US1893031A - Control circuits for space discharge devices - Google Patents

Description

jam. 3, 1933. F. B. LLEWELLYN CONTROL CIRCUITS FOR SPACE DISCHARGE DEVICES Filed April 12. 1930 1 v, L mm m "I M A n i 3 Y% B F. n m m w E F. M m Q T0 Patented Jan. 3, 1933 UNITED STATES PATENT OFFEQE FREDERICK B. LLEWELLYN, OF MON'ICLAIR, NEW JERSEY, ASSIGNOR TO BELL TELE- PHONE LABORATORIES, INCORPORATED,

NEW YORK OF NEW YORK, N. Y., A CORPORATION OF CONTROL CIRCUITS FOR SPACE DISCHARGE DEVICES Application filed April 12,

This invention relates to circuit arrangements for the eflicient and quiet operation of space discharge devices and particularly to the operation of these devices as modulators and detectors.

In the operation of a space discharge device it is generally desirable to secure a high degree of amplification and a large output of useful power together with relatively little generation of disturbing currents which will give rise to noises in a receiver connected to the device.

It is well known that the disturbing effect of noise producing currents is dependent upon their relative intensity compared with the intensity of the useful currents with which they are commingled in a receiving device. It is evident that any measures taken to re duce noises which originate in space discharge devices and their attached circuits must, in order to be effective, either increase the intensity of the useful currents or not reduce them to the same extent that the noise producing currents are reduced.

In the present invention the desired eflicient and quiet operation of a space discharge device is secured by a particular adjustment of the direct current biasing potentials which areapplied to the various electrodes of the device together with a readjustment of the intensities of the alternating currents applied to the device. The manner in which these adjustments provide for increased eificiency and quiet operation will be pointed out more particularly hereinafter.

In the proceedings of the Institute of Radio Engineers for February 1930, Volume 18, N0. 2, pages 243 to 265, I have published the results of a study of the causes of noise in vacuum tubes and attached circuits. The principal sources of noise which I have found and those which are most susceptible of control are the small shot effect in vacuum tubes and the thermal agitation of conductors.

The small shot effect is due to the existence of slight irregularities in the stream of electrons passing from the cathode to the anode ofa vacuum tube, giving rise to fluctuations in the plate current. The fluctuations are due to the arrival of electrons at the plate at a 1930. Serial No. 443,696.

non-uniform rate. This effect is most pronounced when a vacuum tube is operated in a condition in which temperature saturation is not complete.

The thermal agitation of electricity in a conductor is a state due to the irregular motion of the electric charges in the conductor and is produced by the heat energy in the conductor. This irregular motion of the charges produces slight irregularities in the current through the conductor and gives rise to noises in the receiver. The effect of thermal agitation is greater in large resistances than in small ones and increases with the temperature at which the resistance is operated.

In many commercial tubes the amount of noise is reduced to a satisfactory amount by operating the tube in a condition of temperature saturation to reduce the small shot effect and by using fairly large direct current plate potential to reduce the plate filament resistance and its accompanying thermal agitation to a small value.

In certain tube structures it has been found that the condition of temperature saturation is not readily obtained with the high values of direct current plate potential commonly used. The four-electrode type or screen grid tube which is at present available commercially is particularly disadvantageous in this respect. Tubes of this type are generally sufliciently quiet in operation when employed as amplifiers, for then the useful currents are amplified to a sufficient intensity so that the noise producing currents are relatively weak and hence unobjectionable. lVhen the tube is used as a modulator or detector, however, the degree of amplification obtainable is less and the noise producing currents are relatively strong, requiring the use of special measures to reduce their relative intensity.

In order to improve the temperature saturation in a tube of the type above mentioned and thereby decrease the small shot effect I reduce the value of the direct current potentials employed. I use a relatively small positive bias on the screen grid electrode and preferably apply no direct current potential whatever to the plate. In reducing the direct currentbiasing potentials the degree of am- V plification obtained in a tube is decreased.

t is possible, however, to compensate to a large degree for this decrease of amplification by correspondingly increasing the. in-

tector this loss of amplification is notof great importance. While the reduction of amplification necessarily means that the intensity of the useful currents is reduced, it has been found that the intensity of the noise produc I ing currents isreduced to a still greater extent. The result is a net improvement in the ratio of useful currents to noiseproducing currents. p

' I The invention will now be described indetail with reference to the drawing in which Fig. 1 shows an embodiment of the invention employing a four-electrode space discharge device. Fig. 2 shows the application of the invention to a three-element tube and Fig. 3 shows its application to a two-element tube while Fig. 4 shows theoretical" curves which will be employed in explaining the invention.

Referring to Fig. 1, the terminals 1 and 2 arerinput terminals to which may be con nected a source of incoming electrical waves or currents. Theinput circuit 1, 2 is coupled by means of a transformer Sto an input tuned circuit 4. *The tuned circuit 4 is connected to the input side of a fourele'ctrode space dischar e device 5 havin a cathode 6 a an anode or plate 8, and a control grid 7,

One side of'the circuit 4 is screen grid 9.

- connected'to the cathode Giandthe other side isconnected to the control grid 7. The screen grid electrode 9 is connected to the positive terminal of a battery 10. An adjustable connector '15 is connected to another portion of the battery 10 and to the cathode 6. By ad justment of the connector a positive direct current potential of any desired value may be impressed upon the screen grid 9.

' A'local source of alternating currents for us'e in heterodyne reception of the incoming waves is provided at 16. This source is adjustable as to frequency by any suitable means as indicated by the arrow in the diagrammatic representation. The source 16 isconnected to the plate circuit of the tube 5-through an adjustable potentiometer 50 and any suitable coupling means, which latter is represented by the transformer 17. An output circuit 18 is provided which is tuned for the frequency of the useful current to be derived from the tube 5. The circuit 18 is coupled by means of a transformer 19 to the outputterminals and 26. a

The plate 8 is preferably operated without any direct current potential being applied thereto, but a small potential may be applied without departing from the scope of the invention. By varying the position of the con tact 15 in the screen grid circuit and the intensityof the'alternatin'g current from the which gives a large degree of amplification and a large output of useful current together with a relatively small amount of disturbing currents or noises. i I r I The circuit shown in Fig. 2 is similar to that shownin Fig. l 'ericept that a three electrode tube 20 is employedin place of the four-electrode tube 51 The plate 8110f. the tube 20 is provided withan adjustable biasing batterylt) and {is preferably operated with relatively little or no direct current biasing potential. V The effective potential of battery 10 and the intensityof the alternating current from the local'sourc'e 16 maybe adjusted to such values that the circuit will give a maximum amplification and a large amount of useful current with a minimum amount of noise.

InFig. 3a two-elementtube fis-illustrated. The input circuit ,4 is connected in series with the output'circuit 18. As in the caseofthe circuit shown in Fig. 2 theeifec 'tive potentialof battery 40and the intensity 01"- the current from the local source 16 may be adjusted to obtain the most quiet and efiicient operation of the system.;

The principle underlying the adj ust-ment bf i ent values of the direct current biasing potential of the tube in decreasing order.

Each of these curves has a rising portion for the small values of the cathode temperature and a nearly horizontal portion, for the higher cathode temperatures. The r sing portion of the curve represents a condition in which the plate electrode is drawing electrons directly from the cathode and is a condition in which temperature saturation does not exist. In this condition any irregularity in the emission of electrons from the cathode produces a corresponding irregularity in the plate current. In other words, this region'is one in which the smallshot effect is most pronounced and gives rise to noise in the plate circuit.

The horizont'al portion of the curve represents a condition of temperature saturation. In this condition a change in the cathode temperature has practically no effect upon the intensity'of the plate current. At these high temperatures the cathode emits electrons greatly in excess of'the number'required to constitute the plate current. The excess elec-' these circumstances is composed of electrons drawn from the space charge and not directly from the cathode. The irregularities in the stream of electrons emitted from the cathode are not transmitted directly to the plate and consequently it appears that the plate current is made more uniform by operating the tube under the condition of temperature saturation.

The curves 21, 22, 23 and 24 represent the manner in which the tube noises depend upon the cathode temperature. Curve 2 1 corre sponds to the same direct current plate potential as the curve 11. In the same Way curves 22 and 12 correspond and also 23 corresponds with 13 and 24 with 14. Each of the curves 21, 22, 23 and 24 shows a maximum amount of noise in the region where the plate current curve is rising and each of these curves also falls to a relatively small value at the higher cathode temperatures.

The maximum value of noise is in each case due to the small shot effect while the noise observed at the higher cathode temperatures,

that is, in the region of temperature saturation depends principally upon the thermal agitation within the tube. Since the noise of thermal agitation increases when the plate filament resistance increases, the noise curve 21 which corresponds to the smallest value of plate filament resistance falls to a lower value than the curves 22, 23 and 24. The latter curves, in order, come down to increasingly higher final values because they correspond to larger values of plate filament resistance.

A tube which cannot be operated in a condition of temperature saturation will have its normal operating temperature represented approximately by the value T The vertical line corresponding to the temperature T intersects the noise curves 21, 22, 23 and 24 at the points 31, 32, 33 and 34 respectively. It will be seen that as the direct current biasing potential is reduced the values of noise obtained in the tube will correspond to the ordinates of the points 31, 32, 33 and 34. A minimum amount of noise is obtained in the region of the points 33 and 34. By adjusting the effective potential of the battery 10 in Fig. 1 or battery 40 in Figs. 2 and 3 an operating condition corresponding to the point 33 may readily be obtained.

As hereinbefore mentioned, the decreased values of direct current biasing potential corresponding to the operating conditions of points 33 and 34 are accompanied by a reduction of amplification and a lower in tensity of useful current. The noise, however, is reduced in a greater degree than is the useful current so that there is a material improvement, that is, a diminution in the net disturbing effect of the noise.

What is claimed is:

1. A circuit comprising a space discharge device having an anode, a'cathode, a control grid and a screen grid, the anode being at the same direct current potential as the cathode, means for applying a positive direct current bias to said screen grid, a source of alternating current coupled to said anode, and means for adjusting the intensity of said alternating current whereby the ratio of useful currents to noise producing currents in said device is increased. I

2. A circuit comprising a space discharge device having an anode, a cathode, a control grid and a screen grid, the anode being at the same direct current potential as the cathode, means for applying a positive direct current potential to said screen grid, a source of alternating current coupled to said anode, and means for adjusting the intensity of alternating current from said source.

3. A circuit comprising a space discharge device having an anode, a cathode, a control grid and a screen grid, the anode being at the same direct current potential as the cathode, means for applying a positive direct current bias to said screen grid, a source of alternatin g current coupled to said anode, input and output circuits for said space discharge device, and means for adjusting the bias of,said screen grid whereby the ratio of useful currents to noise producing currents in said output circuit is increased.

4. A circuit comprising a space discharge device having an anode, a cathode, a control grid and a screen grid, the anode being at the same direct current potential as the oathode, means for applying a positive direct current bias to said screen grid, a source of alternating current coupled to said anode, input and output circuits for said space dis charge device, means for adjusting the bias of said screen grid, and means for adjusting the intensity of the alternating current from said source, whereby the ratio of useful currents to noise producing currents in said output circuit is increased.

5. A mod ulating system comprising a space discharge device having an anode, a cathode, a control grid and a screen grid, a source of carrier current to be modulated, means for coupling said source to said anode, a source of modulating current connected to said control grid, an output circuit for utilizing modulated currents, the anode of said discharge device being at substantially the same direct current potential as said cathode, and the screen grid having such a value of direct current potential with respect to said cathode that the ratio of useful currents to noise producing currents in said output circuit is substantially a maximum.

6. A modulating system comprising a space discharge device having an anode, a cathode, a control grid and a screen grid. a source of carrier current to be modulated, means for coupling said source to said anode, a source of modulating current connected to, said control grid, anoutput circuit for utilizing modulated cur-rents fromsaid device, said anode being at substantially the samedirect current potential as said cathode, and the direct current potential of said screen grid and the alternating current potential of said anode being at suchvalues respectively that the 7 ratio of usefulcurrents to noise producing currents in said output circuit is substantially a maxlmum.

IrIn wltness whereof, I hereunto subscribe my name this 11th day of April, 1930. r I FREDERICK B.,LLEWELLYN.

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