US2441969A - Electric carrier wave system - Google Patents

Description

May 25, 1948. G. J. LEHMANN ELECTRIC CARRIER WAVE SYSTEM 4 SheetS -Sheet 1 Filed April 13, 1943.

CIRCUIT AM/ER mvv: WIPES Milli/Fl m WWW/470R RAID 0mm IMPULJE GEN! AMPLITUDE MODULATION SOURCE May 25, 1948. G. J. LEHMANN ELECTRIC CARRIER WAVE SYSTEM Filed April 15, 1943 4 Sheets-Sheet 2 52 25; f I I l I!!! I! III May 25, 1948. G. J. LEHMANN 2,441,969

ELECTRIC CARRIER WAVE SYSTEM Filed April 13, 1943 4 SheetsSheet 3 INVENTOR GE fill/F011. f/v'MA/V/V ATTORNE') Patented May 25, 1948 2,441,909 anaemic caaama'wavs srs'mu Gerard J. Lehmann, Bouiogne-Billancourt. France, assignor to International Standard Electric Corporation, New York, N. Y., a

corporation of Delaware Application All!" is, 1943, Serial No. 482,910 In France September 12, 1941 6 Claims. 1

The present invention relates to systems for the on of modulated electric waves and particularly of carrier waves modulated by impulses In a general way, the invention aims at providing methods and means for the transmission and reception of telephone or telegraph communications by employing impulse modulated waves.

More specifically, the invention aims at providing systems for the transmission of impulse modulated electric waves in which a transmission path modulated in amplitude is superposed on the impulse modulated on path so as to transmit two independent paths over the same carrier wave.

According to certain features of the invention, an impulse modulation can be produced by displacing in time successive impulses that form a carrier wave, and in such a way that their interval will vary as a function of the modulation, while the mean recurrence frequency of the impulses remains constant. The position of the impulses varies on either side of mean positions which are selected once for all, and the successive impulses are displaced in opposite directions to each other, the even numbered impulses being displaced in one direction and the odd numbered impulses in the other direction. At first, when there is no modulation, the even numbered carrier impulses are staggered with respect to the -odd numbered carrier impulses by a constant quantity that is suflicient for the intervals between the successive impulses to be equal only to the maximum value of the displacements that correspond to the complete modulation of two successive impulses.

According to certain features of the invention, an impulse modulation method of this kind may be effected by a double position relaxation circuit. or trigger circuit that is controlled by the modulated carrier impulses so as to always return to one of its positions of stability, it being brought.

numbered impulses of a series with respect to to its other position of stability by another series of impulses that act on the trigger circuit alternately with the carrier impulses and outside of the limits of modulation in time of the said carrier impulses. These impulses for returning the trigger circuit are omitted in the subsequent circuits of the transmitter. and only the modulated carrier impulses are transmitted to the receiver.

An impulse modulated wave transmitter that employs features of the invention essentially comprises means for generating two series of impulses alternating in time, means for constantly staggerlng the even numbered impulses and the odd each other, means for advancing or retarding the generation of the impulses of this series in the rhythm of a useful modulation, a trigger device to which the impulses of the two series are applied alternately, means for suppressing in the output of the trigger device the impulses of the series that has fixed spacing in time, and means for modulating by the remaining i nalling impulses a high frequency generator whose waves are then radiated into space or transmitted by cable to one or more receivers.

According to another feature of the invention, the said high frequency generator may also be modulated in amplitude by another transmission path so as to insure an amplitude modulation of the impulses that are radiated or transmitted.

At the receiving end, after frequency changing and detection of the received impulses and eventual separation of the two paths modulated in amplitude and by impulses, the impulse modulation is transformed into an amplitude modulation according to certain features of the invention by means of a resonant circuit that is tuned to the mean recurrence frequency of the impulses or to an odd numbered harmonic of this frequency; the received impulses as well as a frequency voltage as abovementioned are applied to this circuit and the resulting amplitude modulated voltage is then treated in the usual way for the purpose of restoring the original signals.

I These features as well as others are described in detail in the-following specifications given with reference to the appended drawings. in which:

Fig. 1 illustrates schematically the type of impulse modulation provided by the present invention;

' Fig. 2 illustrates schematically one example of a transmitting station that makes use of features of the invention:

Fig. 3 is a schematic illustration of the connection arrangements of the trigger and modulating circuits provided for use in the transmitting station of Fig. 2; v

Figs. 4 to 8 inclusive show various graphs used in the explanation of the operation of the station of Fig. 2;

Fig. 9 illustrates schematically one example of a receiving station that makes use of features of the invention, and p I Figs. 10, 11 and 12 show the manner in which the impulse modulation that is employed is transformed at the receiving end into an amplitude modulation.

As shown in Fig. 1, the "carrier impulses I, II,

III drawn in heavy lines have between them intervals that are alternately wider and narrower, but of the same periodicity, which gives the impulses a quite definite and constant mean recurrence frethe odd numbered mean positions I, III, V, etc.,

and the even numbered mean positions II, IV, VI, etc. spacings between impulses of the even numbered series or the odd numbered series equal to 40 microseconds, and spacings between alternate limits I, 2', 3, 4', etc., equal to microseconds, which may accordingly be considered as the mean recurrent frequency of the modulated impulses. and individual widths (or modulation depths) of the impulses equal to 2 microseconds on each side of the mean position I, II, etc.

The triggering impulses with fixed spacing that are provided as abovementioned for bringing the trigger device to the position of stability that is alternate to the one to which it is brought by the modulated impulses may then suitably consistof a series of uniformly spaced impulses, e. g. equal to 20 microseconds, disposed soas to be produced in the intervalsbetween the modulation limits of the successive impulses |'2, 2'-3, etc. It must be understood however that this last series of impulses may be of any desired recurrence, and the only condition they must comply with is that of being produced alternately to the modulated impulses.

When complying with these conditions, the two series of impulses can be utilized in the manner shown in Fig. 2 for the emission of an impulse modulated carrier wave.

7 Inthis Fig. 2, the reference number l0 designates an alternating generator stabilized in any suitable way, e. g. by quartz, and having an oscillation frequency suitable for regulating the mean recurrence frequency of the desired impulses. In the example shown in the drawing, its frequency is taken to be kilocycles, and this, by frequency doubling in the modulator circuit H and in the auxiliary triggering impulse generator circuit l2, brings the mean frequency of the impulses acting on trigger circuit to a value of 50 kilocycles for each series of impulses, this 50 kilocycle frequency corresponding to impulses 2 0 microseconds apart, as shown in Fig. 1.

The auxiliary impulse generator circuit 12 may, for example, be of any construction suitable for generatingbrief impulses at regular intervals of 20 microseconds. For this purpose, it may comprise adouble diode tube to rectify the 25 kilocycle voltage that proceeds from the generator ID. The rectified voltage has backing points at a frequency of50 kilocycles and these are accordingly at spaced intervals of 20 microseconds, as desired. These backing points are suitably deformed by two successive electric derivations according to well known methods and circuits and, after amplification, they permit the obtaining of short impulses or negative impulses,

As an example, the drawing shows mean e. g. of a duration of 0.5 microsecond and of an amplitude of 20 volts. I

The modulator device ll may be designed in the manner shown schematically in the bottom portion of Fig. 3. Two grid type tubes 13 and I4, e. g. pentodes, are connected in a. symmetrical connection arrangement so as to operate alternately at the 25 kilocycle frequency generated by the oscillator l0. However, a certain phase displacement between the voltage applied to the modulator II and the voltage applied to the impulse generator |2 is provided at the output of generator I0, this displacement in time being greater than the useful modulation interval ll etc., of the modulated impulses of Fig. 1.

The screen grids of these two tubes I3 and H are fed by the 25 kilocycle output alternating voltage of generator l0 across transformer l5. Since the two tubes l3 and I4 have a common cathode resistance, the voltage a that is tapped at the terminals of this resistance 16 is of the kind shown at I! in Fig. 4. Each of the tubes l3 and I4 accordingly only generates a short impulse at two terminals of resistance l Tube l3 will generate the even numbered impulses and tube l4 the odd numbered impulses in the series.

When no modulation is applied by transformer 13 to the control'grids Of tubes l3 and H, the impulses generated in resistance l6 under the control of the 25 kilocycle alternating voltage would be uniformly spaced. However, by means of the adjustment of the potentiometer IS inserted in the feed line of the screen grids, it is possible to obtain a slight asymmetry between tubes l3 and 14, as shown at 20 and 2| in Fig. 4. This asymmetry can be adjusted in such a way that the series of even numbered impulses is systematically staggered with respect to the series of odd numbered impulses so that these impulses may occupy in time the positions indicated by the heavy lines in Fig. 1.

When a useful or signalling modulation is applied to the control grids of tubes 13 and H by transformer l8,'the carrier impulses obtained in the above described manner are modulated in amplitude as shown at 22 and 23 in Fig. 4, the limits of variation of the amplitude of each impulse at the terminals of resistance l6 being such that it displaces in time the impulses on both sides of their mean position in dependence upon the modulation, and in opposite directions for the even numbered and odd numbered series of impulses, as explained above.

Since the frequency of the impulses of each series is 25 kilocycles, it follows that the impulses of same series are regularly spaced 40 microseconds apart, depending on the size of the modulation. The stagger due to the modulation may suitably be equal to one tenth of the spacing be tween successive even numbered or odd numbered impulses, i. e. 4 microseconds in the example under consideration, and the maximum amplitude of the displacementdue to the modulation should consequently not exceed 1-2 microseconds.

The modulated impulses that are generated at the terminals of resistance I6 of the modulator circuit II are applied to resistance 25 of the trigger device, of which a schematic example of embodiment is shown in the top portion of Fig. 3.

This device, which makes it possible to obtain from fixed impulses other impulses that can be staggered in time-consists of a multivibrator or double stability circuit 26. It comprises two tubes 21 and 28, e. g. pentodes, and the plate of tube 21 is connected to the control grid of tube 22, but without the plate of tube 22 being connected to the grid of tube 21 as in conventional multlvibrator circuits.

The control grid of tube 21 receives the uniformly spaced impulses that are generated by circuit l2. The impulses generated by the modulator circuit II are applied across resistance 25* to the control grid of tube 22.

The constants of the circuits are selected in such a way that in the state oi equilibrium the plate currents of tubes 21 and 22 are equal. When the control grid of tube 21 received a negative impulse from circuit l2; the plate current of this tube dies out abruptly, and this results in the feeding of a positive impulse to the control grid of tube 22 by means of the condenser 29 which becomes charged to a certain extent. The

plate current of tube 22 accordingly increases, abruptly up to a certain value which is dependent on the constants of the circuits. This current variation is represented by the line 20 in Fig. 5. Then the condenser "discharges across the grid resistance 25, and the positive voltage of the grid of tube 22 decreases. The plate current of tube 22 consequentiy'decreases as shown by the curve 2| oi Fig. 5, this curve depending on the time constant of the resistance 25-condenser 22 circuit.

During this first operation, the voltage drop due to the current that passes into the resistance 22 brings the cathodes of tubes 21 to a positive potential which decreases at the same time as this current. As long as this positive potential is above a certain critical value, the plate current of tube 21 will remain nil and tube 28 will operate alone.

When the cathode potential drops to a value below the cutoif point, a plate current will appear in tube 21 and, since the equilibrium of the device is upset, a violent tilting will occur and the plate current of tube 22 will die out abruptly, as shown by line 22 in Fig. 3. The occurrence will then begin again with each impulse applied to the grid of tube 21, i. e. every 20 microseconds in the described example.

All other things being equal, the time X at the end of which the tilting occurs will depend on the value of resistance 22. This time can accordingly be adjusted within very wide limits by adjustment of the value of this resistance.

However, since impulses generated in the modulator H are also applied to resistance 25 and consequently to the control grid of tube 22 at moments in between the moments of application of the impulses proceeding from the generator l2 to the control grid of tube 21, the time X at the end of which the tilting occurs becomes shortened or lengthened according to the modulation in time of the impulses proceeding from the modulator ll. As a result of this, if the abrupt variations of plate current such as those represented by the line 22 occur at uniform intervals in time under the control of the impulses proceeding from circuit l2, the abrupt cut-oil variations such as those represented by the line 22 in this same Fig. 5 will depend on the impulses proceeding from the modulator II and will consequently have variable spacings dependent upon the modulation.

- The output voltage U of the tilting device 26 is, for example. shown in Fig. 6. Aftershunting into a suitable circuit, there is obtained a curve like the one in Fig. 7 which comprises negative impulses such as 24, which are fixed in time, and positive impulses such as 22 having variable spacing in time.

The fixed impulses are then eliminated by detection and the short remaining impulses, such as those indicated at 26 in Fig. 8, are amplified in a short impulse amplifier 21 of any well known suitable type, which, .for example, effect the abovementioned detection that eliminates the negative impulses with fixed spacings shown in Fig. 7. This amplifier, for example, comprises two amplification stages and the output of the second stage is eiiected in the low impedance cathode circuit in the way well known to the art. The duration of the impulses obtained in this way is about 0.5 microsecond in the example under consideration.

The short impulses issuing from the amplifier 21 then serve for modulating a high frequency oscillator 28 which,'for example, may consist of a conventional self-oscillator that comprises two triode tubes in push-pull arrangement and that furnished an average power, e. g. of about 2 watts, which corresponds to a peak power of about 80 watts. The frequency employed is 50 megacycles, for example.

In the interval between the impulses, the oscillators grids to which the modulation impulses have been applied are blocked by the voltage drop produced by the grid current across a high resistance.

In the example of a transmitter that has just been described, other features of the invention provide for the impulses already modulated in time being also modulated in amplitude in the high frequency oscillator 28 by an independent transmission channel having stages of conventional type. 22 indicates a low frequency modulator for the amplitude modulated transmission channel, this amplifier modulating the plate voltage of the high frequency oscillator 28.

The high frequency modulated waves that issue from the oscillator 38 are then fed to a suitable aerial to be radiated into space, or else to be transmitted along a cable adapted to the frequency and modulation characteristics.

Fig. 9 illustrates an example of a receiver that is specially suitable for use in association with the transmitter shown in Fig. 2. The type of receiver shown is a superheterodyne and first of all it comprises high frequency amplifier stages, the number of which depends on the desired sensitivity. These stages are designated by the reference number ll and, for example they may consist of two symmetrical amplification stages tuned to the 50 megacycle frequency. They are followed by a frequency changing stage 42 which may also be of the symmetrical type and operate in association with a local oscillator 22 of good stability.

The subsequent mean frequency stages 22 have a large band pass width on account of the slight duration of the impulses to be received, 0.5 microsecond in the described example. For example,

" the total band pass width may be 4 megacycles for an attenuation of 4'decibels, the mean frequency being 14.5 megacycles. These stages may be of any desired number and of any well known suitable type of construction.

Detection of the mean frequency signals is eilected by a threshold detector 45, the threshold being controlled, for example, by means of an adjustable cathode bias in the detector circuit.

The purpose served by the threshold is that of permitting elimination of the background noise and of strays, with consequent improvement of the signal-to-noise ratio. Its adjustment may be :described for the'transmitter. These signals are amplified by means of a resistance-coupled audio frequency stage 46. The two modulation channels are separated at the output of this preamplification stage. The amplitude modulated channel is treated by a conventional audio Irequency stage 41 whichmakes it possible .to raise the level of the amplitude modulation and to reproduce the original signals, e. g. by means of an earphone or a loudspeaker 48.

The modulation channel due to impulse displacement must not be influenced by variations of amplitude of the impulses. limiter stage 59 is provided for this purpose at the output of the pre-amplifier 46 in order to bring all the impulses to one same level, and this i can be adjusted to any desired value .by means of a potentiometer acting on the amplitude limiter stage.

This stage, for example, may consist of a tube whose grid receives the negative impulses of variable amplitude. The tubes cut-off point is adjusted so as to be reached .by the impulses of minimum. amplitude, and passed over by all the impulses of greater amplitude, through variation of this tubes plate voltage, as this results in displacement of the plate characteristic. V

This amplitude limiter stage also acts as a protection device against strays whose amplitude cannot add to thatof the useful signals, and it consequently improves the signal-to-noise ratio.

At the output of the amplitude limiter device 59, the received impulses are of the same amplitude but staggered in time, and means must be provided for transforming this modulation into amplitude modulation.

According to one feature of the invention, this transformation can be effected by applying the impulses to a circuit that resonates at the recurrence frequency of the series of impulses, i. e. 25 kilocycles in the described example. Figs. 10, 11 and 12 make it easy to understand the operation of such a tuned circuit to which the impulses are applied.

When the series of impulses consists of regularly spaced impulses 6| (Fig. 10), the resultant of these impulses and of an alternating voltage of their mean recurrence frequency will be zero.

If the impulses are staggered in the manner shown in Fig. 1, then when there is no modulation, there will be a resultant which will be all the larger according as the staggering is greater.

This will cause the appearance of a resultant carrier wave. This is shown at 63 in Fig. 11.

When this staggering varies in dependence upon the modulatiomas shown at 64 in'Fig. 12, the carrier wave will be modulated in amplitude, and this insures transformation of the impulse modulation into an amplitude modulation, as shown at 65 in Fi 12.

According to anotherfeature of the invention, a variant of this method consists in applying the impulses to a resonant circuit that is tuned to an odd numbered harmonic of the recurrence frequency of the impulses instead of to that frequency, but the mode of operation is still the same. In one example of an embodiment, use has been made of this arrangement by utilizing a modulation transformation circuit that consists An amplitude of two circuits tuned to the fifth harmonic of the' said recurrence frequency, 1. e. 125 kilocycles, and capacity coupled. The pass band was about 1 kilocycles.

The amplitude modulated wave obtained in this way is then detected in a detector circuit 88 and amplified as shown at 61, and the original signals are reproduced 'by a suitable device, a loud speaker or earphone, as indicated at 58.

It is evident that in the above specification no detailed description has been given of the circuits or devices which are well known per se. It must be understood however that they may be of any suitable design and are not necessarily special types that might have been mentioned as examples.

It is also evident that the invention is not limited to the examples of embodiments shown and described, nor to the numbers cited as examples, but that on the contrary it is capable of numerous modifications and adaptations without departing from its scope.

What is claimed is:

1. A transmission system comprisingv a source of carrier wave energy, means for deriving equally spaced impulses from said carrier wave source, a first modulator, means for applying energy from said source to said first modulator, means for applying modulating energy to said first modulator to phase modulate said applied energy, a trigger circuit operative into two positions, means for applying said equally spaced impulses to said trigger circuit to operate it into one position at fixed time intervals, means for applying said p'hase modulated energy to said trigger circuit to operate it into other positions in response to the frequency modulated signal, and means for deriving an output wave from said trigger circuit.

2. A transmission system according to claim 1 further comprising differentiation means for said output wave to produce pulses representing the beginning and end of each impulse of said output wave, and means for suppressing the pulse representing the beginning of said impulse.

3. A transmission system comprising a source of energy of predetermined frequency, means for producing a series of impulses spaced apart a half wavelength at said predetermined frequency, a first modulating means forfderiving from said wave, pulses for each half-wave thereof, a source of signal energy, means for applying energy from said signal energy source to said modulator to phase modulate said pulses, a trigger circuit operative into two positions of stability, means for applying said impulses to said trigger circuit into a first of said positions of stability, means for applying said phase modulated pulses to said trigger circuit to trigger, it into its other position of stability, means for deriving an output wave from said trigger device, said wave having one edge constantlyspaced in time under control of said impulses and its other edge variable in time under control of said phase modulated pulses, and means for deriving fromsaid output wave time modulated pulses positioned in accordance with said variable edge, whereby pulse signals modulated in time by said signal are produced.

4. A pulse modulation system comprising a carrier wave source, a push-pull modulator circuit,

unbalanced means for applying said carrier wave energy to said modulator to produce pulses corresponding to half-wave position of said carrier wave, balanced means for applying signals from said source to said modulator to eflect the degree of unbalance thereof, whereby phase modulated pulses are produced in said output, means for producing a train of constantly spaced impulses from said carrier wave, a double stability trigger of said double stability circuit for deriving a series of time modulated pulses positioned in accordance with the alteration in said interval, a source of radio frequency energy, and means for circuit, means for applying said impulses to said 5 modulating a radio frequency energy from said trigger to periodically trigger it into one state of stability at fixed intervals of time, said circuit normally operating to return to its other position of stability in a predetermined interval, and means for applying said phase modulated pulses to said circuit to alter said predetermined interval in accordance with said signals.

5. A pulse modulation system according to claim 4 further comprising means in the output of said double stability circuit for deriving a series of time modulated pulses positioned in accordance with the alteration in said interval.

6. A pulse modulation system according to claim 4 further comprising means in the output source in accordance with said time modulated pulses.

GERARD J. LEHMANN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Luck Jan. '7, 1941

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