US2534508A - Electron tube - Google Patents

Description

DCC. 19, W, R. FERRlS 2,534,508

ELECTRON TUBE Original Filed April 3, 1944 oww@- Patented Dec. 19, v1950 ELECTRN TUBE Warren R. Ferris, Kingston, N. J., assignor to Radio Corporation of America, a corporation of Delaware original application April 3, 1944, serial No. 529,281. Divided and this application February 28, 1946, Serial No. 650,957

This application is a division of my application filed April 3, 1944, Serial No. 529,281, now Patent No. 2,422,088, dated June l0, 1947. The invention relates to electronic phase and frequency discriminators useful in the .detection of frequency-modulated (FM) waves and in various other circuits in which phase variation may be useful, and more particularly to a frequency modulation discriminator of the purely electronic type.

An object of my invention is to provide an electron tube of plural electron paths of different length for convection currents having the same modulation.

Another object is to provide an electronic methd of obtaining from an ultra high frequency alternating current two or more currents of the same frequency but with components of alternating current which differs in phase.

Another object is to provide an electron tube for converting phase and frequency modulation into amplitude modulation.

Another object is to provide an electron discharge tube which is particularly useful as a phase splitter for high frequency alternating currents.

Another important object of my present invention is to provide a novel and improved device for deriving amplitude modulated carrier wave currents from angle modulated carrier waves.

Another object of my invention is to provide a tube having two electron beams of different time delays with modulated carrier currents and deriving from secondary emission currents from the time-.delayed beams. i

. Still other features of my invention will best be understood by reference to the. following description, taken in connection with the drawing, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect, but these are given by way of example only.

In the drawing:

Fig. 1 shows schematically the networks of an FM receiver adapted for use with an electronic discriminator of my invention, the discriminator device being shown in cross-section.

Fig. 2 graphically shows the ideal characteris tic of the discriminator device of Fig. 1.

Fig. 2a represents an oscillogram of the two currents at the 180 degree point of Fig. 2.

Fig. 3 shows a modified form of discriminator device, preferable at ultra-high frequency.

7 claims. (ci. 25o-27.5)

Fig. 4 illustrates the manner of utilizing secondary emission in an electronic discriminator device of thetype shown in Fig. 1.

Fig. 5 shows a modification of the device shown in Fig. 4, wherein a pair of independent collector circuits are employed to provide currents of split phase.

2v Referring now to the accompanying drawing, wherein like reference characters in the different figures correspond to similar circuit elements, my novel electronic discriminator device is shown applied to a system for receiving FM signals.

Since those skilled in the art of FM communication are fully acquainted with the manner of constructing an FM receiver system, the latter is shown in purely schematic form. The FM signals are assumed to be located in the 42 to 50 megacycle (mc.) band and have a maximum deviation'range up to 75 kilocycles (kc.) on each side of the transmitter mean or carrier frequency.

My invention is not restricted to any specific frequency range, nor to any particular frequency deviation range. The FM signal energy is collected at collector I which is preferably a dipole, although any well-known source of signals may be used. The modulation on the carrier may be sound, video or code. The converter 2 may-be of a well-known form and is fed with collected FM signal energy and local oscillations. The latter are produced by a local oscillator 3 capable of generating oscillations whose frequency is chosen to combine with the FM signal mean frequency to provide the converter operating output frequency. For reasons to be later explained, I prefer to employ the sum frequency of the FM signal frequency and oscillator frequency as the operating frequency at the converter output. As-

`suming the oscillator is adjustable for a frequency range of 58 to 50 mc., there will be provided FM signal energy at the input terminals of amplifier 4 whose mean frequency is, for example, mc. l

Where the converter 2 and oscillator 3 are concurrently varied over the respective frequency ranges of 42-50 mc. and 58-50 mc., the converter output energy will have a mean frequency of substantially 100 mc. throughout said frequency ranges. For operation at xed frequencies, oscillator 3 may be crystal-controlled in the usual and well-known manner. Of course, the conversion step may produce a difference beat frequency where the collected signal energy is suiciently plied directly to the amplifier 4 without usingy conversion. It is to be clearly understood that the 100 mc. value is purely illustrative and may readily be higher or lower in magnitude.

, After amplification at amplifier 4, the FM signal energy may be applied to amplitude limiter 5.

The latter can be of any well-known construction and generally functions in the manner of a readily-saturatable amplifier thereby to eliminate,

or greatly reduce, amplitude variations in the FM signal energy applied to the resonant input circuit 6 of the discriminator device. Circuit 5 is tuned to 100 mc., which is the predetermined desired mean frequency Fc of FM signal energy to be applied to the discriminator device. It is to be understood that theV selective circuits of networks 2, 4, and 6 are to be given pass band characteristics sufficiently wide to pass the maximum frequency deviations of the signal energy;

Before describing in detail the constructional and functional features of the electronic discriminator device, it is pointed out that numeral 'I denotes an output circuit tuned to the mean or carrier frequency Fc of 100 mc. Across this tuned output circuit thereV will bev developed amplitude modulated CAM) carrier wave energy of 100' mc. Any FM effects remaining are purely incidental and' donot affect the operation of a conventional detector. The amplitude modulation will correspond to, and be representative of, the frequency deviations of the FM signal energy collected at collector I. Thel AM carrier- Wave energy at out'- put circuit l will beY applied to any desired form of rectier device, such as a simple diode. The rectified modulation may" then be `amplified in any' well-known manner and utilized. y Of course, the electronic discriminator about to' be described can' be' used with' direct application ofirequency, cr phaseV variable energy to input circuit t. Also, the' genericY expression angle modulated employed herein is to be understood as including frequency modulation, phase modulation and o'ther'relat'ed` forms of modulation. My invention intended to cover the utilization of any source of" angle modulated carrier waves' in connection with the electron discriminator device.

Thediscriminator device is generally shown in cross-section. It isv an electron discharge tube of the so-called orbital beam' type. The dimensions and spacing of' the electrodes are not critical and the construction may' be along the lines of the tube disclosed in the patent to H. M. Wagner, No; 2,293,418, granted August 18, 1942. Since the latter patent gives precise and detailed information as' to the manner of constructing tubes of the orbital beam type, it is believed sufiicient for the purposes of this invention to show the genera-l relations of the tube electrodes in the crosssectional manner used herein. The tube envelope 8 is provided Iwith concentric cylindrical metallic focusing electrodes 9 and Ill. The outer electrode 9` may be at' ground potential or even positive thereto, but it is preferably connected to a potential point which is relatively negative with respect to ground. The inner cylindrical electrode i is connected to a source of relatively high positive potential.

Thet'wo cylindrical electrodes have a common axis. The flat electron emitter or cathode, indicated by numeral I, is grounded and is constructed to emit electrons from each opposed ilat face thereof. The cathode II is indicated as hollow and having a rectangular cross-section. The heater element is located within the walls of the emitter. The cathode element' is equidistantly spaced between concentric focusing electrodes 9 and I0 and provides a pair of oppositely directed electron beams from its opposed flat emission faces. The beams pass through the mesh of thecontrol grid I2 which surrounds the cathode with its walls parallel thereto and spaced substantially uniformly therefrom.

f The control grid I2 is connected to one side of the input: circuit 6. Thev other side of circuit 6 is connected to a negative potential point, approximately the same negative potential as that oi electrode 9. The flat collector electrode i3 is located in spaced relation to grid i2 and is inclined relative thereto. The collector electrode I3 is equidistantly spaced between the focusing electrodes. The electrode i3 is connected to one side of the primary circuit e oi output transformer 1, and the other side of circuit 'i' is connected toV apositive potential point which is relatively more positive than the focusing electrode lil. It is to be understood that a common direct current source may be used to supply the negative and positive voltages for the electrodes within the envelope 8. The actual values of positive and negative potentials employed may in the light of the description herein bev left to the discretion oi' those skilled'Y in the art.

The orbital beams ern'` ted in opposite directions aredenoted' by dotted circular lines itl and I5. The electronV beam It is the long path beam and flows from cathode li through grid I2 in a clockwise direction to the right-hand face oi co'llector electrode i173. The beam l5 is the short path beam and travels but a relatively short distanceY from cathode il through grid l2 in a counter-clockwise direction to the left-hand face of electrode i3. The dottedV arcuate lines l and i5 schematically represent the electron beams flowing toY electrode I3. The beam I4 is caused to follow a constantly curved path in a clockwise direction by virtue of the electrostatic force eX- erte'd by the positive inner focusing electrode It. The highly positive collector electrode I3 provides acceleration for the electrons.

Inv order to explain the functioning of the electronic discriminator tube, there will be first set' forth certain facts relative to the electron beams Ill and l5. it is known that a progressive change of phase is suiiered by the alternating convection current component of an electron beam asA its constituent electrons traverse a given path. So long as the electrons in the beam keep in step, this is the only effect of a relatively long path upon the alternating convection current component.

The term convection current is peculiar to electronics and is the current representing the actual quantity of electricity, usually carriedY by electrons, entering a conductor per unit of time. The symbolic representation of the convection current is zpo, where p is the charge density (say in couloinbs per cubic centimeter), v is the beam velocity in centimeters per second and i is` the current density in amperes per square centi'- meter. The displacement current is the current induced in a real or hypothetical electrode at a given location by the change of potential gradient on the surface of the conductor with time. Whenever modulated electron clouds or streams are encountered, currents of both above kinds are foundl in all conductors capturing electrons, but only displacement currents exist in negative electrodes capturing no electrons. The displacement current associated with the electronic charges is small at low frequencies, but may become very large at ultra-high frequencies. In the present invention displacement current is disregarded and is readily eliminated by assuming an electrostatic screen I5 is placed around collector I3, as indicated in Figs. 1 and 3. It is to be then understood that only convection current contributes substantially to the net output current.

lD two steady electron beams are varied bythe same alternating control voltage and subsequently collected by a single collector element, their instantaneous algebraic sum determines the net alternating current owing in the collector element circuit. Further, if the electrons in two streams having the same alternating current component are caused to differ in transit time in periods per second by an odd number of half periods, the term designating a complete cycle of alternating current or voltage, the net effect of the two streams will be to produce in the co1- lector circuit only direct current and no alternating current. If now both streams are varied by an alternating voltage of a somewhat different frequency from that producing zero alternating current in the collector element or electrode, the alegbraic sum of the two alternating components will no longer always be zero. The sum of the components will have a steady alternating current component proportional in some fashion to the difference in transit angle resulting from the fact that the transit angles no longer diler by exactly an odd number of half periods.

The discriminator tube may have its parameters so chosen that at the highest frequency of modulation the short electron beam would arrive at the collector electrode I3 at approximately a zero half period point and the other, or long, beam would arrive at the collector a half period later or as large an odd number of half periods as possible. Thereby, the arrangement is made sensitive to small frequency deviations of the signals from the mean or carrier frequency. It is now seen why it is desirable to operate the tube at a very high frequency, of the order of 100 mc. The eiect sought after is best secured at such frequencies unless the dimensions of the tube are made excessively large, since transit angles of even one-half period require a fairly long path unless the control frequency is very high. On the other hand, if the control frequency is made exceedingly high (say for a wave length of the order of 1 centimeter), it becomes difficult to keep even the short path less than one-half period.

Hence, I propose to permit the short path I5 to go where it will and merely lengthen the path I4 by an amount diiering from that of the path I5 by an odd number of half periods. For example, if the short path transit time is 1/10 period, the long path transit time would be l/io-I-n/z, n being an odd integer. The limit to the useful electron path length in any beam tube is reached when the effect of variations in the initial velocity (or secondary emission scattering) of the electrons in the beam smears the collected beam out in time phase so that its original modulation is seriously decreased. This requires several periods in a well-constructed tube.

So far as actual dimensions and proportions of the discriminator tube are concerned, a tube suitable for use in a system adapted to receive signals at 40 mc. would be little more than onehalf inch in diameter and about an inch to one and one-half inches long. For example, it is commercially feasible to use a cathode and grid assembly in which the indirectly heated cathode hasa diameter or thickness of about mils; the control grid wire, about l mil in diameter, is spaced about 5 mils from the cathode surface and the screen grid is spaced about 5 mils from the control grid. With such a cathode grid assembly the tube may be made so that the curved or circular path midway of the cylindrical electrodes, Band I0 and along which the beam` I4 manner, and by way of explanation only, the

phase relations existing between the alternating currents reaching the output electrode I3 from `the short path beam I5 and the long path beam I4, respectively. The curves a and b of Fig. 2a represent an oscillogram of the two alternating currents at the 180 point of Fig. 2. It will be observed that the short path current, represented by solid line curve a, goes through one complete cycle in one-one hundred millionth of a, second at mc. The electron current of the long path, represented by broken line curve b, starts in electrode I3 one-two hundred millionth of a second after the short path current reaches that electrode. The net current collected at electrode I3 depends on the relative amplitudes and phases of the two beams at the instant of arrival. So long as the principles of the invention are not departed from, electrode I3 can be variously 1ocated to suit the desires of the designer.

Fig. 2 relates the Net Output Current as ordinates in the collector circuit to Relative Phases of Electron Beams as abscissae. It will be observed that the net alternating output current through electrode I3 is zero at 180. Assuming that the predetermined frequency Fc of circuit 6 is such that the net alternating output current will be somewhat as shown in Fig. 2 for the instant that the signal energy has a frequency of Fc, it can readily be recognized that the slope of the characteristic of Fig. 2 permits discrimination. As the applied signal energy deviates from Fc the net alternating output current flowing in circuit 'i' will vary by virtue of the variable relation in the phases of the alternating components of electron beams I4 and I5. The variable net alternating output current is an amplitudemodulated wave whose eiective frequency is, in the example given, 100 mc. and whose amplitude modulation corresponds to the frequency modulation on the original FM wave. Operation at the zero point on the characteristic c of Fig. 2 may be used for telegraph code, but is not so satisfactory for telephony as the curvature near the zero point is likely to produce some distortion, which is absent when the operation is had around an intermediate point as Fc. rIhe location of collector electrode I3 to secure the mean net current at Fs may be determined by mathematical calculation to get about 71/4 periods between the transit times. Also, by trial and error in adjusting the overall voltage, there can be determined the location of electrode I3. In practice it would probably be convenient to make the short transit time about 1/10 period (assume 11:1) and the long period 1/1o-{-1/4=7/2o period.

If desired, and as shown in Figs. 1 and 3, an electrostatic screen such as a grounded suppressor grid l5 can be employed around the collector I3. The function of the screen, as stated above, is to prevent the displacement current components of the beams from affecting the net output current. Further, a screen grid element I6, somewhat less positive than electrode I0, may surround the control grid I2. The remaining l'construction of the device will be as .shown in Fig. 1.

.It is to be understood that a very long path might be given to one beam by the lapplication of a magnetic iield so that a tube could be designed to operate at fairly low radio frequencies. Itis, again, emphasized that the tube shown in Fig. l is but one mode of providing the two electron beams of different transit times. Adjust- 'ments for frequency could be made either by varying the electrode voltages of the tube, or by varying the frequency of local oscillator 3. If a crystal-controlled oscillator is used, then adjustment of the electrode voltages of the discriminator tube will be employed for frequency adjustment.

Merely by way of illustration vthe following direct current voltages are specified for the tubes 8 of each of Figs. .1 and 3. The cathode voltage may be Zero or ground; control grid i2 may have a bias Aof --2 volts; inner cylinder I0 may be at +300 volts; outer cylinder 9 may be at a voltage between +100 and h400, Zero potential is convenient; collector I3 may be equal or greater in voltage than cylinder IE if suppressor I' (Fig. 3) ,is omitted. If the suppressor l5' is used around collector I3, then any positive voltage above 100 volts may be applied to the collector. I prefer an additional (screen) grid surrounding the control grid i2. This is shown in Fig. 3, where I designates the positive screen grid which may be at +100 volts. Further, as shown in Fig. 3, the suppressor I5 may be at ground potential. At ultra-,high frequency the screen grid 'I5 is highly desirable, as is also the suppressor grid I5.

My invention is readily adapted to a beam tube employing secondary emission multiplication. In Fig. 4 I have shown the collector electrode i3 located opposite a pair of rangularly related secondary emission electrodes 2? and 2i. Beam I4 will impinge on electrode 2I, while the short path beam I5 will fall on electrode 20. The surfaces of electrodes 2B and ZI are treated with any well-known emission coating to promote secondary electron emission. In the aforesaid Wagner patent such secondary emission is used in conjunction with the orbital beams. The collector electrode I3 will, of course, be located to be out of the path of either of the primary orbital beams I6 or l5, but will be at equal distances from the faces of electrodes 2E! and 2l. In this way the collector output current will be essentially the secondary emission currents of electrodes and 2i, but the relative phases of the currents depend on the relative phases of the beams IG and i5. The relative electrode polarities will be as indicated in Fig. 4. The collector I 3' will be at a higher positive potential than the secondary emitter electrodes 20 and 2 l while the outer focusing electrode 9 may be operated slightly positive relative to the grounded primary electron emitter. Secondary emission provides a good way to provide a high output from a small tube structure, and is used for that purpose herein.

My invention is readily adapted to derive two separate currents of different phases from the applied signal currents. This is shown in Fig. 5. The phase splitting action is produced by utilizing secondary electron emissions from electrodes 2G and 2i and respectively associated collector electrodes di! and 4i. In other words, the tube construction' is similar to that of Fig. 4, except that instead of a single collector electrode I3 there are-used separate collectors 40 and 4I. The collector electrode It is located adjacent the secondary emitter 2e and out of the primary electron stream I5 from cathode to electrode 25. The collector electrode il is located adjacent secondary emitter electrode 2i and out of the primary electron path I4. A resonant circuit 3B is connected to collector electrode e! and a separate resonant circuit Si may be connected to collector electrode bili. rEhe resonant circuits 30 and 3I are each tuned to the same frequency as input circuit (i.

Since the primary beam paths I4 and I5 are of different transit times and the respective secondary electron beam paths have phase characteristics corresponding to those of the primary paths, the phases of the alternating currents iiowing in the output circuits 33 and 3l will be different, and are generally designated by the symbols P1 and P2.

The differentially phased currents flowing in circuits Se and Si may be used in any desired manner. One manner of use would be to combine them to produce the effects illustrated in Fig. 2. The electrodes l5 and di will be at the highest positive potential of the entire electrode system and secondary emitters and EI will be connected to a point of positive potential Vless than that to which electrodes il and il are connected and more positive than inner focusing electrode Iil.

It is desirable to have the secondary beams in each of Figs. i and 5 of equal strength, but the secondary beam transit time is merely to be added to that of the primary beam so that the odd half period condition is satisfied by the total primary plus secondary transit times for the long path and short 'path electron beams. If desired, phase splitting can be secured by using two separate primary beam collector electrodes instead of employing secondary beams as in Fig. 5.

It is to be understood that in each of Figs. 3, 4 and 5 the tube envelope 8 has been omitted to preserve simplicity of disclosure. rEhe following voltages are given in connection with the tube shown in Fig. l (and it is to be understood that the stated voltage values are purely illustrative) z Cathode lI=0 volts Control grid I2=2 volts Inner cylinder l'l=+300 volts Outer cylinder S=between and +100 volts,

say zero Secondary emitter 2f:+2'75 volts Collector IS=+350 volts In the case of the tube shown in Fig. 5 the same voltages may be used for like electrodes with reference to Fig. d. Each'of collectors lil and 4I will be at +350 volts in Fig. 5. In the tubes of Figs. fr and 5 I would, also, prefer a screen grid, at +100 voits for example, surrounding control grid i2. lio suppressor grid need surround the collectors these forms of the invention.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope my invention.

What I claim is:

l. An electron discharge device comprising a pair of electrodes spaced apart to provide a passage therebetween, a cathode having surfaces facing in opposite directions in said passage, a

control electrode adjacent said cathode surfaces and an electrode in said passage spaced unequal distances from said cathode surfaces adapted to provide transit time periods for the currents between it and said surfaces differing from each other approximately an odd number of half periods of the frequencyof the control electrode voltage.

2. An electron discharge device comprising a pair of curved electrodes spaced apart to provide an annular passage, a cathode having surfaces facing in opposite directions in said passage, a control grid around said cathode surfaces and an electrode in said passage spaced unequal distances from said cathode surfaces adapted to provide transit time periods for the currents between it and said cathode surfaces differing from each other approximately an odd number of half periods of the frequency of the grid voltage.

3. An electron discharge device comprising a pair of electrodes spaced apart to provide a passage therebetween, a cathode having surfaces facing in opposite directions in said passage, a control electrode adjacent said cathode surfaces, an output electrode in said passage spaced unequal distances from said cathode surfaces adapted to provide transit time periods for the currents between it and said surfaces differing from each other approximately an odd number of half periods of the frequency of the control electrode voltage and a screen electrode around said output electrode.

4. An electron discharge device comprising an envelope containing a pair of curved electrodes spaced apart to provide an annular passage, a cathode having surfaces facing in opposite directions in said passage, a control electrode adjacent said surfaces and an output electrode in said passage spaced unequal distances from said cathode surfaces adapted to provide transit time periods for the currents between it and said cathode surfaces differing from each other approximately an odd number of half periods of the frequency of the control electrode voltage.

5. An electron discharge device comprising a pair of curved electrodes spaced apart to provide an annular passage, a cathode having surfaces facing in opposite directions in said passage, a control grid around said cathode surfaces, an anode having a secondary electron emissive sur- 10 face in said passage spaced unequal distances from said cathode surfaces adapted to provide transit time periods for the currents between it and said cathode surfaces differing from each other approximately an odd number of half periods of the frequency of the grid voltage and two output electrodes for secondary electrons equally spaced from said anode.

6. An electron discharge device comprising a pair of electrodes spaced apart to provide a passage therebetween, a cathode having surfaces facing in opposite directions in said passage, a control electrode adjacent said cathode surfaces, an electrode having a secondary electron emissive surface in said passage spaced unequal distances from said cathode surfaces adapted to provide transit time periods for the currents between it and said surfaces differing from each other approximately an odd number of half periods of the frequency of the control electrode voltage and an output electrode for secondary electrons spaced from said secondary electron emissive surface.

'7. An electron discharge device comprising a cathode having two surfaces facing in different directions, a control grid adjacent said cathode surfaces, an electrode spaced from said cathode for receiving electrons therefrom, deflecting means adjacent said cathode and said electrode for causing electrons to travel from said two cathode surfaces to said electrode along two separate paths of unequal length providing transit time periods for the two electron currents between said cathode surfaces and said electrode differing from each other by approximately an odd number of half periods of the frequency of the control grid voltage.

WARREN R. FERRIS.

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

UNITED STATES PATENTS Number Name Date 2,220,841 Metcalf Nov. 5, 1940 2,254,096 Thompson Aug. 26, 1941 2,293,418 Wagner Aug. 18, 1942 2,299,619 Fritz Oct. 20, '1942 2,422,088 Ferris June 10, 1947

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