US2584144A - Positive pedestal switched video tube - Google Patents

Description

Feb. 5, 1952 P. "r. MAREScA POSITIVE PEDESTAL SWITCHED VIDEO TUBE Filed Sept. 7, 1949 E 5 wumsom Em uumaom INVENTOR.

PETER T. MARESCA ATTORNEY Patented Feb. 5, 1952 a z gsscmt rosrrrvu gwiroaen-vmco Beter-T.-;Mare soa-, iAsiini-v Bark,Npl assignor to ithel Unitect States ,of America as; represented: -by,the};Secretary 1"the Army. 'Application' -septemberir, I849;Ser ial Noi1 1-'4;448

5,0laims; rename- 26i (Granted undel tthe act o't Ma'rch '3, 'I 8 83*,'::'as

The? invention described hereimma ybemannr *factured" and used-by orftorthe Government-for governmental purposes;,-withoutithe "payment any" royalty thereon.

My-invention relates to improvements in:='e1ec-- Itronic switching circuits; and" more particularly to such a circuit as 'embodied in 'akeyed amply fier. V

' In certain radar systems; it' is sometimes necessary to" switch' electronically a video stage handling positive video signals. One o'fthe qnired characteristics" of s'uch'a system or--"circui t,

'isitha't the *videosignai "be ofthe' samefi-poiarity 'as'" the accompanying-pedestal; some radar "applications, it is important?to intercepftw video signals forsimu'ltaneousobservation an, a comparison 'iofthef'samm; (m me-"screen ofifa cathodei-ray-tube; Therrespectiverpedestals ac--: "companyingthe twovideosignais cair be eiiniiinated' or blanked out; butfofthe purposes ofobservation-and comparisonat 'thc 'point"of -re! ceptioznf they are needed inf-"effect" displacement Ordinarilwwhena 'tubeisswitch'edon; ane ga tive "pedestai is produced irr the--p1ate circuit Izhereofi Therefore; *'where*a p0'sitivevideo signal i is being haindldand ntr'inversion is desired; it

becomes' necessary to empioy-"an extrag inverting tube:

"With th 'fOregOing-in mind; it is one of 'the objects-of my'invention to provi'de an-improvfed e1ectronic-switching-oireuit of the ehar-acterree' f erred to, *whereinthdesired conditions of vid'eo switching are satisfied, withoutuse -of -an extra invertingstage: I

I 1 Another obj ect-i o'f 'my invention is to provide animproved'- system orciIcuit ofthe-character referi ed to; "-wherein there is a; display of two video-"signals going: respectively; in opposite in:

'40 sharingonacommon' cathode-ray--tube;*and with rections;-dispiaced -one below=theother;-with tiine the intrinsic characteristic that each image'f-"ds 'freetoact-independently of; orwithontinfiuence from the other.

" Another object of my: invention isathe pro vision of an improved circuit-o'f the character referred to, which has advantages over-"those proposeci heretofore in the way o'f being? com- "sistently reliable; and positive -in periormance. 'j 1 "GthenObjects andadvantages will-hereinafter appear;

- For-*the-rpurpose'rofillusttatingQmy "invention;

an embodiment thereof is shown inane-ermine wherein I lFig it 'isaisimplifled. fiiagrammatic' view ot an T electrom'cfswitching circuit; constrircted operating hi aecordancefwith my invention; and 2 is'ar'graphica!f illustration" of? the'operatringectiozrin Figfl v *With reference to theidrawing; the nun'ierals "I5 iand' ZI designate, respectively; two screen-grid switching; tubes-."Two sources o'f-"video'--signais, shownfor" simplification" as the two 1 input lines orconnections I4 and I5, impress-the intercepted video signals I 0" and" I -I -througlt'therespective condensers I1 an'ci I Bfon; the cathodes" IBa and 'ZIaof" the switching'tubes; r

The tubes I-Sb andII'U-"are associatediirespecti'vely', with switching tubes I 9 'and 2;l an'difunc- 'tion'toj produce or'iorm pedestals-of the same, positive'polarity-asthevideosigna1s I0 and] I".

Timing oi? the "switching? action i's- "effectedby the-"two rectangular voltage -waves:2'-'I -"an d-- 28 which are supplied from-"the two sources'or wave generators 21a"and"'28a-, respectively.- These voi'tage-wavesi-are impressedon the grids'of tubes 19c; HdQZIc-and ZId; asshownz' "The two-generators 21a and" 28m may be: "ofaconventional "design; and the respective output voltage waves '21 "andi'a therefromere" Iv degrees out-of phase. as" represented in- Fig". 1.

' v Iir-the operation ofmy improved-"circuityttie video signal I I 'i's"-app1ied'-tothezathdd I Bel ef appears*"as--=signai "output m theplate circuit thereof. Pedestals of positiveand the same polarity as "the 'video signal I0; "appear in the plate circuit of them edestai-forming tube 19b 'dnr-ing the' active; =condi1ctingperiod of tube' I9. The*-output video-signal from tube--19 combines with the-positive rpedestaHrom-tube= I 9brand the we appearing irr the common piate oroutput conneeticn, are abpliedas shown to "the flipper vertical deflecting'plate I2 oi.- a: cathode ray tube I32 "Accordii'iglygthe'visnai indication I Ilawhich i's that of-- the video signer-; appeals 'on the fluorescent screen- "of-thejcatl'iode-ray "tube. When" the 'voltage -of rectangular "wave* 21 "from the tnbe -Ific is -positive on the 'screen"gri&= IQ'go'i switching tube I 9,- the i latter conducts; and the amplified vide'o signal I0? appears i'r'r'its "output plate -circuitz' "-During the interveningperiods, 'whenrthe voitage'of wavewl fromtube I96: drops to; practically" zero tube I9 is: keyed; or 1 made non condnctivea v In like manner, thevoltage --wave 2'8 afipl'ied Will seen that the pedestais frow tube 1 I91) cathode 2 la of switching tube 2 I, is amplified by the latter, and appears as signal output in the plate circuit thereof. Pedestals of positive sign and the same polarity as the video signal ll,

4 produced on the face of the cathode ray tube, which is determined by the potential differences existing between vertical deflecting plates l2 and l2a, will be asshown in Fig. l. r

From'the foregoing, it. will be understood that the switching tubes l9 and '2l constitute the respective, keyed video stages. Tube l9 receives on its cathode l9a the positive video signal l0, and

. delivers positive video signal riding on positive appear in the plate circuit of the pedestal-formon'positive pedestals from tube 2 lb.

ing tube 2 lb during the active-conducting period of tube 2l. The output video signal from tube 2l combines with the positive pedestal from tube 2 lb, and the two, appearing in the common plate or output connection, are applied as shown to the lower vertical deflecting plate l2a of cathoderay tube l3. I la, which is that of thejvideo signal I l appears on the fluorescent screen of the cathode-ray tube. When the voltage of rectangular wave 28 from the tube Zlc isv positive on the screen grid 2 lg of switching tube 2 l, the latter conducts, and the amplified video signal ll appears in its Output plate circuit. During the intervening. periods, when the voltage of wave 2.8. fromtube 2 I is practically zero, tube 2l is keyed, or made non-conductive. In like manner, the voltage 21 applied from tube 2 Id to the grid of the pedestal-forming tube 2lb, keys the latter. As above, since the wave 2'! is 180 degrees out of phase with the wave 28, the pedestals from tube 2lb appear on the vertical deflecting plate In at spaced periods which intervene the spaced periods during which the video signal ll from tube 2| appears on plate l2a.

Fig. 2 shows the potential at vertical deflecting plate l2 relative to ground potential. Time t1 represents the negative half cycle of switchingv .wave 21 and the positive half cycle of switching wave 28. .It will be seen that during the time h tube l9b is conducting and tube l9, which then has a negative potential on the screen grid l9g,

is cut off. Resistor I9) is adjusted so that tube l9b draws milliamperes. This 10 milliampere current causes a 200 voltage drop across resistor l9e, which has 300 volts applied thereto. This leaves vertical deflecting plate l2 at a,,p0tential of 100 volts, as shown in Fig. 2. At the,end ;of .time t1, switching waves 21 and 28 change in .polarity, whereby for an instant both tubes #9 and l9b are cut ofi and the entire 300 volts are applied to vertical deflecting plate l2. Then, for

:therest of time ta tube l9b is cut off and tube l9,

which now has a positive voltage on screen grid l9y, becomes conducting. The 10,000 ohm resisstor in the cathode circuit of tube I9 is adjusted so that tube l9 draws a quiescent current of 5 milliamperes. The 5 milliampere currentcauses a 100 voltage drop across resistor l9e so that the quiescent voltage on vertical deflecting plate I2 is now 200 volts. Video signal In and. noise applied as an. input to tube l9 causes the 200 volt quiescent potential at vertical deflecting plate l2 to vary in the manner shown in Fig. 2.

Since tubes 2l and 2 lb are rendered conducting 180 out of phase with tubes l9 and lab, re-

spectively, the potential on vertical deflecting plate l2a will vary in a manner similar to the potential on deflection plate l2, shown in Fig; 2,

.except that vertical deflecting plate l2a will have a quiescent potential of 200 volts during time h and a potential of 100 volts during time is.

It will be seen, therefore, that the vertical'trace pedestals from tube l9b. In like manner, tube 2| receives on its cathode 2la the positive video signal I l, and delivers positive video signal riding 'The cathode-ray tube I3 is provided with the usual horizontal-deflecting plates (not shown). To these may be applied a saw-tooth voltage wave having a sweep-frequency, for example. of 500 cycles.

Accordingly, the visual indication:v

Pedestal height is controlled by adjustment of the variable resistors i9) and 2 If. In each of the ,two channels, there is but the one tube I91) and 2lb,- respectively, for controlling pedestal height. Signal inversion isavoided by driving the respective cathodes l9a and 2la of tubes l9 and 2|, instead of their respective grids I99 and 2 lg.

Other advantages of my improved circuit reside in the fact that there is continuous control of 'the separation x between the two phenomena appearing on the screen of tube l3, and the fact that no fixed cut-off bias on either of the switched amplifier stages l9 and 2l is required. There is also, elimination of the pulse transformers. Unless they are of a special design, these transformers are unable to pass a long'switching wavewhich is necessary for eflicient operation. Otherwise, the operating action of the circuit is limited to very short switching intervals.- In my improved circuit; a saving-of tubes is effected by employing tubes as amplifiers, thereby providing voltage gain in addition to theotherfunctions. Also, by adjusting either or both of the potentiometers l9f and 2 If, there is the possibility of continuous adjustment of pedestal height tochange theimage separation, represented at a: in Fig. 1.

The various circuits proposed heretofore and which have a cathode output, rely uponsignal not being present in the cathode circuit in the absence of plate voltage. However, on account of diode action, it is possible to pass signal in the absence of plate voltage, should the input voltage approach the bias. In my improved circuit, on the other hand, screen-grid switching is employed, which definitely controls signal passage through the tube. V

In the prior art complete elimination of pedestal is the object, whereas in my improved circuit, there is the provision for choice of pedestal or transient, or completeelimination thereof byadjustment of the tubes l9 and 2l. Another-important feature in my improved circuit resides in the elimination of leakthrough of undesired sig- .na 1s I i v The broader aspect of my invention resides in the improved method whereby a positive pedestal may be produced in the plate circuit of an electronically switched video tube.

Some of the practical applications of my im proved method and circuit would be in radar display circuits where time sharing techniques are used, in experimental electronic switching cir cuits, for dual display purposes for testing IFF systems, and generally, in systems requiring dual sweep displays.

The respective values of resistors, condensers and voltage'given in Fig. l; are not critical, and

may vary over a substantial range to suit particular requirements. The tubes which might be used in my improved circuit are identified in Fig. 1 by their respective typ numbers and letters, as used in the trade.

The embodiment of my invention disclosed in Fig. 1 is considered to be a preferred form thereof, but it will be obvious to those skilled in the art that various modifications are possible without departing from the spirit of my invention, or the scope of the claims.

I claim as my invention:

1. An electronic switching circuit comprising a first signal amplifier including first and second electron tubes and a first common impedanc in the output circuit of said first and second electron tubes, a second signal amplifier including third and fourth electron tubes and a second common impedance in the output circuit of said third and fourth electron tubes, means for applying a-first signal to the input circuit of said first tube, means for applying a second signal to the input circuit of said third tube, a source of square-wave voltage, means for rendering said first and fourth tubes conducting only during alternate half-periods of said square-wave voltage, means for rendering said second and third tubes conducting only during half-periods of said square-wave voltage intermediate said alternat half-periods, and means for applying the potential difierence between the voltage appearing across said first common impedance and the voltage appearing across said second common impedance as an input to a utilization means.

2. In combination with a cathode ray oscilloscope having an electron beam and a. screen, wherein the electron beam thereof is periodically deflected along a first coordinate; an electronic switching circuit for providing a dual display of first and second signals on said screen, said switching circuit comprising a first amplifier includin first and second electron tubes and a first common impedance in the output circuit of said first and second electron tubes, a second amplifier including third and fourth electron tubes and a second common impedance in the output circuit of said third and fourth electron tubes, means for applying said first signal to the input circuit of said first tube, means for applying said second signal to the input circuit of said third tube, a source of square-wave voltage, means for rendering said first and fourth tubes conducting only during alternate half-periods of said squar wave voltage, means for rendering said second and third tubes conducting only during half-periods 6 intermediate said alternate half-periods, and means for applying the potential difference between the voltage appearing across said first common impedance and the voltage appearing across said second common impedance to said cathode ray oscilloscope to effect the deflection of said electron beam along a second coordinate perpendicular to said first coordinate in response to said potential diiference.

3. The combination according to claim 2., wherein said first amplifier includes means for varying the gain of said first tub relative tothe gain of said second tube, and said second amplifier includes means for varying the gain of said third tube relative to the gain of said fourth tube.

4. The combination according to claim 2, wherein said first and third electron tubes include a screen electrode, and wherein said means for rendering said first and fourth tubes conductive include means for applying a positiv potential to the screen electrode of said first tube relative to the cathode thereof only during said alternate half-periods of said square-wave voltage, and said means for rendering said second and third tubes conductive include means for applying a positive potential to the screen electrode of said third tube relative to the cathode thereof only during said intermediate half-periods of said square-wave voltage.

5. The combination according to claim 2, wherein said first and second signals are injected at the cathodes of said first and third tubes, re-

spectively.

PETER T. MARESCA.

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

UNITED STATES PATENTS Number Name Date 2,221,115 Shepard Nov. 12, 1940 2,303,924 Faudell Dec. 1, 1942 2,345,932 Gould Apr. 4, 1944 2,366,357 Schlesinger Jan. 2, 1945 2,420,516 Bischoif May 13, 1947 2,452,655 Hecht et al Nov. 2, 1948 2,455,992 Goldsmith, Jr. et al. Dec. 14, 1948 2,464,822 Mallet Mar. 22, 1949 2,471,530 Lobel May 31, 1949 FOREIGN PATENTS Number Country Date 579,973 Great Britain Aug. 22, 1946

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