US2336258A - Carrier current apparatus - Google Patents

Description

13% 7, 1943- E; w. KENEFAKE 4 2,336,253

7 CARRIER CURRENT APPARATUS Filed March '7, 1942 3 Sheets$heet 1 l9 :9 czwisfir cz'a'a'ss-r APPARATUS APPARATUS m I7 I CARRIER 5 CURRENT m APPARATUS 2 CARRIER CURRENT n 7 APPARATUS 2 23 I4 f CARRIER CURRENT APPARATUS 1/ r5. AWL 1 Hi ttorney v Filed March 7, 1942 3 Sheets-Sheet 2 Fig.5.

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mwmuuwmuuauwmmmw V L m CABLE LENGTH- FEET Inventor: Edwin W. Kenefake,

His ttorney Patented Dec. 7, i943 CARRIER CUR RENT API'ARATUS Edwin W. Kenefake, Schenectady, N. Y., assignor to General Electric New York Company, a corporation of Application March 7, 1942, Serial No. 433,824

Claims; (01. 177-352) My invention relates to carrier current appa- 1 ratus, and more particularly to means for coupresents a purely resistive impedance to the car- I rier current apparatus. Such anarrangement is satisfactory from an operating standpoint where the carrier current apparatus is in close proximity to the power transmission line since tuning means for the coupling capacitor and transmission line may then be in the same enclosure as the carrier current apparatus and is thus readily accessible for easy adjustment without special precautions to protect it from the weather.

In many cases carrier current apparatus is located at a considerable distance from the power transmission line, making it necessary to use a signal transmission line, such as a coaxialcable, to transmit signals between the carrier current apparatus and the power transmission line. Where such a signal transmission line has been utilized, it has been the practice in the past to provide tuning means and impedance matching means both between the carrier current apparatus and the signal transmission line and between the signal transmission line and the power transmission line.

In such systems a coupling capacitor is used to insulate the signal transmission line from the power transmission line, and an inductance is provided in series with the coupling capacitor between these two lines to tunethe capacitor and power line to series resonance at the signal frequency. There is also provided a transformer for matching the characteristic impedance of the signal transmission line to the impedance of the coupling capacitor and power transmission line, so as to provide for the maximum transfer of signals therebetween. A similar tuning means and transformer has been used between the signal transmission line and the carrier current apparatus. These tuning and impedance matching means have reduced attenuation to the low levels.

It is an object of my invention to provide in such systems requiring a signal transmission line improved and simplified connecting means between the carrier current apparatus and the power transmission line to which it is connected. It is a further object of my invention to provide such improved connecting means which requires 'a' minimum of adjustment and maintenance except at the'carrier current apparatus.

It is another object of my invention to provide such improved and simplified connecting means including a signal transmission cable between carrier current apparatus and the power transmission line, which means requires a minimum of space anda minimum investment and which is rugged and reliable in operation.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, both as to its organization and manner of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings in which Figs. 1, 2, 3 and 4 illustrate four modifications of my invention and Figs. 5, 6, 7 and 8 are graphical representations of the various characteristics thereof.

veniently be the center conductor of a coaxial transmission line, of which the outer conductor is grounded. At that end of the transmission line ll adjacent the power transmission line iii, the line II is connected through one winding M of a transformer 55 to ground. One terminal of the other winding 56 of the transformerifi is grounded, and the other terminal is connected through a suitable coupling capacitor ll to the conductor I2 of the power transmissionline it.

An adjustable inductance i8 is connected between the carrier current apparatus it and the signal transmission line H, and is so adjusted as to be series resonant with thecapacitive im-= pedanceof the system including the signal transmission line H, the transformer it, the coupling capacitor ll andtthe power transmission line it.

The windings l4 and it of the transformer 5 have a turn ratio equal to the square root of the ratio of the impedance of the power transmission line H as measured through the condenser H to the characteristic impedance of the signal transmission line. That is, the square of the number of turns in the winding l6 corresponds to the impedance of the power transmission line is, as measured through condenser ii, in the same way as the square or the number of turns of the winding it corresponds to the characteristic impedance of the signal transmission line.

' Such a relation between impedances and turn ratio of the transformer i5 is desirable because attenuation in a transmission line .is minimum when it is connected at its ends to devices whose impedance, including both resistance and reactance, is equal to the characteristic impedance of the transmission line. While it is true that such attenuation may be further minimized by making the impedance of the connected devices purely resistive, it has been found that the attenuation is satisfactorily low even though reactance be present, if impedances are matched, as stated above. The characteristic impedance of the transmission line need not be matched exactly, as the attenuation does not increase rapidly as the connected impedances vary from a value equal to the characteristic impedance of the line.

In a typical example, where a coaxial cable of -70 'ohm characteristic impedance is selected as accuses at oneirequency, and a-receiver operative at a difierent frequency. The transmitter in the apparatus it is connected to the adjustable inductthe transmission line, the carrier current appara'tus operates at 100 kilocycles and the coupling condenser H has a capacity oi 0.002 microfarad, the impedance of the-power line is may be assumed to be 400 ohms of pure resistance, and the impedance to which the transmission line must then be matched is 882 ohms. The impedance ratio is then 12.6 and-the turn ratio of transformer 65 is 3.4.

it has been determined that the elements shown in Fig. .1, adjusted as described above, may be utilized for the eficient transmission of signals between the carrier current apparatus iii and the power transmission line it with reasonably small attenuation through thesignal transmission line H over substantial distances. The actual relation between attenuation through the signal transmission line i i and the length oi that line is set forth hereinafter for certain situ= ations encountered in practice.

In Fig. 2 the carrier current apparatus ill is connected through the inductance iii, and the transmission line it directly to one terminal of the coupling condenser ll, whose other terminal is connected to the conductor it of the power transmission line it. The inductance it is in this case adjusted so that the whole system including the inductance it, the transmission line H, condenser ii, and the'power transmission line it is resonant at the signal frequency. When this arrangement of elements is used, omitting the impedance matching transformer is oi Fig. 1, there is somewhat more signal refiection' and consequently greater attenuation through the signal transmission line ii. It has been iound;however, that even such greater attenuation is reasonably small over some distances, and, in fact, not lntolerably large. The

relation between attenuation and the length of the signal transmission line it for this arrangement or elements inFig. 2 is also set forth hereinafter for certain situations encountered in practice.

The apparatus illustrated in Figs. 3 and a are modlcations oi the arrangement illustrated in Fig. i. In Fig. 3 the carrier current apparatus it is arranged for operationat two different frequencies. For example, the carrier current apparatus it may include a transmitter operative ance i3 and transmission line H through two paths, one of which includes an inductance is and condenser 2b in series, and the other of Which includes an inductance 2i and condenser 22 in series. The receiver in the apparatus it is connected through a conductor 23 to a point between the condensers 2t and 22 and the inductance it.

The arrangement of such a transmitter and receiver together with means for coupling them to the power transmission line it including inductances i3, iQand 2i and condenser i1, 2% and 22 forms no part of the present invention, and is described and claimed in my application for United States Letters Patent, Serial No. 357,- 274. filed on Sept. 18, 1M0, for Coupling apparatus, and assigned to the same assignee as the present application.

Briefly, the adjustment of these elements is as follows. 'lhe inductance is is adjusted to resonate in series with the condenser 28 at the transmitter frequency, and the inductance it is adjusted to resonate with the condenser H and transmission line 53 are series resonant at the transmitter frequency and present a low impedance to the transmission of carrier current signals from the apparatus it to the transmission line l3.

At the frequency'to be received, the two series paths, one including the transmitter in the apparatus it, the inductance it and condenser 26, and the other including the inductance it and condenser ll, exhibit inductive or capacitive reactance, depending on whether the receiver frequency is higher or lower, respectively, than the transmitter frequency. The overall reactance of the series path including the adjustable inductance 2i and condenser 22 is adjusted to such a a value that the total reactance of the group including the transmitter, the inductances ii and 2t and the condensers so and 32 is equal in value, and opposite in character, to the reactance of the remainder of the path from the apparatus it to the power line i3, such path including the ad-= justable inductance it and condenser ii. The inductance it and condenser ll therefore. form one branch of a series tuned circuit, the other branch being formed by the transmitter in the,

apparatus iii, the inductances iii and 2! and the condensers 2t and 22, so that the potential at a point between these two branches tends to rise to a high value at the receiver frequency. The conductor 23 whichis connected to such point therefore transmits such high potential to the receiver in the apparatus it.

In other respects the apparatus illustrated in Fig. 3 performs in a fashion similar to that illustrated in Fig. 1, and provides emcient transfer of signals with small attenuation over substantial lengths of signal transmission line it between the apparatus it and the power transmission line it. The data presented hereinafter corre lating such attenuation and line lengths for Fig. l is equally applicable forFig. 3.

heretofore, the connection shown in Fig. 3 and described in the above mentioned application has been used only where the carrier current apparatus has been so near the power line i 3 that a transmission line has not been needed,

since only in such arrangements could the connection 23 be made to a point between the coupling condenser IT and the adjacent tuning inductances and condensers I8, I9, 20, 2I and 22.

through transmission line II being satisfactorilysmall and the connection 23 may easily be made, thereby resulting in more efllcient operation of the receiver, as well as easier adjustment and maintenance of the apparatusas a whole.

In Fig. 4 the transmission line II, transformer I5 and condenser II are utilized to transfer carrier, current signals between the power transmission lines I3 and two carrier current apparatus, including the apparatus I0 and another apparatus 30. Each such apparatus may include a carrier current transmitter, and a receiver operating at the same frequency, each apparatus as a whole being operative at a different frequency.

The apparatus I0 is connected to the adjustable inductance I8 and transmission line II through a condenser 3| shunted by an adjustable inductance 32. The apparatus 30 is connected to a point between the inductance I8 and transmission line II through a path serially including an adjustable inductance 33 and a shunt combination of a condenser 34 and an adjustable in-. ductance 35.

The adjustment of the apparatus of Fig. 4 is as follows. The parallel tuned circuit including the condenser 3I and inductance 32 is made parallel resonant at the frequency of the apparatus 33. Similarly, the parallel circuit including condenser 34 and inductance 35 is made parallel resonant at the operating frequency of the apparatus Ill. The inductance I8 is then adjusted so that the apparatus Iii, inductance 32, condenser 3I, inductance I8, transmission line II, transformer I5, coupling condenser I1, and the power line I3 are resonant at the operating frequency of the apparatus Ill. At this frequency the parallel resonant circuit including condenser 3d and inductance 35 ofiers a high impedance so that signals transferred between apparatus Iii and the power line I3 are not dissipated in the apparatus 30.

In similar fashion, the inductance 33 is adso that the grounded terminals of the carrier current apparatus may in each case be connected, instead of to ground, to a second conductor of the power transmission line I3.

In many situations it is entirely satisfactory to ground one terminal of a carrier current apparatus, where the attenuation caused by such connection is not unduly great. By connecting the two terminals of the carrier current apparatus to two different conductors of the power line I3, signals may be transmitted through the power line I3'with somewhat less attenuation, so that longer transmission distances may be achieved.

, In the case of the apparatus illustrated in Figs. 1, 3 and 4, it is not necessary to duplicate all the equipment for connecting the carrier current apparatus to the power line I3, since in each case,

the two terminals of the secondary I6 of transformer I5 may be connected through separate coupling condensers to two different conductors of the power line I3. In other respects the apparatus for interphase transmission may be exactly the sameas that illustrated in Figs. 1, 3

justed so that the apparatus 30, the inductance 35, condenser 34, the inductance 33, the transmission line II, transformer I5, coupling condenser I1, and power line I3, are all resonant at the frequency of operation of the apparatus 30. At this frequency the circuit including condenser 3I and inductance 32 offers a high impedance so that signals transferred between the power line I3 and apparatus 30 are not dissipated in the apparatus I0. Insofar as the present invention is concerned, the apparatus I0 and 30 respectively cooperate with the inductances I8 and 33, and with transmission line II, transformer I5, coupling condenser I1 and the power transmission line I3 in the same-way as corresponding elements .in the apparatus of Fig. 1. The relation between attenuation and signal transmission line length is the same as that set forth hereinafter for Fig. 1.

While the apparatus in Figs. 1 through 4 has been illustrated as impressing a signal between a single conductor I2 of the power line I3 and ground, it is within the scope of my invention to and 4.

One of the conditions termining how a simplified arrangement according to my invention may be utilized is the capacity of the coupling condenser II. This capacity to a substantial extent determines the reactance presented to the end of the transmission line II adjacent the power line I3, and hence is important in considering how much attenuation may be produced through the transmission line II. In all cases hereinafter considered, it is assumed that the impedance between one conductor I2 of the power line I3 and ground is 400 ohms, which value is substantially correct for any,

transmission line. Although power'transmission lines may have a somewhat higher impedance, the impedance is primarily resistive and consequently higher impedance lines reduce attenua tion losses inthe transmission line I I and make the resulting operation more efficient.

As a practical matter, the capacity of the coupling condenser I! is determined primarily by the amount of insulation it must afford between the conductor I2 and ground. That is, the higher the voltage of the power line I3, the more insulation there must be between the capacity elementsof the condenser II, and consequently the less the capacity of the condenser I1. At the present time certain sizes of condensers are available for.

use as coupling condensers on power lines of particular voltages, as indicated in the following tabulation:

' Number Capacity Line voltage. kv. condensers for conden- 2?? in series ser, pf. p y

I In Fig. 5 values of attenuation between the carrier current apparatus I0 and the power line I3 are plotted as ordinates against correspond-' 'ing lengths of the transmission line II, plotted as abscissae, for several conditions of operation of the apparatus of Figs. 1 through 4. All the to be considered in de-- I l. Accordingly, a

trated by the curves ti, the attenuation which is realized when neither the transformer to nor tuning apparatus adjacent the condenser it is utilized is reasonably small even though transmission line it is as much as 1000 it. long. The attenuation realized when the transformer i5 is utilized, as indicated by the curve dd, is even smaller and is well within usable limits for cable lengths greater than 2000 ft, even though no tuning device is used adjacent the condenser ii.

For purposes of the general discussion herein,

it has been assumed that there should be no more than two decibels attenuation between the carrier current apparatus i and the power line it for satisfactory operation. Certain operating considerations-may change this value to some extent in certain particular situations, Carrier current apparatus, as generally utilized at the present time, is capable of producing about 50 decibels gain in a received signal when the automatic volume control voltage is smallest. The total attenuation between carrier current transmitter and receiver must, therefore, never exceed this value, even under the worst conditions of weather and the like.

Accordingly, it is common practice to assume that an attenuations of 25 to 35 decibels on. a

clear dry day can be tolerated between a carrier.

10 and the power line it is in the order of one or two decibels.

If, however, the distance along the power line it! between the points where carrier current equipments are connected thereto are shorter than such maximum distance, more attenuation can be tolerated tl rough the transmission line determine the maximum us able length of the transmission line H in any particular case, in view of the above considers.-

utilized where the coupling condenser H has a capacity of .004 microfarads. J Figs. 6, '1 and 8 are identical with Fig. excep that they illustrate theattenuatio'n for various 1 lengths of the transmission line I I when the couthrwehil may be utilized to determine. the maxi-.

mum usable length of the transmission line H. It the transmission line H which is to be used approaches a. length equal to one quarter wave length of the carrier current wave at the operating frequency, additional consideration must be accuses given to the possibility of tuning the apparatus by means of the inductances l8 and If the transmission line it, in any particular case, is made slightly greater than one quarter wave length long, the capacityof the power line i3 and condenser ii appearsat the carrier current apparatus iii as inductance, since a quarter wave length line acts as an impedance inversion trans former. In such acase the inductance Hi cannot be used to tune the equipment, and if it be desired to use such a transmission line, it is necessary to replace the inductance it with a condenser or with a capacitive combination of a condenser and an inductance. In any case, when the equipment is properly tuned by the inductance id, or by any suitable means, th attenuation is as illustrated in the curves of Figs. 5 through 8, even though the length of transmission line it be exactly one quarter wave length, or a multiple thereof.

One form of the transmission line H which is especially suitable iorconnecting the carrier current apparatus 0 to the condenser H is a coaxial cable insulated with a rubber compound. The dielectric constant of one particular rubber compound used in practice is 3.3, so that a quarter wave length of the transmission line I i at 150 kilocycles is 903 ft. at 85 kilocycles is 1,590 ft., and one quarter wave length at 50 kilocycles is 2,710 ft. It should be remembered in making any particular installation that when the length of transmission line H approaches such values an inductance may be incapable of tuning the equipment at the carrier current apparatus i0.

If the exact operating frequency at which the system'is to operate is not known when it is desired to determine how long the transmission line H may be, the determination should be made on the basis of the lowest frequency which may act ually be used, sincesuch' determination generally indicates that the transmission line H is shorter than that which would be determined for .a higher operating frequency. However, when considering whether the transmission line II I may approach a. quarter wave length or multiple thereof in length, it may be necessary to determine the'actual operating frequency.

As illustrated by the curves of Figs. 5 through 8, it has been mind that a. transmission line H may be utilized to connect a carrier current apparatus Hi to a. power line IS without the necessity of providing tuning means at the connection between thetransmission line and the power line, and also that an impedance matching transformenmay be omitted. It is within the scope of tions, the curves 40 and 4| of Fig. 5 may be my invention .to utilize such a transmission line with tuning means only at the' carrier current apparatus ID in any type of carrier current system having a signal transmission line, of which 7 several forms have been illustrated.

While I have shown anddescribed a particular embodiment of my inventiomit .will be obvious to aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent in the United States is:

1. An arrangementfor coupling carrier current apparatus to a power line spaced a substantial distance therefrom comprising, means including a transmission line extending between One quarter wave length accepts power line from said transmission line and matching the impedance of said transmission line to the impedance of said condenser and power line, and means for transmitting signals between said carrier current apparatus and said transmission line, the impedance of said power line measured through said condenser, transformer, and transmission line being reactive, said last mentioned means having a reactance equal and opposite in character to the reactive component of the impedance of said power line measured through said condenser and transmission line, whereby attenuation of carrier current signals transmitted between said power line and said-apparatus is minimized.

2. An arrangement for coupling carrier current apparatus to a power line spaced a substantialdistance therefrom comprising, means for transmitting signals between said apparatus and line comprising 'a transmission line, a coupling condenser and transformer connected between' said transmission line and said power line, the impedance of said power line measured through said condenser and transformer being reactive 1 whereby substantial attenuation is produced in pling condenser and transformer connected between said transmission'line and said power line, the impedance of said power line measured through said condenser being reactive, said transformer having an impedance ratio eflective to match said impedance with the characteristic impedance of said transmission line, substantial attenuation being produced in signals transmitted through said transmission line in spite of said transformer because of the reactivecomponent of said impedance, and means coupling said apparatus to said transmission line for tuning the impedance of said power line and coupling condenser through said transmission line and transformer, whereby said attenuation is reduced.

4. An arrangement for coupling carrier current apparatus including a transmitter and a receiver operative at different frequencies to a power line spaced a substantial distance therefrom comprising, a transmission line, a coupling condenser and transformer connected between said lines, said transformer having a turn ratio effective to match the impedance of said transmission line to the impedance of said-power line and condenser, the impedance of said power line measured through said condenser, transformer, and transmissionline being reactive at both the frequencies of said transmitter and receiver, means for connecting said transmitter to said transmission line, said connecting means havingv a reactance at the frequency of said transmitter equal and opposite in character to the reactance of said power line measured through said condenser, transformer, and transmission line, and

means for connecting said receiver in shunt to i said transmitter and atv least a portion of said connecting means and for adjusting the reactfrequency of said receiver to a value equal and opposite in character to the reactance of the remainder of said connecting means, said transmission line, said coupling condenser, said transformer, and said power line at the frequency of a said receiver.

5. An arrangement for coupling-a first and a second carrier current apparatus, having respectween said lines, the turn ratio of said transformer being suitable to match the impedance of said transmission line to the impedance of said power line and condenser, the impedance of said power line measured through said condensers;

transformer, and transmission line having a reactive component at both said frequencies, means for connecting said first apparatus to said transmission line, said connecting means havmg a reactance at said first frequency qual and opp site in character to said reactive component and having high impedance at said second frequency,

and means for-connecting said second apparatus to said transmission line havingva reacts-nee at said second frequency equal and oppodte in character to said reactive component and having high impedance at said first frequency.

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