US2330593A - High frequency communication system - Google Patents

Description

Sept- 1943- I I: w. KENEFAKE 2,330,593

HIGH Frm uEncY COMIVIUNICATION SYSTEM Filed July 11, 1942 3 2 Sheets-Sheet '1 Inventor: Edwin W Kenefake, b W 19W His Attorney.

Sept. 28, 1943. r E. w. KENEFAKE P 2,330,593

' 4 HIGH FREQUENCY COMMUNICATION SYSTEM Filed July 11; 1942 'Z'SheetS-Sheet 2 7 1 mAmsM/rm? RECEIVE? Inventor; Edwin -W. Kenefa'ke,

His Attorney connection between the Patented Sept. 28, 1943 HIGH FREQUENCY COMIH'UNIOATION SYSTEM 1 Edwin W. Kenefake, Schenectady, N. Y., assignor to General Electric Company, a-corporation of New York Application July 11, 1942, Serial No. 450,499

Claims.

This invention relates to high frequency communication apparatus, and more particularly to that type of such apparatus, called duplex equipment, inwhich the transmission and re ception of signals is accomplished at the same carrier frequency.

Duplex high frequency communication equipment is frequently connected to telephone lines to transmit'and receive signals over such lines. If the input of the high frequency transmitter and the output of the high frequency receiver are both connected to such a telephone line, some means must be provided to prevent oscillations caused by the transfer of energy from the output of the receiver to the input of the transmitter and from the output of the transmitter to the input of the receiver. Hybrid coils are sometimes used for this purpose, between receiver output and transmitter input, and sometimes an arrangement is provided whereby the telephone line may be switched either to the transmitter or to the receiver..

It is an object of my invention to provide a new and simplified high frequency communication system of such nature in which such a telephon line may be connected permanently both to the transmitter input and to the receiver output without the use of hybrid coils or a switching arrangement.

According to my invention a. switching arrangement'is provided whereby the transmitter is rendered operative only when signals are transmitted over the telephone line and the receiver is at the same time rendered inoperative so as to prevent oscillations by reason of the transmitter input and receiver output. The apparatus is also arranged to render the receiver operative and the transmitter inoperative when high frequency signals are received by the receiver, similarly to prevent such oscillations. It is a. further object of my invention to provide improved and simplified means for performing such switching in proper sequence and in reduced time such that no noticeable loss of signals occurs due mg.

It is an additional object of my invention to provide such an improved and simplified high frequency communication system which is easily connected to telephone lines and which is especially adapted for use in providing carrier current communication over high voltage power transmission lines.

The features of my invention which I believe to be novel are set forth with particularity in to such switchthat station through the appended claims. My invention itself, both as to its organization and manner of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings in which Figs. 1a and 1b illustrate schematically an arrangement embodying my invention. These figures are so arranged that the two drawings may be placed side by side, with Fig. Id at the left of Fig. 1b, so that the whole forms a single system with several interconnections between the figures.

In Fig. 1b a high voltage power transmissionline I0 is illustrated as connected at a particular station through a coupling capacitor H to a carrier current transmitter represented by rectangle I 2 and receiver represented by rectangle l3, this transmitter and receiver being arranged respectively to transmit signals from and receive signals for a telephone hand set M. Signals from the hand set l4 are caused to modulate a desired characteristic, such as the intensity, of a carrier wave in transmitter I2, which carrier wave is impressed on the transmission line It at condenser II, for reception at another station. Similarly, signals from such other station are impressed bycoupling condenser II- On receiver I 3 and demodulated to be reproduced by the hand set The coupling condenser ll, transmitter l2, receiver l3 and hand set 14 represent equipment similar to that at such other station. The remaining detailed portions of Figs. 1a and 1b represent such equipment at such other station along the power line In such detailed portions of Fig. 1b, the transmitter includes an electron discharge device l5, which acts, as a master oscillator, and a second such device "5, which acts as a power amplifier for the carrier wave, the output of the device l6 being transferred through a tuning inductance I1 and coupling condenser l8 to the power line H). The transmitter also includes electron discharge amplifierdevices I 9, 20, 2! and 22 for amplifying signals from a telephone hand set 23 and connected telephone line 24, and for modulating the intensity of oscillations in the device IS in accordance with such signals.

'I'he'receiver in Fig. 1b includes an electron discharge device 25, acting as a frequency converter, an electron discharge device 26, acting as an intermediate frequency amplifier, a double diode rectifier 21 for demodulating carrier waves, received from the power line 10 through coupling trated in Fig. lb, which condenser l8 and tuning inductance I1 and amplifled through devices 25 and 26. The receiver also includes signal amplifier devices 28 and 29 which, under certain conditions, amplify signals detected by the detector device 21 and transmit such amplified signals through the telephone line 24 to the hand set 23. l

The transmitter and receiver, so described, are so arranged that they never operate simultaneously. The carrier wave generated by the transmitter therefore is never demodulated by the receiver, and the demodulated carrier intensity variations fed from the output of devices 28 and 29 through the telephone line connections to the input of discharge device l9, so as to modulate the carrier wave intensity in the same sense as the change of intensity which was detected by device 21, thus resulting in continuous oscillation of the entire system. Control .means, provided to prevent such oscillation, is effective to prevent the transmitter and receiver from operating simultaneously. also arranged so as to be capable of operating with sufficient speed in response to the transmission or reception of signals that no substantial part of the beginning of such transmission or reception is lost.

In Fig. 1a, the electron -discharge amplifier device 30 amplifies signals from the hand set 23 and energizes a vapor discharge device represented at 3| so as to initiate operation of the master oscillator device l and to prevent operation of the frequency converter device 25. Details of the manner in which this action is accomplished are set forth hereinafter.

When signals are not present in the system from the hand set 23, the master oscillator device i5 is inoperative and the frequency converter 25 is operative. Under such conditions an electron discharge device 32 in the receiver is effective upon the reception of a signal by the receiver to make the amplifier devices 28 and 29 operative, these devices being normally inoperative, to transmit such received signal to the hand set 23. Such received signal is simultaneo'usly detected by a diode rectifier 33 in Fig. 1a and is amplified by a continuous potential amplifier device 34 to prevent the vapor discharge device 3| from operating. In consequence, reception of signals by the receiver is effective to prevent operation of the transmitter, and at the same time opens up the channel from the receiver to the hand set 23.

The master oscillator of the transmitter illusincludes the discharge device 15, is arranged to be rendered operative or inoperative by switching means in Fig. la. This switching means is connected between ground and a conductor 40, a cathode biasing resistance 4| being connected between the conductor 40 and the cathode 42 of device l5. A bypassing condenser 43 is connected between the cathode 42 and ground, to maintain cathode 42 at ground potential for high frequency currents. The master oscillator circuit in Fig. 1b includes an inductance 44 which has one terminal connected to the anode 45 of the oscillator discharge device l5, and the other terminal'coupled through a suitable coupling condenser 49 to the control electrode 46 of device l5. A tuning condenser 39 is connected in shunt to the inductance 44, thereby forming 'a resonant circuit 39, 44 tuned to the frequency at which the oscillator operates. An intermediate tap 38 of the inductance 44 is connected to a positive This control means is duced if the negative aesaaoa voltage tap of a source 41 of operating potential, whose negative terminal is connected to ground. A grid leak resistance 48 is connected between the control electrode 46 and cathode 42 of device I5.

Whenever conductor 40 is connected to ground, the tuned circuit 39, 44 is thus arranged so as to maintain alternating potentials of opposite phase and of the desired frequency on the anode 45 and control electrode 46 of device l5. The above mentioned. switching means connected to conductor 40 is arranged, when it does not connect conductor 40 to ground to impress a positive potential thereon of such magnitude as to make the cathode 42 positive in potential with respect to ground by an amount at least within ten volts of the positive potential applied to the second, or screen, electrode 31 of device l5. This electrode 31 is connected to a suit able tap 36 of source 41 of substantially less positive potential than the tap to which the anode 45 is connected.

When such a positive potential is impressed on the cathode 42 and correspondingly on the control electrode 46 to which cathode 42 is connected by resistance 48, oscillations smoothly die out as the potential of the cathode 42 becomes more positive, such action taking place within a time determined by the sizes of resistance 4| and condenser 43.

For optimum oscillation of an oscillator, the screen electrode and anode of the discharge device are supplied with positive potentials having a more or less fixed relation therebetween. By the arrangement described above, in which the cathode 4.2 is made less negative with respect to the screen electrode 31 and anode 45 of device I5, this relation between screen electrode voltage and anode voltage of the discharge device is substantially undisturbed, while the oscillation intensity is smoothly decreased by the smooth decreasingof anode voltage and mutual transconductance of the discharge device.

It is of much value to provide such smooth decrease in oscillation intensity of the discharge device l5 whenever positive potential is applied to the conductor 46, to reduce the carrier frequency surges impressed on the power line Ill and consequently on all carrier current receivers connected thereto. Abrupt stoppage of the oscillations produced by device l5 would be probias potential between control electrode 46 and cathode 42 were increased, even if this increase took place very slowly, which abrupt stoppage is avoided by changing the relation between the voltage of cathode 42 and the voltages of the screen electrode 31 and anode 45 as described.

Oscillations produced by the device IS in the tuned circuit 39, 44 are impressed on the control electrode 50 of the amplifier device 16 through a serially connected condenser BI and resistance 62. The device I6 is suitably connected to amplify such oscillationa'thereby producing amplified oscillations across the inductance 63, connected between the anode 64 of device l6 and a tap' 65 on the secondary of the transformer 66, to be described hereinafter. One terminal of the secondary of transformer 66 is connected to the positive tap of the source 41 to which tap 38 of inductance 44 is connected, whereby operating current is supplied to the device I6.

Amplified oscillations appearing across the inductance 63 are coupled'through a condenser 61 .discharge current for the devic I6. flows.

to a tuned circuit 68, from which such oscillations are transmitted through the tuning inductance I1 and coupling condenser I8 to the 23 by reason of the fact that such telephone line 24 connected to 'the hand set 23 is connected across the primary of a, transformer 10, across the secondary of which a voltage di viding resistance II is connected. The first control electrode 72 of the device I9 is connected to an adjustable tap 73 on the voltage dividing resistance II, and one terminal of the resistance "II is connected through a by-passing condenser I4 to the cathode I5 of device I9.

The same terminal of the resistance II is also connected to the third, or gain control, electrode 16 of the device I 9, and through two serially connected resistances I1 and 18 to ground. The cathode I5 is connected through a biasing resistance 80 to ground; a by-passing condenser 8| be-- ing connected in shunt to the resistance 80. A by-passing condenser 82 is connected in shunt to the resistance I8 to prevent voltage changes on resistance 18 from occurring rapidly. When voltage changes occur across the resistance 18. the gain of the device I9 is varied by reason of the fact that the average bias potential on the control electrodes 12 and I6 is'changed.

Discharg current for the device I9 is supplied to the anode 83 thereof through a. resistance 84 from a suitable tap 85 on the source 41 of operating potential. Voltage across resistance 84 is impressed on the control electrode 86 of device 28 through a coupling condenser 6T, electrode 86 being connected to ground through a resistance 88. The cathode 89 of device 20 is con-- nected to ground through a resistance 90, the flow of discharge current through such resistance providing suitable operating bias potential for the control electrode 86.

Discharge current .for the anod 9| of device 20 is supplied through a resistance 92 from the tap 85 of source 41. Similarly, discharge current for the other anode 93 of device 20 is supplied thereto through a resistance 94 from the tap 85. The other cathode 95 of device .20 is I resistance I22 to a point between the 25.

connected to ground through a biasing resistanc 96 by-passed by a. suitable condenser 91, the control electrode 98 which cooperates with the anode 93 and cathode 95 being connected to ground through a suitable resistance 99.

The discharge devices 2| and 22 ar connected in push-pull relation, their respective control electrodes I00 and IM being coupled respectively to the anodes 9i and 93 of device 20 through condensers I02 and I03. The control electrodes I00 and IM are-respectively connected to the control electrode 98 of device 20 through resistances I 04 and I05. The respective cathodes I06 and I 07 of the devices 2i and 22 are connected together and to ground through a suitable biasing resistance I08. The anodes I09 and I I0 are connected to the respective terminals of the primary of transformer 66, the center tap of such primary being connected to.the positive terminal of source 41 thereby tosupply discharge current for the operation of the device's 2i and 22.

So connected the amplifier acts to amplify signals from the hand set 29 and to modulate the discharge current supplied from source 41 to the carrier wave amplifier I6 in accordance therewith. To increase the fidelity of such amplification, a resistance I I2 is coupled through a coupling condenser III between the anode I09 .of device 2I and the cathode 89 of device 20. The resistance 2 is made as small as possible without causing oscillation in the amplifier, in order to produce a maximum amount of degeneration fromthe output of the push-pull stage including devices 2I and 22 to the input of the device 20. Such degeneration is well known to increase the fidelity of amplification of an amplifier.

In order to limit the amount of modulation of discharge current supplied to the device I6 below a predetermined value'such that this discharge current for the device I6 never approaches zero,

the cathode I20 of a rectifier H9 is connected to that terminal of the secondary of transformer 66 opposite the'terminal which is connected to the source 41 of operating potential. The anode I2I of the rectifier H9 is connected through a resistances I1 and I8, described above in connection with the device I9. The resistance I22 and the condenser 82 together form a low pass filter circuit, so that when the rectifier H9 passes current, such currentbeing passed only during alterate sistance 18. r

This modulation control circuit is described and claimed in the copending application of George M. Brown, for Modulation control systems, Serial No. 346,947, filed July 23, 1940, and assigned to the same assignee as the present application. Its operation dependson the fact that the continuous potential from the source 41 is impressed with equal intensity on the cathode I20 of the rectifier I I9 and the anode 64 of the device I6, while the amplified signal potential is impressed on the cathode I20 with greater intensity than on the anode 64, on account of the autotransformer action of the secondary of transformer 66. Accordingly, when the peak intensity of the amplified signals reaches a predetermined percentage of the continuous potential, the rectifier I I9 conducts and reduces the gain of the discharge device I9. I

The transmitter described is effective, when the conductor 40 is grounded, to generate carrier waves which are transmitted over the power line I 0, and to modulate such carrier waves in accordance with signals from the hand set 23. When the transmitter is-inoperative, that is, when the conductor 40 is not grounded, carrier waves may be received over the power line I0 and demodulated by the receiver, the demodulated signals being supplied to the hand set 23.

Such carrier waves coming from a distant station over the power line I0 and through the coupling condenser I8 and tuning inductance H, are impressed across a voltage dividing resistance I30, to which the receiver is connected. A resistance I3I and a vapor discharge device I 32 are connected serially between an adjustable tap I33 of the resistance I30 and ground. The vapor discharge device I32 is of the type through which a discharge current passes only after a voltage greater than a critical value has been applied to the electrodes, and which has very low resistance once such discharge current passes. The device I32 acts to prevent large carrier wave voltages, as from the power amplifier half cycles, a continuous potential appears across the; re-

device I6, from being impressed on the frequency converter device 25.

Carrier voltage across the device I32-is impressed through a doubly tuned circuit I34 between the third or signal control electrode. I35 of the converter device 25 and ground. The converter device 25 is arranged, in well known fashion, to generate, in conjunction with suitable tuning and coupling circuits I36, a local oscillation, and to heterodyne such local oscillation with the signal wave on the control electrode I35 and thereby produce a wave of intermediate frequency. For example, it has been found desirable in certain instances, where the wave impressed on the tuned circuit I34 may be within the range from 50 to 150 kilocycles, to use a local oscillator circuit I36 tunable between 225 and 325 kilocycles and a fixed intermediate frequency of 1'75 kilocycles. Coupled circuits I40 and MI, tuned to this fixed frequency, are arranged to transfer the output of converter device 25 to the control electrode I42 of device 26. Additional fixed tuned circuits I43 and I44 are arranged to transfer the amplified oscillations of such fixed frequency from device 26 to the rectifier, or detector device 21.

The detector 21 has two separate diodes, respectively including cathode I50 and anode II and cathode I52 and anode I53, which perform two separate functions. The first diode including cathode I50 and anode I5I rectifies the amplified carrier wave from tuned circuit I44, when the intensity of that wave is above a predetermined value, and produces a continuous potential for automatic volume control purposes. The other diode including cathode I52 and anode I53 rectifies the carrier wave from the tuned circuit I44 to produce audible signals.

In the first of these diodes, the cathode I50 is maintained at a positive potential with respect to ground by a connection from the cathode I50 to a positive voltage tap I54 on a source I55 of potential, whose negative terminal is grounded. The anode I5I is coupled by a condenser I56 to one terminal of the tuned circuit I44, the other terminal of circuit I44 being grounded. The anode I5I is connected through two serially connected resistances I51 and I58 to ground, the resistance I58 being by-passed for high frequency currents by condenser I59.

The resistance-condenser combination I51, I58, I59 serves as a low pass filter, so that voltage variations across resistance I58 correspond only to variations in the average carrier wave 'intensity. The control electrodes I and I42.

which are connected respectively through tuned circuits I34 and MI, are connected through such circuits to ground throughthe resistance I58. When the carrier wave in the tuned circuit I44 is sufficiently intense to cause the anode I5I and cathode I to conduct current, a negative voltage with respect to ground is developed across the resistance I58, which increases the negative bias potential on the control electrodes I35 and I42 with respect to their respective cathodes, so that the gain of the discharge devices 25 and 26 is reduced.

In order that the other diode in the rectifier 21 shall function to detect audible signals modulated upon the carrier wavefrom tuned circuit I44, the anode I53 is coupled to the ungroundedv terminal of the tuned circuit I44 through a condenser I60, and the cathode I52 is coupled to ground through a similar condenser I6I. Four re istances I62, I63, I64 and I65 are serially connected in that order between the anode I53 and cathode I52. Each of the resistances I63 and I64 is by-passed for currents of carrier frequency by condensers I66 and I61, respectively, and a point between these two resistances I63 and I64 is connected to ground. Audible signals appear across the resistances I63 and I64 in push-pull, or balanced, relation.

The balanced signal voltages across the resistances' I63 and I64 are amplified through devices 28 and 29, which are connected in pushpull or balanced relation. The cathodes I10 and HI of. the devices 28 and 29 are connected together, and through a resistance I12 to a positive voltage tap I13 on the source I55. The respective control electrodes I14 and I15 of the devices 28 and 29 are connected to adjustable taps on voltage dividing resistances I16 and I11. One terminal of each of the resistances I16 and I11 is connected through a resistance I16 to the tap I13 on the source I of potential. The remaining terminal of each of the resistances I16 and I11 is connected respectively through coupling condensers I19 and I80 to a point between resistances I62 and I63 and a point between resistances I64 and I65, thereby impressing the detected signal voltages in push-pull relation between the control electrodes I14 and I15.

The anodes I8I and I82 of the devices 28 and 29 are connected to opposite terminals of the primary of a transformer I83, the center tap of the primary being connected to the positive terminal of the source I55. The secondary of transformer I63 is connected to telephone line 24 to transfer signals amplified through the devices 28 and 29 to the hand set 23, where they are reproduced as sound.

The device 32, which is arranged to prevent the transmission of energy through the devices 28 and 29 to the hand set 23 when no carrier wave is received by the receiver, is operated'in accordance with the average potential developed across the resistance I63 in the diode detector circuit of the receiver. To this end, the cathode 190 of the device 32 is connected to ground, and the control electrode I9I is connected to ground through a condenser I92, and through a resistance I93 to a point between resistances I62 and I63. The anode I94 of the device I32 is connected to a point between resistances I16,- I11 and I18, and is thus supplied with discharge ourrent from the tap I13 of source I55 of operating potential. A condenser I is connected in shunt to resistance I18 for a purpose to be e plained in detail hereinafter.

When a carrier wave is received by the receiver, a continuous potential appears across the resistance I63, such that'the point between resistances I62 and I63 is negative with respect to ground. This negative potential appears on the control electrode I91 of the device 32, which is so arrangedthat its discharge current is substantally cut ofi by the presence of this negative potential on its control electrode I9I with respect to the ground potential of its cathode I90. Although the discharge current of device 32 is cut 05 at a speed determined by resistance I93 and condenser I92, the potential of anode I94 returns to the positive potential of tap I13 of source I55 at a speed determined by the resistance'l18 and condenser I95. This speed is adjusted in a manner to be described hereinafter.

When the discharge current of device 32 is thus substantially cut ofl, substantially no po- I ceiver.

tential drop exists across resistance I18, and the circuit between the control electrodes I14 and I15 of devices 28 and 29 and cathodes I and I'll thereof includes no differences of potential, except that across the resistance I12 occasioned by the flow of the discharge current of devices 28 and 29 therethrough. This potential across the resistance I12 is adjusted to a value such as to provide a suitable operating bias for the control electrodes I14 and I15.

When the receiver is not receiving a carrier wave, the potential across resistance I 63 is reduced substantially, and the potential of control electrode I9I accordingly approaches the potential of cathode I90 of discharge device 32. Consequently, discharge device 32 passes discharge current and a potential drop appears across the resistance I18 at a speed determined by the anode to cathode resistance of the device 32 and by the size of condenser I95. That is, the speed with which voltage appears across the condenser I95 is determined primarily by the resistance between the anode I94 and cathode I90 of device 32, while the speed with which this voltage disappears when the device 32 is cut off is determined primarily by the size of resistance I18.

' and 29 in about three milliseconds.

The resistance I10 .was made sufilciently large I that it took about six milliseconds after reception of a carrier wave for the voltage to disappear from condenser I95 suificiently to allow devices 28 and 29 to reproduce signals in the hand set 23. This delay in allowing the devices 28 and 29 to reproduce signals is desirable in order to allow transients in the distant transmitter, in the power line, and in the receiver circuits to die out before energizing the hand set 23 from the re- Such transients are caused by the extremely rapid switching utilized in transmitters and associated equipment according to my invention. The six milliseconds delay in energizing the hand set 23 from the receiver, added to the inevitable delay in starting the transmitter in response to signals to be transmitted is sufliciently short that the loss of even the beginning of the first syllable at the beginning of a trans mission is avoided.

The condenser I92 in conjunction with the resistance I93 serves to prevent very rapid changes in the conductivity of device 32, which might be caused by noise pulses appearing across the resistance I63 and lasting only for very short times. The low pass filter including resistance I93 and condenser I92 serves to prevent any voltage change on the control electrode I9I in response to such voltages of short duratiom'and thus to prevent false operation of the noise suppression circuit including the device 32 in response to such short voltage pulses.

In addition to the above described means for silencing the output of the receiver in the absence of the received carrier, additional means is provided to prevent operation of the transmitter while a carrier wave is being received by the receiver. When a carrier wave is received the receiver so that a voltage of intermediate frequency appears across the tuned circuit I44, this voltage is impressed across the rectifier device 33 in Fig. 1a through a coupling condenser 200 205 connected between suchanodeand cathode.

The resistance 205 is shunted by a condenser 206 whose reactance is low at the intermediate frequency to which tuned circuit I43 is resonant,- but whose reactance is high at frequencies at which the intensity of the intermediate frequency carrier wave changes. Consequently such intermediate frequency carrier waves are rectified in the device 33 and a unidirectional potential appears across the resistance 205 within a short time afterthe reception of carrier waves by the receiver, this time being, for example, in'the order of 1 millisecond or less. This unidirectional voltage across the resistance 205 is utilized to prevent operation of the transmitter, which operation might otherwise be caused by signals from the hand set 23 impressed on the transmitter, or byreceived signals from the receiver.

The apparatus'which initiates operation of the transmitter in response to signals from the hand set 23, and which is blocked by voltage across the resistance 205, includes a transformer 2I0, the primary of which is energized through telephone line 24 by signals from the. hand set 23. That is, the primary of transformer H0 is connected in parallel with the primaries of transformers I0 and I83, all three primaries being connected to the telephone line 24. A voltage dividing resistance 2 is connected in shunt to the secondary of the transformer 2|0 and one end thereof is grounded. The control electrode 5H2 of discharge device 30 is connected through a resistance 2I3 to an adjustable tap 2I4 of the resistance 2| I, and the cathode M5 is connected through a shunt combination of resistance 2I6 and capacity 2H to ground. The anode 2I8 of the device 30 is connected through a pair of serially connected resistances 2I9 and 220 to the positive terminal of a source 22I of operating potential, an intermediate tap 222 of which is ground. A point between resistances 219 and 220 is connected to ground through a condenser 223 to maintain such point at ground potential for signal frequency currents. Another condenser is to prevent the production of parasitic oscillations by the device 30, or the amplification of high frequency energy which may be picked up from the transmitter mounted in close proximity,

and also to reduce thetransmission of high frequency noise components through device 30. In one particular case the device 30 was a 6SQ7, the condenser 224 was 0.001 microfarad and 'resistance 2 I 9 was 100,000 ohms.

The signal frequency voltages developed across resistance 2|9 by the amplifier 30 are impressed on the control electrode 230 of the vapor discharge device 3| through a coupling condenser 23L The cathode 232 of the device 3| is grounded, and the control electrode 230 is connected through two serially connected resistances 233 and 234 to a suitable tap 235 of the source 22| at a negative potential with respect to the tap 222 and ground. The negative potential between the tap 222 and 235 is sufficient, when no signal is applied to the control electrode 230, to maintain the device 3| nonconductive. Discharge current is supplied to the anode 235 of the device 3| through the actuating winding 231 of a relay having an armature 238, and through a resistance 233 from the positive terminal of th source 22 Signals amplified by device 30 which appear across the resistance 2!!! are impressed through the coupling condenser 23| on the control electrode 230 with sufiicient intensity to decrease its negative potential during a portion of each cycle sufficiently so that the device 3| becomes conductive.

The condenser 23| and resistance 233 are so proportioned that signals of all frequencies from the hand set 23 are impressed on the control electrode 230 of device 3| with substantially equal intensities. This is to insure that the transmitter shall be started with equal speed, whether a message be started with a high frequency syllable, such as one including a sibilant, or a low fre quency syllable, such as one of the vowel sounds. It is well known that low frequency sound signals in general contain much more energy than high frequency sound signals. The condenser 23| is therefore made small in capacity so that such low frequency sound signals are not transmitted to the control electrodes 230 in larger relative intensity than high frequency sound signals. In the case discussed above in connection with the condenser 223 and resistance 2 I 9, it was found desirable to make the condenser 23| of 0.001 microfarad and the resistance 233 of 250,000 ohms. The device 3| was a type 2051 gas tetrode. With such values, transmission of signals from resistance 2|9 to the control electrode 230 of device 3| was reduced in increasing amounts at frequencies below 800 cycles.

The device 3| has a second control electrode 231 which, during normal operation of the device 3|, may be maintained near the potential of the cathode 232, but which, when it is made sumciently negative with respect to the cathode 232, prevents discharge current from flowing in the device 3| even though the control electrode 230 is made positive. That is, whenever either of the control electrodes 230 or 231 is maintained sufliciently negative with respect to the cathode 232, discharge current cannot be initiated in the device 3|.

Means is provided to prevent the initiation of the flow of discharge current in device 3| by maintaining the control electrode 231 negative with respect to cathode 232 whenever a signal is received by the receiver. A explained previously, whenever a signal is received by the receiver, a unidirectional potential appears across the resistance 205. This potential across resistance 205 is amplified through the continuous potential amplifier device 34 and, after amplification, maintains the control electrode 231 negative with respect to cathode 232, whenever a unidirectional potential exists across resistance 205.

The continuous potential amplifier device 35 may, as illustrated, include two triode amplifier sections, and may, for example, be a 608G twin triode, the control electrode 240 of one section being connected ot a point between resistances 204 and 205. The cathode 2 of this triode sec tion is connected through a resistance 242 to the cathode 202 of rectifier device 33, and to a tap 243 on the source 22| at a potential with respect to tap 222 and ground, which potential is considerably more negative than that of the tap 235. The anode 244 of this triode section is connected to the control electrode 2450f the other triode section, and is also connected through a resistance 246to a tap 241 on the source 22| of potential more negative with respect to ground than the potential of tap 235, but substantially less negative than the potential of tap 243 to which the cathode 24| is connected.

The cathode 248 of the other triode section is connected to a tap 249 on the source 221 whose negative potential is intermediate in value the negative potentials of the taps 241 and 243. The anode 250 of this other triode section of the device 33 is connected through a voltage dividing resistance 25| to ground. The control electrode 231 of the vapor discharge device 3| is connected to an adjustable .tap 252 on the voltage dividing resistance 25 i In operation, the continuous potential amplifier is supplied with discharge current from the source 22 I. When substantially no voltage exists across the resistance 205, in the absence of a signal in the carrier wave receiver, the control electrode 240 is at a minimum negative bias potential with respect to the cathode 24l, so that a maximum discharge current flows throughthe anode 244. The size of resistance 246 and the potentials of the taps 241, 249 and 243 with respect to each other and with respect to ground are so adjusted that the control electrode 245 is sufliciently negative with respect to the cathode 248 that substantially no discharge current flows in the anode 250, and consequently the control electrode 231 of the vapor discharge device 3| remains substantially at the potential of the grounded cathode 232. Thus, whenever a signal is impressed on transformer 2|0 from the hand set 23, making thecontrol electrode 230 positive with respect to cathode 232, the discharge device 3| is caused to carry discharge current and initiate operation of'the transmitter, and is not prevented by a negative potential on control electrode 231.

Whenever the carrier wave receiver is receiving a signal and a continuous potential exists across the resistance 205, the control electrode 240 becomes more negative with respect to the cathode Ni and reduces the discharge current in' the anode 244. The potential of control electrode 245 then becomes less negative with respect to the cathode 248 and discharge current flows in the anode 250. Since the potential of the cathode 248 is fixed at a negative value with respect to ground, the potential of the anode 250 when discharge current flows therein becomes negative with respect to ground and the voltage dividing resistance 25| consequently impresses a negative potential on the control electrode 231, The intensity of this negative potential on the control electrode 231 may be adjusted by the adjustable tap 252, so as to prevent discharge current from flowing in the vapor discharge device 3| even though the control electrode 230 becomes positive with respect to cathode 232.

Under some conditions of operation it may be desired to break in on a message from a distant station, even though the operator at the distant station is still operating his transmitter, and the receiver is still receiving a carrier wave so that the control electrode 231 of vapor discharge device 3| is still negative in potential. If this type of operation be desired, the negative potential impressed on the control electrode 231 of vapor discharge device 3| may be reduced by suitable adjustment of the tap 252 of voltage dividing resistance 25|, so that, when a particularly intense signal is applied to control electrode 230 through the hand set 23, the control electrode 230 will be made sufiiciently positive with respect to cathode 232 as to overcome the effect of the negative potential on control electrode 231, thereby making the vapor discharge device 3| pass discharge current, even though the receiver is at the time receiving a signal. The adjustable tap 252 on the resistance 25| allows the control equipment to be adjusted, therefore, in either manner, so that the transmitter can never'be operated when the receiver is operating, or alternatively so that the transmitter can be operated by shouting or whistling into the hand set 23 even when the receiver is operating. I v

Such adjustment of the adjustable tap 252 of the voltage dividing resistance 25| may also be made where it is desired to force the transmitter into operation in the presence of received noise of high intensity. Under certain circumstances, random noise voltages and the like may be received by the receiver and detected by the rectifier device 33 so as to apply a negative bias potential on the control electrode 231 of the vapor discharge device 3|. Obviously, it is desirable in the presence of such high noise voltages, to reduce the negative potential applied to control electrode 237 to such a value that the transmitter can be turned on by whistling or shouting in the hand set 23.

The vapor discharge device 3| and the relay including operating coil 231 and armature 238 are so connected and arranged, together with the master oscillator device l5, that, immediately after the vapor discharge device 3| begins to carry current, the master oscillator device I5 produces oscillations, and the relay maintains the oscillator device |5 in operation for a predetermined time after the cessation of discharge current in vapor discharge device 3|. The relay is also arranged so that it is operated in response to the flow of discharge current in the vapor discharge device 3| with minimum time delay, and is further arranged to reduce the negative bias potential on the control electrode 230 of device 3| so as to make it easier for signals from the hand set 23 to maintain the device 3| conductive after it has once become conductive. The relay is also arranged to apply an alternating potential to the anode 236 of vapor discharge device 3| after it has become conductive, so that, during alternate half cycles of the applied alternating potential, the control electrodes 230 and 231 may stop the flow of discharge current whenever a suitable negative potential is applied thereto. The vapor discharge device 3| and relay together are arranged to block the operation of the carrier wave receiver even before the oscillator device I5 is made to operate.

In order that the relay shall open a predeter- I mined time after the vapor discharge device 3| stops passing current, a coil 266 is provided, in

operated by the armature 238. The coil 266, normally open switch 26|, and an adjustable resistance 265 are connected in series, so that, when the operatingcoil 231 is deenergized, current tends to continue flowing in the coil 2'66 and resistance 265, thereby maintaining the armature 236 in operated position for a time determined by the adjustment of resistance 265. A delay of about to 300 milliseconds may be provided between the deenergization of operating coil 23! and the opening of switches 26|, 262, 263 and 264.

Switch 2620f the relay is arranged to reduce the negative bias potential on control electrode 236 of vapor discharge device 3|, so that signals from the hand set 23' may easily maintain the device 3| conductive after it has once been ren- For this purpose the switch dered conductive. 262 and a resistance 266 are connected serially between ground and a point between the resistances 233 and 234. Resistances 234 and 266 act as a voltage dividing resistance, so that, when switch 262 is closed, only a portion of the negative potential between tap 235 of the source 22| and ground is applied to the control electrode 2311.

Switch 264 of the relay is arranged to apply an alternating potential to the anode 236 of vapor the deenergized position of the relay conductor 40 of Fig. lb'is connected through switch 263 to, the anode 236 of vapor discharge device 3|. As explained previously, whenever conductor 46 is maintained at ground potential, the oscillating discharge device l5 produces carrier frequency oscillations. Connection of the conductor 46 7 through switch 263 to the anode 236 of device 3| is also effective, when device 3| is nonconductive, to maintain the cathode 42 and electrode 46 of oscillating device l5 at a positive potential, whose magnitude was explained previously, with respect to ground, by reason of the connection through the operating coil 23'! and resistance 239 tothe positive terminal of source 2-2|. As pointed out thus previously, whenever cathode. 42 of oscillating device I5 is at such a suitable positive potential, the oscillating device I5 is effectively prevented from producing oscillations.

Switch 263 of the relay is also arranged so as to connect conductor 40 directly to ground whenever the relay is in its energized position. By such connection, it is necessary for the vapor discharge device 3| to carry the discharge current of oscillating device I 5 only for a short time immediately after the vapor discharge device 3| is first made conductive. Also, the oscillating device l5 by this direct connection to ground is maintained in operation so long as the relay is in its energized position, which persists for a predetermined time after the vapor discharge device 3| becomes nonconductive by reason of the action of coil 260 of the relay and switch 26| and resistance 265. This delay in turning oil the carrier wave generator of the transmitter is desirable so that the transmitter does not turn ofi between syllables or between words of a message, but turns off only when a message is finished. It is generally found that a delay in turning ofi the carrier wave generator of the transmitter of 150 to 300 milliseconds, as mentioned previously, is suflicient for this purpose.

It is desirable that the relay including switch 263 operate as fast as possible, preferably within 8 milliseconds after its energization, in order that the cathode 420i the master oscillator discharge device shall be connected directly to ground through switch 263 before the noise suppression circuit of the distant receiver shall energize the hand set It at that receiver. If these time relations are not observed, and the noise suppression circuit in receiver I3 opens the channel to hand set !4 before the relay including switch 263 operates, the operation of switch 263 causes the master oscillator device !5 to stop operation .for an instant as the switch 263 passes between its contacts.

In order to provide fast operation of the relay, a higher voltage than necessary to operate the relay is applied to its operating coil 23'! from source 22 and the steady current after operation of the relay is limited by resistance 239. Also, a resistance 28!! is connected between the anode 236 of vapor discharge device 3! and ground, so that, when device 3! is not conductive, a small current flows through the operating coil 23'! of the relay, which current is not quite suficient to operate the relay or even to maintain it operated. By providing this initial current to the operating winding 23'! of therelay and by impressing a voltage across the winding greater than that necessary to operate it, very fast operation may be attained. As pointed out above, it is desired that the relay shall be operated to its energized position before the noise suppression circuit in the distant receiver !3 shall energize the hand set !4.

In addition to the previously described control arrangement, another continuous potential amplifier device 28! is provided which is responsive to changes of potential on the conductor 40 for rendering the converter device 25 in the receiver operative or inoperative. The device 28! may conveniently be a type 6C8G twin triode unit similar to the device 34. The control electrode 282 of one section of the device 28! is connected to the adjustable tap of a voltage dividing re,- sistance 283, which is connected between the conductor 40 and the negative terminal of the source 22!. Another voltage dividing resistance 284 is connected between the negative terminal of source 22! and ground, the cathode 285 of the first section of the twin triode device 28! being connected to a tap 286 on the resistance 284. The anode 28'! of this section is connected to the control electrode 288 of the other triode section of device 28!, and is also connected through a resistance 289 to another tap 290 on resistance 284 which is less negative in potential than the tap 286. The cathode 298 of this other triode section of device 28! is also connected to the tap 290, and the anode 29! of this other triode section is connected through a resistance 292, shunted by a condenser 293, to ground.

The twin triode device 28! and its associated connections as thus described are -so adjusted that when the potential of the anode 236 of vapor discharge device 3! plus the negative potential with respect to ground of source 22! are impressed across resistance 283, substantially no potential exists across the resistance 292. Also, when the conductor 40 is connected to ground, either through the vapor discharge device 3!, when it is conductive, or through the switch 263 directly to ground, a. potential appears across the resistance 292 such that the anode 29! of the second triode unit in device 28! becomes nega tive with respect to ground.

In order to block the operation of the con-' verter device 25 of the receiver in response to negative voltage developed across the resistance 292, this negative voltage is applied to the control electrodes 294 and 299 of device 25, these two control electrodes being connected together and to the anode 29! of the second triode section of device 28! through a grid resistance 295. The control electrodes 294 and 299 are coupled to the tuned circuit I36 of the oscillator section through a coupling condenser 296. That terminal of the resistance 295 opposite the control electrode 294 is connected to ground at the converter device 25 through a by-passing condenser 29'! which is small with respect to the condenser 293. The condenser 29'! should have a reactance which is low at the frequency of operation of the tuned circuit I36.

I The time within which a negative voltage is built up across the condenser 293 depends on the size of the condensers 293 and 29! and the anode to cathode resistance of the second triode section of device 28! including the anode 29! and cathode 298. It is desirable that the time of charging this condenser 293 to a sufiiciently negative potential to prevent the operation of converter device 25 be small, as for example 2 milliseconds.

Of course, as the connections for device 28! are illustrated, the voltage dividing resistance 284 also contributes to the time constant of the circuit including the condenser 293. If the power delivering capability of the source 22! be made relatively small, so that the voltage dividing resistance 284 must be relatively large, it may be that the resistance 284 is larger than the anodeto-cathode resistance of one section of the device 28!. In any case the charging time of the condenser 293 should be small, and preferably 2 milliseconds or less.

In a particular case where the source 22! delivered about volts negative potentia with respect to ground, and the voltage dividing resistance 284 was of the order of 50,000 to 75,000 ohms, the device 28! being a 6086- twin triode unit, the condenser 293 was made 0.1 microfarad. The condenser 29'! was much smaller than condenser 293, condenser 296 was, about 200 micromicrofarads, and resistance 295 was about'l00,- 000 ohms.

It is desired that, after the conductor 40 has been disconnected from ground and returned to a positive potential, the converter device 25 shall remain inoperative for a relatively long period, in the order of 25 to 50 milliseconds. In order to accomplish this the resistance 292 must be relatively large, so that the condenser 293 discharges therethrough relatively slowly. In the particular case described, this resistance 292 was 1 megohm.

It was found that the oscillator section of the converter device 25 would operate properly, even with this high resistance 292'in its control electrode cathode circuit, provided the condenser 296 be sufficiently small. If the condenser 296 be too large, blocking of the osci. ator section results, producing a kind of superregenerative action or an audible blocking. The necessity for making condenser 295 small can be avoided by using a smaller resistance 284 as the voltage divider, so that the negative potentials of the mized.

taps 286 and 290 are more nearly fixed, whereby the condenser 293 may be made larger and its charge time still remain small, with the result that the resistance 292 may be made smaller, thereby reducing the possibility of superregenerative blocking action in the oscillator section of converter device 25.

The blocking of the receiver is brought about by impressing a large negative bias potential on the control grid of the oscillator section of the frequency converter device 25 as well as on the control grid of the mixer, or heterodyne, section of the frequency converter device 25. By providing this negative blocking. potential on both of these control electrodes, the possibility of the production of intermediate frequency potentials is made small, and the possibility of any carrier wave reaching the detector device 21 is mini- As described, there are four difierent control arrangements, each of which performs two control operations. Control means is provided which responds to signals from the hand set 23 to start and stop the generation of a carrier wave. Control means is provided, responsive to signals from the hand set 23, to make operative or inoperative the frequency converter device 25 in the receiver. Control means is provided responsive to signals received in the receiver for preventing or allowing the first control means to control generation of a carrier wave in response to signals from the hand set 23. Finally, control means is'provided responsive to signals received in the receiver for transferring such signals from the receiver to the hand set 23.

The eight operating times of these four control means are so correlated as to make it possible to connect the hand set 23 directly with the transformers Ill and I83 without producing oscillation between the transmitter and receiver. There are two conditions under which the entire apparatus at a particular station is used, one condition being that of the transmission of a message, and

the other condition being that of reception of a message. Under each such condition two sets of operating times are involved, one occurring at the beginning of the message and another at the end of the message. In each case an operation is performed both on the transmitter and on the receiver to make one operative and the other inoperative, or, at the end of a message, to put the receiver into condition to receive a signal.

The operating sequences of the entire apparatus may now be described. When the apparatus is in standby condition, and a sound impinges on hand set 23 of sufiicient intensity to cause transmission, this sound is amplified through amplifier device 30 and causes vapor discharge device 3 to become conductive. Within a very shorfl'time and before relay 231, 260 operates to close any of its contacts the continuous potential amplifier device 28| produces a large negative bias potential on the control electrodes 294 and 299 of the frequency converter'device 25 in the receiver, to prevent/operation of the receiver. Shortly, and

- as' soon as possible thereafter, the potential of the cathode 42 of oscillating discharge device l5 reaches a value such that oscillations are generated, and are modulated in the carrier wave amplifier device 64 in accordance with the signals from the hand set2'3 which initiated operations. As pointed out previously, the receiver may, for

' example, be rendered inoperative in response to signals from hand set 23 in about 2 milliseconds,

oscillations to be modulated in accordance with such signals in about 3 milliseconds.

So long as signals continue to be impressed from the hand set 23 upon the vapor discharge device 3| to maintain it conductive, transmission continues. Transmission of the carrier wave also continues in the absence of such signals, so long as such signals from the hand set 23 are not absent for more than the time required after deenergization of the operating coil 231 for th relay to move to its unoperated position. As pointed out previously, the relay operates after about eight milliseconds and it may be adjusted so that it remains in its operated position for 150 to 300 4 milliseconds after the. vapor discharge device 3| becomes nonconductive. Thus, if signals from the hand set 23 are not interrupted for periods longer than 150 to 300 milliseconds, the relay does not move to its unoperated position, and the transmission of carrier waves is continued. At the end of the message, a period occurs without signals from hand set 23 whose duration is greater than the period required for the relay to move to its unoperated position, and at the end of that time transmission of the carrier wave is discontinued.

When transmission of the carrier wave is discontinued, the continuous potential amplifier device 28| allows the condenser 293 to discharge again. The device 25 may, for example, become operative within 25 or 30 milliseconds after the transmission of the carrier wave discontinues.

If, during the reception of a message or noise, the transmitter be started by shouting or whistling into the hand set 23 as explained previously, the frequency converter device 25 in. the receiver is first blocked and then the oscillating device i5 is caused to begin the transmission of a carrier wave. Under no COllQitiOIl can the receiver be operative when a carrier wave is being transmitted. There is thererore no possibility of oscillation through the transmitter and receiver even though both are connected directly to telephone line 24.

Upon the reception of a carrier wave over the power line It], as from a distant station including the hand set M, the receiver first energizes the rectifier device 33 in the control apparatus of Fig. la, and, through the continuous potential amplifier device 34, blocks the vapor discharge device 3| from becoming conductive. Thereafter,

set 23 from received signals aiter reception of the carrier wave by the receiver should be in the order of 6 milliseconds in order to allow surges of various sorts in the system to die out.

electrode 23'! of device 3| in less timethanthis short period to prevent device 3Lrf ron'i becoming nonduotiver For exampleraft? reception of the carrier wave by the receiver, a bias potential may 7 appear on the control electrode 231 of the vapor discharge device 3| in about 1 millisecond.

The period after reception of a carrier wave by the receiver before the device 32 operates should be as short as possible, so that the time and the transmitter may b made toj genrate elapsing between initiation of a signal at the The continuous potential amplifier device 34 should be effective to put a bias potential on the control hand set l4 and its reception at hand set 23 is small enough to avoid losing any significant part of the beginning of a message. This time, however, should be sufficiently long to allow surges in the carrier wave transmission system and in the automatic volume control circuits of the receiver to die out substantially before the device 32 makes the amplifier devices 28 and 29 operative. In view of the desirability of allowing time for such surges to die out, it is desirable that they be initiated in minimum time. Hence, it is highly desirable that the transmitter be turned on in minimum time after the impression of a signal on the associated hand set. If, as explained previously, the transmitter is effective to transmit a modulated carrier wave in about 3 milliseconds after impression of a signal on the associated hand set, the receiver may be made to demodulate such signal and impress it on the associated hand set in about 6 milliseconds after reception of the carrier wave, this period being sufiicient to allow surges to die out substantially. Under such conditions, the total time of transmission of a signal from one of such hand sets to the other is only about 9 milliseconds, which is an acceptably short time, within which no appreciable portion of the beginning of a syllable is lost.

When the receiver no longer receives a carrier wave from the distant station, the noise suppression circuit including the device 32 first operates to block the amplifier devices 28 and 29. This action may take place in about 3 milliseconds after the carrier wave is no longer received. Thereafter, for example, in about 6 milliseconds, the bias disappears from the control electrode 231 of vapor discharge device3l, thereby leaving the apparatus as a whole in its standby condition. The time within which the negative bias disappears from the control electrode 231 is determined by the sizes of resistance 205 and condenser 206. The larger the resistance 205, the longer it takes for the charge to disappear from condenser 206.

By thus arranging carrier wave transmitting and receiving apparatus according to my invention it is possible to operate it in response to signals to be transmitted to cause transmission and reception of such signals, and to operate it with sufiicient speed that no significant portion of the beginning of any signal is lost. For example, the total time from the point of time when sound impinges on the hand set until such sound is reproduced by the hand set l4 in a distant station includes only the time required for the cathode 42 of the oscillating device l5 to reach its proper potential, and the time for the noise suppression circuit including the device 32 in the distant receiver l3 to energize the hand set [4 at that station.

In the example of time given where such total time is about 9 milliseconds, no appreciable portion of the beginning of a message is lost. Assuming that the average frequency of the first syllable of a message is 200 cycles, in 9 milliseconds only two cycles of the signal are lost. These first two' cycles do not contribute substantially to the intelligibility of the first syllable of the message.

Furthermore, no appreciable pause between the cessation of transmission from one station and the beginning of transmission from the distant station is necessary because, after the cessation of transmission at one station, the carrier wave is turned off in from 150 to 300 milliseconds, long before the operator at the distant station realizes that the message is ended. Conversation can therefore be carried on over this system as over an ordinary telephone system, with the exception that a message from a distant station normally is'not interrupted.

It should be noted that adjustable tap 252 of the voltage dividing resistance 25l, which determines the intensity of negative bias potential on the control electrode 231 of the vapor discharge device 3|, should never be adjusted so that this bias potential is smaller than a certain critical value. That is, this bias potential on the control electrode 231 of the vapor discharge device 3| should be at least large enough to prevent the discharge device 3| from becoming conductive in response to received signals reproduced in the hand set 23, and therefore impressed on the transformer 2|0. If the bias produced by such received signals on the control electrode 231 through the agency of the rectifier device 33 and continuous potential amplifier 34 be smaller thanthis critical value, these same signals impressed on transformer 2||| as well as on hand set 23' are impressed on control electrode 230 of device 3| in sufiicient intensity to make the device 3| conductive. Under such conditions the receiver continues operation and the transmitter starts, thereby producing an oscillating circuit through both transmitter and receiver to block the operation of the entire system.

The high speed starting of the transmitter is accomplished by connecting the cathode 42 of the oscillating device I5 to ground directly through the vapor discharge device 3| until the relay operates to connect that cathode to ground through the switch 263. This high speed operation is made possible by the even higher speed operation of the continuous potential amplifier 28| which blocks the receiver by rendering the frequency converter device 25 inoperative whenever the discharge device 3| becomes conductive.

The high speed operation of the receiver i accomplished by the high speed operation of the noise suppression circuit including device 32, which is possible because the amplifier stage on which the noise suppression circuit operates is balanced with respect to ground, and therefore does not reproduce the rapid grid bias potential changes caused by device 32 in making operative or inoperative the amplifier devices 28 and 29.

This high speed operation is also aided by the rapid operation of the rectifier device 33 and continuous potential amplifier device 34 in preventing operation of the vapor discharge device 3|.

Although I have described my invention as applied to-a carrier current transmission system in which the carrier wave is transmitted over a power line' I0, it is equally applicable to a radio transmission system including a radio transmitter and afradioeregeiver connected to the same antenna or to adjaceiit antennas. In fact, my invention may be employed witlTanysignalling system in which messages must be transmitted" What I claim as new and desire to secure by Letters Patent of the United States, is:

1. The combination in a signalling system of a signal transmission channel, a signal transmitter and signal receiver at each of two spaced points along said channel, means responsive to signals in one of said signal transmitters for rendering inoperative the associated signal receiver andv for subsequently connecting said signal transmitter to said channel in a time sufiiciently short that no appreciable part of said signal is lost, means associated with the other of said signal receivers for detecting the presence of a signal in said channel and for rendering the associated signal transmitter inoperative and subsequently-causing said signal receiver to reproduce said signal after a period sufiiciently short that no appreciable part of the beginning of said signal is lost and sufficiently long that disturbances in said system are substantially attenuated beforesaid receiver reproduces said signal.

2. The combination in a signalling system of a signal transmission channel, a signal transmitter and signal receiver at each of two spaced points along said channel, each signal transmitter and the associated receiver being connected to signal conductors over which signals are impressed on said transmitter and upon which signals are impressed by said receiver, means responsive to a signal on said signal conductors associated with one of said signal transmitters and receivers for rendering said receiver inoperative and subsequently connecting said transmitter to said channel within a time sufiiciently short that no appreciable part of said signal is lost, means associated with the other of said receivers for detecting a signal in said channel and for preventing the operation of the associated transmitter and subsequently connecting said receiver to said channel to reproduce said signal on the associated signal conductors within a period sufiiciently short that no appreciable part of the beginning of said -signal is lost and sufilciently long that undesirable transients in said channel are substantially attenuated before said signal appears on said last mentioned signal conductors, said signal responsive means and signal detecting means being arranged to operate said transmitters and receivers in such a sequence that the common interconnections with said signal conductors are not effective,

continuous potential arranged to supply discharge current to said discharge device, means for impressing-signals from said first source on said control electrode to render said device conductive whenever the intensity of said signals exceeds said bias potential, means for reducing said bias potential on said control electrode in response to conduction of said discharge device to make said signals more effective in rendering said device conductive, and means responsive to conduction of said discharge device for impressing upon the discharge path thereof an alternating potential whose peak intensity exceeds the continuous potential of said second source, whereby, upon the cessation of signals from said signal source, the bias potential on'said control electrode is effective to render said discharge device nonconductive.

5. The combination in a signalling system having a signal transmission channel and a signal transmitter, of means for connecting said transmitter to said channel in response to signals from a signal source, said means comprising a vapor discharge device arranged to connect said transmitter to said channel when current flows through its discharge path and having a control electrode, and a relay arranged to connect said transmitter to said channel in the operated position, means for impressing a negativ bias potential on said control electrode to maintain said discharge device nonconductive and for impressing signals from said source on said control electrode to render said device conductive in the presence of said signals, means for operating said relay in response to conduction of said discharge device, and means for maintaining said relay operated for a substantial time after said device hecomes nonconductive in the absense of signals.

6. In a signal transmission system the combination of 'a signal transmitter and receiver, a

signal transmission channel, a source of signals, a vapor discharge device having two control electrodes, means for impressing signals from said signal source on one of said control electrodes to upon the discharge path thereof an alternating potential whose peak intensity exceeds the continuous potential of said second source, whereby, upon the. cessation of signals from said signal source, the bias potential on said control elec trode is effective to render said discharge device make said discharge device conductive, means responsive to conduction of saiddischarge device for operating said transmitter to transmit signals from said source through said channel, and means responsive to signals received by said receiver through said channel for impressing a bias potential on the other of said control electrodes to prevent said device from becoming conductive to operate such transmitter and make said receiver inoperative in the presence of signals simultaneously from said transmitter and channel.

7. The combination in a signal system of a signal transmitter and receiver, a signal transmission line, means responsive to a signal from said transmitter for rendering said receiver inoperative and subsequently connecting said transmitter to said channel, and means responsive to a signal in said channel for rendering said first.

produced in the output of said balanced amplifier.

8. A signal transmission system having a signal channel and a signal transmitter including an oscillating discharge device, said device having an anode, a cathode and a control electrode, said anode and control electrode being connected to opposite terminals of a tuned circuit of which an intermediate tap is maintained at fixed potential, means for impressing a positive potential on said anode, and means for reducing the potential of said cathode to initiate oscillation in said device and tuned circuit and for substantially uniformly increasing the potential of said cathode with respect to said anode to decrease uniformly the intensity of oscillations of said device and tuned circuit, said increase in potential of said cathode being effective to reduce such oscillations to small intensity before extinction.

9. A signal transmission system having a signal channel and a signal transmitter connected thereto and including an oscillating discharge device, said device having an anode, a cathode. a control electrode and a screen electrode, said anode and control electrode being connected to opposite terminals of a tuned circuit of which an intermediate tap is maintained at a fixed potential, means for impressing positive potentials on said anod and screen electrode in such intensities as to provide optimum conditions for oscillation of said device and tuned circuit when the potential of said cathode with respect to said anode is decreased, means for decreasing the potential of said cathode with respect to said anode to initiate oscillation in said device and tuned circuit, and means for increasing the potential of said cathode with respect to said screen electrode and anode at a substantially uniform rate to reduce the intensity of oscillations insaid device and tuned circuit substantially uniformly to a low intensity before the extinction of such oscillations.

10. A signal transmission system comprising a signal transmitter and a signal receiver connected to the same signal transmission channel, said receiver being of the superheterodyne type and including a frequency converter having a 10- cal oscillator and a mixer, said local oscillator and mixer each including an electron discharge device having a control electrode, a source of signals, means responsive to signals from said source for impressing a negative bias potential on both of said control electrodes to block operation of said receiver and for subsequently initiating operation of said transmitter to transmit such signals through said channel.

EDWIN W. KENEFAKE.

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