US2461120A - Signal generator - Google Patents

Description

Feb ,3, 1949. B. D. LoUGHLiN 2,461,120

SIGNAL GENERATQR Filed Nov. 2, 1944 Potential at Tap 2O Control Electrode ./0 Potential of Tube Il Ou'tput Potential O Cathode Potential IN VEN TOR.

BERNA D D. LOUGHLIN ATTORNEY Patented Feb. 8, 1949 SIGNAL GENERATOR Bernard D. Loughlin, Bayside, N. Y., assig'nor, by mesne assignments, to Hazeltine Research, Inc., Chicago. Ill., a corporation o! Illinois Application November 2, 1944, Serial No. 561,613'

(Cl. 25o-36) 2 Claims. l

This invention relates generally to generators for generating signals of periodic wave form and of variable periodicity and particularly to a pulse generator of this type having an adjustable period and in which the ratio of the pulse duration to the interval between pulses is substantially constant.

For some purposes, such as in keying circuits, it is necessary to utilize a signal of periodic wave form which has a periodicity which may be varied over a relatively wide range of values. It is often desirable, during the time that the frequency or periodicity of this wave signal is being altered, to maintain substantially constant the ratio of the succeeding time intervals of the signal. For example, in a signal having la pulse-type wave form, it may be desirable to hold substantially constant the ratio of pulse duration to pulse separation with changes in frequency. Signal generators of the prior art for developing a signal of this general character are subject to several disadvantages which, in some cases, may limit the extent of their usefulness. For example, in some signal generators which heretofore have been used, the frequency of operation is 'controlled by means of a variable resistor or variable energy-storage device, such as an adjustable inductor or condenser, which comprise elements in a time-constant circuit. Control arrange"- ments of this type, in varying the frequency of the generator over a substantial range, tend to upset the ratio of the succeeding portions of individual periods of the generated signals. Another method frequently employed in generators of periodic signals to alter the periodicity thereof comprises varying the magnitude of the unidirectional operating potential supplied tothe system. Generators which are controlled in this manner are generally capable of developing a signal of a periodicity which is variable only over a relatively small range of values while maintaining substantially constant the ratio ofl the succeeding intervals of the generated signals.

An additional disadvantage is inherent in a generator of the last-mentioned type when a signal of variable periodicity but of substantially constant amplitude is desired, since a change in the potential of the source of supply also results in acorresponding change in the amplitude of the generated signal. Prior art signal generators of the character under consideration employing electron tubes in the circuits thereof periodically require the replacement of tubes. Since the characteristics of individual tubes of the same type usually vary to some extent, it is .generally necessary to readjust the control elements of the generator whenever a tube is replaced. This procedure is often rather complicated, thus making -it more desirable to accomplish this readjustment by means of a simple arrangement such as a unicontrol adjusting device.

It is an object of the invention, therefore, to

' provide an improved generator of the type under consideration which is substantially free from one or more of the above-mentioned disadvantages or limitations of generators of the prior art.

It is another object of the invention to provide a signal generator for generating a signal of periodic wave form and of variableA periodicity, including relatively simple means for adjusting the periodicity of the generated signal over a relatively wide range of values while maintaining substantally constant the ratio of the succeedingtime intervals of the generated signal.

It is a further object of the invention to provide a signal generator for generating a signal i of periodic wave form and of variable periodicity,

including means for. adjusting the periodicity of the generated signal over a relatively wide range of values while maintaining substantially constant the ratio of the succeeding time intervals of the generated signal and for maintaining substantially constant the amplitude of the generated'signal.

In accordance with the invention, a generator for generating a signal of periodic wave form and of variable periodicity comprises arst electron tube having an anode-cathode space-current path and at least two operating conditions and a second electron tube having a control electrodecathode space-current path. The generator includes means including the second electron tube, a first time-constant circuit including an energystorage device and the aforesaid control electrede-cathode space-current path, and a second time-constant circuit including an energy-storage device and the aforesaid anode-cathode spacecurrent path for causing the first electron tube to alternate during succeeding time intervals from one of the operating conditions to the other to complete for successive pairs of time intervals operating cycles. The generator additionally includes an impedance included in an energizing circuit of the rst electron tube to develop an output potenti-al having an amplitude varying with the two operating conditions thereof. An adjustable contact engages the impedance for connecting a different portion of the impedance in each of the first and second timeconstant circuits and for applying to the aforelsaid means a selectable portion of the output pothereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing, Fig. 1 is a schematic circuit diagram of a signal generator in accordance with the invention; Figs. 2 and 3 comprise curves which are utilized in explaining the operation of the generator oi Fig. l; while Figs. 4 and 5 are elementary diagrams utilized in summarizing the operation of the generator of Fig. 1.

Referring now to Fig. 1 of the drawing, the generator for generating a signal of periodic wave form and of variable periodicity comprises means having at least two operating conditions. This means comprises an electron tube Ill which constitutes one element of a relaxation-type oscillator. The signal generator also comprises means including at least one time-constant circuit for causing the rst-named means to alternate during succeeding time intervals from one of the above-mentioned conditions to the other to complete operating cycles for successive pairs of the intervals, the ratio of the time intervals of each of these pairs having a predetermined value. In addition to the aforementioned time-constant circuit, to be described in detail subsequently, this means comprises an electron tube II coupled to the input and output circuits of tube I for alternately rendering tube II) conductive and nonconductive. The cathodes of tubes I0 and II are interconnected and are grounded through a common cathode resistor I2, while the control electrode of tube lll is connected directly to ground and the control electrode of tube Il is grounded through a resistor I3 in series with a resistor I4. Resistor I3 has a small value, say 100 ohms, whereas resistor I4 preferably has a. large value of the order of several megohms. The resistance of element I 4 is also materially higher than the conductive interelectrode impedances of the tubes and particularly the conductive anode-cathode impedances of tube I Il and also the control electrode-cathode impedance of tube 'II when the latter is drawing grid current. An impedanceV having a xed value is included in an energizing circuit for the tube I0 to develop an output potential having an amplitude varying withl the two operating conditions of the tube I0. This means comprises a. resistor I6 having a high impedance. One of the iixed terminals of the resistor I6 is connected to the anode of tube I0 and the other xed terminal is connected to a terminal of a resistor i1, the latter also having a high resistance. The anode of tube II is connected to the other terminal of resistor I'I and also to a source Iof potential indicated as +B through a resistor I9.

The signal generator also comprises an adjustfor applying to the aforementioned means which includes at least the one time-constant circuit a selectable portion of the output potential developed by the resistor I6 while at the same time providing with adjustments of the tap an adjustment of the time constant of the aforesaid time-constant circuit, whereby the duration of each of the succeeding time intervals is adjusted in substantially the sameproportion while the periodicity of the operating cycles is adjusted over a relatively wide range of values. The adjustable tap 20 is connected to the common junction of the resistor I3 and the resistor I4 through a portion of a time-constant circuit including a resistor 2| and an energy-storage device such as a condenser 22. A pair of output terminals 23, 23 may be connected between any two suitable points in the signal generator circuit such as between the anode of tube II and ground. It will be understood, however, that an output signal of a desired magnitude and wave form may be derived from other points in the multivibrator circuit.

In considering the operation of the signal generator of Fig. l, it will be assumed that at time to, indicated in Figs. 2 and 3, the tube Il becomes conductive. The flow of current through resistor I 9 at time to causes the potential between the anode of tube II and ground, and hence the potential across the terminals 23, 23, to drop from the +B value, indicated in curve A of Fig. 2, to a value designated e. Also at time to, the ow of current through cathode resistor I2 increases in magnitude so that the potential of the cathodes of tubes I0 and II rises in a positive direction above the original value f, which is illustrated in curve B of Fig. 2, and approaches a new value g. This increases the bias on the control electrode of tube I0 thereby driving tube Ill to cutoff, Whereupon the potential at tap 20 initially rises sharply in a manner similar to that shown in curve C of Fig. 3 so that a more positive potential is applied to the control electrode of tube II through the resistor 2i, condenser 22, and resistor I3. This causes an increased current flow through tube I I and resistor I2 so that the cathode potential reaches the value g mentioned above at about the time anode current saturation is reached. Tube Il thereupon draws grid current between the interval to-ti, and the voltage developed across the resistor I2 decreases as illustrated in curve B, whereupon at time t1 this voltage has dropped to the value h while the signal developed at terminals 23, 23 increases slightly to the value i indicated in curve A of Fig. 2. During the interval to-ti, the potential between tap 20 and ground rst rises abruptly from a predetermined positive value 2', as shown in the curve C of Fig. 3, and then asccnds more gradually until it reaches a value n approaching that of the +B level at time t1. The wave form of the signal which is applied to the control electrode of tube II also rises abruptly at time to to the value k, as shown in curve D of Fig. 3, and then, as a result of the iiow of grid current, declines slightly to the value l at time t1. At instant t1, since the potential of the cathode of tube I0 has dropped tothe critical value h as represented in curve B in Fig. 2, the bias on tube I is suiliciently reduced so that tube Il! becomes conductive. A sudden decrease in the cathode potential to the original value f then follows. The iiow of current in tube I0 and in the anode load resistors I6 and II causes the vpotential between tap 20 and ground to decrease. As a result, a negative signal is translated to the control electrode of tube II through resistor 2I, condenser 22, and resistor I3, thereby operating tube II to cutoff at time t1 and increasing the potential developed across terminals 23, 23 to the +B value, as shown in curve A of Fig. 2.

At time t1 the potential between tap 20 and ground drops from the value designated n to the value i illustrated in curve C of Fig. 3, while the voltage developed at the control electrode of tube II drops suddenly in a negative direction to the negative value m illustrated in curve D of Fig. 3. The magnitude of this potential change is determined by the setting of tap 20, and its effect will be more fully explained hereinafter. Since tube II becomes non-conductive at time tl, the negative charge which is accumulated on the lower manner illustrated by curve D of Fig. 3 over a period determined by the time constant ofithe circuit comprising the large resistor I4, lthe resistor 2|, the portion of resistor Il between tap "and the anode of tube Il, resistor I2, and the anode-cathode resistance of tube I0. The potential of the'control 'electrode` of tube II therefore becomes more positive until at'time tva when it reaches the p-plevel, indicated in*l connection with curve D of Fig. 3, tube II 'again becomes conductive and its cathode acquires a positive potential, 'as shownin curve B of Fig. 2. Tube 'I0 is again driven to cutotl, as previously explain'ed, and thecycle of operation is repeated.

The mannerv in which the period of an operating cycle is'a'djusted over-a relatively wide range while -maintaining substantially constant' the ratio ofthe `succeeding time intervals T1 and T2 of the generated wave will now be explained. Tube II is conductive during 'period T1 and also draws grid current.' Since tube III is nonconductive at this time, the potential at' tap -20 on vthe resistor I6 risestoward the +B value. -During this 'portion of the cycle the condenser 2211s being charged through resistor I1, the upper -portion of resistor I6, resistors 2Iand I3, Raku and resistor I2, where Raku is 'the grid-cathode resistance of tube II. As-the charge on condenser -22 increases exponentially, the current through the aforementioned resistors decreases exponentially until the voltage drop across resistor I2 is low enough that` tube I becomes conductive. The voltage drop e of curve C of Fig. 3 occurs when tube I0 conducts current and the negative voltage mgshown in curve D of Fig. 3, appears on the grid of tube II. Thus, tube II` is biased to cutoff at time t1 .and tube I0 becomes conductive, thereby terminating the period T1. When the tap is adjusted in the directionV of the junction between resistor I8 and resistor I1, the charging time-constant is reduced thereby decreasing the duration of period T1.

' Movement of tap 20 toward the junction between resistor IB and resistor I1 also decreases the variation inl voltage between tap 20 and groundduring period T1 (i. e., reduces' `the voltage step e'), since tap 20 is thus positioned at a point nearer the potential of the source +B.

A corresponding reduction in the magnitude of the signal which is translated to the control electrode of tube II, and accordingly Ithe charge which is developed on the lower plate of condenser 22, iseffected. As a result, a shorter interval of time T3 is required before the charge on condenser 22 leaks off and tube II again becomes conductive. Thus, the adjustment of the tap 20 on" the A`'resistor I8, which comprises the common adjusting element in the above-men#- tioned charging and. discharging time-constant circuits, is e'ec'tive to increase the generator frequency while maintaining the ratio Ti/T2 substantially constant.

Conversely, adjustment of tap 20 on the resistor IB toward the anode of ttube I0 increases proportionately the duration of both the periods Ti and Ta. Thus, this direction of adjustment is effective to decrease the generator frequency while maintaining the ratio Ti/Ta substantially constant.

It will be seen, therefore, that a, simple adjus'tment of the tap 2Ii'fengaging the resistor I6 is effective to alter the duration of succeeding time intervals T1 and Te in the same' sense while maintaining substantially constant the ratio of 4plate of yconeqlenser 22 decays exponentially In the these periods. Also, it win be apparent niet sistor I1 is employed to provide suiilcient voltage feedback to the input circuit of tube II regardless of the setting of the adjustable tap 20 of voltage divider I8. With some generator arrangements oi' the type under consideration, it

may be unnecessary to include the resistors I3 ceeding time intervals of the signal. When considera'bly greater accuracy in the ratio of the succeeding periods is desired, resistors I3 and 2| may be selected to provideV a charging time constantfor the condenser 22 which varies appropriately with the magnitude of the feed-back voltage applied to condenser 22, as in the preferred embodiment described above.

To provide a clearer picture of the time con stants involved during the periods T1 and T2, reference is made to Figs. 4 and 5. In these equivalent circuits the small resistors I3 and 2| have been omitted since forjmany applications of the generator they are unnecessary.

During the shorter interval T1 when tube II is fully conductive and tube III is nonconductive, the circuit condition is -shown by the equivalent circuit of Fig. 4. The condenser 22 is charged from'battery +B (the upper plate positive and the lower plate negative) through resistors I1, IBa, Rgkn and I2 in series, where I6a represents the portion of resistor IIi between the tap 20 and the resistor I1. As the charge on condenser 22 increases exponentially the current, and Lhence the voltage drop across the several resistors, decreases exponentially until the voltage across resistor I2 no longer biases tube I0 beyond cutoff. At this point tube I0 becomes conductive and the regenerative action previously described cutsoi tube II. V

The longer time interval T2 thereupon vstarts and the equivalent circuit during the interval T2 is shown'in Fig. 5. The effective voltage from the battery is now divided by the several resistors I1, I6a, |617, Rpkio and I2, where Rpkio is the plate-cathode resistance of tube III and I6b represents the portion of resistor I6 between the tap 2li and the anode of tube I0. The source voltage for condenser 22 is therefore thatiacross resistors ISb, Rpkm and I2 in series. The voltage charge on the condenser 22 is originally higher than the voltage of this new source and, hence, condenser 22 starts to discharge through resistors I6b, Rpkio, I2, and I4 in series. Resistor I4 is 'very large and is the principal resistor in determining the larger time constant during the interval T2.

It is evident from an examination of Fig. 4 that if resistor Ita is made smaller the time constant. and'hence interval T1, is decreased. Conversely, it is evident from Fig. 5 that, in making resistor I6a smaller, the resistor I 6b is made larger, thus increasing. the time constant ldurin': interval T2. However, this change is relatively small lbecause resistor I4 isv very large and the major effect is the reduction of the source voltage@ by the voltage step e'. Hence, the time required for the condenser 22 to discharge to a point l where tube Il becomes conductive, resulting in the circuit conditions of Fig. 4, is shorter when resistor Ila is made smaller. That is, the time T: is made shorter. l

It is noted, therefore, that, in both Figs. 4 and 5, the eect of decreasing the size of resistor 10a is to decrease both the time intervals T2 and Tx, which results in an increase in the generator frequency while the ratio Ti/Tz' is maintained substantially constant. Conversely, an increase in the value of resistor lia decreases the generator frequency while maintaining the ratio Ti/Ta substantially constant.'

In summary it is noted that the time interval Ti is ailected primarily by a change in the charging time constant due to a change in the value of resistor la, while the time interval T2 is aiected primarily by the change in the voltage step e' due to a change in the same resistor Isa. For a given change in resistor |6a. therefore, the corresponding changes in the periods Tl and T2 are in the same sense.

While applicant does not wish to limit the inventionto any speciilc circuit constant, the following circuit constants are given as illustrative of values of circuit elements which may be utilized in the circuit of Fig. 1:

Tubes I and Il 6J6 (twin triode) Resistor I2 10,000 ohms Resistor 'i3 100 ohms Resistor I4 2.2 megohms Resistor I6 500 kilohms (max.) Resistor Il 470 kilohms Resistors i9 and 2| 330 kilohmsv Condenser 22 0.05 microfarad +B 400 volts Frequency range of generator- 21/2 to 10 cycles Der second Ratio of periods T1/T2 V3 While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modications as fall within the true spirit and scope ot the invention.

What is claimed is: v

1. A generator for generating a signal of periodic wave form and of variable periodicity comprising: a tlrst electron tube having an anodecathode space-current path and at least two operating conditions; a second electron tube having a control electrode-cathode space-current path; means including said second electron tube, a ilrst time-constant circuit including an energy-storage device and said control electrodecathode space-current path, and a second timeconstant circuit including said energy-storage device and said anode-cathode space-current path for causing said first electron tube to alternate during succeeding time intervals from one of said conditions to the other to complete for successive pairs of said time intervals operating cycles; an impedance included in an energizing `circuit of said ilrst electron tube to develop'an output potential having an amplitude varying with said two operating conditions thereof and an adjustable contact engaging said impedance for connecting a different portion oi said impedance in each of said rst and second means a selectable portion of said output potential while at the same time providing with adjustments of said contact an adjustment oi the time constants of said first and second timeconstantcircuits, whereby the duration of each of said succeeding time intervals is adjustedin substantially the saine proportion while the periodicity of said operating` cycles is adjusted over a relatively wide frequency range.

2. A generator for generating a signal of periodic wave form and of variable periodicity comprising: a iirst electron tube having an'anodecathode space-current path and at least two operating conditions; a second electron tube having a control electrode-cathode space-current path; a cathode impedance common to both ot said electron tubes; means including said second electron tube, a rst time-constant circuit including an energy-storage device with said cathode impedance and said control electrodecathode space-current path, and a second timeconstant circuit including said energy-storage device and said anode-cathode space-current path for causing said irst electron tube to alternate during succeeding time intervals kfrom one of said conditions to the other to complete for successive pairs of said time intervals operating cycles; an impedance included in an energiztime'constant circuits and for applying to said ing circuit of said first electron tube to develop an output potential having an amplitude varying with said two operating conditions thereof; said energizing-circuit impedance having a xed value which is substantially greater than that of said cathode impedance, the conductive anode-cathode impedance of said rst electron tube, and the conductive control electrode-cathode impedance of second electron tubes during one of said operating conditions; and an adjustable contact engaging said energizing-circuit l 40 impedance for connecting a different portion of said impedance in each of said iirst and second time-constant circuits and for applying to said means a selectable portion of said output potential while at Ithe same time providing with adi justments of said contact an adjustment of the time constants of said first and second time-constant circuits, whereby the duration of each of said succeeding time intervals is adjusted in substantially the same proportion while the periodicity ofsaid operating cycles is adjusted over a relatively wide frequency range.

BERNARD D. LOUGHIJN.

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

UNITED STATES PATENTS Number Name Date 2,185,363 White Jan. 2, 1940 2,193,868 Geiger Mar. 19, 1940 2,346,396 Rider Apr. 11, 1944 2,356,248 Koren Aug. 22, 1944 2,375,950 Schlesinger' May 15, 1945 2,411,062 Schade NOV. 12, 1946 2,414,323 Moe Jan. 14, 1947 2,416,201 Nagel et al. Feb. 18, 1947 2,438,638 Lakatos Mar. 30, 1948 FOREIGN PATENTS Number Country Date 485,934 Great Britain May 26, 1938 352,401 Great Britain July 3, 1931 528,806 Great Britain Feb. 14, 1940

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