US 2692372 A
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1954 H. E. GOLDSTINE WIDE BAND RADIO FREQUENCY CHOKE COIL 2 Sheets-Sheet 1 Filed July 19, 1951 0 40. an 12.0 100 20.0 z4.0 20.0 52.0 500 FREQUENCY /A/ MEG/V6255 HBIIaQJ E. file R H m m w 0 0 3 m Oct. 19, 1954 H. E. GOLDSTINE 2,692,372
I WIDE BAND RADIO FREQUENCY CHOKE CQIL Filed July 19, 1951 Q 2 Sheets-Sheet 2 "5/11; R I 5 flall aaol sm INVENTO ATTORNEY Patented Oct. 19, 1954 WIDE BAND RADIO FREQUENCY CHOKE COIL Hallan Eugene Goldstine, Port J eiferson Station,
Y., assignor to Radio Corporation of America, a corporation of Delaware Application July 19, 1951, Serial No. 237,646
This invention relates to radio frequency choke coils, and particularly to radio frequency choke coils useful over a wide frequency range.
A conventional radio frequency choke coil comprising a single winding has a highly uneven impedance characteristic over a wide frequency range, with anti-resonant peaks spaced from resonant dips occurring at difierent frequencies within the range. As is well known in the art, the capacitance between adjacent turns of a winding acting together with the series inductance of the winding produces anti-resonant peaks and resonant dips in the impedance characteristic of a single wound coil, causing the impedance of the coil to vary widely over a frequency range of any considerable extent. This characteristic is highly undesirable in transmitters which must operate on more than one frequency.
It is an object of this invention to provide a radio frequency choke coil useful over a wide frequency range without self-resonant dips in the impedance characteristic.
Another object of the invention is to provide a novel choke coil construction having a low resistance to direct current and low frequency components, and which has a relatively high and substantially uniform impedance over the entire wide high frequency band which it is desired to attenuate.
A further object of this invention is to provide a radio frequency choke coil having two closely coupled windings in which one winding is of low resistance wire and the other winding is of relatively high resistance wire compared to the first winding.
Briefly, in accordance with the present invention, there is provided a choke coil having a winding of low resistance wire which is adapted to be electrically. connected in series in the utilization circuit, and a second winding very closely coupled to the first winding but made of high resistance wire to dissipate the high frequency components and reduce the peaks of selfresonance of the choke coil. The coil of the invention may be considered to be a bifilar winding constituting a two-terminal network in which one of the windings is of low resistance wire and connected at both ends to terminals, and the other winding is of high resistance wire closely coupled to the low resistance winding but electrically connected at only one end of the high resistance winding.
A more complete understanding of the operation and features of the invention, together with additional objects thereof, may be gained from a reading of the following description in connection with the accompanying drawings, in which:
Fig. 1 shows, schematically, the electrical circuit diagram of the choke coil of the present invention;
Fig. 2 illustrates an embodiment of a choke coil, partly in cross section, in accordance with this invention;
Fig. 3 shows an impedance versus frequency curve for a choke coil of the present invention compared with a choke coil without the high resistance winding;
Fig. 4 illustrates a cross-sectional view of a second type of choke embodying the principles of this invention;
Fig. 5 illustrates, partly in cross section, another form of the invention, and
Fig. 6 is a circuit diagram of a push-pull amplifier showing, by way of example, locations where the choke coils of the present invention may be used.
Referring now to Fig. 1 of the drawings, which is a schematic representation of a choke coil according to this invention, there is shown a low resistance winding II which is provided at each end with terminals [3, l4. Wound on the same core and closely coupled to the low resistance winding I l is a high resistance winding I 5, which may be connected to one of the terminals [4 of the choke coil. The close coupling between the high resistance winding l5 and the low resistance Winding Il may be obtained by winding the two conductors side by side on a single coil form in insulated relationship, or one winding may be wound in a single layer and the other wound on top of the first in another layer, separated only by a thin insulating covering.
Close coupling may also be attained with pie windings. A pie Winding is one composed of many turns which are built upon one another to form a structure Which has an outside diameter several times the width of the coil winding, and results in a flat disc shape coil. Individual coils of this fiat construction are termed pies. A construction utilizing pie windings employs a low resistance winding ll, each pie section of which is in very close physical relationship and tightly coupled to adjacent pie-shaped sections of a high resistance winding [5, and is described below in connection with Fig. 4.
In the choke coil construction shown in Fig. 1, a very low resistance is ofiered to currents of low frequency and especially to direct current through the low resistance winding II. On the other hand, by virtue of the tight coupling between the low resistance winding I I and the high resistance winding [5, high frequency currents are coupled into the high resistance winding I and are dissipated in the high resistance winding.
In Fig. 2 there is shown a choke coil in accordance with the principles set forth above with respect to Fig. 1 having a high resistance winding I5 wound in a single layer and a low resistance winding II in a second layer over the high resistance winding I5 but insulated therefrom by a thin insulating material II. It will be noted that both of the windings I i, I5 are connected to. one of the terminals I4 at one end whereas only the low resistance winding I I is connected to the terminal 13 at the other end. In this form of the invention, the high resistance winding I5 is not terminated at one end. A coil form or core I5 is provided which, depending upon the band of frequencies for which the choke coil is designed, may be of a plastic material such as Bakelite or polystyrene, either hollow or solid. If the coil form is hollow, a magnetic core material may be inserted therein. In one embodiment of the invention successfully tried out in practice, a choke coil which was found to work satisfactorily over the band of frequencies from 5 to 3,0 megacycles, a core of insulating material inch in diameter and i long was first wound with Nichrome wire of No. 40 gauge, 0.003 inch diameter, 40 turns, per inch and 12% turns total. The Nichrome wire had a D. C. resistance of 1400 ohms. A thin insulating sleeve of low-loss material was placed overthe Nichrome winding and a winding of copper wi Of No. 28 gauge, 0.015 inch diameter, was wound 66 turns per inch, 198 turns total. over the Nichrome winding and insulating sleeve. The copper wire had a D. C. resistance of 2 ohms. Both ends of the low-resistance, copper winding were connected to the terminals it, I4 while only one end of the Nichrome winding was, connected to a terminal Hi, the other end being left unterminated.
The high resistance winding I5 may be of other metals besides Nichrome. For example, climax metal, chromax, constantin, German silver, manganin, or some other alloys of high specific re sistance can be used. Nichrome is preferred since it has a higher specific resistance than any of the b re.
In Fig. 3 there is shown an impedance versus frequency curve for a, choke coil in accordance with the teachings of the. present invention compared with a choke coil of conventional form without a high resistance winding. The impedance of a wide band radio frequency choke coil in accordance with this invention is shown by the solid line 243, while the impedance of a radio frequency choke coil with a single layer conventional winding is indicated by a dash line 2i. Both curves extend over a frequency range from 5 to to megacycles.
It will be noted that the choke coil of conventional construction gave a nearly zero impedance in two places 23, 25 at frequencies of 24.6 and 37 megacycles and had two very high impedance points at places corresponding to approximately and 26 megacycles. These high impedance points are anti-resonant peaks which are not shown in the drawing in order to conserve space. On the other hand, the biiilar coil of the present invention gave a characteristic which varied from a minimum of about 4,000 ohms to a maximum of 8,500 ohms over the entire frequency band from 5 to 30 megacycles. No points of zero or very low impedance were found in the choke coil of this invention, and the anti-resonant peaks of very high impedance are greatly reduced. The resultant impedance characteristic is smoother over the wide range of frequencies tested.
The choke coils used for comparison in Fig. 3 had similar low resistance windings of 198 turns of No. 28 gauge copper wire on a inch polystyrene rod. The double wound choke coil whose curve is indicated by the solid line 2E3 had a high resistance Nichrome winding closely coupled to the low resistance winding like that described in connection with Fig. 2 above.
Series resonance points occurring in choke coils, such as those shown at 23, 25 in Fig. 3, produce impedances which are so low that it takes only a small amount of radio frequency voltage at those frequencies of resonance, whether it be the fundamental or a harmonic of the signaling frequency or current arising from spurious oscillations within the radio frequency equipment, to produce currents which are large enough to burn out the low resistance winding and destroy the choke coil or cause improper operation of the radio frequency equipment.
In Fig. 4 a cross-sectional view of a multisection pie-wound choke coil is shown utilizing the principles of this invention. A low resistance winding H is composed of several sections of a pie shape universal winding. Such a universal winding is made by winding the wire upon a cylindrical form accompanied by an oscillatory motion with respect to the cylinder. The resulting coil structure is self-supporting and can be made very narrow with respect to the outside diameter. A multi-sectioned pie-shape high resistance winding i5 is wound on the same form and the individual pies of the high resistance winding are inter-leaved with the pics of the low resistance winding II. The spacing between adjacent pie sections is made small to increase the coeiiicient of coupling between the low resistance winding I I and the high resistance winding I5. The individual pics of the low resistance winding I I are connected in series and both ends are brought out to terminals in the usual manner as shown in Figs. 2 and 5. A coil form I9 provides the support for the entire assembly and may be of an insulating material, either solid or hollow. The coil form It} is preferably a hollow tube to allow a core of magnetic material to be inserted therein.
An advantage which may be pointed out with respect to the multi-section, inter-leaved pie wound choke coil is that a greater number of turns of wire can be put into a very small space, and a reasonable value of the coefficient of coupling can still be obtained. It may be noted at this point that the spacing between the individual pics in the design shown in Fig. 4 is much smaller than that usually encountered in either radio frequency chokes wound with multiple pie sections on a single core or coil form I9. The reason for the narrow spacing between the pieshaped sections is to increase the coefiicient of coupling between the low resistance winding II and the high resistance winding I5. Further, the pies containing the high resistance winding I5 may have a great many more turns in each section than those pies of the low resistance winding ii. In a choke coil according to this invention having the form shown in Fig. 4, for greater current carrying capacity and lower direct current resistance, a low resistance winding may be made of wire of a larger diameter than that of the high resistance winding l5.
Fig. 5 shows, partly in cross-section, a bifilar wound choke coil in accordance with the present invention in which the high and low resistance windings are in side-by-side relationship. The low resistance winding 1 l, which i preferably of copper or similar low resistance conductive wire, is in insulated relationship with, but tightly coupled to, the high resistance winding [5. Both windings are wound on a coil form ll; of insulating material, which in this illustration is shown as being tubular. Two terminals l3, M are provided on the coil form. Only the low resistance winding II is connected to one of the terminals IS; the high resistance winding [5 may be left unterminated at one end. Both high and low resistance windings ll and I 5 are connected to the other terminal M. A multifilar winding having more than one high resistance winding l5 may also be utilized to carry out the present invention. In such an arrangement, the impedance of the separate high resistance windings will appear in parallel, and the damping effect of the high resistance winding will be reduced.
A core of magnetic material may be inserted into the interior of the coil form IS. The advantage of inserting a core of magnetic material into the coil form I9 is to effectively lengthen the winding for all frequencies of alternating current. Greater choking action may thus be obtained by utilizing a core of magnetic material.
In Fig. 6 there is shown by way of example a push-pull amplifier circuit utilizing the wide frequency band choke coils of the present invention. Two vacuum tubes, VI and V2, which for the purpose of illustration are shown as tetrodes, have their grids 21 and 29 fed in phase opposition from a pair of input terminals 3|. The anodes 33, 35 of the vacuum tubes VI and V2 are coupled together in phase opposition or push-pull relation by means of an output tank circuit having a center-tapped inductor 31. Anode voltage is supplied from the positive terminal 13+ of a source of unidirectional potential, not shown, to the center point of the inductor 3'! through a wide band choke 4| in accordance with this invention. Such a choke 4! may take the form of Fig. 2, Fig. 4, or Fig. 5. Voltage for the screen electrodes 43, 45 of vacuum tubes VI and V2 respectively may also be supplied from the terminal +Sg of a source of screen unidirectional potential, not shown, through wide band choke coils 4| also constructed in accordance with the present invention. The output voltage i coupled through an inductor 4i and appears across a pair of output terminals 5!, in turn coupled to any suitable utilization circuit, such as another amplifier stage or a transmission line.
Since the operation of a push-pull amplifier circuit such as that shown in Fig. 6 is well known, no detailed explanation of the operation of the circuit will be undertaken. The utility of the wide band choke coils 4| in series in the anode voltage supply and in the screen electrode voltage supplies to the two vacuum tubes VI and V2 will be set out. In amplifiers of this design, where a great deal of output power is desired, the amplitudes of the anode currents may reach large values of current. Since any resistance inserted in series in such a current-carrying lead will dissipate power, the ohmic resistance of such a choke becomes of paramount importance. The choke of the present invention enables a connection which dissipates a minimum amount of direct current power, and at the same time offers a high resistance to radio frequencies.
Also, such high power push-pull amplifiers draw considerable screen current, and the screen isolation resistors necessary for the high frequency components of current must dissipate large amounts of power. Such resistors and chokes which are required to dissipate large amounts of power are expensive to manufacture, and since the power dissipated in such components i lost, the overall efficiency of the amplifier is consequently reduced.
By utilizing the wide band choke coil of this invention, the screen isolation circuits are not required to dissipate large amounts of direct current power, and at the same time high impedance is offered to radio frequency components, and the tendency toward instability and oscillation arising from screen current variation is greatly diminished. Although some of the advantages listed above can be obtained with a choke coil of conventional design if care is taken to insure that none of the points of self-resonance coincide with harmonics of the frequency being transmitted, such precautions are unnecessary with the choke coil of the present invention. An even greater source of failure of conventional choke coils arises in transmitters in which the frequency must be changed from one frequency to another. If care is not taken to insert the proper choke coil, particularly in power amplifier stages, the choke coil may be burned out or seriously damaged. In the choke coil of my invention, on the other hand, it is unnecessary to change the choke coil because the points of self-resonance are so clamped by the losses in the high resistance winding that no points of nearly zero impedance occur. Operation of transmitters which must transmit at different frequencies is therefore simplified and the chance for accidentally burning out an important component is eliminated.
1. A wide band choke coil comprising a twoterminal network, a coil form, a high resistance winding wound in a single layer on said coil form, and a low resistance winding wound in a single layer over said high resistance winding in tightly coupled hut insulated relationship to said high resistance winding the ends of said low resistance winding constituting said terminals of said network, and at least one end of said high resistance winding being unterminated.
2. A wide band two-terminal network radio frequency choke coil comprising a coil form, a pair of terminals, a multifilar winding on said coil form, said multifilar winding having a plurality of conductors in side by side relationship, one of said conductors of said multifilar winding having a low resistance to direct current, the other con ductors of said multifilar winding having a high resistance, the said low resistance conductor having its ends connected to different ones of said terminals, and the said high resistance conductors each having only one end connected to one of said terminals the other ends being maintained. in insulated relationship and unterminated.
3. A wide band two-terminal network radio frequency choke coil comprising a pair of tightly coupled windings, one of said windings having a low resistance and the other of said windings having a high resistance, a core of magnetic material within and extending throughout the length of both of said windings, a pair of terminals, said low resistance winding having its ends connected to different ones of said terminals, and said high resistance winding having only one end connected to one of said terminal, the other end being open and unterminated.
4. In combination, an electron discharge device having an electrode, a source of unidirectional operating potential for said electrode, and a radio frequency choke coil connected between said electrode and said source, said choke coil including a pair of tightly coupled windings, one of said windings having a low resistance and the other of said windings having a high resistance, said low resistance winding being connected in series between said electrode and said source of operating potential, and said high resistance winding having only one end connected to one end or" said low resistance winding the other end of said high resistance winding being maintained in insulated relationship to said low resistance winding and unterminated.
5. A wide band two-terminal radio frequency choke coil comprising, a pair of terminals, a pair of tightly coupled windings, one of said windings having a low resistance and the other of said windings having a high resistance, said low resistance winding having its ends connected to different ones of said terminals, and said high resistance winding having only one point thereon connected to said low resistance winding, the remainder of the high resistance winding being maintained in insulated relationship with respect to said low resistance winding.
6. A choke coil as defined in claim 5 in which said low resistance winding is composed of a plurality of pie sections, and said high resistance winding is composed of a plurality of pie sections, said high resistance sections being interposed between the said low resistance sections.
7. A choke coil as defined in claim 6 in which the said low resistance winding is of copper wire, and the said high resistance winding is of Nichrome wire.
8. A choke coil as defined in claim 5 in which the said low resistance winding is composed of a plurality of pie sections, and the said high resistance winding is composed of a plurality of pie sections, the said low resistance sections being alternated with the said high resistance sections and spaced closely thereto in tightly coupled inductive relationship.
9. A choke coil as defined in claim 5 in which the said low resistance winding is of copper wire and the said high resistance winding is of Nichrome wire.
References Gited in the file of this patent UNITED STATES PATENTS Number Name Date 1,761,110 Dijksterhuis et a1. June 3, 1930 1,791,236 Drake Feb. 3, 1931 1,835,015 Crouse Dec. 8, 1931 1,837,413 Dcbson Dec. 22, 1931 2,088,454 Whisk July 27, 1937 2,539,926 Rainwater Jan. 30, 1951 FOREIGN PATENTS Number Country Date 21,646/29 Australia Mar. 25, 1925