US2470063A

US2470063A - Modulator system

Info

Publication number: US2470063A
Application number: US523303A
Authority: US
Inventors: Henry T Winchel
Original assignee: Bendix Aviation Corp
Current assignee: Bendix Aviation Corp
Priority date: 1944-02-21
Filing date: 1944-02-21
Publication date: 1949-05-10
Anticipated expiration: 1966-05-10

Description

May 10, 19.49. H. T. WINCHEL' MODULATOR SYSTEM Filed Feb. 21, 1944 Driver Modulaiing Source INVENTOR. fikmyf WZ'ncfieZ ATTORNEY Patented May 10, 1949 UNITED STATEE t' ATENT OFFICE MODULATOR :SYSTEM Application February 21, 1944, Serial No. 523,303

4'Claims. 1

This invention relates to systems for the generation of modulated waves, and more partioularly 'tosuch va system in which the percentage of modulation is maintained substantially con' stant despite improper loading and variation in the anode supply potential.

In a grid leak biased radio frequency class C amplifier, With .constant excitation, the anode current and the alternating current anode impedance presented to the source of modulating voltage are considerably influenced by the magnitude of radio frequency load in the anode circuit of the amplifier. If, for some reason, the coupling'to'the load is decreased, the alternating current potential at the anode of the amplifier increases, causing an increase in the grid current thereby increasing the grid bias and plate impedance of the amplifier. As the modulating source has anappreciable impedance, this causes an increase in the .rnagn'itude of the modulating potentialat the anode and/or screen of the tube bringing about deeper modulation of the radiated signal. Further, should the direct current anode supply potential decrease while the modulator output remains constant the death of modulation will again increase. There are certain anplications in which it is desirable to radiate a tone modulated signal with a substantially constant percentage of modulation, but only unskilled operators are available for handling the equinment. "Under such conditions, it has been found impossible to secure satisfactory operation with the personuclavailable, as they frequently fail to proper'lyadjust the anodepotential and the loading reflected by the antenna circuit.

Accordingly, it is an object of this invention to provide new and novel radio transmitting apparatus in which the percentage of modulation is substantially independent of the coupling of the antenna circuit to the transmitter output circuit.

A further obiectof the invention is to provide new and novel radio transmitting apparatus in which the percentage of modulation is substantially independent of the anode voltage of the modulated stage.

.Still another object of the invention is to provide a modulating system in which the gain of the modulating potential amplifier is controlled by'variations'inthe'load impedance in "the anode circuit of the modulated stage.

Yet'another object of the invention is to provide a new'and novelrn-odulating system invvhich the modulating potentials applied to the modulated stage are "regulated in accordance with 2 variations in the load impedance in .the anode circuitof saidstage.

The above objects and advantages of the invention are substantiallyaccomplished.by acircuit arrangement in which the voltage .drop across .a resistor in .the anodesupply circuit of a plate and screen moduiatedclass C final radio frequency power.amplifieriisemployed to regulate the gain of an amplifier system supplying .the modulating energy .for said .final amplifier.

Other objects and advantages of the invention will in part be described and .in part beobvious when the following specification is .read in conjunction with the drawing which is a schematic diagram of transmitting iapparatus embodying the invention.

A source it of radio frequency energy having any desired vfrequency energizes the .primary vLl of the driver rcoupling transformer L2 and .is shunted by the split stator tuning capacitor .13 having .a grounded rotor. The source All will be hereinafter referred to as the driver. The center-tapped secondary winding M of transformer I2 is likewise shunted by a split stator tuning capacitor 15 and the center tap of winding It is connected to ground through thegrid leak resistor [6. In operation, the values iof capacitors l3, .15 are adjusted to resonate .the transformer 52 atthe operatingjfrequency ofthe driver [0.

The output voltage from winding I14 across capacitor l5 is'balanced toground and is applied to the control grids tl, It of the dual .radio'irequency power amplifier tube I 9 'havingtheheater Ell thermally associated .withthe cathodes 2l,,ji2'2.

Each of the sections of the tube 19 may be of the pentode type, and in the embodiment shown, the cathodes 21! and '22 are connectedtogether Within the envelope and thenceltogroundthrough the cathode resistor "23 shunted by the bypass capacitor 24. The suppressor grids 25 and '26 may also be internally connected to the cathodes of the dual sections, while the

space charge grids

21 and 23 are connected together and to the positive terminal of the direct current source 29 through the dropping resistor '39 and the secondary winding 3! of the modulation transformer 32. The direct current supply circuit is completed by the connection of the negative terminal of source 29 toground. Radio frequency potentials are eliminated from space charge grids 21,28 by the bypass capacitor'33 connected from the grid end of resistor 30 togroun'd. Theanodes Maud of thetube i=9 are connected to either end of the .center tapped primary winding 1316 of the radio frequency output transformer 31 whose center tap is also connected to the positive terminal of the source 29 through the secondary 3| of modulation transformer 32 and the parasitic suppressor including the parallel connected resistor 38 and inductor 39. Radio frequency potentials are eliminated from the winding 3! by the connection of plate bypass capacitor 48 from the tube side of Winding 3| to ground.

The winding 36 is tuned to resonance with the desired frequency in the output circuit of the tube [9 by the split stator capacitor 4! connected across the end terminals thereof and the radio frequency energy appearing in this portion of the circuit is transferred to the antenna 42 through the secondary winding 43 connected across the series connected variable capacitors 44 and 45. The junction of capacitors 44 and 45 is grounded and antenna 42 is fed from the ungrounded terminal of capacitor 44. As is well known, the antenna circuit is tuned to resonance by the combined action of capacitors 44, 45 and the loading reflected into the anode circuit of tube I9 is determined by the ratio of the capacitance of 44 to the capacitance of 45 in addition to the coupling coefiicient between the primary winding 36 and the secondary Winding 43. Since the coupling coefficient is constant when

windings

36 and 43 are fixed with respect to each other, the loading may be regarded as under the control of capacitors 44 and 45, decreasing as capacitor 44 is increased.

The system as thus far delineated operates in the conventional manner. The driving voltages on the control grids I1 and I8 cause grid current to flow, producing a voltage drop across the resistor l6 to provide a substantial portion of the operating bias for the power amplifier stage. Stray capacitances afford sufficient bypassing around resistor |6 when the apparatus operates at fairly high frequencies of the order of 10 megacycles per second. Upon tuning the anode circuit 36, 4| to resonance, the grid current and the grid bias increase, causing the anode current to decrease, with a resultant increase in the alternating current impedance presented to the secondary 3| of transformer 32. As the antenna excitation circuit including capacitors 44 and 45 is tuned to resonance, the anode current rises, the final value of the current depending upon the relative values of capacitors 44 and 45 being higher the greater the ratio of capacitance 45 to capacitance 44. These anode current changes are indicative of variations in the aternating current impedance viewed by the secondary winding 3|, which variations alter the modulating voltage delivered by winding 3! and change the percentage of modulation in the output of amplifier l9. It is clearly seen, therefore, that in the conventional system thus far described the percentage of modulation is, to a material extent, afiected by variations in the tuning and reflected load of the antenna circuit. This effect is eliminated by the additional apparatus now to be described.

The modulating source 46 is connected to the primary 4! of the audio coupling transformer 48 having the center tapped secondary winding 49. The center tap of Winding 46 is grounded through capacitor 58 and is connected to the cathodes 2|, 22 of power amplifier |9 through the series connected resistors and 52 whose junction point is also grounded by capacitor 53. The end terminals of winding 49 are connected respectively to the

control grids

54 and 55 of the push-pull connected

amplifiers

56 and 51 having the

anodes

58 and 59 connected to the end terminals of the center tapped primary winding 6|] of the audio coupling transformer 6|. The electron emission necessary for the operation of the

tubes

56 and 51 is provided by the

emissive cathodes

62 and 63 associated respectively with the heaters 64 and 65. There is also provided a voltage

divider comprising resistors

66, 61 and 68 connected in the order named between the positive terminal of source 29 and ground, the junction point of

resistors

66 and 61 being grounded by bypass capacitor 69, while the junction point of resistors 6'! and 68 is grounded by the bypass capacitor 16. The

cathodes

62 and 63 are connected together and to the junction of

resistors

61 and 68 and the center tap of winding 68 is supplied with anode excitation energy from the tap between

resistors

66 and 61. The tubes 56 and 5'! may be of the type havin a variable pitch grid winding, commonly known as variable mu tubes, and are arranged in push-pull to minimize the production of distortion during the control actions to be later described. Resistor 68 and the bleeder current of the associated voltage divider are proportioned to maintain the

oathodes

62 and 63 of amplifiers 56 and 5'! somewhat positive with respect to the cathodes 2| and 22 of the radio frequency power amplifier l9.

The output energy in the anode circuits of

tubes

56 and 51 appears in the center tapped secondary winding 1| of the transformer 6|, Whose central tap may be connected directly to ground and the end terminals of which are connected to the

control grids

12 and 13 of the modulation power amplifier 14. Amplifier 14 may also be of the dual pentode type having the

cathodes

15 and 16 connected together internally and to the suppressor grids 11 and 18, the combination being grounded through the bias resistor 19. The space charge grids 86 and 8! are also connected together and to the junction of the resistors 82 and 83' forming a voltage divider connected between the positive terminal of source 29 and ground. The grids 88 and 8| are maintained at ground potential for modulation frequency energy by the bypass capacitor 84 connected from these grids to ground. A heater 85 is provided for the elevation of the

cathodes

15 and 16 of the modulation amplifier 14 to the required operating temperature. The

anodes

85 and 86 of the power amplifier 14 are connected to the respective ends of the primary winding 81 of modulation transformer 32, said primary winding 81 being center tapped and the center tap thereof connected directly to the positive terminal of the source 29. The modulation energy appearing in the output circuit of amplifier 14 is transferred by magnetic coupling from primary 81 to secondary 3| to carry out the well known modulation process.

The operation of the complete system may now be easily understood.

Amplifiers

56 and 51 are voltage amplifiers of the controllable gain type amplifying the output of modulating source 46 and supplying this amplified energy to the modulation amplifier 14, whose output in turn appears in the secondary winding 3| of transformer 32 to modulate the potentials appearing on the

anodes

34, 35 and space charge grids 21, 28 of the radio frequency power amplifier l9. If for some reason the antenna system is detuned or decoupled from the anode circuit of amplifier I9, the anode current drops, decreasing the voltage across the resistor 23. This change in voltage is transmitted through the low pass filter including the resistors 5i and 52 to the

control grids

54 and 55 of the amplifiers 56 and 5'! causing them to become more negative with respect to the

cathodes

62 and 63, thereby decreasing the gain through this stage to reduce the modulating power appearing in the secondary winding 3| and prevent an increase in the percentage of modulation of the radio frequency energy in the tank circuit including primary 36 and capacitor 4|. Should the antenna load on the amplifier I9 increase, the reverse action occurs to again maintain the modulation percentage constant.

In the event that the voltage supplied by the source 29 drops, it is desirable to decrease the available modulation potential delivered from transformer 32 to prevent increase in percent of modulation or over-modulation of the radiated energy. This correction takes place in the following manner: A decrease in the anode potential applied to the amplifier l9 causes a decrease in the anode current flowing through resistor 23 with a corresponding diminution of the voltage drop thereacross. This change is again applied to the

control grids

54 and 55 of the

amplifiers

56 and 51 to bias them more negatively with respect to their associated cathodes and decrease the gain of the amplifier. As a result the modulating potential delivered to the power amplifier I9 is decreased in the right amount to maintain the percentage of modulation substantially constant. An increase in anode potential effects a similar correction of reversed sign.

While the heater elements of the various vacuum tubes have been shown in the drawing, the circuits for energization of same have been eliminated from the showing to simplify the presentation, it being understood that any of the many available circuits, parallel, series, etc., may be employed for the purpose.

It will be obvious that many changes and modifications may be made in the invention without departing from the spirit thereof as expressed in the foregoing description and in the appended claims.

I claim:

1. In apparatus for modulation of a carrier wave by a signal wave; a modulation amplifier tube having a cathode, a control grid and an anode; a source of carrier waves and means for applying them between said cathode and said grid; a load circuit coupled to the anode and cathode of said tube; a variable mu signalamplifier tube having a cathode, a control grid and an anode, the amplification of which increases with increasingly positive bias on its grid; a source of signal waves and means for applying them between said grid and cathode of said signal amplifier tube; a source of unidirectional anode potential and means connecting its positive terminal to the anode of said modulation amplifier tube; means negatively biasing the grid of said modulation amplifier tube to such extent that the average cathode current increases with increasing signal output; means intercoupling the anodes of said tubes for applying amplified signal waves from said signal amplifier tube to said anode of said modulation amplifier tube; resistance means connected between the negative terminal of said potential source and the cathode of said modulation amplifier tube for developing a potential on said cathode that is increasingly positive relative to said negative terminal with increasing cathode current; and means for applying said cathode potential to the grid of said signal amplifier tube to bias it; whereby variations in the cathode current of said modulation amplifier tube continuously produce corresponding variations in the amplitude of the signal wave.

2. Apparatus as described in claim 1 in which said means for biasing the grid of said modulation amplifier tube comprises a grid leak resistor.

3. Apparatus as described in claim 1 including means for maintaining the cathode of said signal amplifier tube at a substantially constant potential positive with respect to the negative terminal of said potential source.

4. In an anode modulated transmitter, a modulation amplifier tube having input and output circuits with means for supplying a carrier wave to the input thereof, and a signal wave source, the output of which is connected to the output of said modulation amplifier tube and to a load circuit, in combination with means for maintaining constant modulation despite variations in amplitude of the output wave of said modulation amplifier tube, said means comprising biasing means for so biasing the modulation amplifier tube that its average cathode current increases with increasing amplitude of output wave, a variable mu discharge device in said signal wave source, and means continuously responsive to variations in the unidirectional component of the cathode current of said modulation amplifier tube for varying in the same direction the amplification of said variable mu device in said signal wave source.

' HENRY T. VVINCHEL.

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

UNITED STATES PATENTS OTHER REFERENCES Radio, March 1938, Automatic Modulation Control, by L. C. Waller, pages 21-26 and 72.