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.Jan. 6, 1942.

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W. R. HEWLETTVARIABLE FREQUENCY OSCILLATION GENERATORFiled July 11, 1939

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2,268,872

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INVENTORW/LLIAM R. HEWLETTB ~ ~ ATTORNEY

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Patented Jan. 6, 1942 2,268,872

UNITED STATES PATENT OFFICEz.za.nzVARI&Bf.E :nEQlJEllfCE'OSCIU.ATIONGENEKA'.rOK .

William :LBewlel&, 'Pale .au.. Calif., . . . . . . . . . toHewleU-f'ulii:IIN c . m ~ . Pale .Aile, calif., ae o - ~ c : 'hrcof'WIIIIam.:LIIewleUa D C l D a Y I I l ~ App1lcatleaJ'. t , ' 113t, Sc:dal No. 213,111

I CW.S.. (CI. zs t -31 )My invention relates to variable :freque!lcy os - osciDaUons in the circuit. By properly propor-cQiation generators an d more partlcularb' to os- tioDIDg th e value of th e resistance capacity net-clllation generators wherein th e freq1leDCJ" Is de- works Rt, Ct, Ra, C2. the desired frequency maytermined by means of resistance capacity n e ~ be obtained If Rt, and Ct, respectively, equalworks.. 6 Ra. and Cz. the ratio of the voltage Eb at point bOscillation generators capable of tuning mer a to th e voltage :& at point a varies with frequencywide range of frequencies ar e useful fo r :many In a manner similar to a resonance curve. In

pUI"poSeS, for eXample, as test oscillation genera- Pig, 2 this curve is shown in solid lines withtors in place of beat frequency generators. In voltage ratkls of :& and Eb plotted as ordinatesgeneral such oscillation generators have been 10 against the relative frequency. At th e maximumprovided with a variably tuned resonant circuit of this curve, th e frequencyto maintain th e oscillations at a frequency to 1which th e tuned circuit is adjusted. I ...~ - ~ ; 1~ ~ ~ ; ; : According to my invention, I :Provide an oscil- L.r-y1lhR,ChCtlation generator with a variable resistance. ca - 16 and the volmges at points a and b have th e samepacity network forming a positive feedba.Ck 11Bth phase . Accordingly, oscillations tend to occurfor the purpose of determining the frequency of at the frequency I for this circuit. oscillation. :rn. Pig. 2 the broken line curve indicates th eAccording to another feature of my inven- :pllase shifts as ordinates with th e change in rela-tion an oscillation generator with IeSistance a . lve frequenCY. I t can be seen that at zero phasecapacity frequency determining arrangements shift , th e maximum of th e Ea, Eb ratio occurs. Ifan d with a negative feedback circuit for reducing the various c o n d ~ r s and resistances are notdistortion caused in th e amplifier c i r c u i ~ is pro- made equal as they were fo r plotting tbls curve,vided the :Duaimum would occur at some other pointAccording to a further feature of my invention 25 and a symmetrical curve would not be obtained.I provide an automatic amplitude control ar- However, the principles of my invention apply torangement in a negative feedback path of an '!5-:. the circuit regardless of equality of these ele-cillation generator to maintain th e amplitude of ments. in practical construction, however, i t isth e generated oscillations constant despi te genf'.rally desirable toniake th e separate elementschanges in the supply voltages for the tube:>. 30 equaLOther advantages and features of my ln'vention As shown in Pig. 1, frequency shift in th e os-will be apparent from th e particular description cillation generator is accomplished by adjustingthereof made in connection with th e accompany- ~ b l e capacitieS Ct an d C2. I t is clear, bow-ing drawing, In which ever, that since the resistances as well as th e

Fig. 1 shows a diagrammatic circuit of the pre- 35 capacities serve to control th e frequency of theferred form of an oscillation generator in accord- oscillator, either th e resistances alone or bothance with my ~ v e n t i o n , and th e resistances and capacities may be adjustablePig. 2 shows curves for th e purpose of illus- fo r varying the tuning,trating th e operation of the circuit. Por the type of oscillators illustrated in Fig. 1,In Fig. 1 is shown an oscillation generator com- 40 i t is necessary In order to obtain satisfactoryprising two amplifying tubes II and I I , coupled operation that the ampliflers comprising tubes IDtogether by resistance capacity means in a known an d I I have phase shift independent of changesmanner. Th e voltage snpply sources and the in snpply, etc., an d furthermore, there should becathode heating supply are omitted fo r simplify- provided some means fo r controlling th e ampli-in g th e circuit. The anode of tube II is coupled 45 tude of the osclllatlons so that they do not exceedto th e input grid of tube II an d the anode of tube the range over which tubes II and II will op-11 is then connected to a circuit which provides . erate as class A ampliflers. A constant amplifiera feedback path through a couPling condenser 12 phase shift is necessary to insUre a constant fre-and series connected resistance Rt and variable quency, because the phase angle of th e transfercapacity Ct, and a second circuit comprising R2 50 impedance of the . esistance capacity networkand variable condenser C2, to th e input of tube from point a to point b varies only slowly withII . The circuit including R1, Ct, R2, C2, provides _ frequency. Accordingly, a small cha,nge In ampli-a regenerative or positive conpling between the :fler phase shift such as might be produced by aoutput and input circuim of th e amplifiers II and variation in voltage supply requires a compara-11, so that feedback occurs sumcrent to maintain 55 tively large change In the frequency of opera-

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2 2,268,872tion to produce a compensating phase shift inthe resistance capacity coupling system.In the circuit of Fig. 1, I provide a negativefeedback arrangement for stabilizing the amplifier. This feedback is made from the output ofamplifier I I over coupling condenser 12 and theresistance RJ , R4 to the cathode of tube 10. Thepotential drop across the resistance produces anegative feedback to the input cf amplifier 10.This negative feedback operates in a mannerknown per se to reduce the amplification of thesystem and at the same time to stabilize theamplifier gain in magnitude and phase with respect to variations occurring within the amplifier, such as tube changes, temperature variations and changes caused by fluctua.tion in thevoltage supplies to the electrodes of the amplifier system.Amplitude control to prevent the oscillationsfrom building up to such a large value that distortion occurs. is obtained according to my in vention by non-linear action in the amplifier circuit. In order to produce this non-linear variation, I provide for reSistance R3 a small incandescent lamp, or similar device in which the resistance increases rapidly with increased currentflow, the lamp being heated by th e plate current of the tubes 1"0 and II or by an auxiliarymeans, so such a temperature that its resistancewill vary rapidly with a small change in current.Thus, when th e oscillation amplitudes tend toincrease, the temperature of the lamp R3 in creases with a resulting increase in resistancethereby causing a greater negative feedback, thusreducing the amplification. Similarly, as the oscillations decrease in amplitude, the currentthrough th e lamp is reduced permitting the lampto cool with an accompanying decrease in resistance and reduction of th e negative feedback, thusincreasing the amplitude of the generated oscillations. As a result the system operates at substantially a constant amplitude which is preselected to be below the value at which grid currentflows. As a result no distortion of .the wave formtakes place.While I have described the amplitude controlsystem in connection with my particular resistance capacity tuned oscillation generator, it isclear that this feature of my invention is similarly applicable to various types of oscillationgenerators, wherein frequency stabilization is obtained by means of negative feedback. Since thelamp itself responds relatively slowly to currentchanges it tends to average out the voltage peaksand does no t cu t off the peaks of the generatedwaves, maintaining th e amplitude constant without producing distortions in wave form.In the construction of oscillation generators inaccordance with my invention, it is generallyconvenient to make the resistance Rt equal toR2, and capacity Ct equal to C2. Under theseconditions the frequency of oscillation is

1! t = 2 1 C ' R ~ , C t I t will be noted that this frequency is inverselyproportional to the capacity instead of being in versely proportional to the square root of th ecapacity, as in the case of the tuned circuit arrangement. AccordinglY. th e frequency may bevaried over a much wider range, utilizing thesame type of condensers than in th e case of oscillation generators utilizing a tuned circuit feedback. A frequency range of 10 to 1 ca n be easilyobtained on a single dial in accordance with this

system. If other ranges are desired, decimalmultiplying factors ca n be obtained by changingresistances Rt , R2, in decimal values.An experimental arrangement employing a5 four gang broadcast condenser with the sectionsconnected in parallel in pairs, so that one paircorresponds to Ct , and the other pair correspondsto C2, has been found capable of covering thefrequency range of 20-20,000 cycles in three sub-10 divisions, namely, 20-200, 200-2000 and 2000-20,000 cycles, by employing three sets of resistances. The output voltage on this circuit wasfound to be constant within approximately 10%over the entire frequency range, while it was15 found that the output waves had a distortion inthe order of .25% only. The frequency stabilitywas such that only a negligible frequency shiftin the order of less than .1% with quite a largevariation in voltage supply occurred. It can,20 therefore, be seen that an oscillation generatorin accordance with my invention is capable ofcovering an extremely wide frequency range andmaintaining substantially pure sine wave outputthroughout the entire range.25 While I have illustrated my invention merelyby showing a preferred embodiment thereof, itis clear that many variations thereof may bemade by those skilled in the art within the teachings of this disclosure. The particular connec-30 tions of the amplifier circuit may be varied considerably and as above stated, the capacitiesand/or resistances may be varied in. order tochange the tuning. Although I have shown anordinary filament lamp as the volume control re-35 sistor, any known arrangement may be utilizedfor this purpose, for example, these lamps usedfor such purpose are generally termed ballastlamps.What I consider to be my invention Is defined40 in the accompanying claims.What I claim is:1. In an osc1Ilation generator for producingsustained electrical oscillations of a desired substantially constant frequency and amplitude, vac-

4; ) uum tube means having control, anode and cathode elements, and a frequency determining electrical network connected to th e vacuum tubemeans for generation of electrical oscillations,said network including a substantially non-in-50 ductive positive feed back path and also a degenerative circuit including a path for osc1Ilatorycurrent of said generator, oscillatory current flowthrough the last named path serving to reducethe amplitude of oscillations generated, said path55 in said degenerative circuit having an impedancevariable in response to variations in th e amplitude of the oscillatory current. of the generatorto tend to maintain the amplitude of oscillationsconstant.

GO 2. In an osc1llation generator for producingsustained electrical oscillations of a .desired substantially constant frequency and amplitude,vacuum tube means having control, anode andcathode elements, and a frequency determiningC;J electrical network connected to the vacuum tubemeans for generation of electrical osc1llatlons,said network including a substantially non-inductive positive feed back path and also a negative feed back path for oscillatory currents, os-iO cillatory current flow through the last namedpath serving to reduce the amplitude of oscillations generated, said negative feed back path including a resistance element which increases itsresistance with .an increase in osclllatory currenti; ; flow through the same, thereby serving to auto-

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2,268,872 3matically stabilize the amplitude of oscillationsgenerated and to maintain the amplitude of os-cillations constant. 3. In an oscillation generator for producingsustained electrical oscillations of a desired sub- 5stantially constant frequency and amplitude,vacuum tube means having control, anode andcathode elements, an d an electrical network connected to the vacuum tube means for generationof electrical oscillations, said network having a 10non-inductive frequency determining positivefeed back path including resistive an d capacitative elements variable for varying th e frequencyof operations, an d also having a separate noninductive negative feed mick path having an im- 15pedance variable in response to variations in theoutput current of th e generator, thereby servingto automatically stabilize the amplitude o f oscillations generated an d to tend to maintain theamplitude of oscillations constant. 20

4. In an oscillation generator for producingsustained electrical oscillations of a desired substantially constant frequency an d amplitude,vacuum tube means having control, anode andcathode elements, and an electrical network con- 25nected to the vacuum tube means for generationof electlical oscillations, said -network having anon-inductive frequency determining pasitivefeed back path including resistive an d capacitative elements variable for varying the frequency 30of operations, an d also having a separate noninductive negative feed back path including aresistive element connected to pass anode currentthrough the same an d said element being adapt-ed to increase it s resistance with an increase in 35oscillatory current :flow through the same, there-by serving to stabilize the amplitude of oscillations generated and to maintain the frequency ofthe. same constant.

5. In an oscillation generator for producing 40sustained electrical oscillations of a desired sub-

stantially constant frequency and amplitude, atleast two vacuum tubes each having control,. anode an d cathode elements, and a frequency determining electrical network connecting the tubesin cascade, said network including adjustablesubstantially non-inductive reactive means forming a path for positive feed back of oscillatoryenergy from said second tube to said first tubewhereby oscillations are generated at a selectedfrequency, and means forming a path for negative feed back of oscillatory energy from saidsecond tube to said first tube whereby energytransferred by said path tends to reduce the amplitude of oscillations generated, said last meansincluding an impedance connected to pass anodecurrent for the first tube and adapted to increaseit s resistance with an increase in :flow of oscillatory current through the same.6. In an oscillation generator for producingsustained electrical oscillations of a desired substantially constant frequency and amplitude, atleast two vacuum tubes each having control,anode and cathode elements, a non-inductivefrequency determining electrical network connecting the tubes in cascade, said network in cluding means forming a path for positive feedback of oscillatory energy from said second tubeto said first tube whereby oscillations are generated. and means forming a path for negative feedback of oscillatory energy from said second tubeto said first tube whereby energy transferred bysaid path tends to reduce th e amplitude of oscillations generated, said la.St means including aresistance element connected to pass oscillatoryanode current for the first tube and oscillatoryanode current from said second tube and whichincreases its resistance with an increase in oscillatory current :flow through the same therebyserving to stabilize th e amplitude of oscillationsgenerated. 'WILLIAM R. HEWLETI'.

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