US1653837A - Hod ttlated-carrier-wavb signaling system - Google Patents

Description

Dec. 27, 1927. 1 1,653,837

H. 5. BLACK MODU LATED CARRIER WAVE SIGNALING SYSTEM Filed se tfze. 1925 2 Sheets-Sheet 1 5cm fxaqqeraTed As Comf pmd Wl'Th Curve b. ;urfl6\' VOWlqt; ------g;(7g)dc 'dToCri. Time E W 1 //7 van/0fa/ddfi/dak by my Dec. 27, 1927.

H. S. BLACK MODULATED CARRIER WAVE SIGNALING SYSTEM Filed Sept. 26. 1925 2 Sheets$heet 2 Grid Resisfance all) Patented Dec. 27, 1927.

UNITED STATES PATENT OFFICE...

HAROLD'S. BLACK, 03? EAST ORANGE, NEW' JERSEY, ASSIGNQR T0 WESTERN ELECTRIC COMPANY, INCORORATED, OF NEN YORK, N. Y., A CORPORATION OF NEW YORK.

MODULATEDGARRIER-WAVE SIGNALING Application filed September 26, 1925. Serial No. 58,744.

This invention is related to modulated carrier wave signaling systems, and to space discharge modulators, oscillators, and combination oscillator-modulators adapted to be used therein.

Although the invention may perhaps be most usefully employed in line-wire carrier wave systems the principles are applicable to carrier wave systems generally, including systems for signaling by radiant energy. A two-way circuit, that is, the minimum prerequisite for conversation, will be assumed as the practical signaling unit, the term channel being used to indicate one of such units.

Until recently the development of single channel modulated carrier wave signaling systems has been prejudiced by the fact that such a system has commonly been found to require the use of circuits and apparatus that couldbe used in common for a plurality of such systems, that is, multi-channcl systems have been justly regarded as inherently more economical or plant.

It is an. object of this invention to make the economy and other advantages of modulated carrier wave signaling systems generally realizable in smaller units than has heretofore been possible and specifically to provide a single channel modulated carrier wav signaling system in which the elliciency of operation, first cost, simplicity, stability, etc, approximates that of an equivalent single channel circuit comprised within a multi-channel modulated carrier wave signaling system. The over-all performance of the system to be hereinafter disclosed compares favorably with physical circuits routed over short lengths of non-loaded open Wire cop per lines and maintenance has been reduced to a minin'ium.

A. subsidiary object is to provide a system having the characteristics above indicated and which additionally is adaptable with relatively slight modifications to extension into a nmlti-channel system when traflic outgrows the limitations of single channel operation.

Modern transposition practice, in most circumstances, requires the operation of multi-channel systems on the group frequency basis, that is, an operation in which all the channels used for each direction respectively employ frequencies grouped to gether. In other words, the groups of frequencies occupy a space in the frequency spectrum indlvidual to their directon of transmission and distinct from that used for transmission in the opposite direction. Such a system is described in U. S. Patent 1,548.- 260, August 4, 1923, to Espenschied. A single channel system adaptable to extension into such a multi-channel system must accordingly use different frequencies for opposite directions of transmission. In its simplest conception it must have circuits at a terminal station individual to the two oneway portions comprising a complete single channel two-way system, each such portion utilizing the carrier frequency individual thereto.

One way to secure economy in the use of material and of energy in such a system is to transmit a pure modulated wave in each direction. An ellective way of accomplslr ing this is by suppressing the unmodulated carrier by a balanced circuit including space discharge modulating devFces.

A still further way of promoting economy in the system, so far described, is to operate each discharge circuit to effect modulation and also as an oscillator, that is, to use a self-oscillatory balanced modulator. Since a local receiver source must be used at each terminal, in the absence of a transmitted unmodulated carrier, the demodulator ma similarly be made self-oscillatory to avoid the necessity of using a separate source sup plying oscillations of carrier frequency, and it may be of the balanced type to suppress the locally produced carrier. The resultant receiving circuit at each terminal is accordingly similar in structure and, in a broad sense, in function, to the corresponding transmitting circuit The type of single channel system characterized by the above, described features is the invention of others. The present invention, although related to features that may be usefully employed in systems generally employing oscillators, self-oscillatory modulators, and balanced self-oscillatory-modulators, is an improvement over such type of single channel system whether used alone or as a component unit of a multi-channel sys tem into which it may be extended.

A specific object of the invention is to improve the frequency stability of a space dis charge circuit as affected by space battery potential variations.

This object is achieved in part by inserting a resistance in the grid-filament circuit of each space discharge device When the grids become positive, as they must by rea son of the feed-back that is necessarily incidental to the oscillating operation, rectified current will flow through this resistance, the average value of which will determine the average resultant negative bias which is added to the biasing potential impressed upon the grids by a separate polarizing source. The amount of feed-back is adjusted so that this additional bias so occurs as to compensate for exceptionally Wide variations in the source of potential of the space current.

A further object is to improve such a system by eliminating the separate grid polarizing source altogether, that is, to relay upon the negative bias turnished by a grid stabllizing resistance, and to accomplish this result consistently with frequency stabiliza tion.

This result, or these results, is, or are, achieved by increasing the amount of feedback so that the grids are normally made positive once in each cycle, so that a normal negative biasing potential applied to the grids.

Besides the increase of frequency stability resulting from the arrangement described above, it has been found that the stability of the system is increased in other Ways. One measure of the eiiicient operation of a modulation system, or of its stability, is the constancy of the modulator gainirrespective of variations of the signal input, of the space battery potential, of the filament current, and also of variations in all elements com prising the circuit With the single exception of the frequency determining circuit. Further, frequency stability is affected, in addition to the eil'ect of variation oi space-battery potential, by variation of modulation input and filament current. It has been found that the arrangement t the invention insures a remarkable degree of stabilit as affected by each oi? these factors. This result follows "from the, "fact. that each of them atl'ects, in some way, the amplitude of the resultant potential in'ipressed on the input circuit iron] the teed-back circuit, or from the external, 1110dulating circuit, and thus the stabilizing operation is responsive to this resultant potential.

The remarkable stability that has, been realized in practice may be illustrated by mentioning that, in the case of the first of the circuits to be disclosed in detail further on,- it the B battery voltage is reduced from 150 volts to 18 volts. the frequency oi? the oscillations only changes by a small fraction at one percent,

Fo the particular case where the cathodes ot the modulator or demodulator are in series with respect to aconunon source the re sulting tendency toward dissymmetrical operation may be compensated by the use of individual grip resistances for thetwo tubes. However, it has been found that it a single resistance is used in a circuit common to the grid-iathode circuits of the two devices, While symmetry of operation is sacrificed, the same ultimate result is achieved. With weak input current levels only one of the space discharge devices, the one having the smaller negative biasing potential, cooperates ivith the resistance to perform the stabilizing and biasing functions for both devices. Any dissimilarity of the resultant output currents of the two devices, as to either amplitude or phase, can be corrected by impedance loading in the anode lead of one device. In general it is preferable to correct dissyinmetry in this manner than to acconnplish it by changing the arrangement of the grid circuits.

Additional features closely related to the resistance biasing means comprise:

1. The use of a very large biasing resistance, that is, oi? the order of infinity, the teed-back being adjusted accordingly. It has been "found that this arrangement serves to increase the stability of the system, since the operation of the system is not affected by variations of the grid-cathode characteristic, or by those of the biasingresistance itselt.

2. .lleans to insure eificient plate modulation, and to prevent, or to render innocuous, grid modulation; which tends to occur due to the operation of the grid biasing means, and which would be highly detrimental to the etiicient operation of the circuit as a plate n'iodulator.

In the preceding description the terms modulation and modulator are frequently used. It is to be understood that these and similar terms, as so used and as they will be used throughout the specification and claims, except Where another meaning is indicated by the context, comprehend the so-called demodulation operation and the demodulator since the operation of the two device considered muilitatively, is essentially the same.

Other objects and features oi the invein tion and its principles of operation will be clearly understood from the following detailed description when read in connection with the accompanying drawing in which:

Fig. 1 illustrates the circuits constituting one embodiment of the invention;

Figs. 2 and 3 are graphical devices that are aids to the understanding of the operation of the invention; and

Fig. i illustrates the circuits constituting aimtner embodiment of the invention,

In the description which follows the elements of the system will be identified so far as practically possible by literal designations w rich are su 'estive of their respec Far-1 ti've functions.

.l ig, 1 illustrates the circuits at one terminal of a modulated carrier current system of the invention, which is also composited so as not to interfere with or limit in any manner, the use of tern'iinal apparatus for the trans nission of voice frequency or telegraph signa or the association of phantom circuit ith the transmission circuit 2. Compositing is accomplished by what is commonly denominated a carrier composite set comprising a low pass filter FLP in the low frequency circuit 1, and the high pass filter Fill in the high frequency circuit. The high and lowfrz-iqucncy circuits are connected to the transmission circuit 2, the high frequency circuit by means of transformer 23, which may be replaced if desired by a conductive connection.

The filters FLP and FHP effectively separate the currents corresponding to the frequencies used in the respective types of transmission. Obviously, the system could be used for modulated carrier wave transmission alone,in which case these filters could be dispensed with. hen used in this manner, the transmission circuit 2 could be, or could be connected with, a radio antenna, to adapt the system for radio signaling. As modified to permit the transmission of low frequency, of course the circuit 2 would have the form of the usual conductive con nection between con'nminicating stations.

The circuits, adapted for the superimposed modulated carrier wave transmission, constitute the necessary elements whereby two-way communication may be accomplished between low frequency circuit 41, which may be connected, for example, with a central oflice of the usual telephone .system, and the transmission circuit 2.

The transmitting portion of this two-way circuit comprises the following elements which function in like sequence; low frequ-ency line 4, balanced oscillator-n'iodula tor BUM, output transformer 5, transmitting band lilter FTB, high pass filter FHP, transforn'ier 3. and transmission circuit 2. This portion of the apparatus operates as follows: The low frequency modulating current in line l: is interi'nodulatcd with a carrier current in the device ROM which con-- stitutes a unitary self-oscillating balancedmodulator, and the proper resultant modulated currents are selected by filter FTB and transmitted through lilter FHP and transformer 3, whose functions have been described above, to thctransmission circuit.

The portion of the two-way circuits adapted for receiving comprises the following elements which function in like Se quence; transmission circuit 2, transformer 3, high pass filter Fill, receivlng band pass filter FEB, transformer 6, balanced-oscilhu toi demodulator BUDM, output transformcr 7, low pass filter FLP, and low frequency circuit 4;. The operation of this portion of "the two-way apparatus is as follows: The

account of the self oscillating character of the device to reproduce, by a demodulating operation, the modulating current transmit ted from the distant communicating station. This demodulated current is then transmitted through transformer 7 and low pass filter liiil tothe low frequency line 4-. Filter FLP, insures that the only current transmitted therethrough is such demodulated current, that is, it separates this cur rent from certain high frequency currents whose production is incidental to the de modulating operation.

Circuit N is a network designed to balance low frequency line i. This network, the low frequency line 4t, and the transmitting and receiving low frequency circuit are connected in energy transfer relation through hybrid'coil or balanced transformer 8 in such a manner as to conjugately connect the transmitting and receiving circuits to the low frequency line. The function of this hybrid coil and the specific circuit for carrying it out are illustrated conventionally. T hey are sufficiently well understood by those skilled in the art as to require no further description.

Selection, rather than balance, is relied on at the iunctions of the high frequency portions of the transmitting and receiving circuits to preventinterference therebetweeu. This is made possible by proper choice of different frequencies for the two directions of carrier transnnssion; The use of different carrier frequencies for thetwo directions of transmission in the single channel. circuit described makes it possible, without change of frequency or of the interconnected circuits including the filters, to extend the sys-' tern into a desirable type of multichannel system, that is, a multichannel system whose frequencies are arranged. on a group basis. i

In the single channel system described,

ill

economy in the use of material and economy of energy used is effected by the use of the balanced-oscillator-modulator and balancedoscillator-demodulator, since the use of these devices in the circuit avoids the necessity of using oscillator circuits distinct from the modulator and demodulator circuits, and insures that the outputwave is pure, that is,

contains no unmodulated carrier component.

The particular circuit features which characterize the system of the invention constitute elements in such balanced-oscillator modulators and balanced-oscillator-demodulators or their immediately associated circuits. Their presence results in a marked improvement, over prior circuits, of stability of frequency and of modulator gain as effected by, for example, space battery po tential variation, variation of signal input, and variation of filament current. The description from this point will be confined to the details of these two circuits, the balanced-oscillator-modulator and the balanced-oscillator-demodulator.

The common translating circuit BOM, used to operate as an oscillator and as a balanced modulator, or in other words, as a balanced self-oscillatory-modulator, comprises a balanced circuit including two electric discharge tubes 9 andv 10 which may be of the usual three-electrode type, including a cathode, an anode, and a control electrode or grid. The cathodes of these tubes are connected serially and in series with the similarly related cathodes of the electric discharge tubes of the balanced-oscillatordemodulator BODM by means of circuit 11 and ground. The cathodes of the two tubes are so related to source 12 that the cathode of tube 9 is adjacent the negative end of such source, whereby the average potential of this cathode is lower than that of the cathode of tube 10. v

The junction point of the cathodes of tubes 9 and 10 is connected to the mid-point of .the secondary of the input transform-er or hydrid coil 8, the grids of the two tubes being connected to the respective opposite terminals of the secondaryby conductors 13 and 14. The same point in the cathode circuit is also connected. through tuned feedback circuit 15, feed-back resistance 16, and blocking condenser 17, to the mid-point of the primary of output transformer 5 and thence through the two halves 18 and 19 of this transformer to the anodes of the respective tubes.

Space current is supplied to these anodes from direct current source 20, through choke coil 21 and the two halves 18 and of the primary of transformer 5. Condenser 17 and choke coil 21 insure that the space current and the high frequency feed-back current are confined to their individual paths. The common portion of the grid-cathode circuits of the two tubes contains secondary winding 22, which couples the tuned circuit to this common portion. Resistances 23 and 24: are connected across the two halves of the secondary winding of the input transformer 8 so as to provide a suliiciently high stable impedance for the input circuits of the two tubes. The use of impedances in this relation conduces to an efficient operation of the two tubes as translating devices.

The grids of the two tubes are given a proper iiXcd negative bias by means of large resistances 25 and 26 each of which is made individual to the grid of one of the two tubes by means of blocking condensers 27 and 28. The use of this type of biasing means, and its function in stabilizing the operation of hese circuits is an important feature the invention and will be considered separately later. Resistance 29, which is connected across the tuned circuit 15 and feedback resistance 16, contributes further to the etlicient operation of the output circuit-s. Its specific manner of accomplishing this will be described later.

It is a necessary characteristic of a properly balanced circuit of the type illustrated in this figure that the output circuit, con nected to the secondary of output transformer 5, and the input circuit, connected to the primary winding of transformer 8, should be balanced with respect to voltages applied through the feed-back circuit to the common. portion of the grid-cathode circuits of the tubes. This is accomplished by virtue of a certain relation of the windings of the several transformers. For example, the two halves 18 and 19 of the primary of transformer 5 should be wound in the same direction with respect to their common axis. A similar relation applies to the primary and secondary of transformer 8. However, the voltage Variations impressed on the common portion of the grid-cathode circuits are repeated in amplified form in the two anode branches of the output circuit, which includes the common portion, comprising tuned circuit 15, and the portions of the output circuit individual to the two anodes. By tuning the circuit 15 to the desired frequency, the circuit as a whole may therefore be caused to oscillate indruiend-ently at this frequency, since the output energy in the tuned circuitis returned inpart to the inputcircuits where it is again. amplified. Since the path which the oscillations follow is neutral with respect to the input and output circuits connected to transformers 5 and 8, the locally generated oscillations are confined to the translating circuits, and are therefore prevented from passing into either of the input and outputcircuits coupled to the translating circuits by the transformers 5 and 8.

The operation of the translating circuit -lll as a modulator will be clear from the following description of the path taken by currents in the course 0f translation with relation to the path of the oscillating current.

Modulating current from low frequency circuit 4i flows through on the prim ary win d ing of transform-er 8 and, on' account oi? the connection of its secondary winding to the grids of devices 9 and 10, causes the p0tentials of the grids to vary in an opposite manner to produce coinplen'iental impedance variations in the anode-cathode paths '01 these tubes. In this manner, the translating circuit is unbalanced to an extent dependent on the amplitude of the impressed potentials and the n'iodulating current is accordingly transmitted to the output circuit, connected to the secondary transformer 5, as variations in the amplitude of the hi gh frequency current produced by the oscillatory action of the translating devices. The hi gh frequency carrier currents themselves are balanced out so that they do not appear in such output circuit.

The operation of the circuitas a TQOCllP later, as thus tar described, is similar to that disclosed in U. S. patent to Carson 1.3 13306, June 15, 1920, to which reterence made for a more complete description. it will he rec'- ognized after reading this patent and the re i'uainder or" the present specification that other arrangements of transformer windings and of the input and output circuit with respect to the circuits more iii'nnediately associated with the tubes, may equally well be used in the system of the present invention.

It is a characteristic of modulating; cir cuits generally, including the balauced-n'iodulator illustrated in Fig; 1, that for e'liicient operatioi'i the external path of the output circuit nuist have an impedance at the desired output frequencies which is substantially equal. to the ii'npcdancc of the space current path within the devices. This is accomplishcd by proper design of the output transiiori'uer 5. It is also required or cilicient operation, that the output circuit have small or zero iinpialance at.the two impressed l re-- qucncies, that is. the carrier and the modu lating frequencies. The balanced relation oi? the output transtorn'icr ofliig. 1 insures that such outputcircuit will have a very low e'tiective impedance for the carrier frequency. Additional means for insuringa very low impedance for the carrier frequency is provided by the resistance 29 which is con nected in shunt to the teed-back circuit in the coniin'ion portion of the anode-cathode circuits of the two tubes. The use of this resistance with the balanced relation of the winding of the output transformer insures that every part of the circuit, as a whole through. which carrier frequency currenttcudr o flow has a "very low eilectiye impedanceat the carria' frequency;

of Fig. 1.

this grid current curve are greatly Although the amplified modulating current flows in such directions through the windings 18 and 19 of the output transformer as not to make the balanced relation efi ective in the same manner for these eurrcnts, nevertheless, low impedance is insured by a design of the transformer, since a design which is of high impedance for the side bands is consistent with a low impedancei or the modulating currents. In this connect-ion it is necessary to design the filter FTB so that its impedance across the terminals, connccted to transtormcr 5,'is substantially zero for all frequencies lying within. the band comprising the modulating current.

The function. of the grid resistances 2-5 and 26 may be best explained with reference to Fig.- 2. In this figure curv s (2 and Z) are respectively the well known grid-potentialspace-current and grid-potential-grid current characteristics of a typical space dis charge tube, adapted to be used in the circuit These curves are plotted with reference to the intersection of the coordinate axes, indicated at 0. It is desired to operate the tubes at a substantially constant negative grid potential corresponding to the potential E The shaded areas associated with the carrier Voltage curve indicate quantitatively the variations of the grid potential that are etllective in varying the electron emission from cathode to grid. The fixed negative bias results from the fact that when the grids are periodically made positive by the energy fed back to coil 22 through tuned circuit 15, current flows from the grids to the cathodes, or, which is the same thing, negative electrons pass from the cathodes to the grids. This current which is indicated by L; plotted on a time axis near the lower right-hand corner of the figure flows impulsively, since the grids can become positive only during the positive alternations of the current ted-back. The ordinates-i of exaggerated as compared with the corresponding values taken from curve 7).

Obviously this curve l also indicates the characterci the compensating currents flow- 111g in the grid circuit. It may be considered as made up of a direct current component on which is superposed alternating current.

The direct current component determines the fired negative bias. if desired a levelling condenser may be placed around each of the grid biasing resistances to further accentuin the circuit, so that with very large grid resistances, there will be sutlicient feed-back to produce the desired fixed negative biasing potentials. The resistances may then be adjusted relatively to each other and the feed-back coupling and shunting resistance readjusted until there is an exact balance consistently with the desired biasing poten tials.

The advantages accruing from the use of very large grid resistances is illustrated in Fig. 3 in which the curve discloses the relation, for any particular value of fed-back potential, between the, grid resistance and the resultant grid biasing potential. The curve is in no sense quantitative since the scale of ordinates depends altogether on the value, for example, of the fed-back poten tials. The figure is intended to illustrate only the variation characteristic with changes in grid resistance. It shows that for a resistance approaching infinity, the curve is nearly horizontal. This means that variations of this grid resistance, or of the grid-cathode characteristic which effectively forms a part of it, have relatively slight effect when a resistance of approximately infinite value is used. Qbviously, the stability of the system is increased. by using a resistance of that value. I

In addition to the functions described above, thegrid resistances also serve to compensate for effects which tend to produce changes in the amplitude of the resultant potential impressed on the grids, whether due to the feed-back circuit or the modulating wave circuit. That is, they compensate for changes in input potentiahchanges in potential of the source 20, and changes in cathode current and also prevent variations in frequency and gain that would otherwise attend such changes. For example, consider the effect of change in potential of source 20. An increase of the potential of this source causes the circuit to oscillate more violently and therefore to correspondingly increase the fed-back potential and hence the resultant potential on the grids. The effect is exaggerated where an air core coupling transformer, instead of an iron core transformer. is used in the feed-back circuit, since this type of core does not have the storage characteristics that an iron core transformer has. The use of an air core transformer has certain advantages in thesystem described, including that of low cost and an impedance which does not vary with changing temper ature, which would in certain circumstances determine its use as a standard element in such a system. The biasing potential correspondingly becomes more negative and therefore tends to oppose the tendency to such increase in violence of oscillation. Since the frequencyand gain of the tubes as translating devices have a direct relation to the degree of violence of the oscillations, the resultant effect is to stabilize these quantities, that is, to prevent them from changing.

The function of stabilizing the system may be made independent of the function of negative grid biasing. F or example, the amount of feed-back may be adjusted so that grid current normally does not flow, but flows only when the potential of thespacc source 20 increases above normal. F or this alternative operation a separate normal. neg ative grid biasing source may be used.

The structure and characteristic modes of operation of the transmitting portion of the two-way. circuits at tne terminal station illustrated in Fig. l have been described above. The structure and mode'of operation of the receiving portion of the channel illustrated in the lower part of the figure is similar. In fact, the continuity may be, as it is in the instance illustrated by Fig. 1, identical. he mode of operation is also identical in its theoretical aspects the only dif ference being in the numerical values of the quantities involved. For instance, in the modulator, tl e waves impressed, that is, the modulating and carrier waves, have radically different frequency values, and the resultant side bands have frequency values comparable with that of the carrier, whereas, in the case of the demodulator, the waves impressed upon it, namely, the side bands and local oscillations have frequencies of the same order, and the resultant low frequency demodulated wave has a frequency of a radically different order.

The d scription of the balanced-oscillatormodulator will be understood to apply within these limits to the circuits of the balancedoscillator-demodulator, which is therefore shown largel without reference numerals.

The condenser is related in identical manner with the modulator and the demodulator, and is necessary only because common anode source, which necessarily must contain some impedance, is used for both devices. Its purpose is to insure that the total impedance in the circuit of this source is at all times negligible. The ultimate result is to preventthe modulator and demodulator reacting upon one another, which would otherwise be caused by the variable drop across the impedance contained in the circuit of the common space source.

Fig. 4 illustrates an alternative circuit differing from that of Fig. l in certain elements. The elements which are similar in structure and function to those of Fig. 1 are similarly labeled, but onlythose elements which are dilferentwill be described in detail. I y

This circuit illustrates a simplified form oi the circuit of Fig. 1 in that it makes use oil a single grid biasing and stabilizing resistance 31 placed in the common portion of the grid-cathode circuits of the two tubes, instead oi a resistance individual to each tube included in its input circuit. Most of the differences in operation of the two cir cuits result from the use oithis feature. For example, with this arrangement of grid cathode circuits, since the cathodes are in series with each other, and with the energizing sourc as in Fig. 1, the free potentials oi the grids ot' the two tubes with respect to their individual cathodes, that is, the POtQlltlttl independent of any biasing poten tial resulting from the use of the resistance 31, will dill'er by an amount corresponding to the drop across one cathode. This dissymmetry with respect to the plate-filament circuits is exaggerated by an amount corresponding to the amplification constant of the tube. The use of a single biasing resistance therefore tends to result in a dissymmetrical operation of the tubes. In a particular practical arrangement, this difference in the grid potentials was iiound to amount to about 4 i in general, it is so great that when r ed-bacl potential is so adjusted as to provide a steady negative biasing potential for tube 9 through the operation of the biasing resistance 81, the grid of tube 10, when there is no signal. current supplied to the input circuit, fails to become positive at any point on the cycle oi? the ted-back potential. This means that the tube 9 performs the biasing and stabilizing function for both tubes so far as it is determined by the bias ing 1 sistance 31. As compared with tube 10 the steady iizegative biasing oi tube 9 is a com osite of the biasing determined by the re r1l ll1Ct3 31 and the biasing determined by the potential across its cathode. The tube 9 therefore functions in the conventional manner oi? selt-exeited oscillators, whereas tube 10 functions as a hybrid oscillator and amplifier, or perhaps better stated, as a nonoscillatory regenerative, or separately e2:- cited aniiplilier.

The expedient oi using a single biasing resistance is in the direction of economy of plant and simplicity of circuit. The resultdissymmetry does not affect the ultimate result, since it does not detract fromthe value of the circuit as a means for suppressing or balancing out the unmodulated carrier component although it may result in a slight dillerence of phase or amplitude, or both, of the output currents flowingthrough coils 18 and 19. This is compensated by the resistance-inductance combination 32 in the anode lead of one of the tubes.

Since, in this figure, the biasing resistance is in the common lead through which the current due to feed-back connection flows, provision must be made to insure that the in'ipedance of the input circuit for the cur rent fed-back is not too great. This is accomplished by connecting in shunt to the biasing resistance a condenser 33 having low in'ipedance at the frequency of the oscillations generated. This condenser also functions to improve the operation of the resistance by levelling the impulsive current flowing therethrough. lt will be charged at the potential impressed across the combination during each alternation when the grid becomes positiv e. Its capacity must not be too great or it will not have time to discharge through the very high resistance 31 during the negative alternation. The time of this discharge is determined by the product of its qpacity and the value of the biasing resis ance. On the other hand, the capacity must not be too small or it will unwarrana ably impede the carrier current. In practice it has been found possible to choose a value of capacity which meets both of these conditions.

l hen the grid periodically becomes positive the conditions in the input circuit are favorable to grid current modulation. This form of modulation results from the nonrectilinearity of the grid-potential-gridcurrent characteristic instead of the gridpotentalanode-current characteristic. The non-rectilinearity results from the relatively great change of impedance when the grid becomes positive. The theory of this type of modulation as distinguished from plate current modulation, which is the much more usual form, is described in British Patent 2 1-? ,626, April 1%, 1926. Since plate current modulation is relied upon in this instance, grid current modulation is detrimental, iuasmuch as when the umdulacted products resulting from grid i'i'iodulation have been auuplilied and appear in the external plate circuits of the tubes, they are in phase opposition to the otherwise similar side bands resulting from plate current i'i'iodulation. Grid current modulation in the circuit of ig. 4: is substantially prevented, or at least so reduced that its eltect is made negligible by the insertion of condenser 34 between the grids of the two tubes. This condenser provides a low impedance path between the points at which the side bands resulting from grid current modulation tend to occur and therefore prevents the potentials of such side bands from reaching a value which would materially reduce the amplitude of the side bands resulting from plate current modulation.

In Fi l, the stabilizing input resistance 35 is shown connected between the two grids. As a practical matter, it is immaterial whether this arrangement or the arrangenient involving resistances 23 and 2a of 1 is used. 'lhe receiving circuits are related to high and low frequency transmission circuits 2 and i in the same manner in Fig. 1. The only feature therein which has not been already suliiciently described in connection with Fig. l is the use of ed circuits 36 and 3'7 in the feed-back 'ircuits of the two tubes. These circuits are for the purpose of providing a low impedance path for the side band frequencies impressed on the demodulator. it has been Sli( vn above that etiicient operation of a modulator (of course the principle is equally true ot a demodulator) requires that the output circuit have a low impedance at each of the impressed fro-- que icies. In the case of the modulator, the output transform *1 and associated band filter are carefully and purposely adjusted so that the output circuit has high iinped ance at the side bandfrequencies and a very low impedance for the modulating frequencies. However, in the case of the demodulator'the output transformer, which must have high impedance for the demodulated low frequencies, would have a very much higher impedance for the impressed side bands, regardless of the impedance of iilter FLP, at these frequencies, so that a.

separate circuit means must beeinployed to provide a low impedance path therefor. the functions of the circuits 36 and 37 are ribed in an article by J. R. Carson, pul lisued in the Proceedings of the A. I. E. E. for June, 1921, the only difference being that this publication describes the effect of making the output impedance low for the impressed carrier, instead of the impressed signal, wave, the theoretical principle being identically the same.

These tuned circuits have an additional function which is specific to an arrangement, of which the circuit of Fig. t is an example, in which a perfection of balance of circuits arranged in push-pull relation cannot be completely secured. This function is not suggested in the Cars-on application. It may be .-..plaiiietl as follows:

:is has been described the eiiicient opern of modulators (including den'iodulato; s) requires, among other things, that the output circuit have low or Zero impedance at the carrier frequency. Incidentally also i an oscillator, and therefore in an oscillator-modulator, the amplitude of the oscillations is great in proportion as a low impedance path is provided for the oscillation currents. it has also been explained that, on account of the series relation of cathodes and common sources, the two tubes function somewhat differently so as to tend to produce a slight dissynui'ietry in the output circuits of the two tubes. To the extent that there is such dissymmetry, the balanced relation of output circuits does not, as it otherwise would, effectively provide a Zero impedance path in the individual plate filament circuits of the two tubes. The tuned circuits provide a low in'ipedance path for the carrier currents and accordingly ameliorate the condition resulting from such dissymmetry. The tuned circuits are obviously properly located to permit this function to be effected, since each of them provides a direct path from the plate of one of the tubes to the oscillation circuit.

In a iimi ting case the connection from the j unctiou of these tuned circuits to the transformer T, which'now includes condenser 38, may be omitted altogether, the tuned circuits alone being relied on to provide the low iu'ipedance paths. in order that these tuned circuits can perform their function with respect to the carrier frequency consisteutly with their somewhat similar function with respect to the received side bands a compromise tuning may be adopted, that is, the circuits may be tuned to a frequency at or near the inner edges of the side bands. Obviously separate tuned circuits may be used, one set tuned to the carrier and the other to the side band frequencies. Consistently with the function of these tuned circuits to compensate for an unbalance or dissymmetry of the output circuits, each of them should be entirely independent of the other, that is, there should be no mutual im- .pedancc therebetween.

Although the general plan of the invention has been described above, there are certain additional aspects of it that need treatment. 1t should first he noticed that there is no feed-back resistance in the circuit of Fig.

This can be explained as follows:

Although a method has been described, in connection with Fig. 4, for decreasing the e'tlects of grid n'iodulation, or for decreasing the relative amount of grid modulation, obviously a complemental method of decreas ing the relative amount of grid modulation by increasing the amount of plate modulation by providing a low impedance path for the carrier frequency. As has been described this may be done by balancing the individual portions of the iilate-cathode cir cuits and decreasing the impedance of the common portion as much as possible. In the system of Fig. 1 the impedance of the common portions of the circuits was made low by means of resistance 29 and the requisite stability was secured by the use of the re sistance inthe grid-cathode circuits and by the use ot a feedback resistance.

In the system of Fig. an equivalent ultimate result is secured without the use of the shunting resistance 29 or the feed-back resistance and, further, with decreased costof the oscillation circuit 15.

lhis result is accomplished in part by making the inductance in this oscillation circuit very low and the capacity'very high. Thus the impedance of this circuit to carrier frequency is very much less than if the inductance Were large and the capacity Were small, assuming in each case the presence of a small amount of resistance. This may be expressed inthe language of circuit design by saying that it results in making the Q of both the inductance and capacity, and accordingly the Q, of the combination, small. This means that the impedance is small. 'The specific design of the feed-baclrcircuit Will be considered later. As a practical matter the impedance can be made small ascompared with the joint plate resistance of the tWo tubes in parallel, and small enough to obviate the necessity for the shuntingresistance 29 of Fig.1. Additional advantages of this arrangement are that the load carrying capacity of the circuit is increased, its design is greatly simplified and its cost is materially reduced. V] hen to these advantages there is added those resulting from the elimination of the feed-back resistance, the efficiency, load carrying capacity and economy are cor respondingly still further increased. To overcome any attendant loss of stability the stabilizing resistance is made extra large. Accordingly the stabilizing resistance in Fig. 4: tends to be larger than either of the stabilizing resistances in Fig. 1. It is, especially, because of the very great stabilizing resistance used in]? l that the shunting condenser 33 is required to bypass the high fre quency carrier currents.

The elimination of the feed-back resistance and the particular design of the oscillation circuit as described is a radical departure from conventional practice, which requires an oscillation circuit of very high impedance and a large feed-back resistance.

In addition to increasing the stability, as in the manner pointed out immediately above, by compensating for the effects caused by the omission of the feed-back. resistance, and in the manner pointed out With reference to Fig. 8, the use of a very large stabilizing resistance has a pronounced effect in decreasing grid modulation. That it has this result is evident when it is considered that with an infinite biasing resistance the grids cannot be made to go positive, so thatthere can be no grid modulation. In a practical case the resistance Would be made so great that the grid current flowing would be re duced a negligible value and hence be insufficient to cause an appreciable amount of 1nodulation in the grid-circuit.

An interesting feature of thesystem of either Fig. 1 or Fig. i is the use of a normal biasing potential having a much greater negative value than is customary in conventional pract ce. This is disclosedin Fig. 2.

As a matter of practice a normal negative; biasing potential to give the self-oscillatory circuits of Figs. 1 and 4: amaximum frequency stability, using a grid biasing resistance, was found to be about 9 volts. The negative bias necessary in order to secure optimum modulator operating characteristics was found to be about 27.5 volts. WVhat Was actually done Was simply to choose a value of negative bias highly satisfactory from the standpoint of modulation and then to increase the biasing resistance to restore the oscillator stability.

In this connection it was found that the stability of frequency, stability of gain, and load carrying capacity Were enormously 1ncreased if a voltage as great as was used, instead of 27.5. The value of stabilizing resistance for the conjunction of conditions just described is consistent with its value as governed by the more or less independent considerations stated in the preceding paragraphs.

In order to further teach a person skilled in the art to build a circuit possessing the characteristics of the oscillator-modulator circuit of Fig. 4 the following data is taken from a circuit Which was used under practical operating conditions:

The inductance of the feed-back coil (the primary of the feed-back transformer) is 314: micro-henries. Its resistance is .34 ohms.

The quantity 113 for this coil, known by designersof this type of circuit as the Q lOl) of tht coil 59.5. The inductance of the secondary of the feed-back transformer is 39.15,

mil-henries. The coetlicient' of coupling is 89.5%. The voltage step-up is 10. The capacity of the tuning condenser of the feed back circuit is .757 micro-farads. The Q -ML of the condenser is 150. The .sta

bilizing resistance is of the order of l megohms. The capacity of the condenser shunting the stabilizing resistance is .001 microfarads. The tubes used are known in the trade as lOl-D tubes. The average amplification constant a of such a tube is 5.8. 7 Its plate resistance R, under operating conditionsis 7 500 ohms. The static characteristic of such a tube is indicated by the equation in Which e is plate voltage, 2', is space current in mils and e is the base of the natural system of logarithms. The design constants of the corresponding oscillator-demodulator are readily, derivable from the above values and from the known differences in the frequencies concerned in the operation of the two devices. From the general principles governing the design of the circuit of Fig. l as outlined above, similar design constants may be deduced therefor. 7

While the invention has been illustrated as embodied in a limited number of forms, it will be understood that it is equally well adapted to other forms and may embody variations in details, without departing from the spirit of the invention as defined inthe appended claims.

The principle governing the advantage of the use of very large biasing resistances that is illustrated by Fig. 3 and described with reference 'tothe system of Fig. 1 is" also applicable to the system of Fig. l to the extent that it illustrates the shapeof the portion of curve corresponding to the large resistances that would be used in practice. However, the portion or the curve corresponding to the lower value of grid resist ance tor' the system 0t Fig. l is quite radir cally different from that shown, since therein the asymptotic approach to the horizontal proceeds from a maximum point higher than the finally attained value 01 biasing potential it the resistance is increased to infinity.

. lVh-at is claimed is: I

l. The method of stabilizing the operation of a space discharge translating device having a cathode, a space discharge path, an input electrode and means for energizing said cathode and space discharge path, as at tected by abnormal changes in condition of said elements and abnormal changes in the impressed variations, which comprises teeding back a portion of the translated electrical variations from said discharge path to the input electrode and utilizing said variations to Vary the normally fiiXed negative potential applied to said electrode in proportion to the change from normal in the amplitude ofthe variations impressed on said input electrode. I

2. The method ot operating asp-ace dis-.

charge translating device, having a cathode, a space discharge path, an impedance control electrode, and means for energizing said cathode and discharge path, to produce a normal negative bias for said electrode which varies so as to stabilize the operation of the device, as affected by abnormal changesin the condition of said elements and by abnormal changes of the impressed variations, which comprises :leeding back a portion of the translated electrical variations from said discharge path to the control electrode and utilizing said variations to impress upon said electrode a normal negative biasing potential which varies proportionally to the amplitude of the variations impressed thereon.

3. Themethod ot operating a space di 7 charge oscillator having an input circuit including a control electrode which comprises ut lizing the variations fed back to. impress on said control electrode a potential which varies in proportionto the change in the am-' phtude from normal of said fed-back, varia tions, whereby the operation of the oscillator is stabilized with respect to those variations of the characteristics of the electrical element comprising the oscillator which tend to produce variations in the amplitude of the oscillations.

l. The method of operating a space dis charge oscillator having an input circuit including a control electrode which comprises utilizing the variations ted back to the input circuit to impress on said control electrode a normal negative bias which varies proportionally with the amplitude of the fed-back which tend to produce abnormal variations of the impressed wave.

6. The method recited in claim 5 in which the impressed variations are utilized additionally to produce the normal negative bias oil the control electrode.

7. In combination a space discharge translating device having a control electrode and a cathode, means whereby the control elec trode periodically becomes positive with respect to said cathode when the translated current increases above normal and in proportion to said increase, and a circuit including a resistance connecting said control electrode and said cathode and through which current is adapted to flow from said cathode to said control electrode to compensate for theaccumulated negative charge on said control electrode, whereby said control electrode is given an increment of negative potential bias )roportional to the increase of translated current above normal.

8. The combination recited in claim '7 including additionally a condenser in shunt with said resistance, whereby the lieu of current through said resistance tends to be more nearly constant.

9. In combination itself-oscillatory modulator comprising an oscillatory circuit, a space discharge tube including an anode and cathode and an impedance varying electrode, means for impressingmodulating waves on said cathode and impedance varying electrode, a resistance connecting said cathode and impedance varying electrode, and means whereby, due to the control electrode periodically becoming positive with respect to said cathode when the amplitude lUU ill)

of the resultant impressed waves including the oscillation component increases above normal, said control electrode is given an ii'icremcnt of negative bias proportional. to the amplitude of such imynfessed waves.

1.0. The combination. recited in claim f), including additionally a condenser connected in shunt to said resistance.

11. The combination recited in claim 9 in which the last mentioned means is mljusted so that said control electrode periodically becomes positive during the normal as well as abnormal operation of the circuit, where by said control electrode is given a normal negative bias which is adapted to increase proportionally with the increase in amplitude of the impressed waves.

12. The combination recited in claim 9 in which the last mentioned means is adjusted so that said control electrode periodicallybecomes positive during the normal as Well as abnormal operation of the circuit, whereby said control electrode is given a normal negative bias which increases proportionally with the increase in amplitude of the impressed waves, and a condenser connected in shunt to said resistance.

13. In combination a pair of space dis charge translating devices, each having a cathode, an anode, and a control electrode, a circuit including an energizing source connecting said cathodes in. series, an input circuit connecting the cathode and control electrode of each device, an output circuit connecting the cathode and anode of each device, a feed back circuit containing frequency determining elements connecting the output circuits collectively with the input circuits collectively, and an individual circuit including a resistance, connecting the control electrode of each device to its cathode, the values (it said resistances with relation to the feedback adjustment being such that the control electrodes-periodically go positive when the amplitude of the waves in'lpressed on the input circuits, including the fed-back wave, increases beyond normal, by equal amounts and by amounts proportional to such increase in amplitude l-il. The combination specified in claim 13, including additionally means for impressing modulating waves on said input circuits.

15. The combination specified in claim 13 in which the resistances and the feedback are so adjusted that the control electrodes periodically go positive during normal operation of the circuit loy-an amount suilicient to provide a desired, substantially equal, normal negative bias for said control electrodes.

16. The combination specified in claim 13, including means for impressing modulating waves on said input circuits and in which further the resistances and feed back are so adjusted that the control electrodes periodically go positive during normal opera provide a desired, substantially equal, nor

mal negative biasing potential for the said control electrodes.

17. The combination specified in claim 13, including a circuit for in'ipressing modulat ing waves on said input circuits, and an mdependcnt circuit for deriving resultant carrier modulated waves from said. output circuits, said input, out1: ut and feed back circuits being in balanced relation to each other and to said independent output circuit so that when no modulating waves are being impressedno current flows in said independent circuit.

18. The combinationspecified in claim13 in which the resistances and the feed back are so adjusted that the control electrodes periodically go positive during normal operation of the circuit by an amount suflicient to provide desired, substantially equal, normal negative biasing potentials for the said control electrodes, and including a circuit for impressing modulating waves on said input circuit and an independent circuitfor deriving modulated carrier waves, said input, output and feed back circuits being in balanced relation to each other and to said independent output circuit so that when no modulating waves are being impressed no current flows in said independent circuit.

19. In combination a pair of space discharge translating devices, each having an input and an output circuit, the output circuits, including a common portion, a feed back circuit containing frequency determin ing elements connecting the output circuits collectively with the input circuits collectively, a circuit for impressing modulating waves on said input circuits, an independent circuit for deriving modulated carrier waves from saidoutput circuits, said input, output and feed back circuits being in balanced relation to eacl'rother and to said independent circuit so that when no modulating waves are" being impressed nocurrent .flows in saidindependent circuit, and a relatively low impedance in shunt with said feed back circuit, thereby a low in'ipedance path is offered to the carrier frequency, as required for efficient modulation.

20. In combination, a pair of space discharge translating devices, each having a cathode, an anode, and a control electrode, a circuit including an energizing source con necting said cathodes in series, input circuits connecting the cathodes and control electrodes of the pair of devices and including a common portion and a portion individual. to each control electrode, output circuits associating the cathodes and anodesof the pair of devices, an independent cirraiit for derivin energy, from said output circuits, and

reed back circuit containing frequency determining elements connecting said output circuits collectively with the common portion of said input circuits and a resistance in the common portion of the input circuits, the value of said resistance with relation to the teed back being such that the con ,trol electrode ot one of the devices periodically goes positive when the amplitude of the waves impressed on the input circuits, including the feed-back waves, increases be yond normal, by an amount proportional to the increase in the amplitude of said impressed waves.

21. The combination specified in claim 20 in which the resistance and feed back are so adjusted that the control electrode or" one device periodically goes positive during normal operation of the circuit by an amount sutiicient to provide a normal negative bias for both devices, one different from the other by the potential drop across one cathode.

22. The combination specified in claim 20 in which the resistance is between substantially tour megohms and infinity.

23. The combination specified in claim 20 including means for impressing a modulating current on said input circuits.

24. The combination specified in claim 20 including means "for impressing modulating waves on said input circuits, said input, output and teed back circuits being in' balanced relation to each other and to said independent circuit so that when no modulating wave is being impressed no current flows in said independent circuit.

25. In combination a pair of space discharge translating devices, each having a cathode, an anode and a control electrode, a circuit including an energizing source con necting said cathodes in series, input circuits associatingthe cathodes and control electrodes ot the pair of devices and including a common portion and a portion individual to each control electrode, output circuits associating the cathodes and anodes or the pair of devices and including a common portion and a portion individual to each anode, an independent circuit for deriving energy from said output circuits, means for impressing modulating waves on said input circuits, a teed back circuit containing frequency determining elements connecting said tommon portions of the output and input circuits, a. resistance in said feed back circuit, impecance means in the individual portion ot at least one output circuit, said input, output and teed hack circuits, and said impedance means being in balanced relation to each other and to said independent circuit'so that when no modulating wave is being impressed no current flows in said independentcircuit, and the value of said resistance, with relation to the teed back adamplitude of the resultant waves impressed on the in'iput circuits, produces a proportional change in the resultant positive potential impressed on the control electroue of at least one of the devices and therctore produces a. proportional change in the negative bias potentials applied to the control electrodes of both devices.

26. The combination recited in claim 25, including a condenser. connected across the control electrodes of the two devices and. adapted to constitute a low impedance path thcrcbetween for the side bands produced by grid modulation, whereby the production of said side bands in the output circuit is materially reduced.

27. The combination recited in claim 7 in which said resistance has a value of the order of infinity.

28. In combinationa modulator, comprising a space discl'iarge tube, an input path, and an output path, means for impressing modulating currents and carrier currents together on said .input path, means for deriving the resultant modulated products from the output path, means whereby the input path becomes periodically conductive, means cooperating with said first means and including a resistance connected in shunt to said input path to normally render said input path non-conducting and means tor sub stantially preventing the production of modulation products by grid current modulation as aii'ected by the operation of the first two mentioned means. I

29. The combination recited in claim 28 in which the last recited means comprises a condenser, having low impedance to the frequencies, due to grid current modulation, connected across said input path.

30. In combination, a pair of space discharge translating deviccs, each having a cathode, anode, and a control electrode, energizing means for said cathodes, input circuits associating the cathodes and control electrodes of the pair of devices and including a common portion and a portion individual to each control electrode, output circuits associating the cathodes and anodes of the pair of devices and including a common portion and a portion individual to each anode, an independent circuit for deriving energy from said output circuits, a feed back circuitincluding frequency detern'iining elements connecting said output and input circuits, means for impressing modulating currents on the input circuits of said devices, means producing dissymmetry of the currentsin the individual portions of said output circuits when no modulatingcurrents are impressed, said individual portions of said output circuits being arranged in push-pull relation with said independent circuit and said output circuits being proportioned relajustment being such that a change of the tively to said independent circuit so as to cllicient'ly"transduce the resultant low fre quency modulatedproducts, and an additional circuit having low impedance at the 1. equency determined by said feed back circonnecting the cathode and anode of each device, whereby the output circuits have lOWlll'lPQClEtllCG for the unbalance currents re-' sulting from saiddissymmetry.

31. The combination cited in claim 30, in which the common portion of the input circuits contains a resistance and in which the means for producing dissymmetry is a series connection of said cathodes and the energizing means, said resistance and the feed back being so adjusted that the control electrode otouc device iieriodically goes positive during normal o aeration of the circuit an amount suiiicient to produce a normal nega tive bias for both devices, one diltering from the other by the potential drop across one cathode, and which varies proportionally to, the amplitude oi the variations impressed on said input circuit.

[an oscillation-generator comprising an -041- electric discharge device, including an input circuit and an output circuit, a feed back circuit connecting said input and output circuits and having an impedance at the oscillation irequency which is small as compared with the impedance of said output circuit and means for stabilizing the frequency of said oscillationgenerator to compensate for the inherent loss of stability resulting from the low feed back impedance.

The con'ibination recited in claim 82, in which the feed back circuit consists substantially wholly of a tuned circuit connected in antiresonant relation therein, the Qs of whose inductance and capacity are so small that the antiresonant impedance of the tuned circuit at the oscillation frequency is small as compared with the impedance of the internal output circuit oi the device.

3 1'. ,l.l1c combination specified in claim 32, in which the feed back circuit consists sub stan ti ally wholly of a tuned circuit con-i ncctcd in zuitiresonant relation therein, the 1113" of whose inductance and capacity is so fI-JlIltll that the antiresonant impedance of the tuned circuit at the oscillation frequency is sn'ia'll compared with the impedance oi the internal output circuit otthe device, and a resistance connected across the internal input circuit of said device having such value as to stabilize the oscillation frequency.

35. In combination a pair of space discharge translating devices each having a cathode, an anode and a control electrode, a circuit including an energizing source connecting said cathodes in series, an input circuit connecting the cathode and control electrode 01'? each device, a means for impressing potentials on said input circuits collectively, an output circuit connecting the cathode and anode of each device,and an individual circuit including a resistance, coiinectingthe control electrode of each device to its cathode, the values of said resistances with relation to the related circuits being such that the controlelectrodes periodically go positive, when the amplitude of the waves impressed on the input circuits increases beyond normal, by equal amounts and by amounts proportional to such increase in amplitude. I

v 36. The combination specified in claim in which the resistances are so adjusted that the control electrodesperiodically go positive during normal operation of the circuit by an amount sutlicient to provide adesired, sul'istantially equal, normal negative bias for said control electrodes.

37. In combination, a pair oi? space discharge translating devices, each having a cathode, an anode and a control electrode, input circuits connecting the cathodes and control electrodes of the pair oi devices and including a common portion and a portion individual to each control electrode, output circuits associating the cathodes and anodes ol the pair of devices, an independent circuit for deriving energy from said output circuits, and a resistance in the common portion of the input circuit, the value of said resistance with relation to the feed back being such that the control electrode of at least one of the devices periodically goes positive whenthe amplitude of the waves impressed on the input circuits increases beyond normal, by an amount proportional to the increase in the amplitude of said impressed Waves. e

38. The combination specified in claim 37 in which the resistance isSO adjusted that the control electrode of at least one device goes positive during normal operation of the circuit by anamount sufficient to provide a normal negative bias for both devices.

39. In a demodulator, a pair of space dis charge devices connected in push-pull rela tion, a low frequency divided output winding connected to the output electrodes ot' said devices, a circuit for supplying space current for said devices connected to the output circuits of said devices, a high-frequency "ill put electrodes of said; devices, a high-freconnected from a point between the portions quency path connected betwcenthe SLIIHB'OUtof said output Winding and a point in said put electrodes that are connected to the first path. 10 outermost terminals of said output winding, In witness whereof, I hereunto subscribe :1 high-frequency path from a point Within my name this Q ith day of September, A. 1).,

said first path to the othei" output electrodes 1925. of said devices, and a high-frequency path HAROLD S. BLACK.

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