US2570875A - Sweep wave generating circuits - Google Patents

Description

Oct. 9, 1951 R. L. SINK SWEEP WAVE GENERATING CIRCUITS Filed May 13, 1947 Inventor Robert Lsmk, b ,0M

His Attorney.

Patented Oct. 9, 1951 SWEEP WAVE GENERATING CIRCUITS Robert L. Sink, Altadena, Calif.,-assignor to General Electric Company, a corporation of New York Application May 13, 1947, Serial No. 747,824

3 Claims. (01. 25027) My invention relates to circuits for generating sweep waves, such as are commonly utilized in television, radar, oscillographic and similar apparatus. From another aspect, it relates to constant current charging networks which can be utilized to produce sweep waves having a high degree of linearity. 7

This application is a continuation-in-part of my prior application Serial No. 543,235, filed July 3, 1944, entitled Pulse Echo System, and assigned to the same assignee as the present invention.

It is an object of the present invention to provide an improved sweep wave generator which may be adjusted to give a substantially linear sweepand which is readily synchronized from an external source.

It is a further object of my invention to provide a charging network, for producing a linear sawtooth wave or similar wave by charging a capacitor through an electron discharge device, which exhibits a substantially constant current characteristic even though the discharge device may have relatively low internal impedance.

Still another object of my invention is to provide an improved generatorof linear sweep waves which is readily synchronized from an external source, which is extremely stable in operation and which has a low impedance output.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in. connection with the accompanying drawings, in which Figs. 1 and 2 are circuit diagrams of two different types of sweep circuits embodying the principles of my invention.

Referring first to the circuits of Fig. 1, there is shown an electron discharge device III, illustrated as a triode, having an anode II connected to the positive side of a sou' e of ope-rating potential (not shown but indicated conventionally by 3+), and a cathode I2 connected to ground through a circuit serially comprising a resistance I3, an electronic switch in the form of a unilateral conducting device I4, poled in the same direction as the di charge device Iii, and a resistance I5. The unilateral conducting device I4 is shown as a diode. This latter device I4 and resistance I5 are shunted by one or the other of two series resistance-capacitance combinations I6, I I or I8, I9 depending upon the positions of a switch 20. With switch 20. in the position shown, they are shunted by the second mentioned series combination I8 and I9 respectively.

The control electrode 2| of the discharge device ID is connected through a resistance 22 to a movable contact 23 on a potentiometer 24, by means of which the operating bias of device I0 may be independently adjusted. Potentiometer 24 is connected in a resistance voltage-divider between B+ and ground consisting of a resistance 25, potentiometer 24 and a resistance 26. he contact 23 is adjusted so that the discharge device I8 is normally conducting to an extent dependent upon the position of the contact 23 on potentiometer 24. The control electrode 2| is also connected through a relatively large coupling capacitor 21 to the anode 28 of the diode I4, so that the control electrode 2| is at substantially the same alternating current potential as the anode 28. The cathode 29 of the diode I4 is connected through a blocking capacitor St to any suitable source of synchronizing potential (not shown but indicated by terminals 3 I). This source may'supply positive pulses of the form generally indicated by the wave 35, each of which is effective to render the diode I4 non-conductive.

In analyzing the operation of the circuit of Fig. 1, first assume that a positive pulse has just been applied to the cathode of diode I4, rendering it non-conductive. The capacitor I9 now begins to charge through the triode I0 and the resistors I3 and I8. The wave shape of the voltage at the point 36 in the charging circuit is generally indicated by the wave form 31. Itwill be noted that theinitial portion 38 of thewave rises abruptly upon the occurrence of the positive pulse 35; This initial abrupt rise at point 38 is due to the charging current of capacitor I9 flowing through resistor I8, the remaining rise in voltage being entirely due to the voltage accumulated on the capacitor I9. If the resistor I8 is omitted, there will be'no pulse component in the voltage wave at point 38 and the wave form will be substantially a pure sawtooth.

The discharge device I0 operates substantially in the manner of acathode follower. When the value of the resistance I3 is large compared to the electrode 2| is substantially that present at point 36. It therefore follows that, since the alternating potentials at the points 38 and 39 are almost equal during the charging of capacitor IS, the current flowing through resistor l3 must be almost constant. This, of course, results in a linear sweep voltage at either point 36 or point 39. The output may be taken from either of these points. As indicated in Fig. 1, the connections to the output terminals 4| are between the point 39 and ground.

The values of resistor l6 and capacitor I! may be so selected that other different values for the pulse components of the output wave (indicated by the rising portion 38) and of the rate of sweep (indicated by the slope of the portion 31) may be secured when the switch 20 is placed in its alternate position.

It will thus be apparent that the device I functions as a substantially constant current device'in the manner of a cathode follower with a gain of very nearly unity. If the gain were exactlyuni ty, then the sweep would be perfectly linear. Neglecting the relatively low resistance of resistor it, an analysis of the operation shows that the equivalent source impedance R whenviewed from the point 36 is:

where Tp is the plate resistance and p is the amplification factor of the tube l0, and where R1 is the value of resistor l3.

Merely for the purpose of illustration, some quantitative relations are of interest. For example, if triode lfi'is one-half of a type 6SL7GT tube and resistor [3 equals 100,000 ohms, then the equivalent source impedance R is:

R'=7000-lf- (704-1) 100,000-=71 megohms This figure of about 7 megohms compares very favorably with. the. one-megohm source impedance ofv a pentode tube which is. commonly employed as a constant current charging. device in sweepcircuits. Now, if the bias adjustment of the triode i0 is such that the direct current equals one milliampere, then the linearity of the sweep is-the. same as that which would be obtained by charging the condenser 19 from a direct current source of 1,100 volts through a 7-.1-megohm resistor; and: in this particular illustration, the departure from linearity is only 0.3 when the peak of the sweep wave has reached a value of about 20 volts.

In addition to the advantages of constant currentoperation. and linearity of sweep, the circuit possesses great stability of operation, due to the degenerative characteristicof a cathode follower. The output voltage may also be taken from a relatively low impedance source, i. e., either from the point 36-or the point 39, which is a definite advantage in many sweep systems.

Fig. 2 illustrates a modified form of -my invention which is particularly useful in such applications as television sweep circuits. Certain circuit components which generally correspond in function to those of Fig. 1 have been indicated by corresponding reference numerals with the sufiix letter a added. In this modification, the constant current charging triode comprises the left-hand section Illa of a dual triode. Charging current flows from the anode potential source B+ through triode [0a and resistor 13a into the sweep capacitor [9a. The grid 'Zldis supplied with suitable biasing potential from the voltage divider network 25a, 26a through the resistor 22a,

in the manner similar to the circuit of Fig. 1; and it is also connected to the point 36a for alternating potentials through the relatively large coupling capacitor 21a.

In the circuit of Fig. 2, the trigger tube for discharging the sweep capacitor [9a is represented as a pentode l4a instead of a diode. This tube receives its. anode potential through the triode Hla and resistor Mia and its operating characteristic is so determined, by adjustment of its operating potentials and the values of the bias resistors 4| and 42 in its grid circuit, that it is cated by the wave form 44.

normally inoperative. It operates to discharge capacitor lSa, across which it is shunted, whenever positive synchronizing pulses 43 are supplied to its grid through blocking capacitor 30a. Thus, its operation is functionally similar to that of the diode l4 in Fig. 1.

The general form of the sawtooth waves developed at points 36a and 39a in Fig. 2 is indi- These waves are supplied directly to the grid of the right-hand triode section lllb which operates as a conventional cathode follower circuit and supplies these waves'from its cathode resistor 45 through a blocking capacitor 46 to the control grid of a sweep power amplifier 41. The amplified sweep waves are supplied to the output terminals 48 through a coupling transformer 49. The terminals 48 may be connected to any suitable load (not shown) such as the sweep circuit of a cathode ray discharge device.

In order to improve the linearity of sweep even further, the switch 50 in Fig. 2 may be placed in its alternate position. In this case a certain amount of negative feedback, derived from a potentiometer 51 across the secondary of output I transformer-'49, is injected into the sweep charging circuit through resistor 52. The function of this circuit connection, and its advantages, will be apparent without detailed explanation to those familiar with negative feedback amplifier design.

While I have shown a particular embodiment of my invention, it will, of course, be understood thatI do not wish to be limited thereto since various modificationsmay be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent in the United States is:

1. In combination, an electron discharge device having an anode, a cathode, and a control electrode, a source of operatingv potential connected to said anode, a source of unidirectional bias potential connected to. said control electrode, a first condenser connected between said cathode andthe negative terminal of said source, a switch connected across said condenser to discharge it, a second condenser connected between said control electrode and the electrode of said first condenser nearest said cathode whereby when said switch is opened said first condenser charges and the potentials of said cathode and control electrode relative to said negative terminal vary together to produce a linear charging of. said first condenser, and an output circuit connected to utilize the linear variation in voltage across said first condenser.

2. A generator of sweep waves comprising a source. of direct operating potential, a capacitor charging circuit comprising an electron discharge device having an anode connected to the positive terminal of said source and a cathode'connected through a resistor and capacitor in series to the negative terminal of said source, a control electrode in said device connected to said cathode through a coupling capacitor of relatively low impedance at frequencies present in said waves, means independently biasing said device to conductive condition, a capacitor discharging circuit comprising a second electron discharge device connected in parallel circuit relation with said capacitor, and means for rendering said second device alternately conductive to discharge said capacitor and non-conductive to initiate charging thereof.

3. A linear sweep generator comprising, an electron discharge device having an anode, cathode and control electrode, a source of operating potential for said anode, a charging resistor and a sweep capacitor serially connected in the order named between said cathode and the negative terminal of said source, means comprising a source of unidirectional potential for independently biasing said control electrode to render said device conductive, a second electron discharge device having an anode and a cathode connected across said capacitor poled to discharge it and a control electrode biased to render said second device normally non-conductive, means for impressing periodic positive synchronizing pulses on the control electrode of said second device to render it alternately conductive and non-conductive, whereby said capacitor alternately discharges through said second device and charges through said first device and resistor to produce sweep Waves, and means comprising a coupling capacitor of relatively low impedance at the sweep frequency for impressing said sweep waves on the control electrode of said first device in degenerative phase to maintain the charging current substantially constant.

ROBERT L. SINK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,237,425 Geiger Apr. 8, 1941 2,439,324 Walker Apr. 6, 1948 2,452,683 Rieke NOV. 2, 1948

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