WO1994007319A1

WO1994007319A1 - Telephone line interface

Info

Publication number: WO1994007319A1
Application number: PCT/US1993/008551
Authority: WO
Inventors: Uzoma Ulugbo Agbaje-Anozie
Original assignee: Agbaje Anozie Uzoma Ulugbo
Priority date: 1992-09-14
Filing date: 1993-09-10
Publication date: 1994-03-31
Also published as: AU4856493A

Abstract

A telephone line interface circuit (10) including a diode bridge for electrically coupling on-hook and off-hook signals between a telephone line and an intermediate signal path, and two optical couplers serially connected within the intermediate signal path for simultaneously optically coupling incoming and outgoing signals therefrom and thereto, respectively. An incoming signal, either on-hook or off-hook, can be coupled from the telephone line while an outgoing signal is simultaneously being coupled to the telephone line. The solid state design of the circuit (10) allows for integration thereof into a single integrated circuit chip.

Description

TELEPHONE LINE INTERFACE

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electrical interface circuits, and in particular, to circuits for interfacing with telephone lines.

2. Description of the Related Art

A basic telephone line interface circuit needs to perform two basic functions. It must provide for reception of an on-hook signal from a public switched telephone network, such as a ring signal, for detection thereof by a user's telephone. It must also provide for reception and transmission of off-hook signals, such as voice communications or electronic data communications, by a user device via the public switched telephone network.

Several such basic telephone line interface circuits are well known in the art. Most such circuits rely on the use of transformers for coupling the on-hook and off-hook signals between the telephone network and the user's telephone or data transceiver. However, with the increased use of various forms of electronic data transceivers, such as facsimile machines and data modems, more sophisticated telephone line interface circuits are needed.

When providing a telephone line interface for facsimile machines and data modems, the interface circuit, often referred to as "data access arrangement" ("DAA"), must be capable of providing an efficient and reliable interface for both on-hook and off-hook signals. Further, as such interface circuits are increasingly integrated into their host devices, compactness of the design becomes increasingly critical.

In particular, elimination of the transformer is desireable in order to reduce the size, weight and cost of the interface circuit. Although some recent interface designs have successfully eliminated the transformer, such designs nonetheless include complex circuitry for reception and transmission of on-hook and off-hook signals. Further, in order to maximize data communication efficiency, it would be desireable to reliably receive and transmit data through the interface simultaneously.

Accordingly, an improved telephone line interface circuit design would be desirable, wherein no coupling transformer is needed and a simplified, solid state design provides for simultaneous reception and transmission of on-hook and off-hook telephone signals.

SUMMARY OF THE INVENTION A telephone line interface in accordance with the present invention includes a coupler, receiver and transmitter. The coupler receives and couples an on-hook or off-hook signal from a telephone line for the receiver, which in accordance therewith provides an incoming signal. The transmitter receives an outgoing signal and in accordance therewith provides an output signal to the coupler simultaneously with the reception of the coupled on-hook or off-hook signal by the receiver. The coupler couples the output signal to the telephone line simultaneously with its reception and coupling of the on-hook or off-hook input signal. Thus, a telephone line interface in accordance with the present invention provides for simultaneous reception of on-hook or off-hook input signals from a telephone network and coupling thereto of output signals (either on-hook or off-hook) .

In a preferred embodiment of the present invention, the coupler includes a diode bridge circuit with multiple interconnected diodes for electrically coupling signals to and from the telephone line. The receiver and transmitter each include an optical coupler for receiving and optically coupling their respective signals.

These and other features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings .

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a simplified functional block diagram illustrating the interconnection of a telephone line interface between a telephone network and a user device. Figure 2 is a simplified functional block diagram illustrating the basic functional components of a telephone line interface in accordance with the present invention.

Figure 3 is a schematic diagram of a telephone line interface circuit in accordance with the present invention. DETAILED DESCRIPTION OF THE INVENTION Referring to Figure 1, a telephone line interface 10 receives signals via the "tip" (negative) 12 and "ring" (positive) 14 lines of a public switched telephone network, and in accordance therewith, provides a receive signal 16 for the user device. Additionally, the telephone line interface 10 receives a transmit signal 18 from the user device, and in accordance therewith, transmits a signal into the public switched telephone network via the tip 12 and ring 14 lines.

Referring to Figure 2, a telephone line interface 10 in accordance with the present invention includes a line coupler 20, current hold circuit 22, receiver/coupler 24 and transmitter/coupler 26. In a receive mode, an input signal received by the interface 10 via the tip 12 and ring 14 lines is coupled by the line coupler 20 to provide a coupled input signal 28 for the receiver/coupler 24, which in accordance therewith, provides the receive signal 16. In a transmit mode, the transmitter/coupler 26 receives the transmit signal 18, and in accordance therewith, provides an output signal 30 for coupling by the coupler 20 onto the tip 12 and ring 14 lines. Referring to Figure 3, a preferred embodiment of a telephone line interface 10 in accordance with the present invention is schematically illustrated. As discussed further below and listed below in Table 1, the telephone line interface 10 of Figure 3 includes the following active (e.g. operational amplifier t"op-amp"] , optical couplers, transistors and diodes) and passive (e.g. switch, resistors and capacitors) components: TABLE 1

As is known in the art, and as used herein, the terms "on-hook" and "off-hook" signals refer to those types of signals conveyed by the tip and ring lines of a public switched telephone network in which AC, or both AC and DC signals, respectively, can be conducted. In other words, an "on-hook signal" includes AC signals (e.g. AC currents), and an "off-hook signal" includes either AC or DC signals (e.g. AC or DC currents), or both. An "on-hook condition" provides an open DC signal path and a closed AC signal path, while an "off-hook condition" provides a closed DC signal path and a closed AC signal path.

As discussed above, the telephone line interface 10 connects to the tip 12 and ring 14 lines of the telephone network, provides a receive signal 16 and receives a transmit signal 18. The tip 12 and ring 14 lines are connected to the diode bridge DBl via a resistor-capacitor-switch network for coupling input and output signals between the tip 12 and ring 14 lines and the remainder of the telephone line interface 10. With the switch SI in its open position (as shown) , the interface 10 is in an on-hook condition. Only an on-hook input signal, e.g. a ring signal, can be received by diode bridge DBl, via the series RC network of resistor RI and capacitor Cl.

The first signal port (e.g. "receive-input" and "transmit-output" port) of diode bridge DBl consists of the node connecting the cathode and anode of diodes Dl and D2, respectively, and the node connecting the cathode and anode of diodes D3 and D , respectively. The second signal port (e.g. "receive- output" and "transmit-input" port) consists of the node connecting the cathodes of diodes D2 and D4 and the node connecting the anodes of diodes Dl and D3. An on-hook input signal received at the first signal port of the diode bridge DBl is electrically coupled (e.g. rectified) therefrom to the second signal port of the diode bridge DBl. This coupled on-hook input signal 28, rectified by diode bridge DBl, is applied across and clamped by zener diode ZD1, and used to bias the remainder of the current hold circuit 22, which includes transistors Ql and Q2, resistors R2, R3 and R4, and capacitor C2. This coupled on-hook input signal 28, in the form of an electrical current through the light emitting diode of optical coupler 0C1, is optically coupled thereby to produce an electrical incoming (e.g. ring) signal 32 in accordance therewith at the node connecting the base of the darlington transistor of optical coupler OC1, resistor R6 and capacitor C4.

This incoming signal 32 (e.g. as a voltage across resistor R6) is coupled by series capacitor C4 and buffered by a series of inverting buffer amplifiers, beginning with a first buffer amplifier

34 (op-amp U1A, and resistors R9 and RIO) . The first buffered signal 36 is then buffered by second buffer amplifier 38 (op-amp U1B, resistors Rll and R12, and capacitor C7) . The second buffered signal 40 is further buffered by third buffer amplifier 42 (op-amp UlC and resistors R13 and R14) to produce the receive signal 16 discussed above.

With switch SI in its closed position, the interface 10 is in an off-hook condition. Off-hook signals, received via the tip 12 and ring 14 lines, are applied at the first signal port of the diode bridge DBl, just as described above. In this off-hook condition, the negative and positive polarities of the tip 12 and ring 14 lines, respectively, forward bias diodes Dl and D4 of the diode bridge DBl, thereby causing the aforementioned current holding circuitry 104 to be biased on. This allows incoming off-hook signals and outgoing signals to be freely coupled through diode bridge DBl from and to, respectively, the tip 12 and ring 14 lines. In the transmit mode, an outgoing signal, i.e. the transmit signal 18, is applied to the input of optical coupler OC2 via resistor R7 and capacitor C3. This input signal applied to optical coupler OC2, in the form of an electrical current through its light emitting diode, is optically coupled through the optical coupler OC2 to produce an electrical signal at its output. This output signal 30 is applied to the second signal port of diode bridge DBl and electrically coupled (e.g. rectified) thereby onto the tip 12 and ring 14 lines.

From the foregoing it can be seen that, regardless of whether the input signal from the tip 12 and ring 14 lines and applied across diode bridge DBl is on-hook or off-hook, an outgoing signal 18 can be simultaneously coupled (via optical coupler OC2 and diode bridge DBl) onto the tip 12 and ring 14 lines. The biasing of the diodes D1-D4 within diode bridge DBl by either an on-hook signal (e.g. an incoming ring signal) from the tip 12 and ring 14 lines or an off-hook condition provides for simultaneous coupling of input signals to the interface 10 and output signals therefrom. It can be further seen that input signals coupled from the tip 12 and ring 14 lines via diode bridge DBl can be received via optical coupler 0C1 simultaneously with coupling of outgoing signals by optical coupler OC2 for coupling onto the tip 12 and ring 14 lines via diode bridge DBl. However, this simultaneity of the receive and transmit functions of the interface 10 causes virtually no interference with or degradation of the receive signal 16. As discussed above, the transmit signal 18 is coupled via optical coupler OC2 and diode bridge DBl to the tip 12 and ring 14 lines. However, due to the serial, or totem-pole, connection of the output circuit of optical coupler OC2 and input circuit of optical coupler 0C1, the interim coupled transmit signal, i.e. the output signal 30 of optical coupler OC2, is also coupled back into the receive signal path via optical coupler OC1 due to the current flow established thereby through the light emitting diode of optical coupler OC1. This "back-coupled" transmit signal forms part of the aforementioned incoming signal 32 and is time-shifted, i.e. phase-delayed, due to the time, or phase, delays introduced by optical couplers OC1 and OC2. However, the interference potential presented by this back-coupled transmit signal is effectively eliminated by cancelling it out.

Interference with or degradation of the receive signal 16 by the transmit signal 18 is minimized initially by coupling a first portion 44 of the outgoing, or transmit, signal 18 to the buffer amplifiers 34, 38, 42 via resistor R7 and capacitor C6. The first coupled signal 44 is a voltage which appears at the node connecting the non-inverting inputs of op-amps U1A, U1B and UlC and to which a voltage reference VREF (VREF=VCC/2) is applied. The first coupled signal 44 voltage is phase-shifted, or phase-delayed, with respect to the transmit signal 18 due to the capacitive impedance established primarily by resistor R7, capacitors C5 and C6, and the source impedance of the voltage reference VREF (e.g. the Thevenin equivalent resistance of resistors R17 and R18 in parallel) . The phase delay introduced to the first coupled signal 44 is selected empirically (e.g. by selecting the values of resistor R7, capacitors C5 and C6, or the source impedance of the voltage reference VREF) to closely approximate that which is introduced to the back-coupled transmit signal by the optical couplers OC1 and OC2 (discussed above) . The first coupled signal 44 voltage is amplified, or buffered, differentially with the incoming signal 32 which includes the back-coupled transmit signal. Thus, due to the aforementioned substantially equal phase delays introduced into the first coupled signal 44 and incoming signal 32, the back-coupled transmit signal is substantially subtracted out. Further subtraction, or cancellation, of the back-coupled transmit signal is done by further differential amplification, or buffering, thereof with the first 36 and second 40 buffered signals by the second 38 and third 42 buffer amplifiers.

Further reduction of the back-coupled transmit signal is achieved by coupling a second portion 46 of the outgoing, or transmit, signal 18 to the second buffer amplifier 38 via resistor R7 and capacitors C3 and C8. The second coupled signal 46 is a current which enters the node connecting the inverting input of op-amp U1B, resistors Rll and R12, and capacitor C7. The second coupled signal 46 current is phase- delayed (approximately 180 degrees) with respect to the transmit signal 18 due to the capacitive impedance established primarily by resistors R7 and R5, and capacitors C3 and C8. The second coupled signal 46 current sums with the signal current caused by the first buffered signal 36.

From the foregoing, it can be seen that a signal current loop is formed between the cathodes of diodes D2 and D4 and the anodes of diodes Dl and D3 of diode bridge DBl via the input circuitry of optical coupler OC1 and output circuitry of optical coupler 0C2. Within this single signal path, or current loop, simultaneous conduction of the input and output signal currents occurs. This contributes to the simultaneous coupling of input and output signals by diode bridge DBl, and the simultaneous reception and transmission of input and output signals by optical couplers 0C1 and 0C2, respectively, as discussed above.

It can be further seen that optical couplers OC1 and 0C2 simulate the operation of a coupling transformer. For example, the signal path formed by the light emitting diode of optical coupler OC1 and output circuit of optical coupler OC2 simulate the primary side, or winding, and the signal path formed by the output circuit of optical coupler 0C1 and light emitting diode of optical coupler OC2 simulate the secondary side, or winding, of a coupling transformer.

As seen in Figure 3, and in accordance with the foregoing discussion, a telephone line interface 10 in accordance with the present invention is of a solid state design. The design requires no coupling transformer, and therefore allows for integration of the circuit into a single integrated circuit chip, albeit with some off-board passive components, such as resistors and capacitors. Further, this design is also well suited to a hybrid implementation, wherein chip and wire techniques are used. Chip components could be used for all circuit elements, both active and passive, or alternatively, chip components could be used for the active elements and capacitors with thin-film or thick-film resistors.

Various other modifications and alterations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments.

Claims

WHAT IS CLAIMED IS:

1. A telephone line interface, comprising: coupler means for receiving and coupling an on-hook input signal from a telephone line to provide a coupled on-hook input signal, and further for receiving and coupling an output signal to said telephone line simultaneously with said reception and coupling of said on-hook input signal; receiver means coupled to said coupler means for receiving said coupled on-hook input signal and in accordance therewith providing an incoming signal; and transmitter means coupled to said coupler means for receiving an outgoing signal and in accordance therewith providing said output signal to said coupler means simultaneously with said reception by said receiver means of said coupled on-hook input signal.

2. A telephone line interface as recited in

Claim 1, wherein said coupler means comprises a plurality of diodes interconnected as a diode bridge.

3. A telephone line interface as recited in Claim 1, wherein said receiver means comprises an optical coupler for optically coupling said received, coupled on-hook input signal to provide said incoming signal.

4. A telephone line interface as recited in Claim 1, wherein said transmitter means comprises an optical coupler for optically coupling said received outgoing signal to provide said output signal.

5. A telephone line interface as recited in

Claim 1, further comprising signal buffer means coupled to said receiver means for receiving and buffering said incoming signal.

6. A telephone line interface as recited in Claim 1, further comprising signal buffer means coupled to said receiver means and said transmitter means for receiving and buffering said incoming signal and a portion of said outgoing signal.

7. A telephone line interface, comprising: a diode bridge circuit which includes a plurality of interconnected diodes and first and second signal ports for receiving and coupling an on-hook input signal from a telephone line to provide a coupled on-hook input signal, and further for receiving and coupling an output signal to said telephone line simultaneously with said reception and coupling of said on-hook input signal, wherein said first signal port receives said on-hook input signal from said telephone line and couples said received output signal to said telephone line, and further wherein said second signal port provides said coupled on-hook input signal and receives said output signal; a first optical coupler circuit coupled to said diode bridge circuit for receiving therefrom said coupled on-hook input signal and in accordance therewith optically coupling therefrom an incoming signal; and a second optical coupler circuit coupled to said diode bridge circuit for receiving an outgoing signal and in accordance therewith optically coupling thereto said output signal simultaneously with said reception by said first optical coupler circuit of said coupled on-hook input signal.

8. A telephone line interface as recited in Claim 7, further comprising a buffer amplifier circuit coupled to said first optical coupler circuit for receiving and buffering said incoming signal.

9. A telephone line interface as recited in

Claim 7, further comprising a buffer amplifier circuit coupled to said first and second optical coupler circuits for receiving and buffering said incoming signal and a portion of said outgoing signal.

10. A method for interfacing with a telephone line, comprising the steps of: receiving and coupling an on-hook input signal from a telephone line to provide a coupled on-hook input signal; receiving said coupled on-hook input signal and in accordance therewith providing an incoming signal; receiving an outgoing signal and in accordance therewith providing an output signal simultaneously with said reception of said coupled on-hook input signal; and receiving and coupling said output signal to said telephone line simultaneously with said reception and coupling of said on-hook input signal.

11. A telephone line interface method as recited in Claim 10, wherein said step of receiving and coupling an on-hook input signal from a telephone line to provide a coupled on-hook input signal comprises: receiving said on-hook input signal from said telephone line via a first signal port of a diode bridge circuit which includes a plurality of interconnected diodes; and providing said coupled on-hook input signal via a second signal port of said diode bridge circuit.

12. A telephone line interface method as recited in Claim 10, wherein said step of receiving said coupled on-hook input signal and in accordance therewith providing an incoming signal comprises receiving and optically coupling said coupled on-hook input signal to provide said incoming signal.

13. A telephone line interface method as recited in Claim 10, wherein said step of receiving an outgoing signal and in accordance therewith providing an output signal simultaneously with said reception of said coupled on-hook input signal comprises receiving and optically coupling said outgoing signal to provide said output signal simultaneously with said reception of said coupled on-hook input signal.

14. A telephone line interface method as recited in Claim 10, wherein said step of receiving and coupling said output signal to said telephone line simultaneously with said reception and coupling of said on-hook input signal comprises: receiving said output signal via a first signal port of a diode bridge circuit which includes a plurality of interconnected diodes; and coupling said received output signal to said telephone line via a second signal port of said diode bridge circuit simultaneously with said reception and coupling of said on-hook input signal.

15. A telephone line interface method as recited in Claim 10, further comprising the step of receiving and buffering said incoming signal.

16. A telephone line interface method as recited in Claim 10, further comprising the step of receiving and differentially amplifying said incoming signal and a portion of said outgoing signal.

17. A method for interfacing with a telephone line, comprising the steps of: receiving an on-hook input signal from a telephone line via a first signal port of a diode bridge circuit which includes a plurality of interconnected diodes; providing a coupled on-hook input signal in accordance with said received on-hook input signal via a second signal port of said diode bridge circuit; receiving said coupled on-hook input signal with a first optical coupler circuit and in accordance therewith optically coupling therefrom an incoming signal; receiving an outgoing signal with a second optical coupler circuit and in accordance therewith optically coupling therefrom an output signal simultaneously with said reception of said coupled on-hook input signal; and receiving said output signal via said second diode bridge circuit signal port and coupling said received output signal to said telephone line via said first diode bridge circuit signal port simultaneously with said reception of said on-hook input signal and said providing of said coupled on-hook input signal .

18. A telephone line interface method as recited in Claim 17, further comprising the step of receiving and amplifying said incoming signal with an amplifier circuit coupled to said first optical coupler circuit.

19. A telephone line interface method as recited in Claim 17, further comprising the step of receiving and differentially amplifying said incoming signal and a portion of said outgoing signal with an amplifier circuit coupled to said first and second optical coupler circuits.

20. A telephone line interface, comprising: signal path means for receiving and conducting a coupled on-hook input signal and an output signal simultaneously; coupler means coupled to said signal path means for receiving and coupling an on-hook input signal from a telephone line to provide said coupled on-hook input signal, and further for receiving and coupling said output signal to said telephone line simultaneously with said reception and coupling of said on-hook input signal; transmitter means coupled to said signal path means for receiving an outgoing signal and in accordance therewith providing said output signal; and receiver means coupled to said signal path means for receiving said coupled on-hook input signal simultaneously with said providing of said output signal by said transmitter means, and for providing an incoming signal in accordance with said received, coupled on-hook input signal.

21. A telephone line interface as recited in Claim 20, wherein said signal path means comprises a current loop and said coupled on-hook input signal and said output signal comprise first and second signal currents, respectively.

22. A telephone line interface as recited in Claim 20, wherein said coupler means comprises a plurality of diodes interconnected as a diode bridge.

23. A telephone line interface as recited in Claim 20, wherein said transmitter means comprises an optical coupler for optically coupling said received outgoing signal to provide said output signal.

24. A telephone line interface as recited in Claim 20, wherein said receiver means comprises an optical coupler for optically coupling said received, coupled on-hook input signal to provide said incoming signal.

25. A telephone line interface as recited in Claim 20, further comprising signal buffer means coupled to said receiver means for receiving and buffering said incoming signal to provide a buffered signal which is substantially unrelated to said outgoing signal.

26. A telephone line interface as recited in Claim 20, further comprising signal buffer means coupled to said receiver means and said transmitter means for receiving and buffering said incoming signal and a portion of said outgoing signal to provide a buffered signal which is substantially unrelated to said outgoing signal.

27. A method for interfacing with a telephone line, comprising the steps of: receiving and coupling an on-hook input signal from a telephone line to a signal path to provide a coupled on-hook input signal thereto; receiving an outgoing signal and in accordance therewith providing an output signal to said signal path; receiving and conducting said coupled on-hook input signal and said output signal simultaneously on said signal path; receiving said coupled on-hook input signal from said signal path and in accordance therewith providing an incoming signal; and receiving and coupling said output signal from said signal path to said telephone line simultaneously with said reception and coupling of said on-hook input signal from said telephone line to said signal path.

28. A telephone line interface method as recited in Claim 27, wherein said step of receiving and coupling an on-hook input signal from a telephone line to a signal path to provide a coupled on-hook input signal thereto comprises: receiving said on-hook input signal from said telephone line via a first signal port of a diode bridge circuit which includes a plurality of interconnected diodes; and providing said coupled on-hook input signal to said signal path via a second signal port of said diode bridge circuit.

29. A telephone line interface method as recited in Claim 27, wherein said step of receiving an outgoing signal and in accordance therewith providing an output signal to said signal path comprises receiving and optically coupling said outgoing signal to provide said output signal.

30. A telephone line interface method as recited in Claim 27, wherein said step of receiving and conducting said coupled on-hook input signal and said output signal simultaneously on said signal path comprises receiving and conducting a coupled on-hook input signal current and an output signal current simultaneously in a current loop.

31. A telephone line interface method as recited in Claim 27, wherein said step of receiving said coupled on-hook input signal from said signal path and in accordance therewith providing an incoming signal comprises receiving and optically coupling said coupled on-hook input signal to provide said incoming signal.

32. A telephone line interface method as recited in Claim 27, wherein said step of receiving and coupling said output signal from said signal path to said telephone line simultaneously with said reception and coupling of said on-hook input signal from said telephone line to said signal path comprises: receiving said output signal via a first signal port of a diode bridge circuit which includes a plurality of interconnected diodes; and coupling said output signal to said telephone line via a second signal port of said diode bridge circuit simultaneously with said reception and coupling of said on-hook input signal from said telephone line to said signal path.

33. A telephone line interface method as recited in Claim 27, further comprising the step of receiving and buffering said incoming signal.

34. A telephone line interface method as recited in Claim 27, further comprising the step of receiving and differentially amplifying said incoming signal and a portion of said outgoing signal.