US2055992A

US2055992A - Reflex superheterodyne receiver

Info

Publication number: US2055992A
Application number: US668406A
Authority: US
Inventors: Yolles Jacob
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1933-04-28
Filing date: 1933-04-28
Publication date: 1936-09-29
Anticipated expiration: 1953-09-29
Also published as: DE658909C

Description

Sept. 29, 1936. J. YoLLEs 2,055,992

REFLEX SUPERHETERODYNE RECEIVER Filed April 28, 1955 s sheets-sheet 1 N ASE..

hm-Ix MW Ql ATTORNEY s sheets-sheet 2 Filed April 28, 1935 ,WENN

INVENTOR JACOB YOLLS w@ ATTORNEY J. YOLLES Filed April 28, 1935 5 Sheets-Sheet 3 REFLEX SUPERHETERODYNE RECEIVER Sept. 29, 1936.

Patented Sept. 29, 1936 UNITED STATES PATENT OFFICE Jacob Yolles, Brooklyn, N. Y., assignor to Radio Corporation of America, a corporation of Dela- Ware Application April 28, 1933, Serial No. 668,406

8 Claims.

My present invention relates to reflex radio receivers, and more particularly to improvements in superheterodyne receivers utilizing the refieX principle in the amplification of the intermedi- .5 ate frequency energy.

There has been disclosed, and claimed, by G. W. Pickard, in U. S. Patent 1,770,143 issued July 8, 1930, a method of operating a superheterodyne receiver in such a manner that, before reproduc- 10 tion, the heterodyne beat energy is transmitted through at least one stage of radio frequency energy amplification. He has pointed out that in this way there is obtained the combined advantages of superheterodyne reception and reflexing. In general, it may be stated that the essential principle disclosed in the aforesaid Pickard patent comprises the heterodyning of the amplified collected radio frequency signal energy with locally produced oscillations, and the reflexing of the resulting intermediate frequency energy through the radio frequency signal amplifier.

Now, after considerable experimentation in, and development of, the aforesaid reflex superheterodyne method, I have discovered certain criteria of operation which must be considered if such a method of operation is to be satisfactorily used in a commercial broadcast receiver. 'I'here exists a condition which makes feed-back and instability possible in a superheterodyne system wherein the intermediate frequency energy is refiexed through the radio frequency amplifier. An ideal frequency changer would have only intermediate frequency energy in its output. However, there does exist a considerable amount of energy at the received signal 'frequency. If this radio frequency energy gets back Ato the tuned radio frequency circuit in the input of the reflex stage there exists a possibility for 40 oscillation at radio frequency. Again, oscillation may take place at the intermediate frequency when there is insufficient attenuation of the intermediate frequency energy by the radio frequency load in the output of the reflex stage;

this may be due to its lack of selectivity, or other causes. The efficiency of the circuits, thestage gain, and the phase of the feed-back voltage are further determining factors.

Accordingly, it may be stated that it is a prime desideratum of the present invention to provide a superheterodyne receiver wherein the intermediate frequency energy is, in general, reflexed for additional amplification through an amplifier, the latter, additionally, being utilized .to amplify venergy of a frequency other than the intermedi,-

.load and thence to the input of the refiex stage through the associated radio frequency input 15 circuit, and to, additionally, maintain a high impedance to the radio frequency signal, and extremely low impedance to the intermediate frequency, across the input of the frequency converter. I

Another important object of the present invention is to embody the reflex amplification principle in a superheterodyne receiver in such a manner that the intermediate vfrequency energy produced in vthe output of the frequencyv25 changer network is not only transmitted through ,a preceding radio frequency signal amplifier but is additionally transmitted through at least one stage of audio frequency amplification, before demodulation, the audio frequency amplifier stage 30 Ibeing particularly characterized by its including a vhigh gain, high plate impedance tube of the pentode type, provided with a suppressor grid at cathode potential, and generally low grid-anode `capacitance to insure stability.

Still another object of the present invention is toprovide a superheterodyne receiver wherein a composite detector-local oscillator tube is utilized, and wherein the intermediate frequency energy output of the aforesaid tube is transmit- 40 ted through two stages of amplification, one of these lastmentioned stages consisting of the radio frequency signal amplifier.

Still-.another object of the present inventionis .to permit the use of a fairly high order of .inter- 45 mediate frequency to be employed in a reflex circuit in which the tuning of the radio frequency circuits does not cause the tuning of the intermediate frequency resonance to be disturbed.

`And still other objects ofthe present invention V50 are to improve generally the simplicity and efficienoyof superheterodyne receivers, and to particularly provide a receiver of this type, employ- -inga radio frequency amplifier for amplification of intermediate frequency energy, which is not .55

only reliable in operation, highly economical in its use of electron discharge tubes, and stable in operation, but satisfactorily adapted for use in connection with modern broadcast receiver requirements.

The novel features which I believe to be characteristio of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings,

Fig. 1 schematically shows a functional diagram of the route taken by the high frequency currents through a system embodying the present invention,

Fig. 2 diagrammatically shows a superheterodyne receiver embodying the invention illustrated in Fig. 1,

Fig. 3 is a circuit diagram of a modification embodying the present invention,

Fig. 4 shows a modification of the circuit shown in Fig. 3, the modification including the use of an audio amplifier stage for the amplification of intermediate frequency energy.

Referring now to the accompanying drawings, wherein like reference characters in the different figures designate corresponding circuit elements, attention is first directed to Fig. 1 wherein is shown a functional diagram of the flow of currents in the arrangement shown in Figs. 2 and 3, as well as partly in Fig. 4. It should be noted that the input circuits disposed between the input electrodes of the reflex tube I are resonant simultaneously and independently, to both the signal frequency, which is of a radio frequency, and the intermediate frequency. Thus, the network resonant to the signal frequency is designated as Rh F. response circuit; and the network resonant to the intermediate frequency is designated by the expression 1. F. response circuit.

The output of the reflex stage contains circuits which are likewise resonant to the signal and intermediate frequencies, and these circuits permit amplification at these frequencies. The Voltage which excites the frequency changer 2 (converter, or first detector) is taken from the R. F. load" in the anode circuit of tube I; the voltage across the I. F. load, which is likewise in the anode circuit of tube I, is impressed across the input' electrodes of the second intermediate frequency amplifier tube 3. The output of tube 3 is, of course, transferred in the usual manner to the second detector circuit and demodulated thereby.

Hence, it will be appreciated that the general method of operation involves the amplification, at radio frequency, of the collected signal energy, the utilization of the amplified signal energy in the production of intermediate frequency energy, the amplification of the intermediate frequency energy in the aforementioned signal amplifier, and the detection of the amplified intermediate frequency energy, after optional additional amplification.

There exists a condition which makes feedback and instability possible and which can best be understood by reference to the generalized circuit shown in Fig. l. An ideal frequency converter 2 would have only intermediate frequency energy in its output circuit. However, there does exist a residual quantity of energy at radio frequency in the anode circuit of tube 2. If this radio frequency energy gets back to the radio frequency responsive circuit in the input circuit of the reflex tube I, there exists a possibility for oscillation at radio frequency. The criteria are the efficiency of the circuits, the gain, the phase of the feed-back voltage and its magnitude which depends upon the degree of attenuation by the intermediate frequency circuit through which it must pass.

Oscillation may, also, take place at the intermediate frequency when there is insufficient attenuation of the intermediate frequency by the radio frequency load in the output of the reflex tube I. It can readily be seen, therefore, that the design of the system should be such as to minimize the transmission of radio frequency energy from the frequency changer 2 to the intermediate frequency response circuit in the input of the reiiex stage, and thence to the associated radio frequency response circuit. The radio frequency load in the output of the reflex stage should further be designed with a view to offering high impedance to the radio frequency signal, and extremely low impedance to the intermediate frequency.

Again, referring to Fig. l, it is evident that the intermediate frequency response circuit is in intimate relationship electrically to the radio frequency response circuit, and since the former is adjusted to respond to a predetermined fixed intermediate frequency, it is essential that the variation of the radio frequency respo-nse circuit which is tunable over a wide band of carriers shall not disturb the tuning adjustment of the intermediate frequency circuits.

In Fig. 2, there is shown a conventional superheterodyne receiver embodying the method of operation shown in the generalized circuit of Fig. l. As stated, the receiving circuit shown is a purely conventional one, and possesses novelty only in so far as it includes various novel circuit elements which are essential to the proper operation of the reflex arrangement which the circuit embodies. The electron discharge tube ll is of the screen grid type, and preferably of the variable mu type. Such a tube is well known to those skilled in the art at the present time, and essentially, possesses an anode current-grid voltage characteristic which is substantially exponential. electrodes of tube 4 are coupled, through a network to be later described, to the usual grounded antenna circuit 5, the reference character M designating the coupling between the antenna circuit and the tuned input circuit of tube 4.

This tuned input circuit consists of a coil L1 connected across the variable tuning condenser C1, one side of the coil L1 being connected to the stator, or high potential, side of the condenser Ci, while the other side of the coil is grounded through a condenser c1, it being noted that the rotor side of the condenser C1 is additionally grounded. The control grid of tube Il is connected to the high potential side of the tuned circuit L1C1 through a condenser c2. The grounded cathode lead of tube 4 includes in series therewith the usual control grid biasing network 6, and positive potential is applied to the screen grid and anode of tube 4, the potential sources not being shown; the anode of tube 4 being connected to its source of positive potential through coil Z4.

The network including the coil L1 and the variable condenser C1 is adapted to be resonated toa desired radio frequency signal, such signal being selected in the usual manner by adjustment of The input the condenser C1. In this way radio frequency signal energy is impressed on the amplifier tube 4, and the amplified output of this tube is impressed upon the resonant input circuit of the first detector tube 1, which is preferably a screen grid tube. It will be noted that the anode of tube 4 is connected to the control `grid of tube I through a condenser C3, and that the resonant input circuit of tube 'I consists of a coil Lz connected across the variable tuning condenser C2, as in the case of the signal input circuit of tube 4. The low potential side of the coil L2 is grounded through a condenser c3; and the rotors of condenser C2 are grounded, while both the control grid of tube 'I, and one side of condenser C3, are connected to the high potential side of the circuit Lz-Ca The local oscillator circuit is designated by the `numeral 8, and is conventionally shown. It does not form a part of the invention, as shown in Fig. 2, and therefore is generalized in its coupling to the first detector circuit. That is, the tunable oscillator circuit, including the variable tuning condenser 9, is shown coupled, as at M1, to the grounded cathode lead of tube 1. The rotors of the condensers C1, C2 and 9 are mechanically connected, in any'well known manner, for uni-control tuning, and this well known uni-control arrangement is conventionally designated by the dotted lines generally denoted by the numeral I 0.

The intermediate frequency energy produced in the output of the first detecte-r tube 'I is reflexed back to the input circuit of tube 4, and after ampliiication of this energy by tube 4, the amplified intermediate frequency energy is further amplied in an additional stage of intermediate frequency amplification conventionally symbolized, and designated by the expression 1. F. amplifier. The output of this latter stage is then impressed upon the input circuit I I of the conventional second detector circuit, and the output of the second detector may be directly reproduced, since it is at audio frequency, or it may be amplified at audio frequency prior to final reproduction. These last named steps in the treatment of the intermediate frequency energy after its reflex amplification are purely conventional, and are believed so well known to those skilled in the art as to render it unnecessary to require further eX- planation.

The intermediate frequency load in the output of tube I comprises the anti-resonant network including the coil Z1 and the fixed condenser c4, one side of this network being connected by a lead I2, to the anode of the rst detector tube 1; the other side of this network is connected to a source of proper positive potential (not shown), and this connection serves to provide the proper positive potential for the anode of the iirst detector tube 1. The network l1-c4 is magnetically coupled to a second similar resonant network comprising the coil Z2 and the shunt condenser c5. The coupling between these two networks consists of the mutual inductive coupling between the coils Z1 and l2. The coil Z2 is inductively coupled to a coil la, which with condenser c2 forms the I. F. response circuit, and which has one side thereof connected to the control grid of tube 4; the other side thereof is connected to ground through a path which includes the automatic volume control filter resistor I3, the adjustable lead I 4, and the grounded automatic volume control resistor I5.

It will be observed, additionally, that this same path constitutes the normal biasing path for the control grid of tube 4, as well as the automatic biasing path for this control -g-rid. 'Ihe automatic volume control arrangement is not shown in detail sinceit is of a type well known to thoseskilled in the art. It is merely necessary to show in conventional form the control resistor I5 connected to the second detector, and the connections from the resistor I5 to the control grid of amplifier tube 4, and to the control grid of the first detector tube 1. The connection to the latter tube is provided through` a path which includes the signal frequency tuning coil L2, the resistor I6, and the adjustable lead I'I.

The control resistor I5 is, of course, disposed in the anode circuit of the'second detector; potential variations across the resistor I5 thus automatically bias the control grids of tubes 4 and 'I in such a manner that the input to the second detector input circuit is substantially constant, regardless of fiuctuations in the intensity of the signal energy collected by the antenna circuit 5. It is to be clearly understood, however, that while it is desirable to employ automatic gain control of the tube 4 and first detector tube l, it is not essential to the present invention.

In explaining the operation of the receiving arrangement shown in Fig. 2, it is to be clearly understood that the iirst detector circuit reiiexes back intermediate frequency energy upon the radio frequency amplifier tube 4. The path including the coil L1 and the condenser ci is serially resonant to the operating intermediate frequency, and the condenser c1 has a magnitude of about 0.0032 mf. Each of the tuned circuits Z1-c4 and lz--ce is resonant to the operating intermediate frequency, and the path including the coils ls and the condenser c2 is resonant to the operating intermediate frequency. Each of coils Z1, lz and la has a magnitude of about 9.8 mh. while each of condensers c2, c4 and c5 has a magnitude of about 7 0 mmf.

The intermediate frequency energy reilexed back from the iirst detectortube 'I is transmitted to the input electrodes of the radio frequency ampliiier tube 4 through several cascaded resonance networks, all tuned to the operating intermediate frequency. Inthis way there is insured the impression across the coil Z3 voltage variations due solely to the intermediate. frequency currents, the radio frequency currents being iiltered out by the passage of the energy from the anode circuit of tube 'I through the cascaded selective circuits all resonant to the intermediate frequency.

The series resonant path Li-ci functions to prevent amplified intermediate frequency energy stray currents feeding back into the rst tube by capacity coupling to the antenna. ls-cz constitute the intermediate frequency response circuit of Fig. 1.

rThe construction of the coil Z3 is somewhat critical. That is to say, it is preferred thatl the same be multi-layer wound on a wooden core having a diameter of about iive-eighths of an inch and a thickness of about one-eighth of an inch, the wire employed `for winding this coil being No. 38 D. S. C. It is to be clearly understood, however, that the construction for the coil Z3 is described as an illustration of a coil that has been found satisfactory; further, the various values given herein for the coils and condensers are purely illustrative, and are in no way to be considered as the only values permissible. The special design is to prevent the introduction of resistance into the Lr--Ci circuit due to losses in the coil I3.

The intermediate frequency load circuit for the tube 4 lis composed of coil Z4 and condenser C3.

The elements The coil Z4 has a magnitude of about 10 mh. while the condenser C3 has a magnitude of about 90 mmf. maximum. Another series resonant path, tuned to the operating intermediate frequency, is provided in the signal input circuit of the rst detector tube 1, and this path consists of the coil L2 and the condenser cg. The coil L2 has a magnitude of about 280 mh., and the condenser c3 has a magnitude which is the same as that of condenser c1 in the signal input circuit of tube 4.

In actual operation, then, the condensers C1, C2 and 9 are Varied to select a desired broadcasting station frequency, and the intermediate frequency energy produced in the output of the first detector tube i is refiexed back to the input network of the amplifier 4, this input network including circuits capable of independent response to the intermediate frequency energy and the radio frequency signal energy, Similarly, the output network of tube 4 includes independent radio frequency signal and intermediate frequency circuits. Due to the arrangement of these independent frequency load circuits in the input and output of tube 4, as shown, oscillation at radio frequency or intermediate frequency is substantially prevented.

Furthermore, it has been found that the tube 4 provides sufcient intermediate frequency amplification to eliminate the need for a separate intermediate frequency amplifier tube ahead of the stage designated I. F. amplifier. That is to say, two normal stages of intermediate frequency amplification, and the gain normally secured by such two stages, are secured with the use of a single intermediate frequency amplifier tube. At the same time instability is sufficiently prevented, even though the radio frequency signal amplifier tube is functioning as the other intermediate frequency amplifier. The present receiver construction operates particularly well when the operating intermediate frequency range is in the vicinity of 175 kilocycles, because detuning of the intermediate frequency is negligible as the tuning condenser is operated throughout its tuning band, and the instability tendencies lend themselves to control by the devices described.

In Fig. 3 is shown a modification of the arrangement shown in Fig. 2. The receiving arrangement of Fig. 3 is one preferred for a receiver construction wherein compactness, and economy of space, are determining factors. The receiver is of the superheterodyne type, and embodies several novel constructional features which render the receiver highly desirable for use as an automobile receiver.

For example, the receiver not only embodies automatic volume control and refiex intermediate frequency amplification, but also, includes a composite detector-oscillator of a desirable type. The grounded antenna circuit 5 is coupled, as at M, to the tuned radio frequency signal input circuit of the radio frequency amplifier tube 4 of the screen grid type. The control grid of the tube 4 is connected to a tap on the input coil S1, this coil being coupled to the antenna coil P1. The variable condenser C1 adjusts the input circuit of tube 4 to the desired signal frequency, and the series resonant path, including condenser c and coil S2, is tuned to the operating intermediate frequency. The coil S2 is inductively coupled to the coil P2, and the latter coil is shunted by the condenser cio, the circuit Pz--cio being tuned to the operating intermediate frequency, one side thereof being connected by lead 20 to the anode of the oscillator-converter tube 2|, and the opposite side of this circuit being connected, by a lead 22, to the positive potential source for the anode of tube 2|. The screen grid of tube 4 is connected through a path which includes lead 23, resistor 24 having a magnitude of about 350 ohms, resistor 25 having a magnitude of about 20,000 ohms, and lead 26 to the positive potential source B.

The anode of tube 4 is connected to the control grid of the converter tube 2| through a condenser co. The amplified intermediate frequency energy is taken out of the anode circuit of tube 4 through a path which includes the coils Pi and Pz in series. The lead 2 connects these series connected coils to the positive potential supply lead 22, and therefore this aforementioned path, which includes the coils, serves to supply positive potential to the anode of tube 4.

The tube 2| is a so-called pentagrid converter tube. The tube essentially comprises a cathode, a signal control electrode, an anode, a screen grid, a grid which is at a potential close to that of the cathode, and a positive shield grid between the cathode potential grid and the screen grid. The tube, and its electrodes, are connected to provide a combined oscillator-converter circuit, the radio frequency signal input circuit being connected between the signal control grid and the grounded leg of the cathode, this grounded leg including the usual signal control grid biasing network 28. The positive shield grid is coupled, as at M2, to a tunable oscillator circuit 29, which tunable circuit includes the variable condenser 30. Both the cathode, and the cathode potential grid, are connected to one side of the tunable circuit 29 through a radio frequency by-pass condenser, and the other side of this tunable oscillation circuit is grounded.

The pentagrid converter, and its use as an oscillator-converter in a superheterodyne receiver, is not a part of the present invention, since such an arrangement is disclosed and claimed by J. C. Smith in application Serial No. 654,421 filed January 31, 1933.

It is believed suficient for the purpose of the present application to point out that the action of this tube 2| in converting a radio frequency to an intermediate frequency depends upon independent control of the electron stream by three electrodes, including the cathode, connected in an oscillator circuit, and by a fourth electrode, a grid, to which the radio frequency signal input is applied. The cathode and the first two grids may be regarded theoretically as a composite cathode which supplies a modulated electron stream.

This modulated cathode stream may be further controlled and utilized by means of the addition of other grids and a plate. The control grid placed between the composite cathode and the plate provides for the introduction of the radio signal. The additional grids placed on either side of the control grid shields the control grid Velectrostatically from the other electrodes, and

the tuned circuit includes the coil S'i'as well as the variable tuning condenser 3l. The rotors of the three condensers C1, 3l and 30 aremechanically connected for uni-control, and the dotted Vline 32 symbolizes such mechanical uni-control.

The screen grid tube 32 functions as an intermediate frequency amplifier, and the resonant input circuit connected between its control grid and its cathode includes the coil Sz shunted by the condenser 33. The input circuit of tube 32 is tuned to the operating intermediate frequency, and coils Pz and S2 are tightly coupled. The screen grid of tube 32 is connected to a point between

resistors

24 and 25, while the anode of the tube is connected, through coil 34 and lead 26, to the source of positive potential B. The usual grid biasing network 35 is connected between the cathode of tube 32 and the grounded side of its grid circuit, and an appropriate radio frequency bypass condenser is connected between the cathode and screen grid lead of the tube.

The amplified output of the intermediate frequency amplifier 32 is impressed upon the resonant input circuit of tube 35. This tube is a triple purpose tube inasmuch as it performs simultaneously the second detector function, an automatic volume control function, and an audio frequency amplification function. The tube 35, brieiiy, comprises a pair of independent electron systems disposed within a single glass envelope. One of these systems comprises the cathode and a pair of diode anodes disposed adjacent it; the other electron system comprises another emission portion of the cathode, a grid and the main anode of the tube. One of the diode anodes is connected to the high potential side of the resonant input circuit 36 which is tuned to the intermediate frequency to be detected. The other low potential side of the. input circuit 36 is connected to the cathode of tube 35 through a parallel resistor-capacitor network 3T; the cathode being grounded through a resistor path.

The remaining diode anode is connected to the aforementioned diode anode through a radio frequency by-pass condenser to have intermediate frequency energy impressed thereon; this second diode anode functions as the automatic volume control anode, and is connected back to the grounded side of the cathode of tube 35 through a path including lead 38, resistor 39, and resistor 40. The grid of tube 35 is connected, by a lead 4|, to a point on the diode anode circuit which functions as the second detector for the intermediate frequency energy, the lead 4l terminating rin an adjustable tap which is operable over a resistor; the tap and resistor functioning as a manual volume control instrumentality. The adjustable lead 4l applies on the grid of tube 35 the rectified intermediate frequency energy, and the main anode of tube 35 has iiowing in it the amplified rectified intermediate frequency energy.

The audio frequency energy appearing in the anode of tube 35 is impressed upon a power output tube, of the pentode type, through a conventional resistance-condenser coupling, the tube being designated by the reference numeral 42, and the output circuit of tube 42 is adapted to be coupled to any type of loud speaker. The automatic volume control connections from the control resistor

path including resistors

39 and 40 comprises the lead 43 connected to the low potential side of coil Sz, and the lead 43 to the low potential side of the coil Si.

In this way, in a manner well known to those skilled in the art, the gain of the amplifier tube 4,

and that of the converter 2| is automatically controlled. It is not believed necessary to describe in any further detail the construction of the duplex diode triode tube 35 and its associated second detector, automatic volume control and audio amplifier circuits since such circuits and tube are not a part of the present invention, except in so far as they cooperate with the reflex arrangements and novel circuit elements disclosedl in Fig. 3. Attention is directed to Loy E. Barton application Serial No. 640,946, filed November 3, 1932, for a disclosure, which is claimed, of tube 35 and its associated circuits and functions.

The operation of the arrangement shown in Fig. 3 is quite similar to that described in connection with Fig. 2. That is to say, the intermediate frequency energy in the anode circuit of the converter 2i is reflexed back to the input of the radio frequency amplifier tube 4. An electrostatic shield is interposed between coils S2 and P2. This shield prevents electrostatic coupling between these coils, and the consequent capacitive transfer of energy at radio frequency from the oscillator-converter to the control grid of the reflex tube 4. In the design of the network Pz-cio it is preferable to use a large capacity and a small inductance, the object being to by-pass radio frequency energy.

Either, or both of these features, may be necessary, depending on the particular circuit, its eihciency andthe stage gain. The output of the reflex tube 4 contains two primaries P'1 and Pz in serial connection. The primary P'1 in conjunction with the capacity co represents a conventional design for iiat stage gain at radio frequency. The primary Pz is tightly coupled to the secondarySgz forgain at intermediate frequency. It should be noted that the intermediate frequency stage is thus designed for tuning in the secondary alone; that is, the primary is untuned. If it is desired to use a double tuned intermediate frequency stage at this point, good design calls for a'large capacity and a low inductance in the primary.

In Fig. 2 there was obtained a low impedance at the intermediate frequency between the grid" and cathode of the first detector by tuning the radio frequency signal coil by the automatic volume control condenser ca. The accuracy of such tuning depends uponthe magnitude of the gain achieved in the converter at intermediate frequency. In the cases where the gain is sufficie'ritly low the tuning need not be resorted to at all, and for this reason in Fig. 3 such tuning is not employed. 'Ihe primary P'1, the mutual inductance M', and the capacity co are chosen to give the maximum transmission at signal frequency to the radio frequency signal input circuit Si-SI with the desired radio frequency characteristic with respect to radio frequency, and to obtain a. minimum transmission of intermediate frequency energy.

The signal control grid of tube 4 is connected to an intermediate point on the tap coil S1, through condenser c, for a ldouble purpose. In the rst place image frequency suppression is secured. In the second place, it is an object of the choice of a large value for coil S2 and the small valueof condenser c to render the intermediate frequency circuit Sg-c free from detuning due to the Variation of the main tuning control C1. When a tapped coil is used, such as Si, there is greater freedom from detuning which permits the use of a. larger Value for c and S2. A further advantage lies in the fact that the losses in the coil S2 are not introduced into the Yradio frequency tuned circuit to the same extent as when the tap is at the top of the coil S1.

Similarly, in the case of the condenser co, this condenser could be connected to a tap on the coil Si. In Fig. 3 the condenser co is very small and serves no tuning function. It is possible, however, to provide a coupling between tubes 4 and 2| in which the coil P1 is omitted, and the condenser co tunes the coil P'z to the intermediate frequency. In this case especially, the tapping of coil Si would be important since the lower the tap on coil S'i the greater will be the attenuation of intermediate frequency energy in the circuit S1-3I. Additionally, it is often desirable to employ in the lead a resistance-capacity filter for attenuating radio frequency energy in the output of the oscillator-converter thereby preventing its transmission to the grid of the reflex tube 4.

In Fig. 4 is shown a modification of the arrangement shown in Fig. 3 embodying the modifications described in the last named paragraph. That is, in this receiving arrangement, which employs oscillator-converter 2| with associated circuits (corresponding exactly to that shown in Fig. 3), and a tube 35 with associated circuits similar to the latter figure, the anode lead 20 from the oscillator-converter includes the resistor 5E) and condenser 5| which provides the resistance-capacity filter for attenuating the radio frequency energy in the output of the converter thereby preventing its transmission to the grid of the reflex tube 4. Also, the condenser co is shown adjustably connected to an intermediate point on the coil S'i, the coil Pi being omitted. The condenser co tunes the coil Pz to the intermediate frequency, and the lower the tap on coil S'i the greater will be the attenuation of intermediate frequency energy in the circuit S1-3|.

It is not believed necessary to go over the description of the entire circuit shown in Fig. 4 since it is a substantial duplicate of the arrangement shown in Fig. 3, with the exception of the features to be hereinafter described. For this reason connections not necessary to a proper understanding of the novelty of Fig. 4, and particularly the circuits of the converter 2|, and the automatic volume control arrangement associated with tube 35, have been omitted. The arrangement in Fig. 4, besides eliminating the need for one intermediate frequency amplifier tube by utilizing the radio frequency amplifier tube 4 for one stage of intermediate frequency amplification, additionally eliminates the need for the intermediate frequency amplilier tube 32 of Fig. 3.

The audio frequency Voutput tube 52, of the pentode type, is utilized as an amplifier for the intermediate frequency output of tube 4. The object of. utilizing the audio frequency amplifier tube 52 for additional amplification of the amplified intermediate frequency energy is to keep the intermediate frequency signal at the first tube at a small amplitude to prevent undesired whistles and squeals in the receiver due to beats between harmonics of the intermediate frequency and the radio frequency. The audio frequency power tube may be utilized since it operates at maximum gain, and is comparatively distortionless. The power tube 52 is preferably of a high mu, high impedance type, the grid-plate coupling being small.

The attenuation of intermediate frequency energy in the output of the second detector 35 is accomplished by condensers 53 connected from lead 54 to ground, and the audio frequency output of the detector 35 is impressed on the signal grid of the audio power output tube 52 through the path which includes lead 54, direct current blocking condenser 55, and the resonant network consisting of coil La and shunt condenser Cs. The condensers 53 function to prevent any intermediate frequency energy getting back to the grid `of tube 52 and causing feed-back, and they also permit the intermediate frequency energy in the Le-C circuit to be fully impressed across the input of power tube 52.

As in the case of Fig. 3, the intermediate frequency energy output of converter 2| is refiexed back through lead 20 and the circuit Pz--cio upon the input 0f the reflex tube 4. The amplified intermediate frequency energy is then transmitted through the coupling M4 to the input circuit Ls-Cs of the audio frequency power output tube 52, and the amplified intermediate frequency energy is transmitted to the input circuit 6| of the tube 35 through the transformer primary 60, both circuits 60 and 6| being tuned to the operating intermediate frequency. The anode circuit of the tub-e 35 has iiowing in it the rectified intermediate frequency energy, and the audio frequency energy component is impressed, through lead 54, upon the grid of the power output tube 52.

The amplified audio frequency energy is taken out of the anode circuit of tube 52 by means of the audio frequency transformer 60', and utilized in any desired type of, reproducer. It is highly desirable to have tube 52 a high mu power tube of the type wherein a suppressor grid, at cathode potential, is disposed between the positive screen grid of the tube and the anode thereof, since stability and gain must be realized to make the refiex worth while. When such a pentode tube is used for tube 52 satisfactory operation is secured. It will, therefore, be seen that in Fig. 4 there is shown an arrangement wherein the tube 3 of Fig. l is eliminated by employing for this tube 3 the audio frequency amplifier which follows the second detector; and it will be, additionally, appreciated that this has been accomplished without sacrificing gain or stability.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In combination in a superheterodyne receiver, a radio frequency amplifier, a frequency changer, a first detector, a second detector, circuits inter-connecting said amplifier, said frequency changer, said rst detector and said second detector, said circuits including a tunable input circuit connected between the input electrodes of said radio frequency amplifier, said input circuit including a coil and a variable tuning condenser, and the control electrode of said radio frequency amplifier being connected through a coupling condenser to an intermediate point on said input circuit coil, a coil connected between said tunable input circuit and the control electrode of said radio frequency amplifier, said coupling condenser and said coil providing a series resonant path tuned to the operating intermediate frequency, a reflex path between the anode of. the first detector and said second coil, said reflex path including a resonant circuit tuned to said intermediate frequency, a tunable inputk circuit connected to the input electrodes of said first detector, and a resonant path connected to the anode circuit of said radio frequency amplifier and the tunable input circuit of said detector, said second resonant path being tuned to said intermediate frequency.

2.' In combination in a superheterodyne receiver, a radio frequency amplifier, a frequency changer, a first detector, a second detector, circuits inter-connecting said amplifier, said frequency changer, said first detector and said sec- :ond detector, said circuits including a tunable input circuit connected between the input electrodes of said radio frequency amplifier, said input circuit including a coil and a variable tuning condenser, and the control electrode of said radio frequency amplifier being connected through a coupling condenser to an intermediate point on said input circuit coil, a coil connected between said tunable input circuit and the control electrode of said radio frequency amplifier, said coupling condenser and said coil providing a series resonant path tuned to the operating intermediate frequency, a reiiex path between the anode of the first detector and said second coil, said reex path including a resonant circuit tuned to said intermediate frequency, a tunable input circuit connected to the input electrodes of said first detector, and aA resonant path connected to the anode circuit of said radio frequency amplifier and the tunable input circuit of said detector, said second resonant path being tuned to said inter- .mediate frequency, an audio frequency amplifier coupled to said second detector, and a coupling path, resonant to said intermediate frequency, between the input electrodes of said audio frequency amplifier and the anode circuit of said radio frequency amplifier.

3. In combination in a superheterodyne receiver, a radio frequency amplifier, a frequency changer, a first detector, a second detector, circuits inter-connecting said amplifier, said frequency changer, said first detector and said second detector, said circuits including a tunable input circuit connected between the input electrodes of said radio frequency amplifier, said input circuit including a coil and a variable tuning condenser, and the control electrode of said radio frequency amplifier being connected through a coupling condenser to an intermediate point on said input circuit coil, a coil connected between said tunable input circuit and the control electrode of said radio frequency amplifier, the coupling condenser and said coil providing a series resonant path tuned to the operating intermediate frequency, a reflex path between the anode of the first detector and said second coil, said reflex path including a resonant circuit tuned to said intermediate frequency, a tunable input circuit connected to the input electrodes of said first detector, and a resonant path connected to the anode circuit of said radio frequency amplifier and the tunable input circuit of said detector, said second resonant path being tuned to said intermediate frequency, an audio frequency amplifier coupled to said second detector, and a coupling path, resonant to said intermediate frequency, between the input electrodes of said audio frequency amplifier and the anode circuit of said radio frequency amplifier and a coupling network, resonant to the intermediate frequency, between the anode circuit of said audio frequency amplifier and the input electrodes of said second detector.

4. In combination in a superheterodyne receiver of the type including a radio frequency amplifier, an oscillator-converter, a second detector and an audio frequency amplifier, the improvement which comprises a coupling path, resonant to the operating intermediate frequency, between the anode circuit of said converter'and the input circuit of said radio frequency amplifier, a coupling path betweenA the input of said second detector and the output of said radio frequency amplifier, said coupling path being tuned to both the said intermediate frequency and simultaneously and independently to the radio frequency, means in the input circuit of said radio frequency amplifier for preventing feed-back of radio frequency signal energy from the anode circuit of said converter to the input circuit of said radio frequency amplifier, and additional means in the output circuit of said radio frequency amplifier for preventing feed-back in regenerative phase of radio frequency signal energy to the input circuit of said radio frequency amplifier.

5. In combination in a superheterodyne receiver of the type including a radio frequency amplifier, an oscillator-converter, a second detector and an audio frequency amplifier, the improvement which comprises a coupling path, resonant to the operating intermediate frequency, between the anode circuit of said converter and the input circuit of said radio frequency amplifier, a coupling path between the input of said second detector and the output of said radio frequency amplifier, said coupling path being tuned to both the said intermediate frequency and simultaneously and independently to the radio frequency, means in the input circuit of said radio frequency amplifier for preventing feed-back of radio frequency signal energy from the anode circuit of said converter to the input circuit of said radio frequency amplifier, and additional means, including a circuit resonant to the intermediate frequency, in the output circuit of said radio frequency amplifier for preventing feed-back in regenerative phase of radio frequency signal energy to the input circuit of said radio frequency amplier.

6. A superheterodyne receiver comprising a radio frequency amplifier, a first detector, a second detector, an audio frequency amplifier, means for impressing signal energy upon the input of said radio frequency amplifier, means for combining in the first detector locally generated oscillations with amplied signal energy, means for impressing the intermediate frequency output of the first detector upon the input of said radio frequency amplifier, means for impressing the amplified intermediate frequency output of said radio amplifier upon the input of said audio frequency amplifier, and means for impressing the amplified intermediate frequency output of said audio amplifier upon the input of said second detector.

7. A superheterodyne receiver comprising a radio frequency amplifier, a first detector, a second detector, an audio frequency amplifier, means for impressing signal energy upon the input of said radio frequency amplifier, means for combining in the first detector locally generated oscillations with amplified signal energy, means for impressing the intermediate frequency output of the first detector upon the input of said radio frequency amplifier, means for impressing the amplified intermediate frequency output of said radio amplifier upon the input of said audio frequency amplifier, means for impressing the ampled intermediate frequency output of said audio amplier upon the input of said second detector, and additional means for impressing the output of the second detector upon the input of said audio frequency amplifier.

8. A radio receiving circuit comprising a radio frequency amplier, a first detector coupled to the output of said radio frequency amplifier, an intermediate frequency amplifier, circuits interconnecting said intermediate frequency amplier and said first detector so that the output of the rst detector may be amplified in said intermediate frequency amplifier, a second detector, circuits connecting said second detector with the output of said intermediate frequency amplifier, circuits for reflexing signal variations from the output of the first detector to the input of the radio frequency amplier, and means maintaining a high impedance to the radio frequency signal and an extremely 10W impedance to the low frequency connected across the input of the rst detector.

JACOB YOLLES.