US1921431A - Loading system - Google Patents

Description

Aug. 8, 1933. SHEA 1,921,431

LOADING SYSTEM Filed Aug. 11, 1930 2 Sheets-Sheet 2 FIG. 7 4 IKE LP/F H AMP LPflF AMPL IF/ER GAIN FREQUENCY -K/LOC ICLES HWEN TOR T.E SHE A BVJW Patented Aug. 8, 1933 A'r r o Flc LOADING SYSTEM Timothy E. Shea, Rutherford, N. .l., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., aCorporation of New Yorlr Application August 11, 1930. Serial No. 474,331

7 Claims. (o1. 17s

invention relates to loading systems for telep'-one lines and the like, and has for an object to improve the transmission and impedance characteristics of loaded lines.

This application is a continuation in part of the co-pending U. S. application of 'I. E. Shea,

Serial No. 198,459, filed June 13, 1927, now Patent No. 1,772,558 issued August 12, 1930.

It is well known that the ordinary loaded line has an impedance characteristic which is a practically pure resistance but which varies quite considerably with frequency. This variation is particularly marked in the case of loaded cable circuits. On the other hand the impedance of unloaded open-wire lines is practically independent of frequency so that when an unloaded open-wire line is joined to an ordinary loaded line considerable reflection loses are introduced;

In order to avoid such losses and also to simplify the impedance balancing arrangementat repeaters, it is desirable that the loaded line should have an impedance which is resistive and practically constant with frequency.

.Inone specific embodiment this invention comprises an open-wire line or, cable periodically loaded by means of loading'units comprisinginductance coils and condensers connected in parallel with each other in each side of the line. These loading units may be distributed along the lines at approximately the same distances apart as are the loading coils in lines loaded by the Pupin method. When terminated mid-coil such system has a characteristic"impedance which is a substantially constant resistance throughout-the transmission range. 7

In a modified formthis invention comprises loaded system in which the section or sections at each-terminal are loaded according to the method of this invention as above described while the intermediate sections are loaded according to the ordinary method, i. e., the Pupin method. It ispossible to use this combination, because for frequencies in the transmission range the mid-section characteristic impedance of a line loaded with the loading units of this invention is substantially equal to the mid-section characteristic impedance ofa line loaded in the usual manner.

In certain types of circuits, the loading netof this invention;

viding the improved characteristics mentioned above and in addition for reducing or prevent ing undesired cross-talk and distortion effects due to electrostatic or electromagnetic induction between adjacent portions of the circuits. One application of this'is in connection with loaded ofiice entrance cable connecting a carrier repeater to an open-Wire line. Where the out.- put and input cable pairs of the entrance cable are in close proximity, as is usually the case, and the usual coil loading is applied, the electro static and electromagnetic induction between the adjacent pairs-at high frequencies may be such as to produce circulating currents tending to cause regeneration and singing. If the loading networks of the invention are substituted for the usual loading coils in the entrance cable,

the sharply rising attenuation characteristics of the former above the loading cut-01f frequency erative effectsin the repeater.

@This invention will be better understood by reference to the following detailed description taken in connection with the accompanying drawings in which: 7 7

Fig.1 shows a network substantially equivalent to a section of a cable or line inwhich the distributed series resistance and shunt capacity of the-line are represented by equivalent lumped impedances.

Fig. 2- illustrates the equivalent network-of Fig. 1 combined 'with'the usual'type of loading coils and terminated mid-coil; .Fig, 3 shows schematically a portion of a transmission lineloaded with-the loading units Fig. 4'shows a network equivalent to a single mid-coil'terminated section of the system of -Fig. 3; v

Fig. 5 shows the impedance-frequency characteristics of cables loaded in the ordinary manner'and in accordance with the present invention; 7

Fig. 6 shows schematically a composite type of loaded line of this invention;

Fig. l'illustrates diagrammatically the usual arrangement of oilice entrance cable associatm'. it)

ing two sections of an open-wire line with a two-way amplifying repeater; and,

Fig. 8 shows curves illustrating the application of the invention to a system such as shown in Fig. 7.

If a section of cable or open-wire line in which the series resistance and shunt capacity are the controlling factors, be represented by the equivalent T network shown in '1, where R is the total series resistance of the section and S and C are respectively the length of the cable section and the capacity for'each unit of length, so that the product is the total ca pacity, such a section loaded in the usual manner may he represented by the network shown in Fig. 2, where L is the total inductance of the loading coil. In this case, as is well known,

the cut off frequency of the loaded sectionis approximately given by The two most common methods of terminating such loaded sections are, respectively, mid-section and mid-coil. In the case of the midsection termination, the first loading coil is loand From which it is seen that for mid-coil and mid-l ,ction. to- "nations the characteristic impodance is approximately a pure resistance, r

with the frequencies as a function the ratio of the frequency to the cut-off frequency.

Referring to Fig. 5 curve 13 represents the pedance-frequency characteristic of a cable 1 ded iii-the ordinary manner (Pupin method.)

th mid-coil termination, and asthe imped ce decreases with increasing frequency, -apprcaching zero at the cut-off iiequency which for this case is about 2800 cycles per second. Curve A shows the impedance-frequency characteristic for the'sainc type of cable with midsection termination, as shown, in this the impedance increases with increasing fref uency, approaching infinity at the cuteoff frequency. C *e C is the inpedance-frequcncy characteristlc of a non-loaded open-wire line that is,

characteristic, and its cut-oil frequency is ap proximately Il /L(SC+4C1) (4) Its nominal impedance is the same as for lines loadedin the usual manner and is independent of the capacity of the series condensers. The characteristic impedance of a cable terminatedat mid-section with this typ loading is, for a given nominal impedance and given cut-off frequency, very nearly the same as that of the usual type of loaded line, given by formula (3). While the variation of impedance. taken midsection, throughout the frequency hand is unaltered, this, as hereinafter explained, is advantageous rather than otherwise since it allows a cable of composite loading to be formed. The char cteristicimpedance of a cable tor inatcd at mid-load (mid-coil) -with the antiresonant type of loading is, however, affected by the series condensers and isapproximately where f fthe anti-resonant frequency of the loading network, is

. 1 V g =2m/Ts1 Except where this anti-resonant frequency is very'clcse in value tothe cut-off frequency, the effect will he to make the characteristicim- LOO pedance very uniform throughout the transmission band.

cmparing that for the two types of loading the mid-load impedance differs by'the factor 1 T To and that this factor may be varied over a wide range by a suitable choice of f that by a suitabi choice of capacity for the condensers.

In Fig. 5' curve D shows the impedance frequency characteristic for a loaded line of the type shown in Fig. 3 in which the anti-resonant frequency is 1.25 times the cut-off frequency,

For a fixed value of loading coil inductance in each anti-resonant network and fixed spacing ormulae) and 2), '1: is seen' quency. The dilferencein transmission characteristics may be roughly expressedby saying that the cut-off frequency is somewhat reducedas compared with the ordinary type loading, although this effect may be minimized or prevented by reducing the loading spacing.

The attenuation and phase characteristics are not always controlling, however, and in certain cases it may be desirable, in an effort to obtain uniform impedance characteristics to sacrifice to some extent the obtaining of good attenuation and phase characteristics. This isparticularly true when toll entrance cable'or submarine cable must be joined to open-Wire lines without'causing reflections at the junction points. For example, in a case in which an entrance cable is used to connect a repeater to an open-wire line, the repeater having a cut-off frequency of about 2600 cycles per second and the nominal cut-off frequency of the cable being fixed at 7200 cycles per second, only 36% of the transmission range is used. The impedance of the cable loaded in the usual manner, as shown by curves A and B inFig. 5 will still be considerably difierentfrom that of the open-wireline, as shown by curve C. However, if the anti-resonant network in accordance with the invention is substituted for the ordinary type of loading, and the same value of loading coil inductance and the same spacing are chosen as in the case of ordinary loading sections and the condensers are chosen of such capacity that f =L5fc the cut-off frequency of the cable would be 5400 cycles and the impedance of the cable'would be practically the same as that of the open-wire line throughout the entire effective transmission range. Also the attenuation and phase shift characteristics would be practically identical with those of the cable loaded in the ordinary Fig. 6 shows a composite type of loaded line in which some of the sections are loaded with anti-resonant networks and some are loaded in the ordinary manner. This arrangement is possible because the mid-section characteristic impedence with either type of loading may be made very closely the same by choosing the same cut-off frequencies and nominal impedances. The particular advantage .of this type of composite loadedline is that the uniform impedance characteristics are desiredv primarily at the ends'of the cable span, that is, at repeater and terminal points. By loading a few sections at either end of the cable span with anti-resonant type of network the terminal impedance of the span of cable may be made to closely approximate a constant value so that it may be connected to terminal or repeater apparatus without reflection losses.

There may conceivably be cases where the attenuation characteristic of the line above the cut-off frequency is important in excluding in- .terfering frequencies induced from extraneous sources, and inasmuch as the anti-resonant network has an attenuation characteristic which rises sharply above the cut-off frequency, in the case of the corresponding type of low pass filter, this proposed type of loading may be of considerable value from this standpoint. Furthermore, the sections of this type when inserted in the cable, may have different frequencies of anti-resonance, so that theattenuation ismaintained high for some distance above the cut-off frequency. 1 Fig. -7 illustrates diagramatically the usual arrangement of oflice entrance cable associating an open-wire line with a carrier telephone repeater in a multiplex signal transmission system. The two-way repeater R may be for example of the type disclosed in U S. Patent 1,413,357, to P. A. Raibourn, issued April 18, 1922. It comprises two one-way repeating channels RW and RE adapted to repeat signaling currentsbetween a west section WL'and an east section EL of an open-wire line through entrance cable pairs ECl andECZ.

The signaling currents received at the repeater R over the cable pair ECI from line section EL are amplified in the channel RW and the amplified currents are delivered to the line section WL over the cable pair EC2. Similarly the signaling currents received at therepeater R over the cable pair E02 from the line section WL are amplified in the channel RE and delivered to line section EL over cable pair ECi.

The currents incoming at the repeater from' the line section EL are of higher frequencies than those incoming thereat from the line section WL; Accordingly, the respective repeating channels are provided with frequency discriminating filters which direct the incoming currents to the proper channels. The repeating channel RW includes a'high pass input filter 1, a one-way amplifying device 2 and a I high pass output filter 3. The repeating channel RE includes the low pass input filter 4, the one-way amplifying device 5 and the low pass output filter 6. The directional filters 1 and 3 are designed to transmit currents of theupper group of frequencies to be repeated by the amplifying device 2 and to suppress from transmission currents of lower frequencies, while the directional filters 4 and 6 are designed to transmit currents of the lower group of frequencies to be repeated by the amplifying device 5 and to suppress from transmission currents of higher frequencies. The filters may be of the type disclosed in U. S. Patent No. 1,227,113. to Campbell, issued May 22, 1917.

The repeating paths RE and RW may contain other apparatus than that shown, for. example, attenuation equalizers for compensating forunequal attenuation of the transmitted currents of different frequencies.

As the carrier repeater R ordinarily would be located at some distance from the mainpole leads of the open-wire line, it is the usual practice to bring in the input and output wires of the repeater through entrance cable which is ordinarily loaded periodically with Pupinloading coils L, as indicated in the drawings, to reduce the attenuation to a'minimum. This coil load ing is usually designed to have a cut-off at a frequency which is well above the frequencies in the signaling range to be transmitted, in order to prevent reflections. For a range of signaling frequencies up to 30 K. C. the cut-off frequency would be 50 K. 'C., for example. On the other hand, in order to secure a fiat amplifier characteristic for the range of signaling frequencies up to 30 K. C. it may be necessary to transmit fairly efficiently frequencies for a considerable distance above 30 K. C.

If the two entrance cable pairs EC1 and ECz are in close proximity, as would commonly be the case, some electrostatic or electromagnetic induction will take place between them so that there will be a tendency for currents leaving the repeater at a high level to cross over to the opposite pair and reenter the repeater at a lower level. In this there is a tendency towards regeneration or singing, due to the fact that a circuit may be formed by one side of the repeater, portions of the entrance cable pairs, and the leakage path therebetween.

For the current which travels through the low pass side of the repeater, that is, the channel there is little to worry about from the standpointof circulating currents due to electrostatic or electromagnetic induction, as the entrance cable pairs would be ordinarily welltransposed with respect to each other. At high frequencies, however, the transpositions become less effective and the electrostatic and electromagnetic induction becomes greater. Consequently, it is the current which travels through the high pass side of the repeater, that is, the channel RW (which will be high frequency current) which may be troublesome.

The path of these high frequency circulatory currents may be traced as follows. They will pass as indicated by the arrows from the east line section EL through the cable pair E61 to the junction. pointA of the repeater and thence to the repeating channel RW. In the repeating channel RW these currents will pass through the high pass input filter l, and after being amplified by the amplifying evice 2 will be passed through the high pass output filter 3 to the point B of the repeater. From the point B these currents will pass t rough the loaded entrance cable pair ECz to the west line section WL, and in passing therethrough, will induce a corresponding high frequency current the adjacent entrance cable pair E 31, which will pass, as indicated by thev arrow, into the repeater at the junction point 'A, and will be amplified therein. It may be readily seen that such regenerative and singing effects under certain conditions may be such as to prevent proper opera tion of the system. 1

The high side of the repeater will be effective in transmitting currents to a much higherfreoucncy generally than the maximum conversational frequency. This maximum conversational frequency might be 30 K. C. and the repeater might be efficient up to a frequency in the vici ity of 59 ii. C. it would be possible to prevent currents of greater frequencies than 30 K. C. from flowing through this part of the repeaterby inserting therein a separate low pass electrical filter. However, the same result may be accomplished by changing the type of loading and the entrance cable through which the circulating currents flow the sort of characteristics that a low pass electrical filter would have. In other words, for circulating currents of freauencies between the maximum conversational frequency and the maximum frequency of transmission of the repeater, attenuaticns may be introduced to circulating currents at any point or points along their path. It may be introduced by a low pass electrical filter cutting off at a frequency of about 30 (3., or it may be introduced by the use of properly designed loading networks in the entrance cable pairs.

If the loading networks of the invention either the anti-resonant type shown in Fig. 3 or the composite type shown in 6, are substituted for the coil loading networks L in the system of Fig. l, the sharply rising attenuation characteristic of these networks above the loading cut-oi frequencymay be utilizedto introduce considerable attenuation in transmitted frequencies in the range between 30 K. C'. and

K. C., that is, in the range occupied by the currents will take place in each section of the loaded cable, and, for the induction takingplace in any particular section, the attenuation to which that particular portion of the circulating cu rents will be subjected, is that of one circuit of the entrance cable from the repeater down to the point where induction takes place plus the attenuation taking place in the induction process plus the attenuation in the corresponding portion of the entrance cable between the second point'of induction and the opposite side of the re"eater. For-those elements of the circulating cu rents which are caused by induction at points remote from the repeater, the attenuation will therefore be high; forthose elements for whicinduction takes place at a point close to the r peater the attenuation will be low. On the average, the elements of -circulating current subjected to a considerable amount of attenuation and consequently the circulating currents willbe smaller with the anti-resonant type of loading or the composite type of loading or the invention than with the Pupin type of loading, and the tendency toward regeneration or singing will be minimized.

The advantages which maybe obtained by the use oi loa" g networks of the invention in the system or 7 will be clearer ircm an examination of the characteristic curves of "Fig. 8. In 8, the curve no representsthe fre" quency gain' characteristic of the amplif ing device 2 in the amplifying path RW; As indicated, it may have a fairly fiat gain characteristic over a frequency range extending from the lowest speech of about 58 K. C. Therefore. the amplifiertransmits some distance beyond the maximum'conversational frequency of say 30 K. C. The reason for this is generally inherent in amplifier and transformer des n. The curve AP represents the attenuation equency characteristic for the loadingnetworks L in the system of .Fig. '1 corresponding to the well known Pupin loading coils, this characteristic being similar to t at of a low pass electrical filter of corresponding structure. curve designated AAF. represents the attenuation frequency characteristic of "the.

anti-resonant loading network, such as shown in Fig. 3, where the frequency of maximum attenuation corresponding to fm is selected by design. The curve Ac represents a combination of the anti-resonant and the Pupin type of loading, such as shown "in Fig. 6, and gives an intermediate characteristic. Because this composite loading network maintains a fairly large attenuation at all frequencies a substantial distance above the minimum conversational frequency it would often be used in preference to the anti-resonant type of network as shown in Fig. 3. It may be seen from the curves of Fig. 8 that the characteristics A0 and AAa rise sharply for frequencies immediately above 30 K. C. while the characteristic AP representing the .rupin type of loading rises much more gradually above 30 K. C. It may be seen therefore, that either of the loading networ s of the invention may be advantageously used in place of the Pupin type loading network in the entrance cable in the system of Fig. 7 to minimize cross-talk effects as well as to obtain additional frequencies up to frequency advantages from the standpoint of impedance constancy explained above.

In loading phantom circuits and side circuits in phantomed systems with the anti-resonant type of loading network the condensers forming a part of the loading network in the side circuit produce no effect as far the phantom circuit is concerned, and similarly those used in the phantom circuit produce no effect on the side circuits.

What is claimed is:

1. In a wave transmission system, twoadjacent two-way transmission' paths, means for impressing on said paths signal waves of one frequency range to be transmitted in one direction thereover, and signal waves of a different frequency range to be transmitted in the opposite direction thereover, an amplifier for repeating the waves of said one frequency range over said paths in said one direction while substantially excluding transmission of waves of said other frequency range, a second amplifier for repeating the waves of said other frequency range over said paths in said opposite direction while substantially excluding transmission of waves of said one frequency range, and means for periodically,

loading said paths so as to obtain substantially uniform, low attenuation in the transmitted waves of all frequencies in said ranges while producing high attenuation in transmitted waves of frequencies at least immediately above the highest frequency in said ranges and thereby efiectively reducing undesired interchange of high frequency energy by induction between said adjacent paths.

2. In a wave transmission system, two adjacent two-way transmission paths so arranged that there will be substantially no interchange of low frequency energy by induction therebetween, means for impressing on said paths, waves'of one frequency range to be transmitted in one direction thereover, and waves of a dif ferent frequency range to be transmitted thereover in the oppositedirection, two amplifiers for respectively repeating waves of a different one of the two frequency ranges over said paths to the substantial exclusion of Waves of the other frequency range, and means for periodically loading said two-way paths to obtain substantially uniform low attenuation in transmitted Waves of all frequencies in both ranges while producing high attenuation in wave frequencies at least immediately above the highest frequency in said ranges.

3. The system of claim 1 and in which said loading means consists of a plurality of loading units connected in series in said paths, each comprising inductance and capacitance in parallel and having an attenuation characteristic which rises sharply above the loading cut-off frequency.

4. The system of claim 1 and in which said loading means comprises a plurality of loading units connected in series with said paths and having sharply increasing attenuating properties in the frequency range above the loading cut-off frequency.

5. The system of claim 1 and in which said loading means consists of a plurality of loading units connected periodically in series along said paths, each of said loading units comprising parallel inductance and capacitance of such values as to make the unit anti-resonant at a frequency substantially greater than the loading cut-01f frequency, and to make the unit have a high attenuation for frequencies above the loading cut-off'frequency.

6. In combination, two sections of an openwire line and a two-way repeater interconnecting said sections, means for impressing on said line signal waves of one frequency range to be transmitted thereover in one direction, and signal waves of a different frequency range to be transmitted thereover in the opposite direction, said repeater comprising amplifying means for repeating res ectively waves of each frequency range between said line sections to the substantial exclusion of waves of the other frequency range, two twc-way cable pairs respectively connecting said line sections to said repeater and in such close proximity as to allow'interchange of high frequency energy therebetween and means for periodically loading said cable pairs to obtain substantially uniform lcw attenuation in transranges. I

7. In combination, two sections of an openwire line and a two-way repeater interconnecting said sections, means for impressing on said line signal waves of one frequency range to be transmitted thereover in one direction, and signal waves of a different frequency range to be transmitted thereover in the opposite direction, said repeater comprising amplifying means for repeating respectively waves of each frequency range between said line sections to the substantial exclusion of waves of the other frequency range, two adjacent two-way transmission paths respectively connecting said line sections to said repeater, and arranged so that there is substantially no interchange of low frequency wave energy by induction between said paths, and means for periodically loading said paths to obtain substantially uniform low attenuation in transmitted waves of all frequencies in both ranges while producing high attenuation in transmitted waves of substantially all frequencies above said ranges.

TIMOTHY E. SHEA.

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