US2269784A - High frequency coupling device - Google Patents

Description

Jan. 13, 1942. w. J. POLYDOROFF 2,269,734

HIGH FREQUENCY COUPLING DEVICE I Filed June 19, 1939 Fla/ 5 [Ni ENE? Patented Jan. 13, 1942 vUNITED STATES PATENT OFFICE HIGH FREQUENCY COUPLING DEVICE Wladimir J. Polydorofi, Wilmette, Ill.

Application June 19, 1939, Serial No. 279,893 In Great Britain September 23, 1938 Claims.

The present invention relate to high frequency coupling devices employing inductors having comminuted insulated irondust cores.

In such devices the oscillatory circuits comprise inductors and capacitors adjusted to resohate to a desired frequency, the several oscillatory circuits of a plurality of such circuits being coupled by a common reactance, preferably in the form of mutual inductance. One such coupling device is an intermediate frequency transformer operating at a fixed frequency, used in the superheterodyne receivers. In such a device, adjustment to a desired frequency may be brought about by means of adjustable capacitors, but because of their inherent instability with time and temperature, it has recently been the practice to vary the inductance by means of adjustable magnet cores. Practice shows that such adjustment remains constantin time, and any minute variations, usually positive (with increase of temperature), of induct ance which may occur, can be compensated by the employment of fixed capacitors of certain ceramic composition, having negative characteristics.

One of the objects of the present invention is to provide improved coupling devices which are stable in operation and retain their characteristics during the process of adjustments. Another object is-to provide such coupling devices which are easily accessible for the adjustments in the radio receivers. Still further object is to arrange the components in the device so that it performs substantially alik at different positions of the adjusting means.

The present invention, in one of its forms, is applicable to high quality inductors with a single core member and with a coil of good quality in which: the movement of the core at the frequency of adjustment does not substantially alter the Q value of the inductor; This value Q is defined from the expression:

R=lcL (1) which means that the coil losses decrease more rapidly than the inductance decrease. It is therefore possible to improve the L/R relationship or Q value of a coil, in comparison with that of a corresponding air-core coil, by first reducing the number of turns and then to preserve the thus improved Q by the insertion of a suitable core. It is evident that in such construction the partial withdrawal of the core Will o change the quality of the inductor.

In order to obtain these desirable properties certain relationships, in particular between the coil and core cross-sections, must be closely followed, which in addition allow more complete utilisation of the magnetic material.

This utilisation of the magnetic properties, which will be called permeance and designated as P, is the ratio of the inductance of the coil with the core at the maximum position to the inductance of the air-core coil. If the effective permeability of the certain type core is designated as u' and the cross-sections of the coil and core are respectively designated A and a the following relation exists:

One of the simplest forms of an inductor with adjustable core has an open type cylindrical core,

th length of which is approximately twice thelength of the coil, the coil beingwound with several layers. The apparent permeability of suitable core material for the frequency of 450 kc., is of the order of 8. Practice shows that in order to obtain constancy of Q at that frequency the value of P should be about 2, while the effective permeability of the open core should be counted as u/ 2. With these figures we arrive to the value ofa/A=l/3. In a practical example of I. F. transformer, the coil is Wound with a Litz wire to the inductance of 750 uH on a tubing of 1 cm. diameter and having a square cross-section of 015 cm. The mean diameter of the coil is thus 1.5 cm. and the diameter of the core to satisfy the ratio of cross-sections must be of the order of 0.9 cm'. The wire used is 12/44 Litz.

Very compact, high quality inductance results from the above considerations, still more advantag'e'ous because the losses due to the shielding are diminished.

When two such inductors are associated with suitable value condensers to form resonant circuits', they must be so coupled as to get the desired selectivity with optimum gain, when associated with a thermionic valve. The value of coupling known as critical is often employed such being characterized by a flattening of the top of the combined selectivity curve and giving the maximum gain.

In this case the value of coupling k is determined from:

The formula indicates that the adjustable inductors preserving the value of Q require the coupling to be maintained at the critical value or at a value near to the critical value in order to insure the same selectivity.

In inductors equipped with iron cores, the field density is concentrated near the end of the cores, rather than in the coils, and it i therefore essen tial to maintain a proper distance between the cores, particularly when inductive (magnetic) coupling is solely employed for the convenience of construction. The present invention advocates first to find or adjust the proper distance between the cores and then make th inductance adjustments without changing the distance between the cores.

The invention will be better understood if reference is made to the accompanying drawing in which Fig. 1 shows one form of execution of the invention and the Fig. 2 shows the modification of same.

Referring now to Fig. 1, two oscillatory circuits, each having coils 4 and capacitor 13 form a coupling device enclosed in a shielding container I.

Plate 3 at the bottom of the container carries the coil 4 of self supporting type, wound on the tube 5. A magnet core 6 is arranged to be moved inside the tube 5. which movement changes the inductance of the coil and tunes the circuit to a desired frequency of the adjustment. The core is cemented to a rod 1, having its bottom end 8 threaded into the plate 3, so that the rotations from the top will provide the adjustments. At a certain predetermined distance from the first core another core is cemented to the same rod, so that the movement of both cores is simultaneous and the distance remains the same. Another coil 4a is placed on its own tube 511 in the vicinity of the second core, the tube a being fixed to a sliding plate 9. This second coil is arranged to be displaceable for which purpose a hollow threaded tube I2 is riveted to the plate 9 so that it may freely rotate. Two cylindrical rods l0 act as the guides for the plate 9 and also support the complete assembly, including the upper plate ll, having a threaded hole to engage tube l2. The rotation of tube I2 will cause the coil 4a to slide up and down. The tube l2 may be provided with slots for the adjusting tool.

Adjustment of the transformer to a desired frequency will be effected by first adjusting the anode inductor 4 by the movement of core assembly to the maximum reading. The second coil may be either arranged to remain unaltered in its value, in which case a special adjusting tool will cause the coil to slide together with the core assembly, or the second coil may remain fixed and be detuned considerably off resonance. After the adjustment of the anode circuit is completed, the grid coil is moved up or down the stationed core to produce again a maximum reading.

Fig. 2 shows an arrangement equivalent to the one shown before, in which the complete assembly of the transformer constitutes a more compact and self-centered device. Both cores 6 are again cemented together at a desired distance but they travel in a common tube 5. The tube inductance of the coil 4.

with its bottom coil 4 is cemented to a threaded tube I2, having a shoulder l2a within which another threaded hole engages the common rod with cores, having a threaded portion 8.

The upper coil 40 is placed about the upper core on a tube 5a fixed to a stationary plate 3 or is glued directly on said plate, which may be of insulating material and carries condensers and lead wire terminals. Short rods [0 act as the supporting members and the guides for the sliding plate 9, which is rigidly connected to the adjusting tube l2. Between the plate 9 and an upper assembly plate I l a coil spring 15 is placed, so that the rotation of nut l4 causes the plate 9, which carries bottom coil 4 and both cores to move up and down.

The rotation of rod 1 causes the adjustment of The rotation of nut I 4 causes the displacement of the upper core within the coil 4a, without affecting the adjustments of coil 4. If it is desired to perform the first mentioned adjustment without affecting the upper coil a special combined tool, consisting of a screw driver and a wrench, may be used in such a manner that both screw 8 and the tube l2 are rotated simultaneously, for which reason the screw 8 and the tube l2 are given the same pitch.

The described arrangement and adjustments do not substantially alter the value of k, because the cores have remained at their properly chosen distance. However, the further examination of coupling, from the known formula reveals the fact that an adjustment of the inductance calls for readjustment of mutual inductance, however small this readjustment may be. The usual practice of changing the relative position of cores, when coils are adjusted is entirely unsatisfactory and produces far greater changes, sometimes in the wrong direction, of mutual inductance. In the present invention the greatest part of mutual inductance remains constant and only a small part of mutual inductance between the coils will alter. This small part of mutual inductance may be utilised to correct the coupling so as to increase or decrease M with the changes of denominator /L1L2 by simple arrangement of the relative position of the coils. It is clear that after adjustment of the inductors the relative position of the coils may change. The adjusting movements may be so chosen that, for instance, the withdrawal of the core from the anode coil towards the grid coil will produce a decrease of inductance in the former. Likewise the movement of the grid coil away from the anode coil may also cause a decrease in inductance. In both cases this will be accompanied by a slight decrease of the mutual inductance (coils being further apart when inductance decreases) to satisfy the constancy of coupling in accordance with Equation 4.

On the other hand when the invention is applied to the usual type of inductors, where the withdrawal of the cores causes a decrease of Q, the coupling, in accordance with Formula 3, must be compensated in opposite directions, 1. e. the mtual inductance must be increased in order to get; the same degree of coupling. In this second case the cores and coils are arranged to have their adjusting movements in an opposite sense to that described above.

The above described construction also permits a convenient and speedy method of readjustment of coupling by shortening or lengthening of the tie-rod between the cores, which tie-rod may be in the form of a screw thread.

The present invention is not limited to the construction in which the coils are arranged coaxially, but may also be applied to a transformer of the type in which two coils are arranged on parallel axes as well as to the cores of semi-closed constructions, where outside linkage field exists and changes with the readjustment of the inductors.

What I claim is:

1. A high frequency coupling device including a plurality of inductively coupled resonant circuits, each comprisingv an adjustable inductance coil and a capacitor, ferromagnetic core means in each of said coils, said means forming a structure unitarily movable relative to said coils for frequency adjustments and having its core structure arranged to provide a constant degree of coupling between said circuits, and means for providing further inductance adjustment in one of said coils by displacement of one of said coils relative to the other coil.

2. A high frequency coupling device according to claim 1 in which said ferromagnetic core means and said adjusting means are arranged to move simultaneously.

3. A high frequency coupling device according to claim 1 in which the movement of said ferromagnetic core means is effected by actuation of screw means, and the displacement of one of said coils relative to one of said other coils is effected by actuation of other screw means concentrically disposed relative to said first mentioned screw means.

4. A coupling device for a, given high frequency including a plurality of inductively coupled resonant circuits, each comprising a capacitor and an adjustable inductance coil of a given value of Q of said frequency, ferromagnetic core means in each of said coils, said means forming a structure unitarily movable relative to said coils for frequency adjustment and having said structure arranged to provide a constant degree of coupling between said circuits, the magnetic material and core and coil cross-sectional relationship being suitably chosen for said frequency in order to maintain the same value of Q when the core is displaced in the coil and the means for providing further inductance adjustment in one of said coils by the displacement of one of said coils relative to one of said other coils.

5. A high frequency coupling device according to claim 4 in which the cross-sectional area of said ferromagnetic core means is substantially one-third of the mean cross-sectional area of any one of said coils.

WLADIMIR J. POLYDOROFF.

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