Method and system for increasing subscriber line interface circuit voltage during ringing

A voltage controller increases a DC input voltage in response to a ring instruction received from a processor in a control signal. The increased voltage is converted to an AC ring signal that causes a telephone to ring. The ring signal voltage is generated at a battery high node (“VBH”) and may be routed to a SLIC before being sent to an on-hook telephone.The voltage at VBH is kept low during off-hook and other periods when a ring instruction is not present. Thus, voltages internal to an indoor device that houses the controller are kept below a predetermined safe threshold, thereby allowing the periods of high voltage to be deemed as transient. Accordingly, if the device is unearthed, or not grounded to the earth, the device may nevertheless obtain listing by a certifying and testing organization without being subjected to abnormal testing and possible failure thereof.

FIELD OF THE INVENTION

This invention relates, generally, to communication networks and, more particularly, providing increased voltage at a telecommunications Subscriber Line Interface Circuit (“SLIC”), during ringing.

BACKGROUND

In a plain old telephone service (“POTS”) system, a telephone is designed to receive a “ring” signal, typically an AC wave at 20 hertz (Hz) of approximately 40 V RMS, when it is supposed to ring. It will be appreciated that other ring voltages are often used that fall within a range equal to or greater than 40 V RMS. The ringing signal is/was traditionally sent along the twisted pair telephone wire that connects an individual telephone to the telephony network. A typical telephone still uses the higher ring signal voltage, even if the telephony communications are transmitted along non-traditional pathways, such as, for example, community antennae television (“CATV”) coaxial cable, optical fiber cable, or any other network technology that transmits communications signals. When one of these technologies is used, a telephone is typically connected to a line card that provides an interface between the telephone and the communications network. Such a line card typically contains subscriber line interface circuitry (“SLIC”) that is designed to translate a signal from the network protocol to the traditional telephony protocol. A node within the line card circuitry may be maintained at a constant 90-95 V level, and when an instruction to ring is received, the 90-95 V source is directed to the telephone in accordance with a predetermined waveform shape. Thus, when a line card receives a signal that instructs it to cause a telephone connected thereto to ring, it generates a ringing signal of about approximately 40 V RMS, or equal to or greater than 40 V RMS as discussed above, having a waveform shape similar to the waveform of a traditional telephony signal.

A line card may be part of a subscriber's premise equipment (“SPE”) device inside a consumer's home. To deliver power to the SPE, a typical household AC power supply may be used that provides AC current in the range of between approximately 105 V and 230 V. In order to facilitate compatibility with different styles of power outlets and powering schemes, the power supply conductor, or cord, may be terminated with a two prong plug that works in either a two prong receptacle or a three prong receptacle.

Although the use of a two prong plug facilitates compatibility with a wider range of receptacles, a two prong plug does not provide a safety ground path for the device to which it provides power. Thus, the internal circuitry of the device, while being possibly grounded through other means, a coaxial CATV cable, for example, is not safety grounded from the power supply power source. This may cause difficulty in obtaining listing by an independent, product-safety testing and certification organization, such as UNDERWRITERS LABORATORIES, INC.® (“UL®”). Abnormal testing may be required before certification and/or listing of a device that is not safety grounded is granted. This may cause delay in the introducing of a new product that is not safety grounded. In addition to protracted delay for testing, there is the possibility that a device being tested may not meet the certification's standards, thus delaying further introduction of the device into the stream of commerce until the device can be redesigned and retested. Such delay can be costly at best, and result in the stillbirth of a product at worst in the fast changing realm of modem telephony.

Current UL® standards specify that continuous internal voltages within an unearthed device not exceed a hazardous voltage level. If a device, such as a cable modem telephony line card, is not safety grounded, it is subjected to abnormal testing that impose “double insulation” requirements, for example, before the device becomes UL® “listed.” However, if a device's internal voltages that rise above the predetermined safe level, approximately 50-60 volts, for example, are not continuous, but transient in nature, then the device may not be subject to abnormal testing due to internal voltages exceeding the predetermined safe level. Since line cards typically generate a ringing signal voltage of approximately 40 V RMS, or equal to or greater than 40 V, to cause a telephone to ring, an unearthed SPE having a line card that provides a telephone interface may be subjected to abnormal testing, and the aforementioned associated delay, before becoming listed, if an continuous, approximately 90-95 voltage tap, or node, is provided in the circuitry of the line card.

Thus, there is a need for a method and system that provides a 90-95 V ringing signal that is not continuous, but only high (above the predetermined safe level) during ringing, thereby being deemed transient in nature.

SUMMARY

It is an object to provide a method and system for maintaining telephony line voltages from a SLIC within a line card used in a cable telephony modem system below a predetermined safe level, while providing for increased voltage during ringing. This eliminates the need for meeting double insulation requirements and/or abnormal testing for indoor devices that are not safety grounded. Therefore, such a device can be brought to market rapidly and the design and/or testing costs performed to meet standards in order to obtain listing with a testing and certification organization, UL® for example, are reduced.

Generally described is an indoor, unearthed cable telephony device comprising an input power source, a control signal source, a voltage converter that operates in response to the control signal source and a node for providing a high battery output from the voltage converter. The control signal is provided from a processor that receives an incoming cable telephony signal to the voltage converter. The processor provides translation between the cable network protocol, typically a digital signal, and the SLIC, which is typically configured to provide and receive analog signals to and from a conventional telephone device. The voltage converter provides an energy signal to the SLIC, while the processor provides an information signal to the SLIC, without passing through the voltage converter. The voltage converter may comprise a fly back transformer, or a boost converter. These are merely examples of the means that may be used, as well as others known in the art, of selectively increasing the voltage to a circuit.

DETAILED DESCRIPTION

As a preliminary matter, it will be readily understood by those persons skilled in the art that the present invention is susceptible of broad utility and application. Many methods, embodiments and adaptations of the present invention other than those herein described, as well as many variations, modifications, and equivalent arrangements, will be apparent from or reasonably suggested by the present invention and the following description thereof, without departing from the substance or scope of the present invention.

Accordingly, while the present invention has been described herein in detail in relation to preferred embodiments, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for the purposes of providing a full and enabling disclosure of the invention. The following disclosure is not intended nor is to be construed to limit the present invention or otherwise to exclude any such other embodiments, adaptations, variations, modifications and equivalent arrangements, the present invention being limited only by the claims appended hereto and the equivalents thereof. Furthermore, while some aspects of the present invention are described in detail herein, no specific conductor type, integrated circuit, discrete component, connector, enclosure, circuit board arrangement, capacitor or resistor value, or fuse rating, for example, is required to be used in the practicing of the present invention. Indeed, selection of such parts and components would be within the routine functions of a designer skilled in the art.

Turning now to the figures,FIG. 1illustrates a system2for providing telephony services, as well as television programming, to an end user via a CATV network4. A head end6acts as a central office for directing signals, including telephony signals, over network4. Such signals may be received at subscriber premises equipment (“SPE”) device8outside a user's home or office, for example. SPE8may determine how to route the signal based on the information type of signal, e.g., a television signal or a telephony signal. If the information signal contains television content, the signal may routed to television10as a standard CATV signal. If, however, the signal contains telephony information, the signal is routed to line card device12for further processing.

As the telephony information may have been transmitted over network4as a modulated CATV signal, an internet protocol (“IP”) signal, or preferably as a Data Over Cable Service Interface Specification (“DOCSIS”) signal, the line card translates the incoming signal format to POTS format. As POTS telephone14may be designed to ring when a signal having a level of about 40 V RMS, or equal to or greater than 40 V, is impressed into the twisted pair16that connects it to line card12, the line card must produce this voltage level. Line card12uses power and energy from an indoor wall receptacle18that supplies standard AC house current at about 120 V. This house current is transmitted from receptacle18to line card device12via power cord20. The household current is typically transformed to DC current at approximately 12-15 V by power supply21, which supplies the DC current from its output to line card12via DC power cord22. It is noted that power cord20is terminated with plug23, which has only two prongs. A third prong for providing safety ground protection is not provided from plug23. This allows compatibility with electrical systems that do not provide a safety ground conductor and that typically provide two-prong receptacles, as illustrated by receptacle24.

Although the use of two-prong plug23facilitates use with a greater number of electrical system schemes, safety ground functionality is eliminated. This is so even when line card device12is used in an electrical system that supports a safety ground circuit, and that provides three-prong receptacles, because there is no third prong on plug23to connect the line card device to an existing safety ground circuit. In addition, even if plug23and cord20provide three conductors, cord22may typically contain only two conductors, and thus may not provide a safety ground circuit to line card12. Thus, the generation of a 40 V RMS, or equal to or greater than 40 V RMS, signal within line card12may prevent the certification of said line card if the 40 V RMS, or equal to or greater than 40 V RMS, ringing signal source is constantly live.

Turning now toFIG. 2, a block diagram of line card12is illustrated. Line card12receives power from power cord22, which will typically contain two active conductors, a positive and a negative. The positive and the negative conductors carry DC current that has been transformed from AC household current known in the art for house current distribution. The negative conductor may be grounded to the chassis of line card12, and the positive lead provides current to voltage controller26. Processor28interfaces signals between SPE8and SLIC30. Processor28also provides a control signal to voltage controller26. It will be appreciated that processor28comprises a variety of components, including, for example, a microprocessor, media access control layer circuitry and software, digital-to-analog (“D/A”) and analog-to-digital (“A/D”) circuitry as well as other typical circuitry known in the art of cable telephony. Voltage controller26includes means, which will be discussed later, that can increase the 12 V DC signal from power source conductor22into the ringing supply voltage of approximately 95 V used by SLIC30to generate an approximately 40 V RMS ringing signal, thus causing a telephony device connected to twisted pair16to ring.

When processor28receives an information signal, it decodes the received signal, which may be a DOCSIS data signal, and uses D/A converters to convert it to an analog signal. In addition, if the received signal contains an instruction to ring a telephone connected to SLIC30, a control signal is sent to voltage controller26, thereby causing the means internal to the voltage controller to increase the voltage from the approximately 12-15 V to approximately 90-95 V. This increased voltage signal is produced at VBH node32, which is connected to VBH input34at SLIC30. SLIC30impresses the high voltage signal received at VBH node34into twisted pair lines16, thereby causing a telephone connected thereto to ring. The SLIC30detects when a user picks up the handset of the telephone and then disconnects the high voltage signal from the twisted pair cable16, thereby allowing an analog information signal received from the processor28to pass through the SLIC and on to the handset so that the user may then converse with the calling party.

Voltage controller26, which converts the DC input voltage received from cable22into the higher ringing voltage, may comprise a variety of means. Turning now toFIG. 3, the preferred means for converting the DC voltage to the higher AC ringing voltage includes a fly back transformer, a schematic diagram of which is illustrated with reference to circuit36, which may be included inside voltage controller26, as shown inFIG. 2.

A fly back transformer38is used to store energy received from the supplied voltage Vin, and to discharge the stored energy as a higher voltage when the field in transformer's core40collapses. It will be appreciated by those skilled in the art that a flyback transformer is a special type of transformer that does not continuously transform an input voltage into an output voltage in direct proportion to its windings ratio.

Typically, energy is stored in an air gap in the transformer core. When the field from the primary winding collapses, the stored energy flows within the core, thereby inducing a field, and thus current, into the secondary winding of transformer38. Therefore, current does not flow in the secondary when current flows in the primary, but flows as the field in the primary collapses. It will further be appreciated that flyback transformer38should be connected according to the dots shown in the figure. This arrangement lends itself for use with providing ringing signaling in telephony devices because telephony devices conventionally use a negative VBHto ring a telephone ringer.

In addition to the flyback transformer, various other components are used to provide the functionality of circuit36. To provide on/off switching of current through the primary of transformer38, transistor42, preferably a MOSFET, is used. Transistor42receives its gate voltage from an output pin VOUTof feedback regulator44. Feedback regulator44may be an off-the-shelf component known in the art, and provides a constant reference voltage at a VREFpin. The voltage at VOUTis controlled by a signal received by regulator44at a feedback pin (“FB”) and alternates to provide a constantly increasing and collapsing field in the primary of transformer38. Resistors46and48may be used to form a voltage divider that maintains the voltage level at FB in a predetermined range according to the manufacturer's specifications corresponding to regulator44.

The control signal that provides the ringing instruction is received from the processor shown inFIG. 2by circuit36at port50. This ringing control is typically digital signal inasmuch as it is either at zero volts, indicating a no ring state, or at a logical full voltage, typically 3.3 V, indicating ringing state. The control signal passes through biasing resistors52and54for biasing of feedback transistor56. When the control signal is low, transistor56is off. Therefore, current does not flow through draw-down resistor58, and the voltage applied to operational amplifier (“op amp”) 60 is VREF×R62/(R62+R64). The resistance values of resistors46,48,58,62and64, as well as gain adjusting resistors66and68are selected such that when the control signal voltage at port50is low, the voltage impressed at pin FB of regulator44regulates the voltage at VOUTto maintain the voltage at VBHless that 60 V. It will be appreciated that the voltage at VBHis negative according to the phase relationship indication dots at fly back transformer38.

When the control signal voltage at port50is high, such as 3.3 V, for example, as discussed above, transistor56turns on. This causes the collector of transistor56to draw through resistor58to ground, such that resistor58and resistor62are in parallel between the non-inverting input of op amp60and ground. Since the resistance of resistor62and58in parallel is less that the resistance of resistor62alone, the voltage at the non-inverting input of op amp60drops, and the feedback network comprising resistors66and68causes the output of op amp60to drop proportionally to the drop at the non-inverting input pin.

In order for the output of the op amp60to drop, the voltage at the inverting input drops proportionally thereto. As this inverting input voltage drops, the current through resistor66falls according to Ohm's Law, thereby causing the ringing voltage signal at VBHto drop. Accordingly, a properly selected resistance value of resistor66will cause the ringing signal at VBH to drop to a voltage lower than −60 V (lower because more negative, but having magnitude greater than 60 V), preferably approximately −90 V, when the ringing control signal at port50is high. Conversely, the voltage at VBHwill rise above −60 V (the term rise indicates that the sign becomes less negative, although the magnitude is less that 60 V) when the ringing control signal voltage is low.

These and many other objects and advantages will be readily apparent to one skilled in the art from the foregoing specification when read in conjunction with the appended drawings. It is to be understood that the embodiments herein illustrated are examples only, and that the scope of the invention is to be defined solely by the claims when accorded a full range of equivalents.