US2454132A - Oscillating system - Google Patents

Description

Nov. 16, 1948.

P. F. BROWN OSCILLATING SYSTEM Filed Jan. 11, 1944 All...

FIG-2 INVENTOR. PAUL F BROWN [#forng,

Patented Nov. 16, 1948 7 OSCILLA'TING SYSTEM Paul F. Brown, Cambridge, Mass,, assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application January 11, 1944, Serial No. 517,895

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to "an oscillating system and more particularly to an oscillator for generating a predetermined number of cycles. In the use of certain equipment, it is frequently desirable to generate a predetermined number of cycles and thereafter suppress said oscillations until a new cycle of operation is initiated. Thus in certain types of radio apparatus the pulsing of a transmitter is utilized to initiate a train of waves. In the use of an ordinary oscillator, it is necessary to build up the initial cycles to the final amplitude. When the negative or zero resistance of the oscillating system becomes a positive resistance, then damping sets in to produce a train of waves of decreasing amplitude. However, in certain instances it is necessary to have oscillations begin practically instantaneously at substantially full amplitude, maintain the oscillations at substantially constant amplitude for a predetermined number of cycles and then suppress the oscillati-ons almost instantaneously.

In its more general aspect, the invention provides a shock-excited oscillating circuit having a minimum of damping during the time that oscillations are desired. When the number of waves generated has reached the required value, means are provided for introducing high damping. Unless the shock duration is small compared to the periodicity of the wave train,'there will be a masking effect on the initial cycles and this will effectively result in the train beginning several cycles after the desired time. A small shock duration requires a high shock intensity compared to the energy in a wave train of a naturally resonating system. But high shock intensity involves forced oscillations and transient efieots which will have a deleterious effect on the wave shape of the train. This invention provides means for impressing a comparatively high intensity shock on a resonant circuit and provides means for eliminating shock effects in the circuit so that substantially pure natural resonance efifects result.

Referring now to the drawings, Figure 1 shows one circuit wherein a combination of inductance and capacitance .are used in connection with a crystal. Figure 2 shows a modification wherein resistance and capacitance are used in combination with a crystal.

Referring now to Figure 1, a vacuum tube 10 is provided having a cathode H connected 12 Claims. (01. 250-36) through a bias resistor I2 to ground l3. An input terminal I5 is connected to cathode II. and together with ground l3 forms an input circuit. Control grid I6 of tube It! is connected by a grid resistor I! to ground I3. An accelerating grid I8 is connected through a resistor l9 to junction point and thence through a dropping resistor 2! to lead 22 connected to a source of 3+ potential. Accelerating grid [8 is effectively grounded as'far as alternating current is concerned by condenser 23.

Vacuum tube It) has its anode 25 connected through a load resistor 26 to junction point 20. Shunted across resistor 2B.are condenser 21 and winding 28, forming the primary of a transformer 29. Transformer 29 has a secondary, having ,a center connection '3I connected to ground through a variable resistor 32. The terminals of secondary 30 are connected to variable condenser 33 and crystal 34 respectively, these latter going to a junction point 35. Load resistor 26 preferably has a resistance high compared to the impedance of the resonant circuit 2'|2B for the operating frequency.

The entire secondary circuit forms a bridge system in which excessive shock energy tending to cause forced'oscillationsqis dissipated. The energy left for normal resonance operation remains in the circuit, which, after the initial shock, behaves as a normal resonant circuit. The crystal with its high Q results in the overall circuit Q being high enough so that a wave train of substantially constant amplitude is generated. The response of a crystal to a shock is highly destable in this instance. The crystal appears to follow the shock wave and then continues on with oscillations upon the disappearance of the shock. The inherent sharp frequency response of this type of circuit is desirable since transients are suppressed.

Junction point 35 is connected through a coupling condenser 36 to grid 31 of vacuum tube 38. Grid 31 is connected to ground through a grid resistor 39, while cathode 40 is directly connected to ground. Anode 4| of vacuum tube 38 is connected through a load resistor 42 and dropping resistor 43 to lead 22.

Anode 4| of tube 38 is connected by lead 45 and coupling condenser 46 to control grid 41 of vacuum tube amplifier 48. Control grid 41 has a grid resistor 49 connected between it and ground. Cathode 50 is also grounded. Vacuum tube 48 has its anode 5! connected through a load resistor 52 to the junction of resistors 42 and 43 and is lay-passed bygrounded. condenser 53.

From anode 5I a lead 55 is taken, terminating in an output terminal 56 which together with ground constitute an output circuit.

From anode M of vacuum tube 38 lead 45 is extended to a coupling condenser 51, and the other terminal of this coupling condenser is connected to control grid 58 of vacuum tube amplifier 59. Control grid 58 is connected to ground by means of grid resistor 69, while cathode 6| is automatically biased by bias resistor 62 connected between it and ground. Bypass condenser 63 is shunted across bias resistor 62. Accelerating grid 65 is by-pas-sed to ground by condenser 66 and is maintained at a suitable potential by connection to lead 22 through dropping resistor 61. Vacuum tube 59 has its anode 88 connected through a load resistor 19 to dropping resistor 61 and thence to lead 22. Load resistor I9 is shunted by condenser II and the primary I2 of transformer I3. Transformer I3 has its secondar-y M shunted-by a condenser I5. One terminal -16 of condenser L-is connected to ground by lead 'IJ, this lead also being connected to suppressor.'grid 18 of the vacuum tube. The other terminal 89 of condenser I5is connected to lead 81 :going back to control 'grid I6 of the first vacuum tube. -A feed-b-ack control resistor 82 is connected between lead BI and ground.

"The-operation of the system is as follows. A

positive gate input consisting of a rectangular voltage Wave of suitable amplitude and duration is impressed between .ground and input terminal and-serves to raise the potential of cathode I I. -"As'shown, vacuum tube I 9 isnormally conducting but is rendered non-conducting at the instant that' the 'catho'de potential is-raised by the control gate. lI-he'cutting off ofspace current in vacuum 'tube .19 results in arisein potential at'anode 25, and .thisgeneratesasurge across condenser 21 :anmprimaryL-ZS. The surge in primary 28induces a surge insecondary 39, and this results in shock exciting the crystal abridge circuit. Durin the idiirationof the positive gat.e,'vacuum tube I9 is non-conducting so that the crystal circuit may nscillate with a minimum of damping. This is :du'e to-the'high resistance of. The shock-itself is 'eliminated by the bridge circuit. The sine waves generatedlin :the crystal bridge circuit are impressed-through.coupling condenser 36 on'control grid 31=of'vacuumtube-'38, amplified and impressedon control grid '4I-of amplifier '48. The resulting potential variations at anode 5| of vacuum tube 48 are impressed upon output terminal'5li.

Atthe same time,wavesfrom-anode4I are also i impressed-upon control grid 58 of amplifier 59 and 'result in oscillations being generated in circuit-TI and -12. These amplified waves appear in secondary I4 and are fed backto control grid I6 of first vacuum tube I9. The amplitude o'ffeedback may-be controlled-by variable resister 82. As long'as vacuum tube-l9is non-conductingand thisoccurs during the'duration of the positive gatefee'd-'backto control grid I6 is ineffective. By-careful design of the'various circuits, a substantially uniform calibrated series of waves may be taken off at output terminal'56. At the end of *the positive gate when cathode II drops, vacuum'tube I9 becomes conducting. This provides substantial damping for thecrystal bridge circuit while the'feed back, which is 180out of phase, 'adds'further-damping. The 180 phase angle-of feed-"back is a composite-of the phase angles incircuit 21- 28, tube circuitsand other elements. Thus" phase shift due to'initial crystal response and other unavoidable circuit elements are compensated for. The net result is that when the positive ate is ended, oscillations in the crystal bridge circuit are quickly suppressed. Thus the inverse feed-back potential applied to the bridge circuit functions to accelerate the damping.

In the system shown in Figure 1, the resonant circuit formed by condenser '21 and primary 28 does not introduce much damping resistance into the secondary crystal circuit. This will be true as long as load resistor 26 is high and vacuum tube I9 is non-conducting. As soon as vacuum tube I9 becomes conducting, then a comparatively low resistance is effectively connected across primary 28 and results in a highly damped primary circuit. This high damping is coupled into the crystal circuit.

Referring now to Figure 2, a circuit is disclosed havin a different operation. In this circuit, a vacuum tube 99 has its cathode 9| connected through a high bias resistor 92 to a ground lead 93. Control grid 94 is connected through a grid resistor 95 toground. Vacuum tube 99 has its anode 96 connected through a load resistor 91 and a dropping resistor 98 to alead 99 connected to a suitable source-of B+ potential. The junction of resistors 91 and 98 is connected to .a groundediby-pass condensor I09.

From anode 96 of vacuum tube :99 connection is made to a condenser I M and thence by lead I112 to Ia variable tuning condenser I03. From cathode v9I a connection is taken through a condenser I94 and'lead I95 to crystal I196. A lead I91 connects crystal I96 and variable condenser 193. Between leads I92 andI95 a high resistance I98 is connected and cooperating with this resistance is a. grounded movablecontact I99. Condensers I..9I and I94 may have substantiall equal capacitances, while variable condenser I93 may preferably have a capacitance range somewhat larger than but of the order of the capacitance of crystal I96.

From lead I91 aconnection is made to control grid II9'of a vacuum tube amplifier III having a cathode I IZcOnnected-through a comparatively low bias resistor II3 to ground. Control grid 119 is connected to ground by a high grid re- .sistor ,I'I5. Anode I I6 is-connected toa'parallel resonant circuit consistingof condenser II! and inductance I. I.9,- andfthence-the anode circuit continues on throughadroppingresistor I29 tolead .99. A by-passcondenser I.2-I is connected between ground and the -low-end .of dropping resistor Ir29. Variations in potential of the .anode II6 are impressed upon a "coupling condenser I22 and continueon to control grid 123 of vacuum'tube amplifier .-I24. The gridcircuit is completed bya grounded .grid resistor I25, While'cathode1I26 is connected to ground by -.a .low bias resistor -I- 2'I. .Anode -I28 is .connected through a load resistor .li29 and dropping'resistor I39 to B+-lead 99. A .byepass condenserd 3 I .is connected between dropping resistor 13.9;and ground. An output terminal I32.is taken fromanode I28 andtogether with ground constitutes an outputcircuit.

From anode I28 connectionismade to a con plingcondenser I23 and thence to agrounded resistor I34. Cooperating with resistor I34 is a potentiometer tap.I35 connected to .control grid I36of an amplifier I31. Cathode I38 is connected .to'groun'd'by .a low bias resistor I39. Anacce'l- 'eratinggrid I49 is .connected through a suitable resistor I4I to a dropping resistorl'42 and thence to B+lea'd-9.9. Grounde'd'by-pass condensers I43 and I4-4'are provided on opposite sides of resistor MI. A suppressor grid I45 maybe grounded, while anode I46 is connected through a load resistor I41 to dropping resistor I42. From anode I45 a feedback lead I56 is takenthrough a coupling condenser II, lead i561 going back to con--- trol grid 94 of vacuum tube 96. Input terminal I52'is connected to cathode I36 and together with ground forms an input circuit.

Pentode I31 is biased by potentiometer tap I35 to be normally conducting. When a positive voltage gate is impressed on input terminal I52 .and ground, the potential of cathode I38 is raised with respect to control grid I36 and results in space current being cut off. This results in a sudden rise in potential at anode I46, which rise is communicated to control grid 94 of vacuum tube 90. This tube has a high bias resistor 92 in the cathode circuit which results in little space current under normal conditions. However, the high positive voltage gate impressed upon control grid 94 reducesthe resistance of vacuum tube 86 to a low value. The sudden decrease in tuberesistance and increase in space current cause the potential at anode 96 to drop sharply and thus shocks the bridge circuit IBI, Hi3, I06 and I04.

As long as thepositive gate is impressed upon control grid 94, vacuum tube 90 forms a comparatively low resistance connection between condensers I04 and i8! and completes the crystal circuit. Resistance I03 is so high in comparison to the series react-ances of condensers MI and I84 for the frequency used and resistance of tub-e 90 as to have substantially no effect. The shock impressed upon crystal lUB results in the generation of a series of sine waves having a comparatively low decrement. These sine waves are'impr-essed upon amplifier III and result in oscillations in parallel resonant circuit I H and I I8,.- This circuit is tuned to the same frequency as generated in the shockeXcited-crystal circuit. From vacuum tube I I! the sine waves are further amplified at vacuum tube I24 from which the output may be taken.

In order to provide feed-back for suppressing oscillation after the gate disappears, the output from vacuum tube I24 has a predetermined portion thereof applied to the control grid of pentode I3'I. As long as the pentode is cutoff by the gate there is no feed-back. At theend of the positive gate at input terminal I52, vpentode I31 reverts to its normal conducting condition. Then feed-back in opposite phase is applied to tube 90. At the same time that pentode I31 resumes its normal condition of conductivity, a negative pulse is impressed upon control grid 94 of vacuum tube 98 and causes that tube to be near cut-off. When this happens, there is a high resistance between condensers IllI and I04 by way of vacuum tube 90. Also feed-back eifectively increases this resistance. This leaves the crystal circuit highly clamped by resistance I 08.

Vacuum tube amplifier I II in the absence of signals on its control grid II 0 tends to be normally conducting by virtue of a comparatively low value of bias resistor II3. Hence the comparatively low resistance of vacuum tube III becomes shunted across the parallel oscillating circuit normally consisting of condenser I I! and I I8,

of the circuits will depend on such factors as the number of cycles for each gate cycle, permissible decrement, variation of circuit constants with tube life and other details. If a large number of cycles per shock excitation are required then high Q crystal circuits will be required, care will have to be exercised in coupling through transformers or condensers and the entire system will have to be designed to have a low decrement. In addition, when damping is introduced, it will have to be heavy to dissipate the oscillating energy. However, such details are well known to one'skilled in this art.

What is claimed is:

1. An oscillating system comprisin a resonant circuit having inductance and capacitance, said circuit having some resistance inherent therein, means for shock exciting said circuitso that oscillations are induced therein, an output circuit from which said oscillations may be taken for utilization, and means operative after a predetermined number of shock-excited oscillations for damping said oscillations, said oscillating circuit having negligible damping during its normal operation and having such high damping upon the operation of said last named means as to suppress oscillations completely almost instantly, an amplifier for said shock-excited oscillations, means for feeding said amplified oscillations back into said shock-excited circuit in opposite phase, and means for rendering said feed-back means inoperative during the time that oscillations are desired.

2. An oscillating system comprising a parallel resonant circuit having inductance and capaci- This results in a high dampin of oscillations in I tance, and including a crystal, said circuit normally having a low decrement, a vacuum tube amplifier, said amplifier having cathode, grid and anode circuits, means for connecting said resonant circuit in said anode circuit, said anode circuit forming a low resistance across said res.- onant circuit when said tube is conducting, means for norm-ally maintaining said Vacuum tube amplifier in a conducting condition, means for changing said vacuum tube from a conducting condition to a non-conducting condition rapidly enough to shock excite said resonant circuit, means for maintaining said vacuum tube in a non-conducting condition for the time during which oscillations are desired, and means for returning said tube to its normal conducting condition whereupon said oscillating circuit becomes highly damped to suppress oscillations.

3. The system of claim 2 wherein oscillations from said parallel resonant circuit are fed back to the grid of said amplifier, said feed-back being ineffective while said tube is non-conducting and being out of phase so that when said tube becomes conducting amplifier action in the tube aids in damping.

4. An oscillating system comprising a three element vacuum tube, three condensers and a crystal all connected in series to form a bridge with a crystal in one arm a condenser in a second arm the third and fourth arms having condensers with the cathode-anode tube space forming a switch connection between the third and fourth arms, a high damping resistance connected across the first and second arms, said bridge being resonant to a certain frequency when said tube is conducting, said damping resistance being high enough to have little effect on the bridge under normal conditions, means for biasing the tube grid so that said tube is normally near cut-off,

means for sharply changing said grid bias to render said tube highly conducting and shock excite said bridge circuit, means for maintaining said tube in said highly conducting condition while oscillations are desired and means for restoring said tube to its normal non-conducting condition to suppress oscillations.

5. The system of claim 4 wherein oscillations from said crystal circuit are fed back to the control grid of said amplifier, said feed-back being out of phase to suppress oscillations.

6. An oscillating system for generating spaced wave trains comprising a crystal bridge circuit, means for intermittently shock exciting said bridge circuit, means for dissipating the initial shock in said bridge while leaving energy therein for resonant oscillations, and means for amplifying said resonant oscillations without imposing a substantial load on said resonant circuit.

7. The system of claim 6 wherein the amplified oscillations are fed to said resonant circuit 180 out of phase and means for rendering said feedback inoperative only during the time that oscillations are desired.

8. An oscillating system comprising a symmetrical bridge, only one of the arms of said bridge comprising a circuit resonant to a predetermined frequency and having a relatively low decrement at said frequency, the other arms of said bridge, being so constructed that said bridge is substantially balanced at all frequencies except those near said predetermined frequency, means for applying intermittent, shock-exciting pulses to the input of said bridge, the repetition rate of said pulses being considerably lower than the resonant frequency of said circuit, whereby the initial shock of each pulse is substantially balanced out, while the resonant circuit continues to oscillate at its own frequency after said initial shock.

9. An oscillating system comprising a symmetrical bridge, only one of the arms of said bridge comprising a circuit resonant to a predetermined frequency and having a relatively low decrement at said frequency, the other arms of said bridge, being so constructed that said bridge is substantially balanced at all frequencies except those near said predetermined frequency, means for applying intermittent, shock-exciting pulses to the input of said bridge, the repetition rate of said pulses being considerably lower than, and independent of, the resonant frequency of said circuit, whereby the initial shock of each pulse is substantially balanced out while the circuit continues to oscillate at its own frequency after said initial shock, and means to damp said oscillations a predetermined interval after each shock.

10. A system as set forth in claim 9, wherein said last named means applies an inverse feedback potential to said circuit.

11. The method of generating spaced wave trains which comprises intermittently shock exciting a resonant circuit, alternately shunting said circuit with a low impedance to damp the waves therein, and accelerating said dampingby applying inverse feed-back to said circuit during the shunting periods.

12. The method of generating spaced wave trains comprising shock exciting at spaced intervals a resonant means to cause it to generate oscillations to be utilized, alternately damping said resonant means reversing the phase of said oscillations, and applying the oscillations of reversed phase to said resonant means during each damping period.

PAUL F. BROWN.

REFERENCES CITED The following references are of record in the

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