US2250277A - Coupled circuit regenerative receiving system - Google Patents

Description

July 22, Q. W A, SCHAPER 2,250,277

GOUPLED CIRCUIT REGENERATIVE RECEIVING SYSTEM Filed Aug. 2, 1940 2 Sheets-Sheet l awww@ ATTORNEY July 22, 1941. w. A. sczHAF'ER'` COUPLED CIRCUIT REGENERATIVE RECEIVING SYSTEM Filed Aug. 2, 1940 '2 sheets-sheet 2 INVENTOR Mum/w ,4. 6CH/4PM ATTORNEY Patented July 22 1941 naar COUPLED CIRCUIT REGENERATIVE RECEIVING SYSTEM William A. Schaper, Cicero, Ill., assignor to'Jolinson Laboratories, Inc.,

tion of Illinois 9 Claims.

f The utility of a selective resonant high-fre quency system is determined principally by (1) its resonant performance, 4(2) the range of frequencies over which it may be adjusted, (3) the ease with which it may be adjusted to any frequency within its range, (4) the uniformity of its resonant performance over the frequency range, (5) the degree of difficulty encountered in ganging it with similar systems for simultaneous adjustment, and (6) its cost.

The resonant performance of the system determines the increase in any impressed voltage to which the system is adjusted, which is commonlycalled the resonant gain, and it also determines the degree to which the system discriminates against voltages of undesired frequencies, which is commonly called the selectivity. It is apparent that, within limits, the better the resonant performance the greater the utility of the system, provided the improvement is not secured at disproportionate increase in cost.

Although selective resonant systems have been employed in radio apparatus for many years, it has only recently become possible to design systems which have substantially constant performance over the range of adjustability, and at the same time have relatively high eiiiciency. Such a system is the one disclosed by Polydoroif in United States Patent No. Re. 21,282, in which a resonant circuit having an inductance coil and a capacitor is adjusted over a range of frequencies by movement of a compressed comminuted ferromagnetic core relative to the inductance coil. This method of tuning is commonly called permeability tuning. An improved form of such a system is described in my United States Patent No. 2,051,012. Both Polydoroffs original system and my improved system readily cover an adequate range of frequencies and may easily be ganged to provide multiple unit systems.

' With regard to the matter of resonant performance, however, taken in relation to cost, it has been found that in order to secure adequate gain and selectivity, for example in a. broadcast Chicago, Ill., a corpora- Application August 2, 1940, Serial No. 350,052.

receiver, it is necessary to employ several such systems ganged together, andy electrically connected in'the well-known cascade arrangement maintaining accurate alignment when several such systems are employed, such arrangements are not widely used in spite of the great advantage which they possessof uniformity of performance over a range of frequencies.

Further improvement of the systems themselves to secure an increase in gain and selectivity that would materially decrease the number of systems required in a complete receiver has thus far not been possible. in the eiiiciency of the low-loss inductance coils could be secured only at greatly increased cost, not only of the coils themselves but also of the ferromagnetic cores which had to be employed with them.

It is to the solution of the problem above suggested that my present invention is addressed.

. In accordance with my present invention I employ coupled resonant circuits tuned by inductance variation with movable ferromagnetic cores and associated with amplifying vacuum tubes, but I additionally employ means for very greatly increasing the resonant performance of such circuits, so that even though they may be, and preferably are, constructed with relatively inefficient coils, two or at thev most three pairs of such circuits arranged in accordance with my invention provide adequate gain andselectivity, for example in a broadcast receiver.

The means which I employ to increase the resonant performance of my amplifier is, in some respects, broadly similar to sci-called regenerative arrangements to be found in many forms of apparatus in which oscillations are produced. It is necessary to point out, therefore, that the arrangement here described differs in important structural aspects and in the manner of its operation frorn any earlier arrangement which diligent search in the literature of the art has revealed. Additionally it is pointed out that in arrangements in accordance with my invention no oscillations are produced.

The use of negative resistance or regeneration to improve the performance of selective resonant systems was common in the early days of the receiver art, but was abandoned over ten years ago, at least so far as broadcast receivers in the Even' slight improvements the negative resistance or regenerative effect.

The demand for increased simplicity of control, and finally for uni-control, drove the regenerative receivers off the domestic market.

Many attempts were made in the period from 1921 to 1926 to provide a system of regeneration which would be'uniform in action over the range of frequencies. due principally to lthe non-uniform performance of the circuits then employed, with the resultkk that the art turned to completely. non-regenerative-types such as to-so-called Neutrodyne receivers, and'to the superheterodyne method in which the problem of high-frequency selectivity and gain was avoided by transposing all the signais to a fixed lower frequency.

It might be lsupposed that immediately upon the advent of' the uniform-performance permeability-tuned circuit, it.would have vbeen possible to add regeneration-according to any one of a number of well-known methods, to secure greatly improved performance uniformly over It may be shown, however,

the tuning range.

These attempts failed, however,

and many experiments have adequately verified f the fact, that the rgene'..itive effect itself varies materially with frequency, even when applied to j a circuit of inherently uniform performance, and my present invention is an arrangement in which the highly desirable and advantages result of substantially uniform performance in a regenerative system is secired.

In my co-pending application, Serial Number 339,697, filed June 10, 1940, I describe a system employing unitary rather than coupled resonant circuits, and securing a regenerative vor negative resistance effect through the employment of a vacuum tube whose only function is tuning by inductance variation with a movable ferromagnetic core, however, the inductance-toresistance ratio may be maintained substantially constant, since insertion of the core provides a simultaneous increase in the inductance 'and the resistance of the resonant circuit as the frequency decreases.

In my United States Patent No. 2,106,226, I

, disclose means for coupling two permeabilitytuned resonant circuits, each of which has a substant'ially constant ratio of inductance to highfrequency resistance throughout the frequency range, in such a way as to provide substantially uniform over-all amplification and selectivity. The system described in that patent is intended to operate without an appreciable degree o f regeneration. In accordance with my present invention, however, I employ a pair of permeability-tuned resonant circuits .which are so designed and so coupled as to provide substantially luniform performance even though regeneration is utilized to greatly improve the performance of the system.

Thus my invention provides, for the first time, means for realizing the improvement in gain and selectivity obtainable by utilizing regeneration without requiring additional vacuum tubes or manual controls, and without sacrificing the uniformity of performance which is readily secured by the use of movable ferromagnetic cores for tuning. While the electrical circuit which I employ is supercially similar to earlier varrangements, it is pointed out that only by employing components which are designed in the mannerto be disclosed herein is it possible to realize the y greatly improved performance which my system provides. As will be clear from what is to folto produce this effect in a controlled manner.

In arrangements according to my present invention, on the other hand,"I employ coupled rather than unitary resonant circuits, and I do not employ vacuum tubes solely to produce the regenerative eifect, each of the vacuum tubes in the present system acting additionally as a highfrequency amplifier in the customary manner.

In practicing my present invention I employ one or more systems or stages each including an amplifying vacuum tube, a pair of coupled resonant circuits tuned by inductance variation with a ferromagnetic core movable relatively to the inductance coil in each circuit, means for producing a decrease in the effective resistance of the resonant circuit, and an arrangement for utilizing this means in such a way that its resistance-reducing effect varies in a desired manner with the frequency to which the coupled resonant circuits are tuned. I thus secure a degree of gain and selectivity over' the range of adjustabilitywhich is much greater and more uniform than could be secured by the employment of the resonant circuits alone.

If a resonant circuit is -tnned by capacitance variation, the inductance remains fixed and the resistance increases with frequency. Thus the ratio of inductance to resistance varies widely over the frequency range, the circuit becoming increasingly poor at the higher frequencies. By

which do not, in themselves, provide substantially uniform selectivity over the frequency range, without sacrifice of the highly desirable uniformity of performance which is a feature of the invention.

It is an object of my invention, therefore, to provide a high-frequency amplifying system having a high degree of gain and selectivity which is maintained substantially uniform over a wide range of frequencies.

A further object vide simple and inexpensive means for improving the selectivity and gain of a high-frequency amplifying system while at the same time maintaining its performance substantially uniform over a wide range of frequencies.

Still another object .of my invention is to improve the over-all resonant gain and selectivity of a pair ofy inductively 'tuned resonant circuits by a substantially uniform degree throughout a wide range of frequencies.

The attainment of these and other highlyl de- A out more fully in the specification which follows.

The invention will now-be described by reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of a tuned radiofrequency amplifying stage in accordance with the invention;l

Fig, 2 is 'a graph showing the variation with frequency of some of the parameters of an embodiment of the,v arrangement shown in Fig. 1; and

Fig. 3 is a schematic diagram of acomplete radio receiver embodying the invention.

of my inventionl is to'prol The system shown schematically in Fig. l comprises two, resonant circuits designated generally by numerals I and II, circuit I being effectively connected across the output terminals of a first vacuum tube I and circuit II being effectively connected across the input terminals of a second vacuum tube 2. Circuit I conprises capacitor 3 and inductor 5 with associated and relatively movable ferromagnetic core 1. Circuit II comprises capacitor and inductor 6 with associated and relatively movable ferromagnetic core 8. Cores l and B are arranged for motion in unison relatively to inductors 5 and 6 by means of a suitable uni-control mechanism, as indicated by the dotted line 9. Capacitors 3 and ,4 are indicated by arrows as being adjustable for purposes of initial alignment, but they may alternatively be made variable and may be ganged for rotation in unison with movement of the cores 1 and 6 if desired. Capacitor I is. connected between the output electrodes II and I2 of vacuum tubes" I and 2 respectively.

The high-potential terminals of capacitor 3 and inductor 5 are connected to the output electrode II of vacuum tube I. The high-potemial' terminals of capacitor 4 and inductor 6 are connected to the input terminal I l of vacuum tube 2. The low-potential terminals of capacitors 3 B+ but not shown. Circuit IIv is completed to ground through resistor I6 and capacitor I 8. Automatic volume control potential, if such is employed, may be applied to circuit II through resistor I6. Vacuum tubes i and 2 are shown as of the pentode type, but it will be understood that they may alternatively be of any suitable type. v

Circuits I and II are preferably capacitively coupled, and to this end inductors 5 and 6 are either so positioned or so shielded from one another or both, that the inductive coupling between circuits I and II is negligibly small. With reasonable precautions, such inductive coupling as may remain will not be detrimental `to the proper performance of the system, and in par-V ticular circumstances, in order to achieve a desired performance characteristic, additional amounts of inductive coupling may be employed.

With the inductive coupling between inductors 5 and 6 negligibly small, I prefer to couple circuits I and II only through common capacitor I3 and common resistor I5, which together provide a suitable variation in coupling over the frequency range, remembering that circuits I and II are tuned by inductance variation.l In my United States Patent No. 2,106,226 relating to nonregenerative systems I show various other methods of securing suitable coupling variation between resonant circuits inductively tuned by relatively movable ferromagnetic cores, dependv inglupon the performance characteristics` of the individual circuits. Any of these other coupling methods might be adapted for use in connection with my present invention, but for my present` i the further detailed description of my system it shouldv be understood that I have specific reference to this coupling arrangement although others may alternatively be employed.

Still referring to Fig. 1, in the absence of ferromagnetic cores l. and 8, and if circuits I and II were tuned over any considerable range of frequencies by varying the capacitance of capacitors 3 and 4 respectively, it would be impossible to introduceeven a relatively very small amountof regenerative amplification in one portion of the frequency range without encountering uncontrollable self-oscillation of the system in some other portion of the frequency range. By means of ferromagnetic cores ll and 8, however, and by the manner in which I design and arrange the other circuit components, as hereinafter described, I am able to tune the system entirely by inductance variation and I secure su'ch control over the individual performances of circuits I and II, and consequently over the regenerative amplification produced by vacuum tube 2, that I am enabled to greatly increase both the gain and selectivity of the system so that it equals or exceeds that which would otherwise be obtained only over a very limited portion of the frequency range at one extreme thereof. I achieve 'this highly beneficial control 'first by proper adjust-v ment of the coupling between circuits I and II, and second by appropriate choice of thekferromagnetic materials employed in the cores and 8, and by suitable design of the cores with respect to the characteristics of inductors 5 and 6 with which they are used.

The particular arrangements with regard Vt the `performance-controlling characteristics of cores 'I and 8 which I shall now describe are the preferred arrangements, since they may be readily aplied in the development laboratory, and produce highly satisfactory designs. It will beunderstood however, that within the scope of the teachings now to be given, numerous other arrangements to secure the advantages of the invention may be employed.

With regard to variable inductors of the type employing relatively-movable ferromagnetic cores, such as inductors-5 and 6 with cores .'I and 8 respectively, it is known that as the core enters -the winding it produces simultaneous increases- Vin the .effective inductance and in the effective It is also high-frequency resistance` thereof. known that by appropriate choice of the ferromagnetic material employed, these two increases may be made to have any reasonable desired rela` tion, one to the other, so that, for example, a resonant circuit employing the device may be designed to have av substantially constant ratio of inductance to resistance (L/R) over the' frequency range.

It is also known that the regenerative effect produced by an appropriately connected vacuum tube varies between wide limits over the frequency range, thatthe specific manner of this variation ,will depend in each case upon the specific circuit `arrangement employed, and that, in general, the .variation of the regenerative effect will be markedly different from the variation in the performance of the associated resonant circuits per se. In accordance with the invention. I overcome these difficulties by employing two circuits cuit tol lower frequencies.

It will be apparent that in any specific embodiment, any core chosen, for example, for use with inductor in circuit I, will produce a definite and measurable variation (or constancy) of the performance of circuit I per se over the frequency range. In general, it is possible to then design a. core for use with inductor 6 in circuit II which will compensate for (1) the variation in circuit I,

(2) the variation in the effective coupling be' tween circuits I and II, (3) lthe variation in the regenerative effect of vacuum tube 2, and (4) the otherwise uncontrolled variation in the performance of circuit II. Similarly, ifa core be chosen for circuit II, a core may then be designed for circuit I which will bring about similar compensations. The compensations thus achieved may be such as'to produce substantial uniformity of performance over the frequency range, or, if desired for special purposes, may' be such as tor produce some particular desired variation of performance with frequency other than that which would result without the control provided by the cores.

Assuming that it is desired toachieve substantially constant performance over the frequency range,the two cores 1 and 8 may be made identical, and under these circumstances will require each to have resistance-producing losses somewhat less than those required to produce substantially constant L/R in either of the circuits separately. Similarly, a core having losses higher than those -necessary to produce constant L/R in either of the circuits may be employed in one of them, but in this direction lies a limit beyond which it will be found impossible to secure adequate compensation by designing the core for the othercircuit to have relatively very low losses.

Various combinations of lcore characteristics may be found, however, which while having adequate effective .permeability to tune over the desired -frequency range, will co-act to achieve substantial constancy, or any reasonable desired varia.

tion, of the performance of the system. There-'- fore, while I shall now describe a preferred arrangement according to the invention, various other possible combinations such as those mentioned above, lie within the scope thereof.

I prefer to so construct the core 8 with respect to the material of which it is made and its proportions relative to the winding of inductor 6 that resonant circuit II, measured by itself, will have a-substantially constant ratio of inductance to resistance over the frequency range.

Since the inductance of the circuit will be inversely proportional to the square of the frequency, constant L/Rl requires that the resistance be also inversely proportional to the square of the frequency, with a slight additional resistance increase as thefrequency is lowered to compensistance I5 to the impedance of circuit II, and l sate for the decrease in the resistance of the@ 66 in circuit I is materially improved as the core 1 is inserted into the inductor 5 to tune the cir- The most advantageous law of variation of the L/R ratio in circuit I is not a simple one. In a system tunable from 550 to 1625A kilocycles, for example, the ratio was inversely proportional to approximately the 0.5 power of the frequency as the core first entered the inductor at the high-frequency end of the tuning range, and gradually increased to inverse proportionality to approximately the .with frequency in a system according to Fig. 1

and having cores of the types just described. As will be seen, circuit II has nearly constant L/R and circuit I has L/R increasing sharply with decreasing frequency. The graph also shows curves of selectivity or band width in kilocycles and sensitivity in microvolts per meter, of a radio receiver embodying the above described tuning system. It will be noted that as to both sensitivity and selectivity the performance of the receiver was remarkably constant over the frequency range.

An empiric equation for the curve of L/R. in circuit I as shown in Fig. 2 will have the form f=G(L/RD)u 1) where j is the frequency and D and G are constants. preferred arrangement, with core 8 (Fig. 1) 'giving very nearly constant L/RI in circuit II, the constants for the above equation were D=4.2 and G=1202 and the exponent `was 0.334. This equation reproduced the measured L/R curve of circuit I as shown in Fig. 2 to within 5.5% at all points.

By analysis of the equations of the electrically equivalent circuit, and assuming that the induct where K is the ratiol of the mutual impedance consisting principally of capacitance 'I3 and reinl which the numerical subscripts correspond to the numerals in Figs. 1 and 3, the presence of regeneration being indicated by the plus signs in Equations (2) and (3) and the minus sign in Equation 4.' f

At resonance the inductive reactance of the system is equal to its capacitive reactance. The time constant (L/R) and the dynamic resistance (L/RC) therefore are y && JL3 L5 1+K2 2 RC' R R Cadts-KZRQ) where w=21rf and f is the frequency.

Since the circuit is tuned by inductance variation, L5 is inversely proportional to' the square of thefrequency. The Values of K, Rs and Re, are controllable through the design of cores 1 and 8. It is apparent fromEquations 5 and 6 therefore, that either constant selectivity (L/R) or constant gain (L/RC) may be achieved, or that a compromise between them may be obtained as lthe designer may desire. In the preferred embodiment about to be described, the performance of which is indicated in Fig. 2, such a compromise has been secured.

For a particular embodiment -in the y Wire On a. .200" 'I. D. by .224" O. D. tube, the

core, and comprises hydrogen-reduced iron dust and instructions for constructing a pair of cores Which will perform in accordance with the preierredarrangement above described, it being understood that these constants aregiven merely by way of illustrative example and that they are therefore not to be taken as in anyway limiting the' scope of the invention which is defined in the appended claims.

In Fig. 3, antenna i9 is connected to variable inductor 20and capacitors 2| and 22 to form a tuned input circuit connected to the control grid 23 Ofvacuum tube 24, which is preferably a triple-grid super-control amplifier of the type known to the trade as the 6K7. Vacuum tube 25 is preferably a triple-grid detector amplier tube of the type known to the trade as the 6.17 vand is arranged as shown to function as a detector and to supply audio-frequency voltage to vacuum tube 26, which may be a beam power amplifier of the type known to the trade as the25L6, and

which supplies audio-frequency current to loud 25 speaker 21. Plate and screen-grid voltages are supplied to vacuum'tubes 24. 25 and 26 through a i'llter comprising choke ,29 and capacitors 30,

' 3| by rectifier Vacuum tube 28, which may be of the type knownto the trade as the 25Z5, and 30 which is supplied with 60-cycle 11G-volt energy from the lighting circuit. The source of heater or filament voltages for vacuum tubes. 24, 25, 26 and 21 is not shown but may be arranged in the conventional manner. The regenerative coupled-circuit system of Fig. 1 is connected as shown between vacuum tubes 24 and 25, ferromagnetic cores 1, 8 'and 32 being ganged together throughA a suitable uni-control mechanism for operation in unison to tune the entire receiver 40 to a desired signal. y In an' illustrative embodiment according to Fig. 3 thecoils 5 and 6 maybe alike and may each have 207 'turns of No. 37 single enamelled winding length ,being 1.125". These coils will have an inductance of approximately 47 ph. and will have a quality coeiicient Q of at 1625,' kilocycles when properly made. A suitable arrangement for coils 5', 6 and 20 is to place them side by side with their axes parallel and approximately 1%." apart, with plate coil 5 in the center and with a grounded'conductive shield f aroundit, no other shielding of the coils b eing necessary.

drogen-reduced `iron dust sifted through a 300- mesh screen, insulatedwith a suitable insulating varnish and molded withirml 2 to 3 percent of powdered Bakelite at Vrelatively low pressure.`

Core 8, onthe other hand,- is not homogeneousf but may consist of two different portions. The portion which last enters the coil occupies approximately '10 percent of the totallengthof the sifted through al 1GO-mesh screen. The remaining 30 percent of the length `of the core may consist of approximately 80 percent of 30D-mesh hydrogen iron dust and 20 percent of iron reduced from carbonyl of the grade known to the trade 70 Vhtf' long. Core 1 will weigh approximatelyi 75 y 'grams' and have a density of about 5.8." Core 8 `will weigh about 3.9 grams and will haves. density of approximately 5.65.

' The remaining lcomponents in an embodiment in accordance with Fig. 3 may be as follows:

It will be understood that depending upon the performance characteristics of the particular tubes employed, the regenerative coupling ca-` pacitor l0 is to be adjusted to provide the required sensitivity. By proper development and choice of the constants, in accordance with the instructions herein given, the system is entirely stable in operation, being well away from the condition of incipient oscillations 'at all -frequencies, but will at the same time provide high selectivity.

I prefer to secure 'control of the performance of my system by employing a coil 6 and corev 8 in circuit II, which in themselves produce substantially constant L/R and by employing a coil 5 and core 1 in circuit I, which produce an L/l'iI rapidly decreasing with increasing frequency, as

illustrated in Eig. 2. It will be understood, however, that control may also be achieved by employing a constant L/R coil and core combination in circuit I and by providing a compensating L/R characteristic in-circuit II. It is also to be understood that the characteristic curves of Fig.`

2 are those of a practical embodiment of the invention and do not represent the best that may be achieved, either from -the standpoint of constancy of gain or the standpoint of constancy of selectivity, and that when, in particularcases;`

variation of either the selectivity or the fsens'iif tlvity is desired over the frequency band,'this` may also 'be secured without departing 1 from the scope of the invention.

From the above it appears that by the regenerative effect produced and controlled asabove described, cooperating with the compensations and circuit characteristics described, I am able to maintain the effective high-frequency resistance of the .amplifier at a value only slightly greater than zero throughout the 4tunable range of frequencies of the amplifier.

Having thus described my invention, what I claim is:

1'. A tunable signal-frequency selective ampli- 1 55 iler for use in radio receivers and the like in- Y Core 1 may be homogeneous and made of hy-` cluding first and second resonant circuits veach having an inductive winding, a non-inductive couplingrimpedance common to said circuits, a.

vacuum tube having its input terminals coni nected across said second circuit and having an output electrode connected through a capacitance to the high-potential side of said iirst lcircuit, a ferromagnetic core movable relatively to the winding in said first circuit and having such characteristics as to cause the inductance-'to-re. sistanceratio in said rst circuit to decrease with increasing frequency, and a ferromagnetic core movable relatively to the winding in said sec' ond circuit and having such characteristics Ias to maintain the inductance-to-resistance ratio in said secondA circuit substantially constant, said ,v cores being movable in unison to tune said amplifier over a range of frequencies.

2. AA tunable signal-frequency selective ampliner for use in radio receivers and the like includan adjustable capacitor and an inductive Winding, a coupling impedance comprising a resistor and a capacitor connected in parallel betweenthe low-potential terminals of said adjustable capacitors and ground, a vacuum tube having its input terminals connected across said second circuit and having an output electrode connected through a capacitance to the high-potential side of said first circuit, a ferromagnetic core movable relatively to the Winding in said first circuit and f having such characteristics as to cause the inductance-to-resistance ratio in said first circuit to decrease with increasing frequency, and a ferromagnetic core movable relatively to the winding in said second circuit and having such characteristics as to maintain the inductance-to-resistance ratio in said second circuit substantially constant, said cores being. movable in unison to tune said amplifier over a range of frequencies.

3. A tunable signal-frequency selective amplifier for use in radio receivers and the like including first and second resonant circuits each having an vinductive winding, a non-inductive coupling impedance common to said circuits, a vacuum tube having its input terminals connected across said second circuit and having an output electrode connected through a capacitance to the high-potential side` of said first circuit, a ferromagnetic core movable relatively to the winding in one of said circuits and having such characteristics as to cause the inductance-toresistance ratio in said one circuit to decrease `with increasing frequency, and a ferromagnetic core movable relatively to the winding in the other of said circuits and having such characteristics as to maintain the inductance-to-resistance ratio in said other circuit substantially constant, said cores being movable in unison to tune said amplifier over a range of frequencies. A

4. A 'tunable signal-frequency selective amplifier for use in radio receivers and the like including rst and second resonant circuits each having an adjustable capacitor and an inductive winding, a coupling impedance comprising a resistorand a capacitor connected in parallel between the low-potential terminals of said adjustable capacitors and ground, a vacuum tube having its input terminals connected across said second circuit and having an output electrode connected through a capacitance to the high-pctential side of said rst circuit, a ferromagnetic core movable relatively to the winding in one of said circuits and having such characteristics as to cause the inductance-to-resistan-ce ratioin said one circuit to decrease withincreasing frequency, and a ferromagnetic core movable relatively to the winding in the other of said circuits and having such characteristics as to maintain the inductance-to-resistance ratio in said other circuitsubstantially constant, said cores being movable in unison to tune said amplifier over a range of frequencies.

`5.,A tunable signal-frequency selective amplifier for use in radio receivers and the -like including first and second resonant circuits each having an inductive winding, a non-inductive coupling impedance common to said circuits, a vacuum tube having its input terminals connected across Ysaid second circuit and having an output electrode connected through a. capacitance to the high-potential side of said first` circuit, and a ferromagnetic core movable relatively to the inductive winding in each of said circuits, said cores being movable in unison to tune said amplifier over a range of frequencies, said noninductive coupling impedance having a rising coupling effect with decreasing frequency, and the impedance lof said output electrode connection increasing with decreasing frequency, said ferromagnetic cores having such respective characteristics as to compensate for said rising coupling effect and said increasing impedance effect to establish a substantially constant selectivity for said amplifier throughout the tuning range.

`6. A tunable signal-frequency selective amplifier for use in radio receivers and the like including 'first and second resonant circuits each having an inductive winding, a non-inductive coupling impedance common to said circuits, a vacuum tube` having its input nterminals connected across said second circuit and having an youtput electrode connected through a capacitance to the high-potential side of said first circuit,

and a ferromagneticcore movable relatively to I inductive coupling impedance having a rising coupling effect with decreasing frequency, and the impedance of said output electrode connection increasing with decreasing frequency, said ferromagnetic cores having respective characteristics producing inductance-to-resistance ratios in said circuits compensating for said rising coupling effect and said increasing impedance effect to establish -a substantially constant selectivity for said amplifier throughout the tuning range.

7. A tunable signal-frequency selective amplifier for use in radio receivers and the like including first and second resonant circuits each having an inductive winding, a non-inductive coupling impedance common to said circuits, a vacuum tube having its input terminals connected across said second circuit and having an output electrode connected through a capacitance to the high-'potential side of said first circuit, and a ferromagnetic core movable relatively to the inductive winding in each of said circuits, said cores being movable in unison to tune said amplifier over a range of frequencies, said cores being free from common electrical connections' and producing independent effects upon said circuits and having characteristics coacting with said coupling impedance and with said output electrode connection to establish a substantially constant selectivity for said amplifier throughout the tuning range.

8. A tunable signal-frequency selective amplifier for use in radio receivers and the like iny circuits, said cores being movable in unison to tune said amplifier over a range of frequencies, said non-inductive coupling impedance having a rising coupling effect. with decreasing frequency,

- and the impedance of said output electrode connection increasing with decreasing frequency, said ferromagnetic cores having such respective. characteristics `as to compensate for said rising coupling effect and said increasing impedance effect to establish a substantially constant selectivity for said amplier throughout the tuning range, said cores being free from common electrical connections and producing independent effects upon said circuits.

9. A tunable signal-frequency selective ampliiler for use in radio receivers and the like in cluding first andvsecond resonant circuits each having an inductive winding, a non-inductive coupling impedance common to said circuits, a 10 vacuum tube having its input terminals connected across said second circuit and having an output electrode connected through a capacitance to lthe high-potential side `of said rst circuit, and

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