US1763009A - Loading system - Google Patents

Description

June 10,1930. D. A. QUARLES'} 1,763,009

LOADING SYSTEM Original Filed Aug.14, 1924 F IG. 5

Elmo/Iva COIL UNIFORM LINE 1 l I l4 l5 i ze ke 7e I 526 I PAT/0 INVEN 70/? D. A. QUAPL E8 ATTORNEY Patented June 10, 1930 UNITED STATES PATENT OFFICE DONALD A. QUARLES, OF ENGLEWOOD, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N Y., A CORPORATION OF NEW YORK LOADING SYSTEM Original application filed August 14, 1924, Serial No. 731,909. Patent No. 1,711,653, dated May 7, 1929.

Divided and this application filed September 29, 1928.

This invention relates to loaded lines for the transmission of telephonic currents and the like. The object of the invention is to provide a system of loading applicable especially to long lines which will improve the quality of transmission.

This application is a division of application Serial No. 731,909, filed August 14, 1924, and issued May 7, 1929, as Patent No. 1,711,653.

The Pupin-Campbell system of loading, as originally conceived and hitherto practiced, has as a principal object the reduction of attenuation, thereby increasing the possible range of the telephonic transmission of speech. With the advent of the eflicient telephone repeater, mere reduction of the attenuation by loading ceased to have its former fundamental importance, while at thesame time the possible length of lines for tele phonic communication was much increased. These considerations made it necessary to give more serious attention to the quality of transmission and the received speech because in long lines various factors combined to impair the quality. One of these factors which requires particular attention is the transient distortion, a phenomenon that is observable when all waves are not transmitted over the line with equal velocities.

The part played by the transient distortion maybe illustrated by the consideration of a simple case in which a simple sinusoidal is suddenly impressed upon a loaded line. Although the impressed E.M.F. oscillates at but one frequency, its sudden application corresponds to impressing upon the line a very large number of E.M.F.s of closely spaced frequencies extending through the frequency spectrum, the summation of which 'represents the rapid growth of the amplitude of the principal wave. If all of these waves arrive together at the distant end of the line, the current in the receiving apparatus will be built up with substantially the same rapid- "ity as was the amplitude of the impressed wave; but if some of the components are delayed in transmission more than the others, the building up of the received current will proceed in a quite different manner. The rapidly varying amplitudes of speech Serial No. 809,290.

waves produce effects of a similar character and unless the proper precautions are taken the currents received at the distant end of the line may never build up to anything remotely resembling the form of the impressed wave in the short interval during which the latter eX- ists. The effect produces or may produce serious impairment in the quality and intelligibility of the received speech even when the line is so designed that the steady-state attenuation of all currents within the speech range is substantially constant, and the system, therefore, from the steady-state standpoint substantially distortionless.

The present invention proposes to overcome this difficulty by using instead of the inductance coils of the Pupin-Campbell system, an improved loading unit the effective inductance of which is a variable quantity depending upon the frequency of the transmitted waves and diminishing as the wave frequency increases. By virtue of its diminished inductance at higher frequencies the propagation velocity of the line falls off less rapidly with increasing frequency than it does in an ordinary loaded line and by properly proportioning the loading unit the velocity may be substantially equalized throughout a wide frequency range.

A loading unit embodying this feature is disclosed in my copending application, Serial No. 731,909, filed August 14, 1924. This unit comprises a two-winding coil the mid-points of each winding being connected with a condenser which requires the use of a special coil each winding of which is tapped at the midpoint. The present invention has the advantage that thev same result may be effected by applying condensers to existing coils having nomid-point taps.

Fig. 1 of the accompanying drawings represents a section of a line equipped with loading units of the improved type; Fig. 2 shows in schematic form a network to which this loading unit corresponds; Fig. 3 is a simplified schematic of a loaded section of the improved type; Fig. 4 illustrates graphically the velocity characteristics of lines loaded in accordance with the invention.

The transmission line of Fig. 1 comprises loading units a between which extend equal sect-ions 6 unloaded lines having uniformly distributed constants. The loading units consist of inductance coils 7, the inductance of which equally divided between the two sides of the line, and two condensers 8 of equal capacity connected diagonally bet veen the line terminals l e and 23 respectively. The complete unit constitutes a bridge, or lattice, structure of a type the general properties of which are described in an article entitled Physical theory of the electric wave li ter by G. A. Campbell, published i the Bell System Technical Journal, vol. 1, No. 2, November, 1922.

The characteristics of ave propagation over a line of fie type illustrated may be determined exactly by methods which are described in part in the above noted publication and in part in a paper on the subject of Cissoidal oscillation by G. A. Campbe l, printed in the Transactions of the American Institute of Electrical Engineers, Vol. XXX. page 873. The general method of solution consists in resolving each complete loading sect-ion, con'iprisingone loading unit and a section of the non-loaded line, into T or 7r network of simple impedances. These networks when joined in series constitute a network equivalent to the whole line. the propagation through which may be determined by simple network formulae. Methods and formulae for eliecting the transformation of the loading section into its equivalent forms of network are sl'iven in the above noted paper on cissoidal oscillations, this including; formulae for the transformation of th line with uniformly distributed constants to a network of three sinnile impedances. Formulae for determining the wave propagation through the ladder structure equivalent lines are given in the first of the references mentioned above. In the following analysis of the operation of the invention this method will be used, but to simplify the solution certain approximations will be made, these being of such a nature as will not materially ali'ect the accuracy of the solution, particularly with respect to the velocity of propagation.

Transformation of the lattice structure of the loading unit 5 into its a type equivalent net-work results in the network shown in Fig. 2, which comprises two shunt condensers 13 having fixed capacities and two series inductances 12, each having variable coeflicients dependent upon the value of the frequency. The coeflicients of the impedance elements of the equivalent network are expressed in terms of the constants of the loading unit impedances by the following equations:

in which R L and C represent the total resistance and inductance of the two coils 12 and the capacity of condensers 13 respectively, and in which the corresponding quantitles on the right-hand side of the equations refer to the inductance coil 7 and the condensers 8 of the loading unit. The symbols j and p in accordance with common practice refer to the imaginary quantity 1 and to the em 'valent angular velocity 21.- m frequency, respectively. The terminals 1, 2, 3 and erof the equivalent circuit correspond to the similarly marked terminals of the loading unit. That the two networks are equivalent may be readily checked by comparing their iterative impedanccs as measured at the terminals 12 and their propagation con stants for the flow of current from terminals l2 to terminals 3 l. The first of these quantities is, in the case of a. symmetrical network, equal to the geometric mean of the short circuitand open circuit impedances of the network, the terms short circuit and open circuit referring to the condition of the terminals remote from those at which the impedance is measured. The propagation constant is, in a like case, dependent; upon the square root of the ratio of the open circuit iu'ipedancc to the short circuit impedance.

If in Fig. 1 the network of Fig. 2 is substituted for the actual loading unit it will be seen that the condensers 13 are directly in shunt to the ends of the non-loaded section of the line. To simplify the further solution of the propagation characteristics, it will he assumed that these capacities may be regarded as effective merely to increase the distributed capacity of the line, that is, as though they also were uniformly distributed over the non-loaded section. The justification of this assumption lies in the fact that the series impedance of a single section of the line in practice seldom exceeds in nine 10% of the shunt impedance due to the line capacity. The loaded line may thus be regarded as a uniform line of increased distributed capacity loaded by means of spaced inductance coils, the inductance of which decreases as the frequency increases in accordance with a variation expressed by Equation (1) above. A single section of the equivalent line is shown in Fig. 3, the section being terminated at each end in the middle of a loading unit. The half coil ll has an impedance 7 1/2 Z.. 1/2 (R -M1213 (3) both. R, and L, being variable as expressed by Equation (1). The uniform line section 15 is characterized by an iterative iInpedaiice Z and a propagation constant ya for its full length, these quantities corres'sponding to the iterative impedance Z, and the propagation constant y of the actual line section 6 of Fig. 1, but modified by the effect of the added capacity.

To the equivalent line may be applied the equation for the propagation constant of a coil loaded line first given by G. A. Campbell in the Philosophical Magazine, Vol. V, 1903, page 313, et seq., namely,

. t: Z. Cosh P cosh The propagation constant P is in general a complex quantity having a real component A which represents the attenuation and an imaginary component B which represents the phase shift and is related to the time of propagation. Equation (4) gives the total propagation constant for a single loading section, since the quantities y, and Z are based on the loading section as the unit length of the transmission line.

While Equation (4) is suflicient for the computation of the propagation constant it is more convenient to derive therefrom separate expressions for the attenuation and phase constants. In addition, since the ideal method of loading would consist in adding sinh 7,. (4)

to each section of the line a uniformly distributed inductance having a total value equal to that of the loading coil and having no added capacity, it is of interest to express the propagation constants of the actual line in terms of the corresponding constants of the ideally loaded line.

Let the total resistance of a non-loaded section of the actual line be denoted by B and its total capacity be C the distributed inductance and leakage being assumed to be zero. The total capacity added to each section of line by the loading units is 2G, the capacityof one of the condensers 8 being added at each end. Let the ratio of this capacity to the line capacity be denoted by r. The constants Z and 7., of theenon-loaded section of the equivalent line are expressed in terms of R and C by the following equations:

The expansions of cosh and sinh in a power series and a substitution therein of the values for Z and y given by the foregoing equations gives the following equation for cosh P, only those terms involving the frequency in the second degree and lower being retained:

Cosh P=1 one section of the coil loaded line, the attenuation and phase constants have the following values respectively:

Feds} B=p1/ It is assumed that in the ideal system no resistance is added by the addition of the inductance. By means of Equations (8) and (9), Equation (7) may be transformed into the following:

Cosh P=1 %+jafl(l+r)(l+ in which is the factor (1+l/2p L0) of Equation (1) which relates the effective line impedance of the lattice loading unit to the impedance of the inductance coil, and p is the ratio of the effective resistance R of the loading unit to the resistance R, of the line. This equation, when the components of P are written down has the form:

the right-hand side of which may by standard mathematical process be expanded into two terms, one real and one imaginary, thereby giving expressions for A and B separately. The separation of cosh" (X +jY into its real and imaginary combination is given in the aforementioned paper in the Philosophical Magazine. For small values of Y, which by comparison with Equation (10) correspond to small values of the propagation constant, values of A and B are respectively The critical frequency of a coil loaded line is defined as the cut-off frequency of the structure having equal inductances and capacities but having no resistance in any of its branches. For a loaded line with the lat- To relate the propagation constant to the attenuation and phase constants of the ideally loaded line having the same total inductance these quantities must first be expressed in terms of the line constants. For a length of the uniformly loaded line equal to that of which is the same as that for a line loaded in the ordinary manner with coils of the same inductance.

The time required for a were t o traverse the loaded section is given by the quation IVhen the quantities B and 7a are xpressed in terms of the ritical trequency by means of Equation (1%}, this equation tor the time of propagation becomes The time of propagation over a imifornily loaded line of length equal to one loading section and having a total inductance L and *apacity 0 -1-20 is equal to which in accordance with Equation (16) is also the time of propagation of a very low frequency wave over he lattice loading section. The factor multiplying in the right-hand side of Equation (16), therefore expresses the ratio of the propagation time for a wave of any frequency lower than the critical frequency of the line to the limiting time to in lines having various proportions of added capacity.

The three curves plotted in Fig. 4 show the values of this factor for the particular cases in which the ratio r is O, .5 and .75, respectively; it is evident that values of 1' between about .4: and .8 result in a greatly increased uniformity of the propagation time over a substantial fraction of the range below the critical frequency of the line.

The attenuation of a line loaded in accordance with the invention is increased by the addition of the loading unit capacities, the increase as indicated by Equation (12) being substantially proportional to the square root of the increased values of the total effective capacity. In most cases this may be offset by increasing the amount of gain in the repeaters inserted in the line, and in some cases the fact that a greater proportion of the transmission range of frequencies is available for high grade transmission permits a greater amount of inductance to be used, thereby reducing the critical frequency to a. lower value, but at the same time reducing the attenuation.

In the practical application of the invention it is not necessary that all loading units in av line he of the improved type. A substantially equivalent gain with respect to uniformity of velocity may be had if only the alternate units are of the improved type, the others being simple loading coils of the Pupin-Campbell type. In this case the capacity added in the modified units should be increased in the ratio 1.46 and the inducta-nces should be increased in the ratio 1.37 as compared with the values for a system in which all loading units are of the modified type. Other distributions of the improved units throughout a system may also be used to secure ditto-rent degrees of velocity compensation.

IVhat is claimed is:

1. In an inductively loaded transmission line having two line wires, a loading unit comprising an inductance coil having equal vindings connected in series with said line wires, and equal capacities connected in shunt to said line between diagonally opposite corners of said line windings.

2. A two-wire transmission system comprising a plurality of similar sections, each of said sections comprising equal inductances connected in series with said wires, equal capacities connected in shunt to said wires between diagonally opposite terminals of said inductances, and a capacity connected in shunt to said wires between a pair of adjacent terminals of said inductances.

3. A signaling line comprising inductance coils for periodically loading said line according to the Pupin-Campbell system, and conoensers included in shunt to said line at each loading point for compensating the variation of wave velocity due to periodic loading, said coils and said condensers being arranged in the form of a lattice network.

4. A two-wire transmission line comprising a plurality of equal sections divided by inductive loading units, each of said units comprising an inductance having two equal windings, one in series in each line wire, and two equal capacities interconnecting said coil windings, the sum of said capacities having a value between .4 and .8 the total capacity between the wires of one of said sections.

In witness whereof, I hereunto subscribe my name this 28th day of September, 1928.

DONALD A. QUARLES.

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