ANALOG-TO-DIGITAL CONVERTER UTILIZING A SAMPLING CATHODE-RAY-TUBE

Abstract: A high speed analog sampler using a CRT (21) having a number of target elements (43, 45) arranged in a circle with the electron beam being repetitively scanned over the target elements in a circle. The amplitude of the analog signal to be sampled is applied to a grid (41) of the CRT to modulate the intensity of the electron beam prior to its striking the conductors. Electrons striking the target elements create a detectable voltage proportional to the amplitude of the analog voltage at the various intervals in time that the electron beam struck the various target elements. This output can further be digitized by one or more analog-to-digital converters.

Description: ANA OG-TO-DIGITAL CONVERTER UTILIZING A SAMPLING CATHODE-RAY-TUBE BACKGROUND OF THE INVENTION This invention relates generally to the arts of high speed analog signal sampling and analog-to-digital converting of such samples particularly wherein a cathode-ray-tube (CRT) is employed. There are many applications wherein a series of discrete analog voltage levels are desired to be obtained by sampling a continuous analog signal. In some cases, these samples are directly utilized, but in most in¬ stances the samples are digitized. In the most popular high speed analog-to-digital converters (ADC's), an ana¬ log sample is taken and digitized within the interval between samples; that is, a digital word is formed repre¬ senting the magnitude of the analog sample before the next sample of the continuous analog waveform is taken. The fastest of these ADC's are referred to as "flash con¬ verters" and utilize a large number of semiconductor com¬ parators. Present silicon technology puts an upper limit on ' these devices. They are able to produce digital words with about 6 bits of accuracy at 5 x 10 8 samples per sec¬ ond. A serious limitation of these devices are signifi¬ cant errors that result in regions of rapid change in the analog signal being sampled. Other devices use a cathode-ray-tube (CRT) with at least one analog charge-coupled-device (CCD) that is the target of the electron gun. A CCD stores pockets of charge within a semiconductor, each pocket of charge being proportional to the analog signal at a correspond- ing sample time. After the required number of samples have been stored, they are clocked out at a slow rate, digitized by a low speed ADC, and stored in a low speed memory. Although such an ADC and memory are inexpensive, a significant amount of power is required for driving a CCD at a sample rate comparable to high speed flash con¬ verters. Further, " non-linearity of the CCD's is a sig¬ nificant problem. Therefore, it is a principal object of this in¬ vention to provide an apparatus and method for accu¬ rately, inexpensively and continuously sampling a high frequency analog signal at a rate that is much faster than existing sampling devices and ADCs. SUMMARY OF THE INVENTION This and additional objects are accomplished by the present invention wherein, briefly, an electron beam within a CRT is intensity modulated by the continuous- analog signal to be sampled. The electron beam is de¬ flected repeatedly over the same pattern across a target surface, most conveniently a circular pattern. Placed adjacent one another on the target surface in a closed figure, such as a circle, are a number of electrically conductive targets that produce a voltage proportional to the intensity of the electron beam when sweeping across the targets. The output is a continuous number of suc¬ cessive samples of the continuous analog signal. These samples can be held and subsequently digitized by low speed, accurate and inexpensive available ADCs. Additional objects, advantages and features of the various aspects of the present invention will become apparent from the following detailed description of its preferred embodiments, which descriptions should be taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a continuous analog waveform that is to be sampled by the techniques of the present invention; Fig. 2 shows a preferred form of a CRT sampling device according to the present invention; Fig. 3 is a diagram of an electronic circuit for use with the sampling device of Fig. 2; Fig. 4 illustrates in block diagram form a com¬ plete system utilizing the samples obtained by the system of Figs. 2 and 3. THE PREFERRED EMBODIMENTS Referring initially to Fig. 1, a continuous analog waveform 11 is shown as an example of the type of voltage waveform that is desired to be sampled in a wide variety of electronic instruments and systems. The wave¬ form 11 is shown to be sinusoidal, but the techniques herein work equally well for other analog waveform shapes. It is desired to obtain a voltage at various sample times 13, 15, 17, 19, and so on, at a fixed time interval of X. These analog voltage samples are then either used directly by the instrument or system, but most frequently are digitized, the sampling process being done prior to holding for digitization. - J EΛ J-- O PI Referring to Fig. 2, a typically shaped CRT vacuum envelope 21 contains a standard electron gun 23 at its narrow end and provides at its other end a somewhat larger target surface 25 which supports targets impinged upon by an electron beam 27 from the gun 23. A high voltage power supply 38 provides an accelerating voltage in the order of 10,000 volts, and its positive output line 40 is grounded, as is the anode 42 of the CRT. A pair of plates 29 deflects the beam 27 in a vertical di¬ rection, and a pair of plates 31 deflects the beam in a horizontal direction, a standard arrangement. A standard magnetic deflection system would serve the same purpose. In this example, the beam 27 is driven at a constant speed over a circular pattern across the target surface 25. This deflection is accomplished by generating in de¬ flection circuitry 33 a sine wave in a conductor 35 and a cosine wave in a conductor 37 for the horizontal and ver¬ tical deflection plates, respectively. The analog signal to be sampled is applied through an amplifier 39 to a grid 41 in the path of the electron beam 27 which varies its intensity as a function of time in response to the varying voltage at the grid 41 from the input analog sig¬ nal. On the target surface 25 is positioned a number of individual electrically conductive targets, such as adjacent targets 43 and 45. These targets preferably each have the same dimensions and are positioned equal distance from each other completely around a circular path that is coincident with the * path scanned by the beam 27. The targets are electrically insulated from one another and communicate by one or more wires through the envelope 21 to external processing circuitry. The loads attached to these external conductors of each of the tar¬ gets 43, 45, etc., are capacitive only, so the voltages are proportional to the value of the analog signal 11 at the instant that the electron beam 27 swept across the target. If the electron beam 27 travels at a uniform speed and the targets 43, 45, et al. are equally spaced in a closed path, the samples are periodically and con¬ tinuously taken. The pattern of the conductive targets 43, 45, etc., and the corresponding scanned pattern of the elec¬ tron beam 27 need not necessarily be circular, but such a pattern is the most convenient. So long as the conductive targets 43, 45, etc., are positioned in a continuous pat¬ tern and the electron beam 27 scanned over them in that same pattern, the input analog waveform will be contin¬ uously sampled. Such samples are assuredly made to be periodic so long as a speed of electron beam scan and spacing of the targets is coordinated, most conveniently a uniform speed scan and equal distance spaciag of the targets. Each of the targets 43, 45, etc., can simply be a metal conductor upon which a charge is deposited by the electron beam 27 in an amount proportional to the inten¬ sity of the beam when striking the target and inversely proportional to its velocity across the target. The vol¬ tage developed by each target conductor is generally pro¬ portional to the amount of charge deposited and inversely proportional to its capacitive load. Preferred over or¬ dinary metal conductors because of the higher voltage output are targets 43, 45, etc., of a type which amplify the charge received from the electron beam 27. Such a device is an electron bombarded semiconductor (EBS) -6- diode, which is known and commercially available in var¬ ious forms. Such a diode is characterized principally by generating in the order of 2,000 electrons for each inci¬ dent electron from the CRT beam when the accelerating voltage is in the range of 10,000 volts. When appropri¬ ately reversed biased, the larger number of electrons will be swept out of the diode into external circuitry. Referring to Fig. 3, an example of such exter¬ nal circuitry is shown for use with target electrodes 43, 45, etc., when in the form of EBS diodes. Each of the diodes 43, 45, etc., is connected to an individual sample-and-hold circuit 47, 49, etc., respectively. Each of the diode targets 43, 45, etc., have one terminal con¬ nected to a common positive voltage supply conductor 51, the second terminal of each diode going to its indepen¬ dent sample-and-hold circuit. The sample-and-hold cir¬ cuit 49 will be described as an example. The second ter¬ minal of diode 45 is connected to ground potential through a storage capacitor * 53 which stores the charge generated in the diode target 45 by the CRT electron beam 27. A voltage across the capacitor 53 is thus propor¬ tional to the value of the input analog signal 11 at the sample instant when the electron beam 27 scanned across the diode 45. The capacitor 53 is controllably dis¬ charged by an FET switch 55 which shorts across the capa¬ citor 53 in response to an input pulse in a circuit 57. Operation of the FET switch 55 clears the sample-and-hold circuit 49 and prepares it for receiving another charge the next time the electron beam 27 scans across the diode target 45. A buffer FET amplifier 59 is also connected to the capacitor 53 in a source follower circuit that produces in a conductor 61 a low impedance output with -7- respect to ground potential. The voltage between the conductor 61 and ground is proportional to a sample of the input analog waveform 11. Referring the Fig. 4, the output conductor 61, and the outputs of each of the other sample-and-hold cir¬ cuits, are applied to a multiplex and hold circuit 63 and thence as analog .voltage signal inputs to an analog-to- digital converter 65. An output of the ADC 65 is in the form of digital words which give the magnitude of each of the samples of the analog waveform 11, these words being stored in a digital memory 67. Control circuits 69 coor¬ dinate the cooperative operation of these Fig. 4 ele¬ ments, as well as ' providing the clearing pulse for the sample-and-hold circuit 49 in the line 57 and the synch¬ ronization signal in line 34 for the deflection circuitry 33. The digitized sample values held in the memory 67 can be used in any one of a number of applications. A common one is in a display device wherein the samples are selectively read out of the ffte ory 67 into a digital-to- analog converter 69 and the analog signals then displayed by a visual display device 71. A display device 71 is most usually a CRT display but can also be of other types, such as an X-Y recorder. In most applications of such a sampling device, it is desired to acquire a certain number, such as 1,000, of consecutive samples of the analog waveform. The num¬ ber of targets 43, 45, etc. (Fig. 2), can be made equal to the number of samples desired at one time, each target then having its individual sample and hold circuit, such as the circuit 49 (Fig. 3) previously described. The various outputs of the sample-and-hold circuits are then OMPI applied by the multiplexing circuit 63 (Fig. 4) one at a time to a single ADC 65, thus digitizing each sample in time sequence after the full set has been acquired; how¬ ever, for large numbers of samples, providing a separate target 43, 45, etc., for each sample makes the CRT sam¬ pling device physically large and expensive. It is preferable to utilize a smaller number of targets 43, 45, etc., than the number of anticipated con¬ secutive samples. In this case, the voltage level stored by each sample and hold circuit, such as the circuit 49, is transferred to a similar analog voltage storage device within a circuit 63 (Fig. 4) as soon as possible after the electron beam has impinged upon its target. Once the voltage is transferred from a particular sample-and-hold circuit, such as the circuit 49, its storage capacitor is discharged by receipt of a clear pulse to ready the cir¬ cuit to receive a new voltage. This transfer and clear must occur before the electron beam circulates back around to the given target again. Once the desired num¬ ber of samples are accumulated in the circuit 63, they can then be digitized by serial application to the ADC 65. Of course, multiple ADC's can be used for parallel processing in order to speed up the process, but in any event, the type of ADC 65 that is utilized will be a high quality one. This is possible since speed of digitizing a signal is not critical in this system. Alternative to such an arrangement when fewer targets 43, 45, etc., are utilized than consecutive sam¬ ples to be taken, a plurality of sample and hold cir¬ cuits, such as the circuit 49 previously described, could be provided for each of the target conductors with some type of switching mechanism to successively connect the target to a different sample and hold circuit each time the beam impinges upon the target. In either case, the number of targets 43, 45, etc., must be sufficient that the voltage generated upon each impingement of the electron beam with it can be acquired and stored in less than one revolution of the electron beam. Exactly 199 such targets are advanta¬ geously utilized, in a specific example utilizing the various aspects of the present invention. This is enough for a sampling rate of up to 2 GHz. With a minimum of . two samples per cycle of the analog signal, the maximum bandwidth possible of the signal is 1 GHz. Use of an odd number 199 of target has a fur¬ ther particular advantage that a lesser- sampling rate than the maximum designed rate can selectively be used without changing any of -the geometry, electron beam rota¬ tion speed, etc. To reduce the sampling rate by a factor of two, every second diode is used during the first elec¬ tron beam sweep. The remaining alternate diodes are sub¬ sequently used in the second circular sweep of the elec¬ tron beam. Similarly, for a factor of four (that is, a sampling rate of one-fourth the maximum rate) , a signal from every fourth target around the target array is uti¬ lized, in each of these cases, a number of samples of the analog signal that are acquired are continuous. Each revolution of the electron beam automatically causes every second, fourth, or whatever desired factor, diode to give a desired sample because of the particular number of diodes utilized. This number will usually be odd, and something other than 199 can certainly be utilized, de¬ pending upon the particular sampling reduction factors desired and the speed of rotation of the electron beam. It is standard to provide an ability to select a reduc¬ tion of the maximum sampling rate by the factors of 2, 4, 10, 20, 40, 100, and so on. A specific example of the configuration of diode targets 43, 45, etc., is for them to be 15 mils, wide with a separation between them of 5 mils, around the circle. The cross-sectional size of the electron beam 27 is made to be as small as possible, generally within the range of 5-10 mils, in diameter. The voltage output of the target diodes, because of their width, will integrate that portion of the analog signal 11 (Fig. 1) that occurs while the beam is crossing the finite width of each tar¬ get 43, 45, etc. Although the various aspects of the present in¬ vention have been described with respect to specific im¬ plementing examples, it will be understood that the in¬ vention is entitled to protection within the full scope of the amended claims.

Claims: WHAT IS CLAIMED IS: 1. A method of sampling at defined intervals an electrical analog signal, comprising the steps of: repetitively scanning at a uniform speed over a continuous closed loop path on a surface within a vacuum envelope an electron beam, said surface containing a plurality of electrically independent conductive elements at substantially equal intervals around said path, said electron beam having a cross-sectional dimension in the direction of said path that is less than the elements' extent along said path, intensity modulating the electron beam by said analog signal before striking said surface, whereby each of said conductive elements is charged by said electron beam to a level proportional to the amplitude of said signal during the interval the electron beam strikes said element, and independently storing an electro 'beam induced voltage of each element, whereby said samples may be ob¬ tained continuously at a high rate of speed. 2. A method according to claim 1 wherein the step of scanning the electron beam includes scanning said beam over said elements that are each characterized by providing an output current which is amplified relative to the incident current of said electron beam which strikes the element. 3. A method of sampling at periodic intervals a continuous analog signal, comprising the steps of: scanning substantially in a circular path and at a uniform speed over a target surface in a vacuum en- OMPI WIPO >- velope an electron beam of substantially the size of a point in cross-section, ' positioning a plurality of electrically inde¬ pendent conductive elements at substantially equal inter¬ vals in said circular beam path, intensity modulating the electron beam by said analog signal, whereby each of said conductive elements is charged by said electron beam to a level proportional to the amplitude of said signal during the interval the electron beam strikes said element, and independently storing an electron beam induced voltage of each element, whereby said samples may be ob¬ tained continuously at a high rate of speed. 4. A method of digitizing a continuous analog signal, comprising the steps of: repetitively scanning in a continuous closed loop path over a target surface in a vacuum envelope an electron beam of substantially the size of a point in cross-section, positioning a plurality of electrically inde¬ pendent conductive elements around said closed path, said elements being spaced and said beam being scanned at a speed so that the beam strikes one of said elements at periodic intervals. intensity modulating the electron beam by said analog signal, whereby each of said conductive elements is charged by said electron beam to a level proportional to the amplitude of said signal during the interval the electron beam strikes said element, independently storing for each element a volt¬ age proportional to the intensity of the electron beam when striking the element, and converting each of said stored voltage levels to a digital signal, whereby said analog signal may be digitized at a moderate rate after all samples are stored. 5. A method according to claim 4 which com¬ prises the additional steps of: storing said digital signals, and reconstructing from said stored digital signals a display of said analog signal. 6. A method according to any of claims 3, 4, or 5 wherein the step of positioning the elements in¬ cludes positioning semiconductor diode elements that are each characterized by providing an output current which is amplified relative to the .incident current of -said electron beam which strikes the element. 7. A method according to any of the claims 1, 2, 3, 4, or -5 wherein a number of successive periodic samples of the analog signal are acquired that exceed by a significant amount the number of elements which the electron beam strikes. 8. A method according to any of the claims 1, 2, 3, 4, or 5 wherein the step of independently storing a voltage for each element includes the steps of storing the voltage in a sample-and-hold circuit, and transfer¬ ring the voltage signal out of said circuit and into another analog voltage storage device before the electron beam next strikes that element, whereby a number of con¬ tinuous samples may be taken of the analog signal that is OMPI \ greater than the number of conductive elements within the vacuum envelope. 9. A method according to any of the claims 3, 4, or 5 wherein the step of positioning said elements comprises using an odd number of said elements around said path, such that the use of every Xth target results in using all of them in X completions of the electron beam around said path, whereby the sampling rate may be reduced for some applications while maintaining full uti¬ lization of said elements. 10. A method according to any of the claims 3, 4, or 5 wherein a number of successive periodic samples of the analog signal are acquired that exceed by a signi¬ ficant amount the number of elements which the electron beam strikes, and further wherein the step of positioning said elements comprises using an odd number of said ele¬ ments around said path, such that the use of every Xth target results in using all of them in X completions of the electron beam around said path, whereby the sampling rate may be reduced for some applications while maintain¬ ing full utilization of said elements. 11. A sampling cathode-ray-tube, comprising: an electron gun within a vacuum envelope at one end thereof, said gun generating a beam of electrons and directing them toward another end of the vacuum envelope, a plurality of electrically isolated conductive elements positioned adjacent one another in a circle at said other end of the envelope. means for electrically connecting said conduc¬ tive elements to conductors outside of said tube, OMP means for repetitively scanning said electron beam at a uniform speed in a given circular path over said conductive elements, and means for receiving an analog electrical signal and modulating the intensity of said electron beam in ac¬ cordance with a time varying amplitude of said signal, whereby said analog signal may be periodically sampled at a high speed and said samples presented at said conduc¬ tors outside of said tube. 12. The sampling cathode-ray-tube according to claim 11 wherein said conductive elements are each char¬ acterized by amplifying in a current flowing out of the element the number of electrons of said beam that strikes the element. 13. The sampling cathode-ray-tube according to either of claims 11 or 12 wherein the number of conduc¬ tive elements is exactly 199. 14. In an analog-to-digital converter of the type utilizing a cathode-ray-tube for sampling a continu¬ ous analog electrical signal, said cathode-ray-tube hav¬ ing an electron gun to direct an electron beam of sub¬ stantially a point size in cross-section toward a target area within a vacuum envelope, said converter comprising: means receiving said analog electrical signal for modulating by the varying amplitude of said signal the intensity of the electron beam before striking said target area. means for deflecting with a given speed func¬ tion said electron beam in a regularly recurring closed - 16- loop pattern over said target area, said pattern being independent of said analog signal, a plurality of electrically isolated conductive element positioned at said target area adjacent each other continuously around said closed loop, means connected to each of said conductive ele¬ ments for storing an analog signal proportional to the amplitude of the electron beam when it last passed over the element, and means connected to each of said storing means for converting its voltage level to a digital signal, whereby said analog electrical signal is digitized. 15. The converter of claim 14 wherein said analog signal storing means includes a separate sample- and-hold circuit connected to each of said conductive elements, and means for transferring the stored analog signal into a separate storage means and then clearing the storing means before said electron beam reaches that conductive element again, whereby the number of contin¬ uous samples that can be taken of the continuous 'analog electrical signal are greater than the number of conduc¬ tive elements in the cathode-ray-tube. 16. The converter of claim 14 wherein the total number of said conductive elements positioned in the close looped pattern is an odd number and said analog signal storing means includes means for storing the ana¬ log signal from every Xth element for one traverse of the electron beam around said closed loop, the number of said elements and the analog storing means being configured so that all of the elements are utilized in X number of beam OMPI cycles around said closed loop, whereby the sampling rate of said converter can be changed by changing the value of X. 17. The converter of claim 16 wherein said analog storing means includes means to set X at any of the numbers 1, 2, 4, 10, 20, 40, etc. 18. A display system comprising an analog-to- digital converter according to any of the claims 14, 15 or 16, and additionally comprises means receiving said digital signals from said digital signal converting means for reconstructing and displaying a representation of the original continuous analog electrical signal. 19. A method of sampling at periodic intervals a .continuous analog electrical signal, comprising the steps of: scanning an energy beam substantially in a cir¬ cular path and at a uniform speed over a target surface, positioning on said target surface a plurality of energy responsive elements at substantially equal in¬ tervals in said circular beam path, each of said elements being characterized by generating an electrical quantity proportional to the intensity of said energy beam when striking the element, intensity modulating the beam by said analog signal, whereby each of said elements generates an elec¬ trical quantity proportional to the amplitude of said analog signal at the instant the beam strikes said ele¬ ment, and independently storing a beam induced electrical quantity from each element, whereby said samples may be obtained continuously at a high rate of speed. 20. A method of digitizing a continuous analog electrical signal, comprising the steps of: repetitively scanning in a continuous closed loop path over a target surface an energy beam of sub¬ stantially the size of a point in cross-section, positioning on said target surface a plurality of independent energy responsive elements around said closed path, said elements being spaced and said beam being scanned at a speed so that the beam strikes one of said elements at periodic intervals, intensity modulating the energy beam by said analog signal, independently storing for each element a volt¬ age proportional to the intensity of the energy beam when striking the element, and converting each of said stored voltage levels to a digital signal, whereby said analog signal may be digitized at a moderate rate after all samples are stored.

Inventors: BUCHELE WILLIAM NORMAN, BUCHELE, WILLIAM, NORMAN
Applicants: GOULD INC, GOULD INC.
Application year: 1983
Main classification: H03K13
IPC codes: G01R13, G11C27, H03M1