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March 22, 1938. ' ‘ ‘ ' D. E. FOSTER SIGNAL CONVERTER ' 2,111,764 CIRCUIT ’ Filed April 13, 1956 3 Sheets-Sheet l z + 70 5/6/1641 sax/R05 3 ‘ N LOCAL OSCILLATOR \\ ? ————— __ J1 _ __]':9, LOCAL ' osc/uAro/z _ ATTORNEY March 22, 1938. , D. E. FOSTER 2,111,764 SIGNAL CONVERTER CIRCUIT \ V Filed April‘l3, 1936 1.0. __ 3 Sheets-Sheet 2 l749 ' .4 _._ 1%] 1i \—'|\6 TOJ‘IGA/AL _ U1 + // + é , Z)’ l5 ‘ / . __/ / 4-_0 l0 /6~ l SIGNAL - NETWORK w. 6 sou/e05 LOCAL OJ'C/LLA 70R INVENTOR ' DUDLEY E. FOSTER BY‘ /. -. ' ATTORNEY March 22, 1938. DE“ FOSTER 2,111,764 ' SIGNAL CONVERTER CIRCUIT Filed April 13, 1956 S‘Sheets-Sheet 3 ZKAMPL @152 706761144! SOURCE 0511100014701? ’ v I r ' w’ LOCAL 0361 22 . 8 ' I | ‘Y TOLENETWORK 'd- /,/ 70 .SIGA/AL \ / ' - . .SOURCE J I = ll Q like 1, j (J ' ‘ INVENTQR LOCAL 0fC/UA7'0R ' AVC ‘1 AVG DUDLEY E. FOSTER 2" BY ' ;/ . _ . WVL/ ATTORNEY Patented Mar. 22, 1938 NlTD ‘STATES PATENT OFFICE 2,111,764 ‘SIGNAL CONVERTER CIRCUIT Foster, Orange, ‘N. ‘7.1., assignorf to Radio Corporation of America, a corporation ‘Dudley ‘ ‘of Delaware . Application 'April' v13, 1936, Serial no. 73,998 9 Claims.'- (01.‘ 250-20)‘ ' c., thenthe oscillator frequency will be 1500 My present invention relates to radiov frequency k. 0. when the receiveris tuned for reception of a signal converter circuits, and more particularly k. 1000 k. 0. signal. .2 If nowv the oscillator frequency to converters of the electron coupled type. ‘ , Asis well known, the-frequency converter tube, be slightlychanged to 1501 k. c., then the result» ant I. F. from'the'i?rst order term will be 501 k. c. 5 .5' or first detector, of a superheterodyne receiver. The ‘secondaorder-Itermlwill give a frequency equal p-roducesin its output circuit the sum and differ signal frequency, i. e., the difference ence frequencies of the signal and locally pro, to'twicethe' between ‘2000 k. c. and 1501 k. c.-, or 499 k. c. It duced loscillation‘frequencies. In‘ ‘addition to these sum and'differencei frequencieathere are =10 also produced-frequencies equal to‘ the sum and difference of integral multiples of the signal and oscillator frequencies. These latter frequencies are usually referred to as higher order effects, since they are due ,to terms .inthel expression '1‘5 ‘representing the tube characteristic higher. in order than the term which‘results in. the desired will,.thus, be seen’ that there will be applied to the second detector. two frequencies of 501 k. c'. 10 andl499- kc c.,‘:withi the,‘ result that a beat note equal-totheir difference, 2 ‘k. 0., will be heard. ‘ At signal frequencies other than twice the I. F., multiple response will be‘evidenced instead of audible whistles. Consider again a superhetero- 15 operating intermediate ‘frequency. For example, in the simple type of converter tube, wherein the signal and oscillator voltages are simultaneously 30 applied to a common‘control grid, if. the charac teristic of the .plate current be expressed as a ‘power series, i. e., a series whose, successive terms are ascending powers of the grid voltage, then the‘ second order (voltage squared) term results 05 in the desired conversion frequency‘andhigher orders in undesired frequency terms. . The electron coupled type of frequency con verter, and its many circuit forms, is well known at the present time. In such a converter the os 39 cillator voltage is applied to one control grid whilethe signal voltage is applied-to another grid, dyne receiver using a 500 k. c. I. F. with an input signal of 10201:. c. If the oscillator frequency is'1'520 k. 0.," then the 1.1“. of 500 k. 0. will be ob tained; Furthermore, if the oscillator frequency is 1540 k. .c., the second order term of the signal 20 grid characteristic will give an output of twice signal frequency minus oscillator frequency, or 2040 k. 0., minus 1540 k. c., resulting in 500 k. c. It is seen, therefore, that for any applied signal frequency, there-are at least two oscillator fre- 25 quencies which willresult in the correct I. F. Accordingly, it may be stated that it is one of the main. objects of my present invention sub stantially to reduce undesired responses in a con verter of the electron coupled type, which re— 30 v sponses correspond to terms higher than the ?rst order term in the expression representing the tube characteristic; the reduction in and both grids being disposed in theiel'ectron ‘converter stream flowing between the cathode and out-put undesired responses being accomplished by ap 3;, electrode of the tube. In this typerof- converter plying the local oscillator voltage simultaneously 35 the higher order terms give undesired frequencies, to-the signalrand. oscillator grids, but the oscil just as inythe simple type of converter tube. Only lator voltage being impressed on the two grids in those frequencies falling within the response band out-of-phase relation, and the magnitude of the the latter being usually spaced fromthe former, of the‘intermediate frequency network'will be 4') transmitted to the second detector, or demodu lator. However, any undesired term falling with in the intermediate frequency network response band will likewise reach the demodulator, and thus give an undesired response. This di?iculty 3 is most commonly encountered when twice the intermediate frequency falls within the signal frequency range it is desired to receive. In this case, ‘the intermediate frequency can result not only from the difference between the signal and 50 oscillator frequencies, but, also, from the differ-, ence between twice the signal frequency and ‘the oscillator frequency. The latter problem can be more clearly under stood when viewed in the light of an actual ex ample. Assuming that the operating'I. F. is 500 out-of-phase oscillator voltage being chosen to secure the‘ substantialreduction of the higher 40 orderterms referred to. } Anotherimportant object of the present inven tion is to cancel from the output circuit of an electron coupledtype of frequency converter, un 45 desired responses produced by the second order effect; and this-cancellation being accomplished by applying the- local oscillator voltage simulta neously to the oscillator grid and the signal grid, the oscillator ‘voltage being applied in reversed 50 phase to the respective grids; it being pointed , out that since ‘the second order effect is less than the ?rst order‘ effect, the amount of local oscil lator voltage on the signal grid which causes cancellation of the second order effect will result 55 2 2,111,704 in only a slight decrease in the desired ?rst order frequency conversion. In existing electron discharge tubes of the type employed in electron coupled converters, the amount of local oscillator voltage to be applied to the signal grid for cancellation of undesired re sponses, is a function of the bias on the signal grid since the tube characteristic with respect to the signal grid is not purely exponential. Hence, 10 it may be stated that it is another object of the present invention to apply local oscillator voltage of an electron coupled converter to the signal‘ and oscillator grids in phase and magnitude re ent invention. The numeral l designates a pen tagrid converter tube which is well known to those skilled in the art, and is usually designated as a 2A7. The input electrodes of the tube are con nected across a tuned signal input circuit 2, the latter circuit being coupled to a source of signal energy. The local oscillator is schematically rep resented by the numeral 3, and is connected between the ?rst grid and the grounded side of the cathode bias resistor 4, the latter being shunted by the usual by-pass condenser 5. The fourth grid 6 is connected to the high alternat ing potential side of the input circuit 2, and the lations substantially to reduce the higher order effects resulting in undesired responses in thefre , signal grid 6 and oscillator grid 1 have disposed quency range it is desired to receive, and yet between them a pair of positively biased grids. employ automatic volume control action on the A positively biased grid is also disposed between the output plate electrode 8 and the signal grid 6. converter; this dual result being secured by uti The numeral 9 denotes the usual variable tun lizing a phase inverter tube to impress local os 20 cillator voltage on’ the signal grid, and the bias :ing condenser disposed in the input circuit 2, of the inverter tube being varied concurrently and the condenser 9 is to be understood as being adjustable over a relatively wide frequency range, with that of the signal grid of the signal con verter tube with the result that the'proper amount as for example the broadcast range of 500 to of local oscillator voltage is :applied to the signal 1500 k. 0. Of course, the tuning range of cir cuit 2 may be in the higher frequencies, and it grid for any value of signal grid bias. ' Another object of the invention may be stated is vto be clearly understood that the local oscil lator circuit 3 is simultaneously tunable, by to reside in the provision of an electron coupled converter tube which not only has applied-to its means of its own tuning element, over a fre signal grid a local oscillator voltage in phase and quency range which differs from the signal fre quency range by the operating I. F. The I. F. magnitude substantially to eliminate the-unde sired responses due to the higher orderi e?ects, network is disposed in the circuit connected to but wherein the direct current voltageof the the plate 8. It is not believed necessary to go into the de oscillator and signal grids are varied in depend tails of the functioning of the converter tube I. ence upon received signal amplitude variation. Still other objects of the present invention are Those skilled in the art are fully aware of the to improve generally the e?iciency and operating fact that there is produced in the circuit con nected to the outputelectrode 8, the sum and reliability of frequency converters of the elec tron coupled type, and more especially to provide difference frequencies between the applied signal frequency and the local oscillator frequency, as such’ converters which are not only durable, re well as frequencies equal to the sum and dif ~10 liable and simple in operation, but economically manufactured and assembled in radio receivers ference of integral multiples of the signal and of the superheterodyne type. oscillator frequencies. The electron stream ?ow ing between the cathode and plate 8 is modu The novel features which I believeto be char lated at the frequencies of the signal and oscil acteristic of my invention are set forth in par lator voltages, and the various modulation fre 45 ticularity in the appended claims; thev inven tion itself, however, as to both its organization quencies, as explained above, are produced in and method of operation will best be understood the circuit connected to the plate 8. The I. F. by reference to the following description taken network being resonated to the desired operat in connection with the drawings, in which I ing I. F., only those frequencies falling within have indicated diagrammatically several circuit the response band of the I. F. network will be transmitted to the demodulator. organizations whereby my invention may be car ried into effect. ' However, undesired responses produced by the In the drawings: second, third or higher order effects will like Fig. 1 is a schematic circuit diagram showing wise reach the demodulator, and give undesired responses if any undesired term falls within the the basic circuit employed in this invention, Fig. 2 shows a converter network embodying I. F. network response band. To reduce substan one form of the invention, tially these undesired responses there is im Fig. 3 shows an alternative arrangement of the pressed upon the signal grid 6 a portion of the invention, ' 1 locally produced oscillator voltage, and this im 60 Fig. 4 illustrates another embodiment/of the pression is made through the impedance Z. The invention, impedance Z is designed to provide the proper Fig. 5 illustrates another converter arrange phase so as to produce reduction of the higher ment, 1 order effects. Fig. 6 shows a converter network embodying The impedance Z may be inductive or capaci still another form of the invention, ‘ tative; the various following ?gures illustrate the Fig. 7 shows a converter network employing the impedance in these two aspects. The impedance invention, and utilizing automatic volume con Z must be chosen of such phase and magnitude trol, > Fig. 8 shows a modi?cation of the arrange ment of Fig. 7. . . Referring now to the accompanying drawings, wherein like reference charactersin the. di?erent figures designate similar circuit elements, there is shown in Fig. 1 a purely schematicrepresen tation of the basic principle involved in the pres 15 20 25 30 35 as to cancel out the response due to the second order term. If the third, or higher, order effect results in undesired responses in the desired fre quency-range, then the amount of out-of-phase 70 oscillator voltage applied to the signal grid 6, through impedance Z, isadjusted to cancel higher order responses. In Fig. 2, there is shown a converter network 75 3 2,111,764 In the circuit arrangement of Fig. 6, the con wherein the out-of-phase oscillator voltage is ap plied to the signal grid 6 of the converter tube ' verter tube I is of the type shown in Fig. 2. l (of the 2A7 type) ‘through condenser ID. "The Otherwise, the arrangement is a‘variation of that latter is made adjustable so that it can be varied shown in Fig. 4. - Here the local oscillator has its in magnitude to the point where the undesired normal coupling to oscillator grid 1 through con denser C1; the latter being connected to the high alternating side of oscillator circuit 2'. The har monic cancellation coupling in this case com responses in I. F. network I I can be cancelled. The condenser I0 is connected between the grid 6 and the high alternating potential side of the tunable oscillation circuit 2' of the local oscil'-' 10 lator. The dotted line l2 denotes the usual me prises the mutual inductance M1’ provided be tween cathode coil I1 and the coil of the tunable 10 oscillator circuit 2’. In a practical embodiment of this arrangement it was necessary to decrease the magnitude of condenser C1 to cut down the chanical coupling between the rotors of the vari able tuner condensers 9 and 9'. It is to be clearly understood that the coupling between the oscil normal oscillator injection. With this adjust lator grid 7 and the oscillator is conventional. 15 15 Further, the tuner circuits may be of the multi - menta decrease of approximately 20 db. in the second ‘order effect was obtained upon proper range type, if desired. proportioning of M1. The circuit arrangement shown in Fig. 3 em When AVC action is applied‘to the converter bodies a form of the invention wherein the signal tube the second harmonic cancellation is a?ected. grid 6, disposed adjacent the cathode of the con This arises by virtue of the fact that the signal 20. 20 verter tube l, is connected to the plate of the os grid characteristicris not purely exponential in‘ cillator tube 3’ through condenser In. The oscil lator is of a conventional form, and needs no existing tubes. The amount of oscillator voltage detailed description. The oscillator grid 1, dis—_ to be applied to the signal grid for harmonic can posed in the positive shielding ?eld, is connected 25 to the grid of the oscillator tube. The condenser It] may be‘adjusted to balance out the second harmonic effects in the I. F. output network II. The circuit arrangement of Fig. 4 diifers from the preceding arrangement in the harmonic can 30 cellation coupling between the local oscillator and the signal grid 6. This coupling comprises the mutual inductance M provided between coil l3 and tunable circuit ‘coil 14. The magnitude of M may be varied until the undesired second har 35 monic response in network II is cancelled out. With the oscillator higher in frequency than the signal, the signal circuit appears capacitive to the oscillator frequency. If the signal circuit were a pure capacity, then the feedback coupling 40 and signal tank circuit would act as a simple capacitive voltage divider. However, the tank. circuit appears as capacity and resistance, so for phase as well as magnitude balance the feedback circuit should comprise resistance, which may be 45 in series or parallel with the feedback‘ capacity. In practice, appreciable reduction is secured without phase balance, but better reduction of spurious components will result from phase as well as magnitude adjustment. Similarly a resistance in conjunction with M would produce better balance by adjustment of phase as well as magnitude. Capacitive and inductive couplings are shown to indicate alternative arrangements. The modi?cation shown in Fig. 5 involves the 55 connection of the signal gridB to the high alter nating potential side of ‘signal input circuit 2. The local oscillations are impressed'on the signal grid 6 by connecting the low alternating potential side of oscillation circuit coil l5 to the low alter nating potential side of signal circuit coil [4 through adjustable condenser ill. The junction of condenser H) and coil I4 is grounded through condenser U5. The oscillator grid 1 is connected to the high alternating potential side of oscil 65 lator circuit 2’. Coil l5 of Fig. 5, padding cone 60 densers iii and iii and the variable condensers connected to coil l5 represent the tank circuit of a separate oscillator tube. 70 The common cou pling to the signal circuit is condenser l5. Con denser l6 is the cancellation coupling condenser; condenser It is added in series therewith, the series combination of condensers Hi and Ill act~ ing as an oscillator padding condenser. Adjust ment of It affects padding, but does not affect the coupling magnitude to signal network. cellation is a function of the bias on the signal grid. Since the signal grid bias varies in magni 25 tude when AVC is used, it follows that the'har monic cancellation voltage will also vary. It, therefore, becomes necessary to employ a‘ device for insuring the application of the proper amount of oscillator voltage for any signal grid bias. Fig. '7 shows one arrangement for automatically correlating the harmonic cancellation voltage with the converter signal bias value. The local os cillator applies the oscillations to the oscillator grid 1 through condenser C2. The AVC network 35 20, of conventional type‘, automatically regulates the conversion gain of tube l. The AVC network, in general, may embody a signal recti?er supplied with I. F. energy, and which recti?er furnishes the direct current voltage component for auto 40 matically regulating the signal grid bias of the converter tube I. Of course, the AVG network may also regulate any of the high frequency sig nal transmission tubes in gain. Furthermore, the AVG‘ recti?er maybe an independent recti?er, or may be part of the demodulator of the system, and in the latter case the direct current voltage component of detected I. F. voltage would be used for the AVG action. The harmonic cancellation oscillator voltage is ‘ applied to the signal grid .5 through a phase inverter tube 2!. The electron discharge tube 2| has its plate connected to the high alternating potential side of signal circuit 2 through a con denser C3, the plate being supplied with proper positive voltage through the choke 22. The con trol grid of the inverter tube 2! is connected to the high alternating potential side of the local oscillator circuit 2' through condenser C4, and the cathode of the inverter tube is grounded through bias network 24. When bias increases on tube l, the bias decreases on tube 2|. By taking screen voltage for tube i through resistors 25 and 26, as tube l is biased negatively the screen potential increases in a positive direction, and with it, to a lesser degree, the potential of point 2? of resistor 26. By con necting the control grid of tube 2|, through resistor 23, to point 27, the grid potential of tube 2! becomes less negative'when that of tube I be comes more negative. Resistor 24’ in the cathode of 2! is required so that the cathode of tube 2| is at greater positive potential, relative to ground, than point 21; the grid of tube 2| thus being 4 2,111,764 maintained negative with respect to its own cathode. In this way a change in gain control bias on the signal grid 6 will automatically vary the bias LT on the grid of the inverter tube 2!, and the gain of the latter will be varied. The function of the tube 2| is to invert the phase of the applied tude such ‘that undesired responses due to the second order, or higher, term of the detection characteristic are substantially reduced. 3. In a superheterodyne receiver, a ?rst detec local oscillator voltage so as to secure the proper tor system comprising a tube having a cathode 5 and an oscillation electrode, means electrically connected with said oscillation electrode for vary ing the alternating current voltage thereof at a harmonic cancellation phase. The variation in predetermined locally produced oscillation fre— 10 gain of tube 2| is chosen so that the proper quency, said tube including a control grid and an anode, said control grid being located in the amount of oscillation voltage is applied for every change in converter signal grid bias. In this way the departure of the converter signal grid characteristic from the pure exponential type is compensated for; the compensation involves the automatic correlation of the magnitude of the applied harmonic cancellation oscillator voltage with the value of the converter signal grid bias. At the expense of some conversion sensitivity the phase inverter tube 2! may be dispensed with. The local oscillator, conventionally represented in Fig. 8, is shown coupled, through condensers, to the local oscillator grid T and the signal grid 6. The AVC connections, from the usual AVC network, are made to the signal and oscillator grids. The bias of the oscillator grid is automatically varied with that of the signal grid in this modi?cation. This requires that the oscil lator grid ‘I draw little current for practical AVC action, and it must, therefore, be run negative. Both grids vary in the same polarity sense, but at different rates. In general, grid 6 varies less than grid 1. While I have indicated and described several Li 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 par ticular organizations shown and described, but that many modi?cations may be made without 40 departing from the scope of my invention, as set forth in the appended claims. What I claim is: 1. In an electron discharge tube which is pro vided with at least a cathode, an output elec trode and an electron path therebetween, the method which includes drawing electrons to an intermediate point in said path, retarding said electrons to form a virtual cathode which is be yond said point, modulating the density of the virtual cathode by varying the attraction of said electrons through attracting the part of said electrons from said virtual cathode to said output electrode, causing a received signal to vary the further attraction of said part of the electrons, - the modulation of the density of the virtual cathode thereby causing modulation of the signal in said tube, and simultaneously modulating said part of the electrons in out-of-phase relation with said ?rst modulation and to an extent such 60 as to reduce substantially the appearance of sec ond order, or higher, term effects. 2. In a superheterodyne receiver, a ?rst detec tor system comprising a tube having a cathode and an oscillation electrode near said cathode, means electrically connected with said oscillation electrode for varying the alternating current volt age thereof at a predetermined locally produced oscillation frequency, said tube including a con trol grid and an anode located in the space path beyond said oscillation electrode, a source of sig nal voltage coupled between said control grid and said cathode, and additional means connected between said control grid and said ?rst means for impressing upon said control grid said 10 75 cally produced oscillations in phase and magni space path between the cathode and said oscilla tion electrode, a source of signal voltage coupled between said control grid and said cathode, and additional means connected between said control grid and said ?rst means for impressing upon said control grid said locally produced oscillations in phase and magnitude such that undesired re sponses due to the second order, or higher, term of the detection characteristic are substantially 20 reduced, means for developing a bias control volt age which automatically varies with the intensity of said signal source, means for applying said bias voltage to said control grid, and additional means, responsive to the variation in .said bias control voltage, for automatically correlating the local oscillation voltage impressed on said con trol grid with the change in said bias control voltage. 4. In combination in a ?rst detector network of a superheterodyne receiver, an electron discharge tube provided with at least a cathode, an output electrode, a pair of control electrodes disposed be tween the cathode and output electrode, and means electrostatically shielding said pair of con trol electrodes, a tuned signal input circuit con nected between the cathode and one of the control electrodes, means coupled to the cathode and the other control electrode for varying the alternating current voltage of said other control electrode at 40' a frequency rate which di?ers from the signal frequency by a predetermined intermediate fre quency, and means coupling said input circuit and said varying means for impressing on said ?rst control electrode alternating current voltage 45 varying in frequency at said oscillation frequency rate, the phase of the alternating current voltage impressed on said ?rst control electrode being related to the phase of the voltage on said other control electrode substantially to reduce the e?ect of the second order term of the detection char acteristic on the electron stream ?owing between the cathode and said output electrode. 5. In a ?rst detector network of a superhetero dyne receiver, an electron discharge tube which 55 includes a cathode, an anode and at least two control grids disposed in the electron stream be tween the cathode and anode, a resonant net work in the anode circuit which is tuned to an operating intermediate frequency, a tunable sig 60 nal input circuit connected between the cathode and one of the control grids, a source of local os cillations, variable over a predetermined local oscillator frequency range, coupled between the cathode and the other control grid, and means, 65 electrically connected with said local oscillation source and the said signal input circuit, for im pressing upon said ?rst control grid local 0s cillator voltage which is out-of-phaserwith the oscillator voltage impressed on the other control 70 grid. 6. In a ?rst detector network of a super heterodyne receiver, an electron discharge tube which includes a cathode, an anode and at least two control grids disposed in the electron stream 75 2,111,764 between the cathode and anode, a resonant net work in the anode circuit which is tunedto an operating intennediate‘frequency, a tunable sig nal input circuit connected between the cathode and one of the control grids, a source of local oscillations, variable over a predetermined local oscillator frequency range, coupled between the cathode and the other control grid, and means, electrically connected with said local oscillation 10 source and the said signal input circuit, for im pressing upon said ?rst control grid local oscillator voltage which is out-of-phase with the oscillator voltage impressed on the other control grid, means, responsive to variations in signal ampli tude, for automatically regulating the ?rst con trol grid bias, and additional means responsive to said last named means for varying the magni tude of the oscillator voltage impressed on said ?rst control grid. . '7. A frequency converter tube comprising a cathode, an anode and at least two additional electrodes disposed in succession in the electron 5 stream between the cathode and anode, a source of oscillations connected to one of the said elec trodes to vary the potential thereof at a prede termined frequency, a source of signals connected to the other electrode to vary the voltage thereof at a frequency different from the oscillation fre quency, a circuit connected to the anode to utilize the frequency component of anode current equal to the difference between said signal and oscilla tion frequencies, a high frequency coupling path, 10 independent of said electron stream‘, between said electrodes, said path including a reactive im pedance whose magnitude and phase are chosen so that said oscillations are impressed on said sig nal electrode in out-of-phase relation with the 15 oscillations impressed on said one electrode there by to prevent the production of harmonic re sponses in said anode circuit, and means, re sponsive to signal amplitude changes, to vary the gain of said converter tube. 20 9. In a ?rst detector network of a super heterodyne receiver, anelectron discharge tube stream between the cathode and anode, a source of oscillations connected to one of the said electrodes to vary the potential thereof at a predetermined frequency, a source of signals connected to the other electrode to vary the voltage thereof at a which includes a cathode, an anode and at least frequency different from the oscillation frequency, a circuit connected to the anode to utilize the frequency component of anode current equal to the difference between said signal and oscilla tion frequencies, and a high frequency coupling path, independent of said electron stream, be nal input circuit connected between the cathode and one of the control grids, a source of local oscillations, variable over a predetermined local 30 tween said electrodes, said path including a re active impedance whose magnitude and phase are chosen so that said oscillations are impressed on said signal electrode in out-of-phase relation with the oscillations impressed on said one electrode thereby to prevent the production of harmonic 40 responses in said anode circuit. 8. A frequency converter tube comprising a cathode, an anode and at least two additional electrodes disposed in succession in the electron two control grids disposed in the electron stream between the cathode and anode, a resonant net 25 work in the anode circuit which is tuned to an operating intermediate frequency, a tunable sig oscillator frequency range, coupled between the cathode and the other control grid, and means electrically connected with said local oscillation source- and the said signal input circuit, for im pressing upon said first control grid local os 35 cillator voltage which is out-of-phase withthe oscillator voltage impressed on the other control grid, means responsive to signal amplitude varia tion for controlling the potential of said one grid, and additional means responsive to said signal amplitude variation for controlling the magnitude of said out-of-phase voltage. DUDLEY E. FOSTER.