US2180364A - Cathode ray sweep circuits - Google Patents

Description

Nov. 21, 1939.

SOURCE 0F SYN. SIGNALS FLA TE CURRENT F. R. NORTON 2,189,364

CATHODE RAY SWEEP CIRCUITS Filed Feb. 25. less PLA TE VOL T4 65 TIME /N 5 N TOR ERNORTON ATTORNEY Patented Nov. 21, 1939 S "EES Bell Telephone Laboratories, Incorporated,

New York, N. Y., a corporation of New York Application February 25, 1936, Serial No. 65,607

13 Claims.

This application relates to cathode ray devices cuits suitable for television scanning and for genand more specifically to cathode ray sweep cireral oscillographic purposes.

In order to produce proper linear scanning in cathode ray tubes, it is necesary to have sweep circuits capable of generating saw-tooth wave forms. To produce wave forms of this general shape, circuits in the prior art have been devised which charge up a condenser through a resistance or through a saturated diode, the condenser being discharged at suitable intervals by a suitable gasfilled electron discharge device such as a Thyratron tube actuated by received synchronizing signals or by an oscillator. The wave forms produced by these circuits have deviations from the exact saw-tooth wave form desired because of the fact that neither the resistance nor the diode at saturation produces a constant plate current with changes in plate voltage.

It is an object of this invention to provide improved television scanning means.

It is another object of this invention to provide means for producing an improved sawtooth wave form adapted for use in a sweep circuit.

t is a further object of this invention to provide improved means for maintaining the current in a circuit substantially constant under varying impressed voltages.

In carrying these objects into effect, there is preferably employed a pentode tube in series with a battery to charge up a condenser or condensers in a sweep circuit, these being discharged by the periodic discharge of a 'Ihyratron tube actuated by synchronizing signals. Preferably, the pentode tube is connected so that the current in the output circuit of the tube controls the bias on either the control grid, the screen grid, the suppressor grid, or any combination of these three. It has been found that the pentode gives a flatter plate current versus plate voltage characteristic than a triode and for this reason is preferred. A triode or a tetrode with a feedback connection to a grid or grids may be used but it has been found that the pentode type of tube with feedback on one or more of its grids is particularly effective. If the feedback is to be to a single grid, the control grid is preferred. Somewhat better results are obtained by including also the screen grid or suppressor grid, or both, in the feedback connection. By means of this arrangement of connections, as the plate voltage decreases, the bias on one or more of the grids is 55 reduced and hence the current tends to remain constant. With this feedback hook-up, the pentode tube tends to give a very stable output current regardless of voltage changes. The resultant sweep circuit voltage is thus of the desired saw-tooth wave form to produce correct linear scanning.

The invention will be more readily understood from the following description taken in connection with the accompanying drawing forming a part thereof in which:

Fig. 1 shows a sweep circuit for producing sawtooth wave forms;

Figs. 2 and 3 show modifications of the hook-up for the pentode tube shown in Fig. 1; and

Figs. 4 and 5 are graphic representations included to better explain the operation of this invention.

Referring more particularly to the drawing, Fig. 1 shows a sweep circuit for a cathode ray oscillograph tube in which a pentode tube it is used. The circuit comprises a means such as battery II for charging condensers l2 and it through the pentode tube it, and a Thyratron tube M for discharging these condensers at predetermined periodic intervals, these discharges being actuated by synchronizing signals. A suitable source of synchronizing signals, represented generally by the box IE, may comprise, for example, a receiver for receiving these signals from a transmitting station or it may be a local oscil-v lator at the station where the sweep circuits are located. The source 15 is coupled by means of a transformer l6 and resistances I1 and It to the grid I9 of the Thyratron tube I4. The grid is normally negatively biased by the battery 20 located between the cathode 2i and the grid IS of the 'Ihyratron tube. In the plate circuit of the Thyratron tube I4 is a resistance 22 and one set of sweep plates 23 and 24 of a cathode ray device.

The voltage applied across the sweep plates 23 and 24 is determined by the charging and discharging of the condensers l2 and i3 and the voltage wave form follows the general form of the full line wave shape 50 shown in Fig. 4. The condensers l2 and I3 may be, if desired, replaced by a single condenser. In the preferred embodiment of this invention, however, two condensers in series are used with their common terminal connected to a point of fixed potential in order to produce a balanced sweep circuit. The potentials generated by the sweep circuit maybe balanced with respect to the potential of the accelerating anode 45 of the cathode ray tube by using coupilng condensers 43 and coupling resistor 44. For a more complete description of balanced sweep circuits, in general, reference should be made to an application filed February 25, 1936 Serial No. 65,606, by Frank Gray.

The elements of the pentode tube Ill and the connections therefor will now be described. This tube comprises a cathode 25, a heater element 26 for the cathode, a plate or anode member 21, a control grid 28, a screen grid 29 and a suppressor grid 36. Current for the heater element 26 is supplied by any suitable means such as the secondary 3| of a transformer, the mid-point of this secondary being connected to the cathode 25. A battery 32 places the screen grid 29 at a. positive potential with respect to the cathode, preferably at a potential between that of the cathode 25 and the plate 21. The suppressor grid 30 is connected to the cathode 25 and this connection may be, as desired, made either in the tube envelope or externally. Bias is supplied to the control grid 28 of the tube ID by means of the resistance 33 which may be, if desired, variable. This biasing resistance 33 serves a double purpose. First, it takes the place of a biasing battery, and second, as it is in the circuit including the battery II, condensers I2 and I3 and the plate circuit of the device I0, it serves to change the bias on the control grid 28 as the current in the external circuit changes. If desired, a battery may be included in the circuit including the cathode and the resistance 33 to produce a fixed bias. Thus, it can be said that there is a feedback from the plate circuit to the grid circuit of the device Ill. Its function will now be considered.

In order to better explain the operation of the device ID, reference will now be made to Figs. 4 and 5. In Fig. 5, curve 40 represents a typical plate current versus plate voltage characteristic -of a diode tube which has been customarily used in cathode ray sweep circuits of the past. It will be noted that while the curve tends to flatten out as higher voltage values are reached, it. is still not very close to the ideal constant current characteristic desired. The ideal characteristic is represented by the dotted line 4|. In the ideal sweep circuit, the current in the condenser charging circuit must remain constant even with changes in voltage if the rate of charge with respect to time is to remain linear. In order to bring this out more fully, the operation of the sweep circuit will now be described.

The battery I I charges the condensers I2 and, I3 in series through the device I0. As the potential between points A and B rises as the condensers are charged, the potential drop across the tube I falls because the battery I I supplying the electromotive force for this series circuit tends to produce a substantially constant potential. This drop across the device I0, assuming that it is an ordinary resistance or a diode, would cause a decrease in the current in the circuit which would tend to charge the condensers at a slower rate. The dotted curve 5| of Fig. 4 illustrates the effect of this decrease in the charging rate. This departure from linearity in the saw-tooth wave form will cause distortion of the image on the fluorescent screen of the cathode ray tube. The use of the pentode in place of the diodewill materially improve the linearity of the wave form as pentodes have an inherently flatter plate current versus plate voltage characteristic. In Fig. 5 the curve 42 represents the characteristic of an ordinary pentode tube (without feedback). If the pentode is connected as shown in Fig. 1, the characteristic tends to approach the ideal charproach the ideal saw-tooth form 50, shown in full lines in Fig. 4.

When the charge across the condensers has been allowed to build up during the desired time interval, the condensers are discharged, this discharge producing the fly-back of the cathode ray beam. Let it be assumed, for example, that the sweep circuit of Fig. 1 is to be used to produce horizontal deflection of the beam. At the end of each line of the picture, a synchronizing signal will cause the Thyratron tube I4 to be discharged and the electron beam will fly back to place itself in position to scan the next line. The condensers will then begin to charge again and the entire cycle will be repeated over and over again at a frequency depending upon the frequency of the synchronizing signals generated by the source I5. Once every cycle, therefore, the source I5 supplies a signal which overcomes the negative bias of the Thyratron tube I4 to cause a discharge to be initiated which in turn causes the condensers I2 and I3 to be discharged through the resistance 22 and the discharge path of the Thyratron I4. When the potential between plate and cathode of the Thyratron tube drops to a low value due to the discharge of the condensers, the discharge current stops and the grid I9 regains control of the Thyratron and thus is in a position to initiate the next discharge at the proper time.

The change in potential between the points A and B may be impressed between the deflecting plates 23 and 24 of a cathode ray tube by any desirable means such as, for example, by coupling condensers 43 and resistance 44, the mid-point of which resistance is preferably connected to the accelerating anode 45 of the cathode ray tube.

To produce vertical deflection, a circuit similar to that of Fig. 1 would be used. The frequency of the wave form produced by this circuit would, of course, be much smaller than that of the circuit producing horizontal deflection, as in television the frequency of the deflections produced by this circuit bears to that of the circuit producing horizontal deflection some definite ratio, and the circuit constants would therefore be different. Aside from these differences, however, the description and operation described above in connection with the horizontal sweep circuit apply equally as well to a circuit for producing vertical deflection. The above description applies to the case where horizontal scanning is used. It is obvious, however, that the sweep circuits may be reversed and vertical scanning used instead.

Figs. 2 and 3 show modifications of the method of connecting the elements of the pentode tube I3, shown in Fig. 1. Either of the hook-ups shown in Figs. 2 and 3 may be substituted for the portion of Fig. 1 below the line X--X. In both Fig. 2 and Fig. 3 the pentode tube I0 contains exactly the same elements as the tube used in Fig. 1, but the external connections are slightly modified.

In Fig. 2, the screen grid instead of being at a fixed potential with respect to the cathode 25 (as in Fig. 1) is arranged by means of a battery 46 connectedbetween the screen grid 29 and the biasing resistance 33 to give feedback on the screen grid aswell as on the control grid 28.

The efiect of having feedback on both the con--- trol grid 28 and the screen grid 29 is cumulative and tends to produce a characteristic very close,

to the ideal characteristic 4!.

Fig. 3 differs from Fig. 2 in that feedback is applied to the suppressor grid 30 as=well as to the control grid 28 and the screen grid 29; This feedback is obtained by connecting suppressorchanges the bias on the three grids 28, 29 and 38' with respect to the cathode 25 and has the efiect of keeping the current through the tube substantially constant.

It will be obvious that other combinations may be used as well as those above described and that this invention in certain aspects may be carried out with any electron discharge tube arrangement.

Various other modifications may be made without departing from the spirit of the invention, the scope of the invention being defined by the appended claims.

What is claimed is:

i. A linear time axis generator comprising two condensers connected in series with their common terminal placed at a fixed potential, means comprising a pentode electron discharge device and a source of direct current for charging said condensers at a substantially uniform rate, and means for periodically discharging said condensers simultaneously.

2. A sweep circuit for a cathode ray tube comprising two condensers connected in series with their common terminal placed at a fixed potential, an electron discharge device, having at least one grid, connected in circuit with said condensers, and means for controlling the bias of said grid in accordance with the current through said device.

3. An electron discharge device arrangement for use in cathode ray sweep circuits comprising an electron discharge device having a cathode, an anode, a control grid, a screen grid, and a suppressor grid, two condensers connected in a series circuit with said cathode and anode with the common terminal of said condensers placed at a fixed potential, means for producing a flow of current between said cathode and anode, and means including apparatus responsive to said flow of current and which controls the biasing potential between said cathode and said suppressor grid for controlling said flow of current in such a manner that it tends to remain substantially constant in intensity.

4. An electron discharge device arrangement for use in cathode ray sweep circuits comprising an electron discharge device having a cathode, an anode, a control grid, a screen grid, and a suppressor grid, two condensers connected in a series circuit with said cathode and anode with the common terminal of said condensers placed at a fixed potential, means for producing a flow of current between said cathode and anode, and means including apparatus responsive to said fiow of current and which controls the biasing potential between said cathode and at least two of said grids for controlling said flow of current in such a manner that it tends to remain substantiallyconstant in intensity.

5. An electron discharge device arrangement for use in cathode ray sweep circuits comprising an electronv discharge device having a cathode,

an anode, a control grid, and a screen grid, two condensers connected in aseries circuit with said cathode and anode with the common terminal of said condensers placed at a fixed potential, means for producing a flow of current between said cathode and anode, and meansincluding apparatus responsive to said flow of current and which controls the biasing potential between said cathode and said control and screen grids for controlling said flow of current in such a manner that it tends to remain substantially constant in intensity.

6. An electron discharge device arrangement for use in cathode ray sweep circuits comprising an electron discharge device having a cathode, an anode, acontrol grid, a screen grid, and a suppressor grid, two condensers connected in a series circuit with said cathode and anode with a common terminal of said condensers placed at a fixed potential, means for producing a flow of current between said cathode and anode, and means including apparatus responsive to said fiow of current and which controls the biasing potential between said cathode and said control and suppressor grids for controlling said flow of current in such a manner that it tends to remain substan tially constant in intensity.

7. An electron discharge device arrangement for use in cathode ray sweep circuits comprising an electron discharge device having a cathode, an anode, a control grid, a screen grid, and a suppressor grid, two condensers connected in a series circuit with said cathode and anode with the common terminal of said condensers placed at a fixed potential, means for producing a flow of current between said cathode and anode, and means including apparatus responsive to said flow of current and which controls the biasing potential between said cathode and said suppressor and screen grids for controlling said flow of current in such a manner that it tends to remain substantially constant in intensity.

8. An electron discharge device arrangement for use in cathode ray sweep circuits comprising an electron discharge device having a cathode, an anode, a control grid, a screen grid, anda suppressor grid, two condensers connected in a series circuit with said cathode and anode with the common terminal of said condensers placed at a fixed potential, means for producing a flow of current between said cathode and anode, and means including apparatus responsive to said flow of current and which controls the biasing potential between said cathode and all three of said grids for controlling said flow of current in such a manner that it tends to remain substantially constant in intensity.

9. A linear time axis generator comprising two condensers connected in series with the common terminal of said condensers placed at a fixed potential, means including a constant current device and a direct current source in series with said condensers for charging said condensers, said constant current device comprising an electron said grids to control the bias on at least one of charge device to thereby discharge said consaid grids to thereby keep the flow of current through said device substantially constant, a gasfilled electron discharge device having an input and an output circuit, said output circuit includ- Ling said condensers, and signal responsive means for periodically starting a discharge in said gasfilled discharge device to thereby discharge said condensers.

18. A linear time axis generator comprising two condensers connected in series with the common terminal of said condensers placed at a fixed potential, means including a constant current device and a direct current source in series with said condensers for charging said condensers, said constant current device comprising an electron discharge device having a cathode, an anode, a control grid, a screen grid, and a suppressor grid, means including said direct current source for maintaining a flow of current in saidv discharge device, means including a resistance in circuit between said cathode and said control grid to control the bias on said grid to thereby keep the flow of current through said device substantially constant, a gas-filled electron discharge device having an input and an output circuit, said output circuit including said condensers, and sig nal responsive means for periodically starting a discharge in said gas-filled discharge device to thereby discharge said condensers.

11. A linear time axis generator comprising two condensers connected in series with the common terminal of said condensers placed at a fixed potential, means including a constant current device and a direct current source in series with said condensers for charging said condensers, said constant current device comprising an electron ischarge device having a cathode, an anode, a control grid, a screen grid, and a suppressor grid, means for fixing the biasing potentials of said control and suppressor grids with respect to said cathode, means including said direct current source for maintaining a flow of current in said discharge device, means including a resistance in circuit between said cathode and said control grid to control the bias on said grid to thereby keep the flow of current through said device substantially constant, a gas-filled electron dis charge device having an input and an output circuit, said output circuit including said condensers, and signal responsive means for periodically starting a discharge in said gas-filled disdensers.

12. A linear time axis generator comprising two condensers connected in series with the common terminal of said condensers placed at a fixed potential, means including a constant current device and a direct current source in series with said condensers for charging said condensers,

said constant current device comprising an electron discharge device having a cathode, an anode, a control grid, a screen grid, and a suppressor grid, means for fixing the biasing potential of said suppressor grid with respect to said cathode, means for fixing the biasing potential of said screen grid with respect to said control grid, means including said direct current source for maintaining a flow of current in said discharge device, means including a resistance in circuit between said cathode and said control grid to control the bias on said grid to thereby keep the flow of current through said device substantially constant, a gas-filled electron discharge device having an input and output circuit, said output circuit including said condensers, and signal responsive means for periodically starting a dis charge in said gas-filled discharge device to thereby discharge said condensers.

13. A linear time axis generator comprising two condensers connected in series with the common terminal of said condensers placed at a fixed potential, means including a constant current device and a direct current source in series with said condensers for charging said condensers, said constant current device comprising an electron discharge device having a cathode, an anode, a control grid, a screen grid, and a suppressor grid, means for fixing the biasing potentials of said screen and suppressor grids with respect to that of said control grid, means including said direct current source for maintaining a flow of current in said discharge device, means including a resistance in circuit between said cathode and said control grid to control the bias on said grid to thereby keep the flow of current through said device substantially constant, a gas-filled electron discharge device having an input and an output circuit, said output circuit including said condensers, and signal responsive means for periodically starting a discharge in said gas-filled discharge device to thereby discharge said condensers.

FRANK R. NORTON.

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