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May 29, 1962 c. L. RUTHROFF 3,037,173 HYBRID NETWORK Filed Jan. 23, 1959 FIG. / ,V I D INVENTOR C. L. RUTHROFF BY 1%)”. ?é% ATTORNEY United States Patent O?ice 3,037,173 Patented May 29, 1962 2 1 the other end of the second coil, that is, by joining non adjacent ends of the two coils. Utilizing means are con 3,037,173 nected from each of the remaining free ends and from HYBRID NETWORK Clyde L. Ruthrotf, Fair Haven, N.J., assignor to Bell Tele the interconnection to a common terminal. Additional utilizing means are connected either between the free ends or between the interconnection and one of the free ends, phone Laboratories, Incorporated, New York, N.Y., a corporation of New York Filed Jan. 23, 1959, Ser. No. 788,648 5 Claims. (Cl. 333-11) depending upon the impedance match desired. These and other objects and advantages, the nature of the present invention, and its various features, will ap This invention relates to wave transmission systems, and, in particular, to broad-band coupling arrangements 10 pear more fully upon consideration of the various illus trative embodiments now to be described in detail in con commonly known as hybrid networks for use in such sys nection with the accompanying drawings, in which: tems. FIG. 1 shows diagrammatically a hybrid network con— One of the more useful circuit arrangements employed nected in accordance with the principles of the invention; in communications networks is the so-called hybrid net work. This type of network is of particular importance 15 FIG. 2 is a schematic illustration of the network of FIG. 1; in that it makes possible the two-way operation of a FIG. 3 is a schematic illustration of a hybrid network telephone line. However, because of its nature, the modi?ed for impedance matching purposes, and ordinary hybrid coil is not adaptable to the higher fre FIG. 4 is a schematic illustration of a hybrid network _\ quencies. Thus, as the range of operating frequencies is extended, as is the current trend, the problems of distor 20 modi?ed to accommodate single ended utilization means. Referring to the accompanying drawings, and more tion and unbalance in hybrid networks have become more speci?cally to FIG. 1, there is diagrammatically shown acute. For example, in order to supply gain and to a ?rst embodiment of a broad-band hybrid network con transmit pulses of millimicrosecond duration, ampli?ers nected in accordance with the present invention. The and coupling networks with bandwidths of hundreds of megacycles are needed. While the problem of extending 25 transformer T comprises a pair of insulated conductive ?laments 11 and 12, wound together in a substantially the frequency range of hybrid networks has received the helical form over coil form 10. Insulated ?laments 11 and 12 are arranged so that their insulated coverings are attention of many investigators, current circuit arrange- . ments still fall far short of ful?lling the bandwidth re quirements presently encountered in the communications art. 30 in close juxtaposition substantially throughout their entire lengths. The juxtaposition or contiguous arrangement of these two wires is such as to produce substantially unity coupling between the two windings and, in addition, to produce the equivalent of a uniform parallel wire trans are described which have bandwidth ratios as high as mission line from one end of the coil to the other ‘end 20,000 to 1 in the frequency range of a few tens of kilo cycles to over a thousand megacycles. It is therein in 35 thereof. The double threaded spiral or helical coils de scribed above are known in the art as bi?lar coils and dicated that the frequency limitations in a conventional will be referred to as such hereinafter. The actual spac transformer of a type which might be used in a hybrid ing of the conductive portions of members 11 and 12, network are in great part due to the series self-inductance and the diameter of said conductors, will be considered and parasitic interwinding capacitance of the transformer windings themselves. For example, in a conventionally 40 in greater detail below. The bi?lar coil is mounted upon a coil form 10 which wound transformer, the upper end of the pass~band is gen may be composed of any suitable high permeability, low erally determined by the large interwinding capacity loss core material. For example, a number of trans which resonates at some relatively low frequency, while formers using nickel-zinc ferrite cores have been con the low frequency end of the pass-band is limited by a structed and have given very satisfactory results. While relatively small coil inductance which appears as a low coil form 10 has been shown as a toroidal member, it impedance in parallel with the signal source. While these may assume any convenient shape consistent ‘with the elec limitations have been somewhat overcome by the use of trical requirements of the transformer windings. miniature construction and new and improved magnetic Coils 11 and 12 are serially connected by joining non core materials, this type of approach to the problem has enjoyed only limited success. 50 adjacent ends 2 and 3 to form an internal interconnec tion. The latter is brought out as terminal 5. It is, therefore, an object of this invention to produce In my copending application Serial No. 734,751, ?led May 12, 1958, broad-band bi?lar wound transformers Connected to coils 11 and 12 are the two pairs of con broad-band hybrid network arrangements using a single jugate impedances 13 and 14 ‘and 15 and 16. One pair bi?lar wound coil. of conjugate impedances 15 and 16 connects between ter By applying transmission line theory to the transformer art, broad-band transformers of the type described in my 55 minals 1 and 4, respectively, and the common terminal 6. Impedance 14, of the second pair of conjugate impedances copending application are now available. It has been 13 and 14, connects between the common junction 6 and recognized in accordance with the present invention that the interconnection 5. The other impedance, 13, is con bi?lar transformers of this type may be adapted and nected across terminals 3 and 4. utilized to produce broad-band hybrid networks by the appropriate interconnection of the bi?lar transformers 60 In FIG. 2 there is shown a schematic diagram of the and the external utilization networks. As will be shown network of FIG. 1 in which impedance 13 of FIG. 11 hereinafter, transformers utilized in this manner preserve more speci?cally comprises a signal generator 20 and its their broadband characteristics and produce hybrid cou equivalent internal impedance R1, and impedance 14 com; pling properties. prises a resistor R4. The conjugate impedances 15 and A broad-band hybrid network constructed in accordance 65 16 are represented by the two equal resistors R2 and R3. with the invention comprises a pair of insulated conduc The hybrid operation of the network and its frequency tive wires, uniformly spaced from each other and wound response, may be demonstrated by considering the cur together in a substantially helical form. A coil so wound has the distributed properties of a uniform transmission rents produced as a result of a signal E impressed upon line and the corresponding broad-band capabilities when 70 the circuit by signal [generator 20. Assuming that the reactance of each winding is much larger than the ter The two coils thus formed are minating resistances and that the coupling coefficient k serially connected by joining one end of one of them to used as a transformer. 3,0 0 7,173 3 4 is equal to unity, the network equations may be written Solving fOtl‘ Z0, the characteristic impedance of the trans as: former, gives Zo=\/2R The transmission T3 to load R3 may also be calculated and is found to be T3: 11:12 cos ?l-l-JJZLZ0 sin til where l is the equivalent length of transmission line formed by the bi?lar windings 11 and 12, B is the phase constant of this line, and Z0 is the characteristic impedance of the line. Solving for the currents gives 2 ‘ (1+7 m 10 . - 2 ?l [a 212 cos Bl) 2 +-ls1n R+ZO It will be noted that both T2 and T3 at low frequencies (Blz?) are equal to 1/2, as expected. 15 Letting Z0=\/iI€ and solving for the ratio of the trans mission to loads R2 and R3, gives T3_(1+3 cos BZ)2+2 sing at At low frequencies 51:0 and T2/T3=1, or, equal power is delivered to each of the load resistors R2 and R3. At Bl=1r/ 4, or at a frequency -for which the transformer has an equivalent length equal to one-quarter wavelength, the ratio of the power delivered to the two load resistors R2 and R3 is 1.218, or the power split is unequal by about 1 , decibel. Miriam-R1123] 00s al+iZ0 Sin HZIRIJFRSJFR‘] +j slgflmlmwRlRzRsJrRlRsRd At half a wavelength, the ratio of power de livered to the two resistors is 3:1. The ratio of power delivered to the conjugate resistor R4 to the available power is given as 30 For balanced loads and matched conditions, T _4(cos Bl-1)Z+2 sin2 Bl “(1+7 cos Bl)2+32 sing at At 18l:0, T4 is equal to zero. At ?l=1r/4, T4=.O332 or or the insertion loss from R1 to R4 is about 30 decibels. Ideally, of course, the insertion loss should be in?nite Under these conditions, R1=R4=Rtheand currents can be written as , at all frequencies or T4 should equal zero. In the embodiment of FIG. 1 and FIG. 2, the con jugate resistors R1 and R4 are equal. In some applica tions, however, these resistors are more conveniently made unequal. In particular, for those applications in which R1=4R4, the circuit is modi?ed as shown in FIG. 3, wherein resistor R1 is connected across both of the serially connected coils ‘11 and ‘12. As shown, resistor R1 con nects from the terminal 1 to terminal 4, all other con nections being the same as in FIG. 1. In either arrangement, however, the characteristic im pedance of the transmission line formed by the bi?lar wound coils 11 and 12 is given as Z0=\/_27€. In terms of the Wire size and spacing, Ideally, the current in R4 should be zero. Actually, this current is 50 However, at low frequencies, when the wavelength is large compared to the winding length, BI is small and the current (Ia-I2) approaches Zero in amplitude. The transmission T is given as the power delivered to the load resistors R2 and R3, divided by the available power, E2 4R where n is the effective permeability; e is the effective dielectric constant; b is the distance between wire centers, and a is the wire diameter. An examination of the hybrid networks shown in FIGS. 60 1 and 3, discloses that at best only three of the four loads For load resistor ‘R2, can be grounded simultaneously. In FIG. 2, the common terminal 6 for resistors R2, R3 and R4 is grounded, where as the generator, 20, is double-ended, or balanced with X1? respect to ground. This arrangement may be modi?ed, 65 and all four load resistors operated single-ended, by using or an additional bi?lar transformer as shown in FIG. 4. In T2: 2(3-l-cos (Bl)2 FIG. 4, generator 20 and resistor R1 are connected to 2 transformer I‘; through the second transformer T2 which (1+7 cos sz)2+4 sin” al<-Z—“+-2la R Z0 acts as a simple balanced-to-unbalanced transformer. Be cause of the broad-band properties of this type of trans The transmission is a maximum when the coe?icient of sin2 pl is a minimum or where dZ02R2 217, n+2; =0 ' former there is no appreciable degradation in frequency response. The characteristic impedance of transformer T2 is either R or 4R depending upon whether the arrange‘ ment of FIG. 1 or FIG. 3 is used. In either arrangement, 75 however, all four loads may be operated single-ended, with 3,037,173 5 the grounds as shown in FIG. 4 as a typical arrangement. In all ‘cases it is understood that the above-described arrangements are illustrative of a small number of the fourth circuit is connected between said free ends and has many possible speci?c embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can readily ‘be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention. fourth circuit is connected between one of said free ends and said interconnection and has an impedance R. 4. The combination according to claim 1 wherein said common terminal is grounded and said fourth circuit is having a characteristic impedance Z, comprising two which is unbalanced with respect to ground. an impedance 4R. 3. The combination according to claim 1 wherein said balanced with respect to ground. 5. The combination according to claim 1 wherein said What is claimed is: 10 common terminal is grounded and said fourth circuit com~ prises an unbalanced-to-balanced transformer and a load 1. In combination, a two-element transmission line insulated conductive wires wound together in a substan tially helical form to form a pair of coils, said wires being contiguous and parallel from the ?rst end of each to the 15 second end of each, said coils being serially intercon nected ‘with said ?rst end of one being connected directly to the second end of the other, first and second external circuits each having an impedance 2R connected from each of the remaining free ends of said coils to a common terminal, a third external circuit having an impedance R connected from said interconnection to said common terminal, and a fourth external circuit connected between a pair of ends of said coils wherein said characteristic impedance and said external circuit impedances are re- 25 larted by z,=\/2_RT 2. The combination according to claim 1 wherein said References Cited in the ?le of this patent UNITED STATES PATENTS 1,755,243 2,229,078 2,654,836 2,735,988 2,736,864 2,875,283 Crisson ______________ __ Apr. 22, 1930 Hansell ______________ __ Jan. 21, 1941 Beck __________________ __ Oct. 6, 1953 Fyler ________________ __. Feb. 21, 1956 Sinclair ______________ .__ Feb. 28, 1956 Maione ______________ __ Feb. 24, 1959 OTHER REFERENCES Article, “The Hybrid Coil,” from. Electrical Communi cation by Arthur L. Albert, 2nd ed., John Wiley & Sons, Inc. (1940), page 434 relied upon.