US2963579A - Stair-step voltage function generator - Google Patents

Description

C. E. BERRY STAIR-STEP VOLTAGE FUNCTION GENERATOR Dec. 6, 1960 Filed July 20, 1956 United States Patent vC) 21,963,519 sTAm-STEP VOLTAGE FUNCTION GENERATOR Clifford E. Berry, Altadena, Calif., assignor, by mesne assignments, to Consolidated Electrodynamics Corporation, Pasadena, Calif., a corporation of California Filed July 20, 1956, Ser. No. 599,264

Z Claims. (Cl. 328-186) 2,963,579 Patented Dec. 6, 1960 ice . weighted summing resistors connecting each of the series paratus, including counters, dividers, integraters, digitalto-,analog and analog-to-digital converters, and the like. Various means have heretofore been proposed for generating a stair-step voltage in which the voltage may be changed in incremental steps of equal amplitude. In many applications it is desirable that the stepping voltage be changed in 'incremental steps of considerable accuracy, the total voltage being divided into a large number of increments.

A considerable number of circuits have heretofore been proposed which are capableof generating a stairstep type'of voltage function.l Capacitive storage circuits have been used for this purpose, but generally produce a wave form having exponentially varying steps. Various circuits, employing feed-back or other compensating means have been suggested for producing linear steps of voltage, but such circuits become relatively complicated and are not particularly suited for use where a large number of linear steps are required.

Mechanical circuits utilizing electronic switching have likewise heretofore been proposed forrgenerating a stairstep voltage function. Stepping circuits using mechanically-operated stepping'switches are useful only where their slow speed of operation can be tolerated. Electronic type switching circuits used togenerate a stepping function are generally complicated and frequently are adversely affected in their accuracy by variations in loading, or require a complicated compensating network to provide a high degree of linearity in the stepping voltage.

The present invention is an improvement on such known types of stepping circuits in that i. provides a relatively simple resistive network by means of which a reference voltage can be `derived which is varied in accurately determined incremental steps of equal value. The network can be used in conjunction with a mechanical stepping switch where slow speed operation can be tolerated, or may be used equally as well with a multipleelectrode switching tube where high speed operation is desirable. The accuracy of the network in generating a linear stepping voltage is not affected by loading and hence can be readily cascaded or summed where it is desired to increase the number of incremental steps in the stepping voltage output.

In brief, the invention provides a stepping circuit comprising a plurality of resistors of equal resistance connected in series. Switching means, such as a multipleelectrode switching tube, successively connects one or more of the series resistors across a constant current source. The output voltage is derived across the series of resistors. The output voltage can be summed with the output voltage of similar stepping circuits by means of resistor circuits to a common output terminal to increase the number of incremental voltage steps which can be produced.

For a better understanding of the invention reference should be had to the accompanying drawing, wherein:

Fig. l shows a schematic diagram of a stepping circuit employing the features of the present invention;V

Fig. 2 shows the wave form of the output of the circuit of Fig. 1 where the stepping circuit is triggered at equal intervals of time; and

Fig. 3 is a schematic diagram of the resistor network where a plurality of stepping circuits are summed.

Referring to Fig. 1 in detail, the numeral 10 indicates,

generally, a multi-electrode tube which may be a Sylvania Type 6476 multiple cold cathode gas discharge tube. The tube 10 includes a common anode 12 and a plurality of cathodes 14. While the tube is shown schematically in the drawing asV having the cathodes spaced out in line, in actual construction the cathodes are spaced radially about a centrally disposed anode. The tube 10 further includes two groups of auxiliary electrodes, such as indicated at 16 and at 1S, disposed on either side of the cathode electrodes 14. The auxiliary electrodes 16 are connected by a common lead to one output of atrigger circuit 20, while the other group of auxiliary electrodes 18 are similarly connected by the common lead to a second output of the trigger circuit 20.

The common anode 12 of the tube 10 is connected to a constant current source 22. The source 22 is of conventional design and is regulated `automatically to provide an effective internal resistance which is very high in relation to the total external load hung across the source. Thus, the current through the load remains substantially constant regardless of changes in the external load.

Each of the cathode electrodes 14 is connected to a different junction or tap on a resistor type voltage divider or series resistor circuit indicated generally at 24. The resistors forming the series circuit v24 are preferably equal in value. An output voltage is derived across the entire series circuit 24.

ln operation, the tube 10 provides a low impedance current conductive path between the anode electrode 12 and one of the cathode electrodes 14. Conduction is transferred from one cathode electrode 14 to the next in one direction by means of the pulses from the trigger circuit 20 applied successively to the rst group of auxiliary electrodes 16 and then to the second group of auxiliary electrodes 18. However, if the auxiliary electrodes 1S are pulsed iirst, followed by pulsing of the auxiliary electrodes 16, conduction is transferred in the opposite direction from one cathode electrode 14 to the next. Stepping of the tube 10 is accomplished by means of stepping pulses applied to the input of the trigger circuit 20, the trigger circuit 20 providing means for selectively delaying the pulse coupled to one group of auxiliary electrodes with respect to the other group according to the direction in which it is desired to step the tube 10.

It will be seen that Lif no current is drawn from the output, the voltage appearing across the output is expressed as:

Where I is the current of the constant current source 22, R is the resistance of each of the series resistors, and k is the number of resistors connected by the tube 10 across the constant current source 22.

Since the impedance looking into the output terminals of the circuit is constant, if a load RL is imposed across the output, the output voltage is still divided into equal increments by switching of the tube 10. The output voltage is given by the expression,

where N is the total number of resistors in the series resistance circuit 24.

Fig. 2 shows the resulting Wave form of the output voltage E as a function of time Where stepping pulses are applied to the trigger circuit 20 at equal intervals of time. Each of the incremental steps of the wave form in Fig. 2 is equal regardless of the value of the load impedance connected across the output of the circuit of Fig. 1.

It can be shown that as a consequence of this property of the circuit of Fig. 1, a plurality of such circuits, having identical voltage dividers and identical constant current sources, may have their outputs summed rigorously by means of Weighted summing resistors, as shown in Fig. 3. If the circuits are arranged in decades, as shown in Fig. 3, summing resistors as indicated at 26, 28 and 30 may be used to couple the respective circuits to a common output terminal. It should be noted that while only three such circuits are indicated in Fig. 3, additional decade circuits may be provided if desired. By properly proportioning the respective summing resistors, incremental steps produced in the first decade circuit may be made ten times as large as the incremental voltage steps produced in the second decade circuit, and the incremental steps in the second decade circuit may be made ten times as large as the incremental steps in the third decadecircuit. The respective values of the summing resistors 26, 28 and 30 may be determined from the following expression:

The value R51 is -arbitrary and merely sets an attenuation factor. In fact, the value of R51 may be 0. From the above expression it will be seen that in the circuit of Fig. 3, with the summing resistor 26 eliminated, the value of the resistor 28 should be 81R, where R is the value of the resistors in the decade circuits, and the value of the summing resistor 30 is 891R.

From the above description it will be seen that a stepping circuit is provided by which a stair-step voltage function may be produced in response to a series of input pulses. The circuit may be summed by means of weighted resistors to increases the number of incremental steps by a factor of 10 or 100 or even more, without alecting the linearity of the output voltage steps. The network is relatively simple, yet requires no compensation for variations in external load, such `as is required in the usual voltage divider type circuit.

What is claimed is:

1. A decade stepping circuit comprising a plurality of series circuits, each including nine equal resistors of resistance value R and a summing resistor of resistance value R5, the summing resistance value R5 being different for each series circuit, the series circuits being connected together in parallel, a plurality of constant current Sources, each series circuit having an associated constant current source, and means including a corresponding plurality of multi-electrode switching tubes for coupling each ofthe sources across a portion of the associated series circuits, and means for deriving an output voltage across the parallel connected series circuits.

2. A stepping circuit as describedin claim 1 wherein the summing resistor in the nth series circuit has a value determined by the expression where R51 is the value of the summing resistor in the Iirst series circuit.

References Cited in the le of this patent UNITED STATES PATENTS 2,427,533 Overbeek Sept. 16, 1947 2,472,774 Mayle June 7, 1949 2,531,624 Hanscom NOV. 28, 1950 2,658,139 Abate Nov. 3, 1953 2,679,013 Barnes May 18, 1954 2,700,750 Dickinson Jan. 25, 1955 2,709,770 Hanson May 3, 1955 2,715,678 Barney Aug. 16, 1955 2,731,631 Spaulding Jan. 17, 1956 2,767,908 Thomas Oct. 23, 1956 2,803,818 Wulfsberg Aug. 20, 1957 2,813,199 Sciaky et al Nov. 12, 1957 2,827,233 Johnson Mar. 18, 1958 2,896,112 Allen et al July 21, 1959

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