US3440571A

US3440571A - Inductively coupled unidirectional taps

Info

Publication number: US3440571A
Application number: US574712A
Authority: US
Inventors: Keneth A Simons
Original assignee: Jerrold Electronics Corp
Current assignee: Jerrold Electronics Corp
Priority date: 1966-08-24
Filing date: 1966-08-24
Publication date: 1969-04-22
Anticipated expiration: 1986-04-22
Also published as: GB1187666A

Description

April 22, 1969 K. A. SlMONS INDUCTIVELY COUPLED UNIDIHECTIONAL TAPS Filed Aug. 24. 1966 OTHER LINES SUCH 14$ /N$3'4 VIA COUPLER-5 SUCH A55 AT TORNEYJ United States Patent Int. Cl. H0lp /14 US. Cl. 333- 14 Claims ABSTRACT OF THE DISCLOSURE A directional coupler for inductively coupling CATV subscribe drop lines or the like to a coaxial feeder cable. A coil is disposed adjacent the center conductor of the feeder cable; but insulated therefrom. One side of the coil is connected to the center conductor of the drop line, and to a back-match resistor for the drop line, the other side to the outer conductor of the drop line. The back-match impedance is connected to the secondary of a transformer the primary of which is connected across the feeder cable.

This invention relates to transmission line systems and, more particularly, to coupling arrangements between a main transmission line or feeder line and one or more other transmission lines such as drop lines so arranged that such other transmission lines (or drop lines) may be added to the main or feeder line whenever desired to receive signals from the latter line, without cutting the feeder line or interrupting the normal flow of signals over the main or feeder line while in the process of adding such other drop lines to the feeder line.

In present transmission systems used for community antenna television systems, known as CATV systems, directional couplers have been used to connect the feeder line, usually in the form ofa coaxial cable, to one or more drop lines which are also coaxial cables and are expected to receive programs of TV and the accompanying signals. In such systems,'the structure of the directional coupler has made it necessary to physically cut or open the feeder cable so as to insert in series with the central conductor of the cable either one or more coils of one or more transformers,'or 'one or more other elements used to form a directional coupler.

The physical cutting or opening of the feeder line or cable during the installation and the necessary testing procedures to check the operations introduced an undesired interruption in program transmission over the feeder line to all of the other drop lines connected to the feeder line and this generzfily involved a fairly substantial revenue loss to the proprietors of the system and an inconvenience to the various subscribers. A former arrangement, which has been used fairly widely as a directionalcoupler for this purpose, is shown and described in applicants Patent 3,048,798, issued Aug. 7, 1962, entitled Directional Coupler.

An object of the present invention is to provide an improved arrangement for coupling directionally a feeder line to one or more drop lines so that there will be no service interruption whatever in the normal transmission of signals or other energy via the feeder line to any or all other drop lines while the system is undergoing changes during the installation in coupling additional drop lines to the feeder line or in removing drop lines from the feeder line. In the arrangement according to this invention, the drops previously connected to the feeder line will be continuously fed all of the signals or energy from the feeder line substantially without inter- 3,440,571 Patented Apr. 22, 1969 ruption or distortion during the entire process of adding the new drops or of removing some of the drops. I

Another object of the. invention is to provide a unique unidirectional coupling arrangement which comprises very few components and is relatively inexpensive to manufacture and install. The installation may be performed substantially without interfering with normal and continuous transmission over the feeder line and without any appreciable degrading of the service between the feeder line and the various associated subscribers loops. Thus, in the arrangement of this invention, the feeder line..will serve as a continuous reservoir of signal energy to be tapped unidirectionally by additional drop lines as may be desired, and hence drop lines may be added to or removed from the feeder line without in any way affecting the loops or drops previously connected to the feeder line.

Another object of this invention is to provide an arrangernent for coupling a feeder TV line to a plurality of TV drop lines so that TV (and related) signals may flow freely in the forward direction along the feeder line to the drop lines with a predetermined attenuation, but such signals may flow in the opposite direction along the feeder line to the same drop lines with a very much higher attenuation. This unidirectionality is especially important in color TV systems, where reflections or o'iher'disturbing or. unwanted signals between subscribers lines may severely distort the quality of the picture or of the accompanying sound effects to be reproduced.

Another object is to provide a directional coupler for adding a plurality of TV drop lines, one by one, to a feeder TV line, so designed that there will be no appreciable.refiection from any of the plurality of droplines to the feeder line, both during the installation procedures and,after the coupling has been completed. The arrangement will be designed to avoid any breakage or interruption of the feeder line or of the circuit between the feederline and any and all other drop lines that were previously connected to the feeder line, and it is further designed also to maintain continuous transmission over such other lines.

This invention will be better understood from the following description when considered in connection with the accompanying drawing, in which FIG. 1 illustrates schematically the novel coupling arrangement of this invention, and FIG. 2 shows a sectional view of a general arrangement of one form of coil structure for coupling to a coaxial conductor transmission system of the type shown in FIG. 1.

FIG. 1 illustrates a main transmission line in the form of a coaxial cable of a familiar type which consists of a central conductor 1 and an outer shield 2, both of which are metallic and may be spaced from each other by a dielectric which may be, for example, in the form of foam insulation. The coaxial cable system 1-2 may be used, for example, in a CATV system for feeding the usual TV programs and accompanying sound signals carried by the main or feeder line 1-2 to one or more drop lines, one of which is shown for illustration in the form of a similar coaxial cable consisting of a central conductor 3 and an outer shield 4 which are also insulated from each other by a suitable dielectric. The coaxial cable 3-4 (together with other similar cables which need not be shown in the drawing) are designed to receive the signals carried by the main or feeder line 1-2 for supplying those signals to suitable receiving equipments located in the homes or ofiices of subscribers where such signals may be reproduced. The coupling network, shown schematically within the dotted line boundary 5 representing a housing, includes a coil 6 mounted on a core 7 of magnetic material, a transformer 8 which may be an auto-transformer, and a resistor 9 serving as a matching impedance.

The core 7 upon which the coil 6 1S wound is inductively coupled to the central conductor 1 of the main or feeder line 1-2. There is no physical or conductive connection whatever between any part of the electromagnetic structure 6-7 and the central conductor 1. One terminal of coil 6 is connected to the central conductor 3 of the subscribers line 3-4 which is connected to the receiving equipment such as a TV set (not shown). The other terminal of coil 6 is connected in series with resistor 9 which may be considered to be a reverse terminating impedance or a back-match for the feed system 3-4. Resistance 9 is in turn connected to the tap 10 on the auto-transformer 8, the outer terminals of which are physically connected between the central conductor 1 and the outer conductors 2 and 4.

FIG. 2 illustrates, by way of example, a sectional 'view of one form of the electromagnetic structure 6-7 shown generally in FIG. 1. Sheath 2 has been cut away so to accommodate the structure 6-7. The structure 6-7 is embedded in suitable insulation 11 as shown. There is a slot s in insulation 11 to enable the structure 6-7 to be moved over the central conductor 1. The coil structure 6-7 may be held in place as shown within slot s by conventional means familiar to those skilled in the art, such as plugging the remainder of the open slot with insulation and taping it closed. The positioning and spacing of the core on the central conductor is therefore predetermined, and the core is protected against the weather. This construction would insure against any conductive connection between any portion of the coil 6 with any portion of the central conductor 1, Hence, any vibration or movement of the overall coupling structure would not affect the electrical coupling relation between the electromagnetic structure 6-7 with respect to the main transmission line 1-2.

In operation, TV or other signals flowing over the main or feeder line 1-2 from left to right, for example, will be fed to the drop cable 3-4 so that the signals may be faithfully reproduced by receiving equipment connected at the distant end of the drop cable 3-4. At the same time the level of signals passing through the point of the coupling mechanism 5 to other points further to the right along line 1-2 will be but minutely affected by the tapped connection to coaxial cable 3-4. This is due to the fact that the drop cable receives only a minor portion of the signal energy flowing along line 1-2. Hence, additional taps, which may be similar to-the one shown and described as component 5 of FIG. 1, may likewise be applied at other or more distant points along the main transmission line 1-2, without amplification, for supplying other similar drop cable systems, so that subscribers sets connected thereto may likewise be arranged to reproduce the same signals faithfully.

As transmission takes place from left to right along the main transmission line 1-2, the flow of current over the central conductor 1 will cause voltage to be induced in coil 6 which, at any instant, will have a predetermined polarity. At the same time, the auto-transformer 8, which is electrically connected to the central conductor 1 in a well-known manner, will develop a voltage between tap 10 and the outer conductors 2 and 4 which corresponds to the level of signals traversing the inner conductor 1 of the feeder line. This voltage will remain substantially constant and of a predetermined magnitude and polarity. However, the voltage induced in coil 6 will, with predetermined adjustments of the components of the coupling equipment 5, be substantially equal in magnitude and have the same polarity as that developed by autotransformer 8. In other words, the coil 6 voltage and the auto-transformer 8 voltage will be substantially equal and in phase when transmission is taking place from left to right along the feeder cable 1-2. On the other hand, when transmission takes place from right to left over the feederline l- 2, similar currents flowing in the central conductor 1 w ll nduce in coil 6 an equal voltage which will be opposite m polarity to that de eloped by coil 6 when transmission was taking place normally, that is, from left to right along cable 1-2. a

As already observed, coil 6 is wound about core 7 in such a direction that the resulting voltage applied thereby to the central conductor 3 of the drop coaxial cable 3-4 will have, for signals traveling from left to right, a polarity which is the same as thatof the voltage derived from the auto-transformer 8. The voltage across auto-transformer 8 will be applied through tap 10 and resistor 9 to the same central conductor 3. Under these conditions, these voltages will be additive in effect and hence signals in sufficient amplitude will be transmitted from the feeder line 1-2 to the drop line 3-4. The subscribers radio set connected at the distant end of line 3-4 will receive, and will be able to reproduce, those signals with good fidelity.

On the other hand, signals or other effects flowing in the opposite direction, that is, from right to left, over the feeder line 1-2 will induce a voltage in the structure of coil 6 which will be opposite to that derived from auto transformer 8. Since these opposing voltages are made equal in magnitude, none of those signals or other effects will be relayed to the drop line 3-4. The coil 6 voltage obviously may be adjusted to full equality with the opposing voltage of auto-transformer 8 by any wellknown expedients, such as by moving or rotating coil 6 or by a variable slug in or adjacent to coil 6. Hence the directivity of the device may be optimized after installation.

Those skilled in the art will realize that in order for these opposing voltages to be equal over the broad range of signals herein contemplated, the response of said autotransformer and said coil must be substantially flat for the TV spectrum.

This arrangement clearly discriminates against all signals or effects flowing in the reverse direction over feeder line 1-2 and such effects do not reach the drop line 3-4 in sufficient magnitudes so as to distort the desired signals. A

The voltage across the auto-transformer 8 will remain the same whatever may be the direction of transmission along cable 1-2; that is, whether from left to right or from right to left. Thus, when transmission occurs in the feeder line 1-2 from right to left, the voltages generated by coil 6 and auto-transformer 8 will be, in a proper design, substantially out of phase, whereupon such opposing voltages will substantially nullify each other.

The arrangement of the inductive coil 6 and the tapped auto-transformer 8 serves to reduce, and practically to eliminate, in properly designed equipment, the reflection or return ripple which would otherwise occur in a nonbalanced tap system and would introduce distortion into the signals.

In a properly designed system, the transmission level at the receiver connected to line 3-4 may be in the range of 10 to 50 db lower than the level of signals that may traverse the feeder line 1-2. Such a comparative signal level will be suitable for transmission over the auxiliary line when it is carrying a plurality of TV programs occupying a broad band of the frequency spectrum, as, for example, the band extending from the subaudible frequency range to the UHF range, including that part of the region extending from 54 mHz. to 216 mHz.

Resistor 9 and coil 6 are connected via tap 10 and a part of the auto-transformer 8, as shown, to the terminal of the central conductor 3 of the drop line 3-4 adjacent to the feeder line 1-2. This resistor 9 serves as a backmatch or terminating impedance for line 3-4. This timpedance is designed to match the normal characteristic impedance of line 3-4, for example, 75 ohms. This backmatch, or the impedance of resistor 9, will act to absorb reflections or other signals or effects returned by irregu= larities in line 3-4 or by signals.or other effects generated or picked up by the subscribers set. As a conse quence of the directional properties of the coupler, miscellaneous signals or other undesired effects traveling along line 1-2 in a reverse direction will not be relayed to the drop lines connected to the line 1-2, The suppression of these undesirable voltages will be etfective throughout the frequency ranges of all signals tending to flow in line 1-2. The core 7 of the electromagnetic struc ture may be made, for example, of ferrite material, but any other suitable core material may be appropriate for this arrangement, 1

The arrangement shown in FIG. 1 is a practical unidirectional coupler, that is, it serves to transmit signals from the feeder line 1-2 to the drop line 3-4 when they arrive from one direction on line 1-2, and it does not transmit any substantial amount of energy when signals arrive from the other direction on line 1-2, This unidirectional feature is accomplished without any breakage or interruption of the feeder line at any time. Thus, subscribers loops previously connected to the feeder line 1-2 will be substantially unaffected as additional couplers, such as 5, and their corresponding loops, are installed across feeder line 1-2, Similarly, subscribers loops connected to the main line will be substantially unaffected when specified unidirectional couplers, such as 5, and their corresponding loops are removed from the feeder line. By avoiding breakage or interruption the main transmission line, service continuities are maintained and substantial savings are incurred.

'It is apparent that in the arrangement of this invention, compensating components such as condensers or usual way as would be apparent to those skilled in the other reactive or resistive components may be used in the art. While this invention has been shown and described with reference to a CATV system and to certain frequency ranges, the features 'of this invention are equally applicable to transmission systems for other types of signals and for other frequency ranges,

While this invention has been shown and described in certain particular eriibodiments merely for illustration, other embodiments will be apparent to those skilled in the art. Hence, the invention is to be treated as limited only by the scope of the appended claims What is claimed is:

1. Apparatus for coupling a"-subscribers coaxial cable to a main coaxial cable carrying currents corresponding to TV programs to be transmitted to remote points, the coupling to be effected without breaking the circuit of the main cable and without interfering with the continuity of service to the remote points, comprising a coil structure, which is inductively coupled to the main cable and produces voltages continuously proportional to, and in a predetermined phase relation to the currents in the main cable, and atransformer connected between the center and outer conductors of the main cable and producing voltages proportional to the voltages across the main cable and in a predetermined phase relation to such voltages, and means for combining the two voltages produced and applying them to the subscribers cable.

2. Apparatus according to claim 1 wherein said inductively coupled coil is adapted to fit within a slot cut into the sheath of the main cable.

3. Apparatus according to claim 1, such apparatus also including an impedance matching the impedance of the subscribers cable and connected at the coupler end thereof.

4. Apparatus according to claim 3 including a housing for enclosing both coil structures and the terminating impedance, said housing being positioned at the point where the two cables meet,

5. A coupler for coupling two coaxial cables to each other so as to transmit unidirectionally between said cables a broad band of signals, such as those corresponding to television programs, said coupler being installable without breaking the circuit of either cable, comprising means for deriving from the signals traversing one of the cables a first voltage corresponding to the currents flowing in said cable and a second voltage equivalent to that across said cable, and means for combining said voltages so as to be additive for signals travelling in the normal direc* tion on said cable and subtractive for signals travelling in the other direction, the first voltage deriving means consisting of a coil wound upon a magnetic core and positioned in inductive relation to the central conductor of the cable with which it is directly associated.

6. A coupler for coupling first and second coaxial cables for the unidirectional transmission of signals corresponding to television programs or like signals between said cables, said coupler being installable without breaking the circuit of the first cable, comprising means for inductively generating a first voltage which corresponds to the signal currents flowing over the first cable, means for generating a second voltage which corresponds to the voltage across said first cable, and means for applying both voltages to the second cable, so as to be additive when the signals are transmitted in one direction over said first cable andbeing subtractive when the signals over said first cable are reversed in direction, the first voltage generating means consisting of a single coil mounted 'on a magnetic core which is inductively related to the central conductor of the first cable but has no conductive connection to said central conductor.

7. An arrangement corresponding to claim 6 which also includes a terminating impedance for the second cable, said terminating impedance being connected to the second cable at a point adjacent to the first voltage generating means.

8. An arrangement in accordance with claim 6 in which the first voltage generating means is enclosed within a housing positioned around the sheath of the first cable.

9. A coupler for coupling a first coaxial cable to a second coaxial cable so as to transmit unidirectionally a broad band of signals, such as television signals, from the first cable to the second cable, without breaking the circuit of the first cable, comprising a coil mounted on a magnetic core and disposed within a slot cut therefor in the dielectric of the first coaxial cable, and responsive solely to currents flowing serially over the first cable, said coil having no conductive connection to the central conductor of the first cable, a second coil connected across the first cable and responsive to the voltages across the first cable and having no capacitive or inductive coupling therewith, said coils being so coupled to the first coaxial cable and connected thereto and to each other and to the second cable so that the voltages of the two coils arecombined and applied to the second cable, being additive during signal transmission over the first cable in one direction and in opposition to one another during signal transmission over the first cable in the opposite direction.

10. An arrangement according to claim 9 which includes a terminating impedance connected across the second cable at a point where both cables meet, said terminating impedance serving to substantially prevent any return of energy from the second cable to the first cable.

11. Apparatus for coupling a subscribers coaxial cable to a main coaxial cable carrying currents corresponding -to TV programs to be transmitted to remote points, the

coupling to be effected without breaking the circuit of the main cable and without interfering substantially with the continuity of service to the remote points comprising: means inductively coupled to said main cable for producing a first voltage proportional to the current flow at a selected point within said main cable, means connected across said main cable for producing a second voltage proportional to the voltage across said main cable, means for combining said first and second voltages so as to be additive when the current in said main cable is flowing in the normal direction, and subtractive when the current is flowing back through said main cable, and means for applying the sum of said voltages to the subscriber cable, wherein said inductive means comprises a coil so disposed about the center conductor of said main coaxial cable as to be closely coupled thereto, and one side of which connects to the center conductor of said subscriber cable.

12. The device of claim 11, further comprised of means to block current fiow within said subscriber cable in opposition to that induced within said coil by the normal signal flow in said main cable, including an impedance matching that of said drop line and connected between the other side of said coil and the secondary of said transformer 13, The device of claim 12 wherein said coil is so positioned with respect to the center conductor of said main 8 1%. The device of claim 13 wherein said means for producing a second voltage comprises a transformer the secondary winding of which connects to the other side of said coil.

References Cited UNITED STATES FATENTS 2,531,438 11/1950 Jones 343-s52 x US. Cl, X.R.