US2235131A - Saw-tooth wave generator - Google Patents

Description

March 18, 1941. H.A. WHEELER SAW-TOOTH WAVE GENERATOR Filed 001: 25, 1939 VID O'- FREOU ENCY 3 AMPLIFIER DETECTOR FIGJ.

INTERMEDIATE F REOUENCY AMPLIFIER OSCILLKTOR MODULATOR RADIO- FREQUENCY AMPLIFIER ATTORNEY Patented Mar. 18, 1941 PATENT OFFICE SAW-TOOTH WAVE GENERATOR Harold A. Wheeler, Great Neck, N. Y., assignor to Bazeltine Corporation, a corporation of Delaware Application October 25, 1939, Serial No. 301,103

8 Claims.

This invention relates to periodic wave generators and particularly to periodic saw-tooth wave generators for developing an output, the wave form of which has relatively long trace and relatively short retracedntervals. While the invention is of general utility, it is particularly useful as a scanning wave generator for a television signal receiver. The invention also is of particular utility in a periodic wave repeater adapted to be excited by periodic pulses, such as in a frequency divider where undesired other pulses may occur during the trace interval and only a certain periodic pulse is intended to control the excitation. 7 For some purposes, it is necessary to develop a current of saw-tooth wave form having relatively long trace and relatively short retrace intervals. In a television receiver of the type which utilizes a cathode-ray signal-reproducing tube with magnetic' scanning, this current is delivered to a scanning inductor for deflection of the cathoderay beam. Certain generators for providing such a saw-tooth current include an oscillator, the natural period of which is slightly greater than the period of the desired wave, together with a source of synchronizing signals for synchronizing the oscillator. 'A periodic wave generator of this type is disclosedin applicant's copending application Serial No. 256,033, flled February 13, 1939. Periodic wave repeaters in such generators, adapted to be excited by periodic pulses, may have an undesirable characteristic in that undesired other pulses present with the desired pulses tend to aifect the operation of the repeater and this particularly is true if they have an amplitude comparable with that of the desired pulses. For example, in submultipledrequency generators or frequency dividers, such as those commonly used in the timing system of television tranmnltting apparatus, it often is desirable toeflect synchronization by only particular ones of the pulses from a source of periodic synchronizing signals, it being particularly desirable that the apparatus be insensitive to the other pulses. A periodic wave repeater concerned with this problem is disclosed in applicants copending application Serial No. 264,935, filed March 30, 1939.

It is often desirable in a saw-tooth wave generator that the period of the generated wave be critically determined and, therefore, stable in a free-running condition. Also, as pointed out above, it is usually desirable that the generator or oscillator be insensitive to undesired noise or synchronizing pulses other than the synchronizing pulse which occursat the initiation of the retrace and thereupon trips the oscillator. The former desirable feature is present in an insufllcient degree in the usual self-oscillating systems, while the latter feature is subject to improvement in the usual repeater systems in which periodic till synchronizing pulses are applied to an oscillator. It is known in the prior art how to desensitize a vacuum tube of the generator during the major part of a cycle to prevent disturbance by nudesired pulses such as static or intermediate pulses occurring during the trace interval. The prior art includes arrangements for feeding back sawtooth voltages for this purpose, but they are not completely satisfactory in some cases because their desensitlzing action is maximum only at the beginning of the trace interval.

It is an object of the present invention, therefore, to provide an improved periodic wave generator for developing an output of saw-tooth wave form with relatively long and referably linear trace intervals and relatively short retrace intervals, which overcomes one or more of the abovementioned disadvantages of the arrangements of the prior art.

It is still a further object of the invention to provide an improved saw-tooth wave generator, the period of which is critically determined and therefore stable in a free-running condition.

It is another object of the invention to provide an improved periodic wave repeater or generator which is adapted to be excited or synchronized by periodic pulses applied thereto and which is insensitive to pulses other than the synchronizing pulse which determines the period of the generated wave.

In accordance with the invention, a periodic wave generator, for developing an output of sawtooth wave form with relatively long trace and relatively short retrace intervals, broadly comprises an input circuit adapted to be connected to a source of periodic pulses synchronous with the desired output wave and incidentally subject to undesired other pulses, a vacuum-tube repeater coupled to said input circuit for developing said output wave, means for deriving from the periodic pulses a periodic wave, the form of which provides a maximum rate of change in one sense at the beginning of each trace interval and progressively varies to a maximum rate of change in the opposite sense at the end of each trace interval, and means for applying said periodic wave to said input circuit with such polarity that it does not reduce the effectiveness of said periodic pulses on said repeater but doesreduce the efl'ectiveness oi.- said other pulses on said repeater. The derived wave preferably is a parabolic volt-'- age wave which operates either to stabilize the running frequency of the generator or to protect it against interference over the major portion of its cycle, or both. The running frequency of the generator or oscillator is made more stable by causing the oscillator to trip itself more abruptly at the end of the trace interval.

In a preferred embodiment of the invention, the periodic wave generator includes integrating means coupled to the input circuit for deriving a saw-tooth voltage from the periodic pulses, means including integrating means for deriving a sawtooth output current from the periodic pulses, integrating means for deriving a parabolic voltage from the saw-tooth current, and means for applying both the saw-tooth and parabolic voltages to the input circuit with such polarity that they do not reduce the eil'ectiveness of the periodic pulses on the first-named integrating means but do reduce the effectiveness of undesired other pulses on the first-named integrating means by a maximum amount during the initial portion or the trace interval.

For a better understanding of the invention together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing, Fig. l is a circuit diagram, partly schematic, of a complete modulated-carrier television-signal receiver embodying one of th invention as a line-frequency scanning generator while Fig. 2 comprises graphs illustrating certain ed the operating characteristics of the system of Fig. 1.

Referring now more particularly to the drawirm, the system illustrated in Fig. 1 comprises a n1cdulated-carrier television-signal receiver of the superheterodyne type including an antenna system iii, i-l connected to a radio-irequency amplifier ii to which are connected in cascade, in the order named, an oscillator-modulator it, an intermediate-frequency amplliier M, a detector it, a video-frequency amplifier i6, and an imagerepnoducing device il. A line-frequency generator i8 and a ileldi'requency generator i9 are coupled to the output circuit of the detector through a synchronizing-signal separator and processor 26, the line-frequency generator id being coupled to a line-scanning winding 28 of image-reproducing device El and the output of the field-frequency generator it being coupled to a field-scanning winding 22 of image-reproduc- 8 device H in a conventional manner. The stages or units Ilii'i, inclusive, E8, and 28 all may be of conventional well-known construction so that detailed illustrations and descriptions thereof are unnecessary herein.

Referring briefly, however, to the operation of the above-described system as a whole, television signals intercepted by antenna circuit til, it ar selected and amplified in radio-frequency amplifier i2 and coupled to the oscillator-modulator i3, wherein they are converted into intermediate-frequency signals which, in turn, are selectively amplified in the intermediate-frequency amplifier i4 and delivered to detector it. The modulation components of the signals are derived by the detector 55 and are supplied to the video frequency amplifier I8, wherein they are amplified and from which they are supplied in the usual manner to a brightness-control electrode oi the image-reproducing device ii. The modulation components are also supplied to the synchronizing-control circuits of generators l8 and i9 through separator 20 which separates the line-frequency and field-frequency components from. the video-signal components and from each other. The intensity of the scanning beam of the device I1 is modulated or controlled in accordance with the video-frequency voltages impressed upon i-ts brishtness-wntroll electrode in the usual manner. Scanning Waves a e 8 ated by the line-frequency and field-frequency generators I 8 and I9, respectively, which are synchronized by the output oi synchronizing-signal separator and processor 20 and applied to the scanning elements of the image-reproducing de- 5 vice to produce electric scanning fields, thereby to deflect the scanning beam in two directions normal to each other so as to trace a rectilinear scanning pattern on the screen and thereby reconstruct the transmitted image.

Reierring now more particularly to the portion of Fig. l embodying the present invention, there is provided an improved periodic wave generator It for developing an output current of sa/w-tooth wave form with relatively long substantialiy linear trace and relatively short retrace intervals. The generator .58 is essentially an oscillatter which can be synchronized with a relatively weak pulse of short duration derived from unit it at the beginning of the retrace interval. The generator it comprises an input vacuum tube 23, an output vacuum tube 25 and an efficiency triode 25. The tube 23 is utilized as a driver tube for deriving the required input wave form for controlling the output tube 24 and its control electrode is coupled to. the synchronizing-signal separator and processor unit 20 through the generator input circuit comprising a blocking condenser 28 and leak rwistor N. The tube 23 is normally biased substantially to cutofl by an adjustable cathode-bias circuit comprising a resistor 28 shunted by a condenser 29, the time constant of which has a major influence on, and preferably is of the same order of magnitude as, the period of the generated Wave.

In order to develop an input voltage of the required wave form for the tube 24, there is provided a wave-shaping circuit comprising a seriesconnected resistor 36 and condenser 3! connected in the anode circuit of tube 23. Condenser 3| 4 is shunted by a resistor 32 to carry the unidirectional component of the space current of tube 23, that is, to charge the condenser 3i steadily during the trace intervals to produce a sawtooth voltage component thereacross. The re- 4 sultant voltage across resistor 33 and condenser Ed is applied to the control electrode of tube 24 by means of a coupling condenser 33 and a gridleak resistor 3 Thus, the vacuum-tube repeater 2 is coupled through tube 2310 the generator 5 input circuit comprising the condenser 26 and resistor 27.

The output tube 25. is preferably a high-mu tube and includes some means for suppressing secondary emission from its anode, a beam type 5 of tube being assumed for this purpose in Fig. l. A bias for the tube 24 is obtained by a cathode bias resistor 36 shunted by a condenser Bl which, in the absence of the present invention, would be merely a by-pass condenser. The periodic 5 voltage across condenser 3? is applied to the control electrode of the driver tube 23 by a circuit including the resistor 35 and the resistor 21 in series.

The anode circuits of the high-mu output tube 26 and the efflciency tube 25, preferably a low-mu tube, are coupled, as shown, with opposite polarl ties to the load circuit of the system. The common output circuit of tubes 24 and 25 includes a parallel-resonant circuit comprising shunt capacitance 38, shown in dotted lines for the reason that it may be comprised in whole or in part of the inherent capacitance of the circuit, and the load inductance of scanning coil 2i, coupled into the tuned circuit by means of a step-down trans- 7 former comprising inductively-coupled primary and secondary windings l4 and 4|, respectively.

for converting the relatively small inductance of scanning coil 2i to the larger inductance required 5 in the output circuit or the system. The effective inductance of the resonant output circuit oi tubes 24 and 25 is thus that of the transformer windings 39 and 4. as measured with the scanning coil 2| connected across the winding 48.

A condenser 4|v is connected in series with a resistor 42 directly across the load circuit, the voltage across resistor 42 being applied to the control electrode of the tube 2| through a condenser 43 and voltage-divider resistors 44 and 45. The initial grid bias on the inverted tube 25 is determined by the ratio of resistors 42, 44, and 45. An additional direct current source 46 may be needed in the plate circuit of the tube 28.

'lhe uenerator i2 is provided with a teed-back circuit comprising a direct current blocking condenser 41 coupled between the common terminal it oi the transformer windings I! and 4! and the control electrode of tube 23 through a resistor it. The values of resistors 21 and 3!. which are coupled across the input circuit of tube 22 tocether with that of resistor 4|, determine the relative amount of 'ieedback and the coupling from the synchronizing separator unit to the control electrode of tube 23. The scanning coil ii is coupled to the transformer winding II. as pointed out above, and suitable sources of unidirectional voltage are provided for tubes 22, 24, and 25, as indicated by the several batteries in Fig. l oi the drawing. 7 r

In considering the operation of the periodic saw tooth wave generator ll. neglecting for the moment the particular features or operation which characterize the present invention and assuming a traction of the pulse voltage appearuu luu across the load circuit is returned to the control electrode of driver tube 23 by teed-back path including condenser 41 and resistor 40, at the same time a pulse is received from unit 2|. these components are sufficient to overcome the cutol! in bias oi tube 23 and the resultant pulse current which flows in the anode circuit of the tube 22 churues the cathode condenser 29. The subseuueut discharging of the cathode condenser 28 tlucuuh resistor 2t developsa saw-tooth voltage to u, Eric. 2, in the cathode circuit or the driver tube it. The output pulse current of the tube 28 develops a pulse voltage across resistor II and a sawtooth voltage across condenser II, the resultant oi which is applied to the control elec lsu trade of the output tube 24 to develop an output current of saw tooth plus pulse wave form. llhus, the condenser I! and resistor 22 comprise luteurating means coupled to the input circuit of tube M for deriving in the output circuit thereof w u suvv-tooth current from the periodic input pulses lrom unit 20, this saw-tooth current being synchronous both with the pulses applied to the input circuit of tube 23 and with the pulses developed in the output circuit thereof. The circs cult constants are so proportioned that the pulse voltaue applied to the control electrode of tube ill in sufilcient-to drive the tube 24 to cutoi'i during the retrace interval. The saw-tooth component of current flowing in the cathode circuit 79 oi the tube 24 develops a parabolic voltage wave ll across the cathode resistor 36 and condenser ll. At the beginning of the retrace interval, load inductance 2| is carrying maximum current which is diverted to flow through the capacitance it it because that is the only remaining current path, the high-mu tube 24 being biased beyond catch and the low-mu tube 25 being connected with opposite polarity so that it is nonconductive to this current. The energy of the current in inductance 2! is transferred to capacitance 38, the circuit completing a portion of'a cycle of free oscillation and building up a reverse current in inductance 2| to a value nearly equal to its previous maximum value.

As the voltage across the load circuit reverses during the retrace interval, the low-mu tube 25 becomes conductive to carry the reverse current 0! load inductance 2| and thereby prevents the building up of a substantial reverse voltage across the load circuit and any substantial reverse charging of the capacitance 38. The output current of tube 25 commences to build up in value at the end of the retrace interval, that is, at the end of the pulse from tube 23 which operated the high-mu tube 24 beyond its cutoff point. The current then gradually rises in tube 24 during the succeeding trace interval, the rate of change oi current in its output circuit being determined primarily by the wave term of the voltage onits control electrode, and at the same time the current gradually falls in'tube 25 aesthe voltage across the load circuit reverses polarity as the current builds up in the load circuit. In addition, the voltage on control electrode of tube-25 derived from circuit H, 42 keeps itnear cutoii except during the positive pulse developed in the retrace interval.

By adjusting the value of resistor 28 the frequency oi the generator may be changed substantially independently of its other characterlstics. The amplitude of the saw-tooth output current or the generator may be controlled substantially independently of its other characteristics by adjusting the unidirectional voltages supplied to the tubes, preferably simultaneously.

The input circuit comprising the condenser 28 and resistor '21 is adapted to be connected to the source 2| oi! periodic pulses synchronous with the desired output wave and incidentally sub ject to excitation by undesired other pulses. The particular features oi operation which characteriae the present invention reside in deriving from selected ones oi the periodic pulses supplied to the input circuit of the generator a periodic parabolic voltage, the. wave iorm of which provides a maximumrate oi change in one sense at the beginning of each trace interval, and progressively varies to a maximum rate of change in the opposite sense at the end of each trace interval, and applying such periodic voltage to the input circuit with such polarity that it does not reduce the effectiveness of the selected pulses but does reduce the effectiveness oi? pulses other than said selected pulses, which undesired other pulses may include undesired transients and may occur between said selected pulses. The current flowing in the cathode'circult of tube 24 has a sawtooth component from which the condenser 21 derives a parabolic voltage b which appears across the cathode resistor II and the condenser 31. Thus, the condenser 31 and resistor 36 comprise integrating means for deriving a parabolic voltage from the sawtooth current of tube 24 or, expressed in another way, comprise means for deriving from the periodic pulses of unit 20 a periodic wave the form of which provides a maximum rate of change in one sense at the beginning 'of each trace interval and progressively varies to a max imum rate of change in an opposite sense at the end of each trace interval. The grid return oi the input tube 23 includes the cathode-bias circuit of the output tube 24 so that the parabolic voltage 22 is applied negatively to the control electrode of tube 23. Since the parabolic voltage b has a maximum value during the middle portion of the trace.,-interval, the tube 23 is thus immunized against the disturbing influences oi equalizing pulses d, Fig. 2, or other undesired condenser 29 in the cathode circuit of the tube 23 is of the same order of magnitude as the period of the generator, the output pulse current of the tube 23 periodically charges the condenser 29 during the retrace interval, condenser 29 par- 5 tiaily discharging through the resistor 28 during the trace interval. This produces a sawtooth voltage component a. The condenser 29 and resistor 28 thus comprise integrating means coupled to the input circuit for deriving a sawtooth voltage from the periodic input pulses from unit 20. The third curve c of Fig. 2 is the algebraic sum of the saw-tooth voltage a and parabolic voltage I: and is the net voltage of the control electrode of the tube 23 relative to its cathode during tree-running operation.

It is seen that the voltage wave c is efiective to desensitize the tube 21 during the major part of the cycle. It has the greatest eiiect in the middle of the cycle when equalizing pulses d may appear in the synchronizing signal. It also desensitizes the system to noise or transient pulses during the major part of the cycle. This function is particularly useful in a frequency divider, where several pulses, such as d and e, may

occur regularly during the trace period and only the pulse e is intended to trip the generator.

As pointed out above, the arrangements of the prior art for feeding back a saw-toothvoltage alone for this purpose are also inadequate to provide a stable free-running generator since the feed-back wave has a maximum slope only at the beginning of the trace period and a gradual slope at the end of the trace period when 5 the generator must be tripped. The parabolic voltage. wave b of the present invention is ideal for such purpose because it has a maximum slope just at the point where it is desired to trip the generator. Also, in the case of a driven .generator, the slope of the feed-back wave is a maximum at the point at which the generator should be sensitive to. the synchronizing pulse. There is no problem of resonating a tuned circuit or any other adjustment in order to develop. the voltage wave b because it is derived from an aperiodic circuit including resistor 36 and condenser 31 and, therefore, is inherently in synchronism with the generator.

The dotted pulses d and e, as shown in Fig. 2 in connection with the voltage wave. 0, illustrate the behavior of the present system toward equalizing pulses d which occur in the middle 01' the cycle as well as toward the desired synchronizing pulses e which occur at the end of the cycle. The equalizing pulses d are prevented by the feed-back wave c from effecting operation ofthe system.

Thus, the broad aspect of the invention in- VolVes developing a voltage wave which has a broad extreme value near the middle of the trace interval and progressively varies toward an opposite extreme value at the end of the trace interval, and utilizing said voltage wave to stabilize the free-running frequency of the sawtooth wave generator and to desensitize it against disturbing pulses over the major part of the cycle. The free-running frequency of the periodic wave generator is made more stable by the generator being caused to trip itself more abruptly at the end of the trace interval in the absence oi synchronizing pulses. By combining the parabolic wave with a saw-tooth wave, the generator is made less sensitive to disturbin pulses occurring during the initial portion of the trace period.

The following circuit constants are given as illustrative of values of circuit elements which may be utilized in the circuit of Fig. l:

Inductance 21 1 millihenry Tube 23.; Type 6J5G Tube 24 Type 6L6G Tube 25---; Type 45 Resistor 27- 50 kilohms Condenser 29 0.005 microfarad Resistor 30 10 kilohms Condenser 31 750 micro-microiarads Resistor 32 0.25 megohm Condenser 32 0.001 microtarad Resistor 35 10 kilohms Resistor 36 370 ohms Condenser 37 0.1 microiarad Capacitance 38---. About micro-microfarads Winding 39 420 turns Winding 40 '70 turns Condenser 41 micro-microfarads Resistor 42 1 megohm Condenser 43 0.001 microi'arad Resistor 44 5 megohms Resistor 45 2 megohms Source 46 40 volts Resistor 49 0.1 megohm Ratio 01' retrace period to line period 0.12 Pulses from source 20 10 volts positive While the invention has been illustrated as applied only in the line-frequency generator of the receiver, it will be understood that the circuit oi the field-frequency generator may also incorporate a circuit in accordance with the invention.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims 'to cover all such changes and modifications as fall within the true spirit and scope oi the invention.

What is claimed is:

l. A periodic wave generator for developing an output of saw-tooth wave form with relatively long trace and relatively short retrace intervals comprising, an input circuit adapted to be connected to a source of periodic pulses synchronous with the desired output wave and incidentally subject to excitation by undesired other pulses, a vacuum-tube repeater coupled to said input circuit for developing said .output wave, means for deriving from said periodic pulses a ill lit

periodic wave, the term of which provides a uiuuimum rate of change in one sense at the beuinnlng of each trace interval and progressively varies to a maximum rate or change in an oppocite sense at the end of each trace interval, and means for applying said periodic wave to said input circuit with such polarity that it does not reduce the effectiveness of said periodic pulses on said repeater but does reduce the effectiveness of saidother pulses on said repeater.

*2. A periodic wave generator for developing an output or saw-tooth wave iorm with relatively long trace and relatively short retrace intervals comprising, an input circuit adapted to be com uocted to a source of periodic pulses synchronous with the desired output wave and incidentally subject to excitation by undesired other pulses. o. vacuum-tube repeater coupled to said input circuit tor developing said output wave, means tor deriving from said periodic pulses a parabolic voltage, and means for applying said parabolic .voltage to said input circuit with such polarity that it does not reduce the eiiectiveness oi. said periodic pulses on said repeater but does reduce the eflfectiveuess of said other pulses on said repeater.

l. A periodic wave generator for developing an output of saw-tooth wave form with relatively long trace and relatively short retrace intervals comprising, an input circuit adapted to be conuected to a source of periodic pulses synchronous with the desired output wave and incidentally subject to excitation by undesired other pulses, integrating means coupled to said input circuit for deriving a saw-tooth current from said periodic pulses, integrating means for deriving a parabolic voltage irom said saw-tooth current, and means for app mi said parabolic voltase to said input circuit with such polarity that it does not reduce the effectiveness oi. said periodic pulses on said first-named integrating means but does reduce the eiiectiveness of said other pulses on sold first-named integrating means.

l, A periodic wave generator for developing on output current of sawtooth wave form with volutlvely long trace and relatively short retvuco intervals comprising, an input circuit utluptcvl to be connected to ooource oi. periodic synchronous with the desired output out root onto and incidentally subject to excite tlou luv undesired other pulses, lnteuiutinu moons coupled to said input circuit tor deriving sold ouvv tootlt output current from cold periodic pt on, integrating means for deriving it pure.

. loollc voltuue lroru said sawtooth output current,

cull uiconc lot applying said parabolic voltage to cold input circuit with such polarity that it does not reduce the effectiveness cl. said periodic pulses on sold first-named integrating means but does reduce the effectiveness of said other pulses on said first-named integrating means.

it u periodic wave generator for developing an output of saw-tooth wave form with relative lv long trace and relatively short retrace intervulo comprising, a vacuum tube having a control electrode and an anode circuit, means for uourually biasing said tube substantially to out om, means for developing in said anode circuit periodic pulses 01 current synchronous with the desired output wave and incidentally tending to tlovelop other pulses, integrating means for devlvluu a saw-tooth current from said periodic puloco, integrating means for deriving a parabolic subject to elicitation voltage from said saw-tooth current, and means for applying said parabolic voltage to said control electrode with such amplitude and polarity as to reinforce said cutoi! bias primarily in the interval between said periodic current pulses in said anode circuit.

8. A periodic wave generator for developing an output of saw-tooth wave form with relatively long trace and relatively short retrace intervals comprising. an input circuit adapted to be connected to a source of periodic pulses synchronous with the desired output wave and incidentally subject to excitation by undesired other pulses, a load circuit, integrating means coupled to said input circuit and a vacuum tube repeater and an efficiency triode coupled to said load circuit with reverse polarity i'or deriving a saw'tooth current from said periodic pulses, integrating means for deriving a parabolic voltage from said sawtooth current, and means tor applying said parabolic voltage to said input circuit with such polarity that it does not reduce the eii'ectiveness of said periodic pulses on said first-named integrating means but does reduce the eflectiveness or said other pulses on said first-named integrating means.

7. A periodic wave generator for developing an output current of saw-tooth wave form with relatively long trace and relatively short retrace intervals comprising, an input vacuum tube having a control electrode and an anode circuit, means for normally biasing said tube substantially to cutoil, an output vacuum tube having a control electrode and a cathode circuit, means for developing 111 said anode circuit periodic pulses of current synchronous with the desired output current wave and incidentally tending to develop undesired other pulses, means including a condenser for coupling said anode circuit to said output tube control electrode for deriving from said periodic pulses a saw-tooth voltage for developing the desired saw-tooth current in said cathode circuit, a condenser in said output oi tutu-tooth lvuvo lot-u; with relatively long truce and relatively short votroco intervals compulsion, to input circuit ovloutovl to loo con ucctevl to a source periodic pullout synchronous with the desired output tvuvo curl incidentally luv undesired other pulses, integrating tncuuo coupled to cold input circuit tor deriving u caw tootlt voltage irorn said'pericdic pulses, means including integrating means tor deriving a saw-tooth current from said periodio pulses, integrating means for deriving a parabolic voltage from said sew-tooth current, and means for applying both said saw-tooth and parabolic voltages to said input circuit with such relative polarities that they do not reduce the eflectiveness of said periodic pulses on said firstnamed integrating means but do reduce theeii'ectiveness of said other pulses on said first-named integrating means by a maximum amount dur ing the initial portion-oi the trace interval.

Hill

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