US1739668A - Transmission system - Google Patents

Description

Dc. 17, 1929'. C, w, GREEN ,739,668

TRANSMI SSION SYSTEM Filed Aug; s1, 1926 wur QA EE mf warm/ Patented Dec. 17, 1929 UNITED .srAri-:s PATENT oFFlc-E CHARLES W. GREEN, or MILLRU'RN, 'NEW JERsEY, AssIeNoR 'rofBELL TELEPHONE LABORATORIES, INcoRroRATEn, E NEW YORK, N. Y., A CORPORATION 0E NEW t lrRANsMIss'IoN- SYSTEM Ajpnation mea August' s1, 1926. serial No. 132,670.-

This inventionrelates to transmission systems, and particularly to carrier current s igi naling systems.

An object of the invention is lto reduce re- -tlection effects such 'as are commonly dueto impedance irregularities in Ielect-ric signaling systems. A related object. of the invention is to repeat a plurality of waves of different. frequencies efficiently yand without substantial reflection.

The 4invention is particularly adapted for Ause in multiplex carrier current telephone Whenthe waves in traversing the line meet an impedance irregmllarity, portions'of the i materially'reduced by means of an attenua-` and telegraph systems, where several conversations or messages in the form of modulated carrier waves are transmitted in bothvdirections over a common transmission line."

waves are reiiected ck over the line.- Such reflection usually .occurs at translating dezvices which are connected to`th e line but glo r `not accurately match the impedanceof the line. The reflected waves set4 upcross-talk adjacent v paralleling lines, thus interfering with the normal transmission of signals n thereover.

According "togthis invention .reiiection -is tion netwo'r which is :so situated and proportioned 'with respect 'to the device cansing .reflection that the reliected waves are greatly' attenuated.

' transmission lines, "the waves of higher fre quencles are more greatly .attenuated 1n trav- It is well lmown that systems of this type, particularly those Aemploying 'long designed to have substantially 'the .same highA voltage transformation ratio at all frequencies within the transmitted band. However, in designing thev repeating coil to havefthe desired 'high voltage transformation ratio over a 'wide band of frequencies, i'thas been' 'found that the .characteristic impedance of" -th'e' repeating coil doesnot match the -im pedance of the incoming yline at 'all the transmitted frequencies. It 'is evident, therefore, thatsreflection will occurf-a-t that partof the frequency scale where the impedance of the repeating coil does not accurately matchl the impedance of the incoming line.. Reflection often 'occurs' at that part of the'l frequency scale where the attenuation of the' equalizer` 1s lowest, Aand hence reiiected waves are readily transmi-ttedthrough the equalizer-to the v transmission line. rf According to a feature of this invention the impedance of the' repeating coil is made of' the proper value to tejnlin'ateA the incoming line at that part of the frequency range at which the e ualizer'loss'isflowest,'namely,at l

the higher' requencies, so 'that no reflection vwill occur at the higher frequencies.y Then,

even t h ouh reiiectionoccursat. other fre# tenuated byl traversing the equalizer twice, thus reducing the reiection coetiicient to a minimum for a xdesired'maximum coil step- -up ratio.

The features and advantages of the invention will be described in connection with the accompanying drawing, in which:

Eig. 1 is' a .diagrammatic illustration of a twway amplifying repeater embodying the invention' 5 Fig. 2 shows ciirves illustrating the reduction in the vrei/ection coeiiicient 'obtainable by means of theinvention; and Figs. .3a and'Bb are circuit diagrams used in coninnctio'n with the. curves ofV F ig. 2 in Idescribing` the invention.

The twoway repeater of Fig. 1 comprises two `one-way repeating channels RW and RE adapted'to connect transmission line sections' and E. This repeater is of the general type @disclosed `in U. S. Patent No. 1,413,357

to P.'A. Baibourn, issued April 18 1922.

The currents incomin at 'the repeater from -line section are amp ified in the channel RWand the amplified currents are delivered lto the line .section E. Similarly, the currents incoming from line' section E are amplified in the channelREfandthe'am lified currents are delivered to line section The vcurrents incoming from the line section W are of higher frequencies than those incoming from line section Accordingly, the respective repeaterchannels are provided with frequency discriminating filters which direct the incoming currents to the proper channels. 'Therepeater channel RW fincludes a high passv inputl filter HPIF, an

amplifying structure WA -and ahigh passv voutput filter HPOF. The repeater channel RE includes a low pass input filter LPIF, an

amplifyin structure EA and a low pass output filter POF.

The directional filters HPIFand HPOF are designed to transmit currentsy of the ptosuppressvcurrents of higher frequencies.' These-filters may be of the ty e disclosed in A.`.Cam pbell' U. S. Patent No. 1,227,113 to issued May 22, 1917.

Since the attenuation ofthe transmission line sections W and VE is a f1mction of frequen'cy, the carrier currents incoming at the repeater will vary in magnitude corresponding with the attenuation of the line section which they have traversed. When these currents Yare repeated `into a succeeding linesecf tion the variation in magnitude at the receivfing end of the second line section-is proportionally much greater Vthan for the. first line section, and hence equalizers are-usually employed at the repeater to equalize or compen-` sate for the unequal attenuation of the-incominocarrier currents.

the channel RW, an attenuation equalizer WE is connected to thel hi h 'pass input filter HPIF to :equalize the ifferences of line attenuation at the upper group of carrier i' frequencies transmitted Vfrom west to east.

The-.equalizer WE is coupled to the'inputl circuit of the amplifier WA-by means of a repeating coil T. Similarly, an attenuation 4'equalizer EE is-.connected tof the low pass input filter LPIF in the channel RE, thisi;

equalizer being connected to the input circuit of th am lifier EA by means of a re coil 1. The equalizers WE and E have transmission loss characteristics .which are substantially complemental'to the transmission loss characteristics of -linesections W`K and E, respectively, so that'all the .waves in the output ofeach repeater channel will be at? substantially `the sametransmission level.

The ualizers WE and EE may be of the '.-typejdisclosed in-U. S. Patent No..1,591,073,

issuedV July 6,1926, to D. J. Zobel, having a nst'ant resistance characteristic impedance,

ting

edance to terminate the filters PIF, respectively.- y

In order lto obtain thedesired high` gain at all the 'frequencies 'assigned to the channel RNV, for example, th .repeating coil T isdesigned to have the same high voltage transformation ratio at such frequencies;v -vSimilar 'conditions apply to the design of the repeating coil T1 inthe. channel RE. In one. s ecie instance, a repeating' coil employed or this purpose was designed to have a voltage step-up .ratio of 1:111. When the "repeating coils are designed to have a high 4voltage transformation ratio overa wide band of frequencies, it hasl been found impossible atthe Sametime to give the coils a characteristic impedancef'suitahle for terminating the respectivel input filters, that is, a characteris tic impedance which is constant with frequency. Accordingly, waves of certain frequencles willmeet an impedance irre 'ul'arity lin the` repeating coils 'I and T1 an a portion of these waves willr'be refiected.

According" tothe invention the repeating proper im edan for terminatin the input filter .to w ich it is`cnnected at at part of the frequency band at which the ualizeris' ractically transparent,v that is', att e higher requencies. If, then, the coil impedance-differs from what it should be at the other frequencies, any reflection introduced by the repeating coil due tol impedance mismatching is reduced bythe relatively high loss `in the equalizer. :This is due to the factthat the Y reiiected wavesfmust traverse. the equalizer' twice, whereas the s'gnal traverses' the equ'al-v izer but once, so that the reflected waves are highl attenuated. High resistance elements r an r1 are connected in shunt .to the -primary windings of repeating coils T and T1,

respectively, to-Inakel the impedance of. the

coils correct to a firstapproin'mation.

' Figs. 2, 3 and 3P illustrate how, by'- properly dsigning'the impedance of a repeater input coi 'the overall reflection coetiicient considered as a function of frequency reduced to a minimum for' a desired When a wave in traversing a #.uit such as that shown-in Fig. 3* meets'a'f-*clan'ge in maximum im edance in the repeating coil T such that. 'i

loo ing one,v way the Aimpedance ris `Zland looking the other way the impedance is Z2,

reflection -of the wave occurs. The coeiiiecient is represented by Y Zl-Zz R f Z1 The'coeicient of reflection under these conof refiection R `ditions, when no loss element is employed in the circuit, is high, as shown by th curve A of Fig. 2.

Whe1I,- as shovm .in Eig-3", a loss element :,vaaecs tion reducing standpointA at any frequency. varies as ltwice the loss` through the inserted element at that frequency. Thus, i'

R i with equalizer R l with coil alone varies as'twice the lossV through the, equalizer. The coefficient' of reflection when the equalizer `E is employed between the line and the V*repeating coil 'I is materially reduced, as shown by the curve B of Fig. 2. In an actual installation a 3% fold improvement in reflection coefficient has been obtained by means of this invention with the same repeatergain as has been obtained heretofore. Looking at the advantage of the invention from another viewpoint, vif the same reflection coeicient heretofore obtained is not objectionable in certain installations, the gain can be increased materially' by means of the invention, without changing the reflection coefficient,

While the invention has been described as` 'tions and in other types of systems.

What is claimed is: 1. In a system for'transmitting waves of different frequencies, a transmission line,

means associated with said line causing waves of certain selected frequencies to be reflected, and a loss element-between said line and said .1 means to attenuate'all of said reflected waves,

said loss' element having its .highest transmission loss -characteristicsat the frequencies of said reflected waves and a lower loss at the other frequencies transmitted.'

2. In a system for 'transmittingwaves of different frequencies, -a transmissionv line, means comprising' a. transformer associated with said line causing. waves of vcertain selected frequencies to be reflected, said transformer having a high voltage transformation A, ratio substantiallyconstant.over the range of frequencies transmitted and a loss element in means-'associated with said line causing waves v cf certain selected frequencies to be reflected,

anda losselement associated with said line and-arranged to produce a greater loss for all the reflected waves than for the Waves .which are transmitted without reection.

4. In a system for transmitting waves of 4different frequencies, a line over which said waves ofdiiferent frequencies aresreceive'd,

means for 'reducing reflection effects due to..

limpedance .irregularities including a circuit comprising an electrical network upon which said waves areimpressed, said circuit having a transmission floss characteristic which is high at'certain frequencies and lo'w at other frequencies, and a translating device comprising a transformer having a high voltage transformation ratio substantially constant over the range of 'frequencies transmitted connected to receive the waves from said circuit and havingv the same impedance as said. circuit at frequencies at -whichsaid circuit has a low transmission `loss and a different impedancev at the other frequencies transmitted;

5. In a system -for transmitting waves of different frequencies, a line. over which said waves of different vfrequencies are received, means for reducing reflection effects due to impedance irregularities including a circuit comprising an electrical network upon which said waves are impressed, said circuit having a transmission loss characteristic. which .is high at' certain .frequencies and low at other frequencies, anda translating device connected to receive the waves from said circuit and having ari impedance which varies throughout thera'ige of transmitted frequencies but accurately matches the impedance of 'said circuit at the, frequencies at which said circuit has a low transmission loss.

6. In a system fortransmitting waves o different frequencies,- a' line over which said waves ofv dierent frequencies are received, means for reducing reflection effects due to impedance irregularities including a circuit comprising` an' electrical network -upon which said Waves are impressed, said circuit having a'transinission loss characteristic which is high atv certain frequencies and low at other frequencies, and means connected to receive the waves from said circuit comprising a re- .p'eating coil having a high transmission efiiciency at all of said frequencies and having approximately the same impedance as said circuit at-vfrequencies at which said circuit has a low 'transmission loss and a different ido llU

impedance "at the other frequencies transmitted. L

7. In a system for transmitting waves of different frequencies, a line over which said waves of dierent frequencies are received, means for reducing reflection effects due to impedance irregularities including a circuit comprising an electrical network upon which` impdanceof'said circuit' atthe frequencies at -y/,h'ichfsai-id loss..' 8. In a wave transmission system employing waves of different vfrequencies, aline the transmission loss of which difers'for dif'- erent frequencies,- an attenuation equalizer associated .therewith andv having a trans"- e mission loss characteristic' substantially com-y cies.

plementaltothat 4of the line throughoutthe rangeqof frequencfusI transmitted, and a translatingdevice conn ted; to said equalizer and having an impedance which matches the imi pedance. of said lineJ and equalizer only in the upper region of the transmitted frequentransmission system employ` ing' Waves of different frequencies, a line the transmission loss of which differs for different frequencies, an attenuatlon equallzer associated therewith andl having a transmission loss characteristic substantially complemental to that of the line throughout the range 'of frequencies transmitted, an amplifier, and a transformer connecting said attenuation equalizer and amplifier. and having an impedance which .varies throughout 4the range of frequencies transmitted but' matches the i impedance of said-,equalizer at frequencies r transmission loss.

at which said attenuation equalizer has a low 10. In ,a Wave transmission system, a line l over, which signaling waves are transmitted,

duced by said element a translating device from which waves of certain selected frequencies are reflected, and

'a loss element arranged to be traversed by the reflected Waves a greater' number of .times Wavesv the loss introcing substantially proportional to v'the amount of reflection` at the different frequencies.

11. Ina systemfor transmitting waves of than by said signaling different frequencies, a transmission line, a`

repeater. associated with said line and comprising a frequency discriminating filter, an ampli erand a repeating coil connecting said ampliierto said-filter, and,a loss element Abetween Asaid filter and said repeating coil having a transmission loss characteristic which is high at certain frequencies and low at other frequencies, said repeating coil haw ing an impedance which varies throughout the range of frequencies transmittedby said ilterrbut matches the impedance of said filter at frequencies at which said loss element has aV low transmission loss.

12.v The method of'reducing cross-talk' in signaling' circuits which comprises reducing the reflection of waves inthe upper region of the transmitted frequencies to a greater ex `tent than' in the'loy'ver region, attenu'ating ,circuiti has' ai low transmission frequencies to a ygreater extent'than the upper region.

13. In al system for transmitting waves of Y.

pedance matched to that of the incoming'cir- Y cuit comprising the line4 and equalizerat the frequenciesl at which *said equalizer has the lowestv attenua-tion andmismatched at other frequencies, said repeating coil having its voltage ratio substantially constant lover the lrange ofafrequex'icies transmitted and as high as is consistent' with` th@ permissible extent of'mismatching of the impedance ofsaid'reof the incoming circuit, as4 determined by a substantially constantand negligibly small amount of reiiectionrappearing-on the line side Aof said equalizer throughout the transmitted range. y

14. rIAhe method of reducing reiection eftem for transmitting waves of different fr quencies' which comprises 'attenuating the; waves in an'inverse relation with respect to frequency, reflecting waves in the lower` region ofthe transmittedXfrequenciesto a greater degree than theother waves transmitted, and attenuating certainv reflected Vfects due to impedance irregularities in a sys'n waves after reflection in an inverse relation s with respect to frequency.A l 1 l15. The meth d .of 'securinga high voltage .transformation ratio over a Wide band of frequencies Without substantial. reflection effects in a repeater for a system transmitting-waves of different frequencies which comprises attenuating the waves aninverse` ratio 'to the line attenuation, making thel reflection of waves a minimum. in the region of the transmitted frequencies at Whichfthe line at? ros tenuation is high and attenuating the waves of other frequencies after reflectionin an inverse ratio to said line attenuation.l Y, n In witness whereof, 'I hereunto subscribe reflected waves in said circuits a greater num ber oftimes than lthe effective signaling currents therein and attenuating 'the reflectedl waves in the lower. region of the transmitted` 85 peating coil'with respect to the impedancel lit my name this 30th day of August A. D., 1926'.

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