US2571017A - Electronic switch - Google Patents

Description

Oct. 9, 1951 J. DEMPSEY ET AL ELECTRONIC SWITCH Filed April 27. 195o Patented Oct. 9, 1951 ELECTRONIC SWITCH John L. Dempsey, Atlantic City, and Richard W.

Sonnenfeldt, Camden, N. J., assignors to Radio Corporation of America, a corporation of Dela- Waffe Application April 27, 1950, Serial No.7158,452 l (Cl. Z50- 27) 8 Claims.

This invention relates to improvements in electrical switches, and particularly to an improved swtch of the type commonly designated electronic, as distinguished from mechanical switches.

In an electronic switch, as that term is used herein, electron tubes are utilized to provide very high speed switching or commutating action. In conventional electronic switches, a square wave or the like may be applied to the contr-ol grid of an electron tube to turn the tube 4off and on rapidly while a signal voltage is being applied continuously to another grid in the tube. Where a commutating action is required, several such tubes may be turned on andoi by a plurality of square wave generators operating in sequence (see, e. g., U. S. Patent 2,426,454-Johnson While switch systems of the foregoing type have many useful characteristics, they are not entirely satisfactory in all respects. In general, with a single electronic switch unit, any change in the rate of switching or in thev duration of the switching interval requires a change inrthe switching voltage waveform. Where a-plurality of such switch units are arranged as an electronic commutator, changes in the order or rate of switching, or in the switch intervals, require extensive changes in the switching voltage waveform or in the number of switch waveform generators required.

Assume, for example, that one wishes to display a plurality of different signals sequentially on a cathode ray oscilloscope. In such case.

the cathode ray tube deflection system can rbe connected to the various signal sources through an electronic switch .system of the type just described. However, if one wishes to change the display time allotted to each waveform, or to make other changes in the display, substantial changes must be made in the switching system.

It is, accordingly, a general object of the present invention to provide an improved electronic switch which -is adaptable to a wide variety of switching problems. y

A more specific object of the invention is to provide an electronic switch which is simple to adjust, has a minimum number of elements, yet is extremely accurate and dependable in operation.

separation or signal combining purposes.

In accordance with the invention, certain of the foregoing objects and advantages of the invention areattained by utilizing two varying A further object of the invention is to provide f -an improved electronic commutator for signal control voltages to operate an electronic switch. The control voltages are arranged to vary simultaneously, and each control voltage acts upon a separate control element in the switch. With this arrangement, one of the control voltages can be utilized to open the switch at a predetermined time. At a predetermined later time, the other control voltage can be utilized to close the switch. As will be shown, it is a simple matter to vary the operating characteristics of the switch simply .by adjusting unidirectional voltages applied to the switch electrodes, but without changing the basic switch control waveforms.

In accordance with a preferred embodiment of the invention, control voltages of staircase Y waveform are utilized. A further feature of the invention resides in the provision of an improved negative staircase voltage wave generator particularly suitable for use in a switching system of the type contemplated by the present invention.

A m-ore complete understanding of the invention can be had from the following description of illustrative embodiments thereof, when considered in connection with the accompanying drawing, /whereinz Figure 1 is a schematic diagram of an electron switch arranged in accordance with the invention,

Figure la shows certain of the voltage waveforms utilized in the switch of Figure l, and

Figure 2 is a schematicdiagram of an electronic commutator arranged in accordance with the invention and including individual switch sections embodying a modified form of the switch shown in Figure l. l

Referring particularly to Fig. l of the drawing, there is shown one form of electronic switch illustrative of the invention. The switch of Fig. 1 is connected between an input terminal l2 and an output terminal I4 to open and close the circuit between the terminals I2, i4. As will K be shown hereinafter, either the terminal i2 or the terminal I4 can be connected in common with other input and output terminals to provide a commutator arrangement either for signal combining purposes or for signal separation.

The switch of Fig. l comprises an electronic tube I6 having a cathode-I8, first and second control electrodes or grids 2U, 22, and an anode 24. The cathode I8 is connected to the input terminal i2, while the anode 24 is connectedto the outputv terminal i4. The cathode i8 and the anode 24 are connected 'toground through relatively large resistors, I9 Vand 25, respectively.

In a case Where the signal to be passed through the switch id is of negative polarity, suoli signal would be applied to the terminal l2 and taken 01T at the terminal It. If a positive signal is involved, the designations input and output for the terminals i2, it would be reversed; the terminal l then serving as the input terminal and the terminal i2 as the output terminal. In either event, it will be understood that a signal could be transferred from one to the other of the terminals l2, ill at any given time only if the voltages on both of the control electrodes 23, 22 were such as to allow the tube I8 to conduct current at that time. In other words, the control electrode voltages will determine the tube conductivity in the usual manner characteristic of electron tube operation. However, in the absence of a signal, no current will flow in the tube IS, since the anode voltage for the tube i6 is provided by the signal itself.

In accordance with the invention, the control electrodes 26 and 22 each are connected to separate varying voltage sources, shown as sawtooth wave generators 25,22, through coupling capacitors 2l, 29. One of the generators, 25, is adapted to provide a positive sawtooth voltage; i..e. a voltage wave that increases steadily from a minimum to a maximum value, and then drops abruptly back to a minimum value at the end of the cycle. in the drawing. The other generator, is adapted to provide a negative sawtooth voltage; i. e. a wave B having the same shape as the positive wave A, but inverted in polarity. The

This is shown by the waveform fr :lo

two generators 26, 28 are connected to operate 11:

synchronously in response to pulses from an oscillator or pulse generator 30. No details are given for the generators 25-30 since such generators are well known in the art (see e. g. pgs. 511-515 of Terman-Radio Engineers Hanfbook, i'lrst edition, McGraw-Hill Inc).

In the switch. system of Fig. 1, as thus far described, it is evident that the sawtooth voltages A, B, will tend to vary the conductivity of the tube IB in opposite directions simultaneously. Of course, the exact effect of each sawtooth voltage will depend on the absolute value thereof with respect to the cathode voltage. In other words, due to the coupling capacitors 2l, 29, the

instantaneous absolute values of the voltages A,

B at the grids 20, 22 will depend on the direct current circuits connected to the grids 233, 22. In order to adjust the absolute values of the voltages A, B to vary the operating conditions, the grids 20, 22 are connected to separately adjustable unidirectional voltage sources, 2, 343. For simplicity, the voltage sources 32, 34 are shown as a pair of potentiometers 36, 38 connected in parallel with a common battery 49. Thus, the voltage at either grid, 29 or 22, at any instant will depend on the setting of the associated potentiometer, 3B or 38, as well as on the value of the varying voltage, A or B, applied to the grid at that instant.

For a typical case, assume that the potentiometer 35 is set so that the grid 25J will allow the tube I5 to conduct when the voltage A is above the level A1 in the drawing. Assume also that the potentiometer 38 is set so that the .grid 22 will permit conduction in the tube l5 until the voltage B is below the level B1. When the voltage A rises to the cut-orf level A1, at the time t1, the voltage B will be above cut-01T. Thereforey the tube I6 will be able to pass a signal from cathode to anode until the time t2 when the the signal.

4 voltage B drops to the level B1. This particular combination of potentiometer settings will give a switching interval of duration t1-t2, occurring at the predetermined time t1 in each sawtooth cycle. To change the duration of the switching interval, or to change the time of switching in the sawtooth cycles, it is a simple matter to adjust either or-both of the potentiometers 3%, S8 to attain the desired result. It is especially important to note that these changes in switching interval and time of switching can be made without making any chan-ge in the basic switching waveform. Moreover, it should be noted .that the circuit is adaptable to a wide variety of variable interval automatic switching operations simply through the provision of automatically variable unidirectional Voltage sources, such as carbon-pile resistors, heat actuated resistors, bridge-circuits, and the like. It should also be noted that theV invention is not limited to the use of sawtooth waveshape control voltages. Sinewave and other generally sloping wavefront switch-control waveforms can be used in a manner similar to that already described, all within the scope of the invention. Of course, a square wave switching voltage would not be suitable since it would not permit switch interval adjustment by bias voltage variations.

It was previously noted that the anode voltage for the tube I6 consists of the signal voltage to be transferred between the terminals E2, M. Of course, where the signal to be transferred is not strong enough to carry through the tube i6 without' amplification, conventional anode voltage can be applied to the tube. The signal then can be applied to an additional grid electrode in the tube yto obtain signal amplification in addition to switching action. However, if vthe tube I6 is operated with conventional l' anode voltage, the output signal will contain a unidirectional Vvoltage component which usually must be removed before the signal nally is used. In removing this unidirectional component, it is extremely diflicult to avoid distorting Therefore, the arrangement shown is deemed preferable wherever sufcient signal voltage is available.

While the switch shown in Fig'. l has the advantages already noted, it is subject to certain limitations. In the great majority of electron tubes having two control electrodes, these two electrodes do not have an exactly equal eect on the tube current. This makes it necessary, in some instances, to provide a rather large varying voltage to the control electrode which has the least effect on tube conductivity. Also, where a single tube is used, as in the switch of Figure l, both the varying switching voltage and the unidirectional adjusting voltage must be applied to the same electrodes, or to additional control electrodes. Either of theserarrangements may be undesirable.

In Fig. 2, there is shown an electronic commutator arranged in accordance with the invention, and including a modied form of switch which avoids the foregoing problems. Also, the system of Fig. 2 includes a novel circuit for generating one of the switching voltage waveforms.

To illustrate one use of an electronic commutator embodying the principles of the invention, the system of 2 comprises an arrangement for displayingt on a cathode ray tube 5t samples of the signals from a plurality of signal sources S31-Sr.. While only three signal sources Si, S2, Sn, with accompanying switch units SW1, SW2,

SW, are shown, it will be understood that a much larger number of signals could be accommodated in this way.

The cathode ray tube 50 is provided with the usual electron gun 52, sets of horizontal and vertica1 deflection plates 54, 56, and a fluorescent screen 5-8. For simplicity, other details of cathode ray tube structure have been omitted. The vertical deflection plates 56 are connected to the signal sources Si--Sn through the switch units SW1-SWn. The horizontal deflection plates 54 are connected to a conventional sawtooth sweep voltage generator 60. An oscillator l64 is provided to generate a voltage of assymetrical waveform for timing the various operations of the system, and the sweep generator 60 is connected through a differentiating network 62 to the oscillator 64. The oscillator 64 can be of'any conventional type, such as a socalled blocking oscillator, a multivibrator, or the like, details of which are given in the above mentioned Handbook, pgs. 511-516. Assuming that the oscillator 64 is adapted to provide a voltage of rectangular pulse waveform C, the diierentiator output voltage will consist of positive and negative pulses D, E. Assuming also that the sweep voltage generator 60 will be triggered by the positive pulses D, the cathode ray-beam in the tube 50 will make one horizontal sweep for each output pulse C from the oscillator 64.

It is intended that a sample of the signal from each of the sources Si-Sn be displayed on the tube screen 58 during succeeding portion of each horizontal sweep. 'Ihis can be accomplished by sequentially connecting the sources Si-Sn to the Vertical deflection plates 56 through the switch units SW1-SWn.

Each of the switch units SW1-SWn comprises a pair of serially connected electron tubes 66, 68. Since these switch units are substantially identical, only the first unit SW1 will be described in detail.

In the switch unit SW1, one of the tubes, 66, has an anode electrode which is connected to ground through a relatively large resistor` |2 and also to the associated signal source Si. The same tube 66 has a control electrode |4 connected to a source 'I6 of staircase waveshape voltage (described in detail hereinafter) through a current limiting resistor T8. The cathode electrode 480 of the tube 66 is connected to ground through a relatively large resistor 82.

The other tube giii! has an anode electrode 84 connected to the preceding tube cathode 80, a control electrode 86 connected to a second source of staircase waveshape voltage `88 (also described hereinafter), and a cathode electrode 90 connected to ground through a relatively large resistor 92. The cathode 90 also is connected to the cathode ray tube vertical deflecting electrodes 56 through a coupling capacitor 94.

The cathodes 80, 90 are connected to separate sources 96, S8 of variable unidirectional voltage. The voltage sources 96, 98 are shown as a pair of potentiometers |00, |02 connected in parallel with a battery |04. The potentiometers |00, |02 and the battery |04 provide means for adjusting the voltage level required at the control electrodes .14, 86 to allow conduction through the tubes 66, 68, similar to the function of the volte,

age sources 32, 34 in the switch of Fig. l. However, it is to be noted that the arrangement of the switches SW1-SWU in Fig. 2 avoids common connection of the switching voltage sources 16, 88 and the adjusting voltage sources 96, 98, and

Without requiring additional control elements in the switch. Of course, the voltages selected at the potentiometers |00, |02 should not be more positive than the minimum value of signal v'oltage expected from the signa1 source Si. f"

The switching voltages for the switches SW1-SWn are of staircase waveshape, as shown at E, F, each increasing in discrete steps, but in opposite directions, from a minimum to a maximum value and then returning abruptly to the minimum value. While switching voltages of staircase waveshape do not provide continuous control of the switch action as would sawtootli or similar voltages, they do permit more exact control of the-switches, as will be pointed out hereinafter.

rOne of the staircase switch voltage generators, 16, is of conventional design, and comprises a storage capacitor |06 connected to be charged by positive pulses receivedthrough a coupling diode |08 and a capacitor |09. The source of charging pulses comprises a frequency multiplier` network H0, connected to provide a pulsating voltage at a multiple of the frequency of the oscillator 64. The multiplier ||0 may be any one of a number of well known circuits, such as a class C amplifier having a high Q resonant output circuit tuned to a harmonic of the frequency of the oscillator 64 (see e. g. the above-mentioned Handbook, pgs. 458-462). Assuming that multiplier ||0 is a circuit of the type just mentioned,

the output thereof will comprise a pulsating voltage containing a predetermined number of positive and negative half cycles for each oscillator output pulse. Each positive half cycle from the multiplier will cause current to iiow through the diode |08 to place an increment of positive charge on the capacitor |06. Each negative half cycle from the multiplier ||0 will be shunted around other capacitor terminal.

the storage capacitor |06 by a second diode ||2 connected in parallel with the series path through the diode |08 and the storage capacitor |06. Thus, the voltage on the storage capacitor |06 will continually increase in stepwise fashion.

At the end of a predetermined number of steps, it is necessary t0 discharge the capacitor |06 to begin a new cycle of the staircasewave. To this end, a gas-iilled tube ||4 is connected in parallel with the storage capacitor |06. The gas tube cathode ||6 is connected to one capacitor terminal, and the anode ||8 is connected to the The control grid |20 of the gas tube is connected to receive pulses from the differentiator 62 through a capacitor 63. The gas tube I4 is of conventional gridstart plate-stop type, wherein initiation of conduction, or iiring, is controlled by the grid |20, while conduction is interrupted by a decrease in anode voltage.

The gas tube control grid |20 is connected to an adjustable negative voltage source, shown as a battery |22 and potentiometer |24. As the Voltage increases on the capacitor |06, the gas tube anode voltage also will increase, but the gas tube will not conduct current to discharge the tube anode voltage will cause the gas tube to stop 7 QOndllCling, allowing a new cycle of stepcharging of the capacitor |06v to begin.

rIhe other step voltage generator 88 is similar' inpsome respects lto the positive staircase gen.- erator '16.l However, as a negative-going staircasewave is vrequired from the generator S8, a pair of diodes |28, |33 are connected to a storage capacitor |32 in opposite polarity to the diodes H18, 2- of` the positive staircase generator. That is,r positive half cycles of voltage from the multiplier ||Y will be shunted around the capacitor |32 by the parallel-connected diode |36, while negative half cycles will place a negative charge on the capacitor |32 through the series-connected diode |28. Accordingly, the voltage on the capacitor |32 will increase stepwise in a negativeA direction while the voltage on the capacitor |06 is increasing stepwise in a positive direction. A. gas tube |34 is connected in parallel with the storage capacitor |32, with the gas tube anode |.36 being connected to the grounded terminal of tha-capacitor |32 and the cathode |38 being connected to the other terminal of the capacitor |32.

It can be seen that the problem of discharging thestorage capacitor |32 in the negative staircase generator issomewhat different than that of discharging the storage capacitor |08 in the positive staircase generator IB. In the latter case, the cathode HS of the gas tube lll is grounded, soA there is no change in the grid-tocathode voltage during charging of the storage capacitor |06. Of course, it will be understood that vthe grid-to-cathode voltage is of critical importance as regards firing of the gas tube. In the negative staircase generator 88, thegas tube Cathode .|33 becomes increasingly negative during the stepwave, so that the grid voltage also must` become more negative as the cathode voltage increases. Otherwise, the tube would fire on the first or second step or the negative staircase.

.In accordance with the invention, this difficulty is avoided by coupling the grid |49 to the cathode |38 through a capacitor |42. Means are provided for placing a negative grid-to-cathode voltage on the capacitor |42 shortly after each stepcharging cycle begins. The terminal of the capacitor |42 at the gas tube grid |49 is connected to the diferentiator 62 through a diode 4|44. The diode anode |46 is connected to the capacitor |42, and the diode cathode |43 is connected to the diierentiator t2. The other terminal of the capacitor |42 is connected to ground vthrough a resistor IE to complete a charging current path through the capacitor M2, the differentiator 62, and the diode |48. A coupling capacitor |52 is shunted across the diode M4.

With the arrangement just described, each 4negative output pulse E from the dilerentiator 62 will cause current to ow through the diode |44, placing a negative voltage on the capacitor |42. As long as the gas tube |34 is non-conductive,` the grid-to-cathode impedance will be high and the voltage on the capacitor |42 will not change appreciably. Accordingly, as the gas tube cathode voltage becomes more negative, the grid 14g-will stay more negative than the cathode |48, preventing iiring of the tube |34. Subsequently, when a positive pulse D is generated at the dif- -ferentiator 62, the tube |34 will re'. At that time, the step-charge storage capacitor |32 will discharge through the low impedance anodecathode space in the tube |34. The voltage on the capacitor |42 will reverse polarity temporarily While the positive pulse D is being generc atedat the diierentiator B2. Whether .or not va positive charge will remain on the capacitor |42 at the end of the positive pulse D will depend on the internal impedance of the oscillator S4. In any event, when'the next negative iinpulse lE is generated at the diierentiator 62, a negative charge willbe left on the capacitor |42 as previously described.

Considering, next,` the sequential action of the switch units SW1-Slim it will be noted that each switch unit receives the same positive and negative staircase voltages, E, F, from the generators le, 83. The potentiometers |09, |62 in the various switch units SW1- SW2 each are adjusted so that the tubes 66, 68 controlled thereby will become conductive or non-conductive on selected steps of the staircase voltages. In a simple case, it can be assumed that the tube B6 in the rst switch unit SW1 will be turned on by the rst step in the positive'staircase E, and that thel tube 63 f therein will be cut olic by the second step in the negative staircase F; that the second switch unit SW2 is adjusted so that its rst tube 5G will conduct'on. the second step in the positive staircase E and its second tube 63 will cut-oli on the third step in the negative staircase F; and that the third switch unit SW is adjusted tofclose and open on the third and fourth .steps of thepositive and negative staircase. It is apparent that such adjustment wilprovide the desired sequential action. Any desired change in the switching sequence can be obtained simply by suitable a'djustment of the various bias voltages in the Vswitch unitsSWi-SWn- V As previously stated, vstaircase waveshape switching voltages provide most precise switch action, although they do not provide the continuous range of adjustment obtainable with smoothly varying switch voltages. The abrupt changes from step to step in the staircase voltages insure that the switch tubes will turn on and off exactly on the vertical section of the selected steps, whereas with smoothly varying switch voltages, the transition from conduction to cut-oli and vice versa may vary by a small amount from one switching cycle to the next. The waveshape selected for the switching voltages in any given system will, of course, depend on the characteristics of switch action desired.

Although switching voltages of equal frequency have been assumed in connection with the systems of both Fig. l and Fig. 2, the invention is not 'necessarily limited thereto. By utilizing switching voltages oi different frequency, very complex switching sequences can be obtained.

It will be understood that various other changes could be made in the' systems shown and described, all within the scope and spirit of the invention. By way of example only, it is noted that the frequency multiplier llil of Fig. 2 could be eliminated by providing a frequency divider network between the oscillator 64 and the differentiator 62. In some instances, it may be desirable to have a blocking capacitor betweenthe cathode 36 of the tube 65 and the anode $4 of the tube 68. Provision of such a capacitor would avoid interdpendance between the biasing voltages used i'nany given switch unit. Accordingly, the

foregoing is to be construed as illustrative, and ynot in a limiting sense. f

What is claimed is:

1. Inv an electronic switch for opening and closing a circuit between an input terminal and an output terminal, in combination, electron tube vmeans connected betweenV said terminals and including anode and cathode electrodes connected one to each of said terminals, first and second conductivity control electrodes in said tube means for controlling the conductivity of said tube means as a function of the voltage on each said control electrode, a first source of periodically varying voltage, a second source of Voltage varying simultaneously with said Yfirst source voltage and in a direction opposite to the direction of variation of said rst source voltage at any given instant, said voltage sources being connected one to each of said control electrodes to vary simultaneously and in opposite directions the effect of said control electrodes on the conductivity of said tube means, and means to adjust separately the voltage level required at each said control electrode to establish conductivity between said terminals through said tube means.

2. Apparatus as deflned in claim l wherein said tube means comprises two serially connected tubes each having anode and cathode electrodes and each having one of said control electrodes.

3. Apparatus as dened in claim 1 wherein said rst and said second voltage sources each comprise a staircase waveform voltage generator.

4. As defined in claim 1 wherein said adjusting means comprises first and second variable unidirectional voltage sources connected each to separate ones of said electrodes.

5. In an electronic commutator of the type comprising a plurality of switch units having one common terminal and wherein said switch units each comprises electron tube means having anode and cathode electrodes and two conductivity control electrodes for controlling the conductivity of said tube means as a function of the voltage on each said control electrode, the combination with said switch units of rst and second sources of voltage varying simultaneously and in opposite directions, each Said voltage source being connected to a separate one of said control electrodes in each said switch unit, and means to adjust separately the voltage level required at each said control electrode to establish conductivity in said tube means.

6. Apparatus as defined in claim 5 wherein said voltage sources each comprise a generator of staircase waveform Voltage.

'7. Apparatus as dened in claim 6 wherein one 10 of said generators comprises a kiirst capacitor, a iirst source of pulsating voltage, a iirst diode electron tube connected in series with said first capacitor and said pulsating voltage source, a second diode electron tube connected in series with said pulsating voltage source but in polarity opposite to that of said first diode, said second diode being connected in parallel with the series combination of said capacitor and said lrst diode, a gas-lled electron tube connected in parallel with said rst capacitor and having anode, cathode, and control grid electrodes, a second source of pulsating voltage of recurrence rate which is a submultiple of the recurrence rate of pulses from said rst pulsating voltage source, said second pulse source being connected to said gas tube grid, and a second capacitor connected between said gas tube grid and cathode.

8. Apparatus as dened in claim 6 wherein one of said generators comprises a rst capacitor having a pair of terminals, means to charge said capacitor in discrete steps of predetermined recurrence rate so that one of said capacitor terminals becomes increasingly negative relative to the other of said capacitor terminals, a gas-filled electron tube having an anode, a cathode, and a control grid, said gas tube cathode being connected to said one capacitor terminal and said gas tube anode being connected to said other capacitor terminal, a source connected to said gas tube grid of positive and negative voltage pulses of recurrence rate which is a submultiple of said predetermined rate, and a second capacitor connected between said gas tube grid and cathode for maintaining said gas tube grid negative with respect to said gas tube cathode during intervals between said positive pulses.

JOHN L. DEMPSEY. RICHARD W. SONNENFELDT.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,221,115 Shepard Nov. 12, 1940 2,250,708 Herz g July 29, 1941

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