting elements; 
         FIG. 4  uses two figure elements to show the design of the decoupling circuit between the first two limiter circuits ( FIG. 4   a ) and the decoupling circuits in front of and behind the third limiter circuit ( FIG. 4   b ); 
         FIG. 5  shows the internal design of a preferred gas capsule with end electrodes which extend into the area of the center electrode; 
         FIG. 6  shows a preferred circuit design for the protective circuit shown in  FIGS. 1 to 4 ; 
         FIG. 7  shows an example of a diagram of power limiting by a gas discharge tube; 
         FIG. 8  shows an example of a diagram of power limiting by two gas discharge tubes arranged one behind the other in parallel; and 
         FIG. 9  shows an example of a diagram of power limiting by the protective circuit shown in  FIG. 1   a.    
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  uses two figure elements to show a block diagram of a protective circuit according to one exemplary embodiment of the invention ( FIG. 1   a ) and the arrangement of the protective circuit from  FIG. 1   a  in front of an appliance to be protected ( FIG. 1   b ). The protective circuit  10  in the exemplary embodiment has an input  11  and an output  18 , which may be in the form of coaxial plug connections. By way of example, the input  11  may be a Type N connector, and the output  18  a Type SMA. Plug connectors such as these for frequencies in the GHz range are available, for example, from the applicant. 
     Three limiter circuits  12 ,  14  and  16  are arranged in series between the input  11  and the output  18 , and are connected to one another and to the output  18  by decoupling circuits  13 ,  15  and  17 . The decoupling circuits  13 ,  15  and  17  ensure that, when the fastest reacting third limiter circuit is activated, sufficient voltage is dropped to then subsequently activate the second limiter circuit  14  and the first limiter circuit  12 . 
     The power ranges and response times of the limiter circuits  12 ,  14  and  16  are graduated, with the first limiter circuit  12  having the highest power range and the slowest response time, while the third limiter circuit  16  is distinguished by the lowest power range and the fastest response time. The third limiter circuit  16  therefore extends the scope of protection of the appliance ( 30  in  FIG. 1   b ) toward lower power levels and faster rising flanks, and therefore improves the effect of the protective circuit, irrespective of the actual configuration of the first limiter circuit  12 . As shown in  FIG. 1   b , the protective circuit  10  is connected in front of the input  31  of the electronic appliance  30  to be protected, and therefore protects the appliance against overloading by fast and high-power interference pulses. 
     If just one simple typical gas discharge tube of the conventional type were to be used in the first limiter circuit  12 , this would result in a relationship between the input power present at the input  11  and the output power that appears at the output  18 , as is illustrated in the diagram in  FIG. 7 . The output power rises linearly with the input power with the gradient  1 , until the gas discharge tube ignites at about 60 Watts, and limits the output power to less than 10 Watts. As the input power rises further, the output power then also continues to rise, as a result of which, at input power levels of several 1000 Watts, such as those which can occur in extreme cases, the protection of the appliance  30  is no longer ensured. 
     It has now surprisingly been found that a simple parallel circuit of two gas discharge tubes (GDT 1  and GDT 2  in  FIG. 1   a ) in the first limiter circuit  12  provides protection against 10-times higher input power levels, provided that the ignition responses of the two gas discharge tubes are sufficiently closely coupled to one another. This then results in a relationship between the input power and the output power as is shown in the diagram in  FIG. 8 , that is to say even input power levels of 3000 Watts and more are reliably limited to output power levels of about 30 Watts, before the output power rises again with the input power. 
     In order to achieve close coupling in a simple manner between the two gas discharge tubes (GDT) arranged one behind the other in parallel, a ceramic three-electrode gas discharge tube (gas capsule  19 ) is used as shown in  FIG. 2  and  FIG. 5 , and is connected such that the two end-face electrodes  20 ,  22  respectively form one electrode of the two gas discharge tubes GDT 1 , GDT 2 , and the center electrode  21  forms the other electrode of the two gas discharge tubes GDT 1 , GDT 2 . The type of gas capsule  19  illustrated in  FIG. 5  is preferably used in this case, in which the end-face electrodes  20  and  22  project so far into the interior of the gas space  23  in the axial direction that they are covered by the (annular) center electrode  21  in the radial direction at the ends. This results in very close coupling between the discharge paths of the electrode pairs  20 ,  21  and  22 ,  23 , and this leads to the ignition of the rear gas discharge tube GDT 2  being triggered by the already ignited first gas discharge tube GDT 1 , and in this being done with only a short time delay. 
     As can be seen in  FIG. 2 , the three-electrode gas capsule  19  is installed in the protective circuit  10  such that the path which is formed between the two end-face electrodes  20 ,  22  interrupts the inner conductor of the protective circuit  10 , while the center electrode  21  is grounded. In order to bridge the interruption, a bridging circuit is arranged between the end-face electrodes  20  and  22 , in the form of a T-element with two inductances L 2  and L 3  and a capacitance C 2 . Furthermore, the capacitances C 1  and C 3  of the discharge paths of the gas capsule  19  as well as the inductances L 1  and L 4  of the supply lines to the end-face electrodes  20  and  22  play a part in terms of radio-frequency technology. 
     The second limiter circuit  14  and the third limiter circuit  16  are designed as shown in  FIG. 3 : two PIN diodes D 1 , D 2  (and D 3 , D 4  in  FIG. 6 ) of the same type are connected back-to-back in parallel in them. By way of example, glass-passivated chips from the Tyco Company or M/A-Com can be used as PIN diodes. In this case, the inductances L 5 , L 6  and L 7  of the supply lines are also important for radio-frequency purposes, and these are governed by the bonding wires  27 , which can be seen in  FIG. 6 , of the PIN diode chips. The decoupling circuits  13 ,  15  and  17  which are provided between the limiter circuits  12 ,  14  and  16  and after the limiter circuit  16  are internally designed as shown in  FIGS. 4   a  and  b : the limiter circuit  13  comprises a series circuit of a capacitance C 4  and a transmission line T 1 , and the limiter circuits  15  and  17  each contain a transmission line T 2 . 
     Since the protective circuit  10  must be designed for the GHz range and must process considerable power levels in the event of interference, the design of the limiter circuits  12 ,  14  and  16  is of considerable importance.  FIG. 6  shows one exemplary embodiment of an appropriate circuit design  24 , in a simplified form. The circuit design comprises a preferably metallic cold plate  26  with good thermal conductivity and a high heat capacity, which can absorb and dissipate the heat that is created. A printed circuit board  25  is arranged on the upper face of the cold plate  26 , and is used for wiring of the circuit. The printed circuit board  25  has a cutout  32  which leaves free part of the area of the surface of the cold plate  26 . Thin, electrically insulating ceramic substrates  28 ,  29  are arranged in the cutout  32 , closely thermally coupled to the cold plate  26 , and the PIN diode pairs D 1 , D 2  and D 3 , D 4  of the limiter circuits  14  and  16  are accommodated on their surface, in the form of unpackaged semiconductor chips. The size relationships of the chips shown in  FIG. 6  have in this case been illustrated in an exaggerated form, in order to improve clarity. The PIN diodes D 1 , . . . , D 4  are adhesively bonded or soldered on the lower face to the ceramic substrates  28 ,  29 . The bonding pads on the upper faces are connected by means of bonding wires  27  to corresponding conductor tracks on the printed circuit board  25 . 
     A particular type of installation is envisaged for the gas capsule  19 : corresponding openings are left free one above the other in the cold plate  26  and in the printed circuit board  25 , through which the cylindrical gas capsule  19  can be passed, with its cylinder axis at right angles to the plane of the plate. In this case, the center electrode is located at approximately the same height as the printed circuit board  25  and can be grounded via a very short path. In this configuration, the metallic cold plate  26  represents a shield, which reduces or entirely suppresses the electromagnetic coupling between the electrode paths  20 ,  21  and  22 ,  21 , thus suppressing undesirable crosstalk from the input to the output of the gas capsule  19 . 
     The protective circuit  10  as shown in  FIGS. 1 to 6  results in a considerably extended protective effect for the appliance  30 , as is illustrated by the diagram shown in  FIG. 9 , which shows the relationship between the input power level and the output power level (in decibels with respect to 1 mW) plotted on a double-logarithmic scale. 
     REFERENCE SYMBOLS 
     
         
           10  Protective circuit 
           11  Input 
           12 ,  14 ,  16  Limiter circuit 
           13 ,  15 ,  17  Decoupling circuit 
           18  Output 
           19  Gas capsule 
           20 , . . . ,  22  Electrode (gas capsule) 
           23  gas space 
           24  Circuit layout 
           25  Printed circuit board 
           26  Cold plate 
           27  Bonding wire 
           28 ,  29  Ceramic substrate 
           30  Electronic appliance 
           31  Input 
           32  Cutout 
         C 1 , . . . , C 4  Capacitance 
         D 1 , . . . , D 4  PIN diode 
         GDT 1 , GDT 2  Gas discharge tube 
         L 1 , . . . , L 7  Inductance 
         T 1 , T 2  Transmission line