Tunable klystron oscillator

(i;e. having two cavities The present invention relates to a two-Klystron/used as a tunable self-oscillator.

It is known to use a two-Klystron as a self-oscillator.

A Klystron essentially comprises a first section having a cathodic structure which generates an electron beam, a second section formed by a radio-frequency structure in which interaction between the electron beam and. a radio-frequency field occurs, and a third section having a final structure or collector at which the electrons terminate their travel. In the kind of Klystron described above, the radio-frequency structure comprises 0n input cavity and an output cavity. In such a tube, the electron beam generated by the cathodic section passes through the gap grid of the input cavity to which a variable signal is applied which produces a speed modulation of the electrons.

The said «need modulation results in modulation of the density of the^electrical charges passing through the output / cavity«. Suck a pulsating electron beam energizes, by induction, radio-frequency fields in the output cavity, such fields being fed back to the. input cavity by way of a feed-back circuit at a suitable phase, thereby producing electrical oscillations .

Since the frequency‘ of the output signal from the output cavity is unequivocally determined by the dimensions of the

Λ r two cavities, the Klystrons generally comprise means for modifying the dimensions of such cavities so as to tune the oscillator to a predetermined frequency value.

Such tuning means generally comprise two small pistons by means of which it is possible to tune the self-oscillator by they are changing the extent to which rHr-is introduced into both the input cavity and the output cavity.

A Klystron of this type has the disadvantage of requiring a double tuning and, since for such a kind of tube 5t is usually required a high merit factor of the resonant cavities, the said tuning operation is particularly long and expensive.

To eliminate this drawback known solutions include the elimination of the said small pistons and insertion of a diaphragm facing the two cavities, so that by acting on the diaphragm it is possible to modify the output frequency of the selfoscillator.

Such a solution requires one accurate tuning but has the disadvantage that tuning is particularly critical since it modifies the feed-back circuit, and thus the level of the signal which is fed back to the input cavity is altered.

An object of the present invention is to provide a Klystron which can be tuned by means of a single accurate tuning which is deprived of the abovementioned disadvantages. To this end, according to the invention the input cavity has a tuning frequency substantially different (foi' example 10%) from the tuning frequency of the output cavity. Furthermore, the feed-back circuit as well as the grids of the two cavities are dimensioned so as ratio at the to have a suitable voltage/central timing frequency In this way a small variation (e.g. - 2%) in the frequency of the output cavity (and thus in the outpxit signal) does not appreciably modify the said voltage ratio.

Accordinglyj the present invention relates to a tunable Klystron oscillator of the type having two cavities, i.e. an input cavity and an output cavity, connected by way of a feedback circuit, in which oscillator:

- the input cavity is shaped so as to resonate at a fixed frequency having a value which substantially differs from the value of the frequency at which the output cavity resonates}

- the feed-back circuit and the grids of the said cavities are so shaped that the voltage ratio between the signal passing through the output cavity and the signal fed back to the input cavity has an optimum value in order to sustain the oscillating phenomenon;

- tuning is effected by means capable of modifying only the characteristics of the output cavity.

Further characteristics of the invention will appear from the following description of a non-limiting embodiment with reference to the single figure of the accompanying drawing which shows a cross-sectional view through an embodiment of a Klystron.

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In the drawings,_Atwo-cavity Klystron substantially comprises a cathode 1 which generates an electron beam, the electrons being subjected to an accelerating static force due to the anodic bias voltage when passing through the grids in the input cavity 2, thereby having a predetermined ):inetic energy associated with the speed V.

The grids form electrodes which control the speed and by onic means of which the said electr&s- beam is modulated in accordance with the timing of the signal v^j reaching the input cavity 2.

Such speed modulation results in a modulation of the density of the electronic beam since at a given distance from the grid the said electrical charges are packed owing to the fact that the fastest electrons reach the slowest onej. Accordingly, an elecvariable tron flow having a density vrsrtrte in accordance with the timing of the modulating signal is applied to the grids of the output cavities 3.

The/ flow of electrons thus obtained energizes, by induction, radio-frequency fields in the output cavities, such fields being fed back by way of the feed-back circuit 4 to the input cavity 2 to provide the input signal as ^a positive feed-back.

The electrons then continixe their travel towards a collector electrode 6 where they are collected.

The frequency of the output signal v^ from the output cavity 3 is determined by the dimensions of the two cavities 2 and 3. According to the invention the input cavity 2 is dimen״ sioned so as to resonate at a frequency which differs by approximately 10% from the resonance frequency of the output cavity 3» Furthermore, the feed-back circuit 4 and the grids are dimenratio at the sioned so as to have a suitable voltage/central tuning frequency. 1-at־i״o.

The said grids and the feed-back circuit 4 are dimensioned so that the feed-back signal has a value such as to obtain a stable oscillatory phenomenon and optimum efficiency.

The output cavity 3 is also provided with means 5 capable of modifying the dimensions, and thus the resonance frequency, in order to tune the Klystron at a desired frequency value. By varying by a small percentage (approximately ί 2%) the frequency ox the output cavity, for example by modifying the extent to which the small piston 5 is inserted into the cavity 3, the said voltage ratio does not appreciably vary, whereby the oscillatory phenomenon is kept stable .and‘ optimum efficiency is obtained large within a sufficiently tfgfe frequency band» Some dimensional values which make it possible to attain the aboveinent ioned. object are given below by way of example only.

If for example the input cavity 2 and the output cavity 3 are dimensioned so as to resonate respectively at a frequency of 8*5 GHz and 9*5 GHz and if the feed-back circuit 4 and the grids are so dimensioned that they have a suitable voltage/tatio at frequency equal to 9.5 GHz, since it is possible to experimentally prove that the oscillatory phenomenon remains stable and the efficiency is optimum until the voltage ratio does not - exceeds ί 5% the predetermined value, it will be noted that by varying the frequency of the output cavity 3 by ί 0,2 GHz, the voltage ratio varies to a negligible extent and in any case limits / does not exceed the abovementioned val-nes־.beyond which the I oscillatory phenomenon would be either changed or cancelled.

Accbrding to the invention it is thus possible to vary the frequency of the output signal within a range of 0,4 GHz, which cannot be obtained by a Klystron of conventional type in which tuning is effected on the feed-back circuit having its diaphragm facing both cavities.

The said tuning means 5 can be of mechanical type and in that case frequency variations are obtained by modifying the extent to which a small piston is inserted into the said output cavity 3» or of electrical type and in that case the piston is replaced by a dielectric means or non-linear component coupled to the fields of the output section only.