US2451858A - Controlled frequency oscillator - Google Patents

Description

Oct. 19, 1948. R. MORK 2,451,858

CONTROLLED FREQUENCY OSCILLATOR Filed Jan. 26, 1945 Figl.

w R v Fig. 2

5294? 53/0 525 fa/19f295 Fig. 3. [37 E [if 27 \E37 \EZS Inventor": Raymond P. Mork, by j a/my His AttoPney.

function of a small resistance change. oscillator should be compensated for resistance ingipart of the invention.

Patented Oct. 19, 1948 Raymond P. lVIork, Schenectady, Y., assignor to General Electric Company, a corporation of New York ApplicationvJanuary 26, 1945, Serial No. 574,658

. 1 N My invention relates to controlled frequency oscillators, and more particularly to apparatus for modulating the frequency of oscillations in accordance with the intensity of a desired signal or a quantity being measured or recorded. ,Myinvention is particularly applicable to measuring and recording apparatus where it is desired to control the frequency of electric oscillations in accordance with a condition, such as temperature, humidity, pressure, liquid level, mechanical stress, or the like. thus controlled may be recorded, as by a magnetic wire recorder or the like. 1 In such apparatus, it is convenient to provide an oscillator having a frequency which is a function of the impedance of a'vari'able measuring impedance.

More particularly, it is desirable to provide an oscillator, the output ,frequencyof which is 2. Such an changes in the measuring resistor resulting from changes in ambient temperature, and should provide a large frequency swing in response to small percentage changes in the resistance of the measuring resistor.

Accordingly, it is an object of my invention to provide a new and improved ambienttemperature compensated oscillator the output frequency of. which is variable over a relatively wide range in response to a small percentage change in'the value of one of the circuit impedance elements.

It is a still further object of my invention to provide a new and improved electric oscillation generator having a frequency determined by a phase displacement of electric oscillations.

It is a further object of my invention to provide a new and improved oscillation generator having a natural frequency determinedbythe phase displacement between oscillations at the input and output terminals of aphase shifting circuit interposed in the feedback path of the oscillator.

It is still another object of my invention to "provide anew and improved arrangement for shifting the phase of electric oscillations.

My invention itself will be more fully understood and its objects and advantages further appreciated by referring now to the following detailed specification taken in conjunction with the accompanying drawing in which Fig. 1 is a schematic circuit diagram of a frequency controlled oscillator embodying my invention; and

Figs. 2 and 3 arevectordiagrams illustrating the operating of certain phase. shi ting circuits form'- 7 Referring now to the drawing and particu- The oscillations larly to Fig. 1, I have shown an oscillation generator comprising an amplifier! and a pair of phase shifting networks or circuits 2 and 3. The amplifier l comprises a pair of electric discharge devices-land 5 connected in cascade relation between an input. circuit coupled to the circuit 3 and an output circuit including. a transformer 1. The discharge device 4 comprises a cathode 8, a control electrode 9, screen and suppressor grids IQ and, respectively, "and an anode [2. The screen grid H3 is connected through a resistor I3 to a suitable source of positive potential indicated upon the drawing asl3+, and the suppressor grid H is connected directly to the cathode 8. The anode I2 is connected to the positive potential source 13+. through a load resistor. M. The negative end of the load resistor is connected through a coupling condenser l5 to the input electrode of the amplifying discharge device 5. Suitable grid bias for the discharge device 4 is provided by a cathode bias resistor l6 connected betweenthe-cathode 8 and ground in parallel with a by-pass :.conde nser IS. The amplifier 5 includes a cathode IT, a control electrode l8 connected. to the coupling capacitor [5 and a grid resistor I5, screen and suppressor grids [-9 and 20, respectively, and an anode 2!. The screen grid I9 isconnected through a resistor 22 to thepositive. potential source 3+, and the suppressor grid 20 is connected directly to the oathode IT. The anode 2| is connected through the primary winding 23 of the transformer 1 to the positive potential source B+.

Suitable grid bias for the discharge device 5 is supplied by a cathode resistor 24 connected between the cathode ll and ground in parallel with a by-passcondens'er 24.

The secondary winding 25 of the transformer l is connected to supply energy from the output circuit of the amplifier lback to the amplifier input circuit through the phase shifting circuits 2 and 3 in such phase and with such gain as to supportoscillations. In practice, I have found thatit is preferable in a circuit constructed in accordance with my invention to provide an overall gain through the circuits I, 2 and 3, not'appreciably greater than unity. It may be here noted that the amplifier I is constructed in such a manner that the phase shift between oscillations at the input and output circuits of the amplifier is substantially constant over the frequency range used. Ordinarily, and preferably, "this phase shift is either substantially zero or electrical degrees; depending upon the manner in minals 21 of the bridge being between the re'-" ' sistors 28 and 29 and between-the resistors w and 3!. The resistor 3| is variable in accordance with the value of an impressedsignalor a measured condition, and determines "by its re-- sistance the frequency of oscillation of the oscillator. By way of example, itmay be assumed that the variable resistor 31 is the variable resistance element of a resistance strain gage 32 *connected continuously to measure the stress in a structural member. It 'will beunderstood-that whe n the're sistance bridge is :balancedth'e' ratio of the re sistances of the resistors- 29 and 31- is equal-t the ratio of the resistances of the resisto'rs Z8 and and preferably equal -to unity. Under such balancedconditibngnovoltage component arising from the bridge circuitalone appears at the output terminals 21. On 'the other hand; if' the bridge-is unbalanced bychange in 'the' resistance of the gage resist-or 31; a voltagecomponen't arisingfrom"bridge 'unbalano'e-andin phase with'the voltage applied to i the input terminals 50f the resistance bridge appear's at-the"output terminals 212 In'order to s-hift 'the' 'ph'ase of oscillations at the bridge output terminals 21 withrespect to oscillations supplied-to the input terminals 26, I connect across-theresistor w a capacitor- 33 of small capacitance 'thereby toprovide" an impedance largerelative to the impedance "of the gage resistance '3 l.- 1 By thu's connecting the ca'- pacito'r 33 'and the gage-resistor 3 I -in series circuitrela tio'n' between the bridge input terminals 26, 'there issu-perposed 'upon the" resistor 3l a differentiated voltage-component insubstantially quad-rature relation with" the oscillations "supplid'ito the input terminals 26. This quadrature voltage 1 component combines withthe in-phase output voltage component du'e to 'bridgeunbalance alone' to provide at-the output terminals :21

oscillationsshift ed -i'n phase relative' to oscillation's suppl'ied-to the input-terminals 26. i

For a more complete understanding-roethe operation :of :the' phase shifting circuit 1 2 reference may now-be'h'ad to Fig. 2 wherein is illustrated Vectoriallythe'phase relation between various current and voltage componentswexis't'ing in--the circuit 'AtFigJ-2, the vector Ezsrepresentsthe voltage-applied to i the input-terminals, 2 6: of the circuit-.2. Since the impedance of-the condenser .33,-.-is largescompar-ed tothe impedance of-the gage resistor :3 l the current through the series circuit comprising-the condenser 33 and resistor 3 I may-be represented =by:-a vector-Isa I leading the 4 These voltage vectors are in phase with the an plied voltage E25.

It will now be evident that the output voltage appearing at the bridge terminals 21 is the sum of the voltage vectors E2913, E3113 and E310. Since the voltage vectors E2913 and E313 are in opposite phase relation with respect to the output terminals--2T{ theif differeri'ce 'repre'serits'='the voltage componentat' the terminals 21 due to bridge unbalance alone. It will be understood that this differential voltage Ems-E2913 may vary between zero and a maximum Value determined by the maximum percentage change of resistance of the resistor 3|; and may be either positive or negative depending upon the direction of the resistance change. Thus, the net output voltage E27 being the ve cto'rial sum of E31BE29B and E310, at Fig. 2 is variable in phase from a position in phasewith the voltage E310 when the bridge is balanced to positions of maximum phase displacement 'in 'either direction. The output voltage 'Ez'z from circuit" 2- l'eadsthe input'vol'tage E25 'by'an angle 01: At maximum-bridge"unbalance; the displacement angle '01 -is a minimum, "and I'have found-that, in practice, it 'is desirable to limit the minimu'm phase displacement- 01 to --appro'-ximately'the angle shovi'n at Fig z wherein the magnitudeof the' voltag'e vector-"Ens is approximately twice the magnitude of the differential voltagevector-Ens -Ezsedue to maximum bridge unbalance.

From the-foregoing; it will-be understood'that the'phase angle 01 is variable over-a wide range in response tosm'all percentage resistance changes of-the resistor 3H This property arises'from 'the fact that, due to-thebri'd'ge circuit arrangement, only a small percentage changeof'th'e resistor 3 l is necessary to producean appreciable differential voltage due to bridge unbalance, while the smallpercentage resistance change haslittle effect upon themagnitudeof the voltage componentlilaic. Furthermore, the phase shifting aircuit '2 is compensated for ambien't temperature changes'by' reason'of the "voltage balancing effect of the bridge circuit forming'a part thereof. Preferably ambient temperaturecom'pensa- 'ti-on-is effected byfutilizing pairs of resistors 28 and 3 Oand 29 and 3! of similar characteristics.

Theo'utputvoltage Ezr'fro'nithe phase shifting networkfi is impressed uponthe phase shifting network 3 which comprises a pair of resistancecap'a'city phase shifting circuits connected incascade"relatio'n. Specifically, the circuit '3 comprisesares'istor 34"an'd a c'apacitor'35 connected in series 'cir'cuitrelation across the Output'termin'als 21011 the circuit" 2 anda resistor 3B"and capacitoriifl "connected. in series ci'r'cui't' relatidn acros the capacitor 35. An outputvolt'agafrom the circuit" 3' taken" off across the c pacitor 3Tis supplied t'o 'thecontrdlelectrode Sof 'th amplirying'cis'tnargeuevice 4.

The circuit. 3 shifts the phas'fof oscillation suppliedtofit infa sense'opposite t'othat ii'l'wlii'ch the circuit? hifts'thd'pha'e'of"appliedoscilla tidns, and the amount (if "phase Shift produced by' the circuitii3 is 'depefidentiiipon th'e'frequiicy of oscillations" suppliedTther'et'o'. Thus, whenfthe circuit. constants of the"circ'iiit 3 arepro'p'eny ropor'tioned' and a predetermined freciuencyfis supplied t'o the circuit}, thebutpl'itivoltage from the circuit 3 will. be"in."pha'se"witli"the]inpiit voltage to'th'e' circuit 2.

The manner 'in which phase shifting" takes .place in the circuit 3=Will lifem6f 'vid'ent'by in spectidn of Fig": 3L At Fig. '3, I liave'show'rfthe Fig,2'.; In the circuit 3, -the capacitors 35 and 31 aresmall,'so that their impedan'ces are of the same-order of magnitude as the impedances of the resistors 34 and 36-, respectively. "Accordingly, therefore, the current'through the capacitor 35 maybe represented by a vector I35 leading the vector E27 by an angle i of less than 90 electrical degrees; Thevoltage drop across the capacitor 35produced bythe current I35 is illustrated by a vector-Eat lagging thecurrent vector I35 by 90 electrical degrees: The voltage E35 is that supplied to thesecondstage ofthe phase shifting circuit 3 and, due tothis voltage, a"cur rent In passesthrough the condenser '31 The current Isrleads the voltage E35 by an angle (#2 equal approximately to 1. The voltage drop across-the condenser-31 lags the currentIav by 90 electrical -degrees and is shown at Fig. 2 as 'a vector Ea'z. --'-It will now be observed that the output voltage E37 lags the input voltage E27 byan angle a: and is in phase with thevoltage E25 supplied to the input of the phase shifting circuit 2' Accordingly, therefore, at the frequency assumed, the phase angle 01 of Fig. 2 is equalto the phase angle az of Fig. 3. It will be evident that the angle 02 is determined by the angles 4n and 452. It is well'understood by those skilled in the; artthat the phase angles 1 and as are deter'mined by the frequency of the oscillations supplied to" the phase shifting circuit 3. Ac-' cordingly, therefore,*the angle 02 is determined by the frequency ofthe supplied oscillations.

Where large frequency deviations are encountered, it will be found desirable to provide manual or automatic amplifier gain control means.

In view of the foregoing explanation, the operation of my newand improved oscillator will be fully understood from the following brief description. uAs described hereinbefore, the angle of phase displacement 01 between the input and outputoscillations atthe circuit 2' is determined primarilybythe resistance of the bridge element 35! and is substantially independent of the frequency 'oscillations applied .to the circuit 2, while the. angle of displacement 02 between the input and output oscillations at the circuit sis determined primarily by the frequency of oscillations applied to the circuit 3. The phase shifting circuitsZ and 3v are connected in cascade circuit relation between the output and input circuitsof the amplifier I. 1 have found that an amplifier provided with" s'iiflicient' feedback through a pair of phase shifting circuits of the type described will oscillate at a natural frequency such as the total phase shift through the amplifier and feedback network is a minimum. Specifically, where the phase shifting circuits shift the phase of the regenerative oscillations in opposite senses, the natural frequency of oscillation will be that at which the net phase shift, including the small but relatively constant amplifier phase shift, is zero, for, at this frequency, the energy supplied through the feedback circuit is a maximum.

It will now be understood that, with the resistance bridge of the phase shifting circuit 2 balanced, the circuit 2 produces a 90 phase displacement. Accordingly, therefore, the mean or center frequency of oscillations will be that at which the phase shift through the circuit 3 is plus or minus 90, depending upon the orientation of the transformer 1. As the bridge circuit is unbalanced in one direction or the other by change of resistance of the resistor 3|, the circuit 2 Will effect a phase displacement of the oscillations i supplied to it and, for 7 each each valueof-phase displacement thus produced, a corresponding natural frequency of oscillations will exist at which the opposite phase displacement effective in the circuit 3 is equal in magnitude. Accordingly, therefore, the frequency of oscillationswill be found to be a function of the resistanceof the resistor 3|. While I have described, by way of illustration,- only a preferred embodiment of my invention, many modifications will occur to those skilledin the art and I, therefore, wish to have itunderstood that I intend in theappended claimsto cover all such modifications as fall within. the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

-1.'A frequency con-trolled oscillator comprising an electron discharge amplifier having input and output circuits, and means for supplying oscillationsfrom said output to said input circuit com prisinga network for shifting the phase of oscillations by an amount determined by the fre quency of said oscillations and a second network for shifting the phase of-said oscillations by a selectable amount substantially independent of said frequency.

2." A frequency controlled oscillatorcomprising an electron discharge amplified havinginput and output circuits, and means for supplying oscillationsfrom said output to said input circuits comprising a network for shiftingthe phase of said oscillations in one sense by an amount determined by the frequencyof said oscillations and aisecond networkfor shifting the phase of said oscillations in an opposite sense by a selectable amount substantially independent of saidfrequency. 3; A frequency controlled oscillator comprising anelectron discharge amplifier having input and outputcircuits, said amplifier exhibiting a constantshift in phase between said input and out- 'put circuits, and means for supplying oscillations from said output to said input circuits comprising a network for shifting the phase of said oscillations in one sense by a selectable amount substantially independent of the frequency ofsaid oscillations, said phase shifting network including a variable impedance element, and a second network for shifting the phase of said oscillations in an opposite sense by anequal amount,said second phase shifting network being responsive to the frequency of said oscillations and determining said frequency such'that the net phase shift between said output and input circuits externally of said amplifier is substantially zero.

4. An oscillation generator comprising an electron discharge device having input and output circuits, a network including a variable resistor for shifting the phase of oscillations derived from said output circuit by an amount dependent upon the resistance of said resistor, and a second phase shifting network responsive to the frequency of oscillations from said first phase shifting network for supplying oscillations to said input circuit substantially in phase with said derived oscillations.

5. An oscillation generator comprising an electron discharge device having input and output circuits, and means for supplying oscillations from said output to said input circuit including, a network substantially independent of the frequency of said oscillations for shifting in one sense the phase of oscillations derived from said output circuit by a selectable amount, a second network for shifting the phase of said oscillations in-, ai i opposite sense by: an amount: determined by thefrequency ofsaidoscillations, and means for supplying to said input circuit oscillations derived fromsaid phase shifting. means,

6, An oscillation generator-comprising an electronddischarge device havinginput and output circuits, a network for shifting the phase of oscillations derived from said, output circuit in one sense by an amount substantially independent of the frequencyof said oscillations and dependent upon the value of a variable impedance element ofgsaid network, and a second network connected in cascade relationwith said first network to shift the phase of said oscillations by an equal amount in an opposite sense, said second network being responsive to the frequency of said oscillations thereby to-determine said 'frequency.

7. An oscillation generator comprising an electron discharge amplifier having input and output circuits, means including an impedance bridge for shifting the phase of oscillations from said output circuit in one sense by an amount substantially independent of the frequency of said oscillations, said impedance bridge including a variable impedance element determining said amount, and means connected in series circuit relation with said first shifting means for shifting the phase of saidjoscillationsby an equal amount in an: opposite sense, saidsecond phase shifting means being. responsive to the 1 frequency. of said oscillations thereby todetermine said frequency. f :8; Ant-oscillation generatorcomprising;an elec tron-discharge amplifier having input and output circuits, said amplifier providing asubstantially fixed phase shift between oscillations supplied to saidinput, circuit and oscillations supplied from said output circuit, a network-for shifting the phase of oscillations derived, from said, output circuit in one sense bya variable, amount substantially independent of the frequency, of said oscillations, a second network connectedjn' cascade relation with said first network for shifting t e phase of said oscillations by an; approximately equal: amount. in an opposite sense, said second network being responsive to the frequency of said oscillations, and means for supplying to said input circuit oscillations derived from said second network. r. 9.. An oscillation generator comprisingan elec, trondischargeamplifier having input and output circuits, said amplifier providing between oscillations-supplied to said input circuit :andoscilla: tionssuppliedfrom said output circuit a phase displacement equal to: an integral multiple; including zero ofgone hundred and eighty electri-v 8 ce l de rees, and means for: supplyina osci lations from said output circuit to said inputcircultim cludinga network substantially independent of the frequency; of said oscillations for shifting the phase of said oscillations in one sense by an amount determined'by, a variable impedance element of said network and a second'network re+ sponsive-to the frequency of saidoscillations for shifting the-phase-of said oscillationsan equal amount in the opposite sense, said amplifier and phase shifting networks providing an overall gain of substantiallyunity. 10.-A frequency controlled oscillator-comprise ingan electron discharge device having input and output circuits, phase shifting means for displacing the phase of oscillations derived from said output circuit by a predetermined selectable amount, and means including a frequency re sponsive phase shifting network utilizing said phase displacement ,to determine the frequency of, said oscillations. V I x 11.- A frequency controlled oscillator comprising an electron discharge device having input and output circuits, first phase shifting means for displacing in one sense and by -a predetermined selectable amount, the phase of oscillations de rived from said output circuit, and means for utilizing said phase displacement to determine the frequency of said oscillations including a network arranged to shift the phase of said DSC11181- tions in an opposite sense by an amount dependent upon the'frequency of said oscillations.

RAYMOND P. MORK.

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

UNITED STATES PATENTS

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