US2894213A - Waveform generators - Google Patents

Description

July 7, 1959 G-EWHITHAM 2,894,213

WAVEFORM GENERATORS Filed April 19, 1957 l l 1 E I 6' l /Z3 9 1 3 Q MVEFORM AT PLATE WAVEFOQM AT 7 76.5

% 9 s \J g H g bvvswrore LN Exp-9 GLENN 5. Wl-l/THAM WAVEFOQM AT By r ORA/EV States Patent WAVEFORM GENERATORS Glenn E. Whitham, Wayland, Mass., assignor to Raytheon Company, a corporation of Delaware f This invention relates to a precision waveform generator and, more particularly, to a blocking oscillator having its output connected to a shorted termination delay line wherein the reflected wave from said delay line accurately'controls the formation of the output pulses from said blocking oscillator.

In this invention there is disclosed a waveform generator of the blocking oscillator type having a delay means, such as a delay line, connected to the output of said blocking oscillator. The delay line has a propagation time substantially less than the natural period of the oscillator circuit and is artificially terminated, preferably by a shorted termination device, in order to cause reflected waves from said delay line to be fed back to the output of said blocking oscillator. Means in the form of a first and secondunidirectional means are connected, one in series with said delaying means and one in shunt across the input to said delaying means for preventing recurrent signals generated by said reflected wave from said delay line from being propagated back into said delay line. g The application. of blocking oscillators for controlling pulses of accuratelydetermined duration has in the past required precision production control of the individual circuit parameters. These parameters include the individual electron tubes used together with the transformer usedto couple the energy from the output circuit back into theinput circuit together with the individual load resistors and cathode, resistors. In practice, therefore, the time duration of the output pulsefrom the output circuit in conventionallybuilt blocking oscillators has been determined by such constants as the blocking oscillator transformer which includes the permeability of the core material, distributed winding. capacitances, and self, mutual and leakage inductances of the windings, all of which are diflicult, if not impossible, to control accuratelyin production. In addition to these aforementio ned transformer characteristics: that affect the, pulse duration, there must. also be considered such things as the cathode resistances, thegrid capacitor, the grid resistancesjthe shunt-resistances across any of the transformer' windings, the transcondu'ctances of the .electron tube being used and the plate and grid supply voltages. With such a wide variety of parameters affecting the pulse duration, it has been very difficult on a production basis to build reproducible circuitry having the desired pulse width characteristics. A circuit built according to the principles of this invention eliminates all these aforementioned difficulties, and further, it is possible now to build a blocking oscillator having desired pulse width characteristics that are independent of all these aforementioned diliiculties which previously 'afiected all circuits of this type. Further objects and advantages of this invention will be made more apparent as the description progresses, reference now being made to the accompanying drawing wherein:

Fig. 1 is a blocking oscillator having its output connected to a terminated delay line;

Fig. 2 illustrates a waveform produced at the plate circuit of the oscillator illustrated in Fig. 1;

Fig. 3 illustrates a blocking oscillator built in accordance with the teachings of this invention for producing controlled width output pulses;

Fig. 4 illustrates the waveform of voltage at the plate circuit illustrated in Fig. 3; and

Fig. 5 illustrates the desired output Waveform from the oscillator illustrated in Fig. 3.

Referring now to Fig. 1, there is shown a blocking oscillator consisting of an electron tube 11 having a plate 12, grid 13 and cathode 14. Connected to plate 12 at one end is a first winding 15 of a conventional blocking oscillator transformer 16. Connected across plate winding 15 is a delay line 17 that is terminated by shorting device 18. A second coil winding 19 of transformer 16 completes the feedback circuit by having one end of said coil connected to grid 13 and the other end connected to grid resistor 20 and then to a negative source of grid bias voltagedesignated as E;,. A grid capacitor 21 is connected intermediate grid resistor 20 and first winding19 to ground. A cathode resistor 22 is connected at one end to cathode 14 and the other end to ground. The operating supply voltage for blocking oscillator 10 is connected in the normal manner with the operating plate supply designated as +E being connected at one end to plate winding 15 of blocking oscilla tor transformer 16. The other end of the plate supply voltage (not illustrated) is designated as ground. All curves illustrated are with respect to ground. By assuming a propagation time of T for the delay line 17, it was discovered that if the blocking oscillator could be triggered Off at some precise interval after it is triggered On and at a time appreciably less than its natural period, then the pulse duration would depend only on theoccur: rence of the trigger which turns it Off.

'I his wave is illustrated in Fig. 2 where there is shown the negative plate pulse 23 that is reflected back from termination device 18 with reversed polarty after a perriod 21- with the resultant, positive pulse at the plate that causes the blocking oscillator to betriggered Off. However, it was observed that the positive overshoot 24 61- lowingthe turnoff pulse isreflected back as a negative pulse after the period of 2T'thereby causing the blocking oscillator to be triggered On again. This cycle will'repeat itself and producethe waves illustrated inFigL 2. The dotted curve 24:: shows a normal pulse having'a;

widthT that is, determined by the individual component,

parameters. The repetition rate of this. oscillator has, been found to be erratic due to the positive overshoot 24 being. propagated back into the delay line 17; This se quence. of operations produces recombinedsignals having the tendency to turn the oscillator On and Off at diiferent time intervals which results therefore in spurious oscilla? tions. i r r Referring now toFig. 3, thereis shown a blocking oscillator similar to that illustrated in Fig. 1 for producing a single square wave output of specified pulse width irrespective of the independent parameters of the individual components and free of spurious oscillations.

These desired characteristics are achieved by inserting a first unidirectional means, such as diode 25, in series with plate 12 to delay line 17 thereby allowing the negative plate pulse to be coupled into the delay line and the positive turnoff pulse to be coupled back into the plate circuit, but preventing negative retriggering impulses from being coupled from the delay line to the plate circuit. This wave voltage will be more readily seen when it is remembered that the propagation time of the delay means is substantially less than the natural period of the oscillator. Therefore, the reflected positive wave caused by the termination device of said delay line meets the for- Ward going negative pulse thereby causing the negative pulse "tobe "forcedup to the normal plate supply voltage which is E The reflected Epositive pulse has the efiect, therefore, of cutting oil the negative pulse which thereby starts the cutting-.otfcycle of the oscillator since the ,plate voltage is now foreed backto the "E value due -to the reflected Positive wave. A second unidirectional device in theform of diode Z6 is connected 'in shunt across-the input.to,delay line -17 in order 'to eliminate spurious pulses caused by the capacitive coupling illustrated as 27 across the first .diode.2 5. This waveform, which is the result of capacitance '27, is the positive overshoot illustrated in Fig. 4. This second diode26 was discovered to be desirable since it was found that a portion of the positive overshoot. at the end of the-blocking oscillator pulse was coupled into the delay line 17 by means of the diode capacitance represented as 27. "This positive pulse was then "found to be reflected back as a negative pulse which diode 25 could pass back to the plate circuit of tube '11. Diode 26 eliminates the possibility of having spurious triggering due to the capacitivecou'pling of diode 25, and also said diode '26damps outany positive overshoot of plate voltage exceeding 'the plate supply voltage E The resultant waveform is that shown in Fig. 5.

Indesigning circuits of this type, it was found that the delay line impedance should match the impedance of the transformer windings since if the delay line impedance is Ltoo low it may load the transformer and cause the naturalperiod of the blocking oscillator thereby to 'be too short, whereas, if the impedance is too high, linsuffi'cient energy will be coupled back into the transformer to terminate the blocking oscillator pulse reliably. Flexibility in the pulse width can be obtained by the use of a tapped delay line, and further, the natural period or the blocking oscillator should be approximately 50 to 100 percent greater than the control period to allow for the variations in the circuit parameters. The disclosed idea of diode coupled delay lines for period stabilization is not limited to blocking oscillators but could also be used with other waveform generators such as multivibrators and phantastrons.

This completes the description of the embodiments of this invention. However, many modifications may be made such as the use of other circuit combinations to eifect'the same result. These may include such items as bridging the delay line diode combination across the grid winding .or across a tertiary winding of the transformer.

What is claimed is:

1. In an oscillator for generating pulses of shorter duration than its natural period of oscillation, said .oscill'ator being of the type comprisedby an electron tube,.a transformer having windings coupling the grid and anode circuits associated with said tube, a delay line, and means for coupling a signal from said delay line to said transformer for terminating a cycle ofpulse generation of said oscillator, the improvement comprising a termination for said delay line to cause reflection and inversion of incident wave energy, and a unidirectionally conductive device connected in scrim with said delay line and arranged to inhibit energy reflected from said delay line from initiating a pulse generating cycle of said oscillator.

2. In an oscillator for generating pulses of shorter duration than its natural period of oscillation, said oscillator being of the type comprised by an electron tube, a transformer having windings coupling the grid and anode circuits associated with said tube, a delay line, and

v means for coupling a signal from said delay line to said transformer for terminating a cycle of pulse generation of said oscillator, the improvement comprising means tor terminating said delay line to .causereflection and 'inversion of incident Wave energy, a first unidirectionally conductive device connectedin series with said delay line,

, the grid and anode circuits associated with said tube, and

a delay line having one end connected to the windingin one of said circuits, the improvement comprising means for terminating said delay line to cause reflection and-inversion of incident wave energy, a first unidirectionally conductivejdevice serially connected in said one circuit to said delay line, and a second unidirectionally conduc: tive device shunting said delay .line, said unidirectonallyconductive devices being arranged to inhibit energyreflected from said delay line from triggering said oscillator to initiate apulse generating cycle.

4. A blocking oscillator comprising an electron discharge tube, a transformer having-windings couplingthe grid and anode circuits associated with said tube, a delay line for controlling the duration of generated pulses by. providing a signal which terminates each pulse generating cycle of said blocking oscillator, said .delay line being connected in the anode circuit of said oscillator, saidv energy reflected from said delay line from initiating a pulse generating cycle of said. oscillator.

References Cited in the file of this patent UNITED STATES PATENTS Robinson e't a1. Dec. 26, 1950 MacNicnol et a1 Dec. 14, 1954 OTHER REFERENCES Waveforms by Chance et al., vol. '19., 'RadiationlLaba oratory Series, by McGraw Hill 1949., pages 245-253.

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