EP0036746A2

EP0036746A2 - Method and apparatus for line isolation and interference shielding for a shielded conductor system

Info

Publication number: EP0036746A2
Application number: EP81301118A
Authority: EP
Inventors: Pierre Dobrovolny
Original assignee: Zenith Radio Corp
Current assignee: Zenith Electronics LLC
Priority date: 1980-03-20
Filing date: 1981-03-17
Publication date: 1981-09-30
Also published as: EP0036746A3; JPS56141628A; CA1148626A

Abstract

method and structure is disclosed for isolating the shield of a shielded conductor system from a low frequency 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 magnetically 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 isolation 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

This invention relates generally to the fields of high frequency electromagnetic interference shielding and A.C. power isolation and is more particularly directed to a method and apparatus for the shielding of high frequency shielded conductor systems, such as coaxial cables, from electromagnetic interference and the simultaneous isolation of such conductor systems 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 required by Underwriters Laboratories (U.L.) in the United States to doubly isolate exposed metal parts from the A.C. line which powers the receiver. For example, the 300 ohm twin lead terminals usually situated on the rear of the receiver's cabinet are required to be separately isolated. Such isolation is intended to doubly insulate a consumer from accidental shock which he might otherwise receive either from contact with the exposed terminals or with the metal "rabbit ear" antenna to which such terminals are sometimes connected.
Conventionally, television receivers also include 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 of the coaxial input, evidently because the technology has not been available to television manufacturers to enable them to provide such isolation while simultaneously affording acceptable television reception.
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 permit ambient high frequency electromagnetic 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 conventional capacitors coupling the coaxial cable with the tuner input to A.C. isolate the cable from the tuner. While the isolation thus achieved is satisfactory, the field within the cable is inadequately shielded from electromagnetic interference.
Another prior approach involves a t.v. antenna isolation assembly designed to effect power line isolation at the antenna input of a television receiver. In this approach a number of ferrite beads are loosely fitted around the coaxial cable conductor connected to the 75 ohm input and a number of capacitors are mounted externally of the conductor. While this arrangement provides A.C. line isolation) in fields of strong ambient electromagnetic interference its shielding effect is ineffective.
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.C. 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 electromagnetic interference shielding is of importance.
It is a general aim of the invention to provide a method and apparatus for isolating the shield of a shielded high frequency conductor system from a low frequency power source in such a way that the desired field within the cable is shielded from ambient high frequency electromagnetic interference.
The present invention therefore provides a method and apparatus for accomplishing this aim by 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.
The features and advantages of the invention are more particularly set forth in the following detailed description of a number of preferred embodiments of the invention taken together with the accompanying drawings, in which:

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;
Figure 3 is a lumped-element equivalent circuit diagram useful in explaining the operation of the embodiment shown in Figure 2; and
Figures 4-6 illustrate alternate embodiments of the invention.

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 of 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 22 are selected to provide a high impedance at the low 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 satisfactory, the simple capacitive decoupling of the outside conductor can cause an intolerable increase in electromagnetic interference, particularly when a local signal is broadcast on the same frequency 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 portion 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.absorptive 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 40. Each such element is annular and has a central opening to surround the smaller diameter outer conductor. The elements 36, 38 and 40 may be stacked one against the other and aligned 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 from the leftmost portion 30. Hence, any A.C. line voltage applied to the leftmost portion 30 is inhibited from reaching the rightmost portion 32. In addition, the capacitances formed by the elements 36 and 38 co-operate with the element 40 to shield the field inside the cable 24 from ambient electromagnetic radiation, as described hereinafter.
The magnetically absorptive element 40 serves to absorb electromagnetic interference not bypassed by the capacitive 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 circuit diagram of a two port which may be placed between the cross sections AA (input port) and BB (output port) of Figure 2. The source I represents the current on the outer surface of the outer conductor induced in the vicinity of the cross section AA by the ambient interfering signal. The source E represents the desired signal to be carried by the cable, the resistor Rl represents the nominal output impedance of the source E (75 ohms), and the resistor R2 represents the nominal input impedance (75 ohms) of a television tuner.
The resistor R3 represents the equivalent series resistance (100 ohms, for example) of the magnetically absorptive element 40, the capacitor Cl represents the capacitance due to the effect of the dielectric element 36, and the capacitor C2 represents the capacitance due to the effect of the dielectric element 38. Each capacitor Cl and C2 may, by way of example, have a value of about 1000 picofarads.
At typical television frequencies, the impedance of the capacitors Cl and C2 is much less than the impedance of any of the resistors in Figure 3. Hence, the capacitor Cl shunts the desired signal from source E away from the resistance R3 and toward the input impedance of the tuner. Consequently, the magnetically absorptive material represented 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 substantial 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. However, capacitors Cl and C2 cause the residual electromagnetic 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 lower 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 of interest for a particular application.
Referring again to Figure 2, the arrangement shown therein has been found to provide exceptional shielding from electromagnetic 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 constant of several thousands to provide a total capacitance of about 2000 picofarads. Barium titante is one example of such dielectric material.
The element 40 is made of a magnetically absorp-. tive material whose equivalent series resistance is as high as possible at the frequencies of interest for best absorption of electromagnetic interference. A ferrite material having an equivalent series resistance of about 100 ohms has been found to be acceptable for use at television frequencies. Such a ferrite 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. The 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 interruption or cavity described above need not be completely disposed in the cable alone. For example, in Figure 2, the leftmost 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 the 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 corresponding connection to a tuner input or corresponding structure. In fact, the required isolation and shielding may be effected by disposing the interruption at any practical location in a coupling path between the outer conductor of the cable and the input to the tuner or corresponding structure.
Such a connector and cable as shown in Figure 2 may be disposed with a television receiver's cabinet. In that case, the cable itself need not be flexible as is the case with conventional coaxial cable. Instead, the cable may be constructed of conductive pipe having a center conductor. Such a pipe will be understood to be the equivalent of a coaxial cable, wherefore, references herein to a coaxial cable or a shielded conductor are intended to be inclusive of such pipes.
In some instances, 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 television tuner or corresponding structure. Hence, references herein to a shielded conductor are meant to include such connectors and their equivalents.
The isolator of Figure 2 comprising the elements 36, 38 and 40 is illustrated as employing only one ferrite or magnetically absorptive element disposed between a pair of dielectric elements. However, additional dielectric and ferrite elements may be used in an alternating sequence, as shown in phantom at 138 and 140, respectively. In the illustrated preferred embodiment, the first element on the inside (element 36 in Figure 2) is a dielectric element so that no losses are introduced into the desired signal path. The first element on the outside (element 38 in Figure 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 isolator, the construction of which depends on the main direction in which the electromagnetic interference signal within the isolator is forced to propagate (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 clement 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 propagates axially and the dielectric-fertile 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 ports (left and right, as shown). The ends of the separated parts are interleaved so as to provide a total of at least three spaces between 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.
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 outer 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 fact, 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 limitation 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 that modifications and alterations may be made to the method and structure described herein without 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 claims.

Claims

1. A method of isolating the shield of a shielded conductor 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 electromagnetic interference, said method 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.

2. The method of claim 1, wherein a series of dielectric elements separated by magnetically absorptive elements are situated within the shield interruption to create a capacitive coupling across the interruption to isolate the shield and provide magnetic absorption within the interruption.

3. An isolator for 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, said isolator isolating the conductor's shield from the low frequency power source and shielding the desired field within the conductor from ambient high frequency electromagnetic interference, said isolator including 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.

4. The isolator according to claim 3, wherein said material comprises a series of dielectric elements separated by magnetically absorptive elements disposed in said interruption.

5. The isolator of claim 4 wherein discrete elements of dielectric material are disposed in said interruption in alternating sequence with discrete elements of magnetically absorptive material.

6. The isolator of claim 4 or 5 wherein said dielectric and magnetically absorptive elements are aligned coaxially within the shield's interruption.

7. The isolator of claim 4 or 5 wherein said dielectric and magnetically absorptive elements are aligned radially with respect to the conductor.

8. The isolator of claim 7 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.

9. The isolator of any of claims 4 to 8 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.

10. The isolator of any one of claims 4 to 9, 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 magnetically absorptive material.