Abstract: A splitter has input terminals for interfacing with a subscriber line and output terminals for interfacing with a telephonic device. The splitter includes a low pass filter and a transient suppression circuit. The low pass filter is coupled to the input terminals. The transient suppression circuit is coupled between the output terminals and the low pass filter and is operable to limit the rate of change of current through the splitter.

Abstract: A line driver for connection to a transmission line having a characteristic impedance. The line driver can be arranged in a voltage-mode or a current-mode configuration. In the voltage-mode configuration, the line driver comprises an amplifier, a transformer, a reference resistor and a feedback circuit. The first winding of the transformer has a first end connected to the output of the amplifier and the second winding is connectable to the transmission line. The reference resistor has an end connected to the second end of the first winding at a junction point and the feedback circuit is connected to the input and output of the amplifier and also to the junction point. The reference resistor has a resistance equal to {fraction (1/k)} times the characteristic impedance of the transmission line. The feedback circuit is arranged to produce a voltage at the output of the amplifier substantially equal to −(K−1) times the voltage at the junction point, for a predetermined value of K.

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: The present invention relates to a method and apparatus for separating a low frequency voice signal from a high frequency data signal propagating on an analog loop connecting the central office of the PSTN to a CPE. The apparatus includes a low pass filter connected in series with the tip and ring conductors of the analog loop to extract the voice signal. The apparatus also includes a high pass filter to reject the low frequency voice signal, leaving only the higher frequency components. The high pass filter features a transformer including two balanced primary winding sections, one section being in series with the tip conductor and one section being in series with the ring conductor. A junction circuit is coupled to the high pass filter to provide the signal exchange functionality needed to separate the transmit and receive components of the high speed data signal.

Abstract: A telephone ringing signal generator includes a DC converter for producing high positive and negative voltages that define peak amplitudes of the ringing signal, which is produced by low pass filtering a generally trapezoidal waveform having these peak amplitudes. The generally trapezoidal waveform is produced by a level-shifting integrator at a frequency determined by a low voltage square wave supplied to the integrator. The integrator comprises a capacitor coupled between a virtual ground point and an output of the integrator and arranged to be charged with two opposite polarities in response to opposite transitions of the square wave, and a resistor via which the square wave is supplied to the virtual ground point. Buffers are provided for buffering the trapezoidal waveform supplied to, and the ringing signal produced by, the low pass filter.

Abstract: A current limiting circuit in a battery feed arrangement provides a variable DC resistance for limiting subscriber loop current to a threshold value. The current limiting circuit also presents a low AC impedance so that no imbalance is introduced between tip and ring leads of the subscriber loop. The current limiting component is a low ON resistance power MOSFET in the ring side of the battery feed arrangement. The current limiting circuit is self protecting in the event of lightning strikes and AC power signals either induced or short-circuited.

Abstract: A telephone line interface circuit includes a transformer provided with first, second and third windings. The first winding is connected across the tip and ring wires of the telephone line and the second winding is connected to the output of an amplifier which has an input connected to a receive line. In a first case where the input to which the receive line is connected is the inverting input, a first resistor is connected in series between the receive line and the inverting input. The third winding is connected in series with a second resistor to form a feedback loop which is connected to the inverting input of the amplifier. The transfer function of the interface circuit is dependent on the ratio of the resistances of the first and second resistors and the turns ratio of the first winding to the third winding but is substantially independent of transformer parasitics.

Abstract: In a SLIC in which loop current is supplied via active circuits, feed resistances via which the active circuits are conventionally coupled to the line are constituted by bifilar resistance wire windings of a ferrite core transformer, which may include a third winding coupled to an input of the active circuits.

Abstract: A two-wire telephone line interface circuit includs two transformers each having primary and secondary windings. The primary windings, which are divided into equal halves for balance and d.c. feed purposes, are connected in series between the two wires, and the winding resistance together with optional series resistance provide desired d.c. resistance across the line. To this end one or both of these windings may comprise resistance wire. A signal from a receive line is coupled to the two-wire line via an amplifier whose low output impedance terminates the secondary of one of the transformers. Another amplifier couples a signal from the two-wire line and the secondary of the other transformer to a transmit line, and also provides at its input a transhybrid signal cancellation node, a transhybrid signal being coupled thereto via a balance impedance. A feedback path can be provided for increasing the a.c. impedance presented by the interface circuit to the line.

Abstract: A SLIC includes controlled current circuits (CCCs) which simulate feed resistors for controlling d.c. feed to a two-wire telephone subscriber line which is coupled to two terminals, control being achieved via a feedback loop. A differential amplifier has an inverting input coupled to the tapping point of a potential divider connected between the terminals, and a non-inverting input connected to a reference potential of half of the supply battery voltage. For suppressing longitudinal currents on the line, the amplifier output is connected to each terminal via a d.c. blocking capacitor in series with a resistor, the resistors having closely matched resistances. For maintaining d.c. balance, the output of the amplifier is coupled via a low pass filter to control a current splitter included in the feedback loop, to control the relative proportions of control currents for the CCCs.

Abstract: An integrated SLIC having input pins coupled to the wires of a two-wire telephone subscriber line is protected from excessive voltages on the line by reverse biassed 60 volt zener diodes connected between each input pin and ground, normally reverse biassed diodes within the integrated circuit connected between each input pin and the substrate of the integrated circuit, and a normally forward biassed diode connected between the substrate and a -48 volt (nominal) supply voltage.

Abstract: Battery feed resistors are constituted by thick film resistors on a substrate, electrically in series with switching type positive temperature coefficient (PTC) resistors which are closely thermally coupled to the thick film resistors by being mounted on the substrate behind the thick film resistors or on insulating layers over the thick film resistors. Precise battery feed resistances and matching of pairs of battery feed resistors are achieved by trimming the thick film resistors. The PTC resistors prevent excessively high temperatures being reached in the event that external high voltages are applied to a telephone subscriber line to which the battery feed resistors are connected.