US2626318A - Radio-frequency transformer and inductance element therefor - Google Patents

Description

Jan. 20, 1953 A, TORRE 2,626,318

RADIO-FREQUENCY TRANSFORMER AND INDUCTANCE ELEMENT THEREFOR Filed OCT.. 4. 1947 lnvenor:

lfon Jahn Torre A T TONE Y Patented Jan. 20, 1953 RADIO-FREQUENCY TRANSFORMER AND INDUCTANCE ELEMENT THEREFOR Alton J. Torre, Westmont, N. J., assignor to Radio Corporation of America, a corporation of Dela- Ware Application October 4, 1947, Serial No. 777,941

8 Claims.

This invention relates to radio-frequency transformers and inductance elements therefor, and particularly to tuned radio-frequency transformers and inductance elements employing molded magnetic cores.

Superior performance is achieved, in accordance with the invention, by utilizing more effectively the core and conductor material of the inductance elements to give more inductance and less resistance. The simple, compact arrangement results in better performance and is more economical both as to its own cost and in its space requirements, so that it is particularly useful as a component of radio receivers and other compact devices that are made in large quantities. The improved inductance element is peculiarly adapted for use in combination with a certain type of capacitor to form an improved radio-frequency transformer.

It is well known to those skilled in the art that the purpose of an electrical inductance is to store electromagnetic energy. It does this perfectly in the sense that no energy is wasted in storage, but it functions imperfectly in that energy necessarily is dissipated in the resistance of the conductor in passing to and from the inductance. It is like a water pail that does not leak, but that inherently spills a little water each time it is filled or emptied. Since in the alter hating-current applications herein contemplated, the inductance element is charged with energx7 and uncharged many hundreds of thousands, or even millions, of times each second, it is apparent that the resistance of an inductance element relative to its inductance is a matter of importance. A widely recognized figure of merit of an electrical inductance at a given frequency is the ratio of the energy stored therein to that dissipated during its sinusoidal charge and discharge. and this ratio, when multiplied by a convenient proportionality factor, is known as the Q of the inductance element.

The principal object of the invention is to provide an inductance element having the highest possible Q consistent with the low cost and small size necessary for large quantity production. This object is achieved by arranging the core and conductor material to make the best use thereof as more fully described hereinafter.

The invention is directed to improvements in tuned radio-frequency circuits as distinguished from lower-frequency circuits that usually are untuned and that do not require a high Q. A radio-frequency transformer suitable for use as an intermediate-frequency transformer in a superheterodyne radio receiver will be described as an example of the invention. In addition to the dominant need for high Q, it is necessary in some applications that the intermediate-frequency transformer be shielded and that it be small. For large quantity production it should also be inexpensive,

A further object of the invention, therefore, is to provide a self-shielded, high-Q transformer that is a compact unit and that may be constructed cheaply. This object is achieved by combining with the aforementioned inductance elements a capacitor that dissipates very little energy and that is adapted to encompass and shield the inductance elements to form a compact and effectively shielded transformer. Another object of the invention is to provide a transformer that readily may be tuned to a predetermined freouency, This obiect is achieved, in one modification of the invention, by removing a suitable portion of one of the plates of the capacitor. This expedient is simple, inexpensive, and effective, but it has the disadvantage that the operator sometimes cannot be sure he has removed enough material until too much has been removed. reouiring replacement of some of the plate material.

In another modification tuning is accomplished by sliding in or out an auxiliary core that is not critical in its adjustment because it constitutes only part of the magnetic path, and that advantageously may be of higher permeability than the core which constitutes the remainder of the path.

Tn the accompanying drawings:

Fig. 1 is a longitudinal sectional View of a transformer embodying the invention. certain portions thereof being shown schematically;

Fig. 2 is a longitudinal sectional view of a transformer embodying a modification of the invention, certain portions thereof being shown schematically;

Fig. 3 is a sectional view taken along the line 3-3 in Fig. 1;

Fig. 4 is a sectional view taken along the line 4--4 in Fig. 2;

Fig. 5 is a fragmentary sectional view of one of the inductance elements of Fig. 1;

Fig. 6 is a schematic diagram of a circuit employing a transformer in accordance with the invention; and

Fig. 'l is a schematic diagram of a circuit employing an inductance element in accordance with the invention.

Referring to Figure 1, a. pair of spool-shaped `the capacitor comprising inner plate cores I, I archere shown, each core consisting of a body portion 2 and end flanges 3 integrally constructed as by molding of comminuted magnetic material in accordance with well-known practice. By magnetic material is meant material that is capable of being magnetized and that has a high magnetic permeability. Conductor I is random wound on body 2 between flanges 3 to form coil 5. In a typical intermediate-frequency transformer for a superheterodyne receiver employing a 455 kc. intermediate frequency, lconductor l consisted of a solid Vcopper conductor 0.0035 inch in diameter insulated with a type of enamel commonly known -as Formex to Van overall diameter of 0.0043 inch, -coil 5 being about 0.12 inch wide and 0.06 inch deep. Flanges 3 extend beyond coil 5 sufliciently to forma path of large cross-sectional area, lines of force passing therebetween in the individual cores, interlinking all of the turns of coil 5.

In a transformer ofthe so-called double-tuned type, two .similar inductance coils are placed end to vend, Yas shown in Fig. 1, spaced Aapart suf- HViiciently to giveadequate electromagnetic coupling therebetween. Inductance coil 5 .istunedl byv azcapacitor comprising inner'plate 6 connected to one end of the inductance coil and 'outer plate 'I connected to the vother end thereof, separated by dielectric-8 which preferably lis cylindrical in form and lmadeof some'suitable ceramic material. VInner plate "G is vconnected to theanode 'of preceding vacuum tube @and outer plate 'I is connected to a suitable B+ voltage supply fas indicated in Figure f.

Coil I0 may be like coil ,5 or `maygdiifer therefrom as to the numberof its turns but is random wound of enamel-covered Vwire directly Von the other core I between iianges which extend substantially beyond the coil. Coil I is tuned by Ii connected to one end of the coil and outer plate I2 connected tothe other end thereof. Inner plate Il is connected tothe jgrid of tube I3 which may be followed by any suitable utilization circuit, not shown. Outer plate I2 is connected to ground, either directly as shown in Fig. 6 or'through a low radio-frequency impedance, not shown.

Thus, 4outer plates I and I2 being at low potential or substantially at ground potential insofar as radio frequencies are concernedfserve as an "effective shield to coils E and I0 whose fields .are already *confined more or less closely by the good magnetic path provi-ded by the projecting edges of flanges 3. Outer plates 'i and I2 `also serve to shield .inner-plates 6 and `II which are at `high radio-frequency potential `with respect to ground. It is apparent that coils -5 `and I0-and their tuning capacitors are thusconnected-by the shortest possible leads, .thereby minimizing .the resistance ofthe transformer circuit and also avoidingn the undesirable extraneous electromagnetic fields that would result from longer leadsVv Vextending outside vof shielding Vplates 'I-and I2.

If impedance coupling is to be Vused-instead-of transformer coupling,l the primary ,portionof the transformer shown in either Fig. l or Fig. 2, with `or :without thevsecondarygportion, may loe-employed asan impedance and utilized as .Shown by way .of .exampleinFig V7. .Capacitor ,I4 connects plate 6 to thevgrid of tube I3 and resistor I5 connects the grid of tube I3 toa point in the circuit that Ais at a suitable D.C. potential and shown as grounded. The impedance of capacitor I- is so low and the resistance -of resistor I5is so high that these lelements have a negligible effect on alternating currents owing in the circuit.

Each of the intermediate-frequency transformers in a radio receiver, for example, may, if desired, be pretuned to the same predetermined frequency before installation in the receiver, or each of the transformers may be tuned to the same frequency after installation. This tuning may be accomplished in a simple manner by removing a portion of the outer plates 'I or I2, which may be formed of a thin layer of sprayed silver or other metal, suiiicient in area to give the necessary capacitance to tune the coil to the desired frequency.

Figure 2 shows an alternative embodiment of the invention in which a transformer is tuned to the desired frequency by adjusting auxiliary cores I5 to vary the inductance of coils 5 and I0. Except foropenings in which auxiliary cores Iii are slidable, cores I8 are the same as cores I of Fig. 1. Since auxiliary cores I6 form only a relatively small part of the cross-sectional area of body portion Il of core I0, their movement with respect to coils 5 or I0 results Yin afcomparatively small variation in inductance so that the adjust- -ment ofcores I is not unduly critical, particu- .feasible to employ lmaterial 'ofhigher permeability in auxiliary cores I6, thereby increasing the 'effectiveness of the magnetic path'ofl core I3 notwithstanding that .-most Vpresently known magnetic materials j havingV higher permeability also have greater Vlossesin aradio-frequency eld Y 5 the wire preferably Ashould be guided'by a fixed guide positioned a substantial distance from core I in order that the turns of coil 5 may assume the compact form shown in Fig. 5, whereeach turn rests in a groove between two previously wound turns, or directly against the core.

It has heretofore been supposed that higher Q of magnetic-core radio-frequency inductance elements would be attained with a plurality of universal-wound sections in order to reduce the distributed capacitance of the coil by separating appreciably the individual turns thereof. Frequently vlitz wire, formed of a plurality of vsmall strands each .insulated from the others,;has been used reduce losses due to skin effect by providing a.. greater .proportion of surface' conductor material (an-inherent geometrical property of small conductors compared .to largev ones) because of a strong tendencycf radio-frequency currents `to .,concentrateat .and near the surface of ayconductor.

These expediente havev been objectionable because specialmachines. are required to wind universal coils and, more important, the coils thus wound have been too bulky formaximum inductance. Litz .wire is .expensive and laboriousto apply` because of the diiiiculty of. cleaning the individual strands before soldering. These .objections have been `overcome and, inaddition, substantially higher Q has been obtained, in accordance with the invention, merely by random winding ordinary enamel-covered wire directly in a slot in a magnetic core. By random winding is meant the winding of wire in a slot with whatever distribution naturally occurs when the wire is guided by a stationary guide a substantial distance from the slot. The circular wire thus fills to the greatest possible extent the interstices between turns, as shown in Fig. 5, and the lines of force surrounding each current-carrying conductor thus interlink the greatest possible number of turns. As is well known to those skilled in the art, this interlinkage of lines of force with conductors is called inductance and lines of force that merely occupy the interstices between turns, without interlinking turns other than the ones that produce them, are largely wasted insofar as inductance is concerned.

It is thus apparent that the inductance of a given quantity of wire is greater if the turns are random wound than if they are universal wound, since in universal winding the wire guide moves back and forth rapidly to cause successive turns to cross over those underneath, thereby causing larger interstices between turns and, therefore, greater waste of lines of force. Furthermore, the average turn length is less with random winding because the turns do not encompass unnecessary Waste space and consequently the resistance of the coil is lowered by the same improvement that increases the inductance. But the increase in inductance is greater than that 'due merely to bringing the turns into closer proximity with each other by reducing the size of the interstices between them. Random winding also brings the turns closer to the magnetic core so that the greatest possible number of lines of force are multiplied by passing through the magnetic material, thereby similarly multiplying the inductance.

What has been said of universal winding applies also to other forms of coils heretofore used for high-Q radio-frequency inductances, such as progressive universal, basket-weave coils, layerwound coils with insulation between layers, or multiplenpie coils in which a plurality of separate coil sections are connected in series to reduce the distributed capacitance. In all of these arrangements, the electromagnetic lines of force do not interlink the greatest possible percentage of the conductors and the maximum number kof lines of force do not pass through the magnetic core, thereby failing to conform to the spirit of the present inventionl If litz wire were used instead of solid (singlestranded) enamel-covered wire, there would be more interstices that would decrease the inductance so that a greater length of litz wire would be required to produce a given inductance and it consequently would have greater total resistance notwithstanding that the resistivity per unit length of litz wire may be less than that of solid wire of the same cross-sectional area. It should be noted, however, that this statement and that of the preceding two paragraphs apply only to magnetic-core radio-frequency inductance elements of the type herein disclosed wherein reducing the interstices between turns increases the inductance not only by bringing the turns into closer proximity with each other, but also by bringing the average turn into closer proximity to a high-permeability magnetic core.

It is recognized that random winding of inductance coils, such as loudspeaker elds, has long been practiced merely as a matter of winding expediency. In accordance with the invention, the

simplicity of random winding is of value, but of far more importance is the increased inductance for a given resistance (higher Q) due not alone to random winding, but to the combination of random winding directly on a particular kind of magnetic core that acts on the greatest possible number of lines of force produced by current flowing through the random winding.

The resistance of the coil is minimized effectively in accordance with the invention by making the conductor shorter as a result of reducing the sizes of the interstices between turns, and by making the conductor more effective by placing it in closer proximity to a magnetic core having flanges extending beyond the coil. From another viewpoint, a certain conductor gives more inductance when random wound directly in contact with the bottom and sides of a slot in the magnetic core of the invention, thereby forming a higher Q inductance element than would result from other forms of winding or from winding over the usual paper or cloth insulation.

Inasmuch as the magnetic field is highly concentrated inside the coil and immediately outside its ends, it is important that there be no air gap in the magnetic core in these regions. Even the very short joint formed by two closely abutting core sections in the central region of the coil seriously will reduce the effectiveness of the core. Since it is not feasible to eliminate all joints and air gaps, it is best to place the air gap in the outer portion of the magnetic path where its area and that of the adjacent core face may be large and hence the effect on the magnetic circuit will be relatively small. To this end, flanges 3 extend substantially beyond coil 5, as previously described, in order that the resultant large area between the iianges outside the coil will form an air gap that is large in cross-sectional area in comparision with its length. All of the lines of force passing between these outer portions of the flanges interlink all of the conductors of coil 5, thereby forming a maximum inductance.

Thus, in a cheap and simple construction, high-permeability magnetic material is provided in the place where it will do the most good, that is, in the region where the lines of force are most dense, and a large-area transition member (flange 3) is interposed between the high-density region and the low-density, largearea air gap thereby forming an effective and efficient magnetic path. A random-Wound coil less closely associated with this magnetic core, or one associated with a less effective core not having flanges extending beyond the coil, would not possess fully the aforesaid advantages and might be less efcient than coils hereto known.

It will be understood that the invention is not limited to the precise form depicted herein as an example, but that it includes modifications thereof within the scope of the following claims:

What I claim is:

1. In a high-Q radio-frequency electrical inductance structure a high permeability magnetic core having a hollow narrow body portion and two spaced anges integral therewith extending outwardly from the body portion, and an elongated-insulated conductor random wound directly on said body portion against and between said flanges to form a coil, said anges extending beyond said coil a substantial distance to form a part of a magnetic circuit enclosing all of the turns of said coil, and an auxiliary high permeability magnetic core slidably mounted said, hollowibody portionqto varyy lthe inductancezof; said inductance element, said auxiliary `core having higher Apermeability than Ysaid body portion, andan electrostatic shieldinductance structure ings' structure for said comprising al pair of concentric spaced capacitor electrodes, the inner one of said concentric capacitor electrodes surrounding and engaging at' least; one of the ,flanges of said inductance structure.

2. In a radio-frequency transformer apparatus: a pair of high-Q electrical inductance vstructures each comprising a magnetic core having aynarrow body portion and two spaced anges ,integral therewith; an elongated-insulated conductor random wound directly on said body portion against and between said ilanges to form a coil, said iianges extending beyondsaid coil a substantial distance to form part of a magnetic circuit enclosing all of the turns of said coil; two pairs of concentrically spaced capacitor electrodes, the inner ones of said concentric capacitor electrodes respectively surrounding and engaging the iianges of each of ysaid pair of inductance structures.

3. In a high-Q Aradio-frequency electrical in- Vductance structure: a magnetic core having a high permeability at radio frequencies, said core including anarrow body portion and two spaced iianges integral' with and extending outwardly fromv the body portion, an elongated single-stranded 'insulated conductor wound directlyon; said body portion'A and between said flanges to. form closely packed turns of aV coil, a `ilrst capacitor elementV surrounding and engaging at least one of said ,anges avdielect-ric sleeve surrounding and engaging said rst capacitor element, and a second capacitor element` surrounding and engaging said dielectric sleeve.

4. The combination as dened by claim 2 in which the capacitor structure includes Va dielectric tube having inner and outer surfaces, and capacitor electrodes on the respective surfaces for cooperating with the intervening portions of the dielectric to provide a capacitor rfor each Ainductance structure.

`5. The combination as defined byV claim 4 in which theouter capacitor electrodes are in the form of sprayed metal coatings, portions of which are readily removable for capacitance .adjusting purposes.

6. The combination as defined by claim 5' in which the bodyportion ofl at least one core` is hollow, .and an auxiliary core is slidably held in said hollow body for-adjusting the corresponding `inductance, said auxiliary core having a Vhigherpermeability anda greaterloss thanthe surrounding and engaging said dielectric tube being respectively in concentric relation with said body portions, and connections for each of said coils connecting the opposite ends thereof respectively to the capacitor elements concentric therewith.

8. The combination as dened by claim 7 in whichV the body portion of at least one of said cores is hollow, and an auxiliary core is slidably positioned within said body portion for adjusting the corresponding body inductance.

ALTON J. TORRE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,167,722 Scott Jan. 11, 1916 1,865,256` Johannesen June 28, 1932 2,141,890 Weis Dec. 27, 1938 2,163,775 Conklin June 27, 1939 2,206,250 FillA July 2, 1940 2,251,631 Mayer Aug. 5, 1941 2,283,924 Harvey May 26, 1942 2,302,893 Roberts Nov. 24, 1942 2,323,376 Harvey July 6, 1943 2,338,134 Sands et al. Jan. 4, 1944 2,403,349 Dolberg July 2, 1946 2,430,990 Moore Nov. 18, 1947 FOREIGN PATENTS Number Country Date 433,366 Great Britain Aug. 13, 1935 435,884 Great Britain Oct. 1, 1935 463,348 Great Britain Mar. 30, 1937 479,880 Great Britain Feb. 14, 1938 OTHER REFERENCES Modern Iron-Cored Coils, pages and 91, The Wireless World, August 4, 1938.

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