Note: Descriptions are shown in the official language in which they were submitted.
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LINE ISOLATION AND INTERFERENCE SHIELDING
FOR A SHIELDED CONDUCTOR SYSTEM
BACKGROUNfD OF THE INVENTION
This invention relates genexally to the fields
5 of high frequency electromagnetic interference shielding
and A.C. power isolation. It is particularly directed to
the shielding of high frequency shielded conductor systems,
such as coaxial cables, from electromagnetic interference
and the simultaneous isolation of such conductor systems
10` from sources of A.C. power. The 75 ohm coaxial cable input
to a television tuner is a prime example of one type of
shielded conductor to which such shielding and isolation
is directed.
Television receiver manufacturers are currently
15 required by Underwriters Laboratories (U.L.) to doubly
isolate exposed metal parts from the A.C. line which powers
the receiver. For exampler the 300 ohm twin lead termina]s
usually situated on the rear of the receiver's cabinet are
required to be separately isolated. Such isolation is
20 intended to doubly insulate a consumer from accidental
,i; shock which he might otherwise receive either from contact
with the exposed terminals or wi-th the metal "rabbit ear"
antenna to which such terminals are sometimes connected.
,~ ~ ` Conventionally, television receivers also include
25 an exposed connection for a 75 ohm coaxial cable input to
the receiver's VHF tuner. No U.L. requirement presently
exists providing for double isolation oE the coaxial inpu-t,
evidently because the technology has not been available
to television manufacturers to enable them to provide such
:f 30 isolation while simultaneously afEording acceptable
television reception.
f The problem which arises in connection with the
` 75 ohm coaxial input is that conventional techniques for
isolating the coaxial input from the A.C. line tend to
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permit ambient high frequency electromacJnetic interference
signals to couple with the field within the cable, and
thus to interfere with the desired signal propagating
inside the coaxial cable.
For example, one prior approach utilizes con-
ventional capacitors coupling the coaxial cable with the
tuner input to A.C. isolate the cable from the tuner. i.
While the isolation thus achieved is satisfactory, the
field within the cable is inadequately shielded from
electromagnetic interference.
~ more recent isolation techniq.ue, described
in applicant's co-pending-application Serial No. 378,193,
filed May 25, lg81, employs a feed-through or tubular
type capacitor in the cable for A.C. isolation. The
latter arrangement does provide the required degree of
A.C. line isolation but, in fields of strong ambient
electromagnetic interference, its shielding effect is
less than perfectly satisfactory.
The shielding problems mentioned above may be
particularly evident where the coaxial cable, connected
to the 75 ohm input, carries a CATV signal. If the
cable includes an A~Co isolator which is an inadequate
electromagnetic interference shield, strong co-channel
ambient broadcast fields will not be adequately shielded
from the field within the coaxial cable and will produce
strong co-channel.interference.
For the reasons stated above, presently available
A.C. isolators have not proven adequate where electro~
magnetic interference shielding is of importance.
OBJECTS OF THE IN~IENTION
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It is a general object of the invention to
provide a method and apparatus for isolating the shield
of a shielded h.igh frequency conductor system from
low frequency A.C. power in such a way tha-t the desired
field within the cable is shielded ~rom ambient high
frequency elect:romagnetic interference.
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It is another object of the invention to provide
such isolation and shielding for a shielded conductor
system adapted to carry a television signal to the tuner
of a television receiver.
Tlle invention relates to a method of isolating the
shield of a shielded conductor system from a low ~requency
power source to which the shield may be coupled, and for
shielding the field within the conductor system from
ambient high frequency electromagnetic interference,
comprising: providing an interruption in the shield; and
situating within the interruption dielectric and magnetically
absorptive material selected and disposed to create a
capacitive coupling across the interruption to isolate the
shield and magnetic absorption within the interruption to
absorb energy associated with the ambient electromagnetic
interference.
In its apparatus aspect, the invèntion is ~Ised in
a system employing a shielded conductor which carries a
- desired high frequency signal, and whose shield is adapted
to be coupled to a low frequency power source. The
invention relates to an isolator for isolating the conductor's
shield from the low frequency power source and for shielding
the desired field within the conductor from ambient high
frequency electromagnetic interference, comprising: means
defining an interruption in the shield; and magnetically
absorp~ive and dielectric material situated within the
interruption, the material being selected and disposed to
create a capacitive coupling across the interruption to
isolate the shield and magnetic absorption within the
interruption to absorb energy associated with the ambient
electromagnetic înterference.
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BRIEF D~SCRIPTION OF THE DRAWINGS
The objects stated above and other objects of
the invention are more particularly set forth in the
following detailed description and in the accompanying
drawings, of ~hich:
Figure 1 illustrates a coaxial cable having
conventional capacitive A.C. line isolation,
Figure 2 illustrates a cable-isolator assembly
in accordance with the invention;
n Figure 3 is a lumped-element equivalent circui~
diagram useful in explaining the operation of the
embodiment shown in Figure 2; and
Figures 4-6 illustrate alternate embodiments of
the invention~
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Figure 1, a coaxial cable 10 is
shown which may be used for carrying a television signal
to the tuner of a television receiver. The cable 10 has
an inner conductor 12 disposed coaxially within an outer
conductor 14. The rightmost end 16 oi the cable may be
coupled to a signal source and the leftmost end 18 may be
coupled to the input of a television tuner.
Conventionally, the tuner may be isolated from
the A.C. line which powers the receiver. To doubly
isolate the end 16 of the cable from the A.C. line, it
has been proposed to capacitively couple the ends 16 and
18 of the outer conductor 14. This prior approach is
indicated schematically by capacitors 20 and 22 disposed
in the cable's outer conductor. The capacitors 20 and ~2
are selected to provide a high impedance at the low
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frequencies associated with the A.C. line, thereby to
further isolate the end 16 of the cable from the line
voltage. The inner conductor 12 may also be decoupled
from the A.C. line by a capacitor (not shown).
Although the isolation effected by the technique
shown in Figure 1 is satis~actory, the simple capacitive
decoupling of the outside conductor can cause an intoler-
able increase in electromagnetic interference, particu-
larly when a local signal is broadcast on the same fre-
quency as a CATV signal carried by the cable.
Figure 2 shows a preferred embodiment of the
invention. A shielded conductor system in the form of a
coaxial cable 24 includes an inner conductor 26 and an
outer conductor 28. The cable may include a leftmost
por-tion 30 whose outer diameter is greater than the outer
diameter of the rightmost portion 32 such that a portion
34 of the larger diameter outer conductor overlaps the
smaller diameter outer conductor. The space defined by
such overlap constitutes a gap or interruption in which
dielectric and magnetically absorp-tive material is situated
`~ for purposes of shielding and line isolation.
In the illustrated embodiment, the annular,
cavity-like interruption thus created holds -two discrete
elements of dielectric material 36 and 38 separated by
an element of magnetically absorptive material ~0. Each
such elemen-t is annular and has a cen-tral opening to
surround the smaller diameter ou-ter conductor. The
elements 36, 38 and 40 may be stacked one against the
other and a:Ligned coaxially of the cable as illustrated.
With this arrangement, the dielectric elements
36 and 38 create a capacitive coupling across the gap
between the large and small diameter portions of the outer
conductor to isolate the rightmost portion 32 of the outer
conductor ~rom the le~tmost porti~n 30. Hence, any A.C.
line volta~e applied to the leftmost portion 30 is
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inhibited from reaching the rightmost portion 32. Inaddition, the eapacitances formed by the elements 36 and
38 eo-operate with the element 40 to shield the field
inside the eable 24 from ambient eleetromagnetie radiation,
as described hereinafter.
The magnetically absorptive element 40 serves to
absorb electromagne-tic interference not bypassed by the
eapacitive effect of elements 36 and 38, without any
substantial absorption of the desired field within the
cable.
To more fully explain the shielding effect
achieved, reference is made to Figure 3 which shows an
equivalent eircuit diagram of a two port whieh may be
plaeed between the cross sections AA (input port~ and BB
(output port) of Figure 2. The source I represents the
. eurrent on the outer surfaee of the outer eonduetor indueed
~ in the vicinity of the eross seetion AA by the ambient
,., interferincJ signal. The source E repre~ents the desired
i~ signal to be carried by the cable, the resistor Rl
: ~0 represents the nominal output impedance of the souree E
(75 ohms), and the resistor R2 represents the nominal
input impeda.nee (75 ohms) of a television tuner.
The reslstor R3 represents the equivalent series
:'`3 resistanee (100 ohms, for example) of the magnet.ically
~ 25 absorptive element 40, the capacitor C1 represents the
:~ capacitance due -to the èEfect of the dielectrie element
36, and the capacitor C2 represents the capacitance due
.~ to the effect of tlle dielectric element 38. Eaeh eapaeitor
Cl and C2 may, by way oE example, have a value of about
~',! 30 1000 pieofarads.
At typ.ica.l television frequencies, the impedanee
~ of the eapacitors Cl and C2 is much less than the impedanee
.~s of any of the resistors :in Figure 3~ Hence, the eapacitor
Cl shunts the desired s;.cJnal from souree E away from the
resistance R3 and towarcl the input impedanee of the tuner~
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sented by R3 does not substantially absorb any of the
desired signal.
The capacitor C2 acts to shunt the current I
so that the interference current does not develop a sub-
stantial corresponding voltage in R2 (the tuner input
impedance).
Because the capacitor C2 has only a finite
capacitance, not all the current I will be shunted. How-
ever, capaci~ors Cl and C2 cause the residual electro-
magnetic interference to be absorbed by the magnetically
absorptive material (R3).
It should be mentioned that any magnetically
absorptive material will also produce an equivalent and
frequency dependent inductance which is in series with its
equivalent resistance. Such inductance may help to suppress
interference at lo~er frequencies, but it is not very
desirable at higher frequencies. Hence, the magnetically
, absorptive material should be selected to maximize
interference suppression at the frequencies o~ interest
~; for a particular application.
Referring again to Figure 2, ~he arrangement
shown therein has been found to provide exceptional shield-
ing from electromagne-tic interference while simultaneously
providing isolation from the line voltage. The dielectric
elements 36 and 38 may be of any suitable dielectric
material preferably having a high dielectric cons-tant of
several thousands to provide a total capacitance o~ abou-t
, 2000 picofaxads. Barium titante is one example of such
dielectric material.
The element 40 is made of a magnetically absorp-
tive ma-terial whose equivalent series resistance is as
high as possible at the frequencies of in-terest for best
absorption of electromagnetic interference. A ferrite
material having an equivalent series resistance of about
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lOQ ohms has been found to be acceptable for use at
television ~requencies. Such a f~rrite is available from
Fair-Rite Products Corp., Wallkill, New York, referred
to as material number 43 or 64.
In constructing the isolator, the dielectric
elements 36 and 38 may be silver plated inside and outside
and soldered to the outer conductor 28 on the inside and
to the outer conductor 30 on the outside. ~he magnetically
absorptive element 40 may be in the form of a ferrite bead
disposed loosely between the dielectric elements and
need not be in physical contact with the cable's outer
conductor. It is thought that greater A.C. line isolation
may result if no such contact is permitted, particularly
in the case where ferrite materials with a high D.C.
specific conductance are used.
It will be appreciated that the isolator-cable
combination may be used in applications other than with
television tuners. However, when the cable 24 is designed
to carry a signal to a television tuner, the interrup-tion
or cavity described above need not be completely disposed
in the cable alone. For example, in Figure 2, the lef-t-
most portion 30 of the cable (-the part of larger diameter)
may actually be an input connector to a television tuner.
In that case, the larger diameter portion of the connector
may be extended over the smaller diameter cable so that
an area of axial overlap exists as shown, with the dielectric
and magnetically absorptive material disposed in the gap
defined by l-he area of axial overlap. Hence, when an
interruption is referred to herein as being in the outer
conductor of a cable, it is to be understood that such
terminology is meant to also include an interruption
between the outer conductor of the cable and a correspond-
ing connection to a tuner input or corresponding structure.
In fact, the required isolation and shielding rnay be
~ 35 effected by disposing the interruption at any practical
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location in a coupling path between the outer conductor
of the cable and the input to the tuner or corresponding
structure.
Such a conneetor and cable as shown in Figure 2
may be disposed with a television receiver's cabinet.
In that ease, the eable itself need not be flexible as
is the ease with conventional coaxial cable. Instead,
the eable may be constructed of conductive pipe having a
- eenter conduetor. Sueh a pipe will be understood to be
10 the equivalent of a coaxial cable, wherefore, referenees
herein to a coaxial eable or a shielded eonduetor are
intended to be inclusive of such pipes.
In some instanees, the interruption may be
implemented without the use of either a coaxial cable or a
conductive pipe. Instead, the interruption may be placed
within a connector which is attached directly to a tele-
; vision tuner or corresponding structure. Hence, references
herein to a shielded conductor are meant -to include such
eonneetors and their equivalents.
The isolator of Figure 2 comprising the elements
36, 38 and 40 is illustrated as employin~ only one ferrite
or ma~netically absorptive element disposed between a pair
of dielectric elements. However, additional dielectric
and ferri-te elemen-ts may be used in an alternating sequence,
25 as shown in phantom at 138 and 140, respectively. In
the illustrated preferred embodiment, the first element
on the inside (element 36 in FicJure 2) is a dielectric
element so that no losses are introducecl into the desired
signal path. The first element on -the outside (element
38 in Fi~ure 2) may be either a dielectric element or a
magnetically absorptive element, the former case being
more effective.
There are several alternatives for the design
of an A.C. line isola-tor, tlle construction of which
depends on t:he m~in direc-tion in which the electromaJnetie
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interference signal within the isolator is forced to pro-
pagate (radially or axially). The construction shown in
Figure 2 illustrates a case in which the interference
signal propagates axially and the dielectric-ferrite pairs
are distributed axially.
Figure 4 illustrates an isolator in a coaxial
cable for radially propagating interference signals and
having radially distributed dielectric-ferrite elements.
As shown, the cable 24a has an inner conductor 26a and
an outer conductor 28a. The latter conductor is divided
with upturned edges or radial flanges arranged vis-a-vis
to form a gap or interruption 42ain which dielectric
elements 36a and 38a are separated by a ferrite or other
type of magnetically absorptive element 40a so that the
dielectric and magnetically absorptive elements are
sandwiched between the flanges and concentrically arranged
such that the alternating sequence of elements is in a
direction radial to the cable. Again, as in the
Figure 2 embodiment and other embodiments to follow, a
greater number of dielectric and magnetically absorptive
elements may be employed in alternating sequence in
applications where greater performance is desired in spite
of the necessarily higher consequent cost~
Referring to Figure 5, an alternative design is
shown for the case in which the interference signal pro-
pagates axially and the dielectric-ferrite pairs are
disposed radially. In this design, the cable 24b has an
inner conductor 26b and an outer conductor 28b, the latter
being separated into two parts (left and right, as shown!.
The ends of the separated parts are interleaved so as to
provide a total of at least three spaces be-tween the
interleaved parts. A first space contains a dielectric
element 36b, a second space contains a magnetically
absorptive element 40b, and a third space contains another
dielectric element 38b.
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Another embodiment is shown in Figure 6 in which
the interference signal propagates radially and the
isolator elements are distributed axially. Again, an
outer conductor 28c of the cable 24c is separated into
two parts as shown. The separated parts of the outPr
conductor are interleaved to provide at least three spaces.
A dielectric element 36c is disposed in a first space, a
magnetically absorptive element 40c is disposed in a
second space, and another dielectric element is disposed
in the third space.
The cable shielding and isolation technique
described herein has been found to provide satisfactory
isolation and superior shielding from electromagnetic
interference. In fàctr measurements in television receivers
exposed -to strong ambient fields have shown that an
isolator-cable assembly of the type shown in Figure 2
provides interference suppression which is approximately
equivalent to the interference suppression provided by a
, singly isolated, fully shielded cable, the primary limita-
tion on electromagnetic interference pickup being the
construction and quality of shielding built into the tuner.
Although the invention has been described in
terms of its applicability to television tuners, it will
be understood that the invention is not limited to that
field. Moreover, those skilled in the art will appreciate
tha-t modifications and alterations may be made to the
method and structure described herein wi-thout departing
from the invention. Accordingly, it is intended that all
such modifications and alterations be included within the
spirit and scope of the invention as defined by the
appended c]alms.
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