US2762917A

US2762917A - Frequency stable multivibrators

Info

Publication number: US2762917A
Application number: US372891A
Authority: US
Inventors: Sharin Samuel; Arthur E Canfora; Liguori Anthony; Hajime J Kishi
Original assignee: RCA Corp
Current assignee: MAN AG; RCA Corp
Priority date: 1953-08-07
Filing date: 1953-08-07
Publication date: 1956-09-11
Anticipated expiration: 1973-09-11

Description

sept'. 11, 1956 s. SHARIN ETAL 2,762,917

FREQUENCY STABLE MULTIVIBRATORS Filed Aug. 7, 1955 @HAN/7i J.' A75/W BY @da United States Patent FREQUENCY STABLE MULTIVIBRATORS Samuel Sharin and Arthur E. Canfora, Brooklyn, N. Y., Anthony Liguori, Hackensack, N. J., and Hajime J. Kishi, New York, N. Y., assignors to Radio Corporation of America, a Delaware corporation Application August 7, 1953, Serial No. 372,891

12 Claims. (Cl. Z50- 36) The invention relates to multivibrators, and particularly pertains to means for stabilizing the output frequency of such multivibrators.

It is known that the output frequency of multivibrators depends on such factors as the variation of D. C. anodeto-cathode resistance, and the variation in grid cut-off voltage of individual electron discharge devices of a given type, the change in D. C. anode-to-cathode resistance and the change in grid cut-off voltage of a given electron discharge device due to aging, as well as the time constant of the circuit.

It has been proposed to regulate the operating supply potentials and the clamping of portions of the multivibrator circuit to predetermined fixed potential levels during part of the operating cycle in order to stabilize the output frequency.

An object of the invention is to reduce the effect of the D. C. anode-to-cathede resistance, and the grid cutolf voltage values on the output frequency of a multivibrator and thereby render the output frequcncy`de pendent solely upon the time constant of the circuit.

Another object of the invention is to prevent the replacement of discharge devices of a given type from adversely affecting the frequency of operation of a multivibrator.

A further object of the invention is to minimize or prevent the effects of normal aging of the electron discharge devices used in a multivibrator from affecting the frequency of operation.

Still another object of the invention is to provide a multivibrator affording more stable frequency division by larger numbers than is presently possible with conventional multivibrators.

The objects of the invention are attained by means of a pair of cross-coupled electron discharge devices, arranged so that conduction is alternated between the two discharge devices under the control of other electron discharge structures, preferably having a grid voltageV cutoff characteristic substantially lower than the grid voltage cut-off characteristic of the cross-coupled electron discharge devices, individually coupled between the anode and control electrodes of each of the cross-coupled electron discharge devices.

In order that the invention may be more clearly understood and readily put to practical use, one circuit arrangement embodying the invention is hereinafter described, by way of example only, with reference to the accompanying drawing, in which:

Fig. l is a schematic diagram of a general embodiment of the invention;

Fig. 2 is a graphical representation of the mode of operation of a portion of the circuit shown in Fig. l, useful in explaining the operation thereof.

Referring to Fig. l, there is shown a multivibrator according to the invention having an output frequency which is extremely stable. A pair of electron dischargel devices 11, 12 having cathodes 13, 14,

grids

15, 16 and

anodes

17, 18 respectively have unidirectional operating potenice tials applied thereto from a source (not shown) which is preferably regulated but not necessarily so. The

anodes

17, 18 of the devices 11 and 12 are connected to the positive terminal of the operating potential source by means of

anode resistors

21 and 22, while the

grids

15, 16 are connected to the negative terminal of the operating potential source by means of

grid resistors

25, 26. The cathodes 13, 14 are connected to a source of potential -k which is negative with respect to a point of fixed reference potential or ground which, in turn, is electrically intermediate the positive and negative terminals of the operating potential source. The voltage on the cathode 13, 14 is preferably set for the type of electron discharge device used so that, even in the worst possible case of anode-to-cathode resistance variation due to aging of the discharge device, the potential at the

anodes

17, 18 will tend to go below groundv when the devices 11, 12 conduct.

The electron discharge vdevices 11 and 12 are crosscoupled in conventional manner to form a reciproconductive circuit 30. As employed herein, the term reciproconductive circuit is construed to include all two electrode structure regenerative circuit arrangements in which conduction alternates in one or the other electrode structure. When considering the free running or astable circuit, the term is synonomous with the broad term multivibrato-r, which term is herein limited to the astable reciproconductive circuit. rlfhe monostable reciproconductive circuit (sometimes referred to as a self-restoring trigger circuit), which requires one triggering pulse to switch from the single stable state of conduction to the singleV unstable state andV return, is occasionally referred to as a monostable multivibrator and more often referred to as a trigger circuit, which latter term is sometimes used for a stable reciproconductive circuit. The bistable reciproconductive circuit, which is one which requires two triggering pulses to switch from one stable state to the other stable state and return, is sometimes termed al locking circuit; bistable multivibrator; or, according to use, a binary circuit.

As shown in Fig. 1, the electron discharge devices 11 and 12 are cross connected by means of networks

cornprising resistors

31, 32 and

capacitors

33, 34 to formi a bi-stable reciproconductive circuit, capable of operating in the conventional bistable mod` In many instances the

capacitors

33, 34 maybe omitted without affecting the mode of operation to any practical extent. Diodes' 35 and 36` are connected between the anodes 17,-18 of' the electronv discharge devices 11 and 12 respectively and ground in order to clamp the anodes to ground when the respective electron discharge device is conducting, thus holding the anode voltage swing constant and in'- dependent of the D. C. anode resistance. Triggering of the reciproconductive circuit 30 is accomplished according to the invention by a pair of triggering

electron discharge structures

41 and 42,. each including respective cathodes 4S and 46 at ground potential and control electrodes 51 and 52, coupled to the anodes of the devices 11 and 12 by capacitors C1v and C2, respectively. The grid electrodes 51 and 52 are connected to the positive terrninal of the operating potential sourceby means of resistors R1 and R2, respectively. Anode potential is obtained by means of voltage dividers comprising resistors S3, 55 andV 56, 57 connected between ground' and theI positive terminal of the operating potential source.` The anodes 61, 62 of the triggering structures are vcoupled to the

grids

15, 16 of the controlled electronV discharge devices 11, 12 by means of couplingv capacitors 65 and 66, respectively. According to the invention, the cut-olf voltage' the corresponding electron discharge device 11, 12, and

3 the potentials at the anodes 61, 62 are made low with respect to the potentials at the

anodes

17, 18. The voltage dividers comprising the

resistors

53, 55 and 56, 57 coupled to the anodes 61 and 62 of the

structures

41 and 42 are used to lower the absolute cut-off voltages required for the

triggering structures

41 and 42 by lowering the eiective anode operating supply. The components associated with the reciproconductive circuit in and of itself are chosen so as to obtain triggering and clamping action according to the practiced art.

In explanation of the operation of the circuit of Fig. l, it is assumed that a transition is just occurring such that the reciproconductive circuit device 11 is just starting to conduct, and the device 12 is just starting to block. The voltage at the anode 18 of the controlled device 12 will rise to a value determined by the relative values of the voltage divider comprising the

resistors

22, 32 and 25 and the value of the operating potential supplied according to the formula where E is the operating potential applied between minus and (-1-) plus. This is followed by the complete blocking of the device. The positive swing of the anode 18 of the device 12 is coupled to the grid 52 of the trigger structure 42 which already is conducting. Since the structure 42 is conducting, the coupled positive swing causes no change in the state of conduction of the device 42. At the same time that the voltage at the anode 18 of the device 12 is rising, the voltage at the anode 17 of the device 11 is faling toward a negative value due to the device 11 conducting. The voltage at the anode 17 of the device 11 cannot go to a negative value, due to the clamping action of the diode 35, but is stopped at ground. This limited negative swing of the anode 17 of the device 11 is coupled to the grid 51 of the trigger structure 41, which has been conducting. The trigger structure 41 is thusdriven to cut-oi permitting the anode voltage to rise. Now this stable state of conduction of the device 11 and the structure 42 and the non-conducting state of the device 12 and the structure 41 exists for a time, t1, determined by the values of the resistor R1 and the capacitor C1 after which the capacitor C, associated with the triggering structure 41, is discharged and the grid 51 of the structure 41 rises to the cut-off voltage. The structure 41 then conducts and the anode 61 goes in a negative direction. This negative swing is coupled to the reciproconductive circuit 30 at the grid 15 of the device 11. The reciproconductive circuit 301 changes state so that the device 11 is non-conducting and the device 12 is conducting. The negative swing at the anode of the device 12 drives the triggering structure 42 into cut-oit for a time t2, determined by the values of the resistor R2 and the capacitor C2, after which the capacitor C2 associated with the structure 42 is discharged and the grid 52 of the triggering structure 42 rises to the cut-oi voltage. The triggering structure 42 conducts and the anode 62 goes in a negative direction. This negative swing is coupled to the reciproconductive circuit 30 at the grid 16 of the device 12 and the entire cycle is repeated as described above.

A graphical representation of the waveform obtained at the grid 51 is shown in Fig. 2. The zero voltage axis is represented by the broken line 81; the cutoff voltage level by the dashed line 83; and the positive operating potential level by the dashed line 85. The'heavy line 90 represents the voltage variation of the grid 51 with respect to time. The other grid will exhibit the same voltage variation but displaced in time by the period t1, and the waveform at this grid is therefore omitted. The charging curve 91 of the capacitor C1 follows the usual law and if allowed to charge fully would follow the broken curve 93. The time constant of the capacitor C1 and the resistor R1 determine the time period t1. Like- 4 wise the values of the capacitor C2 and the resistor Rz determine the period tz. These time periods may be made equal by using equal values of resistors and capacitors, or, if an asymmetrical or unequal duty cycle Wave is desired, different values may be used.

The output square wave maybe taken from either anode 61 or 62, but preferably it should be taken from the

anodes

17, 18 as the waveform will be somewhat better. The circuit arrangement as shown in Fig. 1 will not start to oscillate on noise potential, but must be pulsed. The starting pulse may be applied at one of many points; for example, the anodes 61, 62; the grids 51, 52;,the

anodes

17, 18; or the eathodes 13, 14 provided a diode element having a nite back resistance characteristic is interposed in the cathode lead of the tube that is pulsed.

The circuit arrangement according to the invention may vary from that shown in Fig. l. The simplest variation is obtained by omitting the speed up

capacitors

33, 34. The reciproconductive circuit 30 will still be bistable.

The results obtained for low frequency operations willy remain the same, but at the higher frequencies, phase shift problems will be encountered so that the triggering will become less stable.

The reciproconductive circuit 30 can be connected for monostable operation -by omitting the cross-coupling resistor 33 orthe cross-coupling resistor 31. The speed up

capacitor

32 or 34 may be retained for short time period operation, otherwise it may be eliminated'alsoj. The non-conductive tube is then pulsed to start the multivibrator circuit arrangement to oscillating. This circuit arrangement will always be started in the same phase relationship, a feature which is important for many telegraph and counting applications.

A free running multivibrator arrangement, not requiring any starting pulse, is obtained on converting the reciproconductive circuit 30 to the astable form by omitting the

cross-coupling resistors

31, 32. This variation, as in each of the other circuit modiiications, operates in the same manner as the complete circuit arrangement shown in Fig. 1 insofar as the triggering function is concerned. v

Because the monostable reciproconductive circuit requires only one critical timing circuit, the improved results according to the invention may be obtained a monostable circuit, with one-half of the circuit arrangement shown in Fig. 1. For example, the portion of the circuit comprising the tubes 11, 35 and 41 and associated components with the cross-coupling networks 31 33 aud 32-34 disconnected and a resistive element connected between the anode 61 and the grid 15 can be i triggered to the unstable state by application of a triggering pulse to the anode 17, the grid 51, or the anode 61. This circuit will then return to the stable state after a time depending on the values of the resistor R1 and the capacitor C1. The RC circuit in this instance follows the same law as in the conventional reciproconductive circuits..

The voltage swings and cut-off characteristics are `Clifferent, of course, to provide the improved results.

The circuit arrangement according to Fig. 1 can be used as a frequency divider which exhibits improved'performance. Positive pulses applied to the grids 51 or 52 at given recurrence rate will produce square waves of sub-multiples of that rate at the -

anodes

17, 18 of the controlled tubes 11, 12. If an output pulse train is re-4 quired, conventional dilerentiatingand shaping circuitry can be used in known manner.V

The frequency of operation of Fig. 1 is about 600 C. P. S. for R1=R2=1 megohm and C1=C2=ll65 micro-micro-farads. It should be noted that the values for R1 and C1, associated with one-half of the circuit, need not be equal to the values for R2 and C2 for the other half, unless a symmetrical wave shape is desired. Any pulse rate with any pulse interval is possible'within the range of such multivibrators as practiced in the art.A The-following values of other components were used inv a circuit in accordance with the arrangement of Fig. 1 as operated and tested at 600 C. P. S.

Resistors Ref. No.:

Value

21, 22 kilohms-- 75 25, 26 megohms 2.0 31, 32 do 2.7 53, 55, 56, 57 kilohms 100 Capacitors Ref. No.: Value 65, 66 0.001 pid.

Speedup capacitors

33, 34 were not used.

Tubes Ref. No.: Type 11, 12 5963 35, 36

6AL5

41, 42 12AX7 Positive operating potential of 150 volts regulated was applied between ground and those points of the circuit marked by the plus sign and negative operating potential of 150 volts was applied between ground and those points marked by the minus sign, with the cathodes 13 and 14 fixed at a value of 371/2 volts with respect to ground.

The invention claimed is:

l. A monostable multivibrator circuit including an electron discharge device having at least cathode, grid and anode electrodes, and having a given cut-off gridvoltage characteristic, an anode resistor and a grid resistor coupled in series with the individual grid-anode path of said electron discharge device, means to apply positive operating potential between said anode resistor and a point of iixed reference potential, means to apply negative operating potential between said grid resistor and said point of iixed potential, means to apply a potential intermediate the fixed and negative potentials to the cathode of said electron discharge device, an electron discharge structure having a cathode, a grid and an anode and having a cut-oft' characteristic grid voltage substantially less than that of said electron discharge device, means to connect the cathode to said point of fixed reference potential, an anode resistor connecting the anode of said electron discharge structure to said means to apply positive operating potential and to said point of fixed reference potential, means coupling the anode of said electron discharge structure to the grid electrode of said electron discharge device, and a return time determining network coupling the anode electrode of said electron discharge device to the grid of said electron discharge structure, said network comprising a capacitor connected between the coupled anode electrode and grid and a resistor connected between that grid and said means to apply positive operating potential.

2. A frequency stable multivibrator circuit including a pair of electron discharge devices each having at least cathode, grid and anode electrodes, anode resistors and grid resistors coupled in series with the individual gridanode paths of said electron discharge devices, means to apply positive operating potential between said anode resistors and a point of fixed reference potential, means to apply negative operating potential between said grid resistors and said point of fixed potential, means to yapply a potential intermediate said fixed and negative potentials to the cathodes of said electron discharge devices, networks cross-coupling the anode electrode of one of said electron discharge devices with the grid electrode of the other, said networks comprising a resistive and a capacitive element, electron discharge structures each having a cathode, a grid and an anode, means to connect the cathodes to said point of xed reference potential, anode resistors individually connecting the .anodes of said electron discharge structures to said means to apply positive operating potential and to said point of fixed reference potential, capacitors individually coupling the anodes of said electron discharge structures to the grid electrodes of Said electron discharge devices, and frequency determining networks individually coupling the anode electrodes of said electron discharge devices to the grids of said electron discharge structures, said networks each comprising a capacitor connected between the coupled anode electrode and grid and a resistor connected between that grid and means to apply positive operating potential.

3. A frequency stable multivibrator circuit including a pair of electron discharge devices each having at least cathode, grid and anode electrodes, and having a given cut-oli grid-voltage characteristic, anode resistors and grid resistors coupled in series with the individual gridanode paths of said electron discharge devices, means to apply positive operating potential between said anode resistors and a point of iixed reference potential, means to apply negative operative potential between said grid resistors and said point of fixed potential, means to apply a potential intermediate said xed and negative potentials to the cathodes of said electron discharge devices, networks cross-coupling the anode electrode of one of said electron discharge devices with the grid electrode of the other, said networks each comprising a resistor and a capacitor connected in parallel, diode elements individually connected between the anode electrodes and said point of fixed reference potential, electron discharge structures each having a cathode, a grid and an anode, and having a cut-oli characteristic -grid voltage substantially less than that of said electron discharge devices, means to connect the cathodes to said point of fixed reference potential, anode resistors individually connecting the anodes of said electron discharge structures to said means to apply positive operating potential and to said point of fixed reference potential, capacitors individually coupling the anodes of said electron discharge structures to the grid electrodes of said electron discharge devices, and frequency determining networks individually coupling the anode electrodes of said electron discharge devices to the grids of said electron discharge structures, said networks each comprising a capacitor connected between the coupled anode electrode and grid and a resistor connected between that grid and said means to apply positive operating potential.

4. A frequency stable multivibrator system comprising a pair of electron discharge devices each having only cathode, grid, and anode electrodes, means to apply operating potentials to said anode and grid electrodes, crosscoupling networks coupling the anode electrode of each of said devices to the grid electrode of the other to form a bistable reciproconductive circuit, a pair of triggering electron discharge structures each having a cathode, a grid, and an anode, connections from the last-mentioned cathodes to a point of fixed reference potential, connections individually coupling the anodes of said discharge structures to said means, means coupling the anode of each of said discharge structures to the grid electrode of a respective discharge device, and frequency determining networks coupling the anode electrode of each of said discharge devices to the grid of a respective discharge structure and operating to determine the intervals during which said circuit is in each of its two stable states, said last-named means and said networks constituting crosscoupling means, each of said frequency determining networks comprising a series capacitor and a shunt resistor, connected between the anode electrode of one of said discharge devices and the grid of the discharge structure coupled thereto,

5. A frequency stable multivibrator system comprising a pair of electron discharge devices each having only cathode, grid, and anode electrodes, means to apply operating potentials to said anode and grid electrodes, cross-coupling networks coupling the anode electrode 0f each of said devices to the grid electrode of the other to form a bistable'reciproconductive circuit, a pair of triggering electron discharge structures each having a cathode, a grid, and an anode, connections from the lastmentioned cathodes to a point of fixed reference potential', connections individually coupling the anodes of said discharge structures to said means, the arrangement being such that each of said triggering structures has a cutoff grid voltage substantially less than that of said discharge devices, means coupling the anode of each of said discharge structures to the grid electrode of a respective discharge device, and frequency determining networks coupling the anode electrode of each of said discharge devices to the grid of a respective discharge structure and operating to determine the intervals during which said circuit is in each of its two stable states, said lastnamed means and said networks constituting crosscoupling means, each of said frequency determining networks comprising a series capacitor and a shunt resistor, connected between the anode electrode of one of said discharge devices and the grid of the discharge structure coupled thereto.

6. A frequency stable multivibrator system comprising a pair of electron discharge devices each having only cathode, grid, and anode electrodes, means to apply operating potentials to said anode and grid electrodes, crosscoupling networks coupling the anode electrode of each of said devices to the grid electrode of the other to form a bistable reciproconductive circuit, each of said networks comprising a resistor; a pair of triggering electron discharge structures each having a cathode, a grid, and an anode, connections from the last-mentioned cathodes to a point of ixed reference potential, connections individually coupling the anodes of said discharge structures to said means, the arrangement being such that each of said triggering structures has a cutoff grid voltage substantially less than that of said discharge devices, means coupling the anode of each of said discharge structures to the grid electrode of a respective discharge device, and frequency determining networks coupling the anode electrode of each of said discharge devices to the grid of a respective discharge structure and operating to determine the intervals during which said circuit is in each of its two stable states, said last-named means and said networks constituting cross-coupling means, each of said frequency determining networks comprising a series capacitor and a shunt resistor, connected between the anode electrode of one of said discharge devices and the grid of the discharge structure coupled thereto.

7. A frequency stable multivibrator system comprising a pair of electron discharge devices each having only cathode, grid, and anode electrodes, means to apply operating potentials to said anode and grid electrodes, crosscoupling networks coupling the anode electrode of each of said devices to the grid electrode of the other to form a bistable reciproconductive circuit, one of said networks comprising a resistor and the other comprising a capacitor; a pair of triggering electron discharge structures each having a cathode, a grid, and an anode, connections from the last-mentioned cathodes to a point of fixed reference potential, connections individually coupling the anodes of said discharge structures to said means, the arrangement being such that each of said triggering structures has a cutoff grid voltage substantially less than that of said discharge devices, means coupling the anode of each of said discharge structures to the grid electrode of a respective discharge device, and frequency determining networks coupling the anode electrode of each of said discharge devices to the grid of a respective discharge structure and operating to determine the intervals during which said circuit is in each of its two stable states, said last-named means and said networks constituting cross-coupling means, each of said frequency determining networkscomprising a series capacitor and a shunt resistor, connected between theanode electrode of one of said discharge devices and the grid of the discharge structure coupled thereto.

8. A frequency stable multivibrator system'comprising a pair of electron discharge devices each having only cathode, grid, and anode electrodes, means to apply operating potentials to said anode and grid electrodes, cross-coupling networks coupling the anode electrode of each of said devices to the grid electrode of the other to forni a bistable reciproconductive circuit, each of said networks. comprising a capacitor; a pair of triggering electron discharge structures each having a cathode, a grid, and an anode, connections from the last-mentioned cathodes to a point of xed reference potential, connections individually coupling the anodes of said discharge structures to said means, the arrangement being such that each of said triggering structures has a cutoff grid Voltage substantially less than that of said discharge devices, means coupling the anode of each of said discharge structures to the grid electrode of a respective discharge device, and frequency determining networks coupling the anode electrode of each of said discharge devices to the grid of a respective discharge structure and operating to determine the intervals during which said circuit is in each of its two stable states, said last-named means and said networks constituting cross-coupling means, each of said frequency determining networks comprising a series capacitor and a shunt resistor, connected between the anode electrode of one of said discharge devices and the grid of the discharge structure coupled thereto.

9. A frequency stable multivibrator system comprising a pair of electron discharge devices each having only cathode, grid, and anode electrodes, means to apply operating potentials to said anode and grid electrodes, crosscoupling networks coupling the anode electrode of each of said devices to the grid electrode of the other to form a bistable reciproconductive circuit, one of said networks comprising a capacitor and a resistor connected in parallel and the other comprising a capacitor; a pair of triggering electron discharge structures each having a cathode, a grid, and an anode, connections from the last-mentioned cathodes to a point of lixed reference potential, connections individually coupling the anodes `of said discharge structures to said means, the arrangement being such that each of said triggering structures has a cutot grid voltage substantially less than that of said discharge devices, means coupling the anode of each of said discharge structures to the. grid electrode of a respective discharge device, and frequency determining networks coupling the anode electrode of each of said discharge devices to the grid of a respective discharge structure and operating to determine the intervals during which said circuit is in each of its two stable states, said last-named means and said networks constituting cross-coupling means, each of said frequency determining networks comprising a series of capacitor and a shunt resistor, connected between the anode electrode of one of said discharge devices and the grid of the discharge structure coupled thereto.

l0. A frequency stable multivibrator system comprising a pair of cross-coupled electron discharge devices each having only cathode, grid, and anode electrodes, said devices forming a bistable reciproconductive circuit, means to apply operating potentials to said anode and grid electrodes, a pair of triggering electron discharge structures each having a cathode, a grid, and an anode,

individually cross-coupled to Vsaid discharge devices, connections from the last-mentioned cathodes to a point of fixed reference potential, connections individually coupling the anodes of said discharge structures to said means, and frequency determining networks interposed in said individual cross-coupling connections and operating to determine the intervals during which said circuit is in each of its two stable states, each of said networks 9 comprising a series capacitor and a shunt resistor connected between the anode electrode of one of said discharge devices and the grid of the associated discharge structure.

11. A frequency stable multivibrator circuit comprising a pair of cross-coupled electron discharge devices each having only cathode, grid, and anode electrodes and having a given cutoC grid voltage, means to apply operating potentials to said anode and grid electrodes, a pair of triggering electron discharge structures each having a cathode, a grid, and an anode, individually cross-coupled to said discharge devices, connections from the lastmentioned cathodes to a point of fixed reference potential, connections individually coupling the anodes of said discharge structures to said means, the arrangement being such that each of said discharge structures has a cuto grid voltage substantially less than that of said discharge devices, and frequency determining networks interposed in said individual cross-coupling connections, each of said networks comprising a series capacitor and a shunt resistor connected between the anode electrode of one of said discharge devices and the grid of the associated discharge structure.

12. A frequency stable multivibrator circuit comprising a pair of electron discharge devices each having only cathode, grid, and anode electrodes, means to apply operating potentials to said anode and grid electrodes, cross coupling networks coupling the anode electrode of each of said devices to the grid electrode of the other, said networks comprising resistive and capacitive elements; a pair of electron discharge structures each having a cathode, a grid, and an anode, connections from the lastmentioned cathodes to a point of xed reference potential, connections individually coupling the anodes of said discharge structures to said means, capacitors coupling the anode of each of said discharge structures to the grid electrode of a respective discharge device, and frequency determining networks coupling the anode electrode of each of said discharge devices to the grid of a respective discharge structure, said capacitors and said networks constituting cross-coupling means, each of said frequency determining networks comprising a capacitor connected between the coupled anode electrode and grid and a resistor connected between that grid and said first-named means.

References Cited in the le of this patent UNITED STATES PATENTS