Abstract:
An input N-plexer filter stage is susceptible to receive surge energy via an input conductor, when the surge energy occurs. A high-pass filter included in a diplexer filter stage applies a radio frequency (RF) signal in a signal path between the input N-plexer filter stage and an output of a Data Over Cable Service Interface Specification (DOCSIS) transmitter stage. A low-pass filter included in the diplexer filter stage couples a surge suppressing threshold device to the signal path to dissipate the surge energy in the surge suppressing threshold device, when the surge energy occurs. The low-pass filter has a cut-off frequency that is lower than a normal operation frequency range of the RF signal. The low-pass filter isolates the surge suppressing threshold device from the signal path in normal operation frequency range of the RF signal, when no surge energy is applied.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a surge protector coupled to a transmission line connector. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the cable network example, the cable head end typically provides input signals to, for example, a set-top box. A multiplexing filter forms an input/output stage of the set-top box. The input signals, applied via a transmission line, may contain, for example, television signals. In a, so-called, Data Over Cable Service Interface Specification (DOCSIS) return channel, a transmitter generates an output signal at a frequency range of 5-54 MHz that is applied through the multiplexing filter. 
         [0003]    Surges in transmission line voltages can change the operating range of the components and severely damage and/or destroy them. There are many sources which can cause harmful electrical energy surges. One source is a radio frequency (RF) interference that can be coupled to transmission lines from a multitude of sources. The transmission lines act as large antennas that may extend over several miles, thereby collecting a significant amount of RF noise power from such sources as radio broadcast antennas. Another source could be lightning. Therefore, it may be desirable to interpose an RF surge suppression device, forming direct current (DC) short or low impedance when a surge occurs. 
         [0004]    Typical lightning waveforms are approximated by simpler waveforms described in the IEC 61000-4-5 document. Those waveforms have substantial low frequency content. The multiplexing filter at the input of the set-top box passes a substantial amount of energy in the low frequency range. Therefore, without surge protection, the multiplexing filter, disadvantageously, might transfer the energy from the surge to the DOCSIS transmitter output, exposing that transmitter to large voltages that could damage it. The same potential problem may be applicable also to a receiver or a transceiver, not shown, and it would also need to be protected. 
         [0005]    A common practice for protecting consumer electronics is to place a surge protection device directly at the input of the set-top box to be protected. For example, a gas discharge tube will be placed at the RF connector input known as F connector. There is a minimum firing voltage that is necessary to meet and an appreciable length of time required for the protection device to turn on to provide the required protection. During such turn on delay time, the electronics might be exposed to large voltages. Those voltages can cause damage. 
         [0006]    Using an additional, secondary protection that can be provided by a transient high speed diode also has a disadvantage. If such high speed diode is to be connected to an output terminal of, for example, the DOCSIS transmitter, the transmitter output signal itself might cause the diode to conduct, when an output signal of the transmitter is sufficiently large. The diode conduction might, disadvantageously, produce harmonics in the transmitted signal in normal operation. Harmonics in the transmitted signal are undesirable because they cause signal interference to other services. Therefore, it may be desirable to increase the isolation between the surge protection device and the transmitter output. 
       SUMMARY OF THE INVENTION 
       [0007]    An apparatus for performing a method that provides protection against surge energy includes an N-plexer filter coupled to a conductor susceptible to apply the surge energy to the N-plexer filter, when the surge energy occurs. A second filter is coupled between the N-plexer filter and one of a transmission stage and a receiver stage for applying a radio frequency (RF) signal at a normal operation frequency range between the N-plexer filter and the one stage, in normal operation. A low-pass filter has a cut-off frequency that is lower than the normal operation frequency range. The low-pass filter is coupled to the second filter to form with the second filter a diplexer filter for coupling the surge energy to a surge suppressing threshold device to dissipate the surge energy in the surge suppressing threshold device, when the surge energy occurs. The low-pass filter isolates the surge suppressing threshold device from each of the second filter and the N-plexer filter at the normal operation frequency range. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The preferred embodiment of the present arrangement will be described below in more detail with reference to the accompanying drawings in which: 
           [0009]      FIG. 1  illustrates, in a block diagram, a set-top box, embodying an advantageous feature; 
           [0010]      FIG. 2  illustrates, partially in a block diagram and partially in details, a portion of the circuit of  FIG. 1 , embodying an advantageous feature, that includes a surge protector; 
           [0011]      FIGS. 3, 4, 5 and 6  illustrate, each, a corresponding graph obtained from a simulation that depicts variations of a corresponding parameter or parameters of the circuit of  FIG. 2  as a function of frequency; and 
           [0012]      FIG. 7  illustrates a graph characterizing the surge protector of  FIGURE. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  illustrates, in a block diagram, a signal receiving device  100 , embodying an advantageous feature, which includes a set-top box  102 . Signal receiving device  100  receives signals from a cable service provider  101  representing broadcast audio and video programs and content. Signal receiving device  100  includes components that reside inside a user&#39;s premises. Cable service provider  101  communicates an RF signal  113  that includes cable television signal in the range of 54 MHz to 1000 MHz and a Data Over Cable Service Interface Specification (DOCSIS) signal in the range of 5-54 MHz. In addition, RF signal  113  may include multimedia over cable alliance (MOCA) home network system signal that is bidirectional and operates in a frequency spectrum of 1100 MHz-1600 MHz. 
         [0014]    Cable service provider  101  is coupled to set top box  102  via a transmission line  112  having a characteristic impedance of, for example, 75 Ohm. Transmission line  112  is susceptible to apply RF surge energy to set top box  102 , as explained before. One or more components of set-top box  102  may be integrated with a display device, such as a television or display monitor, not shown. 
         [0015]    Within set top box  102 , an input RF F connector, not shown, is coupled to a well-known N-plexer stage, for example, a triplexer filter  103 . Filter  103  forms a load of, for example, 75 Ohm at a dual function, input/output terminal  122 . Filter  103  is coupled to, for example, a tuner  105  via a filter terminal  103 B and, for example, to a tuner  110  via a filter terminal  103   c.    
         [0016]    In a reverse mode operation, filter  103  is coupled to DOCSIS reverse channel  150  in a manner embodying an advantageous feature. The DOCSIS standard specifies that the return link, from the cable end user to the cable head end, a transmitter in the approximate frequency band of 5 to 54 MHz should be used. Accordingly, DOCSIS channel  150  operates in a frequency range of 5-54 MHz 
         [0017]      FIG. 2  illustrates in more details DOCSIS channel  150  of  FIG. 1 . Similar symbols and numerals in  FIGS. 1 and 2  indicate similar items or functions. In reverse operation mode of DOCSIS channel  150  of  FIG. 2 , an RF output signal, not shown, is generated at an output terminal  151   a  of a DOCSIS transmitter  151 . 
         [0018]    In a first advantageous embodiment, output terminal  151   a  is coupled via a conductor  156  to an input port  3  of a passive diplexer  152 . In the first advantageous embodiment, a network  155  that includes a low-pass filter  157  and a protection diode DZ 2  is excluded from any signal path between output terminal  151   a  and port  3  and has no effect. Thus, the output signal that is generated at output terminal  151   a  is applied via conductor  156  to input port  3  of passive diplexer  152 . Therefore, a resulting output signal of passive diplexer  152  at a port  2  of passive diplexer  152  is applied via an input terminal  103 A of triplexer filter  103  of  FIG. 1  and input/output terminal  122  of triplexer filter  103  to transmission line  112 . Thus, terminal  122  forms, in the reverse operation mode of DOCSIS channel  150  of  FIG. 2 , an output terminal of triplexer filter  103  of  FIG. 1 . 
         [0019]    In passive diplexer  152  of  FIG. 2 , a capacitor C 3  and a capacitor C 4  are coupled in series between ports  3  and  2 . An inductor L 2  is coupled between a ground or common conductor G and a junction terminal  152   a,  that is coupled between capacitors C 3  and C 4  to form a T-shaped configuration. Inductor L 2 , capacitor C 3  and capacitor C 4  form a high pass filter for passing DOCSIS channel frequency range of 5-54 MHz from port  3  to port  2 . 
         [0020]    Calculated or simulated parameters of the three-port network diplexer  152 , as a function of frequency, are provided in  FIGS. 3, 4, 5 and 6 . Similar symbols and numerals in  FIGS. 1, 2, 3, 4, 5 and 6  indicate similar items or functions. For the purpose of the simulation transmitter output terminal  151   a  is directly coupled via conductor  156  to input port  3  of passive diplexer  152  and network  155  is excluded. Also, for the purpose of the simulation, the values of the elements of diplexer  152  of  FIG. 2  are, as follows: inductor L 2 =6.8 uH, capacitor C 3 =3.3 nF, capacitor C 4 =1.2 nF, capacitor C 1 =820 pF, inductor L 3 =1.8 uH and inductor L 1 =5.6 uH. In particular,  FIG. 3  illustrates a graph in solid line of transmission characteristic, S( 2 , 3 ), of high pass filter of diplexer  152  from port  2  to port  3  of  FIG. 2 . 
         [0021]    In diplexer  152 , an inductor L 3  and an inductor L 1  are coupled in series between port  2  and a port  1 . A capacitor C 1  is coupled between ground conductor G and a junction terminal  152   b,  that is coupled between inductors L 3  and L 1  to form a T-shaped configuration. Capacitor C 1 , inductor L 3  and inductor L 1  form a low pass filter such that the high and low pass filters of diplexer  152  are joined at port  2 . 
         [0022]    In carrying out an advantageous feature, a surge suppressing threshold zener diode, surge diode DZ 1 , for example, transient voltage suppressor (TVS) SP3021 made by Littelfuse, is coupled between ground conductor G and output port  1 . Additionally, a terminating resistor R 1 =75 Ohm is coupled in parallel with surge diode DZ 1 . 
         [0023]      FIG. 3  also illustrates a graph in dotted line depicting the low pass transmission characteristic, S( 2 , 1 ), from port  1  to port  2 . According to  FIG. 3 , a frequency in which the transmission of high pass filter, that includes capacitors C 3  and C 4 , and that of the low pass filter of diplexer  152  of  FIG. 2  are equal and cross each other, referred to as the crossover frequency, is at 2 MHz. The manner of selecting the values of the inductors and capacitors for establishing the desired crossover frequency between high and low pass filters is well known in the art. 
         [0024]    In carrying out an advantageous feature, low-pass filter of diplexer filter  152  formed by capacitor C 1 , inductor L 3  and inductor L 1  diverts the energy of a surge that may be coupled via transmission line  112  of  FIG. 1  to power dissipating resistor R 1  and surge diode DZ 1 . But for the low-pass filter of diplexer filter  152  formed by capacitor C 1 , inductor L 3  and inductor L 1 , the surge energy could have been coupled from triplexer  103  to transmitter  151  via a signal path that includes the high-pass filter of diplexer  152 . In this way, the energy of the surge, for example, of lightning is prevented from being channeled to DOCSIS transmitter  151 . In the case of very light surges, diode DZ 1  does not conduct and the energy is dissipated entirely in resistor R 1 . On the other hand, in the case of a larger amplitude surge, diode DZ 1  conducts and dissipates internally the surge energy. Thus, for protecting DOCSIS transmitter  151 , inductor L 3  and inductor L 1  of the low-pass filter isolate port  1 , to which diode DZ 1  is connected, from output terminal  151   a  of DOCSIS transmitter  151 . In addition, at the frequency range of interest, 5-54 MHz, in normal operating voltages, inductor L 3  and inductor L 1  of the low-pass filter prevent the output signal from DOCSIS transmitter  151  from partly turn on protection diode DZ 1 . Consequently, inductor L 3  and inductor L 1  of the low-pass filter, advantageously, prevent harmonics from ever being generated at output terminal  151   a  of DOCSIS transmitter. If the surge energy becomes sufficiently large, transient protection diode DZ 1  starts to conduct, thus effectively shorting port  1  to ground potential G. In passive filter  152 , the resulting short on port  1  will also present low impedance at common port  2 , so as to substantially reduce energy from the surge from being channeled through diplexer  152 . If, instead, DOCSIS transmitter  151  was connected directly to a main surge protection diode that is similar to diode DZ 1 , then, undesirably, there could be an increase in the harmonics content at output terminal  151   a.  The reason for the generation of harmonics is that such diode is nonlinear when, as a result of a high level output signal from transmitter  151 , it becomes partly conductive. 
         [0025]      FIG. 4  shows the isolation between ports  1  and  3 , S( 1 , 3 ), as a function of frequency. Diplexer  152  of  FIG. 2  that is of the 3 rd  order Butterworth filter type provides isolation, as shown in the graph of  FIG. 4 , of about 12.5 dB between ports  1  and  3  at the lowest applicable frequency of interest, 5 MHz, for DOCSIS transmitter  151 . An even higher isolation might be possible by using a different type of filter or a higher order filter. 
         [0026]    It may be desirable to maintain the input return loss at each port at least at −10 db. This ensures that any undesirable interaction between different subsystems will be reduced.  FIG. 5  shows the return loss of port  3 .  FIG. 6  shows the return loss from port  2 , in solid line, and the transmission loss between port  2  and port  3 , in dotted line. 
         [0027]    In a second advantageous embodiment, an additional surge diode DZ 2  of the type, for example, TVS SP3021 made by Littelfuse is coupled between output terminal  151   a  of transmitter  151  and ground conductor G. In addition, conductor  156  of  FIG. 2  is removed from the circuit and, instead, terminal  151   a  is coupled via a low-pass filter  157  to port  3 . Low-pass filter  157  includes a capacitor C 11  of 68 pF that is coupled between terminal  151   a  and ground conductor G. A parallel arrangement of an inductor L 11  of 270 nH and a capacitor C 13  of 3.3 pF is coupled between terminal  151   a  and port  3 . In addition, a capacitor C 10  of 68 pF is coupled between ground terminal G and port  3  that is coupled to junction terminal  157   a,  disposed between inductor L 11  and capacitor C 13 . 
         [0028]    The current vs. voltage characteristic of diode DZ 2  is shown in  FIG. 7 . In this example, for voltages lower than about 10V peak-to-peak, there is only an insubstantial conduction. However, because practical devices might have a non-negligible conduction it still could create a nonlinear behavior in transmission channel  150 . This could have generated harmonics of the transmitter frequency. Including low-pass filter  157  in the signal path between transmitter  151  and port  3 , results in an overall band-pass filter. The purpose of such low-pass filter  157  is to attenuate harmonics that may be produced by DOCSIS transmitter  151  and those that may be produced by the nonlinear effect introduced by protection diode DZ 2 . The values of the components forming the resulting band-pass filter are optimized to form constant impedance for port  2  of diplexer  152  that is coupled to triplexer  103 . Furthermore, the impedance presented to DOCSIS transmitter  151  is also optimized by the selected component values. It should be understood that, in an alternative embodiment, diplexer  152  can be coupled to a receiver or a signal processing stage, instead of to DOCSIS transmitter  151 , for providing surge protection to such receiver or signal processing stage.