US2023556A - Frequency selective communication system - Google Patents

Description

y 1935 J. s. STONE FREQUENCY SELECTIVE COMMUNICATION SYSTEM Filed 001:. 3, 1931 4 Sheets-Sheet 1 I Q' Q H Q s3 m -w INVENTOR John/Stone Star/w A ATTORNEY Dec. 10, 1935. J S STONE 2,023,556

FREQUENCY SELECTIVE COMMUNICATION SYSTEM Filed Oct. 5, 1951 v 4 Sheets-Sheet 2 V I I 427331} I 80.6.7.52 I I INVENTOR Jo l/u/o Stone 510m BY W ATTORNEY J. S. STONE FREQUENCY SELECTIVE COMMUNICATION SYSTEM ec. w, 1935.

4 Sheets-Sheet f5 Filed 001:.v 5, 1931 INVENTOR Jofiu/z/ Sjone Stone BY ATTORNEY Dec. 119,1935. J s STONE v 2,023,556

FREQUENCY SELECTIVE COMMUNICATION SYSTEM Filed Oct. 5, 1931 4 Sheets-Sheet 4 ,a M" 3 44 0 A 2' 0 1) gnb o 1' w M M w INVENTOR Lia/em Sim Le Stole e BY ATTORNEY Patented Dec. 10, 1935 UNITED STATES PATENT OFFICE FREQUENCY SELECTIVE COMMUNICATION SYSTEM Application October 3, 1931, Serial No. 566,741

14 Claims.

An object of my invention is to provide a system of communication by means of electric waves with economy of the frequency range of the waves. Another object is to provide a highly selective electric wave signaling system with substantially undistorted operation within a certain range of wave frequency and with substantial exclusion of effects outside that range. In the following specification I disclose a limited number of ways in which my invention may be practiced. It will be understood that the following text relates principally to these specific examples of procedure under the invention and that the scope of the invention will be indicated in the appended claims.

Referring to the drawings, Figure 1 is a diagram of a receiving system with tWo frequency maxima, Fig. 1a is a corresponding curve diagram, Fig. 2 is a diagram of a receiving system with four frequency maxima, Fig. 2a is a corresponding curve diagram, Fig. 3 is a diagram of a receiving system with two frequency maxima and with four stages of tuning and intermediate amplification for each of the two frequencies, Fig. 4 is a diagram of a receiving system with four frequency maxima and with four stages of tuning and intermediate amplification for each of the four frequencies, Fig. 4a is a corresponding curve diagram, Fig. 5 is a diagram which may be compared with Fig. 3 by stating that crystal circuits have been substituted for the four stages of tuning and intermediate amplification of Fig. 3, Fig. 6 is a diagram of a transmitting station having certain advantages, Fig. '7 is a diagram of a more usual type of transmitting station for comparison with Fig. 6; Fig. 8 is a curve sheet of operation for a receiver like Fig. 2 or Fig. 4 receiving from a transmitter like Fig. 6, Fig. 9 is a similar curve sheet for a system involving a certain simple modification of Fig. 2; Figs. 10 and 11 correspond respectively with Figs. 8 and 9, but are for speech and music instead of telegraphic signals, Figs. 12 and 13 are performance curves for the receiver of Fig. 2 respectively without and with alternately reversed connections in its audio output circuit, Fig. 14 is a detail scale notation diagram, Fig. 15

a curve diagram further illustrating the perfcrmance shown in Fig. 12, and Fig. 16 is a diagram of a complete transmitting and receiving system involving certain principles discussed in connection with the preceding figures.

The number of channels of radio communication securable within a given range of frequencies is ultimately limited by the width of the band of frequencies which have to be transmitted in order to secure the requisite clarity of the signals received. This is because the selectivity of the receiver may only be pushed to a point beyond which the extreme frequencies of this band of essential frequencies are suppressed or materially discriminated against in the receiver. This fact is so well understood that it need not be further elaborated here.

It is also well known that the band of essential frequencies consists of two side bands, one of which may be suppressed and transmission of the 1 signals effected with the remaining side band, thus halving the width of the band of frequencies essential to transmission and so doubling the ultimate possible number of channels which may be successfully used. 15

It is further well known that the selectivities of the receivers may be increased through the use at the receiver of a succession or chain of similarly tuned resonant circuits, each such circuit being coupled with the next preceding circuit through a one-way relay, and each successive circuit being carefully screened against inductive action from without itself.

Under all circumstances, it is necessary, in order to make full use of the radio frequency range, that transmitters radiate bands of frequencies only wide enough to secure clarity of reception of the transmitted signals in adequate receivers, and to secure this desideratum band filters are used at the transmitter which preclude the radiation of any but the desired band of frequencies. In the discussion which follows, it will be assumed that the transmitters used are so equipped.

It is my purpose to disclose a radio receiver 35 which shall exhibit a great power of excluding frequencies in the vicinity of those of the band to be received, as well as a great power of excluding frequencies remote from those of that band.

It is also my purpose to disclose means which will 40 not only permit of the use in radio receivers of a cooperative chain of selective unit circuits of the maximum selectivity heretofore attained in such unit circuits, irrespective of the frequency of the carrier waves employed, thus obviating the seri- 45 ous limitations to the selectivity of the unit circuits of a chain of such unit circuits; but also to disclose how such improved means may be employed to permit of the use at radio receivers of selective units of far greater selectivity than that 50 attainable in resonant electric circuits. Reference is here had to crystal resonators.

My invention may be realized by a procedure which consists in the use of independent and differently attuned selective units for the recep- 55 tion of different parts of the signal frequency band; in combining the audio frequency output currents of these several receiving units in the circuit of the ultimate receiving telephone; and in so choosing the relative frequencies, selectivities, and receptivities of the independent selective units that the arithmetical sum of the audio subscript 0, let the frequency'of maximum re-' sponse be f0 and let 1 be any othenfrequency. Then the frequency function 0 :0 is defined by the equation Similarly for any other tunedcircuit designated say by the subscript I; in this case j The current intensity at various frequencies in a tuned circuit designated by subscript 0 will be given -by.an equation of the form Io=F(f) and therefore by an equation of the form I0=F'(ao). The function F may be transformed so that and this equation defines So, which is called the selectivity of the circuit designated by the subscript 0.

A simple form of receiver is shown in Fig. 1, but it cannot be completely illustrated in a circuit 7 diagram, the receiverinvolving as it does the prei ference. V

circuits'attuned to frequencies f1 and f2 respeccise proportionment of the. several selective parts in accordance withthe principle of the invention.

In this diagram (l) and (2) are two equal antennas whose only direct association is the coupling M which is carefully designed to neutralize the mutual inductive effects of the antennas and so render them mutually independent of each other or conjugate, in order thata current in one will not engender a current in the other. This element permits the plural antennas of the receiving stations disclosed in this specification to be juxtaposed without engendering mutual inter- Circuits A and Bare two resonant loop tively. D1 and D2 are'detectors which translate the radio frequency currents of theloops" A and. B into audio frequency currents corresponding to the fluctuations of amplitude of these radio frequency currents. The devices: marked Q are sources for supplying current 'of carrier frequency to the detectors. associated with the audio frequency side of the detectors D1 and D2 that it"responds to the;

arithmetical sum of the outputcurrentsjof these detectors.

The antennas tivity to waves of their respective frequencies f1 andfa'as are also the selective circuits A and B. The detectors Drand D2 are equally responsive to the E. M. F.s impressed upon them by the loop circuitsA andB respectively. The transformers T1 and'Tz are connected in such senses as to cause the equallyintense audio frequency isless than of that of curves I and 2.

sides giving a less favorable receiving character- R is a telephone receiver so' '(I) and (2) are of equal recep currents delivered by the detectors D1 and D2 to be arithmetically additive in their eifects upon the ultimate telephone receiver R.

The mode of operation of this organization will be best understood by having reference to the curves'of Fig; 1a. Curves l and 2 have intensities of direct current developed in the output circuits D1 and D2 respectively, for ordinates, and for the abscissas they have the values of Sue,

' where S is the common selectivity of the resonant loop circuits A and B, while an is the frequency function of the carrier frequency.

7 Curves l and 2 are the receiving characteristics of component receivers lAD1T1 and 2BD2T2 respectively, i. e., they represent the intensities of the direct current outputs of these two component receivers for varying frequency functions of the received waves over the frequency function range of ao=:9/S, and for varying frequencies of these received waves over the frequency range given by the expression ceiver R, so that the receiving characteristic of the complete receiver is given by the sum of these curves, that is, by curve 3, Fig. 1a.

It will be observed that the receiving characteristic is substantially flat over the range Sao=:0.9. Over this range the characteristic does not differ by quite 1% from the mean. For a selectivity of 200, this corresponds to'a frequency function range of a= -0.0045, or a frequency range of I'=(1.0000025303:0.00225) f0. If 1 this band is to be 10,000 cycles wide with a selectivity'of 200, the carrier frequency must be about two and a fifth megacycles.

If the extreme frequencies of the band may be permitted a lossof 17%, then the width of the band becomes Sa:il.5, which gives a frequency range of 0.00750fo. When this band is 10,000 cycles Wide, the carrier frequency is one and a third megacycles.

Theamplitude curve of a simple resonant circuit that gives a characteristic in which the extreme frequencies of the signal band of 10,000 cycles are 17% less intense than the central or carrier frequency is plotted for comparison in curve 4, Fig. la. In this curve A, the selectivity istic than that of curve 3 over the signal frequency bandto be received, the circuit of curve 7 4 is not nearly so exclusive of frequencies beyond tiply the number of equal and independent component receiving units whose output currents are combined to actuate a common or ultimate telephone receiver. Thus Fig. 2 illustrates the case of four equal component receivers combined to forma single receiver with a favorable receiving characteristic over more than double the width of frequency band received by the receiver of Fig.

1. In this-diagram all of the resonant receiving units or component receivers have the same selectivity and receptivity and their equal intensity outputs are additive in their efiects upon the common and ultimate telephone receiver R. The reversing switches S1, S1, S2 and S2 are for a use that will be mentioned in connection with Fig. 6. The four equal antennas are so associated, through the intermediary of the mutual inductances designated by the Ms, as to neutralize any natural inductive effects between them.

The theory of operation of this organization is illustrated in curve i, Fig. 2a. This curve is obtained as the sum of four curves equally spaced along the axis abscissas, each of the character of curves l and 2 of Fig. 1a. They are separated by the interval 80:0:4, which when S=200 corresponds to a frequency function interval of 0.08, and which in turn corresponds to a frequency interval 0.0110.

Curve l on Figs. 2a has the scale of the ordinates so reduced as to give a receiving characteristic of approximately unity maximum intensity. This curve gives the receiving characteristic of the organization of Fig. 2. It varies over a range of about 7% in a range of Soco=i3. For 8:200, this corresponds to a frequency function range of 0.03 or a frequency range of 0.015Jo. If this range is 10,000 cycles, then the carrier frequency f0 is about 670,000.

In the curves of Figs. 1a, 2a, and 4a the posit on on the Sum scale of the resonant units or component receivers is marked by an .r on the scale of abscissas. Curve 2 on Fig. 2a shows the receiving characteristic secured by a chain of four resonant circuits alternately disposed with amplifiers, each such circuit being of selectivity 51:0.94358. and having a receiving frequency band of about the same width as that of the receiver of Fig. 2. Two such chains are shown in Fig. 3, one from A1 to A1' and another from A2 130 A2.

An inspection of the receiving characteristics shown in curve 3, Fig. 1a and curve I, Fig. 2a shows that the parallel resonant unit receiver is capable of giving a fiat top receiving characteristic with very definite cut-offs, so that this type of receiver tends to exclude nearby frequencies to a degree not shared by the series resonant unit receivers of the chain type, though the latter tend to give a much better exclusion of remote frequencies. This latter advantage of the series unit receivers is very marked. The four resonant unit series receiver which has nearly as good a receiving characteristic as curve I, Fig. 2a, over a band of width Soco=i3.5 is one in which the resonant units have the common selectivity 81:0.09435368 and a receptivity of 1.01227. Such a chain has a receiving characteristic given by curve 2, Fig. 2a. This curve has a maximum intensity of 1.05 at Sozo=0, an intensity of 0.9 at Soo=i3, an intensity of 0602+ at Sue -i6, and an intensity of 0354+ at Sao=+9. However, at the remote frequency corresponding to Suo=l00, the parallel arrangement of curve I, Fig. 260, has an intensity of about 0.0201, while the series arrangement of curve 2, Fig. 2a, has an intensity of but about 0.0013.

A careful study of these two systems of combining resonant units shows that they are complementary, each possessing in the greater degree the advantages possessed in the lesser degree by the other. I propose their combination as a possible means of producing the ideal type of receiver having a flat-topped receiving characteristic with sharp cutoffs and great powers of excluding not only remote, but also nearby frequencies.

Such suggested combinations are illustrated in Figs. 3 and 4. In Fig, 3 is illustrated a system composed of two chains, each consisting of four resonant circuits disposed alternately with amplifiers, these two chains combined in parallel so as to act additively on the ultimate receiver R. In this diagram, we have the equal antennas I and 2 and the chains of equal resonant circuits A1, A1, A1", A1" and A2, A2, A2", A2' in which the elements are associated together through one way relays designated by small squares each enclosing an arrow which indicates the direction of action of the relay.

Without further discussion of this case, we may pass to that of the organization of Fig. 4. Here we have four chains of equal resonant circuits and alternately disposed amplifiers combined in parallel so as to cooperate in actuating the ultimate telephone receiver R. The reversing switches S1, S2, S3 and S4 are for a use that will be mentioned in connection with Fig. 6. The receiving characteristic of this organization is illustrated in curve i on Fig. 4a. Here we have a very favorable characteristic over the band of width Soco=j -1.75. This receiving characteristic is compared with curve 2, Fig. 4a, which is that of a receiver consisting of a chain comprising 16 resonant circuits, each of which has a selectivity of 81:0.0657945'. This chain receiver has about the same width of band as that of Fig. 4 but is far from having the power to exclude nearby frequencies possessed by the latter.

It is seen from the foregoing that the combination of the parallel and series chain of resonant units overcomes the disadvantages of those types of chains as well as combining their advantages.

The system of Fig. 3 has 2 chains of 4 stages each, 8 resonant units in all. Similarly, the system of Fig. 4 has 16 such units. It will be instructive to show what advantage there might 'be in putting all these 8 or 16 units in a single chain.

If the resonance characteristic of each tuned circuit of the series of such circuits be expressible as (1+S where S is the common selectivity of the several circuits and or is their frequency function, and if the intercircuit screening be assumed to be perfect, then the resonance characteristic of the receiver, when n such circuits are used, will be expressible as (1+S If the receiving characteristic of the receiver is to be such that the intensities of the extremes of the receiver band are to be of the intensity of the central of carrier frequency, then we have (1+S =1/m, where ia are the values of the frequency function a for the extreme frequencies of the transmitted band. This gives S: (m "l) /a1, from which it is seen that the selectivity of the component circuits is strictly limited by the Width of the band to be received without excessive distortion. For purposes of illustration we may consider the case in which the frequency of the transmitter is corresponding to a wave length of 3000 meters. Then if the signal band transmitted be 10,000 cycles wide, we get oe1=0.09762. If we permit l/m to be as small as 0.80, and if we take n=2, then S=5.122. This illustrates carrier frequency. It shows that the common selectivity of the two resonant circuits may not exceed 5.122 or about 2.5% of the most usual s e- 1Q lectivity of a tuned radio circuit andabout 1.25%

of that of the highest attainable selectivity of such circuits. If the chain in this case consisted of four equal resonant circuits instead of two such circuits, then n=4, and we get from our ex- Tpression 8:3.519 or about 1.75% of the usual selectivity of tuned radio circuits andabout 0.88% of the best attainable selectivity of such circuits. In order that the chain of n equal resonant circuits may-make. use of the most selective pos- Esible circuits of the type used in radio, the relation 1r1 (m i 1)/1.6 10 must subsist. Again taking l/m as of the value 0.8, and the width of the signal band as 10,000cycles, we find, for a chain of two circuits that the-carrier frequency :must be about 8x10 which corresponds to a wave length of 37.5 meters. Taking the same values of m and of the width of the signal'band, but assuming that there are four equal resonant circuits in the chain, then the carrier frequency :must be about 11.5 10 correspondingto a wave length of about 26.1 meters, in order that the most selective circuit may be made use of.

In these examples the width of the signal band is excessive from the point of view of the more usual modern practice, but the width of the effective signal band inradio has been progressively broadened in the past and will doubtless be still furtherbroadened in the near future, particularly in specializeduses of radio. However, the '5prime reason for using a wide signal band in these examples is to bring out most clearly the effect of the width of thisband onthe selectiveness of the receivers and their component circuits. a Further to illustrate the point of the foregoing remarks, curves 3 and 4 have been added on Fig. 4a. In each of the cases represented by curves 2, 3 ands, the equal and effective receptionof a frequency band whose width is given as is predicted. These curves permit the comparison between the receiving characteristics of chains having respectively 4, 8 and 16 equal resonant 55 units.

Individually these curves show that the chain (associated receivers permits or the use of verymuch more selective units'than wouldotherwisebe possible. Though resonant circuits attuned to radio frequencies may, through the exercise of great care, in their construction, attain a selec- 70 'tivity of about 400, a far greater degree of selec tivity, in fact a. selectivity of a higher order is attainable through the use of suitably cut crystals such as are used in stabilizing the frequency of,

transmitters.

15 Ordinarily the use of so high an order of selec- 1 I tivity as is provided by the resonant crystal would give the receiver an impracticably narrow band of received frequencies, but through the use of parallel associated receivers of the type illustrated in Figs. 1 and 2, the width of the received band of 5 frequencies can be made as great as desired.

Fig. 5 illustrates a mode of connection by which crystals may be used with the receiver of Fig. 1. In Fig. 5 the letters of reference which are the same as those in Fig. 1 refer to the same elements 10 as they do in Fig; 1. The condensers K1 and K1 are preferably equal, as are also the condensers K2 and K2. The capacity of the condenser C1 is preferably equal to the effective capacity 1 of and K2 respectively. Other types of balanced bridges than that here shown may be used. For 25 frequencies different from that to which the crystal is resonant, these bridges will cause the ;detector branch to be conjugate to the loop resonant circuit so that there will be no tendency to develop a current in the detector, but when the 30 frequency of the received waves is that to which the crystal is resonant, the crystal branch presents a relatively low resistance instead of a very large capacity reactance. The balance of the bridge is thereby upset, and the detector is ener- 35 gized.

A sufiicient number of parallel resonant units must be used in each receiver to secure for it the requisite width of received frequency band. The principle involved is illustrated by Fig. 5 in which 0 the number of such parallel resonant units is only two.

In all the cases considered heretofore the improvement has resided in the receiver, and transmission has been according to the simple, familiar, 45 single band system with the use of a band-pass filter at the transmitter. I will now disclose a modified transmitter capable of advantageous use with receivers like those described heretofore, or only slightly modified therefrom.

The fundamental distinction between the transmitter hereinafter to be described and those ordinarily in use is that it transmits consecutive parts of the signal frequency band in opposite phases. Thus, if itbe elected to divide the signal frequency band into two equal parts, half of the band is subjected to phase reversal and transmitted, whereas the other halfis not so reversed before transmittal. If it be elected to separate the signal frequency band into three equal divisions, then either the central division is reversed or the other two thirds are reversed before transmittal. 1

This distinctive feature is illustrated in Fig. 6,

which depicts in a purely conventional manner, a

I radio transmitter in which the signal frequency is divided into'two equal parts transmitted in opposite phase. In this diagram, K formally represents a high speed automatically driven circuit control device, B represents a battery, R is a coupling resistance, the symbols HP and LP representrespectively high pass and low pass filters, andthe numerals above these, such as 2000 and 4000, give their cutoff frequencies. The frequency range indicated in Fig. 6 is chosen arbitrarily for purposes of illustration. G represents a high frequency generator whose modulating field coils are designated by M.

Of course, any suitable means may be used for subdividing the signal band into consecutive sections with reversed lace, and any suitable means may be used to gen the carrier frequency and to modulate it by the resulting signal frequency band with a? opposed phases.

We may assiune U the generator G, under unit excitation, gives output represented by sin 2mm, and that the key is opened and closed regularly, the durations of open ng and closing being equal and there bein n complete periods per second. An analytical study of the output from the generator G under these conditions may be made. When this output is resolved into its components according to frequency and these components are compared with the corresponding components of the output from a transmitter of the kind shown in Fig. 7, it will be found that the radio signal band from the system of Fig. 6 shows a natural division into three parts A, B and C, in order of increasing frequency. In the following discussion we shall refer to these several parts or bands A, B and C. The central division B is the same in phase as the corresponding part of. the radio frequency band developed by the transmitter of Fig. '7, while the extreme divisions A and C are of opposite sign or phase to the corresponding divisions of the band radiated by the transmitter of Fig. '7. It will also be noted that the central division B of the radio signal frequency band is twice as broad as either of the end divisions A and C.

The types of receiver adapted to utilize the radiation from the transmitter of Fig. 6 to the best advantage are those illustrated in Figs. 2 and 4. These receivers have four separate selective units in operative parallelism and might therefore seem best adapted to reception from a transmitter radiating four instead of three band divisions, but in this respect the appearances are deceptive as will become apparent later.

In the receivers of Figs. 2 and 4, the output terminals of the four detectors or demodulators are so connected in the circuit of the receiving device R as to produce in that device an arithmetically additive effect when, as is the case of the transmitter of Fig. 7, the signal band impressed upon the modulator of the transmitter is of normal phase throughout, i. e., not subdivided into band divisions of opposing phase. But when these receivers are used to receive signals from a transmitter such as that of Fig. 6, in which the radiated signal band is divided into parts, the consecutive divisions of the band having opposite phases, then the connection of the output terminals of the four detectors must be so changed as still to produce an arithmetically additive effect upon the receiving'device R. Such change may be effected by the use of the switches S1, S1, S2 and S2 in Fig. 2 and S1, S2, S3 and S4 in Fig. 4.

Since the band division B radiated from the transmitter of Fig. 6 is of double the width of the individual band divisions A and C, two selective units of the receivers of Figs. 2 and 4 are assigned to the reception of this band division, while single se ective units are assigned to the reception of the band divisions A and C. Thus, in the case of the receiver of Fig. 2, antennas 2 and 2, and their associated selective circuits are respectively tuned to the central frequencies nc-12n and respectively, while antennas 2 and 3 and their associated selective circuits are respectively tuned to the symmetrically situated frequencies no-4 and no+4 of the central band division B.

Fig. 8 illustrates the mode of operation of a receiver having four selective units in effective parallelism when receiving from the transmitter Fig. 6. In this diagram the selectivity of each unit or component receiver is assumed to be that of a simple resonant circuit. These units are equally spaced in the frequency scale and the receiving characteristic of the receiver as a whole is given by curve 5 which is seen to be not very uniform. The individual effects of the four selective units or component receivers upon the circuit of the ultimate receiving device R, are

illustrated respectively by curves l, 2, 3 and 4, of

which the characteristic curve 5 is the algebraic sum. In this diagram the abscissas are values of the frequency and the ordinates are intensities of the current components in the circuit of the ultimate receiving device B.

Fig. 9 illustrates the mode of operation of a receiver having four pairs of selective units or component receivers in effective parallelism when receiving from the transmitter of Fig. 6. In this diagram the selective units in effective parallelism are assumed to be simple resonant circuits, but they are grouped in pairs along the frequency scale, each pair taking the place on the frequency scale of one of the selective units of the organization whose mode of operation is illusted in Fig. 8. The individual effects of the r pairs of selective units upon the circuit of the intimate receiving device R are given by the curves l, 2, 3 and 4 respectively, while curve 5, which is the algebraic sum of these curves, is the receiving characteristic of the station as a whole.

The receiving characteristic of Fig. 9 is an improvement on that of Fig. 8, but both are characterized by the elimination of the frequency c- Tesponding to that at which the phase reversal tool: place at the transmitter, and a partial suppression of the frequencies in the immediate vicinity of this frequency. The receiver having receiving characteristic given in appended 9 gives a very adequate reproduction of the s gnal sequence assumed for Fig. 6, namely a succession of closures of key K with the same duration for all the closures and for all the alternate openings of the key.

In the foregoing, I have described the invention entirely with reference to the high speed telegraphic signal transmitters of Figs. 6 and 7. was done in order to make the disclosure as complete and clear as possible, but by far the meet important and at the same time the simplest annlication of the principles involved is in connection with the transmission of speech and 0 follows from this that receivers of the types whose characteristics are given in Figs. 8 and 9, may be used to receive speech and music from the usual radio transmitter without material loss of clarity to receive speech or music from the usual radio.

transmitter, then its mode of operation will be illustrated by Fig. 11. In the diagrams of Figs. 10 and 11, the abscissas are frequencies in the radio signal scale and the ordinates are intensities of effects induced inthe circuit of the ultimate receiving device R of the station as a whole.

In Fig. 10, curves' I, 2, 3 and 4 represent respectively the individual effects produced in the circuit of the ultimate receiving device R by the four selective receiving units in effective parallelism, while curve 5, which is the algebraic sum of curves I Z, 3, and 4, represents the characteristic of theresulting receiving station. Similarly,

. in. Fig. 11, curves l, 2, 3, and 4 represent respectively the individual effects produced in the circuit of the ultimate receiving device R by the four pairs of selective receiving units in effective parallelism, while curve 5, which is the algebraic sum of the curves I, 2, 3, and 4, represents the characteristicof the receiving station.

From an, inspection of the curves 5 of these diagrams, we again note the suppression of the frequency at which the reversal of the phase takes place, and the partial suppression of a nar.-

row band of contiguous frequencies. The measure which may be taken to prevent this feature of the characteristic from materially affecting the clarity and quality of the reception will be considered after I have discussed the question of the immunity from: interference secured by the reversed phase receiver.

The advantages to be gained through the use of the reversed phase receiver over that secured through the use of the otherwise identical re-- ceiver disclosedin connection with Figs. 2 and 4 is best exhibited by the two diagrams of Figs. 12 and 13.

In the construction of these diagrams, I have assumed that the receiver of Fig. 12 is of the four selective unit type illustrated in Fig. 2, while that of Fig. 13 is an identical receiver except for the fact that it has the reversed phase feature. In other words, the receivers contemplated in Figs. 12 and 13 consist of four equal resonant units in effective parallelism. In the receiver contemplated in Fig. 12, the output terminals of the four demodulators are connected together in the same sense so as to produce arithmetically additive effects upon the receiving device R of the station, while in the otherwise identical receiver contemplated in Fig. 13, the output terminals of the that the joint effects of these two'pairs of reso- V nant units upon the receiving deviceR is equal to the arithmetical difference of their individual effects.

The effect of reversing the phase in a multiple selective unit receiver is to'greatly reduce its" response to sources of interference such as radio transmitters whose frequency bands are too close to be effectively excluded by the usual selective methods.

In Figs. 12 and 13 I have, for purposes of illustration, assumed that the receivers are acted upon by a transmitter radiating a band of frequencies equal to that normal to the receivers, i. e., 3212, and separated from the receivers frequency band by a frequency interval equal in width to the width of the bands in question. The relation in the frequency scale of the interval between the interfering frequency band and the band normal to the rec eivers is illustrated on a small scale in Fig. 14. The interfering transmitter is assumed to be of the usual type, but radiating a band of frequencies having uniform intensity.

' In Figs. 12 and 13, curves 1, 2, 3, and 4 indicate the individual responses of the four selective units in eifective parallelism as impressed upon the receiving device R of the station, while curves 5, which are the algebraic sum of their respective curves l, 2, 3, and 4 show, in each case, the resultant of the response of the four selective units as impressed on the ultimate receiving device R of the station.

A comparison of the curves 5 of these two diagrams serves to show the great disparity in the intensity of the response of the two stations in question to the interfering transmitter. We see that the response of the receiver having the reversed phase feature is but about 0.5% of that of the receiver, with which it is otherwise identical.

The modeof operation of the receiver contemplated in Fig. 12 when receiving from the transmitter proper to it, is illustrated in Fig. 15. Here, as before, curves l, 2, 3 and 4 represent the individual responses of the selective units in effective parallelism as impressed upon the ultimate receiving device R of the station, and curve 5, which is the sum of these curves, represents the response of the station. In this diagram the assumed intensity of the waves received from the transmitter proper to the station is but one-fifth the assumed intensity of the waves from the interfering transmitter. in Figs. 12 and 13. The mode of operation of the receiver contemplated in Fig. 13, when receiving from the transmitter proper to it and to the receiver contemplated in Fig. 12, is illustrated in Fig. 10.

We have seen that in the reception by a reversed phase receiver, the frequency at which the phase reversal takes place is suppressed and a narrow band of frequencies contiguous there- 'to is partially suppressed. This characteristic of the receiver is more or less detrimental to the clarity and quality of the transmission, depending respectively on how low or high the frequency of reversal is in the scale of signal frequencies. If the frequency of reversal be so chosen as to lie in the extreme upper range of the signal frequency band, little detriment to the clarity or quality of speech or musical transmission need be apprehended. When, moreover, the frequency of phase reversal is pushed to the extreme upper limit of the transmited signal frequency band, then the phase reversal at the receiver exerts but a negligible effect upon the character of the transmission.

It must be remembered that the purpose of the phase reversal at the receiver is solely to minimize interference, and that this purpose can be effected whether or not the phase reversal occurs within the signal frequency band. In the preferred form of the contrivance, therefore, the transmitter is equipped-with a low-pass filter represented as noi-m.

which cuts out all frequencies above the upper limit of the required frequencies, and the phase reversal at the receiver takes place at this frequency or even at a somewhat higher frequency.

This preferred form of the invention is illustrated in a strictly formal manner in Fig. 16. Here the transmitter has a low pass filter which confines the radiation to frequencies below the upper limit of the desired band, which may be Here 110 is the mid frequency of the band and non1 and 7Z0+7Z1 are respectively the lowest and highest frequencies of the band which it is desired to transmit. At the receiver, the component receivers l and 4 are attuned to the frequencies n d;%n respectively, while component receivers 2 and 3 are attuned to frequencies noi m respectively. The output terminals of the detectors D1, D2, D3 and D4 are so relatively connected to the ultimate receiving device R of the station that D1 and D4 act in one sense, while D2 and D3 act in the opposite sense. It should be observed that in such an organization, component receivers l and 4 serve no useful purpose in the reception of the signals, their function being solely to neutralize crosstalk and other interference.

I claim:

1. In a system of signaling by waves of various frequencies within a certain complete frequency band of substantially uniform average intensity, receiving means comprising means to separate such band into respective parts having considerable overlap each with consecutively adjacent parts, respective means for selectively amplifying these parts so as to enhance intensity at and near the main frequency of each part but not at overlapping frequencies, and means to combine the outputs of the said last mentioned means.

2. The method of selectively receiving single band electric wave signals which consists in receiving them in separated parts at several different frequencies within such band and with considerable overlap of frequency between successive parts, selectively amplifying these parts while separated so as relatively to enhance the intensity of each such part at and near its main frequency relatively to its overlapping frequencies and then combining them for ultimate reception.

3. In a system of signaling by waves of various frequencies within a certain single carrier frequency band, a plurality of separate receivers for respective parts of said band, said receivers having respective frequency maxima along said band with considerable overlap into successively adjacent bands, respective selective amplifiers therefor adapted to enhance the intensity at the central frequency of each said receiver relatively to its overlapping frequencies, and a common ultimate receiver associated with said amplifiers.

4. In a system of signaling by waves of various frequencies within a certain single carrier frequency band, a plurality of separate receivers for respective parts of said band, said receivers having respective frequency maxima along said band with considerable overlap into successively adjacent bands, respective selective amplifiers therefor adapted to enhance the intensity at and near the said frequency maxima relatively to the overlapping frequencies, respective detectors associated with said selective amplifiers, and a common receiver for the detector outputs.

5. A radio receiving system adapted to receive signals on waves lying at any and all frequencies within a certain frequency band comprising a plurality of receiving antennae, respective selective amplifying units each adapted to enhance the intensity of its main frequency relatively to frequencies remote therefrom, the frequency, range of each such unit overlapping with diminishing intensity into the ranges of successively adjacent units, the main frequencies of these units being distributed along the said band, respective detectors with said units, and a common audio frequency receiverassociated with said detectors.

6. The method of selectively receiving radio signals carried at any and all frequencies within a definite limited. band of frequencies which consists in receiving them separately at different frequencies within the said band with considerable overlap into successively adjacent frequencies, selectively amplifying at these different frequencies so as to enhance the main frequencies relatively to the overlapping frequencies, separately detecting the signal elements at each such frequency and reducing them all to audio frequency,

and combining the audio frequency outputs to get the signals.

7. The method of selectively receiving electric wave signals lying within a certain complete frequency band of substantially uniform average intensity, which consists in receiving them separately at several different frequencies distributed within that band with considerable overlap between consecutively adjacent frequencies, selectively amplifying these respective separated frequencies so as to enhance intensity at and near the central frequency of each separated part of the band but not at its overlapping frequencies, then detecting them, and then combining the detected outputs for ultimate reception of the signal.

8. In a system of signaling by waves of various frequencies within a certain complete frequency band of substantially uniform average intensity, means at the transmitting end alternately to reverse the phase of the waves for consecutive intervals of said band, separate receiving means at the receiving end corresponding to these intervals, each such receiving means permitting some overlap, respective selective amplifiers adapted to reduce the relative intensity of the overlapping parts, and means to combine the outputs of said amplifiers.

9. In a system of signalling by waves of various frequencies within a certain frequency band on a single carrier, means at the transmitting end to separate the said band into parts with alternate phase reversal along these parts, separate receiving means at the receiving end corresponding to these transmitted frequency intervals, each receiving means permitting some overlap, respective selective amplifiers therefor adapted to reduce the relative intensity of the overlapping parts, respective detectors, and a common receiving circuit connected on the output side of said detectors.

10. The method of electric wave signaling which consists in generating the signal as a restricted single carrier band of frequencies, separating this band into parts, reversing alternate parts in phase, receiving these parts separately with some overlap selectively amplifying them so as to reduce the relative intensity of the overlapping parts, detecting them, and combining the detected outputs additively for reception of the signal.

11. In a system of signaling by waves of various frequencies within a certain frequency band,

a receiver adapted to receive over a wider frequency band to which the first mentioned band is intermediate, said receiver comprising means to receive parts of its band separately and in alternately opposite phase, an ultimate receiver, and means to combine said parts and apply them to said receiver. 7 7

12. The method of reducing interference in electric wave signaling which consists in receiving a frequency band of a certain width, separating the received waves into consecutive narrow smaller bands according to frequency, reversing the phase in alternate narrow bands over an entire frequency band wider than the band intended to be received, said band to be received being intermediately located in the frequency scale with respect to said wider band, and combining the effects of all of said narrow bands.

13. A radio receiving system adapted to receive signals on waves lying within a certain major frequency band on a single carrier current comprising a plurality of receiving antennae, respective chains of tuned circuits and alternately disposed intermediate amplifiers, the circuits of each chain tuned alike and differently from other chains so that each chain will pass a minor band of frequencies, the different central frequencies of the respective minor bands for the different quency receiver associated with said detectors.

'14. In a system of signaling by Waves of various l frequencies within a certain frequency band, re ceiving means comprising means to separate such band into respective narrower bands having considerable overlap each with consecutively adjacent narrower bands, respective selective means Y for reducing the relative intensity of the overlapping parts of these narrower bands, respective amplifying means for these narrower bands, and means to combine the narrower bands after such relative reduction of intensity and such amplification.

JOHN STONE STONE.

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