US2298297A - Radio receiver - Google Patents

Description

Oct. 13, 1942. JACOB 2,298,297

RADIO RECEIVER Filed June 22, 1940 3 Sheets-Sheet 1 i 20 a? :4 23 i \T E 24 T T A JAAAnAnA nnnnn ATTORNEY F. N. JACOB RADIO RECEIVER Filed June 22, 1940 Oct. 13, 1942.

3 Sheets-Sheet 2 INVENTORV, *fmderg'muacob-i aBY" ATTORNEY Oct. 13, 1942. 2 F. N. JACOB RADIO mcmvm Filed June 22, 1940 3 Sheets-Sheet 3 W 5 M Q m H mm 5 1 V m m M R W Q M m j N w Ms Q 8. wag? am L wfl ATTORNEY Patented Oct. 13, 1942 RADIO RECEIVER Frederick N. Jacob, Chicago, 111., assignor to Johnson Laboratories, Inc., Chicago, 111., a corporation of Illinois Application June 22, 1940, Serial No. 341,791

22 Claims.

This invention relates to an amplifying system in which a plurality of vacuum tubes are arranged to selectively amplify a desired high frequency signal, and to simultaneously improve the performance of the resonant circuits associated with the vacuum tubes. The invention is in the nature of an improvement on the invention disclosed in application Serial No. 339,697, filed on June 10', 1940, in the name of William A. Schaper. In said application a regenerative amplifying system is disclosed in which the performance of the tunable circuitsof a radio receiver over the tuning range is substantially improved by providing separate vacuum tubes for the regenerative and the amplifying functions.

In accordance with the invention one or more resonant circuits are employed whose performance as to both gain and selectivity is substantially uniform over the frequency range. Fur thermore, I employ circuits which are, in general, of considerably lower inherent resonant gain and selectivity than the non-uniform circuits heretofore employed, but this deficiency is compensated for by their uniformity and by the means employed to greatly increase their effective resonant gain and selectivity as hereinafter explained. By using circuits having low inherent resonant gain, it is possible to employ inexpensive coils in which the ratio of L/R. is of a relatively low order, the use of coils wound with special very low resistance windings not being required in order to secure an adequate amount of gain. The use of such low Q coils for the resonant circuits tends to decrease the cost of the receiver.

By employing regeneration, however, the selectivity and gain per resonant circuit are very substantially increased so that it is possible to secure a degree of selectivity and gain with a single resonant circuit comparable to that which might be obtained by the use of several resonant circults of the usual type connected in cascade in a radio receiver. The number. of resonant circuits required to secure the desired selectivity and gain is thus reduced, resulting in a further decrease in the cost of construction of the receiver.

By employing regeneration according to my invention, the gain of each stage may be increased to such an extent that fewer cascaded amplifying stages are required, thereby resulting in a further decrease in cost of the receiver with respect to the number of amplifying tubesrequired. According to my invention, it is possible to use resonant circuits of inexpensive construction which without regeneration produce a gain per stage of the order of 50 and by a suitable amount of regeneration to build up the gem t the order of 500 per stage and, since the total gain is substantially equal to the product of the gains per stage according to the number of stages used, it will be seen that by employing regeneration it is possible, by means of my invention, to construct a radio receiver having an adequate amount of gain with only a few amplifying stages.

My invention comprises the use of a circuit having substantially constant resonant gain and constant selectivity over the frequency range through which it is tunable, such a circuit having a substantially constant ratio of inductance to resistance (L/R) and a substantially constant dynamic resistance (L/RC) This circuit is preferably one having an inductance coil and 8. capacitor, the inductance being varied by movement of a compressed comminuted ferromagnetic core relative to the inductance coil to effect its tuning in the manner known as permeability tuning. The inductance of such a circuit is inversely proportional to the square of the frequency and the total resistance must also be inversely proportional .to the square of the frequency in order to secure a constant dynamic resistance. If a negative resistance is introduced into such a circuit, as by regeneration, the effective resistance of the circuit becomes R,R,' where R is the resistance of the circuit and R. is the negative resistance, and it is this total or effective resistance which must now be maintained inversely proportional to the square of the frequency.

Where the tunable circuit is associated with an amplifying tube, I introduce the negative resistance by means of a separate vacuum tube of the regenerative type, and which is coupled to the amplifying tube by means of a coupling system so arranged as to automatically introduce the proper amount of negative resistance to maintain the dynamic resistance of the circuit substantially constant. In the above Schaper application an arrangement is disclosed for introducing the proper amount of negative resistance into the resonant circuit, which is shown connected to the plate circuit of the amplifying tube. This arrangement comprises two capacitive couplings in cascade between the plate and grid circuits of the vacuum tube which effects the regeneration. Since the mutual reactance due to each of these couplings is inversely proportional to the first power of the frequency, the total effective mutual reactance between the plate and the grid circuits due to these two couplings is inversely proportional to'the square of the frequency.

I have found that when this type of regenerative amplifying arrangement is permeability tuned over an extended range of frequencies, as for example, over the range from 1600 to 600 kilocycles, the selectivity of the resonant circuit tends to decrease by a substantial amount toward the low frequency end of the tuning range. This results in the transmission of a substantially wider band of frequencies at this end of the range than at the high frequency end with the accompanying possibility of receiving the carrier of one or more undesired stations along with that of a desired station to which the resonant circuit is tuned. It is an object of the present invention to increase the selectivity and gain of the resonant circuit or circuits of an amplifying arrangement of this type and to secure substantially uniform selectivity and gain throughout extended tuning ranges of the circuits. It has been found possible to secure these desirable results by connecting in the grid-cathode circuit of the regenerative tube a parallel resonant circuit which presents a capacitive reactance at the frequencies to which the permeability tuned circuit is tunable. The addition of such a parallel resonant circuit was found to modify the regenerative effect in such a manner that the width of the band of frequencies transmitted remains substantially constant over the tuning range and, furthermore, tends to prevent the regenerative amplifying system from going into oscillation.

Where an amplifying stage is used in association .with a regenerative tube arrangement, as just described, in a radio receiver provided with means for automaticallycontrolling the gain of the amplifier tubes substantially inversely as the strength of a received carrier, it was found that as the grid bias of the amplifying tubes becomes more negative, the selectivity tended to increase by a substantial amount and, under certain conditions of initial adjustment, the receiver might go into oscillation. It is a further purpose of the present invention to provide means for compensatingfor this tendency toward increased selectivity. I have found that by simultaneously in-- creasing the grid bias of the regenerative tube associated with each amplifying tube by an vention, however, reference is made to the accompanylng drawings in which like reference characters designate like parts throughout the several views, and in which:

Fig. 1 is a schematic circuit diagram of an amplifying stage embodying the invention;

Fig. 2 is a schematic circuit diagram of a radio receiver embodying the invention: and

Figs. 3- and 4 are schematic circuit diagrams of modiiie'dTforms of the regeneration control ar- "rangement of Fig. 2.

Referring to Fig. 1 of the drawings, the basic arrangement, according to my invention comprises, in the illustrative example shown, a resonant circuit I, tunable over a range of frequencies, such as the broadcast range, by a ferromagnetic core 2a movable relatively to the inductive branch of the circuit, namely, the coil 2. The circuit has two parallel capacitive branches 3 and 4, 5, one of which comprises the capacitors 4 and 5 connected in series. As a means for producing a. substantially constant increase in the dynamic resistance of the circuit I, there is provided a vacuum tube 6 having its control grid 1 connected through a path of negligible highfrequency impedance through lead 8 and capaccurrents of the frequencies to be amplified.

Cathode II of vacuum tube 6 is connected to ground through a resistor I6 and inductance coil 23 in series, coil 28 being shunted by a capacitor 24 to form a parallel resonant circuit having a capacitive reactance to currents of the frequencies to which the circuit I is tunable.

The plate circuit of amplifying tube I 6 includes the plate II, variable inductance 2 and voltage source I4, a tap on source I4 supplying a suitable potential to screen grid I8, and a capacitor 20 being connected to the tap point, to provide a bypass between the screen grid and cathode I9. The input signal is applied to the control grid 2I of vacuum tube I6 and the amplified output is taken ofi through any suitable means, herein illustrated as a capacitor 22.

As described in the above mentioned Schaper application, the vacuum tube I6 functions in the usual manner as a radio frequency amplifier. Its plate load comprises resonant circuit I, which may be tuned to a desired frequency by adjusting the value of inductance 2. The path of high frequency currents in the grid circuit of vacuum tube 6 may be traced from grid I through lead 8, by-pass capacitor 9, capacitor 5 in parallel with the series combination of capacitor 4, and inductor 2 and capacitor 3 in parallel, and resistor I2 to the cathode II. Thus capacitor 5 is common to the grid circuit of tube 6 and to resonant circuit I. High frequency currents in the plate circuit of vacuum tube 6 follow a path which comprises capacitor 4 in parallel with the series combination of capacitor 5, and inductor 2 and capacitor 3 in parallel, and resistor I2. Thus capacitor 4 is common to the plate circuit of tube 6 and to resonant circuit I. The grid and plate circuits are, therefore, each capacitively coupled to resonant circuit I, and it. is these two cascaded couplings which cause regeneration by the tube 8 and cause the amount of regeneration to vary automatically with the frequency. Resistor I2 functions to improve the stability of vacuum tube 6. Resistor I5 and coil 23 provide a return path for the direct current component of the cathode current of tube 6 to follow. Due to the effect of regeneration in tube 6, the efiective resistance of circuit I is greatly reduced, with the result that its resonant gain is considerably increased and it provides a much higher degree of selectivity than would otherwise be secured from it.

7 The degree of regeneration at any given frequency depends upon the amount of coupling between the plate and grid circuits of the tube 6 and may be varied by adjusting the value of resistor I2, the regeneration increasing as the value of this resistor is decreased. The amount of coupling depends on the relative values of capacitors 3, 4 and 5, the coupling increasing as capacitors 4 and 5 in series are made larger in comparison with capacitor 3.

The variation of the gain and selectivity of the amplifying stage as the circuit I is tuned over a range of frequencies depends partly on the characteristics of variable inductance 2, that is, the relation of its inductance to its radio frequency resistance over the tuning range. The inductance device is preferably such that the ratio of the inductance to the high frequency re sistance of the resonant circuit remains substantially constant over the tuning range. The defrequencies when the resonant circuit is tuned over an extended frequency range, for example, the broadcast range, and more particularly when the capacitors 4 and 5 are small with reference to capacitor 3, due to the shunting eifect of reslstors l2 and on the capacitor 5. This tendency is overcome, in accordancewith my invention, by the action of the parallel resonant circuit comprising inductance coil 23 and capacitor 24. This circuit is designed'to present a capacitive reactance eflectively in series between cathode Ii and the low potential terminal of circuit I, at the frequencies to which the circuit l.is

' tunable. In an embodiment of the invention in which the resonant circuit i was tunable over the frequency range of from 600'to 1600 kilocycles, the constants of the circuit elements 23, 24 were so designed as to give this circuit a resonant frequency of a proximately 330 kilocycles. By properly proportioning the constants of circuit 23-24, furthermore, it is possible also to compensate for a variable inductance 2 which does not in itself provide constant circuit selectivity throughout the tuning range, in such a way that substantially constant selectivity of the system is obtained.- This additional refinement of performance is one of the features of my invention.

Although a tetrode and a triode are shown in Fig. 1, it will be understood that they were chosen for illustrative example only, and that other types of ,vacuum tubes may equally well be employed. For example, the amplifying tube may be a pentode having a suppressor grid in addi-- tion to the other grids shown. Furthermore, the elements of the tubes 6 and I6 maybe mounted within a single envelope and mounted on a single Such a combination tube may employ a base. single heater to heat both the cathodes II and it, or separate heaters connected in series or in parallel may be employed. a

When in the claims, therefore, I describe my novel structure as including vacuum tubes, it will be understood that the essential elements of these tubes may be incorporated in a single evacuated envelope with the possible elimination of some of the elements which would be required in the case of a plurality of separate envelopes. For

example, it-the several elements were arranged in a single evacuated envelope, only a single heater for the several cathodes might be provided. An embodiment of the invention, in a radio receiver tunable over the broadcast band, is shown in Fig. 2. the receiver comprising seven vacuum tubes arranged as follows: two radiofrequency amplifying tubes 25 and 26 arranged in cascade, two regenerative vacuum tubes 28 and 20, cooperating respectivelywith the reso nant; circuits 30, a combined detector, first audio frequency amplifier and automatic volume control tube iii, a power output vacuum tube 32, and a rectifying vacuum tube 33. with the exception of the novel arrangementsnow to be de scribed, the receiver is of conventional design and operates in the usual manner.

The signal energy is supplied to the control grid 34 of tube 25 from the secondary winding 35' of a transformer 36 whose primary winding is connected to the antenna A. The secondary winding 35 is shunted by an adjustable capacitor 31 to form a circuit 38 tunable to the desired carrier frequency, the inductance of secondary winding 35 being varied by means of'a movable ferromagnetic core 39. The voltage developed across resonant circuit 38 is applied to control grid 34 of tube 25 by means of a capacitor 40, a

suitable bias voltage being supplied to grid 34 through a resistor 4|, this bias voltage being filtered by means of the resistor 42 and capacitor 43. A capacitor 44 is shunted between control grid 34 and ground, and the resistor 45 connected between the tube cathode 46 and ground provides the normal grid bias voltage, this resistor being shunted by a capacitor 41. Screen grid 49 is supplied with a suitable positive potential through the resistor 50 and is by-passed to ground by-the capacitor ii.

The plate circuit of amplifying tube 25 includes the resonant circuit 30, which corresponds to the circuit I of Fig. 1 and comprises the adjustable capacitor 52 which is shunted across the inductance coil 53 and capacitors 54, 55, the inductance of the circuit being controlled by the movable ferromagnetic core 29. Plate voltage for tubes 25 and 28 is supplied through the resistor 56, which is by-passed toground through the capacitor 61. Grid 48 of tube 28 is supplied with direct current bias voltage through a resistor 58 and is maintained at ground potential with reference to radio frequency by the capacitor 58. Cathode 60 is grounded through resistor 6i and inductance coil 62 in series, the coil 62 being shunted by a capacitor 63 to form a parallel resonant circuit.

connections between the tubes 28 and 29 aresimilar to those between the tubes 25 and 28, like circuit components being designated by the same reference characters. The ferro-magnetic cores 39 are preferably connected together for simultaneous operation by means of a single manual control, as indicated by the dash lines.

The amplified signal-voltage which is developed across the second resonant circuit 20 is applied to the diode anodes 86 of vacuum tube 3| by means of the coupling capacitor 65, the rectified signal voltage appearing across the resistors 61, i8 and 69 which are connected in series between the anodes 66. and the cathode 10 of tube 3|. A capacitor H is shunted across the series connection of resistors 68 and 69. The direct current voltage developed across resistors 68 and 68 in series is applied to the control grids 34 of tubes 25 and 26 through the time delay network comprislng resistor 12 and capacitor 13 and the filters comprising resistors 42 and capacitors 43. The direct current bias-voltage developed across resistor '68 is applied to the grids 48 of there- ,generative tubes 28, 29 through a time delay network comprising a resistor 14 and capacitor 15 and the filters comprising the resistors 58 and capacitors 59. The modulation voltage developed across resistors 88 and 69 in series is applied to grid 18 of tube 3| by means of a circuit comprising a capacitor 11 and potentiometer '18. A suitable negative bias voltage is applied to grid 18 through a. resistor 19, this bias voltage being filtered by the capacitor 88. Also, as shown, the cathode III of tube 3| is grounded.

The audio frequency voltage developed across load resistor 8| in the plate circuit of .tube 3| is applied to the control grid 82 of power tube 32 by means of coupling capacitor 83 and resistor 84. The potentiometer 85 serves as a grid leak for grid 82 and also, in conjunction with a capacitor 86, as a tone control device for adjustably attenuating the higher audio frequencies. A self biasing resistor 81 serves to connect the cathode of tube 32 to ground, this resistor being shunted by the usual capacitor 88. Theplate circuit of power tube 32 is coupled to any suitable translating device as, for example, a loud speaker 89 by means of a transformer 80.

Suitable operating potentials are supplied to the several tubes of the receiver from a source which comprises the rectifier tube 33, multiple winding transformer N, a filter circuit of the usual type, as shown, and a voltage divider comprising the resistors 82, 93, 94 and 85, which are connected in series. It will be understood that the heaters of tubes -29, 3| and 32 are energized by winding 98 of transformer 8 I, the heaters and their connecting circuits being omitted from the drawing for the sake of clarity. One side of the heater circuit is preferably grounded, as shown.

In the operation of the receiver, a modulated radio frequency signal voltage impressed on antenna A is amplified successively by the radio frequency amplifying tubes 25 and 26 operating in conjunction with the resonant circuits 38, 38 and 38, each of which is tuned to resonance with the incoming signal frequency. and regenerative tubes 28 and 28. This tuning is preferably accomplished by moving the cores 39 simultaneously with reference to the inductance coils and 53, but it is within the scope of my invention to provide separate actuating means for one or more of the cores 88. The amplified signal current is demodulated-by the diode portion of tube 3| and the resulting audio frequency signal voltage is amplifled by the triode' portion of tube 3| and tube 32 from which it is supplied to the loud speaker 89 by the transformer 88. That portion of the rectified carrier voltage which appears across diode load resistors 88 and 69 in series is utilized to conquency depends upon the transconductance of tube 28 and the amount of coupling between its plate and grid circuits. The amplification of tube 28 may be varied by changing its grid-cathode potential, the amplification and hence the degree of regeneration increasing as this potential is decreased, that is, as the grid is made less negative with reference to the cathode. The amount of coupling between its grid and plate circuits detro] the amplification of amplifying tubes 2.5 and:

pends on the relative values of capacitors 54 and 55 where all other constants of the tuned circuit remain fixed, the coupling decreasing as capacitor 54 is made larger in proportion to capacitor 55.

The amplification of tube 25 is controlled by varying its control-grid bias voltage. Increasing the negative bias voltage causes the plate resistance of the tube to increase by a substantial amount. Since its, plate resistance is effectively in shunt with resonant circuit 30 which, due to the regeneration produced by tube 28, has a relatively high resonant impedance, this variation in plate resistance of tube 25 with variation of its control grid bias voltage has an appreciable infiuence on the selectivity and resonant gain of circuit 30. Upon increasing the grid bias voltage, the transconductance decreases but the plate resistance of tube 25 increases, causing an increase in the selectivity of circuit 30 possibly sufficient to produce sustained oscillations through the action of tube 28. This highly undesirable effect is avoided, in this embodiment of the invention, by applying to grid 48 of tube 28 only a portion of the bias voltage which is applied to the control grid 34 of tube 25, namely, only that portion of the bias voltage which is developed across resistor 69. In this manner, the regenerative effect of tube 28 upon resonant circuit 30 is decreased sumciently to compensate for the increase in selectivity due to the increase in the plate resistance of tube 25. In case of exact compensation, the selectivity of the system remains substantially unchanged as the bias voltage, which is applied to grid 34 of tube 25 by the automatic volume control arrangement described, is varied over the full range.

The compensation may be carried further, however, if desired, to provide for a decrease of selectivity under conditions of a large bias voltage as,for example, during the reception of extremely strong signals merely by applying to grid 48 a larger portion or even all of the biasing potential applied to the grid 34 of pentode tube 25.

Because the reactance of capacitor 55 increases as the frequency decreases, the gain and selectivity tend to become appreciably worse at the lower frequency end of an extensive tuning range, as, for example, the broadcast range due to the shunting eifect of resistors 6| and 66. This tendency is overcome by the action of the parallel resonant circuit comprising coil 52 and capacitor 83 which is designed to present a capacitive reactance effectively in series between cathode 60 and ground at the frequencies to which circuit 88 is tunable. By properly proportioning the constants of the circuit 62-63, it is also possible to compensate for an inductance device 3538 which does not in itself provide constant circuit selectivity throughout the tuning range, in such a way that substantially constant selectivity of the system is obtained.

Measurements were made of the performance of a stageof the receiver shown in Fig. 2 comprising the tubes 25 and 28 and their associated circuits to determine the effect of the addition of the resonant circuit 62-68 on the selectivity of the tunable circuit 30 as it was tuned over a frequency range from 1600 kc. to 600 kc. The results of these tests show clearly that a Considerable improvement in the selectivity and gain toward the low frequency end of the range is secured by the addition of the circuit 62-63 in the cathode return. By suitable choice of the constants of this circuit, it was found possible to secure a substantially constant and high order of selectivity of th tunable circuit throughout its tuning range without causing the system to go into oscillation.

Measurements were also made to determine the effect of the change in grid bias voltage of the triode grid 48 other than the self bias developed across resistor 6|, on the selectivity of the tuned circuit for different bias voltage of the pentode grid 34. Denoting the bias voltage of grid 34 other than that present across resistor 45 by VI and that of grid 48 other than that present across resistor Si by V2, the following results were secured at an applied signal frequency of 1600 In the tables the band width of the response curve was measured at a height equal to one tenth of the peak output voltage. The results indicate clearly that by the application of a grid bias voltage to the regenerative tube grid which is directly proportional to that applied to the .amplifying tube grid, it is possible to secure a substantially constant degree of selectivity for different amplifying values of the pentode. It is thus possible by tapping oil on the automatic volume control resistor (68-69, Fig. '2) at the proper point to secure a constant band width or an increasing or decreasing band width depend- ;ng upon the amount of automatic volume control voltage utilized.

The circuit arrangement of Fig. 3 is similar to that of Fig. 2 except that the regenerative tube 91 includes a separate diode portion comprising a pair of auxiliary anodes 98 and a cathode 98a which is separate from the cathode 91a of the triode portion of the tube. The signal voltage is applied to the anodes 93 by means of a capacitor 99 and the anodes 98 are connected to the diode cathode 98a by means of the resistor I00. It followsthat the direct current voltage which is developed across the load resistor I is proportional to the strength of the signal voltage and this voltage is applied to the grid ml of the triode portion through the resistor I02, the capacitor I03 serving to filter out the alternating-component of the bias control voltage. By means of this arrangement and that disclosed in Fig. 4, each stage of the receiver supplies its own source of bias potential for the grid of the regenerative tube and is thus made independent of external sistor GI, a direct current bias voltage is applied to the auxiliary anodes 98 through the resistor I00 in addition to the radio frequency voltage applied through the capacitor- 99. This results in a delay in the application of the automatic control voltage applied to the grid IOI of the triode portion of the tube upon a change in the strength-of a received carrier.

While I have shown my invention in the particular embodiments above described,.-I ,do not limit myself thereto as I may employ equivalents thereof without departing from the scope of the appended claims.

Having thus described my invention, what I claim is:

1. A selective high frequency resonant system comprising, in combination, a resonant circuit containing inductance and capacity and having an effective resistance, said circuit being provided with means whereby the inductance and eflective resistance may be varied substantially in proportion to each other, and means for increasing the selectivity of said circuit comprising a regenerative tube having its anode coupled to said circuit and a circuit connecting the grid and cathode of said tube and comprising a parallel resonant circuit.

2. A selective high frequency resonant system comprising, in combination, a resonant circuit containing inductance and capacity and having an effective resistance, said circuit being provided I with means whereby the inductance and effective resistance may be varied substantially in p'ropor' tion to each other, and means for increasing the selectivity of said circuit comprising a regenerative tube having its anode coupled to said circuit and a circuit connecting the grid and cathode of said tube and comprising a parallel resonant circuit disposed in the plate circuit of, the regenerative tube.

3. A selective high frequency resonant system comprising, in combination, a resonant circuit containing inductance and capacity and having an effective resistance, said circuit being provided with means whereby the inductance and efiective resistance may be varied substantially in proportion onto each other, and means for increasing the selectivity of said circuit comprising a regenerative tube having its anode 'coupled to said circuit and a circuit connecting the grid and cathode of said tube and comprising a parallel resonant circuit having a capacitative reactance at the frequency to which the circuit containing inductance and capacity is reson'ant.

4. A selective high frequency resonant system comprising, in combination, a resonant circuit containing inductance and capacity, means for tuning said circuit over a range of frequencies, a regenerative vacuum tube device, means for sup plying energy from said regenerative tube to said resonant circuit, a circuit connected to the cathode of theregenerative tube and in shunt across at least a portion of the capacity of the resonant circuit whereby the selectivity of the resonant circuit tends to decrease as it is tuned toward the lower end of its frequency range and means for compensating for such decrease in selectivity comprising a parallel resonant circuit connected in series with said shunt circuit.

5. A selective high frequency resonant system comprising, in combination, a resonant circuit containing inductance and capacity, control means for adjusting the inductance of said circuit to tune it over a range offrequencies, a regenerative vacuum tube device, means for supplying energy from said regenerative tube to said resonant circuit, said means being arranged to automatically decrease the energy supplied as said control means is adjusted to tune the resonant circuit toward the low frequency end of the tuning range, and means for automatically increasing the energy supplied by the regenerative tube to the resonant circuit in response to the tuning,of the resonant circuit toward the low ;-;;;frequency end of its tuning range by the ad- Justment of said control means to thereby com- {f pensate for the decrease-in energy supplied to the resonant circuit by the regenerative tube. ,,-6, Aselective high frequency resonant system said control means is adjusted to tune the resonant circuittoward the low frequency end of the tuning range, and means for automatically increasing the energy supplied by the regenerative tube to the resonant circuit in response to the tuning of the resonant circuit toward the low frequency end of its tuning range by the adjustment of said control means to thereby compensate for the decrease in energy supplied to the resonant circuit by the regenerative tube, said energy increasing means including a. parallel resonant circuit connected to one of the elsetrodes. of the regenerative tube, the reactance of said parallel resonant circuit being capacitative at the frequencies within the tuning range of the .resonant circuit.

'7. In combination, a resonant circuit having resistance and comprising a coil and a capacitor, means for varying the inductance of said coil to tune'said circuit over a range of frequencies, an electron discharge tube having coupled grid and .plate circuits and a parallel resonant circuit which is common to the grid and plate circuits. and means for capacitively coupling one of said coupled circuits to said resonant circuit in such a manner that the degree of coupling therebetween is automatically varied to maintain the ratio between the inductance and resistance of said resonant circuit substantially constant as dynamic resistance being substantially constant throughout the range of said tunable resonant circuit.

9. A selective high frequency resonant system including a tunable resonant circuit having a relatively low but substantially constant dynamic resistance and having a portion of its capacitance arranged to provide a midtap, and means for producing a substantial increase in said dynamic resistance including a vacuum tube having a cathode, a parallel resonant circuit connected between said cathode and the low potential terminal of said tunable circuit, a direct connection between the anode of said tube and the high potential terminal of the tunable circuit, and a resistor connecting the tube cathode to said'midtap, said increase in dynamic resistance being substantially constant throughout the range of said tunable resonant circuit. a

10. A selective high frequency amplifying system comprising, in combination, an amplifying vacuum tube and a regenerative vacuum tube, each of said tubes having an anode, a cathode and a grid, a resonant circuit tunable over a range of frequencies and having its high potential terminal directly connected to the anode of said tubes, a lowimpedance connection between the lowpotential terminal of said resonant circuit and the cathode of said amplifying tube, means connected between the cathode of the regenerative tube and the low potential terminal of the resonant circuit including a parallel resonant circuit having a. capacitive reactance at the frequencies to which the resonant clrcuit'is tunable, a direct current connection from an intermediate point of the tunable circuit to the cathode of the regenerative tube, and means for applying negaaive bias voltage to the grids of said tubes which is proportional to the strength of the received signal current.

11. A selective high frequency system comprising, in combination, a resonant circuit comprising an inductive branch, a ferromagnetic core adjustable with reference to said inductive branch to tune said circuit over a range'of frequencies. means shunted across a portion of said circuit acting to decrease the selectivity thereof toward one end of the frequency range, a regenerative tube having its plate connected to the high potential terminal of said resonant circuit, and a circuit connected between the cathode of the reenerative tube and the low potential terminal l of the tunable circuit, the impedance characteristic of said circuit being such as to increase the selectivity of said resonant circuit as it is tuned toward said one end of the frequency range.

12. A selective high frequency amplifying system comprising, in combination, an amplifying vacuum tube and a regenerative vacuum tube. each of said tubes having an anode, a cathode and a grid, resonant circuit tunable over a range of frequencies and having its high potential terminal connected to the anodes of said tubes, means connected between the cathode of the regenerative tube and the low potential terminal of the resonant circuit tending to cause an increase in the selectivity of the resonant circuit as it is tuned toward one end of the frequency range, a connection between an intermediate point of the tunable circuit and the cathode of the regenerative tube, and means for simultaneously and proportionally reducing the gain of the amplifying tube and the amount of regeneration in the regenerative tube.

13. A selective high frequency amplifying system comprising, in combination, an amplifying vacuum tube and a regenerative vacuum tube, each of said tubes having an anode, a. cathode and a grid, a resonant circuit tunable over a range of signal frequencies and having its high potential terminal connected to the anodes of said tubes, means connected between the cathode of the regenerative tube and the low potential terminal of the resonant circuit tending to cause an increase in the selectivity of the resonant circuits as it is tuned toward the low frequency end of the tuning range, and means for applying negative bias voltstant throughout the range of saidtunable resage to the grids of said tubes which is proportional to the strength of the received signal-current.

14. A selective high frequency amplifying system comprising, in combination, an amplifying vacuum tube and a regenerative vacuum tube, each of said tubes having an anode, a cathode and a grid, a resonant circuit tunable over a range of signal frequencies and havingits high potential terminal connected to the anodes of said tubes, means connected between the cathode of the regenerative-tube and a low potential terminal of the resonant circuit tending to cause an increase in the selectivity of the resonant. circuits as it is tuned toward the low frequency end '01 the tuning range, and means for applying negative bias voltage to the grids of said tubes which is proportional to the strength the received signal current, said last named means being arranged to apply a larger negative bias voltage to the grid of the amplifying tube than to the grid of the re generative tube.

15. Means for amplifying received signal currents comprising, in combination, a resonant circuit tunable over a range of frequencies, a regenerative vacuum tube having a cathode, a grid, an anode and a diode plate, a connection between said anode and the high potential terminal or the resonant circuit, connections respectively between said grid and said cathode and different and lower potential points of said resonant circuit, means coupling the resonant circuit to the diode plate, a circuit conductive to direct current connected between said cathode and the diode plate, and means for controlling the voltage of said grid in accordance with the strength of the direct current passing through said last named circuit.

16. A selective high frequency resonant system including a resonant circuit tunable over a range of frequencies and having a substantially constant dynamic resistance, means for producing a substantial increase in said dynamic resistance including a vacuum tube having an anode connected to the high potential terminal of said circuit and a parallel resonant circuit connected between the cathode of said tube and the low potential terminal of said resonant circuit, said parallel resonant circuit having a natural resonant frequency outside of the range of frequen cies over which said first named resonant circuit is tunable, said increase in dynamic resistance being substantially constant throughout the range of said tunable resonant circuit.

17. A selective high frequency resonant system including a resonant circuit tunable over a range of frequencies and having a substantially constant dynamic resistance, means for producing a substantial increase in said dynamic resistance including a vacuum tube having an anode connected to the high potential terminal of said circuit and a parallel resonant circuit connected between the cathode of said tube and the low potential terminal of said resonant circuit, said parallel resonant circuit having a natural resonant frequency outside of the range of frequencies over which said first named resonant circuit is tunable, the natural resonant frequency of said parallel resonant circuit being lower than the lowest frequency of the range over which the first named resonant circuit is tunable, said increase in dynamic resistance being substantially cononant circuit.

18. The method of increasing theselectivity of a resonant circuit tunable overarange of frequencies which comprises the steps of feeding energy. from a. vacuum tube into said circuit in phase with the current therein in such a manner that the gain in selectivity of the circuit is appreciably greater at one end of its tuning range than at the other end thereof and introducing a capacitive reactance betweenanelectrode of the tube and a terminal of the tunable circuit, the magnitude of said reactance being such as to cause a substantial increase in the selectivity of the circuit at said other end of its tuning range.

19. A radio frequency amplifier system, including an amplifier tube, a resonant circuit con,- nected with said tube, a second tube regeneratively associated with said resonant circuit and having a control electrode, a variable bias system connected with said amplifier tube and developing voltage to control said amplifier tube, and a connection between said bias system and the control grid of said second tube substantially maintaining a desired selectivity of said resonant cir cuit for variation in the voltage developed in said bias system.

20. Ar'adio frequency amplifier system, including an amplifier tube, a resonant circuit connected with said tube, a second tube regeneratively associated with said resonant circuit and having a control electrode, a variable bias system connected with said amplifier tube-and developing voltage to control said amplifier tube, and a connection between said bias system and the control grid of said second tube substantially maintaining a desired selectivity of said resonant circuit for variation in the voltage developed in said bias system, said connections with said bias system being such as to impress on the control electrode of said second tube a voltage smaller than the voltage impressed by said bias system on said amplifier tube.

21. A radio frequency amplifier system, includ'-' ing an amplifier tube, a resonant circuit con-- nected with said tube, a second tube regeneratively associated with said resonant circuit and having a control electrode, an automatic volume control system connected with and controlling said amplifier tube by voltage variation, and a connection between said automatic volume control system and the control grid of said second tube for applying to the control grid of said second tube a voltage less than and proportional to the voltage applied by said control system to said amplifier tube.

22. A radio frequency amplifier system, including an amplifier tube, a resonant circuit connected with said tube, a second tube regeneratively associated with said resonant circuit and having a control electrode, an automatic volume control system connected with and controlling said amplifier tube by voltage variation, and a connection between said automatic volume control system and the control grid of said second tube such as to apply to the control grid of said second tube a control voltage of an amount and kind substantially compensating for regenerative change otherwise incident to applying automatic volume control voltage to said amplifier tube.

FREDERICK N. JACOB.

y CERTIFIECA'TE OF CORRECTION. Patent No. '2,298,297.- October. 15, 19m.

FREDERICK N. JACOB.

It is hereby certified that error appears in the printed specification ofthe abo-ve numbered patent requiring correction as follows: Page 2, second column, line 52, for pathbf read -path for-; page 5, second column,

line lfl, claim 5, for "onto" read --to--; page 6, first column, line 16, claim 6, after "capacity" strike out the period and insert insteadacomm'a;

and second column, linel+9,'cle1m l2,'before "resonant" insert --e.--; and that the said Letters Patent should be read with this correction therein that the sange may conform to the record of the case in the Patent Office.

Signed and seal'ed this 1st d y of December, A. D. 19u2.

' Henry Van Arsdale, (Seal) Acting Commissioner of Patents.

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