Abstract:
An impedance blocking filter circuit is provided for use in telecommunication systems for interconnecting between incoming telephone lines and customer&#39;s terminal equipment so as to unconditionally block impedances above 20 KHz  a desired frequency range due to the customer&#39;s terminal equipment from an ADSL  a DSL network unit and/or home networking interface unit. The filter circuit includes first, second, and third inductors connected in series between a first input terminal and a first common point. A first resistor has its one end connected also to the first common point and its other end connected to a first output terminal. Fourth, fifth and sixth inductors are connected in series between a second input terminal and a second common point. A second resistor has its one end also connected to the second common point and its other end connected to a second output terminal. A capacitor has its ends connected across the first and second common points. In another aspect, the filter circuit also includes current limiting protection circuitry for reducing ring trip, dial pulse and off-hook transient current spikes.  In one exemplary embodiment, the filter circuit is adapted to block impedances above 20 KHz, and comprises a series of inductors disposed electrically between respective ones of first and second input terminals and output terminals. At least one capacitor and first and second resistors are also present in the circuit.

Description:
The present application is a reissue of U.S. Pat. No.  6 , 188 , 750  issued Feb.  13 ,  2001  of the same title. This application is also related to co- pending reissue application Ser. No.  10 / 355 , 897  filed Jan.  30 ,  2003 , Ser. No.  10 / 737 , 736  filed Dec.  12 ,  2003 , Ser. No.  10 / 408 , 030  filed Apr.  3 ,  2003 , and Ser. No.  10 / 748 , 729  filed Dec.  29 ,  2003 , all of the same title.   
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to telecommunication systems and more particularly, it relates to an impedance blocking filter circuit used in telecommunication systems for interconnecting between incoming telephone lines from a telephone company&#39;s central office (C.O.) and subscriber or customer telephone equipment such as a telephone set located at a subscriber&#39;s premises so as to unconditionally block telephone impedance above 20 KHz. 
     2. Description of the Prior Art 
     The prior art appears to be best exemplified in the following U.S. Letters Patent which were developed in a search directed to the subject matter in this application: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 4,613,732 
                 4,823,383 
               
               
                   
                 4,742,541 
                 5,642,416 
               
               
                   
                 4,743,999 
                 5,802,170 
               
               
                   
                   
               
             
          
         
       
     
     In U.S. Pat. No. 4,823,383 issued to Cardot et al. on Apr. 18, 1989, there is disclosed a protection device for terminal equipment on telephone subscriber premises which includes a voltage surge protection circuit and/or a filter for providing protection against radio frequencies and interference. The filter is comprised of series inductors L1, L2, L3 and L5 interconnected between terminals E1 and S1 and series inductors L′1, L′2, L4 and L′5 interconnected between terminals E2 and S2. A capacitor C5 is connected between the junctions of the inductors L2, L3 and the inductors L′2, L4. The surge protection circuit includes thermistors TH1, TH2 and voltage limiters D1-D3. 
     In U.S. Pat. No. 5,802,170 issued to Smith et al. on Sep. 1, 1998, there is disclosed a customer bridge module for connecting telephone company wiring and subscriber telephone wiring in a telephone network interface apparatus. In one embodiment, the customer bridge module includes overcurrent protection and an RFI filter. The overcurrent protection is formed by positive temperature coefficient resistors 220, 222, and inductors. The RFI filter is formed by inductors 224a-224c, 226a-226c and capacitors 236a-236c. The inductors and capacitors are used to form a multi-pole low pass filter. 
     In U.S. Pat. No. 5,642,416 issued to Hill et al. on Jun. 24, 1997, there is disclosed an electromagnetic interference by-pass filter which suppresses RF noise currents conducted over the tip and ring leads of a telephone line-powered instrument. The filter includes first and second inductors 51, 53 and first and second capacitors 41, 43. 
     It is generally well-known these days that many telephone subscribers or customers also have a personal computer located on their premises. At times, the computer user receives ADSL (an acronym for Asymmetric Digital Subscriber Line) signals from the Internet over the same telephone lines via an Internet Server Provider (ISP). In order to increase the speed of downloading of information from the Internet, an ADSL network interface is typically purchased and installed between the incoming telephone lines and the user&#39;s computer. However, since one or more telephone subscriber terminal equipment such as telephone sets, facsimile machines and/or answering devices are also connected to the same incoming telephone lines via internal house wiring, ADSL interface problems may be caused by the terminal equipment which can significantly limit or reduce the data rate. In one situation, it has been experienced that the change of state from “on-hook” to “off-hook” of the telephone equipment and sometimes the telephone terminal equipment even being “on-hook” can create a resonance effect to occur so as to drop the impedance value to less than 10 Ω(Ohms) at a frequency as high as 500 KHz. 
     Accordingly, it would be desirable to provide an impedance blocking filter circuit for connection to the telephone terminal equipment causing the erratic input impedances. The impedance blocking filter circuit of the present invention is of a modular design so as to be easily connected in series with the offending telephone terminal equipment. The impedance blocking filter circuit blocks unconditionally any telephone impedances (e.g., open, short, capacitive, inductive, resonant, or any combination thereof) above the frequency of 20 KHz. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide an impedance blocking filter circuit which effectively and efficiently eliminates ADSL interference caused by telephone terminal equipment. 
     It is an object of the present invention to provide an impedance blocking filter circuit for connection to telephone terminal equipment causing the erratic input impedances. 
     It is another object of the present invention to provide an impedance blocking filter circuit used in telecommunication systems for interconnecting between incoming telephone line and customer&#39;s terminal equipment so as to be unconditionally block impedance above 20 KHz due to the customer&#39;s terminal equipment from an ADSL network interface unit and/or home networking interface unit. 
     It is still another object of the present invention to provide an impedance blocking filter circuit which is of a modular design so as to be easily connected in series with the offending telephone terminal equipment. 
     It is still yet another object of the present invention to provide an impedance blocking filter circuit which is comprised of six inductors, two resistors, and a capacitor. 
     In accordance with a preferred embodiment of the present invention, there is provided an impedance blocking filter circuit used in telecommunication systems for interconnecting between incoming telephone lines and customer&#39;s terminal equipment so as to unconditionally block impedances above 20 KHz due to the customer&#39;s terminal equipment from an ADSL network interface unit and/or home networking interface unit. The filter circuit includes first, second and third inductors connected in series between a first input terminal and a first common point. The first inductor has its one end connected to the first input terminal and its other end connected to one end of the second inductor. The second inductor has its other end connected to one end of the third inductor. The third inductor has its other end connected to the first common point. A first resistor has its one end also connected to the first common point and its other end connected to a first output terminal. 
     The filter circuit further includes fourth, fifth and sixth inductors connected in series between a second input terminal and a second common point. The fourth inductor has its one end connected to the second input terminal and its other end connected to one end of the fifth inductor. The fifth inductor has its other end connected to one end of the sixth inductor. The sixth inductor has its other end connected to the second common point. A second resistor has its one end also connected to the second common point and its other end connected to a second output terminal. A capacitor has its one end connected to the first common point and its other end connected to the second common point. 
     In one aspect of the invention, a filter circuit used in telecommunication systems is disclosed, the filter circuit generally comprising: at least three first inductors electrically disposed between a first input terminal and a first common point; at least three second inductors electrically disposed between a second input terminal and a second common point; at least one capacitor disposed electrically between the first and second common points; at least one first resistor disposed electrically between the first common point and a first output terminal; and at least one second resistor disposed electrically between the second common point and a second output terminal. 
     
       In another aspect of the invention, an improved telecommunications filter circuit is disclosed, the circuit generally comprising: at least one first inductor electrically disposed between a first input terminal and a first common point; at least one second inductor electrically disposed between a second input terminal and a second common point; at least one capacitor disposed electrically between the first and second common points; and a substantially transistorized current limiter disposed electrically between the first and second common points and first and second output terminals.  
     
     
       In another aspect of the invention, an improved telecommunications signal filter circuit is disclosed, generally comprising: at least one first inductor electrically disposed between a first input terminal and a first common point; at least one second inductor electrically disposed between a second input terminal and a second common point; at least one capacitor disposed electrically between the first and second common points; and a current limiter disposed electrically between the first and second common points and first and second output terminals, the current limiter comprising first and second transistors, first and second varistors, and first and second resistors, the first transistor having its conduction path electrodes disposed electrically between the first common point and a first end of the first resistor, the second transistor having its conduction path electrodes disposed electrically between the second common point and a first end of the second resistor, the first varistor having its one end electrically communicating with the first common point and its other end electrically communicating with the first output terminal, the second varistor having its one end electrically communicating with the second common point and its other end electrically communicating the second output terminal.  
     
     
       In yet another aspect of the invention, an improved impedance blocking DSL filter circuit is disclosed, generally comprising: a plurality of first terminals; a plurality of second terminals; a plurality of common points disposed electrically between respective ones of the first and second terminals; a first filter stage disposed electrically between the first terminals and the common points and adapted to block impedances in a first band, the first filter stage comprising a plurality of inductors, at least two of the plurality of inductors being formed as separate inductors so as to at least partly block differential impedances; a second filter stage disposed electrically between the common points and the second terminals and adapted to block impedances in a second band, the second stage comprising a plurality of resistors; and a capacitance disposed electrically between the common points, the capacitance being adapted to displace resonance generated within the circuit to a third band. 
     
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and advantages of the present invention will become more fully apparent from the following detailed description when read in conjunction with the accompanying drawings with like reference numerals indicating corresponding parts throughout, wherein: 
         FIG. 1  is an overall block diagram of a telecommunication system for interconnecting a central office and a subscriber&#39;s premises, employing an impedance blocking filter circuit of the present invention; 
         FIG. 2  is an exploded, perspective view of one form of a module housing the impedance blocking filter circuit; 
         FIG. 3  is a schematic circuit diagram of an impedance blocking filter circuit, constructed in accordance with the principles of the present invention; 
         FIG. 4  is a schematic circuit diagram of a second embodiment of an impedance blocking filter circuit, in accordance with the principles of the present invention; 
         FIG. 5  is a plot of input impedances of the impedance blocking filter circuit of  FIG. 3  for various telephone equipment impedances as a function of frequency; 
         FIG. 6  is a schematic circuit diagram of current limiting protection circuitry for use with the filter circuit of  FIG. 3 ; and 
         FIG. 7  is a schematic circuit diagram of a home network demarcation filter for use with the filter circuit of FIG.  3 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now in detail to the drawings, there is illustrated in  FIG. 1  an overall block diagram of a telecommunication system  10  for interconnecting a telephone company&#39;s central office (CO)  12  and a subscriber&#39;s premises  14  over a transmission media such as a conventional twisted pair of telephone lines  16 . The telecommunication system  10  employs a plurality of impedance blocking filter circuits, constructed in accordance with the principles of the present invention, in which each is contained in a modular housing  18 . 
     The central office  12  includes a telephone office switch  20  and an Internet Service Provider (ISP)  22 . The telephone office switch  20  is used to send voice signals via a low-pass filter  24  and a surge protector  26  to the telephone line  16 . The ISP  22  transmits ADSL data signals to a modem  28  which are then sent to the telephone lines  16  via a high-pass filter  30  and the surge protector  26 . It should be understood that the voice signals from the telephone office switch  20  and the ADSL data signals from the ISP  22  can be transmitted simultaneously to the telephone lines  16 . Further, the voice signals (speech) are in the frequency band between 300 and 3400 Hz, and the ADSL data signals are in the frequency band between 30 KHz and 2 MHz. 
     The subscriber&#39;s premises  14  includes a Network Interface Device (NID)/surge protector unit  32  which is connected to the incoming telephone lines  16  on its input side and is connected to the subscriber&#39;s internal wiring or house wiring  34  on its output side via demarcation RJ-11 jack and plug unit  36 . As can be seen, the subscriber&#39;s premises further includes a number of terminal equipment such as a plurality of telephone sets  40 . At times, the computer user will be downloading information to a personal computer  38  from the Internet by receiving ADSL data signals transmitted by the ISP  22 . 
     In order to optimize the downloading of this information from the Internet, the user can purchase and install an ADSL network interface unit  42  for connection between the computer  38  and a RJ-11 jack and plug unit  44 . The ADSL network interface unit  42  includes a high-pass filter  41  connected to the RJ-11 unit  44  and an internal modem  43  connected to the computer  38 . The RJ-11 unit  44  is connected to the house wiring  34  for receiving the ADSL signals from the telephone lines  16 . However, it will be observed that the plurality of telephone sets  40  are also connected to the same house wiring  40  via RJ-11 units  46 ,  48  and  50 , respectively. 
     If it were not for the impedance blocking filter circuits  18  in the present invention, the output impedance from each of the telephone sets  40  would be connected in parallel with the input impedance of the ADSL unit  42 . Since the output impedances from the telephone sets are subject to wide variations due to, for example, changing from “on-hook” to “off-hook” so as to present either an open, a short, capacitive, inductive, resonant, or any combination thereof at frequencies above 20 KHz, this erratic impedance can significantly affect the rate of the ADSL data signals being received by the computer  38  via the ADSL network interface unit  42 . 
     Therefore, the main purpose of the impedance blocking filter circuit of the present invention is to isolate the terminal equipment (telephone sets) impedances from the ADSL unit  42  and the house wiring  34  so as to eliminate degradation of the performance of the ADSL unit  42 . Further, the impedance blocking filter circuit serves to attenuate the ADSL data signal from being received by the telephone sets  40  in order to prevent nonlinear conversion to voice band signals. Moreover, to facilitate the installation required by the customer, the filter circuit is contained in the modular housing  18 . 
     As can best be seen from  FIG. 2 , one form of the modular housing  18  includes a base  52  and a snap-on removable cover  54 . The base has a printed circuit board  56  which is fixedly secured thereto by screws  58  and has mounted thereon the electrical circuit components for the filter circuit  59 . One end of the modular housing  18  has a RJ-11 jack  60  formed integrally therewith for connection to the telephone set. This connection is achieved by plugging a RJ-11 plug (not shown) from a telephone set into the jack  60 . The other end of the modular housing  18  has a short length of cable  62  extending therefrom and terminating in a RJ-11 plug  64  which is connectable to the house wiring. In particular, the plug  64  is connected to the house wiring  34  by plugging the same into a wall socket (not shown) having a RJ-11 jack. 
     In  FIG. 3 , there is shown a detailed schematic circuit diagram of the impedance blocking filter circuit  59  of the present invention for connection in series between the house wiring  34  and the terminal equipment (telephone set) of FIG.  1 . The filter circuit  59  includes two input (tip and ring) terminals  66 ,  68  which are connectable to the house wiring  34  via the RJ-11 plug  64  and two output (tip and ring) terminals  70 ,  72  which are connectable to the telephone set  40  via the RJ-11 jack  60 . The filter circuit  59  is comprised of inductors L 1 -L 6 , a capacitor C 1 , and resistors R 1 , R 2 . 
     The inductors L 5 , L 3 , L 1  and the resistor R 1  are connected in series between the first or tip input terminal  66  and the first or tip output terminal  70 . Similarly, the inductors L 6 , L 4 , L 2  and the resistor R 2  are connected in series between the second or ring input terminal  68  and the second or ring output terminal  72 . The inductors L 5  and L 6  are each preferably formed of a ferrite toroid. The inductors L 3  and L 4  have the same inductance values, and the inductors L 1  and L 2  have the same inductance values. The inductor L 1  and the first resistor R 1  are connected together at a common point A and to one side of the capacitor C 1 . The inductor L 2  and the second resistor R 2  are connected together at a common point B and to the other side of the capacitor C 1 . The resistors R 1  and R 2  also have the same values. 
     As previously pointed out, the primary purpose of the impedance blocking filter circuit  59  is to block the impedances from the telephone set at above the frequency of 30 KHz from reaching the house wiring  34 , thereby preventing adverse performance of the ADSL network unit  42  (FIG.  1 ). In particular, the ADSL data signals being in the frequency range of 30 KHz and 2 MHz are mainly blocked by the inductors L 1  and L 2 . However, it has been experienced that some telephone sets have an input capacitance of less than 5 nf which can cause resonant impedances to occur within the ADSL band. In order to eliminate this undesirable effect, the capacitor C 1  is used to lower any resonance into an acceptable dead band at around the 10 KHz frequency. Further, the capacitor C 1  also provides additional attenuation of the ADSL signals so as to prevent driving the telephone impedance into a non-linear region and converting the high frequency ADSL signals into audible signals which can be heard by the subscriber or converted to another ADSL band and cause ADSL interference. While there may still exist other minor resonances in the telephone set in the frequency range of between 20 KHz and 60 KHz, their undesirable effect is significantly reduced by the resistors R 1  and R 2  which produce a de-Q effect. It should be noted that the inductors L 1  and L 2  are formed as separate inductors so as to avoid longitudinal impedance problems as well as blocking differential impedances. 
     Since the inductors L 1  and L 2  have their own frequency limitations (e.g., self-resonant frequency), the inductors L 3  and L 4  are provided so as to block the telephone impedances in the frequency band of 1 MHz to 20 MHz. These inductors L 3 , L 4  are necessary when phoneline home networking interface units ( FIG. 1 ) are being used in conjunction with the ADSL network interface unit  42 , as will be explained hereinafter. The inductors L 5  and L 6  are provided so as to block the telephone set impedances in the frequency band of 20 MHz to 500 MHz, which will prevent any problems caused by TV/FM interference. 
     For completeness in the disclosure of the above-described filter circuit but not for purposes of limitation, the following representative values and component identifications are submitted. These values and components were employed in a filter circuit that was constructed and tested, and which provides high quality performance. 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 PART 
                 TYPE or VALUE 
               
               
                   
                   
               
             
             
               
                   
                 L1, L2 
                 10 mH 
               
               
                   
                 L3, L4 
                 220 μH 
               
               
                   
                 L5, L6 
                 ferrite toroid, 75 μH 
               
               
                   
                 C1 
                 20 nf 
               
               
                   
                 R1, R2 
                 22Ω 
               
               
                   
                   
               
             
          
         
       
     
     With these above values being used, the input impedance of the impedance blocking filter circuit  59  was plotted for various telephone equipment impedances (e.g., open, short, capacitive, inductive, resonant, or a combination of these conditions) as a function of frequency and is illustrated in FIG.  5 . As can be seen from the various curves, the input impedance across the input terminals  66 ,  68  of the impedance blocking filter circuit  59  for any telephone impedances connected across its output terminals  70 ,  72  is equal to or greater than 2K Ohms at frequencies above 40 KHz. 
     The impedance blocking filter circuit  59  of  FIG. 3  is basically a second-order filter and has been found to minimize adequately voice band transmission effects when up to eight (8) filter circuits are installed into the telecommunication system of FIG.  1 . In order to provide higher attenuation at frequencies above 20 KHz, there is shown in  FIG. 4  a schematic circuit diagram of a second embodiment of a third-order impedance blocking filter circuit  59 a of the present invention. The third-order filter circuit of  FIG. 4  is substantially identical to the second-order filter circuit of  FIG. 3 , except there has been added an inductor L 7  and an inductor L 8 . The inductor L 7  is interconnected between the common point A and the first resistor R 1 , and the inductor L 8  is connected between the common point B and the second resistor R 2 . The inductors L 7  and L 8  have the same inductance values. 
     Based upon tests conducted on the third-order filter circuit of  FIG. 4 , it was observed that higher attenuation was provided at frequencies above 20 KHz. However, it was found that the number of such third-order filter circuits which could be connected to the telecommunication system of  FIG. 1  was limited to three or four. This is due to the fact that the inductor values of L 1 , L 5 , L 7  and L 8  of  FIG. 4  are smaller (on the order of 5-10 mH) than the ones in  FIG. 3 , the capacitor value of C 1  of  FIG. 4  is larger (on the order of 33-47 nf) than the one in  FIG. 3 , and the additive capacitive loading caused by each added filter circuit will adversely affect the voice band performance. Thus, the optimized operation between voice performance and ADSL performance was found to exist when only three or four filter circuits  59 a were installed. 
     While the filter circuit of  FIG. 3  performed adequately, the inventor has found based upon further testing that a transient problem will occur when the telephone set goes “off-hook” at the peak of the ring signal. This “off-hook” transient condition may cause current spikes to occur which are higher than 600 mA. As a result, the high current will tend to saturate the inductors, thereby momentarily lowering the input impedance of the filter circuit and thus adversely affects the data on the ADSL signal being transmitted to the interface unit  42 . 
     In order to overcome this current transient problem, the inventors have developed fast current limiting protection circuitry  74  for providing protection against the “off-hook” transients. In  FIG. 6  of the drawings, there is shown a schematic circuit diagram of the current limiting protection circuitry  74  which is comprised of depletion mode N-channel field-effect transistors (FET) Q 1 , Q 2 ; resistors R 1 a, R 2 a; and varistors RV 1 , RV 2 . The FET Q 1  has its drain electrode connected to a first input terminal  76 , its source electrode connected to one end of the resistor R 1 a, and its gate electrode connected to the other end of the resistor R 1 a. The common point C of the gate electrode of the transistor Q 1  and the resistor R 1 a is also joined to the first output terminal  78 . Similarly, the FET Q 2  has its drain connected to a second input terminal  80 , its source connected to one end of the resistor R 2 a, and its gate electrode connected to the other end of the resistor R 2 a. The common point D of the gate of the transistor Q 2  and the resistor R 2 a is also joined to a second output terminal  82 . One end of the varistor RV 1  is connected to the drain of the transistor Q 1 , and the other end thereof is connected to the common point C. One end of the varistor RV 2  is connected to the drain of the transistor Q 2 , and the other end thereof is connected to the common point D. 
     In use, the current limiting protection circuitry  74  replaces the resistors R 1  and R 2  of FIG.  3 . The first and second input terminals  76 ,  80  of the protection circuitry  74  are connectable to the common points A and B of  FIG. 3 , and the first and second output terminals  78 ,  82  thereof are connected to the tip and ring output terminals  70 ,  72  of FIG.  3 . The transistors Q 1 , Q 2  may be similar to the ones commercially available from Supertex Corporation under their Part No. DN2530N3. The varistors may be similar to the type ZNR which are manufactured and sold by Panasonic Corporation. The resistors R 1 a and R 2 a have the same resistance value and are on the order of 5-20 Ohms depending on the thresholds of the transistors Q 1 , Q 2 . It should be understood that the transistors Q 1 , Q 2  have a large tolerance on current limit and the resistors R 1 a, R 2 a permit the desired current limit value to be adjusted. Alternatively, the resistors R 1 a, R 2 a may have a value of zero Ohms or be entirely eliminated. 
     In normal on-hook operation, the transistors Q 1  and Q 2  are rendered conductive and have an on-resistance value of about 10 Ohms. When the telephone set goes “off-hook” into high ringing voltage, the gate-to-source voltage of the forward conducting FET will become more negative due to the resistors R 1 a, R 2 a. As a result, the resistance of the transistors Q 1 , Q 2  will go very high which will limit the current spikes to approximately 70-100 mA. The transistor Q 1  serves to limit the current flowing in a first direction, and the transistor Q 2  serves to limit the current flow in a reverse direction. Further, the varistors RV 1 , RV 2  defining transient protection means function to clamp transients caused by lightning and power shorts from damaging or destroying the FETs Q 1 , Q 2 . 
     In view of continuing increased use of home computers and the high demand for accessing of information from the Internet in the last decade or so, many of the subscribers will be multi-PC homes. As shown in  FIG. 1 , the subscriber&#39;s premises or small business will typically have a second computer  38 a also connected to the same internal house wiring  34 . In order to effect high-speed data transfer in the multi-PC environment, there will be required phoneline home networking interface units  42 a for using the internal house wiring in the frequency band above 5 MHz so as to interconnect the multiple computers  38 ,  38 a or other devices at data rates above 10 MB/s as illustrated. While the impedance filter circuit of the present invention adequately filters and blocks the telephone impedances from the home networking signals, which are in the frequency band of 5-10 MHz, it will be noted that the home networking signals from the telephone company&#39;s C.O. are however still connected to the house wiring via the NID/surge protector unit  32 . 
     In order to solve this problem, the inventor has developed a home network demarcation filter  84  as shown in dotted lines in  FIG. 1  for connection at a point of demarcation (NID/surge protector unit  32 ) between the telephone company&#39;s incoming lines  16  and the subscriber&#39;s internal house wiring  34  via the demarcation unit  36 . A schematic circuit diagram of the home network demarcation network is depicted in FIG.  7 . The demarcation filter  84  includes two input (tip and ring) terminals  86 ,  88  which are connectable to the incoming lines via the jack side of the demarcation unit  36  in the NID/surge protector unit  32  and two output (tip and ring) terminals  90 ,  92  which are connectable to the internal house wiring via the plug side of the demarcation unit  36 . The demarcation filter is comprised of six inductors L 9 -L 14  and two capacitors C 2 , C 3 . In use, the demarcation filter is transparent to the ADSL data signals having the frequencies between 30 KHz and 2 MHz but will produce an attenuation of more than 40 dB for frequencies above 5 MHz. The demarcation filter will also provide an inductive input impedance for above 5 MHz frequency band so as to prevent loading down the home networking signals on the incoming phone lines and also adds data security benefits. 
     From the foregoing detailed description, it can thus be seen that the present invention provides an impedance blocking filter circuit used in telecommunication systems for interconnecting between incoming telephone lines and customer&#39;s terminal equipment so as to unconditionally block impedances above 20 KHz due to the customer&#39;s terminal equipment from an ADSL network interface unit and/or home networking interface unit. The impedance blocking filter circuit is comprised of six inductors, two resistors, and a capacitor. 
     While there has been illustrated and described what is at present considered to be a preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the central scope thereof. Therefore, it is intended that this invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out the invention, but that the invention will include all embodiments falling within the scope of the appended claims.