Abstract:
A method and apparatus for a telephone line interface or data access arrangement (DAA) which includes a shunt regulator in series with a line modulator. A sense resistor is placed in series between the shunt regulator and the line modulator to provide a measurement of the amount of current distortion in the DAA. The line modulator contains a Darlington pair which reduces the amount of current drawn by the line modulator, allowing the sense resistor to sense a majority of the system current. The voltage across the sense resistor is fed back to the line modulator. The line modulator is capable of adjusting the AC modulation and the DC termination presented to the telephone line. The method includes drawing power from the telephone line using a shunt regulator, modulating the telephone line in series with the shunt regulator, sensing a level of distortion in the DAA, and feeding the sensed level of distortion to the line modulator.

Description:
FIELD OF THE INVENTION 
     This invention relates to a low noise telephone line interface for data access arrangements (DAA). Specifically, it relates to a low distortion line powered DAA having improved linearity and accuracy. 
     BACKGROUND OF THE INVENTION 
     The telephone lines to a residence in the United States and elsewhere can have common mode voltages of over 100V, and the FCC requires the telephone lines to be isolated from any electric main powered device (such as a PC) connected to the telephone lines (through a modem for example) to prevent damage to the telephone network. 47 CFR 68.302,4 (10-1-97 Edition). A data access arrangement (DAA) is specified by the FCC to isolate the telephone lines from electric main powered devices, such as illustrated in FIG.  2 . Since the voice band modem signal is limited to the 100 to 3600 Hz band, a DAA can be constructed using a transformer which operates as a bandpass filter to isolate the electric main powered device from the telephone lines. 
     A smaller size and potentially lower cost solution uses active circuits to communicate with the central telephone offices and various modulation techniques to couple the DAA through small capacitors to the PC. 
     FIG. 3 shows a known line powered telephone line interface circuit for modulating a data signal onto a telephone line using active circuits. The circuit of FIG. 3 is disclosed and described fully in U.S. patent application Ser. No. 09/028,061 filed on Feb. 26, 1998, entitled Low Noise Line Powered DAA With Feedback, assigned to the same assignee as the present application, and incorporated herein by reference. The circuit is designed in low voltage CMOS technology and can handle only a small amount of voltage. The main function of the circuit is to take the incoming current I LINE  supplied by the telephone company and modulate it with a data signal developed by processing a differential data source signal V D  with a line modulator so as to place the data signal on the telephone line. The circuit uses transistor Q 1  as a line modulator, and contains a shunt regulator in series with the line modulator Q 1 . A sense resistor R S1  is placed in series between the line modulator Q 1  and the shunt regulator to monitor the current through the shunt regulator. 
     The circuit depicted in FIG. 3 works by monitoring the current through sense resistor R S1  with a feedback loop around the amplifier A. Resistors R T1  and R B1  sense the differential voltage across R S1 . By setting R T1 =R B1 , the current through R T1  and R B1  will accurately model the current through R S1 . The desired signal to be modulated is introduced by a differential data source signal V D . The differential signal is created by adding signal V D /2 to V CM  to create V P  and subtracting V D /2 from V CM  to create V N . This differential signal then drives the input resistors R IP  and R IN  to provide a differential signal input current. The generation of the differential signal current is well known in the art and will not be further discussed herein. The control amplifier operates to force the current through resistor R S1  to equal the desired signal current by regulating transistor Q 2  to control the base of transistor Q 1 , which in turn regulates the current through the source-emitter path of transistor Q 1  and thereby through resistor R S1 . In this circuit, the collector current of transistor Q 1  is well controlled by the control amplifier A. However, this arrangement incurs a degree of error which is problematic for new communication devices such as high speed data modems. The source of the error is due to current that is outside of the path containing the sense resistor R S1 . This stray current will be discussed after a brief discussion of FIG.  4 . 
     FIG. 4 depicts an alternative circuit arrangement similar to the circuit depicted in FIG.  3 . However, the circuit in FIG. 4 uses the output of amplifier A to control the emitter of transistor Q 2 , rather than the base of transistor Q 2 , and thereby the collector current of transistor Q 1 . As in the circuit depicted in FIG. 3, the collector current of transistor Q 1  is well controlled by the control amplifier A. This arrangement also incurs a degree of error which is problematic for new communication devices such as high speed data modems. 
     The error associated with the previously mentioned circuit designs of FIG.  3  and FIG. 4 will now be discussed. Ideally, the current through R S1  would equal the current, I LINE , introduced to the system by the telephone company. This would allow amplifier A to take all of I Line  into account when modulating the differential signal source onto I Line . An error exists in the line modulation devices of FIG.  3  and FIG. 4 due to the inclusion of only part of the total current I LINE  through sense resistor R S1 . In both circuits, the current from the telephone company is introduced to the system through the emitter of transistor Q 1  (hereinafter “I E1 ”). In the circuits depicted in FIG.  3  and FIG. 4, I E1  is equal to I LINE , the resistances of R T1  and R B1  are a couple hundred thousand ohms, and the resistance of R S1  is 10-20 ohms. Because of the relatively high level of resistance of R T1  and R B1 , the current that flows through R T1  and R B1  can be neglected in the circuit analysis. As current flows through the circuits, I E1  is divided into the transistor Q 1  base current (hereinafter “I B1 ”) and the transistor Q 1  collector current (hereinafter “I C1 ”). The collector current I C1  through the resistor R S1  is used by amplifier A in a feedback loop to modulate the desired signal onto I LINE . Since the current I B1  is outside the feedback loop, an error term in the amount of I B1  is introduced to the circuit, that is, I C1  through resistor R S1  is not equal to I LINE , but is equal to I E1 −I B1  or I LINE −I B1 . 
     An additional problem arises from I B1  being outside the amplifier feedback path. Since I C1  and I B1  are related by the β of Q 1 , and the β of a transistor is a function of the actual signal level, the error term introduced by not accounting for current I B1  in the feedback loop is signal dependent. Signal dependent error terms are a source of harmonic distortion which is problematic for communication devices. In order for current 56 k modems (V.90 standard) to function, a signal to distortion ratio greater than 80 dB is needed. Unfortunately, due to the error term introduced by neglecting I B1 , the circuits of FIG.  3  and FIG. 4 can provide a signal to distortion ratio of only about 75dB, even when high quality components are utilized. 
     One method which has been used to reduce distortion is depicted in FIG.  5 . The circuit is disclosed and described fully in U.S. patent application Ser. No. 09/280,473 filed on Mar. 30, 1999, entitled Method and Apparatus for Decreasing Distortion in a Line Powered Modulator Circuit, assigned to the same assignee as the present application, and incorporated herein by reference. 
     The circuit in FIG. 5 reduces distortion by incorporating a larger portion of I LINE  into the feedback path of the control amplifier A. A larger portion of I LINE  is incorporated by including a second sense resistor R S2  in a second feedback path to amplifier A in order to sense current introduced to the system by I LINE  which does not flow through the first sense resistor R S1 . The operation of the differential signal source and the shunt regulator are similar to the differential signal source and shunt regulator discussed above. In addition, as with R T1  and R B1 , R T2  and R B2  have a relatively high level of resistance and the current that flows through R T2  and R B2  can be neglected in the circuit analysis. 
     In FIG. 5, the output of amplifier A is electrically connected to the emitter of transistor Q 2  through the additional sense resistor R S2 , the collector of transistor Q 2  is electrically connected to the base of transistor Q 1 , and the base of transistor Q 2  is electrically connected to the collector of transistor Q 1 . In this configuration, the original sense resistor current I S1  through the primary sense resistor R S1  is equal to the transistor Q 1  collector current I C1  less the transistor Q 2  base current I B2 . Accordingly, the transistor Q 2  base current I B2  equals the transistor Q 1  collector current I C1  less the original sense resistor current I S1 . The current through the additional sense resistor R S2  is the transistor Q 2  emitter current I E2 , or equivalently the sum of the transistor Q 2  base current I B2  and collector current I C2 . Since the transistor Q 2  collector current I C2  equals the transistor Q 1  base current I B1 , the current through the additional sense resistor Q 2  can also be said to be the sum of the currents I B1  and I B2 . Therefore, the sum of the currents through both sense resistor R S1  R S2  equals (I B1 +I B2 +I C1 −I B2 ), or equivalently I B1 +I C1 , which equals I LINE . This arrangement results in a circuit which is virtually free from distortion. The circuit is free from distortion because the first sense resistor R S1  senses the current and its associated distortion through the shunt regulator, and the second sense resistor R S2  senses all other significant currents and their associated distortion, allowing the amplifier to control I LINE  by incorporating all of I LINE  in a feedback path. 
     This method and apparatus for reducing distortion in a line powered DAA requires the amplifier to sense the level of current at multiple locations. Since the level of current through a resistor depends on the resistance of the resistor, the resistors at the various locations must be carefully matched in order to obtain an accurate relationship between I LINE  and the current sensed by the amplifier. In addition, the introduction of each additional sense resistor requires the addition of high value resistors such as R T2  and R B2 . 
     SUMMARY OF THE INVENTION 
     The present invention provides a novel method and apparatus for increasing the signal to distortion ratio in a line powered telephone line interface or data access arrangement (DAA). The invention accomplishes this task by reducing the amount of error which is inherent to prior art circuit designs by adapting the circuits to incorporate more of the total line current supplied by the telephone company into a single feedback circuit containing a sense resistor. 
     The invention modifies the prior art circuits by replacing the primary modulation transistor used to modulate the telephone company line current, I LINE , with a Darlington pair. By using a Darlington pair to modulate I LINE , much less current flows through the modulator portion of the circuit. Since less current is flowing through the modulator circuit, more of I LINE  flows through the single sense resistor. This allows the amplifier to modulate I LINE  taking into account a majority of the current introduced by I LINE , resulting in enhanced distortion characteristics in the circuit, without the need of introducing additional sense resistors and feedback paths. The amplifier can sense substantially all of I LINE  within the circuit and process accordingly to remove noise and distortion. This arrangement results in a modulation circuit capable of producing a signal to distortion ratio that is greater than 80 dB. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a low noise line powered DAA in accordance with a preferred embodiment of the present invention. 
     FIG. 1A is a circuit diagram of a low noise line powered DAA in accordance with an alternative embodiment of the present invention. 
     FIG. 2 is a block diagram of a conventional interface between a telephone network and an electric main powered device in accordance with the prior art. 
     FIG. 3 is a circuit diagram of a data access arrangement (DAA) in accordance with known art. 
     FIG. 4 is a circuit diagram of an alternative data access arrangement (DAA) in accordance with known art. 
     FIG. 5 is a circuit diagram of an alternative data access arrangement (DAA) in accordance with known art. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention discloses a line powered data access arrangement (DAA) device having a desirable signal to distortion ratio. The present invention is particularly useful for modern telephone modems (V.90 standard) which require a signal to distortion ratio greater than 80 dB. 
     FIG. 1 illustrates a telephone line interface  10  having a differential signal source  12 , line modulator  14 , shunt regulator  16 , and a first sense resistor  18 . The line modulator  14  comprises a Darlington pair  21  made up of transistors  21 A and  21 B, an amplifier  24 , and a transistor  22 . 
     Referring to FIG. 1, the differential signal source  12  functions by adding half of the desired signal voltage  31  to the common mode voltage  34  to create voltage signal  30  and subtracting half of the signal voltage  31  from common mode voltage  34  to create voltage signal  32 . These differential signals  30  and  32  then drive the input resistors  26  and  28  to provide a differential signal input current into the amplifier  24  at the inverting input  29  and at the non-inverting input  27 . The generation of the differential signal currents can be made by other means which are well known in the art, and thus will not be further discussed. 
     The shunt regulator  16  provides power drawn from the telephone line to the line modulator circuit  14  as well as to other modem and/or data processing circuitry necessary to provide DC termination and AC modulation of the telephone line. The shunt regulator  16  limits the voltage across system components which are in parallel with the shunt regulator  16  to voltage level V DDA . The shunt regulator  16  is especially important if the amplifier  24  and other circuitry is fabricated in low voltage CMOS technology that cannot withstand voltages above 5 volts (or other fabrication technologies with low voltage requirements). Since the voltage difference between the telephone line tip voltage  36  and the telephone line ring voltage  38  can range from 5 to 56 volts, the DAA  10  could be destroyed in the absence of shunt regulator  16 . 
     The sense resistor  18  is used in a feedback loop by the control amplifier  24  located within line modulator circuit  14 . By monitoring the current through sense resistor  18  with a feedback loop, the amplifier  24  can compensate for distortion in the DAA  10 . Resistors  17  and  19  sense the differential voltage across sense resistor  18 . By setting the resistance of resistor  17  equal to the resistance of resistor  19 , the current through resistors  17  and  19  will accurately model the current through resistor  18 . In a preferred embodiment, the resistance of resistors  17  and  19  is several hundred thousand ohms, while the resistance of the sense resistor  18  is approximately 10-20 ohms. Because of the relatively large resistance of resistor  17  and  19 , the current through these resistors can be neglected in the circuit analysis. 
     The present invention is directed toward reducing distortion in a line powered DAA  10  using a single feedback path. Low distortion is achieved in the present invention by incorporating most of I LINE  into the single feedback path of amplifier  24  and using amplifier  24  to compensate for the distortion in the path. The present invention accomplishes passing most of I LINE  through a single feedback path by using a Darlington pair  21  comprised of transistors  21 A and  21 B in the line modulator. The use of a Darlington pair  21  results in only a small amount of current falling outside of the path containing sense resister  18 , allowing the majority of I LINE  to pass through the single sense resister  18 . By sensing most of the current in DAA  10 , the amplifier  24  can incorporate most of the total DAA current with a feedback loop and thereby reduce the harmonic distortion in the DAA  10 . 
     The Darlington pair  21  comprised of transistors  21 A and  21 B, as depicted in FIGS. 1 and 1A will be referenced in the specification using the naming convention which follows: the base of transistor  21 B will be referred to as the base of the Darlington pair  21  or the control terminal of the Darlington pair  21 , and the emitter and collector of transistor  21 A will be referred to as the emitter and collector of the Darlington pair  21  or the current flow terminals of the Darlington pair  21 . 
     The control amplifier  24  senses the current through sense resistor  18  with a feedback loop and attempts to control the circuit in the following manner. Resistors  17  and  19  sense the differential voltage across sense resistor  18 . By setting resistor  17  equal to resistor  19  the current through resistor  17  into the non-inverting input  27  of control amplifier  24  and the current through resistor  19  into the inverting input  29  of control amplifier  24  will accurately model the sum of currents through sense resistor  18 . This approximately models I LINE  and is the parameter to be controlled. The feedback action of the loop comprising amplifier  24 , transistor  22 , transistor  21 A, and transistor  21 B adjusts the current through resistor  18  such that the current through resistor  17  equals the current from the differential signal source  12  through resistor  26 , and the current through resistor  19  equals the current from the differential signal source  12  through resistor  28 . 
     In a preferred embodiment of the invention, as depicted in FIG. 1, the output of amplifier  24  is electrically connected to the emitter of transistor  22 , the collector of transistor  22  is electrically connected to the base of the Darlington pair  21 , and the base of transistor  22  is electrically connected to the collector of the Darlington pair  21  through sense resistor  18 . In this configuration, the telephone line current I LINE  is introduced to the DAA  10  through the emitter of Darlington pair  21 . I LINE  is equal to the transistor  21 A emitter current I E1 . Current I E1  is then split into the transistor  21 A base current I B1  and collector current I C1 . The transistor  21 A base current is then split into the transistor  21 B base current I B1A  and collector current I C1A . The sense resistor current I S  through the sense resistor  18  is equal to the transistor  21 A collector current I C1  plus the transistor  21 B collector current I C1A . Since current I C1  is equal to I LINE  minus I B1 , and I C1A  is equal to I B1  minus I B1A , the current I S  through sense resistor  18  equals I LINE  minus I B1A  (i.e., I LINE −I B1 +I B1 −I B1A =I LINE −I B1A ). Although there is still a degree of error in the system, the error is reduced from I B1  to I B1A . The difference in magnitude between I B1  and I B1A  is the β of transistor  21 B (β is typically greater than 50). 
     This arrangement results in a circuit which is virtually free from distortion. The circuit exhibits low distortion because the sense resistor  18  senses most of the current and its associated distortion through the DAA  10 , allowing the amplifier  24  to control I LINE  by incorporating most of I LINE  into the feedback path and compensating for the associated distortion. Even though this arrangement still contains a small amount of error, it produces a signal to distortion ratio suitable for present communication devices (&gt;80 dB). 
     In an alternative embodiment of the present invention, as depicted in FIG. 1A, the output of amplifier  24  is electrically connected to the base of transistor  22 , the collector of transistor  22  is electrically connected to the base of transistor  21 B, and the emitter of transistor  22  is electrically connected to the telephone line ring voltage  38 . The collector of the Darlington pair  21  is connected to the shunt regulator  16  through sense resistor  18 . In this configuration, the sense resistor current I S  is equal to the telephone I LINE  minus I B1A , as in the preferred embodiment. As in the preferred embodiment there is still a degree of error in the system, however, the error is reduced from I B1  to I B1A . The difference in magnitude between I B1  and I B1A  is the β of transistor  21 B (β is typically greater than 50). 
     This arrangement also results in a circuit which is virtually free from distortion. The circuit exhibits low distortion because the sense resistor  18  senses most of the current and its associated distortion through the DAA  10 , allowing the amplifier  24  to control I LINE  by incorporating most of I LINE  into the feedback path and compensating for the associated distortion. Even though this arrangement still contains a small amount of error, it also produces a signal to distortion ratio suitable for present communication devices (&gt;80 dB). 
     Accordingly, the present invention provides a low noise DAA  10  that is particularly useful for modern modems which require a high signal to distortion ratio. By significantly reducing the amount of distortion in the DAA, the present invention allows transistors  21 A,  21 B and  22  to be specified based on breakdown voltage, with minimal β requirements. Reducing the need for transistors with very specific β requirements allows for the use of less expensive components to achieve overall cost reduction. 
     To facilitate discussion, bipolar junction transistors (BJTs) are shown in the figures and used to describe the preferred embodiments. However, the present invention may incorporate bipolar junction transistors (BJTs), field effect transistors (FETs), or a combination of BJTs and FETs. Therefore, the terminology used in the claims will be as follows: the base in a BJT and the gate in a FET will be referred to as the control terminal of the transistor, and the collector-emitter terminals of a BJT and the drain-source terminals of a FET will be referred to as the current flow terminals of the transistor. 
     Having thus described a few particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.