US2186130A - Detecting system - Google Patents

Description

w. T. WINTRINGHAM .nmzc'rme SYSTEM Jan. 9, 1940. 2,186,130

Filed Aug. 19, 1936 2 Sheets-Sheet 1 FIG 2 g INVENTOR W. 7. W/NTR/NGHAM v ATTORNEY.

Jan. 9, 1940.

W. T. WINTRINGHAM DETECTING SYSTEM FIG. .5

Filed Aug. 19. 5

2 Sheets-Sheet 2 FIG. 6

//V VF N TOP l4. 7: W/NTR/NGHAM sygvw ATTORNEY Patented Jan. 9, 1940 units S DETECTING, SYSTEM William T. Wintringham, Chatham, N. J assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 19, 1936, Serial No. 96,774

19 Claims. (Cl.2 50--27) The invention relates to a detector of modulated waves and more particularly to a detecting system employing a vacuum tube. I

An object of the invention is to improve the fidelity of reproduction of the envelope form of the modulated wave. f

Another object is to improve the sensitivity of 1 a vacuum tube detector without sacrificing fidelity of reproduction of the envelope form.

A particular object is to'combine thebeneficial effects of a negative grid bias to prevent absorption of energy by the tube from the incoirning I modulated Wave with a negative or degenerative feedback connection selective to detected signal currents to minimize the distortion of the detccted signal currents. I

When a carrier wave of constant frequency and amplitude is amplitudemodulatedby a signaling wave, the envelope of the modulated wave a faithful reproduction of the signaling wave if were is no distortion in the modulation process. Since detection of a modulated wave is the process of obtaining a replica of the original signaling wave from the modulated .wave, a condition for detection without distortion is that the detected wave be a reproduction of theenvelope of the modulated wave. If the modulated wave were applied to a particular kind of ideal circuit 'element, the current through which is-directly-proportional to the high frequency voltage-whenever thisvoltage is. positive and the current zero whenever the voltage is negative, the average current would be anexact replica of the envelope of the higher frequency wave. Even if such an ideal circuit element existed, stray reactances in circuits connected to it in any practical case would so alter the potentials across itthat disk, tortionless detection would not be produced:

Since impedances external to an apparently ideal detector element affect the operation of such a device as-a detector, a fruitful mode of attack of the problem of distortionlcss detection lies in controlling the effect that these impedances have upon detection, If a circuit element which conductive when the voltage across it is positive and non-conductive when the voltage is negative is connected in series with asource of modulated Waves and a resistance which is very large compared to the resistance of the circuit element in the conducting condition, sub stantially all of the applied voltage will appear. across the resistance during the positive half cycles and substantially all of the voltage dropin the circuit will take place across the non-linear -circuit element during the negative half cycle.

ZJStdEli mm oFFicE The average voltage across either the resistance lor' the non-linear circuit element is a replica of the envelope of the impressed modulated wave;

1 i. e., such a circuit'appr'oachesthe ideal performance of a distortionless detector. I

The averaging mentioned in the previous paragraph implies energy storage from cycle to cycle ofthe high frequency wave; age takes place withinthe detector circuit, 1. e.,

if thevoltage to be averaged is not first trans- .fip

If such energy storierred'through a one-way device to the averaging means, the operation of the. detector becomes quitedifierent'irorn. that described, but the performance is not changed to any appreciable degree; A suitable means for energy storage and consequent averaging is a condenser connected tector circuit.

, in parallel with theexternal resistance in the de- To avoid discrimination against the higher modulation frequencies the magnitudes of the resistance and of the capacitance should be small. To minimizejthe distortion arising in detection, the resistance should be large, and to obtain the most efiective. averaging the capacitance should be large.

It is seen that the dimensions of theresistor and capacitor used for this type of detector in any practical case represent a compromise.

When the detector circuit includes a condenser for averaging the voltage drop across the resist when the non-linear circuit ance, this voltage drop is only slightly different during the portions of the radio frequency cycle element is conducting and non-conducting, respectively.' The mag: nitude" of the current through the non-linearcir cuit elementduring the conducting portioncf the cycle is only that which is suflicient to main- As the effwportion of the cycle during which current must flow tomaintain this charg e is decreased with the consequence that the shape of the. efiective resistance-voltage characteri ear circuit element has a l stic offthe non-linessened effect upon the voltage across the external resistance. When the detected wave is modulated, the identity between the voltage across the external resistance and the envelope of the modulated wave depends upon reducing this interval of current flow to a i minimum. This effect can be obtained by increasing the voltage of the modulated carrier circuit as a whole; by increasing the value of the external resistance;

wave applied to the detector or by' increasing the capaci condenser. v

tance of the shunt A two or three electrode vacuum tube is frequently used as the non-linear circuit element for detection in circuits of the type just described. With the grid and plate elements of a triode connected together and used as one electrodeand the cathode as the other, the equivalent of the usual diode detector results. With reasonable values of external resistance and capacitance, and large carrier voltages, the distortion arising in detection may be decreased to within quite satisfactory limits. The introduction of a modulated carrier voltage wave into such a detector circuit presents difficulties however, requiring, in general, the use of a radio frequency transformer the impedance of which is considerable. Such an impedance in series with the detector element not only gives rise to a reduction in voltage applied to the detector element, but likewise the distortion is changed from the value it would have with low carrier-frequency impedance in the circuit. To avoid these difficulties, the three electrode tube has been used as a plate circuit. de-' tector, where the grid is polarized negatively with respect to the cathode and operates as a control electrode only. Use is made of the plate-voltage plate-current characteristic of the tube in this type of detector. It has the advantage that the carrier frequency impedance in series wih the detector element may be made as small as is desired; butthe equivalent resistance of the platecathode circuit of the tube used in this manner is many times greater than the equivalent resistance of the circuit within the tube between the anode and grid in parallel and the cathode when the tube is used as a diode detector. It is difficult therefore to make use of external impedances sufliciently large to control the detection in the plate detector case, and as a consequence the distortion in this type of detector is excessive.

The detecting arrangement of the present invention, working on the feedback principle, combines the advantages of both earlier types of detector. Since voltages applied to the grid electrode of a triode are more effective than voltages applied to the plate electrode in changing the plate current (in the ratio of the amplification factor of the tube) the voltages appearing across a resistance-condenser combination in the plate circuit of a triode can be made much more effective by feeding them back to the grid circuit.

The invention may be more fully understood from the following detailed description thereof, together with the accompanying drawings.

In Fig. 1, there is shown an application of a feedback detector in a radio receiver.

Fig. 2 illustrates one method of connecting a signal amplifier to the feedback detector.

Alternate circuits for producing feedback are shown in Figs. 3 and 4.

The circuits shown in Figs. 5 and 6 illustrate how the feedback may be extended to be operative on a radio-frequency amplifier preceding the detector, as well as upon the detector.

Referring to Fig. 1, the primary coil of the input circuit of a radio receiver is connected be tween an antenna 2, and a ground 3. Inductively coupled to coil 1 is a tuned circuit 4, the voltage across which is applied to the input terminals of a radio-frequency amplifier. The output terminals of this amplifier are connected to the' primary winding 5 of a radio-frequency transformer. One terminal of the secondary winding 6 of thistransformer is connected in multiple to the grid 1 of a three-element vacuum tube V1 and to the grid IQ of a three-element vacuum tube V2. The other terminal of winding E3 is connected through a condenser 14 to anode 8 of triode V1, and through a resistance I2 to a common ground connection 26. The cathode 9 ,of the triode V1 is connected to ground 26 by the parallel combination of a resistance 13 and a condenser 23. The connections to the remaining electrodes of the triode V including the return path from the cathode H to ground 26, are not shown, as they are not considered essential to the invention and can readily be supplied by one skilled in the art of radio reception. The anode 3 is connected to the cathode 9 by a radiofrequency by-pass condenser it. A resistance I 5 is connected between the anode 8 and one terminal of the primary winding of a signal output transformer 24. A condenser 2| is connected between the other terminal of the primary winding of the signal output transformer 24 and the cathode 9. A resistance 58, inparallel with a series circuit consisting of a resistance [9 and a condenser 26 is connected also across the primary winding of the transformer 24. A'condenser H is connected between the junction of the resistance [5 and the primary winding of transformer 24, and the cathode 9 of the triode V1. A battery Eb is connected between ground 26 and the junction of the condenser 2| and the primary winding of the transformer 24, the negative. end of this battery being connected to ground. A resistance 22 is connected between the positive end of the battery Eh and the cathode 9. The telephone receivers 25 (or other signal responsive means) are connected to the terminals of the secondary winding of transformer In the operation of the system of Fig. 1, modulated radio signals are intercepted by means of antenna 1 and are selected and amplified in the conventional manner in the apparatus preceding the vacuum tubes V1 and V2. In the absence of a signal, the direct current from battery Eb through resistances l3 and Z2. is so adjusted that the drop in potential across resistance I3 is of I result that the relative grid-cathode potential of V1 is increased in the negative direction. Such increase of negative grid potential tends to decrease the space current over the value it had in theabsence of this'direct current feedback. The net result of these two tendencies is the establishment of a new value of steady voltage drop across resistance It, somewhat greater than the value in the absence of the unmodulated carrier wave. When this wave ismodulated with a signal wave, a similar balancing takes place, but in a different manner. is sufiiciently large that the voltage across resistance i3 is substantially unaltered by the signaling currents flowing in the anode circuit of triode V1, with the result that the cathode 9 is practically at ground potential for signaling currents. The anode load circuit, made up of resistances l5, l8 and i9, condensers i! and 20 and output transformer 2 is proportioned, however, in such a way that it offers impedance to currents of the frequencies of the signal wave. As a The by-pass condenser 23 consequence, signal frequency currents flowing in the anode circuit of V1 as a result of detection of the modulated carrier wave, produce voltages of the same frequencies between the anode and cathode'oi triode V1. -By making condenser l4 that the distortion at-the output of a triode tube sufiiciently large, practically identical voltages will also appear across resistance l2. Voltages of the signaling frequencies across resistance 12 1d produce variations of the grid-cathode potential,.

and these variations are in such direction that upon amplification in the triode; they tend 'to' reduce the amplitude of the signalwave 'com-' ponents in the anode circuit. If the detected wave in the anode circuit contains distortion.

components introduced inthe detection process, these components are reduced by the feedback action to a greater. extent than=are those components which are present in the envelope of the incoming modulated wave. It has been found used as a plate circuit detector is reduced ap preciably when this feed-back action: is applied.

The signal frequency componentsof the anode current flow from the anode ii throught the resistance l5, the parallel circuit consisting of resistances l8 and Hi and condensers l1, 2G and 24, and thefprimary winding. of signal output transformer-24 to the cathode 9. As, is well-knovmin the art, the relative magnitudes of the various resistances and'condensers and the impedence 1 presented by the primary winding of the output transformer determine --the ratioof the current flowing through this primary winding to the total anode current, at each frequency in the signal frequency band. -Disc'rimination against or. ac-.

centuation' of certain signaling frequency components in the radiorfrequency amplifier may be corrected by adjustment of this division of current. The portion of the anode signal current flowing through the-primarywinding of the signal output transformer 2 induces corresponding voltages in the secondary circuit and the currents corresponding to these "induced voltages actuate the telephonerec'eiver 25.

Vacuum tube Vagrid iii of which is connected in parallel with grid 'l'of-Ivacuum tube V1 is included to indicate a suitable way in which a d'etector for automatic gain control may be connected.- As pointed cutjabove, the potential dif- 1 ference between grid ier grid {.5 and groundis made up of two part the modulated carrier wave which appears across the secondary 6 of the radio-frequency transformer, and the signaling wave fed back from the anode circuit of triode V1 and which appears across resistance !2. These two waves are sopoled that when the envelope of the modulated wave increases in amplitude,

the fect-backsignaling wave tends t o 'keep the total positive excursion of grids l and It) relative 60 to ground constant. v 7

Referring to Fig. 2. the primary it?! of a radio frequency transformer is connected between an antenna .2! and a ground 28. and is inductively I coupled to a tuned secondary circuit One sideof the tuned circuit '29 is connected to the grid'electrode 3!; of a triode V3 which is usedas a feedback detector, and the-other side of the tunedcircuit 2 9 is connected through the parallel combination of a resistance 33and a radio-frequency by-passcondenser 34 toground d9. This latter terminal of tuned circuit 29 is connected also through a signal feedback condenser 4! to the anode $1 of a triode V3 anda fourth connec-' tion is made to the control electrode 42 of amplifier triode V4. Thecathode 31 of the triode by-pass condenser 41.

win

V3 is connected to a ground 49 through the parallel combination of a biasing resistance 35 and 2. by-pass condenser 36., The anode} 32 of the triode V3 is connected tothe cathode 3! of this same'tube by two parallelcircuits connected inseries. One of the parallel circuits comprises an anode load resistance 48 and a radio-frequency by-pass condenser 39, and the other'parallel circuit consists of an initial bias resistance 31 and a large by-pass condenser 5|. The positive terminal of a plate battery 38 is connected to the common .connection between thesetwo parallel circuits, and the negative terminal 'Of this battery is-conne cted to ground 49. "The cathode 43 of thesignal amplifier triode V4 is connected to ground 45 through the parallel combination of a biasing resistance 44 and a by-pass condenser (45. A plate battery .6 is connected to anode 50 of triodevr'through the primary winding of a signal output transformer 48 and'is shunted by a The secondary'windin'g of the output transformer 48 is'connected to the quencies is large compared to the resistance 33.

Across resistance 33. therefore, only signal frequency voltages exist to any appreciable degree.

Similarly across resistance 40, radio-frequency voltages are reduced, while both signal frequency and zero frequency voltages predominate. Due to the presence of by-pass condenser: 36. the principal voltage across resistance 35 is direct cur- I rent, although it is unavoidable that some small signal frequency and radio-frequency potentials likewise be built up across this circuit between cathode 3! and ground 49. Condenser 4! is chosen of such size in relation to resistances 33' and M3 that so far as signal frequencies are concerned. these two resistances are effectively in parallel and the same signal frequency voltages appear across both resistances, The grid cathode bias of triode V3 is adjusted in the absence of signal to a value near to space current cut-off, by adjustment of the value of resistances. 35 and 31 in conjunction withthe voltage of battery 258. Under this condition, the change in space current of V2 produced. by the application of an unmodulated radio-frequency carrier increases the direct current voltage drop across biasing resistance This increase in negative grid-cathode bias is such that space current flows only during the small portion of the positive halfofeach radiofrequency cycle, i.e.,-.the new bias is very near to space current cut-off. When the radio-frequency carrier is modulated. a further change in space current takes place, this change being nal. This change in space current-at the modulation-rate produces voltagesvarying at the same rate across resistances Stand M. The effect of the voltage drop across resistance it upon the operation of the detector is small; in general, but the voltage drop across resistance 33 adds directly to the negative grid-cathode bias already existing in the circuit. The polarity of these vollargely at the frequencies of the modulating sigtage-variations across resistance 33 is again such that the total grid-cathode bias varies with variation in the carrier amplitude during the modulation cycle in the proper direction that the space current flows only during a small portion of each positive half cycle of the high frequency wave. The signal frequency amplifier tri'ode V4 is selfbiased in the usual manner by the drop in potential due to its own space current flowing through biasing resistance. Ml. As pointed out above, the voltages across resistance 33 are practically the same as the voltages corresponding to the signal wave across resistance 4%. Instead of coupling the control grid of triode V4 by means of 5, a condenser to resistance adding a grid leak to apply negative bias to this electrode, condenser a! and resistance 83 serve a double purpose and replace these customary elements. The remainder of this circuit operates in the usual wmanner. The circuit shown in Fig. 3 is an illustration of the use of inductive feedback coupling at signal requencies between the anode and grid circuits oi the detector instead of the use of capacity- 5;; resistance coupling shown in Figs. 1 and 2. One

terminal of the secondary of a tuned radio-frequency transformer 53 is connected to the control 5 3 of a three electrode vacuum tube V5 The other terminal of the secondary of transformer 53 is connected through the parallel combination of a radio frequency lay-pass condenser 53 and a resistance 5'1, which is shunted by the tertiary winding 56 of a signal frequency output transformer, to a grid biasing batteryii i. This biasing battery is shunted by a by-pass condenser 62. A biasing resistance 59 shunted by a by-pass condenser til is connected between this biasing battery and the cathode 55 of triode V5. The anode 56 of triode Va is connected to one terminal of a radio-frequency by-pass condenser 68, the other terminal of which is connected to the cathode 55 of triode V5. The anode 58 is connected also to one terminal of a primary winding 85 of the signal. output transformer. The 5r, other terminal of the primary winding 55 is connected to the positive terminal of a plate battery The negative terminal of this battery is connected to the common connection between the id "iasing battery El and the biasing resistance 59. The plate battery 83 is shunted by a large by-pass condenser 66. winding til of the signal frequency output transf4 qer connected to a load circuit, illustrated by a resistance 1%.

In the absence of any signal, batteries t l and Gil are adjusted to such voltages that the space current of triode V5 is practically zero. Upon the reception of an .unmodulated carrier, the change in steady anode current through triode V 60-, prod ices an increase in the potential drop across resistance This increase in negative gridcathcde biasing potential produces a shift in the operating point so that the anode current is reduced from the value it would have without 55 t direct current feedback, but only to a value sl ghtly larger than it in the absence of the modulated carrier. When the received carri r wave is modulated, the component of the anode cur ent correspondingto the modulation fl. we through primary winding of the output sioriner, and by induction a corresponding volt is produced in tertiary Winding M5. The voltage in the tertiary winding causes curent to in the series circuit consisting of the tertiary winding and resistance 5?, with the result The secondary that voltages corresponding, to the modulating.

wave components of the anode current appear across resistance 5?. By; properly poling windlugs 65 and 56, the voltageacross resistance '51 will be of such phase as to increase the negative bias on the control grid 54 of triode V5 relative to cathode 55 when the envelope of the modulated wave increases in the positive sense. cuit operates, therefore, to reduce the distortion of the modulation-wave-icrm arising from de' tection in the same manner as the circuits shown in Figs. 1 and 2.

The circuit shown in Fig. l represents a further variation inasmuch as direct feedback coupling from the anode circuit to the grid circuit is accomplished by utilizing feedback circuit elements common to both circuits. In this circuit, the grid electrode H of a threcelement vacuum tube V6 is connected to one terminal of the tuned secondary circuit ill of a radio-frequency transformer. The cathode 12 of tricde V6 is connected through the parallel combination of a radio-frequency by-pass condenser it and a signal frequency output transformer primary winding l5- and through the parallel combination of a bypass condenser '58 and a biasing resistance 79 to the common connectionof the plate and the grid batteries. .A grid bias battery 8% is shunted by a bypass condenser l, and a plate battery 82 is shunted by a by-pass condenser 83.

In the circuit shown in Fig 4, the initial adjustment of the space current almost to zero is made by varying the relative potentials of batteries fit and Rectified space current flowing when an unmodulated carrier is received, produces a steady increment in the negative gridcathode bias by means of the drop in potential across resistance 19. When the received carrier is modulated, detected signal frequency components of the anode current flowing through the primary winding 55 of the signal frequency output transiorrner'produce si nal frequency voltages across this windinm Since these voltages are produced within the grid-cathode circuit, the

of each positive half cycle of the modulated radioirequency wave.

Referring to Fig. 5, a method is illustrated feeding back the signaling wave, not only to the input of a detector, but also to the tubes in radio-frequency amplifier preceding the detector. One terminal of the secondary of a radio-frequency transformer il-i isconnected to the control electrode 85 of a five-element vacuum tube V7. The other terminal of the secondary 84 is connected through a grid bias battery 3i? to the cathode 86 ofpentode V7. A large icy-pass condenser Si is connected between the s reen electrade 8? and the cathcdeiifi of pentode V7. The screen 3'? is connected also to one terminal of choke coil 92, the other terminal of which is connected to the positive terminal of. a battery The negative terminal of the battery 93 is connected to the cathode i363 of the pentode V7. The suppressor grid electrode 880i the pentode V7 is connected to one terminal of choke coilv 98, the other terminal of which is connected to the negative terminal of a battery H2. The positive terminal of the battery 1 i2 is connected to cathode 86 of the pentode V7. A radio-frequency bypass condenser I HE is connected between the suppressorgrid electrode88 and cathode 3'3 of This cir-' wave is carried out by applying the output sig-' nal. wave from thedetector to the suppressor Q grid 88 of pentode V1 in Fig. through the couthe pentode V7. The suppressor grid electrode 80 is connected also" through a signal feedback cone denser'90 to the anode electrode I4 of a triode tube V3; The anode electrode 89 of the pentode- V7 is connected to one terminalof a primary -winding 96: of a radio-frequency transformer;

Theotherterminal of the primary winding 06 is connected both to one terminal of a large bypass condenser 95, the other terminal of which is connected to the cathode-86 of the pentode VT', and ,to the positive terminal ofa battery 94. The negative terminal of the battery 94 is connected to the positive terminal of the battery 93. The circuits connected to the secondary winding QI of the radio-frequency transformer maybe thoseof vany de'tector. The preferred form chosen for'representation in Fig. 5 is an improved feedback type of detector of the same type illustrated in -Fig. 2; In Fig. 5, Vs isa detector triode, the grid E02 of which is connected toone. terminal of the secondary winding 01.

The other terminal of the winding 91 is connected through a signal feedback condenser I0I to the anode I04, andalso through a resistance I00 to ground li t? The cathode" I03 isconnected to ground H4 throughthe parallel combination of a by-passcondenser I05 and a bias resistance I05, a .The radio-frequency by pass condenser I'I is connected between the anode I04; and the oathodefil03 of the'triode Vs. One terminal of a resistance I08 is also connected to the anode I04 the other terminal of this resistance being connected-to one terminal ofan initial bias resist- ,ance I09, one terminal of a large by-pass condenser, l I I and the positive terminal of a plate batteryfl I0. Theother terminals of the resist- :denser 99, the 7 operation of the amplifier-detectorshown in. 5 would be preciselywhat would be expected from a conventional radio frequency amplifier followedby a feedback detectorfof the sort described above. A well-known property of a pentode tube containing a suppressor grid in a radio frequency amplifier is the change in gain of such an amplifier when the potential difference-between suppressor-grid and cathode is varied. If the tube is operating as an amplifier of a radio frequency wave and a signaling voltage is applied to the suppressor grid, the output wave from the amplifierwill ,bemodulated by the signal wave. It is this property of the suppressor grid pentode of which use is made in the-circuit shown in Fig. 5. If the modulation index of a modulated wave be defined .as the ratio of half the difference between the maximum and minimum amplitudes of the modulated wave to the average amplitude, the distortionof the signaling wave due to detection of a modulated wavewitha conventional detectorisgreatest when'j'the modulation index has its maximum value. The feedback detector here disclosed tends to'reduce this distortion by a balancing action. Thefeedback amplifier circuit shown in Fig.- 5. represents one 'method of I reducing this distortion. still further by modulating, the modulated wave with the signaling. wave in a reverse sense so that the modulation index at the input to the detector is reduced. This remodulation of the received modulated The negative ter- I20 of the pentodeVa.

is connected to the cathode Ill.

pling condenser s During the portion of the modulation cycle when the amplitude of the modulated wave at the grid I02 of thetriode Va is increasing, the anode current of this tube increases also.

relative to ground H4 is equal tothe sum of the The potential of the anode I04 electromoti ve force of'the battery IIO andthe voltage drop across the resistance I08 for'frequencies sufficiently low that the reactance of the condenser Hill is much larger than the value of the'resistance I08. Where-the anode current vincreases at the signaling rate the voltage dropv across the resistance increases at the same rate and, therefore, the potential difference between the anode Hi4 andground H4 decreases at the same rate. 'A decrease in positive potential is equivalent to an increase-in negative potentiaL. .so it may be'said that asthe envelope of the Wave attgrid E02 increases in magnitude, the anode, potentialtends to increase in a negative direction. By'means of coupling condenser. 99

this increasing negative potential is applied to I suppressor grid 88 of pentode V7; Cathode 86 is at the samesignal potential as ground H4, so that the suppressor'v-grid to cathode potential of pentode V7. is made increasingly negative. Such an increase in suppressor-grid negative potential decreases the gain of the amplifier stage of which pentode V7 is a part, with the'result that theamplitude of the modulated Wave at grid I02 vof detector triode Va is decreased. vSimilarly,

duringthe- -part of the modulation cycle when the amplitude of the received wave is decreased, thegain of this amplifier stage is increased, and

the amplitude of the wave at grid I02 is increased. Effectively then-the modulation index of the received Wave has been decreased, with; a corresponding desirable effect upon the distortion produced. in the detector. 'obviouslythiscon'trol action may beextended over as many stages of a radio; frequency amplifier as may seem desirable.

The circuit shown in Fig. 6 represents a modification of thatshown in Fig. 5. In this case modulation in the radio-frequency amplifier is effected by usingthe control gridv electrode of the pentode instead of the suppressor grid electrode. One terminal of a radio-frequencytransformer secondary H5 is connected tothe control grid II 6 of a pentode Vs. The other terminal of the secondary Winding I I5 is connected through a grid biasing battery I2! to the' junction between a feedback condenser I33 and a feedback resistance I32. A large by-pass condenser I22 is connected between the screen elecv.trode II8 and the cathode Ill. The screen II8 is connected also to one terminal of a choke coilI23, the other terminal of which is connected to the positive terminal of a battery I24. The negative terminal of the battery I24 is connected to the cathode .I ll of'the pentode V9. The suppressor-grid electrode H9 of the pentode V9 is connected directly to the cathode II'I.'. One terquency transiormer is connected to j the anode The other terminal of the primary'winding' I2! is connected both to one terminal of a large by-pass condenser I26 and. to the positive terminal of a battery I25.

minal of a primary winding I21 of a radio-ire:-

Theother terminal of the by-pass condenser I26 I The negative terminal of the battery I25 is connected to the positive terminal-of the battery 82d. -cuit 'beyond the secondary winding 28 of the ra- The cirias-is supplied by the battery i225, and when the received Wave is modulated, signal frequency voltages-appear across the resistance i321, due to feedback in the detector stage. This circuit therefore, is suitable for making use of pentode V9 as a modulator. As has been explained in connection with the feedback detectorcircuits shown in Figs; 1 and 2, due to the presence of feedback condenser I33, the signal frequency voltages across resistance it? are practically the same as those across resistance iS'l. When, due to modulation, the radio frequency voltage applied to grid i129 of triode V10 increases, the potential aoove ground Hit of the junction between resistance 532 and secondary winding comes increasingly negative at the modulation rate. This increasingly negative potential is applied to grid ill; of pentode V9, with the result that the gain of the amplifier stage including pentode V9 is decreased. Such decreased gain results in a decrease of the amplitude of the radiofrequency wave at grid E29 of triode V10. Similarly, when the radio i'requency potential at grid decreases due to modulation, the gain of the amplifier stage including pentode V9 is increased,

increasing the amplitude of the Wave at grid H9. The net result of this remodulation of the original modulated wave is adecrease in the modula tion index of the wave applied to the detector, with a consequent decrease in distortion arising in detection.

What is claimed is:

1. A detecting system comprising a space discharge device having a cathode, an anode and a control grid, a source of signal modulated carrier waves connected to the grid circuit or" said space discharge device,means for biasing the grid to a negative potential sufficient to prevent the flow of grid current during normal operation, and a resistive load selectively coupled at signal frequencies both to the grid circuit and to the anode circuit in such relation to each that a change of potential across the load has compensatory efiect's upon said two circuits.

2. A detecting system comprising a space discharge device having a cathode, an anode and a control'grid, a source of signal modulated carrier waves connected to the grid circuit of said space discharge device, negative biasing means for preventing the flow of grid current, a resistance coupled to the anode circuit, and lying in a path traversed by detected signal currents, and coupling means for selectively transferring signal variations from said resistance into the grid circuit, in phase opposition to the envelopevariationsof the signal modulated carrier waves impressed in said circuit. V

3. A detecting system comprisinga space discharge device having a cathode, an anode and a control grid, 2. source of signal modulated carrier wavesconnected to the grid circuit of said space discharge device, negative biasing means for preventing the flow of grid current, and a resistive load selectively coupled at signal frequencies both to the grid circuit and to the anode circuit, the

couplings being so poled respectively that a change of potential in the load-affects both the grid and anode potentials in such direction as to counteract said potential change in the load.

4. A detecting system comprising a space discharge device having a cathode, an anodeand a control grid, a source of modulated carrier waves connected to the grid circuit of said space discharge device, a source of negative biasing potential connected to the grid, and a resistive load selectively coupled at signal frequencies both to the gridcircuit and to the anode circuit in such relation to each that a change of potential across the load has compensatory effects upon said two circuits. I 1 I 5. A detecting system comprising a'space discharge device having a cathode, an anode and a control grid, a source or" signalinodulated carrier waves connected to "the grid circuit of said space discharge device, means for biasing the grid to a negative potential sufficient to prevent the flow of grid current during normal operation, and a resistance oi large value relatively to the plate cathode resistance of the tube, said resistance being selectively coupled at signal frequencies both to the grid circuit and to the anode circuitv in such relationto each that a change of potential 1 across the load has compensatory eifects upon said two circuits.

A detecting system comprising a space discharge devicehavlng a cathode, an anode and a control grid, a sourceof signal modulated carrier waves connected to the grid circuit of said spacedischarge device, means for biasing the grid to a negative potential suflicient to prevent the flow of grid current during normal operation, and a resistive load selectivelycoupled by conductive coupling at signal frequencies both to the grid circuit and to the anode circuit in such relation to each that a change of potential across the load has compensatory effects uponsaidtwo circuits.

7. A detecting system comprising a space dis- 1 charge device having a cathode, an anode 'and'a control grid, a source of signal modulated carrier waves connected to the grid circuit of said space discharge device, means for biasing the grid to a negative potential sufficient to prevent the how.

of grid current during normal operation, and a resistive load coupled by capacitive coupling at signal frequencies both to the gridcircuit-and to the anode circuit in such relationship to each that a change of potential across the load has compensatory efiects upon said-two circuits.

8. A detecting system comprising a space dis charge device having a cathode, an anode and a control grid, a source of signal modulated carrier waves connected to the grid circuit of said space discharge device, means for biasing the grid toa negative potential sufiicient to prevent the how, of grid current during normal operation, and a resistive load selectively coupled by inductive coupling at signal frequencies both to the grid circuit and to the anode circuit in such relation to each that a change of potential across the load has compensatory effects upon said'two circuits.

9. In a wave transmission system, the method of operating upon a modulated carrier wave which comprises amplifying said wave in an electron discharge device having a cathode, anode, control grid, screen grid andan auxiliary grid between said'screen grid and said anode, applyfeedback wave. v I a 10. A wave transmission system comprising;

a-r isa rso i-ng-the' outputof saiddeviceto the inputqfa secondelectron discharge device, rectifying said 1, wave in the latter device to produce the-,modulating signal wave in the output of said second device, feeding back a portion ofsaid rectified output across the cathode and the'auxiliarygrid of the firstdischargedevice, and modulating the modulated carrier wave in accordance with the ,modulated carrier wave amplifying means ina eluding an electron discharge device I having a cathode,an anode, a-control grid,"a screen grid and a third grid, the latter grid being positioned betweensaid screen grid and the anode and .con-

nected to said cathode, meansfor rectifying the amplified output of said'amplifying means, and means for feeding hack to said-third grid 2. portionof the rectified output wavelof said rectifying means so that said feedback wave modulates the modulated carrier wave in sense opposite to s the carrier wave is initially modu-f that inwhich latedv r 11. A wave transmission system comprising an electron discharge devicef'having a cathode, an anode, a control grid, 'a screen grid, and a third grid positionedbetweenthe screen grid and the anode and connected'to said cathode, means for biasing said third gridto a potential negative with respect to the cathode, means for impressing a modulatedcarrierwave between said control grid and cathode, a-second electron discharge device havinga cathode, an anode and a control grid, means coupling the anode-cathode circuit of said first device with the grid cathode circuit of said second device, and means for feeding back to thegrid-cathode circuit of: said second device. and to the third grid-cathode circuit of said first. device a portion of the energy in the cathodeanode circuit of said second device. i

'12. A wave transmission system comprising an amplifying electron discharge device having a cathode, an anode, a control grid, a screen grid,

and a third grid positioned between the screen grid and the anode and connected to said cathode, means for biasing said third grid to a potential negative with respectv to the cathode, means for impressing a modulated carrier wave betweens'aid control grid and cathode, a rectifyingelectron discharge device having a cathode,

- of the energy in the cathode-anode circuit of said seconddevice.

13. A wave transmission system comprising an 7 amplifying electron" discharge device having a cathode, an anode, a control grid, a screen grid,

and a third grid positioned between the screen grid and the anode and connected to said cathode, means for biasing said third grid to apobetween said controlgrid and cathode, a rectifying electron discharge device having a cathode,-

tential negative with respect ,to the cathode, means for impressing amodulated carrierwave an anode and a control grid, means coupling the anode-cathode circuitof said first device with the'grid-cathode circuit of saidsecond device,

and means for feeding back to the. grid-cathodecircuit of said second device and to the third grid-cathode circuit of said. first device a portion of the energy in the cathodeanode circuit of said second device, saidmeansqcomprisin'g .a resistance in the grid-cathode circuit of the second device and a capacityconnecting the resis ance with the ,anode of said second device, and a capacity connecting the thirdgrid of the first device and the anode of said seconddevica.

14. A Wave transmissionsystem comprising an electron discharge device having a cathode, an anode, a control gridfa screen grid aithird grid positioned between said screen grid and said anode; means for impressing a modulated carrier wavebetween said control gridfiand'cathode, a

second-- electron discharge device having acathode, an= anode and a control grid impedance means couplingthe cathode-anode circuit of said first device with the control grid-cathode circuit of said second device, and means for feeding hack to the. grid-cathcde circuit 'of said-second device and'to a grid-cathodecircuit of said first device a portion of thelen'ergy in-thecathode-anode circuit of saidsecond' device;

- ;.15."-In combination, anelectron vice comprising a cathode,f.an anode and agri'd, means for impressing electric wave energy between said 'cathode and grid, {afresistor connected to said cathode and "common to the gridcathode andthe cathode-anode circuits of said device, a source of direct current potential for said anode, a local circuit including said resistor andfsaid potential source for producing a 'potential drop in said resistor to bias the control grid negative with respect to the cathode, the space current of'said device also producing a grid biasing negative potential drop, asecond resistor connected between the low potential end of said cathode resistor and said grid, and means for impressing a portion of the energy in the anodecathode circuit on said second resistor inpreassigned phase relation to the electric wave energy impressedacross said cathode and grid.

' .16. In combination, an electron "discharge device comprising a-cathode, an anode and a grid,

fmeans for impressng electric wave energy bedischarge detween said cathode and grid, a resistor connected to said cathode and common to the grid-cathode and the cathodeeanode circuits offls'aid device, a

source of direct current potential for said anode, I

a local circuit including said resistor and said potential sourcejfor producing a potential drop in said resistor to bias the control grid negative with respect tothecathode, the space current of said device also producing a grid biasing negative 110- tential drop,1a second resistor connected between the low potential end of said cathode resistor and said grid, a second electron discharge device having a cathode, an anode and a grid,"and

means for impressing a portion of the energy in the'anode-catho de circuit of the first deviceon said second resistor in preassigned phase relation to the electric Wave energyimpressedacross the cathode and grid of the first device, and on the cathode-grid circuit of said second device.

1 7. In combination, an electron dis-charge device comprising a cathode, an anodeand a grid,

means for impressingelectricwave energy between said cathode and grid, animpedance connected to said cathode and common to the grid-- a cathode and cathode-anode circtuts, a source of potential for said anode, means independent of space current flow through said impedance for producing a potentialdrop in said impedance to bias thegridnegative with respect to said cathode, asecond impedance connected between the low potential end of said cathode impedance and saidgrid, and means for impressing'a portion of the energy in the cathode-anode circuit on said second impedance in preassigned phase relation to the electric Wave energy impressed across said cathode and grid.

18. In combination, an electron discharge device comprising a cathode, an anode and a grid, means for impressing electric Wave energy between said cathode and grid, an impedance connected to said cathode and common to the gridcathode and cathode-anode circuits, a source of potential for said anode, means independent of space current flow through said impedance for producing a potential drop in said impedance to bias the grid negative with respect to said cathode, a second impedance connected between the low potential end of said cathode resistor and said grid, a second electron discharge device having a cathode, an anode and a grid, and means for impressing a portion of the energy in the anode-cathode circuit of said first device on said second impedance in preassigned phase relation to the electric, wave energy impressed across the cathode and grid of the first device, and on the cathode-grid circuit ofsaid second device.

19. A Wave translating system comprising an electron discharge device having a cathode, an anode and a grid,,an impedance connected to the cathode and common to the grid-cathode and cathode-anode circuits, means for producing space current flow through said device, means including said impedance but not the space current path for biasing the grid substantially to space current cut-off, said impedance providing a grid bias of the same character on application of an unmodulated carrier wave across a pair of said electrodes to limit the space current resulting therefrom substantially to that for the initial grid bias condition, the resultant grid bias being such that, if the carrier Wave has been modulated by a signal wave, space current flows for only a small part of each positive portion of the cycle of the modulated wave, an impedance connected between said grid and the end of said first impedance remote from the cathode, and means connecting the cathode-anode. and the gridcathode circuits for feeding back from the oathode-anode circuit to said. second-mentioned impedance a portion of the signal Wave energy in preassigned phase relation,

1 WILLIAM T. WINTRINGHAM.

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