GB2057817A - Telecommunication system - Google Patents

Abstract

In a telecommunication system which uses a pair of radiating transmission lines A, B and has amplifier and filter stations S1, S2, S3, at intervals along its length, the stations have the amplifying elements provided as two distinct amplifiers G, H, one for signals at a first frequency (f1) transmitted from a base station (C) and the other for signals at a second frequency (f2) transmitted from a mobile station. <IMAGE>

Description

SPECIFICATION Telecommunication systems This invention relates to telecommunication systems of the kind in which signals are transmitted from a transmitter base station to a mobile station, and signals from the mobile station are transmitted to a receiver base station through a radiating transmission line.

Such a system is described in British Patent Specification No. 1371291 where an open braid coaxial cable is disclosed as the radiating transmission line. This type of line is known as a 'leaky feeder' and although the braided form of cable is the most popularly used other forms such as a coaxial cable having a cylindrical outer sheath with a longitudinai slit along it can be used for particular applications. In the specification above referred to the communication system disclosed employs a series of unidirectional repeaters to amplify the signals and to compensate for attenuation caused principally by the 'lossy' nature of the radiating transmission line.

This type of telecommunication system is now used successfully for communication purposes in mines and tunnels where conventional radio systems would be useless over any material distances due to the walls of the tunnel or mine absorbing the signals propagated from a transmitter. In practice it has been found necessary to improve the basic system to compensate for defects which have become apparent in use. One significant improvement has been the application of a 'tailback' technique where an additional length of radiating cable has been connected to one side of each repeater and then led back parallel to the main radiating cable on the other side of the repeater in a direction towards the next adjacent repeater for a distance usually equivalent to about half the distance separating the repeaters.This has enabled a boost to be given to the radiated signal strength in the region adjacent the repeater and to compensate, to some degree, for attenuation of the radiated signal. Such an improvement is illustrated in the description of British Patent Specification No. 1485156.

A further modification is described in British Patent Specification No. 1497288 which discloses a system in which a pair of parallel leaky feeder lines are used with one of the lines being connected to the base receiver station and the other to the base transmitter station. The lines are connected at spaced intervals by unidirectional amplifier elements which are arranged so that alternate elements conduct in the opposite direction to adjacent elements. In addition routing elements are connected in each line between each adjacent pair of amplifier elements in such a manner that the elements route signals first along part of the one of the lines and then along part of the other line with the transfer between lines being effected through one of the amplifier elements.

It is our object of the present invention to provide an improvement in a telecommunication system which employs unidirectional amplifier elements connected between a pair of radiating transmission lines and which employs routing elements in a manner which enables higher electrical efficiences to be obtained than has been possible hitherto with known systems.

According to the present invention a telecommunication system of the kind described includes a pair of parallel radiating transmission lines joined one to the transmitter base station and the other to the receiver base station, the lines being interconnected at intervals by stations each including a pair of unidirectional amplifier elements connected to conduct between the lines, and filter means arranged to direct a signal having a first frequency through one of the elements and to direct a signal having a second different frequency through the other of the elements, the elements in alternate stations along the line being connected to conduct in the opposite direction to adjacent stations.

The filter means is preferably a hybrid transformer arrangement, and the output from the stations is preferably arranged to be connected in opposite directions along the line.

In order that the invention may be readily understood one example of a communication system in accordance therewith will now be described by way of example only with reference to the four figures of the accompanying drawings.

In the drawings Fig. 1 is an example of a prior art system while Fig. 2 is a schematic representation of the system of the invention.

Fig. 3 shows a detail part of Fig. 2 and Fig. 4 is a circuit diagram of a combined filter and amplifier unit suitable for use with the system of Fig. 2.

Referring first to the prior art system of Fig.

1, this system shows in schematic form a system similar to that disclosed in British Patent Specification No 1497288. Two braided leaky feeder transmission lines A, B extend parallel to each other along a mine tunnel. Line A is connected at a base station C to a transmitter TX which transmits signals at a frequency fl over the system for radiation from the lines and for reception at mobile stations such as W situated somewhere along the length of the line. A receiver RX at the base station C is connected to line B and receives signals transmitted from the mobile station W and picked up and carried over the lines at a frequency f2. The frequencies fl and f2 are different and are not harmonics of each other.Unidirectional amplifying elements R1, R2, R3---are connected between the lines and are arranged to conduct in alternate directions; ie R1, R3---etc. conduct from line A to line B while R2, R4,---etc conduct from line B to line A. All amplifying elements R amplify both frequencies f1 and f2. The effect of the amplifying elements is to conduct signals of frequency fl, from TX on a path switching between lines A and B. In order to prevent closed loop paths developing filters are located in lines A and B about mid-way between elements R at the points indicated at Al, A2, A3---at Bl, B2, B3---in Fig. 1.

The A filters with an odd numbered suffix and the B filters with an even numbered suffix (eg Al, B2, A3. . etc) are designed to pass signals of frequency f2 while blocking signals of frequency fl. The A filters with an even numbered suffix and the B filters with an odd numbered suffix (eg B1, A2, B3 ctc) will pass signals with a frequency of fi but block those with a frequency f2. The effect of the filters is to give a 'tail-back' effect to the system, where the tail-back extends half the distance between adjacent amplifying elements.

In order to achieve a higher electrical efficiency it is required according to the invention to route the different frequencies f1, f2 through separate amplifier elements. This is so even when the frequencies fl, f2 are closely spaced, in this example 41 MHZ and 46 MHZ, or denote two groups of closely spaced frequencies. The effect of channel intermodulations is thereby reduced. In order to reduce the number of assemblies in the system it is also possible to combine the filters in a common unit with a repeater. This is illustrated in the system of the invention which is shown at Fig. 2 and to which reference is now made.

In the figure the base station C comprises transmitter TX operating at frequency f1, and connected to line A, and receiver RX operating at frequency f2, and connected to line B.

Spaced along the lines at similar intervals to elements R in Fig. 1 are stations S1, S2, S3---etc. Each station comprises two filter units E, F and two separate amplifying eiements G, H for the frequencies f1, f2 respectively. Each filter unit E, F includes a combined filter circuit and hybrid transformer.

It will be appreciated that the filter units of Fig. 1 have effectively been moved along the line towards the base C and then incorporated into a single station with the separated amplifying elements. If reference is also made now to Fig. 3 the functional operation of one of the stations S can be clearly followed. The cores of the coaxial cables A, B have been labelled green and black for ease of reference and so that signal paths can be read. Terminals J, K, L and M have also been marked to correspond with the terminals of the circuit to be described with reference to Fig. 4.

In a normal transmission the signal fl transmitted over A from TX is fed from terminal J of the station S to unidirectional repeater G through filter E. There it is amplified and fed to green core via output terminal M. Some part of the signal is also fed to terminal K along the black core to provide a 'tail-back' effect. If it is assumed that the system is receiving a signal from a mobile station W situated to the right of the diagram of Fig. 3 the signal at a frequency f2 will be received in the black core and (combined with any residual fl frequency) will be fed to terminal L of the station and to filter E where it is filtered to block the f1 frequency and to feed the f2 frequency to element H.In H the signal is amplified and fed on through filter F to the black core via terminal K and combined with any fl signal from element G for transmission to receiver RX of the base station C.

If the mobile station had been situated to the left of the diagram of Fig. 2 the f2 frequency signal picked up in the cores would have been transmitted to receiver RX directly along the black core and indirectly along the green core through terminal J of repeater H.

Referring now to the circuit diagram of Fig.

4 this shows the detail of a station S. The terminals J, K, L and M correspond with those of Fig. 3. The unidirectional amplifiers G, H are formed by the transistors TR1, TR2 respectively and the filter elements E, F are formed by hybrid transformers T1, T2, T3 and T4. Other circuit components associated with these elements are conventional. The signal at frequency f1 in this example 46 MH is fed to terminal J and to transformer T1 onto transistor TR1 where it is amplified to compensate for line losses of 1 6 d B between adjacent stations. The stations S are spaced at 500m intervals in this example.

Continuity of DC supply is maintained between all four terminals via inductors L2 and L3 which each have, in this example, a value of 4.7p,H. These function as RF chokes and are not resonant with any capacitors. 1000 pF feedthrough capacitors C3, C4, C5, C7, C8 and C11 are for decoupling purposes. They represent a very low impedance to the RF signals.

The diodes and resistors are purely for bias purposes, the current through TR1 and TR2 being adjusted to 2.5mA by means of adjustable resistor RV1. The collector current of TR2 is the same as the emitter current of TR 1, the transistors being operated in series across a nominal 1 2 volt supply. Transformers T1 and T3 are tuned to resonance at frequency f1, 46 MHz by capacitors C1 and C9, and T2 and T4 are tuned to resonance at frequency f2, 41 MHz by capacitors C6 and Cm 2.

In operation f1 signals arriving at terminal J are coupled into T1. TR1, T3 and via capacitor C10 routed to terminal M. Since transformer T4 is tuned to resonance at a lower frequency, f2, its 1 turn secondary represents a negligible impedance to signals at fl. Con sequently terminal K is effectively connected in -parallel with terminal M at frequency fl, and about one half of the output power is delivered to terminal K as a 'tail-back' signal.

The main f1 signal continues from terminal M to the green line along to the next repeater's terminal J.

The repeater is required to accept f1 signals only from terminal J. For maximum sensitivity the lower end of the 1 turn primary of T1 must be an RF earth point to f1 signals. This is achieved as follows: transformer T2 is tuned to frequency f2, 41 MHz so that at frequency f1, 46 MHz, its 1 turn primary represents a very low value of capacitive reactance. Inductor L4 forms a series tuned circuit with this primary reactance at the frequency f1, thus effectively short circuiting terminal L to earth at this one frequency f1.

Signals received from a mobile station at frequency f2 may arrive at the repeater from either direction, appearing at either terminal L (main receive path) or terminal J (tailback receive path). They are coupled through L4 to T2, TR2, T4, and to terminal K only. Since T1 is tuned to f1 (46 MHz), its 1 turn primary represents a negligible impedance to signals at f2. Terminals J and L are therefore effectively connected in parallel as sources of signal f2.

The repeater is required to deliver amplified f2 signals to terminal K only. To do this the upper end of the 1 turn secondary of T4 must be an RF earth point for signals at f2. This is achieved as follows: transformer T3 is tuned to f1, 46 MHz, so at f2, 41 MHz, the output tapping point represents a very low value of inductive reactance to earth. Capacitor C10 forms a series tuned circuit with this inductive reactance at f2, thus effectively short circuiting terminal M to earth at one frequency, f2.

The whole of the f2 signal induced in the one turn secondary of T4 is therefore delivered to terminal K, and none appears at terminal M.

The Circuit is designed, in this example, to have circuit components with values which give intrinsic safety to the system and allow it to be used in hazardous atmospheres.

Furthermore due to the incorporation of hybrid transformers rather than separate filters and amplifiers the system is very stable with respect to gain when unwanted and unexpected signals occur. This stability is achieved by the attenuation of signals arriving at terminal L and leaving at terminal M.

Claims (5)

1. A telecommunications system of the kind in which signals are transmitted from a transmitter base station to a mobile station, and signals from the mobile system are transmitted to a receiver base station through a radiating transmission line including a pair of parallel radiating transmission lines joined one to the transmitter base station and the other to the receiver base station, the lines being interconnected at intervals by stations each including a pair of unidirectional amplifier elements connected to conduct between the lines, and filter means arranged to direct a signal having a first frequency through one of the elements only and to direct a signal having a second, different, frequency through the other of the elements, the elements in alternate stations along the line being connected to conduct in the opposite direction to adjacent stations.

2. A telecommunication system as claimed in Claim 1 in which the filter means include a hybrid transformer arrangement.

3. A telecommunication system as claimed in Claim 1 or Claim 2 in which the output from a station includes a combination of signals of the two frequencies and in which the signals are transmitted in opposite directions along the line.

4. A telecommunication system as claimed in any preceding claim in which the unidirectional amplifying elements are designed to produce an amplification factor equivalent to the loss between adjacent stations.

5. A telecommunication system substantially as hereinbefore described with reference to the accompanying drawings.