LT3385B - Circuit arrangement for ring tripping in an electromic subscriber line circuit - Google Patents

Abstract

Electronic subscriber line interface circuits contain sensitive current identification circuits (S) which monitor the current (I) in the subscriber loop. The ringing of a subscriber is signalled by rhythmic interruption of an AC ringing voltage which produces an alternating current in the subscriber loop. AC ringing voltages are normally de-activated near to the zero transition, but, in the switching phase, there is the risk that, as a result of the capacitive load of the connected subscriber line interface circuit, a current flows which deceptively simulates a loop current and leads to erroneous loop identification. This erroneous response is eliminated according to the invention by means of a comparator (K2) with a very low reference voltage (UV) whereby, following evaluation of the loop current (I) by a current identification circuit (S), the ring tripping is carried out in the immediate vicinity of the zero transition of the loop current and the cause of the erroneous loop identification is eliminated. <IMAGE>

Description

The invention relates to the field of telephone communications, namely, electronic subscriber circuits with a call signal circuitry which disconnects a subscriber line from a telephone switching system. The start and end of a telephone call shall be signaled by the connection or disconnection of the electrical circuit. For telephones that respond to impulses to a constant current, the current circuit is interrupted rhythmically by a selection process. In the subscriber circuit, the current changes are determined by the detector circuitry and the process by cycle changes. The desired call signal voltage is pulsed, which pulsates with the detected presence of a signal AC voltage in the subscriber line, characterized by the signal phase.

characterize a telephone connection through its alternating rhythm. Invitation

It is known that switching on and off the alternating voltage of the call signal close to zero prevents interference in the line voltage and false cycle changes, and the loading of the relevant circuit elements at the time of switching uses low current.

The on and off time of the cycle change procedure during the call phase is described in German Patent DE 3231573 A1. If the subscription is unresponsive, this means that there is no cycle change, depending on the call clock cycle in normal mode, the subscriber circuit is triggered over a period of time, when the ac signal voltage drops to zero. The low-pass filter of the cycle change scheme, which has a time constant of r = 50ms ... 80ms, must be activated during the call signal phase, in order to sufficiently dampen the known corresponding signal alternating voltage, this time constant is disconnected as unacceptably slowing cycle changes. At the end of the DC call, the low-pass filter is disconnected by the signal command in Normal mode, so that in normal mode each change of current is recognized in the subscription cycle without any deceleration.

There is a problem with the signal current cut-off. Disconnecting the alternating voltage of the call signal close to the zero crossing of voltage causes the capacitive load on the subscriber side to cause a phase shift between the voltage and the current, resulting in a current that mimics the cycle voltage and leads to false cycle changes. The time constant r = 50ms ... 80ms is ineffective in this case.

In addition, this account switching effect makes mathematical provisioning of calculating machines impossible due to the additional cycle losses.

The invention seeks to eliminate the source of a false loop detection by means of a simple integrated circuit.

This object is accomplished by the scheme described in the first claim of the invention.

The essence of the invention is that through a comparator the current switch communicates with the logic signal so that the signal switching is close to the zero current transition of the cycle current so that virtually no loop current loss occurs during the switching phase.

The diagram shows the circuit diagram of the signal.

The proposed technical solution is illustrated in the example below. The circuit diagram consists of a series connected current switch 1, a subwoofer 2 and a first comparator 3, and another circuit of a series connected second comparator 4, a strobing scheme 5 and a bistable switch 6. The output of the current switch 1 is also connected to the first input of the second comparator 4. and the second inputs of the strobing scheme 5, the bistable switch 6 and the filter 2 are interconnected and supplied with a control voltage RU.

The current switch 1 triggers the pulses of the cycle current I and feeds the signal to the first comparator 3 through the low-pass filter 2, where it is compared to a reference voltage U _etal , for example U _etaX = l V. If the voltage of the low pass filter 2 is higher than the reference comparator 3 forms a cycle signal U _s . In parallel to a circuit consisting of a low pass filter 2 and a first comparator 3, a second circuit consisting of a second comparator 4, a strobing scheme 5 and a bistable switch 6 is connected. The output RU 'of the bistable switch 6 is connected to a standard unit. The second comparator 4 compares the output voltage of the current switch 1 with a negligible offset voltage U _v , e.g. U _v = 50 mV. If the output voltage of the current switch 1 is less than the bias voltage U _v , then the second comparator 4 affects the bistable switch 6 via the strobing scheme 5. The logic control signal RU controls the bass filter 2, the gating scheme 5 and the bistable switch 6.

Logical signal commands RU = H lead to the connection moment. Bistable switch 6 is actuated by this command and RU '= H appears at its output. The high output signal of the bistable switch 6 amplifies the subscription signal.

At the same time, the RU = H command makes the subwoofer filter 2 an effective time constant.

When the logic signal command RU obtains a low level, that is RU = L, then the time constant of the lowpass filter 2 strobing skips the switch and the signal path through circuit 5 is ready. The second comparator 4 output signal only when the current 1 output voltage is less than displacement voltage; in practice, this is the zero crossing of the cycle current. The output signal 4 of the second comparator then propagates through the strobing scheme 5 to the bistable switch 6 and is then returned. During output RU '= L the account signal is deactivated.

Claims (2)

DEFINITION OF INVENTION 1. A circuit diagram for a call signal in an electronic subscriber circuit comprising a series of current switches (1), a subwoofer (2) and a first comparator (3) for detecting a loop current and generating a loop signal Us, characterized in that consisting of a low-pass filter and a first comparator, a circuit connected in series with the second comparator (4), the strobing scheme (5) and the bistable switch (6), and the subwoofer (2), the gating scheme (5) and the bistable switch (6) ) is provided with a logic signal command (RU) and an output of the bistable switch (6) is provided with a logic signal (RU ') to enable and disable the account signal. 2. Scheme according to claim 1, characterized in that, by means of the second comparator (4), the bias voltage, in addition to a voltage that deviates slightly from zero potential, serves as a control voltage.