US2461871A - Relaxation oscillator capacitance multivibrator - Google Patents

Description

. H. A. BAss 2,461,871

RELAXATION OSCILLATOR CAPACITANCE MULTIVIBRATOB FiledAug. 1, 1947 Feb. 15 1949.

SYNC. SEPAPATOR AND FILTER J8 r- 1 B I FEED BACK \N /Z l CIRCUIT YNC. PULSES I A F/ A.EC. A

HORIZON TAL DEFLEC TION AMPLIFIER CIRCUIT MULTlV/BRATOR OSCILLATOR CIRCUIT INTEGRATING CIRCUIT I I 61am Patented Feb. 15, 1949 RaLAXA'rioN o'sorLLA'ro anaemia-res MUL'rIvIBRA'roR Harland A. Bass, Mount Healthy; Ohio, assigno'r' to Avco Manufacturing Corporation, Cincinnati, Ohio, a corporation of Delaware Application August 1, 194'7,Se1-ia1No;'765,362

2 Claims. (01. 25 --3.6)

is adapted to have its recurrence rate or frequency controlled by a unidirectional voltage, such as an automatic frequency control voltage. The invention residesgenerally in the provision of a feedback net o'rkin such a niultivibrato'r for reducihg'j th susoeptibility' of the multivibrator circuit tc'inoise; for minimizing undesired variations in the eohtrervoitage and for increasing the stability of the multivibrator.

In stating the problem which is solved by the present invention, reference is made to indirect synchronizing systems of the type commonly em ployedjin television receivers. A typical super- -heterodyne television receiver of the superheterodyne type comprises a first detector or oscillator modulator, an intermediate frequency amplifier, a seconddetector for deriving the modulation components of the intermediate frequency carrier; and a" synchronizing signal separator and filter forsepa'rating the horizontal synchronizlug-pulses and the vertical synchronizing pulses from each other and from the signal carrying the picture elements. The horizontal synchronizing pulses; for example, are applied to an automatic fr e'quencycontrol circuit, which compares their phase relationships to periodically recurring saw-tooth voltages" produced by integration of voltages'appearing in the horizontal beam-defleeting 'c'oil circuit of the television tube, in order to deriyea' resultant unidirectional control voltage. This control voltage is applied to a blocking oscillator and the output control signals of the blocking oscillator are in turn utilized for triggering a'saw-tooth wavegenerating circuit. This last-mentioned saw-tooth wave circuit generatesa current of recurring saw-tooth wave form; in the horizontal beam-deflecting coils. The function of the A. F. C. circuit is to synchronize the saw-tooth current waves with the horizontal synchronizing pulses applied to .the A 3. ;C'.' circuit; from the output of the synchronizing'signal separator and filter. The frequency of the oscillator output control signals and consequently the frequency of the saw-tooth current waves is functionally related to the magnitude of the unidirectional control voltage appearing at.

the output of the automatic frequency comer circuit. It has heretofore been considerdnec essary or highly desirable to provide an Rt-C. fil ter combination between the output of the'*-frequency-control circuit and the input of the 0s: cillator. Such a combination is represented -by' the elements 69, appearing in Iigi-Z of Ui-SZ Patent No. 2,358,545, issued September 19', 1944'; for exam'ple. The functions of this filter are" to reduce the susceptibility of the oscillatoncircuit tonoise, to minimize hunting-and to smooth 'out abrupt or fast changes in the A.-F. C. control voltage. The performance ofzthis function has heretofore been considered to require a relatively large capacitance of the order of one microfarad. It is desirable that there be provided a means and method whereby this-large capacitor Lmay be dispensed with and a smaller capacitor utilized in lieu thereof.

' In the radio manufacturing industryit-isfdesirable that unnecessary circuit componentswbe eliminated wherever possible and that essential circuit components which: are. capable" of per forming several functions be fully exploited; It is, accordingly, an object of the present invention to provide an oscillator for the purpose set forth, said oscillator being adapted to be con trolled by a direct currentvoltage, such as that derived from an automatic frequency control circuit, and at the same time to dispense with the above-mentioned large capacitor.

Another object of the invention is to providea relaxation oscillator of the multivibrator type; in which the hereto-fore unrealized potentialities of a multivibrator as a multiplier of capacitance are fully exploitechby providing a feed-back network between the anode and control electrode circuits of that one of the multivibrator tubes to which the control voltage is applied.

Briefly described, the invention comprises'a D. 0. controlled multivibrator having a feedback network and so arranged as to function also as a capacitance multiplier. For a better understanding of the present invention, together with other and further objects, capabilities and advantages thereof, reference is made to the accompanying drawing, in the single figure herein there is symbolically represented a television beam-deflecting system of the indirect synchronizing signal type including an illustrative form of capacitance-multiplying multivibrator in accordance with the invention.

Referring nowspeoifically to Fig. 1, thetele' v ments thereof which" are individually considered A, B of an oscillator I2. The function of the oscillator is to generate saw-tooth voltage waves.

A. F. C. voltage) is applied to the input terminals These saw-tooth voltage waves are applied to a j horizontal deflection amplifierld which produces saw-tooth current waves in the horizontal deflecting coils I5, I5 of the cathode ray television receiver tube Ifi. The output of amplicoupled to the cathode-control electrode input circuit of tube by a common cathode resistor 22. The anodes are connected to the high potential terminal (indicated as plus B) of an appropriate source of space current (not shown) by individual anode resistors 23 and 24. The anode circuit of tube 20 is coupled to the input circuit of tube 2| by an R.-C. network comprising a coupling capacitor 25 and a grid resistor 26, that terminal of the grid resistor remote from the grid of tube 2I being connected to the junction of the cathodes and resistor 22. Connected across the output of tube 2 I, specifically between its anode and ground, is a capacitor 21. This capacitor is periodically charged along a linear saw-tooth voltage characteristic and it periodically discharges through tube 2|, thereby to generate the signals applied to the horizontal deflection amplifier I4.

fier I4 is coupled through an integrating network I! back to an input circuit of the A. F. C. network II. This integrating circuit I1 shapes the kick back voltage pulses applied to the deflecting coils I5, I5 into saw-tooth voltage waves. Thelast-mentioned saw-tooth voltage and the horizontal synchronizing pulses from unit I0 are n applied to the automatic frequency control network II with such polarity and -are therein so utilized that there is applied to the terminals A, B a unidirectional control voltage. This A. F. C. voltage is utilized to control the frequency of the control signal output of oscillator unit I2, to the end that the saw-tooth current waves appearing in coils I5, I5 may be properly phased with respect to the horizontal synchronizing signals' originating at the transmitter and finally appearing at the output of unit I0. The system units so far specifically described correspond to those collectively shown in the prior art, with the exception that a cathode-coupled free-running multivibrator oscillator is here employed in lieu of the blocking oscillator generally employed. Accordingly, it is neither necessary nor desirable that such units be further described herein. Such units as II), II, I2, I4, I5, I6 and I! are well known to those skilled in the art.

It has heretofore been considered necessary to provide aseries R.-C. network including a very high capacitance on the order of one 'microfarad across the output circuit of unit II and between the output of A. F. C. circuit II and the input of oscillator'I2, as mentioned above. The present invention, in one aspect, provides a means and method for eliminating this large condenser. Specifically, a multivibrator is employed in oscillator I2. cathode-coupled multivibrator circuit is prior art, not only per se but also as used in sweep generation. The present invention, however, is premised in part on the novel directive concept and perception that a multivibrator has capacitance-multiplying potentialities which have heretofore not been realized, appreciated, or exploited. It is further recognized that other tube circuits have heretofore been employed as multipliers of capacity for other purposes. However, a circuit employing one tube section of a multivibrator as a multiplier of capacitance is believed to be novel. Accordingly, one improvement provided by the present invention resides in a feed-back network, generally indicated by the reference numeral I8, for intercoupling the anode and control electrode circuits of the first multivibrator tube 20.

Referring now to the multivibrator network, it'comprises a pair of triode electronic vacuum tubes 20 and 2I. The anode circuit of tube 2I is It is recognized, of course, that a Those portions of the multivibrator so far described, with the exception of the feed-back network presently to be described, are generallysimilar in construction, arrangement and operation to the sweep circuit multivibrator described in the article by J. L. Potter, page 713 et seq. of the proceedings of the Institute of Radio Engineers for June, 1938, vol. 26, No. 6, published by Institute of Radio Engineers, New York, New York.

In accordance with the invention a feed-back circuit comprising a resistor 28 and a capacitor 29 is connected between the anode and control electrode of the multivibrator tube 20. The A. F. C voltage is applied to the input circuit of the same tube 20 to control the frequency of the saw! tooth signals.

In one successful commercial embodiment of the invention, employing separate sections of a 6SN7GT type tube for triodes 20 and 2|, the following parameters were found satisfactory:

Element Value 82,000 ohms.

0.1microiarad.

82 micromicrofarads. 47,000 ohms.

. 1 mcgohm.

. 470,000 ohms.

1,000 ohms.

micromicrolamds.

It will be noted that a capacitance of 0.1microfarad was found suitable for element 29. In prior art circuits a capacitance of the order of 1 microfarad was provided between terminals A and B. or between some equivalent points. In accordance with the invention, this large capacitor .is dispensed with and the first tube section of the multivibrator is employed to perform an additional function as a multiplier of capacitance, whereby there is presented at the input terminals A, B an effective capacitance (equivalent to that formerly furnished by a 1 microfarad capacitor, for example) greatly in excess of the capacitance (element 29, 0.1 microfarad, for example) in-' cluded in the feed-back network I8.

In operation, the effective capacitance appearing between the terminals A, B smooths out abrupt or rapid changes in the A. F. C. control voltage. As indicated, this high effective capacitance is obtained by connecting a capacitor of a radically lower value between the grid and plate of tube 20. The resistor 28 in series with the capacitor 29 is preferably included for the purpose of minimizing high frequency feed-back and aiding in the anticipationof sudden changes in the A. F. C. control voltage.

In operation, capacitor 21 is charged along a linear voltage characteristic when tube 2| is nonconductive through a long time-constant circuit comprising ground, capacitor 21, anode resistor 24 and the space current source plus B. During the trace period, when capacitor 21 is being charged through its relatively long time-constant circuit, capacitor 25 will discharge the energy (placed therein during the retrace period) through its relatively long time-constant circuit comprising capacitor 25, resistor 26 and the cathode-anode circuit of tube 20, producing a current flow of such polarity in resistor 26 as to cut off current flow through tube 2|. As the charge on condenser 25 decreases exponentially to the cut-off point of tube 2|, that tube tends to and finally does become conductive. As tube 2| becomes conductive at the beginning of the retrace period condenser 21 discharges rapidly through a short time-constant circuit comprising elements 22 and 2? and the cathode-anode circuit of tube 2|. At the initiation of the retrace period, since the plate current of tube 2| increases, the voltage across resistor 22 also abruptly increases whereby the positive bias applied to the cathode of tube 20 is increased. Tube 20 becomes less conductive and its plate voltage rises. By reason of the action of the R.- C. coupling network 25, 26, the voltage across capacitor 25 not being able to change instantaneously the potential applied to the grid of tube 2| abruptly becomes more positive and the conductivity of tube 2| is increased. The increase in the conductivity of tube 2| in turn increases the flow of current through resistor 22 and the bias on the cathode of tube 20. The cumulative effect of this regenerative action is such that the anode current in tube 20 is substantially decreased and tube 2| rendered conductive for a short interval of time. The anode current in tube 2|] is reduced by the voltage developed across resistor 22 when the high plate current of tube 2| flows through it. During this retrace interval, grid current being drawn by tube 2| when the voltage across resistor 26 exceeds that across resistor 22, capacitor 25 is charged through a short timeconstant circuit comprising ground, resistor 22, the cathode-grid circuit of tube 2|, capacitor 25, resistor 23 and the space current source. Next tube 2| becomes less conductive as the grid of tube 2| approaches cathode potential and the current through resistor 22 decreases, whereby the bias on the cathode of tube 20 is decreased and the plate current in tube 2|] tends to increase. The plate voltage of tube 20 falls and the cumulative action of the R.- C. network 25, 26 is such that the bias on the grid of tube 2| is increased until tube 2| is cut oif, whereupon trace occurs and the cycle of events is repeated.' The magnitube of the control voltage applied by unit II to tube 20 determines the length of each time interval during which tube 2| is cut off and therefore the frequency of the saw-tooth voltage output of unit I 2. That is, during each cycle of operation tube 2| remains cut off as long as the voltage across resistor 26 is suflicient to prevent space current fiow in this tube. Since the magnitude of the voltage across resistor 26 is determined by the plate current in tube 20 it is evident that the A. F. C. voltage will control the frequency of the multivibrator. When the bias in tube 26 is such that the plate current is low, the capacitor will initially be charged less, the cut-off level will be reached more quickly and the frequency of the multivibrator will increase. The oscillator including feed- back circuit 28, 29 functions degeneratively in multiplying capacitance in a manner analogous to the behavior of a feed-back circuit in an amplifier-type reactance tube circuit as pointed out in the last paragraph on page 214 and on page 215 of Theoryand Applications of Electronic Tubes by Reich, 2nd edition, McGraw-I-Iill Book Company, New York, 1944.

While there has been shown and described What is at present considered to be the preferred form of capacitance-multiplying relaxation oscillator in accordance with the invention, it will be obvious to those skilled in the art that various modifications and substitutions of equivalents may be made therein without department from the true scope of the invention, and it is accordingly intended in the appended claims to cover all such changes and variations as fall within the true scope of the invention and without the proper scope of the prior art. Such construction of the claims is desired as will best protect the true invention.

Having thus described my invention, I claim:

1. In an indirectly synchronized sweep-signal generating system for a television receiver, the combination of a source of unidirectional automatic frequency control voltage, a multivibrator comprising a pair of electron tubes of which one has a control grid-cathode input circuit, coupled to said source, and an anode circuit, and an inverse feedback network intercoupling said anode and said input circuit for providing. an effective shunting capacitance across said input circuit, said capacitance being employed as a filter to steady the value of said automatic frequency control voltage, whereby to stabilize the output frequency of the generating system.

2. In an indirectly synchronized sweep-signal generating system for a television receiver, the combination of a source of unidirectional automatic frequency control voltage, a cathodecoupled multivibrator oscillator comprising a pair of electron tubes of which one has a control gridcathode input circuit, coupled to said source, and an anode, and a resistance-capacitance network intercoupling said anode and said grid for providing an effective shunting capacitance across said input circuit, said capacitance being employed as a filter to steady the value of said automatic frequency voltage, whereby to stabilize the output frequency of the generating system.

' HARLAND A. BASS.

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

FOREIGN PATENTS Number Country Date 528,806 Great Britain Nov. 7, 1940

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