GB2179502A

GB2179502A - Radio frequency choke

Info

Publication number: GB2179502A
Application number: GB08619982A
Authority: GB
Inventors: Richard C Marshall
Original assignee: EMC Datacare Ltd
Current assignee: EMC Datacare Ltd
Priority date: 1985-08-22
Filing date: 1986-08-15
Publication date: 1987-03-04
Also published as: GB8619982D0; GB8520997D0

Abstract

A magnetic core 1 is arranged in portions that may be secured around a cable 5 so as to impede the flow of common-mode high frequency current. The portions are aligned by clamps 2 and secured by a strap 3. A number of cores may be assembled together to provide increased impedance. <IMAGE>

Description

SPECIFICATION Radio-frequency choke The invention relates to inductive devices intended to provide a high impedance to the flow of radio-frequency currents. Such devices, commonly called "radio frequency chokes" are used to impede the flow of highfrequency electric currents in cables, so as to prevent electrical interference.

Signals may be propagated along a multicore cable either in differential mode (wherein the algebraic sum of the flow in the various conductors is zero) or in common mode, wherein a non-zero algebraic current flows along the cable and returns to the source by some other route. Intended signals are usually propagated in differential mode, whereas unwanted interference signals are propagated in common mode. The choke described herein is intended to impede common-mode current flow without disturbing differential-mode signals.

It is well known that common-mode currents may be reduced by winding all the conductors of the cable in the same direction on a common magnetic core. See, for example, "Electronic Engineers Reference Book", 5th Edition, edited by F F Mazda, Figs. 50.19(b) and 50.23(c). It is also well known that this core may be a toroidal ring core of magnetic ferrite material. However, toroidal cores are difficult to wind with bulky or semi-rigid cables, and so special windings with associated interconnections have been used. In the publication CISPR16 (1977) of the International Electrotechnical Commission there is described in section 32 and in Figs. 15 and 16 a measuring system utilizing split toroidal core. In contrast to this the present invention provides a kit of parts adapted for easy installation on a wide variety of cable sizes and allowing an appropriately variable number of turns.

According to the present invention there is provided a kit of parts for forming a highfrequency choke in co-operation with an electric cable, said kit including one or more magnetic cores each having the topology of a ring that is split into portions so as to facilitate fitment around one or more turns of said cable, and means for securing together the portions of each magnetic core.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is an exploded perspective view of a first embodiment of a kit according to the invention; Figures 2 and 3 are perspective views of assembled alternative embodiments of kits according to the invention; Figure 4 shows how a plurality of the components in Fig. 2 may be assembled according to one aspect of the invention; Figures 5, 6 and 7 show how a high-frequency choke may be formed from kits according to the invention co-operating with electric cables of various kinds; and Figure 8 shows how a flat cable may be folded so that it may be accommodated within a kit according to the invention.

Fig. 1 is an exploded view in which two Ushaped core portions 1 may be clearly seen.

According to the frequency range required these may comprise manganese-zinc or nickelzinc ferrite, or powdered iron material. To exploit the intrinsic magnetic properties of the core portions their mating faces are aligned by the channel-shaped members 2, which may be fabricated from a plastic extrusion. Said faces are forced into contact by the plastic tie 3, which may also be used to mount the assembly by threading it through holes 8 in a convenient structural sheet. When a plurality of components are assembled together (as shown in Fig. 6) they may be aligned by double-sided adhesive pads positioned at 4.

Referring to Fig. 2, a pair of U-shaped core portions 1 are secured together by a pair of identical, interlocking, retaining members 9.

The retaining members 9 may be injection moulded from a plastics material, and each comprise a channel-shaped portion 14, for containing the arm of the core portion 1, and an extension 15 which before assembly may extend generally parallel with the channelshaped portion 14. The extension 15 may be hinged to the channel-shaped portion at the corner 16, for example by means of a thinning of the plastics materials at this point, so as to simplify the manufacture of said retaining member. The extension 15 may thus be bent around the base of the core portion 9 as shown. A retaining tab 17 at the end of the channel-shaped portion remote from the extension 15 is secured in a slot 18 formed in the end of the extension 15 of the other retaining member 9.The extension 15 is bowed inwardly to provide a central part 19 which acts like a leaf spring when the two retaining members are secured together, urging the mating surfaces of the core portions 9 into close contact with one another.

Inward protruberances 10 on the outer edges of the channel-shaped portions 14 serve to retain each core portion 1 in its retaining member so as to facilitate assembly.

The central part 19 of each extension 15 is formed with a peg 12 on one side and a corresponding recess 13 on the other side, the pegs and recesses co-operating to align and secure together a plurality of the assembled chokes when desired.

Fig. 3 shows an embodiment in which the magnetic core portions 1 are of a semi-cylindrical tube shape that is preferred for use with a straight electric cable.

The core-portions are aligned and retained by adhesive tape 11.

Fig. 4 shows how a number of the assembled chokes of Fig. 2 may be aligned and secured together by pegs 12 co-operating with recesses 13.

Fig. 5 shows how a number of loops may be formed in a cable 5, and the components of Fig. 1 assembled over the loops. The impedance is closely proportional to N2C where N is the number of turns and C the number of cores. Thus, 4 turns on 1 core-pair has the same impedance as 1 turn on 16 core-pairs; clearly it is most effective to have the largest possible number of turns.

Fig. 6 shows how a number of core-pairs may be assembled together to provide increased impedance when the cable is too large to allow a winding of many turns. The core-pairs may be held together by doublesided adhesive pads positioned as shown at 4 in Fig. 1. Over a wide range of frequencies the losses in ferrite material result in a constant impedance region. The phase angle will vary due to resonance within the material.

However, the effect of this is minimised by the use of a number of co-operating core pairs spaced slightly apart as shown in Figs. 4 and 6.

At very high frequencies the impedance is strongly affected by the shunt capacitance, which is itself a function of the arrangement of the winding. To get maximum high-frequency performance with windings of more than six turns these should be divided between the two legs of the core, with the leads 6 and 7 diagonally opposite, as shown in Fig. 7.

Fig. 8 shows how a flat cable 20 can be arranged to co-operate with the choke kit of the invention (not shown). The cable, which it is assumed is too wide to pass through the assembled core, is folded longitudinally over an appropriate part of its length, then formed into a loop, as shown. A choke is then assembled around a folded part of the cable.

Claims

1. A kit of parts for forming a high-frequency choke in co-operation with an electric cable, said kit including one or more magnetic cores each having the topology of a ring that is split into portions so as to facilitate fitment around one or more turns of said cable, and means for securing together the portions of each magnetic core.

2. A kit according to claim 1 wherein each magnetic core comprises a pair of rectangular or semi-circular 'U' section portions.

3. A kit according to claim 1 or 2 wherein said securing means comprise channel-shaped sections that grip the core portions so as to align their faces at the split line.

4. A kit according to claim 3 wherein an additional member provides a force to maintain contact between the core faces at the split line.

5. A kit according to claim 3 wherein the channel-shaped members are adapted to provide a force perpendicular to the split line so as to maintain contact between the core faces.

6. A kit according to claims 3, 4 or 5 wherein channel-shaped members co-operate by adhesive or mechanical means to form into a single assembly a plurality ff magnetic cores.

7. A kit according to claims 1 or 2 including adhesive members for securing together a plurality of cores.

8. A kit according to any of the preceeding claims wherein the magnetic core is made of a manganese-zinc ferrite material.

9. A kit according to any of the preceeding claims wherein the magnetic core is made of a nickel-zinc ferrite material.

10. A kit according to any of the preceeding claims wherein the magnetic core is made from iron powder.

11. A kit comprising parts substantially in accordance with Figs. 1, 2 or 3 of the accompanying drawing.

12. A high-frequency choke formed from a kit according to any of the preceeding claims co-operating with an electric cable of any kind.