Note: Descriptions are shown in the official language in which they were submitted.
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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.
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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-
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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
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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
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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
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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
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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
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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
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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
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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.
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