US2431511A - Signal level control system - Google Patents

Description

NOV. 25, 1947. I w, s m z s 1 2,431,511

SIGNAL LEVEL CONTROL SYSTEM Filed D90. 1, 1945 UTILIZATION CIRCUIT ROBERT w. SANDERS ATTORNEY Patented Nov. 25, 1947 2,43l,5ll

SIGNAL LEVEL CONTROL SYSTEM Robert W. Sanders, Fort Wayne, Ind, assignor, by mesne assignments, to Farnsworth Research Corporation, a corporation of Indiana Application December 1, 1945, Serial No. 632,154

9 Claims. (01. 179-171) This invention relates to signal amplifiers and particularly to the facilities for efiecting a signal level control of such amplifiers.

In many cases the intelligence signals which it is desired to amplify, vary appreciably in their level or average peak-topeak value by reason of corresponding variations in the intelligence which is to be conveyed by such signals. One in stance of this nature is in a television system wherein the level of the video signals may vary appreciably in accordance with variations of the average illumination of the television subject. For known reasons of amplifier design and transmission requlrements, it is desirable to maintain the level of such signals reasonably constant irrespective of corresponding variations of the subject intelligence and at the same time to avoid discrimination of the signals at one end. or the other of the range. In other words, it is desirable to effect a substantially linear amplification of the signals.

In a television system, the average peak-topeak value of the video signals representative of a given television subject varies in direct proportion to the general illumination of the subject. The signal generating device usually is of a character which responds to light reflected from the subject. Regardless of the intensity of the subject illumination, totally black portions of the subject will reflect no light. Consequently, the video signal generated to represent a, black elemental area of the subject will invariably have the same value. However, lighter portions of the subject will reflect light varying in intensity in proportion to the intensity of the general subject illumination. For example, under relatively low intensity subject illumination'su'ch as encountered out-oidoors on a cloudy day, the'light reflected from a white portion of the subject, and consequently the representative video signal, will be of relatively small magnitude, differing in a relatively small degree from a black representative video signal. In other words, the peak-to-peak value of such video signals is said to be low. However, the sames'ubject in bright sunlight will cause the reflection from a white portion thereof of relatively high intensity light, as a result of which representative video signal of. relatively large magnitude will be generated. This signal will differ in a relatively large degree from the black representative signal and the peak-to-peak value of such video signals isfsaid to be high.

If video signals such as the two extreme signals describedwere to be ampl fied by means of apparatus which is "not provided with an automatic,

gain control, a corresponding disparity between the peak-to-peak values of the signals would 'be' produced in the amplified signals. It is, however, desirable that the amplified signals of the same television subject, for example, under varying conditions of general illumination, have'substantially the same peak-to-peak values. This end may be achieved by providing the amplifier with automatic gain facilities whereby the gain or amplification factor is suitably reduced when signals having a relatively high peak-to-peak value are impressed thereon.

In one type of television transmission system, the initially generated video signals representative of a subject are in the form of free electrons. It is customary to amplify this electron signal current by means of a multistagestat'ic electron multiplier. A multistage multiplier is essentially a device which is operated by impressing positive potentials of increasing values upon successive ones of the secondary emissive multiplier electrodes or dynodes as they are sometimes called. It ordinarily is convenient to provide the electron accelerating potentials for the dynodes from taps on a voltage divider to which is connected one or more sources of relatively high unidirectional voltage. The multiplied stream of electrons which ultimately is collected by a collector electrode is caused to traverse a load impedance device for the purpose of developing a signal voltage representative of a television subject.

When an electron multiplier is operated in some of the arrangements of the prior art, there is effected a signal level control of the output circuit voltage by means of the values selected for the voltage divider components. When the voltage divider consists entirely of relatively high impedance components, the overall multiplication ratio of the device is less at high electron current levels than at low levels. It is seen that, while in effect a level control of the intelligence signals may be efiected by such means, the signals developed in the output circuit will be distorted by reason of the fact that the high current signals will not be multiplied as much as the low signal currents.

It is the object of this invention, therefore, to provide an improved signal amplifier incorporating means whereby a levelcontrol of the amplified signals may be effected and at the same time a linear amplifier response may be maintained.

Another object of the invention is to provide a novel coupling between an electron multiplier and a vacuum tube amplifier whereby to linearize the output of the amplifier and at the same time to maintain a substantially constant signal level.

In accordance with this invention, there is provided a multistage electron multiplier having at least one but preferably a plurality of dynodes and an electron collector electrode. Electron accelerating potentials are impressed upon the dynodes by means including one or more impedance devices. In a preferred form of the invention, the dynodes are connected to diiferent points on a voltage divider which is furnished with energy from a source of unidirectional current. In order to linearize the output signal voltage of the multiplier, the voltage divider components connected to the higher stage dynodes are of relatively low impedance as compared to the relatively high impedance of the divider components connected to the lower stage dynodes. There also is provided a vacuum tube amplifier having an input circuit coupled to the output circuit of the multiplier which is derived from the collector electrode. The input circuit of the amplifier tube also is connected to one of the voltage divider impedance devices which is coupled to the multiplier in such a manner to be traversed by the electron space current flow in the multiplier. A biasing voltage for the amplifier thus is developed by the divider impedance device. The output circuit of the amplifier tube is connected to a utilization circuit. The biasing voltage which is impressed upon the input circuit of the tube fluctuates in accordance with variations of the electron space current and thereby functions to automatically control the gain of the amplifier in a suitable manner to effect the desired Signal level control thereof,

For a better understanding of the invention together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

The single figure of the accompanying drawing is a schematic circuit diagram of an illustrated embodiment of the invention.

Having reference now to the drawing, the source of signal effects is illustrated as an electron multiplier which may be assumed to be associated with an image analyzing tube em bodied in a television system. Such a tube functions to convert an optical image of a television subject systematically into electronic signal effects representative of different elemental areas of the subject. Only a few of the stages of such an electron multiplier are shown for the reason that the invention relates principally to the higher stages of such a device and to the coupling facilities between a multiplier of this character and a vacuum tube amplifier. The multiplier may comprise any number of stages required to meet the needs of the television system in which it is incorporated. The multiplier is mounted within an evacuated envelope 1 and comprises a series arrangement of a plurality of secondary electron emissive electrodes or dynodes, such as the dynodes 2 to 8 inclusive. These dynodes are illustrated as having conventional box-like forms but it is to be understood that they may have any other configuration desired without departing from the scope of the present invention. Adjacent the last stage dynode 8, there is disposed the electron collecting electrode 8 which may have the form of a substantially flat plate, as illustrated,

Each of the multiplier dynodes is maintained at a more positive potential than its l q$59- These potentials or electron accelerating potentials are derived from a voltage divider H which comprises a series connection of a plurality of impedance devices such as resistors 12 to It inclusive. Energy is supplied to the voltage divider from any convenient source of unidirectiona1 current. In the present instance, this source is illustrated as two batteries 19 and 2| connected in series. The positive terminal of the battery 2 l, which is grounded, is connected through the resistor 2! to the voltage divider resistor I! which is connected to the last stage multiplier dynode ii. In the present instance, the divider resistors 55, I6 and H, which are associated with the higher stage dynodes 5, 6 and"! respectively, are of relatively low impedance, of the order of 30,000 ohms. The divider resistors I2, I3 and M, which are associated with the lower stage dynodes 2, 3 and 4 respectively, are of relatively high impedance, of the order of 200,000 ohms. The divider resistor is, associated with the last stage dynode S and the collector 9, also is of relatively high impedance and is shunted by a condenser 22 to bypass the high frequency component of the video signals for a purpose to be described. The junction point between the batteries l9 and 2! is connected to the junction point between the high impedance resistor I4 and the low impedance resistor IS. The battery I9, therefore, may be of lower output capacity than the battery 2!.

The collector electrode 9 is connected to ground through an output load impedance device, such as a resistor 23, and a source of negative biasing voltage, such as a battery 24. The collector electrode 9 also is coupled to the control grid of a vacuum tube 25. The cathode of this tube is connected to a sliding contact 25 which is en gaged with the voltage divider resistor it. The tube cathode also is connected to ground through a high frequency video signal by-pass condenser 27. The anode of the tube 25 is coupled to one input terminal of a utilization circuit 28, the other input terminal of which is connected to ground.

Referring now to the operation of the apparatus embodying this invention, assume that it is to be used in a television signal transmitting system. In this case the electron multiplier may be associated with an image analyzing tube of the dissector type. In an apparatus of this character an optical image of the television subject is projected onto a photoelectric cathode at one end of the tube, whereby to efiect an electron emission from various parts of the cathode corresponding in magnitude to the intensity of the subject-reflected light projected onto the cathode. The emitted electrons are formed into an orderly arrangement of a plurality of elemental dynodes are most conveniently derived from taps on a voltage divider such as the divider l I. These accelerating potentials are produced in the divider components by the flow of current therethrough. In the absence of any electrons in the multiplier, the current which flows through the voltage divider is solely the result of the connection of the batteries I9 and 2| thereto, and its magnitude is determined only by the impedance of the voltage divider components and the voltage of the power supply battery. However, when an electron space current flows between the series of multiplier dynodes, the path between any two adjacent dynodes is effectively connected in shunt with one of the voltage divider components. In a multistage electron multiplier such as that described, the current flow through the voltage divider is exceedingly complex by reason of the multiplicity of such path around the components of the divider.

For an understanding of the present invention, however, it is not necessary to deal specifically with the details of the current distribution in the voltage divider. It is well known in the art that, when using in connection with a multistage electron multiplier a voltage divider consisting entirely of relatively high impedance components, the voltages developed in the individual components will vary with variations in the electron space current flow in the multiplier. With no space current flowing in the multiplier and using a divider made up of a number of high impedance components which are all of the same value, the voltages developed are equal for all components. In other words, the voltage distribution in the electron multiplier is linear. In the case where an appreciable electron space current flow is produced through the multiplier, the voltages developed across the divider components associated with the low stage dynodes, wherein the electron current density is relatively low, are greater than the normal voltages developed therein when there is no electron current flow in the multiplier. Conversely, with a substantial electron space current flow in the multiplier, the voltages developed across the divider components associated with the higher stage dynodes, wherein the density of the electron current is relatively great, are less than the voltages developed therein when there is no electron space current flow. The total voltage drop across the entire divider, of course, is the same in both cases, being equal to the total voltage of the power supply such as the batteries [9 and 2!.

So long as the electron space current flow in the multiplier is not too great, the overall multiplication factor of a multiplier energized in accordance with prior art practice is not changed appreciably as the result of the described variations in the individual electron accelerating potentials applied to the respective dynodes. However, when the electron space current flow in the multiplier rises to a relatively high value, the multiplication factor of the multiplier decreases. The device no longer functions in a linear manner but, instead, discriminates against the elec-, tron current flow in the higher region oithe range of variation with the result that the signal efiects produced are distorted at the high end of the signal range.

It is considered that the reason for the decrease in the accelerating voltages developed across the voltage divider components associated with the higher stage multiplier dynodes is that the electron current flowing in shunt with the divider components is too great in proportion to the normal current flow through the associated ivider components.

However, by making the divider components such as l5, l6 and 11 associated with some or all of the higher stage dynodes of a much lower impedance than the components such as the resistors l2, l3 and M, the ratio of the shunting electron current to the normal current through the associated divider component is smaller. The result is that the accelerating voltage developed in a resistor such as l5, 16 or i! is maintained sufficiently high to effect the necessary electron multiplication in the higher stages to maintain the overall multiplication factor of the device substantially constant and thereby maintain a substantial linearity of response by the device.

Consequently, in accordance with one feature of the present invention, the voltage divider resistors l5, l6 and I! are of much lower impedance than the resistors I2, [3 and M as described. Then, when an electron multiplier such as that described is used in conjunction with an imageanalyzing tube of a television system, the signal voltages developed in the output resistor 23 bear a substantially linear relationship to the light values of the television subject irrespective of the intensity of the general illumination of the subject. As a consequence, however, the signal voltages developed in the output resistor 23 will have an average peak-to-peak value which, under conditions of bright illumination of the television subject, are greater than it is convenient to amplify and transmit to a receiving point.

Therefore, in order to compensate for such a variation in the average peak-to-peak value of the generated signals, there is provided, in accordance with another feature of this invention, a facility for automatically varying the gain or amlification factor of an amplifier inversely to variations in the average peakto-peali value of the signals. This facility consists of a means for impressing upon the cathode of the vacuum tube 25 a biasing voltage which varies in a suitable manner in response to variations of the average peak-to-peak value of the signals. The voltage divider resistor i8, which is by-passed by the condenser 22 for the high-frequency instantaneous signal values representative of elemental areas of the television subject, develops, therefore, a voltage which is representative of the average peak-to-peak value of the signals. The voltage developed in the resistor i8 varies inversely to the fluctuation of the electron current in the multiplier. As previously explained, this results from the fact that the resistor has a relatively high impedance and is associated with one of the higher stage dynodes. As the average light of the television subject increases, the voltage developed across the resistor it decreases. A decrease in this developed voltage makes the slider 25, and therefore the cathode of the tube 25, more positive, thereby biasing the tube in such a manner to decrease its gain. The video signals developed in the resistor 23, even though they have an average peak-to-peak value which is greater than normal, will be amplified to a lesser degree by thetube 25 as a result of the cathode biasing so that, in the output circuit of the amplifier, the average peak-to-peak value of the signals will remain substantially constant irrespective of variations of the average subject illumination.

Of course as the average peal -to-peak value of the signals developed in the resistor 23 de creases, the low-frequency voltage developed in the divider resistor l8 increases. The voltage impressed upon the cathode of the tube 25 by the slider 26 becomes more negative, thereby increasing the gain of the amplifier tube 25 suitably to maintain the signal level in the output circuit thereof at the desired substantially constant value.

The biasing voltage which is impressed upon the cathode of the tube 25 in the manner described for effecting an automatic gain control of the amplifier is caused to vary at the relatively slow rate of the low frequency component of the video signals. In other Words, this biasing voltage represents the average illumination of the television subject. The biasing voltage is developed in the resistor 58 to represent the average subject illumination by reason of the connection of the by-pass condenser 22 in shunt with this resistor. By this means the relatively high frequency component of the video signals representing the instantaneous light values of the subject are caused to traverse the by-pass condenser which has a suitable value for achieving this result. Consequently, only the relatively low frequency component of the video signals representing the average illumination of the subject traverses the divider resistor l8 for the development of the biasing voltage.

By reason of the illustrated connection of the vacuum tube 25 to a portion of the divider resistor 18, the space current in the tube also traverses a part of this divider resistor. The instantaneous fluctuations of the space current in response to the relatively high frequency component of the video signals impressed upon the control grid are by-passed around that portion of the resistor I8 traversed by the space current by means of the condenser 21. This condenser also is required 7 to have a value suitable to provide a relatively low impedance for the high frequency component of the video signals.

The most satisfactory values of the by- pass condensers 22 and 27 are such that, in conjunction with the respective portions of the divider resistor associated therewith, the time constant of the circuits is relatively great. For optimum results this time constant should be of the order of the time required to scan several fields of the television picture.

While the instant invention has been described in a preferred embodiment wherein the cathode of the vacuum tube is biased to eifect the desired gain control of the amplifier, it is considered to be within the scope of the invention to employ other facilities incorporating the underlying principles of the invention to effect the gain control of the amplifier in the manner desired. For

example, it is known that the current variations in some of the other voltage divider components, particularly those associated with some of the lower stage dynodes are in a sense opposite the current variations in the resistor I 5. In other words the current in some of these other divider resistors increases with an increase in the average light reflected from the subject. The voltage developed in such a resistor also may be employed to effect the desired gain control of the amplifier. However, in such a case, the voltage should be impressed upon the control grid of the tube 25 in any manner within the knowledge of those skilled in the art.

While there has been described what, at present, is considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it, therefore, is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An amplifier for television video signal effects having a relatively low frequency component representative of average subject illumination comprising, an electron multiplier having a dynode and an electron collector electrode, an impedance device connected to said dynode, an output circuit for said multiplier including a load impedance device coupled to said collector electrode, a vacuum tube having an input circuit and an output circuit, means for coupling said input circuit to said multiplier output circuit, means for coupling said input circuit to one of said impedance devices for varying the amplification factor of said tube oppositely to variations of said low frequency signal component, and a utilization circuit coupled to the output circuit of said tube.

2. An amplifier for television video signal effects having a relatively low frequency component representative of average subject illumination comprising, an electron multiplier having a dynode and an electron collector electrode, an impedance device connected to said dynode for developing a voltage depending upon the magnitude of the multiplier space currents, an output circuit for said multiplier including a load impedance device coupled to said collector electrode, a vacuum tube having an input circuit and an output circuit, means for coupling said input circuit to said multiplier output circuit, means for coupling said input circuit to the impedance device connected to said dynode for biasing said tube oppositely to variations of said low frequency signal component, and a utilization circuit coupled to said tube output circuit.

3. An amplifier for television video signal effects having a relatively low frequency component representative of average subject illumination comprising, a multistage electron multiplier having a plurality of dynodes and an electron collector electrode, a plurality of impedance devices connected respectively to said dynodes and traversed by electron multiplier currents, an output circuit for said multiplier including a load impedance device coupled to said collector electrode, a vacuum tube having an input circuit and an output circuit, means for coupling said input circuit to said multiplier output circuit, means for coupling said input circuit to one of said impedance devices for varying the amplification factor of said tube oppositely to variations of said low frequency signal component, and a utilization circuit coupled to the output circuit of said tube.

4. An amplifier for television video signal efiects having a relatively low frequency component representative of average subject illumination comprising, a multistage electron multiplier having a plurality of dynodes and an electron collector electrode, a source of accelerating potentials for said dynodes including a voltage divider comprising a plurality of impedance devices, an output circuit for said multiplier including a load impedance device coupled to said collector electrode, a vacuum tube having an input circuit and an output circuit, means for coupling said input circuit to said multiplier output circuit, means for coupling said input circuit to one of said impedance devices for varying the amplification factor of said tube oppositely to 9 variations of said low frequency signal component, and a utilization circuit coupled to the output circuit of said tube.

5. An amplifier for television video signal effects having a relatively low frequency component representative of average subject illumination comprising, a multistage electron multiplier having a plurality of serially arranged dynodes and an electron collector electrode, a source of accelerating potentials for said dynodes including a voltage divider comprising a plurality of impedance devices, an output circuit for said multiplier including a load impedance device coupled to said collector electrode, a vacuum tube having an input circuit and an output circuit, a coupling between said input circuit and said multiplier output circuit, a coupling between said input circuit and one of said voltage divider impedance devices for biasing said tube oppositely to variations of said low frequency signal component, and a utilization circuit coupled to said tube output circuit.

6. An amplifier for television video signal effects comprising, a multistage electron multiplier having a plurality of serially arranged dynodes and an electron collector electrode, a source of accelerating potentials for said dynodes including a voltage divider consisting of a series connection of a plurality of impedance devices, the impedance devices associated with the lower stage dynodes being respectively of relatively high value and the impedance devices associated with some of the higher stage dynodes being of relatively low value, an output circuit for said multiplier coupled to said colector electrode, a vacuum tube having an anode, a cathode and a control grid, a coupling between said control grid and said output circuit, a coupling between said cathode and the impedance device associated with one of said higher stage dynodes, a condenser connected between said cathode and a point on said cathode coupled impedance device to provide a low impedance by-pass circuit for relatively high frequency signal components of said signal effects, and a utilization circuit coupled to said anode.

7. An amplifier for television video signal efiects comprising, a multistage electron multiplier having a plurality of serially arranged dynodes and an electron collector electrode. a source of accelerating potentials for said dynodes including a voltage divider consisting of a series connection of a plurality of impedance devices, the impedance devices associated with the lower stage dynodes being respectively of relatively high value and the impedance devices associated with some of the higher stage dynodes being of relatively low value, an output circuit for said multiplier connected to said colector electrode, a vacuum tube having an anode, a cathode and a control grid, a coupling between said control 10 grid and said output circuit, a coupling between said cathode and the impedance device associated with the last stage dynode, a condenser connected between said cathode and the positive terminal of said voltage divider to provide a low impedance by-pass circuit for relatively high frequency signal components of said signal effects, and a utilization circuit coupled to said anode.

8. An amplifier for television video signal cfiects comprising, a multistage electron multiplier having a plurality of serially arranged dynodes and an electron collector electrode, a source of accelerating potentials for said dynodes including a voltage divider consisting of a series connection of a plurality of resistors, the resistors associated with the last stage dynode and with the lower stage dynodes being respectively of relatively high impedance and the resistors associated with the intermediate dynodes being of relatively low impedance, a load impedance device connected to said collector electrode, a vacuum tube having an anode, a cathode and a control grid, a coupling between said control grid and said load impedance device, a coupling between said cathode and the voltage divider resistor associated with the said last stage dynode, a condenser connected in shunt with said last stage dynode divider resistor to provide a low impedance by-pass circuit for relatively high frequency components of said signal efiects, and a utilization circuit coupled to said anode.

9. An amplifier for television video signal effects comprising, a multistage electron multiplier having a plurality of serially arranged dynodes and an electron collector electrode, a source of accelerating potentials for said dynodes including a voltage divider consisting of a series connection of a plurality of resistors, the resistors associated with the last stage dynode and with the lower stage dynodes being respectively of relatively high impedance and the resistors associated with the intermediate dynodes being of relatively low impedance, a load resistor connected between said collector electrode and the positive terminal of said voltage divider, a thermionic amplifier including a vacuum tube having an anode, a cathode and a control grid, a coupling between said control grid and said load resistor, a coupling between said cathode and the voltage divider resistor associated with said last stage dynode, a condenser connected in shunt with said last stage dynode divider resistor to provide a low impedance by-pass circuit for relatively high frequency components of said signal effects, a condenser connected between said cathode and the positive terminal of said voltage divider to provide a low impedance by-pass circuit for said relatively high frequency signal components, and a utilization circuit coupled to said anode.

ROBERT W. SANDERS.

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