US2450445A - Modulation - Google Patents

Description

Oct. 5, 1948. c. A. ROSENCRANS MODULATION Filed Jan. 18, 1944 2 Sheet s-Sheet l MODULATlON some:

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INVENTOR. CHARLES A. ROSENCRANS;

Patented Oct. 5, 1948 2,450,445 MODULATION Charles A. Rosencrans, Haddonfield, N. J assignor to Radio Corporation of America, a corporation of Delaware Application January 18, 1944, Serial No. 518,700

This application concerns an improved modulation system wherein carrier wave energy is linearly controlled as to amplitude in accordance with control potentials, such as potentials representing voice programs or potentials covering a wider band of frequencies such as, for example, video signals resulting from the scanning in some manner of a subject.

A general object of my invention is an improved and simplified modulation system.

Another object of my invention in an improved grid modulation system wherein the amplitude of carrier wave energy may be controlled linearly in accordance with control potentials which cover a Wide range of frequencies such as, for example, video signals.

A further object of my invention is an efficient and linear modulation system for use in television and similar systems wherein the so-called direct current component of the modulation signal may be maintained in its original strength or built up, or the direct current component may be partially removed or entirely removed.

Thus in my system means is provided for setting the carrier or average level desired so that when the modulation represents scanned subject matter the average brightness of the picture may be set as desired to correspond to the original subject or to vary therefrom in density in either direction.

An additional object of my invention is an improved efficient modulator of simple nature wherein the modulation range is considerably greater than can be obtained in systems of this general nature known heretofore. I

It will be obvious that my improved modulation method and means is applicable to any radio frequency system which is capable of being grid modulated.

The manner in which the above objects and others are attained will appear from the following description and therefrom when read in connection with the drawings, wherein Figs. 1 and 2 illustrate two embodiments of my improved modulation system, and. Figs. 3 and 4 are curves illustrating the relation of various voltages and currents in the system under different operating conditions.

In Fig. 1, a single stage radio frequency amplifier and grid modulation arrangement is shown. The circuit of Fig. 2 is essentially as in Fig. 1, except that in Fig. 2 a pushpull stage radio frequency amplifier is controlled by the modulation.

In Fig. 1, I indicates a source of carrier wave energy of the desired carrier wave frequency which is to be modulated. This source of radio Claims. (Cl. 179-1715) frequency current or potential is coupled to a circuit including an inductance LI and condenser C2 connecting the cathode of a radio frequency amplifier stage V2 to ground. The amplifier tube V2, in other words, is a cathode driven radio frequency amplifier. The anode of the tube V2 is connected to an output circuit including condenser C4 and inductance L3 from whichmay be derived the modulated carrier. The control grid GI of tube V2 is connected substantially directly to the cathode of tube VI, while the anode of tube VI is connected to ground by two paths, one of which includes the adjustable resistance R2 and the source of direct current potential, and the other of which includes the condenser CI. Note that this completes a direct current path from the cathode of tube VI through the output impedance of tube VI, resistor R2,, the direct current source, inductance LI, and from the cathode to the control grid of tube V2 which is connected to the cathode of tube VI, and an alternating current path through the output impedance of tube VI, condenser CI inductances LI and condenser C2 in parallel, and from the cathode to the control grid of tube V2 which is connected to the cathode of tube VI. The anode of tube VI is also connected to the grid G by resistance RI.

A source of modulating potentials in the rectangle I 6 is coupled by condenser C6 between the control grid G of tube VI and ground. The modulator tube VI is essentially a cathode follower stage using the grid circuit of the radio frequency amplifier stage V2 as its load.

In operation assume that the carrier Wave energy to be modulated is applied from the source I0 to the circuit including inductance LI, and condenser C2, and the said circuit and the anode circuit including condenser C4 and inductance L3 are broadly tuned to the said carrier wave energy. The tube V2 may then be considered a grounded grid radio frequency amplifier, since the radio frequency impedance between the grid and ground is very low due to capacitive effects represented by Cg between the grid and ground and because of the bias circuit connections and its coupling to ground. Now assume that the tube V2 is being modulated in a normal manner, that is, has modulation applied to the grid thereof. Then the stage V2 radio frequency excitation and loading is adiusted in a normal manner to produce linear modulation of the carrier through a given range in accordance with the signals. From the results thus obtained the bias on the grid GI of the power amplifier V2 at which carrier or average output,

- so that the grid modulation circuit of tube V2 now includes as a variable impedance the output impedance of tube VI, and as additional impedance the resistance R2 and as a series alternating current of modulation frequency supply path the condenser CI. By adjustment of R2 or the amount of direct current potential applied to the anode of VI the negative potential on the grid of V2 (no modulation on the grid of VI) is set at its normal value, as described above, that is, at the point at which carrier output from tube V2 is obtained. Since the DC. potential on the grid of V2 depends on the potential drop across the output impedance of VI and/or the value of R2, and/or the value of the direct current potential at +B by adjustment of R2, and/or the radio frequency excitation voltage e from the carrier wave source ID, a negative bias potential at which carrier output is obtained from V2 may again be set up on GI in the absence of modulation from the source in IS. About the same results may be obtained by adjustment of the value of the direct current potential at +13.

The modulator VI, as stated above, is essentially a. cathode follower stage using the grid circuit of the radio amplifier tube V2 as its load. The grid bias of the radio frequency stage V2 is a function of the plate resistance of the modulator tube VI as well as the radio frequency excitation. Since the grid G of modulator tube VI is at zero bias with no signal because of RI, R2 can be used to set the plate resistance of the modulator tube to a value which gives the modulated radio frequency amplifier V2 its normal bias for zero modulation. The purpose of RI is, first, to bring the grid potential of VI to essentially zero in the absence of the modulating signal from source I8, and, secondly, to permit a negative potential to be developed at the grid of VI in the presence of a modulating signal, the value of this negative potential being proportional to the average of the modulating signal. Thus the time constant (rate of discharge) of the RIC6 network is made sufficiently long so that the negative bias on the grid of tube VI is held essentially constant over at least one cycle of the lowest modulation frequency it is desired to pass through the tube VI to grid modulate the grid of tube V2.

When modulation is applied to the grid G of tube VI and say, for example, the instantaneous modulating potential swings positive the tube VI tends to draw more current, but this current through the tube causes the average direct current bias on the grid G to become more negative because of rectified grid current in RI and the alternating current leakage through 06. On the application of this positive swing the instantaneous drop in potential through VI becomes greater and the bias potential on the grid GI of tube V2 becomes more negative and the alternating current output (R. F.) of the tube V2 decreases, since in this tube the alternating current (R. F.) output changes in direct proportion to changes in the negative bias on the grid thereof. The modulation potential then swings around this average negative bias, with the radio frequency output intensity following the modulation potential on the grid of tube VI.

The resistance R2 is made such that the tube VI output resistance is of the desired value for.

4 no signal or carrier condition in the radio frequency amplifier tube V2. The output resistance discussed here is the plate to cathode resistance of this tube VI included in the grid direct current circuit in the amplifier circuit tube V2.

R2 controls the amount of change in direct current through the tube VI that I can get, while the potential at +3 controls or sets the minimum resistance to which VI can be modulated by signals.

As R2 is made smaller the grid GI of V2 becomes less negative and the no signal output of the amplifier tube V2 is shifted towards maximum value, i. e., maximum modulation capability under linear operation. The system may be made a direct current system by omitting R2, in which case the bias of the grid of the modulator tube VI sets the zero signal radio frequency output and the radio frequency amplifier is modulated from this level as a maximum downward. The operating conditions for different values of R2 are shown in Figs. 3 and 4.

In Fig. 3, R2 is zero and the values of plate direct current Ip in the tube V2, and the average bias EG on the grid G of tube VI correspond to various percent modulation. EB is the plate potential on the tube V2. In Fig. 4 all of the curves indicate modulation for different values of R2, including R2 equal zero.

The curves of Figs. 3 and 4 show the combined modulator and radio frequency amplifier performance. These curves are the result of experimental data but are presented to describe a purely hypothetical case.

Of the curves in Fig. 3, the curve EGIO is obtained by setting up the radio frequency amplifier V2 with normal radio frequency loading and excitation for the desired grid modulation condition. Thus any point on the curve EGI-O shows the value of radio frequency tube V2 direct current plate current for any value of grid bias on the radio frequency tube V2 and if the system is to be capable of linear modulation the curve will be a straight line. Then EGI is the cutoff bias and EG5 is the necessary bias for maximum output, so the modulation voltage will have these limits. In my system it is not possible to quite reach the value EGI but the maximum negative bias which can be developed is EG2 (the bias developed with the direct current grid circuit of the radio frequency amplifier open). Therefore, the maximum modulation percentage m will be EG2-EG5 EGl-EGE) and the peak-to-peak volts for this modulation percentage is EG2-EG5.

Since all conditions shown are for conditions of linear modulation, the curves a2-O and b2-O are the loci of the minimum and maximum modulation voltages, respectively, the average values of which are on the curve EGI-O. Thus, at maximum modulation. the peak-to-peak grid swing on V2 is a2-b2, the corresponding average grid bias and plate current for V2 being EG3 and I103 respectively (for sine wave modulation). For some lower percentage of modulation the grid swing a1b1 will result in an average grid bias on GI of EG4 and a corresponding plate current in tube V2 of I114; With no modulating voltage the grid swing will be 0, the grid GI bias EG5 and plate tube V2 current Ip5. Note that the maximum positive grid swing for any moduiation level 'is EG5, i. e., the peak power from the radio frequency amplifier is constant.

Fig. 4 shows the effect of varying R2. In Fig. 3, as stated above, R2 is zero and conditions for changes in percentage modulation are shown, while in Fig. 4 percentage modulation is constant at 100% and conditions for different values of R2 are shown. The curves of Fig. 4 are similar to those of Fig. 3, in many respects. For example, points EG5, O and Ip5 (maximum) corresponding to the zero signal conditions of Fig. 3. The difference is mainly in the fact that the locus of the maximums of the positive grid GI swings is no longer a straight line but the curve EG2, bI, 0 instead, since R2 now limits the maximum modulator tube VI plate current. I have shown curves for four values of R2;

For any curve, say a4O1b2, the no signal plate current, Ip can be found by projecting the point 0 to the Ip axis and in the same manner projecting 0 to the EG axis gives the corresponding value of EG. The maximum modulation swing, i. e., grid swing, is 113-431. The average values of Ip and EG are found by projecting C2 as before.

Thus R2 does effectively change the modulation range, the maximum range being obtained for R2=0 for any condition of R. F. amplifier loading and excitation.

Thus with R2 omitted maximum radio frequency output is always obtained for zero or no modulation. Now modulation of the transmitter can only be down from this value, and maximum direct current insertion is obtained, that is, full direct current is amplified through the entire circuit.

When modulation is applied to the grid G of tube VI this varies the plate resistance of the tube VI between infinity and a low value to change the distribution of voltage across thetube VI and R2 to lower the average bias on thegrid of V2 and modulation takes place around this average bias.

The arrangement is such that modulation or control of V2 in accordance with the modulation from source I6 is linear over a very wide range. With R2 inthe circuit the maximum current in VI and hence the voltage on the grid of V2 is limited by R2. That is, for positive modulation swings the bias on the grid G2 which then swings in the positive direction is limited by the value of R2. As to the negative swings of the modulation the bias on the grid of V2 then swings negative (impedance of VI increases) and the negative swings of the grid of V2 are limited'by the impedance to which VI can be raised as well as the maximum bias on V2 in the absence of VI. This is EG2 and is a function of the radio frequency excitation on V2.

The function of CI is the same irrespective of whether the initial bias on the grid of GI is set by varying R2 or by use of the proper direct current potential on the anode of VI. CI may have any value, keeping in mind that CI is for the purpose of adjusting the alternating component of the current in the grid GI circuit of V2 and as a consequence, the potential on the grid GI. CI controls the gain of the modulation amplifier VI with respect to the alternating current component of the modulation, and is in series with the alternating current output of the modulator tube VI. As the value of CI is decreased the gain of the modulator decreases at the low frequencies. The low modulation potential frequencies are then suppressed or attenuated relative to the higher modulation frequencies. By making CI larger the modulator characteristic may be made fiat. If a very low value of CI is used, say 10 to 20 mmfd. high frequency peaking is obtained in a video system.

In the embodiment of Fig. 2, I use a pushpull amplifier stage comprising tubes V2 and V2 and the modulator stage VI. In this modification the arrangement and operation is substantially as described above in connection with Fig. 1. In Fig. 2, reference characters as used in Fig. 1 or the same primed are used to designate the various circuit elements and it is believed that a detailed description of the arrangement of Fig. 2 is unnecessary. It will be noted that-the description of Fig. 1 given above applies fully to Fig. 2, when the pushpull stage V2V2 is considered as a single stage.

In both modifications I have shown the radio frequency stage input and output circuits as comprising windings. In practice these circuits may comprise windings as shown or lines or conductors disposed and coupled as desired.

Fig. 2 illustrates schematically an embodiment which gave good results. The tubes V2 and V2 were of the type 8025, and had their cathodes coupled by linear conductors (represented at LI, Fig. 2) associated with linear conductors (represented at L5, Fig. 2) connected to the anodes of a two-tube buffer stage, in turn somewhat similarly coupled to the anodes of generator-tubes all of which may be included in unit II]. The radio frequency input is to the cathode circuit which in the embodiment using lines is a more or less aperiodic coupling system. The linear lines represented at LG, between the anodes of the tubes in the buffer stage, which it may be assumed is included in the rectangle I are tuned by a short circuiting strap to the desired operating frequency. In the embodiment tested the buffer stage tubes are of the 8025 type. The cathode line represented by LI in the input to the power amplifier stage including tubes V2 and V2 is set for the desired excitation condition by means of a slider on the line, and is not thereafter tuned and in this respect the coupling of the power amplifier to the preceding stages may be considered aperiodic. The circuit including L6 and the circuit L304 covers a range from 264 me. to 372 megacycles, the experimental data being taken at 312 me.

The radio frequency impedance from grid GI to grid GI is very small and the stage is of the grouhded grid type. The couplings at L3 and L! are also lines. The coupling arrangement is as described more in detail in my U. S. application #525,542, filed March 8, 1944, now Patent Number 2,419,793.

The tube VI is of the type 6V6 with its anode and screen grid tied together and connected to the resistance R2 of 15,000 ohms and to RI of 4.7 megohms. C6 is 1500 mmfd. and CI is 20 mfd.

The normal operation, as described above, for carrier output and no modulation resulted in the application of 60 volts to the grids GI and GI, with 750 volts direct current on the anodes of tube V2 and V2", and with a power output of 25 watts. Under the same conditions the cutoff bias for grids GI and GI was found to be volts. With the grid circuit of V2, V2 adjusted as described in the preceding sentences open, the grid bias would not go down to -90 volts but goes to approximately --85 volts, thus giving an output for this stage of 4 or 5 watts. This means modulation will never reach My improved modulator arrangement including tube VI was then added and adjustments made, as described above, by means of R2 and/or the direct current plate source for VI, so that the voltage on G! and GI was again 60 volts. R2 then had a value of 15,000 ohms. C! was equal to 20 mfd. This gave a fiat gain characteristic for tube Vl over the video range. The modulation input on the grid of VI was video signals, obtained by scanning a subject, combined with synchronizing voltages. A full modulation radio frequency output of 30 watts was obtained and a minimum or negative peak modulation radio frequency power output of 4 watts was obtained.

A 4 to 1 improvement results from the use of my improved modulation system. Compared to the power needed to be supplied, the modulator tube in a conventional alternating current coupled circuit, this system requires only about A; as much input, 1. e., the modulator efficiency can be about 4 times as great as the conventional system.

In.- the embodiments illustrated a grounded grid radio frequency amplifier stage is shown, but any other type radio frequency amplifier stage may be used. That is, instead of grounding the grid for radio frequency potentials at condenser Cg and running the cathode at high radio frequency potential any conventional grid excitation arrangement may be used, such as, for example, one wherein the radio frequency input is between the grid and ground, with the cathode grounded for radio frequency.

I claim:

1. In a modulation system, an electron dis'-,

charge radio frequency amplifier tube having a control grid, and means for modulating the potential on said control grid including, a source of modulating potentials, a modulator tube having a control grid and output electrodes includ, ing an anode and a cathode between which an electron discharge takes place, a coupling between said source of modulation potentials and the control grid of the modulator tube, a circuit including a source of direct current potential connecting the output electrodes of said modulator tube between which said discharge takes place, to the control grid and cathode of the amplifier tube to apply to the control grid of said amplifier a corn trollable negative bias, a resistance in a direct Z current circuit between the anode of the modulator tube and the control grid of said modulator tube to set up on the control grid zero bias in the presence of Zero modulating potentials, and an impedance in series in said circuit including said direct current source for relating the bias on said grid of said amplifier tube to zero bias on said modulator tube grid.

2. In a modulation system, a radio frequency amplifier tube stage having a grid electrode and a cathode, and being arranged to amplify radio frequency energy, a modulator tube having an output electrode, a cathode and a control grid, a series circuit including a resistance, a source of direct current potential, the output impedance of the modulator tube and the grid to cathode impedance of said amplifier tube stage, said resistance and source of direct current potential being dimensioned to set up a negative potential on the' rid of said amplifier tube such that carrier output is derived from said amplifier tube when zero potential is applied to the control grid of said modulator tube, connections for applying modulating potentials to the control grid of said modulator tube to control the impedance thereof 8 and thereby control the bias on the grid of said amplifier tube stage, and an alternating current path in shunt to said resistance for relatively attenuating modulation potentials of a selected frequency range.

3. In a modulation system, a radio frequency amplifier tube stage having a grid electrode and a cathode, and being arranged to amplify radio frequency energy, a modulator tube having a control grid, a cathode and an anode, a direct current circuit including in series a resistance, the anode to cathode impedance of said modulator tube, a source of direct current potential and the grid to cathode impedance of said amplifier tube stage, the source of direct current potential and resistance being of such dimensions that a negative potential is applied to the control grid of said amplifier tube, connections for applying modulating potentials to the control grid of said modulator tube to control the impedance thereof and thereby control the bias on the grid of said amplifier tube stage, and a biasing resistance connecting the anode of the modulator tube to the control grid of the modulator tube to relate negative bias on the grid of the amplifier tube at which carrier output is obtained from the amplifier tube to substantially zero modulation on the control grid of the modulator tube.

4. In a modulation system, an electron discharge amplifier tube having a control grid, and output electrodes including a cathode, an output circuit coupled to said output electrodes, a connection to the cathode electrode of the amplifier tube for exciting said cathode electrode by volt ages of carrier wave frequency to be modulated, a modulator tube having electrodes between which an electron discharge takes place, a source of modulating potentials coupled to said modulator tube to control the discharge therein between said electrodes, a direct connection between the cathode of the modulator tube and the control grid of the amplifier tube, a source of direct current potential connecting the anode of the modulator tube to the'cathode of the amplifier tube, said source of direct current potential being of a value such that the potential on said grid oi said amplifier tube provides normal carrier output therefrom in the presence of zero modulation.

5. Elfin a modulation system, an electron dis charge amplifier tube having a control grid, and output electrodes including a cathode, an output circuit coupled to said output electrodes, connections to electrodes of the amplifier tube for exciting the said electrodes by voltages of carrier wave frequency to be modulated, a modulator tube having electrodes between which an electron discharge takes place, a source of modulating potentials coupled to electrodes of said modulator tube to control the discharge therein between said electrodes, a direct connection between the cathode of the modulator tube and the grid of the amplifier tube, a source of direct current potential connecting the anode of said modulator tube to the cathode of said amplifier tube, the arrange ment being such as to apply a biasing potential to said control grid, a resistance in said direct current circuit of such a value that the biasing potential on said grid of said amplifier tube provides a selected power output from said amplifier tube in the carrier output condition, and a condenser in shunt to said resistance.

5. In apparatus for modulating radio frequency current in accordance with control potentials, an electron discharge amplifier tube having a control grid, and output electrodes including a cathode, an

output circuit coupled to said output electrodes, a source of radio frequency current coupled to the cathode electrode of said amplifier tube to excite the same by radiofrequency currents to be modulated, a modulator tube having electrodes between which an electron discharge takes place, a source of control potentials coupled to electrodes of said modulator tube to control said discharge, a direct current circuit including a resistance and a source of direct current potential connecting the electrodes of said modulator tube between which said discharge takes place in series with the control grid to cathode impedance of the amplifier, tube to apply operating bias to said control electrode, and a resistance between the anode and control grid of the modulator tube for adjusting the discharge through the said modulator tube to establish a selected bias on the control grid of the amplifier for zero modulation, the resistance and source of potential being such that the control grid of said amplifier tube is biased to provide the carrier output from the amplifier tube at zero modulation.

7. In apparatus for modulating radio frequency current in accordance with potentials covering a wide band of frequencies and using during said modulation the desired amount of the direct current component of the said potentials, a discharge amplifier tube having a control grid, and output electrodes including a cathode, an output circuit coupled to said output electrodes, a source of radio frequency current coupled to electrodes of said tube to excite the sameby radio frequency currents to be modulated, a modulator tube having a cathode and an additional electrode between which an electron discharge takes place, a source of modulation potentials coupled to electrodes of said modulator tube to control said discharge, a direct connection between the cathode of said modulator tube and the control electrode of said amplifier tube, and a variable resistance and a source of direct potential connecting the additional electrode of said modulator tube to the cathode of the amplifier tube.

8. In apparatus for modulating radio frequency 45 current in accordance with potentials covering a wide band of frequencies and using during said modulation the desired amount of the direct current component of the said potentials, a discharge amplifier tube having a control grid, a cathode, and an anode, an output circuit coupled to said anode, a source of radio frequency current coupled to electrodes of said tube to excite the same by radio frequency currents to be modulated, a modulator tube having input electrodes including a control grid and output electrodes, including an anode and cathode, a source of control potentials covering a wide band of frequencies coupled to the input electrodes of said modulator tube, a direct connection between the cathode of the modulator tube and the control grid of the amplifier tube, a -variable resistance and a source of direct current potential in series connecting the anode of the modulator tube to the cathode of the amplifier tube, a condenser in shunt to said resistance, and asecond resistance connecting the anode of the modulator tube substantially directly to the grid offthe modulator tube.

9. Anarrangement as recited in claim 1, wherein said impedance is shunted by a condenser which relatively attenuates modulation potentials in a selected frequency range.

10. In a modulation system, an electron discharge amplifier tube having a control grid, and output electrodes including a cathode, an output circuit coupled to said output electrodes, connections to electrodes of the amplifier tube for exciting the said electrodes by voltages of carrier wave frequency to be modulated, a modulator tube having electrodes between which an electron discharge takes place, a source of modulating potentials coupled to electrodes of said modulator tube to control the discharge therein between said electrodes, a direct connection between the oathode of the modulator tube and the grid of the amplifier tube, a source of direct current potential connecting the anode of said modulator tube to the cathode of said amplifier tube, the arrangement being such as to apply a biasing potential 5 to said control grid, and a resistance in said direct current circuit of such a value that the biasing potential on said grid of said amplifier tube provides a selected power output from said amplifier tube in the carrier output condition.

CHARLES A. ROSENCRANS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,684,012 Appleby Sept. 4, 192B 1,800,536 Jobst Apr. 14, 1931 1,859,024 Buschbeck May 1'7, 1932 1,935,342 Zetelzky Nov. 14, 1933 1,999,892 Buschbeck Apr. 30 1935 2,101,438 Lindenblad Dec. '7, 1937

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