US2462110A - Demodulation of time-modulated electrical pulses - Google Patents

Description

Fb. 22, 1949. 7 M. M. LEVY 7 2,462,110 I DEMODULATION OF TIMEMODULATED ELECTRICAL P ULSES Filed May 1944 I Q 2 Sheet-Sheet l n+/ I n+2 H TIME a I I TECEIVEIC DETEET III-i] Feb. 22, 1949. M. M. LEVY ,1 1

DEMODULATION OF TIME-MODULATED ELECTRICAL PULSES Filed May 30, 1944 2 Sheets-Sheet 2 F/G. 3A.

DEL/I) NET aR/I IN I 451.4 A Zrwamr 3R5 Inventor By I I I, Attorne Patented Feb. 22, 1949 ITED STATES AT-ENT; OFFICE DEMODULATIQN F TIME-MODULATED ELECTRICAL PULS-ES I Maurice Moise Levy. London, England, ass ignor,

by mesne assignments, to International Stand- 'a'rd Electric Corporation, New York,"N. Y., a

corporation of Delaware Application May 30, 1944, Serial No. 538,062 I In Great Britain June 1, 1943 13 Claims.

The present invention relates to arrangements for receiving and demodulating electric pulse trains which are phase-modulated in accordance with a signal wave.

By phase-modulated-pulse trains are to be understood those in which the time interval between successive pulses is varied according to the signal, the duration of each individual pulse being constant. By the :method of the invention, an auxiliary train of pulses is derived from the phase-modulated train by means which com bines each pulse with the preceding pulse -delayed by a constant time slightly larger or smaller than the repetition period. The derived train is then applied to charge a condenser, the

potential of which then varies according to the' amplitude of the modulating signal, as will be demonstrated. The potential of the condenser may be applied to an amplifier from which the signal-may be obtained in the usual way.

According to the invention, there is provided an arrangement for demodulating a phase modulated train of short electrical pulses COmP-I'lSlIl means for obtaining from the said train a delayed train of which each pulse overlaps in time the following pulse of the original train, means for combining each pair of overlapping pulses in order toderive a train of pulses whose durations vary in accordance with the modulating signal wave, and'means for extracting the said signal Wavefrom'the derived train of pulses.

The invention will be described with reference to the accompanying drawings in which? Figs. 1; 2 and 4 give diagrams used in explaining the principles of the inven'tion;

Figs. 3 and 3a show schematic diagrams 'of two embodiments; and

Fig. 5 shows a schematic diagram of an arrangement for lengthening the pulses before :demodulation.

- In Fig. l sketch a shows three successive pulses "from a radiated train of regularly repeated short pul'se's assumed to be of rectangular outline. "The pulses really consist of short packets or bundles of radio frequency oscillations of rectangular form. Sketch b shows diagrammatically what may occur when the pulse'train is phase-modulated. The nth pulse is supposed not'to be affected, so that the two nth pulses insketches a and b occur at the same time. 'In 12, however, the (n+1)th pulse is shown as arriving slightly earlier'than usual, while'the (n+2)th pulse is a little late. Thus the interval between the nth and (n+1)th pulses is reduced by the modulaticn andthat between the (n+1) th and (n+2)th 'pulsesisincreased. It will, of course, be'understood that the modulation may make any pulse late or early by anyamount.

Fig. 2 shows 'at i a sinusoidal wave component of a'modulating signal, the frequency being w/21r. The solid vertical lines 2 represent the centre lines of the pulses "of an unmodulated train, the

period, or time interval between successive pulses, being to, which will, of course, be small compared with the period 21r/w oi the signal wave. The duration of the pulses will furthermore be small compared with to. The "dotted vertical lines 3 represent the centre 'linesof the pulses as they would be displaced ,by the modulating signal represented by the curve 1. The pulses which coincide with the points where the wave l cuts the time axis are unaffected, but the others are advanced or-retarded proportionally to the amplitude'of the wave l at the times of occurrence of the pulses. The pulses thus behave rather like a condensation wave in a fluid medium.

According to the invention, .the modulated pulses 3 are received on the aerial of a radio receiverRR, Fig. 3, and the pulse outlines are derived in an appropriate detector D which is connected to a delay network DN, which preferably comprises an artificial non-dissipative transmisof a batteryB through high resistances R2 and R3 (or they may be biassed in any other convenient way), and the anode is connected to the high tension supply through a condenser C shunted by a resistance 'R.

-Referringnow to Fig. 4, sketch a shows four of the phase-modulated pulses after detection, which are drawn so that the spacing progressively decreases. These pulses should'be of positive polarity and are applied to the grid G1. The grid G2 is connected at "a point in the network DN such that any pulse reaches it after a delay To which is a little greater than to. Curve 2: in Fig.- 4 shows how the'pulses are applied to G2.

'Itwill be seen that "the nth delayed pulse overlaps the (n l llth original ulse, and the obtained by combining the nth delayed pulse with the (n+1) th original pulse, and so on.

If desired, as shown in Fig. 3a, the grid'Gz may be omitted (together with R3 and the tapping connection to DN). R1 is removed, the output terminals of DN being left open. The original pulses are then reflected at the output terminals and re-appear at the input terminals as delayed pulses. If the length of the networkDN is such as to correspond to a delay of T/2 then the same derived pulses c of Fig. 4 are obtained. The

grid G1 should in this case be biassed so that the anode current is cut off except when two pulses are simultaneously applied to the grid. Alternatively, the valve may be biassed as a normal amplifier so that when the original and delayed pulses are applied the anode current varies in the manner indicated in Fig. 4, sketch (1. By

, following the valve V with another suitably biassed valve (not shown), the wide lower part of the combined pulse may be cut away leaving only the narrow upper part which will be the derived pulse desired.

The manner in which the demodulation of the pulses is obtained by the arrangement of Fig. 3 is most easily understood from a mathematical analysis.

Let t be the time when the nth unmodulated pulse is received. Then t:tc+nto, where to is a constanttime which is determined by the time origin chosen. Since w/21r is the frequency of the signal wave I (Fig. 2), the amplitude of this wave at time t will be proportional tosin wt and the time displacement of the nth pulse from its unmodulated position in time will therefore be a sin wt, where a is a parameter which depends on the depth of modulation.

Thus the time tn when the nth modulated pulse appears will be given by tn=ta sin wt =tc+ntca.-sin[wnto+wtcl Similarly the time t(n+1) corresponding to the (n+1)th modulated pulse is given by t(n+1)=tc+ (n+1) to-d sin [auto (n+1) +wtc] Thus As already stated, to will be small compared with 21r/w, so that wto will be a very small angle.

Hence, approximately,

Let P be the duration of each of the pulses of the modulated train (sketch a, Fig. 4), and let in be the duration of the nth derived pulse (sketch 0, Fig. 4), then 4 Thus pn=p-awtucos (ncto-l-B) in which :3 has been written for w(tc+t0/2) If iis the amplitude of the anode current pulses in the valve V, and if these pulses are applied to charge a condenser C, then the condenser will be raised by n pulses to a potential Since to is small compared with 21r/w, it will be regarded as an infinitesimal quantity dt. The time of occurrence t of the nth unmodulated pulse will be taken as the time variable. It has already been stated that t=tc+nto, so that nwto+fi=wt+wio/2.

Thus approximately neglecting the small phase angle etc/2.

The potential of the condenser thus has a component C 4 which steadily increases, and also another component which is proportional to the original modulating signal voltage. The first component is effectively removed by shunting the condenser C by the resistance R (Fig. 3) which provides a leak which limits the rise-of potential across the condenser.

Thus the potential variation across the condenser C correspondsto the original modulating signal, and may be filtered if necessary to remove traces of the pulses and amplified by any suitable arrangement (not shown).

It should be pointed out that To could have been chosen to be a little less instead of a little greater than to. Similar derived pulses c (Fig. 4) would be obtained. To should not, however, be made equal to to, for then the derived pulses would not distinguish the positive and negative loops of the modulating signal.

It has been assumed that the duration P is at least equal to twice the maximum time displacement of any pulse productd by the phase modulation. If this were not so certain of the derived pulses would disappear altogether. This requirement will usually be met, but if not, the received pulses may be lengthened before demodulation by any method, for example in the manner shown in Fig. 5. The pulses to be lengthened are applied to the input terminals IN of a delay network DNI terminated at the other end by a resistance R5 equal to its characteristic impedance. Any number of valves V1, V2 Vn have their anodes connected together, and their control grids are connected respectively to intermediate tapping points along the network. The anodes are supplied with current from the high tension source through a common resistance A1 and are con nected through a blocking condenser C1 to the control grid of a collector valve CV supplied with anode current through another resistance A2. The output is taken from the anode of CV through a blocking condenser C2, and the usual grid resistance G is shown. The valves V1 to V2: should be apprcpriately biassed below the cut-.off by any venie-n t means (not shown).

Thebontrol grids of the valves V1. to Vn' are tapped at points such that the pulses applied at the terminals IN reach these points after successive intervals a little shorter thanthe duration? of *each pu lsefl so that the'yalves V1 to Vn are unblockedi-n turn, each remaining unblocked-until iiist after the next hasbecome unblocked.

' ;'=I he-'valve CVshould be biassed so that a mod-; e'rate anode current flows, but so that when any one of the valves V1 to Va is unblocked, the correspondingfall in-anode potential takesthe control gri'doi (W -below the cut-off. Thus CV will be blocked until the pulse has travelled past the last va lv'e Vnl' Thus inverted pulses having a duration a little less than nP will be obtained at the terp The number of the valves V1 to Vn may be halved trol grid i V to the open endshould correspond toadelay a little less than P/2, and care should 1 be; taken; to seethatthe input terminals of the delay network are terminated with its characteristic impedance in order to avoid multiple reflectita It will be understood that when it is desired to lengthen the original pulses before demodulation, the circuit of Fig. 5 is interposed between the detector D and the delay network DN (Fig. 3), the terminals IN being connected to D and the terminals OUT to DN.

Fig. 3 which has been described above, gives one embodiment of the invention, which however, may be carried out in other ways. Although the preferred form of the delay network DN is a nondissipative transmission line, any type of network which will delay the pulses by the required amount can be used. Moreover, any suitable arrangement may be used for combining the original and delayed pulses.

What is claimed is:

' 1. A source of a train of short phase-modulated electrical pulses of substantially constant duration and an arrangement for demodulating said phase modulated train of short electrical pulses comprising delay means for obtaining from the said train a delayed train of which each pulse overlaps in time the following pulse of the original train, the delay being of a value difiering from the unmodulated time spacing of the pulses of said train by a predetermined amount, means for combining each pair of overlapping pulses thus producing a train of pulses whose durations vary in accordance with the modulating signal wave, and means for extracting the said signal wave from the derived train of pulses.

2. An arrangement according to claim 1; in which means is provided for applying the original and delayed trains of pulses simultaneously to a mixing valve from the anode circuit of which the derived pulses are obtained.

3. An arrangement according to claim 1 in which means is provided for applying the original and delayed trains of pulses simultaneously to a mixing valve, and means for blocking said valve.

except when at least two pulses are simultaneously applied thereto.

4. An arrangement according to claim 1 in which the delay means comprises a valve having two control grids one of which is connected to an input terminal of the. delay means, andthaother to an intermediate point thereof,the output terminals of the delay means being connected, toa resistance equal to its characteristic impedance 5. A source of a train. of short phase-modulated electrical pulses of substantially constant .duration and an arrangement for demodulating said phase-modulated train of short; electrical pulses comprising delay means for obtaining from the said'train a, delayed train of which each pulse and means for extracting the said. signal.v wave I from; the derived train of pulses comprising anondenser adapted to be charged by the DUISBSJiOf the derived trainso that: itspotential varies; in

.ing rids to which the original and delayed pulfies are applied, and an anode circuit. comprising said condenser.

p 8:. An arrangement for, demodulating a phase modulated train of short electrical pulses,,- comprising means for lengthening the duration of pulses, means for obtaining from the train of lengthened pulses a delayed train of which each pulse overlaps in time the following pulse of the original lengthened pulse train, means for combining each pair of overlapping pulses thus producing a train of pulses whose durations vary in accordance with the modulating signal wave, and means for extracting the said signal wave from the derived train of pulses.

9. The arrangement according to claim 8, and in which the means for lengthening comprises a delay network.

10. The arrangement according to claim 8, and in which the means for lengthening comprises a plurality of normally blocked valves and means for unblocking them in succession by the pulses received.

11. The arrangement according to claim 8 and in which the means for lengthening the pulses comprises a delay network to which the pulse train is applied, a plurality of normally blocked valves having control grids connected to different points of said network between which the delay is less than the time duration of the pulses, whereby said pulses reach them after intervals shorter than said duration unblocking said valves in turn, each remaining unblocked until just after the next has become unblocked, anodes for said valves connected in multiple, a collector valve having a control grid connected with said anodes, means for blocking the collector valve until the pulse has travelled past the last of said valves, an output circuit for the collector valve in which pulses are obtained which are inverted with respect to those applied to the network, .and whose duration is less than said certain duration multiplied by the number of said valves.

12. An arrangement for effectively lengthening the pulses of a train of electrical pulses having a certain duration, comprising a delay network to which the pulse train is applied, a plurality of normally blocked valves having control grids connected to different points of said network between a MW w.

which the delay is less than the time duration of the pulses, whereby said pulses reach them after intervals shorter than said duration unblocking said valves in turn, each remaining unblocked 3 until just after the next has become unblocked, anodes for said valves connected in multiple, a collector valve having a control grid connected with said anodes, means for blocking the collector valve until the pulse has travelled past the last of said valves, an output circuit for the collector,

valve in which pulses are obtained which i are inverted with respect to those applied to thenetwork, and whose duration is less than said certain duration multiplied by the number of said valves.

13. An arrangement for efiectively lengthening;

the pulses of a train of electrical pulses having a certain duration, comprising a delay network hav ing input terminals for the pulse train, a resistance in which said network terminates equal to having a control grid connected with said anodes.

said collector valve having also an anode, connected with said source, a blocking condenser in the grid and a second resistance in the anode connection of the collector valve, means for biassing the collector valve to permit the flow of some anode current but to place its grid below the cutoff when any of said valves is unblocked, said collector valve remaining blocked until the pulse has travelled past the last of said valves, an output circuit for the collector valve in which pulsesr are obtained which are inverted with respect to those applied to the network, and whose duration is less than said certain duration multiplied by the number of said valves.

MAURICE MOISE LEVY.

REFERENCES crrEn The following references are of record in the its characteristic impedance, a plurality of valves, 20 me of this patent;

each biassed below its cut-oil, said valves having control grids connected to different intermediate points of said network between which the delay is less than the time duration of the pulses, whereby said pulses reach them after intervals shorter than said duration to unblock said valves in turn, each valve remaining unblocked until just after the next has become unblocked, anodes for said valves connected in multiple, a source of potential, a. resistance over which said source is connected with said anodes, a collector valve UNITED STATES PATENTS

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