US2446025A - Modulation system - Google Patents

Description

July 27, 1948. R. J. ROCKWELL 2,446,025

MODULATION SYSTEM Filed May 10, 1946 2 Sheets-Sheet l Fi l L.V. Rectifier Mciin HM Rectifier Fi 2A Fig. 28 Fig. 2c Fig. 5A

Fi 3A Fi 38 Fi 30 Fig. 55

Ronald J. Rockwell INVENTOR.

/w Fig. 4A Fig. 4B

. July 27, 1948. J RQCKWELL I 2,446,025

MODULATION SYSTEM Filed May 10, 1946 2 SheetsSheet 2 Necessary Stages Ronald J. Rockwell INVENTOR.

Fig.6 3%4904/ Patented July 27, 1948 UNITED STATES PATENT OFFICE MODULATION SYSTEM Ronald J. Rockwell, Cincinnati, ."Ohio,1 assignor, by mesne assignments, to Avco Manufacturing Corporation, a corporationof Delaware Application May 10, 1946, Serial No. 668,699

26 Claims. (01. 179-4715) This invention relates to the amplitude modulation of carrier frequencies and isparticularly concerned with wide band class B plate modulation of a radio frequency carrier using a reactance coupled modulator.

The invention includes a system for achieving wide band class B plate modulation of the final class C "stage of 'a radio transmitter, and permits the use of over-allrectified inverse feedback. With this system a large amount of inverse: feed-back may be applied with essentially full eifect over a band'extendin from approximately to 50;000cycles, with less than 1% dis tortion over the entire transmission band and withnoise-content'of less magnitude than 60jdb.

below 100% modulation.

The many problems involved in the design of amplitude modulated transmitters capable of true high fidelity are familiar to persons skilled 'in'the art. Generally, the most troublesome element in such-a system is the modulation transformer, the inherent eharacteristicsof which limit the system in most cases to uniform gain over a range of from about 20 to 10;000 cycles. Above approximatly 10,000 cycles it is ordinarily mandatory that little if any over-all feed-back be permitted in order to avoidoscillation or singing at some higher frequency.

Various corrective methods have been proposed, the most commonly used being the application of audio frequency feed-back from the modulator plates back to some low level stage, with proper isolation means to remove the effects of the modnlation transformer at the modulator plates. With this arrangement very effective distortion and noise reduction can be realized :over the entire passband at the output of the modulation transformer. These advantagescan-no't be fully utilized at the output of the transmitter, however,

since the class C output circuit follows the-output of the modulation transformer and therefore thefull magnitude of distortion and noise arisvolves the phase rotation and othereffects inherent in the modulation transformenit i'snecessar-y ":to I render the over-all feed-back circuit inoperative above a very *few thousand cycles, gen erally well:within the ipassub'andof the system. "pins means. that high' frequncy distortion and noise occurring within the class C equipment iremain essentially uncorrected, making it virtually impossible to achieve high fidelity. i

It has been demonstrated that certain -types' of audio transformers can be used in amplifiers incorporating large amounts of wide ban'dfee'dback provided provisions are incorporated to eifectively remove the transformer from the circult and by-pass it, at frequencies somewhat above the upper limit of the pass band, assuming of course that the transformer does no'tproduce serious phase rotation within the pass band. Various methods are effective in so removing the transformer at these higher frequencieabutthese only permit the isolation of double endto double end orsingle end 'tosingle endtransformers. To date no effective method has been discovered for correcting thatjpart of the circuit within the feed-back loop which incorporates the modulation transformer, which of necess'itymust be double end"to "single "'en'd', since no "known new ing means will result in the desired'amplifier 'operation at frequencies above which the 'trans- Corrective proposals have been suggestedfl'in which two or even three modulation transformers, each differing in design for progressively in creasing frequency bands, are selectively inserted into the system by appropriate networks "in order to extend the range over which safe conditionsof phase rotationand amplifiergaindbtain. Such systems are unduly complex and in: volvesomanydifficulties that they are impractical.

I have discovered thatit is possible toflprovidea system of the type described which eliminates the modulationtransformer but" whichretains the high electrical efficiency required in 'thefmod ulator output circuit "and which is capable'of proper frequency response and cutoff conditions over a frequency band of several hundredthou= sand cycles. The provision of such a system constitutes one of the important objects of my invention. i

The novel features that 'I consider-characteristic of my invention are set forth in the appended claims; the invention, itself; however, bothas to its organization and its method of operation, together with additional objects and advantages 3 thereof, will best be understood from the following description of a specific embodiment when read in conjunction with the accompanying drawings, in which- Fig. 1 illustrates a circuit according to my invention, for wide band class B plate modulation, employing rectified radio frequency inverse feedback; Figs. 2A to 20 illustrate oscilloscope traces of output wave forms under varying conditions, showing how I equalize the magnitudes .of the positive and negative output waves of the respective output tubes shown in Fig. 1; Figs. 3A to BC show the same wave forms shown inFigs.

' .former H and into the usual amplifying stages 2A to 2G when feed-back according tothe present invention has been applied; Figs. 4A and 4B show the effect of improper adjustment of the phase control shown in Fig. 1; Figs. 5A and 5B show different forms of distortion that are commonly encountered in improper design of feed-back systems, said distortion being in the form of oscillation or singing on a kc. wave; Fig. 6 shows frequency response, noise level and distortion curves; Fig. '7 illustrates a circuit for obtaining 100% modulation by returning the modulator and auxiliary driver cathodes to a negative low voltage supply.

Briefly describing the invention with reference td-Fig. 1, modulator tube l5, operating as a class B amplifier (with its grid statically biased substantially to cut off) supplies negative half waves to the class C load, to which its output or anode circuit is directly connected as indicated by the arrow 39. Modulator I5 is fed with plate currentthrough reactor [8 which also supplies current to the class C stage. Modulator I! is similarlyoperated as a class B amplifier, but supplies positive half waves to the class C load through coupling condenser 20, the cathode of modulator l1 being grounded through reactor I9. Thus, I obtain double end (sometimes called pushpushfl to single end operation without the conventional coupling transformer.

.It should also be noted that the grid of modulator I5 is driven in conventional manner from driver l4, and the grid of modulator I l, which ishighly degenerative, being a cathode follower, isdriven from driver [6 which derives its plate supply from the A. C. output of the modulator stages at point 22. Thus, the plate supply at driver I6 is obtained from a voltage source which at the time modulator I l is delivering its positive output wave, is actually higher than the voltage obtainedfrom the high voltage supply. E2. In other words, tube l6, at the time modulator I! is delivering its positive output wave, requires a higher plate voltage supply than can be derived from plate supply E2 and it is therefore connected at 22 in order to obtain the additive effect of the modulator output and the voltage supply E2. It is well known to those skilled in the art that the cathode of a cathode follower tube becomes more positive as its grid becomes more positive. In order to obtain a wide positive swing on the grid of tube 11, I, connect the anode resistor of tube l6 to point 22, so that the effectiveplate potential of driver [6 is increased in a. positive direction as the cathode of tube I! goes positive. In this manner, I increase. the positive swing on the grid of tube ll. Conversely, when tube I5 is delivering its negative output wave, capacitor 20, being connected between point 22 and the cathode of tube ll, applies a negativesignal to the cathode of tube II. This factor tends to render tube l1 conductive at a time when it should be non conductive. However, tube H is held out and circuits I2 and 13, only the first-tube Ha of which is shown, to a driver tube M which drives modulator tube IS. A second modulator I! is connected as a cathode follower and is driven by driver Hi whose grid is driven in phase with the grid oftube i5. Tube Hi is arranged as a phase inverter.

Grid excitation of tube l6 need not be obtained from the output of tube H but may be derived from any appropriate source of proper'phaserelation and voltage magnitude. Tubes I and it constitute push-pull-output signal translating means for driving tubes [5 and I1. Grid bias is applied to the modulator grids from voltage supply E1 through resistors 3| and 32 as shown and plate voltages are applied from voltage supply E2. Air core reactors la and |9a are incorporated respectively in series with reactors IB and i9 in order to obviate iron circuit losses, capacity efiects or resonance effects at frequencies well above the pass band. Resistors may of course be used in place of these reactors if the inherent power losses can be tolerated.

The output of the modulators is taken off at terminal 22 and applied to the class C load at 36 in the form of modulated D. C. It is preferable that the class C load be connected directly to point 22 although it is possible to connect it indirectly by the use of suitable isolating means, in which case the plate voltage for the class C load might be supplied independently of E2. Rectified R. F, is fed back from the class output in known manner through line 33 and, antiesing filter 34 to the grid of the first tube [2a, it'being understood that the feed-back circuit is carried through the secondary 35 of transformer H. A by-pass capacity 36 is provided for minimizing the phase rotation efiects of the secondary winding 35. One or more anti-sing filters may of course be included in preliminary stages .12 or I3.

As previously indicated, the plate supply voltage of tube It is not obtained wholly from a steady D. C. source in the conventional manner, but instead is connected to the modulator output at 22. Thus, during the negative gridcxcursions of driver l6 its positive, plate excursion is augmented by the dynamic positive excursion at point 22, thus producing a positive bias excursion at the modulator tubev ll, actually in excess of the positive cathode excursion of modulator IT. This requirement is made necessary because tube I1 is highly degenerative and therefore requires a positive grid excursion with respect to ground Which is substantially equal to its cathode-excursion plusits grid bias. If desired, the plate voltage on tube [6 may be derived from a separate source of high voltage substantially in excess of Eawbut this would involve sacrifice in utilizing the full benefits of the invention. Thus it will be seen that I have provided a cathode follower tube I! having an un-bypassed reactive cathode impedance l9, l9a, an output reactor [8, 18, a capacitor 20 for coupling said impedancev to said output reactor, and a network 22, 2|, 30, .32 for regeneratively coupling said reactor to the input of said tube.

Resistonfl is an equalizing resistor inserted in thecathodecircuit of driver IS in order to adjust the degeneration of the tube and equalize or balancethe output waves appearing in the output; circuits of modulators i5 and i1. It should benoted that the only bias applied to tube It is through resistor 43 in'its cathode circuit. This resistor has an adjustal'aleeondenser 23 in shunt therewith for adjusting the phase relation of the output waves of tubes l5 and H 5 Resistor isa grid return resistor for tube Hito keep the grid circuit closed. Resistors 38-and 21' are plate circuitresistors conventionally used in resistance coupled amplifiers. Reactors may replace these resistors provided adequate antising precautions are followed. Resistor 38a is a conventional screen grid resistor to insure propor voltage on the screen grid of driver M. The circuit contains the usual blocking capacitors 28, Hand 30, cathode by- pass capacitors 44, 45 and screen by-passcapacitor 46,

' The constants of the circuit of Fig. lshould be designed as far aspracticable in accordance with television practice for wide band operation. The following components and values are illustrative of what may be used in the novel modulation system described.

I Vacuum tubes it -6 l6- -6L6 le -A3150 17--AB150 Other components 18:30 h. a -1 m1". lint-.918 mh. 301 mf. I I I 19.-30 h. I 31--100,000 ohms Illa- .918 mh. 32--100,000 ohms 20 -30 mt. 36.0001mf.

25 10,000 ohms 44-50 f.

27 150 ohms 45-50 mf. 4

28:45 mf. 46-.5 mf. Voltages iii-+750 E1-- 140 The foregoing data were obtained froma low. r power set-up in which the principles andopera tion ofjthe invention were embodied. Figs. 2A, 2B and 20 represent oscilloscope traces of an output wave without feed-back for:

three conditions of adjustment of. cathode resistance 63 of driver [6 showing how the amplitudes of the positiveand negative waves may be equalized. In Figs. 2A and 20 the results of im proper adjustment are shown. Anexcess bias; was put on tubes I5 and I! for the purpose of emphasizing' the cross-over points. T,

Figs. 3A, 3B and 30 represent the same waves.

under identical conditions except with inverse} romeo wave: I I Figs; 5 and 5A show typical distortion of the output waves with improper filter constants in the feed-back: or amplifier circuits, or both. in

each figure a 20 kc. fundamental wave is shown;

kc. with feed-back applied. Noise level was constant at'about 68 db. and distortion was approximately constant at less than of 1% up to 10,000 cycles, which was the limit of the meas u-ring equipment. The curves of Fig. 6 were ob-' tained from data taken on the set up-described in detail above.

For 100% modulation it is necessary to operate the modulator tubes at a plate voltage higher than that used for the class C amplifiers. This can be accomplished by returning the ground end of the modulator circuit to a positive grounded low Voltagerectifier. A circuit for this purposeis shown in Fig. '7, which issimilar to Fig. 1 with thefollowing exceptions. I

Instead of connecting the cathodes of tubes l5 and I! to ground they are returned to an intermediate tap -C' of the bias rectifier E1; the full tap-C of which supplies bias to the modulator I 100% modu lation is thus obtained at substantially the same grids through a potential divider.

rectifier power as required in a conventional circuit, since the increased demand from the low voltage or bias rectifier is of the same order of magnitude as the decreased demand from the;

main high voltage (plate) rectifier. This decrease is brought about by the improved modulator efficiencies at higher plate voltages and by the elimination of modulation transformer losses.

It will benoted that the cathode return of tube f6, shown grounded in Fig. 1, is now connected at point 4| with point 40 in order to effectively increase the plate voltage on tubes l5, l6 and i1. Thus, the voltage betweenthe modulator plates and cathodes is increased by an amount substan tially equal to that effected by the turns ratioof the modulationtransformer that would otherwise have been necessary in order to achieve 100% modulation.

It will also be noted that 100% modulation may be obtained with the circuit of Fig. 1 either byoperating the class C tubes on an. additional rectifier o f lowervoltage than E2, or by inserting a resistance inseries with the load 39 and by-passing it for A.C. These measures, however, entail very considerable additional capital investment in high power equipment, lower efliciencies and concomitant higher operating costs.

Having thus described my invention, I claim:

1. A modulation system comprising an input voltage amplifier, a driver, a negative half cycle modulator, anoutput circuit for said modulator,

a load, a conductive connection between said output circuit and said load, a second driver having a grid connected to the output of said first driver and consistingof a phase inverter, said second driver having its plate conductively connected to said output circuit, a positive half cycle modulator, having a cathode circuit, said second driver being connected to and driving said positive half cycle modulator, an inductive reactance in said cathode circuitacros's which the output voltage of said positive half-cycle modulator is developed,

and a source of negative voltage, the cathodes of said modulators and said second driver being" connected to said source I for supplying cathode voltage to said modulators and said second driver,

said second driver having a cathode circuit ineludingv a resistance-capacity net-work for adjusting the amplitude and phase of the output of the positive half cycle modulator with respect to that of the negative half cycle modulator.

2. A modulation system. comprising an input voltage amplifier, a driver, a modulatonan output circuit for said modulator, a load, a direct connection between said output circuit and said load, a second driver having a grid connected to the output of said first driver and consisting of a phase inverter, said second driver being connected directly to said output circuit, a second modulator, said second driver being connected to and driving said second modulator, said second modulator supplying half cycles of voltage to said load opposite in phase to those supplied by said first modulator, said second modulator having a cathode circuit, an inductive reactance in said cathode circuit across which its output voltage is developed, and a coupling capacity connecting the cathode of said second modulator to said load, the said output voltage being fed to said load through said capacity. 7

v-3. A modulation system comprising a first modulator, a second modulator, a load, a resistance-coupled driver for said first modulator, said driver having a plate, said first modulator having its plate output directly connected to said load, said second modulator having a cathode circult, an inductive reactance in said cathode cir-V 4. A modulation system comprising a first mod- 1 ulator, a second modulator, a load, a resistance coupled driver for said first modulator, said driver'having a plate, said first modulator having its plate output directly connected to said load, said second modulator having a cathode circuit, an impedance in said cathode circuit, across which an output voltage is developed, a capacitive reactance connecting said second modulator to said load for impressing said output voltage thereon, a second resistance coupled driver for said second modulator, said second driver having a grid, a connection between the plate of said first driver and the grid of said second driver, and connections between the output of said modulators for impressing a plate voltageon said second driver, a source of negative voltage, and a connection thereto from the cathode returns of said modulators and said second driver.

5. In a modulation system, a driver, a modulator, an output circuit connected to said modulator and a load, a direct connection from said output circuit to said load, a second modulator, a second driver coupled thereto and a common source of plate voltage for said modulators and said second driver, whereby the said second driver has impressed thereon a plate voltage including all the AC. components in the plate circuit of the output of said modulator.

6. In a'modulation system, a driver, a modulator having an anode circuit, and a load, a direct connection from said anode circuit to said load, a second modulator having a cathode circuit, a second driver coupled to said second modulator, a'common source of plate voltage forsaidmodulators and said second driver and means ulator, and a load, a direct connection from said.

' output circuit to said load, a positive half wave modulator, capacitive coupling means from said positive half wave modulator to said load, and afeed-back connection from the output ofv said.

load to said input voltage amplifier.

9. A modulation system comprising the tam-1 bination of a first and a second modulator, drivers therefor for driving same, an output circuit for said modulators, and a load, a capacitively reactive coupling between the output 'circuitof the second modulator and said load, and a direct connection between the output circuit of the first modulator and said load.

10. A modulation system comprising the com: bination of an input amplifier. and a first and a second modulator, drivers therefor for driving said modulator, an output circuit for said modujlators, and a load, a capacitively reactive coupling between the output circuit of the second modulator and said load, a direct connection be: tween the output circuit of the first modulator and said load, and a feed-back connection between the output of said load and said input voltage amplifier. I j 11. A modulation system comprising the combination of first and second half-wave modulators, an output circuit therefor, and a load, a

capacitive coupling between the output of the' second of said modulators and said load, and a direct connection between the first of said modulators and said load.

12. A modulation system comprising the combination of opposite first and second half-way modulators, an output circuit therefor,- and a load, a capacitive coupling between the output of the first of said modulators and said load, a direct connection between the output of the second of said modulators and said load, and a feed back connection from the output of said load to the inputof said system."

13. A signal translating circuit comprising in combination with a load, means for producing positive and negative half wave output voltages, and means for applying the same to said load, the last mentioned means consisting of a direct connection from the output of the negative half wave voltage producing means, .and a capacitive connection from the output of said positive half wave voltage producing means.

14. A modulation circuit comprising in combination with a load, means for producing posi tive and negative half wave output voltages, and means for applying the same to-said load, the last mentioned means consisting of a direct connection from the output of the negative half wave voltage producing means, a capacitivecon nection from the output of said positive half, wave voltage producing means, and a feed-back connection from the output of said load to the input of said circuit' V I 15. A system for modulating a carrier wave comprising a pair of vacuum tubes for supplying a, modulating voltage, means for supplying the negative half wave of said voltage from the plate of one of said tubes, and means for sup-plying the positive half-wave of said voltage from the oath.- ode of the other of said tubes.

16. A system for modulating a carrier wave comprising a pair of vacuum tubes for supplying a modulating voltage, means comprising a conductive connection for supplying the negative half wave of said voltage from the plate of one of said tubes and means comprising a capacitive connection for supplying the positive half-wave of said voltage from the cathode of the other of said tubes.

17. In a, plate modulation system. a class C amplifier having a plate circuit and a pair of vacuum tubes for supplying modulating voltages thereto, means comprising a direct connection from the plate of one of said tubes to said plate circuit for supplying thereto half of the modulating Wave, and means comprising a capacitive connection from the cathode of the other of said tubes to said plate circuit for supplying thereto the opposite half of the modulating wave.

18. A signal network comprising a pair of electron tubes so arranged that the anode circuit of one of said tubes and the cathode circuit of the other of said tubes are coupled to a common output, and means for returning the cathodes of said tubes to a common point.

19. A signal network comprising a pair of vacuum tubes, one of which has an impedance in its anode circuit and the other of which has ,an impedance in its cathode circuit, said impedances being in D. C. parallelism, and common output circuit means for the alternating current signals developed in said impedances.

20. A signal network comprising a pair of vacuum tubes, an impedance in the anode circuit of one of said tubes between its anode and one terminal of a space current source common to both of said tubes, the junction of said impedance and said anode constituting an output terminal, another impedance in the cathode circuit of the other of said tubes between its cathode and the other terminal of said space current source, the junction of said impedance and said other terminal constituting the other output terminal,

and means for coupling said other impedance to the first-mentioned output terminal.

21. The combination of a cathode follower having an impedance in its cathode circuit whereby its cathode tends to follow its grid and to develop a cathode bias of positive polarity in response to applied signals of positive polarity, a driver tube having an output circuit coupled to an input circult of said cathode follower tube, a common space current source directly coupled to the anode of said cathode follower, a load including an A.-C. impedance for coupling said space current source to said driver tube anode, and means for increasing the anode potential of said driver tube comprising means for coupling the first mentioned impedance to the second mentioned or A.C. impedance whereby in response to A.C. signals of positive polarity applied to said input circuit, the A.C. signals developed across said first-mentioned impedance are applied to said anode in addition to the potential available from said source, thereby to increase the potential applied to the input circuit of said cathode follower over the potential which would be supplied by said driver when the 10 driver anode is supplied by said space current source only.

22. A signal network comprising a pair of vacuum tubes for supplying a voltage, coupling means for supplying a part of said voltage from the anode circuit of one of said tubes and coupling means for supplying another part of said voltage from the cathode circuit of the other of said tubes.

23. A modulator comprising a pair of class B arranged vacuum tubes for supplying a modulating voltage, the cathode and anode of one of said tubes being individually coupled to output terminals, means for supplying one half wave of said voltage from the anode circuit of one of said tubes comprising a reactor in its anode circuit between one of said terminals and a space current source, means for supplying the other half wave of said voltage from the cathode circuit of the other of said tubes comprising a reactor in its cathode circuit between its cathode and the other of said terminals and a capacitor between its cathode and said one of said terminals, the anodes of both of said tubes being coupled to a common space current source, whereby the modulating voltage and the potential from said space current source appear across said terminals.

24. A signal translating circuit adapted to be employed in conjunction with a common space current source for modulating a radio frequency amplifier comprising the combination of a pair of vacuum tubes, one of which has an anode impedance and the other of which has a cathode impedance and means including a common output circuit for both of said impedances for coupling them to a load, the anodes of said tubes being coupled to a common space current source, whereby there appear across said output circuit the potential of said source and the output signals from said tubes, and means for applying to the cathodes of both of said tubes a. negative bias whereby the total effective potential impressed on the anode circuits of said tubes is greater than that appearing across said output terminals.

25. A modulating circuit comprising a pair of vacuum tubes and a pair of impedances arranged in a loop, one of said impedances being in the anode circuit of one of said tubes and the other of said impedances being in the cathode circuit of the other of saidtubes, the cathode and anode of said one tube being connected to output terminals, and means for coupling said other impedance to the output terminal connected to the anode of said one tube.

26. A cathode follower tube having an un-bypassed reactive cathode impedance, an output reactor, a capacitor for coupling said impedance to said output reactor, and a network for regeneratively coupling said reactor to the input of said tube.

RONALD J. ROCKWELL.

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

UNITED STATES PATENTS Number Name Date 2,085,011 Pitcham June 29, 1937 2,290,553 Haanties July 21, 1942 2,340,617 Sanford Feb. 1, 1944 2,401,573 Kozanowski June 4., 1946 2,402,148 Crosby June 18, 1946 2,428,295 Scantlebury Sept. 30, 1947

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