US2571676A - Circuit arrangement embodying electron discharge device employing hollow resonator - Google Patents

Description

Oct. 16, 1951 c. s. BULL 2,571,676

CIRCUIT ARRANGEMENT EMBODYING ELECTRON DISCHARGE DEVICE EMPLOYING HOLLOW RESONATOR' Filed Jan. 17, 1946 ELE C TRN ,2 Hl Y A #vm/me manif/wwwa Patented Oct. 16, 1951 UNITED STATES PATENT OFFICE CIRCUIT ARRANGEMENT EMBODYING ELECTRON DISCHARGE DEVICE EM- PLOYING HOLLOW RESONATOR Application January 17, 1946, Serial No. 641,842 Y In Great Britain May 21, 1941 Section 1, Public Law 690, August 8, 1946 Patent expires May 21, 1961 Cl. Z50-20) Claims.

This invention relates to circuit arrangements embodying electron discharge devices employing hollow resonators.

It is known that the usual circuit arrangements used for mixing oscillations at relatively low frequencies such as tens of megacycles/sec. are unsatisfactory for mixing oscillations of much higher frequencies such as hundreds or thousands of megacycles as difficulties arise due to damping of the circuits by transit time effects and radiation loss. If hollow resonators are used to prevent radiation loss and if a diode is incorporated in such a resonator said diode having a very small gap between its cathode and anode lthere remains the disadvantages that valves with small inter-electrode clearances are difficult to manufacture and also that it is diicult to effect a satisfactory connection between the cathode and the hollow resonator without adding losses to the circuit and causing loss of heat by conduction between the cathode and the resonator.

It is the object of the present invention to provide an improved circuit arrangement for mixing high frequency oscillations which will avoid the diculties or disadvantages referred to above.

According to the invention a circuit arrangement for mixing high frequency oscillations is provided comprising an electron discharge device having a hollow resonator, a cathode for projecting an electron stream through an aperture formed in a wall of said resonator, said wall constituting an accelerating electrode, and an anode for collecting electrons emitted by said cathode, means for applying the high frequency oscillations to be mixed to said device, at least one set of high frequency oscillations being applied so as to excite said resonator, at least a part of said anode being positioned within said resonator so that when the latter is excited a high frequency oscillatory field is set up which extends between said anode and said accelerating electrode, means for maintaining said accelerat ing electrode at a positive potential with respect to the potential of said cathode and for maintaining said anode at a lower potential than that of said accelerating electrode and substantially at cathode potential and an output circuit coupled with said anode in which an output of a desired beat frequency can be established, the arrangement being such that, during operation, part of the electron stream passing said accelerating electrode is reflected due to the high frequency -leld set up before reaching said anode so that the accelerating electrode and anode function as a diode.

The anode may be integral with a wall of the resonator and such wall may be insulated from the accelerating electrode to enable the anode and accelerating electrode to be maintained at diierent potentials. Alternatively, the anode may be a separate cylindrical element capacity-coupled to the resonator or the anode may be located inside the resonator. In all cases the high frequency oscillatory eld extends between the accelerating electrode and anode and thus is distinguished from the conventional type of velocity'modulation mixing device employing a hollow resonator and a reecting target electrode since in this latter type the oscillatory eld does not extend to the target electrode.

In order that the said invention may be clearly understood and readily carried into effect, it

- will now be more fully described with reference to the accompanying drawings, in which:

Figure 1 illustrates one form of electron discharge device and circuit according to the invention,

Figure 2 illustrates an improved anode construction suitable for use with the arrangement shown in Figure l,

Figures 3 and 4 illustrate modified forms of discharge devices for use in the invention,

Figure 5 illustrates diagrammatically a preferred form of discharge device for use in the invention, and

Figures 6 and '7 illustrate diagrammatically two different views of a further embodiment of a discharge device for use in the invention.

As shown in Figure l, the reference numeral 5 indicates a cylindrical cathode having its flat end made electron-emissive. The hollow resonator is of toroidal form having the cross-section shown and comprising a disc 6 of conducting material and a hollowed plate 'l also of conducting material. The disc 6 has a central aperture which may or may not be covered by a conducting grid and forms the accelerating electrode and the disc E is separated from the plate 'i through the intermediary of a ring of dielectric material 8. The central part of the plate l is in proximity to the aperture or grid in the accelerating electrode 6 and constitutes the anode proper. Where the device is arranged to constitute the mixing circuit of a superheterodyne receiver the aerial, signal is fed from a dipole aerial 9 through a coaxial line Il) to a coupling loop Il projecting into the interior of the resonator. The local oscillation may be applied between the accelerating electrode E and the anode plate 'l or may be injected into the resonator via another coaxial line lil and coupling loop II' similar to the line I 0 and loop II. The beat frequency output is set up in a tuned circuit I2 which is connected on the one hand to the anode plate 'I and on the other hand via a blocking condenser I3 to the accelerating electrode 6. In operation, the anode is maintained at a zero or negative, or only slightly positive potential with respect to the cathode whilst the accelerating electrode is maintained at a more positive potential which may be 4of the order of hundreds of volts by suitable means, such as the battery B shown in Fig. 1. With such an arrangement, the accelerating electrode and anode of the device shown in Figure 1 function as a diode and the characteristic of these two electrodes is just as steep at low currents as that obtained in the retarding eld region of an ordinary diode, providing that the current leaving the cathode is not so large that space charge effects will interfere with the operation. It will be observed that kthe cathode 5 constitutes a unit independent of the diode circuit arranged outside the hollow resonator and is not required to carry high frequency currents so that no difficulties are presented in effecting contact between the cathode and the hollow resonator such as would be the case were only a cathode and anode employed. Further, due to the acceleration of the electrons from the cathode, their transit time between electrode and the anode 1 can be made suciently small when the spacing between these electrodesy is much greater than the maximum permissible spacing between the electrode of a simplev diode. The capacity between the periphery of the plate 1 and the periphery of the accelerating electrode 6 may-form part or the whole of the capacity tuning the output circuit I2. When oscillations are applied as above described, the resonator is excited and a high frequency oscillatory field is set up which extends between the accelerating electrode and the anode. Electrons are drawn from the cathode 5 and are projected towards the anode and some of the electrons depending on the high frequency field are collected by the anode and excite the full shot noise and the remainder return almost as soon as they pass the accelerating electrode and excite little or no noise in the resonator. The noise in the arrangement shown in Figure 1 as between anode and accelerating electrode is therefore the same as that of an ordinary retarding field diode and affords good sensitivity, that is to say, the arrangement is capable of detecting signals only 5-10 times greater than the Johnson noise of the resonator. 'Ihe changes in potential of the electrode S and the anode l' under the influence of the high frequency voltages applied thereto are in opposing phase and, therefore, effect only small changes in the cathode current. If it is desired to reduce such changes, the distance from the accelerating electrode to the cathode may be increased or auxiliary electrodes or grids maintained at suitable potentials may be inserted between the electrode 6 and the cathode. I

It may not be advantageous in some cases to separate the accelerating electrode and the plate 'I at their peripheries as shown in Figure 1 and where the high frequency currents are large it may therefore be preferred to effect such separation at a point closer to the anode proper where the high frequency currents are less. It will be appreciated in Figure 1 that the capacity between the accelerating electrode 6 and the plate 'I at their peripheries is a lay-passing capacity for the high frequency currents circulating on the interior surface of the resonator and hence if these currents are large at said peripheries as, in general, will be the case with the arrangement shown in Figure 1, then the bypassing capacity must of necessity be correspondingly large. By making the separation between the plate I and the accelerating electrode in the vicinity of the' anode proper such a by-passing capacity can, since the currents are smaller, be correspondingly reduced without appreciable detriment to the resonant properties of the resonator. A suitable construction for this purpose is shown in Figure 2 which also illustrates a different shape of hollow resonator. In this construction, the holiow resonator is of a substantially rectangular shape in cross-section and may also be of toroidal form. The anode in the arrangement shown, indicated by the reference numeral I4, is in the form of a rod provided with an insulating coating I5 on its exterior and arranged to form a close fit in a tubular sleeve I 6 projecting into the resonator. Preferably, the anode I4 is made of aluminium and the insulating material is an aluminiumoxide film, preferably made by the anodisation process. A suitable thickness of film may be of the order of one thousandth of an inch, suoli a lm thickness being capable of withstanding a potential difference of, say, volts due to the intermediate frequency voltage without imposing substantial loss on the high frequency circuit. In the arrangement shown in Figure 2, the capacity between the anode I and the sleeve I6 may also constitute substantially the whole of the tuning capacity for the output circuit I2.

In some cases it may be desired to provide a construction in which the by-passing capacity between the anode and the accelerating electrode is omitted. One such construction is shown in Figure 3. In this figure the hollow resonator IT is of rectangular form in cross-section and in two opposite walls apertures are provided which may be covered by grids I8 in proximity to which a pairof cathodes I9 is provided, electrons from which are collected by an anode 20 arranged entirely in the interior of the resonator I'I. Provided that the gaps between the anode 20 and the grids I8 are made equal no high frequency currents will flow along the lead 2| which supports the anode 20. In this arrangement the capacity tuning the intermediate frequency circuit I2 is roughly four times the effective capacity of the hollow resonator I1. The arrangement shown in Figure 3 functions similarly to that described with reference to Figure 1 with the eX- cetpion that it operates in push-pull fashion and enables an output circuit of higher impedance to be obtained.

With the arrangement shown in Figure 3, the selectivity of the hollow resonator may be too high to permit both the signal and the local oscillation to be applied to the resonator satisfactorily. It may therefore be desirable to apply one of the oscillations by effecting velocity modulation of the electron stream owing from the cathode to the anode. Such velocity modulation can be effected with the arrangement shown in Figure 1, but for the purpose of illustrating a more suitable construction the form of device shown in Figure 3 is modified as shown in Fig- 'ure 4. In this construction two further hollow resonators 22 are provided on the sides of the resonator I1 having the grids I8 and the re-entrant yportions of the resonators 22 extend in proximity to the grids I8 and are formed with apertures or grids 23. The signal is applied to the resonator |1 by coupling loop |1. 'Ihe local oscillation is applied to the resonators 22 by coupling loops 24, as shown, the effect of the resonators 22 being to impart a velocity modulation to the electron streams emanating from the cathodes I9 so that these velocity modulated streams are then influenced by the voltages set up on the grids I8, each grid I8 and the anode 20 functioning as a diode. The resonators I1 and 2'2 may of course be spaced from one another. It is not essential to employ the hollow resonators 22 since, in some cases, it maybe possible merely to insert grids in the path of the electron stream between the cathode and the grids I8, but such an arrangement has'the disadvantage that it may require the generator of local oscillation to be of high power.

A further and preferred form of discharge device for use in the invention is shown in Figure of the accompanying drawings. In this form of the invention the hollow resonator is in the form of a coaxial line composed of a tubular outer conductor 25 of cylindrical shape and an inner conductor 26. One end of the outer conductor is connected to aV plate 21 which is sealed into an evacuated envelope 28 the aperture in ythe plate 21 being covered by a grid 29 forming the accelerating electrode. One end of the inner conductor 26 is also sealed into the envelope 28 and is provided with an anode 30 invproximity to the accelerating electrode. In order to tune the resonator to a desired frequency an axially movable rod 3| of insulating material is provided having a bore 32 to enable the rod 3| 'to slide over the inner conductor 26, the position of the rod 3| with respect to the conductor 26 determining the resonant frequency of the resonator. If desired, the coaxial line maybe tuned by means of a sliding piston in known manner. The oscillations may be injected into the hollow resonator by coupling loops II and I| or by a capacity coupling to the conductor 26. The operation of the device shown in Figure 5 is similar to that described with reference to Figure 1 of the drawings. The beat frequency output is taken from the inner conductor 26 by means of a conductor 33 which contacts the conductor 26 and which is arranged to be moved along the length of the conductor 26 to enable an appropriate tapping point to be selected. To permit the conductor 33to be moved relatively to the conductor 26 the cylindrical outer conductor 25 is provided with a longitudinal slit, and the conductor 33 is supported by an insulating block 34 which is arranged to slide on the outer surface of the conductor 25. The conductor 25 should be connected to earth through a condenser.

It is found in operating devices such as have been described above that undesirable noise occurs when the local oscillator is switched on and when the anode current is adjusted to its normal value and when the resonator is detuned. It has been found possible to reduce this noise by adjusting the operating potentials so that the device always operates on an exponential portion of its characteristic. In such a case the accelerating electrode should be maintained at a positive potential with respect to the anode of about 20-40 volts.

If desired, with a view to reducing the additional noise referred to above, the construction of deviceshown in Figures 6 and 7 may be employed. In these figures the'resonator is similar 'to that shown in Figure 5 but in Figures 6 and 7 the device is arranged to operate in push-pull. For this purpose a pair of anodes 35 and 36 is provided, these two anodes being connected to inner conductors 31 and 38 which are inter-connected by a further conductor 39. A single cathode 40 is provided which provides two electron streams which pass to the anodes 35 and 36 located within the resonator, the accelerating electrode 4| in this case being in the form of a plate closing one end of a tubular outer conductor 4| and provided with apertures adjacent the anodes 35 -ancl 36, the apertures being provided with suitable grids. `In order to focus the electron streams from the cathode 40 a focussing electrode 42,'Figure 7, of known form may be provided; this electrode 42 is omitted fromr Figure'6. The beat frequency output is derived via a conductor 43 which is arranged to'slide relatively to the conductor 39 for the purpose of providing a neutral tapping point for the beat frequency output. The conductor 43 may be arranged to slide relatively to the conductor 39'in a manner similar to that described with reference to the conductor 33 of Figure 5. If desired, of course, separate cathodes may be employed for'providi'ng the two electron streams.

In order to prevent damage to the grids of the accelerating electrodesV in the constructions of devices described above, the grid-cathode distance must be increased as the voltage between the grid and the cathode 'is increased. -For example, the distance between the grid and cathode may be 3 mms., Vwhen a potential difference of 400 volts is employed, and 10 mms., when a potential difference of 1500 volts is employed. The distance between adjacent wires of the grid should be small compared to the distance between the anode and the grid.' For example, the distance between adjacent wires of the grid should not be greater than .005" when the potential difference between the grid and the cathode is about 1500 volts and the anode-grid rseparation is about 0.3 mm. It is found that whilst theoretically the characteristic of f the device should be just as steep at low anode currents as that of an ordinary diode, nevertheless, in practice, it is found that the characteristic maybe considerably impaired unless the following precautions are taken. The reason for the deficiency in the characteristic is not at all .apparent and whilst at rst it may be thought that the unsatisfactory characteristic may be due to the effect of magnetism in the elements of the vdevice it would, however, normally be considered that the heat treatment of the elements of the device attendant upon the evacuation of the device would destroy magnetic effects to such an extent that any residual magnetism would not affect the operation. It has, however, been found, particularly in the case of the anode, that the heat treatment above-mentioned is not sufficient to overcome the difficulty. In an experimental device the anode was supported on a copper clad nickel iron-wire sealed into an evacuated envelope, the anode being electrically welded to the end of the wire. It was found, however, that the casual magnetisation of theanode and the support wire and also the control electrode and its support which was not removed duringrthe heat treatment considerably reduced the quality of the characteristic which should havebeen obtained. Whilst normally in the manufacture of devices of the kind ldescribed above the heat treatment given to the elements during the 'evacuation process may be sufficient to remove any .magnetic effects arising in the .cathode and any focussing electrodev employed, nevertheless, the anode and its support Wire and the accelerating electrode and its vsupport should either be subjected to a separate demagnetisation process or else should be .made of non-magnetic material. Preferably, all of the electrodes at least in the region close to the electron path and, in particular, in regions close tothe anode should be so tneated vor madeof non-magnetic material. This separate de-mag'netisation process can be effec- .tive either by heating the parts to a sufiiciently high temperature .for asufhcient time before assembling in the envelope, or else by means of an A. C. field before or after the elements are assembled in the fenvelope.

Focussing electrodes associated with the cathodes may also be employed, as shown inFig. 7, the apertures in the focussing electrodes being provided, if desired, with suitable grids. If desired, the accelerating electrode may be arranged on a re-entrant portion of the hollow resonator, as shown in Fig. 4, particularly in cases where a large separation between the cathode and the control grid is desired.

What I claim is:

1. A circuit arrangement for mixing high frequency oscillations comprising an electron discharge device having a hollow resonator, a cathode external to said resonator and arranged to project an electron stream through an aperture formed in a wall `of said resonator, said wall constituting an accelerating electrode, and an anode for collecting electrons emitted by said cathode, means for applying the high frequency oscillations to be mixed to said device, at least one set of said high frequency oscillations being applied so as to excite vsaid resonator, at least apart of said anode being so positioned within said resonator that when the latter is excited a high frequency oscillatory field is set up which extends between said anode and said accelerating electrode, means for maintaining said accelerating electrode at a positive potential with respect to the potential of said cathode and for maintaining said anode at substantially Vthe potential of said cathode, and an output circuit coupled to said anode in which an output of a desired beat frequency can be established, whereby, during operation, part of the electron stream passing said accelerating electrode is reflected due to the high frequency field in said resonator before reaching said anode, so that the accelerating electrode and anode function as a diode.

2. A circuit arrangement according 'to claim 1, wherein the anode comprises a rod provided with an insulating coating and supported in a second aperture in the hollow resonator.

3. A circuit arrangement according to claim l, wherein the anode is mounted entirely within the resonator, and a second cathode is arranged to direct a stream of electrons inwardly through an aperture in another wall of the resonator, said anode being arranged to collect electrons emanating from said pair of cathodes.

4. A circuit arrangement according to claim l, comprising means for applying to said stream one set of oscillations to be mixed to effect velocitymodulation of the electron stream prior to the passage of Said stream through the accelerating electrode.

5. A circuit arrangement according to claim 1,

18 wherein the hollowresonator comprises a tubular outer conductor connected at one end to an apertured disc forming the accelerating electrode, and an inner conductor to which the anode is connected.

6. A circuit arrangement according to claim 1, wherein the parts of said device, at least in the region close to the electron paths, are of nonmagnetic material to avoid undesired effects which would otherwise occur.

7. An electron discharge device for mixing high frequency oscillations comprising ahollow resonator, a, cathode external to said resonator-and arranged to project an electron stream through an aperture formed in a wall of said resonator, said Wall constituting an accelerating electrode, an anode for collecting electrons emitted by said cathode, means for applying to said device the high frequency oscillations to be mixed, at least one set of said oscillations being applied so as to excite said resonator, at least va part of said anode being arranged within said resonator so that when the latter is excited a high frequency oscillatory field is set up between said anode and said accelerating electrode, and means coupled to said anode in which an output oscillation of a desired beat frequency can be established, said accelerating electrode being insulated from said anode whereby it is adapted to be maintained at a positive potential with respect to said anode and said cathode and said anode being adapted to be maintained at substantially the potential of said cathode, whereby during operation part of the electron stream passing said accelerating electrode is returned due to said high frequency field before reaching said anode, so that the accelerating electrode and the anode function as a diode.

8. An electron discharge device according to claim 7, wherein said means for applying high frequency oscillations comprises a coaxial line coupled into said resonator.

9. An electron discharge device according to claim '7, wherein said resonator comprises two insulated metal wall members which include, one of said wall members being apertured and forming said accelerating electrode, said anode forming a part of the other of said wall members.

10. An electron discharge device according to clai-m 7, wherein said anode is arranged entirely within said resonator.

11. An electron discharge device according to claim 10, wherein the resonator wall opposite the first-mentioned aperture is provided with a second aperture, and a second cathode is mounted adjacent said second aperture.

12. An electron discharge device according to claim 7, including means for effecting velocitymodulation of the electron stream priol` to the passage of said stream through said accelerating electrode.

13. An electron discharge device according to claim 7, wherein the hollow resonator comprises a tubular outer conductor connected at one end to an apertured plate forming said accelerating electrode, and an inner conductor to which said anode is connected.

14. An electron discharge device according to claim 13, wherein said anode is arranged entirely within said resonator and said hollow resonator is in the form of a cylindrical coaxial line including said outer and inner conductors.

15. An electron discharge device according to claim 7, wherein said hollow resonator comprises a tubular outer conductor connected at one end to a plate having a pair of spaced apertures andl 9 l forming said accelerating electrode, and a pair UNITED STATES PATENTS of spaced inner conductors aligned with said aper- Number Name Date tures and connected to a pair of anodes including 1 98's 621 Hansen Jan 22l 1935 said anode located Within said resonator adjacent 2 080128 Grundmann May 11, 1937 t0 Sad apertufes- 5 2,167,201 Danenbach July 25, 1939 CABOT SEATON BULL. 2,190,515 Hahn Feb. 13, 1940 2,257,795 Gray Oct. '7, 1941 2,269,688 Rath Jan. 13, 1942 REFERENCES CITED 2,284,405 McArthur May 26, 1942 The followmg references are of record in the 1o 2,237,345 Varian June 30 1942 me of this Patent: 2,293,151 Linder Aug. 18, 1942 2,314,794 Linder Mar. 23, 1943

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