US1695000A - Method and apparatus for testing telephone circuits - Google Patents

Description

Dec. i1, 192s.

W. WOLFF y METHODAND APPARATUS TESTING TELEPHONE CIRCUITS Filed April 24, 1926 Pig. 2.

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Receiving 5h: fion` Patented Dec. 1l, 1928i.

A UNITED STATES PATENT oFFlcE.

COMPANY, A CORPORATIOlQ OF NEVI YORK.

METHOD AND APPAEIATUS FOR TESTING TELEPHONE CIRCUITS.

Application filed April 24, `1.926, Serial No. 104,492, and in 4Germany May 2, 1925i..

This invention relates to electrical testing systems, and particularly to amethod and arrangements in such systems for measuring theattenuation in telephone lines.

Arrangements for measuring the attenuaLV tion of telephone lines are'well-known.l Suchv arrangements are described inan article by F. H. Best in the Transactions ofthe American Institute of Electrical Engineers for February,l921l, beginning at pagel25 of that article. One arrangement is shown diagrammatically in Figi) of that article, and this arrangement is fairly complicated, as will be obvious from the description relating thereto. The important draw-back of this arrange` ment is that the impedance of the receiving circuit, -as stated on page 426, must be equal to the" impedance of the sending circuit, and that, in order to obtain 4practical values, bothv the impedance of the receiving circuit and the impedance of the sending lcircuit must be equal tothe impedance of the telephone line vat the n'ioment `that it is being measured. That arrangement includes switching means for connecting at willany one of three'fiXed impedances, namely, 600, 1200and lSOOohms, these threeA values being merelythe mean values of the types of telephone lines that are most frequently used` in practice. The im-,-

pedance of aftelephone line in general deviates somewhat from these values. Furthermore,

the impedance of every telephone line is also dependent upon the frequency transmitted thereo'ver. Consequently, when higher frequencies are used, there are errors in ythe measurements which are dueto the deviations between the impedance of the telephone line to be measured and the impedance ofthe sending or receiving circuit. y n

The present invention provides an arrangement which makes it possible to measurein a very simple manner corr-ect valuesl for .the impedance of a; telephone line.- Moreoven the arrangement of this` invention is substantially simpler than that of the descrip tion above referred to.y Forinstance, the

calibrating network mentioned inthe above article is quite complicatedy so far as anali-.fof the ratio of the'characteristic impedances brating net work itself isconcerned,the cali brating` network being theY measuring Yapparatus whichis-adjusted for 'every measurement. Y

Y, The features ofmy invention whichare be-Y lowing when read in connection with the accompanying drawing in which Fig. l represents a network which is the equivalent of ya four-terminal network composed of elements of resistance, and Fig. 2 represents a specific embodiment of the invention.

Referring to Figi, there is shown a fourte-rmina-l network including resistances R1, R, and R3 arranged so as to form a T network. These resistances may be adjusted so asto' have any desired attenuation. Letit be assumed that, when looking at vthe network from the side l, the'network has the characteristic impedance Z1 and that, when looking at the network from the side 2, the network has the .characteristic impedanceV ZZQ Asis well known in the art, thek characteristic impedance Zlinay be made equal to thecharacteristic impedance Z2. If this network is closed onside 2 by means of an impedance which vis equal vto its corresponding characi teristic impedance, namely, the impedance Z2, the impedance of the network measured from the side l will be equal to" the ch'ar'acfv teristicr impedance correspondingto thatside, namely, the impedance Z1. Moreover, when a voltage is impressed on the terminals of the side l, no reflection will be produced atfthe sid-e 2, since-.thenetwork is terminated by its characteristic impedance. The. current and voltage `distribution of the network closedlin ai manner described herein will be thesame 'as if the network possessed an in` finitely great attenuation. Consequently,jif

be the attenuationof the network, J 1and J2 andfV,L and V2" thevaluesof the currentand voltage at the input andthe output terminals of thenetwork, respectively, the` ratiofofthe current and ,voltagel at v the. input terminals at its input terminals and its output termi- ,nalsl ,TheV ratio, however, is independent of the magnitudes 4of the characteristic imv pedances of the fnetwork itself. In the case' ofga symmetrical networlni. e., a network in ioo which the characteristic impedances are equal,

In this case the attenuation b depends only on the ratio that the current or voltage at the input terminals of the network bears to the current or voltage at the output terminals of the network. The attenuation of two networks of any characteristic impedances may therefore be compared, in accordance with this invention, by terminating the two networks in the manner described hereinabove and by applying the same voltages or currents to their input terminals.

Fig. 2 shows a specific embodiment of the invention particularly ada ted for the measurement of attenuation o a telephone line. G1 andG2 are two alternating current generators which supply pure sinusoidal waves free from distortion. The generator G2 is at the transmitting station, and the generator G2 is at the receiving station. A telephone line L, shown in dotted lines, is connected to the generator Gr1 through a shielded trans former T1. A calibrating network N of adjustable attenuation is connected to the gen-l erator G2 through a shielded transformer T2. This Calibrating network N comprises resistances R5, R6, R2 and R2 in series relationship with the secondary winding of the transformer T2 and a resistance R2 in shunt relationship with the secondary winding of the transformer T2. This network is shown comprising pure resistances but it will be understood that the network may also be made to include inductances and capacities of suitable magnitudes. There are included in the secondary windings of the transformers T,L and T2 meters Ml and M2, respectively, these meters being used to aid in adjusting the current or voltage at the input terminals of the telephone line L and the Calibrating network N, respectively, to the same values.

Bridged across the input terminalsof the telephone line is arnetwork Nl which has an impedance equal to the characteristic impedance ofthe Calibrating networkN. Thus this network N1 is included between the secondary Vwinding of the transformer T1 and the telephone line L. Bridged across the input terminals of the Calibrating network N is a network N2 which has an impedance equal to the characteristic impedance Z1 otthe telephone line L when looking at the line from the transmitting station. Thus this network N2 is connected between the secondary winding of the transformer T2 and the Calibrating network N. Furthermore, a network`N2 having a characteristic impedance of the telephone line is bridged across the output terminals of the telephone line L so that the telephone line is terminated by its own characteristic impedance; and a net- ,put terminals r2-7),.

work N1 having a characteristic impedance of the Calibrating network N is bridged across the output terminals of the calibrating network N, so that the calibrating network is terminated by its own characteristic impedance. y y

A switch S 1s arranged so as to connect either the output terminals of the telephone line L or the output terminals of the calibrating network N to an indicating meter V or other measuring device of high impedance. `This indicating meter V is used for comparing the conditions at the output terminals of the telephone line L with those at the output terminals of the Calibrating network N. i

The switch S is moved iirst so as to connect the indicating meter V with the output terminals of the telephone line L. The generator G1 transmits a current through the transformer T1 tothe input terminals L1-b1 of the telephone line L. The pointer of the indicating meter V will then be deflected and its deflection may be read. The exact value of the drop in potential over the telephone line need not yet be known. The switch S is then moved so as to connect the output terminals of the Calibrating network N with the indicating meter V. The generator G2 then supplies a current to the input terminals t2-b2 of the Calibrating network N through the transformer T2. The current transmitted by the generator G2 is then adjusted until the current flowing to the input terminals of the calibrating network N equals that which previously flowed to the input termi nals oi' the telephone line L. In other words, the currentsare adjusted until the meter M2 reads exactly the same as the meter M1 reads, thus making the conditions at the input terminals a2--b2 the same as those at the in- The Calibrating network N is then adjusted until the deflection of the indicating meter V equals the deflection of that meter when the switch-S was connected to the output terminals of the telephone line L. When this adjustment is made, the conditions at the output terminals of the Calibrating network N are made the same as the conditions at 'the output termi nals of the telephone line L. Since the calibrating network N and the telephone line L may be said to have the properties of infinitely long lines because they are terminated by their own characteristic impedances, then the attenuation as determined from the calibrating networkN will be equal to the total attenuation of the telephone line L without regard to the characteristic impedance of the telephone line L or the characteristic impedance of' the Calibrating network N.

It seems hardly necessary to state that when the loss of the network N is large,'the resistance R, is small and forms only a small part ofthe characteristic impedance ofthe las network N. The resistance R9 may be varied over a considerable range without materially aii'ecting the magnitude of thecharacteristic impedance of network N. Thus, the resistance N, has a magnitude always substantially equal to the characteristic impedance of network N, even when the impedance of this network is varied by the manipulation of the resistance R9.

It will be understood, however, that although the arrangements of this invention have been described particularly'with reference to the adjustment of voltage conditions at the input and output terminals of both the telephone line and the Calibrating network, respectively, the same arrangements may be employed for adjusting the current conditions ot both the telephone line and the Calibrating network.

ldhile the arrangements of this invention have been shown and described with respect to the particular embodiment of Fig. 2, it is to be understood that the invention is capable ot embodiment in other and widely Varied organizations without departing from the spirit ot the invention and the scope of the appended claims.`

llVhat I claim as new and desireto secure by Letters Patent of the United States, is:

l. The method of determining the attenuation of a telephone line with an adjustable artificial network and suitable impedance elements, which consists in setting up a predetermined electrical condition across the input terminals of a telephone line, setting up the same electrical condition across the input terminals of the artificial network, terminating said telephone line and said artificial network by their respective characteristic impedances, adjusting the attenuation of the articial line to equal the attenuation of the telephone line, and observing at the output terminals of the telephone line and at the output terminals or' the artificial network when the attenuation of the artificial line equals the attenuation of the telephoneline.

2. The method oi determining the attenuation of a telephone line with apparatus including an artificial network, the input and output terminals ot the telephone line being respectively shunted by the characteristic impedance of the artificial vnetwork and the characteristic impedance of the telephone line, the input and out-put terminals of the artiiicial network being respectively shunted by the characteristic impedance yof the telephone line and the characteristic impedance of the artiiicial network, which consists in setting up a predetermined voltage across the input terminals of the telephone line, setting up the same voltage across the input terminals of the artificial network, adjusting the attenuation of the artificial line to equal the attenuation of the telephone line, and observing when the attenuation ot the artificial line equals the attenuation 0i the telephone line.

3. A system for measuring the attenuation of a telephone line comprising an artificial network, means for setting up the same voltages across the input terminals of the telephone line and the input terminals of the artiiicial network, the telephone line being terminated by its corresponding characteristic impedance, the vartiiicial network being terminated by its corresponding characteristic impedance, means for adjusting the attenuation of the artiiicial line to equal the attenuation of the telephone line, and means for observing when the attenuation of the `artificial line equals the attenuation of the telephone line. v

4. In combination, a telephone line, an articial network, two networks each having the characteristic impedance of the artiiicial line, one bridging the input terminals of the telephone line and the other bridging the output terminals of the artilicial vline, two networks each having the characteristic inl-- pedance et the telephone line, one. bridging the output terminals of the telephone line and the other bridging the input terminals ot the artificialnetwork, means for impressing the same currents on the input terminals oli-the telephone line and on the input terminals of the artificial network, means for adjusting the attenuation of the artiiicial network to equal the attenuation oi the telephone line, and means for indicating the equality of the attenuations of the telephone line and the artiiicial network. Y

5. ln combination, a telephone' line, an artiiicial network, two networks each having the characteristic impedance of the artiiicial line, one bridging the input terminals of the telephone line and the other bridging the output terminals of the artificial line, two networks each having the characteristic impedance of the telephone line, one bridging the output terminals ot the telephone line and the other-bridging the input terminals oi."- the artiiicial line, two alternating current generators each so arranged as to transmit equal currents through f the t-elephone line and through the artiiicial network, means for adjusting' the attenuation ot the'artiiicial network to equal theattenuation vof the telephone line, and a measuring device of high impedance for indicating when the attenuation of the artiiicial network equals the attenuation of the telephone line.

ln witness whereof, l have hereunto set y myhand this 15th day of March, 1926;

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