GB2068196A

GB2068196A - Repeaters for digital transmission systems

Info

Publication number: GB2068196A
Application number: GB8001632A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1980-01-17
Filing date: 1980-01-17
Publication date: 1981-08-05
Also published as: DE3047906A1; ES8202217A1; ES498535A0; SE442933B; IT1194020B; FR2474260A1; FR2474260B1; AU6564280A; AU538667B2; JPS56106439A; IT8119191A0; CA1164548A; GB2068196B; NL8100055A; CH651977A5; JPS6352502B2; SE8100095L

Abstract

A repeater for p.c.m. or digital transmission systems includes a secondary feedback control loop consisting of a low-pass filter and a peak detector to derive a control signal for a controllable impedance in the equaliser circuit. This control loop optimises the repeater performance for different cable types and characteristics with regard to the low frequency energy content of the eye diagram.

Description

SPECIFICATION Repeaters for digital transmission systems This invention relates to repeaters for p.c.m.

or digital transmission systems, and more particularly to arrangements for compensating therein for transmission line characteristics.

Conventional automatic line build out (ALBO) networks provide variable equalisation within a primary equalisation feedback control loop to correct for mainly frequency dependent losses due to line length variations. However, changes in cable type, e.g. paper insulated, plastics insulated etc., in cable characteristics and signal ievel variations all effect the performance of repeaters. The use of high efficiency line codes, such as that known as 4B3T and disclosed in British Patent 1,156,279 (D.B. Waters-1), requires accurate equalisation of line losses, especially at lower frequencies where this code contains more energy than codes such as that known as HDB3 (High Density Bipolar 3), whether or not the signals are scrambled.

A known equaliser design for heavily multiplexed p.c.m. systems, e.g. 48 channels at 2.37 M bit/sec, for example, is shown in Fig.

1. This design incorporates a primary equalisation control loop to keep the height of the so-called "eye diagram constant, with the ALBO network carefully designed to give correct equalisation for a range of different line lengths. The equalised signal is taken from a decision point X and applied to an ALBO peak detector 10, which detects the peak or "height" of the eye diagram. This peak value is amplified and fed to a diode network 11.

The output of the diode network is used to control the ALBO network equalisation. When the repeaters are manufactured a manually adjustable variable resistor is included in the equaliser 1 2 to enable the repeater, once installed, to be adjusted for the low frequency content of the eye diagram.

According to the present invention there is provided a repeater for a digital transmission line system, the repeater having an automatic line build out (ALBO) network with a primary equalisation feedback control loop therefor and an additional equaliser circuit, the repeater including one or more secondary feedback control loops to which the digital signals are applied, the or each secondary loop having a filter followed by a peak detector the output of which produces a control signal for a controllable impedance means in the equaliser.

In a preferred embodiment a secondary loop filter is a low pass filter.

An embodiment of the invention will now be described with reference to Fig. 2 of the accompanying drawings, which figure depicts a block diagram form a repeater according to the invention.

The repeater of Fig. 2 is basically the re peater of Fig. 1 with the addition of the secondary control loop. The equalised signals from decision point X are fed to a low-pass filter 20 which is followed by a peak detector 21. The output of the peak detector 21 is amplified and fed as a control signal to a variable impedance 22 in the equaliser 23.

The low pass filter 20 ensures that the sec ondary loop only responds to the low fre quency content of the eye diagram for control of the equaliser. This automatic control of the equaliser optimises the equalisation for the best eye diagram whatever cable type or char acteristic the input comprises.

1. A repeater for a digital transmission line system, the repeater having an automatic line build out (ALBO) network with a primary equl isation feedback control loop therefor and an additional equaliser circuit, the repeater in cluding one or more secondary feedback con trol loops to which the digital signals are applied, the or each secondary loop having a filter followed by a peak detector the output of which produces a control signal for a control lable impedance means in the equaliser.

2. A repeater according to claim 1 wherein the or one of the secondary loop filters is a low-pass filter.

3. A repeater substantially as described with reference to Figs. 2 of the accompanying drawings.

4. A method of equalisation for a digital trasmission line repeater including the steps of filtering the digital signals, detecting the peak values of the filtered signals and deriving from the detected peak values a control signal for controlling equalisation in the repeater.

5. A feedback control loop for a digital transmission line repeater including a filter to which the digital signals are applied, and a peak detector circuit to which the filter output is is applied, to produce a control signal for a controllable impedance in the repeater equali sation network.

CLAIM (15 Jan 1981) 6. An equalised repeater for a digital transmission line system, the repeater having an automatic line build out (ALBO) network with a primary feedback control loop therefor, the reapter including one or more secondary feedback control loops to which the digital signals are applied, the or each secondary loop having a filter followed by a peak detec tor the output of which produces a control signal for a controllable impedance means in the repeater equalisation circuitry.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Repeaters for digital transmission systems This invention relates to repeaters for p.c.m. or digital transmission systems, and more particularly to arrangements for compensating therein for transmission line characteristics. Conventional automatic line build out (ALBO) networks provide variable equalisation within a primary equalisation feedback control loop to correct for mainly frequency dependent losses due to line length variations. However, changes in cable type, e.g. paper insulated, plastics insulated etc., in cable characteristics and signal ievel variations all effect the performance of repeaters. The use of high efficiency line codes, such as that known as 4B3T and disclosed in British Patent 1,156,279 (D.B. Waters-1), requires accurate equalisation of line losses, especially at lower frequencies where this code contains more energy than codes such as that known as HDB3 (High Density Bipolar 3), whether or not the signals are scrambled. A known equaliser design for heavily multiplexed p.c.m. systems, e.g. 48 channels at 2.37 M bit/sec, for example, is shown in Fig. 1. This design incorporates a primary equalisation control loop to keep the height of the so-called "eye diagram constant, with the ALBO network carefully designed to give correct equalisation for a range of different line lengths. The equalised signal is taken from a decision point X and applied to an ALBO peak detector 10, which detects the peak or "height" of the eye diagram. This peak value is amplified and fed to a diode network 11. The output of the diode network is used to control the ALBO network equalisation. When the repeaters are manufactured a manually adjustable variable resistor is included in the equaliser 1 2 to enable the repeater, once installed, to be adjusted for the low frequency content of the eye diagram. According to the present invention there is provided a repeater for a digital transmission line system, the repeater having an automatic line build out (ALBO) network with a primary equalisation feedback control loop therefor and an additional equaliser circuit, the repeater including one or more secondary feedback control loops to which the digital signals are applied, the or each secondary loop having a filter followed by a peak detector the output of which produces a control signal for a controllable impedance means in the equaliser. In a preferred embodiment a secondary loop filter is a low pass filter. An embodiment of the invention will now be described with reference to Fig. 2 of the accompanying drawings, which figure depicts a block diagram form a repeater according to the invention. The repeater of Fig. 2 is basically the re peater of Fig. 1 with the addition of the secondary control loop. The equalised signals from decision point X are fed to a low-pass filter 20 which is followed by a peak detector 21. The output of the peak detector 21 is amplified and fed as a control signal to a variable impedance 22 in the equaliser 23. The low pass filter 20 ensures that the sec ondary loop only responds to the low fre quency content of the eye diagram for control of the equaliser. This automatic control of the equaliser optimises the equalisation for the best eye diagram whatever cable type or char acteristic the input comprises. CLAIMS

CLAIM (15 Jan 1981)

6. An equalised repeater for a digital transmission line system, the repeater having an automatic line build out (ALBO) network with a primary feedback control loop therefor, the reapter including one or more secondary feedback control loops to which the digital signals are applied, the or each secondary loop having a filter followed by a peak detec tor the output of which produces a control signal for a controllable impedance means in the repeater equalisation circuitry.