Abstract:
A system and a method selectively switch between two different voltage supplies supplying 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 comprises: a feedback network comprising a high-pass filter for filtering a signal at the common node and outputting the filtered signal as a feedback signal; a timing controller coupled to at least one of the first and second voltages supplies for determining when to switch between said first and second power supplies; a ring switch controller for applying a first control signal to the first switch for selectively enabling and disabling the first switch in response to the timing controller and the feedback signal; and a battery switch controller for applying a second control signal to the second switch for selective enabling and disabling the second switch in response to the timing controller and the feedback signal. The method comprises the steps of: receiving a request to switch between the first and second voltage supplies; gradually removing the voltage supply coupled to the common node; gradually applying the other of said voltage supplies; and filtering a signal at said common node for detecting noise.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    NOT APPLICABLE  
         STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    NOT APPLICABLE  
         REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK.  
         [0003]    NOT APPLICABLE  
         BACKGROUND OF THE INVENTION  
         [0004]    The present invention relates generally to the field of ringing controllers, and specifically to ringing controllers for providing switching between battery feed and ringing states.  
           [0005]    With increased demand to deliver high-speed data services to subscribers, many techniques have been developed to utilize existing telephone loops to carry data signals simultaneously with normal voice band telephony services. Generally, data signals are carried in a frequency range, referred to as a data band, that is above the voice band. However, signals associated with plain ordinary telephone service (POTS), such as on-hook and off-hook transitions and ringing, generate high noise levels at frequencies above the voice band and, thus, this noise falls in the data band causing interference. 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 data services.  
           [0006]    The generally accepted method of alerting a subscriber that an incoming telephone call has arrived is to apply a high magnitude 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 alternating current 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.  
           [0007]    The process of ringing a subscriber&#39;s line can be considered as a transition between two states. A first state is providing a battery feed to the loop, which may include on-hook transmission or supervision, and when the subscriber is off-hook and connected to another subscriber. Second, the state of providing the power-ringing signal to the loop to alert the subscriber that another subscriber is calling.  
           [0008]    Data transmission in the data band is provided over the subscriber line at all times during these two states, and during transitions between these two states. However, data signals require very good signal to noise ratios to achieve the high throughputs required by applicable industry standards and are quite susceptible to noise, both in the time and frequency domains. Thus, it is desirable that transitions between the battery feed and power-ringing states cause a minimum amount of noise in the data band.  
           [0009]    Traditionally, mechanical relays have been employed to switch power-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 data signals. Thus, it is desirable that the change between the battery feed state and the ringing state be continuous and smooth to avoid creating noise artefacts that interfere with transmission in the data band.  
           [0010]    More recently, solid-state relays have been employed to switch power-ringing signals. In a co-pending application, the need for a traditional POTS splitter filter is reduced by timing the removal and application of the battery state and the ringing states through monitoring the voltage zero crossing and applying feedback techniques. This approach makes the ringing transitions contribute little interference in the data band. Such an implementation is illustrated in FIG. 1. However, in order for this to operate properly, the switching process may begin prior to the next zero crossing of the power-ringing signal and battery supply, thus prediction and timing of this trigger event should be taken into account. This implies that adaptation for different ringing frequencies would need to be implemented, which may vary from country to country.  
           [0011]    Further, the approach described with reference to FIG. 1 is implemented in a “make before break” manner. That is, the battery feed is applied to the subscriber line before the ring source is removed. The disconnection of the ring source from the subscriber line is timed precisely in a short interval, to within {fraction (1/20)}th of the period of the ring source, from the voltage crossing of the ring source and the battery. However, any residual currents in the load, due to inductive elements, will flow into the battery. These currents can be quite large and require a large low impedance device to pass these currents between the battery and the load. Although large discrete devices are available, in certain applications a large device may be expensive in terms of silicon area, and may be difficult and expensive to integrate.  
           [0012]    Therefore, it is an object of the present invention to obviate or mitigate at least some of the above-mentioned disadvantages.  
         SUMMARY OF THE INVENTION  
         [0013]    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 comprises: a feedback network comprising a high-pass filter for filtering a signal at the common node and outputting the filtered signal as a feedback signal; a timing controller coupled to at least one of the first and second voltages supplies for determining when to switch between said first and second power supplies; a ring switch controller for applying a first control signal to the first switch for selectively enabling and disabling the first switch in response to the timing controller and the feedback signal; and a battery switch controller for applying a second control signal to the second switch for selective enabling and disabling the second switch in response to the timing controller and the feedback signal.  
           [0014]    In accordance with another aspect of the present invention, there is provided a method for selectively switching between two different voltage supplies supplying 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 method comprises the steps of: receiving a request to switch between the first and second voltage supplies; gradually removing the voltage supply coupled to the common node; gradually applying the other of said voltage supplies; and filtering a signal at said common node for detecting noise. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    A preferred embodiment of the invention will now be described by way of example only with reference to the following drawings in which:  
         [0016]    [0016]FIG. 1 is a schematic diagram of a ringing controller (prior art);  
         [0017]    [0017]FIG. 2 is a schematic diagram of a ringing controller in accordance with an embodiment of the invention;  
         [0018]    [0018]FIG. 3 is a flow chart of the operation of the present embodiment of the ringing controller while applying ringing; and  
         [0019]    [0019]FIG. 4 is a flow chart of the operation of the present embodiment of the ringing controller while removing ringing. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    For convenience, like numerals in the description refer to like structures in the drawings.  
         [0021]    Referring to FIG. 2, a ring controller in accordance with an embodiment of the present invention is illustrated generally by numeral  200 . The ring controller  200  includes a ring switch  202 , a battery switch  203 , a ring switch controller  204 , a battery switch controller  206 , a feedback network  208 , and a timing controller  210 . An output current sense  228  (optional) may be used in some applications.  
         [0022]    The ring switch  202  is coupled between an output  211  of the ring controller  200  and a power-ringing supply  212 . The battery switch  203  is coupled between the output  211  of the ring controller  200  and a battery supply  214 . The ring switch controller  204  controls the operation of the ring switch  202 . The battery switch controller  206  controls the operation of the battery switch  203 . The feedback network  208  has an input coupled to the output  211  of the ring controller  200  and an output coupled to both the ring switch controller  204  and the battery switch controller  206 . The timing controller  210  has inputs coupled to the output  211  of the ring controller  200 , the power-ringing supply  212 , the battery supply  214 , and the output current sense  228 . The timing controller  210  has outputs coupled to the battery switch controller  206 , the ring switch controller  204 , and the feedback network  208 .  
         [0023]    The application and removal of both the battery supply  214  and the power-ringing supply  212  are gradual, timed, and modified by the feedback network  208 . In the present embodiment, the application of ringing is timed relative to a particular voltage of the power-ringing supply  212 , as detected by the timing controller  210 . The removal of ringing is timed relative to a particular voltage of the power-ringing supply  212 , or timed to a particular output current, such as the zero current crossing, as detected by the timing controller.  
         [0024]    In the present embodiment, the ring switch  202  comprises back-to-back Field Effect Transistors (FETs)  220  and  222 , although other devices may be used as will be apparent to a person skilled in the art.. Back-to-back FETs  220  and  222  are used and each transistor blocks one voltage polarity of the power-ringing signal. Thus, to block an alternating current (AC) waveform, two n-channel FETs  220  and  222  are used, one for each polarity. If the power-ringing signal voltage stays either always above or always below the ring controller output  211  voltage, then it is possible to use only one FET as will be appreciated by one skilled in the art. Each of the FETs  220  and  222  is driven by a low power control signal supplied by the ring switch controller  204 . Thus, the power-ringing signal, which is 86 Vrms in the present embodiment, is effectively controlled by the low power control signal driving the ring switch  202 . The control signal is adjusted to gradually increase or decrease the impedance of the ring switch  202 .  
         [0025]    The battery switch  203  may also be comprised of back-to-back FETs  224  and  226 . Similarly to the ring switch  202 , if the ring controller output  211  voltage always stays above the battery supply  214  voltage then it is possible to use only one FET. Each of the FETs  224  and  226  is driven by a low power control signal supplied by the battery switch controller  206 . Thus, the battery signal is effectively controlled by the low power control signal driving the battery switch  203 . The control signal is adjusted to gradually increase or decrease the impedance of the battery switch  203 .  
         [0026]    The feedback network  208  comprises a high pass filter. The high pass filter is designed to pass noise on the output that would fall in the data band frequencies and block the ring frequency, which allows the ring controller  200  to attenuate switching noise in the data band without attenuating the fundamental power-ringing signal. The feedback network  208  senses the output signal  211  and outputs a feedback signal, which is used to modify the control signals to both the ring switch  202  and the battery switch  203  to attenuate any high frequency signals that may interfere with the signals in the data band.  
         [0027]    The timing controller  210  effectively monitors the voltages at the power-ringing supply  212 , battery supply  214 , and output  211 . The timing controller  210  also, in some applications, monitors the output current through the output current sense  228 . As will be described with reference to the operation of the ringing controller  200 , the timing controller  210  receives a request to switch between states. The timing controller  210  signals to the ring switch controller  204  and battery switch controller  206  to begin to open or close their respective switches  202  and  204  as required.  
         [0028]    The operation of the ringing controller  200  will now be described with reference to FIGS. 3 and 4. The operation can be implemented by a processor, software, digital hardware, analog hardware, a combination thereof, or by other means. An example application of the ring controller  200  is a line card, which combines POTS and data transmission over a subscriber loop, for minimizing the impact of the application and removal of the power-ringing signal on data signal transmission.  
         [0029]    Referring to FIG. 3, a flow chart illustrating the operation of the ringing controller while switching from the battery state to the ringing state is shown generally by numeral  300 . In step  302 , the ring controller  200  is instructed to invoke the change of connectivity to the output  211  from battery  210  to power-ringing supply  212 . In step  304 , the timing controller  210  waits for a predefined trigger event. In the present embodiment, the trigger event is defined as approximately the next voltage crossing of the power-ringing supply  212  and the battery supply  214 .  
         [0030]    In step  306 , the timing controller  210  enables the feedback network  208 . Thus, only signals higher than a predetermined frequency are fed back to the ring switch controller  204  and the battery switch controller  206 . In step  308 , the timing controller  210  causes the battery switch controller  206  to reduce the control signal such that the battery switch  203  effectively turns off. In the present embodiment, the time period is approximately {fraction (1/10)}th of a ring cycle. The timing controller  210  may wait until the connection of the battery voltage at the output  211  falls below a predefined threshold or until the battery supply  214  is completely cut off before proceeding with step  310 .  
         [0031]    In step  310 , the timing controller  210  causes the ring switch controller  204  to gradually increase the control signal over a portion of a ring cycle to turn on the ring switch  202  and connect power-ringing supply  212  to the output  211 . In the present embodiment, the time period is less than half a cycle of the power-ringing supply signal.  
         [0032]    The feedback signal  209  is used by the ring switch controller  204  to modify the transition to the ringing state in such a way as to attenuate any frequency content that will interfere with data transmission. That is, the feedback signal  209  is indicative of noise in the data band. Thus, ideally the feedback signal  209  is zero. If the feedback signal  209  is greater than zero, then the ring switch controller  204  reduces the rate at which the ring switch  202  turns on. In turn, the feedback signal  209  is reduced. This continuous feedback to modify the ring switch control reduces the feedback signal  209  and hence the noise in the data band.  
         [0033]    Referring to FIG. 4, a flow chart illustrating the operation of the ringing controller while switching from the ringing state to the battery state is shown generally by numeral  400 . In step  402 , the timing controller  210  is instructed that a power-ringing signal is to be removed from the output  211  and the battery supply  214  is to be applied. In step  404 , the timing controller  204  waits for a predefined trigger event. In the present embodiment, the trigger occurs when the output current crosses a predefined threshold of 0 mA. Alternatively, the trigger could occur at the next voltage crossing of the output  211  and the battery supply voltage  214 .  
         [0034]    In step  406 , the timing controller  210  causes the ring switch controller  204  to gradually reduce the control signal, effectively turning off the ring switch  202 , over a period of approximately one fifth of a ring cycle in the present embodiment. The feedback signal  209  is used by the ring switch controller  204  to modify the transition from ringing in such a way as to attenuate any frequency content that will interfere with data signals. That is, the feedback signal  209  is indicative of noise in the data band. Thus, ideally the feedback signal  209  is zero. If the feedback signal  209  is greater than zero, then the ring switch controller  204  reduces the rate at which the ring switch  202  turns off. In turn, the feedback signal  209  is reduced. The timing controller  210  may wait until the connection of ringing voltage at the output  211  falls below a predefined threshold or until the ringing supply  212  is completely cut off before proceeding to step  410 .  
         [0035]    In step  410 , the timing controller  210  causes the battery switch controller  206  to gradually increase the control signal, effectively turning on the battery switch  203 , over a period of approximately one fifth of a ring cycle. The feedback signal  209  is used by the battery switch controller  206  to modify the transition to battery in such a way as to attenuate any frequency content that will interfere with transmission in the data band. Thus, ideally the feedback signal  209  is zero. If the feedback signal  209  is greater than zero, then the battery switch controller  206  reduces the rate at which the battery switch  203  turns on. In turn, the feedback signal  209  is reduced.  
         [0036]    In step  414 , the timing controller  210  gradually disables the feedback network  208  over a period of approximately one fifth of a ring cycle. During this period, the voltages that have built up in the high-pass filter in the feedback network  208  are discharged.  
         [0037]    In the present embodiment, the feedback network  208  is enabled just prior to applying the power-ringing signal and not disabled until the power-ringing signal is removed and the battery signal reapplied. Alternately, it may be advantageous to disable the feedback network  208  after each transition of connecting the output between battery and power-ringing supplies is complete. This would be advantageous, for example, for power reduction reasons. In such a case, the feedback network  208  would need to be enabled preceding a transition and disabled after a transition. According to another embodiment, leaving the feedback network  208  active whenever the battery  214  or power-ringing supply  212  is connected to the output may be advantageous, for example to reduce the effective noise of these supplies in normal operation.  
         [0038]    Since the transition between states is gradual, the timing and accuracy of the trigger event is relaxed, to the point of not requiring prediction of the trigger event prior to the next zero crossing, thus not requiring knowledge of the ring frequency.  
         [0039]    Further, a “break before make” transition approach when switching the output between power-ringing and battery supplies can be followed when the transitions are applied gradually. A “break before make” approach can be used that will not cause abrupt discontinuities in the output current, which would in turn cause voltage transients capable of disrupting data signals. It is not necessary that at least one switch be very low impedance in order to maintain a constant, noise free, output voltage while the other switch opens or closes abruptly. Thus, there is little or no period of time where current flows between the power-ringing and battery supplies, allowing the ring switch and battery switch to be smaller. Alternately, a “make before break” transition approach with very little overlap can be applied with similar results, as will be appreciated by a person skilled in the art.  
         [0040]    Yet further, as the transition is applied gradually, for example half the power-ring cycle as compared to {fraction (1/20)}th of the ring cycle in the prior art, the current required to initially drive the load is integrated over a longer period. Therefore the currents are not as large, once again allowing smaller switches, and reducing the amount of noise introduced during the transitions. When the ringing is removed gradually, the residual ringing currents in the load decay more slowly into known current paths to the power-ringing and battery supplies, as opposed to prior art where due to the use of a fast transition these currents had to be conducted rapidly with low impedance large switches to handle the large currents and limit the resulting noise, and additionally require “make before break” to ensure a current path is maintained to ensure further noise is not generated.  
         [0041]    Embodiments of the present invention provide several advantages and potential advantages over the prior art. Primarily, the ringing controller  200  described herein reduces the high frequency noise that falls into a data band, that is higher in frequency than a voice band, during transitions between the battery state and the power-ringing state.  
         [0042]    A further advantage of the present invention is the absence of a requirement for prediction of a trigger event, for example a point in time prior to the next zero crossing of the battery supply  214  and the power-ringing supply  212 , which would require knowledge or estimation of the ring frequency. Thus, the ringing controller  200  can properly operate independent of the ringing frequency.  
         [0043]    Yet further, due to the gradual application of the transitions between battery and ringing states, the requirement for timing accuracy of the trigger events is reduced, which simplifies the implementation.  
         [0044]    Yet further, due to the gradual application of the transitions between battery and ringing states, power-ringing and battery supplies can be applied and removed from the output without excessive currents flowing to and from battery, allowing smaller switches to be used and less noise to be generated.  
         [0045]    Yet further, embodiments of the present invention provide the option of applying the transitions using a “break-before-make” process, wherein a connection to one of either the battery or ringing supplies is severed before a connection to the other of the supplies is made. This avoids a period of time where current conducts between the battery and power-ringing supplies, allowing the use of smaller switches.  
         [0046]    Yet further, embodiments of the present invention allow the use of smaller switches, which allows integration of the invention into a single integrated circuit, resulting in lower cost, and improved reliability.  
         [0047]    Yet further, embodiments of the present invention allow the continuous application of a feedback network  208 , which reduces the high frequency noise of either the battery supply  214  or power-ringing supply  212 , whichever is currently connected to the output, effectively acting as a battery filter or power-ringing supply filter.  
         [0048]    Additionally, embodiments of the present invention may provide additional benefits by limiting and/or eliminating high currents in the ring controller  200 , thus reducing the capacity, complexity, and noise requirements of the battery supply  214  and power-ringing supply  212  that are connected to the ringing controller  200 .  
         [0049]    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.