US2753532A - Variable mutual double tuned i. f. transformer - Google Patents

Description

July 3, 1956 R. M. ASHBY ETAL VARIABLE MUTUAL DOUBLE TUNED I.F. TRANSFORMER Filed April 1, 1953 FIG.2

FIG.

OOOOOOQ FIG.4

FIG. 3

INVENTORS ROBERT M. ASHBY EUGENE A. HOLMES III 774% C fink/Ins VARIABLE MUTUAL DOUBLE TUNED I. F.

' TRANSFORMER Robert M. Ashby, Pasadena, and Eugene A. Holmes III, Whittier, Calif.

Application April 1, 1953, Serial No. 346,292

1 Claim. (Cl. 336--75) (Granted under Title 35, U. S. Code (1952), see. 266) This invention relates to improvements in high frequency resonant systems such, for example, as those which are generally used for operation at a particular frequency in radio frequency amplifier circuits. In particular, this invention relates to a double tuned intermediate frequency transformer having a suitable band width for radar and television receivers.

In practice, where intermediate frequency amplifiers are operated in the region of thirty megacycles with a desired band width as high as ten megacycles, it is difiicult to obtain suificient coupling between the primary and secondary windings of a conventional intermediate frequency transformer to achieve the required band width and gain. Therefore, the usual practice has been to substitute an equivalent T or pi network which is critical to design, adjust and align, since any change in a network component inherently affects the required values of the other components.

The general purpose of the present invention is to provide a compact and flexible radio frequency transformer having a large coupling coefficient and an easily made adjustment of center frequency and band width.

Variable inductors having a fixed winding and a movable ferromagetic core for inductance adjustment have been in common use to obtain resonance at a predetermined frequency. Likewise, the idea of using link coupling between two separate coils has been often used to provide an inductively coupled circuit in which the effective mutual inductance is adjustable over a considerable range.

In the present invention, the primary and secondary windings of an intermediate frequency transformer are wound on D-shaped coil forms of plastic material. Tuning is accomplished by adjusting the amount of insertion of Polyiron screws tapped into the coil forms, thereby providing independent tuning adjustments for the primary and secondary windings. The coil forms are mounted by screws to a metallic base which also supports a shield across its center which acts as an electrostatic shield, reducing the capacity coupling between the primary and secondary windings. Coupling between these windings is obtained by means of a metal ring which is movable axially of the coil forms to intercept varying amounts of the leakage flux.

The invention further resides in the features of construction and arrangement hereinafter described and claimed. For an understanding of this invention and illustrations of embodiments thereof, reference is made to the accompanying drawings in which:

Fig. 1 is a perspective view of a preferred embodiment of an interstage transformer constructed in accordance with the present invention;

Fig. 2 shows a cross section in elevation of the transformer of Fig. 1;

Fig. 3 shows a cross section in elevation of another embodiment of the invention; and

Fig. 4 shows a section of the transformer of Fig. 3 taken at line 4-4.

f nited States Patent ice Referring to Fig 1, the primary winding 12 and the secondary winding 13 of an interstage transformer 11 are wound respectively on D-shaped plastic insulating coil forms 14 and 15 which are fastened in a backtoback relationship to a supporting metallic base member 16. Base member 16 also supports a metallic electrostatic shield member 17 across a diameter of the base between the coils, thereby providing a grounded metallic insulation between the two Ds. The outer edges of shield member 17 extend beyond coil forms 14 and 15 and are formed with notches 18 spaced to form a particular screw thread.

Coupling between the primary winding 12 and a secondary winding 13 is made by a link in the form'of ring 21. Referring to Fig. 2, ring 21 is seen to be of composite structure having an outer band 22 of a metal having good conductivity, such as aluminum, and an inner band 23 of suitable high frequency insulating material. The inner surface of band 23 is provided with a screw thread 24 adapted to cooperate with the notches 18 cut in shield member 17. Ring 21 is seen to be adjustable axially of the coil forms 14 and 15 by a screwing motion imparted to the ring to vary the amount of flux intercepted by the ring and, hence, thereby to adjust the amount of inductive coupling between the primary and secondary windings 12 and 13. By utilizing D-shaped coils, the nonlinear portions of which form arcs of a circle that is concentric with the coupling ring, rather than conventional coils of circular shape, higher coeflicients of coupling can be obtained since, with this arrangement, more than half of the leakage flux of coils 12 and 13 can be intercepted by link ring 21. This may be seen from the fact that the arcuated sides of coils 12 and 13 are of greater length than their complementary linear sides and from the fact that each half of ring 21 cooperates with the total length of these arcuated sides.

Each coil form is also provided with a tapped opening 25 into which threaded cores 26 may be inserted. The threaded cores may be made of magnetic material, such as powdered iron, or of nonmagnetic material and may be composed in part of a dielectric material, depending upon the frequency at which the transformer is to operate. A preferred core material is known commercially as Polyiron, which consists of powdered iron held in the desired shape by a plastic binder. Thus, the midband frequency of I. F. transformer 11 can be accurately determined by independently adjusting the amount of insertion of cores 26 and the band spread of this transformer can be precisely regulated by varying the axial position of link coupling ring 21.

Fig. 3 illustrates a simplified version of the transformer of Fig. l in which the metallic base member and the electrostatic shield are eliminated. In this transformer two complementary D-shaped coil forms 32 and 33, similar to coil forms 14 and 15 but having intermediate sections 34 and 35 of reduced cross-sectional area on which the primary and secondary windings 40 and 41 are wound, are held in a back-to-back relationship by means of sleeve member 36. This sleeve may be made of any suitable insulating material and its outer surface may be provided with a screw thread similar to that carried by the inner surface of metallic ring member 37. With this arrangement, ring member 37 may be threaded axially of windings 40 and 41 to intercept varying amounts of flux and to provide different coefficients of coupling. Since sleeve 36 isolates ring member 37 from direct electrical contact with the primary and secondary windings, this ring may be of complete metallic composition. Access to the primary and secondary windings is had via port 38 formed in the lower portion of coil form 32. This transformer assembly may be conveniently mounted, if desired, by means of screw members 39 which cooperate with the screw threads cut in the coil forms 32 and 33 for the slug tuning of the primary and secondarywinding. if desired, the screw threads maybe omitted from 1ements $6 and 37' and the variable coupling obtained by slidahiy displacing the coupling ting along the surface of the cylindrical sleeve member. The transformerof Fig '2 possesses a relatively high coefficient of coup mg and therefore it finds particular application in control cireuits where wide bandass characteristics are desired.

' Obviously many modifications and variations are possible in the lightof the above teachings; Itis therefore to be understood that within the sco e of the ap ended claims the invention may he practioe'd otherwise" than as specifically described.

The invention described herein may be manufactured and used by or for the Government of the United States of America for" overnmental-purposes without the payment of any royatties thereon or therefor.

is claimed is:- r I An intermediate fre uency transformer com rising incombination, a conductive base member; first" and sec: ond' D=shaped coil forms attached to said base member With tliif" planar" SYi'if'Els iii 5; 'jiiiilll fifi'ai'd' relationship and their nonlinear s i'lrfaces lying along the circumference of a circle, an electrostatic shield member having screw threads cut in a pair of opposite edges thereof disposed between said surfaces and secured to said base member, primary and secondary coil windings on said 'coil forms, and a conductive ring circumventing said windings', said conductive ring having an inner band of insulating material bearing a complementary screw thread whereby said conductive ring may be axially moved along said shield member by a threading motion to intercept varyin amounts; of primar and secondary leakage fluxand= whereby the coefiicient of coupling of said transformer may be adjusted.

References Cited in the file of this patent UNITED STATES PATENTS

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