Patent Publication Number: US-2015077889-A1

Title: Protective device

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to devices for transmitting electromagnetic signals of a desired frequency range and, more particularly, to devices for transmitting electromagnetic signals of a desired frequency range that additionally provide overvoltage protection to the transmission line. 
     BACKGROUND OF THE INVENTION 
     A transmission line, or signal path, is a structure designed to efficiently transmit electromagnetic signals, such as radio frequency (RF) signals, from a signal source to a load. The transmission line formed between the signal source and the load is commonly established using one or more electric devices, such as coaxial cables, connectors and switches. 
     Electric devices of the type described above are well known in the art and are widely used to transmit electromagnetic signals over 10 MHz with minimum loss and limited distortion. As a result, these types of electric devices are commonly used to transmit and receive signals in telecommunications, broadcast, military, security and civilian transceiver applications, as well as numerous additional uses. 
     Electric devices used to transmit electromagnetic signals are often provided with means for protecting the load from any potentially harmful, transient, high-voltage electromagnetic energy present along the transmission line (e.g., as the result of a lightning strike or electro-static discharge). In particular, electric devices with overvoltage protection, referred to herein simply as protective devices, are particularly needed for loads that include voltage sensitive circuitry that operates at a frequency range above approximately 10 MHz, such as radio receivers, low-voltage control circuits and low-voltage communication circuits. 
     It has been found that low-voltage circuits of the type described above are susceptible to a wide variety of different destructive energy including, but not limited to, (i) oscillating ring waves with a frequency between 10 kHz and 100 MHz, and (ii) impulses with a rise time of approximately 1 ns or more and a pulse width in the range from 30 ns to 500 microseconds, both types of energy having a peak current that ranges between a few amperes to a few tens of amperes. Generally, the lower frequency energy is more destructive to the low-voltage circuit, since the lower frequency energy exists for longer durations and the fundamental frequency of impulses is commonly of the highest spectral content. 
     For low-voltage circuits connected to a transmission line operating at a frequency range above approximately 10 MHz, it has been found that most of the destructive transient energy falls below the operational frequency band. This energy that falls below operational frequency band is often blocked using conventional circuit protection techniques. 
     For example, protective devices commonly rely upon gas discharge tubes (GDTs) and/or shunting components to treat undesirable, below operational frequency, electromagnetic energy that is present along the transmission line. 
     Although gas discharge tubes can operate over a wide range of frequencies (even well over 1 GHz) and can exhibit very high transient current shunting capabilities, gas discharge tubes respond too slowly to fast rise time transients. For example, when destructive electromagnetic energy with 5 KV/microsecond edge rates is present on the transmission line, a 90 volt nominal gas discharge tube will pass through to the load a 600 volt impulse that last for about 100 ns. Moreover, faster pulses will pass through to the load an even higher transient impulse. This high-voltage residual pulse passed through the transmission line by the gas discharge tube is substantial enough to permanently damage sensitive electrical equipment. 
     Shunting protectors, which are incorporated into protective devices to shunt to ground undesirable electromagnetic energy present along the transmission line, are typically provided in the form of a quarter wave shunt or an inductor. 
     Quarter wave shunts, or stubs, have been found to be particularly effective in applications with an operational frequency range of over 500 MHz. In fact, the higher the operational frequency of the application, the more efficient the quarter wave shunt becomes at removing the undesirable energy from the transmission line. However, it has been found that quarter wave shunts are ineffective in passing frequencies below 400 MHz, and therefore are not generally utilized in signal transmission applications with a lower operational frequency range. 
     Similarly, inductors utilized to shunt undesirable energy from the transmission line suffer from certain performance limitations. Specifically, inductors can only be incorporated into protective devices that operate at frequencies below 100 MHz and, in addition, have been found to experience severe limitations in removing fast rise time transients. 
     Although both types of shunting protectors described in detail above are commonly utilized in the art to treat electromagnetic energy that falls beneath the operational frequency band, it has been found that the aforementioned shunting protectors are not similarly capable of providing significant attenuation of potentially destructive energy that falls within the operational frequency band. As a result, destructive transient energy that falls within the operational frequency band poses a significant risk to relatively sensitive, low-voltage circuits coupled to the signal path. In fact, most conventional protection devices have been found to be incapable of limiting, or otherwise treating, transient energy that falls both below and within the operational frequency band without compromising the quality of the desired electrical energy (i.e., the desired signal that falls within the operational frequency band). 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a new and improved protective device for transmitting electromagnetic signals of a desired frequency band. 
     It is another object of the present invention to provide a protective device of the type as described above that is designed to treat any potentially harmful, transient, high-voltage electromagnetic energy present on the transmission line. 
     It is yet another object of the present invention to provide a protective device of the type as described above that is particularly well suited for use in protecting a low-voltage circuit in electrical communication with the transmission line from the transient electromagnetic energy. 
     It is still another object of the present invention to provide a protective device of the type as described above that is effective in treating transient electromagnetic energy that falls both below and within the operational frequency band without compromising the quality of the desired electrical energy. 
     It is yet still another object of the present invention to provide a protective device of the type as described above that has a limited number of parts, is inexpensive to manufacture and is easy to use. 
     Accordingly, as a feature of the present invention, there is provided a protective device for transmitting electromagnetic signals of an operational frequency band, the protective device comprising (a) a transmission line connecting an input terminal to an output terminal, (b) a filter for blocking any transient electromagnetic energy received by the transmission line that has a frequency that falls below the operational frequency band, the filter comprising a capacitor located in series on the transmission line between the input terminal and the output terminal, and (c) a semiconductor-based clamping component for limiting any transient electromagnetic energy received at the input terminal that has a frequency that falls within the operational frequency band, the semiconductor-based clamping component connecting the transmission line to a ground terminal, the semiconductor-based clamping component being connected to the transmission between the capacitor and the output terminal. 
     Various other features and advantages will appear from the description to follow. In the description, reference is made to the accompanying drawings which form a part thereof, and in which is shown by way of illustration, various embodiments for practicing the invention. The embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings wherein like reference numerals represent like parts: 
         FIG. 1  is a front perspective view of a first embodiment of a protection device constructed according to the teachings of the present invention; 
         FIG. 2  is a front perspective view of the protection device shown in  FIG. 1 , the protection device being shown with the cover removed therefrom; 
         FIG. 3  is a schematic representation of the electrical circuit shown in  FIG. 2 ; 
         FIG. 4  is a front perspective view of a second embodiment of a protection device constructed according to the teachings of the present invention, the protection device being shown with the cover removed therefrom; 
         FIG. 5  is a schematic representation of the electrical circuit shown in  FIG. 4 ; 
         FIG. 6  is a front perspective view of a third embodiment of a protection device constructed according to the teachings of the present invention, the protection device being shown with the cover removed therefrom; 
         FIG. 7  is a schematic representation of the electrical circuit shown in  FIG. 6 ; 
         FIG. 8  is a front perspective view of a fourth embodiment of a protection device constructed according to the teachings of the present invention, the protection device being shown with the cover removed therefrom; and 
         FIG. 9  is a schematic representation of the electrical circuit shown in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Construction of Protective Device  11   
     Referring now to  FIGS. 1-3 , there is shown a protective device for transmitting electromagnetic signals of a desired frequency range, the protective device being constructed according to the teachings of the present invention and identified generally by reference numeral  11 . As will be described in detail below, protective device  11  is designed primarily to protect a low-voltage circuit to which it is coupled from any potentially harmful, transient, high-voltage electromagnetic energy. 
     As referenced briefly above, protective device  11  is designed primarily for use in protecting low-voltage circuits from transient electromagnetic energy. In particular, it is envisioned that protective device  11  would be particularly well suited for use in protecting circuits that (i) normally operate at relatively low voltages (e.g., under 50 volts), such as radio receivers, low-voltage control circuits and low-voltage communication circuits, and (ii) operate within a desired frequency range between 1 MHz to 3 GHz or greater. 
     It has been found that low-voltage circuits of the type described above are rendered susceptible to a wide variety of different destructive energy including, but not limited to, (i) oscillating ring waves with a frequency between 10 kHz and 100 MHz, and (ii) impulses with a rise time of approximately 1 ns or more and a pulse width in the range from 30 ns to 500 microseconds, both types of energy having a peak current that ranges between a few amperes to a few tens of amperes. 
     For low-voltage circuits connected to a transmission line operating at a frequency range above approximately 10 MHz, it has been found that most of the destructive transient energy falls below the operational frequency band. However, destructive transient energy that falls within the operational frequency band can also be present along the transmission line. Accordingly, as a principal feature of the present invention, device  11  is designed not only to reduce unwanted transient energy that falls beneath the operational frequency band but also limit the magnitude of unwanted transient energy that falls within the operational frequency band without compromising the integrity of any of the desired, or operational, energy. 
     Protective device  11  comprises a generally enclosed protective casing, or housing,  13  into which is disposed an electrical circuit, or protection circuit,  15 . As will be explained further in detail below, the particular design and operation of electrical circuit  15  serves as a principal novel feature of the present invention. 
     Casing  13  is preferably constructed out of a rigid and durable material, such as metal, and includes a generally rectangular base  17  which is shaped to define a shallow interior cavity  19  dimensioned to receive electrical circuit  15 . A flat, rectangular cover, or lid,  21  is mounted onto base  17  so as to enclose cavity  19  and render protective device  11  a unitary component. 
     Together, base  17  and lid  21  provide casing  13  with a generally rectangular, block-like construction. However, it is to be understood that the shape of base  17  and/or lid  21  could be modified, as deemed necessary, to provide casing  13  with alternative configurations without departing from the spirit of the present invention. For instance, it is envisioned that casing  13  could be alternatively constructed as a generally cylindrical enclosure. 
     In the present embodiment, a plurality of fastening elements  23 , such as screws, are driven transversely through the periphery of cover  21  and into threaded engagement into corresponding bores  25  formed in base  17 . However, it should be noted that protective device  11  need not be limited to the use of fastening elements  23  to releasably secure cover  21  onto base  17 . Rather, it is to be understood that cover  21  could be mounted onto base  17  using a wide range of different coupling techniques, such as through soldering, welding or press-fit mounting, without departing from the spirit of the present invention. 
     Additionally, casing  13  is provided with a plurality of mounting holes  27  that extend transversely through both base  17  and cover  21 , each mounting hole  27  being preferably located within a corresponding corner of casing  13 . In use, mounting holes  27  facilitate securing protective device  11  to an item, such as a fixed electrical panel, and may be internally threaded to receive corresponding mounting screws (not shown). 
     As seen most clearly in  FIG. 2 , the various electrical components for circuit  15  are preferably mounted on a printed circuit board (PCB)  29  for ease of construction and assembly. Printed circuit board  29 , in turn, is fittingly disposed within interior cavity  19  and is permanently secured to base  17  by a plurality of fastening elements  31 , such as screws. 
     Referring now to  FIGS. 2 and 3 , electrical circuit  15  includes a transmission line, or through path,  33  that extends in electrical communication between an input, or exposed, terminal  35 - 1  and an output, or treated, terminal  35 - 2 . During routine operation if device  11 , transmission line  33  provides a circuit path for passing radio frequency (RF) signals of a designated frequency range between terminals  35 - 1  and  35 - 2 , with the remainder of circuit  15  provided, inter alia, to treat potentially harmful, high-voltage, transient electromagnetic impulses present in transmission line  33 , as will be explained further in detail below. 
     Transmission line  33  is represented in  FIG. 2  as a conductive trace that extends laterally across the width of printed circuit board  29 , with the ends of the trace defining input and output terminals  35 - 1  and  35 - 2 . The particular width of the conductive trace is preferably determined based on the corresponding thickness and dielectric constant of the dielectric layer for PCB  29  to ensure the conductive trace has the proper impedance. 
     A ground plane  37  is preferably mounted on PCB  29  in a spaced apart relationship relative to the conductive trace that forms transmission line  33 . As can be appreciated, ground plane  37  serves as a common ground terminal for various components of electrical circuit  15 . Although not shown herein, the opposite surface of PCB  29  (i.e., the side without components that directly abuts against base  25 ) may additionally include a substantially solid, ground plane mounted thereon, unless features are required to manipulate impedance or to provide additional functionality, as is traditional in microwave microstrip design. 
     Electrical circuit  15  includes a filter  39  for treating high-voltage, transient electromagnetic impulses that fall primarily below the operational frequency band. In the present embodiment, filter  39  includes a capacitor  41  located in series on transmission line  33  between terminals  35 , with one of its terminals connected to input terminal  35 - 1  and the other of its terminals connected to output terminal  35 - 2 . The aforementioned schematic configuration can be achieved, for example, by conductively connecting capacitor  41  across a gap in the conductive trace that forms transmission line  31 . 
     Preferably, capacitor  41  has a relatively high voltage rating. Optimally, capacitor  41  has a higher voltage rating than highest voltage transient expected. As a result, capacitor  41  would be able to safely handle a transient impulse of any realistic voltage. Furthermore, if any additional component (e.g., a gas discharge tube, quarter-wave stub, or other similar shunting-based protector) is incorporated into electrical circuit  15  to assist capacitor  41  in the treatment of high-voltage transient impulses, the voltage rating of capacitor  41  may be lowered accordingly. 
     As referenced briefly above, filter  39  operates as a high-pass filter that blocks transient electromagnetic impulses that have frequency content below the filter cut-off frequency. For example, if filter  39  is designed to pass a minimum operational frequency of 30 MHz, any disturbing ring waves (generally with a value at or below approximately 10 MHz) which are received by input terminal  33 - 1  are attenuated by capacitor  41 , thereby protecting the desired low-voltage circuit. 
     Lower frequency ring wave transients (e.g., of the type described above) tend to produce inversely longer pulse durations. For constant current or voltage input, the energy available in lower frequency pulses is typically higher. However, filter  39  can increase insertion loss at a rate of approximately 40 dB/decade. As a result, device  11  is able to reduce energy at lower frequencies which is adequate to compensate for longer pulse durations. 
     It should be noted that device  11  is not limited to the particular construction of filter  39 . Rather, as will be set forth in detail below, the construction of filter  39  could be modified without departing from the spirit of the present invention. For instance, a higher order filter could be used in place of filter  39  to further attenuate below pass band energy. In fact, any filter with a low frequency block and/or shunt will provide the desired effect of treating transient impulses with energy that falls beneath the operational frequency band. 
     Electrical circuit  15  additionally includes a semiconductor-based clamping component  43  to limit high-voltage, transient, electromagnetic impulses that fall within the operational frequency band. Clamping component, or voltage limiter,  43  connects transmission line  33 , at a location between output terminal  35 - 2  and capacitor  41 , to ground  37 . 
     As defined herein, semi-conductor clamping component  43  represents any silicone or solid-state voltage limiter that preferably has (i) a low capacitance so as not to hinder the highest frequency operation of protective device  11 , (e.g., a capacitance of 3.5 pF for operational frequencies up to 500 MHz, a capacitance of 1.0 pF for operational frequencies between 500 MHz and 2.0 GHz, and a capacitance of no greater than 0.5 pF for operational frequencies between 2.0 GHz and 6.0 GHz), (ii) a fast acting time (e.g., less than 1 ns response time), and (iii) low-voltage capabilities (e.g., less than 50 Vdc). Examples of suitable clamping components include, but are not limited to, diode-based components, metal-oxide varistor (MOV)-based components, silicon-controlled rectifier (SCR)-based components, protection thyristor-based components, and triode for alternating current (TRIAC)-based components. 
     In the present embodiment, semiconductor-based component  43  is represented herein as a diode-based component that includes a first diode array  45 - 1  in reverse parallel with a second diode array  45 - 2 , as seen most clearly in  FIG. 3 . Accordingly, the opposite polarity configuration of first and second diode arrays  45 - 1  and  45 - 2  provides component  43  with bipolar voltage protection (i.e., protection against both positive and negative polarity voltage transients), which is highly desirable. 
     First diode array  45 - 1  includes a rectifier diode  47 - 1  which is connected in series with a zener diode  49 - 1  in order to increase the breakover voltage. As can be seen, the positive terminal of rectifier diode  47 - 1  is connected to transmission line  33  at a location between output terminal  35 - 2  and capacitor  41 , the negative terminal of rectifier diode  47 - 1  is connected to the negative terminal of zener diode  49 - 1 , and the positive terminal of zener diode  49 - 1  is connected to ground  37 . 
     Similarly, second diode array  45 - 2  includes a rectifier diode  47 - 2  connected in series with a zener diode  49 - 2  to increase the breakover voltage. As can be seen, the positive terminal of rectifier diode  47 - 2  is connected to ground  37 , the negative terminal of rectifier diode  47 - 2  is connected to the negative terminal of zener diode  49 - 2 , and the positive terminal of zener diode  49 - 2  is connected to transmission line  33  at a location between output terminal  35 - 2  and capacitor  41 . 
     It should be noted that the construction of each diode array  45  could be modified without departing from the spirit of the present invention. For instance, each diode array  45  could consist only of rectifier diode  47 , particularly if operational voltages fall below a few hundred millivolts. However, it is to be understood that the utilization of a pair of series diodes for each diode array  45  is preferred in higher frequency applications, as the lower resultant capacitance has a less harmful effect on the desired RF through signal. 
     The inclusion of voltage-limiting component  43  into electrical circuit  15  provides two notable advantages. 
     As a first advantage, component  43  limits, or clamps, transient pulses that fall within a predictable, or defined, frequency range. In particular, voltage-limiting component  43  is designed to primarily treat transient energy that falls within the operational frequency band. Transient electromagnetic energy that falls beneath the operational frequency band is treated primarily by filter  39  (based on the particular specifications for filter  39 ), thereby protecting component  43  from that potentially harmful energy. 
     As a second advantage, component  43  serves to limit the amount of voltage applied across additional electrical components connected in parallel therewith. For example, alternative configurations of electrical circuit  15  may incorporate an inductor in parallel with component  43 , as will be shown in detail below. In this situation, component  43  would limit the voltage across the conductor. As a result, the particular characteristics of the inductor could be selected with less regard to peak voltages, but instead, based on inherent RF properties and size. 
     Lastly, electrical circuit  15  includes an optional gas discharge tube (GDT)  51  to treat very high electrical current introduced to transmission line  33 . As can be seen in  FIGS. 2 and 3 , GDT  51  connects transmission line  33 , at a location between input terminal  35 - 1  and capacitor  41 , to ground  37 . 
     As referenced above, GDT  51  is an optional component that may be incorporated into circuit  15  to increase the ampere capacity, or ampacity, of current diverted to ground  37 . Although shown herein as being directly incorporated into electrical circuit  15 , it is to be understood that GDT  51  could be located at any position along the signal path in need of overvoltage protection. In fact, when utilized to treat transient currents resulting from lightning strikes, GDT  51  may operate more effectively if located in closer physical proximity to the actual strike (e.g., closer to the entry of the building). In this capacity, lower (i.e., more modest) current threats in need of treatment would be more suitably diverted to either filter  37  or clamping component  39  for handling, rather than GDT  51 . 
     Referring back to  FIGS. 1-2 , an input connector  53 - 1  and an output connector  53 - 2  extend orthogonally out from opposing sides of base  17  and enable circuit  15  to be externally coupled to the signal path in need of protection from transient impulses. In other words, input connector  53 - 1  is designated to receive an untreated input signal from a signal source. Upon treatment of the input signal by protection circuit  15 , the resultant input signal, which has been treated to reduce any undesirable electrical energy associated therewith to an acceptable level, is transmitted to a protected low-voltage circuit via output connector  53 - 2 . 
     In the present example, each of input connector  53 - 1  and output connector  53 - 2  is represented as a standard, press-mount type, SMA jack connector. Accordingly, as seen in  FIG. 2 , each connector  53  includes a conductive inner pin, or center conductor,  55  that extends coaxially within a conductive outer sleeve  57  and is electrically insulated therefrom by an annular insulator  59 . Conductive inner pin  55  for input connector  53 - 1  is electrically connected to input terminal  35 - 1  and conductive inner pin  55  for output connector  53 - 1  is electrically connected to output terminal  35 - 2 , thereby establishing a conductive path between connectors  53 - 1  and  53 - 2  via circuit  15 . 
     It should be noted that protective device  11  is not limited to the particular type and arrangement of connectors  53  represented herein. Rather, connectors  53  represent any means for electrically coupling circuit  15  to a signal path in need of overvoltage protection. Accordingly, it is to be understood that the type and arrangement of connectors  53  could be modified without departing from the spirit of the present invention. 
     Operation of Protective Device  11   
     Protective device  11  is designed to pass RF signals of a designated frequency band along a signal path, protective device  11  being disposed at any location along the signal path defined between the signal source and a low-voltage circuit in need of over-voltage protection. As a feature of the present invention, protective device  11  treats any potentially harmful, transient electromagnetic impulses present in signal path, thereby protecting the low-voltage circuit. 
     To initiate protection, protective device  11  is installed in the signal path between the signal source and the low-voltage circuit. Specifically, an RF cable in electrical connection with the signal source is connected to input connector  53 - 1 . Similarly, an RF cable in electrical connection with the low-voltage circuit in need of over-voltage protection is connected to output connector  53 - 2 . With device  11  installed in the manner set forth above, RF signals within the operational frequency band can be delivered to the low-voltage circuit from the signal source via transmission line  33 . 
     Upon receiving any very high electrical current along the signal path (e.g., as the result of a lightning strike), gas discharge tube  51  suppresses the potentially harmful energy and thereby protects the low-voltage circuit coupled to output terminal  35 - 2 . More modest, transient electromagnetic impulses received along the signal path are preferably treated by either filter  39  or clamping component  43 , as will be explained further below. 
     Specifically, any presence in the signal path of transient electromagnetic impulses that fall beneath the operational frequency band are blocked by capacitor  41  and thereby limited from being carried to either clamping component  43  or output terminal  35 - 2 , which is in turn connected to the low-voltage circuit in need of overvoltage protection. In this manner, by treating the lower frequency transient energy with filter  39 , less service is ultimately required of clamping component  41 , which is highly desirable. 
     By contrast, any presence in the signal path of transient electromagnetic impluses that fall within the operational frequency band (i.e., above the pass band frequency of filter  39 ) are efficiently limited by clamping component  43 . Furthermore, as a feature of the invention, the reverse polarity configuration of component  43  provides overvoltage protection against both positive and negative polarity voltage transients. 
     Because filter  39  operates as a high-pass filter that blocks transient electromagnetic impulses that have frequency content below the filter cut-off frequency, it is to be understood that adjustments to the filter cut-off frequency can be made by simply modifying the performance characteristics of filter  39 . 
     By optimizing the cut-off frequency of filter  39 , preferably with ample separation from the pass-band frequency, the size of the various diodes in clamping component  43  that protect output terminal  33 - 2  from in-band transient energy can be minimized. In particular, voltage-limiting component  43  is specifically rated to treat transient electromagnetic energy that falls within the operational frequency band. As a result, component  43  can be designed using relatively low ampere capacity diodes, which in turn have lower capacitance and thus higher maximum frequency capabilities. 
     Additional Embodiments and Design Modifications 
     It is to be understood that the embodiment described in detail above is intended to be merely exemplary and those skilled in the art shall be able to make numerous variations and modifications without departing from the spirit of the present invention. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims. 
     For instance, referring now to  FIG. 4 , there is shown a front perspective view of a second embodiment of a protective device constructed according to the teachings of the present invention, the protective device being identified generally by reference numeral  111 . 
     As can be seen, protective device  111  is similar to protective device  11  in that protective device  111  includes an enclosed housing, or casing,  113  into which is disposed an electrical circuit  115 , with electrical circuit  115  being designed principally to provide overvoltage protection to a low-voltage circuit. 
     Referring now to  FIG. 5 , electrical circuit  115  is similar to electrical circuit  15  in that electrical circuit  115  comprises (i) a transmission line, or through path,  133  that extends in electrical communication between an input, or exposed, terminal  135 - 1  and an output, or treated, terminal  135 - 2 , (ii) a filter  139  for treating high-voltage, transient electromagnetic impulses that fall primarily below the operational frequency band, (iii) a diode-based clamping component  143  to limit high-voltage, transient, electromagnetic impulses that fall within the operational frequency band, and (iv) a gas discharge tube (GDT)  151  to treat very high electrical current introduced to transmission line  133 . 
     Electrical circuit  115  differs from electrical circuit  15  in the construction of filter  139 . Specifically, filter  139  is represented herein as a second order L-C filter that includes a capacitor  153  located in series on transmission line  133  between terminals  135 - 1  and  135 - 2 , and an inductor  155  connecting transmission line  133 , at a location between input terminal  135 - 1  and capacitor  153 , to ground  137 . 
     During normal operation of electrical circuit  115 , the desired operational frequencies pass though filter  139  with minimal attenuation. The disturbing electrical energy present along the signal path that falls beneath the operational frequency range is blocked by series capacitor  153  and is conducted directly to ground  137  via inductor  155 , thereby preventing the energy from being carried to clamping component  143  or output terminal  135 - 2 . Furthermore, it should be noted that the inclusion of inductor  155  enables capacitor  153  to be more limited in voltage rating in many applications. 
     Additionally, inductor  155  serves to share any transient current shunted by GDT  151 , since inductor  155  and GDT  151  are connected in parallel. Sharing of the transient current by inductor  155  can increase the life of GDT  151  in the application. 
     Optimization of GDT Performance Using Inductive Beads 
     It has been found that the incorporation of gas discharge tubes into protective circuits (e.g., circuit  115 ) introduces a notable performance shortcoming, which will be explained in detail below. For illustrative purposes only, the shortcoming will be explained in connection with electrical circuit  115 . However, it is to be understood that the performance shortcoming to be explained below is not limited to electrical circuit  115 , but rather, is prevalent in various types of protective circuits that rely upon gas discharge tubes for treating unwanted transient energy. 
     Referring now to  FIG. 5 , gas discharge tube  151  functions to reduce the peak energy of disturbing transient pulses received at input terminal  135 - 1 , which is highly desirable. GDT  151 , as is the case with most conventional gas discharge tubes, is designed to ultimately clamp at a relatively low voltage level. However, it has been found that during the process of reducing the peak energy of disturbing transient pulses, gas discharge tubes often add frequency content to the reduced voltage output waveform. In other words, the resultant, or treated, transient energy received at output terminal  135 - 2  in response to operation of gas discharge tube  151  has a significantly lower peak voltage level but is often shifted into a different frequency range. 
     In fact, it has been found that the output waveform generated in response to activation of GDT  151  has a frequency that is shifted into the operational frequency band, particularly into the bands from 10 MHz to 1 GHz. Although electric circuit  115  is designed to handle unwanted transient energy that is shifted into the operational frequency band, it is nonetheless desirable to prevent signal content from being shifted into the operational frequency band for performance optimization purposes (e.g., to limit interference between the wanted and unwanted signal components). Higher order filters or band pass filters may be employed to achieve even more dramatic energy blocking, as will be illustrated in subsequent embodiments. 
     It has been found that the shift in signal content caused by operation of gas discharge tube  151  is largely the result of its short fall time when a GDT changes to the conductive state in response to a high voltage. Accordingly, by incorporating a component with a specific type of inductive and resistive impedance in series with GDT  151 , the output pulse generated in response to activation of GDT  151  will have a longer fall time. Consequently, by managing the fall time of GDT  151 , a reduction in the frequency shift of the remaining energy can be achieved, thereby maintaining the energy content closer to the original, lower frequency level, which is highly desirable. 
     For instance, referring now to  FIG. 6 , there is shown a third embodiment of a protective device constructed according to the teachings of the present invention, the protective device being identified generally by reference numeral  211 . 
     As can be seen, protective device  211  is similar to protective device  111  in that protective device  211  includes an enclosed housing, or casing,  213  into which is disposed an electrical circuit  215 , with electrical circuit  215  being designed principally to provide overvoltage protection to a low-voltage circuit. 
     Referring now to  FIG. 7 , electrical circuit  215  is similar to electrical circuit  115  in that electrical circuit  215  comprises (i) a transmission line, or through path,  233  that extends in electrical communication between an input, or exposed, terminal  235 - 1  and an output, or treated, terminal  235 - 2 , (ii) a filter  239  for treating high-voltage, transient electromagnetic impulses that fall primarily below the operational frequency band, (iii) a diode-based clamping component  243  to limit high-voltage, transient, electromagnetic impulses that fall within the operational frequency band, and (iv) a gas discharge tube (GDT)  251  to treat very high electrical current introduced to transmission line  233 . 
     Electrical circuit  215  differs from electrical circuit  115  in the construction of filter  239 . Specifically, filter  239  includes a capacitor  253  located in series on transmission line  133  between terminals  235 - 1  and  235 - 2 , and an inductor  255  connecting transmission line  233  to ground  237 . However, it should be noted that inductor  255  is connected to transmission line  233  at a location between capacitor  253  and output terminal  235 - 2 , rather than between capacitor  253  and input terminal  235 - 1 , so as to form a C-L low pass filter. 
     More significantly, electrical circuit  215  differs from electrical circuit  115  in that electrical circuit  215  includes an inductive component  257  in series with gas discharge tube  251 , with inductive component  257  being located between GDT  251  and ground  237 . 
     It should be noted that the inclusion of inductive component  257  runs counterintuitive to traditional circuit design, since the inductance, or impedance, of component  257  could limit the shunting capability of GDT  251 . However, as will be explained further below, inductive component  257  is preferably constructed of a material that (i) does not compromise either the performance or response time of GDT  251 , and (ii) increases the fall time of the treated transient energy. 
     Specifically, inductive component  257  is preferably constructed, at least in part, of a ferrite material. For instance, component  257  may be constructed using a nickel-zinc (NiZn) ferrite material currently available for sale by Fair-Rite Products Corp., of Willkill, N.Y., under the brand name 43 Material. 
     As can be appreciated, ferrite material exhibits two principal characteristics which are particular significance, namely, (i) the ferrite material functions as an inductive element when conducting signals of lower frequencies and, in turn, transitions to a resistive element with a nearly constant resistance when conducting signals of higher frequencies, and (ii) the ferrite material is a high permeability material, which in turn causes high current to saturate the core and thereby reduce inductance to a conductive element passed therethrough, such as a wire. 
     In the present embodiment, inductive component  257  comprises an annular ferrite bead  259  through which a wire  261  is fittingly passed in coaxial alignment therewith, as seen in  FIG. 6 . The free ends of wire  261  are connected to GDT  251  and ground  237 , as shown in  FIG. 7 , thereby rendering component  257  in series with GDT  251 . However, although not shown herein, it is to be understood that ferrite bead  259  could be alternatively mounted directly on a conductive lead for GDT  251 , thereby eliminating the need for wire  261  entirely. 
     In use, inductive component  257  interacts with GDT  251  in the following manner. Specifically, prior to the activation (i.e., shunting) of GDT  251 , any impulse voltage received at input terminal  235 - 1  is impressed substantially across GDT  251  due to its relatively high resistance. At the same time, any voltage generated across ferrite bead  259  from the input pulse is negligible, and does not interfere with GDT  251  initiating its short circuit shunting operation. 
     Upon initiation of activation by GDT  251 , the initial current flow associated with the transient energy is impressed across GDT  251  due to its resistive impedance. As a result, the fixed resistance of ferrite bead  259  would not produce an unlimited voltage drop, as an inductor would. Consequently, the increase in the total voltage across both GDT  251  and ferrite bead  259  would be adequately managed upon initial shunting of GDT  251 . 
     As current continues to flow to shunted GDT  251 , the impedance of ferrite bead  259  drops because (i) the frequency content for longer duration pulses is lower, and thus the impedance of bead  259  drops accordingly due to its inherent operational characteristics, and (ii) the increase in current saturates the ferrite material for bead  259 , and thus lowers its impedance. Ultimately, at very high currents, the only inductance exhibited by component  257  is the self-inductance of wire  261 . As a result, ferrite bead  259  does not dramatically increase voltage drop at high currents. 
     As noted briefly above, the characteristics exhibited by ferrite bead  259  in response to the activation of GDT  251  serve to preserve the initial rise time of the treated transient energy, and then, extend the fall time of the treated transient energy. This slowing of the fall time has the effect of dramatically shifting the frequency content of the output waveform to lower frequencies (i.e., frequencies beneath the operational frequency), which can therefore be blocked using common filtering techniques (e.g., by series capacitor  253 , which is connected to transmission line  233  after GDT  251 ). 
     For the most efficient operation of inductive component  257 , the inductive voltage of ferrite bead  259  should be selected based on the characteristics of gas discharge tube  251 . In particular, the inductive voltage of ferrite bead  259  selected for use in circuit  215  should be approximately the same, or somewhat less than, the peak voltage for GDT  251 . 
     Referring now to  FIG. 8 , there is shown a third embodiment of a protective device constructed according to the teachings of the present invention, the protective device being identified generally by reference numeral  311 . 
     As can be seen, protective device  311  is similar to protective device  211  in that protective device  311  includes an enclosed housing, or casing,  313  into which is disposed an electrical circuit  315 , with electrical circuit  315  being designed principally to provide overvoltage protection to a low-voltage circuit. 
     Referring now to  FIG. 9 , electrical circuit  315  is similar to electrical circuit  215  in that electrical circuit  315  comprises (i) a transmission line, or through path,  333  that extends in electrical communication between an input, or exposed, terminal  335 - 1  and an output, or treated, terminal  335 - 2 , (ii) a filter  339  for treating high-voltage, transient electromagnetic impulses that fall primarily below the operational frequency band, (iii) a diode-based clamping component  343  to limit high-voltage, transient, electromagnetic impulses that fall within the operational frequency band, and (iv) a gas discharge tube (GDT)  351  connected in series with an inductive component  357  to treat very high electrical current introduced to transmission line  333 , inductive component  357  comprising an annular ferrite bead  359  through which a wire  361  is fittingly passed in coaxial alignment therewith, as seen in  FIG. 8 . 
     Electrical circuit  315  differs from electrical circuit  215  in the construction of filter  339 . Specifically, filter  339  is provided with enhanced signal filtering capabilities and includes (i) a first capacitor  353  located in series on transmission line  333  between terminals  335 - 1  and  335 - 2 , (ii) a second capacitor  354  connected between transmission line  333 , at a location between capacitor  353  and input terminal  335 - 1 , and ground  357 , (iii) a first inductor  355  connected in parallel with second capacitor  354  (i.e., connected between transmission line  333 , at a location between capacitor  353  and input terminal  335 - 1 , and ground  357 ), and (iv) a second inductor  356  located in series on transmission line  333  between first capacitor  353  and second terminal  335 - 2 . As can be seen, filter  339  is a second order band pass filter, with the pass band selected according to the operational bandwidth. 
     As referenced briefly above, each of clamping components  43 ,  143 ,  243  and  343  need not be limited to a diode-based component. Rather, it is to be understood that each of clamping components  43 ,  143 ,  243  and  343  could be in the form of a metal-oxide varistor (MOV)-based component, a silicon-controlled rectifier (SCR)-based component, a protection thyristor-based component, or a triode for alternating current (TRIAC)-based component, all of which are silicone or solid-state voltage limiters with low capacitance so as not to hinder the highest frequency operation of the protective device in which it is incorporated (e.g., a component with a capacitance of 3.5 pF for operational frequencies up to 500 MHz, a capacitance of 1.0 pF for operational frequencies between 500 MHz and 2.0 GHz, and a capacitance of no greater than 0.5 pF for operational frequencies between 2.0 GHz and 6.0 GHz). In particular, each of clamping components  43 ,  143 ,  243  and  343  preferably represents any clamping component that is characterized as having a fast acting time (e.g., less than 1 ns response time) and low-voltage capabilities (e.g., less than 50 Vdc).