Title:
Modulation
United States Patent 2220201
Abstract:
This invention pertains generally to modulation in electric circuits, and specifically to communication. The invention relates more particularly to the modulation of radio-frequency transmitters. An object of the invention is to provide a new and Improved modulator. A further object is to...


Inventors:
Roderic, Bliss William
Application Number:
US14768837A
Publication Date:
11/05/1940
Filing Date:
06/11/1937
Assignee:
Roderic, Bliss William
Primary Class:
Other Classes:
332/183, 455/102
International Classes:
H03C1/00; H03C3/38
View Patent Images:
Description:

This invention pertains generally to modulation in electric circuits, and specifically to communication. The invention relates more particularly to the modulation of radio-frequency transmitters.

An object of the invention is to provide a new and Improved modulator.

A further object is to improve upon the modulation of radio-frequency transmitters.

Still another object is to provide a new and improved system for the modulation of a radiofrequency transmitter in a stage other than the final output-supplying, radio-frequency amplifier, without sacrificing efficiency in any stage. 1'3 Another object still is to provide a new and improved system for converting amplitude into phase modulations and then to reconvert the phase modulation into amplitude.

Other and further objects will be explained 2n hereinafter and will be particularly pointed out in the appended claims.

In the drawings which accompany and form a part of the specification, Fig. 1 is a diagram illustrative of a circuit that may be used for intro2; ducing modulation in accordance with the present invention; Fig. 2 is a vector diagram illustrating the phase shift that may take place in the amplifier occasioned by amplitude modulation in accordance with this invention; Fig. 3 :, is a diagram illustrative of an output circuit that may be used according to the present invention; ig. 4 is a vector diagram similar to Fig. 2 illustrating the relation that may exist between phase angles and radio-frequency amplitudes in :i-> the output circuit; Fig. 5 is a similar diagram illustrating the corresponding relation between phase angles and radio-frequency amplitudes in the input circuit; Fig. 6 is a diagram similar to Fig. 3, illustrating an output circuit connected to Ii utilize the modulation frequency present in the circuit for operating a loud-speaker or other suitable device; and Fig. 7 is a diagram of a radiofrequency transmitter employing the principles of this invention.

4:J All terms used in this specification have meanings as defined in the 1933 Report of the Standards Committee of The Institute of Radio Engineers. In accordance with conventional radiofrequency transmission, a source 2 of constantri) frequency oscillations, shown in Fig. 7 as a piezoelectric oscillator, may be coupled, through various stages, to an output circuit 4 that is, in turn, coupled to a radiating antenna 6. Three stages are illustrated: a first stage 8, a second stage 10, and a third stage 12. The modulating signal may be introduced into the system in any desired way as hereinafter described. It is customery to modulate the output circuit 4, since modulation of one of the intermediate stages 8, 10 and 12 introduces loss of efficiency. In order to amplify without distortion a radio frequency modulated in amplitude, it is customary to employ a class B linear amplifier or other lowefficiency means of amplification. Class B amplifiers may have an efficiency as low as 30 percent and they operate under smaller radio-frequency voltages in a more sensitive portion of the characteristic than is the case with class C amplifiers which, under high-efficiency conditions of operation, are inherently insensitive to small changes 1, in either the grid circuit or the plate circuit, because of the relatively large voltage swing on the plate and the grid of the tube.

According to the present invention,an improved system is provided for modulating an intermediate stage without any sacrifice of efficiency whatever. A small radio-frequency signal may be modulated in amplitude, in any desired conventional manner, sent through an amplitudedistorting amplifier, as in the stage 10, and it may be reproduced in the antenna 6 or other load without distortion of the audio-frequency or other lower-frequency modulation. The term "audiofrequency" will be employed in the specification and claims in this broad sense, to denote any modulation frequency. This may be effected, in accordance with a feature of the invention, with the aid of a high-efficiency class C amplifier, and yet without producing distortion in the amplifier output circuit. Linear amplifiers may, however, from certain points of view, be employed without departing from the spirit and scope of the invention, as defined in the appended claims.

According to the preferred embodiment of the invention, the stage 8 is illustrated as comprising two vacuum-tube amplifiers 20 and 22 containing radio-frequency oscillations, the respective input circuits 24 and 26 of which are respectively coupled at 28 and 30 to the output circuit of the piezo-electric oscillator 2. The piezo-electric crystal 34 is shown, for illustrative purposes merely, as connected in the input circuit of the oscillator 2. The tuned output circuits 36 and 38 of the amplifiers 20 and 22, which are thus supplied with a radio-frequency voltage of substantially the same constant frequency, are both shown coupled at 40 and 42 to the input circuits 44 and 46 of two high-efficiency class C distorting amplifiers 48 and 50, so as to supply substantially equal voltages to the grids 58 and 60 thereof. The description may be more easily understood by reference to Fig. 1, in which are reproduced, among other things, the output circuits 36 and 38 of the amplifiers 20 and 22 and the inr; put circuits 44 and 46 of the amplifiers 48 and 50, but omitting, for clearness, many of the elements shown in Fig. 7.

Though the voltages of the amplitude-modulated output circuits 36 and 38 are of the same constant frequency, they always have phase differences of substantially 90 degrees. The sum of their outputs in the circuits 44 and 46 has little amplitude variation, but is correspondingly phasemodulated, the phase modulation or variation being dependent upon the amplitude variations in the circuits 36-62 and 38-70. In the circuits 36 and 38, there is an amplitude shift, but no phase shift.

The desired phase difference may be produced in any desired way, as will be understood from what follows. One or more phase adjusters may, if .desired, be introduced into the coupling elements 28 and 30; a phase-adjusting condenser 52 is shown, by way of illustration, in the coupling element 30. As it is only the phase difference between the voltages as finally applied to the grids of the tubes 48 and 50 that is of importance, the circuit element 28 is shown unprovided with a phase adjuster. The desired phase difference of 90 degrees may be attained by adjusting the condenser 52. The phase difference may be produced in any one or more of the circuits 28, 30, 24, 26, 36, 38, 44 and 46 by suitable tuning adjustment.

The coupling element 40 couples the output circuit 36 of the amplifier 20 directly to a tuned circuit 62, and the coupling element 42 couples the output circuit 38 of the amplifier 22 directly to a tuned circuit 64. These tuned circuits 62 and 64, respectively comprising a coil 66 and a tuning condenser 68, and a coil 70 and a tuning condenser 72, are each connected in both input circuits 44 and 46 of the amplifiers 48 and 50.

The connections of the tuned circuit 64 in the input circuit of the amplifier 50 may be traced from the cathode 74 of the amplifier 50, which cathode 74 is grounded at 76, and through a biasing battery 78, to the tuned circuit 64; from the tuned circuit 64, by way of a conductor 80, to a midpoint tap 82 of the coil 66; and from the midpoint tap 82, through the lower half of the coil 66, and by way of a conductor 84, to the grid 60 of the amplifier 50. The connections of the tuned circuit 64 in the input circuit of the amplifier 48 may similarly be traced from the cathode 86 of the amplifier 48, which is similarly grounded at 88, through the same biasing battery 78 and tuned circuit 64, and by way of the same conductor 80, to the same mid-point 82; and thence, through the upper half of the coil 66, by way of a conductor 90, to the grid 58 of the amplifier 48.

Though the tuned circuit 64 is thus connected in both the input circuit 46 of the amplifier 50 and the input circuit 44 of the amplifier 48, it will 05 be noted that the lower half of the coil 66 is connected in series therewith in the one case, and the upper half of the coil 66 is connected in series therewith in the other case. The same connections that introduce the tuned circuit 64 into the input circuits 44 and 46 of the amplifier tubes 48 and 50, therefore, introduce also the tuned circuit 62 into those input circuits, but with a phase reversal of 180 degrees. It follows that the voltages from the amplifier 20 thus applied to the grids 58 and 60 are 180 degrees out of phase, Through their output circuits 36 and 38, therefore, the amplifiers 20 and 22 impress upon each of the control grids 58 and 60 of the amplifiers 48 and 50 radio-frequency voltages of substantially the same constant frequency, but having a phase difference, the size of which is dependent upon the relative amplitudes of the voltages from the amplifiers 20 and 22 applied to these grids.

The amplitude modulation produced by the signal voltages is applied to the amplifiers 20 and 22. The vector sum of the modulated radio frequencies transmitted through the coupling links 40 and 42 on the grids 58 and 60 is maintained constant. A phase modulation, but no amplitude shift, appears on the grids 58 and 60. The connections of the amplifiers 20 and 22 to the grids 58 and 60 are, however, such that the voltage amplified by the amplifier 22 is applied to these grids with phase unaffected, while the voltage of the amplifier 20 is applied to the grids with phase reversed 180 degrees.

The amplifiers 48 and 50 are shown respectively provided with screen grids 54 and 56. This is for the purpose only of simplifying the circuit diagram of Fig. 7. Any other type of amplifier tube may equally well be employed in any stage, but it might then be necessary to introduce neutralization.

The amplifiers 20 and 22 are modulated in amplitude by a source 92 of audio power. The modulation of one of the amplifiers 20 and 22 is 180 degrees out of phase with the modulation of the other amplifier. This may be effected in any desired way, as illustrated, for example, in Fig. 7.

The modulating signal may be introduced through the medium of an amplifier 94 which may, if desired, be excited by a piezo-electric-crystal microphone 202. The output circuit 96 of the microphone system is shown coupled, through a transformer 98, to two vacuum tubes 100 and 102, connected together, in push-pull, relation, with a half 104 of the secondary winding of the transformer 98 connected in the input circuit of the tube 100 and the other half 106 in the input circuit of the tube 102. The output circuits of the tubes 100 and 102 are respectively provided with windings 108 and 110 that are properly connected together in series, and that are coupled, through condensers 112 and 114, to suitable series-connected resistors 116 and 118. Other impedances than resistors may be employed, but resistors are preferred because the direct-current component of the current in the hereinafter-mentioned tube 140 is needed for modulation, due to the unsymmetrical output of the tube.

The resistor 116 is connected to the output circuit 38 of the amplifier 22, and the resistor 118 to the output circuit 36 of the amplifier 20. The connections for the amplifier 22 are from the cathode 120 of the amplifier 22, which is grounded at 122, through the plate battery 124 and the resistor 116, and by way of a conductor 117, to the output circuit 38; and thence, by way of a conductor 126, to the plate 128 of the amplifier 22. The connections for the amplifier 20 are from the cathode 130, which is grounded at 132, and through the same battery 124 and the resistor 118, and by way of a conductor 134, to the output circuit 36; and thence, by way of a conductor 136, to the plate 138 of the tube 20. In this manner, two modulators are provided,-through the means of which by suitable impedances and connections to the impedances, the modulating signal will be applied to the plates 128 and 138 one hundred and eighty degrees out of phase with each other.

The phase relations will be understood from Fig. 2, where E3 represents the voltage from the tuned circuit 64 on the grids 58 and 60 of the tubes 48 and 50, and Ei and E2 represent the voltages, differing by 180 degrees in phase, on the respective grids from the tuned circuit 62. The vector sum of Ei and E3 is E4. If E4 represents the voltage on, say, the grid 58 of the tube 48, Es, the vector sum of E3 and E2, will then represent the voltage on the grid 60 of the tube 50. The difference of phase, represented by the angle a, between the voltage E4 or Es on either grid and the voltage E3 depends upon the relative amplitudes of Ei, E2 and E3. As El is always equal to E2, but opposite in sign, the difference in phase between E4 and E3, and between Es and E3, is the same but in the opposite direction. Since the amplifier 20, supplying the voltages Ei and E2, and the amplifier 22, supplying the voltage E3, are modulated in amplitude, as above described, the difference in phase between the voltages E4 and Es will change with the amplitude and the frequency of modulation. In ordinary forms of modulation, obviously, this means that the phase difference can vary between 0 degrees and 180 degrees.

If the tubes 48 and 50 are class C amplitudedistorting amplifiers, it is desirable to keep con80 stant the radio-frequency amplitude on their respective grids 58 and 60. This condition may be obtained in any preferred way, as by the use of an amplitude-detecting device, comprising the said tube 140, which is back-coupled to the modulator of the amplifier 20 in such a way as to oppose any change of amplitude on the grids 58 and 60 of the tubes 48 and 50 by modulation of the amplifier 20. One scheme for accomplishing this back-coupled modulation is illustrated in the drawings. The plate coupling circuit of the detector 140 extends from the cathode 146 of the tube 140, which is grounded at 150, through the battery 124 and the resistor 118, to the plate 148 of the detector 140. The tube 140 is shown provided with a biasing battery 142 by means of which it is biased so as to operate as a detector, and its grid 144 is coupled to the grid 60 of the tube 50 through a condenser 204. The voltage on the plate 148 of the tube 140 is thereby controlled by the amplitude of the radio-frequency voltage on its grid 144, and this voltage is directly applied to the modulating voltage for the amplifier 20. Since the tube 140 is biased negatively, an increase in radio-frequency amplitude on its grid 144 will cause the mean grid potential to rise.

This, in turn, will cause the mean plate current, as averaged over any radio-frequency cycle, to increase. When this current increases, the IR drop through the plate resistor 118 increases, lowering the voltage on the plate 148 of the tube 140, and hence on the plate 138 of the tube 20. This decreases the push-pull voltage component on the grids 58 and 60 of the tubes 48 and 50 and, ,. through condenser 204, on the grid 144 of the tube 140. Analysis shows that voltage-amplitude fluctuation on the grids 58 and 60 of the tubes 48 and 50 is thereby divided by (1+p), where a represents the effective amplification of the tube 140. The size of / may be made large by proper choice of the circuit components and the tubes, reducing the voltage-amplitude fluctuation on the grids 58 and 60 of the tubes 48 and 50, and hence the distortion, to a negligible quantity. The tube 140 is unaffected by phase shifts in its radio-frequency grid voltage, and operates only upon amplitude fluctuations The amplitude-detecting device schematically represented by the tube 140 may, of course, be constituted of several tubes in parallel or cascade, so connected as to increase the effective power or amplification of the back-coupled detector system.

Fig. 5 represents the relation between the phase angle a and the modulation of voltage V3 supplied by the tuned circuit 64 to the grids 58 and 60 of the tubes 48 and 50. V4 and Vs represent the voltages, reversed in phase, supplied to the grids 58 and 60 by the tuned circuit 62, as before described. If tube 140 were inoperative, the envelope of all the vectors representing the radio frequency voltage on one of the grids 58 or 60 would be the triangle ABD. That is, during the modulation process, the radio-frequency amplitude at any time during the modulation cycle may be represented by a vector drawn from D to a point on the line AB. VI, Vs, and V9 are examples of such voltages. On the other grid, 60 or 58, is a voltage represented by a vector within the envelope ACD, which may be drawn sym- 23 metrical to the first vector about the line AD.

If Vi is the voltage on, say, grid 58, V2 is the voltage at the same time on grid 60. The phase difference is represented by the angle a between the vector and the line AD. In order to keep the length of all these vectors a constant, and the voltage they represent unvarying, the envelope should not be a triangle, such as ABD, but a circular sector, such as AFD.

The tube 140 produces this change by increasing or decreasing the voltage on the amplifier 20; or, in other words, by varying the horizontal component of the vectors of Fig. 5. Thus, for example, if one radio-frequency amplitude Vs be present at one instant, the tube 140 will supply a voltage .V to increase the length of Vs to Ve, which is correct. Likewise, if a voltage V9 appears, which is too large, the tube 140 will decrease it by an amount Vio to the size Vi. This correction changes the phase angle of the vector to the correct angle for faithful reproduction of the modulated voltage V3 as P3 in the output circuit, as indicated in Fig. 4.

With no modulation, the voltage V3 should be half of its peak value; hence, the amplitude of the radio-frequency voltages delivered to the tuned circuits 62 and 64 should be so adjusted that a is 60 degrees, or the total phase difference between the voltage on the grids 58 and 60 of the tubes 48 and 50 is 120 degrees when there is no modulation of the amplifiers 20 and 22. The phase angle 2a may thus vary between zero and 180 degrees. The absolute value of the vector V3 of Fig. 5, divided by the absolute value of the vector Vi, is the cosine of half this phase angle 2a. Since the absolute value of the vector Vi is the radius of the circle, a constant, the absolute value of the vector V3 is proportional to the cosine of the angle a. The cosine of half the phase angle 2a of the resultant voltages applied to the (;.: respective amplifiers 48 and 50 is thus maintained substantially proportional to the modulating voltage produced by one of the modulators. The factor of proportionality, that is, the radius of the circle, is of no materiality because Fig. 5 is but a transition stage to the vector diagram of Pig. 4, which represents conditions in the output circuit 4.

The voltages Vi and V2 of Fig. 5 need no correction because the bias of the tube 140 is so ad- ;5 justed as to hold constant the voltage of an amplitude equal to line DA of Fig. 5. Since arc EAF has a radius equal to DA, the vectors Vi and V2, drawn to the points where the arc EAF meets the sides AB and DC of the triangles DAB and DAC, need no correction. For all angles greater than the angle a of the vector VI, the amplitude must be decreased, and for all less than the angle a of the vector Vi, they must be increased, except when a=0, which is correct. It will be understood that the particular size-ratio of triangle to are chosen in Fig. 5 is for convenience only.

Other ratios may be used, if desired. In no case may the arc radius be larger, however, than DA of the triangle.

The amplifier 20 need not be modulated at all by audio from the tube 102. All of its modulation may be obtained from the detector tube 140.

Its modulation must consist of merely the correct changes of amplitude to keep the radio-frequency level on the grids 58 and 60 constant. Modulating with audio 180 degrees out of phase will give a rough approximation of the required amplitude change, and the tube 140 corrects for small deficiencies thereby produced. The invention is not, therefore, dependent for its operation upon reversed-phase signal voltages.

A separate amplifying channel must be used, following each of the tubes 48 and 50. These amplifying channels are shown at 152 and 154 in the stage 12, and they may comprise the ordinary high-efficiency class C type of amplifier.

The input circuit of the amplifier 152 is shown connected to the output circuit of the amplifier 48, and the input circuit of the amplifier 154 is shown connected to the output circuit of the amplifier 50. The only substantial difference in radio-frequency voltage between the two channels at corresponding points is the phase angle before described. The selectivity of an ordinary class C amplifier is broad enough to permit lowfrequency changes of phase to take place without any distortion being occasioned by this phase shift. The present invention permits the use of class C amplifiers after modulation and of class C radio-frequency amplifiers for audio amplification. Since, as before stated, a class C amplifier is insensitive to small changes of amplitude in both the grid and plate circuits, a further advantage of the present invention is the reduction of noise which may be occasioned by the power supplies to the amplifier tubes. The invention further provides for high-plate power efficiency and filament power efficiency throughout the amplifier.

The output circuit 4 of the transmitter is shown comprising two shielded-grid tubes 156 and 158 connected in a differential relation, which becomes a push-pull relation when the phase difference is 180 degrees. The output circuit of the amplifier 152 is coupled through a variable condenser 160 to the control grid 162 of the tube 156, and the output circuit of the amplifier 154 is similarly connected through a variable condenser 164 to the control grid 166 of the tube 158. The grids 162 and 166 are respectively connected, through resistors or other impedances 168 and 170, to the cathodes 172 and 174 of the respective tubes 156 and 158. These cathodes 172 and 174 are connected, by way of a grounded conductor 176, through a battery 178, and by way of a conductor 180, to one side of a tuned circuit 182. The other side of the tuned circuit 182 is connected, by way of a conductor 184, to the midpoint 186 of a coil 188 the ends of which are connected to the plates S192 and 194 of the respective tubes 156 and 158.

A tuning condenser 196 is connected in shunt to Sthe coil 188. These output connections are shown in a simplified form in Fig. 3.

The radio-frequency voltage appearing upon the plates 192 and 194 of the tubes 156 and 158 is substantially free of amplitude variations, but the phase relation of the voltages on these two plates is the same as the relation in phase between the voltages on the grids 58 and 60 of the tubes 48 and 50. The tuned circuit 188, 196 will form a high impedance to all components of voltage on the plates 192 and 194 of the tubes 156 and 158 which are 180 degrees out of phase. The tuned circuit 182 presents an impedance to all of the components of radio-frequency voltage from the tubes 156 and 158 which have no difference in phase.

The amplifier 22 and the amplifiers 50 and 154 and related circuits may be shielded at 200 from the rest of the transmitter.

In Fig. 4, which is much like Fig. 5, P4 and Ps represent the radio-frequency voltage present in the tuned circuit 188, 196. P3 may represent the radio-frequency voltage appearing across the tuned circuit 182. In the input circuit of Fig. 1, the amplitude of the voltage V3 was described as controlled by the modulating voltage. In the output circuit, the voltage V3 will appear as P3 in the same proportion, but amplified by the class C amplifier channels 152 and 154. The useful voltage developed across the tuned circuit 182 is therefore equivalent to a large radio-frequency voltage amplitude modulated by a large audiofrequency or other modulating-frequency voltage. ";5 To the tuned circuit 182 a load 198 (Fig. 3), such as the antenna 6 of Fig. 7, may be coupled. The whole output circuit, comprising the circuits 182, 198, 188 and 196, may thus be characterized as a phase-to-amplitude modulation converter. 41 If the tuned circuit 182 is replaced by an audiofrequency transformer, or other means of supplying the audio-frequency component of current to the tubes 156 and 158, the whole amplifier may be used as a high efficiency audio amplifier. 4: Fig. 6 represents the output circuit of Fig. 3 so connected. In place of the tuned circuit 182 and the load 198, there is illustrated an audio or other modulating-frequency transformer 206, the secondary of which is connected to a loud-speaker 208 or other means of using the audio-frequency power. When the radio-frequency voltages in the tubes 156 and 158 are in phase, the tubes will draw a great deal of current through the transformer 206. When the radio-frequency voltages on the plates of the tubes 156 and 158 are 180 degrees out of phase, the tubes will draw very little current. This phase and current fluctuation take place at an audio rate as controlled by the original modulation in the low-power stage 8. The current change through the transformer 206 supplies the audio power desired.

In practice, a suitable filter (not shown) may be inserted in the lead 184 for by-passing the radio-frequency component. i5 The invention has many uses. Among the advantages of using this system of modulation for radio-frequency transmitters are: 1. The elimination of high-power modulating equipment in large transmitters; 7u 2. The improvement in audio-frequency response characteristic of the transmitter by elimination of losses invariably encountered in large transformers and chokes; 3. The improvements in plate-circuit efficiency 7r and filament-power efficiency over any other method now known for amplifying a modulated radio frequency without distortion; 4. The reduction of noise caused by power supplies through the use of class C amplifiers throughout the radio-frequency channels; and 5. The ease of adjustment. The class C radiofrequency amplifiers are not critical with respect to plate, grid, or excitation voltages.

Due to the inherently wide modulated frequency response of class C amplifiers, the present invention is extremely well adapted also to television transmitters.

The present invention provides a new and improved greatly simplified system for converting and reconverting phase and amplitude modulation, employing a plurality of modulated amplifiers 20 and 22, without the introduction of unfavorable complex power factors, critical adjustments or losses, and without admitting amplitude changes.

Further modifications will occur to persons skilled in the art, and all such are considered to fall within the spirit and scope of the invention. What is claimed is: 1. An electric system comprising two circuits containing radio-frequency oscillations of substantially the same frequency but differing in phase by substantially 90 degrees, means for 80 modulating the amplitude of the oscillations of the two circuits, two amplifiers, means for applying to both amplifiers the voltage of one of the circuits unaffected in phase, means for applying to one of the amplifiers the voltage of the 85 other circuit unaffected in phase, means for applying to the other amplifier the voltage of the other circuit but substantially reversed in phase, an output circuit coupled to the amplifiers, and means for preventing distortion of the amplitude modulation of the radio-frequency voltage in the output circuit.

2. An electric system comprising two radio-frequency circuits containing oscillations of substanStially the same radio frequency but differing in phase by substantially 90 degrees, means for modulating the amplitude of the oscillations of the two circuits, two amplifiers, means for applying to both amplifiers the voltage of one of the said two circuits unaffected in phase, means for applying to one of the amplifiers the voltage of the other of the said two circuits unaffected in phase, means for applying to the other amplifier the voltage of the other of the said two circuits but substantially reversed in phase, means for maintaining the radio-frequency amplitude applied to the amplifiers substantially constant, a phase-to-amplitude-converting output circuit coupled to the amplifiers, means for reconverting the phase modulations into amplitude modulations in the output circuit, and means for assuring linearity of phase to amplitude conversion in the output circuit.

3. An electric system comprising two radio-frequency circuits containing oscillations of substantially the same radio frequency but differing in phase by substantially 90 degrees, two amplifiers, means for applying to both amplifiers the voltage of one of the said two circuits unaffected in phase, means for applying to one of the amplifiers the voltage of the other of the said two circuits unaffected in phase, means for applying to the other amplifier the-voltage of the other of the said two circuits but substantially reversed in phase, two modulators, means comprising one of the modulators for applying signal voltages to one of the said circuits, means comprising the other modulator for applying the signal voltages substantially reversed in phase to the other of the said circuits, an amplitude detector, and means for coupling the amplitude detector to one of the said other modulator to oppose any tendency for modulation in one of the said circuits to introduce any changes of amplitude in the amplifiers.

4. A radio-frequency transmitter comprising a substantially constant-frequency oscillator, two circuits, means for applying to the said two circuits the voltage of the oscillations of the constant-frequency- oscillator but differing in phase by substantially 90 degrees, means for modulating the amplitude of the oscillations in the said two circuits, two amplifiers, means for applying to both amplifiers the voltage of one of the said two circuits unaffected in phase, means for applying to one of the amplifiers the voltage of the other of the said two circuits unaffected in phase, means for applying to the other amplifier the voltage of the other of the said two circuits but substantially reversed in phase, means for maintaining the radio-frequency amplitude applied to the amplifiers substantially constant, 2 an amplifying channel connected to each amplifier, an output circuit to which the channels are connected, and means for reconverting the phase modulations into amplitude modulations in the output circuit. 80 5. An electric system comprising two amplifiers each having an input circuit and an output circuit, means for impressing electric oscillations upon the amplifiers, two modulators for modulating the amplitude of the electric oscillations, an 85 amplitude detector having an input circuit and an output circuit, means for coupling the input circuit of the detector to the input circuits of the amplifiers, and means for coupling the output circuit of the detector to one only of the modulators.

6. An electric system comprising two first circuits, two second circuits for applying radio-frequency voltage to the first circuits, means for applying a voltage of predetermined frequency and phase to one of the second circuits, means for applying to the other second circuit a voltage having the same frequency but differing in phase by substantially 90 degrees, means for applying one of the voltages unaffected in phase to both first circuits, means for applying the other voltage to one of the first circuits unaffected in phase and to the other first circuit substantially reversed in phase, means for applying to the respective second circuits signal voltages substantially reversed in phase, an output circuit coupled to the first circuits, and means for maintaining the voltage amplitude from the second circuits substantially constant.

7. An electric system comprising a source of radio-frequency oscillations, means for modulating the amplitude of the oscillations, an amplifier having an input circuit, means for impressing the modulated electric oscillations upon the amplifier, means for converting the amplitude modulations in the amplifier into phase modulations, an output circuit, means for delivering the phase modulations to the output circuit, means for converting the phase modulations in the output circuit into amplitude modulations, and means for preventing distortion of the amplitude modulations in the output circuit comprising amplitude-controlling means connected between the modulating means and the input circuit of the amplifier. / 8. An electric system comprising a source of radio-frequency oscillations, means for modulating the amplitude of the oscillations, a linear amplifier, means for impressing the modulated electric oscillations upon the amplifier, means for converting the amplitude modulations in the amplifier into phase modulations, an output circuit, means for delivering the phase modulations to the output circuit, means for-converting the phase modulations in the output circuit into amplitude modulations, and means for preve6ting distortion of the amplitude modulations in the output circuit comprising amplitude-controlling means connected between the modulating means and a portion of the system that contains phase-modulated oscillations.

9. An electric system comprising two radiofrequency circuits containing oscillations of substantially the same radio frequency but different phase, means comprising one or more modulators for producing modulating voltages to modulate the amplitude of the oscillations of the two circuits, two amplifiers, means for applying to both amplifiers the voltage of one of the said two circuits unaffected in phase, means for applying to one of the amplifiers the voltage of the other circuit unaffected in phase, means for applying to the other amplifier the voltage of the said other circuit but substantially reversed in phase, and means for maintaining the cosine of half the phase angle of the resultant voltages applied to the respective amplifiers substantially proportional to the modulating voltage produced by one of the modulators.

10. An electric system comprising two radiofrequency circuits containing oscillations of substantially the same radio frequency but different phase, means comprising one or more modulators for producing modulating voltages to modulate the amplitude of the oscillations of the two circuits, two amplifiers, means for impressing the modulated electric oscillations upon the amplifiers, means for converting the amplitude modulations in the amplifiers into phase modulations, and means for maintaining the cosine of half the phase angle of the voltages of the respective amplifiers substantially proportional to the modulating voltage produced by one of the modulators.

W. RODERIC BLISS.