US2554925A - Saw-tooth wave generator - Google Patents

Description

May 29, 1951 K. SCHLESINGER SAWTOOTH WAVE GENERATOR Filed Oct.

Patented May 29, 1951 UNITED STATES ATENT OFFICE SAW-TOOTH WAVE GENERATOR Kurt Schlesinger, Maywood, Ill., assignor to Radio Corporation of America, a corporation of Delaware This invention relates to the generation of elec trical voltages and currents of complex wave forms and more particularly it relates to the production of alternating currents and voltages of sawtooth and related wave forms for use in oscilloscopes, television systems and the like.

A most common type of sawtooth wave generator employs an inductive coupling between a control electrode and anode of an electron discharge tube for controlling the charge and discharge of a capacitive element. A typical sawtooth wave generator of this type is shown and described in the U. S. patent to Rhea, No. 2,165,815, issued July 11, 1939.

Circuits of this type generally have in common what may be calledffeedback through an electron discharge tube. During the fiyback or short slope time interval of the sawtooth wave,

.the circuit elements function like an oscillator which performs one-half of a complete cycle. During the long sweep phase, however, the tube is blocked and the system behaves like an ordinaryclassA amplifier for a sawtooth grid voltage. I

Heretofore, certain disadvantages have appeared in the employment of sawtooth wave generators of the type referred to. First of all, the feedback condition of the circuit arrangements depends largely on adjustments in the deflector circuit and is greatly aifected by alterations thereof.

Secondly, the yoke circuit has A. C. potential with respect to ground. This results in crosstalk between the two deflection circuits when a plurality ofdeflection circuits are closely asso- .ciated with each other.

This, of course, is undesirable, especially in ccnnection with television scanning of the popular interlaced type. It also results in a slowing down of fiyback (i. e., an increase in -;beam return time) When large ,capacitiesare encountered between the; deflection coil and ground.

trol electrode or screen grid feedback arrangement, the system becomes more independent of variations of either frequency or amplitude of the deflection voltage or current.

A primary object of this invention is to provide .an improved sawtooth wave generator.

:injtervals.

ciated cathode 26.

Other and incidental objects of the invention will be apparent to those skilled in the art from a reading of the following specification and an inspection of the accompanying drawing in which Figure 1 illustrates by circuit diagram one form of this invention; and

Figure 2 illustrates also by circuit diagram another form of this invention employed, for example, as the scanning circuit of the television system.

Turning now to Figure 1, a cathode ray tube I has associated therewith deflection coils 3' and 5 which are connected to the secondary of transformer l. Deflection coils 9 and H may, for example, be connected to another circuit arrange,- ment for the provision of sawtooth waves, but for the purpose of explanation of the operation of this invention in one of its forms, no circuit connections will be illustrated as connected to deflection coils 9 and l I. 1

The primary of transformer l is connected to ithe'anode 13 of electron discharge tube l5. Tube 15 contains a control electrode I! and an auxiliary control electrode [9. Control electrode H is 25 connected to condenser 2| through a resistance 23. A positive potential is connected to condenser 2| through resistances 25 and 23.

Control electrode I! also has connected thereto a rectifier such as a diode 2'! whose anode 29 is connected to control electrode I! and whose cathode 31 is connected to winding 33 of transformer winding 4| of transformer 35.

In order'to have a better understanding of how .the circuit arrangement illustrated in Figure 1 "operates, it is desirable to consider first of all the two phases of a sawtooth wave. The sawtooth wave may be broken down into two distinct time First of all, the sweep phase time interval and secondly, the fiyback phase. 1 It may be stated that the circuit arrangemen shown in Figure 1 behaves during the sweep phase like an ordinary class A amplifier. Assume for the purpose of explanationthat condenser 2| contains a charge such that control electrode H is negative with respect to its asso- Condenser 2! is then discharged in the conventional manner through resistances 23 and 25. This results in a substantially linear rise in potential in a positiver'direction of control electrode [1, providing-" 0f :course, that the time constant of the resistance'ca'pacity circuit involving resistances 23 and 25 and condenser 2| is made large, as compared to the total sweep period. During the beginning portion of the sweep phase, the voltage of the control electrode I! is maintained at a level which is substantially negative with respect to its associated cathode 26, and therefore very little current flows in the tube l5. There is, however, a gradual increase in current through tube |5 during the sweep phase.

No current flows through diode 21 because anode 29 is maintained negative with respect to its associated cathode 3|. The inter-electrode capacity of diode 27 may be considered as having a negligible eflect at the frequencies of the sawtooth waves under consideration.

As soon as condenser 2| has discharged through resistances 23 and 25 sufliciently to cause anode 29 to become positive with respect to its associated cathode 3|, a current will flow through diode Z? to cause the ungrounded plate of condenser 2| to go rapidly in a negative direction, or increase the potential stress across the plates of condenser 2| which we may consider charging the condenser.

The feedback in the circuit necessary to sustain oscillations is furnished by auxiliary electrode E9 through transformer 35. The coils 33 and ill at the transformer 35 are so polarized that any increase in current by the auxiliary electrode l9 adds a positive voltage to cathode 3| of diode 2i. It follows, therefore, that during the slow discharge of condenser 2| or the sweep phase of the oscillation, cathode 3| is caused to be more positive as a result of the gradual increase in current drawn by auxiliary electrode I9.

As the current through tube l5 approaches saturation of the tube [5, as a result of the approach of the potential of control electrode ll to zero with respect to its associated cathode 26, the current flowing to auxiliary electrode l9 ceases to increase. At this point, as a result of the stopping of an increased current flow through winding 31 of transformer 35, the positive potential on cathode 3| added by transformer winding 33 of transformer 35 ceases, and diode 21 becomes conductive.

As soon as diode 21 becomes conductive and the potential of control electrode l1 drops in a negative direction, the current drawn by auxiliary electrode I9 rapidly decreases to cause through transformer 35 a more negative potential on cathode 3|. This causes the short or retrace time interval. The cumulative effect thus produces an abrupt blocking of tube IS.

The detailed theor of operation of current generators for oscilloscope and television deflection is known to the art and is well described in literature such as, for example, an article entitled "Current oscillator for television sweep by George Clifford vSziklai, published in Electronics" for September 1946.

It may, however, be well at this point to refer briefly to the basic principles Of operation of such an oscillator circuit in its production of sawtooth current variations for the accurate deflection of electron beams.

As a result of the action of the circuit shown in Figure 1 described immediately above, the quick change in current through transformer 1 resulting from the rapid discharge of condenser 2| and sudden blocking of tube l5 produces a sharp voltage pulse in transformer l.

The effect of this sharp pulse in a transformer may be considered analogous to a circuit arrangement involving deflection coil 3 together with a di/ dt Es/ L where EB is the voltage of the theoretical battery,

and L is the inductance of the deflection yoke 3.

This condition exists until the time the theoretical switch is opened. The current stored in the inductance of the yoke 3 will then be discharged through its associated interturn capacity and any auxiliary capacity inserted in the circuit,

such as condenser 38, in an oscillatory manner.

The solution for the equilibrium equation is If the theoretical switch is then closedagain, the inductance returns its energy to the theoretical battery with a current linearly'changing with time until approximately halfway up the long sweep phase, at which time there is a current equilibrium, and from there on, the theoretical battery will deliver current in a, linear manner to the yoke again.

In a theoretically perfect circuit where no resistance is encountered and if a perfect switch could be provided, no external energy would be required to provide a sawtooth current, however, this is not true in practice.

In comparing the above theoretical explanation to the circuit operation of the circuit shown in Figure l, the time interval of the gradual increase in current of tube I5 may be considered that time interval during which a theoretical battery is connected to deflecting yoke 3. The time interval during which the theoretical switch is open may be compared to the time interval occupied during the discharge of condenser 2|.

In circuits of this type used today, there is usually provided a damping arrangement such as condenser 38 and resistance 40, or preferably there is employed a diode or triode for damping purposes, as is well known in the present television art.

If the natural frequency of the transformer 35 is too high, the time interval during which the ungrounded plate of condenser 2| is driven in a negative direction through conduction of the diode is too short. As a consequence, the amount of differential change in the potential at condenser 2| is insufiicient, and the amplitude of the sawtooth wave output is small.

Correct operation of the circuit may be assured either by tuning down transformer 35 through a separate condenser 39, as indicated in Figure l, or by providing suflicient inductance in the circuit with the result that the natural period of the transformer 35 under load conditions approaches the desired flyback period.

The resistor 23, which may be termed a peaking resistor, connected in series with the sweep condenser 2 l, is provided in order that the diode 2? does not have to operate into an extremely small impedance and in order that the desired current wave form be present in transformer I. The value of the resistance 23 can best be adjusted experimentally, but it may be of the order of, for example, about-10,000 ohms. If, however, the resistor 23 is made too large, the charge feedproduc'tion of television images.

back is throttled and the A. Cf-D. C. efiiciency of the generator decreases.

Although there are alternate ways of synchronizing the system illustrated in the circuit diagram in Figure 1, the synchronizing pulses may be applied to winding 4! of transformer 35, as indicated in Figure 1. Alternatively, negative voltage pulses may be introduced across a small impedance in the cathode circuit of the tube I 5. In the form of the invention shown, however, the

synchronizing is done inductively in both directions using coil 4| to introduce pulse energy into the feedback transformer 35. This synchronizing energy may, for example, be derived from a conart.

In the circuit arrangements shown in Figure '2, dual purpose tubes 69 and 5! are employed,

each of which contain a tetrode and a diode. Such a tube is known commercially as the ll7L7/GT. Here again, thetype tube is merely illustrative and it is not intended that this invention should be limited to its use.

The synchronizing signal input is applied to tube 53 which, as illustrated, may take the form of a dual triode such as the 6F8 type tube.

The synchronizing signal which should be applied in a negative direction causes the lower section of tube 53 to provide a horizontal synchronizing signal to winding 55 of transformer 51.

The vertical synchronizing pulse is obtained through the upper section of tube 53 and applied to winding 59 of transformer 6i.

Sync separation is reasonably well understood in the television art, and it is believed unnecessary to go into further detail regarding its derivation for the synchronization of the circuit 7 arrangements shown, except perhaps to mention that condenser 63 and its associated circuit elements act to permit the upper section of tube 53 as illustrated to pass only the vertical synchronizing pulses to winding 59 of transformer 6|.

The operation of the horizontal and vertical sections of the circuit shown in Figure 2 is substantially the same as shown and described in connection with the circuit diagram illustrated in Figure 1 above. It will be noted, however, that no transformer is provided in the output circuit of the vertical deflector and that in the horizontal deflection portion of the circuit, transformation is obtained by tapping down on an output inductance 65. Operation without transformation in the output circuit is generally satisfactory if approximately 750 turns are employed in the vertical deflection coils of the yoke, and approximately 250 turns are employed in the horizontal deflection coils 41.

A quicker retrace time may be obtained by reducin the yoke inductance and increasing the power input.

Circuit damping is furnished through transformer Si by the employment of a variable resistance 69.

Although values of resistances and capacities are given, together with tube types and turns of transformer and inductance windings, it is not intended that this invention should be limited to the values given.

Having thus described the invention, What is I claimed is:

connected to a point of positive potential and said condenser connected to a point of fixed potential a rectifier connected in series with-another winding of said transformer, said rectifier and the latter mentioned winding of the transformer connected between a point on said resistance and a point of fixed potential a connection between said control electrode and a point on said resistance, the connections of said transformer being polarized to impose a potential in series with said rectifier in the direction of-greater rectifier conductivity upon a decrease in auxiliary electrode current, and an output circuit connected between said anode and cathode.

2. An oscillator comprising an electron discharge tube having a cathode, an anode a control electrode and at least one auxiliary electrode, a transformer having a plurality of windings, one end of each of said windings being connected to a point of fixed potential, a connection between a point on a first winding of said transformer and said auxiliary electrode, a serially connected rectifier resistance and condenser connected in series with a second winding of said transformer, a connection between said control electrode and said resistance, the connections of said transformer being polarized to impress across said rectifier a potential in the direction that said rectifier is non-conductive in response to an increase in auxiliary electrode current, and an output circuit connected between said anode and cathode.

3. A signal generator comprising in combination, an electron discharge tube having at least an anode, a cathode and a first and second control electrode, a transformer having at least a primary and secondary winding, a diode having an anode and cathode, connections for placing said diode in series with said secondary winding between said first control electrode and said cathode, the diode anode and cathode being so connected to permit electron flow therethrough only in the cathode-anode direction, a resistance and capacitance connected in series to form a combination between said first controlelectrode and said cathode, means for applying a unidirectional charging potential to said capacitance for charging the first control electrode terminal of said capacitance in a positive direction relative to said cathode at a predetermined rate, a biasing circuit connected between said second control electrode and said cathode, said circuit including a potential source for establishing said second control electrode positively with respect to said cathode, connections placing said transformer primary in series with said biasing circuit with such winding polarity to induce a non-conductively polarized secondary voltage in series with said diode in response to an increase in said second control electrode current, and an output circuit connected between said anode and said cathode.

4. Apparatus according to claim 3 wherein said unidirectional charging means comprises a resistance connected between a source of charging potential, positive with respect to said cathode, and said first control electrode.

5. A sawtooth deflection circuit for driving an electromagnetic deflection yoke winding comprising in combination, an electron discharge tube having at least an anode, a cathode and a first and second control electrode, a transformer having at least a primary and secondary winding, a

diode having an anode and cathode, connections for placing said diode in series with said secondary winding between said first control electrode and said cathode, the diode anode and cathode being so comiected to permit electron flow there- .through only in the cathode-anode direction, a

8 former primary in series with said biasing circuit with such winding polarity to induce a non-conductively polarized secondary voltage in series with said diode in response to an increase in said second control electrode current, a source of positive anode biasing potential referenced to said discharge tube cathode, an electromagnetic impedance matching device having primary and second terminals, connections placing said matching device primary between said discharge tube anode and said positive biasing source, and connections placing said deflection yoke winding across said matching device secondary.

6. Apparatus according to claim 5 wherein one terminal of said deflection yoke is connected by a low impedance path to said discharge tube cathode.

KURT SCI-ILESINGER.

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

UNITED STATES PATENTS Great Britain Mar. 4, 1935

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