US2053415A - Radio receiver - Google Patents

Description

Sept. 8, 1936. G. l.. BEERs RADIO RECEIVER Filed oct. 28,/ 195s Patented Sept. 8, 1936 UITED STATES PATE? @FFQE RAnlo RECEIVER George L. Beers, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware My invention relates to radio receivers, and more particularly is an improvement over the system disclosed in my prior application, Patent #2,010,131, issued August 6, 1935, and assigned tc Radio Corporation of America.

In the application referred to, thesystem disclosed comprises a plurality of thermionie de vices for amplifying and detecting purposes, a plurality of thermionic devices the function of which is to modify the characteristics of coupling networks interposed between the rst mentioned thermionic devices, and a thermionic device the function of which is to supply controlling potentials to certain of the first-mentioned thermionic devices proportionally to the carrier amplitude of an incoming signal. By reason of the inter-connection of the various thermionic devices in the system referred to, automatic volume control or automatic control of the selectivity and fidelity of the system is obtained, at will.

As pointed out in the aforementioned application, in the design of radio receivers, there are two highly desirable characteristics that, in a sense, are mutually exclusive-that is, a receiver must be sufciently selective to differentiL ate between incoming signals under maximum and minimum sensitivity conditions and, at the same time, reasonably high fidelity and freedom from distortion are desirable at all times.

Heretofore, fair delity has been attained through the use of inter-tube coupling circuits having band-pass characteristics. It has been difficult, however, to so design receivers, especially those provided with automatic volume control means, which will exhibit a high degree of fidelity, as Well as reasonable selectivity, when receiving signals from local stations and still be sufficiently selective to receive weak signals from distant stations without an unpleasant amount of background noise.

It is, accordingly, an object of my invention to provide a radio receiver wherein high fidelity may automatically be had during the reception of strong signals, without sacrificing selectivity when receiving weak signals.

Another object of my invention is to provide a radio receiver wherein smoothly continuous selectivity control may be manually accomplished during the reception of signals from strong local stations.

Another object of my invention is to yprovide, in a radio receiver, means for lautomatically increasing the selectivity during the reception of weak signals.

Another object of my invention is to provide a radio receiver wherein the functions of amplification and selectivity control are performed by the same thermionic devices.

Another object of my invention is to provide 5 means whereby thel number of thermionic devices in a system of the type described may be reduced to minimum.

Another object of my invention is to provide a radio receiver that, in addition to the features l0 enumerated above, shall have automatic volume or gain control.

A still further and more specific object of my invention is to provide, in a system of the type referred to, a single thermionic device for the l5 purpose of correlating automatic volume control and automatic fidelity control to incoming signal strength and a single manually operated element or control device through the manipulation of which either of the aforementioned 2()A functions may be made available to the desired degree.

The foregoing objects and other objects ancillary thereto, I prefer to accomplish in part by utilizing in a radio receiver a plurality of screen 25 grid pentode-triode tubes of the type commercially known as 6FT. In each tube, the pentode portion thereof is independent of the triode portion, although it utilizes the same cathode, and it is employed for amplification and other well known purposes, while the triode portion is uti lized for automatic control of the selectivity and i delity of the receiver in response to variations in field strength. Specifically, such automatic control is had through circuit connections whereby the space current path in each triode portion is effectively in shunt to an intertube coupling device, and the bias potentials applied to the grids of the said triode. portions are so automatically controlled in response to incoming radio signals that the triode plate-impedance thereof governs the selectivity, delity, and gain of the system.

Furthermore, to provide for the foregoing functions, and'to attain, at will, automatic volume 45 control coupled with high selectivity, I utilize a single 'thermionic device on the input circuit of which potentials corresponding to the amplitude of incoming signals are impressed and in the output circuit of which is disposed a resistor network including a manually controllable potentiometer whereby the potentials developed therein may be utilized for the desired functions.

The novel features that I consider characteristie of my invention are set forth with particuradio receiver including an alternative embodiment of my invention.

In both of the figures of the drawing, equivalent circuit components and connections are similarly designated.

Although my invention is susceptible of application to radio receivers of many different types, I prefer to illustrate it, referring to the drawing, as applied to a superheterodyne receiver including a radio frequency amplifying tube I, hereinafter called the radio frequency tube, a first detector tube 3, a plurality of intermediate frequency amplifier tubes 5 and 1, later referred to as first and second intermediate frequency tubes and a second detector tube 9. Each of these tubes I, 3, 5, and 1 is of the screen grid pentode-triode type, the pentode portion being constituted by al cathode I3, a control grid I5, a screen grid I1, a suppressor grid I 9, and an anode 2|, while the triode portion includes the cathode |3, a control grid 23, and an anode 25. The second detector tube 9 is preferably of the diode type having a cathode 21 and an anode 29 between which is connected an output resistor 3|.

The screen grids of the tubes 3, 5, and 1 are supplied with positive potential at the requisite voltage over a common conductor 33.

In order to simplify the drawing, the heaters of the several tubes have been omitted and an oscillator 35 is merely indicated diagrammatically.

Any convenient source of plate and bias potentials may be employed and, since substantially all modern receivers are adapted to be energized from house lighting circuits, this source is exemplified in the drawing by a rectier 31 having an output circuit including a bleeder resistor 39.

The pentode portions of the radio frequency tube I and the rst detector tube 3 are impedance coupled, the coupling network including a tuned circuit 4| connected directly to both anodes of the radio frequency tube I and through a coupling condenser 53 to the pentode control grid of the first detector tube 3. The pentode portions of the first detector tube 3 and the first intermediate frequency tube 5 are inter-connected through a tuned circuit 45 and a coupling con-- denser 41. Y

'Ihe pentode portions of the first and second intermediate frequency tubes 5 and 1 are intercoupled through the medium of a radio frequency transformer 49, the primary and secondary windings of which are tuned and the coupling between them so adjusted that the transformer efficiently passes a band of frequencies l k. c. above and 10 k. c. below the chosen intermediate frequency. 'Ihe pentode portion of the second intermediate frequency tube 1 and the second detector tube 9 are inter-coupled through a band-pass radio frequency transformer Anode potential for the tubes I, 3, 5 and 1 is supplied over a common conductor 53, connected to the positive terminal of the bleeder resistor 39. The cathodes of the said tubes are also interfconnected by a common conductor 55 which extends to an intermediate point 51 on the bleeder resistor.

Grid bias for the pentode portion of the first detector tube 3 is provided by a connection 59 including a grid resistor 6| from the pentode grid thereof to an intermediate point 63 on the bleeder resistor more negative than the point 51 to which the cathodes are connected by the conductor 55.

Grid bias for the pentode portion of the second intermediate tube 1 is provided by a connection 6 5 extending from the secondary winding of the l'transformer 49 thereof to a still more negative point 61 on the bleeder resistor.

In order to automatically correlate to incoming signal amplitude the gain in the pentode portions of the radio frequency tube I and the first intermediate frequency tube 5, the pentode grids thereof are supplied with bias over a common conductor 69 connected to an intermediate point 1| on the bleeder resistor 39 more negative than the cathode connection 51 thereto, the grid connection including a source which supplies additional negative potential as the incoming carrier wave increases in amplitude above a pre-determined threshold value determined by the normal bias supplied from the bleeder resistor. The said source comprises a resistor network 12 constituted by a, plurality of resistors 13 and 15 and a potentiometer resistor element 11, the latter being connected by a movable contact member 19l to the anode 8| of an automatic volume control plate-impedance of the automatic volume control tube 83 when no signal is being received.

It is obvious, of course, that the specic network r12 is merely illustrative of means whereby the space current of the automatic volume control tube may be utilized either for potential application to the conductor 69 or for an additional function later to be explained. Many other networks for the desired purpose will be obvious to those skilled in the art and no reason is seen for complicating the disclosure by their illustration.

The resistance of potentiometer resistor 11 must be high with respect to the resistance of elements 1 3 and 15. The said automatic volume control tube may be of the screen grid equipotential cathode type having, in addition to the aforementioned anode, a cathode 85, a control grid 81, and a screen grid 89. The cathode 85 of the automatic volume control tube is connected to a point 9| on the bleeder resistor 39 more negative than the points thereon to which the cathodes of the amplifier tubes are connected and to which the control grids in the pentode portions of the first detector tube and the second intermediate frequency tube are connected.

Unidirectional potentials proportional to incoming signal amplitude are supplied to the control grid and cathode of the automatic volume control tube 83 from the diode output resistor 3| by a plurality of conductors 93 and 95, connected, respectively, to the grid of the automatic volume control tube over a resistor 91, and to the negative terminal of the bleeder resistor 39. The resistor Y,91 in the grid connection to the automatic volume control tube and a condenser 99 connected between the grid and the cathode thereof provid-e a filter which prevents audio frequencies from being impressed on the input ter- Preferably, the resistor B4 has a maximum resistance comparable to theA minals of the tube, thus ensuring that the grid potential is unidirectional and is proportional to the average amplitude of the intermediate frequency signal impressed upon the diode detector 9 from the preceding tubes in the system.

It will be noted, from the drawing, that the grid of the automatic volume control tube is connected to the cathode end of the diode resistor which end becomes mor-e positive as the average rectified signal current increases in amplitude. The negative bias applied to th-e said grid from the resistor 39 is thus opposed, proportionally to signal amplitude, permitting space current to flow therein, which current traverses the resistor network i2 in the output circuit thereof in the directions indicated by the arrows.

if, therefore, the movable element 19 of the potentiometer is adjusted toward the end A of the potentiometer resistor 11, to which the conductor 69 connects, the potential developed acrossV the resistor 13 is added to the normal bias applied to the pentode grids of the radio frequency tube i and the first intermediate frequency tub-e 5, with the result that the gain in the system is always automatically kept inversely proportional to incoming signal amplitude.

In addition to providing the means just ldescribed, whereby the gain in the system may be kept inversely proportional to the amplitude of an incoming signal without appreciably affecting the selectivity thereof, my invention also includes th-e provision of optionally utilizable means whereby the incoming signal may be made use of for automatically controlling the selectivity of the system and for simultaneously giving a measure of gain-control.

As clearly shown in the drawing, the space current path in the triode portion of the radio frequency amplifier tube I is connected eifectively in shunt to the coupling impedance i l, the space current path in the triode portion of the first detector tube 3 is connected in shunt to the output coupling impedance t5, while the space current path in the triode portion of the first intermediate frequency tube 5 is connected effectively in shunt to the primary winding of the output transformer 49.

When receiving weak signals, it is desirable that the system shall be selective, and that the gain therein shall be reasonably high, and the converse is usually true when receiving strong signals. rlhat is to say, when maximum selectivity is required the triode portions of the radio frequency tube I and the first detector tube 3 must draw substantially no current to damp the coupling reactors AI and 65, while the triode portion of the first intermediate frequency tube 5 must draw sufficient current to effectively loosen the coupling between the windings of the transformer 49. This condition necessitates a high negative bias on the triode grids in the tubes I and 3 and zero or slightly negative bias on the triode grid in the tube 5. Furthermore, if the system is to be placed in such condition that incoming signals, through the intermediary of the automatic volume control tube 83, may be utilized to cause the selectivity to decrease in response to increased signal amplitude means must be provided for rendering the triode grids in the tubes l and 3 less negative and for making the bias on the triode grid in the tube 5 sufficiently negative to substantially stop the flow of space current therein. The manner in which this is accomplished constitutes an important phase of my invention and it will now be explained.

Referring again to the drawing, it will be noted that only potentials increasing in the negative direction with increase in signal amplitude can be supplied by the automatic volume control tube 83 with respect to the common cathode connection 51.

The automatic volume control tube, accordingly, of itself, cannot be utilized to reduce the negative bias normally necessary on the triode grids of the radio frequency tube I and the detector tube 3, although potentials applied therefrom over a conductor III may be utilized for biasing the triode grid in the first intermediate frequency tube 5, since they are in the proper direction. The triode grids in the tubes I and 3, therefore, must be connected to the cathodes thereof over a circuit serially including a separate source of potential which normally, or in the weak signal condition, supplies them with high negative potential and which source may automatically be so controlled by the automatic volume control tube that the potential becomes less negative as signal amplitude increases.

To this end, I connect the triode grids in the tubes I and 3 by means of a common conductor |93, to a point I on the bleeder resistor 39, the potential of which is the same as that of the cathode connecting point 51, or more negative than that of the said point as actually illustrated in the drawing and serially include in the connection a resistor |91 across which a variable potential may be. developed by the flow of current therein from a signal-controlled source. The complete circuit in which the current flows includes the secondary winding of a power-transformer H39, the space current path in the triode portion of the second intermediate frequency tube 1, the common cathode conductor 55 and that portion of the bleeder resistor 39 between the points 51 and IIl5 to which the conductor 55 and the conductor IUS, respectively, extend.

From the triode grid in the second intermediat-e frequency tube, a connection III extends to the conductor lI whereby negative bias is applied to the said grid, the bias in value depending upon the amplitude of an incoming signal and the position of the contact element 19.

For maximum selectivity, therefore, when receiving either weak or strong signals, the contact element 19 is adjusted toward the end A of the resistor 11, thus permitting the potential of the triode grids in the first and second intermediate frequency tubes 5 and 1 to assume a bias potential determined by the point of connection 1I of the network 12 to the bleeder resistor 39, which potential is so chosen as to permit space current to flow in the triode portions of the said tubes. Rectified current from the transformer |39 flows in the resistor I01 in the direction of the arrow and biases the triode grids of the tubes I and 3 to the cut-off potential, thus preventing the triode portions of the tubes from drawing plate current and loading the coupling inductors 4I and 45.

For maximum fidelity, the contact element 19 is adjusted toward the end B of the potentiometer resistor 11. In this position, the potentials developed across the resistor 15 are applied to the triode grids in the tubes 5 and 1, which potentials, as hereinbefore explained, increase negatively with increase in amplitude of incoming signals. As the triode grid of the tube 1 becomes more negative, the flow of rectified current in the resistor |01 decreases, thus causing the triode grids in the tubes I and 3 to become less negative with the resultthat space current flows therein and the tuning of the coupling inductors 4I and 45 is broadened. The more negative potential applied to the triode grid in the tube 5 causes the space current therein to diminish, thus increasing the effective coupling between the primary and secondary windings of the output transformer 49 and increasing the energy transfer thereby.

The increased energy transfer, by properly adjusting the constants of the system, may compensate the decrease in energy transfer by the coupling inductors 4I and 45 or it may be so adjusted that the gain in the system, as a whole, is lessened as signal amplitude increases.

From a consideration of the foregoing, it will be apparent that, when the movable contact element of the potentiometer is adjacent to the end A, the system functions substantially the same as automatic volume control receivers of known types. By adjusting the contact device toward the point B, however, the system automatically so adjusts itself that increased signal strength gives rise to decreased selectivity.

It will also be apparent that, through manipulation of the adjustable resistor 84, current may be caused to flow through the resistor 'I3 o r l5, depending upon the position of the contact element 79, independently of the action of the automatic volume control tube. Such being the case,v

the sensitivity of the receiver can be limited to any desired value, thus rendering the delity of the system independent of the setting of the sensitivity control. That is to say, the fidelity of the system will be determined entirely by the strength of the received signal and the setting of the contact device 19.

It will, of course, be obvious that, if interference manifests itself during the reception of strong signals, by reason of the automatic broadening of the tuning, the contact element may be moved awayV from the point B and toward the point A to an extent sucient to increase the selectivity of the system and to introduce a desired amount of automatic volume control.

It will be noted from the drawing that I have provided a single impedance-coupled amplification stage for the amplification of intermediate frequency currents and a later transformer--` coupled stage for the same purpose. It is, however, to be understood that the illustration of the single impedance coupling device 45 is merely for purposes of explanation, since in receivers constructed by me I have usually utilized a plurality of such devices and an extra thermionic It is, however, important to note that the advantages accruing from a plurality of impedancecoupled stages may be obtained through the use of a single transformer stage in lieu thereof, together with proper circuit connections whereby the primary and secondary of the transformer are both loaded in response to the amplitude of an incoming signal. A stage of the type referred to is illustrated in Figure 2 of the drawing.

For the purpose of practicing my invention, as just described, the coupling transformer 50 comprises a tuned primary winding and a tuned secondary winding, the primaryrwinding being effectively shunted by the auxiliary space current path in the first tube and the secondary winding being effectively shunted by the space current path in the second tube.

The primary and secondary windings of the transformer are so designed and spaced apart that the coupling therebetween is adjustedl for a fair degree of selectivity. As an alternative, the coupling may be so adjusted that the transformer passes a band of frequencies narrower than the band normally passed by the hereinbefore described transformer 49, thus providing the requisite selectivity for the reception of distant stations. Stating the characteristics of the transformer 56 in still another way, it may be so designed as to be band-passing when the selectivity of the system is maximum, whereas the transformer 49 is so designed that it exhibits band-pass characteristics when the selectivity of the system is minimum.

It will be noted from an inspection of the drawing that the auxiliary grids 23 in the several tubes are connected to the conductor 103. Such being the case, when the incoming signal increases in amplitude, these auxiliary grids become less negative, and the several space current paths controlled thereby simultaneously exert a shunting action upon the primary and secondary windings of the transformer. The selectivity of the transformer, therefore, de.` creases with increase in signal amplitude while, at the same time, the signal energy transferred from the first to the second tube is reduced, thusl simulating the action of the impedance device or devices 45 hereinbefore described in detail.

In order to make the advantages accruing from my invention more clearly apparent, and to point out the reasons why the separate potential source is utilized for the purposes hereinbefore de-` scribed, a brief rsum of my improved system will now be given.

In the rst place, it is decidedly advantageous to utilize combined pentode-triode tubes for the functions of amplication and fidelity control, thus obviating the necessity of a plurality of separate tubes for the latter function as disclosed in my aforementioned prior application. also advantageous to utilize a single thermionic device for the purpose of correlating automatic volume control and automatic delity control to signal strength, the two functions being made available, in different degrees, through the manual operation of a single control device such as the contact element 19, associated with the potentiometer resistor l'l, the position of which element determines which of the two functions is being made available.

Obviously, therefore, one end of the resistor 'l'l must supply automatic volume control potentials to the control grids of the pentode portions of the tubes l and 5, while the other end of the resistor must supply fidelity-control potentials to the tubes I, 3, and 5. Y As hereinbefore explained, the potential on the triode grids of the tubes l and 3 is caused to become less negative with increase in signal strength while the potential on the triode grid of tube 5 must become more negative. This necessitates some sort of a potential reversing device,V which is supplied by the triode portion of the tube 1, to the grid of which the end B of the resistor 'l1 is directly connected, and in the output circuit of which is included the resistor H11.V

Since one end of the resistor 01 is connected to the triode grids of tubes l and 3 and the other end must be connected to a point in the system at the same potential as the common cathode potential of tubes I, 3, 5, and 1, or negative with respect thereto, in order to provide the necessary fixed negative bias on the triode grids of tubes l and 3 when no current iiows through it, and since the resistor must be included in a cir- It iSA cuit interconnecting the triode anode of the tube 'I and a source of potential, the common source 39 cannot be utilized.

A very important phase of my invention, therefore, consists inthe vrealization of the need for an extra potential source such as the transformer |09, or an equivalent battery (not shown). Naturally, however, in an alternating current powered receiver, the transformer is the logical source, as illustrated.

If the receiver is of the battery-powered superheterodyne type, a transformer, such as is exemplified by the transformer I I 3, may be utilized instead of the power transformer H09, to supply potential to the anode of the triode portion of the second intermediate frequency tube l. A coupling condenser might also be used for the same purpose.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by thespi'rit of the appended claims.

I claim as my invention:

1. In a radio receiving system, a thermionic device having more than one input electrode, means for supplying to one of the input electrodes of said device unidirectional potentials proportional to the amplitude of an incoming signal and means for supplying to another input electrode of the same device unidirectional potentials inversely proportional to the amplitude of an incoming signal.

2. The invention set forth in claim 1 characterized in that the input electrodes are so disposed within the thermionic device as to independently control an equivalent number of space current paths.

3. The invention set forth in claim 1 characterized in that the rst-mentioned unidirectional potential supplying means is constituted by a thermionic device.

4. The invention set forth in claim 1,1characterized in that the second mentioned unidirectional potential supplying means includes a thermionic device.

5. The invention set forth in claim 1 characterized in that the second-mentioned unidirectional potential supplying means includes a thermionc device having a plurality of input electrodes, a common cathode, and a plurality of output electrodes.

6. In a radio receiving system, a thermionic device of the type having a cathode, output electrode structure, and a plurality of input electrodes, the input electrodes being each so disposed with respect to the cathode and output electrode structure as to be capable of independently controlling space current paths individual thereto, an output circuit connected to said output electrode structure, means for supplying incoming signals to one of said input electrodes, means for supplying to said one electrode unidirectional potentials proportional to an incoming signal, and means for supplying to another of said input electrodes unidirectional potentials inversely proportional to an incoming signal.

7. In a radio receiving system, a thermionic device having a plurality of input electrodes, a common cathode and output electrode structure, the input electrodes being each so disposed with respect tothe cathode and output electrode structure as to be capable of independently controlling space current paths individual thereto, thermionic means for supplying to one of the input electrodes unidirectional potentials proportional to the amplitude of an incoming signal, thermionic means for supplying to another input electrode of the same device unidirectional potentials inversely proportional to the amplitude of an incoming signal, a Source of plate potential, a connection from the cathode to a point on said plate potential source, and means for ccnductively connecting the anodes of the unidirectional potential supplying devices to said plate potential source at a point the potential of which does not differ greatly from the potential of the point to which the cathode of the first mentioned thermionic device is connected.

8. The invention set forth in claim 'l additionally characterized in that a source of potential is serially included in the connection from the anode of the second-mentioned unidirectional potential supplying source to the plate potential source.

9. The invention set forth in claim 7 characterized in that a source of potential is included in the connection from the anode of the second mentioned unidirectional potential source to the plate potential source 4and additionally characterized in that the said source of potential is energized from the said plate potential source.

10. Y In a radio receiving system, a diode detector tube having an output circuit, an automatic volume control tube having an input circuit and having an output circuit from which automatic volume control vpotentials may be derived, a plate potential source, a connection from the anode of the detector tube to the negative terminal of said source, a connection from the cathode of the automatic volume control tube to a point on said source positive with respect to the negativev terminal, and a connection extending from the cathode of the diode detector tube to the grid of the automatic volume control tube whereby unidirectional potentials representative of carrier current amplitude may be impressed upon the grid of the automatic volume control tube and whereby the threshold value of a signal below which automatic Volume control action does not take place may be varied by varying the connection of the cathode of the automatic volume control tube to the said potential source.

11. In a radio receiving system, a plurality of thermionic amplifying devices having a common cathode connection, a demodulating device of the diode type having a cathode and an anode and being adapted to cut ofi" when the anode is negative with reference to the cathode, connections whereby the cathode of the demodulating device is normally maintained more negative than the said common cathode connection, gain controlling means for said system including a gain control tube having a. control grid connected to the diode cathode, and means for causing the cathode of the demodulating device to become less negative with respect to said common cathode connection in response to an increase in the amplitude of a received signal.

12. In a radioreceiving system, an amplifying device having a plurality of input electrodes, a thermionic signal responsive device having a cathode and an anode, an output circuit for said device including a resistor, a plate potential source, a connection from one end of said resistor to a point on said plate potential source, a connection from the cathode of said device to a point on said plate potential source, the potential of the last referred to point with respect to the neg- @tive terminal of said source being substantially the same as the potential of the rst mentioned point with respect to the said negative terminal, a connection between the cathode of the amplifying device and the cathode of the signal-responsive device, a connection between the other end of the resistor and an input electrode of said amplifying device, and means whereby output current from said signal responsive device varies inversely with respect to the amplitude of Van incoming carrier current, whereby said input electrode becomes less negative with respect to its cathode upon increase in signal amplitude.

13. In a radio receiving system, a thermionic device having two control electrodes, a cathode and output electrode structure, a signal output impedance device connected to said output structure, said control electrodes being so disposed with respect to said cathode and said output structure as to be capable of independent control of an equivalent number of space-currents, means for applying signal potentials to one of said control electrodes, means for deriving an unidirectional potential varying in response to the amplitude of an incoming carrier wave, and means for applying said potential to the other of said control electrodes in such direction as`to lower the impedance of its associated space current path upon increase in signal amplitude, whereby automatic control of the damping of the output impedance device in response to signal amplitude may be had.

14. In a radio receiving system, a plurality of thermionic devices each having a plurality of input electrodes, a cathode and output electrode structure, impedance coupling means between said devices, an output circuit for one of Ysaid devices including the primary winding of a transformer, the input electrodes in said devices being so disposed with respect to the cathodes and output electrode structures therein as to be capable of independently controlling an equivalent number of space current paths, and signal responsive means for causing corresponding input electrodes in the several devices to be biased in opposite sense proportionately to signal amplitude.

15. In a radio receiving system, a, plurality of thermionic devices transformer-coupled in cascade, each of said transformers being constituted by a pair of coupled tuned windings, means for varying the effective coupling between the primary and secondary windings of each of the transformers, means for controlling the effectiveness of the coupling-varying means in response to incoming signals, means for making the effective coupling between one pair of windings critical when weak signals are being received, and means for making the coupling between the other pair of windings critical when strong signals are being received.

16. In a, radio receiving system, a plurality of thermionic devices transformer-connected in cascade, the coupling between the primary and secondary windings of one of said transformers being such and the constants of the said windings being,r so chosen that the transformer passes a narrow band of frequencies, the coupling between the primary and secondary windings of another of said transformers being such that the transformer passes a wide band of frequencies, means for imposing a load on the primary and secondary windings of the first-mentioned transformer, means for providing a path effectively in shunt to the primary winding of the second-mentioned transformer, and means for increasing the load on the windings of the iirst transformer in response to an increase in signal amplitude and for increasing the impedance of the shunting path associated with the primary winding of the second mentioned transformer in response to an increase in signal amplitude.

17. In a radio receiving system at least three transformer-coupled thermionic devices, each of the transformers comprising a pair of coupled tuned circuits, signal responsive means for increasing the effective coupling between one pair of coupled tuned circuits in response to an increase in the amplitude of a received signal, and signal responsive means for decreasing the effective coupling between the other pair of coupled tuned circuits in response to an increase in the amplitude of a received signal.

18. The invention set forth in claim 16 characterized in that each of the signal responsive means is constituted by a thermionic device.

19. In a radio receiver, means for changing the selectivity of said receiver in response to a change in the amplitude of an incoming signal, means for changing the sensitivity of said receiver in response to a change in the amplitude of an incoming signal, and means for manually adjusting the sensitivity of the receiver without affecting said first means.

GEORGE L. BEERS.

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