US2280990A - Thermionic valve circuits for the generation of saw tooth currents - Google Patents

Description

April 28, 1942. E. L. c. WHITE 2,280,990

THERMIONIC VALVE CIRCUITS FOR THE GENERATION OF SAW TOOTH CURRENTS Filed May 15, 1940 3 Sheets-Sheet l Zhwentor EL-GI 17 656, BB gm Cittorneg April 1942- E. L. c. WHITE 2,280,990

THERMIONIC VALVE CIRCUITS FOR THE GENERATION OF SAW TOOTH OURRENTS Filed May 15, 1940 3 Sheets-Sheet 2 Ihmentor April 28, 1942.

E. L. c. WHITE THERMIONIC VALVE CIRCUITS FOR THE GENERATION OF SAW TOOTH CURRENTS 3 Sheets-Sheet 3 Filed May 15, 1940 June/Bursa ieiruralvca ELL.

attorney Patented Apr. 28, 1942 U STATS THERMIONIC VALVE CIRCUITS FOR THE GENERATION 0F SAW TOOTH CUR- REN TS greases Application May 15, 1940, Serial No. 335,381 In Great Britain May 15, 1939 4 Claims.

This invention relates to electric circuit arrangments for the generation of substantially sawtooth currents such as are required to be passed through the scanning coils of a cathode ray tube for use in television transmitting or receiving systems.

The invention is principally concerned with the so-called resonant return type of sawtooth current generator in which an electron discharge valve is periodically rendered conducting to generate the long flank of the sawtooth wave, the short flank of the sawtooth wave being generated by permitting an oscillatory circuit to execute a portion of an oscillation and at a predetermined time during the execution of the portion of the oscillation the said valve is rendered again conducting to commence generating a further long flank. The oscillatory circuit usually comprises the scanning coils and their inherent capacity.

This kind of circuit is disclosed in the specification of British Patent No. 400,976 and usually a diode is associated with the said valve which is rendered conducting after the oscillatory circuit has executed a half period of oscillation, thus preventing further oscillation and permitting the valve to generate the long flank of a further sawtooth wave.

It is found that with a circuit of this kind the wave form during the long flank has an exponential curvature determined by the time constant L/R of the inductance of the scanning coils or equivalent elements and their resistance and that of the diode. In television systems this time constant may frequently be as low as two line periods, in which case the long flank of the sawtooth wave will be very appreciably non-linear. Circuits of the kind in question-are usually controlled by applying synchronizing pulses to the control electrode of the valve and it has been proposed, in orderto improve the wave form of the long flank, to control the valve by the application of a sawtooth potential so as to supply a sawtooth current greater than that required by the coils. It will, of course, be appreciated that in order to cause the current in the coils to increase linearly with time it is necessary for the potential across the coils to have a constant component on account of the inductance together with an increasing component on account of the resistance,

and in the case where a sawtooth controlling potential is employed, an excess sawtooth current is provided which causes an increasing potential to appear across the diode and the coils which can be adjusted to provide the required linearlyincreasing current in the coils. This method,

however, is not entirely satisfactory in that the valve has to pass more current than is used in the coils, and, furthermore, it is difficult to secure the exact wave form correction at all amplitudes owing to non-linear valve characteristics.

It is therefore the object of the present invention to provide improved electric circuit arrangements for the generation of substantially linear sawtooth current waves.

tential applied to a control electrode to control the potential difference developed across said load, the arrangement being such that the effect of applying said control potential to said control electrode in said further valve is to tend to maintain the desired linearity of the current in said load.

In the case of the embodiment of the invention in a circuit of the resonant return type, the first mentioned valve is arranged to be rendered periodically conducting to generate the long flank of the sawtooth during which the second valve provides the necessary control of waveform.

In the simplest form of the invention the control potential applied to said control electrode in said second valve is maintained substantially constant with respect to the cathode potential of said first valve. Preferably, however, said control potential is arranged to be dependent on the anode currentin the first valve or is derived from the load and amplified. This affords an arrangement by which the undesired effect of resistance in a reactive load such as that presented by scanning coils can be readily eliminated. Also for the purpose of further improving the wave form of the current in the load means may be provided for feeding back potentials in a negative sense from the output of said second valve to the input of the first valve. In a circuit of the resonant-return type, a diode may be inserted between the first valve and the second valve in order to protect the cathode heater insulation of this valve against the high positive potentials developed on the anode of the first during the generation of the short flank of the sawtooth wave.

As applied to a television transmitting sys- 9 and coils 3 bythe resistance I2.

tern, socalled Keystone modulation may be performed by adding a frame frequency sawtooth component to a control electrode of said second valve and slight skewness of the resulting Keystone scanned patch due to unwanted damping of the half cycle of free oscillation causing the return flank of the sawtooth wave may be corrected by adding a frame frequency sawtooth current of correct amplitude to the current of the first valve by means of a third valve having its anode connected in parallel with the anode of the first valve, a frame frequency sawtooth potential being applied to the grid of the third valve of the same sign as thatapplied to the grid of the second valve.

For the purpose of describing the invention more in detail and for describing the method 'of carrying the invention into effect, reference will be made to the accompanying drawings in which-- Figure 1 is a circuit diagram according to one form of the invention,

Figures 2 and 3 show explanatory curves,

Figure 4' is a circuit diagram according to a further form of the invention,

Figure 5 is a circuit diagram of another embodiment of the invention, and

Figure 6 is a circuit diagram of a further embodiment of the invention, and

Figure 7 is a graph explanatory of Figure 6.

All the arrangements shown in the drawings are of the resonant return type.

As shown in Figure 1, the reference numeral 6 indicates the first valve which is arranged to be periodically rendered conducting and nonconducting by the application to its control electrode of negative synchronising pulses through a transformer I; the valve 6' is preferably of the high impedance type and is shown as a pentode valve, but may be of other types. The valve 6 is arranged to generate through the scanning coils, indicated diagrammatically at 8, substantially sawtooth current waves, the-anode current of' the valve 6 beingfed to the scanning coils 8 through a transformer 9. The anode of the valv 6 is connected tothe positive terminal of "a source of anode current through the primary winding of the transformer 9 which is decoupled to earth by a condenser Ill. The resistance of the primary winding of the transformer 9 is indicated by the resistance II and the resistance of the secondary winding of the transformer of valve 6 is also connected to the cathode of a triode valve I3, the control electrode of which is connected to a tapping point on a potentiometer I4 connected across the sourceof anode current. The control electrode of valve I3is also connected through a condenser I5 to earth. The anode'of valve I3 is connected to th positive terminal of the anode current source through a resistance I6 and potentials developed across the resistance I6 and applied through a condenser I! in the negative sense to the control electrode of the valve 6, as shown, which is biased by a battery I8 and resistance I9.

The operation of the circuit shown in Figure 1 will be described with reference to Figures 2 and 3. The synchronising pulses indicated at 5 in Figure l are, as stated above, in the negative sense and serve to cause the valve '6 to be periodically rendered conducting and non-conducting. In Figure 2' the curve A indicates the current flowing through the'valve 6, the curve B the desired saw-tooth current flowing through the transformer 9, and the curve C the current through the valve I3 which for convenience is plotted in the negative sense. B is the sum of 5 A and C, except during the return stroke when The anode r sawtooth wave.

current flowin in the stray capacities is of importance. The curve shown in Figure 3 indicates the potential developed across the scanning coils 8. When the valve 5 is conducting the anode potential relative to cathode tends to increase. Thus, since the anode of the valve 6 is connected to the cathode of the valve I3 and a substantially fixed potential is applied to the control grid of valve I3, the valve I3 tends to pass less current, thus allowing the current in 9 to increase as required. The effect of any fall in the current in valve 13 will produce additional current for 9 by the feedback in the negative sense from across resistance It to the valve 6, tending to decrease the impedance of the valve 6, and so increase the current in valve 6, tending to maintain the anode potential of valve 6 substantially constant. The current through the valve 6 substantially increases linearly as shown in the curve A of Figure 2 and when the valve 6 is rendered non-conducting on the application of a negative synchronising pulse to its control electrode the current through the valve is suddenly decreased, preferably to zero and the oscillatory circuit formed by the coils 8 and their distributed capacity together with other stray capacities perform a half cycle of oscillation generating the short flank of the sawtooth wave B. When the valve 6 is rendered non-conducting the anode po tential suddenly increases'due to the reactance of the load circuit, causing the cathode potential of the valve I3 likewise to increase, rendering such valve non-conducting. As soon as one half cycle of oscillation is completed the potential. of the anode of valve 6 and thus of the cathode of valve I3 is decreased to such a value that the valve I3 is rendered again conducting causing the oscillation to cease and permitting the valve 6 to commence genera-ting a further long flank of the It will be seen from Figure 2 that the current change in the valve I3 is relatively small and maintains the potential change across the scanning coils 8 as shown in Figure 3 at a minimum since the current passed by the valve 6 is substantially of the sawtooth wave form required by the coils 8.

The wave form in the coils is, with the arrangement shown in Figure l, practically dependent on the time constant L/R of the coils 8 and transformer 9. However, the resistance of the coils is not usually small enough to make the time constant L/R larger than ten scanning periods as is desirable in a television system and in order to overcome this defect the circuit shown in Figure 4 may be employed in which parts corresponding to similar parts in Figure 1 are given the same reference numerals. In this figure the valve 6 is of the pentode type in which the current to the screening electrode is a substantially constant fraction of the anode current and the potential developed across a resistance 26 by the screen current is applied to the control electrode of the valve I3 through a condenser 2|, a leak resistance 22 being provided for the control electrode, as shown. In this example the valve I3 is shown as a pentode, and since its cathode is effectively connected to the anode of the valve 6 and thus assumes the potential of the anode of valve 6, the suppressor grid of the pentode valve I3. is not connected to the cathode as is usual but is taken to the point of substantially fixed bias potential for the control electrode of I3 on the potentiometer I4. The potentials developed across the resistance I6 are applied in the negative sense to the control electrode of the valve 6 through a transformer 23. The synchornising pulses in this case are applied through a condenser 24 and leak resistance 25 to the control electrode of valve 6, the cathode of which is biased by a resistance 26 shunted by a condenser 21. If desired, the potentials developed across the resistance I 6 may be fed to the control electrode of valve 6 through a resistance-capacity coupling, as shown in Figure 1, or the arrangement shown in Figure 4 may be employed in place of the arrangement shown in Figure 1. The potential developed across the resistance 26 is of a substantially sawtooth wave form, and thus the correct sawtooth potential is applied to the grid of the valve I3 and hence by cathode follower action to its cathode, s that the potential is effectively applied to the coils 8 to compensate for the increasing potential drop across the coils when the desired increasing current fiows therethrough. If the resistance of the coils and secondary of the transformer 9, referred to the primary side, together with the resistance of the primary of the output transformer 8, totals R ohms and the ratio of anode current to screen current of the valve 6 is k, then the correct value for the resistance 26 is IcR ohms.

As shown in Figure 4 the anode of the valve 6 is connected to the cathode of the valve I3 through a diode 28 which can be specially constructed to withstand the high positive potentials developed on the anode of the valve 6 when the latter is rendered non-conducting, i. e., during the generation of the short flank of the sawtooth wave, the diode during this period being rendered non-conducting likewise rendering valve I3 non-conducting.

To prevent a considerable proportion of the high positive potential being applied to the oathode of valve I3 through the anode-cathode capacity of diode 28, a condenser of say to 100 times that capacity may be connected from the cathode of valve I3 to earth, or a resistance may be used, so that valve I3 is caused to pass a substantially constant additional standing current.

A further form of the invention is shown in Figure 5 which includes means for effecting Keystone modulation and correcting for slight skewness of the resulting Keystone scanned patch. In this figure parts which correspond to similar parts in other Figures 1 and 4 are given similar reference numerals. Figure 5 embodies generally the form of the invention shown in Figure 4 in which the potentials developed across a resistance 26 by the screen grid current of the valve 6 are fed through the anode resistance 29 of a valve 36 (hereinafter referred to) to the control electrode of the valve I3 and potentials developed across the resistance I6 are fed back in the negative sense through the condenser 24 to the control electrode of the valve 6. In order to perform the necessary Keystone modulation a frame frequency sawtooth component indicated at F in Figure 5 is fed to the grid of valve 30 through a condenser 3| associated with leak resistance 32 and the potentials developed across the anode resistance 29 of valve 33 as a result of the wave F are applied to the grid of valve I3 through condenser 33 and leak resistance 34. In order to correct slight skewness of the resulting scanned patch a frame frequency sawtooth wave is applied to the grid of a valve 35, this wave being of opposite phase to the wave applied to the grid of valve 30. It will be seen that the anode of valve is connected through a condenser 36 to a resistance 31 and a tapping point on the resistance is connected to the grid of valve so that there is effectively applied to the grid 35 a sawtooth wave of opposite phase to the wave F. The anode of valve 35 is connected in parallel to the anode of valve 6 so that a frame frequency sawtooth current is eifectively applied to the current of the valve 6. The synchronising pulses instead of being directly applied to the valve 6 are applied to the grid of a further valve 38 through condenser 39 and leak resistance 40, the anode of valve 38 being connected to the anode of valve I3 and through condenser 24 to the control electrode of valve 6.

Figure 6 shows a further form of the invention similar to the arrangement shown in Figure 4, but instead of a controlling potential being derived from the screening grid of valve 6 of Figure 4 a controlling potential is derived directly from the scanning coils 8. Parts in Figure 6 which correspond to similar parts in the previous figures have been given similar reference numerals. It will be seen from Figure 6 that the valve I3 is a triode as is the case in Figure 1 and the grid of this triode is connected to the anode of a triode which, in conjunction with a further triode 5I forms a push-pull amplifier stage as described in the specification of British Patent No.

482,740. The control potential as stated above is derived from the scanning coils 8 and it will be seen that these coils are connected to the grid of the valve 5| through a condenser 52 and leak resistance 53. The grid of valve 50 has normally applied to it from an adjustable tapping point on a potentiometer 54 a steady potential and if the controlling potential derived from the scanning coils departs from this steady potential then the valve I3 operates by the negative feedback principle to control the valve 6 for the purpose of causing the latter to produce in the coils 8 the required linear wave form. The amplifier 56, 5| should have a gain of the order of 100. Assuming that the coils 8 have their centre point earth as shown one side of the coils will have a steady positive potential 2 dt 7 during the long flank of the sawtooth current where is one half the inductance of the coils and i the current through them. Actually, owing to the resistance of the coil there will also be a sawtooth component which can be removed by arranging that the time constant of the coupling 52, 53 is equal to The steady potential which is applied to the grid of valve 50 is arranged to be substantially equal to L di 5 27a and if the latter tends to vary the difference is amplified considerably by the amplifier 50, SI

and applied to the grid of valve l3, thereby reducing the tendency to variation in the usual manner of negative feedback circuits. During the return stroke or short flank of the sawtooth wave a large negative impulse occurs on the grid of valve rendering this valve non-conducting so removing the negative feedback during the return stroke. It will be seen from Figure 6 that a diode 28 is also employed as in Figure 4, this diode also serving to remove the feedback during the return stroke owing to its being then nonconducting. In some cases the diode may be omitted, in which event negative feedback will also be prevented owing to the high positive potential appearing on the cathode of the valve i3, rendering this valve non-conducting during the return stroke.

It will thus be seen that the steady potential applied. to the grid of valve 50' serves to govern the amplitude of the sawtooth current wave form and by comparing this potential with the controlling potential derived from the scanning coils it is possible to produce a current wave form in the coils 8 in which the long flank is more nearly linear. The grid of valve 5|] may also have applied to it a frame frequency sawtooth potential for Keystone modulation and correction for skewness may be accomplished by an arrangement similar to that shown in Figure 5.

The negative feedback circuit shown in Figure 6 takes substantially complete control of the wave form of the saw tooth current in the coils 8, providing, of course, that the current in the valve 6 is periodically interrupted or suddenly reduced and that after the interruption or reduction there is always more current supplied from the valve 6 than that required by the primary of the transformer 9'. Thus it is not necessary to'ernploy negative feedback from the resistance l6 as in Figure 1, though in some cases such an arrangement may be used for obtaining the required current wave form from the valve 6. In Figure 6, however, the required current wave form from the valve 6 is obtained by controlling the valve 6 by a saw-tooth potential indicated at 55, such sawtooth potential being derived, for example, from a blocking oscillator. The valve (5 is preferably so biased that the first part of the forward stroke of the sawtooth potential applied to its control grid is below' the lower bend of the anode current cut-off since current in the transformer 9 during the period following the return stroke is negative and can only be supplied by the diode 28. Thus, any current passed by the valve 6 during this period is extra current for the diode 28 to carry and is unnecessary waste of power. Figure 6 is a graph showing the relationship between the current in the valve 6, the current through the transformer 9 and the current through the valve I3. The curve 56 represents the current through the valve 6 and it will be seen that following the return stroke during which period the current in the valve 6 is reduced to zero, no current flows through valve 6 for a substantial period for the reasons stated above. The curve 51 represents the current through the transformer 9 and is the sum of the current 56 and the curve 58 which represents the current through the valve l3 plotted in a nega' tive sense.

The arrangements shown in Figures 1, 4 and 5 are especially applicable to cases where a low power output is required and is applicable for use with television transmitting tubes, such as those known by the registered trade-mark Emitron. The circuit shown in Figure 6 is applicable: for high efiiciency working, particularly in cases of television receiving tubes, such as the so-called projection tubes, where very high scanning beam velocities are employed.

If desired, where very high currents are required' in the load circuit so that valve l3 must be large or replaced by several valves in parallel thereby presenting a large input capacity, the valve 59 can be connected to the control grid of valve l3 through suitable repeaters, preferably of the cathode follower type.

It is desirable in the circuits described that the width of the negative synchronising pulses of Figures 1 and 4 and the positive synchronising pulses of Figure 5 should be approximately equal to the return time or duration of the short flank of the sawtooth wave, i. e., the duration of the pulses should be half the natural period of oscillation of the scanning coils and their associated stray capacities. With such an arrangement it is possible to employ the same synchronising pulse for switching off the electron beam of the cathode ray tube during the return stroke, as is usually required I claim:

1. A system for producing deflection voltage variations for a magnetically deflected cathode ray beam comprising a first electron discharge tube having a cathode, a control electrode and an anode, a source of potential having positive and negative terminals, means including a load circuit for connecting said anode to said positive terminal of the source of potential and the cathode to said negative terminal whereby said anode may be maintained positive with respect to said cathode, a second electron discharge tube having a cathode, a control electrode and an anode, means for effectively connecting said second electron discharge tube in parallel with said load circuit with the cathode of saidsecond electron discharge tube connected to the anode of said first electron discharge tube and with the anode of said second electron discharge tube connected to said positive terminal of the source of potential, means for applying synchronizing impulses between the control electrode and the cathode of said first electron discharge tube, and means including a potentiometer for maintaining the control electrode of said second electron discharge tube at a predetermined positive potential with respect to the cathode of said first electron discharge tube.

2. A system for producing deflection voltage variations for a'magnetically deflected cathode ray beam comprising a first electron discharge tube having a cathode, a control electrode, a screen grid electrode and an anode, means including an anode load impedance for maintaining the anode positive with respect to said cathode, means for coupling cathode ray beam defleeting coils to said load impedance, a second electron discharge tube having a cathode, a control electrode and an anode, means for effectively connecting said second electron discharge tube in parallel with said load impedance with the cathode of said second electron discharge tube coupled to the anode end of said load impedance and the anode of said second electron .discharge tube coupled to the other end of the load impedance, means for applying synchronizing impulses between the control electrode and cathode of said first electron discharge tube, means for maintaining the screen electrode of said first electron discharge tube at a positive potential with respect to its associated cathode so that the screen current is a substantially constant fraction of the anode current in said tube, means for coupling said screen grid electrode to the control electrode of said second electron discharge tube, and means for maintaining the control electrode of said second electron discharge tube at a predetermined average potential with respect to the cathode of said first electron discharge tube.

3. A system for producing deflection voltage variations for a magnetically deflected cathode ray beam comprising a first electron discharge tube having a cathode, a control electrode and an anode, means including an anode load impedance for maintaining the anode positive with respect to said cathode, means for coupling the cathode ray beam deflecting coils to said load impedance, a second electron discharge tube including a cathode, a control electrode and an anode, means for effectively connecting said second electron discharge tube in parallel with said load impedance with the cathode of said second electron discharge tube coupled to the anode end of said load impedance, means including a load resistance for connecting the anode of said second electron discharge tube to the other end of said load impedance, means to apply synchronizing impulses between the control electrode and cathode of said first electron discharge tube, means to maintain the control electrode of said second electron discharge tube at a predetermined potential relative to the cathode of said first electron discharge tube, and means for coupling the anode of said second electron discharge tube to the control electrode of said first electron discharge tube to superimpose the voltage variations at the anode of said second electron discharge tube upon the synchronizing signals applied to the control electrode of said first electron discharge tube.

4. A system for producing deflection voltage variations for a. magnetically deflected cathode ray beam comprising a first electron discharge tube having a cathode, a control electrode and an anode, a source of positive potential, means including an anode load impedance for connecting said anode to said source of positive potential whereby said anode may be maintained positive with respect to said cathode, a second electron discharge tube having a cathode, a control electrode and an anode, means for effectively connecting said second electron discharge tube in parallel with said anode load impedance with means including a diode for connecting the oathode of said second electron discharge tube to the anode of said first electron discharge tube, means including a load resistance for connecting the anode of said second electron discharge tube to the other end of said anode load impedance so that electronic current may flow through both electron discharge tubes and said diode, means to apply synchronizing impulses between the control electrode and cathode of said first electron discharge tube, means to maintain the control electrode of said second electron discharge tube at a predetermined potential, and means to couple the anode of said second electron discharge tube to the control electrode of said first electron discharge tube.

ERIC LAWRENCE CASLING WHITE.

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