Telephone interface circuit with impedance termination during call pauses

Abstract: A telephone interface circuit with impedance termination during call pauses, comprising: a first and a second terminals of a telephone line; a branch connected to the first terminal, across a decoupling capacitor, and to the second terminal, and furthermore comprising an intersection node; the branch comprises a first switch and a first impedance resistor between the first terminal and the intersection node and a second switch and a second impedance resistor between the second terminal and the intersection node; the intersection node is connected to the ground across a third switch; the first switch, the second switch, and the third switch are controlled by a control unit by means of a first, a second and a third control lines respectively. The circuit furthermore comprises a ring signal detector that is connected to a microprocessor unit capable of activating the control unit in the interval in which the circuit must be terminated for CLID (Calling Line IDentification) transmission.

Description: The present invention relates to a telephone interface circuit with impedance termination during call pauses. New additional functions are required as the performance required of a telephone terminal evolves. An aspect of this performance is the requirement to identify the user who is calling and to optionally show this identification on a display on the set of the called party. This function is known as CID (Call IDentification) or as CLID (Calling Line IDentification) and occurs for example by transmitting, over the telephone line, information Tx at a low amplitude level and at a low bit rate (V23/1200 baud) in the interval A between the two ring signals RING1 and RING2, as illustrated in figure 1. Document WO-A-85/00488 discloses an apparatus performing this function. During the interval A, it is necessary to terminate the telephone line at a very specific impedance (typically 1200 ohm). This impedance termination during the call interval must be performed equally by using either the power supply taken from the telephone line itself or by means of external power supply. It should be remembered that no DC current must be absorbed from the telephone line during ringing. Furthermore, line termination during the call interval requires the insertion of new circuits that must not alter the current performance of the telephone set. The line termination circuit must be capable of withstanding the high voltages that can be present in the various operating configurations. Furthermore, it must be independent of the polarity of the line. Moreover, during the conversation and calling periods the circuit must be completely open (OFF) and must have low consumption. The aim of the present invention is to provide a telephone interface circuit with impedance termination during call pauses that is capable of terminating the telephone line at a preset impedance value during the interval between two ring signals. Within the scope of this aim, an object of the present invention is to provide a telephone interface circuit with impedance termination during call pauses that does not alter the current performance of the telephone set. Another object of the present invention is to provide a telephone interface circuit with impedance termination during call pauses that is capable of withstanding the high voltages that may be present in the various operating configurations. Another object of the present invention is to provide a telephone interface circuit with impedance termination during call pauses that is independent of the polarity of the line. Another object of the present invention is to provide a telephone interface circuit with impedance termination during call pauses that is a fully open circuit during conversation and calling periods and has low consumption. Another object of the present invention is to provide a telephone interface circuit with impedance termination during call pauses that is DC-decoupled from the telephone line. Another object of the present invention is to provide a telephone interface circuit with impedance termination during call pauses that is highly reliable, relatively easy to manufacture, and at competitive costs. This aim, these objects, and others which will become apparent hereinafter are achieved by a telephone interface circuit with impedance termination during call pauses, characterized in that it comprises: a first and a second terminals of a telephone line; a branch connected to said first terminal, across a decoupling capacitor, and to said second terminal, said branch further comprising an intersection node; said branch comprising a first switch and a first impedance resistor between said first terminal and said intersection node and a second switch and a second impedance resistor between said second terminal and said intersection node; said intersection node being connected to the ground across a third switch; said first switch, said second switch, and said third switch being controlled by a control unit by means of a first, a second and a third control lines respectively. The characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment thereof, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is an example of CLID transmission in the interval between two ring signals; figure 2a is a block diagram of the ring signal power supply and detection circuit if a piezoelectric transducer is used; figure 2b is a block diagram of the ring signal power supply and detection circuit if a loudspeaker is used; figure 3 is a block diagram of the line termination circuit according to the present invention; figure 4 is a detailed electric diagram of the line termination circuit according to the present invention; figures 5a-c are electric diagrams of possible embodiments of the control unit of the circuit according to the present invention. With reference to figure 2a, the first and the second terminals of the telephone line are respectively designated by the reference numerals 1 and 2. A decoupling capacitor 3, adapted to decouple the telephone line from the DC component, is arranged on the first terminal 1 of the telephone line. A resistor 4 is furthermore connected to the capacitor 3 and helps to determine the impedance of the telephone line during ringing. The two telephone line terminals 1 and 2 are connected to terminals of a classic-bridge rectifier 5. The input of a ring signal detector 6 is connected to the output of the rectifier 5. The detector 6 is adapted to detect the presence of a ring signal RING1 or RING 2 on the telephone line. According to the result of the detection, said detector sends a signal MPU to the microprocessor unit (not shown) that controls the impedance of the entire circuit. A filtering capacitor 7 is furthermore parallel-connected to the rectifier 5 and is adapted to filter the signal rectified by the rectifier 5. Furthermore, a diode 8 is series-connected to the rectifier 5 and allows the block 6 to detect the frequency of the ring signal. A driving circuit for a piezoelectric transducer 9 of the high-voltage type (approximately 30 volts) is arranged in parallel to the filtering capacitor 7 and is protected by a Zener diode 10 that is parallel-connected thereto. The output of the circuit of figure 2a is designated by the letter S and is sent to the remaining part of the system for power supply. Figure 2b illustrates a power supply extraction circuit for a driving circuit for an acoustic transducer, that is to say, a loudspeaker. Said circuit, like the previous circuit, comprises a decoupling capacitor 3. The terminals 1 and 2 of the telephone line are, as in the previous case, connected to the rectifier 5, whose output is connected to a telephone ring signal converter 11. A control resistor 12 is furthermore associated with the input of the converter 11 and adjusts its impedance. The converter 11 furthermore comprises the ring signal detector 6, which sends the signal MPU to the microprocessor unit. The converter 11 comprises the transistor 16, the capacitor 17, the inductor 13, and the diode 18, which allow a switched mode power supply. A protection Zener diode 19, a filtering capacitor 20, and a loudspeaker 21 are parallel-connected on the same branch. As in the previous case, the output of the circuit of figure 2b is designated by the letter S and is sent to the remaining part of the system for power supply. In the circuit of figure 2a, the impedance is determined by the values of the capacitors 3 and of the resistor 4, whereas in the case of the circuit of figure 2b the impedance is determined by the value of the resistor 12 divided by an internal current mirroring factor K of the converter 11. Figure 3 illustrates the telephone line termination circuit according to the present invention. The terminals of the telephone line, as before, are designated by the reference numerals 1 and 2. The termination circuit can use the same decoupling capacitor 3 and the same impedance resistor 4 as the power supply circuit of figures 2a and 2b. The first terminal 1 is connected, downstream of the capacitor 3, to the second terminal by means of a line termination branch 22. Said branch comprises a first switch 23 and a second switch 24 connected in series. An intersection node 25 is provided between the first switch 23 and the second switch 24, so that the branch 22 is divided into two half-branches. A resistor is arranged in each of the half-branches and is connected in series to each one of the respective switches 23 and 24. In this manner, a first impedance resistor 26 is series-connected to the first switch 23 and a second impedance resistor 27 is series-connected to the second switch. The corresponding resistor can be located on either side of the switch 23 or 24. The intersection node 25 is connected to the ground GND by means of a third switch 28. All three switches 23, 24, and 28 are controlled by a control unit 29 by means of the respective control lines I1, I2, and I3. The control unit 29 is powered by the power supply S and receives a control signal µC from the microprocessor unit 30. The control signal µC is sent to the control unit 29 during the interval A of figure 1, that is to say, when the circuit must be terminated for CLID transmission. The circuit furthermore comprises the rectifier 5, which is connected to the terminals 1 and 2 and supplies the ring signal circuit, generally designated by the reference numeral 31 herein. The microprocessor furthermore has inhibition lines 70a and 70b that deactivate the rectifier 5, if it is provided in the active mode, or the circuit located downstream of the ring signal circuit 31 in order to eliminate any distortions of the signal in the presence of high levels. The hardware embodiment of the circuit of figure 3 is shown in figure 4. Each one of the switches 23, 24, and 28 is composed of an N-channel MOSFET transistor, of a Zener diode, and of a resistor. More specifically, the first switch 23 comprises a first N-channel transistor 32 in which the source and the body are connected downstream of the first terminal 1 and the drain is connected to the node 25 (across the resistor 26). The gate of the transistor 32 is connected to the first control line I1. A first resistor 33 connects the first terminal 1 to the control line I1, and a Zener diode 34 is parallel-connected to said first resistor (the cathode is connected to the control line I1). The transistor 32 furthermore comprises an intrinsic diode 35, illustrated herein for the sake of practicality, which is formed by the semiconductor structure of the transistor itself. The anode of the diode 35 is connected to the source of the transistor 32 and the cathode is connected to the drain of the transistor 32. The second switch 24 comprises a second N-channel transistor 36 in which the source and body terminals are connected to the second terminal 2 and the drain terminal is connected to the intersection node 25 (across the resistor 27). The gate of the transistor 36 is connected to the second control line I2. A second resistor 37 connects the control line I2 to the second terminal 2. The cathode of a second Zener diode 38 is connected to the second control line I2, whereas the anode is connected to the second terminal 2. Like the first transistor 32, the second transistor 36 also comprises an intrinsic diode 39. The third switch 28 comprises a third N-channel transistor 40 in which the source and the body are connected to the ground and the drain is connected to the intersection node 25. The gate of the transistor 40 is connected to the third control line I3. A third resistor 41 connects the third control line I3 to the ground, and a third Zener diode 42 is parallel-connected to said resistor, its cathode being connected to the third control line I3 and its anode being connected to the ground GND. Finally, the control unit 29 comprises an activation transistor 43 driven by the microprocessor unit 30. Said transistor 43 is series-connected to a current source Is that is connected to a current mirroring unit 44, which generates the currents that are sent over the control lines I1, I2, and I3. The operation of the circuit according to the present invention is as follows. Initially, the ring signal detector 6 detects the presence of ring signals and notifies the microprocessor unit 30 of the presence of said signals. The microprocessor unit 30 activates, at the appropriate time, the activation transistor 43, which in turn activates the current source Is. The current source Is induces a current value in the current mirroring unit 44, so that the control lines I1-3 conduct a current that is set by the current source Is. When the currents are activated in the control lines, the current in the control line I3 induces a voltage across the third resistor 41 and causes a voltage drop across the gate and the source of the third transistor 40. In this manner, the transistor 40 is switched on and the intersection point 25 is set to the ground voltage. Then the control line I1 induces a current across the first resistor 33, the intrinsic diode 35, and the resistor 26 towards the intersection point 25, which is at the ground value. In this manner, a voltage drop is produced across the resistor 33, causing the first transistor 32 (which has a low internal resistance R on ) to switch on. Likewise, the second control line I2 induces a current across the resistor 37, the intrinsic diode 39, and the resistor 27 towards the intersection point 25, which is at ground voltage. In this manner, a voltage drop is produced across the second resistor 37, switching on the second transistor 36 (which has a low resistance R on ). Since both the first transistor 32 and the second transistor 36 are switched on, the telephone line formed by the terminals 1 and 2 is terminated with the impedance resistors 26 and 27 and by the internal resistance R on of the transistors 32 and 36. The CLID signal is an AC signal (see figure 1) having a modest amplitude. The values of the impedance resistors 26 and 27 must be correlated to the internal resistance value R on of the transistors 32 and 36, so that the CLID current does not switch on the intrinsic diodes 35 and 39. In practice, the impedance resistors must be much higher in value than the internal resistances R on of the transistors 32 and 36. The control unit 29 can be provided in various manners. Figures 5a-c illustrate three possible embodiments of the control unit 29. With reference to figure 5a, the activation transistor 43 of the N-channel type, activated by the microprocessor unit 30, is series-connected to an adjustment resistor 45, which in turn is connected to the drain/gate of an adjustment transistor 46 of the P-channel type. The gate of the transistor 46 is connected to the gates of a first current source transistor 47, of a second current source transistor 48, and of a third current source transistor 49, all of which are of the P-channel type. The source of the adjustment transistor 46 is also connected to the sources of the transistors 47-49, and said source terminals are connected, across a diode 50, to the supply voltage S. The drains of the transistors 47-49 constitute the respective outputs for the control lines I1, I2, and I3. This current source is useful when the supply voltage S is defined as external and therefore the value of the adjustment resistor 45 sets the value of the absorbed current. With reference to figure 5b, the activation transistor 43 is again provided and is connected to the ground GND. Said transistor 43 is series-connected to an adjustment resistor 51, which is connected to the emitter of an NPN transistor 52. The base of the transistor 52 is connected to the drain of the activation transistor 43 across two diodes 53 and 54. The base is furthermore connected to the supply voltage S across a resistor 55. The collector of the NPN transistor 52 is connected to the gate and to the drain of a P-channel transistor 56. The gate of the transistor 56 is connected to the gates of three current source transistors 57, 58, and 59, all of which are of the P-channel type. The sources of the transistors 57-59 are also connected to the source of the transistor 56 and to the supply voltage S across a diode 60. The drains of the transistors 57-59 constitute the respective outputs for the control lines I1, I2, and I3. The current delivered in this embodiment depends on the voltage V be divided by the value of the resistor 51 and is therefore temperature-dependent, but offers the advantage that it is independent of the supply voltage S. The absorbed current is substantially constant, since the value of the resistor 55 is sufficiently high. With reference to figure 5c, the reference numeral 43 designates the activation transistor, which is driven by the microprocessor unit 30. Said transistor is series-connected to an adjustment resistor 61 and to the source of a MOSFET transistor 62 of the depletion type. The gate and the body terminal of the transistor 62 are connected to the ground. Furthermore, the anode of the Zener diode 63 is connected to the gate of the transistor 62 and its cathode is connected to the drain of the transistor 43. The drain terminal of the depletion transistor 62 is connected to the gate and to the drain of a P-channel transistor 64. The gate of the transistor 64 is furthermore connected to the gates of three transistors of the P-channel type 65, 66, and 67. The source of the transistor 64 is also common-connected to the sources of the transistors 65-67, which are connected to the supply voltage S across a diode 68. The drains of the transistors 65-67 respectively constitute the outputs for the control lines I1, I2, and I3. The transistors 46-49 of figure 5a may be replaced with PNP transistors. The transistors 56-59 of figure 5b may be replaced with PNP transistors. The transistors 64-67 of figure 5c may be replaced with PNP transistors. It is important to stress that the control lines I1, I2, and I3 must be connected to a junction P at the current mirroring unit 44 if there is a negative voltage across the terminals 1 and 2 of the telephone line in the OFF state of the termination circuit. From the above description it is evident that the present invention fully achieves the intended aim and objects. The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept. Finally, all the details may be replaced with other technically equivalent ones. In practice, the materials employed, as well as the shapes and dimensions, may be any according to the requirements, without thereby abandoning the scope of the protection of the appended claims. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: Telephone interface circuit with impedance termination during call pauses, characterized in that it comprises: a first (1) and a second (2) terminals of a telephone line; a branch (22) that is connected to said first terminal, across a decoupling capacitor (3), and to said second terminal, said branch further comprising an intersection node (25); said branch comprising a first switch (23) and a first impedance resistors (26) between said first terminal and said intersection node and a second switch (24) and a second impedance resistor (27) between said second terminal and said intersection node; said intersection node being connected to the ground across a third switch (28); said first switch, said second switch, and said third switch being controlled by a control unit (29) by means of a first, a second and a third control lines respectively. Circuit according to claim 1, characterized in that said first switch comprises: a first transistor (32), in which the source is connected to said first terminal and the drain is connected to said intersection node; a first resistor (33), which connects said first terminal to said first control line; and a Zener diode (34), in which the anode is connected to said first terminal and the cathode is connected to said first control line. Circuit according to claim 2, characterized in that said first transistor comprises a first intrinsic diode (35), in which the anode is constituted by the source of said first transistor and the cathode is constituted by the drain of said first transistor. Circuit according to claim 1, characterized in that said second switch comprises: a second transistor (36), in which the source is connected to said second terminal and the drain is connected to said intersection node; a second resistor (37), which connects said second terminal to said second control line; and a Zener diode (38), in which the anode is connected to said second terminal and the cathode is connected to said second control line. Circuit according to claim 4, characterized in that said second transistor comprises a second intrinsic diode (39), in which the anode is constituted by the source of said second transistor and the cathode is constituted by the drain of said second transistor. Circuit according to claim 1, characterized in that said third switch comprises: a third transistor (40), in which the source is connected to the ground and the drain is connected to said intersection node; a third resistor (41), which connects the ground to said third control line; and a Zener diode (42), in which the anode is connected to the ground and the cathode is connected to said third control line. Circuit according to claim 1, characterized in that it comprises a ring signal detector (6) adapted to detect the presence of ring signals on said telephone line, said ring signal detector being connected to a microprocessor unit (30) adapted to activate said control unit. Circuit according to claim 1, characterized in that said control unit comprises a current mirroring unit (44) adapted to supply power tc said control lines. Circuit according to claim 8, characterized in that said control unit comprises a transistor (43) for the activation of said current mirroring unit, said transistor being activated during the interval in which CLID transmission occurs. Circuit according to claim 8, characterized in that the outputs of said current mirroring unit that are connected to said control lines are constituted by the P-type junction of the transistors that compose said current mirroring unit.

Inventors: ANTONINI CARLO, CONSIGLIO PIETRO, ANTONINI, CARLO, CONSIGLIO, PIETRO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1995
Main classification: H04M1
IPC codes: H04M19, H04M1