US2227596A

US2227596A - Signaling system

Info

Publication number: US2227596A
Application number: US199091A
Authority: US
Inventors: David G C Luck
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1938-03-31
Filing date: 1938-03-31
Publication date: 1941-01-07
Anticipated expiration: 1958-01-07

Description

Jan. 7, 1941. Q Q C. LUCK 2,227,596

SIGNALING SYSTEM Filed March 3l, 193B GAS G45 6,45 maar le/00E JAW roorff T'WDE oJc/L L .4 ro/ lil liti

Patented Jan. 7, 1941 PATENT OFFICE A SIGNALING SYSTEM David G. C. Luck, Haddon Heights, N. i., assigner to Radio Corporation of America, a corporation of Delaware Application March 31, i938, Serial No. 199,091

'i Claims.

rlhis invention relateswto the art of signaling, and more particularly to'a device and a method applicable in the transmission of intelligence by means of radio frequency signals.

In particular, this invention is related to, and is an improvement over, the system disclosed in a U. S. patent issued to Ray D. Kell November 2t, 1936, No. 2,061,734, entitled Signaling system and assigned to the same assignee as the present application.

It is known that sound or picture signals may be transmitted by modulating a carrier wave in such manner that the carrier itself will have constant amplitude and constant frequency, the modulation taking the form of variations in the on and oft periods of the carrier. U. S. Patent 2,083,245, granted June t, 1937, to Shore et al., discloses a system of that sort. My invention, like the Kell invention above noted, is based on the same principle but is directed more particularly to improved apparatus for increasing the ratio between signal strength and the amplitude ci interfering waves or sources of noise edects.

Accordingly, it is among the objects oi my invention to provide a signaling system in which a carrier may be so modulated for the transmission of intelligence that the signals may be received and translated with a minimum oi noise accompaniment.

Further objects of my invention include the provision of simplified apparatus for the transmission of intelligence in which non-linearities oi the amplifiers may be neglected Without causing distortion.

At the present time, it is the general practice to amplitude modulate the carrier wave. The method of modulating a carrier of constant amplitude, as proposed by Shore et al. in the patent above cited, and possibly by others, has been adopted and practiced. It is, therefore, a further object of my invention to reduce the complexity of the apparatus required by the use of improved and simplified structure elements in the system.

My invention will be more clearly understood upon reference to the following detailed description when read in connection with the accompanying drawing, and its scope is indicated by the appended claims. Referring to the drawing,

Figure 1 shows a circuit diagram of one embodiment of a radio transmitter suitable for voice modulation of a carrier wave of constant amplitude, and

Figure 2 illustrates diagrammatcally theform (Ci. 17g-i715) of certain of the voltages and currents present in the-system, and the nature of the carrier wave which is radiated in .the practice oi my invention.

It may be stated at the outset that in the description to follow, my invention will be illus trated as having particular applicability to the voice-modulation oi a carrier wave. lit will he apparent. however, that the apparatus is not limited to this use, because the same principles apply to the modulation ol? a carrier for the transmission of signals oi any type, whether representing sound, pictures, telegraph code, fac similes or other intelligence, or to the direct transmission of the modulating signals over telephone circuits, or the litre.

For faithful reproduction at the receiver of amplitude modulated signals, it has heretofore been considered essential that all the circuit elements should be linear; that is, that they should respond in proportion to the intensity of the applied signal. Under this limitation it has been known for a number of years that any discrimination between desired signal and undesired noise enteringthe signaling system must be the result of selectivity ot the receiving circuits in`1 volved. if the signal occupies the minimum irequency range necessary ior transmission oi the desired intelligence and the selectivity oi the receiving circuit used is such that it passes this signal frequency range freely but suppresses dis0 turbances oi all other frequencies, no further im provement in the ratio of signal to random noise can be hoped for, except, oi course, by increasing the intensity of the transmitted signal.

The possibilities oi improvement of the ratio of signal intensity to noise intensity lie in a departure from ordinary methods oi transmission which utilize amplitude modulation. lt has been found that the improvement may be realized by the use oil non-linear circuits at the transmitter having an output by no means proportional to the input. One embodiment of my system, like that of Kell, cited above, contemplates the transmission of certain timing marks. I am able to greatly intensify` the amplitude of the transmitted signals without increasing the average amount of power radiated because of theshort duration of each timing mark. It will be apparent that the signal-to-noise ratio is in large part improved over Whatis possible when using the generally accepted present day methods.

Referring to Fig. l, I have shown a transmitter network in which the method oi generation of the modulating impulses, with which this invention is particularly concerned, is shown in detail. The carrier frequency generator, modulator, and amplifier, which are conventional, have been indicated by block 43.

A gas filled triode I, such as RCA type 884, is employed in a conventional manner to generate saw-tooth oscillations of the order of 20,000

cycles per second. The frequency of this oscil` lator may be maintained at a constant value by any of the well known methods. For example, a standard frequency signal may be applied to the input of a transformer II whose secondary is connected between the oscillator grid and ground. 'Ihe cathode is connected to ground through a resistor I3. The vanode is connected through a resistor I1 to the positive terminal of a D. C. source 2|. A capacitor I5 is connected from anode to ground. The output voltage of the oscillator I is coupled into the cathode circuit of an amplier 3 by means of a capacitor I9 and resistor 20.

'I'he operation of this oscillator may be briey described as follows: Assuming that at any particular instant a negative voltage from the standard frequency source is impressed on the grid, and that the anode-cathode path at that instant is nonconducting, a voltage will `be built up across capacitor I5 by the D. C. source 2|. When the polarity of the standard signal reverses and the grid voltage of the oscillator becomes positive, at a certain critical value the cathodeanode electron path will break down" and become highly conductive. As a result, the capacitor is almost instantly discharged and, due to the large voltage drop now present in resistor II, the anode voltage drops to a low value, thus extinguishing the discharge. When this takes place, the cathode-anode conductivity is interrupted, the load across the capacitor is removed, and the capacitor I5 is once more charged to its previous value. This process is repeated cyclically. Consequently, the output voltage of the output voltage of the oscillator is represented by the diagram a of Fig. 2. The peak of the sawtooth represents the point at which the oscillator grid reaches the critical value and triggers the cathode-anode path into a condition of conductivity.

The amplifier tube 3 is a conventional pentode amplifier whose purpose is to couple a mixingtube 5 to the oscillator I. The cathode 49 of `this amplifier tube' is connected to a tap on .the coupling resistor 20. Its grid 5I is connected to ground through a tap on a screen grid battery 25. This provides a proper grid bias for the tube, and compensates for the voltage drop in the coupling resistor 20 caused by the steady cathode current of the tube. The amplifier anode 53 is connected to the anode of the mixer tube 5, and to the positive high voltage source 2| through a resistor 23. The grid of the mixer tube 5 is energized by a source of modulating voltage 29 which, as indicated above, may represent sound, pictures, telegraph code, facsimiles, or other intelligence. For the purposes of illustration, the modulating voltage will be assumed to be a sine wave whose frequency is substantially less than that of oscillator I. cycle of this modulating voltage is shown at b of Fig. 2.

Returning yto Fig. l, the anode of the mixer tube 5 is coupled by capacitor 3| to the grid of a second gas triode 1. A resistor 33 is connected from the grid of tube 'I through va biasing battery 55 to ground. The anode of tube 'I is serially connected through a resistor 35 andan inductor 39 to the positive terminal of the voltage This anode is coupled to the anode One supply 2|. of a third gas triode 9 by means of a capacitor 45. The anode of the ltfhird tube B is also connected through a resistor 3T to the junction of resistor 35 and inductor 39. The grid of the third ltube 9 is directly connected to the cathode of tube I.

When the oscillator output voltage increases in a positive direction, the voltagev on the amplifier cathode 49 increases in a positive direction due to the increasing current in the coupling resistor 20. This tends to decrease the anode current of t'he amplifier Atube 3, and to raise its anode potential. Thus the voltage impressed on the grid of the second gas tube 1, due to the sawin a non-conducting condition with its grid at a negative potential. Its ano-de voltage will be at a maximum because there will 'be no current flowing in the anode circuit and thus no voltage drop in resistor 35. In accordance with the rise in voltage of the rst saw-tooth of curve c,

Fig. 2, t'he grid becomes more positive and after a time t reaches the critical value at which the tube breaks down. This critical voltage is represented by the horizontal line 0. The anode current at time t will suddenly rise to its maximum value, as shown in curve e, where it will remain until the next event takes plalce at time ta At this .time "t1 the grid voltage is reduced to its original negative value due to the breaking down of the oscillator tube I. This alone, however, is not sufficient to stop tube I from conducting. As is well known, the anode voltage must be reduced in some manner to extinguis'h the tube. This is the function of tube 9, which is also a gas filled triode.

When the oscillator tube breaks down at time t1" there is a s udden surge of current through resistor I3 `due to the discharge of capacitor I5. Resistor I'I, however, prevents more than a very small current to flow afterthis condenser has discharged, so that the current immediately drops back to a low value. In terms of the voltage across resistor I3, curve drepresents `this surge of current.

The surge of voltage across I3, represented by d of Fig. 2, is impressed on the grid of tube 9, and is of such a polarity as to cause this tube to break down. The anode of tube 9 immediately starts drawing a large current. The sudden decrease in anlode voltage, due to the drop in resistor 31, is impressed on .the anode of tu'be 1 by means of capacitor 45. This tends to lower the voltage on the anode of tube 'I to such a point that, in conjunction with the negative grid voltage mentioned above, the .tube is extinguished, and its anode current stops. 'I'he anode voltage of tube l, represented by curve f, is the reciprocal of the anode current curve e, except for the small dip at time ti impressed on it by CFI 'l again reaches the critical value, as at time ta It is to be noted that the extinguishing impulses generated by the oscillator, curve d, are uniformly spaced.` In the absence of modulation, as in curve a, the critical voltage is also reached at uniform intervals. However, in the presence of modulation, the critical vol-tage is reached at varying time intervals. The result of modulation is, therefore, to vary the intervals during which the anode voltage of tube 'l is a maximum as compared to the intervals during which it is a minimum. See curve f.

Having attained the desired modulating characteristic, it is now only necessary to utilize this non-uniform voltage to modulate the carrier of any type of `radio transmitter. This may be done in several diierent ways. At M, Fig. 1, I have shown a switch connected so as to permit either one o1 two methods to be selected at will. The anode voltage of tube 'l may be used to modulate the carrier, or, alternatively, a modulating voltage may be taken from inductor 39.

in the first case, the voltage represented by f, Fig. 2, is used to control the transmitter output in such manner that the carrier is shut oil when this voltage is a maximum. The voice modulated saw-tooth thus produces a carrier wave envelope such as shown at g in Fig. 2. During a single cycle of voice frequency the on time of the carrier will vary from a maximum to a minimum. The degree of variation represents the percentage of modulation. Thus the power radiated is a function of the modulation just as in normal amplitude modulation, and signals so modulated may be received on a conventional receiver.

in the second case, the switch Il connects lnductor 39 to the carrier frequency modulator system 43. Inductor 39 has a relatively low D. C. resistance and, consequently, the voltage drop across it is substantially uniform for widely diiferent values of anode current. However, as is well known, an inductor tends to oppose a change ci current through it by generating a back electrcinotive force of a polarity tending to oppose such a change. Therefore, each time the anode current of tube 9 is triggered by the oscillator tube l, and each time the anode current of tube l is triggered by the modulated saw-tooth, a surge oi voltage appears across the inductor. These surges are represented by curve It of Fig. 2. It will be seen that they occur each time the anode lcurrent of tube l or 9 changes. and that they thus mark the boundary conditions, or edges, of the modulating voltage shown at f. If these pulses are applied to the radio frequency modulator system d3 in the same manner, the carrier envelope will appear as shown at i of Fig. 2, in which only the boundary conditions, or edges. oi the modulated carrier of curve g are marked by carrier impulses, which are of negligible duration.

If the output from the transmitter is controlled as described in the foregoing paragraph, the "on time of the carrier may be small, for example, 10% of the total time. A transmitter having a given rated continuous power output will, thereiore, be enabled to transmit these discrete impulses at substantially ten times the power of corresponding continuous impulses. It will be seen that the greatly increased power of the carrier impulses provides an improved ratio thereof to the noise level which is represented by a power ratio of approximately 10.

Transmission of sound by the last means described is, of course, unintelligible to the convenvoltage.

tional radio receiver, since the radiated power is constant at all times. It is n to translate the signals by a special receiver. However, it is `notapartoithisinventirmtodiscloseamcthod for receiving such signals. There are various 5 ways of doing it, among which is the method disclosed by Kell in Patent No. 2,061,734, above mentioned.

I claim as my invention:

1. A signaling system comprising means for obtaining a succession of discrete pulses which define the variations of a signal, said means comprising a source of saw-tooth voltage of supersonic frequency, a source of modulating signal voltage, means for adding said modulating signal voltage and said saw-tooth voltage, means for applying said voltages to a gas iilled triode to initiate its conductivity at the nonuniform intervalsat which the sum of said voltages pames through the critical voltage of said gas filled trlode, and means tor extinguishing said triode at uniform intervals.

2. A signaling system comprising means for obtaining a succession of discrete pulses which deline the variations of a signal, said means comprising a source of saw-tooth voltage of supersonic frequency, a source of modulating Signal voltage, means for adding said modulating signal voltage and said saw-tooth voltage, means for applying said voltages to 'a gas iilled triode 30 to initiate its conductivity at the moment the rising voltage of each cycle of the sum of said voltages reaches the critical value of said triode, and means for terminating said conductivity at the regular intervals at which said saw-tooth voltage collapses.

3. A signaling system comprising means for obtaining a succession of discrete pulses which define the variations of a signal, said means comprising a source of saw-tooth voltage of supersonic frequency, a source of modulating signal voltage, means for adding said modulating Signal voltage and said saw-tooth voltage, means for applying said voltages to a gas filled triode to initiate its conductivity at the moment the rising voltage of each cycle of the sum of said voltages reaches the critical value of said triade, and means for terminating said conductivity by a pulse generated by the collapse of said saw-tooth 5u 4. A signaling system comprising means for obtaining a. succession of discrete impulses of substantially uniform amplitude and of negligible duration, alternate impulses being uniformly timed, and intermediate impulses occurring at time intervals determined by a modulating signal, said means comprising a source of saw-tooth voltage of supersonic frequency, a source of modulating voltage, means for adding said modulating voltage and said saw-tooth voltage, means for applying said voltages to a gas lled triode to initiate a period of conductivity at the moment the rising voltage of each cycle of the sum of said voltages reaches the critical value of said triode, and means for terminating said period c5 of conductivity by a pulse generated by the collapse of said saw-tooth voltage, one of said discrete impulses being generated by each change in the conductivity of said gas filled triode.

5. In a signaling system characterized by a succession of discrete impulses of uniform am plitude and of negligible duration, alternate impulses being uniformly timed and intermediate impulses occurring at non-uniform time intervals which are determined by a modulating signal. means for producing a saw-tooth 'voltage of supersonic frequency, a source of modulating voltage, means for adding said modulating voltage and said saw-tooth voltage. means for applying said voltages to a iirst gas iilled triode whose period of conductivity is started when, at nonuniform intervals. the sum of said voltages rises above the critical voltage of said triode, means including a second gas filled triode operable upon said first gas filled triode to terminate its conductivity when, at uniform intervals, said sawtooth voltage decreases. an inductor in a circuit common to said gas filled triodes, and means responsive to the voltage across said inductor for transmitting said impulses.

6. In a signaling system characterized by a succession of discrete pulses of uniform amplitude and nonuniform duration, in which the termination of on periods is uniform with respect to time, but in which the commencement of said "o'n periodsis nonuniform with respect to time, and is determined by a modulating signal, a source of supersonic saw-tooth voltages, a. source of modulating voltage, means for adding said modulating voltage and said saw-tooth voltage, means for applying said voltages to a rst gas filled triode whose period of conductivity ris started at nonuniform intervals when the sum of said voltages rises above the critical value of said triode, means includinga second gas lled triode operable upon said ilrst triode to terminate its period of conductivity when, at uniform intervals, said saw-tooth voltage decreases, and means for transmitting said impulses.

7. A signaling system comprising a source of saw-tooth oscillations of supersonic frequency, a source of signal-representing voltage, means for adding said signal-representing,voltage and said saw-tooth voltage, a first gas filled triode, means for applying said added voltages to said triode, the period of conductivity of said first gas filled triode being started at the moment the amplitude of said added voltages reaches a predetermined value, a second gas lled triode, and means including said second gasA illled triode for extinguishing said ilrst gas iilled triode at intervals corresponding to the collapse of said saw-tooth voltage, means for obtaining from said iirst gas filled triode a series of discrete pulses of uniform amplitude but varying in duration in accordance with said signal representing voltage, an inductor in circuit with said first gas lled triode, means for obtaining from said inductor a series of discrete pulses of uniform amplitude and of negligible duration, alternate pulses being timed to correspond to the collapse of said saw-tooth voltage and intermediate pulses occurring at intervals determined by said signal-representing voltage, and means for selecting a desired one of said series of pulses.

DAVID G. C. LUCK.