US2443958A - Modulation - Google Patents

Description

June 22, 1948. c. N: KIMBALL 2,443,958

ATTO/@NE )4 June 22, 194s.y .N;K1MBA1;L 2,443,958

MODULATION V Filed Sept. 23, 1944 2 Sheets-Sheet 2 IN .VEN TOR. //f/ma Patented June 22,1943 l TENT OFFICE MoDULATIoN Charles N. Kimball, Kansas City, Mo., assignor to Radio Corporation of America, a corporation of Delaware Application September 23, 1944, Serial No. 555,465

6 Claims.

This application pertains to angle modulation of carrier wave energy in accordance with signal-s. ln an embodiment amplitude modulation is accomplished.

In this specicatiou the term phase modula tion is used throughout the description. It will be understood that the modulation may be more generally designated as angle modulation since the same may have characteristics of what is termed in the art phase modulation or frequency modulation or characteristics of phase and fre quency modulation.

The particular nature of the angle modulation will depend on the treatment of the modulating signal before it is used for angle displacement of the wave energy in `accorda-nce with my invention. In any event, in my improved system the angular displacement of the wave energy is 1inear with respect to theamplitude of the `modulation.

An object ci my invention is to increase the amount of angular or phase deviation of carrier wave energy in accordance with modulation currents.

Another object of my invention is to produce increased angular or phase deviation of carrier wave energy, which deviation is linear with re spe-ct to the amplitude of the modulation currents.

lvl-y improved method and means produces a phase modulated signal, in which the phase deviation can be mad-e to vary linearly-with the amplitude of the modulating voltage over approximately 360 at the carrier source frequency. In other "systems known heretofore, not employing this principle, the amount of phase deviation is generally limited to angles for which the tangent and the angle are substantially eclual in Indians. If a harmonic of the output is: taken the phase deviation is multiplied an amount equal to the harmonic-number.

The principle involved` is also applicable to am.- plitude modulation and in an. embodiment of my improved system amplitude modulation 'is accomplished. A further object of m-y invention is an improved amplitude `modulation system wherein the` carrier is modul-ated `linearly' in accordance with control or modulation currents or potentials.

In describing my invention in detail, reference will be made to the attached drawings wherein Figs. l, 2, 4,15 and 6, illustrate targets of an electron beam tube used-iuy my improved modulation system. These figures` are used in explaining the principle of operation of my invention;

Fig. 2 also illustrates vby circuit and block dian gram the essential features of `an arrangement for carrying out my invention;

A Fig` 3 illustrates a preferredarrangement for generating. gawaveifozt s-awftooth form and applying the same to the deflecting elements of the tube. This ilgure also illustrates preferred means for generating a blanking voltage or potential and applying the same to the control grid of the electron ray tube.

\ The target illustrated in Fig. 6 is used when amplitude modulation is accomplished; while Fig. 5a illustrates by block diagram the means for producing and applying the control voltages to the electron beam tube when a target as'shown in Fig. 5 is used.

To illustrate the principle involved consider an electron device similar to the monoscope or also to the iconcscope, in which the usual mosaic surface of the target elect-rode is replaced by la plate of insulating material illustrated schematically lat il, Figs, 1, 2, 4, 5 and 6, to which is `fastened a conductive member such as a metallic bar t; lying in the plane of the surface of the target or insulating kplate and connected to `an extern-al load resistance RL (Fig. 2). Either static or magnetic reection (preferably static by Plates PH) is used to dellect the electron beam horizontally at the carrier frequency rate, say hc. This is accomplished by generating and applying to the horizontal -deflecting elements El a saw-tooth Wave entrained or keyed in with the unmodulated carrier frequency of said 100 kc. The vertical deflection of the electron beam is obtained electrostatically (electro-magnetic deilection may be used) by applying modulating voltages through a linear amplifier 8 to the vertical plates VP.

Assume iirst that `the bar 6 is placed in a vertical plane, i. e., as illustrated in Fig. 1. Each time the beam sweeps horizontally across the contact bar i5 a pulse of current is generated and appears through the load resistance RL. If the return sweep (scan) is blanked out at the car--V rier lfrequency rate, the pulse repetition rate is equal to thecarrier sweep frequency, i. e., the repetition rate is in the example given 100 kc. per second, *lf the conducting member 6 is perfectly vertical the application of modulating potentials tothe vertical plates PV will produce no effect on the output signal which will be pulses of a repe` tition rate of 3.00 kilocycles per second.

However, if `the conducting member or ibar is inclined at an angle to the vertical, as shown in Fig. 2, while still lying in the plane of the insulating plate Il, the time elapsed between the startof a horizontal sweep and the point` of time where the beam hits the bar l will be a function of the vertical displacement of the bar, and/or o the instantaneous value of the modulating voltage from amplifier 8 which controls the vertical deilection;

frequency wave characteristic and with blanking `applied to the electron guns grid GI to prevent Thus, in the ideal case, with zero return time in the horizontal saw-tooth sweep the generation of a spurious pulse during the flyba-ck interval, i. e., the blanking time, a possible change in pulse phase of i180o at the applied carrier frequency exists. With a longer return time of the horizontal sweep frequency, say 10, this range is reduced proportionally.

The wave shape of the output pulse appearing across RL will be essentially square neglecting the nite rise time due to output circuit capacitance limitations and to the aperture eect due to the nite spot size in the cathode ray tube I0. Hence the output wave will contain harmonics of the applied source carrier frequency such that they may be picked off and employed to increase the phase deviation. That is, if i degrees are obtained at the carrier source frequency, inc degree shift will be available, if say a parallel tuned circuit tuned to the 'nth harmonic of the pulse is coupled to RL and the nth harmonic picked off in a resonant fashion to be used as the output voltage of the beam tube I 0. The output appearing across RL may be coupled into a resonant circuit tuned to the fundamental frequency f, or into a resonant circuittuned to a harmonic nf, or a resonant circuit tunable to the fundamental or a harmonic, directly or through a wide band amplifier 30.

A means for producing and applying a sawtooth wave of the desired carrier frequency and applying the same to the beam deflectors PH of the tube I as the sweep frequency and the means for producing and applying the control energy for blanking on the return sweep is shown inFig. 2.

The tube I0 has a cathode, a stream intensity control grid GI, a focusing anode AI, and a second anode A2 which is connected to the positive end of a potentiometer resistance I6, usually grounded at the positive end. The cathode is tapped up from the negative end of potentiometer I6 with respect to the point at which the control grid GI is tapped, so that GI operates at the desired negative bias.

There is a target anode comprising an insulating plate 4 and a conducting member 6. The conducting member 6 is connected with an output load resistance RL across which the phase shifted pulses and harmonics thereof appear.

The cathode ray tube l0 also has a pair of horizontal deflecting plates PH connected with the output of an arrangement in unit 20 for generating waves of saw-toothed form and of a periodicity equal to the frequency of a control source in 20 by means of which the saw-toothed wave is synchronized. Saw-toothed wave generators are known in the art and may take various forms. Several preferred generators are described hereinafter. The unit 20 has an output of a wave form as illustrated along lead 22.

Although the return sweep time is only labout 14% of the saw-tooth wave cycle, the tube l0 stream should be stopped during the return time. In other words, the system should be blanked during the return sweep in order to prevent distortion from occurring in the output, particularly at the reversals in direction of flow of the current producing the saw-tooth wave. In accordance with my invention a pulse for blanking the system is developed by differentiating the sawtooth wave applied as the horizontal sweep control. I have shown at 30 a unit representing a diierentiating circuit by means of which the rst derivative of the saw-tooth wave form may be obtained. This differentiating network may take various forms, some of which will be described in detail hereinafter. It includes means, however, for deriving a wave form as illustrated at the leads between the rectangle 30 and the wide band driving amplifier 3'2. Note that the output of this 1differentiating network has a wave form in which the voltage is constant during the time the saw-tooth wave form is rising at a constant rate, and then drops suddenly in value as the saw-tooth wave falls off rapidly at a constant rate to its original point. The cycle is then repeated. The peaks in the wave at the output of the differentiating circuit occur during the return sweep of the tube beam, and this wave is supplied to a wide band driving amplier in 32, and impressed from 32 by a coupling condenser 34 on the grid GI'.

During operation the cathode ray tube electrodes are biased as shown by connections to a potentiometer I6. The grid Gl is adjustably connected through a grid leak resistance GL to a movable point on I6, and this grid runs negative with respect to the cathode by an adjustable amount such as to provide during the sweep period an electron beam from the cathode through the fields of the various electrodes and to the target 6 by way of the horizontal and vertical deflecting elements. A potential is developed in GL by the grid current and when the sweep frequency voltage has nished rising during the sweep portion of the cycle, to fall suddenly on the return sweep, an amplified potential which is negative is impressed from 32 through the coupling condenser 34 on to the grid GI to bias the same negative with respect to the cathode by a biasing value such that the electron stream in tube l0 is cut ol. The condenser 34 may be considered a short circuit for the high frequency output of the wide band amplifier 32 so that when the amplifier side of the condenser 34 becomes highly negative the other side does also, and this negative potential biases the gri-d GI to cutoff.

The differentiating circuit in 30 is a means for deriving the rst derivative of the saw-tooth wave. In its simplest form this may be a condenser and a small resistance in series with voltage of the saw-tooth wave form applied across the combination. The current through this resistance then will be the first derivative of the voltage across the combination and therefore the differential of the saw-tooth wave form, the voltage of which is applied across the combination. The condenser C'l and resistance RI of Fig. 3 serves this purpose as will appear in detail hereinafter. When this simplified dierentiating means is used in the amplifier 32 comprises two stages, so that at its output the peaks are in the same direction as at its input, i. e., in a negative direction. Of course a single -cathode driven stage wherein the input wave and output Wave are in phase may be used at 32.

A preferred means for producing the saw-tooth wave form voltage for the horizontal deflecting plates PH and the blanking control potential for grid GI is shown in Fig. 3. This means comprises a conventional crystal oscillator including tube 40 having a tuned tank circuit 42 and a crystal element X so arranged and operated that oscillations of sine wave form and xed frequency are developed in the tank circuit 42. As an example, the generated oscillations could be of 100,000 cycles and have an amplitude of 200 volts alternating current. The oscillations are impressed on a pentode tube 50 having its control grid self-biased highly negative by a biasing resistance `52. The anodev potential suppliedthrough resi-stance '54 vand the :bias on the `control grid. etc., 'of .fthis ftube 5l] is such that it is nbiased Yto cutoi, V`except for small fportions of the vpeaks of the vralternating voltage impressed ontheglid `5&3.

"On application zof the sine wave voltagegenerated in 4|] to the grid 56, theigrid leak 52 develops-aa biasnearly `equal to the peak-signal applied tothe grid and `peaks of this sine Wave voltage barely reach :grid current through the tube; This is shown schematically by the graph above ythe'circuit 42. rI'he `plate'current as aconsequence flows Iasmall .part of the .time at 'the peaks of v:the applied sine wave. The tube 2-there fore may becon'sidered to operate as a'fclass'C amplier with a 'capacitive plate load.

1A condenser CI is connected between'the anode and lcathode of the tube l5|), thereby being effectively connected in shunt to the outputimpedance of tube 5l!l (dueto condenser BPC) and in shunt to 'resistance 54. Whenthetube Ell-is cut off the 'condenserCl Vis charged throughresistance 54 by current andfvoltage from the source connected to thefanodes of the tubes.

The Avoltage on the condenser Cl starts rising andfrisesat a constant rate until the voltage of sine wave form on the grid 56 reaches the rpoint at'which cutoff bias is overcome. The plate circuitof thetube 5B-now draws current to discharge the condenser `of Cl rapidly. Thus across ythe condenserCl is produced a voltage of saw-tooth waveaform.

Thestage includingtube 60 isa resistance cou- .pled `class A-.pentode amplifier, feeding `the am- .in-#the art and will not be described rin detail herein. The rpurpose ol this tube is to provide at `itsoutput-across the resistance [i2 amplified voltage of awave-form corresponding tothe vVoltage'A applied at the input of this tube.

.The vvdiiierentiating network for supplying lthe blanking controlpotential comprises the condenser -CI `and a resistance R2 in series therewith. This resistance yR2 Yis small relative tothe impedanceof the condenser Ci. As stated above, the i'current `through the circuit including R2 -is therst derivative of thevoltage across the condenser -Cl and consequently, the differential of the saw-tooth wave .form voltage Vacross `CI. Therefore 41I derive lfrom R2 a Voltage 'which is oft-constant 'intensity during the time the condenser fCI ischarging andthe saw-tooth wave isi-.rising ata constant -rate 'and this voltage 'then dropsabruptlyto a negative value whilexthe 'icondenser 1C I i-s `discharging at a constant but faster rate;.-andthesaw-tooth `wave is on .'the blanking part of the cycle. Thepotential'acrossRZ `is supplied to a wide band driving amplifier 32', and thence rby a coupling v-condenser 34 to the grid of Athe tube Ill.

Of course, saw-tooth ywave-generators of other types such as, for example, shown in Hoover U. S. Patent #1,978,461, filed November 25, 1933, and issued October 30, 1934, may be used. When this patent is used the arrangement of Fig. 1 of the patent is then included with the carrier rfrequency lcontrol source in the rectangle 20. The carrier frequency source may then comprise a crystal controlled generator such as, for example, the generator of Fig. 3 of lthis application, the generator tube 40 anode is then `coupled-to the condenser. 5 lv of the said Hoover patent.

.A differentiating circuit CLFRZ, as described in 6, connectionv :with Figs. `2 y.and 3 L may then kbe -lconnected to the high potential-one of the two-,leads marked Oof the'HooVer et'algFig. `1. Alternatively, =the -voltagerepresented in Fig. 41er the I-Iooverpatentappearing at the'grid of tubesV I5 and l may be picked off atan appropriatefpoint in the 'circuit of Fig. 1 vof Hoover et a1., and fed toa wide :banddriving amplifier and thence applied'byfthe coupling condenser 34 to the gridGI. Note `that in this embodiment the voltage as illustrated` in Fig. 4 haszpositive peaks which occur at the return sweep part of the cycle, and ithe wide lband amplifier then includes an odd number of .stages inforder to frevers'e the wave `form of t'his'voltage toapply thepeaks in` a knegative di'- rection to bias the grid Gl to cutoff during'the blanking period.

`Maximum utilization ofthe effectiveness ofthe system is f attained by making the inclined bar ortargetelectrode-G span both the maximum-hori-l zontal and vertical `dimensions covered by the beam in its sweep across the target as indicated in Figs. 4 to 6. In Fig. 2 it will be noted that the bar 6 is assumed to be so arranged. The bar Width may be 10 to 20% of the distance covered bythe horizontal sweep.

The linearity of the system depends only upon obtaining asaw-toioth wave of linear horizontal formand a linear 'bar as the target 6. The linearity ofthe system'is independent of Vertical and horizontal centering of the beam as long as lthe sweeps doV not extend beyond the extremity `of the `target 6.

The'device may be made to opera-te directly olf a sinusoidal carriersweep by properly shaping themetallic bar l6, plus theuseof blanking, as described above, to eliminate the horizontal return lsweep generated signal. The target 6 `is then as illustratedinFig. 5, having a shapelike a section of a sine wave. In this embodiment then, the :control frequency of sine wave form included in the unit 20 of Fig. 2, or the amplified output of the crystal generator 40 of Fig. 3, or the control sounce to be used with the saw-tooth wave generator of the above identied Hoover patent, may beofed `directly to the `horizontal deiiecting elements PH; and then the blanking controlvoltage -is derivedfbyA other means, such as, for example, any of the numerous circuits known las square wave generators which derive a square wave-from a sine .waveby means of clipping, limiting, etc. (allor which are-knowniirrthe art). The resultant square wave, in proper polarityis appliedlto'the grid GI. `For example,- as illustrated in Fig.-5a, av sine Wave source 140' suchas,.for example, `the crystal-generator `lll-of Fig. 3,:feeds output directly to plates -PH andto afunit32'f. The unit 32".may :include'one or more ytube Astages oper-ating as `amplifiers yand clippers to :produce `arr-output, as shown at .the lead going toG", of square wave form, androf a polarity such` as'fto-bias tube -I to cutoi ori-the return sweeps. "centering of :the sine wave-section-like` target i relative to the position of 4the beam Aspot thereon, -at rest position, should Ebe accurate.

The invention may -|be vused independently as a generator-of amplitude `modulation A-by using a verticalwedge shaped barasithe .target `t. The target would then vbe as illustrated in Fig. f6. Now it `will rbe noted, as the bearnis sweptback and -forthacross the target the peak amplitudeof the current `developed vthrough RL will depend upon .the magnitude of nthe-beam. This Will-.be consideredto loe-constant. However, when modu- 7. lation is applied, the width of the pulse changes as the vertical deflecting elements VP are energized, and it may be shown by Fourier analysis, or actual measurement, that while the peak amplitude of the pulse remains constanuthe relative amplitudes of the Various constituent harmonies change with Variations in pulse Width. The p-ulse width will vary with vertical deflection in accordance with the modul-ating wave form.

When'amplitude modulation is to be carried out the arrangement may be as illustrated in Fig. 2, except that now the target 6 is as illustrated in Fig. 6. The sweep Voltage may be generated and applied as described in connection with Figs. 2 and 3. The same remarks apply to the blanking potential.

I have shown the deflecting means. as comprising plates PV and PH. It Will be understood that electro-magnetic deflecting means in the form of windings excited by the sweepand modulation voltages may be used if desired.

I claim:

1. In a timing modulating system, an electron beam tube having electrodes including an elec- `tron beam source, a control grid, horizontal and vertical deflect-,ing elements, anda target, having a shape like a section of a sine wave, positioned in the beam axis with its surface in a plane substantially at 'right angles to the beam axis and angularly related to the effective eld of a deflecting element, a generator for producing a voltage of sine wave form and of carrier wave frequency and applying the same to deilecting elements to deflect the beam across the target, a wave forming circuit excited by voltages derived from said generator for applying a potential to said grid such as to cut oif the beam during the time at which the return excursions of the beam would take place except for said cutoft` potential, connections for `applying modulating potentials to another deflecting element, an output impedance coupled to said target and a circuit, -parallel resonant at a frequency integrally related to said carrier wave frequency, coupled to said output impedance.

2. In a modulation system, a cathode ray tube h'aving electrodes including an electron beamy source, a control grid, horizontal and vertical deecting elements, and a target including an elongated conducting member in the path of the beam and substantially at right angles thereto,

said elongated member being angularly related to the main or effective eld of the deflecting elements and having ashape like a section of a sine wave, a sine wave generator for producing' a voltage of carrier wave frequency and applying the same to one deflecting element, means for applying a potential to said grid to out 01T the beam during alternate half cycles of th'e carrier wave, means for applying controlling potentials to the other deflecting element, an impedance coupled to said conducting member and a circuit, parallel resonant at a frequency integrally related to said carrier frequency, coupled to said impedance.

3. In a timing modulation system, an electron beam tube having electrodes including an electron beam source, a control grid, horizontal and vertical deecting elements, and a target including a conducting member in the path of the beam and angularly related to the main or effective fields of the -deecting elements, said conducting member having a contour like a sine wave, means for producing a voltage of carrier wave frequency and applying the same to one deflecting element, means for applying a potential to said grid to cut off the beam on one excursion of the carrier wave, means for applying controlling potentials to the other deflecting element, an impedance coupled to said conducting member and a circuit, parallel resonant at a frequency integrally related to said carrier frequency, coupled to said impedance.

4. In a timing modulation system, an electron beam tube having electrodes including an electron beam source, a control grid, horizontal and vertical deecting elements and a target including a conducting member in the path of the beam and angularly related to the main or effective fields of the deflecting elements, a generator' for producing a Voltage of carrier wave frequency and sine wave form and means for applying the same to one deilecting element, apparatus for producing negative potentials of a time duration about equal to one half of a cycle of said sine wave and applying the same to said control grid to cut olf the beam on half cycles of the carrier wave excursions, means for applying controlling potentials to the other deecting element, an `impedance coupled to said conducting member and a circuit, parallel resonant at a frequency integrally related to said carrier frequency, coupled to said impedance.

5. In a modulation system, a cathode 4ray tube having electrodes including an electron beam source, acontrol grid, horizontal and vertical defleeting elements, and a target including an elongated conducting member in the path of the beam and substantially at right angles thereto, said elongated member being angularly related to the main or effective field of the deflecting elements and having a sh'ape like a section of a sine wave, a sine wave generator for producing a voltage of carrier wave frequency and means for applying the same to one deecting element, means for applying a potential to said grid to out off the beam during alternate half cycles of the carrier wave, means for applying controlling potentials to the other deecting element, and an impedance coupled to said conducting member,

6. In a timing modulation system, an electron beam tube having electrodes including an electron beam source, a control grid, horizontal and vertical deflecting elements, and a target including a conducting member in the path of the beam and angularly related to the main or eective fields of th'e deflecting elements, said conducting member having a contour like a sine Wave, means for producing a Voltage of carrier Wave frequency and applying the same to one deflecting element, means for applying a potential to said grid to cut off the beam on one excursion of the carrier wave, means for yapplying controlling potentials to the other deilecting element, and an impedance coupled to said conducting member.

f CHARLES N. KI'MBALL.

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

UNITED STATES PATENTS Number Name Date 2,148,166 Kucher Feb. 21,1939 2,265,145 Clarke Det. 9, 1941 2,265,337 Beatty Dec, 9, 1941 2,298,922 Beatty oct'. .13, 1942 2,300,394 Beatty et al. Nov. 3, 1942 2,308,639 Beatty et al Jan. 19, 1943 2,328,944 Beatty sept. 7, 1943

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