US2421025A - Demodulator system - Google Patents

Description

D. p. GRIEG DEMODULATOR SYSTEM May 27, 1947.

Filed July 29, 1944 2 Sheets-Sheet 1 INVENTOR. OO/VAZD 0. GfifG D. D. GRIEG DEMODULATOR SYSTEM May 27 1947.

Filed July 29,1944 2 Sheets-Sheet 2 INVENTOR. DON/410' 0. G/P/EG 4 AIYUBVEY Patented May 27, 1947 UNITED STATE alums TENT @FHCE DEMGDULATOR SYSTEM Application July 29, 1944, Serial No. 547,124

7 Claims.

This invention relates to radio impulse systems and more particularly to a system for demodulating pulses, which vary in width in accordance with signal modulation, by translating them into amplitude modulated pulses.

A system for translating amplitude variations of a signal into a train of pulses which vary in width from pulse to pulse according to the corresponding change in amplitude of the modulating signal, is disclosed in my copending application. Serial No. 547,122, filed June 29, 1944.

Methods of demodulating a width modulated pulse signal have been employed in the past which utilize the change of average value accompanying the change in pulse width. With this averaging system, the demodulated dynamic change in amplitude is directly proportional to the pulse width. Similarly, the transmitted power is proportional to the pulse width and therefore proportional to the dynamic amplitude change. Thus, for example, an amplitude change of 6 db. would require a doubling or halving of the pulse width with the corresponding doubling or halving of the transmitted power. At the negative limit of the modulating wave form the pulse width is reduced to zero and at the positive limit increased to twice the unmodulated quiescent width. The dynamic range of a system employing an averaging detector is necessarily limited to the order of 6 db. without introducing severe distortion. It can be seen therefore that such a system does not oii'er a satisfactory dynamic range and inherently requires a large change in transmitted power during modulation. Furthermore, since a change in the pulse repetition rate will also afiect the average value, a relatively stable pulse frequency is required in a system of this sort, in order to prevent the introduction of noise thereby.

It is, accordingly, an object of the present invention to provide a novel and improved method and means for demodulating pulses which vary in their width in accordance with the instantaneous amplitude of a modulating signal.

It is another object of the invention to provide a demodulating method and means wherein the total change in pulse width due to signal modulation for the full dynamic range need be only a fraction of the transmitted pulse width for a full variation in signal level to be obtainable.

It is another object of this invention to provide a method and means for demodulation of the type hereinbefore referred to, wherein the change in average power transmitted is of a minimum.

It is a further object to provide an efficient and economical method and means for demodulating width modulated pulses whereby an amplitude modulated envelope is obtained in accordance with a modulating signal which after suitable processing may be applied to an audio amplifier and/ or an audio reproducer.

In accordance with my invention, there is provided means to first limit clip the width modulated pulses as received from a transmitting source to limit these pulses'to a given amplitude. The pulses, thus suitably shaped, are then applied to a pulse width demodulator consisting of a tuned L-C circuit, a damper and a clipper stage, so arranged that only the second undulation is passed when the tuned circuit is shock-excited to produce an oscillatory wave by an externally applied pulse. A maximum amplitude in the wave output is obtained when the incoming pulse width corresponds to the tuned frequency of the damped L-C circuit. A variation of the pulses from this value will produce an output wave of constant period but with a variation in amplitude which is proportional to the change in pulse width. Thus, the series of Width modulated pulses of constant amplitude will be transformed into a corresponding series of pulses of constant width but modulated in amplitude. By use of a low-pass filter, the frequency components corresponding to the pulse sub-carrier may be removed and the audio modulating signal obtained.

It is thus apparent that the absolute audio output is independent of the magnitude of the pulse width change, and is dependent only on the tuning adjustment of the pulse width demodulator, making it possible to utilize more efiiciently the transmitted power by using narrow pulses without reducing the level of the received signal. The width demodulator of this invention will respond only to changes in pulse width and will not be aifected by any changes in pulse frequencies, frequency instability therefore being tolerated without the introduction of noise. This feature may be taken advantage of by a simultaneous pulse frequency modulation or pulse time modulation as a form of an additional communicating channel. In addition to minimizing noise due to frequency or time modulation, the present demodulator may be adjusted so that 0 sponse is obtained for pulses differing appreciably in width (either smaller or larger) from the wanted pulses, and thus, noise produced by such unwanted pulses may likewise be reduced.

For a further understanding of the invention, eierence may be had to the following detailed description to be read in connection with the accompanying drawings wherein:

Fig. 1 is a schematic wiring diagram of the pulse width demodulator system in accordance with my invention; and

Pig. 2 is a set of curves useful in explaining the operation of the demodulator.

Referring to the drawings, a width modulated pulse train as received over a wire or radio frequency transmission link is applied to a control grid I of a limiter tube 2. By being limit clipped, the pulse are shaped to substantially the same amplitude. This is desirable not only for improving the signal-to-noise ratio by removing the amplitude variations due to the noise, but also for accurate demodulation. Thus, the anode output of tube 2 is made to pass pulse energy of constant amplitude. The output pulse energy from the tube 2 is applied through a resistor 3 to a shock-excitable L-C circuit 4. The condenser C of the circuit is preferably adjustable so that it may be tuned to a wave length or a harmonic thereof, which corresponds to the maximum pulse width to which it is anticipated that the pulse will be modulated.

It will be understood, of course, that the inductance coil shown may also be made adjustable either in place of an adjustment for the condenser C or together with an adjustable condenser, whichever may be desired. Connected across the tunable circuit 4 is a vacuum tube 5, the cathode 6 of which is connected to the input side 1 of the circuit 4, while the anode 8 is connected to the opposite side 9 of the circuit. The side 9 is also connected to a source of potential 3+ to provide a suitable positive bias for an anode connection It) for the clipper tube 2.

The energy output from the anode connection I is applied to a grid ll of the tube so as to block the conduction between the cathode 6 and the anode 8, while pulse energy is applied to the circuit 4. The pulse output of the tube 2, combined with the undulations generated in circuit 4, is taken off through a connection l2 for application to a threshold clipping amplifier stage 13 of known characteristics. The negative bias on grid M of the clipper I3 is obtained over a bias resistor l5. The output from the clipper stage I3, as obtainable across a load resistor I6, is then applied to a low-pass filter i! from which the audio modulation may be had for use in a reproducing type of apparatus, as represented by the phones 18.

The operation of the system of Fig. 1 will be better understood by reference to Fig. 2, where curve it represents pulse input energy 2i as applied to the grid I of the limiter tube 2. The pulseenergy is represented by a train of pulses varying in width in accordance with the instantaneous amplitude variations of a sinusoidal modulating signal. These width modulated pulses may be obtained coming from a transmitting source, as hereinbefore indicated, to the detector of a radio frequency receiver or directly over a wire line or other known type of transmission line.

The train of pulses 2| is limit clipped-to a level 19 and applied to the tuned circuit 4 with negative polarity as indicated at 22, for shockexciting a. damped sinuosoidal type of wave train. The oscillatory wave of the L-C circuit 4 to which are added the pulses from the anode connection l0, appear in the connection I2 in the form shown in curve e or it in accordance with the degree of modulation in pulse width and the frequency to which the L-C circuit has been tuned.

Analyzing the resultant undulations with reference to curve (1, there is illustrated the case of a single pulse, where the modulation in pulse width is of the order of the width W1 of the pulse itself, the pulse widths We and Ws representing the extreme width values acquired by the pulse during modulation. In this instance, the L-C circuit is tuned to a frequency which will result in a wave of maximum amplitude for the width Wa As the leading edge 23 of the pulse of curve (2, is applied at negative polarity to the circuit 4, an initial undulation 24 is produced which ordinarily is followed by an undulation 2 3a and so forth in the form of a damped wave. As the negative voltage value of pulse 23 is combined with the undulation 2d, the resultant, as in curve 2 is displaced downward by an amount 23b for the duration of the pulse. A similar condition is indicated in curve h, where the undulation 28 is displaced downwardly by the amount 21?), corresponding to the negative polarity of pulse ed 2?. When the circuit 4 is tuned to a frequency the period of which is twice the width Wa the trailing edge 25 occurs where the initial oscillatory energy wave crosses the zero aXis. Since the trailing edge 26 shock-excites the circuit in the same direction at this point, the undulation 25 produced thereby adds algebraically to the positive portion 24d of the undulation 24 resulting in undulation 25:; which would normally continue as a damped wave were it not for the damping tube 5. The grid H of tube 5 is arranged to receive a negative voltage for the duration of each pulse, and the anode and cathode of the tube are so connected across the circuit that when the polarity of the oscillating current is in one direction it blocks conduction by the tube and when in the opposite direction it unblocks the tube. By this arrangement the system operates to suppress oscillations by becoming conductive when the voltage across the circuit has the right polarity and when the voltage of the applied pulse becomes zero. The system will therefore produce a representative undulation following the trailing edge of each pulse, and thereafter suppress all further oscillations until the next succeeding pulse of the wave is applied to the circuit. It will be clear, therefore, that a representative pulse or undulation is thus produced which represents the relation of each of the input pulse widths to the tuned frequency. To be exact, all oscillations or undulations except those occurring during the application or duration of the pulse and one oscillation following each pulse, are damped out as exemplified in another instance for the two extreme limits of modulation illustrated in curves 9 and h. This last oscillation following the trailing edge of the pulse is modulated in amplitude in accordance with the width modulation of the applied pulse and represents that portion of the oscillatory undulation which delineates, the modulating signal wave is indicated by curves b and 0.

Instead of tuning the circuit 4 to the frequency, the period of which is twice the duration of the modulated pulse at its maximum whole, but rather to a harmonic thereof which corresponds to the period of themaximum change in pulse width, the eifect illustrated in curves 9, h and i will be achieved. Here, the maximum change in pulse width, We, substantially indicates the period of the harmonic frequency to which the circuit 4 is tuned. The leading edge 27 of the negative pulse of curve 9 shock-excites an oscillatory undulation 2-8 which, in the case illustrated, achieves a negative maximum three times within the period of the pulse. If, due to modulation in width, the trailing edge of the pulse occurs at 29 or 36, the negative voltage due to the pulse at this point ceases to act on the grid 4 l of the damper tube 5, and at the same time an oscillation in the positive direction is set up, as represented by the undulations 3| or 32. Due to the absence of the pulse voltage, the damper tube is now able to damp out all but the first undulation following the trailing edge. These undulations 3! and 32 are proportional in their amplitude to the degree of modulation between zero and maximum modulation from a given normal pulse width. They are more or less in phase with the last undulation due to the leading edge 21 and therefore cause relatively smaller or larger resultant undulations following the trailing edges. Curves f and 2' show the type ofundulations obtained after those have been eliminated by clipper tube l3 at the level 33 01135 (curves ,f and h), which occur during the pulse, and which, because of their combination withthgfhegatWe pulse voltage are negatively biased by" the amount of this pulse voltage. The amplitude -of'these remaining undulations is indicative of the fwidth of the modulated pulses, as desired. The clipping level is chosen at such a value as to just eliminate any positive value undulations due to pulse values having zero or minimum modulation as indicated by trailing edges and 29, respectively.

It is also to be noted that, as the negative pulse voltage ceases to act on the grid H, for example, as at 35 (curve d), the undulation 2%, set up by the leading edge 23, tends to revert to zero. Since, however, this return to zero begins at some point on the curved portion of the undulation, that is, starts from a relatively considerable value, the parameters of the circuit are able to intervene to introduce an element of time, causing a somewhat gradual return to zero, as indicated at 36 and 3'! (curve e).

In curve b is shown the type of pulse train output obtainable from th clipper 13, while curve 0 illustrates the audio output of the low pass filter I! which is effective in eliminating the high frequency pulse carrier components. The amplitude modulated audio signal of curve 0 thus'represents a translation of the width modulated pulse train of curve a.

It should also be noted as in illustrations (g) (h) and (2') a small variation in the width modulation of the pulse is capable of producing-in the output of the demodulator as great an'absolute value of pulse amplitude modulation as the larger width variation as at (d) this being a function only of the tuning of the LC circuit.

It becomes apparent, therefore, that demodulating width-modulated pulses in the manner described hereina-bove, makes it possible to economically employ signals having a small change in average value but a large dynamic amplitude range, in accordance with the objects set forth.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope'of my invention as set forth in the objects of my invention and the accompanying claims.

What I claim is:

1. In a demodulator system having a resonant circuit the method of demodulating a pulse train the pulses of which are modulated in width in accordance with a given signal, comprising shock exciting said resonant circuit to produce periodic undulations by means of the leading and trailing edges of the pulses of said pulse train, damping out all undulations but those occurring during the duration of each of said pulses and one undulation following the trailing edge thereof so that undulations produced by one pulse will not affect the undulations produced by the next pulse, and threshold clipping said undulations at a level corresponding to the amplitude of said one undulation for-at least one limit of pulse width modulation.

2. In a method employing a resonant circuit for demodulating a pulse train the pulses of which are modulated in width in accordance with a given signal, the steps including shock exciting periodic undulations by means of the leading and trailing edges of the pulses of said pulse train in said resonant circuit tuned to a frequency having.

a period which is at least twice the maximum width modulation of any of the said pulses of said train, damping out all undulations but those occurring during the duration of each of said pulses and one undulation following the trailing edge thereof so that undulations produced by one pulse will not affect the undulations produced by the next pulse, and threshold clipping said undulations at a level corresponding to the amplitude of said one undulation for the minimum modulated width of the pulses to eliminate all except those undulations following each pulse which are greater in amplitude than said threshold clipping level.

3. In a method employing a resonant circuit for demodulating a pulse train the pulses of which are modulated in width in accordance with a given signal, the steps including shock exciting periodic undulations by means of the leading and trailing edges of the pulses of said pulse train in said resonant circuit tuned to a harmonic of a frequency having a period which is at least twice the maximum Width modulation of any of the said pulses of said train and the half cycle period of Which is of the order of the maximum change in pulse width due to modulation, damping out all undulations but those occurring during the duration of each of said pulses and one undulation following the trailing edge thereof so that undulations produced by one pulse will not affect the undulations produced by the next pulse, and threshold clipping said undulations at a level corresponding to the amplitude of said one undulation for the minimum modulated width of the pulses to eliminate all except those undulations following each pulse which are greater in amplitude than said threshold clipping level.

4. A method of demodulating a pulse train in accordance with claim 3, wherein the dynamic range of modulation of said pulses constitutes a fraction of the unmodulated pulse width.

5. In a method employing a resonant circuit for demodulating a positive type pulse train the pulses of which are modulated in width in accordance with a given audio signal, tuning said resonant circuit to a frequency having a period proportional to the maximum modulate-d pulse width, inverting in phase the pulses of said train, using said phase inverted pulses to shock excite by means of the leading and trailing edges thereof oscillatory undulations for each pulse in said tuned circuit, each undulation having a constant width and an amplitude varying as the variation in the width of the modulated pulse 7 ca-using the undulation', damping out all undulations but those occurring during each pulse and onefollowing the pulse, using said phase inverted pulses to bias said oscillatory undulations for the duration of the pulses, whereby said undulations following the pulses are rendered distinctive, and segregating said following undulations for demodulation thereof.

6. In a method employing a resonant circuit for demodulating a pulse train the pulses of which are modulated in width in accordance with a given signal, the steps including shock exciting periodic undulations by means of the leading and trailing edges of the pulses of said pulse train in said resonant circuit tuned to a frequency proportionate in its period to the maximum modulated Width of the pulses of said train, damping out all undulations but those occurring during the duration of each of said pulses and one undulation following the trailing edge thereof so that undulations produced by one pulse will not affect the undulations produced by the next pulse, threshold clipping said undulations at a level corresponding to the amplitude of said one undulation for the minimum modulated width of the pulses to eliminate all except those undulations following each pulse which are greater in amplitude than said threshold clipping level, and eliminating by filtering out the high frequency components of the resultant of said clipping, whereby an amplitude modulated wave corresponding to the original signal is obtained.

'7. A system for demodulating a train of pulse energy the pulses of which are modulated in Width in accordance with a given audio type signal, comprising a resonant circuit tuned to a frequency proportionate in its period to the maximum modulated pulse width, means to app y said train of pulses to said circuit for shock excitation therein by the leading and trailing edges of said pulses of oscillatory undulations, means for damping out all undulations but those occurring during the duration of any of said pulses and one following the trailing edge of each of said pulses, means for controlling the operation of said last named means by means of energy of said pulses, means for combining the energy of said pulses with the energy of said undulations whereby any undulations occurring during a pulse are biased with respect to their zero axis for the duration of each pulse, and said undulation following each pulse is rendered distinctive, and means for clipping said following undulations at a level which will eliminate all said biased undulations and all said following undulations except those having an amplitude corresponding to pulses having more than a given minimum width modulation.

DONALD D. GRIEG.

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

UNITED STATES PATENTS Number Name Date 2,266,401 Reeves Dec. 16, 1941 2,113,214 Luck Apr. 5-, 1938 2,181,309 Andrieu Nov. 28, 1939 2,391,776 Fredendall Dec. 25', 1945 2,359,447 Seeley Oct. 3, 1944

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