US2750502A - Cathode-controlled wave generators - Google Patents

Description

June 12, 1956 G. w. GRAY 2,750,502

CATHODE-CONTROLLED WAVE GENERATORS Filed Dec. 29, 1950 F) 44I7'ZYNC INPUT I INVENTOR M ATTORNEY United States Patent CATHODECONTROLLED WAVE GENERATORS George W. Gray, Lambertville, N. J., assignor to Radio Corporationof America, a corporation of Delaware Application December 29, 1950, Serial No. 203,317

The terminal 15 years of the term of the patent'to be granted has been disclaimed 12 Claims. (Cl. 250.36)

This-invention relates to electrical wave generators andthasparticular reference to that type of a generator of waves of complex form which is known-as a multivibrator.

Multivibrators have been used extensively to produce waves of complex forms, particularly those known generally as square or rectangular waves. An ideal square wave has relatively steep leading-and trailing edges and substantially'fla-t tops or crests. Most multivib'rators of the prior art. produce waves which only approximate these characteristics. A multivibratorwhich produces a wave most closely approximating these characteristics is-one: known as asymmetrical. cathode-coupled astable multivibrator; Atypical example of such a device is disclosed in the book entitled Wave Forms published by'McGraw-HillBook Co., Inc., New" York, NewYork, and-appearstirnFigures'5-18 at-page- 173. In'this multivibrator the: resistive-capacitive cross-couplings between each of the tube anodes and theopposite control grids are of such longtime constants that the oscillation frequency is:not determined by them. Instead; thisfrequency isestablishedby a' capacitor coupled between the cathodecircuits of the tubes. From the waveforms produced by'this device shown in Figures -19 also at page 1 73 of this book, it may be'seen that-the developed wave: has fairly steep leading and trailing edges. However, only the positive loops of the wave have substantiallyyflat crestsr The negative loops have decidcd slopes with the exponential curvature whichischaracteristic of prior art multivibrators.

Consequently, in order to utilize such a wave for many purposes, it is necessary to shape it additionally so as to produce relatively fiat crests in both loops of the wave. Such anoperation requires additional apparatus which adds to the cost of making the square wave generator, as well-as to the costof operating it.

Accordingly, it is an object of this invention to provide an improved multivibrator which is capable of generating substantially rectangular waves.

Another object of the invention is to provide an improved square wave generator by which the developed waves have substantially flat crests and in which a novel means of synchronizing from an external source is employed.

A further object of the invention is to provide an improved symmetrical cathode-coupled astable multivibrator having anode and cathode circuits suitably related to produce square. waves having relatively flat crests in both halves of the wave.

In accordance with this invention, a-multivibratoris provided which consists essentially of means, such as two electrontubes defining two space discharge paths, each ofwhich includes at least an anode, a cathode, and a control grid. Cross-couplings are provided fromeach anode circuit tothe opposite control grid in a conventional manner for multivibrators. The time constants of these couplings, however, are long compared with the period of oscillation ofthe device. Respective circuits including impedance devices are coupled to the cathodes of the space discharge paths. The cathode-connected impedances are at least'as large as, and preferably considerably larger than, the anode circuit impedances, in accordance with this invention. The'timing of the multivibrator is determined by a capacitor coupled symmetrically between the two cathode circuits.

In one particular embodiment of the: invention the cathode-connected circuits include deviceswhich are" effectively constant current devices, such as-electrontubes of the pentode type. In this form of the invention; control signals may be impressed upon oneofthe pentodes for the synchronous operation of the'multivibrator.

The novel features that are consideredcharacteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as' well as additional objects and advantages thereof, will best be understood from the following description when read in connection with theaccompanying drawing.

In the drawing:

Figure 1 is a circuit diagram of the basic circuitsof a multivibrator illustratively embodyingthis invention;

Figure 2 is a series ofcurves representing'wave forms produced by apparatus embodying the invention and referred'to indescribing the operation of the, apparatus; and,

Figure 3 is a circuit diagram of another form of the invention, employing jpentode electron tubes as parts of the cathode-connectedimpedances of 'a' multivibrator in accordance with this invention.

Referencefirstwill be made to Figure: l'of the drawing. The. multivibrator embodying this invcntion,, as shown" here, includes twoelectron tubes 11' and 12." It

will be understood that these tubes maybe in separate envelopes as shown. Alternatively, they may be enclosed within the single envelope of a dual triode,,for example. In any case, there are defined two spacedisch'arge paths, each of which includes an anode, a cathode and a controligrid. Space current for. the tubes-1'1 and 12' is derived from a source indicated at +13 through resistors 13and 14'coupled respectively to the. anodes of these tubes. The anode of tube 11" is cross-coupled to the control grid'of tube. 12 by a circuit including-,a capacitor 15Iand a resistor 16. Similarly,,thevv anode of the tube llis cross-coupled to the control gridof, tube ll b'y aciicuitwhich includes a. capacitor 17 anda.resis= tor 18. In each case, the resistors. areconnectedtto the respective cathodes ofthe tubes. Also,.in.each.case, the cross-coupling, has .a long time constant ascompared to the period of oscillation of.- the multivibraton. Con-. se quently, the. oscillation frequencyv off the multivib rator is notdetermined by,v these cross-couplings;

The. cathodesmf. the tubes 11 and12- are-symmetrically coupled/by atiming capacitor 19. Also, theacathodes ofthetubes .11 and 12. are connected to ground through resistors l-laand. 22 respectively.- In accordance-witha feature oft the present invention, theimpedance of. the resistors-21 and 22. is at leastequal-to, and. preferably greater than, .the impedance of. the anode-.connectedlres sistors. 13 and 14. Typical magnitudeszfor.thetresistors 21', 22 mightbe thirtyrtimesigreater thanzthefmagnitndes of'the anode-connected'resistorst13:, 14.

The'operati'on': of the multivibratonshown iniFignrezl is somewhat. similar: to that ofcthe multivibrator rdiselosed in: the-publication:referred to; Theranodestwgridtzcrosscouplings and: the: cathode: coupling: between the: tubes provide two'paths-fors. regenerative: action-a The: cross!- couplings provi'd'es a.- regenerative: action which: is :iinsui; ficient in itself to cause oscillation because of the fact that the cathode-connected resistor 21 is made larger than the anode-connected resistor 13. Accordingly, the apparatus will not oscillate at all without the cathodecoupling capacitor 19. The reason for this is that, the gain produced from the grid circuit to the anode circuit of each tube is less than unity. Therefore, the gain of the entire loop, which includes both of the cross-couplings between anodes and control grids is less than unity. Consequently, no oscillations will occur even though the phase of the feed-back energy through the cross-couplings between the tubes is of regenerative character.

The inclusion of the cathode coupling capacitor 19 in the circuit, however, provides another regenerative feed-back loop which is efiective to make the apparatus oscillate. The way in which the circuit, including the coupling capacitor 19, operates to produce this result, may be explained in the following manner. Assume that the tube 11 is conducting and that the tube 12 is nonconducting. A portion of the space current, which is conducted by the tube 11, traverses a circuit including the capacitor 19 and the resistor 22. As the capacitor 19 becomes charged, this current flow decreases in magnitude thereby causing the potential of the cathode of tube 12 to decrease toward ground potential. This potential change tends to render the tube 12 conductive. The rate at which this cathode potential charges depends upon the time constant of the circuit and is determined principally by the capacitor 19 and the resistor 22.

Another portion of the space current in the tube 11 traverses the cathode-connected resistor 21. This portion of the current, however, does not vary substantially. The resultant decrease in the total space current through tube 11 causes a slight increase in a positive sense of the anode potential of the tube 11. By means of the crosscoupling, including the capacitor 15 and the grid resistor 16, this increased potential is impressed upon the control grid of the tube 12. This potential change also tends to render the tube 12 conductive.

sloping crest 26. The sloping crest 26 is produced principally because of the substantially exponential decrease of the charging current through the cathode coupling capacitor, corresponding to the capacitor 19 of the present apparatus. As a consequence, in order to utilize a wave such as 23, it is necessary to provide apparatus for clipping off the portion of the wave including the sloping crest 26.

The multivibrator, in accordance with the present invention, however, by virtue principally of the employment of anodeand cathode-coupled resistors in the described relationship produces a wave such as 27 in the anode circuit of either of the tubes 11 and 12. It is seen that this wave has very steep leading and trailing edges 28 and 29. It also has a lower, or negative polarity,

It is seen, therefore, that the described potential changes of the control grid and cathode of the tube 12 both are of the character necessary to render this tube conducting. The regenerative feed-back loop, including the coupling capacitor 19, provides more than enough gain to produce oscillation. The grid-to-anode loop adds to the regeneration and thereby accelerates the described operation of the device, even though by itself it is insuflicient to cause oscillation. As soon as space current begins to flow in the tube 12, it increases to a maximum substantially instantaneously and space conduction in the tube 11 is terminated substantially instantaneously in the characteristic manner of multivibrators. The described cycle of operation is repeated during the next succeeding interval of time substantially in the manner described except that the current flow through the coupling capacitor 19 is in a direction opposite to that described. This current then traverses the cathode-connected resistor 21, as well as the space discharge path of the tube 12.

As described, the operation of the present multivibrator is somewhat similar to that of the multivibrator disclosed in the publication referred to. There is one very important difference, however. This is produced by the described relationship between the anodeand cathodeconnected resistors 13, 14 and 21, 22 respectively. The materially improved results obtained by the present devices are graphically illustrated in Figure 2 to which reference now will be made. The curve 23 represents the typical output wave which is derived from the anode, for example, of one of the multivibrator tubes of a device such as that shown in the publication referred to. The leading and trailing edges, such as 24 and 25 respectively, of the'pulses are not as steep as frequently desired.

Aside from the fact that the wave 23 of the prior art apparatus does not have leading and trailing edges as steep as might be desired, it is seen that, in one half of the cycle, the wave has a substantially exponentially crest 31 which is substantially fiat. Actually, the illustration in Figure 2 is quite exaggerated. It has been found to be well within the capabilities of apparatus in accordance with this invention to generate a square wave in which the slope of the crest 31 is less than 5% of the total amplitude of the wave. By this is meant that the difference in amplitude between the leading and trailing edges 23 and 29 of the wave 27 is less than 5% of the amplitude of the wave as measured at the trailing edge 29, for example.

The slopes of the crests 26 and 31 of the waves 23 and 27, respectively, are produced as a result of a variation in the current through the coupling capacitor such as 19 of the apparatus of Figure 1. As described previously, this current flows through the conducting tube, such as 11, and also through the cathode-connected resistor, such as 22, associated with the non-conducting tube, such as 12. As is well-known, the potential at the point in this circuit to which the cathode of the nonconducting tube, such as 12, is connected varies, in a negative sense, toward ground potential as the coupling capacitor 19 changes from a charged condition of one polarity to a charged condition of the other polarity.

Consequently, as a result of this potential change, the current through the capacitor 19 and also through the conducting tube such as 11 decreases. A decrease in the current through the tube 11 causes the potential at the anode of this tube to increase in a positive sense. The potential at the cathode of the tube 11 at the same time tends to change in a negative sense toward ground potential. The later effect, however, in most cases, is negligible because it is such a small percentage of the potential variation at the anode of the tube.

In rnultivibrators of this general type used previously, the cathode-connected resistors have been of relatively low impedance with respect to the anode-connected resistors. Consequently, the total voltages developed at the cathodes of the tubes, such as 11 and 12 relative to ground, have been relatively small. The absolute magnitude of the voltage changes occurring at the cathode of either of the tubes as a result of the charging and discharging of the coupling capacitor such as 19 are approximately the same as in the present invention. In the prior art multivibrators these voltage changes have been of such magnitudes that they represent relatively large percentages of the total voltages developed across the cathode-connected resistors.

In the present case, however, the total voltage developed across the high impedance cathodeconnected resistors is very much greater than is possible with the same tube currents in the prior art arrangements. Ac cordingly, the voltage changes occurring at the cathodes of the tubes as a result of the charging and discharging of the capacitor 19 are a relatively small percentage of the total voltages.

It is seen, therefore, that in the present arrangement, the voltage variations occurring at the cathodes of the tubes 11 and 12 are produced by operating on the extreme upper portion of the exponential curve, which is characteristic of the operation of a circuit having capacitance and resistance. This portion of the curve is substantially linear. In multivibrators of the prior art where the voltage variations of the cathodes of the tubes represents a relatively large percentage of the total voltage developed, operation occurs over asubstantial portion of the characteristic curve and, consequently, includes a considerable portionwhich is'non-linear. These. voltage variations produce the slopes in the negative loopsof the waves'developed' at the anodes of' the multivibra'tor tubes as previously described.

The wave'i32 of Figure 2.represents the voltage at'the cathode of either'of. the tubes 11' and 12; his seen that, while the tube is in aconducting state, the cathode potential is maintained substantially constantas'indicated by the portion 33'. As soon as a conducting tube is rendered non=conducting, the cathode voltage substan-v tially instantaneously increases in axpositive sense to a point such as 34. As a result of the described change in the charging-of the coupling: capacitor 19, the voltage at the cathode of the non-conducting tube decreases substantially linearly toward ground until a point such as 35 is reached at which time the switching action between the tubes is initiated.

It will be appreciated that the switching actionbetween the two tubes 11 and 12 occurs very rapidly because of two principal facts. One is that no grid current isdrawn by -virtue of the describedcharacter of the cross-couplings between theanodesand controlgridsofthe-tubes: The other isthat there are provided two positive feed-back loops bothacting in aregenerative capacity. Immediately after the switch has been made'from one tube to another; thetube whichis rendered nonconducting-is; in a sense, doubly cut oh. Its control grid? is driven negative by means of the coupling to-theanode ofthe conducting tube. Also, its catlfrode'is rendered positive by means of the coupling capacitor I9- -which impresses upon-itthepositive impulses produced at the cathode of the conducting tube at the time of its being rendered conducting; This cathode potential'is represented at the point 34 0? the curve 32 of Figure 2.

Theslope such as-3 l of thewavelflshown in Figure-2 may be minimized by making'the cathode connected resistors: 21 and: 22 as large: as: possible compared to the cathode impedance. of either of the tubes when: it is conducting; Thecathode impedance of1aconductingtube may-be representedzby the expression 1 R1. gut I where gm is the transconductance; u is the amplification factor'and RL- is the anodeimpedance of'thetube. If the cathode-connected resistor is made too large; the transconductance of the tubebecomes excessively low because the space current of" the tube is reduced too much' and the ratio betweenthe cathode-connectedre sister: of the non-conducting tube and the cathode imped ance-of the-conducting tube is reduced; optimum cathode-connected resistor value produces a minimum slope of the negative loops of the generated wavet As=.iirdicated' previously, the frequency of the cathodecontrolled multivibrator in accordance with thisinvention is" a function of the value of. the couplingcapacitor" 19; It hasbeen foundthat-the frequency at'whi'ch this multivi'bratoris capable of operating covers arelativelitlarge range. Furthermore, the frequency variation is substantially linear-with values of the couplingcapacitor over mostzof' the range. For example, in a particular case, it: was found that the frequency of oscillation-ole the multivibrator could be varied'linearly from a-frequeney inexcessof one megacycle down'to afrequencywhich corresponds generally with the time constant of the anode-to-grid cross-couphng circuits. In' one case, the capacitors 15 and 17'were .005 microfarad' andlthe. grid resistors 162 and 18 were 10 megohms. The time constant of the cross-coupling circuits; therefore, is substantially ,50 of a secondi As a consequence, it was found that 6. the oscillation frequency of" the multivibrator began to depart from a linear curve at ,about'60 cycles-per second.

It alsohas been foundthat the range of the linear relationship between the oscillation frequency and the value. of the coupling capacitor 19' may be extended by increasing substantially the value of the grid resistors 16 andi18; In a particular case, these resistors were entirely removed. Leakage currents in the remaining circuits'were' found tolbe suflicient to enable the apparatus to operate" substantially in the manner described: The essential difference,.however, is. that the time constant of the cross=coup'ling' circuits between the. anodes and control" grids of the tubes.11 and'12'is much longer. As a consequence; the linearity of thetfre'quency curve may be'extended into the frequency region of substantially one cycle per second.

In view of the foregoing-description of the mode of operation ofthe multivib'rator apparatus of Figure 1 whereby it has beendemonstrated' that-the negative loops of the generated; wavemay' have minimumv slopes, it is seen' that what is ideally requiredascathode-connected resistors such as 21a'nd' 2'2 of' Figurel' are devices having maximum impe'dances' and' alsothefacility of conducting appreciable current. Also, it should be clear from the foregoing description that, in order to substantially eliminate'all slop'e from-the crests-of" the negative loops of tlie'developed wave, it is necessary that the current flowing" through the coupling capacitor and the asso-' elated resistive circuits be maintained substantially constant. Ih=accordancewith another feature of this invention, this is accomplished by employing electronic devices having a substantially constant currentconducting capabilityaspart', at least,- of-the cathode connected resistors.

Accordingly, reference nowwill bema'de to Figure-3 ofthe drawing showing another embodiment of the in vention' in which substantially all slope' of' the crests of thedevelop'edwave is eliminated and'also one which is susceptible of synchronous control from an external source; In this-form oftheinvention, pentode electron tubes 36 and 37 are connected fromthe cathodes of tubes 1'1 and I1, respectively, and through a common variable resistor 38 to ground. The suppressor grids of thepentodes 36'- and 37 areconnected to their asso-' ciated cathodes ina conventional manner. Also, the screen grids of these tubes are-connected to oneanother by a differential voltage dividingresistor' 39; Positive voltage is ir'npressed througha resistor 40upon a variable tapping point on the resistor 39 so as to differentially vary-the currentcapabilitiesof tlie tubes 36' and 37. The screen grids ofthe p entodes-'36* and 37 also are conventi'onallyby-passed togroundby capacitors 41. The cone trol' grid of the tube 36 is directly connected" to ground as shown. The control grid of-"the' tube 37 is coupled by a capacitor 42' and" a: resistor 43 to the synchronizing signal input terminals- '44.

The anode-connected resistors for the multivibrator tubes l'l and I'Z are-efiectively'th'e same as'in the embodiment of the invention shown'in'Figure" 1. The resistor 14 associated with-thetube 12, however, is indicated as comprising two serially-connected resistor elements 14a and'14b; It will beunderstood that these two elements together may-have substantially the'same value as the resistor 13 a'ssociated withthe tube 11. The junction point-between the'resistorelements'l la. and 14bis connected-to one of'a pair of output terminals 45.

The'multivibrator, in accordance with this form of the invention, operates substantially in the samema'nner as that shown in Figure" 1. In" this case, however, the resistance connected to the cathode of the multivibrator tubes" 11" and 12' consists essentially of the respective anode-cathode impedances" of the pentodes 36 and 37. Theimpedancesofthesetubes is affected by the adjustment'oflthe variable contact'onthe resistor 39. For any given ,value, offim-pedance, however, .the tubes 36. and 37 are essentially" constant current devices. Consequently,

as the potential of the cathodes of the non-conducting ones of the multivibrator tubes 11 and 12 varies with the change of the charging of the coupling capacitor 19, the current conducting capabilities of the associated pentodes do not change. Therefore, the current traversing the conducting one of the multivibrator tubes 11 and 12 does not vary. Hence, there is no voltage change produced at either the cathode or anode of the conducting tube which would produce a slope in the crest of the negative half cycles of the wave developed for impression upon the output terminals 45.

A variation of the resistor 38 provides a fine frequency control of the multivibrator. This control is alfected because the value of the resistor determines in some measure the magnitude of the current drawn by the pentode tubes 36 and 37. Accordingly, the time constants of the frequency determining circuits which include the capacitor 19 may be varied sutficiently to produce relatively small changes of the operating frequency.

As in the case of most multivibrators, the one in accordance with the present invention is susceptible of external control whereby it may be made to operate synchronously with other apparatus. In this case, the synchronizing pulses are impressed upon the control grid of the pentode tube 37 as described. A control of one of the pentode tubes by such means operates as a switch by which the multivibrator may be started and stopped. Accordingly, even though it is of the astable type whereby it will operate in a free running manner exclusive of any external control, it may be synchronously operated in a manner such as that described.

It will be appreciated that a multivibrator in accordance with the present invention is not necessarily limited for use in generating so called square waves in which the positive and negative half cycles are of substantially equal time durations. It is considered to be within the scope of the invention to operate the device in such a manner that other rectangular wave forms may be developed in which the time durations of the positive and negative half cycles are not of the same time durations. Such operation in accordance with well-known multivibrator practice requires that one of the electron tubes be maintained in a conducting state longer than the other tube. Ordinarily, such type of operation may be achieved by unbalancing the two halves of the multivibrator, particularly the frequency determining circuits coupling the two tubes. This type of operation may be achieved in the present case by substantially similar means. For example, the cathode-connected resistors, such as 21 and 22 of Figure i, may be of difierent values. The time constants of the two frequency determining circuits both of which include the coupling capacitor 19 accordingly may be made sufficiently different to cause one of the tubes to be conducting for longer times than the other tube. In the form of the invention shown in Figure 3, the unbalance is produced by positioning the movable contact of the voltage dividing resistor 39 so that a higher positive potential is impressed upon the screen grid of one of the pentodes than that of the other pentode.

It may be seen from the foregoing description of the illustrative embodiments of the invention disclosed herein that the proper relationship between the cathode anode connected resistors enables the operation of a multivibrator in such a way as to develop good square or rectangular waves having steep leading and trailing edges and substantially no sloping crests in any part of the cycle. Accordingly, the output wave may be employed directly without the necessity of employing clipping or other wave shaping apparatus.

Also, in accordance with the foregoing description, it will be appreciated that the multivibrator in accordance with this invention is susceptible of operating over a relatively wide range of oscillation frequencies. Furthermore, such a device is of a character that a substantial linearity exists between the oscillation frequency and'the value of the coupling capacitor such as 19 throughout the entire operating frequency range. Moreover, as previously indicated, the device is readily susceptible of external control of synchronous operation.

What is claimed is: a

l. A multivibrator comprising, means defining two space discharge paths, each of which includes an anode, cathode and a control grid, external circuits coupled respectively to said anodes and cathodes, cross-couplings between said space discharge paths, each of said cross couplings including a capacitor connected between one of said anode circuits and the opposite control grid, said capacitors having values such that said cross-couplings have relatively long time constants compared to the period of oscillation of said multivibrator, a resistor connected between said control grid and said cathode of each of said space discharge paths, whereby said cross-couplings do not determine the oscillation frequency of said multivibrator and the regenerative action provided by said cross-couplings is insufiicient in itself to cause oscillations to be produced by said multivibrator, impedance devices connected respectively in said cathode circuits for traversal by currents flowing in said space discharge paths, the impedance of said devices being at least as great as the impedances of said anode circuits, and a capacitor connected between said cathode circuits for elfecting alternate operation of said space discharge paths in a manner to develop a rectangular wave having substantially fiat crests, whereby the difference in amplitude of the leading and trailing edges of the developed rectangular wave of said multivibrator is less than 5% of the amplitude of said wave as measured at the trailing edge of said wave.

2. A multivibrator comprising, means defining two space discharge paths, each of which includes at least an anode, a cathode and a control grid, respective crosscouplings between each of said anodes and the opposite control grids, each of said grids being connected to its respective cathode, said couplings having relatively long time constants compared to the period of oscillation of said multivibrator, whereby said cross-couplings do not determine the oscillation frequency of said multivibrator and the regenerative action provided by said crosscouplings is insufiicient in itself to cause oscillations to be produced by said multivibrator, external circuits coupled respectively to said anodes and cathodes, impedance devices connected respectively to said cathodes for traversal by currents flowing in said space discharge paths, the impedances of said devices being materially greater than the impedances of said anode circuits, and

a capacitor connected between said cathode circuits for effecting alternate operation of said space discharge paths in a manner to develop at one of said anodes a square wave having substantially fiat crests, whereby the difference in. amplitude of the leading and trailing edges of the developed rectangular wave of said multivibrator is less than 5% of the amplitude of said wave as measured at the trailing edges of said wave.

3. A multivibrator comprising, means defining two space discharge paths, each of which includes an anode, a cathode and a control grid, external circuits coupled respectively to said anodes and cathodes, cross-couplings between said space discharge paths, each of said crosscouplings including a capacitor connected between one of said anode circuits and the opposite control grid, said capacitors having values such that said cross-couplings have relatively long time constants compared to the period of oscillation of said multivibrator, impedance devices, each including a pentode electron tube connected respectively in said cathode circuits for traversal by currents flowing in said space discharge paths, the impedances of said devices being greater than the impedance of said anode circuits, and a timing capacitor connected between said pentodes for effecting alternate operation of said space discharge paths and also of said pentodes in such a manner that the current flowing in either of said space discharge paths also traverses the pentode tube coupled to the cathode of the oppr site space discharge paths, whereby to develop at one of said anodes a rectangular wave having substantially fiat crests.

4. A multivibrator as defined by claim 3, and including additionally means for varying the current conducting capability of at least one of said pentode electron tubes whereby to control the respective time durations of the positive and negative half cycles of the developed rectangular wave.

5. A multivibrator as defined by claim 3, and including additionally means including the screen grids of said pentode electron tubes for differentially varying the current conducting capabilities of said pentode tubes whereby to control the respective time durations of the positive and negative half cycles of said developed wave.

6. A multivibrator as defined by claim 3 wherein, the screen grids of said pentode electron tubes have positive potentials impressed thereon, and including additionally means for differentially varying the positive potentials impressed upon said screen grids for effecting a control of the wave form developed by the multivibrator.

7. A multivibrator as defined by claim 3, and including additionally means for similarly varying the current conducting capabilities of both of said pentode electron tubes whereby to effect a variation of the oscillation frequency of the multivibrator.

8. A multivibrator as defined by claim 3, and including additionally a resistor connected in series with both of said pentode electron tubes, said resistor being variable whereby to effect a variation of the oscillation frequency of the multivibrator.

9. A multivibrator as defined by claim 3, and including additionally means for impressing synchronizing signals upon the control grid of one of said pentode electron tubes.

10. A multivibrator as defined by claim 3 wherein, said cross-couplings between the two space discharge paths include only capacitors.

11. A multivibrator comprising, a pair of triode electron tubes, each including an anode, a cathode and a control grid, capacitors respectively cross-connected between each of said anodes and the opposite control grids thereby providing cross-couplings having relatively long time constants compared with the period of oscillation of said multivibrator, resistors connecting said triode anodes to a source of space current, one of said resistors being provided with an intermediate terminal constituting an output terminal of said multivibrator, a pair of pentode elecfion tubes, each having an anode, a cathode, a con trol grid, a screen grid, and a suppressor grid and constituting constant current impedance devices, the anodes of said pentode tubes being connected respectively to the cathodes of said triode tubes, a frequency-determining capacitor connected between the cathodes of said triode tubes, a single resistor connc .ted in common between the cathodes of said pentode tubes and a point of relatively fixed potential, means including a variable resistor con nected between a point of positive potential and the screen grids of said pentode tubes for varying the oscillation frequency of said multivibrator, means connecting the control grid of one of said pentode tubes to a point of relatively fixed potential, and means coupling the control grid of the other of said pentode tubes to a source of synchronizing signals.

12. A multivibrator circuit arrangement including a pair of triode tubes each having a cathode, a grid and an anode, impedance devices individually connected to the anode circuits of said tubes, capacitors cross-coupling the anode of each of said tubes to the grid of the other tube to form an astable multivibrator circuit, at least one impedance device including at least one electron discharge device having cathode, control, grid and anode electrodes connected in the cathode circuit of one of said tubes with the anode electrode connected to the cathode of said one tube, another device connecting the cathode of the other tube to a point of fixed reference potential and having a value of impedance at least as great as that of the impedance device connected to the anode of said other tube, means connecting the cathode and grid electrodes of said electron discharge device to a point of fixed reference potential, and means to impress a signal between the control and cathode electrodes of said electron discharge device to control operation of said astable multivibrator circuit.

References Cited in the file of this patent UNITED STATES PATENTS 2,106,342 Doba Jan. 25, 1938 2,321,269 Artzt June 8, 1943 2,381,920 Miller Aug. 14, 1945 2,456,089 Shenk et a1. Dec. 14, 1948 2,494,353 Newman Jan. 10, 1950 2,550,116 Grosdoii Apr. 24, 1951 2,562,530 Dickinson July 31, 1951 2,564,559 Canfora Aug. 14, 1951 FOREIGN PATENTS 584,740 Great Britain Jan. 22, 1947 928,923 France Dec. 11, 1947 486,550 Canada Sept. 16, 1952 OTHER REFERENCES Text entitled Waveforms by Chance et a1., first edition, 1949, published by McGraw-Hill Book Company, Inc., New York, N. Y., pages 164-194; note particularly pages 172, 173, and 181.

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