US2603772A - Modulation system - Google Patents

Description

l atented July 15, 1952 MODULATION SYSTEM Lester M. Field, Palo Alto, Calif., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New. York Application April 6, 1948, Serial No. 19,231

16 Claims.

,Copendingf application Serial No. 704,858, filed October 22, 1946 by J. R. Pierce describes an electronic amplifying apparatus for high frequencies in which the transmission path of the wave to be.v amplified is incorporated into the amplifying apparatus.) The traveling wave follows a path such that the associated electric field may be traversed by; an electron stream having a velocity of the, same order of magnitude as that at which the traveling wave moves through the amplifying apparatus. Under such conditions the electron stream reacts, on the electric field and the electricfieldreacts on the electron stream ingsuch a manner that the wave traveling along the path in thesame direction as the electron stream, increasesin amplitude with distance while a wave travelingin a direction opposite to the stream is little affected by the presence of the electrons. Thus, thedevice acts as an amplifier 'for'waves traveling in the same direction as the electron stream. 1 v

;, In .a wave amplifying device of the character described, the wave propagated along the trans-. mission path as an increasingwave has a phase velocitywhich, in the absence of an electron stream, -.is dependent upon the physical configuration and characteristics of the transmission path.'- However. in the present of an electron stream, the phase velocity of the traveling wave is also dependentupon the velocity of the electrons of the electron stream. Thus, while under such conditions the phase velocity of the propagated wave is diiferent from the velocity of the electrons, the phase velocity of the wave is increased when the velocity of the electrons is increased, and, conversely, the phase velocity of the wave is decreased when the velocity of the electrons is decreased.

' The present invention contemplates circuit arrangements whereby the effect upon the phase velocity of the wave produced by changing the electron velocity may be utilized to provide an improved means for the phase modulation of very high fr'equency waves. In one embodiment of the invention the potential gradients along the path of the electron steam are varied in accord- V ance with the modulating potentials .to produce an electron velocity variation and a consequent phase variation of the wave to bemodulated. More particularly, as applied to the preferred structure described in the J. R. Pierce application to which reference has been made, the variable potential gradient may preferably be obtained by varying the potential of the helix which serves as the wave transmission path, with respect to the potential of the electron source or cathode. Alternatively, the modulating potentials may be utilized to vary the potential of either the focussing electrode of the electron gun or the collector anode with respect to the potential of the cathode. Other possible embodiments of the invention are pointed out in connection with the detailed description of the invention.

The features-of this invention are pointed out with particularity in the appended claims. The invention itself may be better understood by reference to the following detailed description, taken in connection with the accompanying drawings, in which: i

Fig. 1 illustrates an embodimentof the invention utilizing a preferred form of amplifying device; and' Y Figs. 2 and 3 illustrate alternative circuit arrangements useful in connection with the embodiment of Fig. 1.

Referring now particularly to Fig. 1, there is shown an illustrative embodiment of a-discharge device adapted to be used as an amplifier for ultra-high frequencies. Thearrangement shown comprises an electron beam tube l0 including an evacuated envelope having an elongated portion II. This portion, which is of uniform diameter along its length, connects with an enlarged electrode containing portion l2. The envelope is constituted of a low loss insulatingmaterial such as quartz or glass. s a

The discharge device it is provided with a means such as a known type of electron gun for producing an electron beam. The combination shown comprises a heater 13, which is supplied with energy by a source M, a cathode l5 connected to the circuit of the heater l3, and a focussing electrode [6 having a configuration which provides field patterns suitable to accelerate and focus theelectron stream when the electrode is biased to a suitable potential. The

, electron stream is further concentrated and 7 above that of the cathode l by a potential source conventionally represented as a battery 22. A

conductor 23 serves to connect the negative poleof the potential source 22 to the cathode l5 while a conductor 24 serves to connect a-zpos itive to the focussing anode l6 and a conductoi 2-5gcon nects the positive pole to the helix l1. The collector anode 2| is connected through a conductor 26 to apply a positive potential somewhat lower than that of the helix for the purpose ofdecelerating the electron stream before it strikes the anode; This isby no means essential -=to the operation of the device' and the collector maybe maintained at a potential 'higher'thanthat of the helix, a

The portions of the device just described are related onlyto the generation of a stream of electrons and guiding them through the' envelope. The transmission elements which' make up the high frequency portionof the device serve to introduce the-high frequency waves to be amplifled into the envelope, provide a path along Which the waves may travel to interact withthe electron stream and to transfer theamplified waves to the output-circuits associated withthe device. The device-is normally intended for the uniform-amplificationbf a Wide-bandof high frequency waves at wavelengths in the centimeter range and the high frequency transmission elements of the device must necessarilyfhave dimensions suitable for use at the particular wavelength to beamplifi'ed. Since the'band -width-of thef waves to be amplified may well be of the order of several hundred me'gacy'cles, the --expressions wavelength to be amplified or "carrier --f're-' 'quency will'be understood-to denote {the wavelength orthe frequency at the center of the-band;

The elongated helix l1 serves as a conductive path along which; the waves =to-" be amplified-are propagated in order that they may react witli the electron stream flowing therethr'ou'gh. {-In' order} to perform this function, 'the'dimensions-ofthe helix must be such'th'at' thelinear velocity 'at which 'a'wave traveling along the circumference of the helix-advances along 'the axis is vo f'sorne value to which the electron 'stream' may be accelerated by the'use ofmoderate' voltages. For accelerating voltages in theorder of 1,500 'to 2,000 volts, the helix may be wound wi-th several turns per wavelength along-the axiswhich may have a length of the order of thirty or forty "wavelengths of the wave to be amplified.

There are providedwave guide paths external to the amplifying device for conducting tofthe device the waves to be amplified and the amplified waves to a transmission" or load circuit. At'the input end: of the helix l6=an incoming-wave path conventionally represented as-a wave-guide 21 of rectangular cross-section is coupled'to a source 28 "of carrier waves. The coupling-isrepresented as acoaxial'transmissionline 29 and'a "probe- 3!), but may be of any'lrnown 'type'suitable for use at 'the frequencies atwhich the device is to'jbe fused. Where'it'is intended that the amplifying arrangement; as iisedwith the present invention, be useful over a-"Wide'band of'frequencies with to the helix l1. :metallic section 32 which supports the input cou- 4 a minimum of adjustment the coupling may be of the type described in application Serial No. 585,096 filed March 27, 1945, now U. S. Patent No. 2,527,146, dated October 24, 1950, by W. W. Mumford or in United States Patent No. 2,408,032 issued September 24, 1946 to A. C. Beck. The amplifying device is inserted transversely through the input wave guide '21, coupling-between the input wave guide and the-helix 'l 'l-being achieved by means of an input coupling strip 3| connected There is provided a cylindrical pling strip 31 and cooperates with a cylindrical metallic section 33 attached to the input wave guide"21 ,to effectively form an open-circuited "transmission line which has an electrical length of one-fourth of the wavelength of the waves to be amplified. The transmission line thus formed acts as a low impedance path across the opening in the wall of the wave guide through which the envelope II is inserted and asa lowfi impedance support point forthe coupling.'strip'- 3jl. -At the output end of the;he1ix 11, an output coupling strip 34 is supported byaacylindrical-section 35 which cooperates with a section 36 ofkan' out'put waveguide 31 in anzarrangement similar s-toithat at the input end ofthe helix. The helix .l is supported by a series of-non-conductive ceramic rods-'38 which are disposedbetween the helix and the envelope I I. The ceramicrods- 38 also cooperate with ceramic spacers 39 and'i40- toi'rn'a'intain a longitudinal spaced relation'betw'e'en-the focussing electrode I 6; the input 'c'oupling strip support section 32, the output coupling strip support section -35 and theano'de"=2l. Themu'tput wave guide 3'! is coupled to -a--1oad--circuit"41 M- means of a probe 42 and a transmission line fl.

In the operation of the 'dev ice; the input wave guide'2-1 is excited s0 as to' produce a mode of wave propagation having-an electric field parallel to the input coupling 'strip tl A corresponding wave is thus generated along the coupling strip and-imparted to the helix "H. 'The wave -th'en travels along the circumference of the helix -at a speed approximating that" of 5 light, *but at a linear velocity along the axisof the -=tube which issmaller in proportion -to the "ratio of zthe' dis-' tance between turns to -the'circumference' per turn. The initialinteractionbetween thtraveling wave and the electron stream is-very slight, the wave serving initially only to produce waves of charge density and velocity in the electron stream. However, as the'wave and the' 'electron streamtravel along the axis of thefihelix and a wave is established-in the electron stream; a 'condition is established in which the-wave a little slower than the electronsforining the modulated electron streamand the electrons imrived from the source areapplied througlr a coupling means; conventionally representedasa aveis 7 stream of. electrons traveling lengthwise of and inth'e; field -re gion of said helix, a sourc'e of low frequencymodulating signals, andv means for varying the velocity of the electrons of said stream in their forward travel in said helix in accordance with the amplitude variations of the said modulating signals whereby waves traveling over. said helix are phase modulated in accordance with the modulating signals from said source. Y 7 a 5. The invention in accordance with 'claim 4, said velocity varying means comprising means for varying the potential between said helix and said electron stream producing means in accordance with the said modulating signals.

6. A modulating apparatus for high frequency waves which comprises a source of electrons, electrode means for focussing electrons from said source into..a concentrated stream along an extended path, a collector electrode for the said 'electro'nstream, wave transmission means for propagating high frequency waves to be modulated along the pathofflthe said electron stream and at'the same order of velocity as the electrons of the said electron stream, a potential source connected between the said source of electrons and the remaining electrodes for establishing a potential gradient along'the path of the electron stream, a source of low frequency modulating signals, and means for varying the velocity of electrons of the said stream in their forward travel along said path in the region of said wave propagation in accordance with the variations of the said modulating signals;

7. The inventionin accordance with claim 6, said velocity varying means comprising means connecting said modulating signal source between said electron source and said potential source. 1

8. The invention in' accordance with claim 6,

' said velocity varying. means comprising means for varying the potential of the said focussing electrode in accordance with the variationsof the said modulating signals.

9. The invention in accordance with claim 6, said velocity varying means comprising means for varying the potential of the said wave transmission means in accordance with the variations of the said modulating signals.

10. In a modulation system, a discharge device comprising an evacuated envelope, means within" said envelope for producing an electronv beam; wave transmission means disposed along the pathof the said beamiand so predesigned as to permit 'propagationof high frequencyelectrical waves thereover in the direction of propagation of said beamjmeans to impress high frequency waves to be'modulated upon an input end of said wave transmission means, a source of low frequency modulating signals, and means responsive to said signals for varying the velocity of the electrons of the said beam in their forward travel along said path in the region of said wave propagation.

11; In combination, a source ofhigh frequency electrical waves, means for forming a concentrated stream of electrons along an extended path, means for propagating the Waves "from said source along the path of said electron stream in such a manner as to cause an increase in the amplitude of the said waves, a'source of low frequency modulating signals, and means for varying the velocitiesof electrons of the said stream in their forward travel along said path in, the region of said wave propagation in accordance with the modulating signals from the said source.

12. In apparatus in which amplification, of high'frequency waves is achieved through iii;- teraction between an electron stream and a high frequency electricv field associated with the waves to be amplified over an extended distance along the transmission path of the wave, the method of modulation of the wave'to be amplified which comprises the variation of the velocity of electrons of the electron stream in their forward travel in the region of said interaction in accordance with-the amplitude of a low frequency modulating signal. V v

13. A device for modulating the frequency of a high, frequency electromagnetic wave in accordance with a low frequency modulating signal comprising means for propagating said wave to be modulated and its associated electric field along an extended path at a velocity within the range of practical electron velocities, means for projecting a stream of electrons along said path in said field in the direction of travel of the field at a velocity substantially the same as the velocity of the field and means for varying. the velocity of projection of the electron stream along the path in said direction in said field in accordance with variations in the said low frequency modulating signal, whereby the high frequency wave becomes frequency modulated in accordance with the modulating signal during its propagation along the path.

14. The method of modulating the frequency of a high frequency electromagnetic wave in ac cordance with a low frequency modulating signal which comprises propagating said wave to be modulated and its associated electric field along an extended path at a velocity within the range of practical electron velocities, projecting a stream of electrons along said path in said field in the direction of travel of the field at a velocity substantially the same as the velocity of the field and varying the velocity of projection of the electron stream along the path in said direction in said field in accordance with the varia-' tions of said modulating signal. 7 A I 15. In a modulator, in combination, a wave amplifying device comprising a conductor in the form of an elongated helix, means for coupling an input wave source to said device, a source of electrons, means for forming electrons from said source into a concentrated stream traveling lengthwise of and in the field region of said helix, a source ofrmodulating signals, and means for varying the velocity of the electrons of said stream in accordance with the amplitude varia-' tions of the said modulating signals, said velocity varying means comprising means for varying the potential between said electron stream forming means and the said helix. V i v 16. In combination, a wave amplifying device comprising a conductor in the form of an elon gated helix, a source of high frequency waves to be modulated coupled to an input end of said helix, means for producing a concentrated stream of electrons traveling lengthwise of and in the field region of said helix, a source of modulating signals, and means for varying the velocity of the electrons of said stream in accordance with the amplitude variations of the said mod'ulating signals whereby waves traveling over said helix are phase modulated in accordance with the modulating signals from said source;

- 9 10 varying the potential between said helix and said Number Name Date electron stream producing means in accordance 2,239,677 Jobst Apr. 29, 1941 with the said modulating signals. 2,254,036 Gray Aug. 26, 1941 LESTER M. FIELD. 2,276,247 Hahn Mar. 10, 1942 5 2,300,052 Lindenblad. Oct. 2'7, 1942 REFERENCES CITED 2,409,608 Anderson Oct. 22, 1946 The following references are of record in the 2,521,760 Starr Sept- 1950 file of this patent: OTHER REFERENCES UNITED STATES PATENTS 10 Article: Traveling Wave Tubes by J. R. Number Name t Pierce et a1., proceeding of the I. R. E., February 2,233,779 Fritz Mar, 4, 1941- 1947, pages 108-111,. vo1. 35, No. 2.

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