GB2036956A

GB2036956A - Improvements in or relating to electrical screening arrangements

Info

Publication number: GB2036956A
Application number: GB7847234A
Authority: GB
Original assignee: Marconi Instruments Ltd
Current assignee: Marconi Instruments Ltd
Priority date: 1978-12-05
Filing date: 1978-12-05
Publication date: 1980-07-02
Also published as: GB2036956B

Abstract

An electrical screening arrangement for use with high frequency circuit 1 which is liable to produce interference signals is enclosed in conductive box 2. Electrically conductive tubes 4, 5 pass through a wall of the box, each tube being dimensioned as an optical waveguide having a cut-off wavelength which is less than the wavelength of interference signals and information is passed along the tubes in an optical form by means of LED's 6, 8 and phototransistors 7, 9. <IMAGE>

Description

SPECIFICATION Improvements in or relating to electrical screening arrangements This invention relates to electrical screening arrangements. In certain kinds of electrical circuits which operate at high frequencies, it is often necessary to provide electrical screening to contain the electro-magnetic radiation associated with the high frequencies to prevent it adversely interfering with neighbouring circuits or equipment. Many electrical circuits, particularly those containing electronic digital components or those which are required to provide a very pure output signal, are susceptible to external high frequency interference. This interference may cause undesirable noise signals to be superimposed on an otherwise pure signal or it may even cause circuits to malfunction completely.

To provide high frequency screening it has been customary to enclose the high frequency circuits in an electrically conductive enclosure, usually a metal box, and to make electrical connection to it via low pass filtering circuits for the purpose of applying power and control signals to it and for obtaining required output signals from it. For example, in signal generators there is generally a requirement for a high level of screening of the circuits which produce and process the output signal, and to achieve this each power line and control line to the high frequency circuits customarily pass through suitable low pass filters.

This form of screening is satisfactory if signals into and out of the screened enclosure have a sufficiently low frequency. However, increasingly this is not the case and it is necessary to pass useful information to and from such high frequency circuits having frequency components of the same order as the undesirable interference.

The present invention seeks to provide an improved screening arrangement in which this difficult is reduced.

According to this invention an electrical screening arrangement includes an electrically conductive screen to one side of which is situated a high frequency circuit which, in operation, generates high frequency interference signals, an electrically conductive tube mounted so as to pass through said screen, the tube being dimensioned so as to constitute a waveguide having a cut-off wavelength which is less than the wavelength of the interference signals, a light source located to one said of said screen and which is operable to illuminate, via said tube, a light sensor located to the other side of the screen so that high frequency signals can be passed from one side of said screen to the other in an optical form.

Preferably the tube is in the form of.a hollow cylinder having a length which is greater than its diameter.

Preferably again the light sensor and the light emitter are located closely adjacent to opposite ends of said tube. To enable high frequency signals to be passed in both directions across the screen, preferably at least two electrically conductive tubes are provided which pass through said screen and for which tubes the light sources are located at mutually different ends. Additional tubes can be provided as necessary.

Preferably again the high frequency circuit which, in operation, generates high frequency interference signals is completely enclosed within an electrically conductive enclosure of which the said screen forms a part.

Preferably means are provided for modulating the intensity of the light generated by the light sources in dependence on the high frequency information which is to be transmitted to its cooperating light sensor. Although alternative forms of modulation are possible, such as frequency or polarisation dependent modulations, these are not preferred since they would result in more complex arrangements.

Although optical isolators have been used in the past to provide electrical isolation between two circuits, they generally operate at extremely low frequencies since usually they are provided to provide d.c. isolation. In the present invention the ratio of the length of the tube to its diameter can be increased to the extent necessary to provide the required level of attenuation for the undesirable high frequency interference signals.

The invention is further described by way of example with reference to the accompanying drawing which illustrates an electrical screening arrangement in accordance with the present invention.

Referring to the drawing, a high frequency circuit 1 which in normal operation produces undesirably high frequency interference signals is located within an.electrically conductive metal box 2. In one side 3 of this box 2, hollow wave-guides in the form of cylindrical tubes 4 and 5 are mounted. A light emitting diode 6 is mounted at the inner end of the tube 4 so as to illuminate a phototransistor 7 located at its outer end.

Similarly, a further light emitting diode 8 is positioned at the outer end of tube 5 so as to illuminate another phototransistor 9 located at the inner end of the tube. In practice, the circuit 1 typically represents a micro-processor which, in operation, generates broad band radio frequency interference. Interference of this kind is extremely undesirable and often quite unacceptable and this particularly so when the microprocessor forms part of a signal generator which produces a very pure output signal, since the interference could adversely affect the frequency stability of the output and could cause noise to be superimposed upon it. The microprocessor 1 is provided with an interface circuit 10, which receives information from the microprocessor 1 in serial form as a train of pulses at a rate of approximately 10 KHz.The interface circuit 10 converts these pulses into a form which is suitable to intensity-modulate the light input of the light emitting diode 6. Typically, the diode 6 generates infrared light. The colour of the light is not of great importance provided that the phototransistor 7 is sufficiently sensitive to the colour emitted by the light emitting diode. Current variations generated within the phototransistor 7 by variations in intensity of light received by it are passed to a further interface circuit 11, which provides a re-constituted train of output pulses for utilisation by other circuits (not shown). Similarly, an interface circuit 12 feeds the light emitting diode 8 with a train of pulses as necessary and these are re-constituted by another interface circuit 1 3 from the phototransistor 9.This train of pulses is applied to the microprocessor 1 to control its operation as necessary.

If is of course not possible to supply power to the rnicroprocessor via the optical links, and for this purpose a conventional low pass filter circuit 14 is mounted within a wall of the box 2. The filter 1 4 is arranged to pass the d.c. power, but has a cut-off frequency which is well below the frequency of any interface signals.

The light emitting diode 6 and the phototransistor 9 are either so screened or arranged that they do not respond to high frequency interference signal present within the box 2. Instead, the light emitting diode 6 responds only to signals fed to it from the interface circuit 10, and to facilitate this either the interface circuit 10 is mounted very closely adjacent to the diode 6, or it is coupled to it via a screened lead. Similar considerations apply to the phototransistor 9 and the interface circuit 13.

The tubes 4 and 5 should be of small a diameter as possible, so that their wave propagation cut-off frequency is as high as possible. Even for frequencies below the cut-off value of a waveguide, some energy will propagate a certain distance along it, although it is rapidly attenuated. For a one to one ratio between diameter and length the attenuation is approximately 30 dB, and for a length twice the diameter the attenuation is about 60 dB, both these figures applying if the applied electrical frequency is well below the cut-off frequency of the waveguide. Although cylindrical waveguides have been described, tubes having other crosssectional shapes can instead be used, e.g. square or rectangular.

Claims

1. An electrical screening arrangement including an electrically conductive screen to one side of which is situated a high frequency circuit which, in operation, generates high frequency interference signals, an electrically conductive tube mounted so as to pass through said screen, the tube being dimensioned so as to constitute a waveguide having a cut-off wavelength which is less than the wavelength of the interference signals; a light source located to one side of said screen and which is operable to illuminate, via said tube, a light sensor located to the other side of said screen so that high frequency signals can be passed from one said of said screen to the other in an optical form.

2. An arrangement as claimed in claim 1 and wherein the tube is in the form of a hollow cylinder having a length which is greater than its diameter

3. An arrangement as claimed in claim 1 or 2 and wherein the light sensor and the light emitter are located closely adjacent to opposite ends of said tube.

4. An arrangement as claimed in any of the preceding claims and wherein at least two electrically conductive tubes are provided, which pass through said screen and for which tubes the light sources are located at mutually different ends.

5. An arrangement as claimed in any of the preceding claims and wherein the high frequency circuit which, in operation, generates high frequency interference signals is completely enclosed within an electrically conductive enclosure of which the said screen forms a part.

6. An arrangement as claimed in any of the preceding claims and wherein means are provided for modulating the intensity of the light generated by the light sources in dependence on the high frequency information which is to be transmitted to its co-operating light sensor.

7. An electrical screening arrangement substantially as illustrated in and described with reference to the accompanying drawing.