US 2379899 A
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Patented July 10, 1945 RADIO COMMUNICATION SYSTEM I Clarence W. Hansell, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application November 29, 1940, Serial No. 367,688 Claims. (Cl. 2501'l) The present invention comprises a pulse type radio communication system.
By operating the radio communication system of the invention in such manner as to transmit short pulses separated by relatively long spaces, I am able to achieve the following desirable results: (1) Higher peak power and much higher frequencies than would otherwise be possible, because of limitations due to heating of transmitting vacuum tubes, (2) an improvement in signal to noise ratio in view of the fact that the receiver is responsive only during time periods which may be occupied by the transmitted pulse, and (3) a degree of secrecy in signaling.
In brief, I propose to transmit short pulses separated by relatively large spaces, and to modify the relative timing of successive pulses in accordance with the useful modulation. Thus, if successive pulses are numbered I, 2, 3, 4, 5, etc., one polarity of modulation potential moves pulses I and 2, 3 and 4, 5 and 6, etc. closer together but moves pulses 2 and 3, 4 and 5, 6 and 1, etc. further apart by an equal amount. Reversing the modulation potential will reverse the time displacements of successive pulses. With this type croseconds (equivalent to width of a pulse) for conditions B and C of Fig. l, with respect to the -timing for the unmodulated condition illustrated in A. It should be noted that for condition B, representing one polarity of modulation, the pulses I and 2 are closer together while pulses 2 and 3 are further apart relative to their respective times of occurrence in condition A which represents the unmodulated condition. Similarly, for condition B, pulses 3 and 4 and also pulses 5 and 6 will be closer together, while pulses 4 and 5, as well as 6 and I, will be further apart. For condition C representing the other polarity of modulation, pulses I and 2 and also pulses 3 and 4 are further apart, while pulses 2 and 3 as well as pulses 4 and 5 are closer together in point of time relative to the timing for condition A.
of pulse timing modulation, there is no change in I the average pulse rate but only a variation in time spacing between adjacent or successive pulses. By means of this timing variation or modulation, I am able to carry out any kind of communication, provided the pulse rate or frequency is sufiiciently greater than the highest si nificant modulating frequency.
A better understanding of the invention may be had by referring to the following description which is accompanied by the drawings wherein:
Fig. 1 illustrates graphically three conditions for the transmitted pulses which are obtained in the transmitter of the invention;
Fig. 2 illustrates a pulse type transmitter circuit in accordance with the invention;
Fig. 3 graphically illustrates the action of the pulsing tubes of Fig. 2; and
Fig. 4 illustrates a receiving system in accordance with the invention for use with the'transmitter of Fig. 2-.
In Fig. 1 there are illustrated three pulse conditions A, B and C which might be obtained in a system if we assume that the shift in timing of each pulse is a maximum equal to the time length of each pulse. If the pulse rate is say 15,000 cycles per second, and the length of each pulse is (l+l50,000) X (10) :6.67 microseconds, then use- Fig. 2 illustrates schematically one transmitter circuit in accordance with the invention for producing and transmitting pulse signals of the character illustrated in Fig. 1. In Fig. 2, electrical direct current from a transmitter power source T is held at a substantially constant value by means of a constant current high inductance reactor X, in a manner similar to that found in the Heising constant current transmitter modulation system. Source T supplies anode potenful modulation may be assumed to have caused shifts in timing of individual pulses: of 6.67 mitial for the anode of the power amplifier vacuum tube in the radio transmitter RT. The current from the reactor divides itself at point A into two paths. One path is to the radio transmitter RT and other path is to a pair of pulsing vacuum tubes I and 2.
When proper electrical constants are used, the pulsing tubes I, 2 pass current at a relatively low potential for most of the time but both tubes cut off their currents to cause source T and the reactor X to send short current pulses at relatively high potential to the radio transmitter. This combination, when properly designed and constructed, results in good power efficiency. While the pulsing tubes are passing current, i. e., when they are conductive, there is a tendency for current through the reactor X to increase. While the tubes are not passing current, i. e., when they are non-conductive, there is a tendency for th 'current through the reactor X to decrease. The low potential to ground from the reactor output terminal between A and ground for relatively long time periods, and the high potential the high potential supplied to the radio trans-.
mitter RT greatly exceeds the normal voltage supplied by T. Putting it another way, while vacuum tubes I and 2 pass current, there is a voltage drop through these tubes which is small, and, in effect, there is a low resistance shortcircuit across reactor X for which reason the current through the reactor tends to increase. when the pulsing vacuum tubes I, 2 become nonconductive for spaced relatively short time periods, the short-circuit path across reactor X is removed, thus permitting the potential from anode source T to be applied to the radio transmitter RT with even greater effect than from a steady source. The reactor X, by holdin substantially constant current, forces a peak current into the transmitter RT having a value substantially equal to the peak current from terminal A through tubes I and 2 even though this may require a sharp rise in potential. The current in reactor X stores energy in the magnetic field of the reactor so that the current cannot be diminished except by using up some of the stored energy.
The control electrodes of the pulsing tubes are supplied with a direct current biasing potential from a rectifier 5, and also supplied with alternating current from two sources 3 and I. Source 3 is a "source of constant frequency current" which may operate at 7500 cycles per second if it is desired to transmit 15,000 pulses per second. Current from this source 3 is applied to the pulsing tube control electrodes in push-pull or 180 phase relation. Source 4 is a source of modulating current which is to be transmitted. It may be made up of telephone, multiplex-telegraph, facsimile, or other types 01' signaling current. If ordinary conversational telephone modulation is employed for source I, it is preferred that the range of telephone frequencies be between 150 and 3000 cycles. Input from source I is also applied to the 40 except when the input from the source of con-.
stant frequency current 3 is near the zero value of each cycle. At the time when the input from source 3 is near the zero value of each cycle, both pulsing tubes I and 2 cut oil? current at the same time, i. e., are simultaneously non-conductive, and cause a short and sharp pulse of current at relatively high potential to be passed into the radio transmitter. For this unmodulated condition, the pulses t ansmitted from the radio transmitter RT are iformly spaced with respect to time. Fig. 3 illus ates qualitatively the action of the tubes in p oducing the pulse Curve D of Fig. 3 represents the potential i ut with respect to time supplied to pulsing tu e I by source 3 while curve E represents the entlal input supplied to pulsing tube 2 by the 5 me source 3. Both of these curves are shown as sine wave curves. The dotted lines L and L1 indicate the current cut-off potential point for tubes I and 2 respectively. It should be noted that both tubes are simultaneously non-conductive only during a short time interval S near the zero value of both curves. Graph F indicates that pulses are produced only during the short times that both pulsing tubes are simultaneously non-conductive.
If, momentarily, modulation fro. source 4 pro duces a differential variation in his potential on the grids of pulsing tubes land 2, t en alternate pairs of pulses will draw together. e tubes will act as though the bias on one pulsing tube was increased and the bias on the other pulsing tube was decreased. Reversing the differential potential will push these same alternate pairs of pulses apart. A complex wave form of modulating potential will produce a corresponding pulse timing modulation which may be utilized for communication purposes.
It is preferred that transmitter RT employ a carrier frequency above 30 megacycles.
To receive signals or modulations transmitted from the equipment of Fig. 2, I may employ a receiving system of the kind illustrated in Fig. 4. In this system a superheterodyne type of receiver is utilized to provide an output current made up of the time modulated pulses from the transmitter. An antenna 6 collects the pulse signals and passes them on to a heterodyne detector and amplifier 1. A local oscillator 8 supplies the beating frequency. The intermediate frequency output from I is further detected, amplified and reduced to an audio frequency signal in 9. The pulses from 9 are delivered to two pulsing oscillators I3, I4, similar to those used to produce saw tooth potentials in television receivers, each adjusted to have a natural period of oscillation substantially equal to half the average frequency of the pulses from the transmitter, i. e., 7500 cycles. The pulse oscillators I3 and II are provided with a common anode circuit resistance R for the purpose of making them tend to operate substantially out of phase. The received pulses are utilized to synchronize the operation of the pulse oscillators and, obviously, alternate received pulses are automatically effective in controlling the timing of each oscillator I3 or II. However, each pair of received pulses produces opposite effects upon the timing of the oscillators I3 and I4 and these effects balance out so far as the pulsing rate of the combined oscillators is concerned. When a received pulse advances the time of tripping of one oscillator I3 or I4. it retards the time of tripping of the other II or I 3. Both pulse oscillators are locked together through the common anode circuit resistance R an the other circuits so that, in effect, the received pulses modulate the phase or timing of oscillations of each pulse oscillator but not the oscillation frequency. During the time when,
both pulse oscillators I3 and II are not passing current, the condensers II, II are being charged up through resistors I2, I2. The rate oi charging of these condensers controls the timing of the operation of the pulse oscillators. In effect, the time constants of condensers II, II and resistors I2, I2, in combination with the tube potential adjustments, controls the operating frequency. The received pulses serve to differentially vary the tripping time of the oscillators, thus difierentially varying the average current passing through the oscillator tubes. The common anode resistor R also affects the operating frequency to some extent because the drop in the resistor affects the input potential and therefore the charging rate of condensers II, II through resistors I2, I2.
Once the oscillations have dropped into synchronism with the received pulses, the average anode currents to the pulse oscillator tubes I3 and M will be differentially modulated by the pulse modulation and this differential current may be utilized to provide the useful modulation output from the receiving system which can be heard in the headphones I0 or recorded by some equivalent circuit.
The polarity of the audio output from the pulse oscillators I3, I4 with respect to the pulse modu lation, will be in one direction or the other, depending upon the phase relations which happen to be established when the locking of the receiving oscillators l3 and Il by transmitted pulses is started.
The system just described provides a considerable degree of privacy or secrecy since any ordinary amplitude, phase, or frequency modulation receiver now known or utilized in the art will be substantially unresponsive to the type of modulation provided. Intelligible reception is, in general, only possible with equipment specially designed and adjusted to operate at the pulse frequencies utilized at the transmitter and with the type of modulation proposed.
Because the receiver pulse oscillators l3 and H are in a condition to be tripped by received power for only brief time periods including the; time periods occupied by transmitted pulses, there will be an improvement in the signal to noise ratio and also less interference from undesired signals. Noise occurring during spacing periods will have no efi'ect because the oscillators l3, I4 are not in a, condition to trip during these periods.
Circuits ahead of the receiver pulse oscillators must be made broad enough in frequency pass' band to pass the pulses. If the carrier frequency is 30 megacycles, for example, then these circuits should have a pass band of 30 megacyclesi15,000 cycles or more. This is a band width greater than that required to pass double side bands of the useful modulation alone. As a result of the broadness in band width, ignition and similar man made noises are much shorter in time duration than they would be in the common type of receiver now in use. This reduces the probability of a noise pulse afiecting the receiver pulsing oscillators.
The effective or narrowest selectivity of the system appears in the action of the receiver pulse oscillators which may have an effective selectivity as great as desired by controlling the amount 01' energy from the receiver utilized to control the oscillators and by controlling the degree to which each oscillator can determine its own timing. The degree of locking between the two oscillators, tending to make them operate 180 out of phase, is controllable to control the sensitivity of the pulsing oscillators as detectors. To summarize, by controlling the pulse input power, the degree of locking between the pulse oscillators, and the other electrical adjustmental can control the effective selectivity and sensitivity of the final detector of the system over a large range.
What is claimed is:
1. A communication system comprising a radio transmitter, a source of polarizing potential connected to said transmitter through a high inductance reactor, a pair of vacuum tubes each having an anode, a cathode and a grid, 2. direct connection between the anodes of said tubes, a direct connection between that terminal of said reactor which is nearest to said radio transmitter and said anodes, a direct connection between said cathodes, a source of constant frequency current, and a source of modulating current, means for coupling said grids in opposed phase relation to both of said last sources of current, whereby the currents supplied to one grid has a 180 phase relation relative to the currents supplied to the other grid, means for biasing said grids negatively with respect to said cathodes to such a value that the tubes are alternately conductive for a large portion of each positive half cycle of constant frequency alternating current supplied to the grids, and both tubes are nonconductive simultaneously for only short periods of time during which the currents from said source of constant frequency current are near their zero values, whereby said source of olarizing potential supplies relatively short high polarizing potentials to said radio transmittter solely during the time both tubes are non-conductive.
2. The method of communication which comprises producing pulses of carrier frequency current at a rate of substantially 15,000 cycles per second, each pulse having a constant length of substantially 6.67 microseconds, and differentially varying the timing of successive pulses by an amount never exceeding the time of one pulse in accordance with the intelligence to be conveyed and without changing the average pulse rate.
3. The method of communication which comprises producing equal length pulses of carrier current, and shifting the time of occurrence of individual equal length pulses by an amount never exceeding the time of one pulse and without changing the average pulse rate, in accordance with the intelligence to be transmitted 4. A system for producing signal pulses modulated in timing but not in mean frequency comprising an alternating current transmitter, a direct current connection from said transmitter to a source of anode polarizing potential, two parallel electron discharge devices in circuit with said connection, a source of constant amplitude and constant frequency current and a source of modulating currents, and connections coupling both of said sources to said devices for excitin said devices differentially from each of said sources, whereby there is produced a pulse of output potential whenever the potentials resulting from the combination of the voltages derived from said two sources pass through the condition of substantial equality.
5. A system for causing a carrier wave transmitter to transmit power in the form of time modulated pulses comprising a parallel combination of said transmitter and two electron discharge devices, and means to deliver a substantially constant current to said parallel combination, said means including a source of constant amplitude and constant frequency current, a source of modulating currents, and connections for applying currents from both sources to said discharge devices differentially.
6. A system for producing signal pulses modulated in timing but not in mean frequency comprising an alternating current transmitter, a direct current connection from said transmitter to a source of anode polarizing potential, two parallel electron discharge devices in circuit with said connection, means for supplying a direct current negative bias to the grids of said devices relative to their respective cathodes, a source of constant amplitude and constant frequency current connected to said grids for supplying potentials of opposite polarity thereto, and a source of modulating currents also coupled to said grids for supplying potentials of opposite polarity thereto, whereby there is produced a pulse of output potential whenever the potentials on said grids pass through substantial equality.
7. A system for producing signal pulses modulated in timing but not in mean frequency comprising a radio transmitter, a direct current connection from said transmitter to a source of anode polarizing potential, two vacuum tubes in circuit with said connection, said tubes each having anode, grid and cathode electrodes, a direct connection between the anode of said two tubes, a direct connection between the oathodes of said two tubes, means for supplying a direct current negative bias to the grids 0! said devices relative to their respective cathodes, a source of constant amplitude and constant frequency current connected to said grids for supplying potentials of opposite polarity thereto, and a source of modulating currents also coupled to said grids for supplying potentials of opposite polarity thereto, whereby there is produced a pulse of output potential whenever the potentials on said grids pass through substantial equality.
8. A communications transmitter for sending out pulses of constant average frequency of which alternate pulses are modulated in timing difierentially, said transmitter including a radio frequency system, a source of unidirectional energy 101' operating said system, and two electron discharge tubes in circuit with the output from said source, said tubes having control electrodes, means to supply a constant frequency potential and current and also modulation potential and current to the control electrodes of the tubes differentially, and means to derive output pulse currents resulting from the variations in current through the tubes, whereby there is produced a pulse of output potential whenever the potentials on said control electrodes pass through substantial equality.
9. A system Ior producing signal pulses modulating in timing but not in mean frequency, comprising a transmitter, a power source for said transmitter, a connection between said transmitter and said power source, a pair of electron discharge devices having their anodes connected together and also connected to said connection, a source of constant amplitude and constant frequency current, and a source of modulating currents, and connections coupling both of said sources to said devices for exciting said devices differentially from each of said sources.
10. A system for producing signal pulses modulating in timing but not in mean frequency, comprising a transmitter, a power source for said transmitter, a direct current connection including a reactor between said transmitter and said power source, a pair of electron discharge devices having their anodes connected together and also connected to said connection, a source of constant amplitude and constant frequency current, and a source of modulating currents, and connections coupling both of said sources to said devices for exciting said devices differentially from each of said sources.
CLARENCE W. HANSELL.