° 2, 2,644 3,037,175 United States Patent Office Patented May 29, 1962 1
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May 29, 1962 C, L RUTHROFF 3,037,175 BROADBAND TRANSFORMERS Filed May 12, 1958 A/G 3 A/G, 4. ° 2, 2,644 3,037,175 United States Patent Office Patented May 29, 1962 1. 2 some saving in space and material is affected by winding 3,037,175 BROADBANDTRANSFORMERS all the coils on a common core. This may be done since Clyde L. Ruthroff, Fair Haven, N.J., assignor to Bell the net magnetic field resulting from the flow of signal Telephone Laboratories, Incorporated, New York, current is zero, and consequently there is no magnetic N.Y., a corporation of New York coupling among the several coils even though they share Fied May 12, 1958, Ser. No. 734,751 a common magnetic core. The magnetizing current, 3 Claims. (C. 333-32) however, does produce a net magnetic field. Conse quently, by winding the several coils series aiding for This invention relates to impedance transforming de the magnetizing current, there is a net increase in the vices and, more particularly, to broadband bifilar wound number of coupled turns, and the corresponding increase autotransformers. O in the effective transformer inductance. This has the The problem of distortionless transmission of signals effect of extending the low frequency end of the trans over wires is an old one, encountered often in electrical former pass band. The difficulty with such an arrange communications systems. As the range of operating fre ment, however, resides in the care which must be taken quencies is extended, as is the current trend, this prob in winding the transformer so as to minimize the inter lem has become more and more acute. For example, in 15 winding capacitance among the coils since this capac order to supply gain for pulses of millimicroseconds itance will adversely affect the high frequency response. duration, amplifiers and coupling transformers with band It is, therefore, an object of this invention to produce widths of hundreds of megacycles are needed. While broadband impedance transformation ratios greater than the problem of extending the frequency range of trans 1:1 using a single bifilar wound coil. formers has received the attention of many investigators, 20 It is a further objective of this invention that said trans and some suggestions and improvements have been made, formation ratios be obtainable in a transformer used to currently available transformers still fall far short of full couple two unbalanced systems. filling the bandwidth requirements presently encountered If the length of the conductors making up the bifil in the electronic arts. coil is an appreciable part of a wavelength of the signal The difficulties in providing broadband transformers 25 transmitted through the transformer, the input impedance is due in great part to the series self-inductance and to the transformer will be a function of the length of the parasitic interwinding capacitance of the transformer conductors as well as the load impedance. Since it is windings themselves. For example, in a conventionally desirable for matching purposes that the terminal im Wound transformer, the upper end of the pass band is pedances of the transformer be only a function of the generally determined by the large interwinding capacity 30 ioad impedance and the transformer turns ratio, it is an which resonates at some relatively low frequency, while additional object of this invention to provide broadband the low frequency end of the pass band is limited by a impedance transformation means which are substantially relatively small coil inductance which appears as a low independent of the signal frequency over an extended impedance in parallel with the signal source. While range of operating frequencies. these limitations have been somewhat overcome by the 35 A transformer constructed in accordance with the in use of miniature construction and new and improved mag vention comprises a pair of insulated conductive wires, netic core materials, this type of approach to the problem uniformly spaced from each other and wound together in has enjoyed only limited success. a substantially helical form. A coil so wound has the The theory of transmission lines, which has been well distributed properties of a uniform transmission line and developed in the communications art, on the other hand, 40 the corresponding broadband capabilities when used as a has indicated how this problem may be attacked by means transformer. An impedance transformation ratio of 4:1 of a more fundamental and promising approach. It is is obtained by serially connecting the coils by conduc known that a pulse can be propagated down a trans tively joining one end of one of the conductors to the mission line without distortion, and that the input imped other end of the other conductor and by making connec ance of any length of properly terminated line is a pure 45 tions to and from the transformer whereby one of the resistance. Thus, for instance, a Lecher wire line having external circuits is connected across only one of the the desired characteristic impedance can be wound on a coiled conductors and the other external circuit is con core form so that the conductors are distributed over the nected across both of the coiled conductors. surface of the core with the two conductive members It is a feature of the present invention that by merely forming a bifilar wound transformer having a 1:1 turns 50 rearranging the ground connections, the transformer may ratio. One end of the bifilar coil comprises the input or be used either to connect two unbalanced networks or to primary end of the transformer, and the other end the connect an unbalanced network to a balanced network output or secondary end. In such an arrangement, the without the further cascading of an additional coil, which series inductances and inter winding capacitances, which addition tends to decrease the overall bandwidth. normally set the frequency limits for the conventional 55 It is a further feature of the invention that by using lumped parameter transformer, are now part of a dis core material of sufficiently high permeability, the num tributed transmission line system. If the line is properly ber of turns may be reduced for a given low frequency terminated with a pure resistance, which may readily response, and the length of line constituting the bifilar be computed in terms of the line constants, a 1:1 imped coil made small. Since the transformer output is zero at ance transformer of unusually broad bandwidth is ob 60 the high frequency end of the transformer pass band tained. when the electrical line length is a half of a wavelength Such distributed transformers have been used in the of the signal frequency, using fewer turns has the effect past, but since the impedance transformation ratio is only of extending the upper end of the transformer response 1:1, it is necessary to use several such coils where im characteristic. Furthermore, since the input impedance pedance transformations greater than 1:1 are sought. 65 of a short length of transmission line is approximately In particular, where an impedance transformation of equal to the load impedance terminating the line, the 4:1 is desired, two bifilar coils have to be used. Fur transformer is substantially independent of the line char thermore, where an impedance transformation of 4:1 is acteristics, and free of transmission line effects over a sought to connect two unbalanced networks, an addi major portion of the transformer pass band. A trans tional, or third coil, has to be added to the two above 70 former constructed and operated in accordance with the mentioned coils. In such multiple coil arrangements, teachings of the invention has the broadband character 3,037, 175 3 4. istics of a parallel wire transmission line and the im quency operation separately. At low and at intermediate pedance transformation characteristics of a center-tapped frequencies, ordinary lumped parameter network analysis autotransformer. - is used. In the high frequency region, transmission line These and other objects, the nature of the present in techniques are resorted to. vention, and its various features and advantages will ap 5 Designating the reactance of each half of the bifilar pear more fully upon consideration of the various specific winding as X= wL, and the mutual inductances as -kX, illustrative embodiments shown in the accompanying where k is the coefficient of coupling, the low frequency drawings and analyzed in the following detailed descrip and intermediate frequency mesh equations are given tion of these drawings. as follows: In the drawings: O FIG. 1 shows diagrammatically a bifilar wound auto E= (R.--iX) I--(R-ikX).I. (1) transformer connected in accordance with the principles E= (R-ikX) 11--(R-RI-iX).I. (2) of the invention; where FIG. 2 is a schematic illustration of the transformer of FIG. 1 when used to connect two unbalanced networks; 5 I is the load current FIG. 3 is a schematic illustration of the transformer and of FIG. 1 when used to connect an unbalanced network and a balanced network; I-I-I is the generator current. FIG. 4 shows diagrammatically a bifilar autotrans Solving for I and I gives: former using two cascaded coils wound on a common 20 magnetic core. Referring to the accompanying drawings, and more specifically to FIG. 1, there is diagrammatically shown X(1-- a transformer wound and connected in accordance with 1- Elix-hk) (4) the present invention. The transformer comprises a pair 25 of insulated conductive filaments 11 and 12, wound to where A=X2(k-1)--iX(2R (1-1-k) --RL--RRL gether in a substantially helical form over coil form 10. Examining the load current, I2, more carefully, it foll Insulated filaments 11 and 12 are arranged so that their lows that: insulated coverings are in close juxtaposition substantially throughout their entire lengths.