US2538541A - Saw-tooth wave generator - Google Patents

Description

Jan. 16, 1951 s. l. rouRsHou 2,538,541

SAWTOOTH WAVE GENERATOR Filed Sept. 27, 1947 ATTORNEY the target electrode.

Patented Jan. 16, 1951 UNITED STATES PATENT OFFICE SAW-TOOTH WAVE GENERATOR Simeon I. Tourshou, Philadelphia, Pa., assigner to Radio Corporation of America, a corporation of Delaware Application September 27, 1947, Serial No. 776,465

3 Claims.

The present invention relates to generators for producing non-sinusoidal waves, and more particularly, but .not necessarily exclusively, to such generators for producing waves substantially of sawtooth form.

Where a beam of electrons is generated and directed against a target electrode, some l'means is normally provided for causing deflection of this cathode ray beam in two different directions in order that the cathode ray beam may be caused to scan or traverse the desired' portion of In a television camera or receiver, the electron beam is deflected at a slow speed, usually in the vertical direction, and at A Va high speed, usually in the horizontal direction.

The high speed deection provides line scanning and the low speed deflection provides eld or frame scanning. In `oscillographic devices, the

cathode ray beam is usually deiiected horizontally at a predetermined rate, while the beam is Viiected uniformly with respect to time, the applied deiiection voltage must appear approximately as a sawtooth wave form or be of such shape as to produce a current of sawtooth wave form `in an inductance. In accordance with this invention, a recurring sawtooth of voltage, suitable for slow speed orhigh speed scanning, ,is

produced by a novel oscillator which is generally control the free running speed of the oscillator.

An lobject of this invention is to provide a sawtooth wave generator consisting of simple parts so that it is cheap to manufacture.

Another object is to provide a sawtooth wave 1 generator which does not require a transformer having a laminated iron core and separate primary and secondary windings. A further object of the invention is to provid a sawtooth wave generator, the free running speed of which mayy be controlled in a novel manner by providing means for changing the inductance of a coil.

Other objects and advantages of the present invention will, of course, become apparent and immediately suggest themselves to those skilled in the art to which the invention is directed from a reading of the following specication in connection with the accompanying drawings in which:

Fig. 1 is a, diagrammatic illustration of a sawtooth wave generator embodying the invention;

Fig. 2 shows a series of curves;

Fig. 3 shows a curve obtained during operation of the generator with a different degree of damping;

Fig. 4 shows a modification of the generator vof Figure 1; and

dit

Fig, 5 shows a further form .which is preferred -for certainA applications.

Referring now to the drawing, and for the present to Fig. 1 thereof, there is shown a single coil i2, which may be regarded as an autotransformer having a secondary or grid section I4 and a primary or plate section It. The plate section i6 is connected to the anode I8 of a discharge tube, such as the three electrode vacuum tube I9. The grid 2l of this tube is connected through a capacitance 23 to the end of the grid section It. synchronizing impulses, such as television sync signals, usually in the form of timed recurlring pulses 26, are fed into the cathode 28 of the tube i9. The cathode is returned to ground by way `of an unbypassed resistor 3 I.

A capacitance 32 is connected to a suitable vsource of positive potential (not shown), which ,may be connected to a terminal 33 through resistors 36 and 31 in series. Under certain conditions of operation of the generator of this invention, the capacitance 32 is charged through the resistors 31 and 36. Discharge, or partial discharge, of this capacitance is brought about by a sync pulse 26, which initiates an oscillation slightly before the would-be free-running oscillation which, as the grid swings positive, discharges er partly discharges the capacitance 32. A resistor 38 serves as a damping resistor, and in Fig. l it is shown as being connected across the entire autotransformer coil I2. This resistor may be connected across any portion of the winding.

The output voltage of sawtooth wave 'form is taken from the resistor 36, which serves as a potentiometer having a, movable contact el. With the potentiometer connection shown, rthe frequency of oscillation of the generator `is independent of the setting. A change in position which,

of the contact 4l provides forY variable high peaking so that the output wave may havera variable impulse part.

In the operation of the arrangement of Figure 1, the capacitance 32 is charged through the resistors 3S and 31 in series, the charge curve 44 (Fig. 2) being substantially linear, initially. The sync pulses 26, shown as extending in the positive direction in Fig. 2 for convenience, bring about an abrupt discharge of the capacitance 32 as represented at 46. is in the discharge path of the capacitance, as

Awell as in its charging pathhas the function of providing the pulse portion 48 of the Wave, since the capacitance 32 cannot be completely discharged due to the time constant of the capacitance 32 and the resistor 3B as well as the plate resistance of the tube. When the tube i9 is blocked, as will be explained hereinafter, the plate Voltage does not have to rise slowly from the cathode potential, but instead rises immediately to an appreciable value, due to the charge that remains on the capacitance 32. The plate voltage then slowly rises along the curve 44 asV indicated, is substantially linear. Upon the occurrence of a sync pulse 25, the grid voltage is raised immediately and increases in the positive direction due to transformer action, as indicated at (Fig. 2). Following this, the grid is driven abruptly negative when the capacitance 32 is nearly discharged and a negative charge is stored in the capacitance 23, which leaks olf asIv represented by the portion of the curve designated 52.

The negative condition of the grid, which'is lassumed abruptly, as stated, is a result of several factors, the discharge of ythe capacitance 32 being only one of these factors.' The principal reason for the grid going negative isV the oscillatory circuit comprising the inductance andi distributed capacity of the coil I2, as well'as the tube and stray capacity of the circuit, so that having reached the positive peak, the voltage starts decreasing and then swings negative,

The sectending toY follow a damped sine wave. ond reason is the collapse of the plate current which induces a voltage in the coil tending to accelerate the negative swing and even making it greater than positive. The third is the negative charge stored in the capacitance 23.

The discharge of the capacitance 23 occurs through a grid resistor 53 in series with anadjustable resistor 54. The timgof discharge can be regulated by varying the resistance ofthe resistor 54. Just before the conduction point of the tube is reached, the occurrence of the next successive sync pulse 26 causes the cycle to be repeated. Fig. 3 of the drawings shows the wave form when the degree of damping, whichis determined by the resistor 38, is less.

Fig. 4 of the drawings shows a slightly modied arrangement of the apparatus of Figure 1 in which the damping resistor, designated 38a,

bridges the plate section la of the coil, krather ,than the entire coil, as shown in Fig. l.

The damping resistor 38a may be connected across any portion o1 the winding. A core 56 is also indicated, which may be of powdered and com- :the sync input is supplied to an auxiliary Wind- The resistor 36, which lfrio l'ent is the following:

4 ing 58, which is coupled to the main winding 12b. The direction of winding of the auxiliary coil 58, or the connection of the sync input to the terminals 6I takes care of the polarity of the applied sync signal. An adjustable core 62 is mounted in any suitable manner (not shown) so thatvit may be moved into or out of the coil, somewhat like the tuning inductance on a radio receiver. This change in inductance changes the resonant frequency and the impedance of the circuit, as Well as the coupling between the plate and grid sections of the winding, and through any one, or any combination of these changes, Varies the peak voltage developed on the grid, as well as the plate. The frequency of oscillation is varied since the charge on the capacitance is varied and the time for this charge to leak off is varied. This method of control eliminates the cost of the variable resistor 54 of Fig. 1.

The windings I2, |20. and l2?) are preferably of the universal type with low distributed ca- Dacity with parameters essentially independent of "voltage In general, this coil is more stable than the laminated iron core transformer.. When the universal'type of coil is connected' as an autotransformen it maintains the ratio of plate currentY to grid current in the positive grid region. The frequency is a function .of this ratio. By making the amplitude of oscillations and therefore the average bias developed by oscillations, large enough, the effect of variation of the contact potentialfof the tube is minimized.A

Having nowdescribed the invention what is claimed and desired to be secured by Letters Patl. A blocking oscillator comprising'a vacuum tubehaving a controll grid, a plate and a cathode, an autotransformer comprising two inductively coupled-coil sections,- one of said coil sections being connecteddirectly to the plate of said tube,

-a capacitor, a connection from the other of said coil sectionsV to said grid through said capacitor, means including a resistor connected. to the junction of said coil sections to maintain said plate positive with respect to said cathode, a second capacitor connected to the junction lof said coil sections, said second capacitor also being eifectively connected to the point of connection of the cathode in said oscillator, said second capacitor being charged'from said positive potential ap- ,plying means and being discharged by said tube upon occurrence of a pulse of grid voltage, and a damping resistor connected across at least apart of said autotransformer.

2. The oscillator of claim 1, wherein said coil sections consist of windings having low distributed capacity.

3. A blocking oscillator comprising a vacuum tube having a control grid, a plate and a cathode, an autotransformer comprising two inductively coupled coil sections, one of said coil sections being connected directly to the plate of said tube,

-a capacitorfa connection from the other of said coil sections to said grid through said capacitor, means including a resistor connected to the junction of said coil sections to maintain said plate positive with respect to said cathode, a second capacitor connected to the junction of said coil sections through a second resistor,'said second capacitor also being effectively connected to the -point of connection of the cathode in said oscillator, said second capacitor being charged from said positive potential applying means and being -discharged by said tube upon occurrence of "a V5 `pu1se of grid voltage, and means for obtaining an output wave having a variable pulse component from said oscillator, said means comprising a connection to said second resistor which is adjustable along said second resistor. f

SIMEON I. TOURSHOU.

REFERENCES CITED The following references are of record in the file of this patent:

Number 6 UNITED STATES PATENTS Name Date Smiley June 8, 1937 Faudell et al. Aug. 20, 1940 Smith Nov. 3, 1942 Lyman Oct. 17, 194A Gottier Apr. 29, 1947

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