US2852681A - Setting of circuits for high frequency, high efficiency oscillators - Google Patents

Description

p 1958 P. M. G. TOULON 2,852,681

SETTING OF CIRCUITS FOR HIGH FREQUENCY, HIGH EFFICIENCY OSCILLATORS Filed July 9, 1947 FIG. IA.

FIG. 18.

FIG. IC.

8 LIVI FIG. ID.

FIG. IE.

FIG. IF.

INVENTOR.

Pierre Mv G. TouIon ATTORNEYS SETTING F CIRCUITS FQR HIGH FREQUENCY,

HIGH EFFICIENCY OSCILLATORS Pierre Marie Gabriel Toulon, Paris, France, assignor, by mesne assignments, to Moore and Hall, Washington, D. C., a partnership Application July 9, 1947, Serial No. 759,810

4 Claims. (Cl. 250-36) My invention relates to improved circuits designed for high frequency oscillators and amplifiers, intended for considerably increasing the efiiciency and the output power obtainable from the said circuits, and characterized particularly by a new process of control of the grid in the said tubes.

Various processes have already been devised in order to obtain the said improvements. For instance oscillating circuits have been already realized in which the output is limited to a very small fraction of the duration of a cycle (the said circuits being called Class C circuits) but it is well known that in such circuits the A. C. plate voltage cannot be increased beyond certain limits for fear of causing a deformation of the plate current liable to lower the proportion of the fundamental, which is the useful component. In order to make the plate voltage at every moment larger than (or at least equal to) the grid voltage, it has already been suggested to change the shape (initially sinusoidal) of the high frequency plate voltage, so as to reduce its crest amplitude while maintaining the same amplitude of the fundamental. To that efiect, various processes are already known which mainly consist in introducing in the plate circuit the third harmonic, so as to give it a proper phase and amplitude with reference to the fundamental.

While the introduction of a circuit (which could be called,damping circuit) in series with the oscillating plate circuit makes possible a small reduction of the voltage drop in the tube and a longer duration of flow of the current, thus allowing to obtain a better efliciency this latter yet remains rather poor, because the power dissipated by the grid is very important. On the other hand, the series connection of two oscillating plate circuits meets with practical difficulties, due to the fact that the phase of the voltage in the circuit of the harmonic is not easy to define with precision and it depends on the load.

In a general way all the systems belonging to the class C type offer very serious difiiculties. The first one is due to the very high value of the voltage amplitude which has to be applied to the grid, wherefrom are likely to result important drawbacks, for instance a failure in the insulation between grid and plate or between the grid and the ground, with important electrostatic effects on the grid connections. The second source of diificulty results from the very large grid current which comes out when the grid is positive while the plate voltage is very low. The third source of difiiculty lies in the considerable relative importance of the instantaneous output current of the tube, which necessitates a very large electronic emission, and consequently the use of cathodes absorbing large amounts of current.

My invention provides for the overcoming of all those difficulties, in supplying to the grid a complex periodic voltage by an improved application of the technical principles described in several prior patent applications (third addition filed June 15, 1937, to French Patent 819,199

2,852,681 Patented Sept. 16, 1958 ice The technical. means outlined in the above patents consist in. generating a voltage of a desired shape, by a synthetic operation carried out on the components of the said desired voltage.

According to the present invention, a complex voltage, synthesized for example by the techniques described in the aforementioned French patents, is applied to the grid (or grids) of the tube (or tubes); and the said synthesized complex grid voltage varies in conformity with the following sequence of conditions: I

(a) The applied grid voltage remains, during the largest fraction of its cycle, at a value negative enough to prevent any production of plate current.

(b) The applied grid voltage suddenly assumes a positive value, at the very moment when an output current is desired.

(0) The grid voltage remains at a relatively low positive value, relatively low, when the plate voltage is minimum (in order to avoid any useless flow of grid current of too high an intensity).

(d) The applied grid voltage rises to a higher positive value when the plate voltage increases again, and

(e) The applied grid voltage then falls abruptly to a negative value in order to cut ofi the plate current at the proper time.

Thanks to the above operating conditions, the power dissipated bythe grid is reduced to a minimum, the rela-' tive duration of action of the triode is brought down to a very low value (generally expressed in angle and called its opening angle) and the efiiciency obtained is excellent.

My invention applies particularly to constant power oscillators (working in frequency modulator systems) or in connection with some treatments involved in high frequency industrial heating operations, in which the efficiencies can attain extremely high values, for instance to For a better understanding of the invention various curves have been included in Fig. 1.

An oscillating triode belonging to the Class C type can be employed, for example, to convert direct current to alternating current. Figure 1a illustrates a D. C. voltage 4 which must be converted by such an oscillating triode into an alternating current, and the numeral 1 designates the alternating voltage of the load or output circuit. In order to realize the said conversion with a high efiiciency, the triode must obviously realize during a short fraction of the cycle, a true commutation; and it is necessary therefore that the 'conductibility of the tube be shifted abruptly from zero to as high a value as possible at time 2 (the voltage drop in the tube must be exceedingly reduced between times 2 and 3) the said conductibility returning abruptly to an as low as possible value at time 3. 9

During the following cycle the same successive changes must be repeated integrally at times 2' and 3'.

For securing a very abrupt conduction and cutting of the triode, one should apply to the grid a voltage which varies very suddenly at the opportune time. If the grid is fed with an alternating voltage having the same frequency as the load circuit, use must be made of an extremely high amplitude.

In Fig. lb has been shown the variation curve of that voltage as a function of the time. The drawbacks of that system so frequently used can be readily understood. At first the very high negative voltage, in comparison with the voltage applied to the grid at time 8", threatens to injure the insulation. Moreover, the high positive grid voltage at time 8 results in extremely large grid current, the more dangerous at that time as the lower is the corresponding instantaneous voltage of the plate.

According to my invention, the sinusoidal voltage 8 how i d tt d lin s is repl c d s a arid pp y y a periodic complex voltage designated 6-56' 7 etc. in

1b, this latter voltage resulting from the superposition on the fundamental frequency 1 of one or several harmonic frequencies of appropriate rank, amplitude and phase. As can be seen in Fig. 11'), the slope of the composite waveform in the zone of progressive control of the triode, extending between zero triode current and triode saturation current (i. e. between the straight lines XX and YY) is practically the same as the slope of the sinusoid 8. On the other hand, the total voltage amplitude between grid and cathode is relatively very low; and particularly the negative maximum value 7 is much smaller in magnitude than the negative maximum value of the sinusoid 8. This reduction of maximum negative grid voltage is of the highest importance in maintaining the life of the triode. Moreover, at the time of passage of the plate voltage through its minimum (e. g. between times 2 and 3) the curve of the grid voltage does not rise beyond all measure (as would be the case at time ,8 if a conventional grid voltage such as that illustrated at 8 were employed) but, to the contrary, the grid voltage actually falls to a minimum 5. By referring to the characteristic curves of the triodes, it will be seen that a given current is obtainable with a lower grid voltage when the plate voltage decreases. lt'has already been seen that grid cu rrent commences to appear for a less positive value of the yoltage. Thus, when the composite grid voltage of the prescntjnvention is employed, one is ableto limit gridcurrent to arelatively very low value for the whole duration ,of the unhlocking of the grid (as shown at points ,6, 5 6') and the power dissipated in control of the grid is thus maintained at a very low value. In distinction to ,ordi-. nary Class ,C oscillators, for which it is generally admitted that nearly /8 of the power supplied must be slost foroperatingthe grid, a circuit constructed in accordance with the present invention permits the same result 'to .be secured with a much lower amount of power (approximately A of that previously required). The relative duration of action of the triode, which the usual circuits had not allowed to bring down under 70, can nowbe considerably reduced (down to to for instance), and it becomes possible to use without any inconvenience tubes oliering a very low internal drop of voltage (the cathode of \vhich may be indirectly heated, provided with a very large surface, and most convenient for the very large outputs).

The periodical curve given as an example in Fig. 1b (points 6, 5, 6', 7, 6", 5', .6") results from the superposition of a fundamental frequency with one or several harmonies of anappropriateorder (which in the ease'of the figure is the sole harmonic 5). The harmonic or harmonies actually superposed on the said fundamental must obviously be chosen in conformity with the opening angle to be adopted. The said angle should be chosen as small as possible for a given saturation current of the triode.

On the Figs. 1c and 1d the fundamental frequency has been designated 9 and the 5th harmonic of said fundamental has been designated 9. By addition of the two curves 9 and .9, the desired complex grid voltage curve has been obtained as shown in 65-6-7. It is obvious that the relative phases of the two frequencies 9 and 9' must be determined with very high precision. The exact superpositionofthe two frequencies must be made withall the necessary accuracy and the resultant voltage must then be amplified up to the desiredamount, by-an application of the technical means already described in various occasions, particularly in the above mentioned French patents and applications. The grid circuit of the oscillator includes several series connected oscillating circuits tuned to the selected harmonic frequencies to be superimposed. For producing in the said harmonics the desired voltages having the proper amplitudes and phases, use is preferably made of such circuits as the cathodyne or other counter-reaction circuits under the lead of a special master circuit in conformity with the technique already developed in the above patents.

in order to generate the above said harmonics (to he later amplifiedto a proper degree and with a properly selected phase), my invention provides a very convenient means of operation. It consists in connecting with the main frequency a relaxation oscillator circuit producing a sawtooth voltage such as the one designated 10 in Fig. 12. By means of that variable voltage, and by using a transformer which gives the derivative of the current, a periodic pulse is generated, such as shown at 11 in Fig. If; and therefore are finally extracted, by means of appropriate filters, the various harmonics needed.

The carrying out of my invention is illustrated by means of a sole instance a few figures only, but its application is in no way limited to such a case'an'd such a particular device. Other appropriate means may indeed'be called to secure the control voltage of the grid of the tube provided that a shape be obtained for that voltage which reproduces with a ,sufiicient approximation the'shape of the complex grid waveform curve shown in Fig. lb.

My invention, moreover, does not exclusively apply to high vacuum tubes, but it can also find application, under certain conditions, to circuits involving the useof'gas l h s- Wh tl c a ms;

1. The method of operating an electronic tube in:

cluding applying to said electronic tube a'cont rol voltage of predetermined frequency for generating in its output circuit pulses of current of predetermined duration and of saidpredeterrnined frequency,comprising the steps of applying a negative bias to such control electrode sufiicient normally to maintain said tube substantially beyond Cut-Off, applying ,to said control electrode a signal composed of a first sinusoidal voltage at said predetermined frequency anda further sinusoidal harmonic of said predetermined voltage which is so phase relative tosaid first sinusoidal voltage that a trough of said harmonic concurs in time with a crest of saidhrst sinusoida'l voltage.

2. Aqnethod in accordancewith claim 1, wherein said harmonic voltage is an odd harmonic of said predetermin d se 3. A ela-5s C oscillator comprising an electronic tube including a control electrode, a voltage source so .connected to said control electrode and of such value as to maintain said tube substantially at cut-off, an oscillatory circpitnetwork connected to the out-put of said tube, a network coupling said oscillatory network to said control electrode whereby a fundamental sinusoidal voltage is applied to said control electrode in phase opposition to the output voltage of saidtube, a network adaptedto gen erate a sinusoidal voltage at an harmonic of said fundamental voltageandanetw orkfor applying said harmonic to .said control electrodein such phase that a trough of said harmonic voltage coincides with the crests of said f n am nta v lt i l 14 A class C oscillator comprising an electronic tube including a control electrode, a voltage source so connectedio said control electrode. and of such value as to m ntain s nb subst m a c -o a os at y i i znst qrk. on c e t t u -P1 s d u networkconpling said oscillatory networkto said control stmdevherebr a und m nta uso a voltage i applied to said control electrode in phaseoppositipn to the output .voltage of said tube, a relaxation oscillator adapted to generate a saw-tooth voltage at a harmonic of said fundamental voltage, a network for transforming said saw tooth. voltage to a sinusoidal voltage, anda ne'twprk for applying saidharmonic sinusoidal voltage to said control electrode whereby a trough of said harmonic voltage coincides with the crests of said fundamental voltage.

References Cited in the file of this patent UNITED STATES PATENTS 5 Sprague Ian. 3, 1928 Rettenmeyer July 24, 1928 Schafier July 2, 1929 Peterson Feb. 7, 1939 10 Moore Sept. 7, 1948 6 Beurtheret Ian. 2, 1951 Andrew Apr. 1, 1952 FOREIGN PATENTS Great Britain July 20, 1920 Great Britain Feb. 8, 1923 Great Britain July 5, 1923 OTHER REFERENCES Radio Physics Course, by Ghirardi, second edition,

pages 664 and 665.

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