US2743357A - Sweep circuit - Google Patents

Description

April 24, 1956 CASEY 2,743,357

SWEEP CIRCUIT Filed April 26, 1952 2 Sheets-Sheet l INVENTOR. ROBERT F. CASEY ATTORNEYS April 1956 R. F. CASEY 2,743,357

SWEEP CIRCUIT Filed April 26, 1952 2 Sheet-Sheet 2 fio 3| 5?:

Fig. 3

INVENTOR. ROBERT F CASEY ATTORNEYS 2,743,357 Ratentecl Apr. 24, 1956 Robert F. Casey, Ilompton Mains; N. 3., :a'ssignor' to Allen B. Du Mont Laboratories; Inc., Clifton, N. 3., :a cor- .poratiouofDelawar-e Application Apfili'zfi, '19 5'2, Serial No. 284,611 7-.Claims. ,(Cl. 250- 27) 'This-invention-relatesto signal forming electrical circuits and particularlyto circuits for producing sawtooth voltage waves. Such Waves are useful, for example, in causing'an electron beam to scan a phosphor screen in a cathode'raytube. I

object .of theinvention is to provide a circuit for producing sawtooth waves having improved linearity.

Anothenobject is to provide such a circuit which .has a wide range fof adjustablesweep speed rates.

A'fur'ther object jis'to obtain a relatively large time duration Tsweep'by charging a relatively small value of capacitance.

Other objects will in pear hereinafter.

The invention accordingly is embodied'in asweep circuit which includes .a.condenser for generating sawtooth voltage waves, the condenser being connected to a charging circuit and through .a coupling -circuitto output terminals, the coupling circuit providing nonelinear signals opposed to .non-linearitywof the signals generated :by the charging condenserso that the output obtained from .the circuit is -a .corrected linear .sweepsignal, the invention beingembodiedinthe combination ;of elements and arrangementsof parts hereinafter set forth inrdetail.

.Inrthe drawing,

Figure 1 he schematic diagramof apreferredembodimentcf the invention and Figures 2 and 3 are schematic diagrams of :circuits incorporating modifications :ol? thetcircuit ofv Figure 1.

In Figure 1,- a-source-M of keyingsignals isconnected to a controlclectrode 1-2 .of .a discharge tube'dia having -.a cathode Mtwhich is, groundedtand an anode 15 connected to-a sawtooth Waue-formingcondenser 16. 'The condenser 16-1-is connected ito .aicontr'ol -electrode-21 "of :an output tube 22 having an anode 23 connected-to-assource 18 of voltage and a- -cathode v24 connected through an output :load resistance :26 to a source 27 of negative volt- .age. Anoutputterminalfi l is connected to the cathode 24 of-the output tube -22. -Th6'0011d6118fi116 is connected through a -fi-Xed resistance 32and variable resistance 33 to an adjustable "tap :34 one potentiometer 35 which is part be obvious and in part apconnected between the output terminal 31 and a resistance I .36which in turn -.is connected :to the voltage source 18. A constant voltage device-37 is connected between the output terminal 31 and the junction 38 between the potentiometer 35 and rthe ifixcd resistance 36. A'screen grid .39 in theoutput tube .22 is connected .to the junction 38.

In Figure .2sthe circuit elements whichcorrcspond to the .elements of Figure .1, are. given the same reference numerals asinFigure 1. "Figure 2 includes a feedback tube 41 :having an anode 42 connected through a resistance 43 to the voltage-source ls, and acathode 44 connected to theremaining end of .the condenser 16 and through a resistance 46 to ground. Series-connected resistances 47 and 48 are connected between the output terminal 31 and ground, and the junction therebetween is connected to control electrode 49 of thefeedback tube 41. "These resistances 47,, 48 may, if desired, .be a potentiometer.

In Figured, the circuit elements which correspond to the elements shownlin Fig res .1 @and .2 are given the same reference numerals as .they have .in Figures 1 and 2. Figure includes .alffeedback tube .41 having .its control electrode \49..connected.to the tap 134,. an anode A2 connectedito the voltrgesourcelti, .and'a cathode '44 con- .nected to .ancndof .the variable resistance 33. In -this embodiment, the variable resistance .33 is ..no.t-;directl,y connectedtotheitap .34 as it isinsthe embodiments of Figures .1 and .2. .flihezscreen grid '39-0f theoutputtube 223s connected to .thecat'hode -44 of the .--feedback tube 41.

The circuit of Figure v-1-.o1:verates2asfollows: fIPhc synchronizing-source 1 1 applies a synchronizing .or, gating signal .56 to the-control electrode .12-of the discharge tube '13. The .-s ync'hroni-zing signal 356 is preferably in :the form of a square wave sor 'rectangular pulse. Normally, the discharge tube 13 is conducting and thus maintains theisawtooth wave'tor'ming condenser 16 in atsubstantiall -y discharged-condition. 'When the voltage of the control electrode 12 is nna'cle relatively :negative .bythe synchronizing signalpthe-zdischarge tube 13 ceases to be conductive and 'rthe sawtooth wave forming condenser -16 z-hegins to charge from the voltage source 18 through .the resistances 32,33, .35 and .316. While .the condenser i61is'ithus charging, at. positively increasing I voltage. is applied by it to -ithetcon'trolelectrode.21:of:the:output 'tube 22. This soutput Ytube, being-connected in .azcathode- :=foIloWer.:cir-cuit provides :at iitszcathode 2 4 and "the output termtrral;31,'.a=.positively increasing vo1tage::57.

The:positivelydncreasing voltage 57 .is applied through the constant voltages-device L3712to the junction 38, =there'b-y improving linearity as will be described hereinafter. liheccon'stant :volta'geidevice '37 maybe a gasdilled regulator tube as is shown schematically in the drawing, or may'be an equivalentdevice such asihyrite.

\llhen the negatively increasing voltage applied to the contro'bel'ectrode IZ-Gf'the discharge tube by the synchronizing signal source 1-1 is interrupted, the discharge tube-13 becomes conductive, and the sawtooth'wave' forming condenseriti is rapidl-ydischarged, thus forming a steeplyshaped negatively increasing vdltageSS in the cut put vo'ltage wave. in the output'voltage wave, the sawtooth iportion sl may be-utilized to control the sweep 'or de'fiection ofan electron'bea'in in a cathode ray tube; "thesteep portion $8 provides the-return sweep.

The constant oltage device '37 functions to provide improved linearity of the output signal, as'follows: Normally, the sawtooth wave forming condenserlo doesnot charge in -a linear'manner; it tends to charge relatively rapidly atfirst andthen more slowly as it becomes more tully'charge'd. The rising portion '57 of'the outputsignal would thus tend to flatten out near its apex 59. This undesirable effect is minimized because of the novel circuit connection whereby the output "voltage is coupled through- =the/constant voltage device 3'7 to the junction 38. The positively increasing output voltage is thus fed back to the condenser 16. In 'e'ifect, the charging voltage for the condenser '16 becomes increased inaccordancewith the positive polarity rise in the output voltage, due to the feedback. This increased charging voltage tends to cause the condenser 16 tocharge faster than it normally would, the increase 'in charging rate becoming continuously greater duringth'e'progress of the charging. This increasedcharg'ing rate counteracts the normal tendency of the charging ratelto decrease. .Thus, improved linearity isachieved.

.The constant voltage device 37, having afixed voltage drop thereacross allowsthe various circuit elements to 3 have the desired values of voltage, and at the same time is capable of feeding back the entire magnitude of output voltage to the junction 38. This linearity correcting feedback occurs at all sweep speeds, including very slow speeds.

The negative voltage source 27 functions to provide a greater magnitude of output voltage to regulator tube 37 than would otherwise occur. The rate of charging of the condenser 16 and hence the steepness of the positively increasing portion 57 of the output signal, depends upon the adjustment of the variable resistances 33 and 35, and also on the magnitude of the voltage source 18 and of the capacitance 16. Adjustment of the tap 34 provides a degree of control over the amount of feedback through the constant voltage device 37, in addition to controlling the sweep speed. An advantage of adjusting the charging rate by means of the potentiometer 35, is that this potentiometer can have a considerably lower value of resistance than the variable resistance 33, and hence is more stable and more easily calibrated in terms of sweep speed. The fixed resistance 36 is preferably small enough in value so that the constant voltage device 37, if a gas filled tube, is conductive at all times, and does not tend to extinguish during the most positive excursion of the output voltage when the variable resistance 33 is set at its minimum value.

When the synchronizing signal 56 is as shown in Figure 1 of a substantially square wave shape, the output signal will have non-sweep portions 61 between each sweep and return portions 57 and 58. The non-sweep portions 61 may be substantially eliminated if desired, by providing a synchronizing signal 56 having relatively narrow positive portions as is shown in Figure 2.

It will be seen from the above that the circuit of Figure 1 provides an output sawtooth wave voltage which is substantially linear. However, there is a very small percentage of the wave form which is non-linear, the wave deviating slightly from complete linearity immediately to the left of the apex 59 of the waveform of Figure 1. In order to eliminate this minute non-linear portion of the waveform the circuits of Figures 2 or 3 may be utilized.

The circuit of Figure 2 operates in substantially the same manner as the circuit of Figure 1, but improved linearity is provided by the feedback tube 41 which is connected as a cathode follower and functions as a wellknown starved amplifier. A portion of the output signal, obtained from the junction between the- cathode load resistances 47 and 48 of tube 22, is applied to the control electrode 49 of the feedback tube 41. A corresponding sawtooth shaped voltage wave with the teeth spaced and occurring during the non-linear portions of the output thus is produced at the cathode resistance 46; this feed back voltage is applied to the lower side of the sawtooth wave forming condenser 16. Therefore, as the output signal becomes increasingly positive in polarity, a corresponding positive polarity voltage is applied to the lower side of the condenser 16, thereby tending to increase the voltage on the condenser 16 at an increasingly faster rate as it becomes increasingly charged. This feedback voltage to the lower side of the condenser 16, combined with the feedback voltage to the higher side thereof through the constant voltage device 37, provides better linearity of the positively increasing portion 57 of the output signal. The resistance 46 forms a part of the resistance through which the condenser 16 is charged from the voltage source 18.

The circuit of Figure 3 operates in substantially the same manner as does the circuit of Figure 2, except that the feedback tube 41 is in the feedback loop going to the higher side of the condenser 16, along with the constant voltage device 37. The positively increasing feedback voltage applied through the constant voltage device to the junction 38, is applied through the potentiometer 35 to the control electrode 49 of the feedback tube 41. The corresponding feedback voltage which is developed at the cathode 44 of the feedback tube 41 is applied through the fixed resistance 32 and the variable resistance 33 to the condenser 16. The condenser 16 is charged from the voltage source 18, through the feedback tube 41 between its anode 42 and cathode 44. The tube 41 forms a part of the charging resistance for the condenser 16, the output electrode 44 thereof being effectively connected to a tap on the total charging resistance.

The feedback voltage is applied to the screen grid 39 of the output tube 22, in the embodiments of Figures 1 and 3. This positively increasing feedback voltage at the screen grid 3? causes the output tube to draw increasingly greater current during the time in which the output wave is changing in a positive direction, thereby reducing the tendency for the output voltage wave form to flatten out when it becomes more positive in polarity.

This connection has been found to improve the linearity of the output sawtooth signal. The output linearity can be further improved by operating the feedback tube 41 on a curved part of its operating characteristic so that the feedback voltage produced at its cathode 44 is non-linear in a manner opposite to the non-linearity of the output voltage. The novel circuits herein described are capable of providing overcorrection so that, if desired, the output voltage may be made non-linear in a manner complementary to its uncorrected non-linearity.

While a preferred embodiment of the invention and modifications thereof have been described, still other modifications will be apparent to those skilled in the art. The scope of the invention is defined in the following claims.

What is claimed is:

1. An electrical circuit comprising a condenser, a voltage source, a charging circuit comprising resistance connected betweensaid voltage source and said condenser, a coupling circuit connected to said condenser and having an output terminal, and a constant voltage device electrically connected between said output terminal and a point on said resistance of said charging circuit.

2. An electrical circuit comprising a condenser, a voltage source, a charging resistance connected between said voltage source and said condenser, a cathode follower circuit having a control electrode connected to said condenser and having an output terminal, and a constant voltage device electrically connected between said output terminal and a point on said resistance.

3. The circuit in accordance with claim 2, in which a potentiometer having an adjustable tap is connected in a parallel with said constant voltage device, and said tap is connected through apart of said charging resistance to said condenser, the part of said potentiometer between said tap and an end thereof forming another part of said charging resistance.

4. An electrical circuit comprising a condenser, a voltage source, a charging circuit connected between said voltage source and said condenser, a coupling circuit connected to one side of said condenser and having an output terminal, a constant voltage device connected between said output terminal and said charging circuit, and a cathode follower feedback circuit having an input electrode connected to receive signals from said output terminal and having an output electrode connected to said charging circuit.

5. The circuit in accordance with claim 4, in which said cathode follower feedback circuit includes an electronic tube having an anode and a cathode connected in series within and forming a part of said charging circuit.

6. The circuit in accordance with claim 4, in which said cathode follower feedback circuit includes an electronic tube having a cathode connected to one side of said condenser other than the side thereof to which said coupling circuit is connected.

7. An electrical circuit comprising a condenser, a source of voltage, a resistance connected between one side of said condenser and said source of voltage, an output coupling tube having an input electrode connected to said side of said condenser and having an output electrode, and a feedback tube having an input electrode connected to receive signals from said output electrode and an out put electrode connected to the remaining side of said condenser.

References Cited in the file of this patent UNITED STATES PATENTS 6 Newsam Feb. 18, 1941 Rosentreter Dec. 3, 1946 Sunstein Aug. 31, 1948 Custin May 22, 1951 Higinbotham May 20, 1952 McCoy Aug. 5, 1952

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