GB1600395A - High frequency screening of electrical systems - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO HIGH FREQUENCY SCREENING OF ELECTRICAL SYSTEMS (71) We, UNITED KINGDOM ATOMIC ENERGY AUTHORITY, London, a British Authority do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to high frequency screening of electrical systems. The importance of screening against extraneous noise in an industrial environment is well recognised with the result that component design and layout aims at high efficiency screening which is quantified by a low transfer impedance across the conducting members forming the screen surrounding a sensitive circuit. The present invention is concerned with preserving this property where an otherwise continuous screen is interrupted for either connection to a further screen as in connections between cables and components or connection to a terminal screen structure such as between a component screen and a closure plate. Present techniques which involve careful design and manufacture between the mating faces of the screens at the interface have been discussed in a paper entitled Screened Coaxial Cable Connectors for High Sensitivity Systems by E P Fowler presented at the IEEE Symposium on Electromagnetic Compatibility at Montreaux in May 1975. In this respect reference should first be made to Figures 1 and 2a, 2b, 3a, 3b, of the drawings accompanying the provisional specification filed with Application No. 21911/77. of which Figure 1 (a reproduction of Figure 3 of the above mentioned IEEE Paper) shows an axial cross section of the separated parts of the mating faces of the screen of a conventional coaxial connector.

Figures 2a 2b 3a 3b are approximate equivalent circuits describing the transfer impedance defined later. In Figure 1 the split skirt 1 of the right hand part of the connector screen is axially aligned but withdrawn from socket screen 2. When mated, electrical contact is ade along the two circumferential contact lines which are as indicated at 3a 3b and refer wed to below as contact rings or contact paths. To provide a low impedance and giv good screening a substantial axial pressure is usually applied to the mated parts by earns of screw-threaded locking ring (not shown). It should be emphasised as stated ab,bve, that screening effectiveness is governed in the main by transfer impedance (symbol/L,1). The The latter property relates the voltage generated in the screened circuit formed by the connector inner conductor (not shown in Figure 1) and the connector outer screen to the disturbing current flowing only in the connector outer screen.

In Figure 2(a) is shown a complete equivalent circuit for a coaxial connector which is being disturbed by a current I, so resulting in a voltage V5 being generated in the screened circuit. The quotient of V5 and I1 is defined as the transfer impedance (symbcll Z21) of the connector. An essential feature of any coaxial system is that the greater part of the inductance of the outer conductor at 6 is coupled to the inner conductor at 7. Circuit elements 8 and 9 represent the distributed inductance and capacitance of the screened circuit. Part of the voltage Vs generated in the screened circuit appears across each load impedance 23, 24 which complete the circuit but are not relevant in the description. The disturbing current Ii from any external generator 10 flowing in the outer conductor (of which the connector forms part), generates voltages across the contact resistance 4 and the reactive impedance 5. This reactive impedance is described as uncoupled inductance and occurs if the outer conductor at the connector is not circumferentially uniform. The same disturbing current flowing in the coupled outer conductor, inductance 6 will generate equal voltages across 6 and 7 and so have no effect on the screened circuit. Therefore the equivalent circuit can be considerably simplified by repositioning 6 in the disturbing circuit and completely omitting 7. By also omitting 8 and 9 which contribute nothing to the screening, the simplified equivalent circuit of Figure 2(b) is obtained with the transfer impedance (Z21) simply the impedance of 4 and 5 in series.

Good screening performance of the coaxial connector as described above can be achieved if the contact resistance is sufficiently low and the contact adequately uniform all round its circumference. Extremely high axial pressure on the mating faces can impose this desirable state, but it is now appreciated that another factor makes a significant contribution. Close examination of Figure 1 shows that the mating face has not one but two possible concentric contact paths at 3a 3b. The inductive impedance of circuits through these two parallel contact paths will differ by a small but significant amount. A more accurate equivalent circuit is given in Figure 3(a) in which 11 represents the contact resistance of the outer contact path 3b and 12 its uncoupled inductance. Similarly 13 represents the contact resistance of the inner contact path 3a and 14 its uncoupled inductance. The coupled inductance 6 is the same as the coupled inductance in Figure 2(a) but is here coupled to the inner contact path 3a at 15 as well as to the inner conductor at 7. A further inductance 16 on the inner contact path 3a is coupled only to the inner conductor at 17 and represents the difference of the inductive impedance referred to above. As before the screened circuit is completed with load impedances 23, 24. Figure 3(a) can be simplified by eliminating the coupled inductances as in Figure 3(b) whence it is readily seen that the magnitude of inductance 16 can play a large part in governing the quotient V which is the transfer impedance Z2l.

Il If the symbol Z1 is used for the impedance of 11 and 12, Z2 for the impedance of 13 and 14 and ZM for the impedance of the coupled inductance 16, then it can be shown that: Z1 = Z' Z2 Z1 + Z2 + ZM and that increase in the impedance of 16 will reduce Z21 and hence improve screening. The inductance 16 in Figure 3 has a value dependent on the axial length of the two contact paths 3a 3b, where these are separate, and on their diameter ratio. In Figure 1 the gap between concentric rings of contact is comparable with the thickness of the split skirt 1 and the axial length of the gap is very short.

According to one aspect of the invention there is provided an interconnection between two parts of an annular electrically conductive screen incorporating a means of reducing the magnetic reluctance of the magnetic path between inner and outer surfaces of the screen at the region of the interconnection. Significant improvement in connector screening is possible by deliberate separation of the two contact rings combined with a reasonable length of separate conducting paths. Much more improvement is obtained when the reluctance of the magnetic flux path involved is reduced resulting in greatly increased conductance. This can be achieved by inserting high permeability magnetic materials such as a small toroid of laminated mu-metal in the volume between the circuits. The interconnection between the two parts is electrically conductive along two, coaxial or concentric contact paths physically spaced apart and electrically commoned at each end and ferromagnetic material is located between said contact paths to reduce the reluctance of the magnetic path between them.

The effect is to increase the inductive impedance of the inner contact "tube" thereby forcing a large part of the disturbing current to flow in the outer concentric "tube".

Although here described in terms of the improved screening to disturbing current flowing in the connector screen, the principle of superposition can be applied to show that it will be equally effective in guarding against the egress of signal from the screened circuit.

Advantageously the magnetic material is of laminated form and conveniently this may be a toroid formed from tape of mu-metal or other high permeability material. In some embodiments of the invention the toroid is introduced at the interface in an annulus existing between the two concentric tubes forming the end face of one part of the connector, the said end face being of that form to effect mechanical inter-connection of the screened parts.

The invention also relates to a method and means for securing the outer, wire braid, conductor of a screened co-axial cable with two or more braid layers to a co-axial cable connector or to a terminal. The securing means which should afford a good mechanical connection is generally termed a cable clamp, and the electrical connection should afford a good screen to the cable in terms of the above-mentioned transfer impedance. The outer conductor of a well screened co-axial cable to which the invention relates incorporates screening in the form of two or more co-axial braids with interlayers of thin magnetic tape interposed between the wire braid layers. In electrical circuits where a high degree of screening is desirable or necessary, there is a risk that, at the region of the cable clamp, the screening may be degraded below that obtaining elsewhere.

The present invention therefore includes in another aspect a method of electrically connecting the outer conductor of a screened co-axial cable to a co-axial cable connector or to a terminal, and resides in making two connections between the outer conductor of the cable and the connector or component terminal and interposing a magnetic toroid between said connections. The effect is to cause any high frequency disturbing currents which flow in the outer wire braid conductor of the cable to flow through the connection made between the outer co-axial braid connection and the connector or component terminal whilst the inner braid connection forms the screened circuit. It is unnecessary therefore for each braid to connector, or component terminal connection to be a high performance connection, hence a low pressure co-axial contact for each braid connection becomes acceptable, the principle and equivalent circuit being described in more detail in relation to said one aspect of the present invention.

One simple method of implementing this aspect of the invention is to form a toroid of magnetic material around the inner braid from a length of deformed magnetic tape stripped from a length of the cable. The toroid exhibits resilience in a radial sense and this is used to hold the inner braid, which is folded back over the outside of the toroid, in co-axial contact with the bore of the hole in the connector which accepts the cable at its point of entry. The outer co-axial braid connection is made in a similar manner using the springiness of the braid wires and/or other part of the joint to maintain contact to the bore of the aforesaid hole, the hole being dimensioned appropriately.

Co-axial cables used with high sensitivity instruments enjoy a higher shielding efficiency where three layers of braid are employed in the outer conductor, interleaved with two layers of mu-metal tape. This is the form of cable which is described below in the exemplary embodiments of this aspect of the invention. With this cable the inner braids are treated as above. Good mechanical retention of the cable may be achieved by crimping a metal sleeve over the outer part of the connector and the outer two of the braid layers.

Embodiments of both aspects of the invention will now be described by way of example only with reference to the drawings accompanying the provisional specification filed with application No. 21911/77 in which: Figure 4 is an axial cross section of a screened coaxial connector on which the screens interengage by two coaxial split skirts, Figure 5 shows a cross section, similar to Figure 4, in which one mating part has an annular socket formed between the two coaxial rings or split fingers which mates with an annular plug of the other part; Figure 6 is an axial cross section of a design applicable to a small screening box or a large diameter connector screen and Figure 7 is a graph showing the comparative screening performance of the screen of Figure 6 with and without the magnetic toroid. and, the drawings accompanying the provisional specification filed with patent application No 30752/77 in which: Figures 1 to 5 are similar views in axial cross-section of different forms of inter-connection between a screened co-axial cable and co-axial connector. The Figures show connection to a triple braided cable in all cases. The connection is valid for all cables with two or more braids with or without the distributed interleaf of magnetic material. In applying the invention to a double braided cable the arrangements shown in Figures 1, 2, 4 and 5 omit the outer braid and tape. When omitting the middle braid and outer tape the arrangement shown in Figure 3 would not be used. Where more than three conducting braids are present the additional braids may be considered as either middle or outer braids. In the drawing the same reference numerals are used for the same parts in each Figure. The same form of cable is employed throughout although other forms of cable may be used at the discretion of those of appropriate skill.

Referring firstly to Figure 4 of application No. 21911/77, the plug and socket connector shown has two outer screen parts 25 26 coaxial with an inner conductor 27 28. One part 25 terminates in two solid skirts 25a 25b which engage with two parallel split skirts 26a, 26b of part 26 with a push fit. The free ends of the skirts of the right hand part 26 are so shaped as to make ring contacts 29, 30 with the inner circumference of the skirts 25a 25b of the left hand half 25. A toroid of laminated mu-metal tape 31 is retained between the split skirts 26a, 26b.

Referring now to Figure 5 the plug and socket connector depicted here in a decoupled condition employs two coaxial rings of split fingers 32, 33 on the left hand half arranged to define an annular socket to be engaged by single tube 34 on the mating right hand half of the connector simultaneously with the plug 36 and socket 35 inter-engagement of the inner conductor. A toroid 37 of mu-metal tape is retained at the base of the recess formed between the coaxial rings of fingers 32, 33. When the connector is engaged, two contact rings are formed at 38, 39.

In Figure 6, the cylinder 40 shows part of a right cylindrical screen of a screened enclosure 4()1 only part of which is shown. The base is closed by a circular cup 41 within which are ring contacts 42 and 43 of a resilient conductive material.

The ring contacts 42, 43 are spaced axially in a recess in the cup 41. In the same recess between the rings is located a toroid 44 of magnetic material. Preferably the toroid 44 is of laminated construction being formed from mu-metal tape.

Figure 7 is a graph showing transfer impedance Z21 (in ohms) against frequency for the enclosure 40a sketched in Figure 6. Curve A of Figure 7 shows the transfer impedance without magnetic material in Figure 6 while curve B shows the transfer impedance with the magnetic material present and demonstrates the lower transfer impedance which comes from incorporating the magnetic toroid. The improvement is such as to obviate the need of applying axial force to the connector at the interface which is otherwise found necessary to obtain good shielding.

Referring now to Figures 1 to 5 of application No. 30752/77, the cable 1 comprises a centre conductor 2 insulated by a layer of insulation 3 from an outer conductor and screening feature 4. The cable's outer cover is indicated at 5; for the present purposes metal wire braid layers 14, 15, 16 are to be regarded as the outer conductor in conjunction with metal tape layers 17, 18.

The drawings show only the rear end of the cable connector 6 for receiving the centre conductor 2 and, to which connector, the feature 4 is to be connected. In Figures 1 and 2 the rear end of the connector has a counterbore 7 whose internal shoulder is machined to an annular knife edge 8. An internal screwthread is formed at 9. An externally threaded metal back nut 10 screws into the screwthread at 9 and urges the end face of a ferrule 11 against the knife edge 8 to give good coaxial electrical contact and hence a good electrical screen.

This is a well known technique used on several connectors. A small diameter hole 12 in the front of the ferrule 11 leads the insulated centre conductor 2 into the body of the connector 6 whilst in Figure 1 an enlarged diameter rear portion 13 of ferrule 11 receives the outer conductor and screen feature 4.

The feature 4 is common to Figures 1-5 and terminates in a specially prepared end of the co-axial cable. In more detail the co-axial cable comprises three co-axial tubular layers of copper wire braid 14, 15, 16 interleaved by layers 17 and 18 of mu-metal tape formed from partially overlapping helical turns, each layer being applied in a manner which leaves clearances (not shown specifically) between the tape layer 17 and the underlying braid 14.

Such a cable is described more fully in UK Patent No. 1,448,820. Referring firstly to Figure 1, prior to the entry of the outer conductor and shield feature 4 into the larger diameter bore portion 13 of the ferrule 11 a significant proportion of the unwrapped turns of the tape layer 17 are superimposed at 19 and having been very slightly bowed in their initial application to the braid 14, the superimposed turns exhibit resilience in a radial sense with respect to the cable axis. The underlying braid layer 14 is then folded back over the outermost of the superimposed turns care being taken to ensure that the ends of braid 14 cannot touch the braid at 30. The centre braid layer 15 is folded back over both the enclosing tape layer 18 and outer braid layer 16. Thus prepared, the outer conductor and screen feature 4 is radially compressed manually and entered into the enlarged bore of the ferrule where two rings of contact will be maintained by the outward spring force of the superimposed tape turns 19 pressing the braid 14, against the bore of portion 13 and braid 15 being trapped between the bore 13 and braid 16. Retention is assisted by the inner conductor 2 which engages plug/socket fashion with a mating part of the connector (not shown). Any suitable means may be used to effect more positive retention.

The remaining embodiments demonstrate modified constructions which incorporate a more positive means of retention. In all cases however the presence of a substantial volume of mu-metal tape adjacent the contact interface reduces the transfer impedance over a large frequency range thereby lessening the risk of degrading the screening efficiency at a location where a discontinuity of the cable screen occurs.

In Figure 2 the ferrule 11 has a parallel bore and the adjacent end of the feature 4, prepared as before, abuts the end face of the ferrule 11. The superimposed tape turns 19 are secured by means of a copper sleeve 20. The sleeve 20 is crimped at 21 over a knurled end portion of the ferrule 11 which here has its outer diameter suitably reduced to enable a satisfactory crimp of the copper sleeve to be achieved. The sleeve receives the prepared end of feature 4 and is crimped at 22 at its end remote from the ferrule where centre braid 15 is back folded over the outer braid 16.

The embodiment shown in Figure 3 omits the knife edge contact 8, ferrule 11 and back nut 10. Both the inner conductor 2 and the outer conductor 4 enter the bore 7 in the connector 6 and the resilience of the superimposed tape layers 19 urges the inner braid 14 into contact with the bore. The rear end of the connector 6 has a portion 6a of reduced external diameter with an end chamfer at 6b. The middle braid 15 is folded back at 23 over an annular resilient distance piece 24 which maintains contact between the braid 15 and the bore 7. The outer braid 16 is led over the chamfered end 66 of the portion 6a on to its outer surface. The braid 16 and is clamped to the outer surface of portion 6a of the connector by a copper sleeve 26 by the application of a crimping tool. The same tool crimps the same sleeve 26 to a compressable ferrule 27 slipped over the cable cover 5 to give additional mechanical cable grip.

Figure 4 shows a modification of the embodiment shown in Figure 3 from which it differs by dispensing with annular distance piece 24 and the technique of folding back the wire braid 23. In Figure 4 both braids 15, 16 are led over the chamfer 6b and are crimped to the connector by the sleeve 28.

Figure 5 shows a further modification which incorporates a wedge-piece 29 for the two outer braids 15, 16. At the near end of the connector body the parallel bore is followed by a divergent portion 6c followed by an enlarged diameter parallel portion 6d, screw threaded internally at 8. The end preparation of the inner braid 14 and the superimposed tape turns are made up as before and entered into the enlarged diameter, parallel sided, bore, followed by the adjacent part of the cable cover 5, over which has been threaded an externally screw threaded back nut 10 and a wedge piece 29. The latter has a cone angle similar to that of the divergent portion 6c of the connector bore. The tape layer 18 is sheared off but the two braid layers 15, 16 are folded back obliquely over the wedge piece 29. The backnut 10 is screwed into the connector and urges the wedge piece 29 axially so clamping the two braid layers into the connector to provide a mechanical and electrical contact.

WHAT WE CLAIM IS: 1. An interconnection between two parts of an annular electrically conductive screen incorporating a means of reducing the magnetic reluctance of the magnetic path between inner and outer surfaces of the screen at the region of the interconnection.

2. An interconnection as claimed in Claim 1, wherein the means of reducing the magnetic reluctance comprises high permeability magnetic material.

3. An interconnection as claimed in Claim 2, wherein the high permeability material is of toroidal form and is disposed in an annulus between two concentric tubes forming one end of one part of the interconnection, the said one end being of that form to effect mechanical interconnection of the said two parts.

4. An interconnection as claimed in Claim 2, wherein the high permeability material is of toroidal form and is arranged to be juxtaposed between two adjacent concentric surfaces of the said two parts.

5. An interconnection as claimed in any one of Claims 2 to 4, wherein the magnetic material is of laminated form.

6. An interconnector adapted to provide an interconnection as claimed in any one of

Claims (1)

1. Claims 1 to 5.

   7. A method of electrically connecting the outer conductor of a screened co-axial cable to a co-axial connector or to a terminal, the method comprising making two connections between the outer conductor of the cable and the connector or component terminal and interposing a magnetic toroid between said connections.

   8. A method as claimed in Claim 7, the outer conductor comprising co-axial layers of conductive braid interleaved by a layer of magnetic material, including positioning the toroid about a said layer of the braid, and folding the said layer back over the outside of the toroid.

   9. A method as claimed in Claim 8, wherein the said layer comprises the inner layer of the conductive braid.

   10. A method as claimed in Claim 8 or Claim 9, wherein the magnetic toroid is provided by winding a tape of magnetic material about said layer of the conductive braid.

   11. A method as claimed in any one of Claims 8 to 10, including radially compressing the folded layer about the toroid, and entering said compressed folded layer about said toroid into the bore of one part of the connector or terminal, the bore being arranged to inhibit unfolding of the braid.

   12. An interconnection substantially as hereinbefore described with reference to Figure 4, or Figure 5, or Figure 6 of the drawings accompanying the provisional specification filed with patent application No. 21911/77.

   13. A method of electrically connecting the outer conductor of a screened co-axial cable to a co-axial cable connector or to a terminal, substantially as hereinbefore described with reference to any one of Figures 1 to 5 of the drawings accompanying the provisional specification filed with patent application No. 30752/77.

   14. A co-axial cable having an electrical connection to a co-axial cable connector or to a terminal made by the method as claimed in any one of Claims 7 to 11, or Claim 13.