US2176168A - Frequency modulated signal - Google Patents

Description

@CL 17, 1939 H. J. J. M. DE R. DE BELLEsclzE 2,176,168

FREQUENCY MODULATED SIGNAL 3 Sheec's-Sheet l Filed Aug. 2i, 1957 BY di `ATT0RNEY7 Oct- 17, 1939- H. J. J. M. DE R. DE BELLEsclz 2,176,168

FREQUENCY MODULATED SIGNAL ATTORNEY 0Cf- 17, 1939- H. J; J. M. DE R. DE BELLEsclzE FREQUENCY MODULATED SIGNAL* 4 Filed Aug. 21, 1937 s sheets-sheet s T-ED IN VEN TOR.

enr Z JCI/YB @/dea//asczz e) v Y 'nn @A TToR-NEY Patented Oct. 17, 1939 l FREQUENCY MonULATEp SIGNAL Henri Jean `Toseph Marle de Regnauld de Bellesclze, Neuilly-sur-Seine, France- Application August 21, 1937, Serial No. 160,330

14 Claims.

My invention relates to signalling methods and sys .ems, and more particularly to wireless telegraphy in which emitters radiate an electric wave whose frequency is varied in accordance with a signal, and in which the receiver demodulates this wave by transforming its frequencyvari'ationsinto corresponding intensity or voltage changes. The main characteristics of such systems are well known and may be summarized as follows: at the emitter, signalling is effected by varying the frequency of the signal wave insuch a way that each vcode sign is individually represented by a predetermined' value of this frequency; for instance, frequencies F-fs and Fel-fs Will correspond to dots or dashes and spaces respectively; the radiated spectrum is wholly defined by the amplitude fr of the changes imparted to the frequency of the signal wave with respect to the mean value F of this frequency, and by the number fo of dots which may be transmitted during one second; fo. proportional to the keying speed, will hereafter be called signs frequency; from a work published by B. van der Pol in the Proceedings of I. R. E., July 1930, I found that the width of this spectrum is practically 2fi=2(fs+2fo) =2(m+2)f0, where hereinafter called index designates the frequency changes to signs frequency ratio. Outside the central band of width 2f1, the radiated components appear to be much less important than in amplitude modulation telegraphy, because a frequency modulated wave has a constant amplifude and; a phase varying progressively, whereas in other telegraph methods amplitude or phase undergo sudden leaps generating important side components far apart from the carrier frequency thus, frequency modulation telegraphy permits communications on closely adjacent channels without interference. Usually, the receiver comprises frequency changers, lters, means for stabiliz'ing the amplitude of the incoming wave, and an indicator fed through 10W frequency circuits by a device in which frequency changes l fs due to signalling are transformed into corresponding changes iis in the intensity of a detected current; this transformation may be' reached through various ways, for instance by utilizing balanced resonators tuned vto the extreme signalling frequencies respectively as illustrated at Fig. 1b in my Patent No.v 1,612,101 dated Devcember 28, 1926, or a synchronizing current acting as an elastic link for maintaining a. localv genwill be proportional to tained with an index about six times smaller,

(Cl. Z50-8) erator of oscillations in locked nsynchronism with the received wave as disclosed in my Patent No. 2,040,980 dated May 19, 1936.

The first. object of the present invention is to provide suitable means for adapting the charac- 5 teristics ,of the emitter to those of the receiver so as to supply the detection and recording of the signal with the protection required against both Statics and' fading. The receiver will be managed so as to derive from each code sign an accumula- 10 tion effect substantially proportional to the index fo v 15 and thus the index must be increased according as disturbances are growing w'crse; but frequency changes fs and signs frequency fo are transmission characteristics, whereby emitter and receiver are closely dependent on each other. For deriving from each code sign an' effect proportional to the index, the frequency change fs representing this sign will be translated into a detected current is of intensity approximately proportional to fs, and further the electricity supplied by this current will beV stored over a space of time -r proportional to the duration 1/2f0 of thissign; thus the accumulation effect (z'sx'r) derived from the code sign and therefore'to the index. 35

A second object of the invention is to increase the keying speed practicable at the emitter by lessening as much as possible the index required at the receiver for protecting the signal against disturbances. This improvement will be obtained by. inserting between the indicatorvand the device for transforming frequency changes into current changes a local distributor synchronized with the signs frequency, for successively accumulating in a circuit of large time constant the whole elec- 45 tricity supplied bythe detected current over the duration of each code sign and then transferring .this electricity to the indicator. With a given index', the effect thus derived from each code sign is much larger than in usual receivers where time 0 constants of low frequency circuits are limited to a small fraction, commonly one sixth, of the signs duration. Thence it follows that the signal effect required for dominating disturbances will be obwhereby keying speed may be increased at the emitter according to f 2f,

azon-t2) where 2h is the predetermined communication band width and m the smaller index.

Another object of the invention is to obtain a more rapid increase of the index and thus of the protection against disturbances at the receiver, in proportion as these disturbances compel the slow ing down of the keying speed at the emitter. This result will be obtained by proportioning frequency changes f5 to signs frequency fo so as to utilize at all keying speeds the maximumand width 2f1 allowed to signalling; in other words, fs will be varied inversely to ,fo according to 2f1=2(fs+fo) Where 2f; is kept constant, whereby the index t',l of the signs frequency, contrarily to what happens in usual telegraph systems where fs is kept constant.

From the foregoing characteristics it follows that my system is particularly adapted for short wave signalling where statics and fading undergo fluctuations of large amplitude: under favorable conditions, one may use a very small index, for instance m=1, whereby keying speed and signs frequency compatible with the predetermined band width 2h are increased; on the other hand, a moderate slowingdown of this speed will supply the largest index, for instance m=8, required under most unfavorable conditions; from 2f1=2(m+2)f0 where 2f1 is now kept constant, it appears indeed that the signs frequency fo corresponding to m=8 is only 3.3 times smaller than that corresponding to m.='1. Y

A further object of my invention is to improve telegraph codes by composing them of three constitutive signs instead of two; for instance, dots, spaces and dashes will be represented at the emitter by frequencies F-fg, F, F-i-,a respectively, and therefore at the receiver by detected currents +13, zero, -ia This improvement permits the transmission of a larger number of words per minute for given signs frequency and communication band; in combination with the large reduction realized on the index, the number of words per minute lbecomes more than ten times larger than in usual telegraph systems, for a same communication band width and a same signal to noise ratio.

A further important improvement of the invention is to facilitate identification of errors by deriving substantially identical effects from both fading and strong static and placing this common effect in the middle of effects normally due to code signs. This result is obtained by combining several conditions cooperating as follows: A rather large index, for instance m=8, is used at the emitter whereby the signs frequency fo is made small comparatively to the communication band width 2f1. At the receiver, the band of the filter selecting the received wave is adjusted to this communication band width 2h whereby the time constant of the filter is approximately 2f1 and drift neutralization means are provided for anales controlling frequency changes preceding the filter so as to maintain the mean frequency of the signal wave in coincidence with the mean frequency Fe of this lter; thence it follows that frequency changes if, due to signalling become symmetrical with respect to this mean frequency F0 while the oscillations due to static assume all through the duration 2f;a of each code sign several independent frequencies varying at random on both 'sides of F0, since the large index used at the emittercauses the time constant 2f1 to be small comparatively to 2i., Further, the device for transforming frequency changes into current changes is exactly balanced with respect to the same mean frequency Fo of the filter, whereby definite frequency changes -l-fs or -fs caused by code signs with respect to F0 are translated into detected currents -i-i or is of definite directions in accordance with these signs, while frequency changes caused about F0 by static dominating the signal wave are translated into currents of directions reverting at random many times during each code sign. Then, fluctuations undergone by the detected current are limited in both directions to intensities equal to iis or slightly larger. Finally, the electricity supplied by this current is accumulated over the maximum space of time, i. e.,

, over the whole duration of each code sign, and the indicator isenergized by this accumulation effect. Owing to the above characteristics, the accumulation effect due to powerful static becomes comparatively small and thus it takes place in the middle of the extreme effects Y and normally due to code signs, since the means intensity of currents limited to is and reverting at random several ,times is necessarily small comparatively to is. As fading effect is also substantially null, it coincides with the effect derived from strong Statics. It must be clearly understood that this system does not avoid static effect, but merely distinguishes this effect from those of code signs. I

In pursuance with the above explanations, the invention allows to correct errors by repeating the signal and comparing the texts issued from the successive repetitions; this possibility extends to errors due to both static and fading and to telegraph codes using indilerently two or three signs, whereas previous repetition systems, the Verdan system for instance, permitted the correction of errors due to static only or to fading only in two signs codes only. This improvement proceeds from the fact that both static and fading will be caused to annul the accumulation effect supplied tothe indicator; marks due to such disturbances may therefore be confused with the mark due to the central signalling frequency, F at the emitter and Fo after the frequency changers ofthe receiver, when this central frequency is used for representing code spaces; but disturbances cannot produce same marks as extreme signalling frequencies F-h and F-i-f, representing dots and dashes respectively; thus, dots and dashes appearing on the tape are' certainly correct; as the same sign of a signal is rarely injured during successive repetitions, an accurate text may be reconstituted by repeating 'systematically the whole signal and by keeping dots and dashes markingV on one at least of the successive recordings. These recordings may be compared visually or through automatic means.

Further developments will show that the indicator may remain'insensitive to intermittent statics, even when they dominate the signal wave;

thdse' only which happen to pass in phase opposition with this wave influence dangerously the detected current by imparting to this current instantaneous changes of very short durations but of considerable amplitudes. Thus danger-- ous fluctuations of the detected current are rarefled but enlarged, which constitutes a particularly favorable condition for placing amplitude limiters at the output of the device which supplies this' current. Such limiters are operatively quite distinct from those eventually preceding the same device for stabilizing the amplitude of the received wave.

My communication method and system will be best understood from the following description and drawings, in which: y

Fig. 1 illustrates the communicationsystem comprising the emitter and the receiver.

Fig. 2 is a curve showing the characteristics of one of the tube circuits `of Fig. 1.

Figs. 3, 4, 5, illustrate the frequency varia-l tions representing code signs.

Figs. 6, 7, 8, illustrate the useful components constituting the communication band, for a constant Width of this band and three dierent keyj ing speeds.

Fig. 9 represents, as a term of comparison, the components ofA a signal whose signs are transmitted through phase reversals.

Figure 1-0 illustrates side components produced far apart from the carrier frequency whencode signs are represented by frequency changes and phase reversals respectively.

Figs. 11, 11a, 12, 13, 14 give a detailed description of the receiver, in which the signal is demodl ulated by means of a synchronized local generator. Fig. 15 shows the detected current obtained responsively to a code sign, when the signal wave is free from interferences. 1 l

Fig. 16 shows the signal wave received together with a disturbing wave of variable amplitude.

Figs. 17, 18, represent two cases which may occur while the signal wave is dominated by the disturbing Wave, according as these waves do or do not pass in phase opposition.

. Fig. 19 illustrates the accidental change undergone by the detected current, when the disturbing.

waves pass in phase opposition.

Fig. 20 represents another way for obtaining the detected current by causing the received wave to interfere with the synchronized local wave.

Fig. 21 illustrates a simplified receiver for obtaining mere blanks responsively to disturb ances.

Fig. 1 represents the communication system, in which the emitter and the receiver are'arranged to efficiently utilize the band width assigned to the signalling. The emitter radiates a hertzian wave Vlizes entirely the band Width through the amplifier whose frequency is varied for representing the signs of the telegraphic code. The master oscillator I, 2, 3 excites the aerial 4 through an amplifier 5, 6. The circuit 2, 3 of the oscillator is also coupled by a coil 1 to the anode circuit of a tube 8. As indicated by Fig. 2, the resistance' of the anode circuit of tube 8 depends upon the grid bias, which may admit one of the values v1 vo vz according as the circuit of one of the relays 9, I 0, I I is closed in correspondence with the code sign to. be transmitted. Each bias determines a given anode resistance of tube 8, and thus a given frequency of the oscillator. The intermediate bias vn is ad- Justed so that the corresponding frequency F is the average of the frequencies F-f, F+f obtained for the biases v1 v2; furthermore, the grid of tube 8 is connected to the intermediate bias vo through a high resistance I2, whereby the oscillator resumes the mean frequency F when all the relays are at rest. The amplitude fs of the variations imparted 'by the keying to the frequency of the radiated wave depends upon the coupling I3 of the coils 3, l; for each keying speed, this coupling is chosen so as to reach the variation fs which uticommunication; consequently, when the Statics y and/or the fading lead to a reduction of this speed,

and therefore to a reduction of the code signs frequency fo, the index growsvery rapidly, since in f1=fs+2fm fl-2f.,

the numerator increases while the denominator is lessened. Owing to the properties of the receiver, this rapid growth of the index improves the security of the communication, just when the conditions become unfavorable. A small adjustable condenser I4 is provided in the oscillator circuit to maintain thel average frequency F at a constant value whatever the coupling I3 might be. The

' frequency uctuations fs can be observed during the signalling by means of a measuring apparatus, as disclosed in my Patent 2,040,980. In the receiver, which will be more fully treated with the aid of Fig. 1l, the frequency fluctuations fs are l. the intensity variations f linearly transformed into of a current i, obtained by combining in a modulator I 5 the wave received from the aerial I 9 with the wave issued from a. local synchronized source I6; these intensity variations impart to the source I6 the frequency corrections preserving its locked synchronism, and also energize the indicator II acteristics of this communication system consists in that the detected current i gets, under the influence of disturbances, van intensity which is the average of those produced by the code signs; this result is reached by managing the emitter in such a way that the index increases according as 2f1 allotted to the` I8. One of the main char-- the signalling becomes more uneasy, and the ren ceiver -so asto preserve a perfect coincidence between the mean frequency of the signal Wave, the middle of the band selected by thei-llter 38, and the natural frequency of the source I6. The theory of this working will be brieiiy disclosed at the end of the description.

The Fig. 3 represents the frequency variations due to the alternative action of relays 9, II, for transmitting a succession of dots and code spaces whose individual duration is L 2f, This working is the one for which the spectrum of the side components is widest; the number of dots per second,v then fo, will "be named hereinafter sign` frequency. Either 2 or 3 code signs may be uti ized. With the Baudot code, the dots (-1-) are transmitted by the relay 9, the spaces by the relay I I; for the letter fr the frequency variations are illustrated by Fig. 4. The Morse code may be transmitted as usual, dots and dashes by the relay 9, spaces by II, or inversely; but the speed is greatly increased by transmitting the dots with the relay 9 and the frequency F-l-fs, the spaces with the relay I0 and the frequency F. and the dashes with the relay I I and the frequency F-f,; for the letter :z:

the frequency fluctuations are now illustrated by Fig. 5, where the spaces between dots and dashes are suppressed. The index must grow to improve the security according as the communication becomes more diicult; Fig. 6 represents the communication band under favorable conditions, for which the signs frequency fo is'the highest allowed by the mechanical devices of the emitter; the chosen index is m=1,5, which means that the coupling I3 (Fig. 1) is adjusted so as to obtain a frequency variation f3=1,5fo; the interval between the side components, four of them being important, is fo; those of amplitude exceeding one tenth (0.1S) of the signalwave amplitude are practically the only useful ones for an accurate reproduction of the signal at the receiver. The width of the communication band is 2f1=6fo.

Fig. 'l concerns less favourable conditions, compelling to reduce by half the keying speed, whereby the signs frequency and the components interval become 0.51%; with the same communication band 211, the amplitude fs of the frequency variations may grow up to fs=2f, through a suitable adjustment of the coupling I3. index, now

ensures a better protection against the disturbances of any kind.

Fig. 8 concerns very bad conditions, under which the speed must be reduced to a quarter of the maximum; the signs frequency and the interval between the components, some of which happen to be null since the index is odd, is then 0.25f0; for the same band width 21'1, the amplitude of the frequency variations becomes fs=2.25f,

, and the index is Fig. 9 represents, as a term of comparison, the spectrum of the components whose amplitudes also exceed 0.1S, in a telegraphic signal transmitted through phase inversions; this spectrum is in every respect less advantageous than that of Fig. 6, corresponding to the same signs frequency; its width is 10f0 instead of 6f0; it contains only two main, components, and is therefore more liable'to selective fading.

Fig. l0 illustrates the part of the spectrum comprising the components of amplitudes varying from one tenth,(0.1S) to one hundredth (0.018) of that of the signal wave. These components are purely prejudicial, because they are useless for the reproduction of the signal, and moreover produce interference with adjacent communications. The two half curves aa', symmetrical with regard to the carrier frequency F, concern the frequency modulation telegraphy for m=1.5; the

The.

points aa mark the summits of the components also designed by aa on the Fig. 6. The curves bb', whose points bb correspond to the components bb of Fig. 9, concern telegraphy through phase reversals. Thus, as we remove farther from the carrier, the prejudicial components decrease much more quickly with the system of my invention.

Fig. 11 illustrates the receiver: the wave collected by the aerial I9 passes through the r. f. circuits 20, 2I, 22, whose amplification is controlled by a conductor 23 connected to the output of a rectifier 24. A first intermediate frequency is obtained in the circuits 25 by combining in the modulator 2B the signal wave with an oscillation issued from the heterodyne generator 21, 28, 29. It is proven by practice that the frequency of this generator can be badly influenced by the amplification control: the anode resistance of tube 22 varies indeed together with the grid bias, whereby the least coupling between the circuits 2| and 28, 29 would produce, responsively ton the potential fluctuations at 23, local frequency changes which could be confused with those fn of the signalling; this drawback is removed by feeding the tube 21 with separated sources 30, and by connecting the heterodyne oscillator to the modulator 26 through a screen'grid tube 3l. Furthermore, the heterodyne frequency is controlled by an auxiliary tube 32, whose anode circuit is coupled to the circuit 28, 29 by a coil 33, and whose grid bias depends upon the synchronizing current i through a relay 34; as shall be explained, this device neutralizes in the modulator I5 the frequency drift of the received wave with respect to the localsynchronized source I6. There is nothing particular about the circuit 25, tuned to the intermediate frequency; its amplification may also be controlled at 35. The received wave, the respective middles of their bands being the oscillation of a heterodyne generator 31, so as to obtain a second intermediate frequency, such as 30 kczs, allowing an easier construction of the filters 38 whose band width is equal to or somewhat larger than that 2f1 of the communication band; thus, the useful components of this band enjoy the best possible protection against interferences; this result proceeds from the fact that the filter will be exactly tuned to the signal wave,. the respective middles of their hands being brought to coincide with the natural frequency of the local source I6 as explained hereinafter. After its final selection inthe filter 38, the signal wave amplitude is raised by an amplifier 39, 40, to a level allowing its stabilization; this stabilizations is obtained through the combined working of a limiter 4I and of an amplification control ensured by the.rectiiier 24. The tube 4I characteristic curve, represented at Fig. 12, has a lower bend a and an upper bend b caused by the resistance l2; the grid bias c is half way from these bends, and the amplitude of the signal wave in the input of the limiter is normally kept under the control of the detector 24 at a value S coinciding with the straight line ab; the amplitude of the oscillation issued from tube 4I remains therefore constant even if the input amplitude is accidentally weakened, as may happen at the outset of a sudden'fading. The amplification control works in the usual Way: the characteristic curve of the rectifier 24, represented at Fig. 13, has a. bow e, beneath which is the grid bias g ensured by the source 43; as soon as the amplitude S of the signal wave tends to exceed the normal value ge, the detected current ensuing therefrom frequency, harmonics 53; the resonators causes in resistor 44 and conductor 23 a poten- -tial fall whereby the amplification of the tube 22 decreases; the grid of this tube is suitably biased by the permanent current flowing-in 44 through the resistor. 45; the condenser 46 is chosen small enough to ensure a rapid working. A circuit 41, tuned to the second intermediate separates thesignal wave from the caused by the nonlinear characteristics of the limiter 4I; this circuit must be damped to transmit equallyall the components selected bythe 1ter3 The signal wave is then combined in a modulator` 48 with the local oscillation issued from the heterodyne source 49, to raise the carrier frequency to a final value F much higher than the fluctuations fr; for instance, if f=200 czs, the carrier frequency will receive a value 2000'times higher, that is' 400 kczs. This has many advantages: 1 the synchronized source I6, whose timeA constant is now small, obeys more quickly to frequency c-orrections; being relatively small. these corrections are almost proportional to their cause, i. e., to the detected current i; the.lter 38 is shielded from the inuence of the source I6, since tuned to a very different frequency. At the output of the frequency changer 48, the signal-wave selected in a circuit 50 interferes in the modulator I5 with the local oscillation issued from the source I6, 5I and from the amplifier 52, 50, 52, tuned to Fo, are damped enough for being equally responsive to al1 the frequencies eventually undergone by the generator I6; these frequencies are varying between Fo-fm and Fo+fm, fm being the maximum frequency correction. Designating by 90+A the phase difference between the received wave and the local wave, the coaction of these waves is in fact a homodyne frequency changing which linearly transforms the received Wave into a current i=Im sin A; thus, the anode current of tube I5 is I+z', Io being the steady component when no wave is received. The currentz performs three func-tions: it synchronizes the source I6, 5I, for compelling this source to follow the frequency fluctuations of the received wave; it neutralizes the frequency drifts for maintaining a permanent coincidence between the average frequency of the received wave and the` natural frequency F0 of the source; it energizes the indicator I1. g

To preserve the locked synchronism, the potential uctuations caused in a resistor 54 by the synchronizing current i impart to the frequencyV of the source I6 corrections j neutralizing the difference (F0-H) F0 between the instantaneous frequency of the received wave and the natural frequency of the said source, owing to their relative smallness, these corrections are practically proportional to i, whereby the frequency fluctuations are transformed into proportional intensity fluctuations. When the local and received frequencies happen to coincide (f=-0), the synchronization requires no correction; then the syn'- chronizing current is null (z`=0), and the waves interfering in the modulator I5 are at quadrature (A=0). The code signs, represented at the output of the frequency changer 48 by the frequencies Foifs, provide the anode currents Ioiis. The maximum correction fm, reached for i=Im, A=90, is adjusted by the potentiometer 55 so as to be about five times larger than fs(m-=5s).

A milliammeter 56, shunted by a large condenser 51, enables to survey the changes in the I the time constant tween the lower bow synchronizing current; the average intensity of the anode current in tube I6 is adjusted by means of a resistor 58 and read on a milliammeter 59. These features have already been disclosed in my Patent 1,976,877, dated October 16, 1934, except in that the condenser 60 does not contribute to the selection of the signal wave: the synchronizing device having now to be very fast, this -condenser 60 only by-passes the h. f. components; it is therefore very small and does not influence of the synchronizing device; according to a relation published in my Patent 2,040,980, this time constant becomes practically for instance, with fm=5fs, f.=mfo, m=5, it is about 75 times smaller than the duration.

2f, of a code sign.

The coincidencebetween the average frequency of the received wave and the natural frequency Fo of the synchronized source I6 is obtained at theoutset of the signallingby tuning the condenser 28 of the heterodyne generator 21, 28, 29; it is afterwards' maintained by the relay 34 which progressively controls the bias of'tube 32 through a circuit 6I, 62, of very large time. constant (for instance 100 seconds). My Patent -1,990,428, dated February 5, 1 935, has already disclosed the principle' of this method, but a difficulty arises in telegraphy, because the signal wave may assume alternately the frequencies Fo-i-fs and Fo-.fa during the signalling, whereas it keeps the frequency Fo-fi during the pauses between the telegrams; thus, the mean,frequencies are in turn F0 and Fo-fr. This difficulty is mastered by utilizing a relay 34 quick enough to follow the keying, so as to close the contact 63 when the anode current of tube I5 is Io-i-is, and the contact 64 when this current frequency of the received wave deviates slightly from Fo, the alternative values of the current become Io+is+e -and Ia-is-i-e, whereby the contact 63 becomes intermittently closed and 64 always open, or vice versa; whence a slow change in the charge of the condenser 62, the bias of tube 32, andthe frequency of the heterodyne oscillator 21; the shift is thus neutralized. On

the other hand, the signalwave must keep, between the telegrams, the frequency Fo-fs, and the anode current the intensity Ioz`s; this is easily obtained by remarking that one may reverse the direction of the shift in the modulator I5, according as the heterodyne oscillator 21 is tuned to a frequency higher or lower' than that of the signal wave; consequently, it is suiiicient to choose the tuning for which the shift tends to decrease the detected current,-i. e. to voperate the contact 64 which maintains the said current at the intensity Iii-s. Similar rules would also allow to preserve the intensities Io-i-zs during the silences and code spaces, and Io-z's during dots and dashes.

The potential fluctuations v caused in the resistor 54 by the vintensity fluctuations i of the synchronizing current are amplified by the limiter tube I 8, and then energize the indicator I1. The anode characteristic of tube I8 is used in Vits straight part e (Fig. 14), comprised be- 7 and the upper bow h due to the resistor 66; a tap 61 is adjusted on the source 68 in such a way that the grid bias Vo, observed when no wave is received, corresponds is Io-is. When the mean to the middle of this' characteristic, whose width supplied by the synchronizing current responsively to a code sign; for this purpose, a rotor 69, 19, is synchronized with the signs frequency fo, so as to store in a condenser 1I the quantity of electricity due to the sign received at the present time, and to discharge through the relay I1 the electricity already stored in another condenser 12 responsively to the preceding sign. The recording is thus continuous, each condenser working in turn to collect one sign on two. Whatever code based on signs of same duration may be used, the dash of Morse code, for instance, being formed by three consecutive dots. The resistance 13 in the anode circuit of tube I8 is much larger than that of the relay I1, and each condenser 1|, 12, is given such a capacity that the time constant of the circuits 13-1|,

13--12 exceeds the duration 2t',7 of a code sign', whereas the time constant of I1-1 I, I1-12 is lesser than this duration, consequently, the quantity of electricity issued from each sign is indeed wholly stored at first in one of the condensers, and afterwards wholly discharged through the relay. This 'relay is adjusted in such a Way that its armature 14 closes one or another of the contacts 15, 16, responsively to a positive or negative discharge i0.5q which is only about one half of that normally provided by a sign. 'I'he tap 11 is adjusted on the anode source 18 so as to equalize the eifects of the signs which supply the currents -i-is and ia Lastly, the receiver comprises means to bring the natural frequency of the generator I6, 5I, into coincidence with the middle of the band selected by the filter 38 and displaced by the frequency changer 48; the statics more powerful than the signal wave will then leadthe amature of the relay l1 to its neutral position, and produce mere blanks less dangerous than would be the marks of incorrect code signs. Let us observe that the devices previously described do not yet enable the operator to ascertain this coincidence, since the natural frequency of a synchronized source is indeed a purely fictitious and unobservable entity. Special means are therefore necessary; they are based on the fact that the disturbing wave generated in the receiver under the impulses of a musical buzzer 19 takes in turn and at random all the possible frequencies contained in the band of the lter 3B; thus, the average frequency of the wave arriving at the modulator I5 is the average frequency of this lter, displaced by the frequency changer 48; if this average frequency coincides with the natural frequency of .the generator I6 to be synchronized, the average intensity of the synchronizing current i will obviously be null. Therefore, for tuning this natural frequency to the mean frequency of the receiver, we have only to close the circuit of the buzzer and to adjust the tuning condenser 60 so as to annul the synchro-` nizing current read in the milliammeter 56. This adjustment is veried from time to time, during the pauses of the signalling.

My invention is grounded on several new propl erties, mainly on the use of the index to shield the signal' against interferences; their explanation rests on the followingfremark: the synchronizing current iis proportional to the difference f between the natural frequency Fo of the generator I6 and the instantaneous frequency Fo-l-f of the received wave, -which may be the resultant of the signal and disturbing waves; during each code sign, this current produces a quantity of electricity st red in the condenser 1I or 12, and measuring th impulse imparted to the indicator I1; on the other hand, the received wave gets during the s'ame sign a shift with respect to the fictitious wave whose frequency would be the natural frequency F of the generator; as i and j are proportional, so are q and p;. consequently, the impulse imparted to the indicator can also be measured by the shift ip.

When the received wave is merely the signal wave free from interferences, its frequency is for instance Fn-i-fs during a code sign. The synchronizing currentr keeps thon a constant intensity is, represented by the line 1m of Fig. 15, in which illm designate the maximum intensities and ile the intensities above which the tube I8 acts as a limiter. The impulses imparted to the indicator, at 'the end of the sign, may be measuredy at will by the area comprised between the line 1m and the time axis,

or by the shift This effect, hereinafter called normal effect, is therefore proportional to the index m. As already pointed out, the relay I1 is adjusted in such a way that one of its contacts 15, 16 operates responsively to positive impulses exceeding ;LO.5m1r, and the other contact to negative impulses lesser than 0.5m1r.

As illustrated by Fig. 16, the Wave arriving at the modulator I is usually the resultant R of the signal wave S and of a disturbing Wave D whose amplitude varies according to some curve d; such a disturbance may be produced b y the cumulative action of `the Statics on the filter 38 of the receiver; it can be more or less powerful, which will. be represented by placing successively the signal wave at different levels si s2 sa s4. The timev constant of the lter 38, i. e., the reverse .1 2f1 v of its band Width, is about (m4-2) times smaller than the duration 1 2f0 of each code sign, since consequently, the phases observed at two instants t1 i2, Such as are entirely independent of one another, and

the wave d can undergo during each sign (m+2) phase charges varying at random between 1r and +r, with respect to a wave. which would preserve the natural average frequent Fo of the receiver. In other words, during every time interval ti t2, the wave d may get any mean fre quency comprised between (Fo-f1) and (Fu-l-fi), limits of the selected band; relatively to the signal vector- S of frequency Fo-l-fa, the vector D can thus turn either in the ypositive direction :c at any speed comprised between 0 and (f1-Hi), or in the negative direction y at any speed comprised between 0 and U14-fi). That being granted, and recalling that an incorrect lsign would Abe recorded if the normal effect -l-mnwere accidentally replaced by an effect lesser than 0.5m1r, we may determine as follows the' index m to be adopted for removing this lrisk of inaccurate marks: l

As long as the signal wave overtops the disturbing wave (level s1 of Fig. 16), the resulting vector R oscillates at random on both sides of the vector S; the angle a of these vectors is varying with the ratio S but cannot exceed As the flnal shift of the lvector S at the en d of the code sign is -l-mvr, that of .the received wave is comprised between L vectors D, S do not pass or pass through phase opposition. The Fig. 17 illustrates the first case; the small oscillations that vector R performed with respect to S before the instant t3 are merely resumed after t4 without ever reaching the phase difference 1r; the large amplitude momentarily got by the disturbing wave has therefore no influence upon the final shift of the resulting received wave. Fig. 18 represents the other case: D comes to opposition whilst it is more intense than S, whereby R gains or loses an` angle 21r, and its nal shift at the end of the sign becomes mvr-l-21r; to lavoid any risk of incorrect marks, the index must then comply with the condition m1r-21r 0.5m1r, or m '1.33; in fact,a n index m=2 becomes necessary on account of the pendular oscillations preceding t3 and following t4. The Fig. 18 proves also that vector R turns more quickly than the disturbing vector D, chiefly at recticn which is to be imparted for maintaining the generator I6 in locked synchronism with the received Wave R reachesthus very high values,

implying large changes u (Fig. 19) in the synchronizing current; this is quite obvious since the area circumscribed by the hook u represents the large phase shift 21r reached in the very short time Z; as the intensity of the impulse u is considerable, its action upon the indicator will ybe greatly reduced when limited at the level IIe by the tube I8.

One may furthermore observe that the vectorv D can pass through the phase opposition of S by turning either in the positive direction or (in the negative one y; in the first case, whose probability is px, the received wave gains 2n relatively to the signal wave; in the second case, of probability py, it loses 21r; now, px and py can be calculated in function of the time interval Z, since the possible` frequencies of the disturbing vector D with respect to the signal vector S are known; this gives the average value 2mm-py) of all the possible accidental impulses which could be undergone by the indicatorf said average value. proves to be the same one as if the syn-y chronizing current were annulled during the time interval Z; this is easily explainable, since during natural frequency Fo of the generator i6.

When the disturbing wave d rises frequently above the signal level duringa code sign, as seen at sa (Fig. 16), it may happen that the resulting vector Rv which represents the received wave loses several times the angles 21r with respect to the vector S; 1c being an integer, the final impulse becomes then m1r-2lc1r; to avoid the mark of an inexact sign, one must comply to the condition mvr-2lc1r -0.5m1r, that is increase again the index; this increase is however limited, as the larger 7c is, the smaller is-its probability. As an extreme case, we may during local thunderstorms consider the statics which -overtop the signal wavelevel (S4, Fig. 16) during the whole of a code sign; the recording of this sign is obviously impossible, since the generator i6 falls under the exclusive control of the disturbing wave, but false marks can still be avoided: relatively to the fictive wave of frequency Fo, the wave d undergoes invdeed (m-f-Z) phase changes varying at random whose ratio to 0.5 m1r decreases according as the index m is increased; with about m=10, the

chance of a final shift exceeding 0.5 mr is very small; in other words, the powerful staticswill almost certainly bring the indicator to its neutral position and produce merely blanks visible at the rst glance.

To sum up, the present system narrows the communication band and almost suppresses the y prejudicial components outside of this band. The interferences weaker than the signal wave becomes harmless. Most of the intermittent strays do not act upon the indicator, while-the effect of the others is enlarged; these combined rarefaction and enlargement of the disturbing impulses constitute a new and favourable condition for passing the detected current through a limiter.

The very strong and durable Statics, as well as thel fadings, annull the mean intensity of this current, whereby the correction of errors is facilitated. These results are reached by associating a frecomes more dimcult, wi

quency modulated tron, in which the index is increased according as the signalling bea receiver in which the mean frequency of theselected band, the mean frequency of the communication bandi and the natural frequency of. a synchronized generator are maintained in exact coincidence. The indicator is preferably energized by the quantity of electricity supplied by the synchronizing current during each code sign.

As illustrated at Fig. 11a, the indicator I1 could of course be directly moved bythe synchronizing current amplified by the tube I8; but the above explanations prove that this device is less commendable, chiey concerning the conditions. required for obtaining blanks instead of false signs responsively to powerful. statics: as well known, the time constant of the relay Il and condenser 8l circuit can scarcely exceed the third of the duration of each sign; otherwise, the recorded marks would 'be lengthened and indistinct. Thus, the comparison of the phase shifts due to the signal and interfering waves rests on a time interval three times shorter than that obtained with the device of Fig. 11; consequently, a three times larger index becomes necessary to avoid incorrect signs; owing to the relation f1= (m+2)fo the width of the communication band and of the filters must grow approximately in the same proportion, whereby the ether is badly utilized and the signal to noise ratio loses about 5 decibels at the output of the lter.,

Various minor modifications could be introduced in the above description without departing from the scope of my invention, for instance: The modulator l5 which supplies the synchronizing current may be balanced, as disclosed in the Patent 1,450,966 of Aifel, dated April 10, 1923; this suppresses the permanent component Ia of the detected current. The coincidence of the mean frequency of the signal wave, the mean frequency of the receiver and the natural frequency of the synchronous generator, could be ensured by combining two frequency control systems, the one based on the Patent 1,712,051 of Round, dated May 7, 1929, for neutralizing the frequency drift of the received wave with respect to the filter 38, the other based on the Fig. 4 of my Patent 1,990,428, for neutralizing the frequency drift of the generator I6 respectively to the received wave. As illustrated by Fig. 20, the generator I6 can also be maintained in locked synchronism with the received wave, by means of a direct coupling to the r. f. circuit 82 of the receiver; then, the indicator I1 is moved by a detected current issued from the coaction of the synchronized Waves in a detector 83 placed outside of the synchronizing device: this modification, disclosed by J. B. Woodyard in the Proceedings I. R. E. of May 1937 (pages 612- 619) comes merely to reproduce the image of the synchronizing current in an auxiliary circuit;

when correctly realized, it leads therefore to the same results as my Patent 2,040,980; yet, one

qualities may be more or less injuredwhen the image reproduced in detector 83 is unfaithful. for

'instance if the upungs u, u, are not purely inductive. y

The method above described for annulling the detected current responsively to the strong statica, whereby errors at the recording become mere blanks, is one of the possible applications of a general rule which may be stated as follows: in a receiver managed so as to produce responsivelyl to statics o r strays a detected current at random positive and negative, the accumulative eect of these impulses in a final device of given time constant is decreasing according as the ratio of this time constant to the time constant of the filters preceding the detector is increased, provided that the amplitude of these impulses be limited at a constant level, for instance that (S) reached by the signal current. 'I'his is easy to prove: lc being the ratio of the final time constant to the time constant of the r. i'. filters, the nal device stores k disturbing impulses at random positive or negative; in other words, the average duration of each impulse, and therefore the quantity of electricity produced by this impulse whose amplitude is S, are inversely proportional to k; consequently, the final accumulative effect of the k consecutive impulses varies as and may be reduced at will by increasing the ratio lc. Of course this method does not suppress the effect of the Statics, since the intermodulation occurring in the limiter causes the signal to disappear, the whole of the received wave (signal strays) producing a blank, but this blank is preferable to an important variation of the detected current and chiefly to the mark of an incorrect code sign. Though yetl new, this method can be used in every receiver managed so as to produce responsively to strays a detected current at random positive and negative. For instance, the synchronizing device disclosed in my Patent 1,976,877, dated October 16, 1934, may be improved by inserting anamplitude limiter between the r. f. filters tuned-to the signal wave frequency and the low pass filter selecting the synchronizing current. Another example is illustrated at Fig. 21, comprising two balanced detectors 86, 81, fed by the lters or resonators 88, 89, respectively tuned to the frequencies Fo-l-fs and Fn-fs representing the code signs; owing to this arrangement, the potential difference between the connexions 90, 9i, is systematically positive or negative responsively to the signalling, whereas it is at random positive or negative responsively to strays, for cause of symmetry; by limiting at 66, I8, the accidental fluctuations of the detected current, and accumulating the quantity of electricity resulting therefrom in a final device 92 which may comprise the elements 13, 69, 1|, 10, 12, I1 of Fig. 11,'the statics will produce mere blanks, provided that the time constant of the filters 88, 89, be small enough with respect to the duration of each code sign. Thissimplifled receiver enjoys therefore some of the qualities which characterize'that of Fig. 11.

Having thus described my invention and the operation thereof, what I claim is:

1.l In a receiver which comprises a demodulator, means for receiving electric waves, a local source of electric oscillations synchronized with the received electric wave by means of an interference current produced by beating the synchroni'zed oscillations and received electric waves in the demodulator, the intensity of said current being null when the received wave frequency coincides with the natural frequency of the source of oscillations, a system for minimizing the eil'ect of disturbances on the interference current which comprises, in the input circuits of the demodulator a selector tuned to the received wave frequency, in the output circuits of the demodulator a device of time .constant much larger than that of the selector foi accumulating the fluctuations thirdly the changes imparted to the frequency of Athe signal wave for individually distinguishing these signs, the method of protecting the signal against disturbances which comprises at the receiver demodulating the signal wave so as to derivefrom each code sign a current of intensity substantially proportional to the frequency change representing this sign and further accumulating this current over a space of time proportioned to the duration of this sign, whereby the effect of each code'sign is proportional to the index, ratio of frequency changes to signs frequency, and proportioning the transmission characteristics within the communication band so as to increase this index according as disturbances increase in intensity. l

3. In frequency modulated telegraphy wherein transmission characteristics are the maximum Width assigned to the communication band, the code signs frequency proportional to the keying speed, and the frequency changes imparted to the signal wave for individually representing these signs, the ratio of frequency changes to signs frequency being hereafter called index, the method of assuring under all conditions the highest keying speed compatible with the width of the communication band which comprises demodulatlng the signal wave so as to transform its frequency changes into substantially proportional changes in the intensity of a current and deriving from this current responsively to each code sign an accumulation effect extending over the whole duration of this sign, whereby the index m'ay be decreased since a same index will supply a larger effect than when the accumulation time constant is a small part of the signs duration,and further proportioning the transmission characteristics by varying the frequency changes inversely to the signs frequency so as to utilize at all keying speeds the maximum band Width assigned to the communication, whereby the signs frequency may be increased since appearing at the denominator of an index which is made smaller owing to its larger effect at the receiver while its numerator is varie-dat the emitter inversely to this sign frequency.

4. In frequency modulated telegraphy wherein transmission characteristics are the maximum communication band Width, the code signs frequency proportional to the keying -speed and the frequency changes imparted to the signal wave for individually representing these signs, 4and. wherein the receiver is arranged to derive from each code sign an accumulation eifectsubstantially proportional to the index, defined as the vratio of frequency changes to signs frequency,.

the method of obtaining a rapid increase` in this effect according as conditions require reduction of the keying speed'which comprises proportioning frequency changes to keying speed so asto use a small index under favorable conditions allowing' the maximumkeying speed. and propor- I tioning the amplitude of frequency changes inversely to the keying speed for preserving at any speed a predetermined communication band width.

5. In frequency modulated telegraphy wherein code signs are transmitted at a given cadence and translated into dennite changes in the frequency of a signal wave, a receiver which comprises means for collecting the signal wave, means for transforming the frequency changes of the collected wave into changes lin the intensity of a current, a circuit of time constant larger than the duration oi' each code sign, an indicator, and a local distributor synchronized tothe cadence of signs for successively accumulating in the circuit of large time constant the electricity supplied by the current over the whole duration of each sign and then transferring this accumulation effect to the indicator.

6. Av method of telegraphy which comprises employing a three signs code, respectively representing these signs by three approximately equidistant frequencies of the signal Wave whereby" the mean frequency of this wave represents a sign and the two symmetrically distributed frequencies represent the other signs, demodulating the signal Wave so as to transform its changes with respect to the mean frequency into corresponding changes in a current with respect to a zero intensity, recording each sign by means of the accumulation effect supplied by the current over Vthe whole duration of this sign, and proportioning this duration to the disturbances met at the receiver.

7. In wireless telegraphy wherein signalling consists in radiating a signal Wave of constant amplitude and individually representing vcode signs by imparting to a characteristic of this Wave definite changes with respect to a predetermined mean value of this characteristic, the method of reception for facilitating correction of errors by deriving from bothstatics and fading the same effect and placing this effect approximately in the middle of the effects normally due to code signs, which comprises proportioning the time constantsof circuits preceding the demodulationy to be substantially smaller than the duration of each code sign whereby oscillations caused by tions the intensity fluctuations of these currents, andv deriving from the limited currents'an laccumulation eect extending over the whole dura-` tionA of this sign, whereby the code signs supplyw normally ilnal effects of directions in accordance with these signs,` while Statics' dominating tlietzifn signal wave substantially annui this effect similarly to fading, since the mean intensity of currents of limited intensities and of directions changing many times is necessarily small com.

paratively to the mean intensity of currents of definite directions.

8. Atelegraph system for facilitating correction of errors by deriving from strong statics and fading a similar predetermined effect and placing this eect approximately in the middle of the eilects individually representing code signs which comprises at the and radiating a signal wave of constant amplitude, means responsive to keying for varying a characteristic of this wave with respect to a predetermined mean value of this characteristic, and means for proportioning the keying speed and thus the duration of each code sign so as to provide a signs frequency substantially smaller than the maximum width assigned to the communication band; at the receiver. filters of band width adjusted to this maximum communication band width whereby it follows that the time constant of said filters is substantially smaller than the duration of each code sign as proportioned at the emitter and that oscillations forced by statics in filters assume over this duration several independent characteristics succeeding each other at random, a demodulator balanced with respect to the mean value of the variable characteristic of the signal wave whereby each code sign is normally translated into a detacted current of definite direction in accordance with this sign while oscillations forced by statics dominating the signal wave are translated into detected currents of directions changing at random several times during each code sign, means for limiting in both directions the maximum intensities of the detected currents whereby the mean intensity of limited currents whose directions change many times at random is necessarily small comparatively to the intensities of currents definitely directed according to code signs, and a distributorsynchronized with the signs frequency for successively accumulating in a device of large time constant the effect of the detected current over the whole duration of each code sign and then transferring this accumulation eiect to the indicator.

9. A telegraph system for facilitating correction of errors due to both static and fading by deriving from these disturbances a similar approximately constant effect, this effect being the average of the extreme effects due to code signs, which comprises at the emitter, means for generating a signal wave, means-responsive to keying for varying the frequency of this wave with respect to a predetermined average value of this frequency whereby code signs are individually represented by frequency changes of definite directions, and means for proportlonlng the duration of each sign for providing a signs frequency substantially smaller than the width of the communication band; at the receiver, a illter of band width adjusted to the communication band widthl whereby it follows that the time constant of this filter is substantially smaller than the duration ascribed to each code that oscillations caused by static dominating the signal wave assume in turn during a sign several independent frequencies' distributed at random about the average frequency of the filter, drift neutralization means for maintaining the average frequency of the signal wave in coincidence with the average frequency of the filter,

emitter, means for generating sign at the emitter and' a demodulation device for transforming frequency changes of the received waves into changes in the intensity of a detected current. the said device being balanced with respect to the average frequencies both of the filter and of the signal wave whereby the definite frequency changes due to signalling supply currents of definite directions in accordance with the signs represented by these changes while frequency changes undergone at random by oscillations due to strong statica supply currents of directions reverting many times during each sign, means for hunting in both directions the intensity changes of the detected current, a circuit of time constant substantially larger than the duration of each sign. an indicator, and a. local distributor sychronized with the signs frequency for successively accumulating in the circuit of large time constant the effect of the detected current over the whole duration of each sign and then supplying this accumulation effect to the indicator.

10.The system as claimed in claim 9 characterized by the fact that said means responsive to keying impart in turn to the frequency of the signal wave twovchanges substantially symmetrical with respect to the mean frequency of this wave for respectively representing two code signs. whereby the accumulation effects supplied to the indicator of the receiver responsively to these signs are of opposite directions while the accumulation effect of strong statica and fading is approximately null.

11. 'I'he system as claimed in claim 9 characterized by the fact that said means responsive to keying impart in turn to the signal wave three frequencies, one of them coinciding with the mean frequency of this wave and the two others symmetrical with respect to this mean frequency, whereby the adcumulatlon effects supplied to the indicator responsively to these signs are, the one null and the othersof opposite directions.

12. The system as claimed in claim 9 characterized by the fact that said demodulation device balanced with respect to the average frequency ofthe ilter for transforming frequency changes into current changes consists in two resonators respectively tuned to two frequencies symmetrical with respect to this average frequency and feeding two detectors in opposition.

13. The system as claimed in claim 9 characterized by the fact that said demodulation devic`e balanced with respect to the average frequency of the nlter consists in a local generator of oscillations tuned to this average frequency, this generator being maintained in locked synchronism with the received wave and coacting with this wave in a demodulato 14. The system as claimed in claim 9 characterized by the fact that said means for limiting in both directions the intensity changes of the detected current comprise in combination means for substantially stabilizing the amplitude of the received wave at the input of the demodulation device and limiters in the output of the said device for limiting instantaneous changes caused in the intensity of the detected current by the rapid frequency changes undergone by this received wave responsively to strong static.

HENRI JEAN JOSEPH MARIE Dl REGNAUID DI BELLEBCIZE.

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