Solid state isolation device using opto-isolators

Abstract

An improved solid state isolation device that uses opto-isolators to provide an electrical barrier between a telephone network and user telephone equipment includes a load and current control circuit, and an adjustable termination circuit to eliminate or reduce signal distortion and insensitivity to low signal levels resulting from low loop current, and lack of complex impedance compensation/termination for diverse telephone system parameters. An improved trans-hybrid return loss circuit reduces internal losses; while loop termination losses are reduced by the use of Shottky diodes in the line coupler circuit. A high pass filter improves common mode rejection ratios in the transmit and receive paths, and adjustable compensation is provided to account for manufacturing variances in the opto-isolators. An additional connection is also provided to adapt the opto-isolator circuit for use with 3-wire ring circuitry.

We claim: 
     
       1. In an isolation circuit for connecting analog transmit and receive channels in a user device to a line pair for full duplex communications on the line pair while providing an electrical isolation barrier between the user device and the line pair, comprising: transmit optical isolation means, having an input located on the user device side of the barrier and an output on the line side of the barrier and being responsive to signals at its input, for generating analog signals at its output linearly corresponding to analog signals at its input;   said transmit optical isolation means comprising . .a monolithic.!. .Iadd.an .Iaddend.opto-isolator having an LED and first and second photodiodes. ., each photodiode, on being reverse biased, delivering a current proportional to the optical flux incident thereon,.!. and a driver coupled between the transmit channel and said opto-isolator in a feedback configuration . .such that each of said first and second photodiodes delivers a current proportional to a voltage signal from the transmit channel.!.;   receive optical isolation means, having an input located on the line side of the barrier and an output on the user device side of the barrier and being responsive to signals at its input, for generating analog signals at its output linearly corresponding to analog signals at . .it.!. .Iadd.its .Iaddend.input;   means for coupling the input of said transmit optical isolation means to said transmit channel;   means for coupling the output of said receive optical isolation means to said receive channel; and   duplexing means coupled to the line pair for directing signals at the output of said transmit optical isolation means onto the line pair, for directing signals on the line pair to the input of said receive optical isolation means; and for preventing at least a portion of any signals on the line pair that correspond to signals generated at the output of said transmit optical isolation means from reaching the input of said receive optical isolation means;   the improvement comprising: load and current control means for providing a high AC impedance regulated reference voltage derived from loop current to a receive path optical isolation means photodiode, whereby said photodiode receives .Iadd.adequate .Iaddend.bias . .adequate.!. current to avoid receive path distortion independently of line current level;   said load and current control means providing a regulated reference voltage derived from said loop current to a transmit path line driver circuit, whereby transmit path distortion is avoided under conditions of varying load impedance.     
     
     
       2. The circuit of claim 1, further comprising: an adjustable load DC impedance circuit for selectively shaping an isolation circuit load DC impedance slope.   
     
     
       3. The circuit of claim 1, further comprising: high pass filter means associated with said optical isolation means in at least one of said transmit path and said receive path, and adapted to provide selected common mode rejection in said path.   
     
     
       4. The circuit of claim 1, further comprising: adjustable termination means for selectably matching said isolation circuit to complex load impedances.   
     
     
       5. The circuit of claim 1, further comprising: means for providing transmit insertion loss and receive gain compensation for said optical isolation means.   
     
     
       6. The circuit of claim 1, further comprising: means for reducing loop DC termination loss of said isolation circuit.   
     
     
       7. The circuit of claim 1, further comprising: trans-hybrid return loss means for reducing internal isolation circuit losses.   
     
     
       8. The circuit of claim 1, further comprising: pin-out means for adapting said isolation circuit for use with 3-wire ring circuitry.   
     
     
       9. The isolation circuit of claim 1, including opto-isolators for providing an electrical barrier between a telephone network and user telephone equipment, said circuit further comprising: a load and current control circuit for reducing signal distortion and insensitivity to low and varying loop current, and lack of complex impedance compensation/termination for diverse telephone network parameters, said load and control circuit comprising a high AC impedance voltage regulator that derives a constant voltage from a variable loop current, said voltage regulator comprising a Zener diode controlled voltage regulator that establishes a reference voltage for a receive path opto-isolator, and that establishes a reference voltage for a transmit path line driver circuit; said load and control circuit also comprising an adjustable resistive/capacitive network for selectively matching device load impedance to said telephone network.   
     
     
       10. The device of claim 9, further comprising: a line coupler circuit including a bridge Shottky diode rectifier circuit coupled between said device and said telephone network for reducing device loop termination losses.   
     
     
       11. The device of claim 9, further comprising: a high pass filter comprising a resistive/capacitive network in either of said device transmit and receive paths coupled between a respective opto-isolator output and a line driver input for improving common mode rejection ratios in said paths.   
     
     
       12. The device of claim 9, further comprising: an adjustable resistive/capacitive termination network for selectably matching said device impedance to telephone networks having complex load impedances.   
     
     
       13. The device of claim 9, further comprising: a trans-hybrid return loss circuit including a transistor having an emitter and a collector coupled to produce a differential output signal that is proportional to the superposition of signals along said receive path and transmit paths.   
     
     
       14. The isolation circuit of claim 1, further comprising: a high AC impedance load and current control circuit for reducing signal distortion and insensitivity to low signal levels resulting from low circuit current, said load and control circuit including a Zener diode controlled voltage regulator that derives a reference voltage for said opto-isolator from a varying loop current.