US2220201A

US2220201A - Modulation

Info

Publication number: US2220201A
Application number: US147688A
Authority: US
Inventors: Bliss William Roderic
Original assignee: Individual
Current assignee: Individual
Priority date: 1937-06-11
Filing date: 1937-06-11
Publication date: 1940-11-05
Anticipated expiration: 1957-11-05

Description

W. R. BLISS MODULATION Nov. 5, 1940.

Filed June 11, 19157' 5 Sheets-Sheet l NOV. 5, 1949. Wl R BLISS MODULATION Filed June ll; 1957 3 Sheets-Sheet 2 Nov. 5, 1940.

w. vFe. Buss MODULATION ZS'Sheets-Sheet 3 Filed June 11. 1957 E i iK.

Patented Nov. 5, 1940 UNITED STATES PATENT OFFICE MODULATION William Roderic Bliss, Middletown, Conn.

Application June 11, 1937, Serial No. 147,688

Claims.

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.

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 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 introducing 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 anoutput circuit that may be used according to the present invention; Fig. 4 is a vector diagram similar to Fig. 2 illustrating the relation that may exist between phase angles and radio-frequency amplitudes in 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 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 principlesA of this invention.

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 constantfrequency oscillations, shown in Fig. 7 as a. piezoelectric oscillatcr, 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 l0. and a third stage I2. 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, I0 and I2 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 o r otherlowefficiency means of amplification. Class B amplifiers may have an efficiency as low as 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 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 I0, 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 whichy are respectively coupled at 28 and 38 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 3 8 of the

ampliers

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

tothe input circuits

44 and 46 of two high-efiiciency class C distorting' amplifiers 48 and 50, so as to supply substantially equal voltages to the

grids

58 and 60 thereof. 'I'he 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

input 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`|0. 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 '|0 and a tuning condenser '12, are each connected in both

input circuits

44 and 46 of the amplifiers 48 and 50. 'I'he connections of the tuned circuit 64 in the input circuit of the amplifier 50 may be traced from the cathode 'I4 of the amplifier 50, which cathode 14 is grounded at 16, and through a biasing battery 18, 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 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 18 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 80, 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 be noted that the lower half of the coil 66 is con- `nected in series therewith in the one case, and

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 86 of the microphone system is shown coupled, through a transformer 88, to two vacuum tubes |00 and |02, connectedtogether, in push-pull. relation, with a half |04 of the secondary winding of the transformer 98 connected in the input circuit of the tube |00 and the other half |06 in the input circuit of the tube |02. The output circuits of the tubes |00 and |02 are respectively provided with windings |08 and ||0 that are properly connected together in series, and that are coupled, through condensers ||2 and ||4, to suitable series-connected resistors ||6 and ||8. Other impedances than resistors may be employed, but resistors are preferred because the direct-current component ofthe current in the hereinafter-mentioned tube |40 is needed for modulation, due to the unsymmetrical output of the tube.

The resistor ||6 is connected to the output circuit 38 of the amplifier 22, and the resistor ||8 to the output circuit 36 of the amplifier 20. The connections for the amplifier 22 are from the cathode |20 of the amplifier 22, which is grounded at |22, through the plate battery |24 and the resistor ||6, and by way of a conductor to the output circuit 38; and thence, by way of a conductor |26, to the plate |28 of the amplifier 22. The connections for the amplifier 20 are from the cathode |30, which is grounded at |32, and through the same battery |24 and the resistor |8, and by way of a conductor |34, to the output circuit 36; and thence, by way of a conductor |86, to the plate |38 of the tube 20. In this manner, two modulators are providedythrough the means of which by suitable impedances and connections to the impedances, the modulating signal will be applied to the plates |28 and |38 one hundred and eighty degrees out of phase with each other.

The phase relations will be understood from Fig. 2, where Ea represents the voltage from the tuned circuit 64 on the

grids

58 and 68 of the tubes 48 and 58, and E1 and E2 represent the voltages, differing by degrees in phase, on the respective grids from the tuned circuit 62. The vector sum of E1 and E3 is E4. If E4 represents the voltage on, say, the grid 58 of the tube 48, E5, the vector sum of E3 and E2, will then represent the voltage on the grid 68 of the tube 58. The difference of phase, represented by the angle a, between the voltage E4 or E5 on either grid and the voltage E3 depends upon the relative amplitudes of E1, Ez and E3. As E1 is always equal to E2, but opposite in sign, the difference in phase between E4 and E3, and between E5 and E3, is the same but in the opposite direction. Since the amplier 28, supplying the voltages E1 and E2, and the amplier 22, supplying the voltage Es, are modulated in amplitude, as above described, the difference in phase between the voltages E4 and E5 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 58 are class C amplitudedistorting amplifiers, it is desirable to keep constant the radio-frequency amplitude on their

respective grids

58 and 68. This condition may be obtained in any preferred way, as by the use of an amplitude-detecting device, comprising the said tube |48, which is back-coupled to the modulator of the amplifier 28 in such a way as to oppose any change of amplitude on the

grids

58 and 68 of the tubes 48 and 58 by modulation of the amplifier 28. One scheme for accomplishing this back-coupled modulation is illustrated in the drawings. The plate coupling circuit of the detector |48 extends from the cathode |46 of the tube |48, which is grounded at |58, through the battery |24 and the resistor ||8, to the plate |48 of the detector |40. The tube |48 is shown provided with a biasing battery |42 by means of which it is biased so as to operate as a detector,

vand its grid |44 is coupled to the grid 68 of the tube 58 through a condenser 284. The voltage on the plate |48 of the tube |48 is thereby controlled by the amplitude of the radio-frequency voltage on its grid |44, and this voltage is directly applied to the modulating voltage for the amplifier 28. Since the tube |48 is biased negatively, an increase in radio-frequency amplitude on its grid |44 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 I8 increases, lowering the voltage on the plate |48 of the tube |48, and hence on the plate |38 of the tube 28. This decreases the push-pull voltage component on the

grids

58 and 68 of the tubes 48 and 58 and, through condenser 284, on the grid |44 of the tube |48. Analysis shows that voltage-amplitude fluctuation on the

grids

58 and 68 of the tubes 48 and 58 is thereby divided by- (l-l-p.) Where y represents the effective amplification of the tube 48. The size of ,u may be made large by proper choice of the circuit components and the tubes, reducing the voltage-amplitude fluctuation on the

grids

58 and 68 of the tubes 48 and 58, and hence the distortion, to a negligible quantity. The tube |48 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 |48 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 68 of the tubes 48 and 58. V4 and V5 represent the voltages, reversed in phase, supplied to the

grids

58 and 68 by the tuned circuit 62, as before described. If tube |48 were inoperative, the envelope of all the vectors representing the radio frequency voltage on one of the grids 58 or 88 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. V1, Vs, and V9 are examples of such voltages. On the other grid, 68 or 58,' is a voltage represented by a vector Within the envelope ACD, which may be drawn symmetrical to the rst vector about the line AD. If V1 is the voltage on, say, grid 58, V2 is the voltage at the same time on grid 68. 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 unkvarying, the envelope should not be a triangle,

such as ABD, but a circular sector, such as AFD. The tube |48 produces this change by increasing or decreasing the voltage on the amplifier 28; or, in other words, by varying the horizontal component of the vectors of Fig. 5. Thus, for ex ample, if one radio-frequency amplitude Va be present at one instant,'the tube |48 will supply a voltageV7 to increase the length of Vs to Vs, which is correct. Likewise, if a voltage V9 appears, which is too large, the tube |48 will decrease it by an amount V10 to the size V11. This correction changes the phase angle of the vectox` 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 yto lthe tuned

circuits

62 and 64 should beso adjusted4 that a is 60 degrees, or the totalphase difference between the voltage on the

grids

58 and 68 of the tubes 48 and 58 is 120 degrees when there is no modulation of the

amplifiers

28 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 bythe absolute value of the vector V1, is the cosine of half this phase angle 2a. Since the absolute value of the vector V1 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 `21:: of the resultant voltages applied to the respective amplifiers 48 and 58 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 Fig. 4,

which represents conditions in the output cir.

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 V1 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 V1, the amplitude must be decreased, and for all less than the angle a of the vector V1, they must be increased, except l when a=0, which is correct. It will be understood that the particular size-ratio of triangle to arc chosen in Fig. 5 is for convenience only. Other ratios may be used, if desired. In n o case may the arc radius be larger, however, than DA of the triangle.

The amplifier need not be modulated at all by audio from the tube |02. All of its modulation may be obtained from the detector tube |40. Its modulation must consist of merely the correct 20 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 oi the required amplitude change, and the tube |40 corrects for small deficlencies 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 |52 and 54 in the stage I2, and they may comprise the ordinary high-efciency class C type of amplier. The input circuit of the amplifier |52 is shown connected to the output circuit of the amplifier 5 48, and the input circuit of the amplifier |54 is shown connected to the output circuit of the amplier 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. 'Ihe invention further provides for high-plate power efficiency and filament power eiciency throughout the amplier.

The radio-frequency voltage appearing upon 5 the plates |92 and |94 of the tubes |56 and |58 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

The amplifier 22 and the amplifiers 50 and |54 and related circuits may be shielded at 200 from 20 the rest of the transmitter.

In Fig. 4, which is much like Fig. 5, P4 and P5 represent the radio-frequency voltage present in 4 the tuned circuit |88, |96. P3 may represent the radio-frequency voltage appearing across the 25 tuned circuit |82. 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 30 C amplifier channels |52 and |54. 'I'he useful voltage developed across the tuned circuit |82 is therefore equivalent to a large radio-frequency voltage amplitude modulated by a large audiofrequency or other modulating-frequency voltage. To the tuned circuit |82 a load |98 (Fig. 3), such as the antenna 6 of Fig. 7, may be coupled. The whole output circuit, comprising the circuits |82, |98, |88 and |96, may thus be characterized as a phase-to-amplitude modulation converter. 4o

If the tuned circuit |82 is replaced by an audiofrequency transformer, or other means of supplying the audio-frequency component of current to the tubes |56 and |58, the whole amplifier may be used as a high efficiency audio amplifier. 45

Fig. 6 represents the output circuit of Flg. 3 so connected. In place of the tuned circuit |82 and the load |98, there is illustrated an audio or other modulating-frequency transformer 206, the secondary of which is connected to a loud-speaker 60 208 or other means of using the audio-frequency power. When the radio-frequency voltages in the tubes |56 and |58 are in phase, the tubes will draw a great deal of current through the transformer 206. When the radio-frequency voltages 3 on the plates of the tubes 56 and |58 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 Go current change through the transformer 206 supplies the audio power desired.

In practice, a suitable filter (not shown) may be inserted in the lead |84 for by-passing the radio-frequency component. if

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;

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 eiiiciency 75 CTI and lament-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. 'I'he 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 fre- .quency 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 modulating the amplitude yof the oscillations of the two circuits, two amplifiers, means for applying to both ampliers the voltage of one of the 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 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 substantially the same radio frequency but differing in phase by substantially 90 degrees, means for modulating the amplitude of the oscillations of 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. y

3. An electric system comprising two radio-frequency circuits containing oscillations of substantially the same radio frequency .but differing in phase by substantially 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 unaected in phase, means for applying to the other amplifier thavoltage of the other of the said two circuits butsubstantially reversed in phase, two modulators,-means comprising one o f the modulators for vapplying 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 ampli- -iier the voltage of the other of the said two circuits but substantially reversed in phase, means for Amaintaining the radio-frequency amplitude applied tothe amplifiers substantially constant, an amplifying channel connected toeach amplifier, an output circuit to which the channels are connected, and means for reconverting the phase modulations into amplitude modulations in the output circuit.

5. Anelectric 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 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 vcomprising 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 rst circuits, means for applying the other voltage to one of the first circuits unaect'ed in phase and to the other first circuit substantially i reversed in phase, means for applying to the respective second circuits signal voltages substantially reversed in phase, an outputcircuit 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 vradio-frequency oscillations, means vfor` 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 con- 1 nected 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 amplier, 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 fortconverting the phase modulations in theA 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 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 orl 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 thesaid two circuits unaffected in phase, means for applying to one lof the amplifiers the voltage of the other circuit unaiected 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 oiV -modulated electric oscillations upon the ampliiiers, means for converting the amplitude modulations inl the amplifiers into phase modulations, and means for maintaining the cosine of half the phase angleof the voltages of the respective amplifiers substantially proportional to the modulating voltage produced by one of the modulators.

W. RODERIC BLISS.