US2453787A - Saw-tooth voltage generator - Google Patents

Description

Nov. 16, 194s.

ATTORNEY GEORGE w. Downs, JR.

Patented Nov. 16, 1948 SAW-TOOTH VOLTAGE. GENERATOR George W. Bovins, Jr., Pasadena, CaIifI, assigner to the United? States of America asrepresentcd by tl'o'e Secretary of the Navy Application May 23, 1944;. Serial No. 536,967'

This invention relates toV a linear sawtooth generator.

En the past, most sawtooth generators have utilized'the charging crpolarizing current of a reactance to establish the slope of the sav/tooth. All such chargng'cr polarizing currents have been found tov vary with v time, and various means have been proposed `to eliminate this variation, with its consequent impairment of they linearity 4of the sawtooth. Previousgenerators have sometimes utilized the high variational' resistance of the plate circuit of a pentode vacuum tube to minimize this Variation, but with only a limiteddegree of succes-s. Other attempts have been made to utilize nonohmic resistors; such as varistors and .thermistors, to obtain such high-variational resistance, and thus a constant current lcharging or polarizing circuit.

It is the purpose of the present invention to utilize vacuum tube arnpliiication` tomultipl'y the variational resistance of a pentode constant current tube by alarge factor.

Thus, one of the objects of the-invention is to provide a voltage which varies linearily with respect to time.

Another off the objects of the invention is a means for providing ya convenient and accurate time base for measurement Vand indication of functions Varying with time.

A further object of the invention is a linear sawtooth generator which mayl be conveniently used to measure time in such applications as television scanning circuits, cathode-ray oscille:- graphs, chronographs, radi-o and. sound-ranging systems, frequ-encyrneters, and any other device in which such a time base is necessary.

A further objectol the invention is a linear sawtooth generator in which a constant current source is utilized to charge .a capacitor, which capacitor is then discharged through a gaseous discharge tube.

Still another object of the invention is a means for maintaining constant current in .a circuit element having variable characteristics.

The drawing is aschematic diagramof a circuit embodying the present invention.

The circuit arrangement of".` the ligure shows a method of obtaining a low-frequency asymmetrical sawtoothwave charactenized by a rise in voltage Ithat is substantially linear with respect to time, followed by a rapid drop in voltage to the startingY point. rPhe rapid drop. or fllyback, lasts for a small fraction or the total sawtooth cycle.

The linear rise in voltage, with respect to time, is achieved by charging a condenser from a high 2l voltage power supply `through a constant current, non-chimio resistance. The rapid drop in voltage,

or fllyback, is obtained by suddenly discharging the-previously charged condenser through a gaseous discharge tube. Y'

Referring to the drawing, capacitor i is charged from the'power supply terminals :through resistor 2, vacuum pentode 3 and rheostat 4i. During the charging phase of the sawtooth cycle, .the gas triode 51s non-conducting, being biased past cut oliE by a voltage derived from potentiometer 6. When the chargingvoltage across capacitor I rises tora value equal to the predetermined breakdown voltage of gas triode 5; conduction is established which rapidly discharges capacitor l, reducing the voltage across capacitor l=to the extinction voltage of gas triade-5; at which point it becomes noncon ducting and the charging' phase of the cycle repeats* When -a capacitor is charged from .a constant resistan-cefsourceof voltage, the charging current decreases` exponentially with time. Since the slope oft the voltage-time curve is proportional to the charging current as the charging current appreaches zero; thefslope of the voltage-time curve also approaches zero. In the circuit shown, the Vacuum pentode 3 and its associated circuit com ponents are arranged to prevent the charging current in capacitor! from varying exponentially withitime; andA instead, force the charging current to remain constant. By thus forcing the charging currentv to' remain constant it is found that the voltage across capacitor l incr-eases linearly with time during the charging phase `of the cycle. The arrangement to thus maintain constant' the charging current through capacitor I is accomplished by vacuum pentodes 3,1 and associated resistors and voltage dividers,

Vacuumpentode 3` is a high resistance, high mu, radlotype',.pentode similar tol types 6J7 or GSJ'?. This tube has Va high variational plate resistance which tends .to resist anychangein its plate current'. As the charging current of capacitor l passes through the plate resistance of vacuum pentode 3,` this'high variational plate resistance minimizes charging current variations.

ThisA inherently high variational plate resistance of vacuum pentode' 3 while useful, is not high enough to maintain the charging current of capacitor I suiliciently constant to obtain satisfacn toryllnearity (with respect to time) of the output voltage. The addition of vacuum pentode 'l to the circuit multiplies the effective variational plate resistance of vacuum pentode 3 by a large factor.

Vacuum pentode 1 ls a. tube of the same type as vacuum pentode 3. Both tubes 3, 'l obtain their screen grid voltage from a tap on the voltage divider made up of potentiometer 6 and resistors 8 and 9. The sta-tic control grid voltage on vacuum pentode 3 is established by a tap on the voltage di vider and coupling circuit made up of resistors l il. il and I2. The cathode voltage on vacuum pentode 3 is established by rheostat 4, vand the cathode voltage on vacuum pentode 'I is established by a tap at the junction of resistors 8 and 9 on the voltage divider made up of potentiometer il and resistors 3 and 9.

The operation of the constant current portion of the circuit can be described by assuming a small variation in the charging current to capacitor l and then examining, step by step, how the circuit opposes this change. Assume that capacitor i is being charged from the external voltage source, and, for example, that the charging current is decreased. It can be seen that resistor rheostat l and the plate circuit of vacuum pentode 3 are all in series with the charging circuit to capacitor i. Therefore, a decrease in charging 4current will decrease the Voltage drop across rheostat i which makes the control grid of vacuum pentode l negative with respect to its cathode. This will tend to reduce the plate current flowing through vacuum pentode 'l which will in turnreduce the voltage drop through resistor lil and thus make the potential at the plate of vacuum pentode 'l more positive with respect to ground. rlIhis rise in positive voltage at the plate of vacuum pentode 'l is coupled to the control grid of vacuum pentode 3 through the coupling resistor H which produces a positive eX- cursion of the control grid of vacuum pentode 3. Making this control grid more positive acts to increase the plate current of vacuum pentode 3, which tendency opposes the decrease in charging current which initiated the above described train of events. Likewise, ii the charging current had increased, all of the eiiects described would have operated in the opposite sense, and the grid of vacuum pentode 3 would have been driven somewhat negative to reduce the plate current of vacl uum pentode 3, thus resisting the initial change in charging current. It should be noted that both the sawtooth amplitude and sawtooth frequency are adjustable so that the invention may be adapted for any desired purpose.

The output amplitude of any relaxation oscillator depends on the voltage diilerence between breakdown and extinction in the gaseous discharge tube used to discharge its associated capacitor. In this invention the gaseous triode 5 is used to discharge the capacitor I. Its extinction is not adjustable but its breakdown voltage can be set at any desired value by means of potentiometer 6, which controls the grid voltage of gaseous triode 5.

The frequency of the sawtooth is in turn determined by its amplitude and the slope of the voltagemtime curve. As has been described, the amplitude is controlled by the characteristics of gas tube 5 and the setting of potentiometer 6; and the slope of the voltage-time curve is determined by the capacitance of capacitor l and the plate current of tube 3, in a manner thatit iS proportional to said current and inversely proportional to said capacitance. Since the current may easily be controlled by the setting of rheostat 4,

4 the frequency of the sawtooth may thus be varied at will.

It is seen that the present invention is extremely useful and has wide application.

As described it enables the production of linear sawtooth waves in a simple eilicient manner. Additionally, it lprovides means for maintaining a constant current in a circuit or circuit element having variable characteristics. As such, the device is very useful in the measurement of high resistane since the current through the circuit is maintained at a constant value independently of the resistance of such cir-cuit. The resistance of the circuit to be measured is proportional to the potential drop across said unknown circuit, and in fact, is equal to the ratio of the potential drop to the current iiowing in it. This will provide .ready means for constructing an ohm meter with a linear scale.

I claim:

l. A sawtooth voltage generator, comprising a rst pentode tube having an anode, a cathode, and a control grid, a first resistor connected to said cathode, a power supply, a capacitor'connected to be charged from said power supply through the tube and the resistor, a gaseous discharge device connected across the capacitor to discharge the same when a predetermined voltage is reached; a second pentode tube having an anode, a cathode, and a control grid, means including a second resistor connecting the anode of the second :pentode to the positive side of the power supply, means connecting the cathode of the second pentode to the negative side of the power F supply, means to apply to the control grid of the second pentode the potential variations at the high potential end of the rst resistor, and means to apply to the control grid of the rst pentode the potential variations at the low potential end of the second resistor.

2. A sawtooth voltage generator, comprising a pentode tube having an anode, a cathode, and a control grid, a rstresistor connected to said cathode, a power supply, a capacitor connected to be charged from said power supply through the tube and the resistor, a gaseous discharge device connected across the capacitor to discharge the same when a predetermined voltage is reached; a second tube having an anode, a cathode, and a control grid, means including a second resistor connecting the anode of the second tube to the positive side of the power supply, means connecting the cathode of the second tube to the negative side of the power supply, means to apply to the control grid of the second tube the potential variations at the high potential end of the rst resistor, and means to apply to the control grid of the pentode tube the potential variations at the low potential end of the second resistoi'.

GEORGE W. DOWNS, JR.

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

UNITED STATES PATENTS Number Name Date 2,281,948 Pieplow a May 5, 1942 FOREIGN PATENTS Number Country Date 758,478 France Nov. 3, 1933

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