US2214077A - Scanning current generator - Google Patents

Description

Patented Sept. 10, 1940 UNiTED STATES PATENT I orrics SCANNING CURRENT GENERATOR corporation of Delaware Application February 10, 1936, Serial No. 63,078

5 Claims.

This invention relates to generators for producing current waves of saw-tooth form incircuits containing both resistance and inductance, particularly as such circuits are used for the generation of scanning fields in oscillight or cathode ray oscillograph tubes as used for television reception.

In my prior copending application, Serial No. 449,984, filed May 5, 1930, entitled "Television scanning and synchronizing system, I have shown that inorder to produce such waves by means of thermionic amplifying tubes the control voltage applied to, the grids of such tubes should be an extremely powerful pulse in one direction, so applied as to cause the sudden change of current from maximum to minimum (or from positive maximum to negative maximum) followed by a gradual and uniform change of voltage to produce a gradual straight-line rise of current from minimum to maximum. This wave form is necessary because the voltage necessary to overcome the inductive reactance of the circuit is directly proportional to the rate mum the inductive component of the voltage is by far the larger, and hence the necessity for the powerful pulse. During the remainder of the cycle, where the rate of change of current is preferablynot over two or three per cent-as great as during theperiod of decreasing current, the inductive component of the voltage may be practically negligible, and the resistance component becomes dominant.

In the earlier copending application above referred to, a current wave of the desired form was passed through a distorting network-in order to produce the desired voltage wave shape. The present invention relates to a method and means for using a succession of relatively short, widely spaced current pulses to produce the required wave form, these pulses being easily generated locally or derived from a received synchronizing p e Among the objects of this invention are: to provide a means and method of generating sawtooth waves of true straight-line form in an ino duct-ive-resistive circuit; to provide a means of generating such waves wherein the fly-back time is a very small part of the entire cycle; to provide a means of converting an oscillation consisting of short widely separated current pulses into a saw-tooth wave; to provide 'a'means "of generating a voltage pulse of the necessary amplitude to overcome the inductive reactance as required to secure an exceedingly short fly-back time; to provide a, means of utilizing received or locally generated synchronizing pulses directly to produce saw-tooth waves, without the intervention of an electronic oscillator; and. to provide a means of correcting the exponentially curved wave form which is derived from the discharge of a condenser to produce a true straightline wave form. l

My invention possesses numerous other objects and features of advantage, some of which, together with the foregoing, will be set forth in the following description of specific apparatus embodyingand utilizing my novel method. It is therefore to be understood that my method is applicable to otherapparatus, and that I do not limit myself, in any way, to the apparatus of the present application, as I may adopt various other apparatus embodiments, utilizing the method, within the scope of the appended claims.

Considered broadly, this invention comprises suitable means for producing widely spaced short current pulses of the type abovedescribed, and a wave-forming network through which these pulses are passed. .This network includes at least one parallel resistance-capacitance circuit which is connected in series with a simple resistor and a suitable source of current and voltage. In its limiting form such a pulse-generating oscillator will alternate between the condition wherein it forms an element of practically zero resistance in series with the wave-forming circuit just described and the current source, and a second condition wherein it forms an open circuit,

resistor which may be utilized as the pulse required to overcome the inductive reactants and current, flowing through .the resistor in the resistance-capacitance circuit, imposes a charge upon the condenser in parallel therewith. when the second condition occurs all flow of current in the resistor ceases, and the only voltage remaining in the wave-forming circuit is that due to the charge upon the condenser. The discharge of this condenser 'causes the gradual .55

- cause the necessary rapid fly-back. This same change of current desired for the second portion of the control-voltage cycle. The voltages thus produced are applied through a suitable network to control an amplifier having a nonlinear characteristic, and these voltages are so applied that, considering equal small increments of time at the beginning and end of the cycle of condenser discharge, the relatively small changes of potential at the end of such cycle produce the same variations in current in the output of the amplifier as do the relatively large changes at the beginning of the cycle of condenser discharge.

It is to be understood that in speaking of the cycle of condenser charge and discharge, what' is referred to is the degree of charge and discharge which are actually achieved by such condenser in the operation of the device, and that this cycle is relatively short compared with the time required for complete discharge of the condenser.

Referring to the drawings:

Figure l is a circuit diagram illustrating my invention as actuated by a kipp-tube oscillator.

Figure 2 is a series of curves showing the voltages and currents appearing in various portions of the circuit of Figure 1.

Figure 3 is a diagram of a modified form of circuit embodying the invention.

Figure 4 is a diagram of the circuit of the final stage of a television amplifier, showing the connection of the oscillight tube and the highand low-frequency scanning-current generators.

Figure 5 is a diagram of the invention as applied to the high-frequency. scanning circuit of a television receiver.

The operation of the system here disclosed can better be understood by reference to a preferred form of the device as shown in Figure 1. In this figure only the essential'circuits are shown, the

' conventional filament-supply circuits being omitbe that type of thermionic tube marketed under the designation of Type 56, and in the description which follows circuit constants will be given which are suitable for the generation of sawtooth waves of fromtwenty to sixty cycles per second with a tube of this character, but it is to be understood that these constants are illustrative only, and that by varying them suitably both widely different frequencies may be generated and widely diiferenttypes of tubes may be used, and that the changes necessary 'to accomplish this may readily be deduced from the values given by the use of well-known engineering principles.

' The cathode 2 of the tube l is shown as grounded. The grid 3 is connected through the secondary winding 5 of a small transformer to a timing circuit comprising a condenser E, which may have a capacitance of the order of 0.25 microfarad, in parallel with a variable resistor 1 whose maximum resistance is conveniently of the order of-250,000 ohms, and thence to ground.

The plate or anode 8 of the tube l connects through the primary winding IU of the above-v mentioned transformer to the wave-forming circuit, which includes the pulse resistor l I and an integrating circuit I 2 comprising a resistor l3 of the order of 200,000 ohms in parallel with adirectly in series with the usual plate-current supply l6 and thus to ground, but I prefer to include The operation of the circuit as thus far described is as follows: When the device is put into operation there is no bias upon the grid 3 and current passes in the plate circuit, including the transformer primary winding I0. This induces a voltage in the secondary winding 5 of this transformer (which preferably has a step-up turns ratio of about 1.5 to l), the transformer being so poled that increase of current in the primary winding swings the grid positive. This not only causes an increased flow in the plate circuit but also causes current flow to the grid, until charging of the condenser 6 and the collection of the negative charge upon the grid causes the current to start to diminish. This at once tends to swing the grid negative, due to the action of the transformer, and causes a complete blocking of the tube which persists until the charge leaks off of the condenser 6 through the resistor l, whereupon the cycle repeats. ii'he curves 20 and 2| of Fig. 2 show the approximate waveform-of the plate and grid currents, respectively. V

The'time required for the charge to leak off of the condenser 6 is controlled by varying the resistor l, and is, in practice, much longer than would be indicated by the curves, the ratio of the length of the current pulses to the interval between such pulses being in practice from to A The voltage on the grid 3 of the tube I is shown in like manner by the curve 22 of Figure 2.

The current I as shown in curve 20, flowing in the integrating circuit l2, causes a voltage drop across the condenser l5 as shown by the falling portion of curve 23, and onto this voltage drop is superposed the drop caused by the flow in the resistor ll. during the period of flow which is shown by that portion of curve 25 between the origin 0 and the ordinate A. At this point the current suddenly ceases to flow, and curve 23 illustrates the This gives a voltage rise of voltage during the period of discharge end 01' the wave-forming network and ground is applied across the input or control circuit of an amplifier through a network which comprises a blockingv condenser 30 having a capacitance of about 1.0 microfarad, in series with two resistors 3i and 32 of a value of about 250,000 ohms each, which form a voltage divider.

The output amplifier, which, is connected across the resistor 32, comprises in the present instance two more type 56 tubes, 35 and 36, whose plates 31 and 3B are connected in push-pull relationship across a deflecting magnet 40, which is illustrated merely as. an iron-core coil. The center tap of this magnet connects through the lead 4| to the supply source l6.

' The grid 42 of the tube 35 is connected to the junction of the resistors 3| and 32, and the filament or cathode 43 of this tube connects to ground through a resistor 45 of about 1000 ohms. The filament 46 of tube 36 is also connected to ground through this resistor, whereas the grid 41 connects directly to ground.

As a result ofthis asymmetrical connection the two tubes are unequally biased and loaded. The major portion of the control voltage is applied across tube 35, whose grid swings positive during a part of the cycle and therefore acquires a bias through the resistor 32. Additional bias is supplied to both tubes through the resistor 45. The unbalanced connection of the tubes and the unequal application of voltage to them results in a curvature of the grid-plate characteristic of the two tubes considered as a unit, and this is .in opposition to the curvature of the discharge characteristic of the condenser l5. It is difiicult, however, to get an exact balance in this manner, and the final correction may be applied by connecting two similar small condensers 49 and 50 across the resistors 3| and 32. The proper size for these condensers can speedily be found" complex as to obscure what actuallytakes place,

but it is possible to break down the circuit into its component elements and so to obtain an approximate picture of what happens. It should be noted first that the frequency of the oscillation is dependent almost entirely on the relationship between the condenser E and the resistor I, and

that the efiect of the coupling circuit upon the oscillation period may be neglected. It should also be noted that the circuit is quite different during the current pulses and in the interval between the pulses.

The pulse period itself is so short in comparison with the time constant of the various circuit elements that the condensers l5, l8and 30 can.

be entirely neglected and thought of as replaced by direct metallic connection. Condensers 59 and 50 in series may therefore be considered as forming one arm of a parallel circuit whose other two arms are respectivelythe pulse resistor II and the resistors 3| and 32 in series. Because of the relatively high resistance of the elements 3| and 32, they too may be neglected and the pulse may be considered as supplied through the resistor (2,000 to 5,000 ohms) in parallel with the capacitance of 0.025'microfarad. The time constant of such a combination is of the order of 5x 10 which is so small that even in theshortest fly-back time contemplated in this arrange-- ment, thepulse voltage is reduced by much less than one per cent by the'presence of these condensers.

When the tube I becomes non-conductive the dominant elements in the network are the integ'rating circuit I2, which has a time constant of approximatelyOl and a branch comprising grating circuit that the presence of the condenser 30=may, as a first approximation, be neglected toether with the resistance Illand the network the tube I biased to cutoff, through the lead 62.

the cycle when the tube is non-conducting are those which may be computed by considering that a short-circuit across'this latter branch of the circuit has suddenly beenfremoved. The solution of this circuit shows it to be in the form 1Ice- The presence of the small condensers 49 and 50 decreases both It and a; that is, it reduces slightly the amplitude of the voltage variations as applied to the amplifier and increases the time constant of the circuit, straightening the curve. The decrease in amplitude is relatively slight for small values of capacitance, but the increase in time constant is of sumcient magnitude to be of material value when applied as a final correction. In the second approximation,

wherein the effect of the series condenser 30 is considered, the complete solution is very complex, but the over-all effect is again an effective increase in time constant, straightening the curve still further with a slight decrease in amplitude.

The final curve of voltage rise across the amplifier may be made as straight as desired, but it is preferable practically tolleave some curvature in the applied voltage curve and to apply a small but definite positive correction by means of the amplifier characteristic as above described.-

With the constants above given the departure from linearity thus to be corrected is less than 1.5 per cent at a frequency of 24 cycles, andis entirely negligible at a frequency of 60 cycles.

The saw-tooth oscillator thus described is selfos'cillatory with considerable stability, but-it may readily be synchronized by injecting a small voltage of the desired frequency into the transformer which couples the grid and the plate circuit of This may be done by an additional small winding 5| coupled to the transformer core and fed from any desired source.

Figure 3 shows a form of the invention which is primarily adapted for use with television transmitters where it is desirable to have the frequency absolutely controlled by external mechanical means. The short, widely separated positive pulses are fed to a tube 6|, which is normally The output circuit of the tube 6| is similar to that of the embodiment first described, and the circuit elements are designated by the same reference characters distinguished by primes. It is assumed, however, that the tubes and 36' are tubes of low amplification factor and high current-carrying capacity. In order to obtain the necessary amplification the tube 35 is therefore used to drive-the tube 36' through a circuit comprising a large blocking condenser 63 in series with a resistor 65, which connects to the grid 41' and to ground through a resistor 66. This connection gives greatly increased amplification, particularly of the-pulse, and the pulse resistor H can therefore be reduced in size. The

biases on the tubes 35' and 36' will usually have to be somewhat different from those shown in the embodiment of Figure 1 in order to get the desired corrective asymmetry of the grid-plate characteristic of the amplifier, but this is de- I cathode 92 is self-biased almost to cutoff by a,

to be utilized directly to efiect scanning without the intervention of any kipp tube or local oscillator whatsoever.

As has been shown in my copending application Ser. No. 449,984, and as is now the universal practice in electronic television systems, a pulse sent out between each two lines of scansion, and another transmitted between each two picture frames, may be utilized for the dual purpose of holding the receiver in step and of eliminating the back lines in the received picture. Figure 4 shows the method of applying the present invention to accomplish this directly. The combined pulses and picture signals, amplified to the required degree by any convenient type of ampli-' fier I0, are fed through a blocking'condenser H to 'the control grid of a final power tube I2 which further amplifies the signals and applies them through a blocking condenser I4 to the grid of the oscillight I5, the output of the amplifier I being so phased that the synchronizing pulses comprise positive swings as applied to the grid of the power tube, by which they are reversed in phase and applied as negative swings to the oscillight, erasing the back lines.

The tube I2 is provided with the usual grid leak I6. Also connected to the grid is a resistor 11 to which the high-frequency and low-frequency scanning units are connected through a pair of conjugate filters, the high-frequency unit connecting to the resistor 'I'I through a small condenser I9, with a resistor 80 as a shunt element across the scanning generator input, while the low-frequency unit is preferably connected through a series resistor 8I with the condenser 82 as the shunt element The output of the highfrequency unit 83 connects to a set of highfrequency scanning coils 84, while the output of the low-frequency scanning unit 85 connects to the low-frequency deflecting coil 86. The circuits of the scanning units 83 and 85 differ essentially only in their circuit constants, and hence only the high-frequency unit 83, as shown more completely in Figure 5, will be described in detail.

Referring to Fig. 5, the positive synchronizing pulse which appears across the shunt resistor 00 is applied to the grid 90 of a triode 9|; whose resistor 94, a large by-pass condenser 95 being shunted across this resistor to maintain the cathode at ground potential insofar as alternating-current components are concerned. The plate of the tube 9| is connected through a pulse resistor 96 and integrating circuit 01, operating as has already been described, to a source of plate potential indicated by B+. The positive pulses render the tube 9| instantaneously conducting, and the pulses charge the integrating circuit 91, as has already been described, to develop the desired wave form. For this particular use, however, where light weight and simplicity are paramount factors, the simplest type of network is used to couple the succeeding amplifier, in this case simply a blocking condenser 99 and grid resistor I00, of the order of magnitude of 2.0 mic-- rofarads and 100,000 ohms, respectively, transferring the saw-tooth wave to a heater-type tube IOI. Thisstube is parallel-fed through a plate reactor I02, its output circuit being through another blocking condenser I04, also preferably having a capacitance of about 2.0 microfarads, and a variable resistor I05, having a maximum resistance of about 200 ohms, to the two highfrequency scanning coils 84 whose combined inductance is about 2.6 millihenries.

In this case the wave form applied to the tube IOI is not as highly corrected as in the embodiments previously described, but since a single amplifier tube is used its plate characteristic will have somewhat greater curvature, so that satisfactory linearity of wave form may be obtained. The final adjustment of this linearity is made by varying the resistance of the element I05.

This same type of high-frequency scanning circuit has also been used for transmitters, but whereas for receiver scanning units a single amplifier tube of the 2A3 type, for example, may be used, for the transmitters several of these tubes will be used in parallel.

Where this type of circuit is used for the lowfrequency scanning, the contants of the pulse resistor and integrating circuit will be of about the same magnitude as those of the first embodiment described, instead of using an integrating circuit with a time constant of about 0.0005 as in the high-frequency unit. The blocking condenser 99 will have the same value as in the high-frequency unit, but the grid leak I00 will be two to three times as great in resistance. The

plate reactor I02 and blocking condenser I04 will.

also be much larger, and I have also found it more convenient to use a type 2A5 output tube than the 2A3 which has proved most satisfactory thus far in the high-frequency units. All of these modifications for change in frequency are, however, exactly what would be expected from ordinary engineering considerations, and many other variants .of the system will naturally suggest themselves to designers in the art. Furthermore, it will be apparent that where the oscillight is sufliciently sensitive to control voltages the phase-reversing amplifier may be ,used in cascade with the scanning generators instead of with the oscillight, the primary requirement being merely that the pulses be applied to the oscillight and generators in opposite phase.

'I claim:

1. The method of producing synchronized sawtooth scanning waves in reactive scanning circuits in a receiver for television signals which include pulses adapted to extinguish return lines in the received picture, which comprises the steps of amplifying said pulses with reversal of phase so that said pulses appear with opposite signs in different portions of said receiver, ap-

plying said pulses in one phase to cause current flow in one portion of said receiver, producing a distorted voltage wave from said current flow of proper form to produce a saw-tooth wave in a circuit containing reactance, applying such saw-tooth wave to scan a picture field,'and applying said pulses simultaneously in opposite phase to suppress flow in another portion of said .receiver.

2. The method of producing synchronized saw-tooth scanning waves in" reactive scanning circuits in a receiver for television signals which .include pulses adapted to extinguish returnlines in the received picture, which comprises the steps of amplifying said pulses with reversal of phase so that said pulses appear with opposite signs in difierent portions of said receiver, applying said pulses in positive phase to cause current flow, distorting and amplifying said current flow to produce a scanning wave, and simultaneously applying said-pulses in negative phase to suppress a current flow to extinguish the return lines in the received picture.

3. The method of producing synchronized saw-tooth scanning currents in an inductive and resistive scanning circuit in a receiver for television signals which include pulses adapted to extinguish return lines in the received picture, which comprises the steps of amplifying said pulses with reversal of phase so that'the pulse appears in diflerent portions of said receiver with opposite signs, applying said pulse in positive phase to cause a current fiow in said scanning circuit substantially coincident therewith in duration, applying a potential in phase with said pulse to cause a sudden change of said current flow, limiting the subsequent changesof current in said scanning circuit to a substantially constant rate until againchanged by a pulse, and applying said pulses in negative phase to extinguish the return lines simultaneously with said sudden change.

4. A receiver for television signals which include pulses adapted to extinguish return lines in the received picture, comprising means for amplifying said pulses with reversal of phase so that said pulses appear with opposite signs in different portionsof said receiver, means for applying said pulses in one phase to cause current flow in one 'portion of said receiver, means for producing a distorted voltage wave from said current flow of proper form to produce a saw-tooth wave in a circuit containing reactance; means for utilizing such saw-tooth wave to scan a picture field, and means for applying said pulses simultaneously in opposite phase to suppress flow in another portion of said receiver.

5. A television signal receiving. system comi prising scanning circuits adapted to be actuated plifier for distorting the amplified pulses to develop therefrom a saw-tooth scanning wave, and means for utilizing said scanning wavelto actuate said scanning circuits.

PHD T. FARNSWORTH.

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