CA1126830A - High frequency screening of electrical systems - Google Patents
High frequency screening of electrical systemsInfo
- Publication number
- CA1126830A CA1126830A CA357,602A CA357602A CA1126830A CA 1126830 A CA1126830 A CA 1126830A CA 357602 A CA357602 A CA 357602A CA 1126830 A CA1126830 A CA 1126830A
- Authority
- CA
- Canada
- Prior art keywords
- screen
- braid
- layer
- toroid
- connector
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Abstract
Title Screening of Electrical Systems Abstract of the Disclosure An interconnection between screened cables and a method of interconnecting screened cables, It is calculated that reduction of magnetic reluctance of the magnetic path between inner and outer surfaces of the screen in the region of the interconnection decreases external interference to the screened cable and the interconnections are constructed to be in accordance with this calculation. Mu-metal can be used to reduce magnetic reluctance.
Description
, This inven~ion rclates to high frequency screening o electriccll systcms. The importance o screcning agains-t extraneous noise in an industrial environment is ~ell recognised with -the rcsult that component design and layout aims at high efficiency screening ~hich is quantified by a low transfer impedance across the conducting members ~ormin~ the screen surroundin r a sensi~ive circuit~ The invention concerns preservation o this property ~here an otherwise continuous screen is interrup-ted 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. Some aspects of design of mating faces at such interruptions are discussed in a paper entitled 'tScreened Coaxial Cable Connectors for lIigh sensiti~itY Systems~by E P Fowler presented at an IEEE SympoSiUm on ~lectromagnetic Compatibility at Montreux in May 1975.
-- .
According to one aspect o~ the present invention there is provided an interconnection between two parts of an annular electrically conductive screen incorporating a means of reducing the ma~netic reluctance of the magnetic path bet-~een inner and outer surfaces of the screen in the region of the interconnectionO Improyement in connector screening is possible by separation of the two contact rings combined with a reasonable len~th of separate con-ducting paths. Further improvement can be made by insertion of a high permeability magnetic material such as a small toroid of laminated mu-metal between the conducting pathsO
According to another aspect of the invention~ a method of electrically connecting an outer conductor o a screened co-axial cable to a co-axial cable connector or to a termina] compriscs ma~ing t~o connections b~-twcen the outer conductor of t}le cable and the connector or component terminal to recluce the ma~netic rel~lctance het~cen themO
Preferably, a ~lagnetic -toroid is interpose~ be~/een the two connectionsO ~ligil frequency disturbing currents flowing in tlle outer ~ire braid conductor of the cable flow throu~h the connection between the ou-ter co-axi braid connec-tion and the connector or component terminal~
^~ Y~ilst the inner braid connection for~ns ~he screened circuit.
- Embodlments of both aspects of the invention will now be described by way of example only with reference to the accompanying drawings~ in which:
Figure 1 is an axial cross section of a screened co-axial connector, Figure 2 is a simplified circuit diagram showing parameters connected wi-th the Fi~ure lg - Fîgure 3 is a simplirication o ~igure 2, Figure 4 is a similar view to Fi~ure 1~
Figure S is an axial cross section of a de~i~n applicable to a small screening box or a large diameter connector screen, _ Figure 6 is a graph showing comparative screening -- --- performance of the screen o~ Figure 6 with _ - and without a magnetic -toroid~
---- Figure 7 is a view in axial cross-section o~ an - interconnection between a screene~ co-axial - - cable and co-axial connector9 and --Figures 8 to 11 are similar views to Figure 7~ but of different forms of interconnect7on~
--- Reference is made firstly to Figure 1, wher~ln a plug 1 is shown to the right and a socket 2 to t~ e leftO
The plug 1 has an outer screen part 3 and an inn~r con-' - ~
: '' 8~
ductor ~!~ The outer conductor screen part 3 terminates in two parallel split s~irts 6 and 7. The socket 2 has an outer screen part ~ and an inner conductor 9. The outer conductor scraen part 8 terminates in two solid s~irts 10 and 11. The skirts 6~ 7 engagc ~ith a push fit into the skirts 10, 11~ The sl~irts 6~ 7 of the socket are shaped so as to make ring contac-ts 12, 14 ~ith the inner cir-cumference of the skirts 10, 11 of tl~e socket 2. A toroicl of laminated mu-metal tape 15 is retained boetween -the split skirts 6 and 7 Reference is no-~ made also to Figure 2 Screening effectiveness is related to the transEer impedance indi-cated by Z21- Transfer impedance relates voltage generated in the screened circuit formed by the inner conductor and the connector outer screen to the dis-turbing current ~lowing only in the connector outer screen. In Figure 2~ there is shown an equivalent circui-t for the co-a~ial connector of Figure 1, which is being disturbed by a current I, so resulting in a voltage V~ being generated in the screen circuit, so that Z21 = VS~/I.
There are t-Yo concentric contact paths 12~ 14 between the plug and socke-t of Figure 1. In this system~ a large part of inductance 16 of the outer conductor outer contact path 14 is coupled to the inner conductor at 17.
Part of the voltage V5 generated in the screen circuit appears across each of load resistances 19, 20 which complete the circuit but are not relevant to screening.
These are also circuit elements (not shown) representing the distributed inductance and capacitance of the screened circuit, but these are omitted for clarity and they do not affect screening. The disturbing current I, -from any e~ternal generator 10 flo-~ing in the outer conductor generates a voltage across contact resistance 21 and reactive impedance 22. This reactive impedance is uncoupled i8~
inductance and occurs if the contact path i~ not circum-fercntially uniform. The samc disturb:;ng current flotrillg in the coupled outer conductor will generate equal voltages across 16 and 17 and so have no effect on the screened circuit.
As ~Yell as the contac-t path 14, there is also the inner contact path 12 to be considered. The induc-tive impedance bet~reen these paths differs and in Figure 2, the contact resistance of path 12 is indica-ted by 25 and its uncoupled inductance by 26. The inner contac-t path has a coupled inductance 27 ~rhich is coupled to the outer path and to the inner conduc-tor~ A further inductance 29 on the inner contact path 12 is coupled only to -the inner conductor at 30 and represents the difference of the inductive impedance.
The circuit of Figure 2 can be simplified by eliminating the coupled inductances as is done in Figure 3. From Figure 3, it can be seen that the magnitude of the inductance 29 can play a significant part in governing the quotient Ys/Il, ie the transfer impedance~
If Zl is taken to be the impedance of resis-tance 21 and inductance 22~ Z2 the impedance of resistance 25 and inductance 26 and Z~l the impedance of inductance 29, -then it can be sho-rn that:
z = Zl Z2 Zl + Z2 + ZM
so that increase of impedance 29 results in decreased transfer impedance and improved screening, The inductance 16 has a value dependent upon axial length of the con-tact paths 12, 1~ and on their diameter ratioO
Reference is now made to Figure ~ the plug and socket connector depicted here in a decoupled condition employs t~ro coaxial rings of split fingers 32, 33 on the left hand half arranged to define an annular socket to be engaged . .
. . ~.
.. ' by s:ingle tube 34 on the mating right hand h~l oE the connector simultancously with the plug 36 and socket 35 inter-engagement of the inne;r conductor. A toroid 37 of mu-metal tape is retained at the base of the recess formed between the coa~ial rings of -Eingers 32~ 33. ~Ihen -the connector is engaged~ two contact rings are -ormcd at 38, 39.
The interconnection bet~Yeen the two parts in both of Figure 1 and 4 is electrically conducti~e along -two co axial or concentric contact paths physically spaced apart and electrically com1ected a-t each end and ferromagnetic material is located between the contact paths to reduce the reluctance of the magnetic path between them. The effect is to increase the inducti~e impedance of the inner contact ~tube~ thereby forcing a large part of the dis-turbing current to flow in the outer concen-tric ~tube~.
Althou~h the present description is applied in terms of the impro~ed screening to disturbing current flowing in the connector screen, the principle o superposition can be applied to sho-Y that it is equally applicable to guarding against egress of signal from the screened circuit.
In Figure ~, there is shown part of a right cylindrical screen 40 of a screened enclosure 41. ~he base of the screen 40 is closed by a circular cup 42, within which are ring contacts 43 and 44 of a resilient conductive material~
~he ring contacts 42, 43 are spaced a~ially in a recess in the cup 42. In the same recess, and between the rings, is located a toroid 44 of magnetic materialO The toroid is of laminated cons-truction9 being formed from mu-metal tape.
In Figure 6 is a graph showing transfer impedance Z21 (in ohms) against frequency for the enclosure 41a s~etched in Figure 5. Curve A of Figure 6 sho-~s the transfer impedance wi-thout magnetic material in Figure S while ~.2~
curve B shows tlle transfer impedance with the magnet:ic material present and demonstrates the lo~er transfer impedance which comes from incorporating the magne-tic toroid~ The improvement is such as to obviate the need of applying a~ial force to the connector at the interface which is otherwise found necessary to obtain good shielding.
If there was only one ring contact then a curve drawn on a similar scale as curves A and B would have a ~ero or positive gradient and not a negative gradien-t at higher frequenciesO Thus~ even provision o an air gap effects an improvementO
Reference i3 now made to Figures 7 to 11~ which are similar views in a~ial cross-section of different forms of interconnection between a screened co-a~ial cable and co-a~ial connector and ~herein like reference nwnerals are used for like parts in the Figures. The Figures show connection to a triple braided cable~ but tlle connection is valid for all cables with two or more braids with or without the distributed interleaf of magnetic material.
For e~ample, in applying the invention to a double braided cable~ the arrangement of Figures 7, ~, 10 and 11 omit the outer braid and tape. The ~rangement of Figure 9 would not be used if the middle braid and outer tape were omitted. If more than three conducting braids were to be used, the additional braids would be considered as either middle or outer braids.
In Figures 7 to 11, the cable 50 comprises a cen-tre conductor 51 insulated by a layer of insulation 52 from an outer conductor and screening feature 53 . The cable7s outer cover is indicated at 5~, for the present purposes metal ~ire braid layers 55, 56, 57 are to be regarded as the outer conductor in conjunction ~ith metal tape layers 5~, 59~
The drawin~s show only the rear end of a cable : ' ' .
~3.2~
connector 60 for receiving the cen-tre con(luc-tor ~land~ to wllich connector~ the eature 53 is to be connected. In Figures 7 and 8 the rear end of the connector has a counter-bore 61 whose internal shoulder is machined -to an anIIular knife ed~e 62. .~n internal screwthread is formed at 63, An externally threaded metal back nut 6~ screws into the screwthread at 63 and ur~es the end face of a ferrule 6S
against the knife edge 62 to give good coaxial electrical contact and hence a good electrical screen, This is a .-technique used on seYeral connectors~ ~ small diameter hole 66 in the front of the ferrule 65 leads the insulated centr~ conductor 51 into the body of t~e connector 60 whilst in Figure 1 an enlarged diameter rear portion 67 oE ferrule 65 receives the outer conductor and screen feature 53.
The feature 53 is common to Figures 7 to 11 and terminates in a specially prepared end of the co-a~ial cable. In more detail the co-axial cable comprises three co-axial tubular layers of copper wire braid 55~ 56, 57 interleaYed by layers 58 and 59 of mu-metal tape formed from partially oYerlapping helical turns~ each layer being applied in a manner whichleaYes clearances ~not shown specifically) between the tape layer 58 and the underlying braid 55. Reference to Figure 7 shows that prior to the entry of the outer conductor and shield feature 53 into the larger diameter bore portion 67 of the ferrule 65, a significant proportion of the unwrapped turns of the tape layer 58 are superimposed at 68 and haYing been very slightly bowed in their initial application to the braid 55~ the superimposed turns exhibit resilience in a radial sense ~Yith respect to the cable axis. The underlying braid layer 55 is then olded back over the outermost of the superimposed turns, care being taken to ensure that the ends of braid 55 cannot touch the braid at 70. The 8~
centre braid layer 56 is folded bacl~ over both -the enclo~in~ tape layer 59 and ou-ter braid layer 57. Thus preparecl, t~le outer conduc-tor and screen feature 53 is radially compressed manually and entered into the enlarged bore of the ferrule where two rings of contact will be main-tained by tne outward spring force of -the superimposed tape turns 68 pressing the braid 55, against the bore oE
portion 67 and braid 56 being trapped between the bore 67 and braid 57. ~etention i3 assisted by the inner con-ductor 51 which en~ages plu~/socket fashion with a mating par-t of the connector (not shown). ~ny suitable means may be used to effect more positive re-tention.
The remaining embodiments demonstrate modified con~
structions which incorporate a more positive means of retention~ In all cases howeYer the presence of a sub~
stantial volume of mu-metal tape adjacent the cont~ct interface reduces the trans-fer impedance over a large frequency ran~e thereby lessening the risk o~ degrading the screening efficiency at a location ~Yhere a discontinuity of the cable screen occurs.
In Figure 8 the ferrule 65 has a parallel borè and the adjacent end of the feature 53~ prepared as before, abu-ts the end face of the ferrule 650 The superi~posed tape turns 68 and 18 secured by means of a copper sleeve 71.
The sleeve 71 is crimped at 72 over a ~nurled end portion of the ferrule 65 which here has its outer diameter suitably reduced to enable a satisfac-tion crimp of the copper sleeve to be achieved. The sleeve receives the prepared end of feature 53 and is crimped at 73 at its end remote from the ferrule where centre braid 56 is back folded over the outer braid 57.
The embodiment shown in Figure 9 omits the knife edge contac-t 62~ ferrule 65 and back nut 6~. Both the inner conductor 51 and the outer conductor 53 en-ter the bore 61 ~3~
in the conncctor 60 and -the resilience of tile super-imposed tape layers 68 urges -tlle inner braid 55 into con-tact witll the bore. The rear end of the connector 60 has a portion 74 of reduced e~-ternal diameter with an end chamfer at 750 The middle braid 56 is folded back at 76 over an annular resilient distance piece 77 whlch maintains contact between the braid 56 and the bore 61. The outer braid 57 i3 led over the chamfered end 75 of the portion 74 on to its ou-ter sur-Eace~ The braid 57 is clamped to the ou-ter surface of portion 74 of the connector by a copper sleeve 78 by the application of a crimping tool.
The same tool crimps the same sleeve 78 to a compressable ferrule 79 slipped over the cable cover 54 to give additional mechanical cable grip.
Figure 10 sho~Ys a modification of the embodiment shown in Figure 9 from ~hich it difEers bydispensing with annular distance piece 77 and the technique of folding back the ~ire braid 76. In Figure 4 both braids 50, 57 are led over the chamfer 75 and are crimped to the con-nector by sleeve 80.
Figure 11 shows a further modification which incorporates a wedge-piece 81 for the -two outer braids 55, 56. .~t the near end of the connector body the parallel bore is follo~ed by a divergent portion 82 followed by an enlarged diameter parallel portion 83~ screw threaded internal~y at 84. The end preparation of the inner braid 55 and the superimposed tape turns are made up as before and entered into the enlarged diameter, parallel sided, bore, follo~ed by the adjacent part of the cable cover 54, over which has been threaded an externally screw threaded back nut 64 and a wedge piece 85. The latter has a cone angle similar to -that of the divergent portion 82 of the connector bore. The tape layer 59 is sheared off but the two braid layers 56~ 57 are folded back obliquely over the .3 wed~,e piece ~5. Tne backnut Gl~ is screwed into the connector and ur~es the wetl~e piece 85 c~tially so clRmping the two braicl layers into the connector to ,rovide a mechanical and electrical contact.
It is to be understood, that the screen described above in connection with ~igure 5 can be part of the screen of a sensitive instrument. Alternatively7 it could be part of a domestic ap,liance such as a microwave oven.
From the above description, it can be seen that an improved interconnection ~or screened electrical cables and screens in general is provided.
. . .
-- .
According to one aspect o~ the present invention there is provided an interconnection between two parts of an annular electrically conductive screen incorporating a means of reducing the ma~netic reluctance of the magnetic path bet-~een inner and outer surfaces of the screen in the region of the interconnectionO Improyement in connector screening is possible by separation of the two contact rings combined with a reasonable len~th of separate con-ducting paths. Further improvement can be made by insertion of a high permeability magnetic material such as a small toroid of laminated mu-metal between the conducting pathsO
According to another aspect of the invention~ a method of electrically connecting an outer conductor o a screened co-axial cable to a co-axial cable connector or to a termina] compriscs ma~ing t~o connections b~-twcen the outer conductor of t}le cable and the connector or component terminal to recluce the ma~netic rel~lctance het~cen themO
Preferably, a ~lagnetic -toroid is interpose~ be~/een the two connectionsO ~ligil frequency disturbing currents flowing in tlle outer ~ire braid conductor of the cable flow throu~h the connection between the ou-ter co-axi braid connec-tion and the connector or component terminal~
^~ Y~ilst the inner braid connection for~ns ~he screened circuit.
- Embodlments of both aspects of the invention will now be described by way of example only with reference to the accompanying drawings~ in which:
Figure 1 is an axial cross section of a screened co-axial connector, Figure 2 is a simplified circuit diagram showing parameters connected wi-th the Fi~ure lg - Fîgure 3 is a simplirication o ~igure 2, Figure 4 is a similar view to Fi~ure 1~
Figure S is an axial cross section of a de~i~n applicable to a small screening box or a large diameter connector screen, _ Figure 6 is a graph showing comparative screening -- --- performance of the screen o~ Figure 6 with _ - and without a magnetic -toroid~
---- Figure 7 is a view in axial cross-section o~ an - interconnection between a screene~ co-axial - - cable and co-axial connector9 and --Figures 8 to 11 are similar views to Figure 7~ but of different forms of interconnect7on~
--- Reference is made firstly to Figure 1, wher~ln a plug 1 is shown to the right and a socket 2 to t~ e leftO
The plug 1 has an outer screen part 3 and an inn~r con-' - ~
: '' 8~
ductor ~!~ The outer conductor screen part 3 terminates in two parallel split s~irts 6 and 7. The socket 2 has an outer screen part ~ and an inner conductor 9. The outer conductor scraen part 8 terminates in two solid s~irts 10 and 11. The skirts 6~ 7 engagc ~ith a push fit into the skirts 10, 11~ The sl~irts 6~ 7 of the socket are shaped so as to make ring contac-ts 12, 14 ~ith the inner cir-cumference of the skirts 10, 11 of tl~e socket 2. A toroicl of laminated mu-metal tape 15 is retained boetween -the split skirts 6 and 7 Reference is no-~ made also to Figure 2 Screening effectiveness is related to the transEer impedance indi-cated by Z21- Transfer impedance relates voltage generated in the screened circuit formed by the inner conductor and the connector outer screen to the dis-turbing current ~lowing only in the connector outer screen. In Figure 2~ there is shown an equivalent circui-t for the co-a~ial connector of Figure 1, which is being disturbed by a current I, so resulting in a voltage V~ being generated in the screen circuit, so that Z21 = VS~/I.
There are t-Yo concentric contact paths 12~ 14 between the plug and socke-t of Figure 1. In this system~ a large part of inductance 16 of the outer conductor outer contact path 14 is coupled to the inner conductor at 17.
Part of the voltage V5 generated in the screen circuit appears across each of load resistances 19, 20 which complete the circuit but are not relevant to screening.
These are also circuit elements (not shown) representing the distributed inductance and capacitance of the screened circuit, but these are omitted for clarity and they do not affect screening. The disturbing current I, -from any e~ternal generator 10 flo-~ing in the outer conductor generates a voltage across contact resistance 21 and reactive impedance 22. This reactive impedance is uncoupled i8~
inductance and occurs if the contact path i~ not circum-fercntially uniform. The samc disturb:;ng current flotrillg in the coupled outer conductor will generate equal voltages across 16 and 17 and so have no effect on the screened circuit.
As ~Yell as the contac-t path 14, there is also the inner contact path 12 to be considered. The induc-tive impedance bet~reen these paths differs and in Figure 2, the contact resistance of path 12 is indica-ted by 25 and its uncoupled inductance by 26. The inner contac-t path has a coupled inductance 27 ~rhich is coupled to the outer path and to the inner conduc-tor~ A further inductance 29 on the inner contact path 12 is coupled only to -the inner conductor at 30 and represents the difference of the inductive impedance.
The circuit of Figure 2 can be simplified by eliminating the coupled inductances as is done in Figure 3. From Figure 3, it can be seen that the magnitude of the inductance 29 can play a significant part in governing the quotient Ys/Il, ie the transfer impedance~
If Zl is taken to be the impedance of resis-tance 21 and inductance 22~ Z2 the impedance of resistance 25 and inductance 26 and Z~l the impedance of inductance 29, -then it can be sho-rn that:
z = Zl Z2 Zl + Z2 + ZM
so that increase of impedance 29 results in decreased transfer impedance and improved screening, The inductance 16 has a value dependent upon axial length of the con-tact paths 12, 1~ and on their diameter ratioO
Reference is now made to Figure ~ the plug and socket connector depicted here in a decoupled condition employs t~ro coaxial rings of split fingers 32, 33 on the left hand half arranged to define an annular socket to be engaged . .
. . ~.
.. ' by s:ingle tube 34 on the mating right hand h~l oE the connector simultancously with the plug 36 and socket 35 inter-engagement of the inne;r conductor. A toroid 37 of mu-metal tape is retained at the base of the recess formed between the coa~ial rings of -Eingers 32~ 33. ~Ihen -the connector is engaged~ two contact rings are -ormcd at 38, 39.
The interconnection bet~Yeen the two parts in both of Figure 1 and 4 is electrically conducti~e along -two co axial or concentric contact paths physically spaced apart and electrically com1ected a-t each end and ferromagnetic material is located between the contact paths to reduce the reluctance of the magnetic path between them. The effect is to increase the inducti~e impedance of the inner contact ~tube~ thereby forcing a large part of the dis-turbing current to flow in the outer concen-tric ~tube~.
Althou~h the present description is applied in terms of the impro~ed screening to disturbing current flowing in the connector screen, the principle o superposition can be applied to sho-Y that it is equally applicable to guarding against egress of signal from the screened circuit.
In Figure ~, there is shown part of a right cylindrical screen 40 of a screened enclosure 41. ~he base of the screen 40 is closed by a circular cup 42, within which are ring contacts 43 and 44 of a resilient conductive material~
~he ring contacts 42, 43 are spaced a~ially in a recess in the cup 42. In the same recess, and between the rings, is located a toroid 44 of magnetic materialO The toroid is of laminated cons-truction9 being formed from mu-metal tape.
In Figure 6 is a graph showing transfer impedance Z21 (in ohms) against frequency for the enclosure 41a s~etched in Figure 5. Curve A of Figure 6 sho-~s the transfer impedance wi-thout magnetic material in Figure S while ~.2~
curve B shows tlle transfer impedance with the magnet:ic material present and demonstrates the lo~er transfer impedance which comes from incorporating the magne-tic toroid~ The improvement is such as to obviate the need of applying a~ial force to the connector at the interface which is otherwise found necessary to obtain good shielding.
If there was only one ring contact then a curve drawn on a similar scale as curves A and B would have a ~ero or positive gradient and not a negative gradien-t at higher frequenciesO Thus~ even provision o an air gap effects an improvementO
Reference i3 now made to Figures 7 to 11~ which are similar views in a~ial cross-section of different forms of interconnection between a screened co-a~ial cable and co-a~ial connector and ~herein like reference nwnerals are used for like parts in the Figures. The Figures show connection to a triple braided cable~ but tlle connection is valid for all cables with two or more braids with or without the distributed interleaf of magnetic material.
For e~ample, in applying the invention to a double braided cable~ the arrangement of Figures 7, ~, 10 and 11 omit the outer braid and tape. The ~rangement of Figure 9 would not be used if the middle braid and outer tape were omitted. If more than three conducting braids were to be used, the additional braids would be considered as either middle or outer braids.
In Figures 7 to 11, the cable 50 comprises a cen-tre conductor 51 insulated by a layer of insulation 52 from an outer conductor and screening feature 53 . The cable7s outer cover is indicated at 5~, for the present purposes metal ~ire braid layers 55, 56, 57 are to be regarded as the outer conductor in conjunction ~ith metal tape layers 5~, 59~
The drawin~s show only the rear end of a cable : ' ' .
~3.2~
connector 60 for receiving the cen-tre con(luc-tor ~land~ to wllich connector~ the eature 53 is to be connected. In Figures 7 and 8 the rear end of the connector has a counter-bore 61 whose internal shoulder is machined -to an anIIular knife ed~e 62. .~n internal screwthread is formed at 63, An externally threaded metal back nut 6~ screws into the screwthread at 63 and ur~es the end face of a ferrule 6S
against the knife edge 62 to give good coaxial electrical contact and hence a good electrical screen, This is a .-technique used on seYeral connectors~ ~ small diameter hole 66 in the front of the ferrule 65 leads the insulated centr~ conductor 51 into the body of t~e connector 60 whilst in Figure 1 an enlarged diameter rear portion 67 oE ferrule 65 receives the outer conductor and screen feature 53.
The feature 53 is common to Figures 7 to 11 and terminates in a specially prepared end of the co-a~ial cable. In more detail the co-axial cable comprises three co-axial tubular layers of copper wire braid 55~ 56, 57 interleaYed by layers 58 and 59 of mu-metal tape formed from partially oYerlapping helical turns~ each layer being applied in a manner whichleaYes clearances ~not shown specifically) between the tape layer 58 and the underlying braid 55. Reference to Figure 7 shows that prior to the entry of the outer conductor and shield feature 53 into the larger diameter bore portion 67 of the ferrule 65, a significant proportion of the unwrapped turns of the tape layer 58 are superimposed at 68 and haYing been very slightly bowed in their initial application to the braid 55~ the superimposed turns exhibit resilience in a radial sense ~Yith respect to the cable axis. The underlying braid layer 55 is then olded back over the outermost of the superimposed turns, care being taken to ensure that the ends of braid 55 cannot touch the braid at 70. The 8~
centre braid layer 56 is folded bacl~ over both -the enclo~in~ tape layer 59 and ou-ter braid layer 57. Thus preparecl, t~le outer conduc-tor and screen feature 53 is radially compressed manually and entered into the enlarged bore of the ferrule where two rings of contact will be main-tained by tne outward spring force of -the superimposed tape turns 68 pressing the braid 55, against the bore oE
portion 67 and braid 56 being trapped between the bore 67 and braid 57. ~etention i3 assisted by the inner con-ductor 51 which en~ages plu~/socket fashion with a mating par-t of the connector (not shown). ~ny suitable means may be used to effect more positive re-tention.
The remaining embodiments demonstrate modified con~
structions which incorporate a more positive means of retention~ In all cases howeYer the presence of a sub~
stantial volume of mu-metal tape adjacent the cont~ct interface reduces the trans-fer impedance over a large frequency ran~e thereby lessening the risk o~ degrading the screening efficiency at a location ~Yhere a discontinuity of the cable screen occurs.
In Figure 8 the ferrule 65 has a parallel borè and the adjacent end of the feature 53~ prepared as before, abu-ts the end face of the ferrule 650 The superi~posed tape turns 68 and 18 secured by means of a copper sleeve 71.
The sleeve 71 is crimped at 72 over a ~nurled end portion of the ferrule 65 which here has its outer diameter suitably reduced to enable a satisfac-tion crimp of the copper sleeve to be achieved. The sleeve receives the prepared end of feature 53 and is crimped at 73 at its end remote from the ferrule where centre braid 56 is back folded over the outer braid 57.
The embodiment shown in Figure 9 omits the knife edge contac-t 62~ ferrule 65 and back nut 6~. Both the inner conductor 51 and the outer conductor 53 en-ter the bore 61 ~3~
in the conncctor 60 and -the resilience of tile super-imposed tape layers 68 urges -tlle inner braid 55 into con-tact witll the bore. The rear end of the connector 60 has a portion 74 of reduced e~-ternal diameter with an end chamfer at 750 The middle braid 56 is folded back at 76 over an annular resilient distance piece 77 whlch maintains contact between the braid 56 and the bore 61. The outer braid 57 i3 led over the chamfered end 75 of the portion 74 on to its ou-ter sur-Eace~ The braid 57 is clamped to the ou-ter surface of portion 74 of the connector by a copper sleeve 78 by the application of a crimping tool.
The same tool crimps the same sleeve 78 to a compressable ferrule 79 slipped over the cable cover 54 to give additional mechanical cable grip.
Figure 10 sho~Ys a modification of the embodiment shown in Figure 9 from ~hich it difEers bydispensing with annular distance piece 77 and the technique of folding back the ~ire braid 76. In Figure 4 both braids 50, 57 are led over the chamfer 75 and are crimped to the con-nector by sleeve 80.
Figure 11 shows a further modification which incorporates a wedge-piece 81 for the -two outer braids 55, 56. .~t the near end of the connector body the parallel bore is follo~ed by a divergent portion 82 followed by an enlarged diameter parallel portion 83~ screw threaded internal~y at 84. The end preparation of the inner braid 55 and the superimposed tape turns are made up as before and entered into the enlarged diameter, parallel sided, bore, follo~ed by the adjacent part of the cable cover 54, over which has been threaded an externally screw threaded back nut 64 and a wedge piece 85. The latter has a cone angle similar to -that of the divergent portion 82 of the connector bore. The tape layer 59 is sheared off but the two braid layers 56~ 57 are folded back obliquely over the .3 wed~,e piece ~5. Tne backnut Gl~ is screwed into the connector and ur~es the wetl~e piece 85 c~tially so clRmping the two braicl layers into the connector to ,rovide a mechanical and electrical contact.
It is to be understood, that the screen described above in connection with ~igure 5 can be part of the screen of a sensitive instrument. Alternatively7 it could be part of a domestic ap,liance such as a microwave oven.
From the above description, it can be seen that an improved interconnection ~or screened electrical cables and screens in general is provided.
. . .
Claims (8)
1. In an annular electrically conductive screen having at least two annular electrically conductive screening paths, An interconnection between two parts of the screen, the interconnection comprising a region of at least one of the parts of the screen whereat the screen is physically divided to define a zone between the two annular paths, in which zone the two paths are physically separated, and an annulus of high permeability material disposed within said zone to reduce the magnetic reluctance of the magnetic path between said electrical paths, said annulus of high permeability material serving to partition a disturbing current so that substantially all of the current flows in that one of said paths which is closest to the disturbing signal.
2. A screen as claimed in claim 1, wherein said region is of such a form as to effect mechanical interconnection of the said two parts.
3. A screen as claimed in claim 1, wherein the annulus of high permeability material is of laminated form.
4. 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 at least two connections between the outer conductor of the cable and the connector or component terminal and interposing between said at least two connections a magnetic toroid for reducing the magnetic reluctance between said at least two connections.
5. A method as claimed in claim 4 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.
6. A method as claimed in claim 5, wherein the said layer comprises the inner layer of the conductive braid.
7. A method as claimed in claim 5, wherein the magnetic toroid is provided by winding a tape of magnetic material about said layer of the conductive braid.
8. A method as claimed in claim 5, including radially compressing the folded layer about the toroid, and entering said compressed folded layer about said toroid into a bore in one part of the connector or terminal, the bore being arranged to inhibit unfolding of the braid.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA357,602A CA1126830A (en) | 1980-08-05 | 1980-08-05 | High frequency screening of electrical systems |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA357,602A CA1126830A (en) | 1980-08-05 | 1980-08-05 | High frequency screening of electrical systems |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1126830A true CA1126830A (en) | 1982-06-29 |
Family
ID=4117571
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA357,602A Expired CA1126830A (en) | 1980-08-05 | 1980-08-05 | High frequency screening of electrical systems |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1126830A (en) |
-
1980
- 1980-08-05 CA CA357,602A patent/CA1126830A/en not_active Expired
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| MKEX | Expiry |