US2505240A

US2505240A - Frequency-modulating apparatus

Info

Publication number: US2505240A
Application number: US743043A
Authority: US
Inventors: Elmer J Gorn
Original assignee: Raytheon Manufacturing Co
Current assignee: Raytheon Co
Priority date: 1947-04-22
Filing date: 1947-04-22
Publication date: 1950-04-25
Anticipated expiration: 1967-04-25

Description

' E. J. GORN FREQUENCY-MODULATING APPARATUS Filed April 22,. 1947 April 25, 1950 /NVENTO/? Patented Apr. 25, 1950 FREQUENCY-MODULATING APPARATUS Elmer J. Gorn, Newton, Mass, asslgnor to Raytheon Manufacturing Company, Newton, Mass..

a corporation of Delaware Application April 22, 1947, Serial No. 743,043

' 2 Claims.

This invention relates to frequency-modulating apparatus, and more particularly to frequency-modulating apparatus for electromagnetic energy in the microwave region of the frequency spectrum.

One of the objects of the present invention is to accomplish frequency modulation of microwave energy by altering the velocity of propagation thereof under the control of a modulatingsignal containing the intelligence it is desired to transmit.

Another object of the present invention is the provision of simple and efiicient apparatus to effectuate the foregoing purpose.

- density. Said space charge may take the form of an electron cloud adapted to move between an electron-emissive electrode, located within the waveguide and functioning as a cathode, and the longer side walls of the waveguide itself, functioning as an anode. In order to obtain a space charge of appreciable density, it isdesirable to prevent the electrons from being swept out of the effective region of the waveguide too rapidly, and for this purpose, it is preferred that a magnetic field be established through the waveguide, in a direction at an angle to the charge path between the emissive electrode and the waveguide itself. Said magnetic field, in conjunction with an electric field set up between the emissive electrode and the waveguide, causes the emitted electrons to travel along spiral rather than direct paths, thereby retaining said electrons in the effective region of the waveguide for longer periods of time.

Now, the density of the initial space charge will determine the dielectric constant of the waveguide and, as is well known, said dielectric constant will determine the velocity of propagation of the electromagnetic energy through said waveguide. Hence, by' varying the density of said space charge, for example, under the control of an intelligence-containing modulating signal, the velocity of propagation and, therefore, the instantaneous phase, of the initial energy will be altered, such variation in instantaneous phase constituting, effectively, a variation in frequency. An increase in the space charge density will increase the dielectric constant of the waveguide,

reduce the velocity of propagation of the energy therethrough, and lag the phase of said energy to result in an instantaneously lower frequency: while a reduction in the space charge density will reduce the dielectric constant'of the wave- 'gulda increase the velocity of the propagation of the energy therethrough, and advance the phase of said energy to result in an instantaneously higher frequency.

In the accompanying specification there shall be described, and in the annexed drawing shown,

an illustrative embodiment of a frequency modulating apparatus made in accordance with the present invention. It is, however, to be clearly understood that the present. invention is not to be limited to the details herein shown and described for purposes of illustration only, inasmuch as changes therein may be made without the exercise of invention, and within the true spirit andscope of the claims hereto appended.

In said drawing, the single figure is a perspective view of a preferred form of the invention.

Referring now more in detail to the aforesaid illustrative embodiment of the present invention, with particular reference to the drawing illustrating the same, the numeral 5 generally designates a waveguide, herein shown as a hollow, rectangular pipe made of electrically conductive material, such as copper. Said waveguide includes longer side walls 6, shorterside walls I and an end wall 8, and may be coupled, at its open end, to a load, such as an electromagnetic horn (not shown).

The waveguide 5 may be excited with electromagnetic energy of a wave length, preferably, of a few centimeters or less, by means of a probe 9 entering said waveguide through one of the longer side walls 6 thereof at an appropriate distance from the end wall 8, said probe being suitably coupled to a source of said energy, herein shown as a'magnetron oscillator Ill.

At a distance of a few wave lengths from the probe 9, the waveguide 5 is, preferably, hermetically sealed, for example, by a low-loss dielectric window II, the output end of said waveguide being coupled, in an appropriate manner, to the load which may, as before stated, consistof a radiating electromagnetic horn.

The purpose of sealing off a portion of the waveguide 5 is to'provide a region which may be evacuated and within which the dielectric con-r stant of the waveguide may be controlled. For this purpose, a space charge may be introduced into said evacuated region, said space charge being obtained, for example, in the following manner. A pair of opposed, elongated electron-emissive electrodes l2 may be mounted within the sealed-off portion of the waveguide 5, parallel with and slightly spaced from the shorter side walls 1 thereof, said electrodes being heated to a temperature of thermionic emission in any suitable manner (herein not specifically shown). Said electrodes 12 may be connected, as shown, to the negative terminal of a unidirectional voltage source IS, the positive terminal of said voltage source being connected to the body of the waveguide itself. The electric field thereby established between the cathode electrodes l2 and the body of the waveguide 5, functioning as an anode, causes the flow of an electron stream between said electrodes and said waveguide, thereby providing the evacuated region of the latter with a space charge.

In order to obtain a space charge of appreciable density and thereby enable better control over the dielectric constant of the waveguide, it is preferred that a magnetic field be established through said waveguide in a direction at an angle to the space charge path between the cathode electrodes l2 and said waveguide. By so doing, the electrons in said space charge may, by appropriate initial adjustment ofthe relative magnitudes of said magnetic field and the electric field set up by the voltage source l3, be caused to follow spiral rather than direct paths, thereby maintaining said electrons in the eiiective region of the waveguide for relatively extended periods of time. Said magnetic field may be obtained by opposed pole pieces l4 mounted externally of the waveguide adjacent to and parallel with the shorter side walls 1 of said waveguide.

In order, after a space charge of suitable density has been initially obtained, to vary the same under the control, as heretofore explained, of a modulating signal, there may be superimposed upon the electric field set up by the voltage source l3 an alternating voltage characteristic of the intelligence it is desired to transmit. For this purpose, a secondary winding i5 of an audio-frequency transformer l6 may be connected in se- -ries with the waveguide 5, voltage source l3 and cathode electrodes l2, said transformer including a primary winding l1 adapted to receive the modulating signal.

Now, upon the application of an alternating voltage to the transformer IS, the magnitude and frequency of the electric field between the cathode electrodes l2 and the waveguide 5 will be varied as a function, respectively, of the magnitude and frequency of said alternating voltage. This will vary the instantaneous space charge density, in other words, the dielectric constant, within the waveguide in step with the frequency of the modulating signal; and this, in turn, will alter the velocity of propagation through the waveguide of the energy fed thereto by the oscillator I0. Such alteration of the velocity of propagation causes alteration of the instantaneous phase of the energy affected, which change in phase constitutes, effectively, a change in frequency. During positive excursions of the modulating signal, the space charge density and, therefore, the dielectric constant, will be increased to retard the phase of the microwave energy, corresponding to a lowering of the frequency thereof, and during the negative excursions of the modulating signal, the opposite phenomena will occur. Thus, the microwave energywill become frequency modulated about its center frequency in accordance with the intelligence incorporated inthe modulating signal applied thereto.

This completes the description of the aforesaid illustrative embodiment of the present invention. I

It will be noted from all of the foregoing that there has been-provided a simple and eifective mechanism for frequency modulating microwave energy through alteration of the velocity of propagation thereof under the control of a modulation signal containing the intelligence it is desired to transmit.

Other objects and advantages of the present invention will readily occur to those skilled in the art to which the same relates.

What is claimed is:

l. Modulating apparatus comprising: a hollow rectangular wave guide having a portion thereof sealed off and evacuated; a fixed-frequency source of microwave energy communicating with said sealed off portion of said wave guide; a cathode disposed in said sealed off portion of said wave guide, adjacent to and insulated from one of the shorter side walls thereof; a source of unidirectional voltage connected between said cathode and said wave guide for introducing into said sealed oif ortion of said wave guide a space charge of predetermined density; means, adjacent the shorter side walls of said wave guide,

' microwave energy through said wave guide; and

means for superimposing an alternating voltage on said unidirectional voltage in response to a modulating signal.

2. Modulating apparatus comprising: a hollow rectangular wave guide having a portion thereof sealed off and evacuated; a fixed-frequency source of microwave energy communicating with said sealed 01f portion of said wave guide; a. pair of opposed cathodes disposed in said sealed oi! portion of said wave guide, respectively, adjacent to and insulated from the shorter side walls of said wave guide; a source of unidirectional voltage connected between said cathodes and said wave guide for introducing into said sealed oil. portion of said wave guide a space charge of predetermined density; means, adjacent the shorter side walls of said wave guide, for establishing a magnetic field through said sealed off portion of said wave guide, in a direction at an angle to the space charge path between said cathodes and said wave guide and at an angle to the direction of propagation of said microwave energy through said wave guide; and means, connected in series with said cathode and said source of unidirectional voltage, for superimposing on the latter an alternating voltage in response to a modulating signal.

EIMER J. GORN.

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

UNITED STATES PATENTS Number Name Date 2,241,976 Blewett et al May 13, 1941 2,256,608 Braden Sept. 23, 1941