US1877561A - Transmitting circuit - Google Patents

Description

Sept. 13, 1932. ,.L AV s 1,877,561

TRANSMITTING CIRCUIT Filed Sept. 13, 1928 2 Sheets-Sheet 1 WIN a, mm

[III I]: llllli INVENTOR Rob err Lflavis.

ATTbRNEY Sept; 13, 1932. R. 1.. DAVIS 1,877,561

TRANSMITTING CIRCUIT Filed Sept. 15, 1928 2 Sheets-Sheet 2 nun t II'IIIII T Q T 'Q Detector "l INVENTOR' R m Robert L.Davi.s.

, I Y E ATTORNEY as having Patented Sept. 13, 1932 UNITED STATES PATENT oF Ic ROBERT L. DAVIS, 0F PITTSBURGH, PENNSYLVANIA, ASSIGNOR I'O WESTINGHOUSE ELECTRIC & MANUFACTURING" COMPANY, A CORPORATION OF PENNSYLVANIA TRANSMITTING GIRCUIT I Application filed September 13, 1928. Serial No. 305,?22.

- This invention relates to radio communication and more particularly to a multiplex system whereby several messages may be sent simultaneously over one system.

It has been frequently proposed to transmit from the sending station a plurality of carrier. frequencies, each modulated in accordance with its own particular message. The messages may then be received either at several different stations or at the same station by means of a plurality of receiving sets, each tuned to its particular carrier frequency.

It has also been proposed to modulate the amplitude of a carrier current of constant frequency with. several modulations each having its own characteristic frequency. The several modulations, being appliedin turn, a plurality of messages may be sent over the one communication channel. I

It has also been proposed to modulatea carrier frequency by changing not theamplitude but the frequency thereof. By my invention, I propose to apply the latter method of modulation to a single carrier frequency andthereby send a plurality of messages over what is, in effect, a single radio channel. j

The method of modulation by changing the frequency differs from the change of frequency made in order to tune the sending set to one or another of several receiving stations in two respects. (1) The changes in frequency are much smaller than the changes used to accomplish said tuning, and (2) they are not great enough to cause the modified frequency to fall outside the range to which the receiving set is sensitive. v The small changes of frequency used for frequency-shift modulation are produced by changing the conditions under which an oscillation generator, of a type which produces a nearly constant frequency operates.

The crystal-controlled vacuum tube and the harmonicgenerator governed by a'tuning fork arethe best known examples of such generators. They were originally described study of them has shown that they change their frequencies slightly in response to large changes in certain conditions. This property a constant frequency but further.

has been used fo producing frequency-shift modulation. r

It is an object of my invention to employ a plurality of different frequency shift-modulations upon 7 mit a plurality of different messages simultaneously over one radio channel. Y

It is a further object of my invention to distinguish with rapidity and certainty between the slightly differing frequencies produced by frequency-shift modulation-of one carrier frequency.

It is a further object of my invention to provide a' plurality of translating devices in one receiving set and to provide means whereby each of the several nearly equal frequencies resulting from the abovedescribed frequency-shift modulation shall control one of. said translating devices.

providea plurality of signal-translating devices and to so arrange the above-mentioned frequency-responsive. translating devices that the operation of one of them will actuate all of said signal-translating devices in one sense and the actuation of anyv of the others will operate the respective signaLtransIating device in the opposite sense;

It is a further object of my invention to provide means whereby intermittent production of any one of said nearly equal frequene cies during a period constituting an element of thesignal shall produce the sameefl'ect in the final translation of the signal or a continuous production of said frequency during said period. T

Other objects .ofthe invention and details of the constructionwill be readily understood from the following description and the accompanying drawings, in which:

Figure 1 is a diagram of the circuits and apparatus at the sending station, and

ig. 2 is a similar diagram for the receiving station. 7

In Fig. 1, the vacuumtube 1 has associated with it the usual circuits to constitute an oscillation generator, the frequency of which is fixed by the piezo-electriccrystal'2. These circuits'include an inductor 3 shunted across the crystal. Across a portion of the inductor 3, vacuum tube 4 is connected. The connection includes a B-battery 5, and both tube and battery are connected to the inductor through a condenser 6 and the usual C-batter 7.

lit has been discovered that, with such arrangement, the frequency of the oscillations generated by the tube 1 will change somewhat in response to changes of the potential upon the grid of the vacuum tube 4. To provide these potential changes, I connect the grid through a resistor 10, and a small portion of the battery 11 to the filament. Additional portions of the batery 11 areconnected to the grid through key-controlled devices.

The keys 12, 13, and 14 are connected, respectively, to points of the battery 11, including successively larger portions thereof. The key 12 is connected, through a slip ring 15, to a segment 16 of a commutator wheel 17. Similarly, the key 13 is connected, through a slip ring 18, to a segment 19, and the key 14 is connected, through a slip ring 21, to a segment 22. The segments of the 4 wheel 17 are insulated from each other and from the central portion of the wheel. The slip rings and wheel are mounted upon a shaft 23 which is intended to represent any suitable driving means. The shaft is driven at such speed that all of the segments will be engaged by the brush'24 several times during the shortest time that the key remains either open or the device. The brush 24 is connected to the resistor 10 and the grid of the tube 4.

The output circuit of the oscillation generator 1 supplies energy to the amplifier which delivers energy to the antenna 27 Instead of an antenna, the device may, if preferred, deliver its energy to a line for carrier-current communication. Instead of a simple amplifier, the device may, if desired, include a frequency multiplier.

In the receiving system illustrated in- Fig. 2, an antenna 31 is shown, which, if preferred, may be replaced by any of the usual devices for receiving high frequency energy from a carrier-current line. The connections from the antenna 31 to the detector 32 include the secondary 33 of a transformer through which a frequency differing slightly from the carrier frequency is introduced. This heterodyne frequency is provided from any suitable source, such as the oscillation generator 34. The connections from the antenna. 31 tothe detector 32 may include any of the usual or known features of a hetero-- dyne receiver which it is desired to employ.

The output of the detector 32 is delivered to a plurality of frequency-selective.devices such as filters, tuned circuits or the like arranged in parallel. The first of these (35) is series resonant to. the beat frequency which arisesbetween the energy received closed in normal operation of from the antenna 27 and the energy from the generator 34 when all of the keys 12, 13 and 14 are open. The next (36) is series resonant to the beat frequency between the energy from the generator 34 and that from the antenna when the key 12 is closed and the rush 24 is on the segment 16. The next (37) is series resonant to the beat frequency which corresponds to conditions when the key 13 is closed and the brush 24 is on the segment 19. The next (38) is series resonant to the beat frequency when the key 14 is closed and the brush 24 is on the segment 22.

From the point of greatest potential change in a series-resonant circuit, a connection is made, over a condenser, to the grid of a vacuum tube. Thus, from the junction between the inductor and the condenser in the first tuned circuit 35, over the condenser 41, a connection is made to the grid of a vacuum tube 42. The grid of-this tube is connected to the filament over a grid leak 43, and the plate circuit of the tube 42 includes the winding of a relay 44 and a B-battery 45.

The tuned circuit 36 is similarly associated.

with a vacuum tube 46, the plate circuit of which includes the winding of a slow-acting relay 47 and the B-battery 45. The tuned circuit 37 is associated in the same way with a vacuum tube 48 controlling a slow-acting re lay 49, and the tuned circuit 38 is associated with a vacuum tube 51 controlling a slowacting relay 52. v I

The contacts of the relay 44 .are connected in series with the left-hand'windings of three differential relays 54, 55 and 56 and to battery 57. This series arrangement may, if desired, also include a rheostat 58 for adjusting the forces exerted by said left-hand windings.

The contacts of relays 47, 49 and 52 are connected in parallel, the parallel circuits being energized from a common battery 61 and including, respectively, the right-handwindings of the relays 54, 55 and 56. Each, of these relays actuates any desirable signalling device. For the purpose of illustration, these are shown asrecorders, whereby the messages are marked upon moving tapes.

In'the operation of the device, the oscillation generator 1 produces a frequency which is nearly constant. Small changes in this frequency occur as described below, but the changes are a very small percentage of the normal frequency which has been referred to herein as the carrier frequency. The carrier frequency is amplified in the amplifier. and delivered to a line or to space over the antenna 27. u

WVhenall the keys 12, 13 and 14 are open, the frequency thus delivered is the carrier frequency, regardless of the positionof the commutator wheel 17. This may, instead of being the frequency generated by the tube 1, be a multiple. thereof, if the amplifier. is

made to deliver some harmonic instead of the fundamental frequency.

The carrier frequency, is received on the antenna 31 and produces, with the energy from the generator 34, a beat note which, amplified, if desired, is delivered by the detector 32. This frequency produces little effect upon the tubes 46, 48 and 51 because the tuned circuits 36, 37 and 38 are not resonant to it. The tuned circuit 35, being resonant to this frequency, impresses a larger potential of this frequency upon the condenser 41 and, thus, upon the grid of the tube 42. V

The tube 42, under these circumstances, will deliver a much smaller plate current than when oscillations are not impressed upon the grid thereof. Consequently, the relay 44 releases its armature, and the circuit through all of the left-hand windings of the relays 54, 55 and 56 is energized. If, at this moment, the right-hand winding of any of these relays is de-energized, the corresponding indi eating device will be moved by the left-hand winding.

When key 12 in Fig. 1 is closed, the potential upon the grid ofthe tube 4, instead of corresponding, all of the time, to the portion of the battery 11 between the filament and the resistor 10, will, at times, correspond to the potential of that portion of the battery between the filament and the connector to the key 12. The latter correspondence occurs each time that the sector 16 comes into contact with the brush 23.

Thus, if the shaft 24 is rotating fifty times per second and the shortest time that the key 12 is closed in normal operation is a fifth of a second, there will be at least ten intervals during which the frequency delivered from the antenna 27, instead of being the normal carrier frequency, differs therefrom by an amount corresponding tothe potential controlled by the key 12.

The shortest time that the key is closed, or open, is a period constituting the signal element. The signal will be made up of com binations of this period with multiples thereof grouped together to form the code characters.

When the key 12 is closed for a longer time, there will be more than ten such intervals during its closure, but, regardless of the number of intervals during which the frequency, characteristic of the key 12, is radiated, the total time which such radiations endure will be one-third of the time the key 12 is closed. During the other two-thirds of the time, the brush 24 is not in contact with the segment 16 and the key 12 is, therefore, without effect upon the radiated frequency.

During each of the intervals that the frequency of the radiations corresponds to the potential controlled by the key 12, the radiations received at the antenna 31 will combine with the energy from the generator 34 to ed through the tuned circuit35 but is readily 'conductedthroughthe tuned circuit 36. This produces a' diminution in plate currentfrom producefln the output of the detector 32, a I beat frequencywhich is not readily conductbeat frequency is therefore, without effect upon-the tube 42 or'the tubes 48 and 51 but the tube'46 and thus a diminution in the cur- 7 rent in the winding of the relay 47. The slow-acting character of this relay preventsithe' armature from moving in reis, therefore, closed as long, or'substantially as long, as the key v12 is closed.

The left-hand winding of theclrelay 54 is energized by current from the battery .57 dure ing not more than two-thirds-ofthe time that the key 12 is closed. Itwill be quite twotllildS, if, and only if, the keys13 and .14 remain open allrthe while that the key 12 is closed. The left-hand winding of the relay V 54 is so designed and the rheostat. 58 is so adjusted that it is only when the currentfrom the battery 57 is present in the left-hand winding all of the time that it is able tocounteractthe current in the right-hand winding. Consequently, the armature of the relay 54 'moves in the direction corresponding to the right hand winding so long as the key 12 is closed. j j l Similarly, the relay 55 will beunder the.infiuence of its; right-handwinding so long as thekey 13 is closed, and the relay 56 will be under the infiuence of its right-hand winding so long as the key 14 is closed. j "Closure of the key 131s without effect upon the relay .54 because it does not influence the movement of the armature of relay 47 and its effect upon the movement of the armature of relay 44 is only to diminish by another third thetime (already diminished by onethird) during which there is current in the left-hand winding.

It will be apparent from the foregoing that the relay 44 is the only one which must act as rapidly as the segments of the wheel 17 2 pass the brush 23. The relays 47 49 and 52 need be no more rapid than the keying freltO quency, and'the relays 54, 55 and 56 need be no more rapid than the keying frequency. If it is desired to operate wheel 17 at a higher a speed than that to which the armature movements of a relay of the type shown at 44 can correspond, a relay of the vacuum tube type may be substituted for the relay 44without difficulty. 7

Because the several messages are conveyed by frequency changes instead of amplitude .modulation, it is possible to receivethem by a heterodyne' instead of an ordinary detector. The greater sensitiveness of the heterodyne method of reception is then combined with multiplex communication.

Other changes in details of the construction will occur to those skilled in the art without specific mention thereof. The description of only one specific form of the apparatus is not to be construed as a limitation. No limitation is intended except as indicated in the accompanying claims or required by the prior art.

- I claim as my invention:

1. In a radio multiplex transmitting system, a crystal-controlled oscillation generator, circuits associated with said generator including a variable impedance for changing the frequency of the oscillations generated thereby, a plurality of separately controlled means for bringing said impedance to respectively predetermined values and cyclic means for operatively relating said impedance to each of'said separately controlled means in turn, the speed of .said cyclic means being sufficient to operatively relate the impedance to any one of said separately controlled means a plurality of times during the normal period of action, of the separately controlled means. 2. In a radio multiplex system, a crystalcontrolled oscillation generator, a plurality of keys, means controlled by said keys for causing said oscillation generator'to produce frequencies which differ from the frequency generated thereby when all said keys are in inoperative position by amounts respectively predetermined for each key, a commutator connecting each key in turn to said means, said commutator being driven at a speed sufficient to connect each key to said means a plurality of times during one period of normal keying frequency and a transmitter controlled by said generator. I In testimony whereof, I have hereunto subscribed my name this 6th day of September ROBERT L. JDAVIS;

Images