US1940228A - Radio-amplifying circuits - Google Patents

Description

Dec. 19, 1933.

W, J. POLYDOROFF RADIO AMPLIFYING CIRCUITS Filed Aug. 26. 1929 3 Sheets-Sheet 2 l I I I I l [III O +/O +20 +30 OFF RESONANCE I: I II :I 'I I II I I K/ZOCJ/CZES OFF RESO/V/V/VCF k I I lllll A TTORNE Y Patented Dec. 19, 1933 PATENT OFFICE 1,940,228 RADIO-AMPLIFYING CIRCUITS Wladimir J. Polydorofl, Chicago, 111., asslgnor to Johnson Laboratories, Inc., Chicago, 111., a corporation of Illinois Application August 26, 1929. Serial No. 388,289 19 Claims. (01. 179-171) The invention relates to the use of iron or other magnetic materials in radio-frequency and other alternating current circuits. More particularly, the present invention contemplates the use of I such materials in tuned radio-frequency amplifying circuits, although the particular new instrumentalities to be described and claimed are not limited in their advantageous application to this particular class of apparatus.

As is well understood in the radio art, a tuned radio-frequency amplifying circuit consists essentially of a relay device such as a thermionic amplifying tube, and an electrically connected resonant circuit consisting of inductance and capacity. This resonant circuit is the portion of the radio-amplifying circuit in which the property of selectivity resides, and it may therefore be referred to as the selective circuit, as distinguished from the complete amplifying circuit including the tube or relay.

Hitherto, many attempts were made to employ iron cores in the transformers used in radio frequency, but owing to great losses introduced by the iron as then employed, efficiency was generally impaired, so that air-core coils and transformers are now exclusively used in the cases where relatively high-frequency oscillations, either modulated or not by voice frequencies, are to be selected and amplified by thermionic tubes and associated circuits.

The invention will be better understood if reference is made to the accompanying drawings where Figure 1 represents a general circuit employing thermionic tubes and embodying the present invention;

Figure 2 is a diagram showing the amplification of a single stage of such a system;

Figure 3 is a diagram showing the resistance variations of difierent transformers or coils;

Figures 4, 5, -6, '7 show various modifications of the present invention;

Figure 8 shows one application of the present invention;

Figure 9 shows the amplification curve of one stage of the circuit, of Figure 8;

Figures 10, 11, 12 and 13 show selectivity curves of amplifiers of various designs, and

Figures 14 and 15 show modifications of the invention.

Figure 16 shows a tuning device including correlated elements.

It is an usual practice to employ several stages of radio-frequency amplifiers in cascade, such as shown on Figure 1, having variable inductances ill REISSUED mac 5 was or capacities, or both, in tuning circuits, so as to cover a certain band of frequencies, say from 1,500

to 550 kilocycles. Such a wide range of frequencies renders those amplifying circuits inefficient and different in operation at different frequencies.

It is the object of the present invention to improve fidelity, selectivity and efiiciency and to secure uniformity of amplification.

Mathematical analysis (Victor G. Smith, Proc.

I. R. E. vol. 15, No. 6, June 1927) shows that amplification of tuned radio-frequency circuits is represented by the ratio of output to input voltages:

n 1 l 2 Vi w/ l Vzancl W 02 I where w-2rrfi providing the circuit is tuned to the resonance, and optimum coupling between primary and secondary circuits is obtained for every given frequency. These formulae prove that, in the case of fixed value of inductance L2 in order to keep the amplification constant, the resistance R2 of the tuned circuit, which is chiefiy composed of the coils resistance, should change proportionately to the square of the frequency.

In the case of a fixed value of mutual inductance, M, the amplification decreases with increase of tuning capacity as represented by the curve a of Figure 2, wherein mutual inductance is adjusted for the high-frequency end of the frequency band. Mutual inductance can be chosen to be at the optimum value somewhere in the middle of the frequency band as represented by the curve b of said figure. In this case, the circuits, being over-coupled at high frequency and under-coupled at low frequency, produce non-uniform amplification. The above formulae also prove that in order to keep the mutual inductance at its optimum value throughout the frequency band, the resistance R2 should be so changed as still to be proportional to the square of thefrequency.

Actual measurements of the resistance of an air coil or transformer, show that the coil changes its resistance with frequency changes, but not as much as would be required to keep the amplification ccnstant and the mutual inductance at optimum. Curve a of Figure 3, shows the re-. sistance for a certain air coil, measured at different frequenciesgwhile curve b represents the resistance required to maintain said resistance proportional to the square of frequency.

One object of this invention is to provide radiofrequency circuits with transformers and coils which will not be subject to the described deficiencies of air coils, to thereby attain even amplification with optimum mutual inductance. It has been found that the use of finely-powdered iron in the field of an air coil, will change the variation of the resistance of the coil relatively to variations in frequency.

A low-resistance air coil was chosen, and its inductance was materially augmented by powdered iron disposed within said coil. The actual measurements of resistance throughout the entire range of frequencies utilized, shown by the curve I) of Figure 3, indicate that the resistance changed approximately nine times while the frequency changed approximately three times. Numerous measurements have verified the fact that the resistance of iron-core coils remains proportional to the square of the frequency, despite frequency changes.

Transformers having powdered iron cores, were connected with thermionic tubes and other elements of the circuit of Figure 1, and it was found that the amplification curve remained substantially even throughout the entire range of frequencies, as represented by the curve '0 of Figure 2. Thus, a close agreement with theoretical considerations was established, that is to say, that when resistance varies as the square of the frequency, the amplification of a tuned circuit remains constant and the mutual inductance adjusts itself to optimum value.

The resistances of transformers having powdered-iron cores, being extremely small at 550 k. c. and the lower frequencies, it is possible to build transformers and coils designed for those low frequencies having an L/R ratio much greater than is practical if ordinary air coils are employed. Furthermore, it is possible to reduce the windings and thereby minimize the size of radio-frequency transformers, thus effectuating a saving of space.

There may be several embodiments of my ironcore coils and transformers, the simplest embodiment being shown in Figure 4 wherein primary and secondary coils 1, 2 are wound on an insulated tube 3, and powdered iron 4 is packed inside the tube 3, and the ends 5 of the tube are sealed.

Figure 5 shows schematically a binocular coil 6 wound on insulated tubes 7, portions 8 of the powdered iron being separated by thin paper discs 9, to prevent readjustment of its particles and consequent packing and slight change in inductance.

Iron cores can also be made by mixing various adhesive and insulating compounds with powdered iron, and giving these cores the desired shape with or without pressure. Also, certain other insulators, such as wax, paraffine and min eral oils, may be used. Substances when melted and mixed with iron particles, while hot manifest very small losses but, when cold and solidified, increase the conductivity of iron cores thousands of times and establish easy paths for eddy currents at high frequencies, resulting in a considerable increase in radio-frequency resistance of the coils equipped with such cores.

However, a coil having either a melted or an unmelted core of wax, without powdered iron, developed substantially no resistance in the coil, while, when powdered iron was mixed with the wax of said core, the resistance in said core was 5 ohms, if the wax was melted, and was 50 ohms, if the wax was solidified. Also, when the core comprised the same mass and quality of powdered iron the resistance of the coil was 5 ohms.

Other insulators preferably of elastic nature,

such as rubber, natural and synthetic gums and certain varnishes mixed with iron and pressed together, will maintain insulating films between the iron particles and, therefore, reduce resultant radio-frequency losses.

The definition iron used in the specification should be applied to any other metal or alloy having magnetic properties, such as silicon-iron, permalloy and nickel-iron. Various powdered metals were tested for radio-frequency transformers and inductances, and it was found that for the most satisfactory results iron reduced by hydrogen, the particles of which had been sifted through a screen of 300 meshes to the inch, should be used for frequencies between 1500 to 1000 k. 0., but" that powders containing coarser particles may be used for frequencies below 1000 k. c., and also that the fineness and the force of compression govern the radio-frequency resistance of the coil. Iron produced by hydrogen in the ordinary way contains particles of various sizes some of which may be too large for use in the production of cores suitable for use in radio frequency circuits. It is, therefore, necessary to eliminate the largest particles. The insulating coat of oxide usually present on the surfaces of iron particles, helps to reduce eddy-current losses. When silicon iron powder is used it is practical to chemically treat the powder with a phosphoric acid solution which creates an insulating film.

When, in this specification, powdered iron is referred to, I mean either incoherent masses of finely-divided iron, or masses of finely-divided iron compressed into bodies in which the individual particles are held together by an insulating 110 binder, but the degree of compression should not be so great as to cause the particles of powdered iron to touch each other and thus exclude the insulating material which should separate them.

To obtain maximum gain in inductance, thus due to the iron cores, the length of a coil should be preferably twice its diameter, and the wire should be space-wound, such arrangement being shown in Figure 6, wherein 10 indicates a spacewound wire, and 11 indicates the powdered iron core. The inductance may be further augmented if the iron core extends around the outside of the coil in the form of cylinder 12. Such arrangement of iron completely closes the magnetic lines around the coil, forming an astatic coil. 125 When iron cores are employed in the coils of the closed or semi-closed magnetic field type, the original astatic properties of such coils are great- 1y enhanced. While the powdered-iron core substantially doubles the individual primary and 130 secondary inductances, it increases the mutual inductance between the windings four or five times.

This phenomenon is especially advantageous when a very tight coupling is required, or when 135 long solenoids are employed for transformation.

Another object of the present invention is to tune a radio-frequency circuit in a new, simple and efficient manner, a movable powdered iron core disposed in the field of a coil being employed 140 for tuning purposes. Depending on the amount of the iron powder inserted in the form of a core, the self-inductance of a coil may be increased four and even six times with a resulting increase 14 in radio-frequency resistance. 5

Figure 7 shows a binocular transformer 13, having a core 14 which can be moved in and out to obtain variations of self-inductance and mutual inductance. This device is capable of tuning the 150 circuit to .a desired frequency when connected with tubes and associated circuits such as shown in Figure 1, or such as are shown in Figure 8 wherein a radio-frequency choke amplifier is represented. In this figure are shown thermionic tubes, 15 and 16, acting as radio-frequency ampliflers, a detector tube 17, choke coils 18 having powdered-iron movable cores 19, suitable resistances 20, coupling condensers 21, a telephonic re-' ceiver 22, and a plate battery 23, filament-heating means being omitted for simplicity. Inthis system it is possible to work the amplifier at its full efiiciency throughout a given range of frequencies by moving iron cores inward or outward. The amplification per stage is shown by curve a of Figure 9. Core movements can be made simultaneous with the movements of the input selector, which selector is usual to secure the necessary selectivity for wave lengths of different broadcasting stations.

When the selecting of a signal accompanies amplification, as in the cascade radio-frequency amplifier shown in Figure 1, the ability to select is called selectivity of tuned circuits. In an ordinary air-core transformer associated with a thermionicamplifier, selectivity varies with frequency. Actual measurements, made for a circuit having L-180 microhenrys variable capacity from 10 to 500 micro-microfarads and total resistance of the circuit varying from 7 to 4 ohms, are shown in Figure 10, wherein, the amplification is plotted against kilocycles off of resonance. Curve a is taken at a frequency of 1400 k. 0., curve b at 1000 k. c., and curve 0 at 600 k. 0. Vertical lines d and g are drawn at 10 k. 0. off of resonance and represent interfering adjacent stations. Curve a shows inadequate selectivity due to high resistance of the circuit and to lack of capacity. As frequency decreases, .the resistance decreases the result being sharpening of the curves b and 0 representing selectivity. By decreasing the inductance and the losses in the circuit and increasing the initial amount of tuning capacity, it is possible to obtain sharper curves as shown in Figure 11. These three curves a, b and 0, represent a very satisfactory selectivity, the amplitudes, as shown, being different because of fixed mutual inductance. The curves of Figure 12 represent the selectivity obtained from a transformer, equippedwith a fixed powdered-iron core as hereinbefore described.

As the radio frequency signal is usually modulated by voice frequencies, three frequencies is, fo-fv, and fo+fv, Where in is carrier frequency, ,fv is voice frequency, have to be passed through a selective circuit with substantially equal intensities in order to avoid distortion. To secure intelligible audio signals, it is usual to modulate carrier frequency with voice frequencies ranging from 0 to 5000 cycles, and, therefore, selective circuits should be capable of passing a band of frequencies 10 kilocycles wide, to obtain the fidelity of reproduction. Two vertical dotted lines it and k in Figures 10, 11 and 12, represent the limits of audio-frequency modulations to be passed through each selective circuit. Referring back to Figure 10, one can easily perceive that curve a shows almost perfect fidelity, curve b shows slightly distorted fidelity, the extreme side bands being attenuated about 15% as compared with carrier frequency, and curve 0 shows attenuation of its side bands and, therefore, introduces distortion. Curve (1. of Figure 11 shows good fidelity, but curves b and 0 show distortion.

Comparison of these curves with curves obtained by the use of powdered-iron core transformers, shows that all of the curves of Figure 12 have very good fidelity, and poor selectivity. It is therefore essential for good selectivity and ndelity, throughout the entire frequency range, to combine the selectivity of a low-loss high-capacity circuit at high frequency with the selectivity and fidelity obtained by powdered-iron cores at low frequency.

An amplifier was constructed with low-loss, low-inductance coils and a movable powdered-iron core and included in the circuit shown by Figure 1. Tuned to high-frequency signals, such as 1500-1200 k. c., said core was entirely withdrawn and the amplifier then developed the characteristics of the curve a of Figure 11. From any high-frequency between 1500 and 1200 k. c., to 550 k. c., said core was gradually moved inside the coil, and, due to increases of self and mutual inductances and of radio-frequency resistance; the curves became broad as shown by curves b and c of Figure 12. The resultant group of curves, taken at three different frequencies, is separately represented in Figure 13 and shows substantially the same selectivity and fidelity throughout the entire range of frequencies.

As it is customary to express the selective properties in terms of band widths in kilocycles at half amplitude, I have chosen to call this quantity the selectance and have used this word in this sense in the appended claims. As determined from Figure 13, the selectance is of the order of 30 k. c., slightly varying at three investigated frequencies. Theoretical analysis of a resonant circuit indicates that selectance expressed in band width is directly proportional to Where R and L are, respectively, resistance and inductance of the circuit, selectance being somewhat impaired by the effects of the plate electrode of an associated thermionic amplifier,should an amplifying stage be employed. The fact that the selectances are substantially equal at three investigated frequencies, indicates that age of said amplifier being fed to a succeeding thermionic. amplifier through a condenser 28 and a grid resistance 27. Figure 15 shows essentially the same circuit 24 at 29, with the exception that output voltage is fed through an additional winding 30 unitarily coupled, to the inductance of the tuned circuit 29.

The amplification may be theoretically expressed as where a is the amplification factor and R1 the plate resistance of the thermionic amplifier, and

R0 the dynamic resistance of the circuit 24, in resonance equal to of the circuit. As the inductance L is varied for tuning, the resistance R of the circuit, resulting from the coil resistance and iron core losses. should proportionately vary so as to maintain with the resultant R0 and constant.

In case of simultaneous variation of capacity and inductance, the amplification of such a circuit can be represented by the above formulae modified as follows:

is dynamic resistance of circuit in resonance. In addition to the change of value of L2 and C2, B2 is also varied by the movement of the iron core. By properly designing the low-loss coils, and properly choosing the quantity, fineness of and pressure on the iron, as well as adjusting the movement of the iron core in conjunction with other tuning elements, amplification may be kept constant.

Using loose coupling between the primary and secondary circuits, it was found favorable to vary the mutual inductance to a very large extent, so that at high frequencies the primary and secondary circuits were under-coupled while at low frequencies they were over-coupled, these variations being produced automatically by the said movements of the iron core.

When movable iron cores are used for tuning purposes in conjunction with other variable devices, such as variable condensers, variable inductances, variometers and the like, the movements of iron cores should be correlated with the movements of said variable devices. It is preferable to so adjust the cores that, at the higher frequencies, the iron is kept away from the coils. As the frequency decreases, the iron cores should be gradually inserted into the coils, with a constant or an accelerated speed, depending on the design of other tuning devices employed.

Figure 16 shows a selective device embodying a movable iron-core which can be used in connection with amplifying circuits, such as shown in Figures 1, l4 and 15. A variable condenser 31 having stationary plates 32 and rotary plates 33, carries a grooved cam 34 firmly connected with the shaft 35. A lever 36, engaging the cam groove, actuates a vertical rod 37 to which a powderediron core 38 is fixed. A transformer 39, which may be an impedance coil, telescopically receives said core 38. When the condenser plates are rotated for tuning purposes, the cam 34, its lever 36 and the vertical rod 37 are set in motion, so that the core 38 travels inward and outward relatively to the coil 39. When the condenser plates 33, are out,'-the core 38 is all the way out, which position corresponds to a higher-frequency limit of the circuit. When the plates 33 are in,

" circuit has selective properties substantially the the core 38 is in the coil and this position corresponds to a lower-frequency limit. The movements of the core in relation to the rotation of the condenser plates is governed by the curvature of the cam 34. It is possible to move the core 38 proportionally to the angular movement of the rotary plates, in which case the condenser plates may be so shaped as to give the desired capacity variations. However, it is preferable to employ semi-circular plates producing straightline capacity variations, and to so choose the curvature of the cam 34 that the speed of the core will be accelerated relatively to that of the rotary plates 33 of the condenser. This train of elements admits of a slow movement of the core 38 when said plates are having their initial angular motion, but causes a gradual acceleration of the speed of the core 38 which reaches its maximum when the plates 32 and 33 coincide. Such combination of semi-circular plates with a progressively moving core, eifectuates the production of equally wide frequency channels throughout the entire range of the selector.

Having thus described my invention, what I claim is:

1. A radio-frequency amplifying circuit including a relay device and a selective circuit electrically connected to the output terminals of said relay device and having an inductance coil, an external capacity across said coil, and acompressed magnetic body disposed in the field of said coil, said body having insulated magnetic particles and being of such characteristic that the amplifying circuit has selective properties substantially the same as when said magnetic body is wlthdrawn.

2. A radio-frequency amplifying circuit including a relay device and a selective circuit electrically connected to the output terminals of said relay device and having an inductance coil, an external capacity across said coil, and a compressed magnetic body disposed in the field of said coil, said body having insulated magnetic particles of such fineness that said amplifying same as when said magnetic body is withdrawn. 3. A radio-frequency amplifying circuit including a relay device and a selective circuit electrically connected to the output terminals of said relay device and having an inductance coil, an external capacity across said coil, and a compressed magnetic body disposed in the field of said coil, said body being of 'a closedmagnetic type and having insulated magnetic particles to maintain the selectance of said amplifying circuit at the 13C desired value.

4. A radio-frequency amplifier including a relay device and a selective .circuit electrically connected to the output terminals of said relay device and having an inductance coil, an external 13! capacity across said coil, and a magnetic body disposed in the field of said coil, said body being compressed and having particles insulated by an elastic substance and being movable relatively to said coil to vary the period of said selective circuit l4( while retaining the desired amplification.

5. A radio-frequency amplifier including a relay device and a selective circuit electrically connected to the output terminals of said relay device, said selective circuit having an inductance 14- terminals of said relay device, said selective cir-' cuit having an inductance coil, an'external capacity and a compressed magnetic body disposed in the field of said coil, said body having insulated magnetic particles and being movable relatively to said coil to vary the period of said selective cir cuit while retaining the desired amplification.

7. A radio-frequency amplifying circuit including a relay device and a selective circuit electrically connected to the output terminals of said relay device, said selective circuit having an inductance coil, an external capacity and a compressed magnetic body disposed in the field of said coil, said body having insulated magnetic particles and being movable relatively to said coil to vary the period of said selective circuit while maintaining the selectance of said amplifying circuit at the desired value.

8. A radio-frequency amplifying circuit including a relay device and a selective circuit electrically connected to the output terminals of said relay device, said selective circuit having an inductance coil, an external capacity and a compressed magnetic body disposed in the field of said coil,

said body having insulated magnetic particles and being movable relatively to said coil to vary the inductance and the radio-frequency resistance of said selective circuit simultaneously and in the same proportion so as to produce substan-' tially uniform amplification and selectance in said amplifying circuit throughout the range of adjustability.

9. A radio-frequency amplifying circuit including a relay device and a selective circuit electrically connected to the output terminals of said relay device, said selective circuit having an inductance coil, an external capacity and a compressed magnetic body disposed in the field of said coil, said body having insulated magnetic particles and being movable relatively to said coil to vary the inductance and the radio-frequency resistance of said selective circuit simultaneously and in the same proportion so as to produce substantially uniform selectance in said amplifying circuit of the order of 30 kilocycles width at half amplitude throughout a frequency range from 550 to 1500 kilocycles.

10. A radio-frequency amplifier including a relay device and an inductively coupled selective circuit, having an inductance coil, a variable capacity and a compressed magnetic body movable in the field of said coil, said body having insulated magnetic particlesthe movement of said body being correlated with the adjustment of said variable capacity so as to produce substantially uniform'amplification throughout the range of adinstability of said amplifier.

11. A radio-frequency amplifier including a relay device and an inductively coupled selective circuit, having an inductance coil, a variable capacity and a compressed magnetic body movable in the field of said coil, said body having insulated magnetic particles, the movement of said body being correlated with the adjustment of said v riable capacity so as to produce substantial y constant effective dynamic resistance between the 7 output terminals of said relay device.

12. A radio-frequency amplifier having a relay device, a selective circuit including the secondary coil of a transformer, an external capacity and a compressed magnetic body movable in the field of said coil, said body having insulated magnetic particles, a primary winding of said transformer being connected to the output terminals of said relay device, the movement of, said body simultaneously varying the secondary inductance and the mutual inductance. of said transformer to tune said amplifier while maintaining the desired amplification.

13. A radio-frequency amplifier having a relay device, a selective circuit including a secondary coil of a transformer, a variable capacity and a compressed magnetic body movable in the field of said coil, said body having insulated magnetic particles, 2. primary winding of said transformer being connected to the output terminals of said relay device, the movement of said body being can-elated with the adjustment of said capacity and simultaneously varying the secondary inductance and the mutual inductance of said transformer to produce desired amplification in said amplifier.

'14. A system including a plurality of radio-frequency amplifying circuits, having selective input and interstage circuits arranged in cascade, said selective circuits eachhaving an inductance coil, an external capacity and a compressed magnetic body disposed in the field of said coil, said body having insulated magnetic particles and being movable relatively to said coil for tuning said system while maintaining the selective properties of said system at the desired values.

15, A radio-frequency amplifying circuit including a relay device and a selective circuit electrically connected to the output terminals of said relay device and having a low-loss inductance coil, a capacity across said coil, and a compressed magnetic body disposed in the field of said coil, said body having insulated magnetic particles and being of such characteristic that said amplifying circuit maintains its selective properties substantially the same as when said magnetic body is withdrawn.

16. A radio-frequency amplifier including a relay device and a selective circuit electrically connected to the output terminals of said relay device, said selective circuit having a low-loss inductance coil, a capacity across said coil, and a compressed magnetic body disposed in the field of said coil, said body having insulated magnetic particles of size small enough to pass through a screen of 300 meshes per inch and being movable relatively to said coil to vary the period of said selective circuit while retaining the desired amplification.

1'7. A system including a plurality of radio-frequency amplifying circuits, having selective input and interstage circuits arranged in cascade, said selective circuits each having an inductance coil, an external capacity and a compressed magnetic body disposed in the field of said coil, said body having insulated magnetic particles and being movable relatively to said coil for tuning said system while preserving substantially uniform amplifying properties in said system.

;/18. A system including a plurality of radiof ziequency amplifying circuits, having selective i put and interstage circuits arranged in casade, said selective circuits each having an inductance coil, an external capacity and a compressed magnetic body disposed in the field of a whereby the variation of the capacity and the movement of the core vary the period of the selective circuit while maintaining the amplification and selectance at the desired value.

WLADIMIR J. POLYDOROFF.

Images