High frequency signal routing devices for use in catv systems

[57] ABSTRACT

Passive devices for signal routing such as directional couplers, hybrid splitters and power inserters, for use in CATV or similar signal distribution systems having improved operating characteristics are obtained by the incorporation of pi-section high-pass filters into such devices. Conventional splitters and directional couplers 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, for example, a leakage inductance of an existing component of such a device. Improved radiofrequency signal return losses and inter-tap isolation are provided over a much greater frequency range than was heretofore possible.

Claims, 13 Drawing Figures

<img file="US3671885A_D0001.tif" />

PATENTED JUN 2 Ο 1972

3.671.885

SHEET 1 OF 4

<img file="US3671885A_D0002.tif" />

<img file="US3671885A_D0003.tif" />

AGENT

PATENTED JUN 2 ο 1972

3,671,885

SHEET 2 OF 4

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PATENTED JUN 2 Ο 1972

3,671,885

SHEET 3 OF 4

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<img file="US3671885A_D0006.tif" />

INVENTOR:

FRANK C. PENNYPACKER

AGENT

PATENTED JUH 2 Ο 1972

3,671,885

SHEET U OF 4

<img file="US3671885A_D0007.tif" />

/25

102

FIG. 13

122^ 104

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20'

106 \ 109.

^13?

108

107

112

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20'

106-⁷

FIG. 12 PRIOR-ART

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s __ INVENTOR:

FRANK C. PENNYPACKER

AGENT

3,671,885

HIGH FREQUENCY SIGNAL ROUTING DEVICES FOR USE IN CATV SYSTEMS

BACKGROUND OF THE INVENTION

The present invention relates to passive signal routing devices for use in community antenna television (CATV) systems in which radiofrequency signals are distributed to individual households or subscribers over a cable system from a community antenna.

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 corresponding wider frequency bands continues to grow. Furthermore, it is desired to provide said passive devices for 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 band widths.

Passive signal routing devices are used in conventional CATV systems for many purposes such as, by way of illustration, directional couplers for proportioning the signals carried by a main trunk line of such a system to branch lines thereof. Furthermore, signal routing devices conventionally known as splitters are used for dividing such signals from branch lines between the individual lines or cables provided for individual subscribers. In many circumstances, it is also desired to provide such devices by means of which low frequency alternating electrical power, for example, 60 Hz power, can be inserted into the cables of the CATV system for the purpose of energizing amplifiers incorporated in the system to maintain signal strength throughout the system or removed from the cables to power amplifiers and the like.

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 tum preventing the transfer of interference signals from one output line to another. Such conventional passive devices generally can be used for the distribution of signals over a frequency range from about 30 to about 300 MHz and provide inter-cable isolation values of about 23 decibels with radiofrequency signal return losses of about 20 decibels.

It is a principal object of the present invention to provide passive signal routing devices 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 of the aforementioned type which provide improved inter-cable isolation when such isolation is required and which provide improved inter-cable matching, when such matching is required, as indicated in the latter case by higher values for radiofrequency signal return losses.

And 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 passive devices, such as directional couplers, hybrid splitters and power-inserters, 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.

More broadly, the invention provides a device having at least three 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 coupled together in said device for the passage of electrical signals therebetween and for the passage of radiofrequency signals between at least first and second ones of said terminals and, 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 radiofrequency signal return loss of at least about 20 decibels between said first and second ones of said terminals over an extended radiofrequency range, a third one of said terminals being inductively or otherwise coupled within said device to said first one of said terminals to provide an isolation between said second and third terminals .- of at least about 25 decibels over said extended radiofrequency range.

The passive devices of this invention can be constructed in many different ways. In the case of the application of the present invention to the construction of power inserters, a 20 ^novel device as hereinbefore broadly defined is constructed so that the said third one of its terminals is coupled through inductors within the device to both the first and second ones of said terminals so as to provide an isolation between said third one of said terminals and each of said first and second ones of 25 said terminals of at least about 25 decibels over the extended radiofrequency range. With this construction, alternating electrical current can be supplied through said device from said third one of said terminals thereof to both said first and second ones of said terminals while maintaining the desired 30 level of radiofrequency signal return loss.

When the invention is applied to the construction of directional couplers and splitters, the second and third ones of the terminals of such a device are inductively and otherwise coupled within the device to the first one of the terminals 35 thereof through the pi-section high-pass filter so as to provide a radiofrequency signal return loss of at least about 20 decibels at each of the second and third ones of said terminals and the first one of said terminals over the extended radiofrequency range.

For use as a directional coupler, the first one of the terminals is serially connected through a first coupling capacitor to a first inductor effectively connected in tum to said second one of said terminals, the first one of the terminals also being effectively grounded through a second inductor to the base ⁴⁵ 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-sec50 ''θ'¹ ^’Sh-pass filter for the passage of radiofrequency signals from the first one of the terminals to the second one of the terminals at a first radiofrequency signal attenuation value, second and third inductors acting effectively as one inductance. Furthermore, the third one of the terminals of such 55 a coupler is inductively coupled to the aforementioned first inductor for the passage of radiofrequency signals from the first one of the terminals to the third one of such terminals at a second radiofrequency signal attenuation value greater than the first radiofrequency signal attenuation value.

In the application of this invention to the construction of splitters, a device in accordance with this invention and as hereinbefore broadly defined is usefully constructed so that the pi-section high-pass filter includes a first coupling capacitor connected between the first one of the terminals and an autotransformer inductor which is effectively serially connected between that first coupling capacitor and the base conductor. A first inductor is effectively connected between the first one of the terminals and the base conductor, and second and third ones of the terminals are coupled to such an au70 totransformer inductor for the passage of radiofrequency signals between the first one of the terminals and each of the second and third ones of those terminals over the extended radiofrequency range.

The manner in which the concept of the invention is applied 75 in practice to the construction of such particular types of

3,671,885 devices 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 in which:

FIG. 1 is a ffagmentrary and schematic diagram of a CATV distribution system showing the manner in which several different embodiments of passive signal routing devices in accordance with this invention can be utilized therein;

FIG. 2 is an enlarged schematic diagram of one of the directional couplers utilized in the system shown in FIG. 1 showing schematically the manner in which that coupler operates;

FIG. 3 is an enlarged schematic diagram of one of the splitters utilized in the system shown in FIG. 1 showing schematically the manner in which that splitter operates;

FIG. 4 is an enlarged schematic diagram of the power inserter shown in FIG. 1 showing schematically the manner in which that power inserter operates;

FIG. 5 is a circuit diagram of one previously known power inserter showing that inserter connected in a CATV trunk distribution line as in FIG. 1;

FIG. 6 is a circuit diagram of a power inserter in accordance with the present invention;

FIG. 7 is a circuit diagram of one previously known two-way hybrid splitter shown as being utilized, for example, in the

CATV distribution system of FIG. 1, and additionally con- ³⁰structed for the passage of 60 Hz electrical power between its terminals;

FIG. 8 is a circuit diagram of an embodiment of two-way hybrid splitter constructed in accordance with this invention;

FIG. 9 is an equivalent circuit of the splitter shown in FIG. 8 and included herein for the purposes of explanation;

FIG. 10 is a graphical representation illustrating the improved transmission characteristics of the splitter shown in FIG. 7; 40

FIG. 11 is a circuit diagram of another two-way hybrid splitter constructed in accordance with the present invention which will allow the passage of 60 Hz electrical power between its terminals;

FIG. 12 is a circuit diagram of one previously known directional coupler for use in a CATV distribution system; and

FIG. 13 is a circuit diagram of a directional coupler in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS ₅₀

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 radiofrequency 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 couplers generally indicated at 21 and by hybrid splitters generally indicated at 22.

For the purpose of maintaining adequate signal strength at all positions along the trunk distribution line 20, it is conven- ¢5 tional 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 ad- 70 ditional power into the trunkTine^20 at one or more positions remotelrom 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 75 being in turn supplied, for example, with 110-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 sub-units 5 of the system 10 are shown in FIG. 1 by single lines, such cables will customarily 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 ¹⁰ 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 ways 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 signal routing devices which can themselves be utilized in the system 10 shown in ^2U 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 ob25 tain the lowest possible extent of radiofrequency signal reflection at any of the components of the system 10, for example, at the directional couplers 21 and at the hybrid splitters 22, 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 sub-units to the maximum possible extent. Furthermore, maximum signal isolation between the individual outputs of each of the hybrid splitters 22 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 radiofrequency signals from the trunk line 20 and to distribute a fraction of such a signal to the branch line 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 ⁴⁵ lines 20 and 20' of about 0.2 to 1.5 decibels as indicated by the arrow 32 and a signal attenuation range of about 8 to 16 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, radiofrequency 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 coupler 21, that sub-unit 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 couplers with or without such power passing means.

The hybrid splitter shown at 22 in FIG. 3 differs from the directional coupler 21 of FIG. 2 in that it is designed and constructed to divide the input signal from the branch line 31 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 37. If it is desired to pass 60 Hz power between the input line 31 and the output lines 36 and 37, power passing connections 38 and 39 are provided

3,671,885 within the splitter 22 as will be explained in greater detail hereinafter.

It should be understood that, although each of the hybrid splitters 22 is shown in FIGS. 1 and 2 as being provided with two output lines 36 and 37, it is equally possible to construct 5 such a splitter with three or more such output lines.

Referring now to FIG. 4 of the accompanying drawings, the power inserter shown schematically at 26 therein is designed and constructed to permit the passage of radiofrequency signals between its terminals to which the main trunk lines 20 ¹⁰ and 20' are connected, such radiofrequency signal passage being indicated by the broken line 41 and being effected with minimum signal attenuation. The passage of radio frequency signals from either of the lines 20 and 20' to the power supply line 40 from the transformer 25 is essentially prevented. The ¹⁵power inserter 26 is also constructed to allow 60 Hz power supplied from a supply line 40 to pass to either of the lines 20 and 20' as indicated by the broken lines 42 and 43.

The manner in which signal routing devices, as exemplified by hybrid splitters, directional couplers and power inserters are constructed to provide the functions hereinbefore described will be more readily understood when specific embodiments of such devices are described in greater detail hereinafter with reference to the remaining figures of the accompanying drawings.

Before proceeding further with such a detailed description, it should, however, be noted that the system 10 is shown in FIG. 1 as additionally being coupled to a radiofrequency signal source 50 intended for feeding radio frequency signals to the trunk line 20 for the passage of such signals to the aforementioned antenna station. Such a signal source can, for example, be provided at a location from which a “programm” may be “transmitted” to the antenna station for subsequent “re-transmission” over the system 10 to the individual subscribers connected to that system.

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 ₄θ to the frequencies of existing television channels 2 to 13. Conventional devices, as exemplified by hybrid splitters, fall short, however, of optimum performance over such a frequency range. In general, it has been customary to obtain about 20 decibels return loss in a hybrid splitter and about 23 decibels 45 isolation between the individual output lines of such a splitter over the same frequency range.

As already stated, it is an important object of this invention to provide a signal routing device, such as a hybrid splitter, by the use of which higher return losses as well as improved isola- 50 tion 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 ³³used for the transmission of radiofrequency signals to the antenna station from a location connected 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 ⁶⁰circumstance, the devices of this invention usefully are designed to operate effectively over an extended frequency range of from about 5 to about 300 MHz.

Referring now to FIG. 5 of the accompanying drawings, there is shown therein generally at 26 a conventional power inserter of the type already described with reference to FIGS. 1 and 4 of the accompanying drawings. The power inserter 26 has first and second terminals 50 and 51 which are directly interconnected within the inserter 26 by a conductor 53. In use, 70 the terminals 50 and 51 of the inserter 26 are connected, for example, in the trunk distribution line as indicated at 20 and 20' to allow the unimpeded passage of radiofrequency signals through that power inserter as indicated by the double-headed arrow A.

A third terminal 52 of the inserter 26 is connected therewithin to the conductor 53 through a high impedance inductor 54 so that 60 Hz power, for example, at 30 volts, supplied to the terminal 52 from the secondary winding of the power supply transformer 25 can pass through the inductor 54 to the main trunk line 20, 20’ as indicated by the arrow B while essentially preventing the passage of radiofrequency signals in the opposite direction.

The terminal 52 of the power inserter 26 is isolated from the trunk line 20,20' by capacitor 55 and inductor 54 to prevent transmission of radiofrequency signals to terminal 52.

A power inserter constructed in accordance with the present invention is shown generally at 58 in FIG. 6 of the accompanying drawings. The power inserter 58 corresponds to the power inserter 26 already described herein in that it has first, second, and third terminals 50, 51, and 52 respectively, terminals 50 and 51 being intended for connection in the main trunk line 20, 20' and terminal 52 being intended for connection to the power supply line from the transformer 25.

Internally, the power inserter 58 differs from the conventional inserter 26 already described in that the first and second terminals 50 and 51 thereof are interconnected by a pi-section high-pass filter including first and second inductors 59 and 60 respectively and a coupling capacitor 61 connected between the terminals 50 and 51. The third or power insertion terminal 52 is connected to the opposite ends of the inductors 59 and 60 as will readily be understood from FIG. 6 and the third terminal 52 is internally grounded through the capacitor 55 as was the case in the power inserter 26.

Typical component values and typical performance characteristics for the conventional power inserter 26 (FIG. 5) and for the novel power inserter 58 in accordance with the invention (FIG. 6) are compared in Table I.

TABLEI

<td></td><td> Prior art inserter 26</td><td> Inserter 58 of the invention</td>

<td> Inductor 54</td><td> 6 microhenries</td><td></td>

<td> Inductor 59</td><td> —</td><td> 6 microhenries</td>

<td> Inductor ¢0</td><td> —</td><td> 6 microhenries</td>

<td> Capacitor 61</td><td> —</td><td> 820 picofarads</td>

<td> Capacitor 55</td><td> 7.5 microfarads</td><td rowspan="2"> . 7.5 microfarads</td>

<td> Operating</td><td></td>

<td> frequency range (1) Minimum signa!</td><td> 30-300 MHz</td><td> 5-300 MHz</td>

<td> return loss (1)</td><td> 20 decibels</td><td> 23 decibels</td>

Notes: (1) Measured at terminals 50 and SI.

(2) Measured over specified f requency range.

The considerable gain in operating performance obtained by constructing a power inserter in accordance with the teachings of this invention will readily be apparent from the figures of Table I.

In general, it can be noted that the coupling capacitor 61 normally will have a capacitance of from about 500 to about 1,500 picofarads, more preferably of from about 500 to about 800 picofarads, while each of the inductors 59 and 60 generally will have an inductance of from about 6 to about 10 microhenries, more preferably of from about 6 to about 8 microhenries. The grounding capacitor 55 usually will have a 65 capacitance of at least about 0.005 microfarads.

Before describing the construction of a typical hybrid splitter in accordance with this invention, the construction of a conventional splitter which is also adapted to pass 60 Hz power will first be described with reference to FIG. 7 of the accompanying drawings.

The conventional two-way hybrid splitter generally indicated at 22 in FIG. 7 includes an autotransformer inductor 68 connected between the first or input terminal 65 of that splitter and a base or ground line or conductor as represented ⁷⁵ by a conductive casing 69 of the splitter. The autotransformer

3,671,885 inductor 68 of such a splitter is generally tapped as indicated at 70 at a position which will give an output voltage equal to about 0.7 times the input voltage and the tap-line 71 is connected to a center-tap position 72 of a power-splitting inductor 73, the tap-line 71 being connected through a grounding capacitor 74 to the base conductor 69. A shunt resistor 75 of appropriate value is connected between the ends of the inductor 73 while second and third output terminals 66 and 67 respectively are also effectively connected to opposite ends of the inductor 73. Together with the flux leakage inductances of the inductors 68 and 73, the capacitor 74 forms a T-section low-pass filter.

If the conventional hybrid splitter 22 of FIG. 7 is to be used for passing 60 Hz power between its input terminal 65 and its output terminals 66 and 67, as actually shown in FIG. 7, blocking capacitors 78, 79, and 80 are provided between the terminals 65,66, and 67 respectively and the inductors 68 and 73 in the manner shown in FIG. 7. Power-shunting inductors 81 and 82 connected between the input terminal 65 and output terminals 66 and 67 respectively to allow the passage of such 60 Hz power across the splitter.

As is the case with conventional splitters, hybrid splitters can be constructed in accordance with this invention with or without such power-passing circuitry. For the sake of simplicity, the construction and operating features of a hybrid splitter in accordance with the invention will first be described herein with reference to FIGS. 8 to 10 of the accompanying drawings in which such power-passing circuitry is omitted. The modification of the splitter of FIG. 8 for its additional use in powerpassing will then be briefly described with reference to FIG. 11 of the accompanying drawings.

Referring, therefore, to the embodiment of the hybrid splitter 85 of FIG. 8, that splitter is generally similar to the splitter 22 in that its input terminal 65 is connected to one end of an autotransformer inductor 68 which is itself tapped at 70 and the tap-line 71 is connected to a center tap 72 of a powersplitting inductor 73 which is, in turn, connected between the output terminals 66 and 67 and in parallel with a shunt resistor 75. A capacitor 74 grounds with aforementioned tap-line 71 to form an internal T-section low-pass filter as already described with reference to FIG. 7.

The hybrid splitter 85 differs from the splitter 22, however, in that a third inductor 86 is connected between the input terminal 65 and the base conductor 69 while the input terminal 65 is also connected to the autotransformer inductor 68 through a first coupling capacitor 87. The inductor 86, the coupling capacitor 87 and the autotransformer inductor 68 effectively form a pi-section high-pass input filter as will be explained in more detail hereinafter with reference to FIG. 9 of the accompanying drawings.

In accordance with a preferred feature of this invention, the output terminals 66 and 67 of the hybrid splitter 85 are connected to opposite ends of the resistor 75 through second and third coupling capacitors 88 and 89 respectively and such output terminals 66 and 67 are also individually connected to the base conductor 69 through first and second inductors 90 and 91 respectively. In this way, pi-section high-pass output filters are effectively provided in the splitter 85. The various components constituting such output filters will be more readily understood when the equivalent circuit of FIG. 9 is hereinafter considered.

Referring, therefore, to FIG. 9 of the accompanying drawings, there is shown an equivalent circuit of the hybrid splitter 85 of FIG. 8. In FIG. 9, the autotransformer inductor 68 is shown as being shunted by an inductor 92 which represents the magnetic coercive force necessary to establish a magnetic field in the core of the autotransformer coil 68. Furthermore, inductances 93 and 94 are shown as being serially connected in the tap-line 71 between the inductors 68 and 73. The inductances 93 and 94 represent inductances due to flux leakages in the inductors 68 and 73 respectively. An inductor 99 shunts inductor 73 and, additionally, a resistor 95 is shown as being connected in parallel with the resistor 75, the resistance 95 representing resistive losses which occur in the inductor 73. The deviations specifically hereinbefore mentioned are those which have proved to be particularly significant in determining the operating characteristics of hybrid 5 splitters of the type in question.

In order to obtain the desired improvements in accordance with this invention in splitter operation, the inductance values of the inductors 86, 90, and 91 and the capacitance values of the capacitors 87, 88, and 89 are selected so as to provide the required pi-section high-pass filters. In particular, the pi-section high-pass input filter already mentioned is formed by the inductance of the inductor 86, the capacitance of the capacitor 87 and by what might be described as the effective self-in15 ductance of the autotransformer inductor 68 as represented by the inductance 92 shown in the equivalent circuit of FIG. 9. Similarly, the pi-section high-pass output filters are effectively constituted by appropriate ones of the inductors 90 and 91 and of the capacitors 88 and 89 as well as by the effective self20 inductance of inductor 73 represented by inductance 99.

Typical component values and operating characteristics for a conventional two-way hybrid splitter as shown in FIG. 7 without the power-passing function described and for a twoway hybrid splitter in accordance with this invention as al25 ready described with reference to FIGS. 8 and 9 of the accompanying drawings are set down in Table II. Unless otherwise stated, identical components were utilized in the two splitters.

TABLE II

Prior art splitter 22

Splitter 8S of the invention

Inductor 99 Resistor 75 Inductors 86, 90,91

Capacitors 87, 4θ 88 and 89

Effective frequency range (1) Minimum return

Loss (1)

Isolation (2)(3) 45 --------------6 to 50 microhenries

200 ohms microhenries

1000 picofarads

30-300 MHz decibels decibels microhenries 200 ohms microhenries

820 picofarads

5-300 MHz decibels decibels (1) Measured at each output terminals 66 and 67 and input terminal 65.

(2) Measured over specified frequency range. 50 (3) Measured between output terminals 66 and 67.

In general, the inductors 86,90, and 91 will each have an inductance of from about 5 to about 8 microhenries, more preferably from about 6 to about 7 microhenries, while the capacitors 87,88, and 89 will each have a capacitance of from about 600 to about 1,000 picofarads, more preferably from about 750 to about 850 picofarads.

The typical operating characteristics of the two hybrid split__ ters reported in Table II are shown graphically in FIG. 10 of the accompanying drawings which shows in broken lines the signal transmission efficiency and signal return losses as represented by voltage standing wave ratios (VSWR) for the conventional splitter 22 and, in solid lines, the same charac¢5 teristics for the splitter 85 constructed in accordance with this invention. The considerable improvement in operating characteristics obtained by the application of this invention will readily be apparent from FIG. 10.

As already explained, a hybrid splitter in accordance with 70 this invention can also be constructed so as to have a powerpassing function and a splitter so constructed is shown generally at 95 in FIG. 11 of the accompanying drawings. To avoid duplication of the description herein, identical components of the splitters 85 and 95 are indicated by the same 75 legends in both figures.

3,671,885

In the power-passing splitter 95, the inductors 86, 90, and 91 are interconnected at 96 to provide 60 Hz current paths from the input terminal 65 to each of the output terminals 66 and 67. Grounding of the inductors 86,90, and 91 and formation of the desired pi-section high-pass filters is effectively obtained by the provision of a grounding capacitor 97 connected between the common connection point 96 and the base conductor 69.

Referring finally to the conventional directional coupler generally indicated at 21 in FIG. 12 of the accompanying drawings, it will be seen that the coupler 21 includes a cunentdividing transformer 100 and a voltage-dividing transformer 101. The current-dividing transformer 100 has a centertapped primary winding 102 effectively connected between the input terminal 103 and the output terminal 104 of the coupler which are connected to the main trunk line sections 20 and 20' respectively. The secondary winding 105 of the transformer 100 is connected between the base conductor as indicated at 106 and a center tap 107 of a primary winding 108 of the voltage-dividing transformer 101 while the secondary winding 109 of the transformer 101 is connected between the base conductor 106 and the center tap 111 of the primary winding 102 of the transformer 100. One end of the primary winding 108 of the transformer 101 is grounded at 110 through a resistor 112 while the opposite end of that winding is connected to the third terminal 113 of the coupler 21. It will be understood that the center tap of primary winding 102 and primary winding 108 can be omitted in certain applications where a degraded return loss is acceptable or where high attenuation between lines 20 and 31 is permissible.

The known directional coupler 21 is also shown in FIG. 12 as being provided with a power-passing shunt inductor 120 interconnecting the trunk line terminals 103 and 104 for the passage of 60 Hz power therebetween, and with blocking capacitors 121 and 122. Since the directional coupler 21 is conventional, it is considered unnecessary to describe it in greater detail herein.

The directional coupler in accordance with the present invention as indicated generally at 125 in FIG. 13 of the accompanying drawings differs from the directional coupler 21 already described in that separate first and second inductors 126 and 127 respectively are connected between the terminals 103 and 104 respectively and an internal connection point 130 which is effectively grounded through a grounding capacitor 131.

The capacitors 121 and 122 and the inductors 126 and 127 together with the effective inductances of the transformers 100 and 101 serve to form a five-element pi-section high-pass filter.

Improved operating characteristics were obtained with the directional coupler 125 as compared to those obtained with the conventional directional coupler 21 exactly as was the case for the power inserters and hybrid splitters already described herein.

It will be understood that numerous variations can be made in the construction of the directional coupler 125 without departing from the scope of the invention. For example, if that coupler does not need to have a power-passing function, the inductors 126 and 127 can be connected directly to the base conductor 106 and the grounding capacitor 131 can be omitted. Similarly, in such a case, one of the inductors 126 and 127 could be omitted so that the coupler includes a threeelement pi-section high-pass filter instead of the five-element filter already described.

In the directional coupler 125 shown in FIG. 13, each of the inductors 126 and 127 generally will have an inductance of from about 7 to about 12 microhenries, more preferably from about 9 to about 10 microhenries, while the coupling capacitors 121 and 122 will generally have a capacitance of from about 330 to about 650 picofarads, more preferably from about 400 to about 500 picofarads. The grounding capacitor 131 will normally have a capacitance of at least about 0.005 microfarads.

Such a directional coupler will usually provide a radio frequency signal attenuation of from about 0.3 to about 1.5 decibels between the input terminal 103 and the first output terminal 104 and a radiofrequency signal attenuation of from about 8 to about 16 decibels between the input terminal 103 and the second output terminal 113. For a typical directional coupler constructed as shown in FIG. 13, a radiofrequency signal return loss of about 20 decibels between each of the output terminals 104 and 113 and the input terminal 103 was obtained with an isolation between the two output terminals 104 and 113 of at least about 20 decibels higher than the said signal attenuation between terminals 103 and 104 over a radiofrequency range of from about 5 MHz to about 300 MHz.

It will be understood that although the foregoing examples and description relate to devices for the frequency range of from about 5 MHz to about 300 MHz, similar devices embodying the invention 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 one-half their values and a frequency range of from about 2½ MHz to about 150 MHz can be obtained by raising the capacitors and inductors to twice their values.

Also, although the foregoing description and examples relate to a 75 ohms impedance, it will be understood that the system can be readily designed by the artisan for other impedances 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.