US1724917A - Signaling - Google Patents

Description

1929- J. F. FARRINGTON I. 1,724,917

' SIGNALING Filed July 51, 1925 3 Sheets-Sheet l Tunz'n 72mm ,eeaciarrqe 1929- J. F. FARRINGTON 1,724,917

S IGNALING Filed July 51, 1923 3 Sheets-Sheet 2 was;

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Filed July 51, 1923 F. Network .6, y 5 F/lfer Y 40 L 45 v 4? 3 Sheets-Sheet 3 HM 7 F w fly. 65.

hvenfon- Patented Aug. 20, 1929.

UNITED STATES PATENT OFFICE.

JOHN F. FARRINGTON,

OF FLUSHING, NEW YORK,'ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, 015 NEW YORK, N. Y., A CORPORATION OF NEW YORK.

SIGNALING.

Application filed July 31,

This invention relates to signaling systems and as illustrated in the embodiments herein described, is particularly adapted for use in radio telephone systems. a

A portion of the subject matter described and claimed herein is disclosed in applicants copending application Serial No. 326,986 filed September 27, 1919.

Duplex working of radio or other high frequency systems has previously been a matter of some difliculty because of the excessive influence of the local transmitter upon the local receiver at each station.

It has often been the practice in desi ming radio systems, to provide a switch for switching an antenna system from a transnitting to a receiving set so that transmission and reception could be carried on alternately. It has also been suggested to provide a transmitter connected to an antenna by an arrangement whereby a minimum amount of the transmitted energy is impressed upon the receiving set. In accord ance with the principles of this invention, however, it is not attempted to completely prevent the energy of the outgoing carrier frequency from acting upon the local receiver. A certain amount of energy of the outgoing carrier frequency is utilized in the receiving set. This causes amplification of the received signals and also produces a side tone effect similar to that in an ordinary telephone substation. The loudness of the out.- going signals or the effect thereof upon the local indicating instrument, will generally be of approximately the same degree of magnitude as that of the lncoming signals.

These results are accomplished by causing the received energy and a certain amount of energy of the outgoing carrier frequency to be combined and detected, thereby producing a local auxiliary carrier current wave. This current of auxiliary carrier frequency will be modulated or varied in amplitude in accordance with both the out oing and the incoming signals and by detecting it a wave will be produced from which may be derived both the incoming and the outgoing signals. The auxiliary carrier frequency may be equal either to the sum orthe difference of the principal carrier frequencies, but since many practical advantages are attached to using the difference frequency rather than the sum frequency, this is the one which may most conveniently be utilized.

The amplitude of 1923. Serial No. 654,823.

the auxiliary carrier frequency current will be proportional to the product of the amplitudes of the local and the distant. carriers. Thus, assuming the same percentage of modulation at each station, the signal heard in the local receiver will be of approximately the same intensity Whether the local or distant station is transmitting. in order to obtain a maximum indication from the distant. station, it is consequently desirable to allow sufficient energy of the local carrier wave frequency to enter the receiving circuit to load the detector or the rectifying device with approximately the maximum energy input which it is capable of handling without material signal distortion. The local energy is thus utilized as a desirable amplifying and selecting agent during the non-transmitting periods.

The object. of the invention, broadly stated, is, therefore, to provide improved methods of and means for duplex signaling, and more specifically for radio signaling.

A particular object is to provide a duplex system wherein the locally generated carrier wave for the outgoing signals aids in selecting and amplifying the incoming signals. The effects of interfering waves on radio systems are thereby, to a considerable extent, overcome.

A further object is to provide a system in which the transmitting operator will at once become cognizant of an inoperative condition of his transmitting or receiving system or that of the distant transmitter. The absence of side tone effect will warn the operator that the apparatus is not working properly.

A further object is to provide improved methods of duplex communication in which a single element such as an amplifier or oscillator of a transmission circuit may function usefully as an element of the receiver circuit. This results in a desirable simplification of duplex signaling systems which are capable of transmitting efficiently and re ceiving selectively.

A further object is to simplify systems for producing modulated Waves and relates especially to producing modulated Waves from which the unmodulated carrier component has been suppressed by means of a single vacuum tube arrangement which is particularly adapted for use in duplex communication systems. v

A still further object is to reduce the num- Cir her of vacuum tubes en'iployed in duplex com munication systems.

Further objects of the invention will be apparent to those skilled in the art from a perusal of the following detailed description with reference to the accompanying drawings,

wherein Fig. 1 represents a system in which an amplifier tube of the transmitting circuit functions as adetectorfor the receiving circuit; Fig. 2 represents a system in which a high frequency oscillator of the transmission system functions as a detector of the receiving circuit; Fig. 3 is a modification of Fig. 2 in which low frequency balance is employed to reduce the side tone introduced into the receiving system; Figs. land 5 are circuit diagrams of systems corresponding respectively. to the arrangements of Figs. 2 and 3 but male ing use of bandpass filters where the arrangements of Figs. 2 and 3 employ coupled tuned circuits; Fig. 6 is a circuit diagram included for the purpose of explaining a single tube carrier suppression modulating system ;'Figs. 6 and 6 indicate specific aJi-angements of impedance elements which may be employed in Fig. 6; Fig. 7 represents an arrangement whereby the completeness of carrier suppre sion may be estimated and the circuit acct.- rately adjusted to increase the completeness of the carrier suppression; Fig. 8 is a simple transmission system employing a single tube carrier suppression modulator; and Fig. 9 is a circuit diagram illustrating the utilization of a circuit, such as that shown in Fig. 8, in a partial carrier suppression difference frequency duplex telephone system.

Description of Fig. 1. Referring specifically to Fig. 1, outgoing speech signal waves 8 from the microphone circuit 1, are in pressed with a high frequency carrier wave f from the source 2, which, for example, may be a frequency of 500,000 cycles per second, on the input circuit of a modulating tube or other equivalent modulator A wave of frequency f modulated by speech 3 is thereby produced in the output circuit f and impressed by means of the transformer 5 upon the amplifier 6, which is preferably of the thermionic type. The output circuit of the amplifier 6 includes a circuit A which is preferably tuned to the mean frequency of the currents resulting when current of frequency f, is modulated by speech waves 5 The frequency of this modulated current is herein designated by the expression f e This symbolic manner of designating a modulated wave will be used throughout this specification. A modulated wave so designated may or may not contain an unmodulated carrier component. iln the operation of the various systems of the invention, can ,cept in the operation according toone method of the system of Fig. 9, the transmitted 1nodulated wave does includes such unmodulated carrier component, as is necessitated by the lack of a local homodyne source in such sys tems.

A. circuit B is tuned to the frequency f I which is the mean frequency of the outgoing modulated wave f s, in order to prevent the outging waves which are impressed upon the antenna 7 from being shunted to any material extent through the shunt circuit 8 in which the circuit B is placed. Any suitable reactance means 9, such as in the present instance a coil 10 shunted by a condenser 11, is provided in the shunt branch 8 as a tuning reactance to give the antenna system a degree of freedom equal to the mean of the frequencies f is which represents the modulated wave received from a communicating station. A tuned circuit C is provided in the output circuit of the amplifier 6. This circuit is tuned to a frequency equal to the difference between the frequencies 7, and f Thus, if frequency f, equals 500,000 cycles and f eq .als 530,000 cycles, circuit C will be adjusted to be resonant at 30,000 cycles. Owing to the detecting or demodulating action of the amplifying tube 6 which has its anodecathode circuit in series with circuit C, there will occur in circuit Coscillations having component with a frequency of 30,000 cycles and a wave of this frequency modulated in accordance with the speech signal 8 or the signal a or by both of themin case both are being simultaneously transmitted. This is represented by the legend f f i 2 adjacent the circuit C. A circuit D tuned to the same frequency as the circuit C, is coupled thereto, preferably loosely. The circuit D is serially included in the input circuit of a thermionic or other suitable detector 12, whose output circuit includes a suitable indicating device such as a telephone receiver 13, shunted by a condenser 1%.

Operation of Fig. 1.The greater part of the high frequency energy which is being transmitted will pass through the circuit C without producing much effect in circuit D. That part of the unmodulated outgoing energy which is impressed upon the circuit C will perform the useful purpose of aiding in the selective reception and amplification of the incoming signal. Since the amplitude of the auxiliary or intermediate frequency impressed upon the detector 12 will have a value dependent upon the amplitudes of the carrier frequencies of the two stations, the signal heard in the local receiver will consist of both the local and the distant signal and these will be of the same order of loudness. The legened s 3 adjacent the receiver 13 indicates that both signals will be heard in this instrument. I

The use of the letters f and a with appropriate-subscripts are applied to the remaining figures of the drawing with meanings similar to those in connection with Fig. 1.

Description of Fig. 2.The transmitting system shown in Fig. 2 comprises an antenna 7 to which is connected a vacuum tube 15 in a well-known manner as illustrated, whereby oscillations are produced in the antenna at a frequency approximately equal to the natural frequency determined by the series arrangement comprising the capacity of the radiating member 7, the inductance of the coil 16, and the capacity of the condenser 17. Space current for the tube 15 is provided by the source 18, through the speech frequency choke coil 19, the circuit 20, and the high frequency choke coil 21. The choke coil 21 functions to discriminate against both the incoming and outgoing high frequency waves and therefore tends to keep them out of the circuit 20. Shunted around the source 18 and the coil 19 is the anode-cathode path of the vacuum tube 22. The input circuit 23 of the tube 22 is coupled to the microphone circuit 24: which represents any suitable source of speech or other signaling frequency electrical waves or variations. The production of speech waves in the circuit 24 will cause a corresponding variation in the impedance of the tube 22, which will in turn cause a corresponding variation in the current supplied to the tube 15 owing to the choking action of the coil 19. This will cause the high frequency waves generated in the antenna by the oscillating tube 15 to vary in amplitude in accordance with the waves produced in the circuit 2%.

Incoming signal waves will be impressed upon the antenna 7 Owing to the detecting action of the tube 15 upon both incoming and outgoin high frequency waves, a variation of potential difference will be produced across the anode and the cathode of the tube 15 having frequencies equal to the differences between the outgoing and incoming frequencies. The tuned circuit 20 will be adjusted to be resonant to these difference frequencies. In selecting the modulated auxiliary carrier frequency wave the circuit 20 functions in the same manner as the circuit designated C in Fig. 1.

A circuit 25 comprising a variable condenser and a variable inductance may be connected in parallel to the circuit containing the source 18 and the coil 19. The circuit 25 may be most advantageously tuned to be resonant at the auxiliary carrier frequency. This prevents a loss of energy of the auxiliary carrier frequency due to the voltage drop across the tube 22.

Coupled to the circuit 20 is the circuit D which is tuned to the auxiliary carrier frequency. The circuit D is in the input cir cuit of the detector 12 in the output circuit of which is the receiver 13. The branch circuit 8 which is connected to the antenna, is intended to give the antenna one natural period equal to the period of the incoming waves from the distant station which are represented by the legend f s An anti-resonant circuit B is serially included in the branch 8 and tuned to the transmitting carrier frequency f, in order to prevent current of that frequency from passing through the branch 8. Any suitable tuning means 10 is also provided in the branch 8 whereby the said branch may be given any desired reactance which is necessary to tune the antenna for the incoming waves.

An important feature of this arrangement is that the frequency of the waves being transmitted is determined by and can be varied by the adjustment of the coil 16 and the condenser 17 After establishing the outgoing carrier at the desired frequency, the tuning reactance means 10 may be varied within considerable limits without producing a material variation of the waves being generated by the tube 15 in the antenna.

Operation 0 f Fig. Q.-When signals are being transmitted from the local station there will be radiated from the antenna 7, a modulated carrier wave f s which is available for combination with the incoming signaling wave 7, is, to cause the transmission through circuits 20 and D of the wave f f,i

2 This produces in the receiver 13 either or both of the signals 8, and 8 depending upon whether one or both are at that instant being transmitted. lVhen no signals are bein transmitted, of course, only the unmodulate component of the outgoing carrier combines with the incoming wave to effect amplification thereof.

Description of Fig. 3.Fig. 3 represents a system in which the elements 7, 16, 17, 15, 18, 19, 20, 21, 22, 23, 24, 12, 13, 14 and D are similar to and function in the same way as the correspondingly designated elements in Fig. 2. In the arrangement of this figure the antenna is tuned for the outgoing carrier wave frequency and hence, incoming signals will be received rather inefficiently. However, a circuit containing elements similar to the circuit 8 of Fig. 2 may be provided if it is desired to provide an antenna having a degree of freedom corresponding to the frequency of the incoming waves. In Fig. 2 the voltage of speech frequency which is supplied to the oscillating tube 15 from the tube 22 is impressed serially across the circuit 20. In some cases, this results in objectionable transients being set up in circuit 20 which cause interference with the operation of the system. In order to avoid this in the arrangement of Fig. 3, the circuit 20, which corresponds to the circuit C of Fig. 1 is balanced for speech frequencies. The lead from the plate ofthe tube 22 is connected to an intermediate point 27 of R of the circuit from the point 27 through the work is so adjusted that the time constant oscillator 15 to ground is equal to the time constant gof the circuit from point 27 for the oscillating tube and comprise an inductance in series with a parallel combination of a resistance and a condenser.

The currents of the intermediate carrier frequency in the circuit D instead of being impressed directly upon detector 12 are first amplified by means of a suitable amplifying device 31 and then passed through any suit ablefilter 32 to the detector 12. The filter 32 1s deslgned to pass only modulated auxiliary I s carrier frequencies f f i 9 m 1 7,, .2 rhe general opei atlon of this system iollows obviously from the described operation of the system shown in Fig. 2. Balancing the circuit for speech frequencies will reduce the number and amplitude of disturbing transients introduced in the receiving circuit and will reduce the amplitude of speech frequency side tone directly introduced into the receiving circuit. 7

Description of Fig. .4.Fig. 4-is similar to Fig. 1 but includes additional means to increase the selectivity and to reduce the side tone. The tuned circuits C and D. of Fig. 1 are replaced by a band pass filter 33 of a well-known type. The function of the filter 33 is to select a band of frequencies equal in width to the modulated wave resulting from the combination of the incoming and outgoing waves and to substantially suppress waves of other frequencies. The band pass filter 33 is connected to the input circuit of an amplifier 34. T o the output circuit of the amplifier. 34 is connected a band pass filter 32 similar to filter 33 or otherwise constructed may be desired.

The filter 32 supplies the input circuit of an amplifier which in turn supplies the detector 12. It will thus be seen that the intermediate frequency which may be equal either to the sum or the difference of the outgoing and incoming carrier waves is first selected by the band filter 33, amplified by the amplifier 34, again selected by the filter 32 and again amplified with the amplifier 35. Such an ar rangement is highly efficient in selection and amplification. Additional stages of amplification or additional selective means may be inserted in the circuit at any point.

The operation of the circuit of Fig. 4 follows obviously from the description of the operation of Fig. 1. However, it may be not-ed that the terminal section of the filter 33, which is serially included in the plate-filament circuit of the tube 6 should offer a reasonably low impedance to the incoming and outgoing carrier waves.

Description of F ig. 5.Fig. 5 is a modified form of Fig. 3 in which corresponding elementsare identified by similar reference characters. The band filter 33, amplifier 34E, band filter 32, amplifier 35 and detector 12 function in the circuit of Fig. 4. By a tap 27 upon the coil 23 a balance of speech frequencies is obtained whereby the amount of speech frequency voltage introduced into the filter 33 is reduced.

Description of Fig. 6'.-Fig. 6 is a circuit diagram illustrating the principle of operation of a single tube carrier suppression modulator circuit which, among other uses, is useful in difference frequency duplex systems. In (3, the input circuit of a modulating tube 40 is supplied with carrier waves from a source ll and signaling waves from a source 12. The plate-filament circuit of the tube 40 is connected to a receiving instrument 43 through a filter 4-1. An impedance network 4-5 is connected between the grid and the plate of the tube 40. The usual batteries and the choke coil 46, together with a stopping condenser 77 are employed in the circuit as indicated. In a circuit'such as illustrated in Fig. 6, it is possible to suppress from the output circuit current of the car rier frequency supplied by source 41 and current of the signaling frequency supplied by source 42.

The physical principles, on which 6.18 operation of the present circuit are, in a broad sense, based, are explained in the description of Fig. 1 of U. S. patent to Nichols 1,558,909, October 2?, 1925. In the patent it'is shown that a three-element electric discharge device may be considered a virtual source, adapted to impress on its output circuit an electromotive force similar to that impressed on its grid but opposite in phase; or, from another point of view, as a means for transmitting an impressed electromotive force wave through a circuit of impedance? the platefilan'ient impedance) with an amplification ,u. and a reversal of phase. It is the function of the network, connected between the'grid and plate, to transmit to the same points in the out-put circuit an electromotiv forc equal and opposite to the electromotive force from the virtual source. In order to transmit through the network 45 an electromotive force which will be equal to that transmitted through the device, the impedance of the network must satisfy the relation Z Since impedance R is substantially a pure resistance, the impedance Z must be effectively a pure resistance, as it is in the Nichols circuit in which the network comprises a pure resistance and a circuit, consisting of reactive elements, tuned to the frequency at Which balance is to be effected. Applicant has improved the Nichols arrangement to the extent of balancing out all the electromotive force waves impressed on the grid-filament circuit, by providing a network whose impedance is substantially a pure resistance for all frequencies. He is thereby enabled to accomplish with a single tube substantially the same results as could otherwise be accomplished by the use of a push-pull arrangement of two tubes as illustrated, for example, b U. S. patent to Hartley 1,419,562, June 13, 1922.

The above relation holds only for frequencies applied to the input circuit and not to those frequencies which result from the curvature of the tube characteristic and which appear in the output circuit. Among such frequencies resulting from the curvature of the tube characteristic are modulated components and multiple frequencies. With the value of impedance corresponding to the above mentioned condition, if the voltage amplitude of waves of carrier frequency f from source 41 impressed on the grid circuit is represented by 6 and if the source 42 is a source of signaling frequency 8 which ap plies to the tube 40 a voltage of amplitude 6 the output voltage of frequency 2f will be isfy the equation Z for the frequencies f ands, it may approximate a pure resistance.

Fig. 6 discloses one arrangement in which a large condenser 47 and a variable resistance 48 are connected between the grid and the plate of the tube 40. In Fig. 6 the variable resistance 48 is placed in series with a source of electromotive force 49 so adjusted as to balance the plate-filament battery of the tube 40. The source 49 has its positive terminal connected to the plate.

The suppressing effect of such an arrangement as illustrated in Fig. 6 may be verified experimentally for audible frequencies by an arrangement such as that of Fig. 7. A source 50 supplies waves of any frequency for which the circuit is to be tested. This current is supplied to the tube input circuit through the transformer as shown and the telephone receiver 43 is connected to the output circuit. The receiver 51 is directly connected to the source 50. By listening at receivers 43 and 51 simultaneously and adjusting the resistance 48, the particular value of the resistance may be found for which the only tone heard in the receiver 43 corresponds to a harmonic or multiple frequency of the tone heard in receiver 51. When speech currents are supplied from source 50, the condition for balance is not so exactly defined as with a single tone because the double frequency of speech in general constitutes intelligible sounds. For speech frequencies, therefore, the actual adjustment is made at that point at which the operator concludes that the sounds produced by receiver 43 consists principally of harmonic frequencies.

Decripzc'on of Fig. 8.A simple transmission circuit involving a single tube carrier suppression modulator is shown in Fig. 8. Source 41 supplies waves of carrier frequency and source 42 waves of signaling frequency to the modulating tube 40. These elements are shown as serially included in the input circuit, which should be taken to indicate diagrammatically any known method of supplying waves of these frequencies to the input circuit by means of inductive, capacitative, or resistance couplings arranged in series or parallel. The network 45 consists of the large condenser 47 and adjustable resistance 48. As mentioned above, the amplitude of the modulated components may be made large as compared to the amplitude of the audible signaling frequencies by making the carrier wave voltage large with respect to the signaling wave voltage. This results in a relatively large amplitude harmonic of the carrier wave frequency, which may be suppressed by the low pass or band pass filter 44, which suppresses multiple frequencies of the carrier wave source and may also suppress multiple frequencies of the signal wave source. If desired, the filter 44 may have its cut-off limit at such a point as to suppress one side band of the modulated wave. The currents trans ferred through the filter 44 are amplified by any suitable amplifying system, such as the amplifier 52, and radiated from antenna 53 or otherwise transmitted or utilized as may be desired.

Fig. 9 represents a duplex system employing a single tube carrier suppression modulator such as is described in connection with *ig. 6. In this circuit the modulator is adjusted so that the unmodulated componentof carrier frequency current is not entirely suppressed for a reason which will be hereinafter stated.

In Fig. 9 the source 60 supplies waves of some convenient base frequency, for example 50,000 cycles per second. A harmonic generator 61 is connected to the source 60 and produces waves of a suitable multiple frequency, for example 500,000 cycles per second, which are-selected by any suitable selective network 62. A. proper voltage of this frequency may be taken off from the potentiometer 63 and applied to the input circuit of th modulator 40. Signaling waves to be transmitted are supplied from the microphone circuit 64 to the-input circuit of the modulator 10, which is so adjusted as to almost but not quite suppress the unmodulated carrier frequency. The resulting modulated waves are transmitted through a filter ll to eliminate multiple carrier frequencies, amplified l y the amplifier 6 and impressed upon the antenna 7 by means of the coupling coil in the tuned circuit A. The tuned circuit B is tuned to the outgoing carrier frequency and the network 10 serves to tune the antenna for the incoming carrier frequency. If desired, a circuit connection 68 may be employed to impress upon the input circuit of the amplifier 6 an adjustable small component of the outgoing carrier wave frequency. For this purpose the connection 68 is provided with an adjustable tap upon the potentiometer 63. There are thus radiated from the antenna 7 modulated carrier waves of the frequencies f is where 7, represents 500,000 cycles and 8 the signal to. be transmitted. A component of the unmodulatedcarrier frequency may be included in the radiated energy and may be regulated to any desired amount. Let it be assumed that incoming carrier waves of similar nature having the frequencies of f -8 where represents 550,000 cycles, are being received. The incoming waves combine in the plate-filament circuit of the amplifier 6 with the outgoing carrier waves and owing to the rectifying action of the tube, a difference frequency wave results. This difference frequency or auxiliary carrier wave comprises in general side bands based upon the outgoing and incoming carrier waves. The side bands and any unmodulated carrier component are selected from other interfering waves by a suitable band pass filter 65. The circuit 66, including an amplifying tube 67, is loosely coupled to the input circuit of the detector 12 to supply thereto Waves having the frequency of the intermediate or auxiliary frequency unmodulated carrier component. These waves are combined with the waves transferred through the filter and the action of the detector 12 is such as to reproduce speech therefrom. The

legend s 8 indicates that during transmission the outgoing signal will be heard in receiver 13 and during reception the incoming signal will be heard and will be of approximately the same intensity.

Having described several forms of the invention, the parts, features and combinations and methods inherent therein which are believed to be patentable are defined in the appended claims.

,VVhat is claimed is:

1. The method of duplex signaling by means of a vacuum tube power amplifier in a system for transmitting messages in one direction between two stations by means of a carrierwave and transmitting a message in the other direction upon a carrier wave of a different frequency and utilizing at one of said stations a portion of the energy of the outgoing carrier wave to reinforce the incoming signal by producing a third wave of frequency diff rent from each of said carrier waves and deriving a wave of signaling frequency from the last mentioned wave, which comprises combining the incoming and outgoing carrier waves in said power amplifier to produce the said third wave and transmitting the outgoing waves by means of said device.

2. In a signaling system comprising two stations each of which comprises a source of signaling waves, a high frequency carrier wave source, means for modulating each of said carrier waves in accordance with the corresponding signaling wave, means for combining the energy of each of said carrier waves modulated at times to produce a similarly modulated intermediate frequency car rier wave, means for deriving the signaling waves from said intermediate frequency carrier wave, and a power amplifier in the outgoing wave channel, said amplifier comprising said combining means.

3-. In a system for duplex radio signaling, a transmission channel for transmitting outgoing waves including an electron discharge device, means for receiving incoming waves and combining them by means of said discharge device with energy of the outgoing wave frequency to produce a wave of carrier frequency modulated in accordance with incoming and outgoing signals, and a common transmitting and receiving antenna impartially resonant to the mean frequency of each of the outgoing and incoming waves.

4:. In a duplex radio signaling system, common radiating and absorbing structure adapted to resonantly oscillate at the mean frequency of both the transmitted and received waves, an electron discharge device having an anode-cathode path, means for impressing the waves to be transmitted upon said anode-cathode path, means for impressing the received waves upon said path, a circuit connected to said path including selective means for transferring electric waves having a frequency equal to the difference of the incoming and outgoing wave frequencies, and means for detecting and indicating the selected waves.

5. In a duplex high frequency telephone system, means for transmitting messages upon one carrier wave and means for receiving messages upon another carrier wave of different frequency, means for combining the two carrier waves to produce an auxiliary carrier Wave, a circuit in which the three carrier Waves are present, and a band-pass filter for selecting from this circuit the auxiliary carrier Wave and supplying it to a receiving circuit.

6. In a duplex radio system for transmission and reception on different carrier waves, a transmitting circuit and a receiving circuit, an electron discharge device for relaying oscillations to be transmitted and for combining incoming oscillations with oscillations to be transmitted to produce combination frequency oscillations, a circuit associated with said device in which said combination frequency oscillations are produced, and a bandpass filter connected to said circuit for selecting the combination frequency oscillations from the received and transmitted carrier frequency oscillations.

7. In a duplex carrier Wave system in which transmission is accomplished on one carrier wave frequency and reception is accomplished on a different carrier Wave frequency, the combination of a circuit adapted to carry waves corresponding to transmitted and received signals, a transmitter circuit and a receiver circuit, comprising a Wave distorting detector, both associated with said first mentioned circuit, and a band-pass filter associated with the receiver circuit to select from and supply to said receiver circuit Waves resulting from the combination of incoming and outgoing carrier waves to the exclusion of other Waves.

8. In a signaling system, means for combining incoming and outgoing carrier waves to produce an auxiliary carrier wave, in combination with means for first amplifying and then selecting the auxiliary carrier Wave energy, said selecting means comprising a band filter.

9. In a signaling system, means for combining received energy with locally generated energy to produce an auxiliary carrier wave, in combination with means for first amplifying and then selecting the auxiliary carrier wave energy, said selecting means comprising a band filter.

In witness whereof, I hereunto subscribe my name this 27 day of July A. D., 1923.

JOHN F. FARRINGTON.

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