Патент USA US2406309код для вставки
v Aug. 20, 1946. - M. _ZIEGLER> ETAL ' ' 2,406,309 ' FREQUENCY STABILIZATION Filed Nov. 3, 1942 Ea. 1., 1 / 0 ‘I I / 4 OSC/ZLATOR 22,’ _ REHCM/VCE ‘ 7085 ' Z0 12\ Ma,” ‘ mmum‘c)? 5 A METER . - / ' 18 . J5 T|~~| l~§+l 14 \ ‘PILOT ' 56 "11-20. " '. 38 00mm 007M ' z J0‘? ' ‘ 51mm’ 00mm 004/272 J14 “Wm” 42 / ' ' > ' 22d 40. / 50pm? 2. - ' B ‘ ' lf/ff ‘ "*7 m4. 1 mam mum: | I 71/5: - I 35 '' , ' ‘ ' 45 MR6’ ZIEGLER ‘ , M4NUFL JUHOKOBILSKX ' INVENTORS. A 7' mRNEY Patented Aug. 20, 1946 2,406,309 UNITED STATES PATENT'I‘OFFICE FREQUENC€ZZTIZEIZATION I Marc Ziegler and Manuel Julio Kobilsky, Buenos Aires, Argentina, assignors to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application November 3, 1942, Serial No. 464,380 ' 9 Claims. (01. 179-1715) The present invention relates to frequency staé _ bilization and more p'articularly'to the‘stabiliza tion at a desired value of the frequency of an oscillator. ., ‘ In connection with the problem of stabilizing the frequency of an oscillator ‘at a desired value, which is of great importance in radio technique, several solutions have heretofore been proposed. For instance, the use of very selective circuits with great physical constancy, such as crystals, spatial circuits and tuning forks, permits ofvthe construction of oscillators the frequency of which which is rigorouslyequal to that of the pilot. The classical phasemeasuring methods are lim ited however to phase differences smaller than 1r/2 which is a given limitation. The second method gives rise merely to a reduction of the frequency, though this reduction can generally be as large as desired. The ,‘present invention is more particularly’ concerned with the frequency reduction meth ods of stabilization. ‘- r > The known systems of frequency control by does not vary more than a small fraction of its nominal predetermined value. 2 two oscillations andto a resultant frequency frequency reduction utilize,~for obtaining the controlling magnitude, the frequency responsive '_ ' The frequency of such an oscillator is, however, properties of selective circuits. Hence the ab solute stability obtainable cannot be better than they absolute stability of the said discriminator desired, from its original value. Only a phase circuits, and this suggests the use, for the dis shift can be forced on the oscillation obtained, either by using the properties of electrical cir 20 criminator, of high quality circuits such as those hereinbefore mentioned.’ While the use of such cuits or by introducing a time-retarding device. circuits is, of course, possible, it will not answer By phase modulation a corresponding frequency the requirements of linear response to frequency modulation may be created at the output, but variations over a relatively wide range, which the frequency of the oscillator itself does ‘not, ,. must be satis?ed in, for instance, frequency mod vary, and the mean frequency of the'output sig ulation problems. If a wide frequency swing has nal remains equal to it. . to be discriminated linearly, only less selective In many cases,'however, it is not desired to circuits can be used, whereby the inherent insta obtain merely a frequency of rigorously known, bility thereof is introduced into the system. 7 value, but to have an oscillatorvwhich shall auto matically adjust itself exactly at a frequency 30 Furthermore, the response of the common dis criminator circuits, and, therefore, the result which is known only-approximately and which ing controlled frequency, depend either on the may be varying in time. Again, it may be de amplitude of the applied signal and on the value sired to modulate directly the frequency of an so well de?ned by the physical constants of the circuits used that it cannot be shifted, even if - oscillator by periodic variation of one of the cir cuit constants. ' ‘ ‘ All these cases necessarily require the use of suitable oscillator arrangements the constants of which may readily be modi?ed, so that the fre quency may be changed as a function of a con trolling magnitude. The control of the circuit may be either mechanical, as by means of vari able condensers, moving cores and the like, or of a compensating voltage, or on the-correct bal ance of a symmetric detector arrangement, all 35 of which factors are possible causes of drift from the exact adjustment. The method of the present invention and the [devices for the realization thereof while shav ing the same basic idea as other frequency sta bilizing arrangements, namely, to measure the frequency diiference between the oscillation to be stabilized or controlled and a pilot oscillation and electrical, as by means of magnetic saturation, to use the magnitude so obtained for automat variation of dielectric constant with temperature, ically reducing the frequency difference as much reactance tubes and the like, and of these meth 45 as possible, utilizes, in contradistinction to the ods, the purely electrical one involving reactance prior art, a compensated frequency meter the tubes is the most generally used nowadays. outputof which is zero for a predetermined fre The controlling magnitude may be obtained in quency independently of the battery tension ap one of two general ways, namely, either by oper ating on the basis of the phase difference be 50 'By'frequency meter is- meant herein a fre tween the oscillation to be controlled and a pilot quency responsive device ‘commonly known as oscillation, or by obtaining it from a frequency plied. responsive device which measures’ the frequency difference between said oscillations. "I ‘ - " . such, whichincludes only resistances/and capacie ties.“ Accordinglythe present invention dispenses " ‘ The ?rst way leads to a synchronization of the 55 with the use of a symmetric discriminator design comprising selective inductance and capacity cir aeoasoe cuits'and with the use of a stabilized compensate ing tension. ~ ' .meter I6 which produces an output proportional ' to said frequency difference. 7 Preferably the mixer 12 includes a convenient output ?lter from which the signal of frequency difference resulting from the mixing of the os Since frequency‘ In ters have already been'dis cussed in detail elsewhere (see Frequenziometri ' electronici a d-esviazione,cAlta Frequenza, F. Vec cillator and pilot outputs is obtained. chiacchi, May 1937) it is deemed su?icient for the understanding of the present invention, to recall that the output current is provided by a number of condenser discharges which is equal, during anytime, to the number of positive phases » 7 It will be clear to those skilled-in the art that, with the, above arrangement any deviation from the desired frequency, due to variation in the constants of the oscillator Ill, as for example a change of capacity with temperature, cannot take place without causing an opposite effect propor tional to the frequency change really taking place ‘; as the result of both effects. Analogously, with of the signal applied to the apparatus. If the RC time of the condenser ClI‘Cllll'! is suf ?ciently small compared withthe period'ofthe. highest frequency to be measured, the condenser . . the ‘well-known .theory of negative feed-back, it can be shown that the frequency drift of the os :'is each time charged tothe sameepotential and discharged completely, so that the discharges are cillator when connected to the correcting link is equal and'the mean value of the; current, talgen; 7 * approximately A times smaller than the frequency over some time including a su?iciently'largenum ber of periods, is proportional to the number-0f drift without such link.to A acenstant the arrangement equal thebeing product of the senof periods and hence to-the frequency. Moreover, sitivity lc-of the frequency‘ meter it in voltsper ‘ if the amplitude of the signalapplied- does vnot kcJand' the in?uenced ‘ofthereactance tube 22' in?uence the loading voltage, the output is in-' _ in kc. per volt. ' ‘ ‘ ' dependent of this. amplitude. It can also be Since for successful; operation of" the: novel '> shown that, if the frequency of the signal varies, 25 method of frequency stabilization the output of the output reproduces the variation correctly so ‘ the" frequency meter must be properly compen ’ long‘ as the variation. is‘ slow as compared with sated at all’ times, variations of the elements of the longest period of the frequencies passed dur- , this'mp’art of a practical arrangement would be ing therexcursion, and that, even if the frequency considered as meriting attention, and indeed, in modulation has a period of the same order as or 30 vestig'a'tion of this 'point'has led to certain valuae even smaller than said longest period, the aver- _ , ble features of the present invention," as will now agegvaluelof the output current, taken over a long time, will still be proportional to the central bemadeclear. frequency. . - I >7 ~ ' I o It can be‘shown that the influence of relative variations da/a and die/k of the constants hand 70 can be made practically negligible, as it is equal to their product with the" frequency "difference divided by the factor 'A=alc, which can be made . These general properties of the frequency me terrwill be utilized in accordance with the present invention for obtaining an improved method and means for frequency stabilization and for ob taining other derivative advantages arising out of the application of'the said improved method large in practice. ' ' , ‘ The influence of undesired variations inithe compensating voltage'Vb, when taken’ separately and means in particular problems, as will here is, though by no means exaggerated,inot at, all inafter be more particularly explained. - negligible, but the present invention provides’ In the drawing, ‘ I ' ‘ means for compensating this effect. As said'be-f i fore,'the output current of the frequency'meter Fig. l is a block diagram illustrating the meth od of- frequency stabilization of the present in—' vention, and I I " _ is prdvided a number of condenser discharges ' equal to the number of periods of the frequency applied. The output tension obtained is there? . VFig. 2' is a diagram illustrating a frequency modulated transmitter embodying frequency sta bilization according to the present invention. ' As already stated, the basic idea underlying the - present method is to measure the frequency dif ' ference between the oscillation to be stabilized fore proportional’ to the frequency, to. the ca pacity of the condenser,v to the’voltage-yup to 59. whichthe condenser has been charged and to and the oscillation of apilot, and it is therefore obvious that‘ the frequency difference which is to be the direct cause of the controlling action to be applied to the oscillator, must be Within the working range, an univocal function of the os cillator frequency. the resistance through which the current is now; If now the compensating voltage is obtained, ing, H . by means of a potentiometer, from the‘ same bat: ' tery, which provides the tension for loading up,’ 5.5.; the condenser, a change of battery tension will not affect the», frequency for which the, compen- ' . sated output tension ofthe frequency meter is . Since,,physically, the difference frequency'can ‘ zero. , , never have attributed to it a negative sign, it 6,0v Such anarr'angement is shown inthe portion of Fig. 2 marked off with chain} lines in which a will be clear that the oscillator frequency will al battery '13 provides the operating tension for the. ways have to be either higher or lower than the frequency meter main tube 245, and the compen pilot frequency. Hence, the oscillator frequency sating tension x13 is taken off a potentiometer mustbe stabilized at. a frequency differing a pre determined amountfrom the pilot frequency, and 65 this constitutes a particular feature of the pres ent invention. The amount of. difference is so chosen that the oscillator frequency will never with an output ?ltercomprising resistances 28 and .36 and condensers 32 and 34-. pass the pilot frequency, taking into-account all' possible factors, including the possibility offree quency modulation. The difference‘ frequency, obtained as‘ shown in Fig. l, by applying the frequency of‘an ‘os 23 across said battery 3. The frequency meter, as shown, likewise comprises a condenser C and an outputresistance r, and is preferably provided 70 o In the operation of the frequency meter the, oscillations present in the outputvof the ‘mixer stage lZa are applied to, the grid of the ,valve'2_§;.¢. During those half periods of the oscillationiori which the gridris»positive'with ,r?specttothe cillator'iil to a mixer l2 simultaneously with the output of a pilot i4, is applied to a frequencyv , cathode-of- the valve. the-latter willbeconductive, ' > 2,406,309 6 and practicallythevfull voltage of battery B will justablev frequency having all the stability ofin-i herently non-variable oscillators, suchas quartz be applied to capacitor C through load resistor r and diode I00. During the other half periods of the oscillation, when the grid of valve 24 is negative with respect to the cathode, capacitor C will be discharged through diode Illl, resistor q and the external ‘circuit including battery B, anoderesistor I04 and potentiometer 26. Resistors I02 and H13, included in the grid cir cult, of valve 24, are designed to transform the oscillation generated in the output circuit of the mixer stage [2a into a square wave. Across re sistor 1' there is generated a discharge impulse of capacitor 0 every time the control grid‘ of the valve 24 is positive with respect to the cathode of this valve, so that for each period of the oscilla tion applied to valve 24, an electrical impulse is obtained in such a form vthat the mean value of the tension developed across resistor r is propor tional to the frequency of the oscillation» applied to the control grid of valve 24. The tension so developed across the resistor r ‘is compensated to zero for a given value of the frequency of the oscillation applied to the grid of valve 24 by means of the compensating tension AB so that oscillators and the like.‘ The desired frequency can be changed, by choosing, for exampleanother position of the potentiometer which determines the fractional compensating voltage. The po tentiometer,v which may be of the continuously or step—by-step variable type, may be calibrated in Y kilocycles for adjustment. ‘ Alternativelythe out put resistance or the .capacitymay be made 31d? justable, and in the latter event, the condenser of the frequency meter would be made variable or comprise a variable portion, as indicated by the varrowin broken lines in Fig. 2. V I ' I ., In the foregoing discussion the arrangement and method have been considered for the case of a non-modulated \ oscillator .7 to be stabilized around a frequencydiifering a ?xed amount from that ‘of a non-modulated pilot. It is obvious that, since the response of the frequency meter does not, within a very large range, depend on the amplitude of the signal applied, either the oscil lator to be stabilized or the pilot oscillator or both may be amplitude modulated, so long as the tive values in proportion to the sense and amount amplitude of the signal applied during the most pronounced amplitude minima, does not fall be of deviation of the frequency'of the oscillation from the ‘given value. If one Or both of the oscillators is frequency the resulting voltage assumes positive and nega ‘ modulated, stabilization is also possible, provided The low pass ?lter following resistor r and con stituted by resistorsl28 and 30 and condensers 32 and 34 is adapted to suppress the alternating components in the mean voltage developed across the resistor r. It can be deduced easily, that low the lower admissible limit. such an ar- . rangement, if a’r’CB>>l, this product having the same signi?cance as the factor A=ak above mentioned, that the input frequency of thexfre quency meter forwhich the compensated out put is zero, that is just the desired frequency dif ference between the pilot and the oscillator to be stabilized, is approximately equal to A/rC. , It follows from this that said desired frequency can be adjusted to a predetermined value by that care is takenv that the predetermined fre quency difference is su?iciently greater than the sum of the maximum excursions of the oscillators for the reasons hereinabove set forth‘. If care is taken to make the response charac teristic of the frequency meter linear between 0 and twice the'desired frequency," and a suitable band pass ?lter, designed to‘cut out all frequencies above SGc/s, is placed between the output of the frequency meter and the input of the reactance tubeasindicated at 28 to 54 in Fig.2",the theoe retical results obtained can be applied without change to the‘ central frequencies of the free » quency modulated signals considered. The frequency stabilization method of the varying either the potentiometer relation A, the output resistance or the capacity of the frequency meter, which is a very interesting feature. present. invention may also be applied to‘ fre factors, moreover, can each be made adjustable ’ ment of such application is shown in Fig. 2. quency modulated transmission, and an embodi with all the precision required and may ‘present Here the transmitteriantenna 36 is connected a high degree of constancy. through a'power output section 38, a driver or doubler 40 and a separator 42 to an oscillator l0a, which is connected to the mixer |2a of the For this reason, and , because the frequency applied to the frequency meter (difference frequency)~ by which the rela-> , tive variations of said factors are multiplied, may ‘ be of the order of only kc. or tenths of kc., a high degree of stability of the oscillator fre quency when compared with the pilot‘ frequency may be expected, even when working at frequen-‘ cies as large as mc./s. frequency meter ‘unit already described, and to. a reactance tube 2211, which in the present in stance acts both as stabilizer. and. as modulator. The output of the frequency meter unit, after. pass ?lter 28 to 34, is , It may further be observed that variations of the proper frequency of the oscillator from the source of intelligence, 60 such as a microphone (not shown). Thus while the frequency meter arrangement maintains the central. frequency constant, the output of the I oscillator is also'frequency modulated by super-' ' output makes it possible and easy to readjust, during operation, the proper frequency of the os-, cillator to the correct value, when it appears that over a long-time, the frequency shift tends to in imposition of the intelligence factor derived from said secondary 44. - " ~ > i Those skilled in the art Will appreciate from the foregoing description that the novel method and means for frequency stabilization of the invention The above-mentioned relationship between the 70 present advantages of great desired frequency and the fractional compensat crease to an inadmissible value. ing voltage, ity of the frequency meter, provides a very con 2,406,309 @7 quantity substantially proportional to the‘. fre merit does :not require more than .-a single quartz ' ' quency of saidjo'scilllation and to :said voltage, ; oscillator. Again, itis possible vto ‘make ‘the sta and ‘simultaneously inje'c‘ting a fraction of said frequency vdepend prac or, the quartz 0's. 1 ‘bility. of . the central . tically only :upon the stability cillator used. In general,"the complete stabiliz- I, ingarrangement does n ot require more ‘than a 3 single adjustment .in'orderto adapt it to work at ‘and a change of the value ‘ the desired frequency, of only one element makes it. possible ‘to ‘stabilize voltage in opposition to said quarrt'ity'to produce 7 an output quantity which is compensated ‘to ‘zero independently of variations in said source at a frequency value jof‘the oscillation equal to ‘the said predetermined ‘mean frequency value and the polarity and value of‘whi'ch areproportional to‘the deviations of said electrical'oscillation from * the oscillator around different well-de?ned fre-' .10. the predetermined frequency value. ' > quencies, a feature which ‘may ‘have many "appli 3. A compensated frequency ‘discriminator for '1 , cations in radio communication. The number "of f vdetecting frequency deviations of an electrical tubesand parts is relatively small and there‘ is i » oscillation varying about apredetermin'ed mean nothing very critical "n' their assembly. More; .‘1-5. frequency, comprising a‘ ‘circuit containing a over, critical elements, such as a tube or a source of tension, may, as 'can'be shown, have variations as‘large 'as 10% without affectingythe validity of the results. . Finally, it should be noted ‘that ‘the ‘ stabilizing arrangement does .notxuse any tuned source of direct current “and a resistance, "means for interrupting the current in said circuit ‘in syn chronism'jwith ‘half-waves‘ of like sign of'said oscillation to obtain voltage ipul'sationsfa'cros’ssa'id resistance, means for transforming said Voltage circuits. Hence it can be utilized at any "fre quency, and vmay be assembled and marketed as a universal frequency modulation and stabiliza pulsations into electrical impulses of like sign having an area substantially’proportional to the voltage ‘of said source, an amplitude independent of said oscillation and a duration smaller than the tion unit. Inclusion of the quartz pilot and of‘ the oscillator gives a frequency modulation trans mitting unit adapted to be directly connected to‘ half-periods of the highest frequencylto be dee tected, means to integrate said impulses to ob tain a potential substantially proportional to the a frequency modulation transmitter. ' Although the present invention has herein above been described with reference to certain preferredembodiments, We do not wish to be lim frequency of said oscillation and to the voltage of to connect a fraction of said voltage in opposition to said pote'ntial'to pro ited thereto as obvious modi?cations will occur 30 duce an output potential which is compensated ' said source, and means to those skilled in the art without departing from ‘ ' the spirit and scope of the invention. We claim: - _ - V 1. The method of detecting frequency deviae to zero independently of voltage variations in said direct current source at frequency values of the oscillation equal to the said predetermined mean frequency value and the polarity and value of, oscillation about a prede~ 35 which ‘are prcportionalto the deviations of said 7 tions of'an electrical termined frequency value by means of a circuit‘ arrangement comprising a direct current source _ means responsive to 7 and current interrupting electrical oscillation from the predetermined fre qu ency value. ' I Li. A ‘compensated frequency discriminator for said electrical oscillation connected in series with ' detecting frequency deviations of an, electrical os said source, comprising thesteps of generating 40 electrical impulses for each period of vsaid os cillation having an area substantially propor " tional to the ‘voltage of said source and a ‘dura cillation varying about a predetermined mean fre quency, comprisingv a thermionic tube having ‘a control electrode, an anode and a cathode, a re sistance and a direct current source‘ serially con nected between theanode and cathode,'means tion smaller than the periods of the-highest fre quency'to be detected, integrating said'impulses‘ ‘ , tov apply said oscillation to said control electrode to obtain an electrical quantity substantially pro to produce voltage pulsations across said resist portional to the frequency of said oscillation and ‘ ance in synchronism with half-waves of like sign to said voltage, and simultaneously injecting ‘an of said oscillation, means to transform saidL volt- , other electrical quantity derived from said source . ' agepulsations into electrical impulses of like sign in, opposition to said first quantity to produce . an output quantity which is compensated to zero independently of variations of said voltage at 'a frequency value of the‘ oscillation equal tort'he said predetermined frequency ‘value and the polarity and value of which are proportional. to the ‘deviation of- said electrical .oscillation fromthe predetermined frequency ‘value. 2. The methodvof detecting frequencyrdevia- I ' having an. area substantially proportional to the voltage of said source, an' amplitudeindependeht of said oscillation and a duration smaller than the-half-peri-ods of the highest frequencyto be 7 having one elec ' detected comprising a condenser trode coupled to the anode circuit of said tube, two half wave recti?ers having anode and cath ' ode electrodes, and a load resistance, the cathode of one of said recti?ers and the anode of the other tions of-an electrical oscillation varying about, a rectifier being coupled‘ to the other electrode of , predetermined mean frequency by'meansof a 60, the condenser, one terminal of theload resistance .~ circuit arrangement comprising a direct current ,being'coupled to the other electrode of one of said source, a resistance and current interrupting r-ecti?ers, and the other electrode of the second means responsive ,to said electrical oscillation of said recti?ers and the other terminal of the connectedin series with said source and resist 65 load resistance being coupled to an intermediate ance, which comprises the steps of interrupting voltage point in said direct current source, and the current in said circuit in synchronism with means to integrate said impulses to produce 'a ' half-waves of like sign of’ saidroscillation to ob potential substantially proportional‘ to the fre tain voltage pulsations across said vresistance, quency of said oscillation comprising a ?lter coil transforming said vvoltage pulsations, into elec-V 1 , pled to. said load resistance, the voltage‘at said tricali impulses of like sign having an area sub— stantially proportional to the voltage of said, intermediate point having a value substantially source, anamplitude independent of said oscil-. potential at a frequency value of the oscillation equal to the said predetermined mean frequency 1 l-ationand a duration smaller thanthe half-pea _ riods'of the highestfrequency to be.deteeted,-in ; telgratme said impulses .to obtain an. electrical equal and opposite in sign to the‘ value of said ‘value; ‘ . ‘ ' 9 2,406,309 10 5. A compensated frequency discriminator ac cording to claim 4, in which the condenser is adjustable and varies the frequency at which the said compensated output potential is zero. 6. In a frequency stabilizing system, means for of the direct current source in opposition to the said potential to produce an output voltage com pensated to zero independently of voltage varia tions in said source at frequency values of the main oscillation equal to said desired value, means generating a main oscillation the frequency of which is to be maintained at a predetermined to connect said integrating means tovsaid fre- , value, frequency adjusting means coupled to said generating means, means for generating a pilot oscillation differing from said main oscilla 10 tion by a predetermined frequency, means for mixing said pilot and said main oscillations to produce a difference oscillation, a circuit includ ing a direct current source and a resistance, quency adjusting means, and means to vary the amplitude of said output voltage proportional to intelligence to produce frequency modulation of said main oscillation. 9. In a frequency modulation system, a main oscillator the frequency of which may vary about an adjustable stabilized value differing by a pre determined amount from the frequency of a crys means for interrupting the current in said circuit 15 tal controlled pilot oscillator, means for radiat in synchronism with the half-waves of like sign a ring the energy of said main oscillation, means for of said difference oscillation to produce voltage pulsations across said resistance, means for trans forming said voltage pulsations into electrical im pulses of like sign having an area substantially proportional to the voltage of said source, an amplitude independent on said oscillation and a duration smaller than the half-periods of the adjusting the frequency of the main oscillator, means for mixing said main and said pilot osoil~ lations to produce a difference oscillation, a ther mionic tub-e having a control electrode, an anode and a cathode, a resistance and a direct current source serially connected between the anode and cathode, a potentiometer connected in shunt with highest frequency of said difference oscillation, said direct current source, means to apply said means to integrate said impulses to obtain a po difference oscillation to said control electrode to tential substantially proportional to the frequency 25 produce voltage pulsations across said resistance of said difference oscillation and to said voltage, in synchronism with half waves of like sign of means to connect said integrating means to said said difference oscillation, means to transform frequency adjusting means to maintain said main said voltage pulsations into electrical impulses of oscillation at said predetermined frequency value, like sign having an area substantially propor and means to inject a fraction of the voltage of 30 tional to the voltage of said source, an amplitude the direct current source in opposition to said independent of said difference oscillation and a potential to produce an output voltage compen duration smaller than the half periods of the sated to zero independently of voltage variations difference oscillation comprising a condenser hav in said direct current source at frequency values 35 ing one electrode coupled to the anode circuit of of the main oscillation equal to said predeter said tube, two half wave recti?ers having anode mined value. and cathode electrodes, and a load resistance, the 7. A frequency stabilizing system, according to cathode of one of said recti?ers and the anode claim 6, in which the relation between said volt of the other recti?er being coupled to the other age fraction and said potential is adjustable, and ‘electrode of the condenser, one terminal of the the frequency at which the main oscillation is load resistance being coupled to the other elec stabilized is variable before and during operation. trode of one of said. recti?ers, and the other elec 8. In a frequency modulation system, means for generating an electrical oscillation the mean fre quency of which is to be stabilized at a desired value differing by a predetermined amount from > the frequency of a pilot oscillation generated in said frequency modulation system, frequency ad justing means coupled to said main oscillation generating means, means for mixing said main and said pilot oscillations to obtain a difference oscil- " lation, a circuit including a direct current source and a resistance, means to interrupt the current in said circuit in synchronism with half-waves of like sign of said difference oscillation to produce voltage pulsations across said resistance, means for transforming said voltage pulsations into elec trical impulses of like sign having an area sub stantially proportional to the voltage of said source, an amplitude independent of said oscilla- - tion and a duration smaller than the half-periods of the highest frequency of said difference oscilla trode of the second of said recti?ers and the other . terminal of the-load resistance being coupled to the movable arm of said potentiometer, and means to integrate said impulses to produce a po» tential substantially proportional to the frequency of said difference oscillation and to the voltage of said source comprising a ?lter coupled to said load resistance, said movable tap being positioned at a point at which the voltage thereat has a value substantially equal and opposite in sign to the value of said potential at a frequency value of said difference oscillation equal to the mean fre quency value of said difference oscillation, said filter circuit being connected to said frequency adjusting means to maintain said main oscillation at the desired stabilized value, and means inserted between said ?lter circuit and said frequency ad justing means for additionally varying the ampli tude of said compensated output voltage propor tional to intelligence to produce frequency modu tion, means to integrate said impulses to produce lation of said main oscillation about said sta a potential substantially proportional to the fre bilized value. . ' quency of said difference oscillation and to said MARC ZIEGLER. 65 voltage, means to inject a fraction of the voltage MANUEL JULIO KOBILSKY.