US1752461A - Transmission network - Google Patents

Description

April 1, 1930. T. SHEA TRANSMI SS ION NETWORK Filed July' 24, 1925 1 0 8 0 Frequency- KHocycles rim/05508 zzy 4/? Patented Apr. 1, 1930 UNITED 'STATES PATENT OFFICE SHEA, OF RUTHERFORD, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A. CORPORATION 01 NEW YORK TRANSMISSION NETWORK Application filed July 24,

This invention relates to electrical networks, and particularly networks comprising elements having natural capacities which act in shunt to the line of transmission electric energy and has for an object to improve the transmission characteristic of such networks.

x .In certain types of electric networks, for example electric wave filters, there may exist natural capacities between elements or between some of the elements and ground which capacities-act in shunt to the network to increase the loss or to otherwise affect the transmission of some of the frequencies. Such-a condition has been found to exist in certain balanced filter structures adapted for use at high voltages where there is a very appreciable capacity existing between each of the high voltage series condensers and ground. These" ound capacities in series are effectively in s nut to the filter and greatly distort the transmission characteristic, increasing the loss, over a large portion of the transmission range. 1 p In accordance with a feature of this invention there is included in the filter a shunt inductance of such value as to resonate with the two ground capacities in series at a frequenc within the transmission 3 impe ance ofthis anti-resonant circuit comprising the two ground capacities and the inductance being very great throughout the transmission range the efiect of the condenser is thereby annulled and the transmission characteristic restored tonormal. The inductance may consist of a separate coil or may be combined with other inductance elements of the filter in one coil to give the same sheet.

In some cases where the disturbing capacities are paralleled by filter condensers, they mayjbe taken into account in determining the capacity of the condensers. 1

This invention may be more readily under stood by referring to the following description in connection with the drawing in which Figs. 1 to 4 show diagrammatically several embodiments of the invention and Fig. 5 the transmission characteristic of the embodiment shown in Fig. 4.

Fig.1 shows a and pass filter of the type hand. The

1925. Serial No. 45,792.

condensers 7, 7 8 and 8, series inductances f 9, 9', 10 and 10 and a shunt inductance 11.

Only a single section is shown and it may be mid series terminated at each end. The filter is of the balanced type each series element being split into two parts, one connected at each side of the line. The series condensers 7 and 7' are. adapted to couple to a high voltage line and each is assumed ,to have an appreciable capacity to ground. These ground capacities are represented by the dotted condensers 12 and 12 which act in series as a shunt across the line and materially increase the loss in the transmission band. In order to counteract 1 their effect an inductance 13, having such a value as to resonate with the capacities 12 and 12 at a frequency within the transmission:

band, (preferably the mid frequency) is shunted across the line. The impedance of the combination comprising the capacities 12 and 12 and the inductance 13 is substantially infinite at the resonant frequency and very great at all frequencies throughout the transmission band so that the resulting effect is the same as though neither the capacities nor the inductance were present.

In order to reduce the number of elements certain of the inductances may be combined;

-The method of combining filter elements in this way is fully explained and is claimed in my copending application Serial No. 91,219, filed February 27, 1926. Fig. 2 shows a filter structure in which the inductances 9, 9,'10, 10 and 11, are replaced-by the equivalent structure consisting of inductances 14, 14, 15 and 16. Inductances 13 and 15 being directly in parallel with one another may be combined in a single coil.

Figure 3' shows another arrangement by which this combination of inductances may be made. In this embodiment the inductances 13, 9,9, 11, are replaced by the equivalent star structure consisting of inductances 17, 17 18, 19, 19. The series inductances 10 and 19 may then be combined in one coil, and similarly inductances 10 and 19'. The annulling inductance 13, does no appea'r physically: but by virtue of the above mentioned equivalence of the star structure, it is'eflectively present and is therefore shown dotted in the position where it effectively acts.

Fig. 4 shows another embodiment of this invention in a band pass filter of the type shown in Fig. 1 of U. S. Patent 1,227,113. This filter is also one of the balanced type comprising series condensers 20, 20, 21, and

21', series inductances, 22, 22,23 and 23, the shunt condenser 24 and the shunt inductance 25. The series condensers 20 and 21' each have appreciable capacities to ground represented by the dotted condensers 26 and 2.6. The effect of these condensers is annulled by the use of the shunt inductance 27. In one particular embodiment in a filter forcoupling a carrier telephone to a high voltage transmission line, the coupling condensers 20 and 20 are each found to have ground capacities 26 and 26 of .001 mf. The inductance 27 is given a value of 6.2 mh. so as to resonate with the condensers at approximately 92 kilocycles.

Fig. 5 shows the transmission characteristic of this filter in which the transmission loss in miles of standard cable is plotted against frequency in kilocycles. CurveA rep resents the transmission characteristic of the filter section in which series condensers of the same value but having no ground-capacities are substituted for the condensers 20 and 20. Curve B represents the characteristic for the condensers 20 and 20 having ground capacities. ment the characteristic is badly distorted, the

' loss being increased over a large portion of the transmission range and the frequency of the lower cut-ofi being greatly reduced.

Curve 0 shows the filter characteristic of the arrangement in which the'annulling inductance 27 is employed.

This is seen to be practically the same as the normal characteristic A of the filter section, the slightly increased loss being largely due to the resistance of the inductance coil.

VVhile this invention has been illustrated with only a few specific embodiments, it is understood that it is equally applicable to other circuit arrangements within the scope of the appended claims.

at is claimed is 1. In a wave filter adapted to transmit selectively a pre-assigned broad band of frequencies, a condenser included in a series branch of the filter and having stray capacity in shunt to the filter, and a star connected group of inductances adjacent to said cor1- denser, said inductances being proportioned with respect to the capacity of said condenser and to the pic-assigned range of frequencies to maintain the desired selective transmission characteristic, and being proportioned also with respect to said stray capacity to provide a substantially neutralizing reactance there- It is seen that with this arrangefor throughout the preassigned frequency range.

2. A wave filter section comprisin a pair of series branch impedances couple by an inductive shunt branch impedance, said series branches including condensers, at least one of which has a stray capacity to ground,

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