US1570770A

US1570770A - Means for and method of reducing interference

Info

Publication number: US1570770A
Application number: US545295A
Authority: US
Inventors: Nyquist Harry
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1922-03-20
Filing date: 1922-03-20
Publication date: 1926-01-26
Anticipated expiration: 1943-01-26

Description

Jan. 26 -1926. 1,570,770

H. NYQUI ST MEANS FOR AND METHOD OF REDUCING INTERFERENCE Filed March 2 1922 J z LI INVEN TOR.

WTTORNEY Patented den. 25, 1923.

' nanny nreuisr, or min-runner. NEW YORK, ASSIGNQR 'ro AMERICAN TELEI'HONE AND TELEGRAPH comranr. A CORPORATION OF NEW YORK.

m ssus .ron AND Marxian or REDUCING mcrnnrnnnucnf Application filed March ,20, 1922. Serial No. 545,295.

To all "iv/mm 2T1 rum concern Be it known that I; HARRY NYQUIST, residing at Elmhurst. in the county of Queens and State of New York, have invented cer- 5 tain Improvements in Means for and .Methods of Reducing Interference, of which the following is a specification.

This-invention relates to multiplex signaling, and more particularly to means for and method of reducing interference between the channels of a multiplex signaling system.

Heretofore, in the operation of multiplex carrier systems, it-has been found that the repeater tubes employed at points along the line for transmitting the carrier frequencies, where the same tube is used to transmit the frequency of a number of channels, tended toimpose a limit on the number of channels that might be employed over a single circuit. This is for the reason that the tubes do not I have a straight line characteristic and a zcertain amount of distortion occurs, which produces second harmonics of the carrier frequencies and also sum and difference frequencies. difference frequencies, in many instances, assume such amplitudes as to cause interference in the channels whose frequencies are near those of the interfering harmonics, etc;

Interference of this character"becomes especially serious in a carrier telegraph sys tem involving a large number of channels employing carrier frequencies within the 35 usual voice range, for in such a case it is almost, essential .to economic working of the system that the channels be narrow and closely spaced, and that all the channels he amplified by the same repeater tube. Even where the circuit is to be used for ordinary voice transmission, the distortion, while not producing results as serious as in the case just. n'ientioned. may be sufficientlyundesirable to require correction, especially where the characteristics of the repeaters depart. materially from the desired linear characteristie. In accordance with the present invention, it is proposed toovercou'i-c this diificulty by pollin'g successive repeaters oppositely, so that the currents under consideration will be substantially neutralized in two successive repeaters. This result may be readily accomplished by reversing the connections between the tube to one repeater and the tube These harmonics and sum and to the next succeeding repeater. In this case, the harmonics and sum and difference frequencies generated by the tube of the second repeater will be substantially 180 degrees out of phase with the harmonics generated in the first repeater" and will, therefore, tend to neutralize them. This will not be true for third harmonies or any distortion arising from the presence of a third power term in the repeater characteristic, but in general, components resulting from the higher power terms of the repeater characteristic are of such small magnitude that they may be neglected.

The invention may now be more fully understood by reference to the following description, when read in connection with the accon'q'ianying drawing, Figure 1 of which illustrates the arrangement of the terminal equipment of a multiplex carrier telegraph system operating within the voice range, and Fig. 2 of which illustrates the arrangement of two successive repeaters in a line-associated with terminal equipment, such as that of Fig. 1. i 4

Before discussing the theory underlyin the present invention, a brief description 0 the circuit. arrangements to which the invention appertains and of the simple change in circuit arrangement for carrying out the invention will begiven.

Referring to Fig. 1, L and Lt, designate two transmission lines operating on a four-'- wire basis, so that one line transmits currents in one direction and the other line transmits currents in the other direction At the. terminal station each of these lines includesa one-way terminal repeater, as indicated at R and R and these repeaters may be of any well-known type, such, for example, as vacuum tube repeaters. The details of the circuit arrangement of the repeaters may be as shown, forexampla in Fig. 2, which will be described later. The inputside of therepeater R, is associated with bus connections '10 from which branches lead through various channels, such asthe transmitting channel TL. In a similar manner, the output side of the repeater R, is connected'to bus-bars 11 'from which branch connections extend to the various channels such as thereceiving channel RL..

, said sections including series resonant elements and shunt anti-resonant elements.

In order to give the filter as great an impedance as possible to frequencies outside of the range which it is desired to transmit, the section of. the filter adjacent the multiple connection to the bus-bars is made a full series section, as indicated. Filters constructed in this'manner will have a fairly sharp cut-0d and the attenuation will be very eat for frequencies on either side of the limits of the band of free transmission,

Thejransmitting channel TL is associated through atransformer 12 with a source of carrier frequency S and the transformer 12 is connected with the'channel TL in such a v manner that the frequency supplied to the channel from the source S may be interruptc'd'by means of a contact 13 of a polar transmitting relay 14, whose windings are arranged one in" the telegraph line L and the other in the connection to the balancing net'- work N. The receiving channel EL includes a detector D, which may be of any well-known type, such for example as a vacuum tube detector, and the output G11- cuit of the detector includes a receiving relay RR whose contact is arranged to relay the received impulses to the midpoints of the windings ofthe polar relay 14 for transmission over the tele aph line L without operating the transmitting relay.

. By means of this arrangement, telegraph signals incoming from the line L will operate the'polar relay 14 to interrupt the con nection of the source S to the channel TL, so that a'carrier frequency interrupted in accordance with the telegraph signals will be t'ransntiitted through the filter TF to the bus-bars l0 and thence throughthe terminal repeater RVV to the line l'l Similar carrier frequencies transmitted in the opposite direction from andis'tant station will be received from thelin e la, and amplified by the terminal repeater RE bcfore being impressed upon the [bus-bars 11. The fret uru relays the Morse signalstothc receiving linerltlj Without interfering with the. terwhere i is the current in the pla.

. sesame line sections 20, 21 and- 22. Each repeater W as shown comprises two tubes'in tandem and a potentiometer associated with the input circuit of the first tube. A transformer associates the input of the first tube with the potentiometer, and another transformer is interposed between the potentiometer and the line section. The output of the second tube of each repeater is similarly connected with the next transmission line section by a transformer. While the circuit arrangezucnt of Fig. 2 has been described as being used in connection with the multiplex telegraph system of Fig. 1, it is obvious that the circuit'may be employed for ordinary voice transmission. In either casethe distortion produced by harmonics and sum and difference frequencies of the various fre quencies transmitted through the repeatersmay be overcome by properly paling the repeaters in the manner about to be described.-

second harmonics and sum and difference frequencies of successive repeaters are caused to oppose each other, may be accomplished in sei'eral different ways. The line wires of the line section intervening between two repeaters might be transposed at any desired point, or the connections of some transformer or other piece of apparatus to the circuit might be reversed. Between the repeater R, and repeater R three

transformers

23, 24 and 25 are interposed. and the any one of these transformers mig t be reversed in order to obtain the desired result. As illustrated in Fig. 2,

transformers

23 and 24 are poled in the-same way, but the connections from the second Winding of the transformer 25 of the grid of the first tube of the repeater R, are transposed, thereby reversing the poling of the transformer This will result in'producing second power effects in the repeater R, which are opposite in phase to the second power effects produced in the repeater R The reason why the simple reversal of connections above described will tend to.

cling ofelimin'ate interference duetof secondharwill be clearfrouithe followingtheoretical discussion of=the principles involved.

Suppose we are 'not consideringdistortion arising from terms higher than" the second power. Thenthe repeater istic may be represented by v and e is the alternating voltage'o'n the rid.

Suppose further that there is no press minalrelay, as. has ,alreadynbecn described. shiftrin,.cither the line or the repc'atcr,--that is, no shifting of the phase with respect to Referring now to Fig. 2, a line is disclosed v which may be considered as typical of lines R, are illustrated at successive repeater stasuch as L, and L Two repeaters R, and

tions and serve to interconnect-successive vocal transmission) at all parts of the we character- 3 I I .120

cuit have the same relative phase with which they started out. Now let us consider the. action of a Wave made up of two frequencies of equal amplitude. Such a wave may be expressed i 6 (cos gate-00551515) (2) where p, and go, represent Q'rr times the fre quencies of the individual carrier waves (or two component volce frequencies). Substituting from equatiqu in equation (1),.

we have In equation (3) the constant a represents a direct current term a hich is not transmitted through the transformer in the output of the repeater from the line section to the next repeater. The expression 6E (cos p ta-cos Ps 0E (cos p t+oos 1 20 r g a: ill (cos pi P2051 ment (which is only approximate) it should be remembered that in equation- (4) the expressmn I 533 (cos p H-cos p 6) is the term representing the an'iplified com- )onent corresmnding to the ori inal wave and the only term with which We "are concorned in amplification. We had a perfect amplifier, that is. an amplifier having a straight line Qhm'acteristic, the second Y power term 0E (cos p rf cos p 2?) algebraic expression (4). Conseqr'rcutly. the

voltage applied to the grid of the succeeding amplifier may be expressed mbE (cos p z+cos 1 'mcE (cos p t-rcos 12 i)" (5) where m a factor representing the relation between the voltage applied to the grid of the succeeding amplifier and the current flowing in the output circuit of the prccediug' amplifier. lt' we analyze the expression itwill be seen that the first term ,thereof is of the same form as the voltage impressed upon the grid of the pryccding' amplifier as given by e nation (2). the only difference being that tie term in equation (5) is multiplied by the factor 7117/. If this factor be made equal to unity the lil'sl term of equation (5), (which represents the voltage applied to the grid of the succeed ing tube) will be identical with the voltage applied to the grid of the preceding tube. The factor Z) is aterm depending upon the amplification of the preceding tube, but the factor m depends upon the transmission charactoristics of'the line and is also controlled by the adjustn'icnt of the potentiometer in the input circuit of the grid of the succeeding amplifier. By proper adjustment of the potentiometer the factor mb ma be made unity, and we then have as the alternating voltage on' the grid in. the second repeater the expression 2 (cos p t+cos gm) (6) andother higher'power terms would not be present at all. Evenfin the practical case now under consideration, where a slight amount of distortion docs actually occur, the second power tern'i is quite small as compared with the first power term. Consequently, it is quite justifiable to sir in connection with equation (6) that, since the first power term of the right-hand side of said equation is identical with the original wave of equation (2), the voltage impressed upon the grid of the succeeding tube. is practically equal to the voltage impressed upon the grid of the preceding tube. Therefore, the gain of the repeater is substantially equal. to the loss in the line. v

The characteristic of the second repeater may also berepresentcd by an equation similar to equation (1). Hence we have, by substituting the values given by equation (6) in the equation corresponding to equation (1) for the second repeater,

inc: BE (cos p t+ cos 9 5);: 0E (cos cos 10 .2 I c[E (cos p 6 cos p t) f p t+cos jp t) 1 (7) l serum/o comes E (cos pJ-l-cos 156 l l terms in 'E and E". e 3) Hence the space current takes the form i=c+bE (cos p t+cos pull-2 0E (cos p,t+ cos umterms in E and E (9) where there. is an even number of reversals. Where there is an odd number of reversals, the space current takes the form terms 111E and E (10) gb'lpifl'lflg equations (9) and (10), it will: bgsecn that the term 7 E (cos p t+eos put) cies will be suppressed. That the expression 1 (cos p t +005 p n 3 is responsible for the second harmonics and sum and difference frequencies will be up parent from a simple algebraic transformation, h which this expression reduces to the fol owing:

Analyzing the four terms on the right-hand side of the equation (11), it will be seen that the first and last of these four terms represent second harmonics of the frequency 12 and 32 while the second and third terms represent the frequencies corresponding to the sum and difference of these two frequencies, respectively. Q s

It will be obvious that the generahprinciples herein disclosed may be embodied 1n many other organizations widely d fl'erent from those illustrated, without departm from the spirit of the invention as define inthe following claims.

What is claimed is:

-1. In a'multiplex system in which 51 plu mlity of carrier frequencies are simultaneously transmitted throu h succcssivcgepcaters'in a transmission line, theme ml of eliminating secondlmrmouics and sum and diiference frequencies produced by the dis,- tortion of the repeaters, which consists in impressing the several carrier frequencies on successive repeaters in opposite phase relution. v

2. in a multiplex system in which u plurality of carrier i'requeucigs are transmitted through successive repeaters in a transmission line, the .nie'thod of eliminating second harmonics, which} consists in lIITPIOSSlHg the severstl carrier frequencies on successive repeaters in opposite ,phase relation.

3. In a, multiplex "system in whidlf'a plurality of carrier ire uencies aresimultm neously transmitted. t iroughsuccessive repeaters inn transmission line, the method of clinfinuting sum and difference frequencies, which consists in impressii'ig the several carrier frequencies on successive repeaters in opposite phase relation.

- rlln s. multiplex system in which a plurality of carrier frequencies are transmitted through successive re esters in s transmission line, the metho of eliminetin even power components due to the distortion of the repeaters which consists in reversing the. connections het ween successive repeaters 5. In a multiplex carrier system, a transmission line, means for transmitting e plurality of carrier frequencies over seidtrens mission line, a plurality of repeaters included in said transmission line, the connections of the successive re eaters being so arranged with respect to one other that the carrier frequencies will be impressed upon each succeeding repeater in'opposite. phase relation to, that in which they were impressed upon the preceding repeater of the series. a ,5;

' 6. In omultiplex carrier system, a transmission line, means for tronsmitting a pinrality of carrier frequencies over said trouss miss on line, a plurality of repeaters included in said transmission line, the connections of each successive repeater 'beiug re versed withrespcct to the repeater imme diately preceding, whereby the currier'frequencies will be impressed upon each] recessive repeaters in. opposite phase relatiom 8, In a. signaling system inwhich a range of frequencies is transmitted through successive re esters in a transmission line, the method 0 eliminating second harmonics and genome sum and difference frequencies producd by the distortion of the repeaters, which consists in impressing the "various transmitted frequencies within the range upon successive repeaters in opposite phase relation.

9. In a signaling system in which a range of frequencies is transmitted through successive repeaters in a transmission vline, the method of eliminating frequencies corresponi'ling to the sums and difierences of cornponent :trequencies Within the transmitted range, which consists in impressing the frequencies to be transmitted upon successive repeaters in opposite phase relation.

10. In :2. signaling system in which a range of frequencies is transn'iitted through successive repeaters in a transmission line, the method of eliminating even-power c0mponents, due to the distortion of the repeaters, which consists in reversing the connections between successive repeaters.

1.1.111 a transmission system, e transmission line, meens for transmitting a bend of frequencies over ssid transmission line, a

plurality of repeaters included in said transmission line, theconnections of the successive repeaters being so arrenged with respect to each other than the component frequencies of the band will be impressed upon each succeeding repeater in opposite phase relation to that in which they were impressed upon the preceding repeater of the series.

12. In a transmission system, a transmission line, means for transmitting a bend of frequencies over said transmission line, a, plurality of repeaters included in such transmission line, the connections of each successive repeater being reversed with respect to the repeater immediately preceding, where-.

by the component frequencies of thebend will be impressed upon each repeater in op posite phase relation to that in which they were impressed upon the preceding repeatelx.

In testimony whereof, I have signed my name to this specification this l8th'day of March, 1922.

Her nroursr.