US2155506A - Controlled carrier modulation - Google Patents

Description

April 25, 1939.

CONTROLLED CARRIER MODULATI ON Filed Feb. 14, 1936 3 Sheets-Sheet 1 1 a L 4 "a INVENTOR JO/M/ 4. 25/4 4272 6 ATTORN EY J. L. REINARTZ 2,155,506

April J. L. REINARTZ 2,155,506

CONTROLLED CARR IER MODULAT 1' ON Filed Feb. 14, 1936 s Sheets-Sheet 2 INVENTOR JOHN L. RE/NARTZ ATTORNEY April 25, 1939. J. L. REINARTZ 2,155,506

CONTROLLED CARRIER MODULATION Filed Feb. 14, 1936 3 Sheets-Sheet 3 RF -z9g' 20 INVENTOR ATTORN EY Patented Apr. 25, 1939 UNITED STATES PATENT OFFHCE John L. Reinartz, Manchester, Conn., assignor to Radio Corporation of America, a corporation of Delaware Application February 14, 1936, Serial No. 63,823

9 Claims.

In operating a short Wave transmitter, say of the order of five meters, it is often desirable that the carrier be suppressed in the absence of modulating potentials and built up when modulating potentials are applied, and remain on the air until modulation stops. Such systems are particularly desirable in police transmitter work.

If one considers a class C stage which is plate modulated by a class B modulating system and where the bias for the class C stage is obtained from a grid leak resistance, one will have a normal radiophone transmitter.

One type of transmitter known today comprises a plate modulated radio wave amplifier stage operating class C and modulated by an audio frequency modulating stage operating class B. The bias for the grid or grids of the class C stage is supplied by a negative bias voltage from a battery and a grid leak resistance. (I have now described a transmitter.)

If one now adds battery bias in series with the grid leak until the plate current goes to zero, the carrier output will also go to zero. Upon modulating, it will be found that because of the increase in plate voltage applied to the class C stage, there will again be a carrier which is modulated for every positive half-cycle of modulation.

I find that such a transmitter has a serious defect that results from the fact that during modulation, good operation is obtained on the positive portions of the modulating potential Waves, but on the negative half of the modulating potential cycle, the high negative bias on the class C amplifier cuts off all of the negative part of the cycle of the modulating potentials.

It occurred to me that this defect could be overcome if a supplemental positive bias could be supplied to the class C stage during modulation, and in the interest of simplicity, efficiency and economy, this supplemental positive bias could be drawn from and produced in the class "B modulator stage. Preferably, the potential is produced in a resistor in the cathode circuit of the class B stage.

In order that the added battery bias may be counteracted and therefore allow normal operation of the class C stage, the moment that any audio syllable is started, we place in the connection to the filament of the class B tubes a resistor of such value that when normal modulation takes place, the voltage drop across this resistor will equal the added battery bias which we added in series with the grid leak so that normal operation of the class C stage again takes place and the carrier is properly modulated and both negative and positive modulation peaks again take efiect.

The time factor which will determine how long after an audio syllable has ceased that the car rier will remain on the air, is determined by the size of the capacitor that is shunted across the resistor.

A trial of this idea completely remedied the fault of negative half-cycle cut-01f. Instead of there being zero carrier half the time and posi tive peak modulation the other half of the time, the positive voltage generated by the cathode resistor completely counteracted the negative battery bias and allowed normal operation of carrier and modulation as soon as any audio tone was allowed to strike the michrophone. The capacity across the cathode resistor can be chosen to give'that time lag desired before the carrier again goes to zero when the microphone is not spoken into.

In describing my invention in greater detail, reference will be made to the attached drawings in which I have shown in Figs. 1 to 6 inclusive, various modifications of my transmitter, each of which includes modulating means, means for suppressing the carrier in the absence of modulating potentials, and means for preventing the class C amplifier from being biasedv to cutoif on the negative portion of the modulating potential cycle.

Fig. 1 shows a typical plate modulated class C amplifier tube 2 having its control grid and cathode connected to a source of high frequency oscillations to be modulated. By the expression class C amplifier applicant has in mind an amplifier so negatively biased that plate current flows only for a small portion of the positive half of the impressed wave. Plate modulation is used and the modulating voltage is obtained from a class B audio amplifier. The audio amplifier comprises two tubes 4 and 6 having their input electrodes coupled to a source of modulating potentials and their anodes coupled to a secondary winding in the plate direct current circuit of the tube 2. The primary winding of the transformer I0 is connected to the anodes of tubes 4 and 5. A point on this primary winding is connected to the cathodes of the tubes 4 and 5 by Way of a source B2 and a resistance R shunted by a condenser C. This resistance R is also included in the grid direct current circuit of tube 2. It is a characteristic of the class B audio amplifier that the average plate current is zero, or at least relatively small, in the absence of audio irequency input voltage, and increases in strength as the audio input increases. Hence, the voltage drop across R is substantially zero in the absence of modulating voltage and rises to a relatively large value when the class B stage is fully loaded. This voltage drop across R is inserted in the grid circuit of the class C amplifier stage in series with the grid biasing means which is usually a bias voltage and grid leak resistance, but may be a source of potential and for convenience has been shown as two batteries Bo and Be. The battery Bo provides the correct operating 'bias for normal operation when-the stage C is fully modulated, while the battery Be is poled to oppose the potential in R and is equal in voltage to the drop across resistance B when the class B stage is Working at full modulation output. The bias supplied by Bo and Be-between the grid and cathode of tube 2 in the absence of modulating potentials on the electrodes of the tubes 4 and 6 is of .such valueas to .bias .tube 2 .for class C operation. Thepotential drop across B when signals are applied to tubes 4 and 6 issuchas to compensate for the. negative bias of Be so that the tube 2 no longer.operates.as a class .C. am-

. plifier-and the negative portion .ofthemodulated carrier wave v.is no longer cut oif .in the. anode circuit of 2. 'Thus, it will be seen thatwhen fully -modulating, the drop. across. R just compensates the voltageof the battery Beandthe stage .C .is .no' longerv cut .oifon the. negativeportion of the modulating potential cycle,.but produces a fully modulated carrier wave output. As the audio frequency input to .the class .B stage is reduced,..the drop across R becomes less .and the operating bias on .the .class 0'. stage increases untilit reaches a maximum value, which ,isthe sumof .the -.two batteries. voltages when the audio frequency .excitation..is .entirely absent. If R ismadelargeenough so that the total voltage of the two batteries .is. sufficient to bias the class C stage. tocomplete cutoff evenon the .positivepeaks .of the RF excitation, then the carrier supplied to theantenna. will .vanish in theabsence of audio frequencyinput to the class ..B...stage. As theaudiofrequency excitation .is reduced, the carrier strength .is .re-

duced somewhat proportionally. so that. both .car-

rier and side-bands diminish.atthesametime as the audio-frequency excitation is reduced.

Fig. 2 shows a similar system wherein the normal operating bias is-obtained from the-grid current flowing through .agrid leak.R ,..instead of the .battery of B0 in Fig. 1. With full. audio frequency input, the drop across resistance. R just neutralizes the-battery Be so that'the stage operates in the improved fashion so that there is-no negative cyclecutofi, but astheaudio frequency -.input .isdecreased, the total bias .on the class C-stage increases and ultimately cuts-off the class C output entirely when :the audio frequency voltage-is entirely .absent. In Fig. 2, a single 7 direct current source B1 replaces the two sources B]: and Bzof Fig. 1.

The :class C stages may beneutralized if desired .by connecting a variable condenser Cn between the anode circuit. and the grid as shown in Figs. 1 and 2. The-modulated Wavemay be-impressedon a line'or on a radiatingsystem 20, as

shown.

Fig. 3 shows a similar system utilizing a pentode type of output tube 2 with modulation on .both the screen electrode l2 and the platezelectrode 14. In this case, the bias of :thezsuppressor grid l6, which is the sum of the battery Be, .30

.stage and the class B stage.

- of audio input and cessation of carrier output will .be determinedby the size of this condenser.

In practice, the single class C stage comprising the tube 2 is often replaced by a push-pull stage comprising tubes 2 and 2' connected as shown in -Fig. 4. Here, as in the prior figures, as for example Fig. 2, the potential supplied in the grid leak Hg is supplemented by a potential supplied from Be and the potential drop in R1. The potential drop in R1 is sufficient to compensate the potential produced by Be in the presence of full modulation so that thecarrier wave linearly modulated throughout the entire modulating potential-cycle is obtained. The operation of the arrangement of Fig. 4 will be understood from an understanding of the operation of the prior figures and a. detailed description thereof is thought unnecessary at this point.

. Instead of using some of the class-9B voltage,

. as developedacross resistance R placed in the the class A stage driving the class B stage.-

The output of this additional tube is dissipated in a resistance R. The voltage appearing across this resistance is used to overcome the added bias of the class C stage as heretofore explained. This method effectivelyremoves any tendency for creating distortion in .the class B .stage.

An arrangementas described briefly in the preceding paragraph has been illustratedin Fig.

-5. In Fig. 5, the radio frequency amplifier class C stage and the audio frequency amplifier and modulating class B stagetis similar to those described hereinbefore and a description thereof is-believed unnecessary here; It is noted, however, that numerals and letterscorresponding 'to those used in -the:description of the prior figures have been applied-to the tubesin the class C An additional thermionic tube of the pentode type has its a control grid 32 connected as shown byway of a biasing potential source Cbto the control grid of a tube 4 in the class B stage. The cathode of the pentodetube 30 is connected to the cathode of tube 4. In this 'manner, modulating, potentials fromany source and preferably from aclass A initial stagesupplying transformer 40 is ap- "plied to the control grid'oftubeflll. The anodeto-cathode circuit-of tube 30 includes a biasing resistance R shunted by the time element condenser C and a source of anode potential EB.

' Voltage for the screen electrode :36 may be sup- .plied from any source, such as the source supplying potential to the anodes of the class C and class B stage. In this arrangement, as in the prior arrangements, the potential produced in R overcomes the added potential Be in the presence of full modulation, thereby insuring complete modulation of the wave throughout the entire modulating potential cycle. The speed with which this control takes place to change the carrier on the air and remove the carrier from the air depends on the size of the condenser O.

The tube 3|] is biased by Ct to operate class B. That is to say, tube 40 is biased by Cb to cutoff when no modulation is taking place so that no modulating potentials are supplied to transformer 40 and from 40 to the control grid 32 of tube 30.

In a preferred modification, the biasing resistor R and condenser C may be placed in the cathode lead of the tube 30 as shown in Fig. 6. When the resistance R. is placed at this point, it is operating nearer ground potential and reduces the precautions necessary with respect to insulation, etc. Fig. 6 is otherwise similar to Fig. 5 and needs no further description.

What is claimed is:

1. In a signalling system, a carrier wave amplifier tube having a grid electrode and a cathode coupled to a source of potentials of carrier wave frequency and output electrodes coupled to a load circuit, a modulator tube having input electrodes coupled to a source of modulating potentials and output electrodes coupled to an electrode of said amplifier tube, biasing means in the direct current grid circuit of said amplifier tube for controlling the gain thereof, and biasing means in a direct current circuit of said modulator tube connected with said first named biasing means to oppose the control thereof in accordance with average modulating potential intensity.

2. In a signalling system, a carrier wave amplifier tube having a grid electrode and a cathode coupled to a source of wave energy and output electrode coupled to a load circuit, a modulator tube having input electrodes coupled. to a source of modulating potentials and output electrodes coupled to an electrode of said amplifier tube, biasing means in the direct current circuit between the grid and cathode of said amplifier tube for controlling the gain thereof, said biasing means being sufficient to bias said wave amplifier tube to cutoff in the absence of modulating potentials, and biasing means in a direct current cicuit of said modulator tube connected with said first named biasing means to oppose the control thereof in accordance with average modulating potential intensity, said last named biasing means being sufiicient to prevent cutofi of said amplifier tube on the negative portion of the modulating potential cycle.

3. In a signalling system, a carrier wave amplifier tube having a grid electrode and a cathode coupled to a source of Wave energy and output electrodes coupled to a load circuit, a modulator tube having a grid electrode and a cathode coupled to a source of modulating potentials and output electrodes coupled to an electrode of said amplifier tube, biasing means in a direct current circuit between the grid and cathode of said amplifier tube for controlling the gain thereof, and biasing means in the direct current circuit between the output electrodes of said modulator tube connected with said first named biasing means to oppose the control thereof in accordance with average modulating potential intensity.

4. In combination with an electron discharge amplifier tube having an input circuit coupled to a source of carrier waves and an output circuit coupled to a load circuit, said tube being negatively biased for class C operation, a modulator tube having its input electrodes coupled to a source of modulating potentials and its output electrodes coupled to an electrode of said discharge tube tomodulate the same, said modulator tube being negatively biased for class B operation, a source of potential connected with the input circuit of said first named tube of sufiicient value to bias said tube to zero alternating current output in the absence of modulating potentials, a resistance connected between the output electrodes of said class B modulator tube, and circuits connecting said resistance in series with said source of potential, said resistance being of such value as to produce a potential suflicient to compensate said source of potential in the presence of modulating potentials.

5. In combination with an electron discharge tube having input electrodes coupled to a source of carrier waves and an anode electrode coupled to a load circuit, said tube being negatively biased for class C operation, a modulator tube having its input electrodes coupled to a source of modulating potentials and its output electrodes coupled to the anode electrode of said discharge tube to anode modulate the same, said modulator tube being negatively biased for class B operation, a source of potential connected between the input electrodes of said first named tube of sufficient value to bias said tube to cutoff in the absence of modulating potentials, a resistance connected between the output electrodes of said modulator tube, and circuits connecting said resistance in series with said source of potential, said resistance being of such value as to produce a potential sufiicient to substantially buck out said cutoff potential in the presence of modulating potentials.

6. In combination with an electron discharge tube having a control grid and cathode coupled to a source of carrier waves and an anode and cathode coupled to a load circuit, said tube being negatively biased for class C operation, a modulator tube having its input electrodes coupled to a source of modulating potentials and its anode electrode coupled to the anode electrode of said an electron discharge amplifier tube to modulate the same, said modulator tube being negatively biased for class B operation, a source of potential connected between the control grid and cathode of said first named tube of sufficient value to bias said tube to zero alternating current output in the absence of modulating potentials, a resistance connected between the anode and cathode of said modulator tube, and circuits connecting said resistance in series with said source of potential, said resistance being of such value as to produce a potential sufficient to overcome said source of potential in the presence of modulating potentials.

'7. In a signalling system an electron discharge tube, a source of waves to be modulated connected to the input electrodes of said tube, a load circuit coupled to the output electrodes of said tube, a modulator tube, a source of modulating potentials connected with the input electrodes of said modulator tube, circuits connecting a pair of electrodes of said discharge tube together and coupling the same to electrodes in said modulator tube, means for supplying a biasing potential to said first named tube to bias the same to cutoff when no modulating potentials are applied to said second named tube, means for producing in a direct current circuit of said second tube a potential which is a function of the mean amplitude of applied modulating potentials, and circuits connecting said first named means to said last named means in a manner such that said potentials oppose.

8. In a signalling system a pair of electron discharge tubes each having input and output electrodes, a source of waves to be modulated connected to the input electrodes of said tubes, a load circuit coupled to the output electrodes of said tubes, a pair of modulator tubes each having input and output electrodes, a source of modulating potentials connected with the input electrodes of said modulator tube, circuits connecting similar electrodes in said discharge tubes together and coupling the same tolike electrodes in said modulator tubes, means for supplying a biasing potential between the input electrodes of said first named pair of discharge tubes to bias the same to cutoff when no modulating potentials are applied to said second named pair of tubes, means for producing in a direct current circuit between the output electrodes of said second named pair of tubes a potential which is a function of the amplitude of applied modulating potentials, and circuits connecting said first named means to said last named means in a manner such that said potentials oppose.

9. In a signalling system a pair of electron discharge tubes each having an anode, a cathode and a control grid, a source of waves to be modulated connected to the cathodes and control grids of said pair of tubes, a load circuit coupled to the anode and cathode electrodes of said pair of tubes, a pair of modulator tubes each having an anode, a cathode and a grid electrode, a source of modulating potentials connected with the grid and cathode electrodes of said pair of modulator tubes, circuits coupling similar electrodes in said discharge tubes to similar electrodes in said modulator tubes, resistive means for supplying a biasing potential between the grid and cathode of the tubes of said first named pair of tubes to bias the same to cutoff when no modulating potentials are applied to said second named pair of tubes, means for producing ina direct current circuit between the anodes and cathodes of the tubes of said second named pair of tubes a potential which is a function of the amplitude of applied modulating potentials, and circuits connecting said first named means to said last named means in a manner such that said potentials

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