US2506644A - Coaxial electron discharge device - Google Patents

Description

May 9, 1950 H. JQHNSON 2,566,644

COAXIAL ELECTRON DISCHARGE DEVICE Filed D60. 18, 1947 o Joao zoog :Mac

wwf/yay [ma/Mya if) ENVENTOR HARWI EK In HNSEJN BYWMM ATTORNEY Patented May 9, 1950 COAXI-AL IELECTRDN DISCHARGE DEVICE Harvvick Johnson, Frinceton, N. J., assignor to Radio Corporation .of America, acorporation of Delaware Application December 18, 1947, SerialNo. 792,469

(Cl. S-39) 15 Claims.

This vinvention relates generally to electron discharge devices and more particularly to an improveddiode for generating noise currents.

.-A diode provides a .convenient known noise source for use :in determining the noise factor of radio circuits .and components, particularly receivers. Asa result of the shot noise currents therein due to the random .transits of the individual electrons comprising the plate current, the diode may :be .considered .asignal vgenerator which furnishes a signal Aincluding .all frequencies simultaneously throughout a large part of the ireqllency spectrum. yIf the diode is operated with temperatureelimited emission from its filament, thenoise currentsquared is accurately calculable..

.For satisfactory use in measurement work, the noise diode must not disturb the circuit to which it is added, or any impedance `added tothe circuit by rthe .diode must be capable of being .incorporated in the circuit in a non-harmful manner. Tosatisfy these conditions at ultra high frequencies, the vuse of a diode vin the orrn of ,a section of coaxial line has been suggested. Such Ya diode could be 'inserted into ultra high frequency coaxial line circuits Without affecting the circuits.

.Inra copending application of Herbert A. Finke, Serial No. 724,208, iiled January 24, 1947, entitled Coaxial electron discharge devices, now ,Patent No. 2,463,363, .dated March l, i949, and assigned to the same assignee ,as the present application,

there is disclosed a noise diode tube Vconsisting of `a section of 50 ohm coaxial transmission line 'wherein the 4outer cylindrical element of the coaxial pair serves ,as the anode and the inner element .provides the .cathode .of the diode. The inner element is .divided .centrally into two portions `separated byamica insulating Washer, and a cathode .lamentis mounted on and electrically connected to the adjacent ends of `said portions, bridging the .mica insulator. One of said portions is made hollow and the other portion is provided with `an extension through the hollow portion, to provide for energizing the rlament from one .end of the tube. A vacuum envelope is .provided .by means of the anode and insulating c v,sealing rings between `the ends of the-coaxial pair elements and between ,said hollow portion and said extension.

In the -course of my Work with the noise diode of said `Finke.application it became apparent that this construction did .not simulate a 50 ohm coaxial line as well as .was desirable and, in fact, could not 'be used as a Aknown noise source over much `of the `ultra Vhigh frequency band. This waszound to be due to the impedance discon- 2 tinuity produced by the break between .the-.two portions of the inner` conductor of the diode. The faces of the inner conductor portions .bearing .on the .mica washer form a condenser which is intended to bypass the R. F. currents and provide continuity for the linner conductor. In addition, the break is shunted by the `lament wire which tends to provide continuity at lower frequencies where the bypass capacity may not :be effective. However, the bypass capacity and the lament :inductance form a `parallel L-C circuit insseries with .the inner conductor portions'which at resonance introduces a very high series iml pedance at this point. This kcauses a. very large reflection ina ohm line connected as a continuation of the coaxial pair. This is evidenced experimentally 'by a high standing Wave ratio measured in the neighborhood of 1750 `mega.- cycles.

In addition to this effect, the lament leads form a second coaxial 'line B, which may Vbe considered 'to 'be eectively in series with the 'inner conductor of main pair A, but shunted by the filament and the bypass capacity. Thus some unknown impedance is present, at the break in pair A, due to the coaxial pair B, which impedance depends on the length of B, the termination of B, the frequency, and the effectiveness 0f the shunting capacity and filament. It was determined experimentally that the bypassing effect of Vthe capacity was not adequate to prevent va reflection from being'introduced into pair A from pair B at this point. In fact, the reilection 'in A even varied with the manner of making connections to coaxial pair B, making `it extremely difficult to bring out the filament leads tothefilament supply Without introducing a reflection into pair A.

The instant invention is an improvement upon the noise diode ldisclosed in said copending Finke application.

A yparticular object of this invention is to provide a therrnionic source of noise voltages having a fixed characteristic impedance which is substantially independent vof frequency.

A further object is .to .improve the .impedance characteristics of the Finke .noise diode While re-v taining the advantageous features thereof. y

A particular feature of the invention is the elimination o1" the break in the inner conductor of the Finke noise diode.

Other objects will be apparent from the iollowing .description or" the invention pointed out in particularity in the appendedclaims and taken'y 3 in connection with the accompanying drawing, in which:

Fig. 1 is a central longitudinal sectional view of a two-element vacuum tube constructed in accordance with a preferred embodiment of the invention;

Fig. 2 is a transverse sectional view taken Ialong the line II-II of Fig. 1;

Fig. 3 is a similar view taken along the line III-III of Fig. 1;

Fig. 4 is a fragmentary plan view taken in the direction IV-IV of Fig. 1;

Fig. 5 is a central longitudinal View of a modied construction; and

Fig. 6 is a graph showing the improved relation between the voltage standing wave ratio and frequency in a 50 ohm line when my diode is inserted in the line ahead of a 50 ohm lossy line, as compared with the Finke diode and a 50 ohm lossy line.

vReferring to Fig. 1, the tube generally comprises a section A of coaxial transmission line having a diametric ratio which will give a characteristic impedance generally encountered in the component elements to which the tube will be connected, for example, 50 ohms. The section A comprises a hollow cylindrical outer conductor I and a cylindrical inner conductor 2 which carries the cathode larnent 3. In order to maintain the coaxial arrangement, insulating rings 4 of glass are sealed between the ends of the outer conductor I and the inner conductor 2. Suitable mea-ns, such as the tube 5, is provided for evacuating the space within the tulbe.

The tube structure just described is generally the same as that of the Finke tube. However, the, structure of my inner conductor 2 is quite different. The conductor 2 consists of two telescoping rods 1 and 8, each of which is undercut, as shown at 9, at each of the rings 4 to compensate partly for the change in characteristic impedance at these points due to the presence of the glass. Rod 1 is hollow to accommodate an elongated rod terminal I supported coaxially therein by an insulating `glass bead II and a seal II. The inner end of terminal I has a lateral lug I2 which extends outwardly through the enlarged part I3 of a key-hole slot III` formed in the wall of the inner end of hollow rod 1. An S-shaped filament support I is secured to rod 1 adjacent the lateral lug I2. The ends of the lament loop 3 are attached to the lug I2 and support I5.

The rod 8 is provided with a reduced diameter inner portion I6 which fits snugly within a counterbore I1 of rod 1, as shown in Figs. l, 3 and 4, thereby providing a low resistance connection therebetween. I

In order to provide a large bypass capacitance between coaxial elements 1 and I0, the reduced portion I6 of rod 8 is formed with a slot I8 in which is disposed a condenser yplate I9 which is welded to lug I2 and insulated from rod 8 by two dielectric strips 20 of mica of greater width than the condenser plate I9. The outer edges of the mica strips are accommodated in the slot I4 andina dia-metrically opposite slot 2| formed in the counterbored portion I1 of rod 1.

The assembly of the tube is preferably carried out as follows. First, the condenser plate I9 and terminal- I!) are aligned and the offset porition I9' is spotwelded to the side of the lug I2. The .plate I9 is then inserted with the two mica 'strips 2Il' 'into the "slot I8 of rod 8, after which the tubular rod 1 is slipped over the terminal I0 and reduced portion I6 of rod 8, the slot I4 providing suicient clearance for passage of the lug I2 and associated elements. The outer end of rod 1 is then sealed to the terminal by means of the glass seal II with the lug I2 centered within the opening I3. The support I5 is spot Welded to rod 1 and the filament loop is spot welded in place. Now, the assembled inner conductor unit is inserted Within the outer conductor I, to which the evacuating tube 5 has been attached, and the glass rings 4 are sealed in place to complete the vacuum envelope.

In the operation of the tube for determining the noise factor of a receiver or other electronic circuit component, the tube is coupled to said component by a suitable section of coaxial transmission line. A source of anode potential is connected between the outer conductor I and inner conductor 2 and a variable source of heating current for the lament 3 is connected between the hollow rod 1 and rod IB, at the end of the tube opposite the coaxial connection with said' component. The method of measurement of the noise factor is the same as that described in the above-mentioned Finke application. With an anode voltage of 300 volts the filament emission'of the tube is temperature-limited up to about inilliamperes. This emission enables the measurement of noise factors up to about 20 decibels at low frequencies. Transit-time ef-J fects reduce this to about 17 decibels near the; upper end of the ultra-high frequency band. A complete analysis of the design and operating characteristics of the diode is embodied in a paper entitled A coaxial-line diode noise source for U. H. F., presented .by me at the 1947 I. R. E. National Convention on March 3, 194'?, and published in RCA Review, March 1947.

It is seen that in my improved structure each of the inner and outer conductors, and hence, the coaxial pair A, is conductively continuous throughout the tube with no gap in either conductor where a series impedence can exist. This eliminates the reflections due to resonance of the series L-C circuit formed by the filament inductance and the bypass capacity in the Finke diode. Reflections which may arise from the magnetic coupling of the filament to the interelectrode space of coaxial pair A are minimized by placing the nia-ment substantially in a radial plane asshown in Fig. 1.

The coupling of coaxial pair B to coaxialpair A is principally eifected through the magnetic coupling of the filament loop which is minimized as stated above. In my improved tube the bypass capacity which shunts coaxial pair B has a much larger capacitance than that available in the Finke tube, and hence, provides a much more eifective bypass.

While it is preferred to construct the diode with the bypass condenser it has been found that the condenser can be omitted without seriously aiecting the performance of the tube.

In the preferred embodiment of Figs. 1-4 the inner conductor 4of the diode `is made up of two closely-fitting telescoped parts, to Afacilitate as' sembly of the parts. However, in the modification shown in Fig. 5 the inner conductor'is a one piece rod 2. One end 1' of the rod 2 is hol-` lowed out to receive the filament terminal '10"'. A longitudinal slot i4 is cut in the solid end 8 of the rod of suicient width and length to permit insertion of the terminal Illin the direction'of the vdashed lines shown. An enlarged opening I3' is provided adjacent the intersection of the slot I4' with the hollow part 1' to receive the lateral lug I 2' of the terminal in spaced relationship. After the terminal lll has been inserted through the slot ld', a glass sleeve I is inserted from the outer end of the part and sealed between the latter and the terminal I9.

The effectiveness of my improved tube compared to that of the Finke tube over a wide frequency range is demonstrated in part by lines b and a, respectively, of Fig. 6. For comparison, the standing wave ratio for a 50 ohm lossy line is shown at c.

Although my improved tube has been described in connection with its use as a noise diode for a certain receiver test and measurement. it is evident that a device of this type may have various other applications wherever it is desirable to maintain a certain circuit impedance and have at the same time an electron discharge device in this circuit without disturbing the transmission characteristics thereof. It will also be evident that, instead of being limited to a two-element tube, other elements may be incorporated in my tube. Therefore, while I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only certain specific applications for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

I claim:

1. An inner conductor for an electron discharge device, comprising a hollow rod, a solid rod having a reduced diameter portion snugly fitting within one end of said hollow rod, the wall of said hollow rod being formed with an aperture adjacent the end of said reduced portion, an elongated terminal insulatedly supported within said hollow rod and having a lateral portion extending through said aperture, and a lament surrounding said hollow rod and supported by said lateral portion and said hollow rod.

2. An inner conductor in accordance with claim 1, including a condenser connected between said terminal and one of said rods.

3. An inner conductor in accordance with claim 2, wherein a slot is formed in said reduced portion, and said condenser comprises a. hat plate disposed in said slot between two dielectric strips and conductively connected to said terminal.

4. An electron discharge device including a section coaxial transmission line consisting of a hollow cylindrical outer conductor and an inner cylindrical conductor, said outer conductor constituting an anode, one end of said inner conductor being formed with a hollow portion and a radial aperture at the inner end of the hollow portion, an elongated terminal supported in said hollow portion and aperture and insulated from said inner conductor, and a lamentary cathode surrounding said inner conductor and supported by said terminal and said inner conductor.

5. An electron discharge device in accordance with claim 4, including a condenser connected between said terminal and said inner conductor.

6. An electron discharge device in accordance iii) with claim 5, wherein said inner conductor is formed with a, longitudinal slot adjacent said terminal, and said condenser comprises a nat plate disposed in said slot between two dielec tric strips and conductively connected to said terminal.

7. An inner conductor for an electron discharge device, comprising a rod having a hollow portion extending from the approximate center thereof to one end, said hollow portion being formed at its inner end with a radial aperture, an elongated terminal supported in said hollow portion and said aperture and insulated from said rod, and a filament surrounding said rod and supported by said terminal and said rod.

8. An inner conductor in accordance with claim 7, including a condenser connected between said terminal and said rod.

9. An inner conductor in accordance with claim 8, wherein said rod is formed with a longitudinal slot adjacent said terminal, and said condenser comprises a fiat plate disposed in said slot bebetween two dielectric strips and conductively connected to said terminal.

10. An electron discharge device comprising a section of coaxial transmission line composed of a hollow cylindrical outer conductor, a cylindrical inner conductor extending through said outer conductor and insulating means supporting said inner conductor coaxially within said outer conductor, both of said conductors being conductively continuous throughout the length of said device, and therrnionic cathode means supported by said inner conductor in the space between said conductors, said outer conductor being adapted to function as the anode element of said device upon thermionic emission of said cathode.

l1. An electron discharge device in accordance with claim 10, wherein said insulating means provide hermetic sealing of said section at each end whereby the section may be evacuated.

12. An electron discharge device in accordance with claim 10, wherein said thermionic cathode means surrounds said inner conductor.

13. An electron discharge device in accordance with claim l, wherein the Wall of said hollow rod is formed with a longitudinal slot extending from said aperture to the end of the rod, to permit assembly of said lateral portion of said terminal within said aperture.

14. An electron discharge device according to claim 4, wherein said inner conductor is a one piece rod.

15. An electron discharge device according to claim 14, wherein said rod is formed with a longitudinal slot extending from said radial aperture toward the other end of said rod, to permit assembly of said terminal within said hollow portion and aperture.

HARWICK JOHNSON.

REFERENCES CITED The following references are of record in the ile of this patent:

UNITED STATES PATENTS Number Name Date 2,145,735 Rice Jan. 31, 1939 2,167,201 Dallenbach July 25, 1939 2,289,846 Litton July 14, 1942

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