US2385566A - Screen-grid modulation control - Google Patents

Description

Sept. 25, 1945. 0' DE GL-RE 2,385,566

SCREEN GRID MODULATION CONTROL Original Filed April 7', 1941 ,4 Sheefs-Sheet 1 gzzzww @a 12 1 v Sept. 2 1 3 f 0. DE GUIRE 2,385,566

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SCREEN GRID MODULATION CONTROL Original Filed April '7, 1941 4 Sheets-Sheet 3 RFC F/GU/Qf A7 3 Sept. 25, 1945. 0. DEV GUIRE 2,385,566

SCREEN GRID MODULATION CONTROL Original Filed April 7, 1941 4 Sheets-Sheet 4 amw/v 5y l/YVE/VTOA Patented Sept. 25, 1945" UNITED STATES PATENT-OFFICE SCREEN -GR1D 2,385,588 MODULATION CONTROL Q OlfanDe Guire, Silver-ton, Oreg.

Substituted for abandoned application Serial No. 387,277, April 7, 1941. This application May 15, 194 3, Serial No. 487,112

90laims. (Cl- 179-1715) This application is a substitute for application 387,277, filed April 7, 1941, and relates to improvement in screen grid modulation and-has particular reference to modulation of a radio frequency amplifier .having a screen grid and a circuit for same by means of a linear modulator, its object being to provide a control means for controlling the direct current and modulating component supplied for the screen grid circuit whereby an auxiliary circuit connected either in parallel or in series with the screen grid circuit is utilized or controlled by the action of the modulating current to either dissipate an excessive voltage swing I from the modulation downward impulse or to counteract same, so as to retard a predetermined amount the rate of reduction of the quantity of the electric vcurrent flowing through the screen rid circuit.

It is proposed in the present invention to provide a condenser having a capacity large enough Byway of explanation it might be stated that because of the increasingresistance of the screen v current oithe radio irequencyampliiier as the to supply the modulated audio peak current from the modulator to the screen grid circuit of the radio frequency amplifier. Since a condenser blocks a direct'current it will be necessary to use an auxiliary circuit to supply the unmodulated direct current component to the screen grid circuit, as well as using said auxiliary circuit to discharge the condenser when under modulation. It the resistance of the auxiliary circuit is controlled in a non-linear manner during modulation similar to the change-in resistance of the screen grid circuit but in an opposite sense, said non-linear characteristic of the screen grid circuit can be counter-balanced by the auxiliary circuit, so as to ive the eil'ect or a constant impedance in as far as the modulator is concerned. Possibly a controlled auxiliary circuit in parallel across the screen rid circuit as a bleeder circuit could be used, and thus not require the condenser, but said condenser simplifies matters and at the same time maintains a balanced load on the modulator, be-

cause it will not allow the modulator to supply power to the screen grid circuit without supplying an equal amount of power to the auxiliary circuit. It is also possible to a limited extent to utilize 'a fixed resistorto act as a balance against the screen grid circuit when a power supply is'used in series with the'resistor. The resistor and power supply circuit is connected so in effectit is across the condenser with the polarity oi the power supply being suchas to increase the discharging voltage of the condenser and to oppose tne'charging voltage applied-to the screen .grid becomes less in relation to the plate voltage and thus requires a current variation having-an increasing nonlinear characteristic as the voltage applied to the screen grid is increased in relation to the plate tage.

In this connection, for a better understanding oi' the problems of screen grid modulation, it might be stated that toobtain a carrier wave etflciency equal to that of plate modulation thescreen grid voltage must be as high in proportion to the plate voltage of the radio frequency amplifler as the ratio used with plate modulation.

Under these conditions the modulated positive peaks -with'screen modulation will be more e1- flcient than the said peaks with plate modulation.

Therefore, to obtain an average efficiency while under modulation equal to that of plate modulation the unmodulated screen grid voltage can be lower than that used with plate modulation.

This method of screen modulation allows a higher ratio ofscreen voltage to plate voltage than existin methods in use and at the same time reduces distortion.

Also byvariations in resistance and voltage flexibility is attained, allowing operation at many different ratios of screen to plate voltage.

Figure 1 shows a modulator, radio frequency amplifier having. a screen grid with both coupled together by means of a series condenser and an auxiliary power supply-with a resistor in series voltage, thus causing a'condition' which can be utilized with the screen grid circuit under certain conditions of operation to balance same.

forming a circuit connected in parallel with the screen grid circuit and the condenser when con- I sidered through the modulator circuit. 1

Figure 2 has the'same arrangement except the resistanceof the auxiliary circuit is not constant during modulation butis controlled by a means which functions due to changes in the quantity of the current flowing t0 the screen grid circuit.

Figure'3 has characteristics similar to Figure 1 with the addition oi another auxiliary circuit con nected directly across the series coupling condenser and consisting of a resistant circuit whose resistance is controlled by the current flowing to the screen grid circuit in combination with the current from the first auxiliary circuit. The

modulator coupling is different but the relation of the condenser is similar.

Figure 4 shows an arrangement with an auxiliary circuit having the same relation to the con-.

,denser as that of Figure 2 the difference being in the modulator coupling which also includes :the plate circuit of the radio frequency amplifier.

without the series condenser, same being in series only with the screen grid circuit.

Referring to Figure 1 in detail, modulator tube ,I is shown provided with a plate audio frequency choke 2, having its plate 3 coupled to the radio frequency amplifier 4 through an audio frequency condenser 5 connected to the screen grid 6 of said.

amplifier. A radio frequency by-pass condenser to plate 8 of the rectifier 1, having .a practically which allows a reduction in the voltage I2 below the modulation peak voltagewithout same leaking back to ground and thus allowing resistor II to be reduced sufiiciently to increase the unmodulated voltage with comparatively less resistance II in the circuit as well as to increase the discharging rate of condenser 5, which will cause an increased dissipation of the negative potential from said condenser and thus reduce the rate of the current reduction in the screen grid circuit and allow a higher unmodulated screen grid voltage 6, because of the increased non-linear characteristics of the modulated current in the screen grid circuit. If the voltage applied tov plate 8 while on open circuit is less than the positive audio peaks which are about twice the unmodulated screen grid voltage, distortion will be A greater. The open circuit voltage at 8 can be about half ways between the unmodulated screen to accomplish this the open circuit voltage at 3 constant positive voltage applied to the resistor at I2.

The unmodulated screen voltage, of screen 5, is determined by voltage I2 and the resistance of resistor II.

The plate 9, of the rectifier I, is connected to a milliammeter l 3 in series with battery I4, thence to ground. The battery I4 is the amount of negative voltage required to cause the plate current;

of the radio frequencyamplifier 4, to drop to zero when its screen grid 6 is disconnected from 5 and I0 and connected to the negative side of battery I4 only. Full radio frequency excitation at I5 is a left on while this is being done.

The purpose, of meter I3, battery I4 and plate 9 is an overmodulation indicator of the negative audio peaks and same has no part in the operation otherwise. i The operation and adjustment of the device may be expla'ined'as follows: With the proper voltages applied to the amplifier 4 and modulator I, at I6 and I1 respectively, radio frequency excitation is-appli'ed to amplifier 4 at I5 and modulator I is connected to a suitable speech amplifier at I8. With a positive voltage connected to re-' sistor II at I2, which should be maintained at. a proximately the modulation peak voltage or about twice the unmodulated voltage supplied for the'screen grid 6, the modulator I is put in operation and the speech gain is adjusted till the plate current just shows overmodulatlon by a suitable plate meter connected between I6 and the plate should be equal to the voltage of the positive audio screen grid peaks. This can 'be accomplished with the circuit shown in Figure 1, only when the screen grid voltage 6 is comparatively low in relation to the plate voltage I6. Plate voltageIIi can be increased, to allow a higher screen voltage at 6 to obtain the full output of amplifier 4, but the efiiciency of same will be reduced, due to the high plate voltage I6 required. Also operation in this manner does not require rectifier 1, except for the operationof meter I3.

Since the resistor I I works well when the screen voltage '6 is comparatively low, it follows, that by using a variable resistance controlled by the rapidly changing screen current 6 at higher screen'voltages, a suitable combination obtained.

Figure 2 shows such a combination with vacuum tube 20 having a plate 2 I connected in series with resistor I I. Cathode 22 is connected directly to audio. condenser 5. Resistor 23 carries all the current supplied by both vacuum tube2ll and would be condenser 5 and needed by the screen grid 6.

Therefore, since grid 24 is across resistor 23 the screen current 6 controls the resistance of tube 20. Since cathode v22 is connected directly to condenser 5 ,'current can fiow between some with- 1 out afiecting the resistance of tube 20. Then, at

of the radio frequency amplifier 4. Resistor I l is then adjusted so the negative audio peaks indicated by meter I3 correspond to the positive audio peaks as indicated by the plate meter near I6. Readjustment of the speech gain may be necessary while doing this. If after making the required adjustments, with the voltage I2 being or the voltage dropping resistor I9 may serve as an aid in maintaining an unmodulated screen voltage which on opening the screen grid eircuit is found to be lower than the modulation peak I The unmodulated screen grid screen voltages. voltage will then be higher because of rectifier I,

the negative audio peaks, the current will be the greatest through resistor, II and tube 20, but condenser 5 will absorb all the current and the screen current 6 will be zero. As the negative audio peaks are reduced by modulator tube I the screen current 5 will increase comparatively slow at first I place from the negative audio peaks to or near because the current will reduce through resistor II and tube 20, as the screen voltage I; raises which will prevent'as rapid a raise in screen current 5, as will occur later when the quantity of the said current through resistor II and tube 20 is reduced. As this reduction in current takes the positive audio peaks and since its quantity is very small at the positive peaks it has very littleefiect in bucking or preventing the rapid rise in current required at this point. The adjustments required with the circuit in Figure 2 are rather easy since the voltage at I2 is made equal to the screen positive audio peaks, with resistor I I being used to adjust the unmodulated screen voltage 6, while resistor 23 is used 4 ldropping resistor I! can be modulation downward impulse, so that the nonlinear characteristics will be suitable to the unmodulated screen grid voltage 6 that is desired. It the voltage availablejior I2 is too high the used as mentioned before. Figure 3 shows a difi'erent arrangement which gives the same results with better efllciency since current through resistor I! is reduced and the efllciency is otherwise increased.

1 This circuit will work without resistor H but the efllciency is then reduced,since the voltage applied to the modulation transformer at 29 cannot be modulated near 100% because a positive voltage will be required to overcome the resistance of tube 25, to supply the current required to prevent condenser 5 from becoming too, negative during the screen grid negative impulse. 'This is due to the fact that the resistance oi! tube 25, is not zero even though its grid 28 is at zero'potential. By using resistor ll connected directly to the screen grid 6, current will fiow between resistor II and condenser I through'bias resistor 23, same being maximum at the negative screen grid peaks, causing grid 28 to become positive, thus reducing the resistance of tube 25, which thus allows a smaller applied voltage at 2'5. v a

The operation with this circuit, as" shown in Figure 3 is also somewhat different since the vacuum tube 25 is connected with its Plate 21 on the other side of condenser 5, which thus allows a positive voltage to be applied to said plate 21. Since the resistanceof tube 25 is controlled by the screen grid current 8 it follows its resistance will be greatest at the positive screen grid peaks thus causing condenser 5 to supply all or most all the current. But as the resistance of tube 25 is reduced, as the screen grid current drops, part of the positive charge on condenser 5 will flow through said tube 25' reducing the negative charge correspondingly on the opposite side of said condenser 5 which thus prevents condenser 5 from 'causing screen grid 6 to go negative too rapidly.

Then by applying practically a constant positive voltage l2 to resistor H, equal to the screen grid positive peak voltage and by using both resistors II and 23 to adjust the screen grid negative peaks and the unmodulated screen grid voltage B, the desired results can be obtained as already explained.

The resistor II can be connected directly to cathode 22 or, to resistor 23 at various points between the cathode 22 and grid 28 for controlling the modulated wave form somewhat if desired.

'audiofrequency choke 2 may be used instead of the modulation transformer oi Figure 3 if desired.

The methods of screen grid modulation described can be combined with plate modulation, the same being an advantage where a high ratio screen grid to pla ducing the high screen grid current at the posivoltage is'used,-thus ref lation under these conditions.

Figure 4 shows a circuit combining-that shown in Figure 2 with plate modulation. r

The class B modulation transformer 29 is shown connected, as if for plate modulation to the radio frequency choke 30 and through some to the tank circuit of the radio frequency amplifier. .Audio frequency condenser 5 is connected to the modulation transformer at 31, and otherwise the circuit is the same as that shown in Figure 2.

With this circuit the percentage of plate modulation can be'lowered either by reduci'ng the unmodulated screen grid voltage 6, or byconnecti5 ing the'plate meter MA to a lower tap on the modulation transformer 29. The operation and adjustment of screen grid modulation, combined with a percentage of plate modulation, are different because the plate voltageis higher on the positive peaks and lower on a I the negative peaks than with screen modulation only.

Having described my invention what I'claim as new is; I

1. In a modulation system, a screen grid radio frequency amplifier havingan anode circuit and a screen grid circuit, an amplified source oi. carrier/wave energy to be modulated coupled to said anode-circuit for making said amplification with the aid of said amplifier, means for applying modulation voltages to vary the current fiow in said anode circuit; and in said screen grid circuit whereby the current flow in each of said circuits is reduced during a portion of the modulation cycle, means for reducing the rate of reduction in the screen grid current relative to the rate at which the anode current is reduced within the limits of said portion of the modulation cycle. said last means including an election discharge tube 40 connected in said screen grid circuit for dissipating excessive screen grid modulation voltages during'a large extent of said portion of the modulation cycle.

2.-In a'modulation system, a screen grid radio 4 frequency amplifier having an anode circuit and a screen grid circuit, an amplified source of carrier wave energy to be modulated coupled to said screen grid current relative to the rate at which.

' the anode current is'reduced within the limits oi said portion of the modulation cycle, said last means including an electron discharge tube connected in said screen grid circuit for dissipating c0 excessive screen grid modulation voltages during said portion of the modulatf on cycle.

3. In a modulation system, a screen'grid radio frequency amplifier having an .anode circuit and a screen grid circuit, anamplified source of carrier wave energy to be modulated coupled to said anode circuit for making said amplification with; the aid or said amplifier, means for applying modulation voltages to vary the current flow in said anode circuit and in said screen grid circuit whereby the current flow in each of said circuits is reduced during a" portion of the modulation cycle, means for reducingthe ratio or screen grid current to anode current at a decreasing rate relative to the rate at which the 7 anode current is reduced within the limits of tive peaks which would occur with screen modu-" 4 I c said portion off the modulation cycle, said last means including an electron discharge tube connected in said screen grid circuit for dissipating excessive voltages in the screen grid circuit for .reducing said anode current ate. more nearly rectilinear rate than the rate at which said screen grid current is reduced when the modulation voltages applied to said means are varied at a more nearly rectilinear rate than the rate at which said screen grid current is reduced.

4. ma modulation system, a screen grid radio frequency amplifier having an anode circuit and a screen grid circuit, means for applying modulation voltages to vary the current fiow in said anode circuit and in said screen grid circuit whereby the current fiow in each of said circuits is reduced during a portion of the modulation cycle, means for reducing the rate of reduction in the screen grid current relative to the rate at which the anode current is reduced within the limits of said portion of the modulation cycle,

said last means including connections in the screen grid circuit to a condenser for applying modulation voltages and to an electron discharge tube having a grid for varying in a ncfi rectilinear manner the current fiow in said electron tube to discharge said condenser sufliciently to dissipate excessive screen grid modulation'voltvaried at a more nearly rectilinear rate than the rate at which said screen grid current is re-v duced. a

5. In a'modulation system, a screen grid radio frequency'amplifier having an anode circuit and I "rent flow, means for reducing the rate of reduction in the screen grid current relative to the rate at which the anode current is reduced within the limits of said portion of the varying action between maximum and minimum current flow, said last means including an electron discharge tube having a grid for varying in a nonrectilinear manner the current flow in said electron discharge tube coupled to said screen grid circuit to dissipate excessive screengrid modulation voltages for reducing said anode current at a more nearly rectilinear rate than the rate at which said screen grid current is reduced when the modulation voltages. applied to said means are varied at a more nearly rectilinear rate than the rate at which said screen grid current is reduced.

6. In a modulation system, a screen grid radio "frequency amplifier having an anode circuit and a screen grid circuit, an amplified source of carrier wave energy to be modulated coupled to said current isreduced withinthe limits of said portion of the varying action between maximum and minimum current flow, said last means including connections in the screen grid circuit to anode current at a more nearly rectilinear rate 7 than the rate at which said screen grid current is reduced when the modulationvoltages applied to said means are varied at a more nearly rectilinear rate than the rate at which said screen grid current is reduced.

'7. In a modulation system, a=screen grid radio frequency amplifier having an anode circuit and a screen grid circuit, an amplified source of modulated carrier wave energy to be produced by said amplifier, means for applying modulation voltages to vary the current fiow in said anode circuit and in said screen grid circuit whereby the anode and the screen grid are utilized to cause areduction in the current flow in each of the respective circuits during a. portion of the varying action between maximum and minimum current flow, means for reducing the rate of reduction in the screen grid current relative to the rate at which the anode current is reduced within the limits of said portion of the varying action between maximum and minimum current flow, said last means including connections in the screen grid circuit to a condenser for applying modulation voltages and to series circuit having resistance and conducting electrlcal energy from said condenser and another supply for discharging said condenser sufficiently to dissipate" excessive screen grid modulation voltages for reducing said anode current at a more nearly rectilinear rate than the rate at amplifier, means for applying modulation voltages to 'vary the current flow. in said anode cirmaximum and minimum current fiow, means for reducing the rate of-reduction in the screen grid current relative to the rate at which the anode which said screen grid current is reduced when the modulation voltages applied to said means. are varied at a more nearly rectilinear rate than the rate at which said screen grid current is reduced. 8. In a modulation system, a radio frequency amplifier having an anode circuit and a grid circuit a'n amplified source of modulated carrier wave energy to be produced by said amplifier,

means for applying modulation voltages to, vary the current flow in said anode circuit and in said grid circuit whereby the anode and the grid are utilized to cause a reduction in the current flow in each of therespective circuits during a portion of the varying action 1 between maximum and minimum current flow, means for reducing the rate of reduction in the grid current relative to the rate at which the anode current is reduced within the limits of said portion of the non-rectilinear manner the current flow in said electron discharge tube coupled to said grid circult" to dissipate excessive grid modulation voltages ior reducing said anode current at a more nearly rectilinear rate than the rate at which said grid current is reduced when the modulation' voltages applied to said means are varied at a more nearly rectilinear rate than the rate at which said grid current is reduced.

9. In a modulation system, a screen grid radio frequency amplifier having an anode circuit and assumes of the modulation cycle, said last means includ ing an electron discharge tube connected in said screen grid circuit for dissipating excessive screen grid modulation voltages for reducing said anode current at a more nearly rectilinear rate than the rate at which said screen grid current is reduced when the modulation voltages applied to said means are varied at a more nearly rectilinear rate than the rate at which said screen grid current is reduced. a B

- OLF'AN DE GUIRE.

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