Abstract:
The present invention is directed towards a low cost Data Access Arrangement (DAA) with improved current control for controlling the current drawn from a telephone line pair to drive the light emitting diodes (LEDs) in an opto-isolator utilized with the DAA, and to an enhanced method of sinking DC line current. Also, an improved method is provided to implement line current draw and battery detect features.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates in general to communication systems, and more particularly to a Direct Access Arrangement (DAA) incorporating optocouplers for interfacing analog devices (e.g. telephone, modem, fax) to a Central Office (CO) or Private Branch Exchange (PBX).  
       BACKGROUND OF THE INVENTION  
       [0002]     DAAs provide an interface between the CO telephone line (tip and ring pair) and analog devices such as a telephone, modem, or fax machine. A DAA is required to perform several functions in accordance with the national specifications of a country. The tip and ring pair is required to be compatible with Plain Old Telephone Service (POTS), giving rise to the requirement that the DAA, for the purposes of coupling with the CO line, must resemble a POTS device.  
         [0003]     In order to resemble a POTS device, the DAA must perform a number of electrical functions. It must provide over-voltage protection from signaling or environmental conditions such as lightning, power cross, or electrostatic discharge, along with polarity definition to mate the appropriate tip and ring voltages with the local circuit. The DAA must present a specific AC impedance (600 ohms in North America, but varying by jurisdiction) to the CO line and, more importantly, sink a predetermined DC current (20-120 mA, depending on the jurisdiction of the CO line). Receiver handset on/off hook control must be provided, as well as conversion of the bi-directional line signal to individual unilateral transmit and unilateral receive signals. Typically, the DAA must also include a detector to ensure line availability and a transhybrid balance network to cancel any transmitted signal that is reflected from the CO line.  
         [0004]     Furthermore, inclusion of a DAA in highly integrated technology (e.g. a PCMCIA modem) requires compact placement of components on a printed circuit board (PCB). It is desirable that the components use a minimum of PCB real estate and that component elevation from the PCB surface is minimal.  
         [0005]     In reference to the DAA requirement of presenting a specific AC input impedance to the line, this is usually accomplished by using a discrete impedance or an active impedance block. However, the use of discrete components conflicts with the need for compact placement discussed above.  
         [0006]     Typically, optocouplers are used to provide isolation between the device and CO line. However, optocouplers require active components (i.e. buffers) on the line side. These active components use power isolated from the device side circuitry. Therefore, the line side circuitry is often line powered. Filtering tip and ring for power usage introduces extra bulky components while directly coupling the tip and ring to the line side circuitry introduces AC current feedback signals that cause distortion and noise.  
         [0007]     The problem of AC current feedback is addressed in U.S. Pat. No. 5,481,606 (Andrieu), U.S. Pat. No. 5,528,686 (Cwynar), and U.S. Pat. No. 5,946,393 (Holcombe). The solutions of Andrieu and Holcombe use a parallel current method to eliminate feedback. The solution of Cwynar uses substantially constant current sink amplifiers that do not generate feedback.  
         [0008]     The above-discussed prior art DAA implementations that address current feedback suffer from extensive use of large or discrete parts. This is also in conflict with the aforementioned goal of compact placement.  
         [0009]     Also, with reference to the requirement that the DAA must resemble a POTS device, the prior art teaches the use of gyrators for sinking CO line current. Such gyrators use large reactive components, or alternatively, introduce high distortion and add to cost and real estate.  
         [0010]     Finally, with reference to the DAA requirement of providing line status detection, the prior art teaches the use of battery detector circuits implemented by inserting an isolated current or voltage sensor in the current path, thereby adding to cost and real estate.  
       SUMMARY OF THE INVENTION  
       [0011]     According to one aspect of the invention, a DAA is configured to resemble a current source for generating an AC current flow in the tip and ring that is proportional to the voltage thereon, with a given proportionality constant. The DAA therefore presents a specific input impedance to the CO line depending on the proportionality constant of the current source.  
         [0012]     According to another aspect of the invention, feedback of AC current on the tip and ring resulting from line side powering of isolation optocouplers is eliminated by monitoring feedback AC current flow through the line and generating equal and opposite current flow in the line to cancel the feedback AC current.  
         [0013]     According to a further aspect of the invention, a shunt regulator is used to draw or sink DC current from the line at 20-120 mA (depending on the jurisdiction). Although the shunt regulator also allows AC current to flow, the AC current is sensed, as discussed above, and an equal but opposite phase current is generated on the tip and ring to compensate for the AC current drawn by the load (i.e. shunt regulator).  
         [0014]     According to yet another aspect of the invention, DC bias current flowing through the receive optocoupler as a result of the application of line power, is sensed to provide a battery detect function. This eliminates the prior art requirement for a separate isolated battery detector. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     A detailed description of the preferred embodiment of the present invention is provided herein below with reference to the accompanying drawings in which:  
         [0016]      FIG. 1  is a block diagram of a Direct Access Arrangement (DAA) of the prior art;  
         [0017]      FIG. 2  is a schematic diagram of a Direct Access Arrangement (DAA) in accordance with the preferred embodiment of the invention; and  
         [0018]      FIGS. 3   a - 3   d  are schematic diagrams showing prior art line current sink circuits. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     Turning to  FIG. 1 , a prior art DAA is shown connected to the CO line.  
         [0020]     The two-wire CO line includes a tip and ring pair that is connected to an over-voltage protection circuit which, in turn, is connected to a polarity circuit (e.g. diode bridge) to define proper line polarity for the DAA circuitry. The purpose of the protection circuit is to prevent over-voltages from damaging the DAA or device.  
         [0021]     A ring detector and a battery detector are connected in parallel to detect ringing voltage on the CO line and line availability, respectively. A hook switch controls on/off hook status of the DAA. A gyrator is connected to the line pair via the hook switch for sinking 20-120 mA DC current, for POTS compatibility.  
         [0022]     A hybrid provides two-to-four-wire signal conversion between the bi-directional line pair and unidirectional transmit (Vtx) and receive (Vrx) lines. Additional circuitry provides the necessary input impedance.  
         [0023]     The line side signals must be isolated from the device side. Conventionally, transformers have been used to provide device isolation from the CO line. A capacitor or capacitors are typically used to block the DC voltages present on the line. Large values of transformer inductance (i.e. &gt;3H) and capacitance (i.e. 2 μF) are required for good low frequency response, resulting in increased size and added cost. The capacitor in most configurations must also be capable of handling high AC line voltages (caused by the ringing signal), resulting in large capacitance size. The use of such large discrete components is not suitable in a device where PCB real estate is limited and component height is constrained. For example, for a PCMCIA standard size card, whose components need to fit within a molded package having a total thickness of less than 0.1″ thick, it is desirable to only use small capacitors (i.e. &lt;0.1 μF).  
         [0024]     In an effort to address the shortcomings of prior art DAAs incorporating transformers in terms of real estate constraints and the requirement for good low frequency response, capacitors and optocouplers have also been used for isolation. The typical optocoupled DAA circuit is divided into a CO line side that is not isolated and a device side that is isolated from the line by the optocouplers. The line side circuitry must be powered to drive the LED of the transmit optocoupler and LDD of the receive optocoupler. This power is typically drawn from the line. However this results in a number of problems such as the necessity of voltage filtering, the necessity of voltage regulation, and the feedback of signals to the CO line.  
         [0025]     When the DAA is in an inactive (i.e. on-hook) state the line is at a high potential (typically −48 V). During the active (i.e. off-hook) state, the CO line is typically at a 5 V to 12 V potential. The DAA draws a specified amount of current from the line (i.e. a minimum of 20 mA and up to 120 mA). Since high AC signals are present on the line, the line signal must be filtered in order to be used as a clean power source. The filters and regulation circuits introduce circuit complexity and increase the component count as well as real estate requirement.  
         [0026]     The tip and ring (after polarity correction by the diode bridge) can be used to provide supply voltage and ground, respectively, for the line side circuitry. However, AC voltages on the supply must not significantly impact the performance of the line side circuitry. Also, the load between supply and ground must not result in an AC current flow from tip to ring. The CO line, without filtering or regulation, is an acceptable power source where these two conditions are met.  
         [0027]     The presence of AC signals on the supply gives rise to power supply noise on the DAA amplifiers. However, this problem is overcome where operational amplifiers having high Power Supply Rejection Ratio (PSRR) are used. Typical operational amplifiers have high enough PSRR that this noise does not significantly impact the transmission characteristics of the DAA.  
         [0028]     However, considerable distortion and gain variation still results from powering the optocouplers with line power due to the feedback currents. Existing solutions to correct this problem either require a DC holding circuit and a high current transistor, or require limits on amplifier selection along with additional components.  
         [0029]     The present invention relates specifically to improvements in prior art use of line power, input impedance and battery detection for providing line availability. Other aspects of the DAA shown in  FIG. 1  are set forth in the prior art literature, and therefore are not further discussed in relation to the present invention, unless strictly necessary to illustrate the improvements.  
         [0030]      FIG. 2  illustrates an exemplary embodiment of a DAA according to the present invention. The DAA interfaces a standard CO line having tip and ring to an analog device via unidirectional receive (Vrx) and transmit (Vtx) lines. For simplification, overvoltage protection, polarity bridge, ring detect and hook control circuits are not shown.  
         [0031]     Electrical isolation between the CO and the device is provided by a pair of optocouplers (TxOpto and RxOpto). In keeping with real estate requirements for DAAs, optocouplers are normally low height components. Each optocoupler contains a Light Emitting Diode (LED) for transmitting light with an intensity proportional to the current traveling through the diode, and a Light Detector Diode (LDD). The incident light from the LED modulates the current flowing through the LDD.  
         [0032]     One drawback of optocouplers is that the coupling between the LED and LDD is not constant over temperature. For this reason, for every LED in an optocoupler, two LDDs are provided wherein one of the LDDs is used to provide feedback to compensate for the coupling variation over temperature. This is possible since the ratio of coupling factor associated with first LDD and coupling factor associated with second LDD remains fairly constant over temperature.  
         [0033]     The device (not shown) communicates via transmission Vtx and reception Vrx lines, with the voltage bias level of the device side circuits being established via Vref applied to the non-inverting input of amplifier U 2 A.  
         [0034]     Active circuits on the CO side are powered from the tip via node Va (CO side power Vcc) and ring via node  0  (CO side common). Active circuits on the device side are powered by device power Vdd and device ground.  
         [0035]     The transmission circuitry of the DAA is characterized by two operational amplifiers. U 2 A and U 1 A. The inverting input of first transmission stage (device side) amplifier U 2 A is connected to Vtx via capacitor C 4  and resistor R 0 , and connected to device ground via resistor R 9 . The output of amplifier U 2 A is connected to the transmission LED of optocoupler TxOpto via output resistor R 10 . Diode balancing feedback is provided by the LDD connected between the inverting input of amplifier U 2 A and device side power Vdd.  
         [0036]     The first and second transmission stages are optically coupled across the isolation barrier from the transmission LED of TxOpto to the second LDD connected between node Vc and Vcc. Consequently, signal output from U 2 A is reproduced across resistor R 3  connected between node Vc and the line side common. The voltage on node Vc (i.e. V(Vc)), is thus responsive to Vtx. Node Vc is connected to Node Vb via resistor R 1 . Node Vb is connected to the second transmission amplifier U 1 A at the non-inverting input thereof via resistor R 13 . The inverting input of amplifier U 1 A is connected to CO line side common via resistor R 17 , capacitor C 5 , and resistor R 15 . The output of amplifier U 1 A is connected to the base of BJT Q 1  base. The collector of Q 1  is connected to the tip and the emitter thereof is connected to feedback resistor R 19 , which is connected to the inverting input of amplifier U 1 A. The emitter of BJT Q 1  is also connected to the ring via resistor ZO.  
         [0037]     The receiver circuitry of the DAA is also characterized by two operational amplifiers U 1 B and U 2 B. The non-inverting input of first receiver stage (CO side) amplifier U 1 B is connected to Vb. The inverting input thereof is connected to CO line side common via resistor R 4 . The output of amplifier U 2 B is connected to the base of BJT Q 2 . Node Vb is connected to the tip via capacitor C 1  and resistor R 2 . The BJT Q 2  collector is connected to CO power Vcc via the transmission LED of RxOpto and the emitter is connected to the junction of capacitor C 5  and resistor R 15 . Optical feedback is provided, as discussed above in connection with TxOpto. CO power Vcc is applied to the inverting input of amplifier U 1 B via the LDD of RxOpto connected to R 4 .  
         [0038]     The first and second receiver stages are optically coupled across the isolation barrier from the transmission LED of RxOpto to the LDD connected between node Vd and Vdd. Consequently, signal output from U 1 B is reproduced across resistor R 5  connected between node Vd and device side ground. The voltage at node Vd is thus responsive to the CO line signals. Node Vd is connected directly to the non-inverting input of amplifier U 2 B. The inverting terminal of amplifier U 2 B is connected to device ground via resistor R 7 , and capacitor C 7 . The output of amplifier U 2 B is connected directly to the Vrx terminal of the device and via feedback resistor R 8  to the inverting input thereof. The output of U 2 B is also connected via resistor R 20  and capacitor C 8  to device ground. The node connecting R 20  and C 8  is connected to a battery-detect output terminal.  
         [0039]     Since the CO line is bi-directional, signal transmissions from the device are fed back to the device for reception, via node Vb. In order to compensate for this feedback the transmitter (at Vtx) is coupled to the receiver (at Vd) via bi-directional feedback resistor R 6  and capacitor C 3 .  
         [0040]     The DAA of the present invention incorporates a shunt regulator from tip to ring including zener diode D 1  in series with a resistor R 18 . The junction of the regulator is node Ve. This node is connected to the inverting input of amplifier U 1 A via capacitor CZ and resistor R 16 .  
         [0041]     The non-inverting input of amplifier U 1 A is connected to CO line side common via resistor R 14  and capacitor C 6 .  
         [0042]     One inventive feature of the DAA according to the present invention is the elimination of AC feedback current in the line by monitoring amount of current flowing and in response generating an equal but opposite current flow resulting in zero net current flow. This approach eliminates the need for a DC holding circuit and any high current transistors. Moreover there is no constraint requiring the use of constant current sink amplifiers, nor is the generation of an invariant bias voltage required. This reduces the part count, parts complexity and real estate requirement.  
         [0043]     Operation of this feature is as follows. As shown in  FIG. 2 , an AC current, I 1 , flows from Vcc through BJT Q 2  and resistor R 15  to the line side circuit common. Hence the AC voltage across resistor R 15  is I 1 *RR 15 . The AC voltage at the emitter of Q 1  is 
 
 V ( QQ   1   e )=− RR   19 / RR   17 * I   1 * RR   15 ,  (1) 
 
 and the AC current flow through Q 1   e  is 
 
 I ( QQ   1   e )=− RR   19 / RR   17 * I   1 * RR   15 / Z   0   (2) 
 
 This requires the current flow through the collector of Q 1  to be 
 
 I ( QQ   1   c )=β/(β+1)* I ( QQ   1   e )=−β/(β+1)* RR   19 / RR   17 * I   1 * RR   15 / Z   0   (3) 
 
 If 
 
 RR   19 / RR   17 =1 and  Z   0 = RR   15 *β/(β+1) 
 
 Then 
 
 I ( QQ   1   c )=− I   1 .  (5) 
 
         [0044]     Since AC current I 1  flows from Vcc to common and AC current−I 1  also flows from power Vcc to common, the net AC current flowing is zero.  
         [0045]     In addition to constraints imposed by equation (4), the values of resistors are chosen such that enough DC current flows through the transmit and receive LEDs for them to operate in a linear region. Since very little current is consumed by the operational amplifiers U 2 A, U 1 A, U 1 B, and U 2 B, sufficient current is available to enhance the linearity of the optocoupler LEDs.  
         [0046]     A second feature of the DAA according to the present invention is the incorporation of an improved transhybrid network. The AC Voltage at Vb passes through the receive optocoupler RxOpto such that voltage on Vd is equal to the voltage on Vb. The voltage on Vb comprises both the transmit and receive signals. However, the amount of the transmit signal at Vb is known. This is subtracted from the voltage on Vd at the device side via compensating resistor R 6 , resulting in a cancellation of transmit signal at the summing node Vd. In this implementation of the transhybrid network, extra amplifier stages are not required.  
         [0047]     The input impedance is produced by the additional feedback path comprising resistor R 2  and capacitor C 1  in conjunction with the 2 nd  transmission stage (the transmission stage incorporating operational amplifier U 2 A) and the ground path resistor R 14  and capacitor C 6 . The existing configuration taps the tip/ring voltage (the tap is the resistor network R 2 , R 13 , R 14 ), and feeds it back to the 2 nd  stage amplifier U 1 A. The amplifier U 1 A is configured with respect to the additional feedback as a voltage controlled current source, where the positive terminal is the voltage input from the tap. The resulting current, determined by the selection of feedback resistor R 19  and output resistor ZO, sources transistor Q 1 , largely determines AC tip/ring current. The ultimate effect is one of apparent impedance, determined by the ratio of tip/ring AC voltage to tip/ring AC current.  
         [0048]     For POTS compatibility, the DAA of the present invention draws at least 20 mA and up to 120 mA of line current. By drawing the holding current from the telephone line, the DAA signals the central telephone office that it is active to either originate or complete a communications connection with the central office (CO). The holding current is generally specified by a national standard, which varies from one country to another in the range of 20 to 120 mA.  
         [0049]     Varying existing solutions have drawbacks, such as requirements for large discrete capacitors or inductors, poor performance over a wide range of frequencies, lower line termination impedance, high levels of distortion in the signals transmitted and received, and/or utilization of current that cannot be used to drive other circuits.  
         [0050]     According to a further aspect of the invention, the line side circuitry is line powered and the driving transistors are biased such that about 20 mA of current is used when tip/ring voltage is at an operating voltage of 6V. Hence, there is no need to activate a separate DC current sink circuit. However, at higher tip/ring voltages the DAA is required to draw more line current (up to 120 mA).  
         [0051]     Zener diode D 1 , in series with a resistor R 18  is connected across the tip and ring. The zener voltage is selected as the operating voltage of the DAA. Where the tip/ring voltage increases beyond the operating voltage, the zener D 1  conducts and a constant resistive load is present across the tip/ring. This resistor R 18  is selected such that it draws any excess current once the tip/ring voltage goes higher than the operating voltage of the DAA. However, since the resistor R 18  is not coupled via a coil it also presents an AC load across the CO line. To eliminate this AC loading effect, the amount of current flowing through R 18  is sensed via a voltage tap at Ve. This tap voltage is fed back via resistor R 16  to the 2 nd  transmission stage (the stage based on amplifier U 2 A). BJT Q 1  subsequently generates an equal current but with opposite phase such that the net AC current flow on the tip/ring is zero.  
         [0052]     As shown in  FIG. 2 , 
 
 Ve=I   2 * RR   18   (6) 
 
 V ( QQ   1   e )=− RR   19 / RR   16 * Ve   (7) 
 
 V ( QQ   1   e )=− RR   19 / RR   16 * I   2 * RR   18   (8) 
 
 I ( QQ   1   e )=− RR   19 / RR   16 * I   2 * RR   18 / Z   0   (9) 
 
 I ( QQ   1   c )=β/(β+1)* I ( QQ   1   e )=−β/(β+1)* RR   19 / RR   16 * I   2 * RR   18 / Z   0 .  (10) 
 
 Therefore if 
 
β/(β+1)* RR   19 / RR   16 * RR   18 / Z   0 =1  (11) 
 
 Then 
 
 I ( QQ   1   c )=− I   1  (as required)  (12) 
 
         [0053]     This solution eliminates the need for large capacitors to be used. Also there is no AC current leakage. Any AC current flow through resistor R 18  is fed back to produce an equivalent opposite phase current on the line. The use of high current transistors, high current inductors, or FETs, as shown in the prior art circuits of  FIGS. 3   a - 3   d , with their attendant real estate requirements and cost, is not required. Only zener diode D 1  is required to implement the DC current sink feature of the present invention. Hence, the component count and cost is reduced as compared to the prior art.  
         [0054]     In some communication devices (e.g. a fax machine) it is desirable to know if a line is connected to the DAA and is available for use. Conventionally, this is implemented by inserting an isolated current sense device including a dedicated optocoupler, in the DAA current path. This approach requires an extra optocoupler and a buffer for the battery detector circuit, thereby contributing to added cost and real estate requirements.  
         [0055]     Therefore, in accordance with yet another aspect of the invention, line availability information is transmitted through the receive optocoupler RxOpto and, hence, a separate detector (i.e. optocoupler etc.) is not required. The line side is powered from the CO line, as discussed above. When the battery voltage is present (i.e. line available), the line side circuitry is active and bias current flows through the receive optocoupler LED. The coupled current through the associated LDD generates a bias voltage on node Vd. This results in a bias voltage charging capacitor C 8 . The device then determines line availability by taking the voltages across C 8 .  
         [0056]     The present invention, as embodied in the DAA of  FIG. 2 , combines an improved receive LED circuit, a transmit circuit, an input impedance circuit, a transhybrid loss circuit, a battery detect circuit, isolation circuit, and a DC line current draw circuit. The described improvements significantly decrease the component count and real estate requirements and thereby drive the DAA cost down while maintaining or excelling the performance of conventional designs.  
         [0057]     It will be appreciated that, although an embodiment of the invention has been described and illustrated in detail, various modifications and changes may be made. The selection of resistances and capacitances may be made by those familiar with the operating requirements of a standard DAA, except where this disclosure provides direction. It will be appreciated that the aspects of the invention may be implemented separately, or cooperatively. Any such choice of implementation may be made without departing from the sphere and scope of the invention as described by the claims appended herein.