US2390009A - Variable tuning system - Google Patents

Description

Nov. 27, 1945. B, STOTT 2,390,009

VARIABLE TUNING SYSTEM Filed May 25, 1944 INVENTOR I A/Maza frorr.

/ +6 BY g/un/M/ ATTORNEY Patented Nov. 27, 1945 VARIABLE TUNING SYSTEM Haroldv B; Stott, Glen Olden, Pa., assignor to Radio Corporation of America, a corporation of Delaware I Application May 25, 1944, Serial No. 537,277

Claims.

The present invention. relates to variable tunnlng systems for radio signal receiving apparatus and. the like, and more particularly to a variable tuning system for the signal input and oscillator circuits of a superheterodyne receiver.

A primary object of the invention is to provide a compact and simplified tuning system for the above-mentioned circuits in a. single unit, whereby the. system is adapted for use in miniature portable receivers of the personal or pocket type.

A further object of this invention is to provide a unitary, relatively wide range, variable tuning system for the. signalinput andoscillator circuits oil a superheterodyne receiver which includes variable capacity tuning of both the oscillator'circuit and the signal input or antenna circuit with unitary tuning means adapted to occupy minimum space in a receiver.

Another object of this invention is to provide a coaxial arrangement for the variable tuning capacitors in order to provide a compact unitary tuning system for high frequency signal circuits. Astill. further object is to provide a tuning arrangement which employs a pair of coaxial.

relatively movable .cylindrical members, one of which has formed thereon a pair of metallized films constituting respectively one of the electrodesof the variable capacitors which are adapted to tune the signal and Oscillator circuits, the

I an improvement over the coaxial tuner described other cylindrical memberconstituting a common electrode which is connected to ground and forms theother electrodes of said variable capacitors.

A still further object is to provide a coaxial capacitor tuner of the type mentioned in which the metallized film constituting the electrode for the oscillator tuning capacitor is so shaped that there is obtained substantially exact tracking between the tunable circuits throughout the tuning-range-of the receiver.

The novel features characteristic of my invention are-setforthwith particularity in the appended claims. The invention itself, however, both as to its organization and mode of operation together with further objects and advantages thereof. will best be understood by reference to Y Fix. 2: is an end view ofthe movable tuning ele- V I mentxas viewed from the left in Fig. 1; t Fig: Bis-a cross-sectional view ofthe movable tuning, element taken onthe line 3-3 of Fig. "2";

and claimed in U. S. Patent N 0. 2,338,134 to Sands and Daley, assigned to the same assignee as the present application, and differs therefrom in that both the signal and oscillator circuits are capacity tuned so that an ordinary solenoid 0r universal wound oscillator coil may be used. As a consequence, the need of a variable pitch oscillator coil necessary in a permeability tuned system is eliminated, tracking of the signal and oscillator circuits in the present application being accomplished by properly shaping the variable plate electrode of the capacitor fo the oscillator along its length in the direction of movement with respect to the stationary plate electrode. Also,

by eliminating the movable magnetic core which effects the tuning of the oscillator coil, the space normally occupied by said core, that is, the center line area of the coaxial cylinders may be occupied by the drive mechanism which in the present case is constituted by an axially disposed screw member.

Referring to Fig. 1, a frame member I adapted to be supported on the receiver chassis or otherwise is provided with a pair of upright arms 2, 3 which carry respectively a bearing 4 and a bearing bracket 5. A screw shaft 6 is adapted to have its opposite ends J'ournalled fo rotation within said bearings, the shaft being extended at the left to provide for manipulation of the shaft, such as by means of a manual control knob, not shown, or other suitable means.

Concentrically arranged with respect. to the drive shaft and adapted for axial movement is a cylindrical member 1 of insulating material, such as ceramic, glass or Lucite. Formed on the inner surface of the movable member 1, by fusing, electro-deposition or other process, are two metal films or coatings 8 and 9 (Fig. 2) which constitute the movable plates of the capacitors which serve to tune the oscillator and signal circuits, respectively. Flexible leads 8 and 9' serve to connect the movable capacitor plates 8 and 9 to their respective circuit parts of the receiver. The films 'orplates need be only about .002 or .003 inch l8 of fibre or Bakelite.

thick and should extend through an are somewhat lessthan 180 degrees in order to provide proper isolation between the two circuits. The oscillator condenser plate 8 is suitably shaped (Fig. 3), tapering along its length from the leading portion Ill, to permit the oscillator circuit to track with the signal circuit over the tuning range of the receiver. Shaping of the plate is accomplished by masking. With this method of tracking, the series padding condenser normally used to obtain tracking in capacity tuned super heterodyne receivers is eliminated.

The means for imparting axial movement to the cylindrical member I .is a .nut II which is threaded on the screw shaft 6 and which is affixed by suitable means, such as a spider l2, to the cylindrical member at the left end thereof. A part l3 depending from the member 1 and arranged for sliding movement within an elongated slot I4 in the frame I prevents the member I from rotating during its axial travel.

The cylindrical member I'is adapted, in its axial movement from left to right, to encompass va ying amounts ofthe surface area of a stationary cylindrical member I5 which serves as the common plate for the two capacitor metallic films or plates 8 and 9. As shown by the cross-sectional view 'of Fig. 4, the stationary or inner cylindrical member comprises a thin metallic sleeve 1 6 which may be of steel. The edge ofthe sleeve at the left is turned down for receiving a grommet I1 which is carried at one end of an insulating tube The other end of the tube through which the screw shaft is adapted to pass is supported by a second grommet l9 carried by the right-hand arm 3. The grommet i9 is spanned by the bearing bracket 5 and a brass collar 6' attached to the right-hand end of the drive screw is adapted to run free in an aperture 5' ofthe bearing bracket. In addition'to serving as a support for the common stator plate or stationary member IS the tube l8 serves as the coil 26 form o support for a coil which is included in the oscillator portion of the system.

The metal sleeve I6 is connected to frame I (or ground) by means of a lead I6 and is provided on its outer surface with a coating 2| of insulating material, such as enamel, in order to insulate the movable capacitor elements 8 or 9 from the stationary capacitor element 16. Preferably, the coating 2| should be of relatively high dielectric constant to provide suflicient capacity for tuning the circuits through a'desired frequency range.

circuit of a superheterodyne receiver. The converter tube is shown at 25. The signal control grid of the tube is indicated at 26 and is connected to a signal input circuit 21 comprising a loop antenna or inductance 28 and the shunt tuning capacitor constituted by the fixed grounded electrode l6 and the movable electrode 9. The oscillator, grid is indicated at 29 and is connected to an oscillator circuit 30 comprising the coil .26 and the shunt tuning capacitor constituted by the fixed grounded electrode I 6 and the shaped movable electrode 8. The coil 3|, connected to grid 32 which acts as an anodeof the oscillator section of the tube, is coupled to the grid coil 20 and provides feedback for the oscillator, and intermediate frequency signals are .derivedthrough the output anode circuit 33 and a suitably-tuned intermediate frequency coupling transformer 34. As the converter circuit is shown only for the purpose ofillustrating a present preferred-use of the coaxial tuner, further description of the circuit is believed unnecessary. v While I have shown and described a preferred embodiment of my invention, it will be understood that various modifications and changes will occur to those skilled in the art Without departing from the spirit and scope of thisinvention. I therefore contemplate by the appended'claims to cover any such modifications as fall, within the true spirit and scope of my invention;

WhatIclaimis: e l. A tuning system for a radioreceiver of. the superheterodyne type having asignal input circuit and an oscillator circuit, a pair of variable capacitors included one in each of said circuits, a

fixed cylindrical member constituting an .electrode which is common to each of said pair of variable capacitors, a second cylindrical member coaxially arranged and movable with respect ,to

said fixed member, and a pair ofarcuate plates formed on said'mo'vable member, one of said plates constituting the variable electrodeiofthe signal circuit capacitor and the other plate constituting the variable electrode of the oscillator circuit capacitor, said latter plate being shaped Disposed within the metallic sleeve 16 is a cyl- 56 to provide a pred rmi ac g relation b inder 22 of magnetite which is adapted to confine the field about the oscillator coil 26 to thereby improve its Q. Styrol tape or other insulating material indicated at 23 is wrapped'around the magnetite cylinder in order to insulate it from ground, and interposed between the layer of insulating tap 23 and the inner surface of the sleeve I6 is a layer 24 of copper which maybe of the order of .005 inch thick in order to 'keep the oscillator field out of the metal sleeve to thereby further improve the coil Q. In order that the drive screw 6 which is disposed in concentric relation with the coil may not have a deleterious effect on the coil Q it should be of high conductivity metal or plated with such metal.

In Fig. 5 there is disclosed a modified form of coaxial tuner in that the inner or stationary cylindrical member is the same as in Fig. 4 except that the enamelled dielectric coating 2! is y omitted, and the outer cylindrical member I i the other plate constituting the variable electrode cuit and an oscillator circuit, a pair of variable capacitors included one in each of said circuits, a fixed cylindrical member constituting a common electrode of said pair of variable capacitorsa second cylindrical member co-axially arranged and movable with respecttosaid fixed member, a pair of'arcuate platesformed on said movable member, one of said plates constituting the' variable electrode of the signal circuit capacitor'and of the oscillator circuit capacitor, and a screwthreaded shaft for effecting movement of the second cylindrical member with respect to the first member.

4. A tuning system as defined in claim 3 wherein the oscillator circuit includes a coil which is mounted within the fixed cylindrical member, and the inner surface of the fixed cylindrical member is lined with magnetite in order to improve the Q of the oscillator coil.

5. A tuning system as defined in claim 3 wherein the oscillator circuit includes a coil which is wound around an insulating coil form concentrically arranged within the fixed cylindrical member, and the screw shaft is extended through the coil form without seriously affecting the Q of the coil.

HAROLD B. STOTT.

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