Abstract:
In a telecommunications system, a switching device selectively applies one of two different voltage supplies to a common node. A first of the voltage supplies is coupled to the common node by a first switch and a second of the voltage supplies is coupled to the common node by a second switch. The switching device includes the following elements. A difference detector detects a difference between the first and second voltage supplies. A control circuit selectively enables one of the first and second switches and disables the other of the first and second switches, for switching between the first and second voltage supplies in response to the difference detector. The control circuit performs the switching between the first and second voltage supplies when the detected difference falls below a predetermined threshold.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
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     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK. 
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     BACKGROUND OF THE INVENTION 
     The invention relates to the field of ringing controllers, more specifically to ringing controllers that provide reduced noise switching between battery feed and ringing states. 
     With increased demand to deliver high-speed data services to subscribers, many techniques have been developed to use existing telephone loops to carry digital subscriber line (DSL) signals simultaneously with normal voice-band telephony services. Generally, the signals associated with plain old telephone service (POTS), such as on-hook and off-hook transitions and ringing, generate high noise levels at frequencies above the voice band and are thus incompatible with high-speed DSL signals. Traditionally, large and costly filters, often referred to as POTS splitters, have been employed at the telephone equipment location to remove POTS-created noise from the data band. This requirement has been an impediment to the large-scale deployment of DSL services. 
     The generally-accepted method of alerting a subscriber that an incoming telephone call has arrived is to apply a high magnitude alternating current (AC) waveform to the subscriber loop in order to ring a bell or similar audible alerting device at the subscriber&#39;s premises. One very common standard in North America is to use 86 V rms AC signals with a frequency of 20 Hz, although other different voltages and frequencies can be employed. This AC waveform is often referred to as the power-ringing signal. 
     The process of ringing a subscriber&#39;s line can be considered as a cycle between two states. First, the state of providing a battery feed to the loop to await activity, which may include on-hook transmission or supervision. Second, the state of providing the power-ringing signal to the loop. Digital Subscriber Loop (DSL) signals require very good signal-to-noise ratios (SNRs) to achieve the high throughputs required by the applicable DSL standards and hence are quite susceptible to noise, both in the time and frequency domains. Hence, it is desirable that transitions between the battery feed and ringing states cause a minimum amount of noise. 
     Traditionally, mechanical relays have been employed to switch ringing signals onto the subscriber loop. It is an unfortunate characteristic of relays that they tend to introduce discontinuities onto the loop voltage due to timing variations, abrupt switching behavior, contact bounce, or open-circuit intervals between states. Large voltage discontinuities manifest themselves as high frequency noise, which interfere with DSL signals. 
     More recently, solid state relays have been employed to switch ringing signals but, as yet, their timing and transition characteristics have tended to induce noise levels incompatible with DSL. 
     It is desirable that the change between the battery feed state and the ringing state be continuous and smooth to avoid creating noise artifacts that interfere with DSL transmission. A need therefore exists for a ringing controller that will allow for reduced noise switching between battery feed and ringing states. Consequently, it is an object of the present invention to obviate or mitigate at least some of the above-mentioned disadvantages. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention, there is provided a switching device for selectively applying one of two different voltage supplies to a common node. A first of the voltage supplies is coupled to the common node by a first switch and a second of the voltage supplies is coupled to the common node by a second switch. The switching device includes the following elements. A difference detector detects a difference between the first and second voltage supplies. A control circuit selectively enables one of the first and second switches and disables the other of the first and second switches, for switching between the first and second voltage supplies in response to the difference detector. The control circuit performs the switching between the first and second voltage supplies when the detected difference falls below a predetermined threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of example only with reference to the following drawings in which: 
         FIG. 1  is a high-level block diagram illustrating a ringing controller in accordance with an embodiment of the invention; and 
         FIG. 2  is a detailed block diagram illustrating a ringing controller in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, numerous specific details are set forth to provide a thorough understanding of the invention. However, it is understood that the invention may be practiced without these specific details. In other instances, well-known software, circuits, structures and techniques have not been described or shown in detail in order not to obscure the invention. In the drawings, like numerals refer to like structures or processes. 
     In general, the ringing controller described herein features reduced noise switching between battery feed and ringing states. Referring to  FIG. 1 , a ringing controller is illustrated generally by numeral  100 . The ringing controller  100  includes a battery access switch  110  and a ringing access switch  120 . The battery access switch  110  couples a subscriber loop  130  to a battery supply  140  (or talk battery) that provides normal direct current (DC) voltage. Typically, the talk battery is a nominal −48 VDC. The ringing access switch  120  couples the subscriber loop  130  to a ringing supply  150  that provides a power ringing signal. In general, when it is necessary to ring the subscriber&#39;s telephone, the battery access switch  110  is opened, removing the normal DC battery supply  140  from the loop  130 , and the ringing access switch  120  is closed, thus applying the power of the ringing supply  150  to the loop  130 . At the end of the power-ringing burst, this sequence is reversed. 
     In the present embodiment, both the battery access switch  110  and ringing access switch  120  are electronic and thus are free from many of the disadvantages of the mechanical relays that are traditionally used. Furthermore, the switches are constructed and controlled in a manner that renders them less noisy than existing solid-state implementations as will be described in detail later in the description. 
     Referring to  FIG. 2 , a detailed block diagram of a ringing controller in accordance with an embodiment of the invention is illustrated generally by numeral  200 . In this embodiment, a commercially available, solid state relay is employed as the ringing access switch  120 . The battery access switch  110  includes first  210  and second  220  MOSFET transistors, first  212  and second  222  drive circuits, a control circuit  230 , and a difference detector  250 . 
     The MOSFET transistors  210  and  220  are coupled in series between the battery supply  140  and the loop  130 . Each of the drive circuits  212  and  222  is coupled between the control circuit  230  and an associated one of the MOSFETs  210  and  220 . The control circuit  230  is further coupled to the ringing access switch  120  and the difference detector  250 . The difference detector  250  is further coupled to the ringing supply  150  and the battery supply  140 . 
     The two-transistor  210  and  220  configuration is used because each transistor is used to block only one voltage polarity of the power ringing signal. Thus, to block an AC waveform, two transistors  210  and  220  are used, one for each polarity. The use of direct drive circuits  212  and  222 , coupled between the MOSFETs  210  and  220  and the control circuitry  230 , allows more control over MOSFET switching behavior. Through the control circuitry  230 , the timing of the MOSFET drive signals  211  and  221  is linked to the timing of the power ringing signal waveform  150  and to the timing of the drive signal  260  to the ringing access switch  120 . 
     Furthermore, the drive circuits  212  and  222  are arranged such that the MOSFETs  210  and  220  can be linearly controlled while in a battery access state. By this arrangement, the DC current flowing in the loop  130  may be controlled by appropriately driving the MOSFETs  210  and  220 . Hence, the battery access switch  110  may be used either as a switch or as a linear control element, as will be described in detail. Further, the drive circuits  212  and  222  allow feedback  240  to be used to modify the AC and DC characteristics of the battery access switch  110  when in battery feed mode and also to control its dynamic switching behavior, thus controlling its noise performance. 
     To control switching noise, it is preferable that the transition from battery voltage  140  to ringing voltage  150  occurs when the two voltages  140  and  150  are equal, that is, at the “zero crossing”. To that end, the difference detector  250  senses the difference between the battery  140  and ringing  150  voltages, and the control circuit  230  uses the difference for controlling the switching process. 
     The operation of the ringing controller  200  will now be described, firstly with respect to the removal of the battery voltage and the application of ringing voltage, and secondly with respect to the removal of ringing voltage and the application of the battery voltage. 
     To initiate a power ringing burst, the control circuit  230  awaits the condition when the ringing supply  150  and battery supply  140  voltages are approximately equal. Both of the MOSFETs  210  and  220  are in an “on” state. When the two voltages  140  and  150  are close, the ringing application operation begins. Both of the MOSFETs  210  and  220  are turned “off” by removing their respective drive signals  211  and  221 . A drive signal  260  for the ringing access switch  120  is enabled, thus applying ringing to the loop  130 . 
     The timing of these events is such that the ringing access switch  120  begins conduction slightly before the battery access switch  110  turns off, thus avoiding periods of time when a high impedance is present to both the battery and ringing supplies  140  and  150 . Such time periods could cause discontinuities in the output voltage, depending on the loop conditions present at that particular time. The control circuit  230  is designed such that the moment of switchover occurs very close in time to the actual zero crossing. Therefore, if the drive circuits  212  and  222 , the MOSFETs  210  and  220 , or the ringing switch  120  possess any inherent delays, the control circuitry  250  compensates by initiating the ringing application process prior to the zero crossing. 
     The drive circuits  212  and  222  and the feedback  240  are arranged such that the MOSFETs  210  and  220  do not turn off abruptly. That is, the drive and control circuitry is sensitive to the rate of change and shape of the transition waveform. If, for example, it is detected that the transition waveform begins to occur too quickly, the control and drive circuits react by slowing down the rate at which the MOSFETs  210  and  220  turn off. This is achieved by decreasing the rate at which the drive signals  211  and  221  are removed from the MOSFETs  210  and  220 , effectively providing a temporary increase in gate drive. Thus, the transition between the conducting and non-conducting states of the MOSFETs  210  and  220  occurs smoothly and continuously over a short time interval, ensuring that the transition to ringing occurs with little or no discontinuities. 
     The smooth, continuous transition also ensures the entire circuit  200  will be reasonably immune to uncertainties in the relative voltage levels of the battery  140  and ringing  150  supplies at the moment of ringing application. These uncertainties may be due to timing errors and component variations. Consequently, the difference detector  250  and the timing of the control circuit  230  need not be exact, and switching noise levels can be sufficiently reduced to not interfere with DSL signals. 
     Typically, the removal of ringing presents the greater noise reduction challenge due to ringing currents that may be flowing in the subscriber loop  130  at the instant of ringing removal. If any reactive elements are present anywhere in the ringing path, including a line card itself, then spikes or other discontinuities may result from an attempt to interrupt the ringing signal, thus creating a large-amplitude noise event. 
     To remove the power-ringing signal  150 , a sequence resembling the inverse of the ringing application sequence described above is followed. The control circuit  230  awaits the condition where the battery supply  140  and ringing supply  150  voltages are approximately equal. When the voltages are close, as indicated by the difference detector, the ringing removal operation begins. Both the MOSFETs  210  and  220  are turned “on” by applying drive signals  211  and  221  to their gate terminals. The drive signal  260  for the ringing access switch  120  is removed, beginning the process of removing ringing from the loop. 
     The timing of these events is arranged such that the battery access switch  110  begins conduction slightly before the ringing access switch  120  turns off. This maintains a continuous current path during the transition, avoiding large voltage transients from reactive loads. The control circuit  230  is designed such that the moment of switchover occurs very close in time to the actual zero crossing. Therefore, if drive circuits  212  and  222 , the MOSFETs  210  and  220 , or the ringing switch  120  possess any inherent delays, the control circuit  230  compensate by initiating the ringing application process prior to the zero crossing. 
     The drive circuits  212  and  222  and the feedback  240  are arranged such that the MOSFETs  210  and  220  do not turn on abruptly. As described above, the drive and control circuitry is sensitive to the rate of change and shape of the transition waveform. If, for example, it is detected that the transition waveform begins to occur too quickly, the control and drive circuits react by slowing down the rate at which the MOSFETs  210  and  220  turn on. This is achieved by decreasing the rate at which the drive signals  211  and  221  are applied to the MOSFETs  210  and  220 , effectively providing a temporary decrease in gate drive. Thus, the transition between the non-conducting and conducting states of the MOSFETs  210  and  220  occurs smoothly and continuously over a short time interval, thus ensuring that the transition to battery feed occurs with little or no discontinuities. 
     The smooth, continuous transition also ensures the entire circuit  200  is reasonably immune to uncertainties in the relative voltage levels of the battery  140  and ringing  150  supplies during the switch from ringing application. Again, these uncertainties may be due to timing errors and component variations. Consequently, the difference detector  250  and the timing of the control circuit  230  need not be exact in order for switching noise levels to be sufficiently reduced to not interfere with DSL signals. 
     During both the transition from battery feed to ringing and the transition from ringing to battery feed, there is a short period of time during which both the battery access switch  110  and the ringing access switch  120  conduct simultaneously. This ensures a seamless transition between ringing and normal battery feed. This simultaneous conduction is often referred to as a “make-before-break” operation and may be accomplished using certain mechanical relays. Mechanical relays, however, tend to exhibit wide variations in the timing of their operation and generally also possess contact bounce and abrupt switching transitions, both of which contribute to high noise levels during the switching process. The electronic switching circuit described in the present invention is more precise and predictable in its operation. Moreover, the effects of component tolerances may be modeled using common simulation techniques to ensure controlled switching behavior over a range of circuit parameters and component tolerances. 
     During the simultaneous conduction interval described above, it is possible for current to flow between the battery  140  and ringing  150  supplies. It may, therefore, be necessary to control or limit the current flowing through the two switches  110  and  120  for this time interval. Current limiting may be accomplished with a resistor  270  coupled in series with the ringing access switch  120 . Other known current limiting devices may also be used. The battery access switch  110  is a low resistance or “strong” switch while the ringing access switch  120  is a high resistance or “weak” switch. Consequently, the battery access switch  110  “overrides” the ringing access switch  120  thus ensuring that only the battery access switch  110  and its feedback  240  control the transition between states of the ringing controller  200 . 
     Through the control circuit  230 , the behavior of the battery access switch  110  is closely controlled. This allows for precise control of the state transitions of the ringing controller  200  state transitions. As a result, switching between the ringing and battery supplies is performed when their voltages are approximately equivalent, reducing switching noise. Further, the switching device described herein provides a smooth transition between the ringing and battery supplies, reducing switching discontinuities that are introduced by other switching devices. Thus, implementing a switching device in accordance with the present invention reduces interference with DSL transmissions that has traditionally been associated with switching between a battery supply and a ringing supply. 
     As previously mentioned, the battery access switch  110  may be used either as a switch or as a linear control element. Thus, the battery access switch  110  may be used for additional applications. For example, it may be used to control current in the loop  130 . By not turning the battery switch  110  fully on, power can be saved in short loops by reducing current flow. In addition, through the use of feedback  240 , the impedance of the battery switch  110  may be controlled, which can be useful for impedance balancing. 
     In summary, the battery switch described above may be controlled in a linear manner as well as in a switching manner. Feedback techniques are employed to control the battery access switch behavior in both the time and frequency domains. A difference detector is coupled between the battery and ringing voltage supplies and the timing of ringing application and removal is coordinated with respect to the ringing waveform so that transitions are as smooth as possible. A common-conduction interval eliminates transients due to reactive load switching. Generally, the battery access switch overrides the ringing access switch, thus controlling the state transition. Further, the switches function without interfering with DSL signals, thus eliminating one cause for bulky and expensive filters or POTS splitters. 
     While the above description has been made with specific reference to MOSFET and technology and solid state relays, it will become apparent to a person skilled in the art that other switching technology may be used. Thus, although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.