US 3747028 A
Combination directional couplers and splitters for extracting controlled fractions of signal power, for use in CATV or similar signal distribution systems and having improved operating characteristics are obtained by the incorporation of pi-section high-pass filters into such integrated devices. Conventional directional taps are modified by the addition of inductances and capacitances so that the required pi-section high-pass filters are formed, with one inductive element of such a filter being constituted by an inductive deviation, specifically a leakage inductance of an existing component of such a device. Improved radio frequency signal return losses and isolation are provided over a much greater frequency range than was heretofore possible while permitting effective distribution of signal power to a plurality of tap lines.
Claims available in
Description (OCR text may contain errors)
Pennypacker DIRECTIONAL TAP COMPRISING PI-SECTION HIGH PASS FILTER FOR USE IN CATV SYSTEM  Inventor: Frank C. Pennypaclrer, Lindsay,
Ontario, Canada  Assignee: Lindsay Specialty Products Limited,
- Ontario, Canada  Filed: Apr. 22, 1971  Appl. No.: 136,443
Related US. Application Data  Continuation-impart of Ser. No. 94,230, Dec. 2, 1970,
Pat. No. 3,671,885.
 U.S. Cl. 333/10, 333/11, 333/73 R  int. Cl. H0lp 5/14  Field of Search 333/6, 7. 10, 11, 333/32, 73 R; 325/308  References Cited UNITED STATES PATENTS [451 July 17, 1973 3,559,] 10 l/i97l Wiley et al ..l 333/10 3,593,222 7/1971 Rizzi 333/73 R Primary Examiner-Paul L. Gensler Att0rneyArne l. Fors, Frank I. Piper, James T. Wilbur and Brian Thorpe [5 7] ABSTRACT Combination directional couplers and splitters for extracting controlled fractions of signal power, for use in CATV or similar signal distribution systems and having improved operating characteristics are obtained by the incorporation of pi-section high-pass filters into such integrated devices. Conventional directional taps are modified by the addition of inductances and capacitances so that the required pi-section high-pass filters are formed, with one inductive element of such a filter being constituted by an inductive deviation, specifically a leakage inductance of an existing component of such a device. improved radio frequency signal return losses and isolation are provided over a much greater frequency range than was heretofore possible while permitting effective distribution of signal power to a plurality of tap lines.
4 Claims, 6 Drawing Figures PATENIED JUL 1 (I915 SHEET 2 [1F 3 PRIOR ART FIG, 4
INVENTOR. FRANK C. PENNYPACKER am L S Azut PATENTED SHEET 3 BF 3 INVENTOR. FRANK C. PENNYPACKER DIRECTIONAL TAP COMPRISING PI-SECTION HIGH PASS FILTER FOR USE IN CATV SYSTEM BACKGROUND OF THE INVENTION This invention relates to a directional tap and is particularly directed to the combination of a directional coupler and splitter for tapping off a portion of the power of a signal from a main line and feeding said tapped power to one or more subsidiary lines.
The present application is a continuation-in-part of application Ser. No. 94,230, filed Dec. 2, 1970 and now US. Pat. No. 3,671,885 issued June 20, 1972.
It is common practice to transmit television signals through a residential area on a main line coaxial cable and to tap off portions of the signal power at spaced apart points along the cable for receiver sets. A device known as a directional tap is used at these tap off points and this invention relates to improvements in such taps that are designed to improve its bandwidth, return loss, directivity, and tap-line to tap-line isolation.
With the ever-increasing acceptance of CATV systems and with the ever-increasing number of television channels and FM radio stations available to the public, the need for CATV system components capable of effectively handling correspondingly wider frequency bands continues to grow. Furthermore, it is desired to provide said directional taps to use in such systems which can not only be used over such wider frequency bands but which will also provide improved operating performance over such bandwidths.
Devices of this type must be directive in the sense that they transfer the required portion of the signal that enters the main line input to the tap output but reject a signal that is reflected back along the line to enter the device at the main line output. Signals reflected in this way which appear at the tap output appear in the case of television signals as ghosts on the television screen. The ability of a signal tap to exclude reflected signals from the tap output is referred to by the term directivity." A tap device with good directivity is efficient at rejecting unwanted reflected signals from the tap out put.
For effective use in CATV distribution systems, it is generally desirable for a device of the aforementioned type to provide the best possible matching between the various input and output signal-carrying cables connected thereto as indicated by the highest possible return loss value, as well as providing maximum isolation between the individual output cables so in turn preventing the transfer of interference signals from one tap line to another. Such conventional passive devices generally can be used for the distribution of signals over a frequency range from about 40 to about 300 MHz and provide inter-cable isolation values of about 26 decibels with radio frequency signal return losses of about 20 decibels.
It is a principal object of the present invention to provide directional taps for use in CATV distribution systems which can be used effectively over wider frequency ranges than those previously known.
Another important object of this invention is the provision of devices ofthe aforementioned type which provide improved inter-cable isolation and which provide improved inter-cable matching, as indicated by higher values for radio frequency signal return losses.
Another object of this invention is to provide passive devices for use in CATV systems which can be constructed, if so desired, for the passage therethrough of alternating electrical current such as 60 Hz power for the purpose of energizing amplifiers or other equipment incorporated in the system.
Other objects of the invention, and the manner in which they can be attained, will become apparent as the description herein proceeds.
SUMMARY OF THE INVENTION The invention is based on the finding that the aforementioned improvements in the operating characteristics of directional taps, for use in CATV distribution systems, can be obtained to a surprising extent by the provision in such devices of pi-section high-pass filters conforming to certain requirements.
In general, the invention relates to a directional signal tap having a main line input, a main line output and one or more tap outputs, said tap comprising one or more directional couplers and one or more splitter circuits to provide additional outputs, with one or more pi-section high pass filters to provide a radio frequency signal return loss of at least 20 decibels at all terminals and a inter-tap line isolation of at least 26 decibels over an extended frequency range.
More particularly, the invention provides a device having at least four two-pole terminals with first poles of all said terminals being electrically interconnected by a base conductor and with second poles of all said terminals being inductively and otherwise coupled together in said device for the passage of electrical signals therebetween and for the passage of radio frequency signals between first and second ones of said terminals at less than about 4 decibels of loss, which device includes a pi-section or multiple pi-section high-pass filter connected between said first and second ones of said terminals to provide a radio frequency signal return loss of at least about 20 decibels at said first and second ones of said terminals over an extended radio frequency range, a third one or more of said terminals being coupled within said device to said first one of said terminals to provide coupling to said first one of said terminals of from about 6 to about decibels from said first terminal to said third or more terminals, and
an isolation of said third or more terminals and said second terminal of at least 15 decibels higher than the coupling between said first and said third or more terminals.
The performance of directional taps can be improved by the incorporation of pi-section high-pass filters in the directional coupler only. This will provide a wider bandwidth and improved radio frequency signal return loss at first and second terminals without unnecessarily improving the tap line to tap line isolation and tap line return loss.
To improve the function of the directional coupler, the first one of the terminals is serially connected through a first coupling capacitor to a first inductor effectively connected in turn to said second one of said terminals, the first one of the terminals also being effectively grounded through a second inductor to the base conductor. The first inductor of such a coupler is also effectively grounded intermediate its ends through a third inductor to the base conductor so that the first, second and third inductors and the coupling capacitor together constitute the pi-section high-pass filter for the passage of radio frequency signals from the first one of the terminals to the second one of the terminals at a radio frequency signal attenuation value, of from about 0.2 to about 2 decibels, second and third induc tors acting effectively as one inductance. Furthermore, the third one of the terminals of such a coupler is inductively and otherwise coupled to the aforementioned first inductor for the passage of radio frequency signals from the first one of the terminals to the third one of said terminals with a radio frequency attenuation value of from about 6 to about 18 decibels while providing an isolation of from about 20 to about 50 decibels from said second terminal to said third terminal.
In the application of this invention to the construction of splitters the device is usefully constructed so that the pi-section high-pass filter includes a coupling capacitor connected between the first one of the terminals and the directional coupler which is effectively serially connected between that first coupling capacitor and the base conductor. A second inductor is effectively connected between the first one of the terminals and the base conductor by the inductance of an autotransformer, and second and third ones of the terminals are coupled to such an autotransformer inductor for the passage of radio frequency signals between the first one of the terminals and each of the second and third ones of those terminals over the extended radio frequency range.
The manner in which the concept of the invention is applied in practice to the construction directional taps will be more readily understood as the description herein proceeds with reference to the specific embodiments shown in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described merely by way of illustration with reference to the accompanying drawings inwhich:
FIG. 1 is a fragmentary and schematic diagram of a CATV distribution system showing the manner in which directional taps can be utilized in accordance with the present invention;
FIG. 2 is an enlarged schematic diagram of a directional coupler utilized in the present system;
FIG. 3 is an enlarged schematic diagram of a splitter shown in FIG. 1;
FIG. 4 is a schematic diagram of a four-output directional tap of a conventional design having internal hybrid splitters to provide four outputs;
FIG. 5 is a schematic diagram of an improved directional tap according to the present invention; and
FIG. 6 shows a directional tap with the splitter formed by a resistive network.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION Referring first to FIG. 1, there is shown therein generally at 10 a CATV distribution system of a conventional type and intended for the distribution of radio frequency television and high frequency radio signals from a community antenna (not shown) to a plurality of households or subscribers indicated schematically at 11 through 18.
The CATV system 10 of FIG. 1 includes a trunk distribution line fragmentarily shown at 20, extending from the community antenna station (not shown) and to which the individual subscribers or households 11 to 18 are connected by means of directional taps generally indicated at 21.
For the purpose of maintaining adequate signal strength at all positions along the trunk distribution line 20, it is conventional to incorporate amplifiers 24 in that line 20. Such amplifiers 24 are conventionally powered by 60 Hz electrical current supplied to the amplifiers through the main trunk line 20. Such power can be inserted into the trunk line 20 at the community antenna station but it is also conventional to insert additional power into the trunk line 20 at one or more positions remote from the antenna station. The system 20 is shown in FIG. 1 merely by illustration as being supplied with such power from a step-down transformer 25 connected to the line 20 through a power inserter indicated at 26, the transformer 25 being in turn supplied, for example, with volt current from a power line 28.
It will be understood that, although the trunk line 20 and the distribution cables between that line and the various subunits of the system 10 are shown in FIG. 1 by single lines, such cables customarily will be in the form of coaxial cables having outer conductive sheath electrodes which are interconnected through the metallic casings of the various sub-units. Such second lines will be referred to herein as ground or base lines but it should be understood that such references are not intended to restrict the invention to systems in which such lines are actually at ground potential.
It must further be understood that the overall system 10 as shown in FIG. 1 is included herein merely for the purpose of explanation and that such system can be modified in many way: without departing from the scope of the invention. Before proceeding further to set down the novel features provided by this invention for use in the directional taps which can themselves be utilized in the system 10 shown in FIG. 1 or in other such systems, some further observations concerning the various sub-units of that system will now be considered.
In the first place, it will be understood that, in order to obtain the lowest possible extent of radio frequency signal reflection at any of the components of the system 10, for example, at the directional tap 29, it iS essential to have optimum impedance matching between the several lines connected to such sub-units. It is also frequently desirable to reduce the signal strength losses which occur on passage of signals through such subunits to the maximum possible extent. Furthermore, maximum signal isolation between the individual outputs of each of the directional taps 29 is desirable as is well known to those conversant with conventional CATV distribution technology.
Referring now to FIG. 2, it will be seen that the directional coupler 21 shown schematically therein is designed and constructed to receive radio frequency signals from the trunk line 20 and to distribute a fraction of such a signal to the terminal indicated at 31 while permitting the balance of the signal to pass to the continuing main trunk line as indicated at 20'. To provide such desired proportioning of the signal intensity, directional couplers, such as directional coupler 21, are usually constructed to give a signal attenuation between the lines 20 and 20' of about 0.2 to 2.0 decibels as indicated by the arrow 32 and a signal attenuation range of about 6 to 18 decibels between the lines 20 and 31 as indicated by the arrow 33. The isolation between the lines 20 and 31 is generally of the order of about 20 to 50 decibels. Although the directional coupler 21 is shown in FIG. 2 as being designed for signal passage from the line 20 to the lines 20 and 31, radio frequency signal flow through that directional coupler in the opposite direction generally is possible with conventional directional couplers and is, in fact and as will be explained in greater detail hereinafter, frequently a desirable feature in CATV distribution systems incorporating devices constructed in accordance with the present invention.
In the event that it is required to pass 60 Hz electrical power through the directional tap 29, the coupler 21 is provided internally with a connection between the lines 20 and 20' as indicated schematically by the broken line 34 to permit such current flow. As will be more fully understood as the description herein proceeds, the present invention embraces directional taps with or without such power passing means.
The hybrid splitter shown at 22 in FIG. 3 is designed and constructed to divide its input signal 31 from the directional coupler 21 equally between its output lines 36 and 37. As was the case for the directional coupler 21 of FIG. 2, maximum signal return loss in the hybrid splitter 22 is desirable as is maximum isolation between its output lines 36 and 3'7.
It should be understood that, although each of the directional taps shown in FIG. 1 is provided with two or four output lines, it is equally possible to construct such a splitter with other combinations of lines by appropriately cascading splitters.
Before proceeding further with such a detailed description, it should, however, be noted that the system is shown in FIG. 1 as additionally being coupled to a radio frequency signal source 50 intended for feeding radio frequency signals to the trunk line for the passage of such' signals through the directional couplers and taps to the aforementioned antenna station. Such a signal source can, for example, be provided at a location from which a program" may be transmitted to the antenna station for subsequent re-transmission" over the system 10 to the individual subscribers connected to that system.
In addition data may be transmitted over the system in digital or some other form providing, for example, data from a monitoring device Indicating which television programs are being watched, data providing a reading of an electric power consumption meter or signals from a tire or burglar alarm.
In conventional CATV distribution systems, it is frequently necessary to distribute radio frequency signals over a frequency range of from about 54 to about 216 MHz, corresponding to the frequencies of existing television Channels 2 to 13. Conventional directional taps often fall short, however, of optimum performance over such a frequency rangef In general, it has been customary to obtain about 20 decibels return loss and about 26 decibels isolation between the individual output lines of such a tap over the same frequency range.
As already stated, it is an important object of this invention to provide-directional taps by the use of which higher return losses as well as improved isolation between the output lines of the splitter can be obtained. A further important object of this invention is, as already explained, to provide such improved performance over a wider frequency range than was heretofore possible, thereby allowing a system incorporating such devices additionally to be used for the transmission of radio frequency signals to the antenna station from a location connection either directly or indirectly to the main trunk cable without interference with the television program signals being distributed over the system as well as to carry more than the regular 12 channels. In such a circumstance, the devices of this invention usefully are designed to operate effectively over an extended frequency range of from about 5 to about 300 MHz.
FIG. 4 shows a basic circuit diagram for a conventional directional signal tap. It has a main line input terminal 40, a main line output terminal 41, and tap output terminals 38. In practice, a coaxial cable along which a signal is being transmitted is broken and connected to the main line input terminal and the main line output terminal so that the signal passes through the device. Portions of the signal are tapped off for use at the said tap output terminals. As indicated above, devices of this nature are used extensively in cable television, where a coaxial cable is extended through a residential district and television signals transmitted thereover. Portions of the signal strength are tapped off at the tap outlets and taken into individual residences along the line.
As discussed above, a problem with taps of this nature is that the signal that is transmitted along the main line is reflected back and a reflected signal enters the device at the main line output 41. The reflected signal can appear at the tap output as a ghost on a television screen or as some kind of an unwanted distortion in other types of cable signal transmissions, such as radio, teletype and facsimile. The tap output is designed to eliminate distortion in the tap output from reflected signals and it is essentially designed with a voltage sensing transformer generally indicated by numeral 43 and a current sensing transformer, generally indicated by numeral 44. The voltage sensing transformer 43 has a relatively large primary winding 45 and small secondary winding 46. The current sensing transformer 44, on the other hand, has a relatively small primary winding 47 and large secondary winding 48. The transformers 43 and 44 are designed such that the voltage on the secondary windings of each of them is about the same. The polarities of the terminals of the transformers 43 and 44 are connected such that the voltage appearing across the secondary windings of the two transformers for a main line input signal are in phase and additive at the tap output. At the same time, voltages appearing across the secondary windings of the two transformers due to a reflected signal entering the device at the main line output are l80 out of phase at the tap output and therefore cancel each other. Thus, the device is theoretically designed to obviate the effect of reflected signals at the tap output and this is essentially the manner in which the reflected signal is prevented from reaching the tap output.
Devices of this type are by usage custom designed to match the circuit into which the tap output 42 feeds. In this connection, the impedance looking back from the tap output by custom in the television industry should be about ohms. This is achieved by means of the terminating resistor condenser arrangement that includes resister 49 and condenser 50. Resister 49 is about 75 ohms. it will be apparent that if the secondary winding 46 of transformer 43 is connected directly to ground, there would be a very low impedance as measured at the tap output 42. Resister 49 is included primarily for this purpose. Capacitor 50 is included to improve an inherent phase shift that takes place with varying frequency as a result of the inter-action of the two transformers 43 and 44. Capacitors 51 and 52 are adapted to correct undesired impedance variations on the main line due to transformers 43 and 44. The design considerations' for designing the resister 49 and capacitor 50 arrangement are well known and not dwelt upon in this specification.
it will be understood that the terminating resister 49 is chosen to be 75 ohms but that this is a matter of choice and that other values could be selected. it will also be appreciated that the basic design of the tap circuit as described above is not new. Moreover, there are other transformer arrangements and designs that achieve the same effect. The basic idea, though, of all directive taps of this type is that they employ a current sensing transformer and a voltage sensing transformer with secondaries arranged to be additive for a main line imput signal and subtractive for a reflected signal that enters the device at the main line output.
FIG. shows one particular embodiment of a directional tap made in accordance with the present invention having splitters designated as A, B and C" incorporated therewith. The directional tap of FIG. 5 differs from the conventional tap of FIG. 4, like reference numerals referring to like parts, in that power passing coil 39 has been omitted and it has been replaced in function with power passing coils 53 and 54 which are by-passed to the ground conductor with capacitors 55 and 56; inductors 53 and 54 along with the leakage inductance of the primary of the voltage sensing transformer 43 as indicated at 45 together with capacitors 57 and 58 together forming a multiple pisection high pass filter. In this embodiment capacitors 59, 60 and 61 have been added, capacitor 59 along with the self inductance of the secondary of currentsensing transformer 44 as indicated at 48 along with the leakage inductance ofstep-down autotransformer of the hybrid splitter as indicated at 62 together forming a pi-section high-pass filter, capacitor 60 along with the self inductance of the step-down transformer of splitter C" as indicated at 63 and the self inductance of the splitting transformer or a portion thereof of splitter A" as indicated at 64 forming a pi-section high-pass filter, and capacitor 61 along with the self inductance of the splitting transformer of splitter A" or a portion thereof as indicated at 64 and the self inductance of step down transformer of splitter B as indicated at 65 together forming a high-pass filter. it is not always necessary that all of these capacitors be used, as for example capacitor 59 may be omitted. This is permissible because isolation between the outputs of splitter C" and the out-put of splitter 8" have included the loss of both of these splitters, which is 3 decibels each, thus permitting less performance tolerated in splitter A."
Also, the performance in bandwidth of splitters is inherently better than the performance in bandwidth of directional couplers and a useful embodiment of this invention might include the power passing circuitry comprising 53 and '54 without addition of the pi-section filters to the splitter circuitry. in some cases where there is a high isolation, that is a low coupling between the through main line and the tap line out-puts, the splitters may take the form of a resistive network 99, well known in the art, as shown in FIG. 6. in this case the splitter will'have an effective bandwidth extending to direct current and no improvement to the splitter is necessary, the improvement of the present invention being incorporated in the directional coupler only.
The usage of directional taps is not confined to distributing signals on a community wide basis in that the same techniques and similar devices are frequently used to distribute signals to a number of television sets within a single building such as a hotel, a school or an apartment building, in the instance of indoor usage, it is not necessary to pass 60 hertz power on the same coaxial cable that carries the television signals and performance improvements can be obtained by forming highpass filters by inductances connected directly to the base conductor instead of through a capacitor.
It will be understood that although the foregoing description relates to devices for the frequency range of from about 5 MHz to about 300 MHz, similar devices embodying the present invention also can be used for higher or lower frequency ranges by decreasing or increasing capacitors and inductors proportionally. For example, a frequency range of from about 10 MHz to about 600 MHz can be obtained by decreasing the capacitors and inductors to about one-half their respective values. A frequency range of of from about 2 5 MHz to about 150 MHz can be obtained by raising the values of the capacitors and inductors by twice their respective values. It will also be understood that the frequency range can be further modified to provide a lower frequency limit below 5 MHz and a device of this type may be usefully employed for certain aplications with decreased performance at a lower frequency limit of 1 MHz.
Furthermore, although the foregoing description and examples relate to a system having a ohms impedance, it will be understood that the system can readily be designed by the artisan for other impedanc es such as 50 ohms. For example, the impedance can be raised by increasing the inductors and decreasing the capacitors, and the impedance can be lowered by decreasing the inductors and increasing the capacitors.
1. A directional tap suitable for use in retrieving a directional radio frequency signal from a transmission line wherein said tap comprises of at least one directional coupler circuit coupled to said transmission line and at least one splitter circuit connected to said directional coupler circuit to provide selective radio frequency signals to a plurality of subscriber stations, said directional coupler circuit including inductive means therein, the improvement comprising in combination: capacitive means connected in a circuit relation with said inductive means in said directional coupler circuit to provide at least one pi-section high-pass filter therein to extend the frequency bandwidth of said directional coupler circuit; and further capacitive means connected in a circuit relation with internal inductive means in said splitter circuit to form at least one pisection high-pass filter therein to extend the frequency bandwidth of said splitter circuit.
2. A directional tap suitable for use in retrieving a directional radio frequency signal from a transmission line wherein said tap comprises of at least one directional coupler circuit coupled to said transmission line and at least one splitter circuit connected to said directional coupler circuit to provide selective radio frequency signals to a plurality of subscriber stations, said directional coupler circuits including inductive means therein, the improvement comprising in combination:
capacitive means connected in a circuit relation with said inductive means in said directional coupler circuit to provide at least one pi-section high-pass filter therein to extend the frequency bandwidth of said directional coupler circuit; said inductive means comprises a first power coil and a second power coil connected in series with each other and in parallel with said transmission line and the primary winding of a voltage sensing transformer coupled to said transmission line; and said capacitive means comprises a first capacitor connected in series with said primary winding to form a first inductor-capacitor circuit which is connected in parallel with said first power coil to form a first pi-section high-pass filter; and a second capacitor connected in series with said primary winding to form a second inductorcapacitor circuit which is connected in parallel to said second power coil to form a second pi-section highpass filter.
3; A directional tap according to claim 2 wherein said inductive means further comprises the secondary winding of a current sensing transformer coupled to said transmission line and the primary winding of an autotransformer in said splitter circuit; and said capacitive means further comprises a capacitor connected in series with said primary winding of said autotransformer to form a third inductor-capacitor circuit which is connected in parallel with said secondary winding of said current sensing transformer to form a third pi-section high-pass filter.
4. A directional tap according to claim 3 wherein said splitter circuit includes internal inductive means comprising the secondary winding of said autotransformer, and a capacitor connected in series with said secondary winding of said autotransformer and the primary winding of an input transformer in a subscriber station to form an output pi-section high-pass filter therein.