US2315848A - Cathode-ray oscillograph - Google Patents

Description

pril 6, 1943. w. A. GEOHEGAN .GATHODE-RAY OSCLLOGRAPH iled Jari. 4, 1940 zysheetsfsheet 2 INVENTOR y -Wr'// .atente PB v 1.943. I

CATHODE-RAY OSCHLOGH William A. Geohegan, New York, N. Y., assigner I to Allen B. Du Mont Laboratories, Inc., Passaic, N. J., a corporation of Delaware Application January 4, 1940, Serial No. 312,358 s l(ci 1v1-e5) My invention relates to improvements in cathode-ray tube oscillographs.

In the various cathode-ray tube osclllographs i proposed heretofore, condenser coupling from the final amplierto the deection plates of the cathode-ray tube is employed, and positioning of the pattern on the iluorescent screen of the cathode-ray tube is accomplished by applying direct voltages or xed potentials ci D. C., to the deection plates through a high resistance. While this method oi' positioning is satisfactory where good low-frequency response is not desired, when the time-constant of the coupling condenser and the resistance through which the positioning voltage is applied is large enough for good Fig. 6 is a simplified illustration of. a. modiiication.

With reference to 1, vthe numeral I designates any triode, tetrode, or pentode, and the numeral 2 kdesignates a gas-glow tube of the type l having the intrinsic characteristic which resides in substantially'constant voltage drop across this tube over a relativelywide range of current variation. In the particular circuit shown in Fig. 1,

low-frequency response, say to one cycle per sec- :cathode-ray tube, Where such amplifier maybe of the type disclosed in my Patent No. 2,208,254, July 16, 1940, positioning may be accomplished by varying the bias on the input grid of the final stage. While this method of positioning provides certain advantages, it too is sluggish when good low-frequency response is desired.

With the foregoing in mind, it is one of the objects of my invention to provide an improved cathode-ray tube oscillograph which not only embodies an amplifier having very low frequency response such as to a few cycles per second, but which also has the characteristic of instantaneous positioning of the cathode-ray spot or pattern on the fluorescent screen.

Other objects and advantages will hereinafter appear.

For the purpose oi.' illustrating my invention,

an embodiment thereof is shown in the drawings,

wherein:

Figure l is a simplied, diagrammatic view of a cathode-ray tube oscillograph constructed and operatingr in accordance with my invention;

Figs. 2 and 3 are partial views similar to Fig. 1, showing modicatlons;

Figs. 4, 4a, 4b, 5, 5a, and 5b are simpiied, graphical views, illustrative of the principle or manner 0I' operation of my improved oscillograph; and

satisfactory results have been-obtained by using for the tube 2 a conventional one-quarter watt neon bulb, Without a resistor in the base. 'I'he tubes l and 2 and the parts or elements associated therewith constitute the positioning circuit,

designated generally by the reference numeral 3. f

In this positioning circuit 3 the cathode 'I of the tube I is shown connected to tube 2 and re-' sistance 8 in parallel. The resistance '8 is a potentiometer to which one end of the lead I2 to an amplifier is slidably connected with a condenser 9 connected between this lead I2 and the upper end of the potentiometer 9.` The plate of the tube I is connected to a. positive potential above ground the same amount as the lower end of resistance 8 is connected to a negative potential below ground so that the lead I2 can be varied from ground potential to potentials above and below ground potential.

The various parts comprising the .ilnal amplifier, designated generallyby the referenceA numerel, 4, have been designated by the same reference numerals as the respective parts in the same amplifier as disclosed in my copendin'g application referred to. This amplifier embodies the two tetrode tubes I0 and II. `The signal or wave-form to be investigated is applied to the control grid I4 Vof the iirst tube I0.

When a positive signal is applied to the control grid I4 of the first tube I0, its plate and screen-grid current tend to increase, making its cathode I5 more positive and its plate I6 and screen-grid I1 less positive. Any increase in the positive potential of the cathode I5 of the first tube I0 applies also to the cathode I8 of the second tube II since these two cathodesU are con-- nected together. Any such increase in positive potential on the cathode I8 will tend to reduce the plate and screen-grid currents of tube II and make both its screen-grid I9 and its plate 20 more positive. Since the respective cathodes I5 on the corresponding elements of the first tube IU by the input signal impressed on its control grid I4. That is, as the potential on the screengrid I1 of tube I0 tends to be driven less positive by the action of the input signal through tube III, the potential on the screen-grid I9 of the tube I I tends to be driven more positive by the action oi' tube II. Likewise, when the potential of the cathode I5 of tube Il tends to be driven more positive by the action of tube I0, the potential o! the cathode II of tube II' tends to be driven i f less pomtlve by the action of tube II. Therefore, if the circuit constants be properly chosen,

the potential changes onthe respective plates Il and 2| of the tubes I0 and I Iwill be approximately equal in amount and opposite in direction, which is the desired effect.

The control grid 2I of these'cond tube II is grounded as shown, and is therefore held ata fixed potential. For other combinations of circuit components and constants, it may be desirable to hold thecontrol grid 2| of tube II'ata vnxed potential above or below ground.

The plate output from the respective tubes I and II is applied, by the connections 22 and 23. across the denection plates 24 of the cathoderaytube 2l.' The input connection I2 maybe connected to a pre-ampliiier.- v

By means of the push-pull ampliner 4 shown.

tbe average potential of each plate ofl thepair oi' vertical deflection plates Vcan be kept approximately constant, thereby minimizing defocusing ofthe cathode-ray spot with deflection.

. The amplifier I gives flat or constant rel spouse, and one hundred per cent response'from D. C. or zero frequency to one me'gacycle.

'I'heinductances 2l and 2l are not essential to the operation of the circuit, but when. properly chosen values are employed, extend the high-frequency range to which the response is nat by eiectively increasing the value of plateload reactance at the proper rate to compensate for the increased effect of stray and output circuit-capacitance.

Pentodes may be used for the tubes Il and Il. in lieu of -tetrodes, in which case the Suppressors would be connected directly to their respective cathodes.

Distinctive features of the push-pull amplifier l reside in the facts that the input signal, taken from the external source to be investigated, is

applied directly to the control grid of one tube; y

that the respective output voltages of the two tubes II) and -II are equalin amplitude'and op- -posite in phase; that the control grid 2| c of the second tube II is held atfixed potential; that the signal developed onvthe second tube II is produced by the combined eiects of tying the two screen-grids I1 and II together and of tying the two cathodes I and IB together; that the two cathodes I5 and I8 are returned to ground I or to the negative side of the high-voltage supply through a common resistor 29; and that the screen grids I1 and I9 are supplied from the positive side of the high-voltage supply through a common resistor 30. In other words, in my improved push-pull amplii'ler, coupling through a common screen-grid resistor and through a Q common cathode resistor 29 is used to transfer the positioning-control potentiometer 8 is held substantially constant by the gas-glow tube 2 connected in parallel relation to this potentiomete'r, the signal appears with equal amplitude atl all points on the potentiometer, and the vsignal input to the ilnal amplifier I is not, therefore, affected by changes inthe positioning-control setting. The unal fixed bias of the amplifier .on which the signal is superimposed, however,

willvary as the positioning controlis moved.

Therefore, the pattern maybe positioned on the f fluorescent screen 3| without 'affecting its amplitude. lit condenser i may be used where high- `frequency yresponse -is desired. to prevent tre-4 quency -discrimination in the positioning control but due to its small size, it exerts no noticeable effect in slowing the positioning which is `instantaneous.

A further advantage of my improved oscillograph over the conventional condenser-coupled arrangement is illustrated in the drawings, in

A whichFigs. 4, 4a and 4b illustrate the operating action in my improved oscillograph, and Figs. 5.

5a and 5b illustrate the operating' action in the ,conventional condenser-coupled arrangement ci the prior art. II'hat is, my improved oscillograph eifcctively provides for handling signals several times as great as those which may be handled pwith prior art arrangements without distortion of the pattern on the fluorescent screen lI-oiv the cathode-ray tube.

` With the pattern centered, it will appear undistorted vwith either arrangement, slncethe overload point of the ampliiler is od-screen." This will apply either whenthe size ci' the pattern iswithin the undistorted range R of the ampliiier, as shown in Figs. 4 and 5, or when it vexceeds the overload point of the amplifier, as

shownin Figs. 4a and 5a. Considering a ycondition, however, where the pattern exceeds the undistorted output range R' of the amplifier, and it is desired to study a portion of it without reducing the pattern size, as shown in Figs. 4a and 5a or 4b and 5b, the difference becomes apparent as follows: With the conventional method. adjustment of the 'positioning-control 8 will shift the pattern, but since the pattern size exceeds the overload pointof the final ampliiier l, the edges of the pattern, when brought onto the screen 3i, will be distorted, as shown in Fig. 5b.

With my improved method, however, positioning is accomplished by effectively superimposing avvariable amount of D. C. on the input signal to the tlnal amplifier. In this way, the overload point oi the amplifier, as shown in Fig. 4b, remains off-screen even though the edge of the pattern is moved into view, and this edge is viewed undistorted.

Fig. 2 shows an arrangement wherein the positioning-control is fed directly from a pre-` ceding amplifier stage comprising a pentode 22, in which case the tube I in Fig. 1 is omitted.

In my improved oscillator, the final ampliiler may be any D. C. coupled amplifier, either of the push-pull type for symmetrical deflection, or of the single-ended type for asymmetric deflection. Such amplier may be connected directly to the deflection plates 24, or may be coupled to them by any means `which provides D. C. coupling, such as through a neo when the signal to be linvestigated is applied to bulb 33, as shown in Fig. 3.

sitioning-control, because of the From the foregoing, it will be seen that in my improved oscillator there is a positioning l circuit'such as 3, providing a positioning-control in the nature of a. potentiometer across which a D. C. voltage exists when no signal is applied, this voltage remaining essentially constant at all times regardless of applied signal,

each point on the potentiometer 8, however,-

varying in its voltage with respect to ground in a manner corresponding totheapplied signal. Also, in my improved oscillograph, use is made of a neon bulb or similar glow-tube across-- such as 3 to a. direct-coupled amplifier, such as 4, feeding the deilection plates of a cathoderay tube. Such a combination permits good low-frequency response with instantaneous podirect-coupled feature. j

The respective values of resistance, capacity and inductance designated in the drawings are given by way of example, and have been found to give satisfactory results, but may be changed to suit particular requirements.

In' the claims, the term "tube is used in the broad sense, and refers to a group or unit of elements such as a cathode, control grid, screengrid and plate, and is intended to embrace a construction wherein two of such units or groups are contained in the same envelope, as illustrated by Fig. 6.

Modiications within the conception of those skilled in the art are possible without departing from the spirit of my invention or the scope of the claims. Y

I claim as my invention:

l. In a System of the character described, a cathode-ray oscillograph, a defiecting plate amplifier-balancer circuit apparatus and a positioning circuit for said oscillograph comprising an amplifier tube with its output load circuit connected to its cathode, said load circuit consisting of a ilrst resistive section and a second resistive section, said load circuit comprising means for passing desired signals undis turbed and comprising means by which variation ofthe direct curent voltage output of the cathode resistor network causes positioning of the cathode-ray beam, said positioning circuit preceding and controlling said deflecting plate circuit.

2. In a system of the character described, a cathode-ray oscillograph, a deflecting plate amplifier-balancer circuit apparatus and a positioning circuit for said oscillograph comprising anampliiier tube with its output load circuit connected to its cathode, said load circuit consisting oi a iirst resistive section and a shunt glow tube in parallel and a second resistive section, said load circuit comprising means for passing desired signals undisturbed and comprising means by which variation of the direct current voltage output of the cathode resistor network causes positioning of the cathode-ray beam, said positioning circuit preceding and controlling said deecting plate circuit.

3. In a system oi the character described, a cathode-ray oscillograph comprising an amplifier connected to the deflection plates of said -oscillograph, means 'for applying signals to the input of said ampliiier which signals are variable both positive and negative with respect to ground, whereby the ampliiied signals from said ampliiier are caused to vary the potential of said deiiection plates both positive and negative with respect to ground, and means comprising a first impedance connected between the plate of a tube of said amplier and a deection plate of said oscillograph, and a second impedance connected between said deflection plate and a source of potential negative with respect to ground, said oscillograph being linearly responsive to both direct and alternating voltage signals.

WILLIAM A. GEOHEGAN.

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