US2482150A - Sawtooth current linearizing system - Google Patents

Description

Patented Sept. 2Q, 1949 isAwroorH CURRENT LiNEARIzINGv s Ys'rEM i Carlo V. Bocciarelli, Elkins Park, Pa.; assigner to' Philco Corporation, Philadelphia, Pa., a cor- `poration of Pennsylvania I Y `pplicatien JuneZ, 1948, Serial No. 30,573

The invention herein lates to improved means for varying automaticallythe ind-uctance oi la circuit in a desired manner. 1

The means provided by` the present invention 7 may be used to important advantage `in the sweep circuits of a cathode-ray'ftube, particularly Where magnetic deflection is employed, to linearize substantially `the beam"deflection.

. Considera television receiver whose horizontal, v-

and/or vertical,deilection system includes a de'- iiection-current:oscillator which is intended to supply a sawtooth Vcurrentof linear-Waveform to the magnetic deilecting` coil or yoke. vIn practice,

thesawtooth current supplied 'to `the deflecting `sweep.` This"^is'due,at least in part, to the in- A ternal resistance Y of.A 'the VoscillatorVT tube, tc` the resistance of the" oscillator Vtransformer andfto 'the resistance'of the deflectingyolre.V Hence, un'- less compensating means 'areemployed the cath'- f s-olaims: (o1. 31a- 27) described and claimed reode-ray `bea-mis deilec't'ed at 'a 'faster rate at the start of the sweep than-during the remaining portion thereof. f The effect, in the'V case of the 4horirfzontal sweep, may' be to stretchv the left- `hand portionfzof the `reproduced pictureV to noticeable :and objectionable extent.V

Inaccordance with the `present invention, a

suicient amount of 'inductance is added to the -plate circuit ofthe' deflection-'current oscillator, at the 'start" of the" conduction period (which corresponds to `the`sweepperiod 'of the cathoderay beam), to retard the rate of increase of the f however,'considerably inferior to those attained by the present invention.V This will become clear from the description which-follows; i

It isa'broadobject of this invention to provide `means for varying automatically, 'in a desired manner, the'inductance of an electr-ic circuit.

VIt is a more specific object oi this invention 'to provide improved compensating means'for linearizing the beam deiiection vin a cathode-ray-tube syst-ern employing magneticdeflection.

It is another specific object of this invention to provide improved Ymeans for varying automatically the inductance of a cathode-ray-tube deflection `system in themanner necessary to achieve substantial linearity of beam deflection.

Y These and other objects, advantages and feature'sof the present invention, andthe manner in which the objects are attained, will become clear from a consideration of the following detailed description and'of the accompanying drawings, wherein:

AFigure 1 is 'a schematic representation of Vthe magnetic deflection system of a cathode-ray tube into Vwhich a preferred embodiment of the `present invention has been incorporated; and Y Figures 2, 30 and 4 are graphical illustrations which will be helpful in describing and understanding the present invention.

The magnetic deflection system shown schematically in Figure l includes a deflection-current oscillator S which, except `for the means added by the Vpresent invention, is entirely conventional. 7

Deflection-current oscillator Vl! includes a pentode l'whose rst three electrodes, II, I2 and I3, comprise the oscillatorsection Aof the tube. In

.\ addition, p'entode I0 includes an input control grid Ill and a plate I5. The input control grid l l may be coupled,'by way of an interstage transformer 2 I, to a source 22, of synchronizing pulses. Plate I'5 of-tube lll may beV connected, by Way of aload resistance 23, to a source of suitable positive potential, B4- Grid-electrode I3, Which functions` as the anode, of the oscillator section of tube Il), is connected to a source of positive potential, B-l-, Vby VWayci plate -coil I1 of the iron-core transformer I9. Winding 28 is part of a two-winding saturable inductor 2li which is added by the present invention and whose function will be described later. The said oscillator 'anode i3 is coupled inductively to the oscillator control grid I2 by means cf plate coil I'I and grid coil 223 of transformer I9. The low potentialend of grid coil I l? is returned to the cathode II by way of an RC .netwo'rhcomprising capacitor IB and resistors 3B, 3 l, .across which a suitable negative bias may be developed.

The operation of the conventional deflectioncurrent oscillator thus far described is well known and need be but briefly considered. The inductance of plate coil I1 is high and the impedance of the oscillator plate circuit is predominantly reactive. Consequently, the current flow through plate coil I`1 builds up gradually. During the building up of the oscillator plate current, a positive potential is induced at the grid end of coil I 3 and the grid circuit draws current. the early stages of the conduction period, the magnitude of the grid current isY considerably larger than that of the oscillator plate current, As the oscillator plate current rises, the oscillator plate potential drops and eventually the potential of plate I3 approaches that of grid I2. When this occurs, a sudden diversion of space current takes place from the plate I3 to the grid I2. The positive potential theretofore induced at the grid end of coil I8 by the increasing flow of plate current in coil I1v is reduced very rapidly and grid I2 swings highly negative in a very short period ofjtime, thus blocking the flow of current through the tube. As soon as plate current ceases to flow, the negative charge on grid I2 rapidly leaks off, the grid swings in the positive direction, and currentl again starts to flow through the tube. The cycle yjust describedrepeats, and a current of sawtooth waveform consequently flows through primary coil I1..

In Figure .1, transformer I'9 is shown to include a third winding 24 which is serially connected Atothe cathode-ray-tube deecting coils 25a, 25h. The deflecting coils are, of course, so positioned, ,with'respect to the cathode-ray tube 2S, that den ection of the cathode-ray beam 21 is effected by lthe magnetic elds established by the sawtooth vcurrent supplied to the deflecting coils from transformer I9. If desired, in lieu of the separate winding 24, a tapped connection to primary winding I1 may be used tosupply a portion of the-sawtooth plate current to the deilectingfcoils. In the absence of compensating means, the conventional deflection-current oscillator, thus farg described, supplies to the deilecting coils a sawtooth currentwhoserising or sweep slope is exponential rather than linear. As previously indicated, this isdue principally -to the internal resistance of tube I0, to the resistance of transformer I9, and to the resistance of deilecting coils 25a, 25h. Itis apparent, if the inductance of the is-not without merit, certain factors prevent it v from being a completely satisfactory solution to the problem, as will be best understood by considering more fullythe actionof a single-winding saturable inductor connected in series with the kplatecoil.` It wouldl seem ythat for best results,

the core of the single-winding saturable inductor should be composed of a readily saturable magnetic material of high permeability, such as an alloyofnickel, iron and molybdenum. The linearizing-eiects actually achieved by the use of such a single-windingsaturable inductor are not,

DuringV however, as good as may be expected from a rst consideration. The reasons may be explained most readily by referring to Figure 2 which shows the upper portion of the hysteresis loop of a commercially available alloy of nickel, iron and molybdenum which is readily saturable and has high permeability. Observe that,in Figure 2, if the applied magnetizing force (in oersteds) is Varied to a substantial extent in the region between zero and a value of positive polarity, the variation in flux density (in gausses) per unit volume will be relatively small. VFor example, in Figure 2, if the m'agnetizing force be varied twenty H'units, i. e. from zeroH units to.+20 H units, the iluX density Will only vary three B units, i. e. from +17 B units to +20 B units, as indicated by the A-to-D portion of the hysteresis curve. The variation in the inductance of the saturable inductor will therefore be small. Consequently, the variation in the inductance of the saturable inductor will be insuflicient to achieve the Variation in the total inductanceof the oscillatorrplate circuit necessary to linearize the beam deflection, unless, of course, a saturable inductor having a very large core be used. The use of a large-core saturable indu-ctor is, however, undesirable from the standpoint of cost and power consumption.

ExaminationY ofthe hysteresis curve of Figure 2 suggests that a biasing magnetizing force, whose polarity is opposedto thatof the magnetizing force intended to be applied, may be advantageously employed to effect operation of rthe saturable inductoron the steep portion of the hysteresis curve. Stated another way, if the applied magnetizing force., could. be caused to vary from say +3 H units to +20 H units, wide variations in inductance could be accomplished, for the `flux density would then vary approximately twenty B units, as indicated by the C-to-D portion of the hysteresiscurve of Figure 2..v The employmentA of-direct current for biasing purposes-is, however, undesirable. In the first place, a large-coreinductor would'have to be used in'l order lto avoid having the D.C. component saturate the core ofthe inductor.` In addition,-a choke would probablyhave to be inserted in series with the low impedance D.-C. Vsupply in order to avoid shorting out of the alternating component.- 'Ihe power consumed by such an arrangement would be undesirably large.

In accordance with the present'invention, the required wide variations in the inductance of the saturable inductor are accomplished by employing a saturable inductor having two windings, of predetermined turns ratio, and passing a different current through each winding in such direction that the magnetizing force due to one current opposes the magnetizing force due to the other, the two currents having such waveforms that the polarity of the net magnetizing force reverses during the sweep'periodof the deection system.

In the circuit of Figure 1, I employ a saturable inductor20 having one winding, 28, in the plate circuit of the oscillator and the other winding, 29, in the oscillator grid circuit. Figure 3 graphically illustrates the manner in which the plate current through winding 28 increases during the conduction period of tube III'. Observe that the platefcurrent builds up gradually to a maximum valueand then, as the tube cuts oi, falls rapidly to zero. Figure 3 also shows the mannerY in which thegrid current inl winding 29 decreases, from a value which at the start of the conduction period is substantially larger than that of the plate aeszgrso current, toa value which just prior to cut-off is substantially smaller.

As indicated hereinabove, the'two windings, 28 and: 29,. of saturable ind-uctor 29 are so connected that the` mag-netizing force applied tof the highpermeability core by the plate current in winding 23 opposes` the magnetizing force applied to the core by the grid current in ,winding 29. That is why, in Figure 3, the grid current has arbitrarily been represented `as being of a polarity opposite to that of the plate current. Thus, the net mag-netizing force applied to thecore of inductor 2O is a function of the diderence between the force exerted by the plate current and the force exerted by the grid current.

In accordance with the present` invention, the turns-ratio of windings 28 and 29 areso chosen that, at thel start of the conductionperiod, the magnetizing force of the grid current predomirnates,v but during the latter portion of the conductionperiod, the magnetizing force of the plate current predominates. This is shown graphically in Figure 4 where the polarity of the magnetizing force applied to the core of the saturable inductor is shown to reverse during the conduction period. It will be understood, of course, that while the graph shown in Figure 4 is related to that shown in Figure 3, the graph in Figure 41 reflects the fact that the applied magnetizing force is a function of the number of turns on each winding as well as of the current through each winding.Y

It has been stated above that, at the start of the conduction period, the magnetizing force Vdue'to the grid current is greater than that due to the plate current. Hence, the net applied magnetizing force, at the start of the conduction or sweep period, may be considered to be of negative polarity. This is consistent with the description of the hysteresis curve of Figure 2 wherein the magnetizing force of Vthe plate current was assumedv to be of positive polarity. Figure 4 shows graphically that, during the conduction period of tube Ill, the net magnetizing force applied to saturable inductor decreases substan- .tially linearly to zero, and then, vas the magnet- Vizing force due to the plate 4current becomes predominant, the net applied magnetizing force reverses polarity and increases substantially linearly in the positive direction.

Referring again to Figure 2 wherein lis Vshown the hysteresis curve of `the readily-saturable high-permeability material of which the core of inductor 2G is assumed to be made, it will be seen that a niagnetizing force which reverses polarity, in the manner hereinabove described, will be effective to vary the flux density of saturable inductor 2l) over a wide range, as for example, over that portion of the hysteresis curve lying ,between points C and D. During the early portion of the oscillator conduction period, the flux density of inductor 20 rwill varyfrom point C to point A. This represents a wide variation, and the Ainductance is consequentially high. During the latter portion of the conduction period, the variation in flux density will be small, as is indicated by the A-to-D portion of the hysteresis curve of Figure 2. The inductance of inductor 2l] during this portion of the cycle is therefore small. It-

will be seen then that the inductance of inductor 2l! is maximum at the start of the conduction period, that thereafter the inductance decreases substantially linearly to a relatively low value in a time period which Icorresponds to a selected portion, say one-third, of the Aoscillator conductionA period, and that for the remaining twothirds of the conduction period, the inductance of saturable inductor 20 is very small.

In the circuit of Figure 1, the inductance of saturable inductor 20 adds to the inductance of transformer [Sto provide a total oscillator-platecircuit inductance which, at the start of the conduction or sweep period, is of desired increased magnitude- 'I'he initial rate of rise of the plate current istherefore retarded. As the plate cur.- rent rises at itsreduced rate, the inductance vof saturable inductor 2l) decreases in a substantially linear manner, as is indicated by the C to A' portion of the hysteresis curve of Figure 2. The retarding force exerted by inductor 20 tothe rise of plate current isgradually reduced, and at the expiration of say one-third of the conduction period, the retarding eifect of inductor 20 is substantially removed. l

AS. a result of the action above described, Ythe excessive rate of rise ordinarily occurring at the start of the sweep portion of the deiiection cycle is corrected .and a substantially linear sweep is r erablybe a toroid, but this is not essential.

obtained.

VIn one of my experiments, I employed a twowinding saturable inductor having a core comprised of molybdenum permalloy tape. The tape had across section of 0.002 inch, and was wound toroidally, forming a ring having an inside diameter of three-quarters of an inch and an outside diameter of about one inch. The plate Winding had turns and the grid winding had 10 turns. I employed this two-winding saturable inductor in a circuit similar to that shown in Figure 1. Prior to theinsertion of Vsaturalole inductor 2D into the circuit, the rate of rise of the deflecting current was 31 per cent faster during the early portion of the sweep than during the later portion. When the two-winding saturable inductor 20 was connected into the circuit in the manner shown in Figure 1, the rate of rise of the deflecting current was less than 2 per cent faster near the beginning -of the sweep than near the end thereof. When a prior art single-winding saturable inductor was employed, in lieu of the twowinding inductor of the present invention, the rate of current rise was 19 per cent faster near the start than near the end of the sweep. It will be seen that the linearizing effect of the twowinding saturable inductor oi the present invention is substantially better than that of the prior art single-winding inductor.

An additional important advantage of the twowinding saturable inductor of the present invention is that the power consumption is Very small, being less than that of any prior art linearizing device known to me.

Various modifications may, of cc-urse, be made without departing from the broad conceptl of my invention. For example, an obvious modification would beto connect winding 28 of saturable inductor 2! in series with the cathode-ray-tube deflecting coils 25a, and 25h, insteadof in seriesY with the plate coil I 'l as shown.

Y The core of the saturable inductor 2@ may'pre'f- However, ifY an air-gap core be employed, the air gap should be small in order to permit the use of less magnetic material and in order that the physical dimensions of the inductor may be small.

I have described my invention in the environment of a sawtooth-current generator of the type having a relatively long period of conduction and a relatively short period-of non-conduction. This type of sawtooth-current generator happens to be particularly .suited to my invention since the grid and plate currents vary in the desired contrasting manner, i. e. currents are available for vestablishing a net magnetizing force whose polarity reverses during the sweep period. Other types of blocking tube oscillator or sawtooth generator may, however, be employed. IfY the deflection system employed is such that, in normal operation, only a single srawtooth current is produced, it Will be necessary to derive a second current whose slope is sufficiently different from that of the first-mentioned sawtooth current to produce the required reversal in the polarity of the net magnetizing force.` This may be done readily by known means.

Having desc-ribed my invention, I claim:

1. In a deflecting circuit for a cathode-ray tube having magnetic deflecting coils and a rst source of non-linear sawtooth current for energizing saidfdeecting coils; a second lsource of current, the phase and waveform of said second current being such that said second current is decreasing in magnitude during at least a portion ofthe time said first current is increasing in magnitude; an inductor having a core of high-permeability material and having first and second windings of predetermined turns ratio; means for passing'said first sawtooth current through said rst winding; means for passing said second current through said second winding in such direction that the magnetizing force resulting from said second current opposes the magnetizing force resulting from said first current, the phase of said first and second currents and the turns ratio of said first and second windings being such that the net magnetizing force reverses polarity during the time said savvtooth currentis increasing in magnitude.

2. In a deflecting circuit for a cathode-ray tube having magnetic deflecting coils and a first source of non-linear sawtooth current for energizing said deilecting coils, the improvement which constitutes the `provision ofv linearizing means, said linearizing means comprising: a second source of current, the phase and waveform of said second current being such that said second current is decreasing in magnitude during at least a portion of the time that said first current is increasingin magnitude; an inductor having a core of readilysaturable high-permeability material whose hysteresis curve is steep in a region lwhere the magnetizing force is of one polarity and substantially flat in an adjacent region where the magnetizing force is of opposite polarity, said inductor having iirst and second windings of predetermined turns ratio; means for passingv said rst sawtooth current through said rst winding; means for passing said second current through said second winding in such direction that the magnetizing force resulting from said second current opposes the magnetizing force resulting from said .rst current, the phase of said first and second currents and the turns ratio of said rst and second windings being such that the net magnetizing force in said inductor core during the iirst part of the rising portion of said sawtoothcurrent waveform has a polarity which corresponds to that of the region in which the steep portion of the hysteresis curve is located, while during the remainder of the rising portion of said sawtooth-current waveform the net magnetizing force in said core has an opposite polarity corresponding to that of said adjacent region in which the relatively flat portion of said hysteresis curve is located.

3. In a deflection circuit for a cathode-ray tube 8 'utilizing magnetic deflection coils and a saw@- tooth-current oscillator, said Sawtooth-current oscillator including a plate circuit and a grid circuit inductively coupled together by means of a coil in each circuit, the improvement which comprises the provision of a linearizing device for linearizing the sawtooth' plate current, said linearizing device comprising: Van inductor having a core of high-permeability easily-saturated 'material v` whose flux-density-versus-magnetizingforce characteristic is steep in a region where the magnetizing force is of one polarity and relatively flat in an adjacent region Where the magnetizing fcrce is of opposite polarity, said inductor having a iirst winding and a second winding; means for connecting said rst winding in series with the platefcir'cuit of said oscillator; means for connecting said second winding in series with the grid circuit ofy said oscillator, said first and second winding of said inductor being so connected that the magnetizing force in the core of said inductor resulting from the oscillator plate current op- 'poses themagnetizing force in said core resulting from the oscillatorgrid current, the turns ratio of said iirst and secondA windings being so chosen that the net magnetizing force in said core during thepiirstpart of the rising portion of said sawtooth plate current has a polarity corresponding to that of the region in which the steep portion of said flux-density-versus-magnetizing-force characteristic is located and during the remaining part of said rising portion of said sawtooth plate current the net magnetizing force has an opposite polarity corresponding to that of the said adjacent regionv in which the relatively flat portion of the said characteristic is located.

' 4. Inan inductive circuit having a source of non-linear sawtooth current, the improvement which comprises Ythe provision of means for substantially linearizing a slope of said sawtoothcurrentV 'i/vaveforin, said linearizing means comprisingfan inductorhaving a core of readilysaturable high-permeability material whose hysteresis characteristic is steep in aregion where the lmagnetizing force isof one polarity and relatively flat in an adjacent region where Ythe magnetizing force is of opposite polarity, said inductor having a rst winding and a, second winding; means for connecting said iirst Winding in series with said source of sawtooth current; a second 'source of current, the phase and waveform of said second current being such that said second current decreases in magnitude at a time when said rst current is increasing in magnitude; means for connectingsaid second Winding of said inductor in series with said second source of current in such manner that the magnetzing force in the core of said inductor resulting from said second current opposes the magnetizing force in said core resulting from said rst current, the turns ratio of said iirst and second windings being such that the net magnetizing force in said core, during the first part of the to-be-linearized slope Yof said savvtooth-current waveform, has a polarity corresponding to that of said region in which the said steep portion of said hysteresis characteristic is located and during the remaining part of said Vslope the net magnetizing force has Van opposite polarity corresponding to that of said adjacent region in which said relatively flat portion of said hysteresis characteristic is located.

5. A relaxation` oscillator for generating sawtooth current waves of great linearity, said oscillator comprising: a/vacuum tube-having at least triode elements serving as cathode, grid and anode electrode respectively; a first transformer having a pair of windings for coupling the grid-cathode circuit of said vacuum tube to the anode-cathode circuit thereof; and a second transformer having its windings connected in series, respectively, with the windings of said first transformer in the said grid-cathode and anode-cathode circuits of said tube, said second transformer having a saturable ferromagnetic core, the windings of said second l0 transformer being so connected in their respective circuits that the magnetizing force produced by the flow of plate current in one of said windings No references cited.

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