Abstract:
A high power band pass RF filtering device having a housing for containing a printed circuit board with filtering components for achieving strong attenuation of out-of-band signals. An input port and an output port on the housing electrically connect to a respective input node and output node on the printed circuit board. Surge protection elements are connected at the input port and at the output port for dissipating surge conditions present at the input port or the output port to the housing before the surge travels through the printed circuit board. A non-surge signal present on the input port can travel through the filtering components on the printed circuit board towards the output port. An oil or other fluid is disposed and completely contained within the housing and contacts the printed circuit board for cooling the printed circuit board or the filtering components.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit and priority of U.S. Provisional Application No. 61/331,292, filed on May 4, 2010, the entire contents of which are hereby incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field 
     The present invention relates generally to band pass RF filters and improvements thereof. More particularly, the invention relates to high power band pass RF filters with surge protection elements and improvements thereof. 
     2. Description of the Related Art 
     Band pass RF filters for use in electronic circuits or between systems or devices are known and used in the art. In-line RF filter devices are similarly known and used in the art. Often in electrical systems, it is desirable to control signal frequencies to a desired range of frequency values. Band pass filters can be used for such purposes by rejecting or attenuating frequencies outside the desired range. In-line band pass filter devices connected along a conductive path between a source and a connecting system will only pass the desired range of frequencies to the connecting system. Signal frequencies outside of the desired range would ideally be highly attenuated. A band pass filter should have as flat of a pass-band as possible so passed signals experience little to no attenuation. A band pass filter should also transition from the pass-band to outside the pass-band with a sharp roll-off, narrow in frequency, to limit the passing of partially attenuated signal frequencies existing outside the pass-band. 
     As systems and electronics increase in complexity and size, power requirements can increase as well. Even in simple systems or devices, large amounts of power may be required or transmitted along signal wires or transmission cables. Operating frequency requirements are often still present in such systems, illustrating the need for frequency filtering devices capable of operating at these increased power levels. Surge events, particularly in such high power applications, necessitate additional considerations since the filtering electronics may be subjected to significant over-voltage or over-current conditions. Thus, an ideal electronic filtering device for such applications would strongly attenuate out-of-band signals while performing little attenuation to in-band signals, operate in high power applications, manage surge conditions present at the device to prevent damage and have a low manufacturing cost. 
     SUMMARY 
     A preferred embodiment of the present invention is an electronic filtering device including a printed circuit board for filtering a signal connected to the electronic filtering device. Signals operating outside of the device&#39;s designed frequency band are highly attenuated while signals operating within the frequency band experience little attenuation. The electronic filtering device includes a fluid-sealed housing defining a cavity therein for containing the printed circuit board. Two connector assemblies acting as connection terminals are secured to the housing. One connector assembly is connected as an input to the printed circuit board and the other connector assembly is connected as an output to the printed circuit board. Thus, a signal present on one connector assembly can travel through the printed circuit board to the other connector assembly for filtering of the signal. A fluid, such as oil, is disposed in the cavity with the printed circuit board and makes contact with the printed circuit for cooling purposes. Additionally, surge protection elements, such as gas tubes, are integrated with the connector assemblies for dissipating any surges seen at the connector assemblies before the surges can be transmitted through to the printed circuit board. 
     By positioning the printed circuit board in the cavity of the housing with the cooling fluid, the electronic filtering device can operate with higher power capabilities than traditional filters due to dissipation of the additional heat from the increased voltage or current levels by the cooling fluid. Use of the cooling fluid also helps keep manufacturing costs down since the electronic filtering device can dissipate heat without being substantially expanded in size to accommodate fans or other bulky heat-sink devices coupled to the printed circuit board. Moreover, as power levels increase, surge protection becomes more desirable and the easily serviceable surge protection element integrated into the device protects the filtering circuit from damage, making the electronic filtering device attractive for use in industry. 
     The electronic filtering device is also easily adaptable to alternative filtering circuits. With both the cooling provisions and the surge protection capabilities separate from the manufacturing or design of the printed circuit board, alternative circuit designs can easily be incorporated onto a printed circuit board for inclusion in the housing without requiring substantial redesign of other components making up the electronic filtering device. This not only allows for the possibility of designing customer-specific filtering circuits for incorporation into the housing at a lower cost, but also allows for alternative circuit product line expansion at lower engineering or manufacturing expense. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other systems, methods, features, and advantages of the present invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. Component parts shown in the drawings are not necessarily to scale, and may be exaggerated to better illustrate the important features of the present invention. In the drawings, like reference numerals designate like parts throughout the different views, wherein: 
         FIG. 1  shows different sealed views of an RF surge protector according to an embodiment of the invention; 
         FIG. 2  is a schematic circuit diagram of a high power band pass RF filter according to an embodiment of the invention; 
         FIG. 3  is a disassembled view of an RF surge protector housing the circuit described in  FIG. 2  according to an embodiment of the invention; 
         FIG. 4  is a disassembled view of a connector assembly according to an embodiment of the invention; 
         FIG. 5  is a top graph of the input in-band return loss and a bottom graph of the input in-band insertion loss of the RF surge protector of  FIG. 3  according to an embodiment of the invention; 
         FIG. 6  is a top graph of the output in-band return loss and a bottom graph of the output in-band insertion loss of the RF surge protector of  FIG. 3  according to an embodiment of the invention; 
         FIG. 7  is a graph of the input out-of-band insertion loss of the RF surge protector of  FIG. 3  according to an embodiment of the invention; 
         FIG. 8  is a graph of the output out-of-band insertion loss of the RF surge protector of  FIG. 3  according to an embodiment of the invention; 
         FIG. 9  is an alternative schematic circuit diagram of a high power band pass RF filter according to an embodiment of the invention; 
         FIG. 10  is a disassembled view of an RF surge protector housing the circuit described in  FIG. 9  according to an embodiment of the invention; 
         FIG. 11  is a top graph of the input in-band return loss and a bottom graph of the input in-band insertion loss of the RF surge protector of  FIG. 10  according to an embodiment of the invention; 
         FIG. 12  is a top graph of the output in-band return loss and a bottom graph of the output in-band insertion loss of the RF surge protector of  FIG. 10  according to an embodiment of the invention; 
         FIG. 13  is a graph of the input out-of-band insertion loss of the RF surge protector of  FIG. 10  according to an embodiment of the invention; and 
         FIG. 14  is a graph of the output out-of-band insertion loss of the RF surge protector of  FIG. 10  according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , a sealed RF surge protector  100  is shown from three perspectives: an angled perspective, a side perspective and a front perspective. The RF surge protector  100  has two connection terminals positioned on a housing of the RF surge protector  100 . By connecting a first cable to the first connection terminal and a second cable to the second connection terminal, voltages and currents can flow from the first cable, through the RF surge protector  100  and to the second cable or vice versa. In the preferred embodiment, the housing is approximately 13 inches tall, 6 inches wide and 3.5 inches deep. 
     Surge conditions at the connection terminals are responded to by dissipating the surge to the housing of the RF surge protector  100 , as described in greater detail herein. In this manner, only the desired current and voltage levels are passed between the two connection terminals and helps prevent damage to any filtering components of the RF surge protector  100 . The RF surge protector  100  contains various electronic and mechanical parts as part of its manufacturing, these electronic and mechanical parts shown and discussed in greater detail herein. 
       FIG. 2  shows a schematic circuit diagram  200  of a high power band pass RF filter. The band pass filter includes a number of different electrical components, such as capacitors and inductors, attached or mounted to a printed circuit board  313  (see  FIG. 3 ). For illustrative purposes, the schematic circuit diagram  200  will be described with reference to specific capacitance and inductance values to achieve specific RF band pass frequencies of operation and power requirements. However, other specific capacitance and inductance values or configurations may be used to achieve other RF band pass characteristics. Moreover, other electronic filters (e.g., low pass filters, high pass filters or band stop filters) may also be achieved in place of the band pass filter. Characteristics of the band pass circuit described by schematic circuit diagram  200  include an operating frequency range of 160 to 174 MHz, a nominal impedance of 50Ω, an average input power of 200 W, a max peak insertion loss in bandwidth of 1.5 dB, an average insertion loss ripple in bandwidth of 0.7 dB, a max return loss in bandwidth of 17 dB, an operating temperature of −40° C. to 85° C. and a turn-on voltage of ±300V±20%. 
     An input port  202  and an output port  204  are shown on the left and right sides of the schematic circuit diagram  200 . Various components are coupled between the input port  202  and the output port  204 . A signal applied at the input port  202  travels through the various components to the output port  204 . The schematic circuit diagram  200  can also operate in a bi-directional mode, hence the input port  202  can function as an output port and the output port  204  can function as an input port. 
     The schematic circuit diagram  200  operates as a high power band pass filter with an operating frequency range between 160 MHz and 174 MHz. Signals outside of this frequency range or pass-band are attenuated. For example, the schematic circuit diagram  200  provides greater than 80 dB of attenuation at 15.4 MHz and greater than 50 dB of attenuation at 1 GHz, as described in greater detail for  FIGS. 7 and 8  herein. In addition, the schematic circuit diagram  200  produces sharp roll-offs of signals at the pass-band transitions, which is desirable for band pass filters. 
     Frequency performance of the schematic circuit diagram  200  includes a desirable high return loss of greater than 20 dB within the operating frequency range of 160 to 174 MHz. Likewise, a desirable low insertion loss of less than 0.4 dB is obtained within the operating frequency range of 160 to 174 MHz. By contrast, for signals at frequencies outside the operating range, the insertion loss is greater than 80 dB at 15.4 MHz and is greater than 50 dB at 1.0 GHz as stated above. Thus, the out-of-band frequencies are highly attenuated. 
     Turning more specifically to the various components used in the schematic circuit diagram  200 , the input port  202  has a center pin  203  connected at an input node of the circuit and the output port  204  has a center pin  205  connected at an output node of the circuit. The connection at the input port  202  and the output port  204  may be a center conductor such as a coaxial line where the center pins  203  and  205  propagate the dc currents and the RF signals and an outer shield surrounds the center pins. The center conductor enables voltages and currents to flow through the circuit. So long as the voltages are below surge protection levels, currents will flow between the input port  202  and the output port  204  and the voltages at each end will be similar. The center pins  203  and  205  also maintain the system RF impedance (e.g., 50Ω, 75Ω, etc.). This configuration is a DC block topology as seen by the series capacitors. By utilizing a different band pass circuit with series inductors and shunt capacitors, a dc pass filter may be achieved. The dc voltage on the center pins  203  and  205  would be used as the operating voltage to power the electronic components that are coupled to the output port  204 . 
     The schematic circuit diagram  200  includes four sets of capacitors ( 206  and  208 ,  222  and  224 ,  238  and  240 ,  250  and  252 ). Each of the four sets is placed in a parallel circuit configuration. The four sets of capacitors are used to increase the power handling capabilities of the circuit. For example, the circuit shown by schematic circuit diagram  200  can handle up to 250 watts of power. The capacitors  206 ,  208 ,  250  and  252  have values of approximately 120 picoFarads (pF) each. The capacitors  222 ,  224 ,  238  and  240  have values of approximately 3.3 picoFarads (pF) each. Additional capacitors are utilized in the schematic circuit diagram  200  for attenuating the out-of-band frequencies or signals. Two sets of series capacitors ( 210  and  212 ,  254  and  256 ) are used for this purpose and have values of approximately 2.2 picoFarads (pF) each. 
     The schematic circuit diagram  200  also includes four inductors  214 ,  226 ,  236  and  246  positioned in series between the input port  202  and the output port  204 . The four inductors  214 ,  226 ,  236  and  246  are used for in-band tuning of the circuit. The inductors  214  and  246  each have a calculated low inductance value, substantially a short, in-air. The inductors  226  and  236  have calculated values of approximately 200 nanoHenries (nH) each in-air. The above inductor values may substantially change when immersed in oil  315  (see  FIG. 3 ) as opposed to in-air. 
     Preferably, three tuning sections  215 ,  225  and  235  are used to tune the band pass stage of the circuit. Additional or fewer tuning sections may be used in an alternative embodiment. The first tuning section  215  includes an inductor  216  and capacitors  218  and  220 . The second tuning section  225  includes an inductor  234  and capacitors  228 ,  230  and  232 . The third tuning section  235  includes an inductor  248  and capacitors  242  and  244 . The inductors  216 ,  234  and  248  have calculated values of approximately 100 nanoHenries (nH) each in-air. Similar to the above, the inductor values may be different when immersed in oil  315  (see  FIG. 3 ). The capacitors  218 ,  220 ,  230 ,  242  and  244  have values of approximately 10 picoFarads (pF) each. The capacitors  228  and  232  have values of approximately 27 picoFarads (pF) each. As shown, the three tuning sections  215 ,  225  and  235  are grounded to a common ground  258 , which can be connected to the housing of the RF surge protector  300  (see  FIG. 3 ). In an alternative embodiment, different components or component values may be used to obtain alternative filter characteristics. 
     Referring now to  FIG. 3 , a disassembled view of an RF surge protector  300  is shown housing the circuit described in  FIG. 2  according to an embodiment of the invention. The RF surge protector  300  has a housing  302  defining a cavity  319 . The components shown by schematic circuit diagram  200  (see  FIG. 2 ) are mounted or included on a printed circuit board  313  and the printed circuit board  313  is positioned within the cavity  319 . The printed circuit board  313  is fastened to the housing  302  by a plurality of screws  312 . In an alternative embodiment, other fasteners may be used to couple the printed circuit board  313  to the housing  302  or no fasteners may be needed. 
     The printed circuit board  313  electrically connects to a connector assembly  301  secured to a portion of the housing  302 . The connector assembly  301  functions as the input port  202  shown on the schematic circuit diagram  200  (see  FIG. 2 ) and as a first connection terminal of the RF surge protector  300 . Similarly, another connector assembly  301  secured to a portion of the housing  302  is electrically connected to the printed circuit board  313  and functions as the output port  204  shown on the schematic circuit diagram  200  (see  FIG. 2 ) and as a second connection terminal of the RF surge protector  300 . Additional details on the connector assembly  301  are discussed herein for  FIG. 4 . 
     One or more walls or sidebars  317  are attached to the printed circuit board  313  and extend in a direction that is perpendicular to a plane defined by the printed circuit board  313 . The sidebars  317  are positioned on one or more sides of the printed circuit board  313  and are used to help isolate the RF signals, enhance the grounding of the printed circuit board  313  or provide a larger surface area for dissipation of heat. In one embodiment, the sidebars  317  are about 0.5 inches high and are made of a copper material. In an alternative embodiment, different dimensions, positioning or materials may be used or the sidebars  317  may be omitted completely. 
     The cavity  319  defined by the housing  302  is filled with an oil  315  for dissipating heat caused by heating of the components (e.g., capacitors and inductors) on the printed circuit board  313 . Preferably, the oil  315  is STO-50, a silicon transformer oil. In an alternative embodiment, the oil  315  may be any silicone, mineral, synthetic or other oil, fluid or substance capable of adequately dissipating the heat generated on or by the printed circuit board  313 . Preferably, the cavity  319  is filled with approximately 23 ounces of the oil  315  and the oil  315  is capable of reducing the temperature of the components from about 120° C. to about 80° C. The cavity  319  or the housing  302  are completely fluid-sealed in order to contain the oil  315  within the housing  302  without leaking. Preferably, the oil  315  substantially fills the entire cavity  319  in order to completely submerge the printed circuit board  313  in the oil  315 . In an alternative embodiment, the cavity  319  may be filled with different volumes of the oil  315 . 
     The RF surge protector  300  includes one or more cylindrical cavities  320  in the housing  302  for the placement of piston springs  305  and pistons  306  that are coupled with O-rings  307  to aid in sealing. In an alternative embodiment, other shapes for the cavities  320  may be used. The piston springs  305  and pistons  306  allow the oil  315  to expand and are used to exert a constant pressure within the cavity  319  when a cover  309  is attached to the housing  302 . The cover  309  is sealed with the housing  302  using an O-ring  308  and a plurality of cover screws  310 . The piston springs  305  and pistons  306  are sealed from the oil  315  using O-rings  307 . Alternatively, the one or more cylindrical cavities  320  can be used as overflow cavities for any excess oil  315  from the cavity  319  due to heating and expanding of the oil  315 . O-rings  303  and additional openings in the housing  302  for containing set screws  304  help secure the connector assembly  301  to the housing  302 . 
     The RF surge protector  300  preferably includes a closed cell foam material  316  attached to a surface of the cover  309  to disrupt the oil&#39;s dielectric constant and keep high frequency out-of-band signals from reflecting within the cavity  319  causing signal interferences. The foam material  316  is sized to cover the entire opening formed by the cavity  319 . The RF surge protector  300  also includes a label  311  attached to the cover  309  with identification, electrical, mechanical, safety or other information or parameters pertaining to the RF surge protector  300 . In addition, a hardware kit  314  is shown with various parts used in the assembly of the RF surge protector  300  to allow for parts replacement. 
       FIG. 4  shows a disassembled view of the connector assembly  301  discussed in  FIG. 3  according to an embodiment of the invention. One connector assembly  301  is attached to each end of the housing  302  as described above (see  FIG. 3 ). The connector assembly  301  has a conductive element or center pin  412  extending from one end of the connector assembly  301 , the center pin  412  connecting to the printed circuit board  313  (see  FIG. 3 ) either as the input center pin  203  or the output center pin  205  depending upon whether the connector assembly  301  is connected as the input port  202  or the output port  204  (see  FIG. 2 ). Preferably, the center pin  412  is electrically connected to the printed circuit board  313  via a solder connection. 
     The connector assembly  301  includes a connector housing  405  defining a connector cavity  414 . A gas tube  402  is positioned within a non-conductive tube  404  (e.g., a plastic or PTFE tube) and both are positioned within the connector cavity  414  of the connector housing  405 . The gas tube  402  is secured in the connector cavity  414  with a gas tube retaining screw  401  and a washer  403 . The non-conductive tube  404  isolates a portion of the gas tube  402  from the connector housing  405  to prevent shorting to ground or unintended contact between the portion of the gas tube  402  and the connector housing  405  (e.g., ground). The gas tube  402  is integrated into the connector housing  405  and does not come into contact with the oil  315  contained within the housing  302  (see  FIG. 3 ). In one embodiment, the gas tube  402  is a three-terminal, dual-chambered device wherein each chamber has a breakdown voltage of approximately 150 volts, each chamber being used serially and thus additive to 300 volts. This serial arrangement puts the capacitances inherent in the gas tube  402  in series, resulting in lower total capacitance and thus better RF performance. In an alternative embodiment, a different gas tube  402  or configuration may be used or determined from transmit power requirements. 
     When the gas tube  402  is within the connector cavity  414 , the gas tube electrically connects with the center pin  412  for dissipating surge conditions present on the center pin  412  through the gas tube  402  and to the connector housing  405 . In an alternative embodiment, other surge protection elements may be used in place of or in addition to the gas tube  402  for dissipating a surge present upon the center pin  412 . The center pin  412  is integrated with the connector assembly  301  by engaging with an internal pin  407  and coupled with a plurality of inserts ( 406 ,  408  and  410 ) and a plurality of O-rings ( 409 ,  411  and  413 ). Preferably, insert  406  is made of Teflon and inserts  408  and  410  are made of PTFE. In an alternative embodiment, other materials may be used. 
     Referring now to  FIG. 5  and  FIG. 6 , graphs are displayed showcasing in-band operating characteristics of the input and the output of the circuit shown by schematic circuit diagram  200 . Graph  500  (see  FIG. 5 ) shows the input in-band return loss and graph  600  (see  FIG. 6 ) shows the output in-band return loss. For signals operating at frequencies within the pass-band of the filter shown by schematic circuit diagram  200 , a high return loss (e.g., at least 20 dB) is desirable. The circuit shown by schematic circuit diagram  200  has been configured for an operating frequency range of 160 to 174 MHz as described above for  FIG. 2 . Input data-point  502  (see  FIG. 5 ) indicates around 25 dB of return loss at 160 MHz. Input data-point  504  (see  FIG. 5 ) indicates around 26 dB of return loss at 174 MHz. Similarly, output data-point  602  (see  FIG. 6 ) indicates around 26 dB of return loss at 160 MHz and output data-point  604  (see  FIG. 6 ) indicates around 24 dB of return loss at 174 MHz. 
     For signals operating at frequencies within the pass-band of the filter shown by schematic circuit diagram  200 , a low insertion loss (e.g., less than 0.4 dB) is also desirable for limiting the attenuation of pass-band signals. Graph  510  (see  FIG. 5 ) shows the input in-band insertion loss and graph  610  (see  FIG. 6 ) shows the output in-band insertion loss. Input data-point  512  (see  FIG. 5 ) indicates around 0.24 dB of insertion loss at 160 MHz. Input data-point  514  (see  FIG. 5 ) indicates around 0.29 dB of insertion loss at 174 MHz. Similarly, output data-point  612  (see  FIG. 6 ) indicates around 0.24 dB of insertion loss at 160 MHz and output data-point  614  (see  FIG. 6 ) indicates around 0.29 dB of insertion loss at 174 MHz. 
       FIG. 7  and  FIG. 8  display graphs showcasing out-of-band operating characteristics of the input and the output of the circuit shown by schematic circuit diagram  200 . Since the circuit shown by schematic circuit diagram  200  has been configured for an operating frequency range of 160 to 174 MHz, data-points at frequencies outside that pass-band are chosen for examples of out-of-band insertion loss. A high insertion loss (e.g., at least 50 dB) is desirable for out-of-band signals since out-of-band signals are to be highly attenuated. 
     Graph  700  (see  FIG. 7 ) shows the input out-of-band insertion loss and graph  800  (see  FIG. 8 ) shows the output out-of-band insertion loss. Input data-point  702  (see  FIG. 7 ) indicates around 85 dB of insertion loss at 15.4 MHz. Input data-point  708  (see  FIG. 7 ) indicates around 68 dB of insertion loss at 1 GHz. Similarly, output data-point  802  (see  FIG. 8 ) indicates around 90 dB of insertion loss at 15.4 MHz and output data-point  808  (see  FIG. 8 ) indicates around 69 dB of insertion loss at 1 GHz. As described above for  FIG. 5  and FIG.  6 , in-band insertion loss for input and output signals with frequencies of 160 to 174 MHz is low as shown by input data-points  704  and  706  (see  FIG. 7 ) and output data-points  804  and  806  (see  FIG. 8 ). 
     Turning now to  FIG. 9 , an alternate schematic circuit diagram  900  of a high power band pass RF filter is shown. Similar to  FIG. 2 , the band pass filter of schematic circuit diagram  900  includes a number of different electrical components, such as capacitors and inductors that are mounted or included on a printed circuit board  1013  (see  FIG. 10 ). For illustrative purposes, the schematic circuit diagram  900  will be described with reference to specific capacitance and inductance values to achieve specific RF band pass frequencies of operation and power requirements. However, other specific capacitance and inductance values and configurations may be used to achieve other RF band pass characteristics. The circuit described by schematic circuit diagram  900  has an operating frequency range of 225 to 400 MHz, a nominal impedance of 50Ω, an average input power of 250 W, a max peak insertion loss in bandwidth of 1.5 dB, an average insertion loss ripple in bandwidth of 0.7 dB, a max return loss in bandwidth of 14 dB, an operating temperature of −40° C. to 85° C. and a turn-on voltage of ±300V±20%. 
     An input port  902  and an output port  904  are shown on the left and right sides of the schematic circuit diagram  900 . Various components are coupled between the input port  902  and the output port  904 . A signal applied at the input port  902  travels through the various components to the output port  904 . The schematic circuit diagram  900  can also operate in a bi-directional mode, hence the input port  902  can function as an output port and the output port  904  can function as an input port. 
     The schematic circuit diagram  900  operates as a high power band pass filter with an operating frequency range between 225 MHz and 400 MHz. Signals outside of this frequency range or pass-band are highly attenuated. For example, the schematic circuit diagram  900  provides greater than 80 dB of attenuation at 10 MHz and greater than 40 dB of attenuation at 1 GHz, as described in greater detail for  FIGS. 13 and 14  herein. In addition, the schematic circuit diagram  900  produces sharp roll-offs of signals at the pass-band transitions, which is desirable for band pass filters. 
     Frequency performance of the schematic circuit diagram  900  includes a desirable high return loss of greater than 17 dB within the operating frequency range of 225 to 400 MHz. Likewise, a preferably low insertion loss of less than or equal to 0.4 dB is obtained within the operating frequency range of 225 to 400 MHz. By contrast, for signals at frequencies outside the operating range, the insertion loss is greater than 80 dB at 10 MHz and is greater than 40 dB at 1 GHz as stated above. Thus, the out-of-band frequencies are highly attenuated. 
     Turning more specifically to the various components used in the schematic circuit diagram  900 , the input port  902  has a center pin  903  connected at an input node of the circuit and the output port  904  has a center pin  905  connected at an output node of the circuit. The connection at the input port  902  and the output port  904  may be a center conductor such as a coaxial line where the center pins  903  and  905  propagate the dc currents and the RF signals and an outer shield surrounds the center pins. The center conductor enables voltages and currents to flow through the circuit. So long as the voltages are below surge protection levels, currents will flow between the input port  902  and the output port  904  and the voltages at each end will be similar. The center pins  903  and  905  also maintain the system RF impedance (e.g., 50Ω, 75Ω, etc.). This configuration is a DC block topology as seen by the series capacitors. By utilizing a different band pass circuit with series inductors and shunt capacitors, a dc pass filter may be achieved. The dc voltage on the center pins  903  and  905  would be used as the operating voltage to power the electronic components that are coupled to the output port  904 . 
     The schematic circuit diagram  900  includes four sets of capacitors ( 906  and  908 ,  922  and  924 ,  938  and  940 ,  950  and  952 ). Each of the four sets is placed in a parallel circuit configuration. The four sets of capacitors are used to increase the power handling capabilities of the circuit. For example, the circuit shown by schematic circuit diagram  900  can handle up to 250 watts of power. The capacitors  906 ,  908 ,  950  and  952  have values of approximately 12 picoFarads (pF) each. The capacitors  922 ,  924 ,  938  and  940  have values of approximately 8.2 picoFarads (pF) each. 
     The schematic circuit diagram  900  also includes four inductors  914 ,  926 ,  936  and  946  positioned in series between the input port  902  and the output port  904 . The four inductors  914 ,  926 ,  936  and  946  are used for in-band tuning of the circuit. The inductors  914 ,  926 ,  936  and  946  have calculated values of approximately 15 nanoHenries (nH) each in-air. The above inductor values may substantially change when immersed in oil  315  (see  FIG. 10 ) as opposed to in-air. 
     Preferably, three tuning sections  915 ,  925  and  935  are used to tune the band-pass stage of the circuit. Additional or fewer tuning sections may be used in an alternative embodiment. The first tuning section  915  includes an inductor  916  and capacitors  918  and  920 . The second tuning section  925  includes inductors  934  and  928  and capacitors  930  and  932 . The third tuning section  935  includes an inductor  948  and capacitors  942  and  944 . The inductors  916  and  948  have calculated values of approximately 75 nanoHenries (nH) each in-air. The inductor  934  has a calculated value of approximately 100 nanoHenries (nH) in-air. The inductor  928  has a calculated value of approximately 15 nanoHenries (nH) in-air. Similar to the above, the inductor values may be different when immersed in oil  315  (see  FIG. 10 ). The capacitors  918 ,  920 ,  942  and  944  have values of approximately 2.2 picoFarads (pF) each. The capacitors  930  and  932  have values of approximately 8.2 picoFarads (pF) each. As shown, the three tuning sections  915 ,  925  and  935  are grounded to a common ground  958 , which can be connected to the housing of the RF surge protector  1000  (see  FIG. 10 ). In an alternative embodiment, different components or component values may be used to obtain different band-pass characteristics. 
     Referring now to  FIG. 10 , a disassembled view of an RF surge protector  1000  is shown housing the circuit described in  FIG. 9  according to an embodiment of the invention. The RF surge protector  1000  is similar in construction to the RF surge protector  300  described in  FIG. 3  and utilizes many of the same component parts. The RF surge protector  1000  includes the housing  302  defining the cavity  319 . The components shown by schematic circuit diagram  900  (see  FIG. 9 ) are mounted or included on a printed circuit board  1013  and the printed circuit board  1013  is positioned within the cavity  319 . The printed circuit board  1013  is fastened to the housing  302  by the plurality of screws  312 . In an alternative embodiment, other fasteners may be used to couple the printed circuit board  1013  to the housing  302  or no fasteners may be needed. 
     The printed circuit board  1013  electrically connects to the connector assembly  301  secured to a portion of the housing  302 . The connector assembly  301  functions as the input port  902  shown on the schematic circuit diagram  900  (see  FIG. 9 ) and as the first connection terminal of the RF surge protector  1000 . Similarly, another connector assembly  301  secured to a portion of the housing  302  is electrically connected to the printed circuit board  1013  and functions as the output port  904  shown on the schematic circuit diagram  900  (see  FIG. 9 ) and as the second connection terminal of the RF surge protector  1000 . 
     The cavity  319  defined by the housing  302  is filled with the oil  315  for dissipating heat caused by heating of the components (e.g., capacitors and inductors) on the printed circuit board  1013 . Preferably, the oil  315  is STO-50, a silicon transformer oil. In an alternative embodiment, the oil  315  may be any silicone, mineral, synthetic or other oil, fluid or substance capable of adequately dissipating the heat generated on the printed circuit board  1013 . Preferably, the cavity  319  is filled with approximately 23 ounces of the oil  315  and the oil  315  is capable of reducing the temperature of the components from about 120° C. to about 80° C. The cavity  319  or the housing  302  are completely fluid-sealed in order to contain the oil  315  within the housing  302  without leaking. Preferably, the oil  315  substantially fills the entire cavity  319  in order to completely submerge the printed circuit board  1013  in the oil  315 . In an alternative embodiment, the cavity  319  may be filled with different volumes of the oil  315 . 
     The RF surge protector  1000  includes one or more cylindrical cavities  320  in the housing  302  for the placement of piston springs  305  and pistons  306  that are coupled with O-rings  307  to aid in sealing. In an alternative embodiment, other shapes for the cavities  320  may be used. The piston springs  305  and pistons  306  allow the oil  315  to expand and are used to exert a constant pressure within the cavity  319  when a cover  309  is attached to the housing  302 . The cover  309  is sealed with the housing  302  using an O-ring  308  and a plurality of cover screws  310 . The piston springs  305  and pistons  306  are sealed from the oil  315  using O-rings  307 . Alternatively, the one or more cylindrical cavities  320  can be used as overflow cavities for any excess oil  315  from the cavity  319  due to heating and expanding of the oil  315 . O-rings  303  and additional openings in the housing  302  for containing set screws  304  help secure the connector assembly  301  to the housing  302 . 
     The RF surge protector  1000  preferably includes a closed cell foam material  316  attached to an inner surface of the housing  302  to disrupt the oil&#39;s dielectric constant and keep high frequency out-of-band signals from reflecting within the cavity  319  causing signal interferences. The foam material  316  is sized to cover the entire opening formed by the cavity  319 . The RF surge protector  1000  also includes a label  1011  attached to the cover  309  with identification, electrical, mechanical, safety or other information or parameters pertaining to the RF surge protector  1000 . In addition, a hardware kit  314  is shown with various parts used in the assembly of the RF surge protector  1000  to allow for parts replacement. 
     Referring now to  FIG. 11  and  FIG. 12 , graphs are displayed showcasing in-band operating characteristics of the input and the output of the circuit shown by schematic circuit diagram  900 . Graph  1100  (see  FIG. 11 ) shows the input in-band return loss and graph  1200  (see  FIG. 12 ) shows the output in-band return loss. For signals operating at frequencies within the pass-band of the filter shown by schematic circuit diagram  900 , a high return loss (e.g., at least 17 dB) is desirable. The circuit shown by schematic circuit diagram  900  has been configured for an operating frequency range of 225 to 400 MHz as described above for  FIG. 9 . Input data-point  1102  (see  FIG. 11 ) indicates around 23 dB of return loss at 225 MHz. Input data-point  1104  (see  FIG. 11 ) indicates around 22 dB of return loss at 400 MHz. Similarly, output data-point  1202  (see  FIG. 12 ) indicates around 23 dB of return loss at 225 MHz and output data-point  1204  (see  FIG. 12 ) indicates around 23 dB of return loss at 400 MHz. 
     For signals operating at frequencies within the pass-band of the filter shown by the circuit shown in schematic circuit diagram  900  (see  FIG. 9 ), a low insertion loss (e.g., less than or equal to 0.4 dB) is also desirable to limit the attenuation of pass-band signals. Graph  1110  (see  FIG. 11 ) shows the input in-band insertion loss and graph  1210  (see  FIG. 12 ) shows the output in-band insertion loss. Input data-point  1112  (see  FIG. 11 ) indicates around 0.18 dB of insertion loss at 225 MHz. Input data-point  1114  (see  FIG. 11 ) indicates around 0.24 dB of insertion loss at 400 MHz. Similarly, output data-point  1212  (see  FIG. 12 ) indicates around 0.18 dB of insertion loss at 225 MHz and output data-point  1214  (see  FIG. 12 ) indicates around 0.24 dB of insertion loss at 400 MHz. 
       FIG. 13  and  FIG. 14  display graphs showcasing out-of-band operating characteristics of the input and the output of the circuit shown by schematic circuit diagram  900 . Since the circuit shown by schematic circuit diagram  900  has been configured for an operating frequency range of 225 to 400 MHz, data-points at frequencies outside that pass-band are chosen for examples of out-of-band insertion loss. A high insertion loss (e.g., at least 40 dB) is desirable for out-of-band signals since out-of-band signals are to be highly attenuated. 
     Graph  1300  (see  FIG. 13 ) shows the input out-of-band insertion loss and graph  1400  (see  FIG. 14 ) shows the output out-of-band insertion loss. Input data-point  1302  (see  FIG. 13 ) indicates around 86 dB of insertion loss at 10 MHz. Input data-point  1308  (see  FIG. 13 ) indicates around 46 dB of insertion loss at 1 GHz. Similarly, output data-point  1402  (see  FIG. 14 ) indicates around 96 dB of insertion loss at 10 MHz and output data-point  1408  (see  FIG. 14 ) indicates around 46 dB of insertion loss at 1 GHz. As described above for  FIG. 11  and  FIG. 12 , in-band insertion loss for input and output signals with frequencies of 225 to 400 MHz is low as shown by input data-points  1304  and  1306  (see  FIG. 13 ) and output data-points  1404  and  1406  (see  FIG. 14 ). 
     Exemplary embodiments of the invention have been disclosed in an illustrative style. Accordingly, the terminology employed throughout should be read in a non-limiting manner. Although minor modifications to the teachings herein will occur to those well versed in the art, it shall be understood that what is intended to be circumscribed within the scope of the patent warranted hereon are all such embodiments that reasonably fall within the scope of the advancement to the art hereby contributed, and that that scope shall not be restricted, except in light of the appended claims and their equivalents.