Abstract:
A data access arrangement (DAA) having a transhybrid circuit for separating a transmit signal from a received signal by providing a transmit cancellation signal to the inverting input of a servo-feedback differential amplifier on the receive channel of the DAA. The cancellation signal is provided by a photodiode optically coupleable with a light emitting diode within a optical isolator on the transmission channel of the DAA. The gain of the cancellation signal can be independently controlled.

Description:
This is a continuation, of application Ser. No. 08/706,858 now U.S. Pat. No. 5,742,417 filed Sep. 3, 1996, which is a divisional of application Ser. No. 08/497,580 filed Jun. 30, 1995, now U.S. Pat. No. 5,579,144. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to a device, known as a “data access arrangement” (or “DAA”), for coupling a data terminal equipment (“DTE”) with a telephone line. In particular, the present invention relates to an optical data access arrangement (DAA) including an optically isolated transhybrid having improved transmit-receive separation. 
     BACKGROUND OF THE INVENTION 
     Telephone signals are provided to subscribers through the public switched telephone network (“PSTN” or “the network”). The subscriber portion of the network has two wires known as “tip” and “ring”. These wires carry information being transferred to and from the subscribers, as well as control signals, such as a connection request (or “ring”) signal. The bandwidth of the network is between about 300 Hz to 3.4 KHz. Accordingly, any data terminal equipment (DTE), such as data modems, facsimile machines, (non-cellular) portable telephones, speaker phones, and message answering machines, for example, must be compatible with the network (PSTN) to function properly. To this end, data access arrangements (DAAs) provide an interface to bridge any inconsistencies between the data terminal equipment (DTE) and the network (PSTN). 
     Furthermore, the network (PSTN) must be protected from damage due to, for example, faulty data terminal equipment (DTE) or inadvertent shorts through the data terminal equipment (DTE) to its power line. Indeed, the United States Federal Communications Commission (“FCC”) requires a 1500 volt isolation between the data terminal equipment (DTE) and the public switched telephone network (PSTN). In the past, data access arrangements (DAAs) used transformers to provide such electrical isolation. Although transformers adequately isolated the network from the DTE and although transformers permitted bi-directional signal transfer (i.e., an AC signal on a primary would induce a signal on the secondary and an AC signal on the secondary would induce a signal on the primary), they have several limitations. First, transformers are costly relative to solid state devices. Second, transformers are relatively large and heavy. Thus, transformers are not well-suited for applications requiring the interface to have minimal volume and weight, e.g., portable DTEs such as portable personal computers, portable facsimile machines, and portable modems. Therefore, an inexpensive, small, and lightweight data access arrangement (DAA) is needed. 
     Moreover, the data terminal equipment (DTE) are typically four wire devices, having separate transmit and receive wire pairs. Accordingly, the data access arrangements (DAAs) must include a duplexing circuit, or transhybrid, to bridge the two-wire network and the four-wire data terminal equipment (DTE). Since data can be transmitted and received simultaneously, the transhybrid must separate the transmit and receive signal paths. This separation is achieved by suppressing the level of the transmit signal at the output of the transhybrid, and inverting this signal to form a transmit cancellation signal. This signal is added to the receive input of the transhybrid, thereby separating the transmitted signal from the received signal. In known DAAs, the transmit cancellation signal is derived from the output of the line drive circuit. Unfortunately, the cancellation signal cannot be independently controlled. Thus, a DAA having an improved transmit-receiver separation circuit is needed. 
     The data access arrangement (DAA) should ideally have a flat frequency response, a constant group delay, extremely small amplitude and frequency distortion, and should match the impedance of the network line. 
     SUMMARY OF THE INVENTION 
     Briefly, the present invention provides a data access arrangement that includes a transhybrid which produces a transmit cancellation signal independent of the line drive circuit to achieve transmit-receive separation. Thus, for example, the gain of the cancellation signal produced by the transhybrid may be independently controlled. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference is made to the following description of an exemplary embodiment thereof, and to the accompanying drawings, wherein: 
     FIG. 1 is a block schematic of a data access arrangement (DAA) provided with a transmit-receive separation circuit constructed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a block schematic of a data access arrangement (DAA) of the present invention. Data to be transmitted from the data terminal equipment (DTE) is provided to a first (non-inverting) input  11  of a first differential (or servo feedback) amplifier  10 . The first differential amplifier  10  produces an output based on a difference between the data provided to its first (non-inverting) input  11  and a feedback signal provided to its second (inverting) input  12 . The output of the first differential amplifier  10  is coupled with the cathode of a light emitting diode (or “LED”)  21 . The anode of the LED  21  is coupled with a supply voltage V s . Thus, the voltage provided at the output of the first differential amplifier  10  controls the amount of current passing through the LED  21 . Alternatively, the output of the first differential amplifier can be coupled with the anode of the LED  21  and the LED  21  can have its cathode coupled with ground, such that the first differential amplifier  10  sources the current through the LED  21 . Accordingly, the LED  21  emits light having an intensity based on the output of the first differential amplifier  10 . However, since the current-voltage characteristic of LEDs is non-linear, the output of the LED  21  is non-linear with respect to its input. 
     The LED  21  is part of a first optical isolating circuit  20 . The first optical isolating circuit  20  also includes a first photodiode  22 , a second photodiode  23 , and a third photo diode  24 , each of which is optically coupleable with the LED  21 . Thus, when the LED  21  emits light based on the signal output by the first differential amplifier  10 , each of the first, second, and third photodiodes ( 22 ,  23 , and  24 , respectively) produce a current based on the intensity of the light emitted by the LED  21 . In the embodiment illustrated in the FIGURE, the photodiodes  22 - 24  are reverse biased depletion layer diodes, operating below the breakdown voltage. However, other types of photodiodes and biasing may be used in alternative embodiments which will be apparent to those skilled in the art. The current produced by the second photodiode  23  is fed back to the second (inverting) input  12  of the first differential amplifier  10 . The feedback current produced by the second photodiode  23  facilitates linear operation of the first optically isolating circuit  20 . 
     The current produced by the second photo diode  22  is provided to a first input  31  of a first operational amplifier (output opamp)  30 . The output of the first opamp  30  is provided to a line drive circuit  50 , via a first capacitor  90 . The first capacitor  90  acts as a high pass filter, blocking the DC component of the output. The line drive circuit  50 , which drives a local telephone line of the public switched telephone network (PSTN), may be a conventional line drive circuit. The line drive circuit  50  may include an impedance buffer, such as a bipolar transistor, for example. A biasing network, such as a voltage divider network for example, may be provided at the gate of the bipolar transistor such that the bipolar transistor operates in its most linear region. 
     The current produced by the third photodiode  24  is provided to a first input  41  of a second operational amplifier (opamp)  40 . The output of the second opamp  40  is provided to a first (inverting) input  61  of a second differential (or servo feedback) amplifier  60 . The gain of the second opamp  40  can be appropriately adjusted to amplify the transmit cancellation signal properly. The second (non-inverting) input  62  of the second differential amplifier  60  is coupled, via a second capacitor  100  to the local public switched telephone network (PSTN). The second capacitor  100  acts as a high pass filter, blocking the DC component of the signal. 
     The output of the second differential amplifier  60  is provided to the cathode of an LED  71  which has an anode coupled with a supply voltage V s . Thus, the output voltage provided by the second differential amplifier  60  controls the amount of current flowing through the LED  71 . As discussed above, the second differential amplifier  60  may be coupled with the anode of the LED  71  so that it sources the current through the LED  71 . The LED  71  is included in a second optical isolation circuit  70 . The second optical isolation circuit  70  also includes a first photodiode  72  and a second photodiode  73 , each of which are optically coupleable to the LED  71 . When the LED  71  emits light, a current based on the intensity of the emitted light is produced by the photodiode  72 . The anode of the photodiode  72  is coupled with a first input  81  of a third operational amplifier (opamp)  80 . The output of the third opamp  80  is provided to a receiver. 
     The second photodiode  73  also produces a current based on the intensity of the light emitted by the LED  71 . The anode of the second photodiode is coupled with the second (non-inverting) input  62  of the second difference (servo) amplifier  60 , thereby providing a feedback signal to facilitate linear operation of the second optically isolating circuit  70 . 
     As discussed above, in the embodiment illustrated in the FIGURE, the photodiodes  72  and  73  are reverse biased depletion layer diodes, operating below the breakdown voltage. However, other types of photodiodes and biasing may be used in alternative embodiments which will be apparent to those skilled in the art. 
     As shown in phantom in the FIGURE, a delay equalizer  200  may be provided between the second opamp  40  and the third opamp  60  for equalizing the transmit cancellation signal with the transmitted signal, i.e., for delaying the transmit cancellation signal such that it is synchronized with the transmitted signal. 
     The embodiments described herein are merely illustrative of the principles of the present invention. Various modifications may be made thereto by persons ordinarily skilled in the art, without departing from the scope or spirit of the invention.