US2793313A - Method and structure for gas tube modulation - Google Patents

Description

May 21, 1957 w. M. WEBSTER, JR

METHOD AND STRUCTURE FOR GAS TUBE MODULATION 2 Sheets-Sheet l Filed Jan. 26, 1953 .l 7, IM// May 21, 1957 w. M. WEBSTER, 'JR 2,793,313

METHOD AND STRUCTURE FOR GAS TUBE MoDULATIoN Filed Jan. 2e, 195s 2 shetS-sheet 2 Lam/.47

INI/E N TOR.

y WzlljamMWebsZ United States Patent O METHOD AND STRUCTURE FOR GAS TUBE MODULATION William M. Webster, Jr., Princeton, N. I., assignor to Radio Corporation of America, a corporation of Delaware Application January 26, 1953, Serial No. 333,257

9 Claims. (Cl. 313-189) This invention relates to gas discharge devices that have continuous grid control. More particularly it relates to such gas discharge devices whi-ch have very high values of transconductance and of lanode current with very low values of output impedance.

The type of gas` discharge devices with which this invention is concerned are described in an article appearing in the IRE, volume 40, Number 6, June 1952, page 645, by L. Malter, E. O. Johnson, and W. M. Webster. These tubes, .or devices, have many advantages over the more conventional types of gas tubes, however, these tubes also have certain limitations.

One limitation upon these devices is that sensitivity of the 4I nodulating capabilities of `the control electrode is limited somewhat by the fact that the control electrode normally must be capable of directly controlling ,a very Ierse magnitude ef Current- .Anether .limitation er1-these devieee .is that, even though the control elect-rode is an excellent method ,of modulating the .Output eurrent ef .the .deviee fer Some ,instances it is desirable to have a plurality of modulating means in the device One instance Where it ie desired to have a p1u relitv ,ef modulating means .in the deviee is fer mixing two or more incoming signals.

,It ,is therefore en Object ef this .invention to provide a gas discharge Kdevice of the .type under consideration hav ing a new and improved means for modulating the output current.

It is another object `of this invention to provide a new and nevel ses diseharce device ef the type under seaside-ration having a plurality lof modulating means.

lt is Aa further object of this invention to provide a modulating means having improved sensitivity `for gas discharge .devices ci the ,type considered .by this invention.

These and other .objects are attained in accordance with the general aspects of this invention by providing a gas discharge device comprising a main cathode andan anode defining a main current path. The main current path may, if desired, include a control electrode. An auxiliary group .of .electrodes is included within .the envelope that comprises an auxiliary lcathode that is sub stantially enclosed by an apertured constricting electrode. intermediate the auxiliary cathode and the constricting electrode is arranged one or more probe type electrodes to modulate the positive ion loss from within the constricting electrode and thus modulate .the main current f low.

These and `other features and advantages will best be understood by referring to the following .descriptions of the illustrated embodiments vread `in connection 'with the accompanying drawings wherein like reference characters designate similar palts throughout the several views and in -Whiehi Figure 'l is a top sectional view through line 1v1-1 lof Figure 2 of a gas discharge device constructed in accordance with this invention;

Figure 2 is a .SeCQDl ViW hIOLlgh the line 2v2 Of Figure l;

Figure 3 is a schematic diagram of a circuit utilizing 2 a gas discharge device constructed in accordance with this invention;

Figure 4 is a graph of the current output versus the anode rpotential for diiferent values of probe potential showing the .characteristics of a gas discharge device constructed in accordance with this invention;

Figure 5 is a graph of the output current versus the probe potential showing the transconductance of a. gas discharge device constructed in accordance with this invention;

Figure 6 is a schematic diagram of a mixer circuit utilizing a gas discharge constructed in accordance with this invention;

Figure 7 is a schematic diagram of another mixer circuit utilizing a gas discharge device constructed in a cordance with this invention; l

Figure 8 is a top sectional view of another embodiment of a gas discharge device constructed in accordance with this invention;

Figure y9 is a schematic circuit diagram of a modulator circuit utilizing a gas discharge device constructed in accordance with this invention; and

Figure l0 is a top sectional View of a further embodiment of a gas discharge device constructed in accordance With this invention.

Referring now tothe drawings and Figures l and 2 in particular,y gas discharge device 1 0 is illustrative of one form which this invention lm-ay take and comprises a gas tight envelope 11 only partially indicated in Figure 2.

The device is provided with the usual stenl 12 through which the lead-in conductors are sealed as indicated. A main cathode 13 is supported between upper and lower insulating supports such as micas 14 and 15. A U-shaped control grid 16 and a U-shaped anode 17 are also supported between micas 14, 15 and, as shown, partially enclose cathode 13, Grid 16 ycomprises a pair of U-shaped support members 18 and 19 between which a plurality of parallel grid wires are supported in spaced relation. Grid 16 is provided with support members 21 which extend through micas 14, 15 and serve to rigidly imount the grid. Anode 17 is made of sheet metal andhas connected thereto support rods 22 which Vserve to rigidly lock the anode between vupper and lower ,micas 14 and 15, Opposite the open ends of grid 16 and anode 17 is mounted a cylindrical constricting electrode 23 supported in a similar manner between micas 14 and 15 by means of support rods 24 connected thereto. The ends of the ycylindrical constricting electrode lare closed by conductive end members 36 and 37 as shown. An ionizing or auxiliary cathode 25 is mounted coaxially and concentric with respect ,to `the cylindrical constricting electrode 23. The auxiliary cathode 25 is insulated from end members 36 an-d 37 by means of insulating material or by being spaced from the end members 3.6 and 37 as shown. Closely spaced from auxiliary cathode 25 is a probe electrode, or wire, 3 3. The probe electrode 38 is also insulated from end members 36 yand 37 .either by means of some type lof insulating material or by being spaced from the end members as shown. The control provided by the probe electrode 38 decreases as the spacing between the auxiliary cathode y25 land probe velectrode 38 increases. It is for this reason that the probeelectrode 38 is preferably closely spaced adjacent the auxiliary cathode 25, i. e. approximately lor 2 mm.

As shown in Figures 1 and 2 constricting electrode 23 is provided with an elongated narrow slot 26, the center of which extends along a plane passing through the axis of probe electrode 3S, auxiliary cathode 25 and main cathode 1 3. Ifhe size of slot 26 is approximately'l to 2 mm' Auxiliary .eafheete 25 ,is .connected te leeden 34 while the leads of heater 2 8 are connected, respectively, to lead-ins 34 and 35. `Probe electrode 38 is 'conw nected to lead-in 39 in the usual manner. One of the constricting electrode supporting rods 24 is connected to supporting conductor 31.

One of the grid support members 21 is connected to lead-in conductor 29 which is sealed through stem 12. One of the anode support rods 22 is connected to a supporting conductor 30. Both supporting conductors 30 and 31 are sealed through stem 12 and serve as the main support members between stem 12 and the electrode assembly. Lead- ins 32 and 33 are connected to the leads .of heater 27 while the lead-in 32 is connected to the sleeve of cathode 13 as shown.

Gas discharge device 10 is processed in the well known manner and is provided with a gaseous atmosphere prior to' sealing off. Any suitable gas or mixture of gases may be utilized. The gas pressure will vary in accordance with the specific envelope and electrode geometry and spacings. Furthermore, it is not believed that the gas pressure is critical except that it is preferable to utilize a pressure which will favor the formation of a selfsustaining ionizing discharge. In the tube now being described as being illustrative of this invention helium was used at a pressure of approximately l mm. or" mercury. However, any suitable gas, preferably a noble gas, may be used with a pressure range of approximately .l to l mm. of mercury.

Referring now to Figure 3 for the operation of gas discharge device there is shown a schematic diagram of a circuit utilizing gas discharge device 10 constructed in accordance with this invention. to the positive side of a variable source of potential 40. The negative side of source 40 is connected through an ammeter 41 to main cathode 13. The main cathode 13 and grid 16 are connected to input terminals 42. Main 'cathode 13 is also connected to the positive side of a potential source 43. The negative side of source 43 is connected to auxiliary cathode 25 through a current limiting resistor 44. Constricting electrode 23 is connected to the positive side of a potential source 45. The other side of source 45 is connected to auxiliary cathode 25 and to one of a pair of input terminals 46. Probe electrode 38 is connected to the other of the input terminals 46.

The sources of potential have been indicated as simple batteries but it should be understood that any suitable potential sources may be used. The potentials are so selected that between auxiliary cathode 25 and main cathode 13 a potential difference exists which is suticient to cause an ionizing discharge to occur therebetween. The potential difference between auxiliary cathode 25 and main cathode 13 (Vaux.) is approximately 40 volts. Between the main cathode 13 and anode 17 is impressed a potential (Vm.) that is less than the ionizing potential of the medium. The magnitude of (Vm.) may vary from approximately 2 volts to 25 volts. The potential difference, i. e. source 45, between auxiliary cathode 25 and constricting electrode 23 is approximately 2 volts. An input signal may be either to terminals 42 or to terminals 46. In the alternative, input signals may be applied to terminals 42 and 46. However, at this time it is assumed that no signal is being applied to Vterminals 42 and thus grid 16 is electrically oating.

When the potentials, as set forth in the proceeding paragraph, are applied to gas discharge device 10 an ionizing discharge occurs between auxiliary cathodeZS and the main group of electrodes through slot 26. Constricting electrode 26 serves to direct the ionizing, or auxiliary, discharge and also intensities the ionization thereof while utilizing a very small current of approximately 5 to l5 milli-amperes.- The auxiliary discharge forms a plasma in device 10 that extends through the grid openings and into the grid-anode region as well. Thus, there is a highly conductive path from main cathode '13 through grid 16 to anode 17. When the auxiliary Anode 17 is connected D discharge occurs, negative electrons emitted by the auxiliary cathode 25 move out of constricting electrode 26 toward the main electrodes. The rate of ion generation in the main electrode group is dependent upon the rate of electron ow out of the region within the constricting electrode 23. The rate of electron ow out of constricting electrode 23 is dependent upon the rate of positive ion flow into constricting electrode 23 'and thus, the plasma electron density is controlled by controlling the ion flow into constricting electrode 23. For equilibrium in the device 10, the rate of ion ow into constricting electrode 23 must equal the rate of ion loss from Within constricting electrode 23. Due to the positive potential on constricting electrode 23, and end members 36 and 37, the positive ions in the constricting electrode must go to auxiliary cathode 25 or to probe electrode 38.

Under normal operating conditions the auxiliary cathode 25 is a prolific emitter of electrons and thus is space charge limited. Due to the fact that the auxiliary cathode 25 is space charge limited, the plasma is slightly negative, say .5 volt, with respect to the auxiliary cathode 25. Because the plasma is negative with respect to the auxiliary cathode 25, positive ions are repelled from the auxiliary cathode 25 thus leaving only the probe electrode 38, Within the constricting electrode 23, which has an attractive force for the positive ions in this area. When a signal is applied to the probe electrode 38, the attractive force for the positive ions is varied in accordance with the signal and thus the rate of ion loss within the constricting electrode 23 is varied. As has been explained, a variation of ion loss within the constricting electrode 23 varies the output or load current of device 10.

Referring now to Figure 4 there is shown a graph of the current output versus the anode potential. This graph, which is plotted for different values of probe electrode potential with respect to the auxiliary cathode potential shows that the characteristic of gas discharge device 10 is pentode-like. The slope of the saturated part of this characteristic is equivalent to a resistance of about ohms but the proper load line depends upon the application of the device.

Referring to Figure 5 there is shown a graph of the output current versus the probe electrode potential. The graph is plotted with a constant anode potential of 6 volts positive with respect to the main cathode 13, 'and with a constant main cathode potential of 30 volts positive with respect to the auxiliary cathode 25. The probe potential is with respect to the auxiliary cathode. The input impedance used is low and is within the range of 150 to 800 ohms depending upon probe potential. The power gain as computed from these curves is approximately 42 db. The transconductance is about 700,000 micromhos.

The internal operation of gas discharge device 10 is as follows: in region 2 of Figure 5, as the probe electrode potential becomes more negative, the ion current to it increases. In order to supply this ion current, the operating arc drop, i. e. the auxiliary discharge voltage within the device, must increase. When the arc drop increases there is less voltage drop across the limiting resistor 44, of Fig. 3, and consequently the auxiliary discharge current decreases. The decrease in auxiliary discharge current reflects itself in a decrease 'in main load current thus resulting in load current modulation. Conversely as the probe Lelectrode potential becomes more positive, the auxiliary discharge voltage decreases and the auxiliary discharge current and main bond current increase.

Another method of modulating device 10, which gives a reverse slope to the output current versus the probe electrode potential, i. e. a negative gm, is as follows: in region 1 of the graph of Figure 5 the main load current is increased by making the probe electrode potential more negative, which increases the auxiliary discharge voltage because the ionization eiciency improves while the auxiliary discharge current isl essentially constant. Eventually, however, when the probe potential reaches the dotted line in Fig. the auxiliary discharge voltage becomes an appreciable part of source 43 and the auxiliary discharge current and main load current begin -to decrease with increased auxiliary discharge voltage. This later phenomena accounts for region 2 of the graph.

Referring to Figure 6 there is shown a schematic circuit diagram for gas discharge device 10 when used as a mixer. In this circuit the grid 16 is left floating or could be connected to anode 17 or omitted. The anode 17 is connected to main cathode 13 through the primary of an output transformer 47 and a source 49. The secondary of output transformer 47 is connected to output terminals 48. Main cathode 13 is also connected to the cathode of a vacuum tube 51 through an vauxiliary source 50. A tap on auxiliary source 50 is connected to constricting electrode 23. Auxiliary cathode 25 is connected to the anode of tube 51 and to one side of the secondary of an input transformer S3. The other side of the secondary of input transformer 53 is connected to probe electrode 38. The primary of input transformer 53 is connected to input terminal 54 and the grid and cathode of tube 51 is connected to a second pair of input terminals 52.

In operation of the circuit shown in'Figure 6 a signal applied to input terminals 54 modulates the main load current of device 10 as does a signal appliedy to input terminals 52. The two signals are mixed in the device and an output is taken lfrom terminals 48.

Figure 7 is a further mixer circuit diagram utilizing a gas discharge device constructed in accordance with this invention. This circuit is similar to Figure 6 except that a constant bias 58 is applied between auxiliary cathode `25 and constricting electrode 23 and the second signal is applied to transformer 56 between the grid 16 and main cathode 13. The other connections of this ligure are similar to those previously described so that further description is not deemed necessary at this time.

In operation of the mixer circuit shown in Figure 7 a signal is applied to input terminals 54 while a second signal is applied to input terminals 57. These two input signals both modulate the main load current of device 10 and thus mix the signals.

Referring now to Figure 8 there is shown a top sectional view of a preferred embodiment of this invention utilizing a pair of probe electrodes 65 and 65 one adjacent to each side of aperture 66 in constricting electrode 67. Auxiliary and main cathodes 68 and 69 respectively as well as anode 70 are similar to those previously described therefore further description of these electrodes is not deemed necessary. If two means for modulating the main load current are desired a control grid (not shown) may be used in the main current path as shown in Figure 9, or in the alternative a different signal may be applied to each `of the probe electrodes 65 and 65. However, it is preferred to apply the same signal to both probe electrodes 65 and 65' so that the one signal will constriction modulate the auxiliary discharge as well as modulate the ion density inside constriction electrode 67. It has been found that when the probe electrodes 65 and 65' bracket the aperture 66 the control obtained from the probes is at a maximum in sensitivity.

Referring now to Figure 9 there is shown a schematic diagram and representation of an amplier circuit utilizing the gas discharge device shown in Figure 8. In the circuit the anode 70 is connected through a load 71 to the positive side of source 72. The negative side of a potential source 72 is connected to main cathode 69 and to the positive side of a potential source 73. The negative side of source 73 is grounded as is auxiliary cathode 68 and one side of the secondary of input transformer- 75. A tap on source 73 is connected to constricting member 67. Probes 65 and 65 lare connected to the other side of the secondary of input transformer 75. The primary of input transformer 75 is connected to input terminals 74.

Referring now to Figure l0 there is shown a top sectional view of a gas discharge device constructed ac. cordance with this. invention that is especially adapted to operate as a mixing device. The electrode structure of this device includesa main cathode 77 and a main anode 76. The auxiliary group of electrodes includes an auxiliary cathode surrounded by a constricting electrode 78 having an aperture 79 therein. Intermediate cathode 80 and constricting electrode 78 and on each side of aperture 79, i. e. bracketing aperture 79, is a pair of probe electrodes 82 and 82', Closely spaced from auxiliary cathode 80, and preferably on the side thereof away from aperture 79 is still another probe electrode 81.

The preferred form of operation of this device is to connect probe electrodes 82 and 82 together and apply a signal thereto. This signal modulates the main anode current as has been described. A second signal is applied to probe electrode 81 which also modulates ythe main anode current and thus mixing occurs.

While I have indicated several embodiments of my invention of which I am now aware and have also indicated several speciiic 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 exact uses indicated, but that many variations -may be made in the particular structure used and the purpose for which it isemployed without departing from the scope of my invention as set forth in the appended claims.

I claim:

l. A gas discharge device, comprising a sealed en ve lope having an ionizable gaseous medium therein, a main thermionic cathode yand an anode defining a main current path within said envelope, means including an auxiliary thermionic cathode surrounded by an apertured constricting electrode within said envelope for producing a plasma including positive ions within a region including said constricting electrode and said main current path, and electrode means within said constricting electrode for modulating the density of positive ions in said plasma whereby the current in said main current path is modulated.

2. A gas discharge device, comprising a sealed envelope having an ionizable gaseous medium therein, a main cathode and an anode defining a main current path Within said envelope, means including an auxiliary thermionic cathode surrounded by an apertured constricting electrode within said envelope for producing an auxiliary discharge resulting in a plasma throughout a region including said constricting electrode and said main current path, and electrode means within said constricting electrode for constricting said auxiliary discharge and for modulating the density of said plasma whereby the current in said main path is modulated.

3. A gas discharge device, comprising a sealed envelope having an ionizable gaseous medium therein, a main cathode and an anode arranged in spaced relationship within said envelope, -an auxiliary cathode for producing a plasma within a region of said device, an apertured constricting electrode surrounding said auxiliary cathode but insulated therefrom and within said region, said constricting electrode being spaced from said main cathode and from said anode within said envelope, and electrode means within said constricting electrode and insulated therefrom for modulating the ion density of said plasma formed by a discharge from said `auxiliary cathode.

4. A gas discharge device, comprising a sealed envelope having an ionizable gaseous medium therein, a main thermionic cathode and an anode arranged in spaced relationship within said envelope, an auxiliary thermionic cathode for producing a plasma within a region of said envelope, an apertured constricting electrode surrounding said auxiliary cathode but insulated therefrom and within said region, said constricting electrode being spaced from said main cathode and from said anode within said envelope, a pair of probe electrodes within said constricting electrode one spaced on each side of the aperture in said 7 constricting electrode and insulated therefrom, and at least one other probe electrode within said constricting electrode but insulated therefrom and spaced adjacent said auxiliary cathode.

5. A gas discharge device, comprising a sealed envelope having an ionizable gaseous medium therein, a main thermionic cathode and an anode arranged in spaced relationship within said envelope, an auxiliary thermionic cathode for producing `a plasma within a region of said envelope, a hollow apertured constricting electrode surrounding said auxiliary cathode but insulated therefrom and within said region, said constricting electrode being spaced from said main vcathode and from said anode within said envelope, and a pair of probe electrodes within lsaid constricting electrode one spaced on each side of the aperture in said constrictingelectrode and insulated from said constricting electrode for modulating the density of said plasma.

6. A gas discharge device, comprising a sealed envelope having an ionizable gaseous medium therein, a main thermionic cathode and an anode arranged in spaced relation within said envelope, an apertured tubular electrode having its ends closed spaced from said main cathode and said anode, an auxiliary thermionic cathode within said apertured electrode and spaced therefrom, and at least one probe electrode within said apertured electrode spaced from said auxiliary cathode and insulated from said apertured electrode.

7. A gas discharge device, comprising a sealed envelope having an ionizable gaseous medium therein, a main thermionic cathode and an anode in spaced relationship within said envelope, an apertured hollow tubular electrode having its ends closed spaced from said main cathode and said anode, an auxiliary therrnionic cathode within said apertured electrode and spaced therefrom, and a probe electrode within said apertured electrode spaced from said auxiliary cathode and insulatedfrom said apertured electrode.

8. A gas discharge device, comprising a sealed envelope having an ionizable gaseous medium therein, a main thermionic `cathode and an anode in spaced relationship within said envelope, an apertured hollow tubular electrode having its ends closed spaced from said main cathode and said anode, an auxiliary thermionic cathode within said apertured electrode and spaced therefrom, and a pair of probe electrodes within said apertured electrode one spaced on each side of the aperture in said apertured electrode, said probe electrodes being spaced from said auxiliary cathode and from said apertured electrode.

9. A gas discharge device, comprising a sealed envelope having an ionizable gaseous medium therein, a main thermionic cathode and an anode defining a main current path within said envelope, means including an auxiliary thermionic cathode surrounded by an apertured constricting electrode for producing a plasma within a region of said device including said constricting electrode, and electrode means within said constricting electrode for density modulating said plasma whereby the current in said main current path is modulated.

References Cited inthe file of this patent UNTED STATES PATENTS 1,917,739 Schroter July 11, 1933 2,443,407 Wales June 15, 1948 2,445,679 Lemmers July 20, 1948 2,611,880 Webster Sept. 23, 1952

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