US2290775A - Stabilization of photoelectric electron multipliers - Google Patents

Description

July 21, 1 942.

L. SNYDER, JR 2,290,775

STABILIZATION OF PHOTOELEC'IRIC ELECTRON MULTIPLIERS Filed May 1, '1940 5 Sheets-Sheet l POWER SUPPLY HG. .3. M

l 1942- R. L. SNYDER, JR 2,290,775

sTAE'ILiZ'A'TIQN 0F PHOTOELEC'I'RIC ELECTRON MULTIPLIERS Filed May 1, 1940 5 Sheets-Sheet 2 Po WEE COLLECTOR :S'UPPLY' 16 PHOTO 16 car/mp2 17 P0 WE)? cazzsc'rori JUPPL V PHOTO cmwozw an GLOW 9 105B 18 3nnentor Ezo'hard L. Snyder, J1:

July 21, 1942.- a. L. SNYDER, JR 2,290, STABILIZATION F- PHOTOELEGTRIC ELECT-RON MULTIPLIERS Filed Ma 1, 1940 Sheets-Sheet 3 0 2o do .120 160 200 220 240 260 E k 8 -1 q 10 E R E00 400 600 600 .1000 .1200 .2'400 1600 .1800 E000 Z200 Z400 VOLT/76L BETWEEN PHOTO CflV'HODE [ND .5467 (ST/7GB Jnnentor Richard L.Jn yder Jr:

v ttomeg y 1942- R. L. SNYDER, JR 2,290,775

ISTAB-ILIZA'I'IGN OF PHOTOELECTRIC ELECTRON MULTIPLIERS Filed May 1, 1940 Sheets-Sheet 4 m H, r m 1 y I I I j/VM 0 0 4o as 160 2'00 cbzzus'cron cwswE/w' MICROHMPEHES Snventor Richard L. Snyder; Jr.

- attorney Patented July 21, 1942 STABILIZATION OF PHOTOELECTRIC ELECTRON MULTIPLIERS Richard L. Snyder, Jr., Glassboro, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 1, 1940, Serial No. 332,632

12 Claims.

This invention relates to circuit arrangements for stabilizing photoelectric electron multipliers,

and more particularly to the stabilization of such devices so that they can be satisfactorily 'used in the photoelectric reproduction of sound.

Conventional practice in reproducing sound from motion picture films, or converting other high quality light signals to electrical signals, employs a gas filled photocell in an effort to reduce the effects of the noise in the coupling re sistor. Because of the critical dependence of the gas multiplication on the voltage between the cathode and anode, this voltage must be carefully regulated. The incandescent exciter lamp used with these systems should also be operated on well regulated direct current because a slow change in its intensity affects the level of the signal, and the fluctuation of the light when operated on 60-cycle A. C. causes 120-cycle' interfer ence. The regulation of the photocell voltage may be simply accomplished by means of a glow tube, but the regulated rectifier required by the exciter lamp is both complicated and expensive.

Long period fluctuation of line voltage makes the elimination of the regulation of the lamp supply by the use of degenerative networks in the photocell amplifier extremely diflicult. How-' ever, the development of the electron multiplier, which is equivalent to a photocell and a D.-C. amplifier, whose gain can be controlled by means of its dynodes, makes a simple degenerative system possible. When this tube is used in the place of a photocell, with one or two glow tubes and an auxiliary photocell, the gain can be made independent of reasonable voltage fluctuations and to vary inversely with the exciting light intensity, so that the multiplier output is little affected by disturbances in the voltage or light.

The operation of an electron multiplier depends on the phenomena of secondary emission. Secondary emission is caused by the impinging of a stream of electrons on a secondary emissive target or dynode and, if the surface of the dynode is properly prepared, the current emitted may be more than six times the primary current,

the ratio-depending on the energy of the primary electrons. In a photoelectric electron multiplier, electrons released from the illuminated photocathode are directed by positive electric fields to the first dynode. 'Upon striking the surface of the first dynode the photoelectrons cause the emission-of a new and greater stream of secondary electrons. electric field to the second dynode, where the process is repeated. In this way the signal is conveyed from dynode to dynode, multiplying on each impact until the final group of secondary electrons leaves the last dynode from which it is directed to the collector which is coupled to the load. To provide suitable electric fields, the dynodes are peculiarly shaped and each maintained at a potential positive with respect to the photocathode, and positive with respect to its preceding dynode( The difierence in voltage between successive dynodes is equal to that between the photocathodeand'first dynode, so that the potential of each dynode is proportional to its number.

The construction and operation of electron multipliers'is more generally described, for example, in the Proceedings of the I. R. E., volume 24, for March,'1936, at pages 351 to 375, in an article by V. K. Zworykin, G. A. Morton and L. Malter, and the construction and operation of the circular type of multiplier, which is generally referred .to hereinafter, is further described in an article entitled iThe Electrostatic Electron Multiplier, by V. K. Zworykin and J. A. Rajchman, appearing in the Proceedings of the I. R. E., vol. 27, No. 9, for September, 1939, at pages. 558 to 566.

One object of the invention is to provide an improved circuit and method of operation for a photoelectric sound reproducer which will compensate for variations in voltage of the power supply.

Another object of the invention is to provide an improved apparatus for and method of operation of a photoelectric sound reproducer which These are directed by a different will compensate for variations in brightness of the exciter lamp.

Another object of the invention is to provide an improved circuit for and method of operation of a photoelectric sound reproducer which will maintain reproduction uniform independent of variations in power supply voltage or variations in exciter lamp brightness.

Another object of the invention is to provide an improved electron multiplier circuit and method of operation which will render the operation thereof independent of variations in power supply voltage.

Other and incidental objects of the invention will be apparent to those skilled in the art from the following specification and the accompanying drawings, in which Figure 1 is a schematic diagram of a circuit for a nine-stage electron multiplier,

Figure 2 shows a modification of the circuit of Fig. 1 to compensate for variations in the power supply voltage,

Figure 3 shows a modification of the circuit of Fig. 1 as applied to the photoelectric reproduction of sound to compensate for variations in the brightness of the exciter lamp,

Figure 4 shows the modification of the circuit of Fig. 1 incorporating the improvements of both Figs. 2 and 3 so as to compensate both for variations in power supply voltage and for variations in exciter lamp brightness simultaneously,

I Figure 5 is a curve showing the average characteristics of one type of electron multiplier,

Figure 6 is a curve showing the efiect of variations of voltage of one stage in an electron mu tiplier,

Figure 7 shows a modification of the circuit of Fig. 1 involving the use of an alternating current supply and using a non-linear impedance to stabilize the output, and

V Figure 8 shows a modification of the circuit of Fig. 1 involving the use of an alternating current supply and using a phase shifting network to stabilize the gain.

The circuit shown in Fig. 1 is connected to a nine-stage multiplier [0. It consists of a D.-C. power supply l8 connected to the terminals of a voltage divider. All the resistors 20 are of equal value except the resistor 2| following the last dynode J at the positive end, which may be larger to provide the voltage required by the collector l2 and load 21, if the latter is of high impedance. The voltage divider should pass sufiicient current to prevent the change-of current in the collector circuit from having an appreciable effect on the dynode voltage.

In Fig. 1, as in Figs. 2, 3 and 4, the exciter lamp of a film sound reproducer is-indicated at l5 and the optical system for directing the light therefrom in the form of a fine line upon the sound record is indicated by the lens 16. The light from the exciter lamp I5, which is directed upon the film I1, passes through the sound track area where it is modulated in accordance'with the sound to be reproduced and is directed upon the photocathode. of the multiplier Ill. The emission from the photocathode is directed by means well known to those skilled in the art to the dynode A from which the secondary emission is directed to the second dynode B, this process being repeated through the successive dynodes C, D, E, F, G, H and J, and the emission from the last dynode J is collected upon the collector l2, which is connected to the output circuit and power supply.

Since the ratio of secondary current to primary current of a secondary emitting target depends on the voltage, the multiplication of a stage in a multiplier will be fixed by its potential above its source of primary emission. Therefore, a multiplier having 1!. stages will have a gain of R, where R is the ratio of secondary emission. Consequently, the gain will depend on a power of the voltage.

The curve appearing in Fig. 5 shows how the ain and sensitivity of a nine-stage photoelectric multiplier vary with the overall and stage voltages. This characteristic is the average of those of a number of tubes.

It is desirable to note here that, although the gain changes rapidly with voltage, this characteristic is essentially linear for small sections of the curve. Therefore, for a small change in the operating voltage (say 1%), the gain G can be considered proportional to the voltage V or where c is a constant in this region and Z is a constant dependent on the slope at the operating point chosen.

The curves appearing in Fig. 6 show the effect on the output or gain when the voltage of ijone dynode deviates from its normal voltage.' LT-he data for these curves were observed on 'a ninestage circular multiplier operating at 100 volts per stage. The dynode voltage is referred to that of the preceding dynode, so that 100 volts is the normal voltage. Each of the three curves is for a different stage (the first, fourth, and fifth). In each case, all the stages except the one under observation were at their normal voltages.

It is important for the voltage stabilizingcire cult that certain parts of these characteristics are essentially linear, while, for the light com-- pensating circuit, other parts are hyperbolic so that G =overal1 gain k =8. constant in a particular region 10 =a constant in a particular region V'=control dynode voltage m =a constant fixed by the potential from which V is referred h =a constant fixed by the potential from which V is referred in some part G=7c (V'+m) while in others Figure 2 shows the basic circuit of the first figure modified to stabilize the multiplier gain, independent of over-all voltage changes. Its operation depends on the much greater change in multiplier gain efiected by the voltage variation of a dynode, independent of other electrodes in the tube, than is caused by the same change in the overall voltage when operating on essentially linear sections of each characteristic. The ratio of the voltages which produce the same change in gain lies between 6 and 10 depending on the dynode and the part of its characteristic selected. For example, the change in gain caused by a change of 8 volts in the overall voltage can be compensated by shifting the voltage of a dynode 1 volt. Therefore, if the proper part of any supply voltage fluctuation can be applied to a dynode in the proper direction, the overall gain will not be affected.

This is accomplished by connecting one dynode G to a second voltage divider 23, 24 which includes a source of constant voltage such as the glow tube 22 or a battery. If the second order efiects are neglected and V =the normal overall D.-C. voltage V'=the voltage between the photocathode and the midpoint of the control dynode characteristic v =constant reference voltage E =fluctuation of V e =fluctuation of voltage at V on main divider e'=fluctuation of voltage at V' on control divider G =gain of multiplier C= constant in operating region Ic=; constant in operating region a=- ,constant in operating region Z=arbitrary constant in operating region m =arbitrary constant in operatingregion 8 and then compensation is effected when E b V- v 71 w) 1 v be a V- v and V=v(aab+ 1) It should be noted that the glow tube or battery which supplies 2; can be replaced by any non-linear circuit element, such as a thyrite resistor as described hereinafter in conjunction with Fig. 7. When this is done, of course, the above equations must be altered.

In the circuit of Figure 3, a vacuum photocell 30 is coupled to a dynode C operated on a linear section of its control characteristic. The photocell 30 is exposed to the exciter lamp l5 directly by the lens 29, and passes much more current than the dynode C, while the multiplier cathode ll receives only the light passing through the film. The voltage developed across the coupling resistor 32 by the photocell 30 is proportional to the light on the photocell. If the dynode is operated on an hyperbolic section of its characteristic, it is possible by adjusting this resistor 32 to cause the voltage of the dynode C to change in such a way that the gain of the multiplier will be inversely proportional to the light striking the photocell or that the gain times the light intensity will be constant. Therefore, if the intensity of the exciting light decreases, the gain of the multiplier increases in direct proportion, and the lowering of the light signal striking the photocathode is exactly compensated by the resulting increase in gain so that the signal level remains unchanged. If the light increases, the process is reversed and the result is the same. The process may be described as follows:

Let

G=multiplier gain I,,=multiplier collector current S,,,=cathode sensitivity of multiplier a constant L,,,=light striking multiplier cathode I =photocell current S=photocell sensitivity a constant L=light striking photocell A= and is proportional to the film density The constant n is made equal to zero by the proper adjustment of the tap from the coupling resistor on the voltage divider 3| with normal light intensity.

Thus the output of the multiplier is independent of the exciting light intensity and depends only on A as long as k is a constant. But A is afiected only by the film density so that the output is proportional to the transmission properties of the film.

The combination of a gain stabilizing circuit and an exciting lamp compensating circuit can be accomplished as shown in Figure 4. When this is done, interaction between the circuits, which complicates their adjustment and limits their effective operating ranges, is prevented by using separate and widely separated dynodes C and G for the controls. This is easily done because the current in the stabilizer circuits 22, 23,

24 is large enough to be negligibly influenced by that drawn by the dynode G near the collector [2, whereas the low current passed by the photocell 30 necessitates the use of a low current stage near the photo-cathode II for the light compensating control.

With this circuit, using a nine-stage RCA type 0-4300 multiplier, satisfactory operation was obtained with the stabilizer using the seventh dynode and the light compensator using the third dynode, as shown in Figi'4. The system, which employed two RCA type VR-l05/30 l05-volt regulator tubes in series and a vacuum photocell, maintained constant gain for supply voltages between 750 and 800 volts and constant output with a 20% change in light intensity. When a 7.5- ampere 10-volt lamp, operating on alternating current was used, the use of the compensator reduced the -cycle A.-C. component of the output due to light fluctuations by a factor of 20 to a level within 6 db. of the shot noise. At the same time the stabilizer reduced the efiect of a 1% A.C. ripple in the power supply by a factor of 200, making its level less than 6 db. above the shot noise.

This form of the invention shown in Fig. '7 is quite analogous to that shown in Fig. 2 but the circuit is modified for the use of alternating current supply. If the frequency of the A.-C. supply is superaudible, this form of device can, of course, be used in sound reproducers but it is equally applicable to use on commercial current supplies for purposes where the intermittent output is not objectionable. In this arrangement insofar as the reference numerals are the same, the parts are the same as those shown in Figs. 1 and 2. The A.-C. supply, however, is provided through a transformer having the primary 40 and .the tapped secondary 4|. It will be apparent that instead of the tapped secondary 4| a tapped impedance or tapped resistance may be used, with the proper voltage applied thereacross, and the tapped transformer in this figure as well as in Figure 8 is shown only for the sake of simplicity. In this arrangement the voltage divider 23, 24 is provided as in Figure 2 but the lead 25 may be applied to the dynode F instead of G, as shown in Fig. 2, depending on the manner in which the voltage is divided. The main difference between this form of the invention and that shown in Fig. 2 is that the variable impedance 42 is used instead of a glow tube. This variable impedance is of the type which varies with the voltage impressed across it, such as a saturated iron core choke or a non-linear resistor, such as Thyrite. In order to cause a change in the voltage ratio between the control dynode and its neighbors when the overall voltage changes, such a change can be made to decrease the gain when the voltage increases, or vice versa, or if the nonlinear impedance is properly adjusted the gain will remain constant independent of supply voltage variations.

A second way of accomplishing gain stabilization on A.-C. power supply is shown in Fig. 8 wherein a saturated iron cored inductance 43 is provided and at an appropriate value of inductance, this is tuned to resonance by a supply frequency by means of the condenser 44. Ihe phase of the voltage of the control dynode will then shift with respect to that of the adjacent dynodes thereby producing a desired constancy of output.

It will be apparent that in the forms of the invention shown in Figs. '7 and 8 an additional gain control in accordance with average light intensity from the light source, which affects the photocathode H, may be applied in the same manner as described above in connection with Fig. 3 as the superposition of alternating current will not interfere with the control effect produced by the uni-directional current through the photocell.

I claim as my invention:

1. In a sound reproducer, a light source, means for modulating light from the source in accordance with sounds to be reproduced, a photoelec tric electron multiplier responsive to said modulated light, said multiplier including a photoelectric cathode, a plurality of multiplying electrodes and a collecting electrode connected to an output circuit, a source of power supply connected to all of the electrodes of said multiplier, and a photocell in the path of unmodulated light from said source and connected to said power supply and one of said multiplying electrodes 'for rendering the output of said device independent of variations in the illumination from said light source the said electrodes being connected to said power supply through a voltage dividing network, one of the electrodes being connected to a separate voltage divider from the others and including a source of constant potential in said separate voltage divider for maintaining the output of said multiplier constant irrespective of variations in said power supply.

2. In a sound reproducer, a light source, means for modulating light from th source in accordance with sounds to be reproduced, a photoelectric electron multiplier responsive to said modulated light and provided with a photoelectric cathode, a plurality of multiplying electrodes and a collecting electrode connected to an output circuit, a source of power supply connected to the electrodes of said multiplier, and a photocell responsive to unmodulated light from said source and connected to said power supply and one of said multiplying electrodes for maintaining the output of said device constant irrespective of variations in the illumination from said light source.

3. In combination with an electron multiplier having'a cathode, a collector and a plurality of intermediate dynodes, a source of potential connected to said collector, said cathode and said dynodes, one of said dynodes being connected to said source of potential through a voltage dividing circuit including a variable impedance and the other of said electrodes being connected to said source of potential through separate potential-dividing means.

4. In combination with an electron multiplier having electrodes including a cathode, a collector and a plurality of dynodes between said cathode and collector, a source of potential for said -'electrodes, a voltage dividing circuit including a variable impedance connecting on of said dynodes to said source of potential and separate voltage dividing means connecting the remainder of said electrodes to said source of potential.

5. Apparatus as defined in claim 4 wherein the variable impedance is a glow tube.

6. Apparatus as defined in claim 4 wherein the variable impedance is a battery.

'7. Apparatus as defined in claim 4 wherein the variable impedance is a saturated reactor.

8. Apparatus as defined in claim 4 wherein the variable impedance is a saturated reactor tuned to resonance by a capacitor.

9. Apparatus as defined in claim 4 wherein the variable impedance is a resistor the resistance of which varies as the current therethrough changes.

10. Apparatus as defined in claim 4 wherein the variable impedance is a resistor the resistance of which changes as the current therethrough changes and having a capacitor connected in shunt therewith.

11. In combination, a light source, means for modulating light from the source, a photoelectric electron multiplier responsive to said modulated light, said multiplier including a photoelectric cathode, a plurality of multiplying electrodes and a collecting electrode connected to an output circuit, a source of power supply connected to all of the electrodes of said multiplier, and a photocell in the path of unmodulated light from said source and connected to said power supply and one of said multiplying electrodes for rendering the output of said device independent of variations in the illumination from said light source,

the said electrodes being connected to said power supply through a voltage dividing network, one of the electrodes being connected to a separate voltage divider from the others and including a source of constant potential in said separate voltage divider for maintaining the output of said multiplier constant irrespective of variations in said power supply.

12. In combination, a light source, means for modulating light from the source, a photoelectric electron multiplier responsive to said modulated light and provided with a photoelectric cathode, a plurality of multiplying electrodes and a collecting electrode connected to an output circuit, a source of power supply connected to the electrodes of said multiplier, and a photocell responsive to unmodulated light from said source and connected to said power supply and one of said multiplying electrodes for maintaining the output of said device constant irrespective of variations in the illumination from said light 10 source.

RICHARD L. SNYDER, JR.

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