US2640173A - Suppression of spurious oscillations - Google Patents

Description

y 1953 R. 'r. CAVANAGH 2,640,173

SUPPRESSION OF SPURIOUS OSCILLATIONS Original Filed Feb. 21, 1948 2 Sheets-Sheet 1 HORIZONTAL I 'QTQQ' DEFLECTION GENERATOR l SEPARATOR AND L SELECTIVE DETECTOR SAW MAKER AMPLIFIERS VERTGAL I l DEFLEGTION QSEE' GENERATOR I I AND I AMPLlFlER L J j f N m I5 I\ I 6:

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7:3? Robert T. Cavanagh BY HHI ATTORNEYS May 26, 1953 R. T. CAVANAGH SUPPRESSION OF SPURIOUS OSCILLATIONS Original Filed Feb. 21, 194

2 Sheets-Sheet 2 INVENTOR. Robert T. Cavanagh A TTOR/VE Y3 Patented May 26, 1953 SUPPRESSION OF SPURIOUS OSCILLATIONS Robert T. Cavanagh, Montclair, N. J assignor to Allen B. Du Mont Laboratories, Inc., Passaic, N. J., a corporation of Delaware Continuation of application Serial No. 9,975, February 21, 1948.

1949, Serial No. 75,148

11 Claims.

This application is a continuation of application Serial No. 9,975, filed February 21, 1948.

This invention relates to the prevention or reduction of spurious high-frequency oscillations in radio and electrical circuits, and particularly to the prevention of undesired high-frequency internal oscillations in electron discharge devices.

In radio and electrical circuits employing electron discharge devices, there occur in many cases undesired high frequency oscillations. Many of these oscillations occur within the electron discharge devices due to the electron flow between the elements of the device. While these oscillations may occur at a frequency beyond the audible range and thus go unnoticed in many electrical and radio circuits, they have been a considerable problem in the television industry where video and radio frequencies are involved. The parasitic oscillations have been found to disturb the television picture on the screen of a cathode ray tube. For instance, one of the problems has been a vertical black streak which appeared on the face of the cathode ray tube when television signals were received in the higher television transmission bands. when the received strong.

Accordingly, it is an object of this invention to prevent or reduce the parasitic oscillations. It is a further object to reduce or suppress parasitic oscillations which occur internally in an electron discharge device.

Other objects will be apparent after a study of the following description, claims, and drawing, in which:

Figure 1 is a perspective view of the internal structure of a conventional electron discharge device of the type known as a beam power tube, parts being broken away, together with part of a deflection circuit and a magnetic field in accordance with an embodiment of this invention, as incorporated in a television receiver;

Figure 2 is a side elevational view of an electron discharge device with a diagrammatical sectional view of a current carrying coil coaxially around the device;

Figure 3 is a side elevational view of another embodiment of the invention, illustrating the electron discharge device with magnetic poles disposed adjacent thereto;

Figure 4 is a side elevational view of an electron discharge device with a single bar magnet disposed adjacent thereto, illustrating another embodiment;

signal was relatively not This was especially true- This application February 8,

Figure 5 is a diagrammatical View of the elements of an electron discharge device of the beam power tube type in which the elements of the tube itself are magnetized; and

Figure 6 is a perspective diagrammatical view of the elements of an electron discharge device illustrating a modification in which small bar magnets are attached to an element.

It has been found that the parasitic oscillations causing disturbance on the screen of the cathode ray tube of a television receiver, are produced primarily within the electron discharge device or tube used in the circuit in the receiver generating the horizontal line scanning frequency for the cathode ray tube.

In accordance with this invention, a magnetic field is produced in the interelectrode space within the tube. This magnetic field causes the electrons to follow a curved path within the tube.

In Figure 1 are shown the elements of such an electron tube T together with a source of magnetic energy and a portion of the associated circuit. This tube T is preferably a television beam deflection tube such as the 6BG6-G. Within it are an electron emitting cathode l of the usual type, a control grid 2, a screen grid 3, a beam forming electrode 4 and a plate or anode 5. Shown external to the tube is a magnet 6 whose field indicated at 1 extends into the interelectrode space within the tube.

The cathode I of the tube 'may be grounded. The control electrode 2 may be connected through a suitable self bias source 8 to a source of si nal having a wave form 9 as shown. The screen grid 3 is connected to a source of voltage ID positive with respect to the cathode. The beam forming plate 4 and another one similar to it which has been cut away are connected to the cathode. The plate 5 of the tube is connected to the source of positive voltage l0 through the primary of a transformer l8, the secondary of which is usually connected to a horizontal deflection yoke 50 for a cathode ray tube 5| and some form of damping resistance well known to the art, such as, for instance, a damper tube. This non-linear load, in conjunction with the input voltage 9 produces a plate voltage I9 of the form shown. The horizontal deflection amplifier tube T is connected in the television receiver circuits 52 in the usual manner, well understood by those skilled in the art.

In the usual method of operation, the grid voltage wave 9 contains a sharp negative pulse 20. This pulse 20 renders the tube momentarily non-conducting. The plate current of the tube,

flowing through the primary of the transformer l8, produces a sudden sharp positive peak 2] in the plate voltage wave form [9. Then, in resonance with stray capacitances, the plate voltage l9 swings negative as indicated at 22, producing a voltage momentarily several hundred volts negative with respect to the cathode.

It is during this short negative excursion of the plate that, in the absence of a magnetic field, the troublesome oscillations occur. These oscillations are of the type lmown as Bark'hau'sen-Kurtzor positive grid oscillations such as are described in Sarbacher and Eidson, Hy-per and Ultra- 'l-ligh Frequency Engineering, Chapter 15, John Wiley and Sons, Inc., New York, 1943. In the case of television receivers the screen grid -isthe positive grid and the plate is only negative for 'a short instant although this short instant is sufficiently long in some instances to allow several hundred oscillatory cycles of the very high and ultra high frequencies normally produced.

The normal operation of the tube produces these oscillations as follows. Electrons emitted from the cathode pass through the control grid wires under the attraction of the positively charged screen grid. Being repelled by the negatively charged control grid wires, the electrons are roughly focussed into fiat horizontal sheets 23 and in this form pass through the screen grid Wires which in this type of tube are carefully aligned behind the control grid wires to keep the screen current of the tube low. The stream of electrons is focussed somewhat in the horizontal direction by the control grid support posts and the beam forming plates 4. Within the electron beam the potential is negative, due to the concentration of electron charge, and this acts somewhat equivalent to asuppressor grid, preventing secondary electrons emitted from the plate from going to the screen grid.

This is the operation of the tube. as anamplifier with current flowing andwith the plate at a positive potential. With the plate negative, even momentarily, a new conditionarises, causing, in the absence of a magnetic field, a strong oscillation to take plate. The causeof this oscillation is believed to be as described, below.

With the plate negative,a zero potential surface occurs between plate and screen. Electrons, emitted from the cathode, form roughly into a beam, pass through the screen grid andhead toward the plate, slowing down until they stop at a point fhinear the zero potential surface (the exact point depends upon the initial velocity of the particular electron whenit was emitted from the cathode). The electrons then reverse and swing back through the space between screen grid wires. If for any reason the electrons have lost energy, they will stop shortv of the cathode, reverse again and head back toward the plate. It is possible under conditions found in practice for a single electron to execute several hundred reversals before it drifts sidewise out ofthe beam and collides with some portion of the positively charged screen grid.

Thus far no mechanism for sustained-oscillations has been described, each electron being considered as performing anumber of reversals independent of other electrons. This is actually not-the case, for several electrons swinging-back and forth through the screen grid cause a moving electrostatic field, which in turn causes other electrons to fall in step-and build up sustained oscillations of considerable energy, completely independent ofexternalcircuit conditions. The

mechanism for the building up of oscillations is believed to be as follows:

If a certain electron happens to be emitted when the majority of electrons are on the cathode side of the screen grid, it will be accelerated less than it would be otherwise due to the negative charge ahead of it. While the electron is approaching the screen grid, the potential within the beam is falling, or becoming more negative, due to the passing through of the majority of the electrons. lhe electron approaches point 24 near th zero potential plane where the potential within the beam is lower, due to the motion of the other electrons. The electron in question therefore stops short of the point of zero potential. On the return trip of the electron toward the cathode, energy is again reduced due to a falling off of potential caused by the motion of the majority of electrons. Because of the presence of the other electrons, therefore, this electron which is in phase with the others continues to stay in phase with the majority, thereby add ing to the density of the in-phase swarm.

Let us consider another electron emitted from the cathode out of phase with the majority of oscillating electrons. It will be accelerated while travelling toward the plate and after reversing will be accelerated again toward the cathode, Since it has gained energy on the round trip, it will be thrown into the cathode and will exert no further effect to counteract the fieldof the ma,- jority of the electrons. Thus, electrons starting out of phase with the majority, tend to be discarded from the swarm.

In order for actual oscillation to occur it is of course necessary that the above mechanism drawing new in-phase electrons into the swarm operate faster than the loss of other in-phase electrons to the screen "grid. When this condition holds, oscillations will build up to an equilibrium density of in-phase electrons.

The frequency of oscillations has been found to be high and very diverse, with difierent parts of the tube often oscillating at different frequencies. Oscillations between 60 and 1000 megacycles per second have been observed, with a great diversity between tubes and between different adjustments of the deflection circuits.

The type of oscillations described above require noexternal tuned circuit for oscillation and is of thetype described by Barkhausen and Kurtz in 1920. By attaching a tuned circuit, the frequency of oscillation can be controlledover certain limits. This is the phenomenondescribed by Gill and Morrill in 1922 Reference to these papers is made by Sarbacher and E'dson already cited.

At the high frequencies of these oscillations, it is virtually impossible to prevent the oscillations that occur from radiating. After radiation the bursts of energy can be picked up in the input circuits of the receiver or the antenna itself. From there the bursts of energy are amplified through regular channels producing the black line in the picture already described, and-other undesirable effects.

The operation of the magnetic field in preventing-these oscillations is believed'to, be as follows:

In the absence of the magnetic held the inphase electrons that perform the most reversals are to be found in the center of thebeam. Here vertical repelling forces of the. gridv wires are balanced, the vertical attracting forces of the screen a e an ed. andin thehorizontal plane the repelling forces of the beam forming plates are balanced. Along the center line, withv all these sidewise forces exactly balanced, many reversals of a single electron can occur. If an electron gets ofi the center line for any reason, the forces are no longer balanced, and the electron is quickly drawn into some part of the screen grid which at this instant is the only positively charged element within the tube. The efiect of a magnetic field is to bend moving electrons, the smallest radius of bending being the slow moving electrons. Therefore at the point 24 where the electrons in the center of the beam reverse, they are diverted by a magnetic field so that they do not return along the same path where the electrostatic forces are balanced, but along another path where they are quickly withdrawn from any tendency to reverse. Thus a magnetic field can prevent the iii-phase swarm from building up at the critical rate necessary for oscillation to occur.

The direction of magnetic field that is most effective is at right angles to the direction of travel of the electrons. In the case of the beam power tube shown the most effective field is parallel to the central axis of the tube which axis runs through the center of the cathode in a direction parallel to it. The direction is also not critical, since oscillations will be prevented by a field at almost any angle, but in the case of directions other than the preferred one a stronger field is required.

Such a magnetic field may be produced by a current carrying wire wound coil I3, as illustrated in Figure 2. As may be seen, the coil I3 is located coaxially about the electron discharge device I2 so that the magnetic field produced thereby is along the longitudinal axis of the tube I2. It may be in a direction indicated by the arrow I4, or it may be in the reverse direction. The lines of magnetic fiux established between the tube elements suppress the spurious high frequency oscillations. The current flowing through the coil may be either direct or alternating, provided in the case of alternating current, that current flows during the time when the plate is negative.

In Figure 3 a modification is illustrated. In accordance with this embodiment, a permanent bar or electromagnet I6 may be disposed adjacent the dome of the electron discharge device I2. As illustrated, two poles of the bar magnet I6 are positioned, one near the top and the other near thebottom of the tube I2. In this way the magnetic field is again set up in a direction normal to the electron flow, which is normal to the longitudinal axis of the tube and the cathode.

While both poles are illustrated in Figure 3 as being adjacent the tube I2, it has been found that a single bar magnet may be placed in a similar position as one of the poles illustrated with equally good results. This is illustrated in Figure 4. The magnetic field is established between the poles of the bar magnet along the longitudinal axis of the cathode.

In the usual electron discharge device, the various elements are arranged coaxially. This is shown diagrammatically in both Figure 1 and Figure 6. The control grid 2 is coaxial about the cathode I, the screen grid 3 is coaxial about the control grid 2, and the beam forming plates 4 and the anode 5 positioned in coaxial relationship thereto. Thus, the' longitudinal axis of the cathode is the longitudinal axis of the electron discharge device or tube I2, and this axis is normal to the direction of the electron stream.

While the magnetic field may be produced by an external magnet, such as the coi1 I3, placed coaxially about the tube, as illustrated in Figure 2, or the bar magnet positioned adjacent the dome of the tube I2, as illustrated in Figure 4, the magnetic field may be produced by magnetized internal elements of the tube itself. As shown in Figure 5, the beam forming plates 4' may be made of magnetic material permanently magnetized and having a polarity as indicated by N and S. These magnetic beam forming plates will produce a magnetic field substantially parallel to the longitudinal axis of the cathode I.

Similarly, in accordance with another modification, the anode 5" may be permanently magnetized by small permanent bar magnets I1 attached thereto. These bar magnets I! are aligned to produce a magnetic field parallel to the axis of the cathode I. This is shown diagrammatically in Figure 6.

The strength of the magnetic field is not critical. Good results have been obtained by placing a bar magnet of the Alnico type adjacent the dome of the tube in the position shown in Figure 4. If the axis of the bar magnet is normal to the longitudinal axis of the cathode, it has been found that better results are obtained since this will establish a stronger magnetic field parallel to the axis of the cathode. Such positioning is not critical, however, since the position of the magnet adjacent to the tube will only vary the strength of the magnetic field. This is also true of the coaxial coil illustrated in Figure 2. As an example, with a 6BG6-G tube, 200 ampere turns were sufficient to eliminate the spurious oscillations.

The expedients described have been found to be satisfactory in suppressing the undesired or parasitic oscillations caused by internal or transit-time electron stream, the oscillations taking place between the electrodes of an electron discharge device. A small permanent magnet may be positioned adjacent or near the dome of the electron discharg device used in generating horizontal sweep or scanning lines of a cathode ray tube for a television receiver. The objectionable parasitic oscillations are suppressed quickly and simply, and at a comparatively low cost.

In some instances it may be possible to use the magnetic field created by the coils used with the cathode ray tube, such as for instance, the focussing coil. Thus the horizontal deflection amplifier tube may be positioned close to this coil in a television receiver and thus receive the benefits therefrom.

While preferred embodiments of the invention have been illustrated and described, it will be apparent that further modifications are possible :vithout departing from the scope of the invenion.

What is claimed is:

1. Apparatus of the class described comprising an electron discharge device, said device having a cathode emitting electron, an anode connected to a source of operating potential through a circuit containing inductance, a first grid connected to a source of operating potential and positive with respect to said cathode between said cathode and said anode, a second grid, a source of periodic signals connected between said cathode and said second grid and polarized to interrupt the flow of electrons to said anode whereupon said anode is rendered temporarily negative in polarity with respect to said first grid by means of a self-induced voltage in said inductance, and a magnet 7 adjacent said device creating a magnetic field therewithin suppressing interelectrode oscillations of said electrons.

2. An-electron dischargedevice having a oath-,- ode, an electrode negative with respect to said cathode, a grid positive with respect to said cathe ode between said cathode andsaid electrode, and

a magnetic field in the interelectrode space of saiddischarge device, said magnetic field derived from a coil external to said discharge device.

3. An electrondischarge device having a cath-. ode, an electrode negative with respect to said cathode, a grid positive with respect to said cathe ode between said cathodeand said electrode, and a magnetic field in the interelectrode spaceof said discharge devicesaid magnetic field derived from a U-shaped magnet the magnetic poles of which are positioned respectively at each end of saiddischarge device.

4. An electron discharge device having a cathode, an electrode negative with respect tosaid cathode, a grid positive with respect to said cathodebetween said cathode and said electrode,,and a, magnetic field in the interelectrode space of said discharge device, said magnetic field derived from a magnetized element with n aid di char e device.

5. The apparatus of the class described comprising anelectron discharge device having ele ments comprising a cathode and grid and electrode, sources of operating potentials connected thereto, said electrode being connected to its source of operating potential through a circuit containing inductance, said grid being positive in potential with respect to said cathodeand being positioned between said cathode and said electrode, a source of signals connected between two of said elements and polarized to cut off said tube momentarily, whereupon said inductance generates a momentarily negative voltage on said electrode, anda magnet disposed adjacent said device and-positioned to provide a magnetic field in the vicinity of said positive grid.

6., An electron deflection device comprising an electron tube having a cathode, a control grid, an anode, and a screen grid between said cathode and said anode, a source of periodic signals con nected between said cathode and said control grid and polarized to cut ofi momentarily the electron flow in said tube, a deflection yoke con:- nected in circuit between said anode and said cathode, said yoke generating periodic momentarily negative voltages on said anode in response to said periodic signals, a source of positive voltage connected between said screen grid and-said cathode, and a magnet positioned to provide a magnetic field .in the vicinity of said screengrid.

'7. An electronic device comprising a thermionic tube having a cathode, an anode, and an accelerating grid therebetween, a voltage source connected to render said grid positive in polarity with respect to said cathode, a source of periodic negatively polarized voltage pulses connected to said anode, a path of electrons between said oath! 8: ode and said anode, and a magnet positioned to provide/a magnetic'fieldbetween said cathode and anode, 'said field being substantially perpendice ular to said pathof' electrons.

8. Apparatus containing a thermionic tube havingan electron-emitting cathode, an anode, and a grid therebetween, a source of positive voltage connected to said grid to attract said electrons'to. and through said grid, a source of negative signals connected to said anode to cause said anodetotrepel said electrons. and cause them to reverse their direction, and .a magnet positionedto provide a magnetic field between said anode and cathode. "to-divert said reversed electrons'from their original paths.

'9. In: a circuit for electrical pulses, a thermionic tube containinga cathode, a control grid, a screen grid, and-an anode; .a, source'of pulse signals connected to said control grid, sources of p itive voltage connected to said screen grid .and aid n de, an inductance connected to said an de and prod cin a ne ative. pulse at said anode iii-response tovsaid pulse i nals connected to said c ntrolrid said negative pulse bein of sufiicientunagnitude to bias said anode ne ative withv resp c tosaid screen rid and tendin to reate nar i a senroscillat ons, an a ma n t positi ned t pr v de a ma n tic field between said node and sa dca h-od t ivert said el ctrons. and pr ent said Bar rhausen. os illat ons.

10. The apparatus according to claim 9 in wh said ma ne is a sin l bar-ma n t p sitionedwith n pole' her f: ad ac n said disha g de ce.

1. lec rond scharee device ha in anathode v an e trod and a ridmsitioned etween sa d cathode and said electrode, our es f e ecr cal potential, c nec ed res e t vely o sa ath d sa d gr drend a d el r e sai id er by eing end d p siti p lar t wit esp e aid cath d and sa d-e ectro e the ebvbe e nde ed n ga v in polarity w r h et tosai cath de, and a m gn t ositione to r t a magnet c field h n a d d schar evic betwe aidcathode and electro T- A A AG References Cited'in the file of this patent France pee-syn, Jan. 5., 1931

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