US1603582A - Carrier-wave transmission system - Google Patents

Description

Oct. 19 1926. 1,603,582

L. M. CLEMENT CARRIER WAVE TRANSMISSION SYSTEM Filed May 5, 1921 6 Sheets-Sheet 2 II I W WW

Oct. 19 1926.

L. M. CLEMENT CARRIER WAVE TRANSMISSION .SYSTEM Filed May 5. 1921 6 Sheets-Sheet 5 Oct. 19 1926.

L. M. CLEMENT CARRIER WAVE TRANSMISSION SYSTEM Filed May 5, 1921 6 Sheets-Sheet 4 s Q v p w w $3 Q \MN 1 w (Ill! Oct. 19 1926. 1,603,582

L. M. CLEMENT CARRIER WAVE TRANSMISSION SYSTEM Filed May 5. 1921 s Sheets-Sheet 5 o 1 I I 11/ l l I I Zea:

Patented Oct. 19, 1926.

UNITED STATES PATENT OFFICE.

LEWIS M. CLEMENT, OF MOUNTAIN LAKES, NEW tTERSEY, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, 015 NEW YORK, N. Y., A CORPORATION OF NEW YORK.

CARRIER-WAVE TRANSMISSION SYSTEM.

Application filed May 3, 1921. Serial No. 466,548.

This invention. relates to carrier wave transmission systems and more particularly to radio toll links for connecting ordinary telephone and telegraph systems for twoway communication.

Carrier wave transmission is accomplished by means of high frequency oscillations termed carrier Waves which are modified or modulated in accordance with the impulses or signals to be transmitted. In order to connect two ordinary conductive communication circuits bymeans of a carrier wave link for simultaneous two-way transmission, it is necessary to provide each of the communication circuits with carrier transmitting and carrier receiving apparatus. The carrier transmitting apparatus serves to modify a carrier wave in accordance with the impulse or signal to be transmitted and to transmit the energy of the modified wave over a carrier circuit or to radiate it into space. The carrier receiving apparatus selectively receives carrier waves modified ata distant station and demodulates the received waves to produce low frequency currents corresponding to the signals or impulses by which these carrier waves were modified at the distant station.

A feature of the present invention relates to an adjustment of the amplifying factors of the carrier wave receiving and transmitting apparatus with respect to their unbalance, or tendency for retransmission of reeeived signals, so that there will be no circulation of energy from the distant carrier wave station through the local station and back.

The communication circuits are preferably terminated at the carrier wave receiving apparatus at an operators position from which the various controls are operated and at which the desired connections may be established. The received carrier waves are demodulated and the resultant signal or impulse currents are supplied therefrom to the communicat on circuit.

Circuits are also provided extending from the carrier wave transmitting apparatus to the operators position to permit the signal or impulse currents originating in the com munication circuits terminating at the o erators position to be impressed upon t e carrier transmitting apparatus in order to modulate the outgoing carrier wave.

If it be desired to transmit two different sets of signal impulses simultaneously and to receive two different messages simultaneously as in the case of simultaneous teleph ony and telegraphy, it is necessary to provide means for preventing interference between the simultaneous outgoing transmissions and to separate the simultaneous incoming transmissions and distribute each to its proper communication channel.

A feature of the invention rel-ates to means for preventing intermodulation or interference between independent signals simultaneously transmitted as modifications of the same carrier wave.

A further feature of the invention relates to means for separating different messages simultaneously received as modifications of a common carrier wave.

In radio systems it is frequently desirable to separate the carrier transmitting apparatus and antenna from the carrier recelving apparatus by a considerable distance, in order to prevent localcirculation of high frequency power, or side-tone in the receiving apparatus, in consequence of energy derived from the transmitted carrier wave. In such systems it is highly desirable to provide at the controlling operators position an indicator to show whether or not the radio transmission system is functioning properly and is in condition for use.

An additional feature of the invention relates to an indicator for showing the presence in a radio transmitting antenna of radio frequency current and for giving an alarm when current fails.

Where oscillations from a thermionic oscillator or similar device are supplied to a transmission circuit under control of a key or relay operated in accordance with Morse or other code signals, it is desirable to prevent surges occurring in this circuit when the circuit is closed or opened. One of the features of the invention relates to an arrangement for preventing disturbances of this character.

The carrier oscillations may be produced by a thermionic oscillator provided with the usual tuned frequency determining circuit.

In radio transmission this tuned circuit may be the transmitting antenna itself. It is therefore very desirable, particularly in the case of short-wave transmission, that the capacity and other electrical characteristics of the aerial be made as nearly invariable as possible. A feature of the invention relates to a rigid antenna construction for oscillation circuits of this character.

According to the present invention, two conductive communication systems each consisting of one or more telephone and telegraph lines are connected by a radio toll link which comprises radio transmitting and receiving apparatus associated with each system. The local radio transmitting apparatus and receiving apparatus for one system are designed to selectively operate with carrier waves of one frequency for outgoing transmission and of different frequency for incoming transmission and are sufficiently separated in location so that local circulation of power may be avoided. The circuits of the communication system are preferably but not necessarily terminated near the radio receiving apparatus at an operators position from which the various controls are operated and the connections are established.

It is accordingly possible for the operator at the local radio station to communicate with the operator at the distant radio station or for the operators at the two positions to connect a communication circuit terminating at each position to the radio apparatus so as to provide two-Way communication therebetween.

The novel features which are considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof will best be understood by reference to the following description taken in connection with the accompanying drawing. in which Fig. 1 illustrates by a schematic diagram one terminalof a radio toll link to gether with its associated communication circuits: Fig. 2 a diagram of the circuits of the radio transmitting apparatus. Fig. 3 shows the circuits of the radio receiving apparatus: Fig. 4 the circuits for interconnecting the incoming and outgoing radio channels to the circuits of the conductive communication system: Fig. 5, a telegraph oscillator circuit; Fig. 6, the details of the transmitting aerial and antenna connections. and Fig. 7. a modification of the schematic diagram of Fig. 1.

Like parts are designated alike in the several figures. In the detail Figures 2 to 6 inclusive, the initial numeral of a reference number will in general correspond to the number of the figure.

Figure 1 presents a circuit diagram of one terminal of a radio toll system, and shows merely the main communication channels. The various details of this terminal circuit including the ringing, supervisory, and local intercommunication circuits are omitted to simplify the diagram. Reference to this diagram will be made at this point in the description of the invention in order to present a general idea of the relation between the various elements, the details of which will be later disclosed more fully in connection with Figs. 2 to 6 inclusive.

The principal portions of this terminal circuit are 1) an exchange X at which are terminated the signaling lines of various types, such as telephone circuits 101 and 401 and telegraph circuit 325, (2) an operators station 0 which may serve as an exchange, or merely as a through repeating station for connecting the radio channels to the lines 101, 401, etc., and is preferably but not necessarily situated near the radio receiving station to enable the operator to attend to the radio receiving apparatus, the radio receiving station R which may be at the same location as station 0, or remote therefrom, and (4) a radio transmitting station T illustrated as remote from the operators station. In some instances stations T. R and 0 may be located in the same building.

A telephone message originating at X may be transmitted as speech current over line 401 to station 0, Where, by connections with circuit 402, the speech currents are supplied to an outgoing channel 405 including an amplifier 406. and a low pass filter 407, and are then transmitted over circuit 203 to modulate a carrier wave which is produced and radiated at station T. Tn similar fashion Morse signal impulses set up by key 103 in circuit 125 at station X operate a relay 104 at station 0 to close its contacts, thus completing the outgoing circuit 204 of a 4500 cycle oscillator 501 and causing the outgoing carrier wave at T to be modulated in accordance with the 4500 cycle oscillations.

Receiving station R is equipped with a directive reception loop antenna 301 connected to the radio receiving apparatus by tuned circuits T. C. which enable selection of the desired frequency carrier wave in addition to that given bv the tuning of the antenna circuit it elf. The received carrier oscillations are supplied together with local high frequency oscillations from an oscillator 305 to a demodulator 304 to produce modulated oscillations of an intermediate or lower frequency, which are amplified by intermediate frequency amplifier 311 and impressed upon two receiving channels. The upper channel includes a demodulator 319 to demodulate the intermediate frequency oscillations and produce oscillations of 4500 cycles. an amplifier 320 to amplify these 4500 cycle oscillations, tuned circuits 321 to select the amplified oscillations from speech currents, and a demodulator 322 to reproduce the code signal impulses for operating an armature 324. which controls circuit 325 and the sounders 105 therein at stations 0 and X.

The operator at 0 may communicate with the operator at X by talking over circuits 101. 102, or 401, or by using Morse key 106 and receiving sounder 105. The operator at 0 may also communicate over the radio toll link, or if connections are established at station 0. the operator at X may communicate over the radio toll link through station 0 as a repeater station. With this general view of one of the two similar terminals of the radio toll link, we may consider in more detail the various portions of the terminal circuits.

Referring to Fig. 2, which illustrates the radio transmitting apparatus 201, there is shown a transmitting antenna 202 arranged to transmit carrier waves modulated in accordance with speech and ringing currents received from line 203 of a conductive communication system. and oscillations repre-' senting Morse or other code signals received from another line 204. The radio transmit-- ting apparatus comprises a thermionic amplificr 205, a radio frequency oscillator which includes the electric discharge devices 200 together with their associated circuits and the antenna circuit. and the variable impedance devices 207 which cooperate with the oscillator to produce modulated carrier oscillations according to the well-known Heising constant current modulation system. See proceedings A. I. E. E.,volume :38. No. 3, page 300.

Each of the thermionic devices 205, 206 and 207 is preferably. although not necessarily. highly evacuated and includes a filan'ientary or other thermionic cathode, an anode. and a grid or impedance control element. Each of the filaments is heated by a source E and is provided with a series current regulating resistance 208. i

Space current is supplied to each device by a source E through a network consisting of series inductances 209 and shunt capacity elements 210. This network serves to prevent commutator ripples or other impulses of the source E which may be a direct current generator. from being impressed'on the discharge devices and also protects the source E from the high frequency E. M. Fs. of the radio apparatus.

The space current path of'the amplifier 205 may be traced from its anode through the secondary winding of transformer 211, resistance 212, left-hand inductance 209, source E right-hand inductance 209, resistance 213 and resistance 214 to the cathable impedance devices 207 may be traced from their anodes through constant cur rent choke 215 and left-hand inductance 209, source E and back to the common cathode circuit as before. The space current path for the oscillator tubes 206 may be traced from their anodes through the primary winding of the output transformer 216, a radio choke 217, and thence through constant current choke 215, left-hand inductance 209, source E and back to the cathode terminals in the manner previously described.

The resistance 212 serves to reduce the ef fcctive space current voltage impressed across the electrodes of amplifier 205 to a suitable value and the capacity element 218, in shunt thereto, furnishes a low impedance by-path for the alternating component of the space current which represents the amplified signal waves. 1

The constant current choke 215, which is of such character as to maintain the space current therethrough substantially constant, performs its usual function in the constant current modulation system. .The radio frequency choke 217 freely passes low frequency signal variations of the frequencies of the currents in lines 203 and 204, but serves to prevent radio frequency oscillations from being impressed upon and dissipated in the circuits of the variable impedance devices and the space current source.

The input circuit of amplifier 205 is coupled by a transformer 219 to the circuits 203 and 204 from which mesi'ages and signals are to be transmitted by the radio transmitting apparatus. In shunt to the secondary winding of this transformer is resistance 220 which serves to improve the impedance characteristic of the input circuit of the amplifier 205 with respect to the transformer 219. The lower terminal of this resistance is connected to a point in resistance 213. Since space current traverses the resistance 213 making this point negative with respect to the cathode of the amplifier, the grid of the amplifier will be maintained at a potential negative with respect to its cathode, and the magnitude of this potential will depend upon the magnitude of resistance 214 and the upper portion of resistance 213.

The variable impedance devices 207 are provided with an input circuit which includes the secondary winding of transformer 211. and resistances 213 and 214. The grids or control elements of these devices will therefore be negative with respect to their cathodes by the difference in potential across the series resistances 213 and 214. The secondary winding of transformer 211 is shunted by a resistance 221 which may be of the order of 10,000 ohms and therefore of small magnitude with respect to the input impedance of devices 207, in

order to stabilize the impedance connected to this winding.

'l'he oscillator comprises devices 206, the output circuit of which is coupled by a transformer 216 to the antenna 202. This antenna serves both as a feed-back circuit for coupling the output and input circuits of the oscillator and as a frequency determining circuit. The antenna is provided with a variable iinluctometer 222 which onables its natural frequency to be varied. The antenna circuit is coupled to the input circuit of devices 206 by means of a transformer 223. A variable condenser 224 serves to so change the constants of the interior circuit of devices 206 as to prevent parasitic oscillations therein. The interior circuit comprises the alternating current input and output circuits of devices 206, and the inherent capacity between their anodes and grids.

The capacity element 224 further serves to improve the power factor of the plate current in a measure compensating for the leakage rcactance of the winding of transformer 216 with which it is in shunt.

A grid leak path for the oscillator may be traced from the grids through the Winding of transformer 223, aresistance 225, winding of relay 226 to the common terminal of resistances 213 and 214. The normal potential of the grids of the oscillator tubes will therefore be negative with respect to that of their cathodes by the potential diforeuce across resistance 214.

The grids of the amplifier, variable impedance device. and oscillator are accordingly each maintained at such negative potentials with respect to their individual cathodes as to reduce their respective space currents to extremely small values. Oscillations are not produced under these conditions. There will accordingly be no current traversing the grid leak circuit, and relay 226 will be deenergizcd.

A time-limit relay 227 is connected across the supply conductors for the filament heating circuit. This relay when energized closes its normally open contacts 228 to short-circuit resistance 214. As soon as resistance 214 is short-circuited. the grids of the various discharge devices become less negative and permit more space current to flow. thus rendering each of the devices operative and causing radio frequency oscillations to be produced in the antenna. This action is further increased by the re duction of the external resistance in the space current circuit when resistance 214 is short-circuited.

In order to prevent surges in the space current circuit in consequence of opening or closing of contacts 228, a path including a resistance 229 and a capacity element 230 in series therewith is shunted across the tcrn'iinals of resistance 2l4. A switch 231 may be interposed in the circuit of the source E to control the application of current to the filaments and time-limit relay.

Relay 226 operates armatures 232 and 237 which control circuits 233 and 238 associated with a pair of conductors 234 extending to the operators position. Circuit 233 extends from ground thru source 235, bell or other alarm 236 left-hand contacts of relay 227 and armature 232 to line 233, upper conductor of line 234 to alarm 453 and ground so that upon completion of this circuit an alarm is given by alarm devices 236 and 453. Conductors 234 may be used to transmit information between the operators at the local radio transmitting and operators station in addition to their function as paths for circuits 233 and 238.

Referring to Fig. 3 there is illustrated a loop radio receiving antenna 301 suitably tuned to incoming carrier waves by means of variable. capacity 302. Looped antenna, as is well known, have marked directional properties. The antenna is preferably so mounted that its plane may be oriented with respect to the distant transmitting station in order to receive maximum signals. If properly situated it may at the same time be so oriented with respect to the local transmitting antenna 202 as to receive practically no energy therefrom. Serially included in the antenna circuit is a closed loop 303 resonant to the frequency of the oscillations transmitted by the local transmitting antenna 202.

The receiving antenna circuit is coupled by a transformer 330 to the input circuit of a thermionic demodulator 304 to which a thermionic oscillator 305 is also connected by a transformer 306. The oscillator 305 comprises the usual three-element thermionic device 307 having a grid-filament circuit inductively coupled to its alternating current plate-filament circuit by two windings of the transformer 306. The oscillation frequency determining circuit includes these two windings, the stopping condenser 308, and parallel condensers 309 and 310. The tuning of this circuit is made such that the frequency of the oscillations produced by oscillator 305 will differ from the carrier tions by an inaudible frequency hereafter termed the intermediate frequency.

In an actual example the received carrier frequency was about 640.000 cycles the frequency of the oscillator 305, 590.000 cycles, and their difference or intermediate frequency 50.000 cycles. As is well-known, when oscillations of two different frequencies are simultaneouslyimpressed upon a demodulating device, there will be present in its output circuit, oscillations of the difference frequency. These intermediate frefrequency of the selected incoming oscillaquency oscillations are amplified by an intermediate frequency amplifier 311, comprising three thermionic amplifiers connected in tandem. A variable resistance 312 is shunted across the output circuit of the first of these in order to enable variation of the amplifying factor to meet the varying conditions encountered.

By proper designof the coils constituting the transformer input windings of the intermediate frequency amplifier 311 with respect to the internal characteristics of the t ermionic amplifiers, the intermediate frequency amplifier as a whole may be made selective to a considerable degree for currents of the general range of the intermediate frequency to the exclusion of the hi h frequency radio currents and of low audiofrequency currents.

As is well known, if a high frequency wave is modified in accordance with a low frequency wave of a single frequency, the resultant modulated wave will include an upper side frequency component of a frequency equal to the sum of the frequencies of the high frequency and low frequency waves, and a lower side frequency component of a frequency equal to the difference.

of the frequencies of the modulated wave and modulating wave. 7

If, however, the low frequency signal wave be complex, that is, have components occupying a band of different frequencies, as in the case of telephony, in which the essential speech frequencies may be considered as ranging from 200 to 2,000 cycles, the same principle will apply in analysis of the modulated carrier wave. In this case,

'however, the upper and lower side frequency cdmponents will become upper and lower side bands, each of a frequency extent corresponding to that of the complex signal wave itself. See Journal of A. I. E. E.. volume 40, No. 4, page 309.

The result of the interaction between the received carrier oscillations and the oscilla tions produced by local source 305 is, therefore, in effect, to step down the carrier frequency of the received energy to the intermediate frequency. If the received carrier waves be simultaneously modulated in accordance with speech currents and signal oscillations of higher frequency than the speech currents, as for example, 4,500 cycles, the intermediate frequency wave will be modulated in the same manner. The transmission characteristic of amplifier 311 should be such that it will freely pass the side bands of the modulated intermediate frequency carrier wave.

The amplified intermediate frequency carrier wave is impressed by amplifier 311 upon two ' transmisslon channels 313 and 318 which serve respectively to select and translate the bands corresponding to speech currents and to the higher frequency telegraph oscillations.

.The channel 313 comprisesv a band filter 314, a demodulator 315- and a one-stage speech current amplifier 316. The band filter is of the well-known type described in the-United States Patent to G. A. Campbell 1,227 113, patented May 22, 1917. Its transmission range is such that it freely asses currents of a band'of frequencies incl dding the intermediate carrier frequency and the side bands corresponding to the speech currents. If, for example, the intermediate carrier frequency be 50,000 cycles and the essential range of speech frequencies be taken from 200 to 2,000 cycles, the filter 314 may be so designed as to transmit freely currents of frequencies f om 48,000 to 52,000 cycles and to substantially suppress currentsof frequency outside this range. It follows that the side bands corresponding to the 4,500 cycle telegraph oscillations having effective upper and lower side frequencies of 54,500 and 45,500, respectively, will be excluded from channel 313.

The modulated waves transmitted by filter 314 are impressed upon demodulator 315 which operates in a well known manner to produce in its output circuit waves corresponding to the original speech currents. These speech currents are amplified by amplifier 316 and impressed upon a circuit 317 leading to other low frequency apparatus at the operators position.

If ringing-or calling is accomplished by modulating the carrier wave with oscilla tions. within the same general frequency range as speech currents, 1t will be apparent that the ringing or calling signals will traverse the same channel as speech currents and will be supplied to the circuit 317.

The channel 318 comprises a demodulator .319, a three-stage amplifier 320, coupled tuned circuits 321, and a demodulator 322. The modulated intermediate frequency waves impressed upon demodulator 319 are demodulated, thus yielding both speech currents and telegraph oscillations of 4,500 cycles frequency. These low frequency Waves are amplified by the amplifier 320 and impressed upon the coupled tuned circuits 321 which discriminate sharply against speed currents and select oscillations of 4,500 cycles to which these circuits are tuned.

The demodulator 322 comprises two three- 4 element thermionic devices having their input circuits oppositely connected ,to the channel 318 and so arranged that the changes produced in their respective. space current circuits in response to a train of oscillations of 4,500 cycles are additive. A relay 323 having a winding in each of these space current circuits and provided with an armature 324 servesto open and close a telegraph circuit 325 in correspondence with received trains of telegraph oscillations.

Space current sources E, E and E are connected in series. The common filament connection is grounded at point F. This arrangement permits the use of thermionic devices operated normally with diiferent s ace current voltages. For example, the oscillator 35 is supplied with space current by sources-E and E in series. The amplifier 311 is supplied with space current from all three sources E, E and E in series. A

I common filament heating source E is provided for all of the cathodes.

Fig. 4 shows in detail a portion of the circuits at the operators station including those by which transmission of speech and ringing currents is accomplished between the or inary telephone lines 401 terminating at the operators station and the radio receiving and transmitting channels 317 and 203 respectively.

The operators station is provided with a main circuit 402 adapted to be connected with the lines 401 in the ordinary manner. Circuit 402 includes a balanced transformer or hybrid coil 403 of well-known type and terminates in a network 404, which simulates the impedance of the circuit 402 and a line 401 connected thereto, at the frequencies of all currents transmitted over the circuit 402.

outgoing channel 405 inductively coupled to the series windings of the hybrid coil leads to a two-stage amplifier 406 from which amplified speech currents are supplied to a low pass filter 407 of the general type disclosed in United States patent to G. A.'

Campbell, 1,227,113. The characteristic of this filter is such that it transmits, with substantially negligible attenuation, currents of all frequencies below a fixed limitin frequency as for example 2,000 cycles and substantially extinguishes currents of all higher frequencies. The filter 407 supplies speech currents and ringing currents to the channel 203 leading to the radio transmitter. A potentiometer arrangement 408 permits regulation of the gain or amplification of the speech currents.

Channel 317 impresses speech currents, obtained by demodulating received carrier waves, upon a two-stage amplifier 409 from i the output circuit of which they are impressed by a transformer 410 upon a low pass filter 411 identical in characteristic with filter 407. Received speech currents are therefore impressed from the low ass filter 411 upon the circuit 402 at the bridge connections of the hybrid coil.

Calling or ringing signals for the carrier wave circuit 203 are produced by a thermionic'oscillator 412 comprising the usual three-element electric discharge device having a cathode, anode and grid. The alternating current input and output circuits of this device are coupled inductively by a transformer 413 in such manner as to cause the device to produce oscillations of a frequency determined by the tuning of a closed circuit associated therewith. This closed circuit includes a capacity element 414 and a winding of the transformer 413 included in the grid filament circuit of the oscillator.

second condenser 415 isarranged to be connected in parallel to condenser 414 either by the normally closed armature contacts of an alternating current relay 416 or by the normally open armature contacts of a direct current relay 417. An operators start key 418 serves when closed to complete the circuit of a relay 419 which closes the filament heating circuit for the oscillator 412. Key 418 also serves to complete the circuits of two relays 420 and 424, the functions of which will be later set forth.

An operators ringing key 421 when closed serves to complete the circuit for a relay 422 which when energized erforms two functions. It closes its rig t-hand contacts to apply space current to oscillator 412. -It closes its left-hand contacts to complete the circuit for a source 423 of alternating ringing current which may be of the usual 16 cycle frequency.

The oscillator is accordingly in operative condition and produces oscillations of a frequency determined by the closed tuned circuit including condensers 414 and 415 in parallel. The alternating current relay 416 responding to the ringing current causes its armature to vibrate at the frequency of the ringing current. Accordingly the condenser 415 will be connected in the closed tuned circuit sixteen times per second and will be disconnected therefrom after each connection. By properly adjusting the rela contacts, the periods of connection and isconnection may be made approximately equal. During a connection period, the oscillator 412 will produce oscillations of one freuency, for example, 650 cycles. During the isconnection period immediatel following, the oscillator will produce oscil ations of a different frequency, as for example, 950 cycles per second. The oscillator will therefore produce thirty-two successive trains of oscillations per second, the frequency of the oscillations for alternate trains being 650 and 950 cycles respectively.

The relay 420 opens its right-hand contacts to interrupt the outgoing channel 203 between the amplifiers 406 and closes its lefthand contacts to connect the input circuit of the left-hand amplifier 406 directly across the terminals of condenser 414. The ringing source 412 accordingly supplies the trains of ringing oscillations to this amplifier from which they are transmitted to the outgoin channel 203, through low pass filter 407 Relay 424 opens the circuit 426 leading to the call receiving apparatus and prevents a reover channel 317 and, after transmission by amplifiers 409, transformer 410, and circuit 426, are impressed on amplifiers 427 from which they are supplied by transformer 428 to the input circuit of an arran -ement of oppositely connected thermionic devices 429 an 430. v

Across the input circuit of each of these thermionic devices is connected a closed loop circuit tuned to one of the two frequencies of the oscillations of the ringing trains. For example, circuit 431 may be tuned to 650 cycles and circuit 432 to 950 cycles. Accordingly, currents of 950 cycles frequency will be shunted through the circuit 431, but will meet with an extremely high impedance in tuned circuit 432. Oscillations of 950 cycles frequency will therefore have little effect on the thermionic device 429, but will strongly affect device 430. On the other hand, oscillations of 650 cycles frequency will affect device 429 but will not affect the device 430. It follows that the space currents of devices 429 and 430 will alternately undergo change and that the common armature 433 will be alternately attracted by electromagnets 434 and 435 in the space current circuits of these devices and will therefore vibrate at the alternation frequency of sixteen cycles per second.

The armature 433 is provided with two contacts and is adapted to remain in engagement with the one to which it is finally attracted. The armature and its contacts serve to close the circuit of a normally energized relay 436. When the armature is attracted from one, contact to the other, it opens its circuit, but relay 436 does not release in the time required for the armature to pass over to the other contact and the circuit is again closed.

However, if the armature is maintained in vibration at a sixteen cycle rate, the circuit of relay 436 is open a suflicient part of the time to permit the relay to release. In releasing, relay 436 closes its armature contact to energize relay 437, which pulls up its left-hand armature to control the circuit of the local signal lamp and its right-hand contact to complete the energizing circuit of relay 439. Relay 439 attracts its armatures 440 and 441 to disconnect circuit 402 from the hybrid coil and to connect sixteen cycle ringing source 442 to circuit 402. At the same time the armature 443 is pulled up to connect balancing net work 444 across the terminals of the hybrid coil. This network simulates the impedance of network'404 and so maintains the balance at the hybrid coil,

, thus checking any tendency toward local circulation of energy or singing. v

and indicator lamp alternating current relay 446 to cause ener- When it is desired to communicate over the radio channels from the operators position, keys 418 and 421 are used for ringing 438 indicates incoming calls. When the clrcuit 402 is connected to a circuit 401 for through repeating of signals at the operators position, incoming ringing impulses from the radio receiving applaratus operate the local indicator 438 an also cause relay 439 to apply sixteen cycle ringing current to line 401. Ringing 4 currents of sixteen cycles frequency in the return direction, originating in line 401, are selected by tuned circuit 445 and operate gization of relay 447, the left-hand contacts 448 of which are in parallel to the normally open contacts of start key 418 and the righthand contacts 449 of which are. in parallel to the normally open contacts of ringing key 421. Consequently sixteen cycle current originating 1n or transmitted over the connected line 401 causes oscillator 412 to apply alternate frequency trains of waves to outgoi g channel 203.

e ringing apparatus may be used to furnish an additional communication channel when not employed for calling purposes. For this purpose there are provided a start key 450 and a circuit closin key 451, the

latter of which isvin series with a telegraph key 452. Closure of the start key .450 initlates operation of relays419, 420 and 424 to respectively apply heating current to the filament of oscillator 412, connect the oscilmo lator 412 to the input circuitof the second amplifier 406 and to disconnect incoming call receiving channel 426.

If key 451 be closed, operation of the Morse key 452 causes relay 417 to be cortions at a single frequency, as for example,

650cycles per second, At its left-hand contacts relay 417 supplies space current to the oscillator 412. Accordingly, trains of oscillations of 650 cycles frequency and corresponding in duration to Morse or other code signals, as determined by the closure of the Morse key 452 will be received in the circuit 402 at the distant operators station and may be heard in the operators monitoring set.

The unbalance at the hybrid coil permits the connection of additional windings 460' for monitoring purposes. These windings terminate at contacts of a switch 461, which permits an operators set 462 to be connected directly to windings 460 for monitoring, or to be bridged across circuit 402 for ordinary two-way communication, either with a dis tant radio terminal station or the exchange X. Switch 461 serves to short circuit the transmitter of set 4.62 in the monitoring position, and to introduce an artificial line Z in the connections of the two-way communicating position so as to maintain a balance at the hybrid coil if line 401 be disconnected. In practice, line 401 is usually left connected to line 402, and is so terminated at exchange X that a practical balance is maintained at all times by network 404.

Fig. 5 illustrates circuits of the 4500 cycle oscillator used for tele raphy in connection with channel 204. Thls oscillator 501 comlprises a three-element thermionic device aving a closed tuned circuit 502 for determining the frequency of the generated oscillations. Between the anode and cathode is connected a potentiometer resistance 503 in series with a stopping condenser 504. The input circuit of a thermionic amplifier 505 is connected across a portion of the resistance 503 and the amplifier therefore supplies amplified oscillations of 4500 cycles frequency to circuit 203 through series stopping condensers 506 and a surge absorbing device comprising series resistance 507 and bridging resistance 508. The combination of resistances 507 and 508 each of which is of the order of 200 or 300 ohms serves to prevent accumulation of electrostatic charges on opposite sides of the line and to prevent surges when the Morse key in circuit 204 is closed and opened.

Fig. 6 illustrates the details of the elevated aerial construction. Two poles 600, each mounted on a massive base 601 of reinforced concrete or other masonry construction are closely engaged by four I beams 602 embedded for about half their length in the concrete base. A spiral of steel cable 603 surrounds the I beams 602 to hold them firmly in position and to serve as an additional reinforcement for the masonry base 601. Bolts 604 pass through the I beams and clamp them firmly to the poles.

For additional lateral support, guys 605 are connected between anchors 606 and a collar member 607 which engages pole 600. Double cross arms 608, attached to the pole near its upper end are firmly braced by braces 609. The aerial conductors 610 are tightly stretched between the poles 600 and each is connected to the cross arms 608 by means of an independent I bolt 611 which permits an adjustment of the tension of that particular conductor 610. At each end of the conductor 610, several porcelain strain insulators 612 are inserted in series to provide suitable insulation. The guy wires 605 are likewise connected at each end by strain insulators 613. The lead-in wire 614 from the antenna is connected to each of the conductors 610 approximately at its mid point 615 and is drawn tautly between the aerial and an insulating connection 616 at the top of a supporting pole 617. This construction insures rigidity of the antenna structure as a whole and reduces the tendency for swaying. Accordingly an antenna of this type may be used as a substantially constant capacity element in the frequency determining circuit of an oscillator; The lead-in wire 614 passes through the usual high tension insulating bushing 618 into the building 619. The ground connection 620 is suitably earthed as at 621.

Operation.

Suppose, for example, that a subscriber whose line terminates at exchange X wishes to converse over the radio toll link with a subscriber whose line terminates at the distant exchange corresponding to X. Ringing current of 16 cycles frequency is applied to line 401 at X. If the line 401 is not connected to the circuit 402, an operator at 0 must answer and make this connection. If the operator at O in answering ascertains that the radio toll link is wanted, the operator makes this connection and applies ringing current of alternate frequency wave trains to the radio toll link by closing start key 418, and ringing key 421.

If, however, the line 401 happens to be connected to circuit 402 for through line operation, the ringing current arriving over line 401 energizes alternating current relay 446 to cause relay 447 to close start key 448 and ringing key 449. The start key applies heating current to the filament, connects up the outgoing ringing channel front osci lator 412 and opens the incoming call receiving chan-- nel 426 as previously described. ing key applies space current to oscillator 412 and 16 cycle current to relay 416 to vary the frequency of the oscillator. ing current produced by oscillator 412 consisting of alternately recurring trains of two different Wave frequencies traverses lefthand amplifier 406, filter 407, circuit 203 and amplifier 205.

Assuming that switch 231 is closed, the relay 227 energized and the transmitting apparatus operating ringing currents impressed on the input circuit of variable impedance devices 207 will operate to modulate the outgoing carrier waveaccording to the well known constant current method. If the radio transmitting apparatus is operating properly, there will be a grid leak current in the oscillator grid leak circuit sufiicient to energize relay 226 and to maintain its right hand armature attracted, thus closing circuit 238 and operating a relay at station 0 to cause lam 454 to be lighted. Consequently, the attem ant at station 0 is notified that the radio transmitting channel is in operative condition.

Suppose, however, that switch 231 is closed The ring- The ringproperly, the amplified and rela 227 energizes, but that for some reason t e oscillator is not working properly, and its grid leak current is too small to energize relay 226. The circuit will thereupon be closed and alarms 236 and 453 situated respectively at the transn'litting station and operators station will both operate indicating at the transmitting station that there is trouble, and at the operators station, that the radio channel is not in condition for use.

An incoming call modulated carrier wave will be received by antenna 301 which is tuned to that carrier frequency and is opaque to its local transmitting frequency. The incoming wave is impressed with oscillations from source 305 upon demodulator 304 to produce oscillations of their difference frequency.

This difference frequency or intermediate carrier frequency band is selectively transmitted by the intermediate frequency amplifier 311. Band filter 314 transmits a band of currents of the intermediate frequency range, but suppresses currents of other frequencies. In the example previously given,

-the receiving carrier frequency was 640 kilocycles, and the local reinforcing oscillation frequency 590 kilocycles. If the local transmitting or outgoing carrier frequency be 760 kilocycles, any interference energy from the local transmitter not excluded by the selective antenna circuit will produce intermediate or difference frequency oscillations of 120 kilocycles and 170 kilocycles, respectively, with the received carrier wave and the local reinforcing wave. Consequently, amplifier 311 will discriminate against these difference frequency currents and the band filter 314,

having a transmission range extending from 48 to 52 kilocycles will entirely suppress them.

The modulated intermediate frequency band corresponding to the ringing frequency range will be freely transmitted by filter 314 and will be demodulated by demodulator 315, to produce ringing currents which after amplification are supplied over the channel 426 to ringing receiver amplifier 427, and therefrom to demodulator 429 and 430 to operate relays 436 and 437. The latter relay upon operation causes signal 438 to be energized, circuit 402 to be cut oif from the hybrid coil and a net 444 to be substituted in its place, and 16 cycle ringing current to be applied to line 401, thus completing the incoming ringing operation.

Conversation may now take place. Speech currents originating in line 401 do not aflect tuned alternating current relay 446, but pass through the hybrid coil, low pass filter 407,

and amplifier 406 to the radio apparatus to modulate the outgoing carrier wave in the same manner as rlngin'g currents. In coming speech modulated waves experience the same operations as the call modulated waves until they reach the transformer 410. The demodulated speech currents pass through both channel 426 and through filter 411. Their action upon demodulator 429 is not such as to affect relay 436, since the casual operation of armature 433 has no influence upon the circuit of this relay. Speech currents passing filter 411 are supplied to circuit 402 and to line 401.

The use of low pass filters 411 and 407 enables a simplification of the net work 404, which accordingly is required to balance circult 402 and line 401 only at the speech frequency range. There is therefore no necessity for balancing at other frequencies, such as that of the 4500 cycle oscillations which might otherwise cause disturbing singing if they were inductively impressed upon the speec current channel.

'In practice the hybrid coil or balanced transformer is only approximately balanced. If the amount of energy received from the incoming radio channel and impressed by the hybrid coil upon the outgoing radio channel is large, and there is a large amphfication factor in the outgoing channel singing between the two radio terminal stations may take place. In the circuit arrangement under consideration, the amplification is made very large and the gain controls 312, 408, etc., are then adjusted to reduce the net amplification or gain to a point well below that at which singing would take place.

When the calling circuits are otherwise idle they ma be used for telegraphy between the ra io terminal stations. Closure of the start key 450 causes application of filament heating current to the oscillator, interruption of the incoming call receiving channel 426, disconnection of the outgoing talking channel 203 from the hybrid coil, and connection of oscillator 412.

Closure of key 451 causes space current to be applied to oscillator 412 and causes condenser 415 to be connected into the circuit of oscillator 412. Waves of a single audible frequency are accordingly transmitted in -trains of a duration corresponding to code impulses, and are impressed upon the transmitting apparatus to modulate the outgoing carrier wave. The received modulated waves carrying single frequency wave currents are received and detected in the same manner as speech currents and may give an audible tone indication in the receiver of set 462. This single frequency wave does not affect relay 436, which can be operated only in response to 16 cycle vibration of armature 433.

Telegraph communication over the radio toll link by means of the 4500 cycle wave may be held between exchanges X, operator stations 0, or an operator station and the remote exchange X. The key 103 at station X controls operation of a relay 104 at station 0 to apply 4500 cycle current from oscillator 501 to the radio transmitter. Received oscillations modulated by 4500 cycle current cause return transmission by way of relay armature 3:24, which controls sounder 105 at X. The frequency 4500 is merely typical of any frequency higher than those of the essential speech frequency range. Currents of such frequencies are not transmitted by the low pass filters, and moreover produce intermediate frequency side bands, which are excluded by band filter 314. I

In simultaneous telegraph" and telephony, where the same carrier wave is simultaneously modulated by a plurality of d fferent message waves, it is possible to injure the quality of transmission for one message by reason of the fluctuations introduced in the load on the modulating system by other messages. Especially is this true in the case of telephony with simultaneous audible frequency telegraph oscillations. The arrangement of the present invention prevents this by properly proportioning the percentage of modulation induced by each modulating wave.

Suppose, that the average amplitude of the modulated carrier wave be measured and found to have the magnitude K. Upon modulation, the magnitude of the wave is found to change by an amount C, making the maximum and the minimum of the modulated wave of magnitudes K+C and K-C, respectively. The percentage of modulation is then If in the case of simultaneous telephony and 4500 cycle telegraphy, the modulating speech currents be impressed upon the modulating system at such voltages as to give a modulation of per cent, and the telegraph oscillations to give a modulation of ten per cent, the transmission for each will be satisfactory. The voltages of the modulating currents may of course be regulated by adjustment of the amplifying apparatus of their respective channels.

The circuit arrangement of Fig. 7 is like that of Fig. 1 except that the 4500 cycle telegraph channel is modified to serve for two simultaneous code messages. The outgoing 4500 cycle oscillations produced by source 501 are supplied to a modulator 703 of any preferred type together with oscillations of two different frequencies, for example 300 cycles and 500 cycles produced by sources 704 and 705 respectively.

The connections of sources 704 and 705 to the modulator circuit are controlled by receiving relays 706 and 707 actuated by currents received over duplex Morse lines 708 and 709 from a distant exchange. The

modulated 4500 cycle wave is transmitted out over channel 204, exactly as in the case of the unmodulated 4500 cycle wave of the arrangement of Fig. 1.

Reception with the system of Fig. 7 is Identical with that of Fig. 1 up to the output circuit of intermediate frequency amplifier 311. The intermediate frequency currents modulated both by speech and 4500 cycle oscillations are impressed upon band filters 710 and 725. Filter 710 in parallel has a transmission range suflicient to pass the speech modulated waves. Filter 725 passes a range of currents of 45000 to 55000 cycles thus including both the telegraph and speech modulated waves. The remainder of the speech receiving channel is the same as in Fig. 1, and the detected speech currents and ringing currents after amplification traverse a low pass filter 411. The waves transmitted by filter 725 are demodulated by a demodulator 711 to produce speech currents and modulated 4500 cycle waves. Filter 708 passes a range of frequencies of about 3900 to 5100 cycles thus excluding speech currents. The filter 708 impresses the 4500 cycle modulated oscillations either directly or thru an amplifier on a demodulator 712 which demodulates these currents to produce oscillations of 300 and 500 cycles respectively. Tuned circuits 713 and 714 tuned respectively to 300 and 500 cycles select these oscillations and supply them to amplifiers 720 and demodulator 721 which control transmitting relays 722 for retransmitting code impulses to lines 707 and 708.

The operation of the modified system of Fig. 7 will be apparent from the foregoing explanation. The telephone channel may be arranged to give 60 per cent modulation of the outgoing carrier wave and to give 10 per cent modulation for the telegraph oscillations of 4500 cycles, to be divided equally between the two telegraph messages carried thereby.

It is to be understood that the. invention is not to be limited to the circuits described. Certain of its features are capable of application to carrier wave transmission over conductive circuits, and some are applicable to electric transmission generally without regard to signaling.

What is claimed is:

1. The method of two-way toll transmission which comprises transmitting two modulating waves, one in each of two opposite directions, modulating a separate carrier Wave in accordance with each modulating wave, radiating the two resulting modulated carrier waves in opposite directions, separately receiving said two modulated carrier waves from opposite directions, amplifying the energy of each of said two received waves, adding one resulting amplified wave to an oppositely transmitted wave modulating one of said carrier waves, partially neutralizing the transmission of said amplified waves with said modulating wave in the direction of transmission of said modulating Wave, and maintaining the amplification of said received wave below the point at which said received wave is retransmitted by said last mentioned carrier wave.

2. A multiplex carrier wave transmission system comprising means for producing and transmitting carrier waves, means for simul- 'of audible but higher frequency, means for receiving said modified carrier oscillations, means for reducing the carrier frequency-of said received oscillations to a lower but inaudible frequency, and selective means for separating the modified oscillations of reduced carrier frequency representing speech from those representing the telegraph messa e. V r

A carrier wave system comprising means for transmitting and local means for receiving carrier waves simultaneously modified in accordance with two or more independent signal waves, a signal line for transmitting one of said signal waves, means including a balanced transformer for conjugately connecting said signal line to said transmitting and receiving means, a filter interposed between said transformer and said transmitting means, and a similar filter interposed between said transformer and said receiving means, said filters having a transmission range including the frequencies of one of said signal waves but excluding" frequencies of another of said signal waves whereby said transformer need be balanced only at frequencies included within the transmission range of said filters.

5. An oscillator, a. transmission circuit, means including a switch member for abruptly connecting said oscillator in said transmission circuit, and means between said oscillator and said transmission circuit for reducing surges in consequence of said abrupt connection.

6. A thermionic oscillator, a normally open transmission circuit, means for closing said circuit to connect said oscillator therein, and a network of series impedance and shunt impedance elements connected between said oscillator and said circuit to prevent surges therein when said circuit is closed.

7. A radio system comprising means for radiating outgoing carrier waves of a given frequency, a local loop receiving antenna for receiving incoming carrier waves of a different frequency, said receiving antenna being so oriented with respect to said radiating means as to minimize the reception of energy therefrom and including in series a closed loop circuit of negligible reception power for radiated waves, said closed loop circuit. being tuned to the frequency of said radiated outgoing carrier waves to offer substantially infinite impedance thereto.

8. A. radio terminal station, means for radiating therefrom modified waves of carrier frequency f,, means for receiving thereat modified carrier waves of frequency f,, means for producing local reinforcing waves of frequency f,, a demodulating device, means for impressing received waves and said reinforcing waves thereon to produce waves of their difference frequency, and a selectivereception circuit associated with said demodulating device to transmit therefrom, with substantially negligible attenuation, waves having av frequency of the difference between frequencies f and f but extinguishing waves of the difference frequency between f, and either f or'f 9. An oscillator comprising an electric discharge device having acathode, an anode and an impedance control element, a conductive circuit connecting said cathode and control element, a relay in said circuit, and a circuit; including a remote indicator, controlled directly by said relay.

10. In combination, an electric discharge device having a cathode, an anode and an impedance control element, an input circuit including a series capacity element connected to said cathode and said element, means for applying alternating E. M. F.s to said input circuit, a conductive leak path of high resistance directly connecting said cathode and control element, aremote station and an indicator at said station controlled by unidirectional current in said leak path to indicate the presence of an alternating E. M. F. in said input circuit;

11. A radio transmitting station comprising an electric discharge device having a cathode, an anode, an impedance control element and a conductive leak path from said cathode to said element, an operators station remote from said transmittmg station, a calling line terminating at said operators station, a connecting circuit extending from said operators station to said transmitting station for transmission of signal waves to modify the oscillations produced by said oscillator, and means at said operators station controlled by current in said conductive leak path for indicating that the transmitting oscillator is In operative condition and that said signal l ne may be connected to said connecting clrcuit.

12. A carrier comprising an e nel, an lDCOlTllIlg wave transmission system utgomg carrier wave chancarrier wave channel, a slgnal circuit, a balanced transformer for con- 5 meeting said cha jugate relation, signals received for disconnectin nnels to said circuit in conmeans responsive to calling over said incoming channel g said signal circuit from my name this 3rd day of Ma A. D. 1921.

LEWIS M. CLEMENT.

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