US2248975A - Electrical oscillation generator - Google Patents

Description

July 15, 1941. i c, FAUDELL I 2,248,975

ELECTRICAL OSC ILLATION GENERATOR File'd Feb. 11, 1939 H DEFLECT/ON col/Q DEFL ECTION 1 COIL lNVENTOR CHARLES LES IE AUDELL ATTORNEY Patented July 15, 1941 ELECTRIQAL GSCELLATION GENERATQR (Charles LeslieFaudeil, Stoke Poges, England, assignor to Electric & Musical Industries Limited, Hayes, Middleseg, England, a company of Great Britain Application February 11, 1939, Serial No. 255,7il5

. In Great Britain February 19,.15238 5 Claims.

This invention relates to electrical oscillation generators and more particularly to electrical oscillation generators for generating saw-tooth or substantially saw-tooth electrical waves suitable for scanning deflections in television and like apparatus.

With the normal type of relaxation oscillation generator, say, for example, a blocking oscillator, as employed for the production of saw-tooth electrical waves the output is usually insufficient to deflect the scanning beam, for instance, of a cathode ray tube, to the required degree and it has been the normal practice to connect. a thermionic amplifier or output stage after the oscilla tion generator in order to increase the output. has been proposed to apply negative feedback to such an output stage both to straighten the amplifying characteristic and to linearize the charging curve of the condenser of the relaxation oscillation generator.

Such circuits are usually critical of adjustment and need quite a number of components for their successful operation. It is the object of the present invention. to provide an improved saw-tooth oscillation generator which has a number of advantages over known saw-tooth oscillation generators in operating characteristics and in economy of components and consequent cheapness of manufacture.

According to the invention, a saw-tooth oscilvalve for supplying a substantially saw-tooth electrical waveform to.an output circuit, said output valve being provided with negative feedback for improving said waveform, the output circuit of said output valve being coupled back to the grid-cathode circuit of the thermionic valve used for discharging said condenser for the purpose of producing blocking oscillations.

For the purpose of describing the invention more in detail reference will now be made to the accompanying drawing in which Figures 1 and 2 illustrate particular embodiments according to the invention.

Referring to Figure l, the anodes of the two triode thermionic valves illand 5 are joined by a charging condenser 3, and the anode of valve It is connected to a source of positivepotential through the charging resistance i, the. anode of valve 5 being connected to the same source by means of an output transformer 6 to which the deflecting coil ll of a cathode ray tube is connected. The cathode of valve it is connected to the negative side of the potential source, to which is connected the grid of the valve it by means of grid resistance 2. The grid of valve It is connected to the anode of valve 5 via condenser l and the anode of valve it is connected to the grid of valve 5 via condenser 8. The resistance i3 and condenser i l form the self-biasing circuit for the cathode of valve 5, the resistance 9 being the grid resistance.

For the purpose of describing the operation of this circuit it will be assumed that the cycle commences with the grid of valve it] having a large negative charge, which is leaking away through the resistance i2. The valve it being non-conducting in this condition, the condenser 3 will be charged through the resistance 4 and the resultant positive saw-tooth potential will be applied to the grid of valve 5. The side of condenser 3 remote from resistance 4 being connected to the anode of the valve 5 and to transformer 6 will become increasingly negative with the rise of anode current, the greater part of the rise of potential across condenser 3 being thereby neutralised. If, however, the fall of potential at the anode of valve 5 becomes less rapid, due, for example, to transformer inductance, there will be a corresponding increase of potential at the grid, thereby increasing the current and tending to main tain a linear fall of potential at the anode of valve 5. Meanwhile, the negative charge on the condenser i has leaked away through resistance 52 and the valve ill commences to conduct. Consequently, the potential on' the grid of valve 5 will cease to rise and will begin to fall, which causes the potential at the anode of valve 5 to rise. Now

the grid of valve it being connected via condenser l to the anode of valve 5, will become increasingly positive in potential until grid current flows thereby producing a large negative potential across the resistance i2 and renderingthe valve non-conducting again to re-start the cycle.

It will be noticed that because of the negative feedback connection afforded by condenser 3 between the input and output of valve 5, the long flank of the sawtooth waveform will be substantially linearised and in addition because the output circuit has a greater inductive reactance during the short flank than during the long flank, the short flank is shortened resulting in improved. scanning. In order to shorten the short flank of the saw-tooth wave, condenser I should be appreciably smaller than condenser 3. The short flank of the saw-tooth wave may be further improved if desired, by inserting a suitable resistance in series with condenser 3 or a low-pass filter may be associated with the feedback path for the same purpose.

The frequency of the saw-tooth oscillations is controlled by varying the resistance [2, and the amplitude of the oscillations is controlled either by varying the resistance 4 or, preferably, by varying the potential applied to the resistance. It is found that variation of amplitude tends to vary the frequency, for example, as the amplitude is increased, the frequency is lowered. This effect may be largely overcome by returning the grid leak resistance l2 to the anode of valve H1 or to a tapping on resistance 4, instead of to the cathode of valve 10. By this means an increase in amplitude will also cause an increase in the positive potential on the grid leak I2 and thereby shortening the time of recovery to the critical grid potential. When using this method the time constant of the condenser l and resistance l2 can be increased.

In order that the oscillator may be synchronised with the television transmitter, synchronising signals may be applied in a positive sense to the grid of valve II] or to the anode of valve 5. Alternatively synchronising signals in the negative sense may be applied to the grid of valve 5 or to the anode of valve In. In the latter case the synchronising pulse will be amplified by valve 5 before being fed to the grid of valve l0.

Referring to the embodiment of the invention shown in Figure 2, in which identical numerals are used for the identification of components which have similar functions to those shown in Figure 1, it will be seen that a cathode follower type of circuit has been adopted for the output of valve 5. A cathode follower type of circuit is one in which .negative feedback is obtained by the provision of an impedance in the cathode lead of a valve, which impedance is large compared with the inverse of the mutual conductance of the valve, so that the potential of the cathode follows substantially potential variations of the grid of the valve. The main point of difference between the cathode follower and other negative feedback circuits lies in the fact that the former will reproduce the input wave form almost exactly provided that the load circuit is effectively resistive. In the case where the output load comprises a transformer nonlinearity due to low primary inductance is successfully compensated. Another useful feature is that where the load is partly inductive, as during the short flank period, a larger negative bias than usual can be applied to the valve thus economising in anode current. In practice, the primary winding of the transformer is wound with a suitable D. C. resistance for automatically biasing the valve. It will also be noticed that the circuit of Figure 2 combines a discharging valve In with a cathode follower valve to form an oscillating circuit. In operation it is'similar to the circuit of Figure 1, except that a non-linear input waveform of the valve 5 is not linearised. In Figure 2, the blocking condenser I is connected between the cathodes of the two valves Ill and 5 and the resistance I2 is used to produce the cathode bias of valve Ill. The output to the deflecting coil H is taken from transformer 6 in the cathode circuit of valve 5, the anode of which is taken directly to the positive potential source. The grid of valve I0 is connected directly to the negative potential source.

Assuming that the cathode of valve In is positive and that the valve is non-conducting, it

will be seen that the condenser 3 charges'up through the resistance 4. The application of the increasing positive potential via condenser 8 to the grid of valve 5 causes the cathode potential of this valve to become positive, owing to the cathode follower action. Meanwhile, the positive charge in the condenser l leaks away through resistance I2 until the bias is insufficient to maintain the valve I0 in an insulating state. When therefore valve Ill conducts the potentials at the anode of IO and the grid of 5 will commence to fall, and the cathode of 5 will follow. This fall of potential will be accelerated by the coupling of the cathode of 5 to the cathode of [0 via condenser l and the fall will continue until the current through the valve In passing through resistance [2 is sufficient to overcome the negative charge on condenser and to raise the oathode to such a positive potential that the valve is rendered non-conducting again.

As in the previous circuit the frequency of oscillation is controlled by the time constant of resistance l2 and condenser 7. Alternatively a resistance and condenser l5 may be connected between the grid of tube l0 and the resistance [2 for controlling the frequency of oscillation. An advantage of this modification is that the resistance 12 in the cathode circuit can be made smaller resulting in increased amplitude of the saw-tooth potential at the anode of valve Ill. The short flank of the saw-tooth wave may be shortened by connecting a suitable resistance in series with condenser 3 or a low-pass filter may be employed as stated above.

synchronising signals can be applied in a negative sense to the grid of valve 5 or to the anode of valve l0. Alternatively the synchronising signals may be applied in a positive sense to the grid of valve l 0. In the latter case the combined picture and synchronising potentials may be applied to the grid without separation if the D. 0. component of the signals is present, and a low pass filter is used to cut off frequencies higher than the frame frequency, assuming that the oscillation generator is being used for frame scanning.

The circuit of Figure 2 may be modified by the inclusion of an impedance in the anode circuit of valve 5 and by connecting the condenser 3 between the anode of valve I0 and the anode of valve 5 thus forming a negative feedback circuit which has the effect of linearising the charging curve of condenser 3. This anode impedance may consist of an additional winding on the transformer 6. It will be apparent in the circuit of Figure 1 that transformer 6 may have an additional winding connected in the cathode circuit of valve 5 in place of resistance I3 and condenser I4 for the purpose of additional negative feedback.

Although the circuits have been described with reference to magnetic deflection it will be apparent that electrostatic deflection may be employed. In this connection it is possible to use a resistance in series with a small inductance in' place of the output transformer 6 shown in the figures. Push-pull output potentials of sawtooth waveform can be obtained by utilizing the method outlined in the previous paragraph, whereby the output valve has impedances in both the anode and cathode circuits. A transformer with a centre-tapped secondary may be used for 5 in Figure 2for the purpose of obtaining pushpull output potentials. It will be obvious that although a transformer is shown for the output circuit, that a choke or resistance may be used with a condenser for the purpose of coupling the valve to the load.

It should be noticed that the circuits described are particularly useful for the production of frame scanning oscillations in a television recciver or in other circuits where the resistive component of the output circuit, as, for example, the scanning coils, is greater than the reactive component, at least during the long flank of the saw-tooth waveform.

I claim:

. 1. A sawtooth oscillation generator comprising a first thermionic tube having a cathode, grid and anode, a variable resistor connected between said cathode and grid, a second thermionic tube having a cathode, grid, anode and an electron path between the cathode and anode of said second thermionic tube, at least one source of direct current supply voltage, a serially connected resistor and condenser, said serially connected resistor being connected to the positive terminal of said source, a connection from the junction point of said serially connected resistor and said condenser to the anode of said first thermionic tube, a capacity coupling from said junction to the grid of said second thermionic tube, a resistor connected between the grid of said second thermionic tube and the negative terminal of said source, an impedance connected in series with said source and said electron path, a direct connection between said condenser and said impedance, means to supply output energy from said impedance, and capacitative coupling means connected between said impedance and said variable resistor.

2. A sawtooth oscillation generator comprising a first thermionic tube having a cathode, grid and anode, a variable resistor connected between said cathode and grid, a second thermionic tube having a cathode, grid, anode and an electron path between the cathode and anode of said second thermionic tube, at least one source of direct current supply voltage, a serially connected resistor and condenser, said serially connected resistor being connected to the positive terminal of said source, a connection from the junction point of said serially connected resistor and said condenser to the anode of said first thermionic tube, a capacity coupling from said junction to the grid of said second thermionic tube, a resistor connected between the grid of said second thermionic tube and the negative terminal of said source, a transformer having a primary and secondary winding with said primary winding connected in series with said source and said electron path, a direct connection between said condenser and said primary winding, a, deflection coil connected to said secondary winding, and capacitative coupling means connected between said primary winding and said variable resistor.

3. A sawtooth oscillation generator comprising a first thermionic tube having a cathode, grid and anode, a variable resistor connected between said cathode and grid, a second thermionic tube having a cathode, grid, anode and an electron path between the cathode and anode of said second thermionic tube, a source of direct current supply voltage, a serially connected resistor and condenser, said serially connected resistor being connected to the positive terminal of said source,

a connection from the junction point of said serially connected resistor and said condenser to the anode of said first thermionic tube, a capacity coupling from said junction to the grid of said second thermionic tube, a resistor connected between the grid of said second thermionic tube and the negative terminal of said source, a transformer having a primary and secondary winding with said primary winding connected in series with said source and the anode of said second thermionic tube, a direct connection between said condenser and said primary winding, a deflection coil connected to said secondary winding, and capacitative coupling means connected between said primary winding and said Variable resistor.

4. A sawtooth oscillation generator comprising a first thermionic tube having a cathode, grid and anode, a variable resistor connected between said cathode and grid, a second thermionic tube having a cathode, grid, anode and an electron path between the cathode and anode of said second thermionic tube, a source of direct current supply voltage, a serially connected resistor and condenser, said serially connected resistor being connected to the positive terminal of said source, a connection from the junction point of said serially connected resistor and said condenser to the anode of said first thermionic tube, a capacity coupling from said junction to the grid of said second thermionic tube, a resistor connected between the grid of said second thermionic tube and the negative terminal of said source, a transformer having a primary and secondary winding with said primary winding connected in series with said source and the cathode of said second thermionic tube, a direct connection between said condenser and said primary winding, a deflection coil connected to said secondary winding, and capacitative means connected between said primary winding and said variable resistor.

5. A sawtooth oscillation generator comprising a first thermionic tube having a cathode, grid and anode, a variable resistor connected between said cathode and grid, a second thermionic tube having a cathode, grid, anode and an electron path between the cathode and anode of said second thermionic tube, a source of direct current supply voltage, a serially connected resistor and condenser, said serially connected resistor being connected to the positive terminal of said source, a connection from the junction point of said serially connected resistor and said condenser to the anode of said first thermionic tube, a capacity coupling from said junction to the grid of said second thermionic tube, a resistor connected between the grid of said second thermionic tube and the negative terminal of said source, a transformer having a primary and secondary winding with said primary winding connected in series with said source and said electron path, a direct connection between said condenser and said primary winding, a deflection coil connected to said secondary winding, and capacitative coupling means connected between the cathodes of said first and said second thermionic tubes.

CHARLES LESLIE FAUDELL.

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