Abstract:
There is disclosed a system and method of providing electrical isolation between a telephone line and a connected device, such as a computer. In one embodiment the telephone line is 2-wire and the connected device is 4-wire and includes a feedback elimination circuit. A pair of controlled CTR opto diodes are used in the communication path to effect electrical isolation. The opto diodes are used in conjunction with an electronic inductor constructed using a pair of cascoded darlington transistors to control the telephone line voltage and current and the circuit is designed to maintain the optocouplers within their linear operating range.

Description:
REFERENCE TO RELATED APPLICATIONS 
     The present application is being concurrently filed with commonly assigned, U.S. patent application entitled ELECTRONICALLY SWITCHED OPTICALLY COUPLED LINE INTERFACE, the disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates to a circuit for providing communication between one computer and another via the public switched telephone network while simultaneously providing a high degree of electrical isolation therein between. 
     BACKGROUND OF THE INVENTION 
     In telephone systems there is a requirement that devices, such as modems, which are coupled to telephone lines must provide isolation between the device and the central office. Typically, prior art systems provide isolation by using a transformer between the telephone line and the device. The telephone line is usually connected to the primary side of the transformer and the device is on the secondary side. Signals from the transmission line and from the device are coupled across the transformer so that current does not flow directly from the device to the central office. 
     Instead of using a transformer to couple a device to a transmission line, some prior art systems use optically coupled circuits to provide the required isolation. This type of system is disclosed in U.S. Pat. Nos. 4,190,747 and 4,228,323, both to Feiner et al and in co-pending Patent application Ser. No. 08/957,916, assigned to a common assignee, the disclosures of which are hereby incorporated by reference. Optically coupled systems use photo emitters and photo detectors to couple the two sides of the circuit using light waves. 
     However transformers have disadvantages due to their physical size as well as their cost. Optocouplers are used for PC/MCIA applications where size constraints are severe. However, these circuits are very complicated and costly since they require feedback circuits to cancel errors due to component variation, especially the current transfer ratio (CTR) of the optocoupler. CTR, is the ratio of induced photo current in a photo-transistor to the forward current of an LED. 
     The circuit must provide a load to the telephone network consistent with certain requirements. These requirements are usually in the form of a permissible DC voltage and current. Additionally, the AC input impedance is specified as well, which typically is 600 Ohms in the U.S. 
     The electrical isolation provided by the circuit accomplishes two objectives: 1) isolates the common voltage or ground of the telephone line from that of the computer&#39;s allowing them to be substantially different and helps isolate; 2) voltage surges which may be present on the telephone line which could prove to be destructive to sensitive integrated circuits within the computer. 
     A final requirement for the circuit is that it performs a 4-wire to 2-wire conversion. From the figure, the “4 wires” are the receive signal (Rx) and its reference (typically the computer system&#39;s ground) and the transmit signal (Tx) and its reference. The “2 wires” are the tip and ring, which carry the standard signals which telephones and modems use. 
     SUMMARY OF THE INVENTION 
     These and other objects, features and technical advantages are achieved by a circuit which uses optocouplers that are manufactured in such a way as to guarantee the CTR of the device over a controlled range, while still costing little more than a standard optocoupler. Thus, by employing these devices, a minimal circuit involving only two optocouplers can be used which yields minimum cost. 
     The major blocks of the circuit are the electronic inductor, the receive optocoupler and its associated biasing circuitry, the transmit optocoupler and its associated biasing circuitry, and the transmit canceller, which is often referred to in the industry as a near-end echo canceller. A central concept of the overall circuit is to take advantage of the availability of optocouplers with controlled current transfer ratios, thus eliminating the need for expensive feedback circuits. By using such controlled transfer ratio optocouplers, a DAA circuit is designed to produce a linear response over the entire range of operation. Since feedback is not required, the minimum number of optocouplers can be employed to meet the requirements of the DAA circuit. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a diagram of a system incorporating the present invention; and 
     FIG. 2 is a diagram showing typical DC input characteristics. 
     FIG. 3 is one alternate arrangement of the optocoupler. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 1, the purpose of the Electronic Inductor (EI) is to absorb the excess DC loop current presented by the telephone line while passing nearly all of the AC signal to the coupling circuit. In the present circuit, additional requirements are placed on electronic inductor  102  when compared to those used in transformer-based Data Access Arrangements (DAAs). For our circuit, electronic inductor  102  must bias LED (Light Emitting Diode)  11 - 1  of receive optocoupler  11  and photo-transistor  12 - 2  of transmit optocoupler  12  over the range of conditions provided by the telephone line. This range of operation is defined by the loop current, which can vary from approximately 20 to 100 mA. Additionally, the DC input characteristics must operate within FCC guidelines as shown in FIG. 2. A further requirement on the electronic inductor is to provide a very high AC impedance, such that the maximum signal level is transferred to the receive optocoupler. The EI resides in the circuit after diode bridge  101  and zener diode ZD which clamps the voltage to a maximum level and is used to prevent severe over voltage conditions. 
     The EI circuit behaves inductively in that it presents a very low impedance to DC voltage but a very high impedance to AC signals, especially those in the band of interest. Thus, in the band of interest, nearly all of the signal energy is passed, typically 300 hz to 4000 hz, to receive circuit  103 . 
     As shown in FIG. 1, EI  102  uses two darlington transistors Q 1  and Q 2  arranged in cascode fashion. A cascode circuit is used to obtain very high impedance between nodes RT and RR, which allows most of the signal energy to pass to LED  11 - 1  of opto  11 . Darlington transistors are used because they have very high betas (the ratio of collector current to base current), and consequently less base current and resulting in a smaller voltage drop across biasing resistors R 1  and R 2 . Of course, other transistor arrangements, including single transistors or other active devices, could be used to achieve proper voltage regulation. Three resistors, R 1 , R 2  and R 3  are used to bias the EI  102 . If the base currents of the transistors are negligible, then the voltage between RT and RR is computed as follows. The voltage across R 3  is the base-emitter forward voltage of Q 2 , approximately 1.5V. This voltage sets the current in R 3 , as well as the current in R 1  and R 2 . If the current in R 3  is 1.5V/R 3 , then the voltage between RT and RR is (1.5V/R 3 )*(R 1 +R 2 )+1.5. Clearly, values of R 1 , R 2  and R 3  can be defined so that the voltage across EI  102  is approximately 5V. 
     Capacitors C 1  and C 2  are used to create the inductive effect by providing a shunt for AC signals. Thus, the voltage at the base of Q 2 , which sets the current of the EI, is sensitive only to changes in DC voltage between RT and RR. The values of C 1  and C 2  are set so that in the AC signals in the band of interest are completely shunted. Capacitor C 3  reduces low frequency resonance of EI  102  by reducing the gain of Q 2  at the point of resonance. C 3  may not be required in all applications. 
     The receive circuit consists of opto  11 , R 4 , biasing diodes BD 1 , BDn, and R 5 . While it is possible to configure a functional circuit without biasing diodes BD 1  , BDn and R 5 , optimum performance and greater design freedom are released through their deploy. The DC forward current is set by R 4  and the biasing diodes. R 4  is set to a value such that the overall input impedance of the circuit is 600 Ohms (for U.S.), typically around 700 Ohms because of finite output impedance of opto- 112  and EI  102 . The forward current, (IF) is multiplied in photo-transistor  11 - 2  by its CTR. Maximum linearity is achieved when the emitter of opto  11  is approximately 2.5V. This is achieved by setting R 5  such that CTR*I f *R 5 =2.5. I f  is defined by the voltage across EI  102  and R 4  and the biasing diodes. Biasing diodes are necessary since R 4  must be set to about 700 Ohms to meet input impedance requirements. Thus, I f =(VEI−VD 1 −VD 2  . . . )/R 4 , where VDn is the forward drop across a diode, typically 0.7V. One or more biasing diodes are used in series depending on how much voltage is desired across R 4 . 
     The transmit circuit consists of opto  12 , R 9 , C 6  and R 10 . The circuit is biased by R 9 , which sets the DC forward current of opto  11  LED  12 - 1 , and thus the quiescent collector current of opto  12 &#39;s photo-transistor  12 - 2 . Capacitor C 6  provides DC isolation of the LED from the signal source, Tx such that the input impedance seen by Tx is R 10 . 
     Since the transmit circuit uses opto  12  in a common emitter configuration, the signal is inverted in passing from Tx to the telephone line. This is used to create a transmit cancellation circuit  105 . The purpose of the transmit canceller  105  is to subtract the signal Tx from the signal received at Rx so that the signal input of Tx is not received back at Rx which could yield an objectionable echo back to the terminal. This subtraction could, if desired, be performed in software on a digital signal processor or microprocessor within the DC. Thus the invention does not require a hardware transmit canceller, however a simple means is shown here for completeness. 
     Transmit cancellation circuit  105  consists of op-amp A 1  connected as a summing amplifier. Capacitors C 4  and CS serve to provide DC isolation. The function the circuit performs is to add the voltages Rx 1  and Tx. The textbook equation for the summation is: Rx=(R 7 /R 6 )*Rx 1 +(R 7 /R 8 )*Tx. Note that Rx 1  contains the signal to be received from, TIP/RING plus an inverted form of Tx such that if some factor of Tx is added to this signal, Tx will effectively be canceled. Thus R 8  is set to be a value to provide the factor of Tx to achieve this cancellation. 
     FIG. 2 characterizes the voltage versus current limits of the telephone set, as measured at the tip and ring interface while the telephone set is disassociated from the central office or PBX line. On simulated outgoing calls operation is within the Acceptable Region during the On-Hook to Off-Hook transition and during the make interval of rotary dial pulsing. On simulated outgoing calls operation is within the Conditionally Acceptable Region during DTMF signaling and after called party answer. On simulated incoming calls, operation is within the Acceptable Region for at least one second after answer; and after one second operation is allowed to be in the Conditionally Acceptable Region for the remainder of the call. 
     FIG. 3 shows an alternate arrangement for the receive circuit. In this topology, the receive opto-coupler is configured with its photo-transistor in a common emitter fashion, as compared to the emitter follower configuration of FIG.  1 . In either configuration, the signal level of the output is determined by load resistor R 5  and the input impedance setting resistor R 4 . This is because the current in the photo transistor in either configuration is equal to the current in R 4  times the CTR of the opto-coupler. The difference between the two configurations is that for the common emitter case, there is a signal inversion, while in the emitter follower case there is not. 
     In the preferred embodiment as shown in FIG. 1, optocouplers  11  and  12  have controlled CTRs and could be CNY  17 - 3 , available from Motorola and others, and diodes BD 1  to BDN are  1 N 4004  and Q 1  and Q 2  are advantageously darlington transistors MPS  13 . It is important for manufacturing purposes that the optocoupler&#39;s CTR is controlled within a range of 100%-200% or higher. This circuit should be designed so that linear operation is achieved in a manner that is compatible with the range of the CTR of the optocouplers. It should be noted that while a specific type of optocoupler is shown and described, the circuit could work with many different devices providing they are linearly coupled and electrically isolated from input to output. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.