US2248470A - Device for varying the damping of a tuned oscillatory circuit without affecting the tuning - Google Patents

Description

uly 8, 19 1- 'r. A. sPooR ETAL 2,248,470

DEVICE FOR VARYING THE DAMPING OF A TUNED QSCILLATORY CIRCUIT WITHOUT AFFECTING THE TUNING Filed April 27, 1938 INVENTORS' T. A SPOOR H- RIN/A BY EA. V Y REN ATTbRNEY Patented July 8, 1941 DEVICE FOR VARYING THE DAMPING OF A TUNED OSCILLATORY CIRCUIT WITHOUT AFFECTING THE TUNING Theodorus Antonius Spoor, Herre Rinia, and

Ewoud Adriaan van Yzeren,

Eindhoven,

Netherlands, assignors, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application April 27, 1938, Serial No. 204,596 In Germany June 7, 1937 6 Claims.

This invention relates to a device for varying the damping of a tuned oscillatory circuit without the tuning being at the same time afi'ected by this damping variation.

With band-pass filters, whose band width is controlled by varying the mutual coupling of the band-pass filter circuits the drawback often occurs that with a great width of the frequency band transmitted a very deep dale ensues in the middle of the resonance curve, due to which the lowest modulation frequencies are weakened with respect to the higher frequencies so that the reproduction is distorted. This drawback may be obviated by increasing the damping of the bandpass filter circuits with increase in mutual coupling of these circuits. However, this damping increase of the band-pass filter circuits must take place without varying the tuning of the circuits, since such variation would involve a new distortion of the reproduction or would make it necessary to tune the receiver anew after any variation of the band-width.

With a well known solution of this problem variable resistances are inserted in the tuned oscillatorycircuits of which the band-pass filter consists, the value of said resistance being altered in accordance with the width of the transmitted frequency band by mechanically coupling the ad- J'usting member, by means of which the bandwidth is adjusted, to the adjusting member of the variable resistances. However, this solution has various drawbacks, the device being expensive and bulky, whilst upon adjusting the variable resistances undesirable crackling cannot be avoided.

The present invention provides another solution of the problem of varying the damping of a tuned oscillatory circuit without at the same time affecting the tuning and whilst avoiding the drawbacks concomitant with the known solution referred to above.-

According to the invention the damping of a tuned oscillatory circuit is varied by shifting and/or turning the inductance coil of the oscillatory circuit relatively to a damping circuit whose resistance is large with respect to the inductance and which is inductively coupled to the coil. By a damping circuit is to be understood in this case any conductive body in which currents causing damping of the oscillatory circuit can be induced by the field of the inductance coil of the oscillatory circuit.

The damping circuit may consist, for instance of a coil which is coupled to the inductance coil of the oscillatory circuit, and is wound from resistance wire, whilst its ends are directly interconnected. This damping coil is either rotatably or movably arranged relatively to the inductance coupling between the damping circuit and the oscillatory circuit.

In another form of construction the inductance coil of the oscillatory circuit is movably arranged on a massive core partly consisting of a material having a high specific resistance (for instance,

iron) and partly of a material having a smaller specific resistance (such as copper), the two parts of the core having the same diameter. If the inductance coil is arranged on the core portion consisting of material having a high specific resistance, great losses will occur in the core so that a material damping of the oscillatory circuit will be obtained. If, on the contrary, the inductance coil is provided on the core portion consisting of material having a low specific resistance the damping of the oscillatory circuit is small. The small variation of the self induction of the coil caused by the available core is substantially equal for the two core portions, since the two parts have the same diameter, so that the displacement of the inductance coil over the core involves only a very slight variation of the tuning of the oscillatory circuit which may be further reduced, if desired, by giving the two core parts a slightly different diameter. The losses incurred when the inductance coil is provided on the core portion made from material having a low specific resistance (for instance copper) may be further reduced by lodging a ring of copper or similar material in the screening box containing the coil. As an alternative, the bottom of the screening case may be used for this purpose, if desired.

In another very simple construction, whose effect corresponds to that of the above construction, a segment consisting partly of material having a high specific resistance (for instance iron) and partly of a material having a low specific resistance (for instance copper) is arranged in front of the head of the movably arranged inductance coil of the oscillatory circuit, so that upon rotation of the inductance coil the latter is more closely coupled either to the part consisting of material having a high specific resistance or to the part-made from material having a low specific resistance. The same efiect can also be obtained when two vertical plates consisting of materials having a high and a low specific resistance respectively are arranged inside the rotatably arranged inductance coil of the oscillatory circuit.

In the two last-mentioned forms of construction the portion of the damping circuit made from material having a low specific resistance might be dispensed with if the thickness of the part made from material having a high specific resistance be very small, i. e. small relatively to.

the depth over which the field of the inductance coil can penetrate into the material. On this recognition is based another form of construction of the invention which resides in the arrangement of a very thin plate from material having a high specific resistance such as constantan in the vicinity of the movably arranged inductance coil of the oscillatory circuit. Upon rotation of the inductance coil the plate is to a greater or less degree brought into the field of the coil due to which the damping of the oscillatory circuit is varied.

Although the invention may be used in any case where damping variation of an oscillatory circuit without simultaneously affecting the tuning is desirable, the invention is of particular importance for band-pass filters with variable band-width in which the latter is varied by mutual displacement or rotation of the inductance coils of the band-pass filter circuits, since in such band-pass filters the mutual rotation or displacement of the inductance coils may at the same time be used for coupling in any of the above ways and to a greater or less degree at least one of these to a damping circuit. In this manner the drawback referred to in the preamble, viz. that a very deep dale occurs in the middle of the resistance curve, with a great width of the transmitted frequency band, can be obviated in an extremely simple manner.

The invention will be more clearly understood by reference to the accompanying drawing wherein Figs. 1 and 2 disclose in diagrammatic fashion two embodiments thereof.

Fig. 1 shows two coils L1 and L2 forming part of the two oscillatory circuits of a band-pass filter whose bandwidth can be altered by shifting the coil L2 relatively to the coil L1. The two coils are lodged in a screening box B. The movable coil L2 passes over a massive core K consisting partly of iron (Fe) and partly of copper (Cu). With the weakest coupling the coil L2 occupies the extreme right position and this coil is then coupled almost exclusively to the copper part of the core K. In this case the losses induced in the coil are very small and are further reduced, moreover, by a copper ring R lodged in the screening box B. If the coupling is made closer by shifting the coil L2 to the left, this coil is steadily coupled more closely to the iron portion of the core K which involves a material increase of the losses. Instead of iron and copper it is also possible to use other materials, for instance two kinds of iron having different specific resistances. Any slight self induction variations occurring upon shifting the coil L2 may be reduced by giving the two parts of the core a slightly different diameter.

Fig. 2 shows two coils L1 and L2 forming part of the two oscillatory circuits of a band-pass filter, whose band-width can be varied by rotation of the coil L2 about the axis A. A segment S consisting partly of iron (Fe) and partly of copper (Cu) is arranged opposite the coil L2. In the illustrated position of the coil L2 the coupling is at its maximum and the coil L2 is mainly coupled to the iron part of the segment S thus incurring great losses. Upon loosening the coupling by rotation of the coil L2 in a clockwise direction this coil is steadily coupled more closely to the copper part of the segment S due to which the losses decrease. Instead of the segment shown in Fig. 2 it is also possible to use a very thin plate from constantan or similar material which is arranged in such a manner that in the position illustrated it is closely coupled to the coil L2, the coupling upon rotation of the coil L2 decreasing continuously in the manner set out above.

We claim:

1. A device for varying the damping of a tuned oscillatory circuit without substantially affecting the tuning thereof, characterized in that the inductance coil of the oscillatory circuit is movably arranged on a massive core partly consisting of a material having a high specific resistance and partly of a material having a low specific resistance.

2. A device for varying the damping of a tuned oscillatory circuit without substantially affecting the tuning thereof, characterized in that the inductance coil of the oscillatory circuit is rotatably arranged relatively to a segment partly consisting of a material having a high specific resistance and partly of a material having a low specific resistance.

3. A device for varying the damping of a tuned oscillatory circuit without substantially affecting the tuning thereof, comprising a stationary metallic member coaxially arranged with respect to and inductively coupled to the coil of said oscillatory circuit, said member consisting of portions which have different specific resistances, whereby axial movement of the coil to selectively cooperate with one or the other of said different portions of the core varies the damping of the circuit Without substantially affecting its tuning.

4. A device for varying the damping of a tuned oscillatory circuit without substantially affecting the tuning thereof, comprising a stationary core member coaxially arranged with respect to the coil of said oscillatory circuit and upon which said coil is mounted for relative movement, said core consisting of portions which have different specific resistances, whereby adjustment of the coil to selectively cooperate with one or the other of said different portions of the core varies the damping of the circuit without substantially affecting its tuning.

5. A device according to the invention defined in claim 4, wherein the core portions are iron and copper.

6. An adjustable band-pass filter comprising a pair of oscillatory circuits, the coupling therebetween for varying the band width being changed by altering the relative positions of the circuit coils, and means cooperating with one of the coils for varying the damping thereof, said means comprising a member coaxially arranged with respect to said one coil and having portions which have different specific resistances, the arrangement being such that with variation of the coupling to produce close coupling the movable coil encircles the portion having the higher specific resistance and is affected only by said portion causing the circuit to be highly damped, and with variation of the coupling to produce loose cou pling the movable coil encircles the portion having the lower specific resistance and is affected only by said portion causing the circuit to be slightly damped.

TI-IEODORUS ANTONIUS SPOOR. HERRE RINIA. EWOUD ADRIAAN VAN YZEREN.

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