Abstract: A method and apparatus for measuring signals conveyed by a varying polarity voltage signal across a tip and ring in a telephone system is disclosed. A voltage present across the tip and ring is used to charge a charge storage device, and the charge is periodically discharged for a relatively short amount of time. The burst of discharge current for the short amount of time allows an optocoupler to accurately operate, while presenting a very high impedance to the telephone line.

Abstract: A digital direct access arrangement (DAA) circuitry may be used to terminate the telephone connections at the user's end to provide a communication path for signals to and from the phone lines. Briefly described, a means for providing a proper hookswitch transition for a variety of international phone standards is provided. The invention may also be utilized with means for transmitting and receiving a signal across a capacitive isolation barrier. More particularly, a DAA circuitry may be utilized which satisfies many or all hookswitch transition standards without the use of additional discrete devices. The hookswitch transition standards may be satisfied by ramping down the current flowing through the hookswitch prior to transitioning the hookswitch state. In this manner the hookswitch current change as a function of time (di/dt) may be decreased. Thus, the current through the hookswitch may be actively controlled prior to switching the hookswitch from an off-hook condition to an on-hook condition.

Abstract: A CMOS implementation for a DC holding circuit in direct access arrangement (DAA) circuitry is disclosed that provides desirable inductive behavior while minimizing power dissipation required by the CMOS integrated circuit, particularly at high loop currents. The DC holding circuitry disclosed may include MOS transistors located on a CMOS integrated circuit and an off-chip power dissipating resistor that acts to dissipate power external to the CMOS integrated circuit. The CMOS implementation disclosed also allows a path for drawing DC current to power other CMOS circuits (e.g., ADCs and DACs) in the CMOS integrated circuit.

Abstract: A digital direct access arrangement (DAA) circuitry may be used to terminate the telephone connections at the user's end that provides a communication path for signals to and from the phone lines. Briefly described, the DAA provides a programmable means for the DC termination for a variety of international phone standards. The invention may also be utilized with means for transmitting and receiving a signal across a capacitive isolation barrier. More particularly, a DC holding circuit is provided in which a programmable DC current limiting mode is available. In the current limiting mode, power may be dissipated in devices external to a DAA integrated circuit. Moreover, much of the power may be dissipated in external passive devices, such as resistors.

Abstract: A hook switch circuit is shown wherein two high-voltage bipolar transistors, a PNP transistor Q1 and a NPN transistor Q2. that are connected in a regenerative feedback manner to form a bi-stable latch. The regenerative structure permits the use of low beta transistors that may be turned on with a low control current, but still conduct a sufficient off-hook current. Also shown is a polarity steering regenerative switch (MP1, MP2) that provides a power supply voltage from a telephone line pair and may be adapted for a polarity signal and can be combined with a current mirror (MP7) to produce a current signal proportional to the line voltage (LV1).

Abstract: A telephone line connection and disconnection circuit has a diode bridge coupled to the telephone line which converts different voltages of the telephone line into a voltage referenced to ground. A switching circuit is coupled to the diode bridge which opens and closes a conductive pathway for connecting and disconnecting a telephony product from the telephone line. A resistive element is coupled to the diode bridge and the switching circuit which allows for the conductive pathway to be normally closed. An optical isolator is coupled to the switching circuit and the resistive element for sending a control signal to the circuit when required.

Abstract: The present invention provides an improved method of protecting a subscriber telephone interface circuit from the application of excess voltages and currents. The method uses a standard subscriber telephone interface circuit, including a speech circuit and a ringing detection circuit, in conjunction with updated controlling software. When excess voltage or current is detected by the ringing detection circuit, a CPU disconnects the speech circuit. The speech circuit is reconnected when the excess voltage or current is removed. The present invention further provides a ringing detection circuit that connects a speech circuit when a ringing signal is detected, and disconnects the speech circuit when excess voltage or current is detected.

Abstract: A novel line-current protection circuit for detecting excessive line current from a telephone line and protecting line interface circuitry from the excessive line current is presented. The protection circuit is adapted to detect when an excessive current is present on the tip and ring lines indicative of the misconnection of the modem or other line interface device with a telephone network that presents an incompatible interface standard. The preferred embodiment appreciates the multimode hold current specifications throughout the world and evaluates the connection of the line interface circuit with a telephone line to determine the compatibility therewith. When excessive line current is detected, the line-protection circuit switches the vulnerable line interface out of the circuit. When compatible line current levels are detected, the present invention reconfigures the configurable hold circuit into its original operating (e.g., non-testing) state for call processing.

Abstract: Embodiments of the present invention relate to systems and methods for an electronic hook switch for customer premises equipment. In an embodiment, a system includes a first lead configured to be coupled to a tip wire and a second lead configured to be coupled to a ring wire. A diode bridge is coupled to the first lead and the second lead and has a positive output. A low side switch is coupled to the positive output of the diode bridge without a high side switch between the low side switch and the positive output of the diode bridge.

Abstract: An overcurrent protection circuit for an analog modem which uses passive-device features during normal operation to minimize modem signal noise and distortion, and active-device features when inadvertently connected to a line having excessive line current.

Abstract: A CMOS implementation for a DC holding circuit in direct access arrangement (DAA) circuitry is disclosed that provides desirable inductive behavior while minimizing power dissipation required by the CMOS integrated circuit, particularly at high loop currents. The DC holding circuitry disclosed may include MOS transistors located on a CMOS integrated circuit and an off-chip power dissipating resistor that acts to dissipate power external to the CMOS integrated circuit. The CMOS implementation disclosed also allows a path for drawing DC current to power other CMOS circuits (e.g. ADCs and DACs) in the CMOS integrated circuit.

Abstract: A replaceable plug which provides voltage surge protector redundancy for use in a maintenance termination unit connecting a telephone company line to a subscriber telephone line. The plug includes connecting terminals for connecting input and output telephone lines to a switch within the plug, and a pair of surge protector circuits. The switch is wired so that it selectively connects one or another of two surge protectors between the input and output lines replacing an actuated protector with a back up protector and restoring telephone service to the subscriber. Associated with this device is a method of determining whether a telephone line fault is due to a protector actuation or in the line proper, and for restoring telephone service without having to send a service person to the location of the maintenance termination box, by providing surge protector redundancy and a switching arrangement whereby a protector is replaced in the line circuit by a switching action.

Abstract: A telephone line interface circuit with event detection capabilities is provided that screens out transient signals and provides an indication to the line side that an actual event has occurred; so that appropriate discrimination circuitry is powered up to determine the exact nature of the actual event only when needed. An event detector develops an AC signal that represents the events to be detected. This AC signal is timed to determine its sustained rate. The sustained rate has a timing threshold which is higher than a transient that occurs on the line. If the incoming signal does not meet the required threshold timing, it is disregarded as being transient and no action is taken. If the incoming signal meets the threshold requirement, the line interface circuit switches to the full power mode and facilitates the actual data transmission.

Abstract: An isolation system is provided that is suitable for use in telephony, medical instrumentation, industrial process control and other applications. Preferred embodiments of the invention comprise a capacitive isolation barrier across which a digital signal is communicated. The system provides a means of communication across the isolation barrier that is highly immune to amplitude and phase noise interference. Clock recovery circuitry may be employed on one side of the isolation barrier to extract timing information from the digital signal communicated across the barrier, and to filter the effects of phase noise introduced at the barrier. Delta-sigma converters may be disposed on both sides of the isolation barrier to convert signals between analog and digital domains. An isolated power supply may also be provided on the isolated side of the barrier, whereby direct current is generated in response to the digital data received across the isolation barrier.

Abstract: A telephone line interface circuit is disclosed having intelligent line current control that can adjust to the various requirements of worldwide telephone systems (DC masks), and/or can reduce the power consumed by the interface circuit. A controller reads a voltage value and based on the theoretical model of a telephone line network, and the expected current/voltage characteristic, as dictated by the appropriate DC mask. The controller sets the line current (via an output voltage) to an optimum value and causes the line voltage to adjust so as to comply with the DC mask and/or minimize power dissipation in the circuit.

Abstract: An improved modem architecture and associated method are disclosed that integrate modem and line-isolation circuitry so as to achieve modem functionality and system-side isolation functionality on a single integrated circuit. The combined modem and line isolation system includes a line-side line-isolation integrated circuit which contains caller ID circuitry. The caller ID circuitry includes a caller ID analog to digital converter for processing caller ID data. The caller ID analog to digital converter may be powered by a power supply generated from power provided across a capacitive isolation barrier. The caller ID circuitry may be coupled to the ringer inputs of the line-side line-isolation integrated circuit. The ringer inputs may be capacitively coupled to the phone line.

Abstract: A power management circuit for use in a terminal interface device such as a modem which converts available line side power into usable power for use by the terminal interface device to activate a variety of line status indicators. These line status indicators signaling the off-hook status of the terminal interface device, low line side power, or the presence of excessive line current. The line status indicators draw power from the available line side power without placing a burden on the attached digital device or its power source. Certain line status indicators utilize flasher/oscillator circuits to increase the functionality of the line status indicators by providing a frequency based pulse to more accurately describe the operating environment of the circuit.

Abstract: A protection circuit for a cellular telephone battery includes a first memory for storing battery identification data and a second memory associated with the telephone includes data relating to which battery type may be used by this telephone. When the battery is connected to the telephone, the battery identification data is compared to the battery type data stored in the telephone. The battery is automatically disconnected from the telephone if the two sets of data do not correspond to each other.

Abstract: A digital line-interface circuit (100) presents a constant impedance to a telephone line (102) by emulating a short or a ground connection between the leads of the telephone line when the transmitter (104) is disabled. The receiver (106) includes a pair of differential op amps (U3, U4) connecting the receive line (122) to the outputs of the transmitter's drivers (U1A, U2A) across a pair of switches (K1, K2). The switches may be replaced by a receive enable (RE) signal line connected to enable (EN) inputs of the op amps. Negative inputs of the op amps are connected to outputs of different transmitter drivers, positive inputs of the op amps are connected to a bias voltage source (Vbias), and the output of each op amp is connected to the receive line and is also fed back by a bias resistor (R5, R6) to its negative input.

Abstract: A surge protector having a failsafe mechanism including at least one overvoltage protection element, at least one arm assembly, at least one ground element, at least one resilient member, and at least one protrusion. The at least one resilient member is electrically connected to the at least one ground element and the at least one protrusion is generally positioned between the at least one resilient member and the at least one arm assembly. The at least one protrusion is in thermal contact with the at least one resilient member, prevents the at least one resilient member from electrically contacting the at least one arm assembly during normal operation, and is spaced away from the at least one arm assembly.

Abstract: A fixed wireless terminal such as a wireless local loop terminal comprising: memory (210) storing characteristics for at least one network, means (214, 220) for accepting network information particular to a subscriber and processing means (208) for reading the subscriber network information and, on the basis of the subscriber network information, selecting appropriate network characteristics for subsequent use by the terminal. Preferably the processing means (208) is arranged to configure a subscriber line interface circuit (212) in accordance with the selected appropriate network characteristics, such as ringing tone, dialling tone, busy tone etc.

Abstract: A subscriber loop interface circuit and method for externally programming the on-hook, off-hook, and transition states of the subscriber loop interface circuit. The circuit having an off-hook overhead sufficient for long loop applications and having a continuous loop current to loop voltage relationship. The circuit may have plural means for generating reference currents for comparison with loop currents.

Abstract: A relay-less telephone line interface circuit is disclosed which incorporates a low-voltage transistor to suppress high-voltage, short-term voltage surges. Specifically, a low-voltage transistor such as a Central Semiconductor C2TA44, a Motorola MPSA42, or similar such transistors actually can withstand a high-voltage spike exceeding the manufacturers specified parameters. The present circuit exploits this undocumented feature by employing such a transistor in combination with a metal oxide varistor in order to provide adequate voltage surge protection for the interface circuit. Thus, a zenor diode is not necessary (avoiding on-hook problems) and the relay-less circuit still passes the FCC Part 68 test.

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 first 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 shunt regulator. The voltage across the first sense resistor is fed back to the line modulator. A second sense resistor is inserted into the line modulator circuitry to provide a measurement of the system current distortion that is outside of the path containing the sense resistor used to monitor the current distortion in the DAA shunt regulator. The voltage across the second sense resistor is also 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.

Abstract: A circuit configuration is provided for supplying a telephone subscriber loop with a supply voltage, for transmitting a transmitted signal into the telephone subscriber loop, and for receiving a received signal from the telephone subscriber loop. The telephone subscriber loop has a first line and a second line. The circuit configuration has a supply voltage source with a first terminal and a second terminal between which a supply voltage drops; a first circuit connected with the first line and having first and second supply voltage connections; and a second circuit connected with the second line and having third and fourth supply voltage connections. The first supply voltage connection is connected with the first terminal and the fourth supply voltage connection is connected with the second terminal. A network-side terminal of a telephone subscriber loop and a chip set are also provided.

Abstract: An isolation system for terminating a phone line is provided. The invention may comprise a capacitive isolation barrier across which a digital signal is communicated. Clock recovery circuitry may be employed on one side of the isolation barrier to extract timing information from the digital signal communicated across the barrier. An isolated power supply may also be provided on the isolated side of the barrier, whereby direct current is generated in response to the digital data received across the isolation barrier. A bidirectional isolation system is provided whereby bidirectional communication of digital signals is accomplished using a single pair of isolation capacitors. In preferred embodiments, the digital data communicated across the barrier consists of digital delta-sigma data signals multiplexed in time with other digital control, signaling and framing information.

Abstract: A communication system is provided with a power supply budget such that portions of a phone line side circuit may be powered from the TIP and RING phone lines while using standard electronic devices for the hookswitch circuits and the diode bridge circuit. For example, low voltage converters in the phone line side circuit may be powered from the phone line. The low voltage converters may operate off a low voltage power supply of approximately 2.5 V or less, more preferably may operate off a low voltage power supply of approximately 2.0 V or less, and in one embodiment 1.9 V converters may be utilized. The communication system may further include a capacitive isolation barrier system for isolating the phone line side circuitry.

Abstract: A replaceable plug which provides voltage surge protector redundancy for use in a maintenance termination unit connecting a telephone company line to a subscriber telephone line. The plug includes connecting terminals for connecting input and output telephone lines to a switch within the plug, and a pair of surge protector circuits. The switch is wired so that it selectively connects one or another of two surge protectors between the input and output lines replacing an actuated protector with a back up protector and restoring telephone service to the subscriber. Associated with this device is a method of determining whether a telephone line fault is due to a protector actuation or in the line proper, and for restoring telephone service without having to send a service person to the location of the maintenance termination box, by providing surge protector redundancy and a switching arrangement whereby a protector is replaced in the line circuit by a switching action.

Abstract: During maintenance of telephony equipment the high voltage ring signals for POTS lines must be disconnected or disabled so as to remove the shock hazard potential associated with the substantially elevated ringing voltage present at a line card of a central office. This problem is effectively obviated by installing a three-terminal alarm fuse socket in a circuit path for the ringing generator, and wiring the socket's terminals to enable the fuse to operate as a switch. The switch is closed by inserting a (blown or unblown) three-pin alarm fuse. This enables a ringing voltage generated by the ringing voltage generator to be applied to the span and delivered to the ringing circuit of the handset. The switch is opened by removing the fuse. Ringing voltage functionality for and/or connectivity with the ringing voltage circuit is selectively dependent upon whether a fuse is installed in the receptacle, irrespective of the state of the fuse.

Abstract: A balanced heat coil protector protects telecommunications equipment from excessive sneak currents through a telecommunications circuit having a ring conductor and a tip conductor. An electrically conductive grounding pin is supported within and extends through the base for providing a path to ground potential. A first electrically conductive line pin and a first electrically conductive central office pin are supported in the base and extend therethrough, the first line pin and first central office pin being electrically connected to each other by a first conductive wire to provide a normal circuit current path from the first line pin to the first central office pin.

Abstract: The present invention is embodied in a system and method for protecting an electronic device connected to a telephone line. The system includes a detector, sensor and protector. The detector determines a resistance built-in to the telephone line, the sensor utilizes the resistance of the telephone line to determine if the telephone line is a digital line and the protector prevents the electronic device from prolonged connection with the telephone line if it is a digital line.

Abstract: An isolation system is provided that is suitable for use in telephony, medical instrumentation, industrial process control and other applications. Preferred embodiments of the invention comprise a capacitive isolation barrier across which a digital signal is communicated. The system provides a means of communication across the isolation barrier that is highly immune to amplitude and phase noise interference. Clock recovery circuitry may be employed on one side of the isolation barrier to extract timing information from the digital signal communicated across the barrier, and to filter the effects of phase noise introduced at the barrier. Delta-sigma converters may be disposed on both sides of the isolation barrier to convert signals between analog and digital domains. An isolated power supply may also be provided on the isolated side of the barrier, whereby direct current is generated in response to the digital data received across the isolation barrier.

Abstract: A surge protection device has a modular construction so that a manufacturer can assemble the parts in any combination desired by a customer. This eliminates any need to maintain large varieties of surge protection modules in inventory. The modular construction includes a housing having a plurality of bores formed therein into which is press fit long pins and short pins for electrical communication with an external power source and an internal customer telephone circuit, respectively. The housing further includes a pair of recesses into each of which may be press fit a positive temperature coefficient resistor. A first embodiment of spring clips helps hold the positive coefficient resistors in place and provides needed electrical contacts, and a second embodiment of spring clips further includes an elongate part that extends to a test point opening formed in a cover for the modular device.

Abstract: A line card for coupling to customer premises equipment at a termination for detecting hook condition in a telephone system, the line card comprising an interface for transmitting and receiving an analog signal with the telephone line, an AC ring signal generator for generating a ring signal at the customer equipment, a DC signal generator for generating a DC signal when said customer equipment is off hook, and a circuit for determining the duty cycle of a loop current in the customer equipment (telephone) for determining the hook status of the customer equipmen.

Abstract: In an electronic inductor circuit, an operational amplifier drives the base of the electronic inductor transistor, and receives negative feedback from the emitter of the transistor. The transistor and operational amplifier combine to form a voltage-controlled current source (VCCS) with respect to the loop current. A voltage divider connected across the rectified Tip and Ring voltage (or another node of the circuit at an equivalent voltage) provides a DC reference to the positive input of the operational amplifier, so that the line current automatically increases with an increase in line voltage. A capacitor couples the transmit signal driver to the positive input of the operational amplifier. This electronic inductor circuit can be driven using a low voltage supply and provides sufficient linearity for high-speed modem applications.

Abstract: A power fault testing apparatus for testing telecommunications equipment interfaces to Telcordia's 2nd Level AC power fault standards. The apparatus is powered by a three-phase 480 VAC, 600 A power service. A plurality of transformers, including a variable autotransformer and a fixed transformer, and load resistor banks are coupled to the power source in a network organized as a plurality of selectable and switchable power paths for interfacing with an Equipment Under Test (EUT) operable to be disposed in a test chamber. Power output of the fixed transformer is referenced to a single point ground positioned at a relay bank. The power to the EUT is referenced to building ground that is tied to the single point ground of the fixed transformer. The fixed transformer thereby operates as an isolation transformer, which allows for testing the EUT while operating under power.

Abstract: A fiber to the curb communication system providing telephone service to subscribers using line cards which are pluggable into a broadband network unit. The line cards provide telephone service to up to six lines per card. The cards are programmable from a central location, and the system includes self-testing of the cards, ring generator testing and provides for testing of the telephone lines from the system to the subscriber's location.

Abstract: A surge reset circuit used to automatically reset a surge protection circuit within a telephone line interface circuit. A sensor (e.g., a zenering device) advantageously generates a voltage level representing current draining out of the transient voltage suppression portion of the surge protection circuit after a transient energy surge. If the current is draining, which may indicate a potential lockup situation, feedback circuitry generates a reset feedback signal that is provided to a cutoff device. The cutoff device, in response to receiving the feedback signal, disables a current limiter portion of the surge protection circuit to reduce the current draining from the transient voltage suppression portion. As the current level is reduced, the transient voltage suppression portion will reset when the current drops below a threshold value or is substantially eliminated. As a result, the feedback circuitry removes the reset feedback signal from the cutoff device to re-enable the current limiter.

Abstract: An electrical static discharge (ESD) protection circuit that protects a modem or electronic equipment employing the modem from lockup or malfunction caused by ESD at the secondary winding of a modem transformer as ESD passes from an RJ-11 interface to the transformer. The ESD protection circuit comprises two zener diodes to detect ESD passing through the secondary winding, wherein a ground leg is coupled to a chassis ground and to a ground plane of a PC board via a low impedance connection to the at least one coupling for grounding one side of the zener diodes to divert the detected ESD to the chassis ground.

Abstract: The invention relates to a power supply unit (SCC) for generating a protective current (sealing current), which is superposed on signal currents carried by metal lines for protection against corrosion at tie points or splices of the metal lines, wherein the power supply unit is particularly suited for delivering ANSI T1.601-1992 conformant protective currents (sealing currents) for digital user terminal lines. It exhibits two voltage-controlled power sources (J1, J2), wherein the first power source (J1) is capable, depending on an applied voltage (UC1), of outputting a constant current (I1) having a first value of 0 mA, or a constant current (I1) having a second value of at least 1 mA, in particular 1.

Abstract: In a telephone system having multiple working metallic interface units, a method and a structure provide protection by assigning to each metallic interface unit a partner unit, which is another metallic interface unit. Under this arrangement, when a failure occurs in a working metallic interface unit, its partner unit switches the ring/tip input signals of the failed metallic interface unit to a protection bus. A standby or protective unit then takes over by receiving the rerouted tip/ring signals from the protection bus. The failed working unit can then be removed from the shelf without disrupting service.

Abstract: A digital direct access arrangement (DAA) circuitry may be used to terminate the telephone connections at the user's end to provide a communication path for signals to and from the phone lines. Briefly described, a means for providing a proper ringer impedance for a variety of international phone standards while also providing a proper isolation barrier to the phone line is disclosed. More particularly, a DAA circuitry may be utilized which satisfies many or all ringer impedance standards without the use of additional discrete impedance devices. The ringer impedance standards may be satisfied by use of an impedance structure coupled between the TIP and RING lines and actively controlling the current drawn through the hookswitch devices when a ringing event is detected so as to control the impedance seen at the TIP and RING lines during a ringing event. The detection of the ringing event may be performed on the phone line side of an isolation barrier.

Abstract: An efficient method and circuitry for transferring various levels of power across a high voltage isolation barrier. In one embodiment of the invention, a data access arrangement (DAA) is provided with system side circuitry that provides power to line side circuitry via the high voltage isolation barrier. The line side circuitry includes voltage measurement circuitry, such as an analog-to-digital converter, that periodically measures the line side supply voltage. The resultant information is transmitted across the high voltage isolation barrier to the system side circuitry, where control circuitry or a software process compares the digital representation of the measured voltage level to a predetermined value. Based on the results of this comparison, the control circuitry may alter the amount of power transferred across the high voltage isolation barrier to the line side circuitry in order to optimize power transfer efficiency.

Abstract: A digital direct access arrangement (DAA) circuitry may be used to terminate the telephone connections at the user's end that provides a communication path for signals to and from the phone lines. Briefly described, the DAA provides a programmable means for the DC termination for a variety of international phone standards. The invention may also be utilized with means for transmitting and receiving a signal across a capacitive isolation barrier. More particularly, a DC holding circuit is provided in which a programmable DC current limiting mode is available. In the current limiting mode, power may be dissipated in devices external to a DAA integrated circuit. Moreover, much of the power may be dissipated in external passive devices, such as resistors. Further, the current limiting mode includes a distortion adjustment circuit to limit distortion at a crossover point at which current limiting effects occur.

Abstract: A telephone line interface circuit is disclosed that functions with use of PTC resistors so that temperature increases within the battery feed resistors is minimized. Utilizing current and voltage feedback loops within the line interface circuit, the PTC resistors are implemented without significantly reducing the performance of the overall circuit that would normally be caused by the PTC resistors' inherent impedance inaccuracies. In normal operation, the feedback loops adjust the current and voltage on the telephone line to the desired level without regard for the impedance values of the PTC resistors. In the case that a high voltage is applied to a telephone line, the significant increase in current flow into the line interface circuit causes the battery feed resistors to heat up, and therefore the PTC resistors, that are physically local to the feed resistors, to heat up and significantly increase in impedance.

Abstract: A digital direct access arrangement (DAA) circuitry may be used to terminate the telephone connections at the user's end that provides a communication path for signals to and from the phone lines. Briefly described, the DAA provides a programmable means for the DC termination for a variety of international phone standards. The invention may also be utilized with means for transmitting and receiving a signal across a capacitive isolation barrier. More particularly, a DC holding circuit is provided that is a second order circuit. The use of a second order circuit provides improved distortion characteristics, particularly at low frequencies.

Abstract: A digital direct access arrangement (DAA) circuitry may be used to terminate the telephone connections at the user's end that provides a communication path for signals to and from the phone lines. Briefly described, the DAA provides a programmable means for the DC termination for a variety of international phone standards. The invention may also be utilized with means for transmitting and receiving a signal across a capacitive isolation barrier. More particularly, a DC holding circuit is provided in which a programmable DC current limiting mode is available. In the current limiting mode, power may be dissipated in devices external to a DAA integrated circuit. Moreover, much of the power may be dissipated in external passive devices, such as resistors.

Abstract: An alarm filter circuit used in telecommunication systems for interconnecting between incoming telephone lines and alarm unit located at a subscriber's premises so as to block DSL data signals to and from the alarm unit for preventing interference is provided. In one preferred embodiment, the alarm filter circuit includes a second-order low-pass filter section for blocking the DSL data signals to and from the alarm unit, a first-order high-pass filter section for bypassing the DSL data signals on the incoming telephone lines to the house wiring, and a first-order low-pass filter section for blocking high-frequency signals between the house wiring and the alarm unit. In a second embodiment, a fourth-order elliptical low-pass filter section replaces the second-order low-pass filter section. In a third embodiment, a third-order high-pass filter section replaces also the first-order high-pass filter section.

Abstract: A balanced spectrum limiter for limiting the frequency spectrum of signals transmitted through a telephone or communication system includes one or more filters and an energy surge protection circuit. Each filter includes a pair of inductors wound around a single core and a capacitor connected between the two inductors. The energy surge protection circuit can be any type of energy surge protection circuit known in the art. The range of frequencies which can be transmitted is determined by the filter. The filter is removably connected to the energy surge protection circuit so that filters of varying parameters may be easily interchanged.

Abstract: An alarm filter circuit used in telecommunication systems for interconnecting between incoming telephone lines and alarm unit located at a subscriber's premises so as to block DSL data signals to and from the alarm unit for preventing interference is provided. In one preferred embodiment, the alarm filter circuit includes a second-order low-pass filter section for blocking the DSL data signals to and from the alarm unit, a first-order high-pass filter section for bypassing the DSL data signals on the incoming telephone lines to the house wiring, and a first-order low-pass filter section for blocking high-frequency signals between the house wiring and the alarm unit. In a second embodiment, a fourth-order elliptical low-pass filter section replaces the second-order low-pass filter section. In a third embodiment, a third-order high-pass filter section replaces also the first-order high-pass filter section.