GB2265777A

GB2265777A - Suppressing unwanted signals in a transmission path

Info

Publication number: GB2265777A
Application number: GB9301216A
Authority: GB
Inventors: Clifford Edward Lammas
Original assignee: GPT Ltd; Plessey Telecommunications Ltd
Current assignee: GPT Ltd; Plessey Telecommunications Ltd
Priority date: 1992-04-01
Filing date: 1993-01-22
Publication date: 1993-10-06
Anticipated expiration: 2013-01-22
Also published as: GB2265777B; GB9301216D0

Abstract

Transmission line apparatus terminating in a connector 3 but having a transmission line 4 connecting between the connector 3 and a circuit load 2 includes in the line connection 4, an impedance 5 which present a high impedance to unwanted signals and is transparent to the wanted signals. Coaxial cable 1 may comprise a twin cored shielded cable (Fig 2). The impedance 5 preferably comprises a ferrite core upon which the twin wires forming line 4 are wound. Capacitor 6 is connected between the line 4 and the equipment screen 10. <IMAGE>

Description

METHOD AND APPARATUS FOR SUPPRESSING UNWANTED SIGNALS IN A TRANSMISSION PATH This invention relates to a method and an apparatus for suppressing unwanted signals in a transmission path and particularly a transmission path where signals are passed over a coaxial conductor having a signal carrying central core and a braided outer conductor. The transmission line will terminate in the connector and the connector will interface signals from the transmission line with a secondary transmission line to a load or a source.

The outer of the coaxial cable and the connector will be set at a 0 signal level. Although this may in certain circumstances be at earth potential it is not necessarily so and may float as a basic reference.

In any transmission system there are two types of signals involved. One is the wanted transmission signal which carries information in a preferred form and the other is error or noise signals which arise either from the equipment or from the transmission line. Certain of these unwanted signals are defined in electromagnetic compatibility and electrostatic discharge requirements. It is particularly important to remove or suppress the electrostatic discharge signals (ESD) as these can lead to large voltage spikes which may be as much as 8 kv arising on the transmission system in which the line forms part and which can give a significant corruption to the transmitted information over the line. This corruption would appear in the form of digital error in a transmitted message.

It is an object of the present invention to provide a circuit arrangement and a method of suppressing unwanted signals such as ESD in a transmission path by making the path block the unwanted signals and be transparent and transmit only the wanted signals.

According to the present invention transmission line apparatus terminating in a connector but having a transmission line connecting between the connector and a circuit load includes in the line connection, impedance means which present a high impedance to unwanted signals and is transparent to the wanted signals.

The impedance means may comprise an inductive element in conjunction with a capacitive element.

Conveniently the line connection comprises a twisted or a bonded pair of wires and these may be wound around a ferrite core to provide the said inductive element. There may be a plurality of ferrite cores each wound with a separate wire to form the inductive element.

If the circuit load is a printed circuit board or the like in a data transmission system a further capacitor may be employed to decouple the inductor from the board.

The further capacitor may include a capacitor connected from the transmission line or the connector outer conductor to one side of the inductive means. This connection would be carrying the signal 0 reference level. The actual transmitted signal would be carried on the part of the twin line connected to the inner conductor.

One example of a circuit embodying the invention will now be described with reference to the Figure 1 and a variation of this circuit is shown in Figure 2 accompanying drawing. Referring first the Figure 1, this shows the connection between a transmission line 1 and a printed circuit board 2 comprising a circuit load. The transmission line 1 is of a coaxial nature and has an inner conductor separated by an insulating material from a screened outer conductor with the inner conductor carrying the signal to be transmitted connected to a connector 3 which has a twin wire line 4 which may be a twisted or bonded pair connected to it and to the load 2. It will be seen from the drawing that the inner connector of the connector 3 is connected to one line 41 and this is the line which carries the signal input to the load 2.

The other line 42 is connected to the outer part of the connector 3 which is in turn connected to the screen of the coaxial cable and this line 42 is also connected to the load 2. Both lines 41, 42 are connected through an inductor 5 which in this example comprises a ferrite core upon which the twin wires 4 are wound. The inductor 5 creates a substantial impedance to the unwanted signals on the line particularly the ESD ones and filters them to prevent them from reaching the load 2.

The filtering effect on the line is further enhanced by the provision of a capacitor 6 between the line 42 and an equipment screen 10. The capacitor 6 also assists in reducing unwanted electromagnetic and electrostatic activity from the signal reference level in the load 2. It should be noted that this signal reference level on line 42 is able to float and it is not connected to earth as may be the case in other applications. The screen 10 can be a metallic housing for the circuit board load 2.

A further capacitor 7 can be connected across terminals 8 in the line 42 in order to add to the filter effect and to raise the level of the impedance on unwanted signals. This capacitor 7 has the advantage also of decoupling the load 2 to the connector 3 and isolating any DC components of the signal. The capacitor 7 is not essential to the invention and may be by-passed if necessary by shorting the terminals 8 together. This may be a feature which is of advantage in helping to meet various communication or transmission specifications.

In operation any signal transmitted over transmission line 1 via connector 3 and the connection line 4 to load 2 which carries any electrostatic or electromagnetic element of an unwanted nature meets in the transmission line substantial filtering which causes it to be suppressed. The filter is designed so as to have no effect at all on the signal which is wanted and is effectively transparent to this signal so that it passes freely with a minimal attenuation to the load 2.

It should be particularly noted that any electrostatic voltage spikes which may be generated and which could cause an upset of the operation of the load 2 are suppressed by the high impedance of the line 4.

The use of the inductance 5 in combination with capacitance 6 is highly satisfactory and is equally effective to the alternative of designing a ferrite bead to be placed over the transmission line.

Referring now to Figure 2, this shows a variation of the circuit of Figure 1 where instead of a coaxial cable 1 with a single central core a twin cored shielded cable is used. The same reference numbers are used in Figure 2 as in Figure 1 for convenience. The inductor 5 is provided with a pair of ferrite cores upon which lines 41 which carry the signals to load 2 are wound together with the wire 42 connected to the outer connection to the screen of connector 3. The circuit operates in a similar manner to that described with reference to Figure 1 and clearly can be adapted similarly to a cable 1 having more than two cores.

The invention will find many applications such as in radio multiplex systems, PABX's, telephone exchanges and anywhere where there is the risk of electrostatic discharge building-up which may cause unwanted and spurious signals.

Claims

1. Transmission line apparatus terminating in a connector but having a transmission line connecting between the connector and a circuit load includes in the line connection, impedance means which present a high impedance to unwanted signals and is transparent to the wanted signals.

2. Apparatus as claimed in Claim 1 in which impedance means comprises an inductive element in conjunction with a capacitive element.

3. Apparatus as claimed in Claim 1 or Claim 2 in which the line connection comprises a twisted or a bonded pair of wires wound around a ferrite core to provide the said inductive element.

4. Apparatus as claimed in Claim 3 and including a plurality of ferrite cores each wound with a separate wire to form the inductive element.

5. Apparatus as claimed in any preceding Claim in which the circuit load is a printed circuit board in a data transmission system and in which a further capacitor is employed to decouple the inductor from the board.

6. Apparatus as claimed in Claim 5 in which the further capacitor includes a capacitor connected from the transmission line or the connector outer conductor to one side of the inductive means.

7. Transmission line apparatus substantially as hereinbefore described with reference to Figures 1 or 2 of the accompanying drawings.