Abstract:
A differential pair of a first and a second transistor is used in a tone burst generator circuit for generating tone bursts for periodic pulse metering. The first transistor carries all of the current through the differential pair when no tone burst signal is to be generated. When a tone burst signal is to be generated, the base or gate voltage of the first transistor is reduced relative to the base or gate voltage of the second transistor, thereby causing the second transistor to conduct and the first transistor to be turned off. A continuous tone signal connected to a common current path through the differential pair is then coupled through the second transistor to a telephone connection in order to apply a tone burst signal to the connection. When a tone burst signal is to be decoupled from the connection, the base or gate voltage of the first transistor is raised to a level above that of the second transistor, again causing all of the current through the differential pair to flow through the first transistor, thereby turning off the second transistor. This decouples the continuous tone signal from the telephone connection. A RC delay circuit is employed to control the rise and fall times of the tone burst signal.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates in general to tone burst generators, and more particularly, to such generators used in telephony suitable for periodic pulse metering. 
     In some telephone line circuits, such as those used at public telephones, it is desirable to include tone burst generators. The generators transmit tone bursts to telephone station equipment for the purpose of incrementing call cost meters so that coin collection may be controlled at public or other telephones where toll is charged based on the time duration for calls. A key consideration in the design of such generators is to avoid audible noise caused by the tone bursts. For each telephone line circuit that is periodic pulse metered, a tone burst generator is required. It is, therefore, important to reduce the costs of such generators. 
     Tone burst generators have been proposed using inductors or 4-quadrant linear multiplier circuits which are expensive. Another tone burst generator is proposed in U.S. Pat. No. 5,373,552. While the above tone burst generators are useful for some applications, they are not entirely satisfactory. It is, therefore, desirable to provide an improved tone burst generator with superior characteristics. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is directed towards a tone burst generator comprising means for generating a continuous tone signal and a circuit converting said continuous tone signal into a tone burst signal having predetermined transition times, said circuit including a differential pair of a first and a second transistor, a common current path coupling the continuous tone signal to the differential pair and means for turning on and off one of the transistors to provide a tone burst signal having said predetermined transitions. 
     Another aspect of the invention is directed towards an apparatus for transmission of telephone signals comprising a transmission circuit including at least one first connection transmitting the telephone signals from a telephone company station to a subscriber location and at least one second connection receiving telephone signals from the subscriber location and transmitting such signals to the telephone company station and at least one generator supplying to said at least one first connection a tone burst signal having predetermined transitions. The at least one generator comprises means for generating a continuous tone signal; a differential pair of a first and a second transistor; a common current path coupling the continuous tone signal to the differential pair and means for turning on and off one of the transistors to provide a tone burst signal having predetermined transitions to provide to said at least one first connection a tone burst signal having said predetermined transitions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic circuit diagram of a transmission circuit for transmitting telephone signals between a telephone company station and a subscriber location and a tone burst generator supplying a tone burst signal to the transmission circuit to illustrate the preferred embodiment of the invention. 
     FIG. 2 is a graphical plot of the tone burst signal supplied by the tone burst generator of FIG. 1 to the transmission circuit and of a control signal applied. to the tone burst generator to illustrate the invention. 
     FIG. 3 is a schematic circuit diagram of six tone burst generator circuits, a common bias circuit and a common low noise power rail circuit to illustrate a preferred embodiment of the invention. 
     FIG. 4 is a schematic circuit diagram of a transmission circuit for transmitting telephone signals between a telephone company station and a subscriber location and a tone burst generator supplying a tone burst signal to the transmission circuit, similar to the embodiment of FIG. 1, but where a transformer is used in the transmission circuit to illustrate an alternative embodiment of the invention. 
    
    
     For simplicity, identical components are identified by the same numerals in this application. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in FIG. 1, a transmission circuit  10  includes a coder/decoder (CODEC)  12  receiving telephone signals from a telephone company station (not shown) along time division multiplex bus  14  for transmission to the subscriber location (not shown), and sending telephone signals from the subscriber location to the telephone company station along bus  14 . Transmission circuit  10  also includes a Subscriber Line Interface Circuit (SLIC)  16 , typically an integrated circuit, which is connected to a subscriber location (not shown) by means of TIP line  18  and RING line  20 . CODEC  12  transmits telephone signals received from bus  14  and transmits them through output  12   a  (signals at voltage Vout) along connection  22  to input  16   a  (RSN) of SLIC  16 , where SLIC  16  sends the signal along the TIP and RING lines  18  and  20  to the subscriber. SLIC  16  also receives telephone signals from the subscriber along lines  18 ,  20  and sends such signals from output  16   b  at voltage VTX along connection  24  to the input Vin of CODEC  12 . 
     Connection  24  is connected to connection  22  by means of a resistor  26  with resistance R B  and connection  24  also includes in its path an operational amplifier  28  and resistor  30  with value R TX , where the two resistors and the operational amplifier are used for echo cancellation in the manner described, for example, in the  Telecommunication Circuit Databook  by Ericsson Components AB (1994), at page 4-152. As described by Ericsson, echo cancellation is accomplished if the following holds:                  VTX     R   TX       +       V   OUT       R   B         =   0           (   1   )                                
     where VTX is the voltage applied by SLIC  16  to connection  24  at output  16   b  and V OUT  is the output voltage applied by CODEC  12  to connection  22  at output  12   a.    
     As shown in FIG. 1, operational amplifier  28  has a negative feedback path through a resistor  32 . An impedance matching circuit  34  with impedance ZT is connected between the connections  22 ,  24 . 
     FIG. 1 also shows a tone burst generator circuit  50  for generating a tone burst signal applied to the transmission circuit  10 . A continuous tone from an oscillator (not shown) is supplied to terminal  52  and generator circuit  50  converts the continuous tone and signal into a tone burst having predetermined transition times for application to the transmission circuit  10 . Generator circuit  50  employs a differential pair of transistors  54 ,  56  whose emitters are connected to a common node  58  through one of a pair of resistors  60 ,  62  of small values, such as about 10 Ohms, where the common node  58  is connected to a voltage reference V ee  at terminal  64  through two resistors  66  and  68 . The continuous tone signal at terminal  52  is coupled to node  70  between resistors  66  and  68  through a capacitor  72 . The collector of transistor  56  or Q 2  is coupled to connection  22  through a capacitor  74  and resistor  76  of resistance R PM . Thus, when transistor  56  or Q 2  conducts, the continuous tone signal at terminal  52  is applied to connection  22  of the transmission circuit  10 . By controlling the turning on and off of transistor  56 , it is possible to cause a tone burst signal having predetermined transition times to be applied to connection  22 . The tone burst signal is, in turn, conveyed by SLIC  16  to the wires  18 ,  20  for periodic pulse metering purposes. 
     The turning on and off of transistor  56  will now be described. As shown in FIG. 1, the gate of transistor  56  is biased at a fixed DC level by means of a voltage divider circuit comprising resistors  82 ,  84  and voltage supply V cc . When no tone burst signal is to be applied to the transmission circuit  10 , the gate of transistor  54  is biased at a voltage higher than that applied to the gate of transistor  56 , so that all of the current through the differential pair  54 ,  56  flows through transistor  54 , so that the continuous tone signal at terminal  52  is not coupled to connection  22 . But when a PPM enable signal is applied to terminal  86 , the voltage at the gate of transistor  54  or Q 1  is reduced, thereby also reducing the current flowing through the transistor. Further reduction of the gate voltage of transistor  54  causes the transistor  54  or Q 1  to be turned off so that all of the current through the differential pair now flows through transistor  56  or Q 2 . The continuous tone signal applied to terminal  52  is then coupled through transistor  56  or Q 2  to the connection  22 . 
     In reference to FIGS. 1 and 2 and in the preferred embodiment, the gate  56   g  of transistor  56  or Q 2  is maintained at a constant voltage  56   g ′ of about 350 millivolts, and when no tone burst signal is to be applied to the transmission circuit  10 , the DC voltage is applied to terminal  86  so that the voltage  54   g ′ at gate  54   g  of transistor  54  or Q 1  is at about 750 millivolts. As shown in FIG. 1, this can be accomplished by choosing the appropriate values for resistors  92  and  94  which form a voltage divider circuit and the appropriate DC voltage at terminal  86 . When a tone burst signal is to be sent to the transmission circuit  10 , this DC voltage at terminal  86  is decreased to ground. Since the gate  54   g  is connected to ground through capacitor  96 , the RC delay caused by resistor  92  and capacitor  96  causes the voltage  54   g ′ of gate  54   g  to be reduced gradually, as illustrated by curve  54   g ′ in FIG. 2, and of a tone burst signal  90  at node  91  that is applied to the transmission circuit  10 . In FIG. 2, the curve illustrating the gate voltage of transistor  54  is labelled  54   g ′ and the tone burst signal is labelled  90 . 
     The values of resistors  82 ,  84  are chosen in the preferred embodiment so that the gate  56   g  of transistor  56  or Q 2  is maintained at about 350 millivolts. As the gate voltage of transistor  54  or Q 1  falls to about 375 millivolts, the transistor  56  or Q 2  begins to conduct. In reference to FIG. 2, this happens at about 8 or 9 milliseconds from an arbitrary reference zero seconds. Also, at about this time, the continuous tone signal applied to terminal  52  begins to be coupled to connection  22  through transistor  56 , capacitor  74  and resistor  76  and is illustrated as curve  90  in FIG.  2 . When the gate voltage  54   g  falls below 350 millivolts, transistor  56  or Q 2  becomes fully conducting so that the amplitude of the tone burst reaches its full amplitude at about 10 or 11 milliseconds from time zero. Thus, by controlling the DC voltage applied to terminal  86 , it is possible to cause a tone burst signal to be applied to connection  22  of transmission circuit  10  at a predetermined transition time. By employing a RC delay circuit between the PPM enable signal applied to terminal  86  and the gate  54   g  of transistor  54 , it is also possible to control the tone burst signal so that it has a rise time of not less than about 100 microseconds, although in the preferred embodiment, the rise time of tone burst signal  90  is not less than about 1 millisecond. By causing the tone burst signal to have a minimum rise time, audible noise caused by the tone burst signal to subscribers is reduced. 
     As shown in FIG. 2, the gate voltage of transistor  54  continues to fall until it is close to ground. When the tone burst signal is to be turned off, the DC voltage applied to terminal  86  is raised, so that the gate voltage at gate  54   g  returns gradually to 750 millivolts, again due to the delay effect of resistor  92  and capacitor  96 . When the gate voltage  54   g  rises to above 350 millivolts (the voltage maintained at gate  56   g  of transistor  56  or Q 2 ), transistor  56  or Q 2  begins to turn off, thereby gradually decoupling the continuous tone signal applied to terminal  52  from connection  22  of circuit  10 , until at about 50 milliseconds from time zero, the continuous tone signal is completely decoupled from circuit  10 . Therefore, by controlling the DC voltage level at terminal  86 , it is possible to cause the turning off of the tone burst signal at a predetermined transition time. The RC delay effect of resistor  92  and capacitor  96  causes the tone burst signal to have a fall time of not less than 100 microseconds, and of not less than about 1 millisecond in the preferred embodiment. 
     While in the embodiment above, the turning on and off of transistor  56  is accomplished by maintaining gate  56   g  at a substantially constant voltage and changing the gate voltage of transistor  54 , it will be understood that substantially the same effect can be achieved by maintaining the gate  54   g  of transistor  54  constant and changing the gate voltage of transistor  56 . Such and other variations are within the scope of the invention. 
     In the preferred embodiment, the voltage reference V ee  at terminal  64  is a negative potential such as about −5 volts, although other reference potentials such as ground may be adequate. The two resistors  60 ,  62  of small values render the differential pair more tolerant of differences between the characteristics of the two transistors so that commercial transistors may be used to reduce costs. While bipolar transistors are illustrated in FIG. 1, it will be understood that field effect transistors may be used instead and are within the scope of the invention. When field effect transistors are used, the sources of the transistors are connected to node  58  through resistors  60 ,  62 , their drains connected to V cc  and node  91 , and their gates are at the same positions  54   g ,  56   g  as the bipolar transistors. 
     As in the case of voice signals, an echo of the tone burst signal may also appear on connection  24 . Cancellation of such echo can be achieved if the resistances R TX , R PM  of resistors  30 ,  76  and value ZT of impedance  34  are chosen so that the following holds:                R   PMC     =       R   TX     ×       R   PM     ZT     ×         ZT   1000     +     2        R   F       +     Z   L           Z   L     +     2        R   F                     (   2   )                                
     where ZT is the impedance of impedance  34 , R TX  the resistance of resistor  30 , R PM  the resistance of resistor  76 , R PMC  the resistance of resistor  89  of FIG. 1, R F  the resistance of the line feed or fuse resistor (not shown), and Z L  is the line impedance, and where the ratio ZT/1000 depends on the type of SLIC used and can be different for different SLICs. For a more detailed description related to equation (2) above, please see  Telecommunication Circuit Databook  by Ericsson Components AB (1994), page 4-152. 
     Telephone equipment frequently includes more than one transmission circuit  10  of FIG. 1, so that a tone burst generator  50  would need to be employed for each pair of such connections  22 ,  24  in such circuit. FIG. 3 is a schematic circuit diagram of six identical tone burst generator circuits  50 ′ substantially the same as generator circuit  50  of FIG. 1 for supplying tone burst signals to six corresponding transmission circuits (not shown) of the type shown in FIG. 1 to illustrate the preferred embodiment of the invention, it being understood that the concept can be extended to any number of transmission circuits. Since potentially a large number of tone burst generator circuits may be employed, to reduce costs, it may be possible to design the tone burst generator circuits so that they can share common components. Thus, it may be possible for all six circuits of FIG. 3 to share the common terminals  70  (shown as VIN and  70 ′ in FIG.  3 ),  58 , resistors  66 ,  68  and voltage reference V ee , terminal  64  of FIG. 1 and a continuous tone input is applied through a capacitor (not shown) to terminal  70 ′. 
     It may also be possible for the six tone burst generator circuits to share the same voltage divider circuit  95  for biasing gates  56   g  of transistors  56  at the terminals BIAS, as shown in FIG.  3 . Thus, all six tone burst generator circuits share the same voltage divider circuit  95  formed by resistors  82 ,  84  and a voltage reference of +5 volts, as well as capacitor  102 . In FIG. 1, the collector of transistor  54  is connected to voltage reference V cc  and the collector of transistor  56  is connected to the same reference through a resistor  104 . 
     If the voltage reference V cc  is noisy, such noise will be coupled through resistor  104  to connection  22 . This is undesirable. For this reason, in the preferred embodiment as illustrated in FIG. 3, a low noise voltage reference at node VFIL is used instead and connected to the collector of transistor  56  through resistor  104 . The same low noise voltage reference may be used for all six generator circuits, as shown in FIG.  3 . The low noise voltage reference is obtained by connecting a commercial voltage reference of +5 volts through a low pass filter comprising resistor  106  and capacitor  108 ., and by using a voltage divider circuit comprising resistors  106  and  110  and an operational amplifier  112  in negative feedback mode. The output of operational amplifier  112  is then used as the low noise voltage reference and applied to node VFIL in FIG.  3 . 
     Instead of using a subscriber line interface circuit  16 , which is typically an integrated circuit, it is possible to replace such circuit by a transformer as indicated in FIG.  4 . As shown in FIG. 4, the telephone signals and connections  122  and  124  are coupled to the TIP and RING lines through a transformer  150 . Subscriber loops using transformers are known to those skilled in the art. For example, see  Databook  1997-1998, issued by the Microelectronics Group of Lucent Technologies for Analog Line Card Products. 
     Echo cancellation is accomplished by employing an additional operational amplifier  152  in connection  122 , in negative feedback mode through resistor  154 . By feeding back a portion of the signal at the output of operational amplifier  152  to the positive input of operational amplifier  28  through a resistor  26 , and by choosing appropriate resistor values for resistors  26  and  154 , according to equation 1 above, echos of voice signals appearing in connection  124  is cancelled by such feedback action. Such feedback also cancels the tone burst signal that is coupled to connection  122  from the collector of transistor  56 . Other than such differences, the transmission circuit  200  and tone burst generator circuit  50  operate essentially as described above for transmission circuit  10  and generator  50  to inject a tone burst signal at predetermined transition times and of desired rise and fall times to connection  122 . 
     In the embodiment of FIG. 3, the values of the resistors and capacitors are listed below, where the left column indicates the numbers used in this application to identify the components, and the right column their corresponding values: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 66 
                 1.4 
                 kΩ 
               
               
                   
                 68 
                 2.43 
                 kΩ 
               
               
                   
                 74 
                 1.5 
                 nF 
               
               
                   
                 82 
                 10 
                 kΩ 
               
               
                   
                 84 
                 750 
                 Ω 
               
               
                   
                 92 
                 10 
                 kΩ 
               
               
                   
                 94 
                 1.5 
                 kΩ 
               
               
                   
                 96 
                 4.7 
                 uF 
               
               
                   
                 102 
                 100 
                 nF 
               
               
                   
                 104 
                 2 
                 kΩ 
               
               
                   
                 106 
                 9.09 
                 kΩ 
               
               
                   
                 108 
                 100 
                 nF 
               
               
                   
                 110 
                 80.6 
                 kΩ 
               
               
                   
                   
               
             
          
         
       
     
     While the invention has been described above by reference to various embodiments, it will be understood that different changes and modification may be made without departing from the scope of the invention which is to be defined only by the appended claims and their equivalents.