US2795760A

US2795760A - Amplitude modulators

Info

Publication number: US2795760A
Application number: US334262A
Authority: US
Inventors: Edward C Dench
Original assignee: Raytheon Manufacturing Co
Current assignee: Raytheon Co
Priority date: 1953-01-30
Filing date: 1953-01-30
Publication date: 1957-06-11
Anticipated expiration: 1974-06-11

Description

June 11, I957 E. c. DENCH 2,795,760

AuPLxiunE MODULATORS Filed 30, 1953 3 Sheets-Sheet 1 SOLENOID ,PRODUCING AXIAL MAGNET/C FIELD 7H? SUPPL IMODULAT'OR H54 Y F76. 4 Mow /NVENTOR LAT/ON nvpur SIGNAL EDWARD C. DENCH June 11, 1957 E C, BENCH 2,795,760

AMPLITUDE MODULATORS Filed Jan. 30, 1953 3 Sheets-Sheet 2 MODULATION IIV U T SIGNAL & l6

F'IG. 5 MODULATOR 6mm LOAD MODULATION INPUT SIGNAL FIG. 6 MODULATOR 2 /0 ofl 30 I 3/ I VAR/ABLE VARIABLE GENERATOR "IRANSERMER WANiEg/ZMER LOAD 12; Y 1 In z [-713 7 GENERATOR /oa D i o# I l' "i l VARIABLE us VAR/ABLE -/4 PULL -4 TRANSFORMER MODULATOR TRANSFORMER MODJLAf/aM' //VPUT SIIGA/AL 27 {L 38a V 7 37a 3 INVENTOI? EDWARD C. DENCH June 11,1957 E. C. DENCH 2,795,760

AMPLITUDE HODULATORS Filed Jan. so, 1953 z sheets-sheet :s

RECEIVER 1 I VARIABLE PUSH VARIABLE -/4 puu. 4 mum/2mm MODULATOR TRANSFORMER Mt -A770 l/VWTSEAML INVENTOR EDWARD C. DENCH CZVM- A TORNEY United States Patent AMPLITUDE MODULATORS Edward C. Dench, Needham,Mass., assignor to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application January 30, 1953, Serial No. 334,262

11 Claims. (Cl. 332-) This application relates to an amplitude modulating device and, more particularly, relates to a system for amplitude modulating a signal generator by means of an electrically controllable quarter-wave transformer.

Cylindrical magnetron diodes having an anode and cathode coaxially: arranged with respect to one another and having a magnetic field transverse to the electrical field between anode and cathode are well known. If a source of relatively low voltage is connected between the anode and cathode, a space charge cloud forms about the cathode whose radius is dependent upon the anode voltage E2. and the magnetic field B.

If the magnetic field B is such that the dielectric constant of the space charge is negative, the space charge cloud behaves as though it were a solid cylindrical conductor serving as the inner conductor of a coaxial line. The radius r, of the virtual inner conductor may be varied over a considerable range in response to the applied magnetron anode voltage. Since the anode voltage is preferably maintained below a critical voltage at which anode current becomes appreciable, the maximum radius mlx of the virtual inner conductor is limited to a value corresponding to said critical anode voltage. Inasmuch as the characteristic impedance of a coaxial line is a function of the ratio of the radii of the outer and inner conductors of the line, a variation in characteristic impedance of this variable section of coaxial line is achieved by variation of the anode voltage.

If the variable section of coaxial line is made long where n is any integer including zero and A is the nominal operating wave lengthof the system with which the variable section is to be used, this coaxial line section becomes an electrically variable quarter-wave transformer. By inserting this quarter-wave transformer whose iterative impedance may be varied between the source of electromagnetic energy, such as a magnetron,

charge cloud about the cathode of a coaxial type magnetron diode;

Fig. 2 is a curve illustrating certain characteristics of the device of Fig. 1;

- circular.

and a load, such as an antenna, the impedance matching between the source and loadand hence the load power variation-may be varied in accordance with the aforesaid anode voltage.

If greater modulation is required, two of such variable transformers may be placed-in series between the Fig. 1 indicates the distribution of the electron space i Fig. 3 illustrates certain details of a quarter-wave transformer with pertinent dimensions shown;

Fig. 4 illustrates a fragmentary cross-sectional view of a quarter-wave transformer showing certain structural details omitted for the sake of clarity from Fig. 3;

Fig. 5 is a block diagram of a first embodiment of an amplitude modulation system utilizing the transformer shown in Figs. 3 and 4;

Fig. 6 is a block diagram of a first modification of the amplitude modulation system] shown in Fig. 5;

Fig. 7 is a block diagram of a second embodiment of an amplitude modulation system; and i i Fig. 8 is a block diagram of a third embodiment of an amplitude modulation system.

Fig. 1 indicates a typical cross section of a coaxial cylindrical magnetron 10 having an electron-emitting cathode 11 of diameter r and an anode 12 of'diameter r spaced concentrically therefrom. For purposes of explanation, the cathode may be either hollow or solid. A magnet (not shown in Fig. 1) also forms part of a magnetron and.

produces a strong magnetic field normal to the region between cathode and anode. If a source of voltage Ea of the proper magnitude is connected between cathode and anode, so that the anode is positive with respect to the cathode, a cloud of electrons 14 will form about the cathode; the outer boundary 15 of this cloud is substantially If the magnitude of the anode voltage is increased, the radius r of the space charge cloud likewise increases. V

Collisions occur between electrons in the space charge 12. Because of these collisions an energy exchange between electrons occurs so that some electrons gain energy while others lose energy. Those electrons which gain energy will bombard and heat the cathode. Those'electrons which lose energy can escape from the space charge cloud and will be found at a radius greater than r In order to heat the cathode by bombardment, the power must come from the anode supply and, since, for a small anode voltage, no anode current is assumed, then for small anode voltages the energy exchangemust be such that no cathode bombardment occurs;

At the onset of this cathode bombardment effect, electrons will be released from the cloud] A necessary condition is that these electrons must reach the anode. Since, at the onset of bombardment, the energy of bombardment is quite small, the released electrons fromthe virtual cathode of radius r are small in number, the potential distribution of the space between r and Ta is not affected by their presence and equations assuming a charge-free space in this region are still valid.

In order to determine thegcritical anode voltage at which bombardment begins, it is'necesary to solve-the Hull cutoff condition for a magnetron having a virtual cathode of radius r and an anode of radius-rs.

If E is the voltage existing at the edge of the electron cloud, the Hull cutoff equation becomes where tion between E and E in terms of the parameters r and r Since both

Equations

1 and 2 must be satisfied simultaneously at the onset of bombardment, they can be equated to yield the following equation implicit in From Equation 3 it is evident that for every value of there is a critical value of 0 corresponding to values of "h are shown in Fig. 2.

Referring to Figs. 3 and 4, the variable transformer section 10 comprises a coaxial magnetron diode having a cylindrical inner conductor or cathode 11 and a cylindrical outer conductor or anode 12. As shown in Fig. 4, one end of transformer 10 is connected to a generator and the other end to aload. The inner conductor 11 has a reduced portion 11 which is made a quarter-wave length long for the nominal frequency of operation of the system in which the transformer is to be used. The transformer may, of course, be made any oddnumber of quarter-wave lengths long. An axial'magnetic field is provided by means of an electromagnet 17 surrounding the anode; it is possible to use a permanent magnet in lieu of an electromagnet where variation of field strength is not required.

A modulator or source of variable voltage 16 is connected between the inner and outer conductors as schematically shown in Fig. 4. As the output of modulator 16 varies, the radius of the space charge cloud 14. varies over the range from r to Although a system for varying the cathode-anode voltage of electron discharge device 10 has been described, it is also possible to vary the dimensions of the space. charge cloud'by varying the strength of the magnetic field, as by varying the current flowing in the electromagnet 17. The difference in radii of theportions 11 and 11' of the inner conductor is preferably such that the maximum outer boundary of. the space charge cloud coincides with the normal portion 11. Since r is equal to or less than max the construction provides its own end shields to minimize cathode emission inthe longitudinal direction.

A cathode heater element 18 is inserted within the reduced portion 11' of the hollow inner conductor and heater leads 19, 19 may be brought out through beaded apertures 22 in the coaxial assembly for connection to a source of heater voltage. The magnetron is vacuum sealedby means of

disks

20 and 21 which may be joined to-the inner and outer conductors by conventional disksealing techniques.

In Fig. 5 a typical circuit for amplitude modulation is shown which comprises a generator 25 of electromagnetic energy, such as a magnetron, a load 27, such as an antenna, a quarter-wave transformer 10 of the type shown in Figs. 3 and 4 and a modulator 16.

Generator 25 is connected to the quarter-wave transformer 10 by means of a coaxial transmission line 30 whose length is where n is any integer. A line of this length acts as a 1-to-1 transformer and insures that the impedance at the generator will be the same as that at the junction of line 30 and the quarter-wave transformer 10. The other end of transformer 10 is connected to load 27 by means of a transmission line 32.

As is well known the characteristic impedance Z of a coaxial line is given by 138 D ia- 810 d where D is the diameter of the outer conductor, d is the diameter of the inner conductor and e is the dielectric constant of the dielectric used in the line.

In terms of radii r and r this equation, assuming air as the dielectric, becomes Z 138 logw For purposes of explanation, the ratio'of the anode radius r to the cathode radius r will be assumed to be 3. Referring to Fig. 2, the ratio max corresponding to a ratio of 3 is found to be 1.56. The space charge cloud may thus be varied from without anode current flowing. The iterative impedance of transformer 10 may be determined from Equation 5;

it is seen to vary from 26.6 to 65.8. The input impedance Z5 is given by z. ZR,

26.6 -26.6 ohms and, neglecting losses in the line, all the energy produced by generator 25 will be transferredto the load. If

the inputimpedance' Zs of" transformer 10 from Equation 6 bec'omes In other words, the input impedance :of transformer 10 for a load of 26.6 ohms can be varied from 26.6 ohms to =162.5 ohms (7) 162.5 ohms. If the magnetron operates at constant voltage the load power variation is given by Since the power is a function of the square of the source voltage, the load voltage variation achieved is or 2.47-to-1 The percentage modulation is given by Average envelope amplitudeminimum envelope amplitude Percent m0d Average envelope amplitude X :3)

For a voltage variation of 2.47 the voltage'modulation is length where n is any integer and 7\ is the wavelength at the operating frequency of the system, so that the coaxial line between the two transformers has no effect upon the impedance represented by the serially connected transformers.

The modulating voltage from modulator 16 is applied in push-pull to the individual transformers so that, when the characteristic impedance of one transformer is 26.6, the characteristic impedance of the other is 65.8 and vice versa. Assume that transformer 10a has a characteristic impedance of 65.8 at the time when the modulator voltage applied to it is zero while at the same time the transformer 10b has a characteristic impedance of 26.6 (when the modulator voltage applied to it is maximum). Assume also that the load is 26.6 ohms as before. The impedance of the load is matched to that of transformer 10b and the input impedance of the latter is also 26.6. This impedance is the same as the output or load impedance of transformer 10a, since

transformers

10a and 10b are separated by an integral number of half-wave lengths. The input impedance to transformer 10a then becomes The input impedance to transformer 10a then becomes It is evident, therefore, that a variation of inputirnpedance of 162.5 to 4.36 is obtained.

and the voltage ratio is or 6.1-to-1 this corresponds to a modulation of seventy-two percent.

In Fig. 7, a system is shown for utilizing a pair of quarter-wave transformers for shifting energy from one load to another, as in electronic scanning.

A generator 25 is connected by appropriate transmission means to two quarter-

wave transformers

10a and 10b. For example, generator 25 may comprise a magnetron oscillator with a conventional coaxial output coupling; a coaxial line section can then be connected between the output coupling and the corresponding

coaxial quarterwave transformer

10a or 10b. Energy transferring means are well known in the art and need not be described in detail.

The output of a push-pull modulator circuit 16 is connected to the two

transformers

10a and 10b. By pushpull control of these transformers, power from generator 25 may be continuously and smoothly transferred from one load to the other. As shown in Fig. 7, the tWo loads comprise antennas 27a and 27b, respectively, including radiating elements 37a and 37b located at the focal points of corresponding parabolic reflectors 38a and 3812. As the impedances of the

transformers

10a and 10b change with varying output of modulator 16, the energy radiated from eachantenna cyclically changes. At one instant the radiation from antenna 371: is a maximum while that from antenna 37b is a minimum; degrees later in the modulating cycle, the radiation from antenna 37a is a minimum while that from antenna 37b is a maximum. In both these two conditions the energy radiated from the two antennas is continuously varying. The system of Fig. 7 can thus be used in radar scanning systems.

The system of Fig. 7 may be modified for reception of energy as shown in Fig. 8.

The system of Fig. 8 comprises receiving antennas 27a and 27b, respectively, including receiving elements 37a and 37b located at the focal points of corresponding

refiectors

38a and 38b. It is possible, of course, to use any type of antenna other than the type shown in Fig. 8 for reception.

The signals received by antennas 27a and 27b are fed to variable quarter-

wave length transformers

10a and 10b, respectively, which are inserted between the antennas and a receiver 45. A push-pull modulator circuit 16 feeds each of the aforesaid transformers thereby varying the impedances of said transformers in the same manner as the system of Fig. 7. In this way, the amount of energy received by receiver 45 is cyclically varied.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the sgr pe of the invention within the art.

What is claimed is:

1. In combination: an input transmission line and an output transmission line each having coaxially positioned inner and outer conductors and a coaxial line impedance matching transformer connected between said input and output transmission lines; said transformer comprising an electron discharge device having an anode whose diameter is equal to the diameter of said outer conductor of said transmission lines, a cathode concentrically arranged with respect to said anode and having a diameter smaller than that of said inner conductor of said transmission lines, the ends of said cathode terminating in end shields positioned substantially perpendicular to said cathode, said cathode being any odd number of quarter-wave lengths long at the desired operating frequency, a source of potential connected between said'anode and cathode for producing an electrical field therebetween, and means for producing a magnetic field normal to said electrical field; said electrical and magnetic fields interacting to produce an electron space charge cloud about said cathode whose radius is a function of the strength of said interacting fields, and control means for varying said fields to vary the impedance match between said input and output transmission lines.

2. In combination: an input transmission line and an output transmission line each having coaxially positioned inner and outer conductors and a coaxial line impedance matching transformer connected between said input and output transmission lines; said transformer comprising an electron discharge device having an anode whose diameter is equal to the diameter of said outer conductor of said transmission lines, a cathode concentrically arranged with respect to said anode and having a diameter smaller than that of said inner conductor of said transmission lines, the ends of said cathode terminating in end shields positioned substantially perpendicular to said cathode, said cathode being any odd number of quarter-wave lengths long at the desired operating frequency, a source of potential connected between said anode and cathode for producing an electrical field therebetween, and means for producing a magnetic field normal to said electrical field; said electrical and magnetic fields interacting to produce an electron space charge cloud about said cathode whose radius is a function of said potential, and control means for varying said potential and thereby the impedance match between said input and output transmission lines.

3. In combination: a generator of electromagnetic energy, a load, a transmission line having .coaxially positioned inner and outer conductors interconnecting said generator and said load and a coaxial line impedance matching transformer inserted in said transmission line; said transformer comprising an electron discharge device having an anode whose diameter is equal to the diameter of said outer conductor of said transmission line, a cathode concentrically arranged with respect to said anode and having a diameter smaller than that of said inner conductor of said transmission line, said cathode having a length where n is any odd integer and A is the operating wave length of said generator, a source of potential connected between .said anode and cathode for producing an electrical field therebetween, and means for producing a magnetic field normal to said electrical field; said electrical and magnetic fields interacting to produce an electron space charge cloud about said cathode whose radius is a function of said potential, and control means for varying said potential and thereby the amount of energy transferred from said generator to said-load.

4. In combination: a generator of electromagnetic energy, a load, -a transmission line having coaxially positioned inner and outer conductorsand interconnecting said generator and said load, and a pair of serially connected impedance matching transformers inserted in said transmission :line; said transformers each comprising a magnetron having an anode whose diameter is equal to the diameter of saidouter conductor of said transmission line, a cathode concentrically arranged with respect to said anode and having a diameter smaller than that of said inner conductor of said transmission line, said cathode having a length where n is any odd integer and 7t is the operating wave length of said generator, a source of potential connected between said anode and cathode for producing an electrical field therebetween, and means for producing a magnetic field normal to said electrical field; said electrical and magnetic fields interacting to produce an electron space charge cloud about said cathode and whose radius is a function of the strength of said interacting fields, said source of potential including a push-pull modulator whose output circuit is connected to each of said transformers and whose input circuit is supplied with a modulation input signal for varying said interacting fields and thereby the amount of energy transferred from said generator to said load. V

5. In combination: a generator of electromagnetic energy, a load, a transmission line having coaxially positioned inner and outer conductors and interconnecting said generator and said load, and a pair of serially connected impedance matching transformers inserted in said transmission line; said transformers each comprising a magnetron having an anode whose diameter is equal to the diameter of said outer conductor of said transmission line, a cathode concentrically arranged with respect to said anode and having a diameter smaller than that of said inner conductor of said transmission line, said cathode having a length where n is any odd integer and 7\ is the operating wave length of said generator, a source of potential connected between said anode and cathode for producing an electrical field therebetween, and means for producing a magnetic field normal to said electrical field; said electrical and magnetic fields interacting to produce an electron space charge cloud about said cathode whose radius is a functionof said potential, said source of potential including a push-pull modulator whose output circuit is connected to each of said transformers and whose input circuit is supplied with a modulation input signals for varying said PQtential and thereby the amount of energy transferred from said generator to said load.

6. In combination: a generator of electromagnetic energy, a pair of loads, a transmission line having coaxially positioned inner and outer conductors and interconnecting said generator and each of said loads, and an impedance matching transformer inserted in each of said transmission lines; said transformers each comprising an electron discharge device having an anode whose diameter is equal -to the diameter of said outer conductor of said transmission line, a cathode concentrically arranged with respect to said anode and having a diameter smaller than that of-said inner conductorof said transmission line, said cathode being an odd numberof quarter-wave lengths long at thedesired operating frequency, a source of potential connected between said anode and cathode for producing an electrical field therebetween, and means for producing a magnetic field normal to said electrical field; said electrical and magnetic fields interacting to produce an electron space charge cloud about said cathode whose radius is a function of the magnitude of said interacting fields, said source of potential including a push-pull modulator whose output circuit is connected to each of said transformers and whose input circuit is supplied with a modulation input signal for varying the magnitude of said fields and thereby the amount of energy transferred from said generator to each of saidloads.

7. In combination: a generator of electromagnetic energy, a pair of loads, a-transmission line having a coaxially positioned inner and ,outer conductors and interconnecting said generator and each of said loads, and an impedance matching transformer inserted in each of said transmission-lines; said transformers each comprising an electron discharge device having an anode whose diameter is equal to the diameter of said outer conductor of said transmission line, a cathode concentrically arranged with respect to said anode and having a diameter smaller than that of said inner conductor of said transmission line, said cathode being an odd number of quarter-wave lengths long at the desired operating frequency, a source of potential connected between said anode and cathode for producing an electrical field therebetween, and means i for producing a magnetic field normal to said electrical field; said electrical and magnetic fields interacting to produce an electron space charge cloud about said cathode whose radius is a function of said potential, said source of potential including a push-pull modulator whose output circuit is connected to each of said transformers and whose input circuit is supplied with a modulation input signal for varying said potential and thereby the amount of energy transferred from said generator to each of said loads.

8. In combination: a pair of receiving antennas, a receiver, a transmission line having coaxially positioned inner and outer conductors and interconnecting each of said antennas and said receiver, and an impedance matching transformer inserted in each of said transmission lines; said transformers each comprising an electron discharge device having an anode Whose diameter is equal to the diameter of said outer conductor of said transmission line, a cathode concentrically arranged with respect to said anode and having a diameter smaller than that of said inner conductor of said transmission line, said cathode being an odd number of quarter-Wave lengths long at the desired operating frequency, a source of potential connected between said anode and cathode for producing an electrical field therebetween, and means for producing a magnetic field normal to said electrical field; said electrical and magnetic fields interacting to produce an electron space charge cloud about said cathode whose radius is a function of the magnitude of said interacting fields, said source of potential including a push-pull modulator whose output circuit is connected to each of said transformers and Whose input circuit is supplied with a modulation input signal for varying the magnitude of said fields and thereby the amount of energy transferred from each of said generators to said receiver.

9. In combination, a transmission line having concentrically positioned inner and outer conductors for transmitting energy from an energy source to a load and a coaxial line impedance matching transformer inserted in said transmission line, said transformer comprising an electron discharge device having an anode and a cathode concentrically arranged with respect to one another and with respect to said conductors, said cathode and said inner conductor constituting a single continuous member, said cathode being any odd number of quarter wave lengths long at the desired operating frequency, a source of potential connected between said anode and cathode for producing an electric field therebetween, and means for producing a magnetic field normal to said electric field; said electric and magnetic fields interacting to produce an electron space charge cloud about said cathode whose radius is a function of the magnitude of said potential, said cathode having a radius smaller than that of 10 said inner conductor, and control means for varying said potential to alter the amount of energy transferred by said line.

10. In combination, a transmission line having concentrically positioned inner and outer conductors for conductor by means of portions extending substantially perpendicularly to said cathode, said cathode being any odd number of quarter wave lengths long at the desired operating frequency, a source of potential connected between said anode and cathode for producing an electric field therebetween, and means for producing a magnetic field normal to said electric field; said electric and magnetic fields interacting to produce an electron space charge cloud about said cathode whose radius is a function of the magnitude of said potential, and control means for varying said potential to alter the amount of energy transferred by said line.

11. In combination, a transmission line having concentrically positioned inner and outer conductors for transmitting energy from an energy source to a load and a coaxial line impedance matching transformer inserted in said transmission line, said transformer comprising an electron discharge device having an anode and a cathode concentrically arranged with respect to one another and with respect to said inner and outer conductors, said cathode being joined at both ends thereof to said inner conductor by means of portions extending substantially perpendicularly to said cathode, said cathode being any odd number of quarter Wave lengths long at the desired operating frequency, a source of potential connected between said anode and cathode for producing an electric field therebetween, and means for producing a magnetic field normal to said electric field; said electric and magnetic fields interacting to produce an electron space charge cloud about said cathode whose radius is a function of the magnitude of said potential, said cathode having a radius smaller than that of said inner conductor by an amount at least equal to the radial distance between the outer periphery of said cathode and the outer boundary of said electron space charge cloud corresponding to a critical value of said potential at which substantial anode cur-rent just begins to flow, and control means for varying said potential to alter the amount of energy transferred.

References Cited in the file of this patent UNITED STATES PATENTS 2,153,728 Southworth Apr. 11, 1939 2,241,976 Blewett et a1. May 13, 1941 2,402,184 Samuel June 18, 1946 2,438,367 Keister Mar. 23, 1948 2,602,908 Linder July 8, 1952