US2406720A - Inductance device - Google Patents

Description

Aug. 27, 1-946. c, JAN VAN LQON 2,406,720

INDUCTANCE DEVICE Filed March 1'7, 1943 Fly.

H 4 r 32 2% 3 "22m g. #4 BY Tatented Aug. 27, 19 46 INDUCTANCE DEVICE Carol Jan van Loon, Eindhoven, Netherlands, as-

signor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application March 1'7, 1943, Serial No. 479,465 In the Netherlands March 8, 1941 1 Claim. 1

The invention relates to self-induction coils whose self-induction may be adjusted by a core of magnetic material which is slidable in the field of the coil, said coils being referred to hereinafter as slidable-core coils. These slidablecore coils are utilized inter alia in radio-receiving sets in which they may form part of the tuning means.

In practice it is often desirable that the value of the self-induction of the slidable-core coil as a function of the position of the iron core should have a predetermined course. Such is the case inter alia in superheterodyne receivers if in the tuning circuits use is made of slidable-core coils. As is well-known, with a superheterodyne receiver there must exist over the whole of the wave range a constant difference in frequency between the tuning of the oscillator circuit and that of the preceding circuits so that, given a determined course of the tuning of the preceding circuit, a determined course of the tuning of the oscillator circuit is fixed.

If the tuning is effected by means of a fixed inductance and a variable condenser, a substantially constant difference in frequency may be obtained by connecting a capacity in series with and a capacity in parallel with the variable condenser. These additional capacities should preferably be formed as trimmers in order to ensure that adjustment is possible.

If, however, the tuning is effected by means of a variable inductance and a fixed capacity, as is the case with the use of slidable-core coils, it would be necessary to try in a similar manner to obtain the desired result by connecting an additional inductance in series with and an additional inductance in parallel with the variable inductance. This leads, however, to an expensive and complicated system of connections.

If these additional inductances are omitted, it is possible to obtain the correct difference in frequency between the oscillator circuit and the preceding circuits only at two points of the wave range, for example in the neighborhood of the highest and of the lowest frequencies; in the intermediate region there is produced in this case, however, an inadmissibly large divergence from the desired difference in wave-length, which divergence may amount in the middle of the intermediate wave-length range up to 60 or 70 kilocycles.

It is, therefore, the main object of the present invention to improve the tracking relation between the signal frequency and oscillator circuits of a superheterodyne receiver so that the correct frequency difference will be obtained not only at the limits of the respective frequency ranges of said circuits but also at one or more intermediate points.

In the drawing:

Figs. 1 and 2 are curves showing conditions of tracking and mistracking between a pair of permeability tuned circuits at various positions of adjustment of the tuning cores. Fig. 3 discloses a known construction for effecting inductance variation in coils. Fig. 4 discloses a coil and core combination, in accordance with the present invention for effecting a predetermined inductance variation. Figs. 4a and 4b are cross-sectional views taken respectively, on the lines a a and b-b of Fig. 4 and Fig. 5 discloses curves showing the improved tracking relation obtained with the present invention.

Figure 1 shows the course of the divergence d in kilocycles as a function of the stroke S in millimeters of the slidable core in the absence of precautionary measures for effecting tracking at an intermediate point.

In Figure 2 the curve I indicates the desired course of the self-induction L of the coil as a function of the stroke of the core in millimeters whereas the curve 11 indicates the course which really occurs if no particular steps are taken. In order to obtain the desired course it is necessary, as ensues from Figure 2, to increase the slope of the self-induction variation at the beginning of the stroke and to decrease this slope at the end of the stroke.

According to British patent specification 410,217 it is known to increase the slope of the variation of the self-induction of a slidable core by causing a core of copper and a high-frequency iron core to slide, the one after the other, through the coil. A construction suitable thereto is shown in Figure 3.

In this figure l represents a coil, 2 is a core of high-frequency iron and 3 is a rod or tube of conductive material, for example, copper. Upon assuming that the initial position is that in which the whole of the core 3 is within the coil and the whole of the core .2 is outside the coil, the selfinduction of the coil increases, when both cores are caused to slide from the right to the left, to a higher extent than would be the case with the use of a slidable core since the iron core exerts its action in a coil portion which steadily increases and, in addition, the copper core 3, which acts as a short-circuited winding, encloses a steadily decreasing number of lines of force. Since, however, by utilizing this construction it aicavao is possible to increase the slope of the increase of the self-induction but impossible to decrease this slope, the use of this form of construction cannot give here the desired result.

According to the present invention, in order to obtain with slidable-core coils a predetermined course of the self-induction as a function of the stroke of the core there is provided a body of conductive material which is caused to slide, jointly with the core, into the field of the coil, the shape of the body of conductive material being so chosen that the desired course of the self-induction is obtained.

In one form of construction according to the invention the body of conductive material is preferably provided on the core itself or, if the core is made of pressed material, it is pressed jointly with the core to form a mechanic unit.

The invention will be explained more fully with reference to Figures l and 5 of the accompanying drawing.

Figure 4 shows a slidable-core coil of cylindrical shape wherein the coil body is denoted by I, the turns of the coil by 2 and the slidable core by In Figure 5 the course of the self-induction L as a function of the stroke of the slidable core is shown by curve I.

If for some reason or other it is desired that the curve indicating the course of the self-induction should have a more horizontal course at the beginnin and at the end but a steeper course in the middle, this may be achieved, in accordance with the invention, by providing on the core 3 a body or insert 4 of conductive material which has a determined shape. The conductive material may be for example, copper. The shape of this body or insert may be determined experimentally and may have in a determined case the shape indicated in Figure 4 which diminishes in cross-section from each end inwardly toward the center.

Otherwise, in constructing such a slidable-core coil the fact has to be taken into account that the presence of the copper body 4 necessitates an increase of the number of turns or again an increase of the permeability of the slidable core in order to be able to obtain the same value of the maximum self-induction. Care should be taken in this case to ensure that the minimum value of the self-induction does not increase, which may be effected by increasing the permeability of the core. In the absence of the copper strip according to invention the course of the self-induction curve would become steeper over the whole of the range; by providing the copper body the course of the curve on the desired portions is restored to the desired slope.

What I claim is:

A. variable inductance device comprising a coil of the solenoid type, a core movable axially within th coil and adapted upon movement from a position outside the coil to a position fully contained within the coil to vary the inductance of the coil in a predetermined manner as a function of the coretravel, said core consisting of a substantially cylindrical body of magnetic material and an insert of non-magnetic conductive material embedded in the surface of said body, the surface of the insert being coextensive with that the body, and said insert diminishing in cross-section from each end inwardly toward the center.

CAREL JAN VAN LOON.

Images