TELEPHONE LINE INTERFACE CIRCUIT WITHOUT HOOKSWITCH

Abstract: A novel electronic inductor circuit is used to establish a DC current drawn from the telephone line needed for indicating to a central office (or PBX) that a telephone device (i.e. a modem) has seized the line. The electronic inductor's transistor is used to perform the functions of a hookswitch, thereby avoiding the introduction of unwanted impedance in the AC signal path of the device. In order to go on-hook, the electronic inductor's transistor (upon receiving a control signal) stops drawing current from the central office loop indicating that it is now in the on-hook state. Unlike a switch, the present invention operates by starting and stopping the DC current while the entire circuit remains connected to the line at all times. The line is "seized" by providing a control voltage to turn on the electronic inductor and thereby initialize line current to flow through its transistor - thus achieving the "off-hook" state.

Description: TELEPHONE LINE INTERFACE CIRCUIT WITHOUT HOOKSWITCH BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to the field of telephone line interface circuitry, and more particularly to a telephone line interface circuit without a hookswitch. 2. Description of Related Art A hookswitch determines whether a telephone device, such as a modem, is off-hook or on-hook by allowing or disallowing DC line current through the device. The hookswitch has previously been viewed as an essential element of a telephone line interface circuit. A prior-art line interface circuit employing a relay 6 as a hookswitch is illustrated in Figure 1. The relay 6 is connected to a telephone line (not shown) through a standard diode bridge 4 connected to the Tip and Ring signals. The Tip and Ring signals are also connected to a ring detect circuit (not shown) via terminals A and B. The relay 6 is used extensively as a hookswitch due to the extremely low impedance between its terminals. However, the relay 6 is bulky and requires a fair amount of power in the primary circuit in order to be activated. Additionally, unwanted high-voltage spikes can be generated across the relay's terminals when the device goes on-hook or off- hook. Note that the relay 6 can completely disconnect the modem circuitry 2 from the telephone line. As shown in Figure 2, a transistor 8 is also used frequently as a hookswitch in the prior-art, but a transistor 8 also exhibits undesirable consequences. In particular, in conventional hookswitch applications, the transistor 8 introduces an unwanted voltage drop between the Tip and Ring terminals and the telephone device with consequent dissipation of power, distortion in the transmitted signal, and considerable changes in return loss measurements as a function of DC line current. Note _,„, , PCT/US99/30098 O 00/36813 -2- that although a FET is shown, a bipolar transistor may also be used. Also, as in the relay circuit of Figure 1, the transistor 8 can completely disconnect the modem circuitry 2 from the telephone line. Furthermore, to work effectively, a transistor used as a hookswitch must be operated in the saturation region and maintained as such throughout the full range of line currents. Although a transistor can be kept saturated with little effort when the line current is 20mA, it can be a challenge to keep it saturated when the current increases to 100mA. The impedance introduced by a transistor switch also decreases the DC operating voltage of the typically associated electronic inductor circuit (also known as a "current holding circuit"). Thus, there is a need for an improved hookswitch circuit. SUMMARY OF THE INVENTION According to the present invention, a novel electronic inductor circuit is used to establish the DC current drawn from the telephone line needed for indicating to the central office (or PBX) that the telephone device (i.e. a modem) has seized the line. The electronic inductor's transistor is used to perform the functions of a hookswitch, thereby avoiding the introduction of unwanted impedance in the AC signal path of the device. In order to go on-hook, the electronic inductor's transistor (upon receiving a control signal) stops drawing current from the central office loop indicating that it is now in the on-hook state. Unlike a switch, the present invention operates by starting and stopping the DC current while the entire circuit remains connected to the line at all times. In one configuration, a low-current transistor is added in series with the AC impedance network (normally 600 ohms), in order to disconnect such network from the line when the device is on-hook. According to this approach, the line is "seized" by providing a control voltage to turn on the electronic inductor and thereby initialize line current to flow through its transistor - thus achieving the "off— hook" state. In a second embodiment, two transistors are used to perform the same function as the single transistor. The two-transistor embodiment, however, provides more flexibility with regards to distortion and other design considerations. BRIEF DESCRIPTION OF THE DRAWINGS The exact nature of this invention, as well as its objects and advantages, will become readily apparent from consideration of the following specification as illustrated in the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof, and wherein: Figure 1 is a block diagram of a prior-art telephone line interface circuit employing a relay as a hookswtich; Figure 2 is a block diagram of a prior-art telephone line interface circuit employing a transistor as a hookswitch; Figure 3 is a schematic of a first embodiment of the present invention; Figure 4 is a schematic of a second embodiment of the present invention; and Figure 5 is a schematic of a third embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventor for carrying out the invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the basic principles of the present invention have been defined herein specifically to provide a telephone line interface circuit without a standard prior-art hookswitch. The present invention will now be described with reference to Figure 3. A transistor 10 is connected between the modem circuitry 10 (or other telephone device) and the diode bridge 4. The transistor 10 is part of an electronic inductor circuit, the remainder of which is not shown, but the construction of which is well known in the art. The electronic inductor's transistor 10 is used to perform the functions of a hookswitch, thereby avoiding the introduction of unwanted impedance in the AC signal path of the modem device 2. The transistor 10 acts as a voltage controlled current source. In order to go on-hook, the transistor 10 (upon receiving a control signal from the controller 12) stops drawing current from the central office loop, indicating that it is now in the on- hook state. Unlike a switch, the present invention operates by starting and stopping the DC current while the entire circuit remains connected to the line at all times. According to this approach, the line is "seized" by providing a control voltage from the controller 12 to turn on the transistor 10 and thereby initialize line current to flow through the transistor 10- thus achieving the "off— hook" state. In a typical United States telephone system, the transistor 10 will need to draw approximately 17mA, in order to achieve the off-hook state. The controller 12 may be programmable, in order to set the value of loop current drawn by the transistor 10. For example, many countries have different standards for how much loop current must be drawn in order to indicate an off-hook status. The controller 12 could be programmed by a device designer to operate correctly for a desired telephone environment, without redesigning the whole circuit. A second embodiment of the present invention is illustrated in Figure 4. Instead of only a single transistor, two transistors 16, 18 are used to draw loop current. A controller 20 controls both of the transistors 16, 18 and determines the amount of loop current drawn from the telephone line. Each transistor has a slightly different function, however. A first transistor 16 is used to draw most of the required DC loop current needed to indicate an off-hook status (transistor 16 acts as the "seizure" indicator). The second transistor 18 draws just enough DC current in order to be properly biased. An AC signal from a digital-to-analog (D/A) CODEC is applied to the base of the second transistor 18, and thus the second transistor 18 modulates the DC signal according to the AC input signal and acts as the modem transmitter driver. For cost and performance reasons, the present inventors decided that two transistors are preferred. Since two transistors are used, one can be used as the transmitter driver and one for seizure indication. The first transistor may be a NPN transistor and the second transistor may be a PNP transistor. SSA third embodiment of the present invention is illustrated in Figure 5. For some applications, a third transistor 26 may be used as a switch to isolate some of the circuit components in order to satisfy leakage current requirements and other regulatory compliance issues. The third transistor 26 does not act exactly like a relay or physical switch, since it does not completely disconnect the line. However, it isolates the circuitry enough to provide better leakage protection. Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.

Claims: CLAΓMS What Is Claimed Is: 1. A telephone line interface circuit comprising: a first transistor connected to an electrical connection between a telephone line and a telephone device, such that the first transistor draws a DC current from the telephone line. 2. The telephone line interface circuit of Claim 1 , wherein the first transistor draws a DC current sufficient to place the telephone device in an off-hook state, but does not electrically disconnect the telephone device from the telephone line in an on-hook state. 3. The telephone line interface circuit of Claim 2, further comprising a controller connected to the first transistor for controlling an amount of DC current drawn by the first transistor. 4. The telephone line interface circuit of Claim 3, wherein the controller turns off the first transistor in order to place the telephone device in an on-hook state. 5. The telephone line interface circuit of Claim 4, wherein the telephone device is a modem. 6. The telephone line interface circuit of Claim 4, wherein the controller is programmable to allow for different DC current settings. 7. The telephone line interface circuit of Claim 4, further comprising a second transistor connected to the telephone line and the telephone device and controlled by the controller. 8. The telephone interface circuit of Claim 7, wherein the second transistor modulates the DC current of the telephone line with an AC signal and draws enough DC current in order to properly bias the transistor. 9. The telephone interface circuit of Claim 8, wherein the first transistor is an NPN transistor, and the second transistor is a PNP transistor. 10. The telephone interface circuit of Claim 9, wherein the first and second transistors and the controller are part of an electronic inductor circuit. 11. The telephone interface circuit of Claim 8, further comprising a third transistor, having a base, emitter and collector, wherein the base is connected to a collector of the first transistor, the emitter to the telephone line, and the collector to the telephone device. 12. A method for setting a telephone device into either an off-hook or on- hook state, the method comprising the steps of: arranging a first transistor to act a as a voltage controlled current source to define the value of a DC current; and controlling the first transistor to draw a predetermined amount of DC current to set the telephone device into an off-hook state, and turning off the first transistor to set the telephone device into an on-hook state. 13. The method of Claim 12, wherein the first transistor does not electrically disconnect the telephone device from the telephone line in either the on-hook or off- hook state. 14. The method of Claim 13, further comprising the step of: arranging a second transistor to modulate the DC current with an AC signal. 15. The method of Claim 14, wherein the step of controlling comprises programming a controller to set a predetermined voltage corresponding the predetermined amount of DC current. 16. The method of Claim 15, further comprising the step of: arranging a third transistor between the telephone device and the telephone line to limit leakage current. 17. A telephone line interface circuit for setting a telephone device into either an on-hook or off-hook state, the circuit comprising: a first transistor having a collector connected to an electrical connection between a telephone line and a telephone device, such that the first transistor draws a DC current from the telephone line sufficient to place the telephone device in an off-hook state, but does not electrically disconnect the telephone device from the telephone line in an on-hook state; and a controller connected to a base of the first transistor for setting a voltage to control an amount of DC current drawn by the first transistor and for turning off the first transistor in order to set the telephone device in an on-hook state. 18. The circuit of Claim 17, further comprising a second transistor having a collector connected to the electrical connection between the telephone line and the telephone device, and a base connected to the controller, wherein the second transistor modulates the DC current from the telephone line with an AC signal. 19. The circuit of Claim 18, further comprising a third transistor having a base connected to the collector of the first transistor, a collector connected to the telephone device, and an emitter connected to the telephone line.

Inventors: RAHAMIM RAPHAEL, SACCA FRANK, RAHAMIM, RAPHAEL, SACCA, FRANK
Applicants: CONEXANT SYSTEMS INC, CONEXANT SYSTEMS, INC.
Application year: 1999
Main classification: H04M11
IPC codes: H04M1, H04M11