CA1148626A - Line isolation and interference shielding for a shielded conductor system - Google Patents
Line isolation and interference shielding for a shielded conductor systemInfo
- Publication number
- CA1148626A CA1148626A CA000366011A CA366011A CA1148626A CA 1148626 A CA1148626 A CA 1148626A CA 000366011 A CA000366011 A CA 000366011A CA 366011 A CA366011 A CA 366011A CA 1148626 A CA1148626 A CA 1148626A
- Authority
- CA
- Canada
- Prior art keywords
- interruption
- shield
- conductor
- dielectric
- elements
- 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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Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01P—WAVEGUIDES; RESONATORS, LINES, OR OTHER DEVICES OF THE WAVEGUIDE TYPE
- H01P1/00—Auxiliary devices
- H01P1/20—Frequency-selective devices, e.g. filters
- H01P1/201—Filters for transverse electromagnetic waves
- H01P1/202—Coaxial filters
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B11/00—Communication cables or conductors
- H01B11/18—Coaxial cables; Analogous cables having more than one inner conductor within a common outer conductor
- H01B11/1895—Particular features or applications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01P—WAVEGUIDES; RESONATORS, LINES, OR OTHER DEVICES OF THE WAVEGUIDE TYPE
- H01P1/00—Auxiliary devices
- H01P1/22—Attenuating devices
- H01P1/225—Coaxial attenuators
Abstract
LINE ISOLATION AND INTERFERENCE SHIELDING
FOR A SHIELDED CONDUCTOR SYSTEM
ABSTRACT OF THE DISCLOSURE
A method and structure is disclosed for isolating the shield of a shielded conductor system from a low fre-quency power source to which the shield may be connected.
In a preferred embodiment, the isolation technique includes providing an interruption in the conductor's shield and situating within the interruption dielectric and mag-netically absorptive material so as to create at least a pair of capacitances across the interruption, and such that the capacitances are separated by magnetically absorptive material. In this manner, low-frequency isola-tion is achieved and the field within the cable is shielded from ambient high frequency electromagnetic interference which could otherwise leak through the interruption into a desired high frequency signal path within the conductor.
FOR A SHIELDED CONDUCTOR SYSTEM
ABSTRACT OF THE DISCLOSURE
A method and structure is disclosed for isolating the shield of a shielded conductor system from a low fre-quency power source to which the shield may be connected.
In a preferred embodiment, the isolation technique includes providing an interruption in the conductor's shield and situating within the interruption dielectric and mag-netically absorptive material so as to create at least a pair of capacitances across the interruption, and such that the capacitances are separated by magnetically absorptive material. In this manner, low-frequency isola-tion is achieved and the field within the cable is shielded from ambient high frequency electromagnetic interference which could otherwise leak through the interruption into a desired high frequency signal path within the conductor.
Description
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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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16~6 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
. . _ 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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'26 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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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 , ~ ~ :
.
'' ~ .
16~6 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
. . _ 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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'26 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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2~i 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 ` mg/~ 3a -g 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 :~
` ~14B6X6 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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i -~ Consequently, the magnetically absorptive material repre-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 , .
~ 26 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 '~ 6 g 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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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 ` mg/~ 3a -g 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 :~
` ~14B6X6 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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i -~ Consequently, the magnetically absorptive material repre-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 , .
~ 26 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 :
;
:
~.~
~ 8~
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 '~ 6 g 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.
:
~ 36;~i :
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.
:
`:
~:
;
. ' :
Claims (26)
1. A method of isolating the shield of a shielded conduc-tor system from a low frequency power source to which the shield may be coupled, and for shielding the field within the conductor system from ambient high frequency electro-magnetie interferenee, eomprising:
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.
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.
2. A method of isolating the shield of a shielded conduc-tor system from a low frequency power source to which the shield may be coupled, and for shielding the field within the conductor system from ambient high frequency electro-magnetic interference, comprising:
providing an interruption in the shield; and situating within the interruption a series of dielectric elements separated by magnetically absorptive material to create a capacitive coupling across the inter-ruption to isolate the shield and magnetic absorption within the interruption to absorb energy associated with the ambient electromagnetic interference.
providing an interruption in the shield; and situating within the interruption a series of dielectric elements separated by magnetically absorptive material to create a capacitive coupling across the inter-ruption to isolate the shield and magnetic absorption within the interruption to absorb energy associated with the ambient electromagnetic interference.
3. The method as set forth in claim 2 including situating within said interruption discrete elements of dielectric material and magnetically absorptive material in alternat-ing sequence
4. A method as set forth in claim 3 wherein the shield includes a relatively large diameter portion separated by the interruption from a relatively smaller diameter portion, such that the relatively large diameter portion overlaps the smaller diameter portion, and wherein the dielectric and magnetically absorptive elements are dis-posed between overlapping portions of the shield.
5. A method as set forth in claim 3 wherein said dielectric and magnetically absorptive elements are aligned coaxially within the shield's interruption.
6. A method as set forth in claim 3 wherein the shield is interrupted with a pair of radial flanges, arranged vis-a-vis and wherein said dielectric and magnetically absorptive elements are sandwiched between the flanges and concentrically arranged such that the alternating sequence is in a direction radial to the conductor system.
7. A method as set forth in claim 3 wherein the interrup-tion is provided by separating the shield into two parts, turning the ends of the separated parts and interleaving the turned ends so as to provide a total of at least three spaces between the interleaved parts and wherein magnetically absorptive material is disposed in one of said spaces and dielectric material is diposed in two of said spaces on opposite sides of the magnetically absorptive material
8. A method as set forth in claim 7 wherein said dielectric and magnetically absorptive elements are aligned coaxially in said interruption.
9. A method as set forth in claim 7 wherein said dielectric and magnetically absorptive elements are aligned radially with respect to the conductor system.
10. A method of providing A.C. line isolation between a shielded conductor system and a television tuner input adapted to receive television signals from the conductor system, and for shielding the desired high frequency field within the conductor system from ambient electro-magnetic interference, comprising:
providing an interruption between an outer shield associated with the conductor system, and an outer conductor associated with the tuner input; and disposing within the interruption a series of dielectric elements separated by magnetically absorptive material 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.
providing an interruption between an outer shield associated with the conductor system, and an outer conductor associated with the tuner input; and disposing within the interruption a series of dielectric elements separated by magnetically absorptive material 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.
11. A method as set forth in claim 10, wherein the interruption is established by providing an area of axial overlap between the outer conductor of the tuner input and the shield, and wherein the dielectric elements and magnetically absorptive material are disposed in said area of axial overlap.
12. A method as set forth in claim 11, wherein said dielectric elements and magnetically absorptive material comprises at least two discrete elements of dielectric material separated by a discrete element of magnetically absorptive material.
13. A method of providing A.C. line isolation in a path coupling the shield of a shielded conductor to a conductor associated with the input of a television tuner, and for shielding the desired field within the shielded conductor from ambient electromagnetic interference, comprising:
providing an interruption in the path of coupling between the conductor associated with the tuner input and the shield; and disposing within the interruption dielectric and magnetically absorptive material so as to create a first capacitance which decouples the path at A.C. line frequencies and shunts a substantial portion of electro-magnetic interference induced in the outer skin of the shield, a region of magnetic absorption for absorbing residual electromagnetic interference not shunted by said first capacitance, and a second capacitance to provide additional A.C. decoupling of said path and to couple the signal within the shielded conductor to the tuner input and away from the region of magnetic absorption.
providing an interruption in the path of coupling between the conductor associated with the tuner input and the shield; and disposing within the interruption dielectric and magnetically absorptive material so as to create a first capacitance which decouples the path at A.C. line frequencies and shunts a substantial portion of electro-magnetic interference induced in the outer skin of the shield, a region of magnetic absorption for absorbing residual electromagnetic interference not shunted by said first capacitance, and a second capacitance to provide additional A.C. decoupling of said path and to couple the signal within the shielded conductor to the tuner input and away from the region of magnetic absorption.
14. 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, 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 absorptive and dielectric material situated within the interruption, said 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 interference.
means defining an interruption in the shield;
and magnetically absorptive and dielectric material situated within the interruption, said 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 interference.
15. 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, 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 a series of dielectric elements separated by magnetically absorptive material disposed in said interruption 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.
means defining an interruption in the shield;
and a series of dielectric elements separated by magnetically absorptive material disposed in said interruption 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.
16. An isolator as set forth in claim 15 wherein discrete elements of dielectric material are disposed in said interruption in alternating sequence with discrete elements of magnetically absorptive material.
17. An isolator as set forth in claim 16 wherein the shield includes a relatively large diameter portion separated by the interruption from a relatively smaller diameter portion, such that the relatively large diameter portion overlaps the smaller diameter portion, and wherein the dielectric and magnetically absorptive elements are disposed between overlapping portions of the shield
18. An isolator as set forth in claim 16 wherein said dielectric and magnetically absorptive elements are aligned coaxially within the shield's interruption.
19. An isolator as set forth in claim 16 wherein the shield is interrupted with a pair of radial flanges arranged vis-a-vis,and wherein said dielectric and magnetically absorptive elements are sandwiched between the flanges and concentrically arranged such that the alternating sequence is in a direction radial to the cable.
20. An isolator as set forth in claim 16 wherein the interruption is provided by separating the shield into two parts, turning the ends and interleaving the turned ends of the separated parts so as to provide a total of at least three spaces between the interleaved parts, and wherein magnetically absorptive material is disposed in one of said spaces and dielectric material is disposed in two of said spaces on opposite sides of the magneti-cally absorptive material.
21. An isolator as set forth in claim 20 wherein said dielectric and magnetically absorptive elements are aligned coaxially within the interruption.
22. An isolator as set forth in claim 20 wherein said dielectric and magnetically absorptive elements are aligned radially with respect to the conductor.
23. In a television receiver having a tuner connected via a coupling path to at least the shield of a shielded conductor for receipt of a television signal carried by the conductor, an isolator for isolating the shield from A.C.
line voltages which may be coupled to the tuner and for shielding the field within the conductor from ambient high frequency electromagnetic interference, comprising:
means defining an interruption in the coupling path; and a series of dielectric elements separated by magnetically absorptive material disposed in the interrup-tion so as 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.
line voltages which may be coupled to the tuner and for shielding the field within the conductor from ambient high frequency electromagnetic interference, comprising:
means defining an interruption in the coupling path; and a series of dielectric elements separated by magnetically absorptive material disposed in the interrup-tion so as 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.
24. An isolator as set forth in claim 23, wherein discrete elements of dielectric material are disposed in said interruption in alternating sequence with discrete elements of magnetically absorptive material.
25. An isolator as set forth in claim 24, wherein the tuner has a coaxial connection for coupling to the conductor, wherein said connection has an outer diameter greater than the outer diameter of the conductor for partly overlapping the conductor, and wherein the interruption is provided between the conductor's shield and the overlapping portion of the tuner connection.
26. An isolator as set forth in claim 25, wherein the dielectric and magnetically absorptive elements are aligned coaxially within the interruption.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US13202080A | 1980-03-20 | 1980-03-20 | |
| US132,020 | 1980-03-20 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1148626A true CA1148626A (en) | 1983-06-21 |
Family
ID=22452075
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000366011A Expired CA1148626A (en) | 1980-03-20 | 1980-12-03 | Line isolation and interference shielding for a shielded conductor system |
Country Status (3)
| Country | Link |
|---|---|
| EP (1) | EP0036746A3 (en) |
| JP (1) | JPS56141628A (en) |
| CA (1) | CA1148626A (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH0789623B2 (en) * | 1987-06-06 | 1995-09-27 | 日立電線株式会社 | Tightly coupled signal transmission / reception transmission device |
| US4952896A (en) * | 1988-10-31 | 1990-08-28 | Amp Incorporated | Filter assembly insertable into a substrate |
| CN103354450B (en) * | 2012-10-24 | 2015-12-02 | 杭州祺来电子有限公司 | Radio frequency signal ground return isolator |
| FR3116646B1 (en) | 2020-11-26 | 2023-06-30 | Thales Sa | Power cable with integrated filter |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE902028C (en) * | 1943-06-10 | 1954-01-18 | Telefunken Gmbh | Broadband ultra high frequency line |
| US2962677A (en) * | 1945-10-04 | 1960-11-29 | Bell Telephone Labor Inc | Wave guide joint |
| FR2254864B1 (en) * | 1973-12-18 | 1976-10-08 | Cables De Lyon Geoffroy Delore | |
| JPS5838904B2 (en) * | 1974-04-20 | 1983-08-26 | 日本電気株式会社 | Microhakan |
| FR2275017A1 (en) * | 1974-06-11 | 1976-01-09 | Thomson Csf | VERY SHORT PARASITIC WAVES ATTENUATION DEVICE, USED IN PARTICULAR IN ELECTRONIC TUBES, AND ELECTRONIC TUBES INCLUDING SUCH DEVICES |
-
1980
- 1980-12-03 CA CA000366011A patent/CA1148626A/en not_active Expired
-
1981
- 1981-03-17 EP EP81301118A patent/EP0036746A3/en not_active Withdrawn
- 1981-03-18 JP JP3802481A patent/JPS56141628A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| EP0036746A2 (en) | 1981-09-30 |
| JPS56141628A (en) | 1981-11-05 |
| EP0036746A3 (en) | 1981-12-30 |
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