US1840360A

US1840360A - Selective circuit arrangement

Info

Publication number: US1840360A
Application number: US337614A
Authority: US
Inventors: Herman Joseph
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1929-02-05
Filing date: 1929-02-05
Publication date: 1932-01-12
Anticipated expiration: 1949-01-12

Description

Jan. 12, 1932. J. HERMAN SELECTIVE CIRCUIT ARRANGEMENT Filed Feb. 5, 1929 3 Sheets-Sheet .5 Sectz'om .8 Sectiow Mu wswk mw INVENTOR v JflI lman/ ATTORN EY Jan. 12, 1932. HER AN 1,840,360

SELECTIVE CIRCUIT ARRANGEMENT Filed Feb. 5, 1929 5 Sheets-Sheet 2 fimyaedamce 17' .4, wmmweczm,

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5 x E 8 Q INVENTOR JJFermmI/u Jan-12, 1932. J. HERMAN 1,840,360

SELECTIVE CIRCUIT ARRANGEMENT INVENTOR BY Jfirmalv ATTORN EY -cuit in which it is connected; In accordance 'transmission from the impedance without Patented Jan. 12, 1932 f flj funrrso,sTArEs PATENT OFFICE I Josnrn HERMAN, OF wEsrrIELn, NEW'J'EESEY, ASSIGNOR TO AMERICAN TELEPHONE .AND TELEGRAPH COMPANY, A CQRPOR-ATION on NEW YORK SELECTIVE CIRCUIT ARRANGEMENT ib ing series and shunt impedance elements,

operate to discriminate between the band or hands of frequencies passed and the band or bands'of frequencies suppressed byreason of the attenuation properties of the filter.

f-These attenuation properties are such that within the band orbands of free transmission, the filter has little or no attenuation, but tends to suppress*frequencies lying outside the band or bands of frcetransmission. The degree of selection depends upon'the attenuation to whichthc frequencies outside of the free transmission range are subjected- The 'attenuation in the suppressionrange in turn depends'upon the number of sections of the Jfi-lter, the, attenuation being greater as the number of sections increases. 7

1 In using a filter in the manner above de scribed its selective functions do not depend upon the impedance looking intotlie filter,

' 5513 and the impedance is only of importance from the standpoint of connecting the filter in a circuit in such a way that reflection losses will not occur due to improper matching of the impedances of the filter of tliecirwithjthe present invention, however, aband selecting action is obtained by taking advantage of the fact that the impedance of the filter is different within its range of free the range. This isaccomplished by intercon- 'Jiecting two sectionsof the circuit in which the selective action is to take place through a hybrid coil or other balanced circuit arrangeat with the filter connected on one side of mentand a suitable network or element connected on the other side to balance the1mpedance of the filter in the range over which Application filed February 5, 192

i ,'brid-coil otherbalancing arrange- Serial No. 337,-3i4.

circuit. With such an arrangement, if the impedance of the filter in a given range is balanced by a network whose impedance is the same, or substantially thesame, over that range, while the impedances of the filter and balancing circuit are different for frequencies without the range, the two sections of the transmission circuit will be substantially conjugate over the range of frequencies for which a balance is obtained but, will not be conjugate for frequencies outside the range; Consequently, a selective action will take place, which is due to the impedance characteristics of the filter and its balancing network.

lVith an arrangement such as above described, itwill generally be true that where a band pass type of filter is employed, a suppression of the frequencies corresponding to this band will occur in the transmission circuit through the hybrid coil or balancing ar rangement, while conversely, if a band suppression type of filter is employed, the band pass range of the transmission will substantially coincide with the band suppression range of the filter. In general, the sharpness of the selection and the relative amount of discrimination as between frequencies passed and frequencies suppressed will not require a large number of filter sections, since the discrimination depends upon the impedance balance between the filter and its balancing circuit at various frequencies.

The invention may be obviously embodied in a wide variety of forms. For example, in some cases, a filter on one side of the hy brid coil may be balanced by a network consisting of pure resistance on the other side and a SllfilClBIlt degree of balance and sup prcs"'cn obtained. In other instances, a filter of one type will be connected on one i de of the hybrid coil and it may be balanced bv means of a filter of another type on the other side of the hybrid coil, the two filters ng of such character that their impedances l be substantially the same within acertain range where suppression is to occur, but will be quite different in the range where free transmission is to take place.

has the advantage that if the filter circuit be opened or short-circuited, the selective effect of the filter is eliminated from the transmission circuit without opening the transmission circuit itself. Furthermore, the filter may be effectively removed from the transmission circuit in this manner by the use of a single'contact. The circuit arrangement also en ables a filter network, which normally functions as a band pass filter, to function as a band suppression filter, and vice versa.

The invention will now be more'fully un derstood from the following description, when read in connection with the accompanying drawings, in which Figure 1 is a circuit diagram of one embodiment of the inventionfFigs'. 2 and 3 are curves illustratingthe operation of the circuit of Fig. 1; Figs. 4' and'fi 'are diagrams of filter circuits which may be employed in connection with the circuit arrangement of Fig. 1; Figs. 4a 'ancloa are impedance curves for the filters cf Figs. 4c and 5, respectively; Fig. (iis a curve showing the attenuation characteristics of the selective circuit when filters of the type of Figs. 4 and 5 are employed; Figs. 7

and 8 are circuit diagrams of still further types offilter circuits which' may be emplo 'gied in connection with the circuit of Fig. 1; Figsr'iaa'nd 8a are impedance curves for the filters of Figs. 7 and 8., respectively, and '9 'is a curve showing the attenuation characteristics of the selective circuit when employing filters of the types of Figs. 7 ELIIC 8. l

nected to thermidpoints of the hybrid coil so that when a balance is obtained between terminals 1112 and 13-44 of the hybrid "001i, at any frequency, the circuit TL will be conjugate with respect to the circuit TL,

thereby substantially preventing transmission between the two sections of the circuit 7 at such frequencies as the condition'of balance obtains.

' Terminals and 1 1 are associated througha transformer 15 with a filter BF.

The'filter BF is-ai. known type of filter com-- prising two dissimilar sections, the series element of each'section comprising an induct- V V .nt inductance, terinmates-in inid series, that is, the series resonant element comprising the inductance and capacity has' one -h-alf the value of the full Y series element of a normal section. The lef hand-section, which includes the shuntcac5 pacity element, terminates in an eight-tenth series section, and the distant end of the filter is closed through a resistance R. V

The

opposite terminals

11 and 12 of the hybrid coil are connected through a transformer 16 with a balancing network which, in the case of the filter disclosed, may be a simple resistance R equal in value to the terminatingresistance R of the filter. By means of a switch 21, a short-circuit may be established across the

input terminals

19 and 20 ofthe filter BF to effectively remove the filter from the circuit and unbalance the hybrid coil 12 so that transmission may take place from TL to TL freely. The same result may be obtained by opening the circuit of thefilter BF through switch 22. In either case, the effective elimination of'the band filter BF from the circuit takes place without opening or closing'any contacts in thetransmission circuit from TL to TL, and the operation of a single contact only is necessary to remove the'filter.

As is well known, afilter of the type BF may be designed so that a band of frequencies between its cut-off limits f and 7'; may be transmitted from end to end through the filter without substantial attenuation, while frequencies outside-of the cut-off limitswill be attenuated in passing through the filter. The selective action resulting from transmission end to end through thefilter depends upon its attenuation properties. "Its use in the circuit ofFig. 1, however, depends upon the impedance characteristics of the filter as v r seen from the terminals 19*20.

Referring to Fig. 1, TL and TL desig- 'nate sections of a transmission circuit which are interconnected for transmission through. a hybrid coil 10, the circuit TL being con- 'Theimpedance characteristics'of the filter are illustrated by the curves of Fig. 2, in which 1" is a curve showing the variation of the resistance component of the impedance with frequency, and the curve a" shows the variation of the'reactance component of the impedance with frequency. As will be noted, the resistance r is substantially negligible in the range outside the pass band of the filter which extends from frequency f to frequency f l/Vithin the range of the pass hand, however, the resistance component is equal to the terminal resistance R near the middle of the band and gradually decreases towards zero as the limiting-frequencies f and f are approached; due tothe dissipation effect of the filter." The. reactance, as shown by the curve as, is zero near the middleof the band and gradually becomes more and more negative as the frequency approaches zero, and more and more positive in value as the frequency approaches infinity. The curve B of Fig. 2-represents the constant'resistance component of the impedance of the balancing element R of Fig. 1. Where the balancing element is a. pure resistance, as illustrated in Fig. 1, it will, of course, have no reactance component. 7 V I 1 It will be obvious from'a consideration of thecurves of Fig; 2 that near the middle ofthe band where the react'anc'e components 'iszero and the resistance component 2* is V nent of the filter BF, with the result that the attenuation, as shown by the curve of Fig. 3, dro s, very rapidly, so that itapproches a minimum atfthe limiting frequencies f and At zero frequency and at infinite frequency, a condition of'complete unbalance occurs because at these frequencies the resistance component 1' of the filter is zeroes compared'with' the finite resistance R of the balancing circuit. the same time, the reactance 'of the filter element is infinite at zero frequency and at infinite frequency. The

result;is that for transmission from TL to, TL there 1s practically complete suppres sion of the band of frequencies near the middle-of the pass band of the filter which lies between the frequencies 7' and The sup.- pression band resulting from the use of the filter in this manneifliain this instance, narrowervthanthe pass band of free transmissionhof tlie filter when used normally, due

- to that-the resistancecurve 9 is section.

rounded off nearithe limitingfrequencies of ,thegband, and a substantial.amountcf reactance is alsopresent near the edges of the hanrh. resulting in a considerable degree of unbalance before the. limiting frequencies f and flarereached. e

A- somewhat better and more sharply :defined suppression band characteristic may be obtained by substituting for the; filter BF in Fig, 1 a band filter section of the type BF of Fig. 4 and by substituting for the balanc ing resistance R a band filter section of the type illustrated at BF, of Fig. 5. in this case,

theterminals

19 and 20 of the filter BF will be connected to terminals 19 and 2001i Fig. '1, while the

terminals

17 and 18 of the filter BF wil be connected to the terminals 1?. h11d 18 of Fig.v 1.1 It will, of course, obvious thatthe two filters BF and BF, .may beinterchanged in Fig. 1 without chang- .,ing the result; The filter BF comprises a single-section 't/h 3 I i/ type illustrated on page lof an article entitled-Theory and design of uniform and -,composite electric wave-filters out J. Zobel, published in the Bell System Technical Journal for Januaryl923,nbeginning at 10f Volume II,

o. 1. .This filtorfis-oftheband pass type, .and, as herein used, terminates in m d-shunt "In rder to understand what. is meant by "the 'e'zrpi'fession mid shunt termination, it

should be noted that the inductance and capacities of the series and shunt elements of filter BF are given the same designations as in the aforementioned article by Zobel. If we compare the filter section of Fig. 4 with the filter section shown at IV on page 41 of the-Zobel article, it will be seen that the series element in F l has the same values as the series element in the Zobel filter. Now each shunt resonant element in the Zobel filter, such as, for example, L C may be considered as being made up of two parallel elements having twice the impedance of the single shunt resonant element, or, in other words, of two shunt resonant elements each having inductance 2L and capacity If, now, one-half of each of the shunt resonant circuits of the Zobel filter section as thus constituted is eliminated,- the filter section is terminated in amid shunt section.

If, further, the eliminated half shunt impedances are connected on the opposite side of the series element, we have the arrangement shown in Fig. 4 of this application, with the filter section terminated at both ends in mid shunt. In other words, the filter BF of Figs lSktCOIHPlBtQ filter section equivalent in all respects to the filter IV of the Zobel article, differing only in the fact that the shunt impedance element of the filter is divided into two portions and arranged one ateither side of the series element. A pure resistance R ill be connected across the distant end of the filter section BF as shown. I The variation of the resistance and reactance components of the impedance of the filter section il -F as seen from the terminals 19-20, are shown in Fig. la. In this figure, the curve 1 is the variation of the resistance component of the filter with frequency, while tl e curve a: is the variation of the reactance component with frequency. The straightline curve R represents the constant value of the terminating resistance R of the filter for all frequencies.

The filter section at Fig. 5 is of the type 1V shown on page 4-1 of the Zobel article above mentioned. The filter section of Fig. 5 is terminated in mid series. In other words, if the series elemei'at of the filter section on page 41 of the Zobel article be divided into two halves, arranged in series with each other and one on either side of the shunt element, we will have a complete filter section terminated at each end in mid series, as illustrated in Fig. 5. If now, the filter section of Fig. 5 be terminated by a resistance R at its far end, the impedance looking into the terminals 1718 will be as illustrated in Fig. 5a. In this figure, the curve 1" represents the variation of the resistance component of the impedance with frequency, and the dotted line curve 00 the variation of the reactanee component with frequency, the straight-line curve B, as before, representing the'constant value of the terminating resistance R of Fig. 5. Let us nowcompare the impedance of the lter section of Fig. 4, as shown in'Fig. 4a, with the impedance of the filter section of Fig. 5' as shown in Fig. 5a. The frequencies f and f may be taken as the limiting frequencies of the bands of free transmission, while the frequencies f and f are the frequencies on either side of the band of free transmission at which the attenuation is infinite. If we examine the resistance component of the impedance ofthe filter of Fig. 4, it will be seen that it closely approximates the-value of the terminating impedance R within the band represented by the limiting frequencies f and l gand is approximately zero-outside thezband of'free transmission. The resistance is actually zero at frequencies O,f ,f ,,and atinfinite frequency. 'Near the middle of the bandat frequency the resistance component is just equal to the and does not depart greatly from zero tween frequencies f and f It 1s also zero at frequency f and fz terminating resistance R, rising suddenly at the limiting frequencies f and to a value somewhat greater than B. At frequencies f1 and 7% within the band'and near the limiting hand, is zero at frequencies f f and f 1]2 V Y but approaches aninfinite value as the frequency decreases below f while approaching a positive infinite value as the frequency increases above I 'If we now examme the components of the impedance of the filter section of Fig. 5 as shown in Fig; 5a, we find that at frequenand 7%, the resistance component is just equal tothegvalue of the terminating resistance .R of the filter, and that it does not depart greatly from the value of R in. this range. 'The resistance component drops off'to zero at frequencies f and j and it is also zero at zero frequency and at infinite frequency. The reactanee, on the other hand, is zero at zero frequency graduallyrisesto an infinite positive value at f and gradually decreases from an infinite negative value to zero as frequency f is approached. At frequencies f Thus it will be seen that at frequencies f and f the resistance components of both filter sections are equal to the value of the terminating resistance R and the reactancecomponents in both cases are zero, so that at these frequenciesa perfect balance is obtainedv and infinite attenuation results in transmission from'TL to TL. At zero'frelquency, the resistance components are both equal and have zero value, but ,a complete condition of unbalance exists by reason of the factthat the reactanee component is, in the case of the filter of Fig. 5', zero,'while it is infinitely negative in the case of the filter of Fig. 4:. At zero frequency, therefore, a condition of no attenuation would exist for transmission from TL to TL. The same holds true for "frequencies f and j for at these frequeneies,while the resistance components are equal to each other since they are both zero, the reactanee component of the filter section of Fig. 4 is zero, while that of the filter section of Fig. 5 is infinite, so

that a complete unbalance exists. So also, at

infinite frequency, a condition of complete unbalance exists notwithstanding the fact that both resistance components have zero value. the imbalance being due to the fact that the reactanee component in the one case is zero, and in the other case is infinite.

The completecurve showing the'variation V of attenuation with frequency for transmission from TL to TL, using the filter sections I of Fig. 4 and Fig. 5, is illustrated in Fig. '6.

'It will be seen from this curve that the band pass filters of Figs. 4 andf5, when'connected in a balanced circuit, give .a band suppression characteristic having very sharp cut-off as the cut-oiffrequencies f and f are approached, the attenuation being very small in the region on either side of the suppressed band, and being infinite at three points with in thesuppressed band while being very large atall other points Within the band between the frequencies f, and f2.

. the pure resistance such as R of Fig. 1 having a value equal to the terminating resistance R of the network. For example, consider the network of Fig. 5 and its impedance characteristics as shown in Fig. 5a. The balancing resistance B would then have an impedance such as indicated by the straight-line curve R of Fig. 5a. .with. the impedance curves 2" and an, it is Comparing this impedance evident that there would be an exact balance at frequencies f /f f and f It is also evident that in the range from zero to f and from 7% to infinity, the reactance would be unbalanced. Likewise, the resistance components would be unbalanced in the ranges outside of the cut-off frequencies of the filter. The result is that for transmission from TL to TL the attenuation frequency curve would be of substantially the same shape as that indicated by Fig. 6.

A band passv characteristic for the circuit TL TL may be obtained by connecting "band suppression filters'of the types shown in Fig. '4' and Fig. 8, one on either side of the hybrid coil. The filter section of Fig. 7 is of j the type 1H shown on page 40 of the Zobel article above referred to, the filter section in this case being terminated on each side in the mid shunt section witha resistance R connected across its distant termination.

f Likewise the filter section of Fig. 8 is of the type TEL on page 40 of the aforesaid Zobel T article, this filter section being terminated at J1... i iTfi and f...,, all of which lie.

each side in mid series with a resistance R' connected across its distant termination.

The. impedance components of the two filter sections are shown in Figs. 7a and 8a,

respectively. Comparing the curves of these two figures. it will be seen that at frequencies 7, withinthe suppression band common to the two filters, the resistance components are both equal to each other and equal to zero,

but the-reactance components at each 'ofthese p frequencies is zero in the one case and infinity in the other, so that a condit on of complete circuit TL to circuit TLis zero.

unbalance exists, and the attenuation from At frequencies f and f lying just outside the limiting frequencies f and f of the suppression band of the two filters, the resistance components in both cases are equal to the value: of the terminating resistance R, and

the 'reactance components in both cases are zero, so that perfectbalance results and the attenuation from circuit TL to circuit TL isinfinite. The same condition occurs at zero frequency and at infinite frequency.

The complete attenuation frequency char-- acteristic is indicated by the curve of Fig. 9. From this curve, it will be seen that within the suppression band of the two filters (which lies between frequencies f and f a condition of substantially zero attenuation exists for transmission from circuit TL to circuit TL, while in the frequency ranges on either side of the limiting frequencies f and a suppressionrange occurs, each suppression 'ange being bounded by points wherethe suppression is infinite with a range of value equal to the terminating impedance R of the filter. For example, the filter of Fig. 8, when balanced by a pure resistance, would give accurate balance at frequencies 0 and f and a fair degree of balance at intervening frequencies. Accurate balance would also occur at frequency f and at infinite frequency witha fair degree of balance in tlie'intervening range. In the suppression range of a filter, the-reactance component, as indicated in Fig. 8a, would not be balanced, while the resistance component of the, filter would either be very small or actually zero ascompared with the finite resistance R of the balancing element. The result is that the attenuation frequency curve for-transmission from TL .plete unbalance may be brought about at all frequencies by short-circuiting the input terminals of the one filter and ope'n-circuiting the other filter, so that free transmission "takes place at all frequencies from TL to TL.

Likewise, a condition of perfect balance at all frequencies may be brought about by shortcircuiting the input terminals of both filters or by open-circuiting both filters, in which case transmission from TL to TL will be prevented at all frequencies.

It will also be obvious that various other types of filters and corresponding balancing networks may be employed in connection with the hybrid coil arrangement to produce other types of. band suppression and band'transmission effects, and that the genthe other two, a transmission circuit having two sections associated Wltll said last two points, a network associatedwith one of the balancing pointsof saidbalancing element,

said network having substantially uniform impedance in one range of frequencies and a materially diife'rent impedance outside said range,and an impedance device associated with the other balancing point of said balancing element in balancing relation to sa d network, said impedance device having substantially the same impedance as said neti work in said first mentioned range of frejquencies and a different impedance outside said range, whereby said balancing element is'balanced to prevent transmission of said firstmentioned range of frequencies over said ftrai'ismission circuit, but is unbalanced to perunit transmission of frequencies outside said range.

2. In a selective syste na balancing element having four connecting po nts so arranged that when two of said points are balance'd no transmission takes place between the other two, a transmission circuit having jf two sections associated with aid last two points, a network associated with one of the ba lancing points of said balancing element, said network having substantially uniform impedance in one range of frequencies and a materially different impedance outside said range, and an in'ipedaii'ce device associated with the other balancing point of saidbalancing element in balancing relation to said network, said impedance device havin a substantially uniform impedance at all requencies which is equal to the impedance of said netwoi in said first mentioned range of frequencies, whereby said balancing element is balanced to prevent transmission of "f said first range of frequencies over said. transmission circuit, but is unbalanced to permit transmission of frequencies outside; 'sa'idrange,

, UL I 3. In a selective system a balancing ele-' im ent having four connecting points so arf ranged that when two of said points are balanced no transmission takes place between I the other two, a transmission circuit having I two sections associatedwith' said last two po nts, a network associated with one of the I "balancing points of said balancing element, i said network havin two frequency ranges in one of which its impedance is materially sion in the other range.

e se-em from that of "the other, an impedance device associated with the other balancng point of said balancing elei'ne'nt'in' balancing relation to saidnetwork, the impedance of saidfldevice being substantially the' same as that of said network over'one frequency range but materially different in the other range, whereby said balancing element is balanced to prevent transmission of one range'offrequencies over said transmission circuit and is unbalanced to permit transmisalpIn a selective system a balancing element having four connecting points so arranged that whentwo of said points are balanced no transmission takes place between the other'two, a transmission circuit having two sections associated with said last two points, a band filter associated with one of the balancing points'of said balancing element and having an attenuating range and a range of free transmission, said filter having substantially uniform impedance in one range and a materialiydiiierent impedance in the other range, and an impedance device associted with the otherbalancing point of said balancingelement in balancing relation to said filter, said impedance device having substantially the same impedance as said filter in a range corresponding to one of the ranges of said filter andia different impedance outside said range, whereby said balancing element is'balanced to prevent transmission of one of said frequency ran es over said transmission circuit, but is unbalanced to permit transmission of frequencies outside said range.

In a selective system a balancing ele ment having four connecting points so arranged that when two of said points are bal aiiced no transmission takes place between the other two, a transmission circuit having two sections associated with said last two points, a band filter associated with one of the balancing points of said balancing ele-* ment and having an attenuating range and a range of free transmission, said filter hav ing substantially uniform impedance in one range and a materially different impedance in tlie'other range, and an impedance device" associated with the other balancing point of said balancing element in balancing relation to said filter, said impedance device having a substantially uniform'impedance at all frequencies which is equal to the impedance of said filter in one, of its frequency" ranges, whereby said balancing element is balanced to prevent transmission of one range of frequencies over said transmission circuit but is -unbalanced to permit transmission of frequencies outside of said range.

6. In a selective system a balancing element having four connecting points so arranged that when two of said points are balanced no transmission takes place between "the other two, a transmissionci "transmission in the other range.

*"iit having last two two sections associated with's points, a band filter associatedwith one of the balancin -Joints ofsaid balancin elev c ment and having an attenuating range and a range of free transmission; the impedance of said filter being materiallydifferent in the 'two'ranges, an impedance device associated -with the other balancing'po nt of said balancing element in balancing'relation to said filter, the impedance of said device being substantially the same as that O'f'SllCl filter over one frequency range but'materially different "in the other range, wherebysaid balancing of'one range of frequencies over said transelement is balanced to prevent transmission mission circuit but is unbalanced to permit 7. In a selective system, a hybrid coil having balancing points, a transmission circuit comprising two sections, one of which is associated with a winding of said coil and arranged to transmit through the coil to the other section which is associated with the midpoint of another winding of said coil, a network associated with one balancing point of said hybrid coil, said network having 7 substantially uniform impedance in one range of frequencies and a materially different impedance outside said range, and an impedance device associated with another balancing point of said hybrid coil, said impedance device having substantially the same impedance as said network in said first mentioned range of frequencies and a different impedance outside said range, whereby said hybrid coil circuit is balanced to prevent transmission of said first mentioned range of frequencies from one of said transmission sections to the other, but is unbalanced to permit such transmission of frequencies outside said range. v

8. In a selective system, a hybrid coil having balancing points, a transmission circuit comprising two sections, one of which is associated with the winding of said coil and arranged to transmi' through the coil to theother section which is associated with the midpoint of another winding of said coil, a network associated with one balancing point of said hybrid coil, said network having substantially uniform impedance in one range of frequencies and a materially different impedance outside said range, and an impedance device associated with another balancing point of said hybrid coil, said impedance deance at all frequencies which is equal to the impedance of said network in said first mencomprisin g two sections, one of which is associated with the winding of said 0011 and arranged to transmit through the coil to the other section which is associated with the midpoint of another winding of said coil, a

network associated with one balancing point of said hybrid coil, said network having two frequency ranges in one of which its im edance materially different from that 0 the other, an impedance device associated with another balancing point of said hybrid coil, the impedance of said device being substantially the same as that of said network over on frequency range but materially different in the other frequency range, whereby said hybrid coil is balanced to prevent transmission of one range of frequencies from one of said transmission sections through the hybrid coil to the other, but is unbalanced to permit such transmission in the other range.

10. In a selective system, a hybrid coil having balancing points, a transmission circuit comprising two sections, one of which is associated with the winding of said coil and arranged to transmit through the coil to the other section which is associated with the midpoint of another winding of said coil, a band filter associated with one balancing point of said hybrid coil and having an attenuating range and a range of free transmission, said filter having substantially uniform impedance in one of said ranges of frequencies and a materially different impedance in the other range, and an impedance device associated with another balancing point of said hybrid coil, said impedance device having substantially the same impedance as said filter in a range corresponding to one of the ranges of said filter and a diiferent impedance in the other range, whereby said hybrid coil is balanced to prevent transmission of frequencies in one of said ranges from one of said transmission sections through the hybrid coil to the other, but is unbalanced to permit such transmission of frequencies in the other range.

11. In a selective system, a hybrid coil havother section which is associated with the midpoint of another winding of said coil, a band filter associated with one balancing point of said hybrid coil and having an attenuating range and a range of free transmission, said filter having substantially uni;

form impedance in one of said ranges of frequencies and a materially different impedance in the other range, and an impedance device associated with another balancing point of said hybrid coil, said impedance de; vice having substantially uniform impedance at all frequencies which is equal to the impedance of said filter in one of its frequency ranges, whereby said hybrid coil is balanced to prevent transmission of oneof said frequency ranges from one of said trans- :inission sections through theihybrid coil to the other, but is unbalanced to permitsuch Qtransmission' of the other range of frequenoles. v

- point of said hybrid coil and having an attenuating range and a range of free transmission,.said, filter having an impedance in one of said ranges which is materially different fromits impedance in the other range,

an impedance device associated with an-V other balancing point of said hybrid coil, the impedanceot' said device being substantially the'same as that of said filter over a frequency range corresponding to one of the ranges of said filter, whereby said hybrid coil is balanced to prevent transmission of one range of frequencies from one of said transmission sections through the hybrid coil to the other, but is unbalanced to permit such transmission in the other range.

V 13. A selective system, a hybrid coll having two balancing points, a transmission circult comprising two sections, one of which is associated with the winding of said coil and arranged to transmit through the coil to the other section which is associated with the IIllClPOlIlh of another winding of said co1l,nn- .pedance elements associated with the balanci-ng points of said hybrid coil and having such impedance relative to each other as to produ'ce-a substantial balance over one band of frequencies while being unbalanced for fre Qquencies outside the band.

' 14. In a selective system, a hybrid coil having balancing points, a transmission circuit comprising two sections, one of which is associated with the winding of said coil and arranged to transmit through the coil tothe other section which is associated with the midpoint of another winding of said coil, a

network inductively connected to one balancing point of said hybrid coil, and an impedance device inductively connected to an a other balancing point of said hybrid coil, said" network and said impedance device having a such relative impedances as to produce a balance at frequencies within a band while bemg unbalanced for a range of frequencies lying outside the band.

15. In a selectlve system, a hybrid coil'having balancing points, a transmission circuit 55 comprising two sections, one of which is. assoruary, 1929-,

ciated with the winding of said coil and arranged to transmit through the coil to the other section which is associated with the midpoint of another winding of said coil, a

networkinductively connected to one balancing point of. said hybrid coil, an impedance device inductively connected to another balancing point of said hybrid coil, said neti work and said impedance device having such relative impedances as to produce a balance at frequencies within a band while being unbalanced for a range of frequencies lying outside the band, and means to effectively remove the impedance of said network from thelcircuit without interrupting transmission from one of said transmission sections through the hybrid coil to the other.

In testimony whereof, I have signed my name to this specification this 4th day of Feb- JOSEPH HERMAN.