US2436700A - Cavity resonator oscillator - Google Patents

Description

'Feb 24, 1948. s C, sPlELMAN 2,436,700

CAVITY RESONATOR OSC ILLATOR Filed Jan. 29, 1944 Patente-d Feb; 1

CAVITY RESONATOR OSCILLATOR Sterling C. Spiclman, Huntingdon Valley, Pa., assignor, by mesne assignments, to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application January 29, 1944, Serial No. 520,296'.

2 Claims. (Cl. 315-39) This invention has to do with the problem of maintaining constant, as nearly as possible, the

frequency of electrical oscillations generated by an ultra high frequency oscillator, particularly one of the type comprising a resonant cavity and a so-called lighthouse type triode.

A resonant cavity oscillation generator, such as that hereinafter described, is designed to function at extremely high frequencies and is often employed, as for example on aircraft, under conditions which entail subjection to a wide range of temperature variations while precluding the provision of temperature-controlled housing means capable of shielding the generator against such temperature variations.

The primary object of the invention is to provide simple and dependable frequency-regulating means operative to counteract the inuence of temperature changes tending to cause frequency deviations; and it is a further object to accomplish the above end without adding substantial bulk or weight to the generating equipment.

The invention can best be described and explained by referring to the drawings wherein,

Fig. 1 is a longitudinal sectional view of a resonant cavity oscillation generator incorporate,I ing a preferred embodiment ofthe invention;I and Fig. 2 is a fragmentary sectional view corresponding in general to the upper end portion of Fig, 1l and' illustrating a modification l The ultra high frequency oscillation generator depicted in Fig. 1 includes a resonant cavity, which is identified as a whole by reference numeral I0, and a three-electrode vacuum tube or triode I\I of the so-called lighthouse type. The latter comprises a base I2 having a number of contact prongs I3,a glass envelope I4, an anode I5, a control grid I6, and a cathode iI of the heater type. The anode is a solid cylindrical slug supported on a flat metal ring I8 from which it extends upwardly to form a contact terminal I5a and downwardly within the envelope of the tube to a plane closely adjacent and parallel to but out of contact with grid I6.

The grid consists of a ilat woven wire `network extending crosswise of the envelope, between the anode and cathode, and supported on a surrounding flat metal ring i9 which is spaced from ring I8 by the glass tube Ila constituting a part of the tube envelope. Ring. I8 is soldered or brazed to anode I5 and is sealed to the upper end of glass tube Ila, and ring I9 is sealed to the lower end of glass tube M'a and also to the upper end of a second glass tube Hb upon which it rests and which, together with tube Ila, forms the glass portion of the envelope. The glass of which tubes Ila and IIb are composed is of a variety which does not expand or contract substantially when subjected to the ambient temperature variations to be expected in the course of normal operation. The lighthouse tubewhich may be a standard No. 464 or 446-is characterized by extremely close spacing between grid and anode and between grid and cathodewhich close spacing is necessitated by the requirement that electron transit time beas short as possible. But this close spacing of said electrodes, amounting in each case to only a few thousandths of an inch. is a detriment with respect to the maintenance of constant frequency because any variation of temperature of the anode or cathode or both, such as would be consequent upon marked changes of ambient temperature, or of load, will cause changes in the inter-electrode spacings which, though dimensionally small, amount to a substantial percentage of the total spacing and thus tend to bring about relatively large capacity variations which result in correspondingly large frequency deviations. The purpose of the present invention is to counteract and offset the frequency-varying effect of the aforementioned variations of interelectrode spacing, as well as those arising from other possible variations' in the system,'when.

trally apertured to receive said tube base. Ring 22 includes a band-like annular portion 22a which is slotted axially so that it is capable of being contracted into gripping engagement with the tube base by means of a, split clamping ring 23.

The lighthouse" tube can be withdrawn and rec placed upon loosening ring 23, which is normally held under contraction by a clamping screw 2l. Mounted within the cavity, concentrically with the shell 20, is a metal cylinder 25, known as a grid cylinder. This is slotted longitudinally at its lower end to render said lower end expansible to receive tube II; and it is internally grooved Y v circumferentially at 2t ring I9. The present invention is not ,directly concerned with the function of grid cylinder 25, but it is' thought appropriate to mention that to engage and made electrical contact with the peripheral edge of grid' gage the external thread 28 of an elongate metal rod 29. known as the plate rod, which extends downwardly and is adapted at its lower end to withdrawably engage vthe cylindrical terminal portion Ilia of anode l5 and thereby edect electrical connection with the anode. The adaptation of the lower end of the plate rod to render it withdrawably engageable with said anode terminal Illa. would glve'rise an increase iny the frequency of oscillation and vice versa. I conceived that it might. be feasible to take advantage o'f the longitudinal expansion and contraction jof plate rod 29, which would inevitably accompany rising and fallingambienttemperatures, respectively: and that such movements might be made capable'of and suiiicient to largely or wholly offset the aforementioned frequency deviations; It is apparent that since plate rod 29 and shell were heretofore both composed of copper or brass andtherefore possessed the same coemcient of expansion, little or no movement of rod 29 relatively to anode terminal Ita would occur in response to ambient temperature changes; but I perceived that if a sufficient and appropriate difference of expansion could b e brought about as between shell 20 and rod 29, my objective might be achieved, at least in part.

Bysubstituting Invar in place of copper as the metalv from which shell 20 is composed, I acterminal is accomplished by boring said lower end to form a thin resilient cylindrical wall, in-

serting a contact bushing 30 into the bore and securing the same by silver soldering or brazing, and then longitudinally slotting the bored porcomplished at once a large expansion dierence which resulted in a correspondingly substantial tion, together with bushing 2t, to form an expansible socket into which the cylindrical terminal la is easily insertable. To the upper end of plate rod 29 is attached a knob 3i by means of which the rod can ,be rotated and thus caused to move up or down by virtue of thread 2B; and the purpose of so doing is to eect tuning adjustments of the generator.

Secured to plate rod 22 and concentric therewith, is a cup-like metal element 32 which is known as a plate choke. This constitutes, in conjunction with shelll 20, a quarter-wave line having the characteristics of a short circuit and operating in the same manner as a complete conductive closure of the space between plate rod 29 and the inside surface of the shell 22.

The plate rod constitutes the axial conductor of a co-axial line, of which shell 2@ is the outer conductor, and said rod also forms a second coaxial line section in conjunction with the grid cylinder.

A probe 32, which need not be described, serves as the mediuml for withdrawing high frequency energy from the cavity; and a terminal 36 provides means for connecting a plate current changes, without any supplemental provision, i

such as a temperature-controlled housing, capable of serving as a shield against such ambient temperature variations.

The problem presented was to reduce to a permissible value or, if possible, completely alleviate the aforementioned frequency instability; and to do so without resorting to a protective housing which would increase the weight and bulk of the equipment besides introducing certain inconveniences which it was desirable to avoid.

The solution of the problem arose from my observation that increased ambient temperature caused the frequency to decrease. Having in mind thata movement of plate rod 29 downwardly, as viewed in Fig. l, relatively to anode Alongitudinal movement of rod 29 in the appropriate direction to counteract the previously mentioned adverse effect of ambient temperature changes; and I discovered that the regulation which could thus be effected remained remarkably constant throughout the entire range of temperature variations which experience indicated were likely to be encountered.

The tuning loi" the cavity to correct for deviations from the assigned frequency, which is ordinarily accomplished by moving the plate rod longitudinally, is mainly a function of the movement of the lower end of the rod along the anode terminal l5a and `is not to any substantial extent a direct function of the change in the length of therod within the cavity. But, of course, the change of length of the rod within the cavity is the factor which determines the movement of the lower end of the rod, so far as this invention is concerned, and it will accordingly be apparent that the length of rod 29 within the cavity is a criticalfactor in determining the sumciency of correction. In other words, if the-rod is too long, Within the cavity, it obviously over-compensates the frequency deviation and if it is too short, within the cavity, it will undercompensate said deviation. The length of rod outside the cavity is, manifestly, of no consequence.

To determine upon a design which will afford adequate regulation in any particular case, the most direct procedure is rst to ascertain experimentally the extent of frequency deviation per degree of temperature change for the triode it is intended to employ when operating within the prescribed frequency band, and then determine how much movement of the plate rod is needed to ofsetthe frequency deviation. Having ascertained that data and knowing the coemcients of expansion of the metals composing the plate rod and the shell respectively, it is a simple matsult when employing a structure like that oi' Fig. 1, resort may be had to the modiilcation shown in Fig. 2 wherein the plate rod is made in two parts-the lower portion 3B being composed of essary additional length is afforded by the Invar extension.

If the occasion should arise, it would be entirely feasible to make the shell 20 of two lengthwise adjoining parts, one being composed of Invar and the other of some other metal, such as copper. This would permit of a predetermined lengthwise expansion and contraction 'of the shell which could be used to offset excessive expansion and contraction of the plate rod in the event such an expedient be found necessary. Such a modication would not of necessity involve any structural change but merely a substitution of material throughout a portion of the length of shell 20. This obviously requires no additional illustration.

I claim:

1. An oscillation generator, comprising a tubular metal shell, a vacuum tube secured to one to said grid and extending longitudinally within said shell toward the other end thereof, and a inder and said shellin response to temperature changes, whereby to counteract frequency devi- Y ations due `particularly to the effects of such changes on the electrodes of said tube.

conductive member secured to said other end of shell a coaxial line and also forming with said' grid cylinder a second coaxial line, said shell and said member being composed at least in part of 2. An oscillation generator, comprising a tubular metal shell, a vacuum tube secured to one end-of said shell and extending longitudinally therein, said tube having an electrode terminal disposed within said shell, and a conductive member adjustably secured to the` other end of said shell and extending longitudinally therein in wiping engagement with said terminal, adjustment of said member effecting tuning adjustment of the generator, said shell and said member being composed at least in part of materials having diierent coeilicients of expansion to counteract frequency deviations duel to temperature changes, and said conductive member compris- `ing two serially connected sections composed of metals having diierent coeilicients of expansion.

STERLING C. SPIELMAN.

REFERENCES CITED The following references Iare of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 1,913,978 Ewen June 13, 1933 2,109,880 Dow Mar. 1, 1938 2,146,365 Batchelor ..--1 Feb. 7, 1939 2,251,085 Unk July 29, 1941 2,284,405 AMcArthur May 26, 1942 2,374,810 Fremlin May 1, 1945 2,408,817 Snow Oct. 8, 1946 Y FOREIGN PATENTS Number Country Date 422,869 Great Britain Jan. 2l', 1935

Images