US2161087A - Oscillator of the magnetron type - Google Patents

Description

MAX. MODULATIU Z K. POSTHUMUS ET AL OSCILLATOR OF THE MAGNETRON TYPE Filed Dec. 17, 1935 v I E5 DEMENSION RATIO June 6, 1939.

HIGH FREQUENCY cumsnr M y ANODE VOLTAGE INVENTOR KLAA 5 POST/104105 ATTORNEY C421. 6. MAM/0E2 71-4 Patented June 6, 1939 .UNirEDsTATss PATENT OFFICE OSCILLATOR OF THE MAGNETRON TYPE Klaas- Posthumus and Carl G. A. von Lindern,

Eindhcven, Netherlands, assignors to N. V.

Philips Gloeilampenfabrieken,

Netherlands Eindhoven,

3 Claims.

This invention has reference to an oscillator in which the frequencyof the oscillations generated depends to a marked extent on the voltages supplied to the electrodes of its electron dis- .5.' charge tube. It is a feature of our invention that the characteristic curve of modulation exhibits discontinuities, due to the superposition upon a variable modulating wave of another wave of predetermined frequency.

An example of such an oscillator is of the magnetron type comprising two or more pairs of anodes connected to. an output impedance in such manner that the alternating potential of each anode is in opposite phase with the alternating potential of the adjacent anode and in which the intensity-of the field is higher than the critical value. Such an oscillator is capable of generating oscillations the frequency of which depends on the voltage supplied, to the anodes.

In the employment of the usual method of modulation in-the. circuit arrangement described in which the anode voltage varies in accordance with the low frequency modulating voltages, we obtain in addition to amplitude variations considerable frequency variations which are highly disadvantageous.

In order that the invention may be clearly understood and readily carried into effect it will now be described more fully with reference to the accompanying drawing, in which Figure 1 shows the characteristic curve of modulation of a magnetron oscillator at a given value of the intensity of the field;

Fig. 2 shows diagrammatically an embodiment 3 of the invention; and Fig. 3 is a graph illustrating the theory of the invention.

Referring now to Fig. 1, the high frequency current iw is plotted as a function of the anode voltage Va. Between the values for the anode voltage designated by b and c, oscillations are continuously generated by the circuit arrangement described. When, however, a modulation voltage is applied, according to prior art practice,

such as would increase the anode potential above the value of c, then on this rising value up to the point 11, oscillations would persist in the magnetron discharge tube while with a falling voltage from the value at to c, oscillations would be interrupted. Similarly, the application of modulation voltages such as would counteract the steady current anode potential and reduce the same to a value between b and a, would still maintain oscillations in the magnetron tube down to the value a, but oscillations would be interrupted at the time of an increasing voltage between the value a and the value b. In other words the modulation depth is limited to the ratio ef.'fc in which e is the mean value between the values cc and cg. Furthermore, it will be seen that there is a certain persistence of oscillations on rising potentials above and falling potentials below the range be of oscillation-exciting voltages, and that oscillations are caused to be interrupted by an initial rise of modulation potential within the range a and b as well as by an initial drop of modulating potential between the range (1 and c.

The employment of the usual modulation method in the oscillators to which the invention relates involves therefore considerable difiiculties.

A further example of an oscillator in which the frequency of the oscillations generated depends on the voltages supplied to the electrodes and in which the characteristic curve of modulation exhibits discontinuities is in a Barkhausen-Kurz oscillator.

According to the invention, the modulation is effected by intermittently bringing about at one or more electrodes a potential variation during a series of time intervals the length of which depends on the instantaneous Value of the amplitude of the modulating oscillations.

This method of modulating oscillations per se has previously been indicated, but in connection with transmitters in which the said dimculties are not encountered, for the purpose of eliminating the effect of fading and of limiting the maximum voltages and energies occurring during modulation.

Fig. 2 shows an oscillator 1 of the kind described. The anode voltage is supplied from the source 2 to one or more electrodes of the oscillator I via a resistance 3 and a discharge tube 4 is connected in parallel with the series connection of the source of anode voltage 2 and the resistance 3. The grid circuit of the tube 4 includes a series connection of the source of voltage 5 in which medium frequency oscillations are generated and a microphone 6 in which the oscillations to be modulated are generated.

Referring to Figure 3, I designates a low frequency modulating voltage generated in the microphone and supplied to the grid of the tube 4 and 8 is the result of the superimposition of this voltage generated in the microphone and the medium frequency alternating voltage from the source 5. This frequency of the source 5 is not critical but is preferably higher than the highest modulating frequency occurring. In order to avoid distortion, the maximum amplitude of the medium frequency voltage is made equal to or higher than the maximum amplitude of the low frequency modulating voltage. The resulting voltage designated by 8 is supplied to the grid of the tube 4 which does not allow current to pass until the voltage amplitude supplied to the grid is equal to or higher than the voltage the value of which is designated by the horizontal line 9. A blocking bias source may be provided in the input circuit between'the cathode and the grid of the tube 4. The tube 4 is therefore conductive during those time intervals which in Figure 3 are designated by thick lines.

If the curved shape of the alternating voltages were triangular, there would be a purely linear ratio between the instantaneous value of the voltages generated in the microphone and the time in which the tube is conductive, but even in the case of a sinusoidal alternating voltage the approximationof this linear ratio suffices to obtain in the receiver a practically pure reproduction of the emitted low frequency oscillations.

The tube 4 is a so-called absorption tube the internal resistance of which has a substantially constant value when a grid bias equal to or higher than that designated by the horizontal line 9 is supplied. This ensures that during the time intervals in which the tube A is conductive the electrodes of the oscillator I have supplied to them such an anode voltage that the oscillator does not generate or only generates to a slight extent, whereas during the time intervals in which the tube 4 is non-conductive, the anode voltage at the electrodes of the oscillator I is such that oscillations having a constant amplitude are generated. a

The anode voltage is preferably such that even when the tube 4 is conductive the oscillator keeps generating to a feeble extent, as in this case with the desired variation of the anode voltage the correct frequency is obtained at once. If, on the contrary, generating is entirely interrupted first, an undesired frequency may still become manifest.

In the case of the circuit arrangement described the oscillations generated by the oscillator are emitted in the antenna connected to the oscillator periodically with a constant amplitude during a series of time intervals the length of which depends on the instantaneous value of the modulating voltages.

We claim:

1. An oscillator having an electron discharge tube of the magnetron type andlhaving a plurality of anode elements and a cathode in circuit therewith, a source of potential and a resistor in series between the anodesand the cathode, and modulating means including a source of variable waves and a source of alternating current of predetermined frequency, and means including a second electron discharge tube connected to the oscillator tube circuit for causing the anode voltage applied to the oscillator to vary abruptly between a value which permits of generating oscillations at a substantially constant frequency and a value which permits of generating oscillations of only negligible intensity.

2. An oscillator having an electron discharge tube of the magnetron type and having a plurality of anode elements and a cathode in circuit therewith, asource of direct current potential and an impedance in series between the anodes and the cathode, a second electron discharge tube having a'space path in shunt with the space path of the oscillator tube, means including a microphone in series with a source of alternating current and agrid blocking bias source for varying the impedance of the second electron discharge tube, and means for so adjusting the amplitude of the alternating current component with respect to the modulating component arising in the microphone that said second tube'is caused to intermittently interrupt the oscillatory action of the first said tube by the absorption of energy through its space path.

3. A high frequency oscillator comprising anelectron discharge tube of the magnetron type, including a linear cathode-and a plurality of surrounding segmented anodes, a resonant circuit system connected across the anode segments, a source of operating potential and an impedance connected between the cathode and said resonant circuit system, means for rendering said oscillator substantially intermittent in itsoperation, said means comprising a discharge tube having a discharge path in shunt with the discharge path of said oscillator tube, and means including a source of low frequency modulation potentials superposed upon a source of intermediate frequency potentials, said means being applied to an input circuit for the second said discharge tube for alternately blocking said tube and ren-'- dering the same conductive, thereby to control the anode voltage-applied to'said oscillator over a range sufiicient-tocause said intermittent oscillatory operation.

POS'IVHUMUS. CARL G. A. voN LINDERLN.

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