GB2126045A

GB2126045A - Subscriber carrier system

Info

Publication number: GB2126045A
Application number: GB08223878A
Authority: GB
Inventors: James Robert Williams
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-08-19
Filing date: 1982-08-19
Publication date: 1984-03-14
Also published as: GB2126045B

Abstract

In a carrier subscriber telephone system where the carrier subscriber's terminal unit is powered by a local battery CBC and the telephone is an electronic telephone e.g. the Ambassador type, then an electronic circuit BB giving a battery boost of about 5 volts is incorporated in the circuitry of the carrier subscriber terminal unit to recover the shortfall in battery voltage under "battery low" condition and ensure correct operation of the electronic telephone. <IMAGE>

Description

SPECIFICATION Subscriber carrier system This invention relates to a carrier telephone system, particularly but not exclusively to a system for adding a single carrier channel to a telephone line extending from a central office to a subscriber station or PBX.

British Patent 1,259,302 describes a system which provides a single channel carrier system for doubling the capacity for existing subscriber lines and PBX lines. This patent describes circuits which overcomes certain problems such as difficulties in coping with variations in line impedances and microphonics and position sensitivity of telephone transmitters. In particular interrupted bursts of carrier are used to transmit the ringing signals for the carrier subscriber, the interruptions defining the cadence of the ringing signal. At the carrier subscriber terminal the ringing signal can be generated either by detecting and amplifying a ringing signal modulated on the carrier or by utilising the carrier to trigger a local ringing signal oscillator.

If the carrier subscriber terminal has a local battery to power the circuitry, this battery can be trickle charged by a trickle charging current from the exchange while the carrier subscriber terminal is idle. British Patents 1,185,678 and 1,195,341 describe aspects of trickle charging the local battery, and British Patent 1,387,565 describes an arrangement in which the trickle current can be increased without causing inadvertent operation of the latch relay.

With the advent of new electronic telephones a problem arises of interface compatability with the battery operated carrier system. The terminal voltage supplied to the subscriber handset is insufficient under worst case conditions (i.e. with the battery low) to operate these new electronic phones.

For example, the minimum terminal voltage required to operate the "Ambassador" electronic telephone is about 11 V with an operating current of 30 mA. Our existing "Extraphone" carrier system provides only 7.5 V under "battery low" conditions.

According to the present invention there is provided a battery-operated subscriber terminal for connection to a transmission path in a subscriber's carrier telephone system, comprising means for demodulating an incoming carrier wave from said transmission path to produce an output audio frequency signal for a subscriber's telephone set; means for modulating an outgoing carrier wave with an audio signal from the telephone set for transmission along the transmission path, an amplifier for amplifying the incoming carrier wave, battery supply terminals for connection to a local battery supply for powering the terminal and the telephone set, and a battery boost circuit arranged to generate a supplementary voltage on top of the local battery voltage for the telephone set when an off-hook condition is present.

In order that the invention can be clearly understood, reference will now be made to the accompanying drawings, in which: Figure 1 is a block diagram of a carrier subscriber terminal according to an embodiment of the present invention; and Figure 2 is a more detailed circuit diagram of one version of part of the embodiment of Figure 1.

Referring to Figure 1 of the drawing there is shown a block diagram of subscriber carrier terminal for a double sideband, amplitude modulated single channel carrier system. A central office is connected to subscriber stations A and B by e.g. an open wire line, an underground cable, etc. Here it is assumed to be a non-loaded exchange "cable pair" and the subscriber terminal unit is connected to the added line station A and the "physical" line is connected directly to its associated station B via a low pass filter LPF. Both the "physical" and "carrier" subscribers function in their own conventional manner totally unaffected by the existence of the other circuit. A local battery is trickle-charged during all intervals while such charging will not interfere with telephone services.

The subscriber unit receives transmissions from the exchange on a 64 kHz carrier and transmits information on a 40 kHz carrier.

Appropriate bandpass filters 6 and 7 for, respectively, transmitted signals and received signals are provided. The carrier telephone A is connected to a hybrid circuit 8 and outgoing speech is modulated by modulator 9 onto a 40 kHz carrier provided by a local carrier oscillator 10. This signal is amplified by amplifier 11 and is transmitted to the exchange via the bandpass filter 6.

Incoming carrier signals are selected by bandpass filter 7 and passed to receive amplifier 12. The signal is demodulated in demodulator 13 and passed via a low pass filter 14 to audioamplifier 1 5 and thence to the carrier telephone A via hybrid 8.

The local ringing oscillator 1 6 is provided to provide a ringing signal to the carrier telephone A.

When the telephone is on-hook, all circuits except the receive amplifier 1 2 are switched off in order to conserve the energy stored in the local battery supply 1 7. The battery is trickle-charged from the exchange via local battery charging circuit LBC.

When the telephone is off-hook this causes the off-hook detector 21 to provide power to the transmit amplifier 11, oscillator and modulator 10 and 9 and if a battery saver circuit is used as described in our co-pending application 8114038 (J. R. Williams 1) to save battery power supplied to receive amplifier 12 (which always needs to be sampling any incoming carrier), then the off-hook detector 21 also overrides the battery saver circuit to maintain on the receive amplifier 12. The off-hook detector 21 also acts to inhibit the ringing signal generator 1 6 so that any dialled out pulses do not cause the ringer 1 6 to "tinkle". This is done by the ringer inhibit circuit 22 via the ringer switch circuit 23.

In addition in the off-hook condition the current drawn through the hybrid 8 causes opto-iso switch SW to switch on a 5 V battery boost circuit BB connected in series with constant current circuit CC. This ensures that the voltage supplied at the terminals of telephone A is at least 11.5 V even in the worst case condition of the battery e.g. a battery of nominally 10 V giving only 7.5 V to the telephone (8.8 V at the battery terminals).

Referring now to Figures 2 and 3 the detailed circuitry will be explained. With a high impedance (electronic) telephone as telephone A connected across the output of the hybrid 8, an "off-hook" condition is detected as follows: The "off-hook" condition places a resistance across the output of the hybrid 8 thus completing a current path via the hybrid 8 from battery +ve, through R22 and the diode section of the integrated circuit opto isolator switch SW of Figure 1 , diode D1 the upper winding of hybrid 8, the subscriber handset of telephone A, the lower winding of hybrid 8, zener diode D1 5 transistor TR1 and resistor R1.

The initial current is sufficient to cause the integrated circuit current sensitive switch SW to go into conduction, this in turn allowing power to be switched to the battery boost circuit BB. This comprises an oscillator circuit including transistors TR2, TR3 associated resistor R2 to R5 and the inductance and capacitance inherent in the transformer Tri . The primary of transformer T1 is centre tapped to the +10 V supply. A full wave rectifier Do 1, D12, D13, D14 has its output smoothed by capacitor C2 (zener diode D15 provides an initial current path). This provides a voltage in series with the existing current path described above, such that an initial current of about 6.5 mA is boosted to about 30 mA. This causes sufficient voltage to develop across resistor R22 to allow the off-hook detector switch TR6 to be operated.

The "off-hook" detector switch TR6 also acts as a dial pulse detector for "loop disconnect" dialling pulses. The "break" section of each pulse switches off TR6 and battery boost switch SW so that the previously described battery boost sequence follows each dial pulse.

The off-hook switch TR6 provides power to the various circuits of the subscriber terminal via line L3 when the off-hook condition occurs. Line L4 inhibits the ringer switch when the off-hook detector operates by putting a high positive voltage on the base of transistor TR7.

Claims

1. A battery-operated subscriber terminal for connection to a transmission path in a subscriber's carrier telephone system, comprising means for demodulating an incoming carrier wave from said transmission path to produce an output audio frequency signal for a subscriber's telephone set; means for modulating an outgoing carrier wave with an audio signal from the telephone set for transmission along the transmission path, an amplifier for amplifying the incoming carrier wave, battery supply terminals for connection to a local battery supply for powering the terminal and the telephone set, and a battery boost circuit arranged to generate a supplementary voltage on top of the local battery voltage for the telephone set when an off-hook condition is present.

2. A subscriber terminal as claimed in claim 1, wherein the battery boost circuit is a DC-DC converter connected in series with a constant current circuit and an off-hook detector circuit in the DC supply path for the telephone set.

3. A subscriber terminal as claimed in claim 1 or claim 2, comprising a current sensitive switch responsive to a current change in the DC supply path for the telephone set to switch on the boost circuit when the current increases and switch off the boost circuit when the current decreases.

4. A subscriber terminal as claimed in any preceding claim, wherein the battery boost circuit comprises a transformer having a primary winding centre tapped to a battery supply connector and a multivibrator circuit feeding a rectifier circuit via the transformer, the output of the rectifier circuit being connected in series in a DC supply path for the telephone set.

5. A subscriber terminal as claimed in claim 4, wherein a zener diode is connected to provide an initial current by-pass path.

6. A subscriber terminal substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings, or to Figures 1, 2a and 2b of the accompanying drawings.