US2601444A - Stabilized multivibrator oscillator - Google Patents

Description

June 24, 1952 M. E., MOHR 2,601,444

STABILIZED MULTIVIBRATOR OSCILLATOR Filed Oct. 12, 1949 lNl/ENTOR M. E. MOHR ATT R/VEV Patented June 24, 1952 STABILIZED MULTIVIBRATOR OSCILLATOR Milton E. Mohr, New Providence, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 12, 1949, Serial No. 121,031

9 Claims.

This invention relates to oscillatory circuits and is especially concerned with improved methods and means for stabilizing relaxation type oscillatory circuits against unwanted frequency variations.

It is well known that the characteristics of many types of oscillators and particularly those of the relaxation type are subject to changes arising from variations in the circuit potentials, and also from changes in the relative positions of the electrodes of the included electron discharge devices. It is, of course, desirable that the oscillation frequency of these circuits be made independent of such circuit changes in so far as is possible. It is accordingly an object of this invention to make possible improved multivibrator type of oscillatory circuits in which frequency instability is reduced to a minimum.

It is also an object-of this invention to make possible stabilized relaxation type oscillators, the operation and frequency output of which are substantially unaffected by variations in the anode potential supply.

It is a further object of this invention tomake possible multivibrator type of oscillatory circuits in which the effect of variations in the cut-off and saturation potentials caused by aging of the electron discharge device is effectively nullified.

In accordance with the invention, in one of its preferred embodiments, a pair of electron discharge devices, which may take the form of pentode or beam-power tetrode vacuum tubes, are arranged with the usual resistance-capacitance cross coupling between the anode of each tube and the control-grid electrode of the other tube to form a conventional multivibrator oscillator. The biasing potential for each controlgrid electrode is made variable with changes in the magnitude of the anode potential source. Also, it is so proportioned with respect to the anode potential supply that the voltage changes that are injected into the control electrode circuit of the oscillator substantially offset and cancel the effect of the voltage changes that are injected through the anode circuit of the oscillator, when the anode potential source varies in magnitude. In this manner, although the variations in the circuit potentials are not actually eliminated, their effects are nullified. In addition, certain of the circuit potentials are controlled in such manner that the cut-ofi potential and the voltage drop in the saturated anode-cathode discharge path of each tube are made variable, and eachassumes a minimum value thatis near zero during that portion of the oscillation cycle in which changes in its value would be reflected .in

changed oscillation characteristics.

A more complete understanding of the invention and the manner in which the foregoing objects are accomplished may be obtained from the following description of two embodiments thereof, when considered in conjunction with the drawing, in which:

Fig. 1 is an explanatory graph of the variations in potential on the control electrode of one electron discharge device of a multivibrator. Reference is made to this graph in the following description;

Fig. 2 is a schematic circuit diagram of a multivibrator oscillator in accordance with the present invention which is stabilized against frequency variations arising from changes: in the magnitude of the anode potentialsupply; and

Fig. 3 is a schematic circuit-diagram of a multivibrator oscillator in accordance with the invention in which the oscillation characteristic is stabilized against changes arising both from variations in the anode potential supply and from variations arising through changes in the electron discharge devices as they age.

In the following descriptions and in Figs. 2. and 3 of the drawing, like reference characters have been used to denote the same or similar circuit elements.

A more complete understanding of this invention may be had if it is first noted that several conditions contribute .to instability of "the oscillation frequency in the conventional relaxation oscillator of :the multivibrator type. In general, the oscillation frequency is largely determined by the time constants of the inter- -electrode coupling circuits of the oscillator. Tosa lesser, but still considerable degree, the frequency is also determined by the magnitude of the potential supply source, the load impedance, and the characteristics of the electron discharge devices or tubes themselves. Of these last-named factors, variations in the first are likely to occur at any time in the operating cycle, While variations in the last tend to occur primarily as aging of the tube progresses. It is thought that these factors affect the oscillation frequency of the oscillator through the variations that they introduce into the tubes cut-off potential value Eco, the magnitude of the voltage drop in the anode-cathode path of the tube when it is saturated, and the potential at which the control electrode of the saturated tube rests in the instant before that tube changes to its cutoff condition. The interrelation of these factors,

associated cathode electrodes.

and the manner in which they are controlled through the practice of this invention, may be best understood if the explanatory grid potential wave form diagram of Fig. 1 is first considered, together with the multivibrator oscillator circuit of Fig. 2.

Although in the following discussion the abovementioned factors will be described as affecting only the potential change on control grid ll of electron discharge device [0, it will be understood that they exert exactly the same influence on the potential of control-grid electrode [3 of discharge device l2. Therefore, variations in these factors are effective in causing variations in the oscillation frequency through the control that they exert in each'of the electron discharge devices.

Fig. l is a graphical representation of the various circuit potential differences that influence the oscillatory period of the oscillator. It also forms a' convenient basis for understanding how the stabilization of the oscillator against the efiect of changes in the anode potential supply is accomplished. In this figure, t1 indicates the cut-01f interval of discharge device H! '(see Fig. 2), which'interval it is desired to maintain constant; E1 indicates the difference in potential between a suitable reference potential, such as ground, and the potential of the anode when the discharge device or tube [2 issaturated; E2 indicates the difference in potential between this same reference potential and that of the control electrode of tube I just prior to the time that this tube is cut oil"; the broken line Eco indicates the value of the cut-off potential of electron discharge device or tube Ill for the designed operating conditions; Ep denotes the total potential difierence that is available for affecting the momentary potential of control electrode H and is numerically equal to the negative value of the anode or plate potential source or battery I B; KEp denotes the potential to which the controlgrid electrodes are connected, and which they would tend to assume if current conduction were not started in the electron discharge device when its control-grid electrode acquired a potential in excess of Eco; and (0) indicates the actual ground and cathode potential.

Referring now to Fig. 2, the electron discharge devices I0 and 12, which may suitably be pentode type vacuum tubes, together with the anode load resistors l4, IS; the coupling capacitors 2B, 22; the grid resistors 24, 26; and the source of anode potential or battery I8, constitute a conventional multi-vibrator oscillator in which the controlgrid electrodes II and i3 are connected to a point of potential that is positive with respect to the This positive biasing potential is secured from the movable arm 3| of potentiometer 32, which potentiometer forms a voltage dividing network across the potential source or battery l8. In accordance with one feature of this invention, the arm of potentiometer 32 is so positioned that a fractional part, KEp, of the anode supply potential Ep is supplied to the control electrodes ii and it through grid leak resistors 24, 26. This supplied potential KEp is critical, and is so proportioned that when added to the cut-oii potential Eco a combined potential value Eb (Fig. 1) results. At the time of cut-off, the potential of control electrode I l becomes Ea volts more negative than its cut-off value Eco. When the grid charging potential KE is properly chosen, the ratio Eb/Ea is a constant value for varying values of E Lil potential on control electrode uated The optimum value of potential KEp is secured by properly positioning the movable arm of potentiometer 32, and it be recognized by the fact that if KEp is less than its optimum value, the frequency of oscillation varies inversely with an increment of potential change in anode potential source [8; that is, an increase in the potential of source l8 results in the lowering of the oscillation frequency. Conversely if KE is greater than its optimum value, the frequency of oscillation varies directly with changes in the potential of source H8. The actual value of the optimum grid charging potential KE will vary as the circuit parameters are changed. However, for each circuit configuration there is one distinct value of biasing potential KEp for which the ratio Eb/Ea (Fig. 1) remains constant for all reasonably expectable potential changes in the anode potential supply Ep.

The manner in which the proper adjustment of the control grid charging potential KEp introduces the desired compensatory effect may be best understood if the operation of the circuit (Fig. 2) is examined at a time just priorto and at the time that electron discharge device I0 changes from its saturated-current condition to its cut-off condition. At the time just prior to being cut off, discharge device it is saturated; its control grid II is resting at a potential which may be slightly in excess of ground potential. At the instant of cut-off, the potential on control electrode ll descends Ea volts below its cut-off" potential to a value 5!! that is determined by the charge then existing across coupling capacitor 22 and the change in po tential at the anode of the now conducting device l2. This potential Ell is numerically equal to the anode supply potential Ep minus the combined residual potentials E1 and E2, these latter being the potential difference between the anode and cathode electrodes of the saturated device !2 and the potential on control electrode I! during the current saturation period of device it), respectively. After being driven negatively, the H increases as capacitor 22 charges until a potential of Ea) is reached, at which time conduction is again started in device N). This potential excursion Ea may be evaluated as:

If current conduction were not started in device it] when its control electrode acquired the potential Eco and capacitor 22 continued to charge, the potential on control electrode would become asymptotic to the biasing potential KEp, as denoted by the dotted line 54. This Would represent a further voltage excursion:

Therefore, as the potential of control electrode I I rises from its most negative point or value 58 to its cut-oif potential Eco at point 52, it follows the exponential condenser charging curve evalas g10:KE (Ep+KEp-El-E2.) 6 where T represents the time constant of capacitor 22 and resistor 24 (Fig. 2). If 6 10 is equated to the cut-off potential --Eco, the cut-off interval t1 of device it may be evaluated as:

cut-off interval n will remain constant notwithstanding variations in the anode potential supply t =T log llv E5 iftheresultant changes -that are introduced into potentials E1; E2; and- Ea, are caused to be compensatory; or-if E1,- E2, and Eware caused to have zero values: Inthecircuit configuration shown in Fig. 2,,tl'1'efirst' of'these conditions, namely;- that the caused variations in potentials Er, E2; and 'Eco are compensatory is-brought about bythc proper proportioning of'the control grid biasing-- potential KEp in the manner which has been described. Inthis (Fig. 2) arrangement; the variations that are introduced into potentials-E1; E2; and Eco by changes in the anode potentialsupply IB-may or-maynot be proportional. Itais' not:necessary that they be proportional; it is-only necessary: that theeffect of these variations becom-pensating in such manner that the above mentione'd logarithmic quantity maintains a" constant value; Thiscompensatingcondition is obtainable ineach case when the -grid biasing potential KEp issuitably chosen.

In-the--circuitarrangement of Fig. 3, increased frequency stability is-obtai-ned by combining. the previously described arrangement with afurther improvement. The variations in potentials E1, Errand-"Eco; caused bychanges-in the-anode potential supply; are made compensator-y "through theproper choice of the grid biasing potential KEp." whileat thesame time the potentials E1 and-Em arecaused to haveaminimum'valueduring that por-tionofthe operating cycle of each electrondischarge'device- I0 or -l2during which they-exert-a controlling influence on the oscillation frequency. Sincetheselatter potentialsare caused to assume=minimum values approaching zero vaiue an-y variations in the'n rwill' necessarily also besmaii, and the disturbing effect of such variations will; be minimized if not completely eliminated.

Inthercircuit arrangement of Fig. 3, likereferenceucharacters are'used to-denote circuit elements that are-the sameas or similar to those shown=in themultivibrator circuit of Fig. 2. It will be noted that the portion of the circuit comprising electron discharge devices" I'D and I2 is substantially the same as-in Fig. 2, with the exceptiorrthat the-screen grid potentials -for these devices are made variable-insteadof being fixed quantities as in the formercaser As in the former case the control-grid electrode biasing potential KEsis securedat the movable arm 3| of potentiometer 32. Each screen-grid electrodeis biased from. a variablevoltage-dividing circuit comprisingres-istor 28 o-r 30 andthe anode-cathode discharge path of an auxiliary electron discharge device-34m 36, each of which may suitablybea pentode typevacuum tube orother suit able controll'ably variable-impedance element. The-screen' electrode: potential for auxiliary: de= vices 34; 36 is obtained through a suitably proportioned. resistor *fromrthe-anode potential source: Hi. The control electrodes, 31 of these devices are connected through capacitors 38', 39 torthe anodes of: discharge: devices I01. and I2; Grid-'ileak-.resistors ill; 44 are also aconnectedito these control electrode:..circuits and" are ;of .such magnitude that time-constant 1 circuits. which: are longcrelative to r the .:oscillation'iperiodv. of tubes .1 D or 1:2 result; from their 'combinationt withrtherre spective capacitors :38 33.

The manner in. whichthe multivibratorztype of oscillator of" Fig; 3 operates 'to: achieve increased frequency'stability will besbetter: understood if it is =recalledthat the saturated current conduction condition of oneoscillator tube coincides with the currentcut-off condition of the other tube; and

' zero or thereabouts.

vice versa: It should also be recalle'dlthatvatiations the-potential"difierence Ei =between-: the anodeand cathode --electrodes in the saturated tube are efiective in-infiuencing the oscillation frequency during thesame portionof the cycle that variations -=in -=the= eut-off potential value areefiective in the non-conducting or cut-off tuber From thepreviouslydescrib'ed circuit relations it will be recalled that the cut-off interval" i1 is evaluated! It is, therefore, evident t that: increased stability in th'ecut-ofiinterval 'trvwill result'if in addition tolcausing the variations in potentials E1, E2 and Eco to be compensatory (as was described in con nection with Fig, 2) thepotentialsEr andEca are causeto assume minimum val'uesnearzero during that portion 7 of j the oscillation cycle in which they exert a controllinginfluence. Thesescombined stabilizing influences, are realized in the multivibratorv type? of circuit of Fig. '3 through thecombination of the compensating adjustment of the controlelectrode. potentials, 'and'the ac tion of the variable impedanceelements, pentodes 34, 36; on the screen electrode potentials off'oscillator tubes 10, 12.

Both E1 and Eco are greatly influenced butin oppcsed'directions, by changes in the magnitude of thescreen electrode potential. That is, an in:- creasein this potential reduces the potential 'dif ference Ei between the anode and cathode-electrodesof the tube while ,it is in itssaturated conditicn, which is desirable; but it also increases the cut-off potential Eco, which is undesirable" Since E1 and Eco exist simultaneously butrin different tubes, it is possible .to reduce each to a minimum at thesameti'menby properly. controlling the screen electrode potential of the respective tube.- Accordingly, separate screen electrodebiasing'cir-v cuits including variable impedance devices 34, 36 areprovided for theelectron discharge devices-.10 and I2. When electron discharge device ortube l0 changes from its full conductionto its cut-off condition, a positive voltagetimpul's'e is coupled from its anode circuit through; coupling capacitor 38to control electrode 33 of the variable impedance device or pentode 34. Theltime constantof capacitor 38 and resistor 44 is large, and this positive voltage impulse persistsxon electrode:33 for the duration of the cut-off interval iii to lower the impedance of thexdi'schargepath of pentode 34*, and'therebylower the potential of the screen electrodeof device 10.. This "lowered screen elec trodezpotential reduces the cut-off" potential Eco to a, minimum value which may bem'adeto be Any changes in the anode potential supply Ep or in the spacing of the elements of discharge device 10; which "change would normally tend to affect the-cut-ofi potenthereby-increase thenimpedancel of the.:a'nod'e cath'odev'diseharge:path: of this auxiliaryrpentode: This increased impedance increasesthemotential on: the screena electrode of: devices 12;? 5 which.

change decreases the voltage drop E1 in the saturated anode-cathode discharge path of device I2 to a minimum value that is considerably less than it would be if the screen electrode potential were not increased. Because of the minimum value condition, the effect of variations in circuit potentials or structures, which variations would normally influence or change the anode-cathode potential difference, is eliminated or at least minimized.

The action of the auxiliary discharge devices 34, 36 is thus seen to be to increase the screen electrode potential of the saturated oscillator tube and thus lower the voltage drop in its anodecathode discharge path, while at the same time the'screen electrode potential of the cut-off or non-conducting device is lowered and the cut-off potential Eco of this device is reduced to a mini.- mum value.

In addition to the stabilizing action of the auxiliary pentodes 34, 36, it will also be understood that the selection of the optimum biasing potential KEp for the control grid electrodes H and [3 of devices Hi and I2 imparts a stabilizing effect against variations caused by changes in the magnitude of the anode potential supply E8. The correct positioning of the movable arm SI of potentiometer 32 to secure this optimum biasing potential KEp is performed in the manner previously described in connection with Fig. 2. The empirical method for determining the correct position of this potentiometer arm may be performed as described, but the actual value of KEp will usually be found to be different in the arrangements of Figs. 2 and 3 because of the revised conditions that govern the potentials E11 and Eco in the latter arrangement.

In general, the most satisfactory method of stabilizing the described type of oscillator will employ both the choice of an optimum value of biasing potential KEp and the use of variable screen electrode potentials on the oscillator tubes. Through the combined use of these stabilizing factors, an extremely high degree of frequency stability may be achieved. However, it should be appreciated that this invention is not limited to only the combination of these two stabilizing factors, since a material but lesser improvement in the stability of the oscillator may be secured through the use of only variable screen electrode potentials on the oscillator tubes.

Although this invention has been described as being incorporated in a specialized type of oscillator, it will be evident to those skilled in the art that it is not limited in its application to the described arrangements. Therefore, it is to be expected that variations of this invention, which do not depart from its spirit and scope, will suggest themselves to those skilled in the pertinent art.

What is claimed is:

. l. A stabilized multivibrator type oscillator comprising a first and a second pair of electron discharge devices, each of said devices comprising anode, cathode, control grid and screen grid electrodes, 2. source of positive potential resistively connected across the anode-cathode electrodes of each of said devices, a capacitive coupling between the anode and electrode of each one and the control grid electrode of each other one of said first pair of devices, grid-leakresistors connected in the control grid cathode circuit of each of said devices, the screen grid electrode of each of said first pair of devices being connected to the anode-electrode of a respective one whereby the impedance of of said second pair of devices, and a capacitive coupling connecting the anode electrode of each of said first pair of discharge devices and the control grid electrode of the same respective one of said second pair of devices.

2. A stabilized multivibrator type oscillator comprising a pair of electron discharge devices each including an anode, a cathode, a control grid and a screen grid electrode, a source of potential, an individual resistive connection between the positive terminal of said potential source and each of said anode and said screen grid electrodes, cross-couplings between the anode and each device and the control grid electrode of each other device, a grid-leak resistor in the control grid cathode circuit of each device, an individual variable impedance connected between the negative terminal of said potential source and the screen grid electrode of each of said discharge devices, and means connected to the anode of each discharge device to control the magnitude of the impedance connected to the screen electrode of said device in inverse relation to the changes in potential of said anode.

3. A stabilized multivibrator type oscillator comprising a pair of electron discharge devices, each of which includes an anode, a cathode, 'a control grid and a screen grid electrode, a source of potential having positive and negative terminals, a load impedance connected between said positive terminal and each of said anodes, a grid- .leak resistor connected between each control grid electrode and a point of potential positive with respect to said cathodes, each of which is connected to said negative potential terminal, a pair of voltage dividing circuits each connected be.- twcen said positive and negative terminals and each comprising a fixed and a variable impedance element in series connection, each of said screen grid electrodes being connected to an intermediate point on a respective one of said divider circuits and a coupling between each anode electrode and the variable impedance element connected to its conjugate screen grid electrode said element ,is changed as the potential of said anode is changed.

4. A stabilized multivibrator type oscillator in accordance with claim 3 in which the impedance of said variable impedance element is increased as the potential of said anode is decreased and vice versa.

5. A stabilized multivibrator type oscillator comprising a pair of electron discharge devices, each of which includes an anode, a cathode, a control grid and a screen grid electrode, a source of potential having a positive and a negative terminal, a load impedance connected between said positive terminal and each of said anodes, a grid-leak resistor connected between each control grid electrode and its associated cathode electrode, which latter is connected to said negative potential terminal, a pair of voltage dividing circuits connected between said positive and said negative terminals each circuit comprising a fixed and variable impedance element in series connection, said variable element including the anode-cathode discharge path of an auxiliary electron discharge device which device includes a control grid electrode, a connection between each of said screen grid electrodes and an intermediate point on a respective one of said divider circuit, and a capacitive coupling between each of said anode electrodes and the control grid electrode of the auxiliary discharge device that is connected to its conjugate screen grid electrode whereby the potential of said auxiliary control grid electrode is increased and the impedance of the auxiliary devices anode-cathode path is decreased as the potential of the connected anode of said first-mentioned device is increased, and vice versa.

6. A stabilized multivibrator type oscillator comprising a pair of electron discharge devices each of which includes an anode, a cathode and a screen grid electrode, a source, of current, said source including positive and negative terminals, a load impedance connected between said positive terminal and each of said anodes, a connection between each of said cathodes and said negative terminal, a grid-leak resistor connected between each control grid electrode and its associated cathode, a pair of voltage dividing circuits connected in parallel between said positive and negative terminals, each of said circuits comprising a fixed resistance and the anode-cathode path of i an auxiliary discharge device, which device includes a control grid electrode, a connection between each of said screen grid electrodes and an intermediate potential point on a respective one of said divider circuits, a capacitive connection between the anode-cathode circuit of each of said pair of discharge devices and the control grid electrode of a respective one of said auxiliary discharge devices, whereby the impedance of the anode-cathode path of said auxiliary device is varied in accordance with changes in the magnitude of the current flowing in the anode-cathode circuit to which its control electrode is connected.

7. A stabilized multivibrator type oscillator comprising a pair of electron discharge devices each including an anode, a cathode, a control grid and a screen grid electrode, anode-cathode circuits including a source of anode potential and control grid cathode circuits therefor, crosscouplings between the anode-cathode circuit of each device and the control grid cathode circuit of the other device, an impedance connection between said source and each of said screen grid electrodes, and means for controlling the potential on each of the screen grid electrodes in accordance with the magnitude of the current flowing in its associated anode-cathode path, said means comprising an auxiliary discharge device respective each screen grid electrode, said auxiliary device including an anode, a cathode and a control grid electrode, a first connection between each of said screen grid electrodes and the anode of the respective auxiliary device, and a second connection between the anode-cathode circuit of each of said first-mentioned devices and the control grid cathode circuit of a respective one of said auxiliary devices.

8. A multivibrator type relaxation oscillator comprising a pair of electron discharge devices each including an anode, a cathode, a control grid and a screen grid electrode, a source of positive potential, means comprising a plurality of resistive connections for supplying positive potential from said source to said anode and said screen grid electrodes, a direct connection from said cathodes to the negative terminal of said potential source, means comprising a pair of resistive connections for supplying a lesser positive potential from said source to each of said control grid electrodes, a pair of auxiliary electron discharge devices each of said devices comprising an anode, a cathode and a control grid electrode, a connection between the screen grid electrode of each of said first-mentioned discharge devices and the anode of a respective one of said auxiliary devices, and a capacitive coupling between the anode of each of said first-mentioned devices and the control grid electrode of the respective connected auxiliary discharge device.

9. A relaxation type of oscillator comprising a pair of electron discharge devices each including an anode, a cathode, a control grid, and a screen grid electrode, a source of positive potential, a resistive connection between each anode electrode and said source, a resistive connection between each screen grid electrode and said source, a capacitive coupling between the anode of each discharge device and the control grid electrode of the conjugate device, conductive connections between each control grid electrode and said source of positive potential, and means for con- REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PATENTS Number Country Date 217,614 Switzerland Feb. 16, 1942 563,794 Great Britain Aug. 30, 1944 OTHER REFERENCES Time Bases by Puckle-line 19 on page 26 through line 6 on page 30December 19, 1947.

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