GB2403072A - Electrical isolator - Google Patents

Abstract

An electrical isolator for protecting at least one electronic device (not shown) comprises a housing 5 into which the device is placed and which serves as an electromagnetic interference (EMI) shield. A connector 1, which extends through a gap in the housing, comprises inner and outer conductors 12, 3 in electrical contact with the device and separated from each other by insulators 2, 11. A first ferrite spacer 16 is provided which surrounds said connector and plugs a gap between said connector and said housing and thus impedes EMI from entering through said gap. Capacitors 8, 13 may be provided to attenuate electrical noise entering the housing via the conductor. A second ferrite spacer 10 may also be provided. Also claimed is a device comprising the said electrical isolator and an electrical circuit such as a TV signal splitting circuit.

Description

ELECTRICAL ISOLATOR

FIELD OF THE IWION

The present invention relates to an electrical isolator' which is specially adapted to protect an electronic device from Electromagnetic Interference (MI) and conducted electrical noise in external wiring.

BACKaRo OF LION The exposure of electronic devices to Electromagnetic Interference (EMI) is undesirable and can adversely affect their performance. One method of reduclug the amount of EMI exposure 4s to place electronic devices within housings that function as electromagnetic shields Coaxial cable wiring is used for carrying signals in aenaitiva electronic systems to further reduce noise.

Coaxial cables have an loner and outer conductor separated by electrical insulating material, and are generally circular in cross section. The inner conductor carrisa the electrical signal whereas the outer conductor provides the return path and is typically grounded. The outer conductor completely surrounds the loner conductor thereby shielding it from EMI. The outer conductor, however, la susceptible to conducted BMI whereby electromagnetic waves in Eros apace are converted to circuit currents which contribute to the amount of e1ectrlcal noise in the system.

Electrically connecting the outer conductor of the coaxial cable to tho device housing, thereby grounding the houalug, serves to reduce the noise entering the housing in the outer conductor. Alternatively, coupling the outer conductor to the device housing with a capacitor has the added benefit of blocking OC voltage thereby ) - 2 providing protection against lighting atrikea. Devices having such arrangemntB are termed electrical isolators.

We have previously proposed the electrical S isolator, disclosed in Chinese patent 2h 9$,122,929.2. The electrical isolator disclosed in this patent has a housing coupled to the outer conductor of a connector by capacitors. The connector has an analogous structure to coaxial cable' that is there is provided an outer conductor and an inner conductor separated by insulation material. The electronic device to be protected ia located within the housing.

The connector extends through an aperture in the housing and ha. a termination which can be connected to coaxial cable on the outside of the housing. The connector is positioned within the aperture by a plastic spacer which fills the gap between the connector and the housing.

The foregoing electrical isolator is effective for attenuating conducted electrical nodal entering the housing in the outer conductor of the connector, however, it has a drawback whereby EMI freely enters the housing through tho plastic spacer. Accordingly, there is a need for an electrical isolator which further reduces the amount of EMI entering the housing. - 3

SUMMARY OF INVENTION

According to the present invention, there i s provided an electrical isolator for protecting at least one electronic device, aald isolator having: a housing which functions as an electromagnetic shields a connector extending through an aperture in said housing, said connector having an inner conductor and an outer conductor separated by electrical insulator material, acid inner conductor and said outer conductor being electrically connected to aald at least one electronic device in use; and a [irat ferrite spacer which surrounds said connector and plugs a gap between aald connector and said housing, whereby said fires ferrite spacer impedes electromagnetic Interference (ELI) from entering said housing through said gap.

Preferably, acid houalug is an electrical conductor and said electrical isolator further comprlaes a first capacitor which electrically couples said housing to said outer conductor, aald first ferrite spacer lutroducing a flrat Impedance in said outer conductor, and said first capacitor combines with aald first impedance and aald housing to form an electrical network for attenuating electrical nolao anterlug said housing in said outer conductor.

BRIEF DESCRIPTION OF D=WXNGS

preferred embodiment of the invention will now be described in relation to the accompanylug drawings in which: Plgure 1 is a cross sectional view of an electrical

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isolator according to a first embodiment of the present invention; Figure 2 shows an equivalent electrical circuit of the electrical isolator of Figure l; Figure 3 shows an equivalent electrical circuit of an electrical isolator accordlug to a second embodiment of the preasut invention; and Figure 4 ahowa an equivalent electrical circuit of an electrical Relator according to a third embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

According to a first embodiment of the present tnvention' the electrical isolator has a housing 5 with both electromagnetic shielding and electrical conductivity properties. Alumnium housings are well sulted for this purpose. An electronic device (not shown) to be protected is situated within the housing 5. As shown in Flg. l, the housing 5 has an aperture through which a connector 1 extende. The aperture is a circular hole. The connector 1 is a F-type connector having a circular cross section, and consists of an inner conductor 12 and an outer conductor 3 separated by insulating material 2,11. A high voltage 2s annular plastic ring 4 surrounds the outer conductor 3 and is countersunk into the outer surface of the housing 5.

The inner surface of the plastic ring 4 abuts the outer conductor 3 thereby positioning connector 1 centrally in the aperture.

The outer conductor 3 and inner conductor 12 of the connector 1 are both electrically connected to the electronic device within the housing 5. These connections can be either direct or via coupling components such as capacitors or inductors.

A first annular ferrite spacer 16 surrounds the - 5 outer conductor 3 thereby effectively plugging the gap between the outer conductor 3 and the housing created by the aperture. The inner surface of the first ferrite spacer 16 abuts the outer conductor 3. A first portion of the outer surface of the first ferrite spacer 16 abuts the housing 5. Ferrite material has very good electromagnetic shielding propertiea. External EMI iB therefore impeded from entering the housing 5 through the aperture which is plugged with the first ferrite spacer 16. The first ferrite spacer 16 additionally asaiata with positioning connector 1 centrally in the aperture. r

A second annular ferrite spacer lo aurrounde the outer conductor 3 within the houalug 5 with an inner surface abutting the outer conductor 3.

first capacitor 8 has a fires electrode electrically connected to the housing 5 and a second electrode olectrically connected to the outer conductor 3 at a polat between the first forrito spacer 16 and second ferrite epacor 10. The first capacitor 8 is a high voltage rlug ceramic capacitor and aurroundn a second portion of the outer curved surface of the float ferrite spacer 16 which extends inside the housing 5. There is an outer conductive shell 6 separated from the fires capacitor B by insulation material 7. The second portion of the outer curved surface of the first ferrite spacer 16 abuse the inner curved surface of the fleet capacitor 8.

A screw It applies pressure to the first capacitor such that: the outer conductive shell 6 madntalns contact with the housing 5 thereby establishing a first electrical connoctlon; the first electrode of the first capacitor 8 maintains contact with the housing 5 thereby oatabllehing a second electrical connection' and the second electrode of the fires capacitor 8 maintains contact with the outer conductor 3 thereby eatabliahing a - 6 third electrical connection.

A second capacitor 13 has a first electrode electrically connected to the housing 5, and a second electrode electrically connected to the outer conductor 3 at a point on the aide of the second ferrite spacer 10 which is distal from the first ferrite spacer 16. In Fig. 1, the first electrode of the second capacitor 13 is connected to the outer conductive shell 6 of the first capacitor 8 which, in turn, is electrically connected to ? th. housing 5.

The equivalent electrical network of the first embodiment is shown in Fig. 2. Point A correapoD-da to a lS point on the outer conductor on the aide of the second ferrite spacer 10 which is distal from the firat ferrite spacer 16. Poi nt B corresponda to a pot nt on the outer conductor 3 outside the housing 5, Point G corresponds deco a point on the housing 5, Capacitor C' correeponda to the firat capacitor 8 ant capacitor C13 correaponda to the second capacitor 13. Capacitors have a lower lmpence at higher frequencisa and therefore more readily couple noise into the housing 5.

Ferrite rings surrounding a wile carrying a signal, ntroduce an impedance to that wire which varica according to the frequency of the algnal. The effective impedance introduced by a ferrite increases with increasing frequency of the signal and therefore higher frequencies are attenuated by a greater degree. Therefore high frequency noise la attenuated to a greater extent than desired signals of lower freguencisa. Impedance Z16 corresponda to the impedance introduced by the first ferrite 16 and impedance Z1O corresponds to the impedance introduced by the second ferrite spacer 10.

Electrical aignale are attenuated by the - 7 olectrical network of Fig. 2 as they pass from point B to polut A in the outer conductor 3. The higher the signal frequency, the greater the signal attenuations The network of Fig. 2 is therefore effectively a low pasa filter.

According to a second embodiment of the present invention, the electrical isolator has a plurality of ferrite spacers and capacitors thereby formlug the eloctrlcal ladder network shown in Pig. 3. A nth order ladder network ca" be formed from n capacitors and D ferrite pacers, where 1a an lutege.

According to third embodiment of the preasut invention, the electrical network is formed by a first ferrite spacer 16 and a first capacitor 8 only, as shown in Fig. 4. That is, the second ferrite spacer 10 and the second capacitor 13 of the fires preferred embodiment is not included in the electrical isolators Alternatively, the electronic device to be protected can be alectrlcally connected to the outer conductor 3 at a point between the first ferrite spacer 16 and second ferrite spacer 10 thereby yielding the effective electrical circuit of Fig. it.

The preasut laventlon is well suited for isolator terminals in CAT Y systems and broadband communication syatame, and can provide up to lOdB reduced ELI over the prior art at frequencies greater than lOOMHz. TV/FM splitters utilize such isolators operating between 5- lO0OMHz and have a aingle isolated input connector, and two aspirate output connectors for TV and PM signals respectively.

Various modifications will be apparent to peracns skilled in the art, for example, the plastic ring may Extend further into the aperture, or the housing may be formed from other suitable materials. - 8

Such madificationa Should be considered ae falling within the Scope of the invention. - 9 -

Claims (1)

1. THE CLAIMS DEFINING THg ICON BE AS POLLOWS 1. An electrical isolator for

   protecting at least one electronic devices said isolator having: a housing which functions as an electromagnetic shield, a connector extending through an aperture in said housing, arid connector having an inner conductor and an outer conductor separated by electrical insulator r: material, card inner conductor and said outer conductor being electrically connected to said at least one electronic device in use; and a firat ferrite spacer which surrounds said connector and plugs a gap between said connector and said housing, whereby said first ferrite spacer impedes electromagnetic luterference (EMI) from ontering arid housing through said gap.

   2. An electrical isolator as claimed in claim 1, wherein said housing ie an electrical conductor and said electrical isolator further co2riaea a first capacitor which electrically couples acid housing to said outer conductor, said first fertlto spacer introducing a first impedance in said outer conductor, and said first capacitor combines with acid first impedance And said housing to form an electrical network for attenuatlug electrical noise entering maid housing in arid outer conductor.

   3. The aicatrical isolator as clammed in claim 2 having second ferrite spacer, eald second ferrite spacer surroundlug said outer conductor within said housing, wherein said second ferrite spacer introduces a second impedance in said outer conductor thereby increasing the effective impedance of said outer conductor. À 10

   dO The electrical isolator as claimed in claim 2 wherela.

   said first capacitor is electrically connected to said housing by a first electrode, and has a second electrode electrically connected to acid outer conductor at a point between said {irat ferrite and said second ferrite; and said electrical isolator further compriaea a second capacitor with a first electrode electrically connected to said houslug, and a second electrode electrically connected to asid outer conductor at a point on a aide of said second ferrite spacer which is distal from said first ferrite spacer.

   5. The electrical isolator device as claimed in claim 4, having at least one additional capacitor and at least one additional ferrite to thereby form a ladder network of ferrite impedances and capacitora.

   6. The electrical isolator device as clammed in claim 2 whereto said network is a low pass filter.

   7, The electrical isolator device as claimed in claim 1 having at least one addlt$onal connector.

   8. device comprlaing: eloctrical laolator as claimed in any one of claims 1 to 7; and an electronic circuit.

   9. A device as clammed in claim when appended to claim 7, wherein the electronic circuit is a TV signal splitting circuit.