US2413348A - Signaling system - Google Patents

Description

Dec. 31, 1946.

J. H. HAMMOND, JR

SIGNALING SYSTEM Original Filed Feb. 10, 1940 .4 Sheets-Sheet l PRE-AMPLIFIER JOHN HAY H INVENTOR MMOND JR.

ATT ORNEY Dec. 31, 1946. HAMMOND, JR 2,413,348

' SIGNALING SYSTEM Original Filed Feb. 10, 1940 4 Sheets-Sheet 2 LjTpDE MODULATION QUASI PHASE MODULATION cos a): 7 cos wt l I l gCOSQF Jt cos wt cos 1: cosm Ticrac.

cos on K SIN bi- .s'M an PHASE OR FREQUENCY MODULATION A cos(aJ-p)t A1 CO8(&J+,o)t

2 gamma-amt TERMJ mm 1? 2 (2H 11v (cos flit +1 Slit/pt s/lv at) INVENTOR JOHN HAYS HAMMOND JR.

ATTORNEY 1366- 1946- J. H. HAMMOND, JR

SIGNALING SYSTEM 4 Sheets-Sheet 3 Original Filed Feb. 10, 1940 INVENTOR J OH N H AY SZAMMOND JR. BY ATTORNEY Dec. 31, 1946.

J. H. HAMMOND, JR

SIGNALING SYSTEM 4 Sheets-Sheet 4 Original Filed Feb. 10, 1940 INVENTOR JOHN HAYS HAMMOND JR.

BY ATTORNEY Patented Dec. 31, 1946 SIGNALING SYSTEM John Hays Hammond, Jr., Gloucester, Mass, assignor to Radio Corporation of America Original application February 10, 1940, Serial No.

318,221. Divided and this application November 12, 1942, Serial No. 465,307

- 11 Claims.

This invention relates to phase modulation systems, and more particularly to systems of the above type having phase characteristics adapted to permit the separate detection of the upper and lower side bands. This application is a division cigigy application Serial No. 318,221, filed Feb. 10,

An object of the invention is to provide a system of the above type which is free from fading and from interference by extraneous influences, such as static or interfering amplitude modulated signals.

Another object is to provide a frequency modulation system having characteristics suited to permit separate detection and recombining of various components thereof.

Other objects and advantages will be apparent as the nature of the invention is more fully disclosed.

It is well known in the art to which the present invention appertains that there is no mathematical distinction between phase modulation and frequency modulation. Accordingly, in the following specification and appended claims the term timing modulation has been used generically to characterize phase modulation and frequency modulation as distinguished from amplitude modulation.

In accordance with the present invention, a phase modulated wave is radiated which comprises upper and lower side bands, and a carrier which is displaced from the phase relationship adapted to produce the usual amplitude modulated wave. The carrier is preferably displaced in phase by 90, and is adapted, when combined with the upper and lower side bands respectively, to produce low frequency, or detected, waves which are displaced in phase by 180. These waves are, accordingly, adapted to be combined in opposed relationship to reproduce the original signal. Extraneous noises or amplitude modulated signals, however, are balanced out during the combining process, because such disturbances appear in the same phase in both parts of the balanced circuit, whereas the desired signals are 180 out of phase, and, accordingly, are converted into additive instead of subtractive relationship.

In the receiver, the phase modulated wave is passed through suitable amplifiers and limiting devices so as to obtain a wave of constant amplitude prior to the detection of the signals. In this way the effect of fading, and other variations in signal strength, is eliminated. The system, accordingly, produces a signal of constant intensity 2 which is substantially unaffected by extraneous influences. I Although the novel features which are believed to be characteristic of thi invention are pointed out more particularly in the claims appended hereto, the invention will be better understood by referring to the following description, taken in connection with the accompanying drawings forming a part thereof, in which certain specific embodiments have been set forth for purposes of Like reference characters denote like parts in the several figures of the drawings. In'the'following description and in the claims parts will be identified by specific names for convenience, but they are intended to be as generic in their application to similar parts as the art will permit.

Referring to the drawings, Fig. 1 shows atransmitter which includes a microphone I I connected to a pre-amplifier l2 which operates into the low frequency end l3 of a push-pull modulator l5, the high frequency end Hi of which is supplied with energy from a high frequency source ll. The high frequency source [1, also, supplies energy through a phase shifter l8 to an electronic mixing amplifier l9, which, also, receives energy from the output circuit of the pushpull modulator E5. The combined output of the mixing amplifier i9 is impressed upon a power amplifier 20, the output of which is fed to an antenna 2|.

.In theparticular embodiment shown, the preamplifier i2 is connected through a transformer 25 to the low frequency end l3 of the push-pull modulator I5. Inductively coupled to the transformer 25 are two coils 26 and 27 which are connected to the grids of two tubes 28 and 29. The

secondary of the transformer 25 is connected through chokes 30 and 3| to the plates of the tubes 28 and 29, which, in turn, are connected 3 through blocking condensers 32 and 33 to the high frequency end of the modulator l5.

The high frequency end I6 is shown as comprising a variable condenser 35 bridged by a resistor 36, a variable resistor 31 bridged by a condenser 38, and the secondary of a transformer 39 shunted by a condenser 46. The primary of the transformer 39 is connected to the highfrequency source IT. The output circuit of the modulator I5 includes a tank circuit 4|. The phase shifter I8 is shown as comprising two resistors and 46 in series with two coils 4! and 46 mounted at right angles to each other, a condenser 49 and an output coil 58 being mounted in the space between the coils 4! and 48.

The mixing amplifier I9 is shown as including two pentode tubes 5| and 52 and a tuned output circuit 53. The first grid of the tube 5| is connected to the output coil 50 of the phase shifter l8, and the first grid of the tube 52 is connected to one side of the tank circuit 4|. The output cir cuit 53 is connected through a condenser 55 to the first grid of a pentode tube 56 which forms a part of the power amplifier 20. The cathode of. they tube 56 is connected to ground through a resistor 5! by-passed by condenser 58. A part of the resistor 5'! may be short-circuited by a switch 59. The amplifier 20 is provided with a tuned plate circuit which is inductively coupled through an antenna circuit 6| to the antenna 2|.

Operation Energy from the microphone H is amplified by the preamplifier I2, and is fed through the transformer 25 to the low frequency end l3 of the modulator |5. At the same time energy from the high frequency source I! is fed through the transformer 39 to the high frequency end l6 of the modulator. The modulated energy is then fed from the output circuit 4| to the grid of the tube 52.

High frequency energy from the source I! is, also, fed to the phase shifter 8, and from the output coil 50 the energy is impressed upor the input grid of the tube 5|. The combined output of the tubes 5| and 52, comprising the carrier and side bands, is fed through the tuned circuit 53, and is impressed upon the first grid of the tube 56. The energy is amplified by the power amplifier 20 and is impressed, through plate circuit 60 and antenna circuit 6|, upon the antenna 2|, which radiates energy corresponding to the combined carrier and side band energy in the circuit 53. The bias on the tube 56 may be increased by opening the switch 59 which increases the resistance of the cathode to ground resistor network. In this case, the circuits 60 and 6| may be tuned in the vicinity of twice the frequency of the tuned circuit 53.

Fig. 2A shows the energy in circuit 53 when the phase shifter device I8 is set so as to give an amplitude modulation effect. By a 90 shift of the rotor 50, the energy in circuit 53 at a suitable instant in the operation may be made to frequency modulation radiation of Fig. 2C, and possess the advantage of requiring only the band width required by amplitude modulations as in Fig. 2A.

If it is desired to produce higher order side bands from the energy distribution of 2B and build up a radiation similar to that shown in 2C, distortion devices may be utilized. That is the energy of circuit 53 may be put through abrupt limiting devices, such for example as shown in U. S. Patent 1,560,206 of Emery L. Chalfee which chop off the peaks of energy and reduce the amount of amplitude modulation. Or it may be put through devices such as a square law detector, for which the result can be directly demonstrated as follows:

Let the output of a detector device be given, say, by:

- in which e is the instantaneous impressed voltage, 2' is the instantaneous output current, and a, b, c are fixed quantities independent of e,

If the Voltage e impressed on the detector is made that corresponding to the quasi-phase modulated signal of Fig. 23, that is e: (cos wt+k sin pt sin wt) the complete expression for the output current of the detector specified by the above equation is 0 do t-a+- (Direct current) 2 -|-b(cos wt+k sin pt sin wt) ac? cos 271i) (Detected current) 2 +005- cos (2w2p)t+ cos (2w-p)t (Lower side bands) 2 +c( cos Zwt) (Carrier) k k +c(- cos (2w+p)t+ cos (2w+2p)t) (Upper side bands) That is, in addition to the direct current, repeated current, and detected current, there are currents of five frequencies grouped about twice the frequency of the central frequency of Fig. 213, with the spacing and phase relations precisely as in the frequency modulation pattern of Fig. 2C. This pattern, for k equal 1 as in 2B is shown in 2D, for comparison with the frequency or phase modulation pattern of 2C.

This output energy of the detector may be tuned by the circuit 60 and antenna circuit 6|, centered at double the frequency to which circuit '53 is tuned, with the result that a radiation closely simulating a phase or frequency modulated wave, with at least first and second order side bands will be radiated.

The spectral distributions of Figs. 2A to 2D inclusive, represent the same amount of total energy. The process of producing radiations of double frequency, with second order side bands. is similar to the process of producing additional side band energy by distorting devices of the instantaneously operative type. The creation of additional order side hands by the distortion process may be combined with the doubling process, but it should be noted that frequency doubling can be accomplished without distortion effects. That is, the process here outlined is dis- (Repeated current) tortion in combination with tuning to the double frequency distortion output.

It will be understood, however, that with the power amplifier operating normally and with circuits 60 and GI tuned similar to circuit 53 and. the tube 56 chosen of small energy handling ability, then second order side bands will be produced by the distortion process resulting from overloading the amplifier, without frequency doubling.

Receiver A receiver for use in connection with the radiations produced by the transmitter of Fig. 1, either with first order side bands or with first and higher order side bands, is shown in Fig. 3. This receiver comprises an antenna 64, a frequency lowering converter 65, an oscillatory source 65, a limiting channel 61, an amplifier 58, an amplifier circuit 99, a rectifier circuit 10, two detector circuits H and 72, an amplifier i3 and a reproducer represented generally as a set of head phones 74.

The converter 85 and oscillator 66 may be of any well known construction, and need not be more fully described herein. The output circuit of the converter 65 is connected through a transformer l! to the limiting channel 61. The latter is shown as including two diodes 18 and 19 in a reversed connection with the elements biased by batteries 88 and 8|. Resistors 82 and 83 may be included in the channel 61, if desired. A tuned circuit 85 is provided which may be tuned similarly to the transformer 11, and is connected through a condenser 86 to the input grid of a pentode tube 81 forming part of the amplifier 98. A direct current path is provided from the first grid of this tube by a resistor 88 with a condenser 89 forming an alternating current impedance from grid to ground.

The amplifier 99, which may be of well known construction and need not be more fully de-- scribed herein, is connected through a transformer 98 to the rectifier circuit 19. The latter, as shown, is of the push-pull type and includes two rectifier tubes 9| and 92. Bias batteries 93 and 94 are provided, the former being connected in series with a potentiometer 95 which functions as an output resistor.

The output circuit of the amplifier 68 includes a tuned circuit 99 which is coupled to two secondaries 97! and 98 forming part of the two detector circuits H and 12, which also include two rectifier tubes I98 and It! and two respective resistors 32 and we each shunted by condensers I85 and IE6 respectively. The cathode end of resistor I92 and the anode end of the resistor I93 are bridged by two equal high value resistors I97 and M8, the center point I99 of which is connected to the input circuit of the amplifier 73. The output circuit of the latter includes the head phones 74. The amplifier 13 may be of well known construction, and need not be more fully described herein.

Operation The transmitted radiations are received by the antenna 64 and are impressed upon the frequency lowerin converter 65, which is supplied with local energy from the oscillator 69. The output energy, which is of lower frequency but otherwise similar to the incoming energy, is selectively transmitted through the transformer ll to the limiting channel 61.

, To accomplish the elimination of the undesired 6. effects such as amplitude modulation transmissions, fading and stray static disturbances which pass through the transformer I1, two independent devices may be employed. Firstly, limiting devices may be used to cut down energy represented by amplitude modulations more rapidly than energy represented by phase, or frequency, modulation. That is, the limiter device may be adjusted to be inoperative unless the amplitude exceeds a predetermined limit, so that no effect is produced unless there are amplitude variations of the high frequency energy impressed on the limiting device. Such limiting devices for purgins the amplitude variationswithout disturbing the frequency, or phase, variations are disclosed in my U. S. Patents 1,977,438 and 1,977,439.

Secondly, due to the special properties of the phase modulated signal, quasi-phase modulated signal or frequency modulated signal, it is possible separately to detect energy at the lower end of the spectrum and energy at the upper end of the spectrum, 50 as to produce signal currents out of phase. These detected currents can be combined in a back-to-back manner so that the effects of the signal modulations are additive, while any amplitude efiects common to both portions of the spectrum will be cancelled out. Suitable circuit arrangements for performing this are disclosed in my U. S. Patents 1,935,776 and 1,976,393.

The present receiver provides two methods 0 limiting action prior to the conversion of the desired signals to amplitude modulated signals and detection. In the first method, energy from the secondary of the transformer 11 is impressed through the abrupt type limiting channel 61 upon the grid circuit of the amplifier tube 81; This limiter is so arranged that the devices do not pass current unless the instantaneous impressed voltage is above a certain predetermined" It is to be understood that it is within the energy in the transformer 71, provided that it is suitably designed to pass the higher order side bands. The energy in the circuit may be much freer from variations due to fading than the energy in the transformer Ti, and in any event is a slightly rising function of the energy in the transformer 11. Further limiting devices of this type may be used, if desired, to produce improved effects. The second limiter device may be of a slower acting type, in which the incoming signal controls the gain of a succeeding tube in a manner similar to operation of automatic volume control devices. This circuit may be arranged to provide delayed automatic volume control so asnot to be effective until a predetermined level'is reached. It may, also, be arranged to be quick acting,

For this purpose, amplifier circuit es the amplified energy drives the rectifier circuit 19 operative at suificiently high level to produce negative voltage across bias condenser. 89. This controls the gain of the tube 81, energized from the circuit 85 throughthe condenser 86, withi'the resistor 88 providing a direct current path Withalis .ac-v tuated from the secondary of transformer 17, and

ternating, current impedance from grid to ground. When the impressed voltage from theamplifier circuit 69is sufficient, the rectifier 19 operates to produce a current in the direction of the arrow in the potentiometer 95 thereby changing the grid bias of the tube 81 in a negative direction. The rectifier circuit 16 itself is positively biased by the battery 94, and in the absence of signals the grid of the tube 81 is biased negatively with respect to the rectifier circuit 16 by the delay circuit battery 93. The voltage of the battery $23 exceeds that of the battery 94'so that in the absence of signal the tube 81 is biased negatively to its normal value. When rectification occurs, due to the voltage from the amplifier 69 exceeding a value determined by the battery 93, the bias of tube 31 rapidly becomes more negative and the gain of tube 81 diminishes. The abruptness of control is enhanced by the use of the high voltage batteries 93 and 94 differently connected. The control would be more gradual if the battery as were omitted, and a lower voltage battery substituted for the battery 93.

These two limitin methods may be organized, coordinated and adjusted so that they supplement each other in making the output of tube 81 of very constant level, and substantially free from amplitude modulation or other amplitude variation effects. The various condenser and resistor elements in the rectifier output circuit may be organized, coordinated and adjusted to produce sufficiently rapid response, and, in addition, delay action by circuits introduced between circuit 85 and tube 81 may be utilized to give higher precision operation.

The output of the tube 81 may be tuned by the circuit 96, coupled to the two secondaries 91 and 98, which operate into the linear detectors we and IUI. For example, circuit 95 may be tuned to the middle of the transmitted band, circuit 91' of detectors I 60 and I OI, as regards the desired signal, are 180 out of phase, but as regards undesired signals present in both channels, are in phase. Therefore, the outputs may be differentially connected to succeeding amplifier circuits.

The output of detector I 69 passes through re sistor I62, shunted by condenser I95, in the di rection indicated by the arrow. The output of detector I9I similarly passes through resistor Hi3,

shunted by condenser 06, in the direction of the arrow. These circuits are symmetrical, and the input circuits are so adjusted that with the transmitter unmodulated the currents are equal. As the cathode end of resistor I 92 and the anode end of resistor I03 are bridged by the two equal highvalued resistors I61 and IE8, the center point I99 will be at ground potential when the transmitter is unmodulated. During modulation the upper and lower rectifiers I68 and I ill are operative al-- ternately, so that the potential of the point E89 varies with respect to ground in accordance with the modulation, and a, receiver voltage is produced matching the transmitter voltage in wave form. It is to be understood that various condensers, resistors, etc., may be organized to reinforce certain frequencies at the expense of others to produce fidelity of transmission. The

These circuitsmay be organized for best operation depending on the nature of the incoming signals. The output output energy is then fed tothe amplifier-1'3 where it is amplified and impressed upon a sound reproducing device, such for example as the head phones 14.

It is to be understood that the above described receiver may be used with timing modulated energy that is phase modulated, or frequency modulated energy, or with various combinations thereof. It has been described for convenience in relation to phasemodulated energy. Other methods of accomplishing the same practical results as those described in connection with Fig, 3 lie fully within the scope of the present invention, and can readily be devised by those skilled in the art. Such an arrangement, may, for example, be that shown in Fig. 4, in which the limiting means, distributing means to the detectors, detector arrangements and combining circuits are somewhat diiferent from those shown in Fig. 3, but accomplish the same general purposes.

In the modified form of receiver shown in Fig. 4 the circuits up to, and including, the transformer 11 are similar to those shown in Fig, 3. The secondary of the transformer 11 is connected to the first grid of a dual-grid, amplifying, limiting tube I I5 in the grid return path of which is connected a resistor IIS shunted by a condenser II1. One end of the resistor I I6 is connected to the grid of a triode tube I I8, in the plate circuit of which is a resistor I I 9 and two series plate batteries I 26 and I2 I. The second grid of the tube I I5 is connected to the plate of the tube H8, and the plate of the tube H5 is connected through a resistor I 22 to an intermediate point between batterie I20 and i-ZI.

The resistor I22 is included in a circuit I 25 which, also, comprises a condenser I26 and the primary winding of a transformer I21. The secondary of the latter is included in a secondary circuit I28 comprising a condenser I29 and a resistor I39. The center taps of the primary and secondary 0f the transformer I21 are connected together at point I3I, the primary and secondary circuits I25 and I28 being substantially identical in nature with magnetic coupling therebetween.

The point I32 is connected through a blocking condenser I35 to a rectifier I36 which is shunted by a resistor I31. The point I33 is connected through a blocking condenser I38 to a rectifier I 39 which is shunted by a resistor I46. The lates of the rectifiers I36 and I39 are connected through resistors MI and I42 and blocking condensers I45 and I 46 to the grids of two amplifier tubes I41 and I48. Condensers I49 and I59 are provided to by-pass high frequency currents, and are prevented from short-circuiting the rectifiers I36 and I39 by the resistors MI and I42. Resistors I5I and I52 are provided to establish a direct current conductive connection with alternating current impedance from the grids of tubes I41 and I 48 to ground.

The tubes I41 and I 48 are biased by a cathode resistor I55 and shunt condenser I56. The output circuits of the amplifier tubes I41 and I48 are connected in a push-pull arrangement. The prlmary winding of an output transformer I51 has a center tap which is made positive with respect to ground by a battery I58. The secondary of the transformer I51 is connected to an indicator, such for example as a pair of head phones I 59.

Operation The operation of the modified form of receiver 2 shown in Fig. 4 is similar to that of the receiver shown in Fig. 3 up to, and including, the transformer 11. Energy from the transformer 11 is impressed upon the tube H5 which uses grid rectification to chop the peaks and control of the second grid to change the gain of the tube I I5 in a gradual manner. This is accomplished by the rectified grid current, due to the chopping process, flowing through the resistor H6 in the direction of the arrow to put negative bias on the grid of tube I I8. This results in a decrease of the flow of plate current through the resistor H3 which raises the potential of the second grid of the tube H5 with respect to the potential of the plate, and decreases the ability of the plate circuit to receive current and amplify the grid-impressed energy.

The combined chopping of input energy to the first grid of tube H5 and change of potential on the second grid, under the action of the chopping, serves to make the amplified output of tube H5 substantially independent of input signal strength and amplitude variations.

For distributing the energy differently to the two detectors I36 and I39 use is made of the following principle of coupled circuits. If two identical circuits I6I and I62 (Fig. 4A) are magnetically coupled, and are energized by a voltage c, then a primary and a secondary current i and is will result. The primary current ip is the vector sum of the current ix, that the same voltage e would produce in a suitable circuit I65 tuned higher than circuits I6I and I62, and the current iy, that this voltage would produce in a suitable circuit I66 tuned lower than circuits I6I and I62. On the other hand, the secondary current is the vector difierence of the currents producible in the single circuits.

That is, in: (ix-I-iy) is (ir-iy) This principle is utilized by making the circuit I25, loaded by the tube H5, the primary circuit and the circuit I28, loaded by the elements I29 and I30 the secondary of the transformer I21. The high frequency potental of point I3I with respect to ground is proportional to /2(i=+i and the potential of point I32 with respect to point I33 is proportional to (ix-1y). Because of the connection between the center taps at I3 I the potential of points I32 and I33 with respect to ground (assuming the roportionality factor is unity) are given by The voltages produced by the tube I I5 and impressed on the blocking condensers I35 and I38 are, therefore, proportional to the voltages which would be created by differently tuned single circuits analogous to the arrangement shown in Fig. 3. By the rectifier action of the tubes I36 and I39 direct current components are developed through the circuits in the direction shown by the arrows, but due to the rectifier I36 being driven from one end of the energy spectrum and the rectifier I39 being driven from the other end of the energy spectrum, the detected currents are polarized differently from the direct currents. That is, when the plate of the rectifier I36 is most positive th plate of the rectifier I39 is most negative.

The arrows S and A serve to indicate the polarities of the currents due to the signal and amplitude effects, respectively, due to any in-phase energies impressed upon the two detectors. It will be seen that the signal currents produce additive effects in the head phones I59, while the if currents due to the amplitude effects are neutralized.

The circuits shown in Fig. 4 differ from those depicted in Fig. 3 first with respect to the nature of the limiter circuits; secondly, with respect to the method of distributing energy from diiferent ends of the spectrum to the two rectifiers; third- 1y, with respect to the nature of the rectifier circuits; and iourthly, with respect to the nature of the push-pull connections from the rectifiers to the indicating device. These two circuits are illustrative of differences of design which can be arranged by those skilled in the art, under the broad principles disclosed herein and within the scope of the following claims.

Although only a few of the various forms in which this invention may be embodied have been shown herein, it is to be understood that the invention is not limited to any specific construction, but may be embodied in various forms without departing from the spirit of the invention within the scope of the appended claims.

What I claim is: v

1. In a timing modulated carrier energy transmission system, an electron discharge device comprising at least an electron emission element, an electron collection electrode and an electron control electrode, an energy input circuit connected between said control electrode and emission element, a resistor-condenser network in a series circuit with said input circuit between said control electrode and emission element to develop a unidirectional voltage which increases in a negative polarity sense with energy amplitude increase, an auxiliary electron control element for the electron stream from said emission element to the collection electrode, and means, directly responsive to said voltage, for increasing said auxiliary control element potential in a positive polarity sense thereby to decrease the electron flow to said collection electrode.

2. In a timing modulated carrier energy transmission system, an electron discharge device comprising at least an electron emission element, an electron collection electrode and an electron control electrode, an energy input circuit connected between said control electrode and emission element, a resistor-condenser network in a series circuit with said input circuit between said control electrode and emission element to develop a unidirectional voltage which increases in a negative polarity sense with energy amplitude increase, an auxiliary electron control element for the electron stream from said emission element to the collection electrode, and electron discharge tube means, directly responsive to said voltage, for increasing said auxiliary control element potential in a positive polarity sense thereby to decrease the electron flow to said collection electrode.

'3. In a modulated carrier energy transmission system, an electron discharge device comprising at least an electron emission element, an electron collection electrode and an electron control electrode, an energy input circuit connected between said control electrode and emission element, a resistor-condenser network inseriesbetween said emission element and said input circuit to develop a unidirectional voltage which increases in a negative polarity sense with energy amplitude increase, an auxiliary electron control element for the electron stream from said emission element to the collection electrode, and means, directly responsive to said voltage, for increasing said auxiliary control element potenl1 tial in a positive polarity sense thereby to .decrease the electron flow to said collection electrode and an energy output circuit connected to said collection electrode' 4. In combination, in a phase or frequency modulation receiver, an amplitude limiter network comprising a tube having at leasta cathode, a plate and a pair of control grids in the electron stream to said plate, a signal input circuit, a grid rectification network in circuit with said input circuit between one control grid and the cathode, said rectification network developing direct current voltage from amplitude variations in the signal energy of said input circuit, a signal output circuit coupled to the plate, means for applying a positive'potential to each of the plate and the second control grid, and means, directly responsive to said voltage, for increasing the positive potential of the second grid relative to the plate.

5. In combination, in a phase or frequency modulation receiver, an amplitude limiter net'- work comprising a tube having at least a cathode, a plate and a pair of control grids in the electron stream to said plate, a signal input circuit, a grid rectification network in circuit with said input circuit between one control grid and the cathode, said rectification network developing direct current voltage from amplitude variations in the signal energy of said input circuit, a signal output circuit coupled to the plate, means for applying a positive potential to each of the plate and the second control grid, and means, directly responsive to said voltage, for increasing the positive potential of the second grid relative to the plate, and said last means comprising a tube having an input electrode and an output electrode, 'means applying said voltage to said input electrode and means connecting the output electrode to said second control g id.

6. In combination, in a phase or frequency modulation receiver, an amplitude limiter network comprising a tube having at least a cathode, a plate and a, pair of control grids in the electron stream to said plate, a signal input circuit, a grid rectification network in circuit with said input circuit between one control grid and the cathode, said rectification network developing direct current voltage from amplitude variations in the signal energy of said input circuit, a signal output circuit coupled to the plate, means for applying a positive potential to each of the plate and the second control grid, and means, directly responsive to said voltage, for increasing the positive potential of the second grid relative to the plate, said applying means comprising a current source having its positive potential terminal connected to said second control grid through a resistor, and said responsive means comprising a tube having its cathode to plate path in shunt with the current source and resistor, and a control element, connected to said rectification network, for regulating the conductivity of said cathode to plate path.

'7. An amplitude limiter circuit of the type comprising a cathode, signal grid, positive screen grid and positive output electrode, a signal input circuit, a resistor arranged in series with said input circuit between the signal grid and cathode, a condenser in shunt with the resistor to provide a grid rectification network, an electronic impedance device in circuit with said screen grid, and means, directly responsive to rectified grid current flowing through said resistor, for varying the magnitude of said impedance thereby to vary the screen grid potential.

8. A method of limiting the amplitude of modulated signal energy which comprises passing electrons through successive control grids to an electron collection electrode, applying signal energy to be limited to the first control grid, developing direct current voltage from electron current flowing to said first grid, and increasing the potential of the second control grid in a positive potential sense thereby to divert electrons from said collection electrode in direct response to said voltage.

9. In combination in an amplitude limiter network, a, tube having at least a cathode, a plate and at least two control grids in the electron stream to said plate, a signal input circuit, a grid rectification network in a. series circuit with said input circuit between one of said control grids and the cathode, said rectification network developing direct current voltage from signal energy at said input circuit, a signal output circuit coupled to the plate, means for applying a positive potential to each of the plate and the second control grid, and means, directly responsive to said direct current voltage, for increasing the positive potential of the second grid relative to the plate.

10. In an amplitude limiter network comprising a tube having a cathode, a plate and at least two control grids in the electron stream to said plate, a signal input circuit, a grid rectification network in a series circuit with said input circuit between one control grid and the cathode, said rectification network developing direct current voltage from signal energy at said input circuit, a signal output circuit coupled to the plate, means for applying a positive potential to each of the plate'and the second control grid, and means, directly responsive to said voltage, for increasing the positive potential of the second grid relative to the plate, said applying means comprising a current source having its positive potential terminal connected to said second control grid through an impedance and said responsive means comprising a tube having its cathode to plate path in shuntwith the current source and impedance, and a control element, connected to said rectification network, for regulating the conductivity of said cathode to plate path.

11. A method of limiting which comprises passing electrons through at, least two successive control grids to an electron collection electrode, applying signal energy to the first control grid developing direct current voltage from electron current flowing to said first grid, and varying the potential or" the second control grid in an increasingly positive potential sense in response to said direct current voltage thereby to divert electrons from said collection electrode.

JOHN HAYS HAMMOND, JR

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