GB2349515A

GB2349515A - Isolator for filtering electromagnetic radiation

Info

Publication number: GB2349515A
Application number: GB9905591A
Authority: GB
Inventors: Paul Broadhurst; Richard Chamberlain
Original assignee: Technetix Ltd
Current assignee: Technetix Ltd
Priority date: 1999-03-12
Filing date: 1999-03-12
Publication date: 2000-11-01
Also published as: GB9905591D0

Abstract

An isolator has a metal casing (1) with an inlet aperture (3) and a coaxial connector mounted on a board (7) attached to the casing (1) so as to cover the aperture (3). The coaxial connector has an inner conductor (6) and an outer conductor (5), the outer conductor (5) being electrically connected to the casing (1) by a plurality of capacitors (12), mounted on the board (7). Each capacitor (12) is connected between the outer (5) conductor and the casing (1) such that the resulting array of capacitors (12) conducts radio frequencies but provides isolation for d.c. voltages or voltages at mains frequency. The board (7) can be a printed circuit board (PCB) and have an electrically conductive coating. The inner conductor (6) may also extend through the board (7) into the metal casing (1) where the projecting end of the inner conductor (6) is surrounded by a ferrite sleeve (14).

Description

TITLE: IMPROVEMENTS IN OR RELATING TO VOLTAGE ISOLATORS This invention relates to isolators for providing voltage isolation between a distribution network and customers'equipment (such as television sets) supplied with broad band communications signals by the network.

For example, in every installation of a television set fed by a cable TV network, there must be voltage isolation between the network and the customer's television set. This isolation must prevent dangerous voltages travelling in either direction but must also retain high screening effectiveness to eliminate the effects of external radio fields and prevent radiation from the cable system to external systems. This requirement is particularly difficult to achieve in an isolator.

According to the invention an isolator comprises a metal casing having an inlet aperture, a coaxial connector mounted on a board attached to the casing to cover the aperture, the coaxial connector having an inner conductor and an outer conductor, the outer conductor being electrically connected to the casing by a plurality of capacitors mounted on the board, each capacitor being connected between the outer conductor and the casing such that the resulting array of capacitors conducts radio frequencies but provides isolation for d. c. voltages or voltages at mains frequency.

The board is preferably a printed circuit board carrying an electrically conductive coating electrically connected to the casing, the outer conductor and the coating being separated by an intervening area of the board which is electrically non-conducting and across which the capacitors are electrically connected.

Preferably, the array of capacitors is physically arranged in a star configuration to extend radially outwardly from the outer conductor to the coating, the array of capacitors being electrically connected in parallel, in electrical circuit terms. The inner conductor preferably extends through the printed circuit board and into the metal casing, the projecting end of the inner conductor being surrounded by a ferrite sleeve.

The inlet aperture may be formed by an area which is recessed below the surrounding wall of the casing, the printed circuit board being accommodated in the recess. In a preferred embodiment, the recess is sufficiently deep to accommodate the printed circuit board, together with the capacitors thereon, and a protective cover which has an outer surface flush or substantially flush with the surrounding wall of the casing.

The coating on the printed circuit board may be electrically connected to the casing by means of metal projections, preferably in the form of nibs, which project upwardly from the recessed area of the casing, extend through registering holes in the printed circuit board and are electrically connected, eg by soldering, to the electrically conductive coating.

An isolator according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic cross-sectional view of the isolator, and Figure 2 is a plan view of the isolator, with a protective cover removed.

The isolator comprises a metal casing 1 of cuboid shape. In one wall 2 of the casing is formed a recessed inlet aperture defined by a circular hole 3 in a square recessed wall 4 of the casing 1. A coaxial connector is supported centrally within the recess. The connector has a tubular outer conductor 5 and an inner conductor 6. The outer conductor 5 is mounted on the upper surface of a printed circuit board 7 and it projects outwardly of the casing, presenting a threaded outer surface to receive a coaxial connector supplying an input signal. The printed circuit board 7 has a square shape and fits snugly in the recess, in which it is retained by six metal nibs 8. The nibs 8 are attached to and project upwardly from the metal wall 4, the nibs 8 passing through registering holes in the printed circuit board 7. The upper ends of the nibs 8 are soldered to a metal coating 9 which covers the outer region of the upper surface of the printed circuit board 7. Between the outer conductor 5 and the coating 9 is an uncoated electrically insulating area 10 which is annular in shape and which has a radial dimension of about 3mm.

To provide conduction of radio frequency signals from the outer conductor 5 to the casing 1, while providing safety isolation for d. c. voltages and voltages at comparatively low frequencies (including mains), the outer conductor 5 and the metal coating 9 are spanned by ten radial chip capacitors 12 each of 1 nano Farad. Thus, the capacitors 12 extend radially from the outer conductor 5 in a star configuration but, in terms of electrical connection, form a parallel array of isolating capacitors.

The ten capacitors 12 are mounted on the upper surface of the board 7, being covered and protected by a square plastics cover 11 which is located and secured in the recess, with the upper surface of the cover 11 being generally flush with the surrounding wall 2.

The inner conductor 6 is longer than the outer conductor 5 and extends through a central hole 13 in the printed circuit board 7, the length of inner conductor 6 within the casing 1 being surrounded by a ferrite sleeve 14. The extremity of the inner conductor 6 passes through a hole in an internal printed circuit board 15 carrying further chip capacitors 16 through which radio frequency (but not d. c. or low frequency signals) are conducted to the inner conductor of a coaxial output terminal 17 mounted in the opposite wall 18 of the casing 1. The board 15 is supported in the casing 1 and spans the shorter pair of walls illustrated in Figure 1, but the board 15 does not span the pair of casing walls lying parallel to the plane of Figure 1, so the internal space within the casing is essentially undivided.

Voltage isolation is thus provided for both the inner and outer conductors, combined with high screening effectiveness.

The described isolator has the following advantages: 1. Easy automated assembly of the capacitors on the printed circuit board by means of surface mount technology.

2. Excellent screening effectiveness.

3. Avoidance of complex cable assembly or complex capacitor assembly.

4. A single metal casing with one internal compartment and no complex mechanical structures.

5. No complex wiring assembly.

The described isolator is for supplying a signal to a television set. A distribution network supplies a broadband signal to the conductors 5,6 and the television set is plugged into the coaxial output terminal 17. It will be appreciated that the distribution network could provide a composite broad band signal including a cable television component and a frequency modulated radio component, in which case the input conductor would supply two output sockets one for cable television and the other for frequency modulated radio.

Claims

CLAIMS 1. An isolator compris : ng a metal casing having an inlet aperture, a coaxial connector mounted on a board attached to the casing to cover the aperture, the coaxial connector having an inner conductor and an outer conductor, the outer conductor being electrically connected to the casing by a plurality of capacitors mounted on the board, each capacitor being connected between the outer conductor and the casing such that the resulting array of capacitors conducts radio frequencies but provides isolation for d. c. voltages or voltages at mains frequency.
2. An isolator according to claim 1, wherein the board is a printed circuit board carrying an electrically conductive coating electrically connected to the casing, the outer conductor and the coating being separated by an intervening area of the board which is electrically non-conducting and across which the capacitors are electrically connected.
3. An isolator according to claim 2, wherein the array of capacitors is physically arranged in a star configuration to extend radially outwardly from the outer conductor to the coating, the array of capacitors being electrically connected in parallel, in electrical circuit terms.
4. An isolator according to claim 2 or 3, wherein the inner conductor extends through the printed circuit board and into the metal casing, the projecting end of the inner conductor being surrounded by a ferrite sleeve.
5. An isolator according to any of the preceding claims, wherein the inlet aperture is formed by an area which is recessed below the surrounding wall of the casing, the printed circuit board being accommodated in the recessed area.
6. An isolator according to claim 5, wherein the recessed area is sufficiently deep to accommodate the printed circuit board, together with the capacitors thereon, and a protective cover which has an outer surface flush or substantially flush with the surrounding wall of the casing.
7. An isolator according to claim 5 or 6, wherein the coating on the printed circuit board is electrically connected to the casing by means of metal projections which project upwardly from the recessed area of the casing, extend through registering holes in the printed circuit board and are electrically connected to the electrically conductive coating.
8. An isolator according to claim 7, wherein the metal projections are in the form of nibs.
9. An isolator according to claim 7 or 8, wherein the metal projections are electrically connected to the coating by soldering.
10. An isolator constructed and arranged substantially as herein particularly described with reference to the accompanying drawings.