Abstract:
A signal coupler is provided which decreases the number of discreet elements required to provide low pass filtering for the plain old telephone service (POTS). The low pass filtering is shifted to areas of the signal coupler circuit which do not operate with the high battery voltage present on telephone lines The low voltage filtering reduces the need for components which are capable of operating in the high voltage environment and therefore reduces the space on the circuit board which is occupied by each of the signal couplers. In this way, the number of individual subscriber lines that a given circuit board can accommodate can be increased.

Description:
BACKGROUND 
     1. Field of the Invention 
     The invention relates to a signal coupler for telephone lines containing both plain old telephone service (POTS) and digital signals where low voltage filtering is used in the POTS channel. 
     2. Description of the Related Art 
     Modern data networks commonly use complex digital signal processing (DSP) devices called modems to transport data over communication channels. Data is typically transported via an analog transmission signal which is representative of a synchronous, constant rate bit stream. This form of communication channel is suitable for the transmission of real-time information such as voice or video. 
     Often it is desirable to transmit both Plain Old Telephone Service (POTS) and digital data, either by Asymmetric Digital Subscriber Line (ADSL) or some other method, over the same line. The POTS frequency spectrum ranges from 300 to 3400 Hz. The ADSL frequency spectrum ranges from 24 kHz to 1100 kHz. 
     As shown in FIG. 1, the data and POTS signals are transmitted over standard telephone lines between a central office and a subscriber&#39;s home. The subscriber may have several modems and a POTS service. The subscriber is typically connected to a central office by twisted copper wire pair. At the central office, a signal coupler is used to filter, split and digitize signals coming into the central office from a subscriber. The digital signals are processed through switching networks and then sent through another signal coupler to another subscriber. Alternatively, the digital signal may be transmitted to another central office before being sent through another signal coupler to another subscriber. The signal coupler converts the digital signals from the switching circuits into analog signals for transmission to the subscriber as well as converting the analog input from the subscriber into digital signals which are sent to the switching circuits. 
     The transmission lines between the central office and the subscriber may be twisted copper pairs, as shown in FIG.  1 . Other possibilities for transmission lines include fiber optics. In any case, the equipment at the central office and at the subscriber must be protected against power cross, lightning strikes, or other high voltage events and current surges on the telephone line. The main voltage protection is accomplished outside of the central office. However, secondary voltage protection is usually included in the signal coupler. In FIG. 2, the signal from the subscriber appears on the TIP and RING lines and the secondary voltage protection is shown as the circuit protection  101 . 
     After the voltage and current protection is accomplished, the signal is split into data and voice lines. The data is sent at frequencies in the 20-30 kilohertz range and up while POTS voice information is nominally below 3000 Hz. The splitting, then, is normally done by using a high-pass filter for the data lines and a series of low-pass filters for the POTS lines. The series of filters is further required to remove the noise from the incoming telephone cable. FIG. 2 shows the typical filtering circuit for the POTS. In FIG. 2, the voice filtering is accomplished by a multistage filtering circuit. The typical low pass filter used in the multistage filtering circuit has two to four stages of filtering. 
     The POTS signal from the subscriber, after passing through the protection circuit  101 , is filtered by the multistage low pass filter  102 . The multiple stages allow for filtering of multiple orders (i.e., one stage provides two orders of filter, thus multiple stages provides multiples of second order filtering). The components of each stage include an inductor pair and a capacitor. Stage  1  in FIG. 2, for example, has inductor L 1  connected in series with the TIP line after protection circuit  101  and inductor L 1 ′ connected in series with the RING line after the protection circuit. Capacitor C 1  is connected across the TIP and RING lines after the inductor pair L 1  and L 1 ′. The remaining stages have similar components. In addition, the low pass filter  102  includes resistors R 1  and R 2  connected in series with the TIP and RING lines before they are connected to Stage  1  and resistors R 3  and R 4  connected in series with the IP and RING lines after they exist stage N. In addition, the low pass filtering circuit may include an additional inductor pair acting as a common mode choke which rejects signals common to both input lines. The common mode choke is connected before stage  1  and is connected similar to inductor pair L 1  and L 1 ′. 
     Each component of the filtering circuit (i.e., the resistors, inductors, and capacitors) must be capable of withstanding the high battery voltage (48 V) and high DC currents (typically 25 mA) used in the subscriber loop. This necessitates that each of the components in the filtering circuit be discrete components which require a large amount of space on the circuit board. This space requirement restricts the number of lines that can be placed on a given circuit board. It is desirable, then, to reduce the amount of filtering which must be accomplished in the high voltage mode. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a signal coupler circuit which provides the circuit protection and the filtering in a way that allows conservation of space on the circuit board at the central office. This is done by filtering the voice lines, at least in part, in a portion of the circuit where the high battery voltage is no longer present. 
     An analog signal capable of containing both POTS and data signals is present between the TIP and RING lines. The TIP and RING lines are the components of a standard twisted pair configuration of telephone service. The invention, however, is not restricted to twisted pairs and is useful for any transmission method of telephone service. 
     The signal between TIP and RING is first inputted to a voltage surge protection circuit. The surge protection circuit limits voltage spikes and current surges which could damage other components. The main voltage and current surge protection is connected to the transmission line outside of the central office so that the protection required on the circuit board inside the central office is secondary circuit protection. 
     The output signal from the protection circuit is inputted to a low-pass filter. The low-pass filter occupies a minimal amount of space while being capable of accommodating the high voltage telephone lines. In the preferred embodiment, only one stage of filtering is used. More stages of filtering could be used at this point but, because of the high battery voltage, these stages utilize a great deal of circuit board space. 
     The output signal from the low-pass filter is then sent to a standard subscriber loop interface circuit (SLIC) device. The SLIC is a standard chip which splits the two incoming lines into four lines, a pair of receive lines and a pair of transmit lines. 
     The transmit lines are finally filtered after the SLIC chip to remove the remainder of the noise. When the filtering is done after the signal is passed through the SLIC component, the high battery voltage is no longer present. The filtering components, then, can exist on a single chip and will take up much less space on the circuit board. Alternatively, some of the filtering may be accomplished digitally after the signal has been digitized by the CODEC. 
     The present invention provides a way of dispensing with the multistage high-voltage filtering which typically occurs before the SLIC in favor of low voltage, and therefore smaller and more compact, filtering on the transmit lines after the SLIC. The resulting savings in space will allow more signal couplers to exist on a given circuit board. 
    
    
     DESCRIPTION OF THE FIGURES 
     FIG. 1 illustrates a block diagram of a telephone subscriber loop. 
     FIG. 2 illustrates a circuit diagram of a prior art POTS filtering arrangement. 
     FIG. 3 is a block diagram of a device to accomplish the present invention for POTS filtering. 
     FIG. 4 is a circuit diagram of the preferred embodiment of the present invention of POTS filtering. 
     FIG. 5 shows a second embodiment where part of the low voltage filtering is accomplished on the digital side of the circuit. 
    
    
     DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, most of the POTS filtering is removed from areas of the circuit which require components capable of handling the high battery voltage and DC current of the telephone subscriber loop to areas of the circuit where only low voltage filtering is required. 
     FIG. 3 shows a block diagram of the signal coupler  100  which will accomplish this result. The TIP line  10  and RING line  11  represent the two wires of the twisted copper pair. TIP line  10  and RING line  11  are inputted to the protection circuit  101 . The protection circuit  101 , a standard well-known circuit, provides the voltage and current protection necessary to guard the remainder of the device from high voltage and high current surges typically caused by lighting strikes, power cross, or other unexpected electrical events. The Bellcore administrative standard LATA Switching System Generic Requirements (LSSGR), the standard adopted by all Bell operating companies, has a first level lightening strike test and a second level lightening strike test. (See Lucent Technologies,  Overvoltage Protection of Solid-State Subscriber Loop Circuits,    APPLICATION NOTE, FEB.  1997, at 9-10). In the first level test, the circuit must survive the tests outlined in Table 1. The circuit need not survive the second level test, but no electrical shock or fire hazards can be created. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Bellcore First Level Lightning Strike Test 
               
             
          
           
               
                   
                   
                 Pulse (μs) 
                   
                   
               
               
                   
                 V p  (V) 
                 (Rise Time/Decay Time) 
                 I peak  (A) 
                 Reps 
               
               
                   
                   
               
               
                   
                 ± 600  
                 10/1000 
                 100 
                 50 
               
               
                   
                 ± 1000 
                 10/360  
                 100 
                 50 
               
               
                   
                 ± 100  
                 10/1000 
                 100 
                 50 
               
               
                   
                 ± 2500 
                 2/10  
                 500 
                 50 
               
               
                   
                   
               
             
          
         
       
     
     In a typical subscriber loop, the battery voltage across TIP  10  and RING  11  is 48 V and the loop may typically carry a current of from 23-35 mA. Although the current is nominally 25 mA, it could range as high as 120 mA. 
     The output signal from the protection circuit  101  on lines  12  and  13  is sent to low pass filter  109 . It&#39;s a good practice to use different numbers for components that are different (particularly when one is prior art and one is part of the invention). In addition, the lines DSLT  12  and DSLR  13  are inputted to high pass filter  106 . Low pass filter  109  passes the POTS signal while not passing the data signal. High pass filter  106  does not pass the POTS signal while passing the data signal. In this way, the data path is separated from the POTS path. 
     In addition to splitting the POTS signal and the data signal, low pass filter  109  also provides enough filtering to prevent overloading of the subscriber loop interface circuit (SLIC)  103 . This requires that the low pass filter  109  have at least one stage. In the preferred embodiment, low pass filter  109  has only one stage of filtering. The output of low pass filter  109  is PTIP  14  and PRING  15 . Signals PTIP  14  and PRING  15  are inputted to SLIC  103 . 
     The subscriber loop interface circuit (SLIC)  103  is a standard integrated circuit which includes the function of splitting two lines which carry both transmit and receive signals (i.e., bi-directional transmission) into four lines, two of which carry the transmit signal and two of which carry the receive signal. The SLIC  103  must also handle the 48 volt battery voltage supplied by the central office and 25 mA of current or more, as opposed to the typically 5 V commonly handled by such circuits. SLIC chips are standard chips and may be purchased, for example, from Lucent Technologies, AMD, Harris, or Mitel. The Lucent Technologies chip L7585 is the preferred component used with this invention. With the use of the standard SLIC chip, the filtering circuit shown in FIG. 3 will appear identical to a standard prior art filtering circuit for compatibility with existing equipment. 
     The SLIC  103  outputs the transmitted signals on lines  18  and  19 . The received signals are on lines  16  and  17 . The received signals are those signals received from the central office for transmission through the signal coupler  100 , to the subscriber. Lines TIP  10  and RING  11  will carry both the transmit and the received signals, the transmit signals being those that are received into the coupling circuit  100  and the receive signals being those that are received out of the coupling circuit  100 . 
     The transmit signals, on lines VTX  18  and VRTX  19 , are inputted to a low voltage filter  104 . Filter  104  finishes filtering the incoming transmit signals. The receive signals, RCVP  16  and RCVN  17 , are not filtered in this circuit. These signals would be filtered in another circuit as transmit signals before being switched into the circuit illustrated here as receive signals. The other coupling circuits and the switching circuits are shown in FIG.  1 . 
     The four lines—RCVP  16 , RCVN  17 , VTX  20  and VRTX  21 —are connected to coder/decoder circuit (CODEC)  105 . CODEC  105  receives the digital signal DRX  22  and outputs the analog receive signal between lines  16  and  17 . CODEC  105  receives the filtered analog transmit signal between lines  20  and  21  and outputs the corresponding digital signal on DRT  23 . CODEC  105  may also receive a clock signal  24  and a framing signal  25  in order to coordinate with a digital processing and switching circuit at the central office. 
     The high pass filter  106 , in addition to filtering out the POTS signal, also filters the 48 V battery voltage from between lines DSLT  12  and DSLR  13 . The output lines of high pass filter  106 , lines  24  and  25 , are connected to Hybrid  107 . Hybrid  107  performs the same two-wire to four-wire function that SLIC  103  performs without the necessity of being capable of handling the high battery voltage or the associated high DC current. The Hybrid may need to operate with high current in higher frequency ranges. The four-wire side of Hybrid  107  includes receive signals on receive lines  26  and  27  and transmit signals on lines  28  and  29 . The four-wires  26 ,  27 ,  28  and  29  are connected to CODEC  108 . Codec  108  converts the analog transmit signals on lines  28  and  29  to digital signals output on line  31  and the digital receive signals received on line  30  to analog signals on lines  26  and  27 . The digital lines  30  and  31  are connected to the digital processing and switching circuits at the central office (See FIG.  1 ). 
     FIG. 4 illustrates signal coupler  100  showing in greater detail the circuitry within the circuit protection circuit  101 , the low pass filter  109  and the high pass filter  106 . 
     The circuit protection  101  in FIG. 4 has two components, current protection and voltage protection. The current protection on TIP line  10  is accomplished by resistor R 1  in series with fuse F 1 . On RING line  11 , the current protection is accomplished by resistor R 2  in series with fuse F 2 . The combinations of resistor and fuse adhere to the Bellcore specifications, fuses F 1  and F 2  opening only in second level testing. In the embodiment shown in FIG. 4, R 1  and R 2  are both 5.6 Ω resistors. 
     The voltage protection is provided by component Z, which acts as a triggered voltage shunt. Monolithic protection devices consisting of one or more SCR-type thyristers are commonly available under the trade names such as SURGECTAR (Harris, Inc.), SIDACTOR (Teccor, Inc.) and LB1201 SLIC Protector (Lucent Technologies, Inc.). Although any device which prevents the voltage between lines TIP  10  and RING  11  from exceeding the Bellcore standard could be used, devices such as the SIDACTOR have the advantage of being benign until they are triggered. Preferably, Z is a SIDACTOR. Component Z is connected across lines TIP  10  and RING  11  with an output leg attached to a protection ground GNDPN. After Z triggers, it shunts the voltage across it directly to GNDPN. Preferably, Z should be chosen so that it triggers at around 200 volts. 
     The low pass filter  109  in FIG. 4 is a single-stage LRC circuit. Line  12  is connected in series with resistor R 3 , inductor L 1 , and resistor R 5 . Line  13  is connected in series with resistor R 4 , inductor L 2  and resistor R 6 . Capacitor C 1  is connected across the two lines, from a point between inductor L 1  and resistor R 5  to a point between inductor L 2  and resister R 6 . Inductors L 1  and L 2  comprise a four terminal inductor with an iron core. In FIG. 4, R 3  and R 4  are 15 Ω resistors, RS and R 6  are 20 Ω resistors, L 1  and L 2  are 18 mH inductors and C 1  is a 0.022 μF capacitor. 
     Although the low pass filter illustrated in FIG. 4 is the preferred embodiment, other embodiments will be apparent to one skilled in the art. For example, providing an initial coupled inductor as a common mode choke, adding further stages of filtering or using different combinations of resistors, capacitor and inductor in the filtering are options. The components of the low pass filter, however, operate with the 48 volt battery voltage provided at the central office and the inductors have to pass tens of milliamperes of current without saturation. As such, the components require a large amount of circuit board space. 
     The high pass filter  106  in FIG. 4 includes capacitor C 2  and transformer T 1 . Transformer T 1  has a first side and a second side. The first side is split and a first coil P 1  and a second coil P 2  of the first side are coupled through capacitor C 2 . An input lead of the first coil P 1  is connected to line DSLT  12  and the opposite input lead of the first coil P 1  is connected to a lead of capacitor C 2 . The opposite lead of capacitor C 2  is connected to an input lead of the second coil P 2 . The opposite input lead of coil P 2  is connected to line DSLR  13 . The two leads of the second coil of transformer T 1  are connected to Hybrid  107 . 
     The high pass filter  106  illustrated in FIG. 4 is of a standard type, any circuit which separates the high frequency data input from the DC and POTS signals can be used. After this separation is accomplished, the Hybrid  107  need only operate at low voltage. 
     The low voltage filter  104  can be any continuous time filter which provides the desired filtering. Several of these filters are well known in the art. Among the well known integrated circuit continuous time filters are G m -C filters and MOSFET-C filters. Each of these filter types can be implemented with multiple stages providing for filters of several orders. A standard ladder filter (which employs inductors and capacitors for a multistage filtering circuit similar to the high voltage filter shown in FIG. 2) will accomplish the low voltage filtering, however it is difficult to accurately construct an inductor on an IC chip and implementation with individual components will take a great deal of space on the circuit board and defeat the purpose of the invention. 
     A second embodiment of the invention is shown in FIG.  5 . The embodiment shown in FIG. 5 differs from the preferred embodiment shown in FIG. 4 in that part of the filtering is shifted to the digital side of CODEC  105 . In this embodiment, the filtering is split between a Low Voltage Filter  110  and a Digital Filtering Circuit  111  located on line  23 . 
     The examples illustrated here are representative examples and in no way limit the scope of this application. Other obvious embodiments of the invention will be apparent to one skilled in the art and are included within the scope of this application. One obvious embodiment is to not have a data path so that only POTS signals are processed.