US2435601A - Phase modulation system - Google Patents

Description

Feb. 10, 1948. s. RAMO PHASE MODULATION SYSTEM Original Filed Jan. 30, 1940 CRYSTAL CONTRWLLED OSLILLATOR 28 S/GNAL SOURCE OUTPUT CARRIER SOURCE 4'2 SIGNAL SOURCE ln-vehtor:

Simon Ramo,

is Attorney.

' Patented Feb. 10.1948

vrrmsiz MODULATION SYSTEM Simon Rama} Nisk'a'yuna, N." Y., assignor to General Electric Company, a corporation of New'York Substituted for abandoned application Serial No.

316.339. January 30. 1940.

This application December 31. 1942, Serial No. 470.811

2 Claims. (01. 179-1715) The present invention relates to an improved system of phase modulation. The-term phase modulation" as used herein is intended to'designate recurrent variations in phase of a fundamental quantity, such as a carrier voltage; as produced by the'variations of a secondary or modulating quantity, such as an audio or video signal. It will be understood by those skilled in the art that this definition includes as a special case thereof the type of modulation currently known as frequency modulation."

It is a primary object of the invention to provide an improved system of phase modulation in which the carrier source is sufficiently independent of the modulating means to permit separate stabilization of the carrier frequency. To this end; theinvention utilizes an electron stream of such character that-variations of carrier frequency may be impressed on the stream at a point which is relatively widelyseparated from a point at which energy is to be taken from the stream for excitation of an output circuit.

Bythe additionalprovision of means for vary-' ing the electron transit time between the said 7 two points inaccordance with the variations of a signal voltage, the voltage developed in the output circuit is caused tops of phase modulated character. r At the same time, no adverse or destabilizing influences are exerted on the carrier source as a result of the modulating operation. The features which 'I-desire to'protect herein are pointed out with particularity in the appended claims. The invention itself. together withfurther objects and-advantages thereof; may best be understood by reference to'the: following description takenain connection with the drawing, in which'Fig. .1. represents diagrammatically one embodiment of the invention: and

Figs. 2 and 3 respectively illustrate modifica tions thereof,

Referring particularlyto Fig. 1, the apparatus illustrated comprises-an elongated tubular envelope l suitably constituted of glass. Within the envelope and at one end thereof there is pro-.,

vided means for producing anelectron stream, such means comprising, for example, a cathode part- II, a negatively biased control grid I2, and

an accelerator I3 which is maintainedat a fixed positive potential. After passage through the accelerator, the electron stream traverses a substantially field-free space enclosed by an elongated conductive tube I5 and is finally collected by means of an anode ll. If desired, magnetic focusing'means (not shown) may be employed to prevent spreading of the electrons as they traverse the envelope II].

An appropriate potential relationship may be? maintained between the various elements referred to in the foregoing by the provision of a D. C.voltage source, which is conventionally illustrated in the drawing as a battery. I9. In

this connection the shielding tube I5 is preferably maintained at ground potential while the cathode is maintained at least several hundred volts below ground.

In the use of the apparatus described, it is proposed initially to produce high frequency variations in the electron stream by the application of a voltage of carrier frequency to the grid I2. Such voltage may be derived, for example,-

to produce excitation of an appropriate high; frequency output circuit, conventionally illustrated as comprising a condenser 25 and an inductance 26. In order to determine the character of the voltage thus developed in the output circuit, reference may be had to the. following analysis.

Let it be assumed that the input voltagev applied to the grid I2 is of the form Vm=e1 sin wt.

where the quantity (n represents a carrierfre quency. Under these circumstances the output voltage developed in the circuit 25, 26 may be expected to be of the form In this latter expression, the term 5 represents an angle of lag introduced by the transit time of electrons through the tube I5. '(For present 1 purposes, it is desired that this transit'time be" j equal to at least several and preferably more than ten complete cycles of the carrier frequency.)

For the production of phase modulation effects by the arrangement described it is proposed to provide means for varying the average beam velocity, and consequently the electron transit time through the tube l5. at a signal (e. g., audioor video), frequency. This'may be donein one resented as E=Eo(1+6 sin at) where it represents a signal frequency and 6E0 represents the magnitude of the signal voltage. Under these circumstances, since the beam velocity v is'proportional to the square root of the voltage, 6- is: small, we

may write approximately =v.(1+% sin 2 Also approximately (1 -sin i) (3) 1) v 2 it Since the electron transit" time" angle 0 is" inversely proportional to beam velocity; We may which is in the form of a phase mo'dulated'wave. Due tothef act that the signal voltage acts on the electron stream at a point where the stream.

intensity is already "fixed (1. e., by the combined action" of thegrid i 2 and the accelerator I 3 it is apparent that the voltage developed in the output'circuit'wi-ll be substantially free of amplitude variations of signal 'frequency. For this reason, the apparatus described is Well-adapted for use in those systems ofcommunication inwhich amplitudemodulation e'fiects are intentionally suppressed at some point of the system in order to utilize the superior qualities of noise-freedom associated with phase or frequency modulated transmission.

It is a furth'er' advantage of a'system such as that described that "the source of carrier potential may comprise a circuit which is quite independent of'the modulating system and which is of separately stabilized character. In this respect thesystem differs favorably from certain arrangements used heretofore in which phase or frequency modulation of a carrier is produced by changing the resonance or other characteristics of the source. This latter method of operation makes it diflicult to avoid sporadic and uncontrollable vari'ationsin the normal or unmodulated-ca'rrier frequency.

In Figl 2 there is shown a modification of the:

invention which diifers from that-previously described mainly with respect to the character of the input and output electrodes employed. In this case the electron source consists of an electron gunhaving parts 35' and '36. The electrode system which is traversed by the electron stream afterits issuance from the electron gun comprises a series =of conducting tubes, numbered 31 to 4i inclusive. Of these, the tubes which bear odd numbers-are inter-connected and are conjoint'ly maintained at ground potential. The electrodes 38" andflljori the -'othe'r hand, are free to vary in potential-with respect to the adjacent conductive parts and are thus adapted to serve respedti velyas input andoutput'electrodes. To this en'd', the electrode 38 is connected to a source 42 Y of carrier voltage, while the electrode 40 is connected to an output circuit including, for example, the parallel combination of a condenser 43 and an inductance M.

For most effective operation of the system thus describediit. is' desired that the, average electron transit timethrough theaelectrode 38. shall correspond at least approximately to a half-cycle of the carrier voltage, or, under some circumstances, to an odd multiple thereof. It has been shown that when this'condition is fulfilled, an electrode such-as 38 willzact-to produce cumulative velocity "article bypWl C.-Hahn' and G. F. Metcalf in the modulation: effects at the two gaps which bound the electrode..'.(:& full discussion of this means of obtaining velocity modulation is given in an Proceedings'ofithell R. E. for February 1939 at page 106). Moreover, the mode of propagation of the resultant beam disturbances along the tube is such that additive output effects may also be obtained at the two gaps: which bound the electrodel'os- Excitation of theputput circuit 43;v 44: will thus be realized...

As a result. of thephasedelay eilectv introduced by the presence of therelatively very long shield.-

ing tube. 39, the relationship between'the input voltage applied to the electrode 38 and. the out put voltage developed-across the circuit 43, 44 will be substantially the same as in the device of Fig. 1-. Consequently, phase modulation efiects may be produced in'theoutput of the apparatusizby the. inclusion. of a :source 4-1. of audio or video: signalin the ground-to-cathodex circuitof the apparatuss wherev it acts tovary the average: velocityof the electron'streanr-in accordance with the variations in thewsignal voltage.

After its traversal ofzthe' high-freqnencyelectrode systemthe electron stream. may .be collected by means of ananodefl;

In the various devices described above. the

magnitude of the phase. modulating effect is clearly a'function of thetotal'electron transit time between the input :andoutput electrodes. It is therefore, desirable: to "make this time as long aspossible consistent with" practical considerations of tube design. In 3:there is shown an arrangement :by which'the aforesaid transit time may :be' considerably lengthened without unduly increasing the: total length oilthe tube structure.

In the figure referred to there is providedan envelope '50 which :encloses a conductive shielding cylinder 52 arranged coaxlally therein. An electron stream generated by an appropriate source 53 .is projected more or less parallel to the circumference of the envelope and is given a. component-of-axial motion by the use 01' an appropriately biased deflecting-element 55. By the influence of a: magnetic field which is produced; for example, by-an elongated magnet coil 56, the electrons are caused to traverse a helical path as tliey proceed along the axis oithe tube, such path being indicated by the dot-dash line A. At the end of' the tube remote from the cathode 53 the electrons are collected by means of an appropriately positioned anode '58.

Pr imary modulation of the electron stream at carrier frequency is produced by means of a grid 60 to which is'connected a source of carrier voltage B l. Additional modulation at signal frequency is obtained by the application of a signal voltage to the deflecting plate 55. such voltage being derived, forexample. from-asource 63. The action oftheplate 55'changes the pitch of'the helical-path described by the electron stream within the shielding cylinder 52 and thus causes the stream to follow a new course, indicated for a particular signal voltage by the dotted line B. Due to the great length of the total electron path, it is apparent that a relatively small change of this character will cause a very material shift in the time required for electrons to traverse the cylinder 52.

As a result of beam changes produced in the manner just specified, the current received by the collecting electrode 58 will contain components of phase modulation in accordance with the principles previously described. For eiiective utilization of this modulation the current collected by the electrode is. fed into an appropriate output circuit including a condenser 65 and an inductance 66.

In order to diminish the tendency of the electrode 58 to be affected by the relatively remote components of the electron stream, it may be partially shielded by the provision of unipotential cylinder 68 extending from the end wall of the cylinder 52. This precaution assures the attainment of a higher transconductance than would otherwise be realized. v

This application is a substitute for abandoned application Serial No. 316,339, filed January 30, 1940.

While I have described my invention by reference to particular embodiments thereof, it will be understood that additional modifications may be made by those skilled in the art without departing from the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A system of phase modulation including means for producing an electron stream, means for modulating the stream at a carrier frequency, means defining an elongated space to be traversed by the modulated beam, means for causing the beam to follow a helical path through said space Number so that the electron transit time through the space comprises at least several complete cycles 01 the carrier, means for varying the pitch of the said helical path at signal frequency thereby to vary the said electron transit time through the said space, and an output circuit adapted to me excited by the electron stream after its traversal of the said space.

2. A system of phase modulation including means for producing an electron stream, means for modulating the stream at a, carrier frequency. means defining a space to be traversed by the modulated beam, means for causing the beam to follow a, helical path through said space so that the electron transit time through the space comprises at least several complete cycles of the carrier, means for varying said helical path at a signal frequency thereby to vary the electron transit time through the said space and output means adapted to be excited by the electron stream after its traversal or the said space.

SIMON RAMO.

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

UNITED STATES PATENTS Name Date Ramo June 24, 1941 Gray Aug. 26, 1941 Brown Apr. 14, 1942 Goldstine July 21, 1942 Hansen et a1. May 5, 1942 Smith May 16, 1944 Labin Mar. 27, 1945 FOREIGN PATENTS Country Date Great Britain May 5, 19-12 Number

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