US2440264A - Modulator system - Google Patents

Description

Filed Oct. 15, 1943 INVENTOR. ow/M0 0. (NP/6 MODUZHTO/P Patented Apr. 27, 1948 MODULATOR SYSTEM Donald D. Grieg, Forest Hills, N. Y., assignor to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application October 15, 1943, Serial No. 506,412

6 Claims. 1

This invention relates to carrier communication systems and more particularly to systems for pulse modulation of carrier waves wherein the pulses vary in time and/or width according to a signal wave.

Telegraph keying circuits have been proposed heretofore for modulating carrier Waves for the transmission of time and width modulated pulses but such circuits are not satisfactory for rapid keying such as required in the pulse transmission of voice, television signals, facsimile, etc., nor where the modulated carrier is to be used for testing receiver equipment. Such telegraph keying circuits generally include a keying amplifier tube whereby the keying is performed either by controlling the grid bias or the plate bias thereof. Keying of the amplifier at a rapid rate and at variations either in pulse amplitude, width or timing, produce variations in the output of the tube from signal to signal and this results in carrier pulses or envelopes which vary both in the steepness of the slopes of the leading and trailing edges and in amplitude.

For telegraph keying this is not a problem because the keying is relatively .slow and the variations caused are small compared to the relatively long duration of the .dot-dash signals. The duration of signal pulses required for voice, television, etc., is very shortcompared to the telegraph signals and variations in slope of leading and trailing edges of the pulses may be so great as to render the system substantially inoperative.

Another point of importance is the difficulty of measuring and selecting the desired power output of such keying circuits where the output must be accurately known for testing purposes. This is so because a measurement of power output prior to a keying of the operation of prior systems is not an accurate measure of the peak power of the carrier pulses because of variations that occur in the gain of the amplifier tube and/ or in the plate circuit thereof in response to variations in the timing, width and amplitude of the signal pulses, and changes in frequency of oscillation.

It is one of the objects of this invention to provide a pulse keying or modulating system capable of producing a pulse. modulated carrier wave in response to time and/or width modulated pulse signals, in which the carrier envelopes conform accurately with the duration and timing of the modulating pulse signals.

Another object of the invention is to provide a carrier wave modulating system wherein the leading and trailing edges of the carrier envelopes produced are substantially vertical and consistently formed; also wherein the average peak voltage of the envelopes is the same as the peak voltage of the umnodulated carrier wave.

Another object of the invention is to provide a system for pulse modulation of a carrier wave in which the peak voltage of the output carrier is selectively adjusted.

The above and other objects of the invention will be best understood by reference to the following detailed description to be read in connection with the accompanying drawings, in which:

Fig. 1 is a schematic wiring diagram of a carrier modulating system according to my invention; and

Fig. 2 is a graphical illustrationused for explaining the operation of Fig. 1.

In Fig. 1 a modulating system is shown whereby a carrier wave is produced and pulse modulated by voice signals although signal sources other than voice may, of course, beused. Any form of .pulse generator wherein the pulses are adapted to be displaced or varied in width or time displacement in accordance with the modulating signals may be provided. For purposes of illustration, a known form of cusper modulator I0 is shown to which a source l2 of sinusoidal voltage is applied. The cusper modulator may be of the symmetrical pulse generating type or it may be of the biased type such as disclosed in the copending application of E. Labin and D. D. Grieg, Serial No. 455,897, filed August 24, 1942 since issued as Patent No. 2,416,329.

One manner of operation of this type of modulator is illustrated by curves 15 of Fig. 1 wherein 12a represents the sinusoidal Wave of the oscillator l2. The modulator Ill is of the push-pull rectifier type wherein the wave l a is fully rectified. The modulator In, however, is preferably biased to an offset condition'as indicated by the axis Illa. For modulation operation of the modulator, the full-wave rectification of wave 12a produces cusps 16, ll, l8, l9, etc., which when clipped and shaped by a shaper circuit 20 produces a series of pulses 25, 21, 28, 29, etc., which may be of either negative or positive polarity whichever is desired.

When voice signals are applied to the modulator Hi from microphone 30, these voice signals vary the bias on the modulator to cause the pulses Z6, 21 and 28, 29 to' shift in time displacement toward and. away from each other in push-pull manner such as indicated by arrows 31, cm, Fig. 2. This time displacement of the pulses corre sponds substantially directly to the instantanethe positive feedback type 32 disclosed in Fig. 1.

The oscillator includes a vacuum tube 33 to which an LC circuit is connected in the grid-to-cathode circuit thereof. The plate 33 is supplied with a positive potential at 31 through load resistor 38. By properly tuning the LC circuit, the oscillator 32 is adapted to produce a carrier wave '44), such as shown at curve a, of the desired frequency.

In order to key or modulate the carrier wave- 40 according to the signal pulses 25, 21, etc., I connect a, three-electrode vacuum tube 42 with the plate 43 and the cathode 44 connected across the terminals of the circuit. To the -grid'45 I couple the output'connection 22 of the shaper 20. The grid 45 is provided with a grid leak 41 which may be-connected by contacts 48, '49 to the cathode '44 for zero' bias or by contacts '48, '5il'to a negative source of potential. When the contacts =48; 55 are closed, the negative bias applied to gri'd 45 maintains open the plate-tocathode circuit of the tube 42, thereby permitting the oscillator 32 to function normally to produce the wave 40. When the contacts 48, 49 are closed, the negative bias is removed from the grid '45 thereby causing the tube 42 to operate as'a diode which conducts current each time the plate 43 is made positive with respect to the cathode 44. This zero bias on tube 42 operates to produce current fiow for each positive undulation of the oscillations -43 and thereby absorb abruptly the energy of oscillation of the LC circuit whenever the bias on the tube is changed fromnegative to zero.

Should signal pulses 26, 21, etc., of negative polarity (curve 12) be applied to the grid 45, contacts 48, 49*being closed, each signal pulse will'bias negatively the grid 45 thereby maintaining the tube '42 at cut-01f for the duration of the pulse. This sudden cutting off of the tube "'42 permits the oscillator '32 to resume operation. This resumption of operation is spontaneous, and since the plate potential at 31 is maintained fixed for a given operation; and oscillatory swing of the oscillator will be the same for each of the carrier envelopes as when the grid is maintained negative by closingcontacts 48, 50. This is illustrated by the oscillations of curves 0. and c. The instant thenegative bias on grid 45 is removed, the tube will conduct and thereby absorb the positive oscillatory energy of the LC circuit.

It will be understood that the illustrations of the carrier wave oscillations as well as the pulse width dimensions are given only for the purpose .of illustrating the principles of my invention, the signal pulses normally ranging in duration from about one-third to three or four microseconds, more or. less. microsecond, for example, would include about 30 ormore cycles of oscillation dependent, of course, on the carrier wave frequency. It will be understood, however, that when the tube 42 is biased to cut-off, the oscillations of the circuit 32 will immediately build-up as indicated at 56 to the full amplitude of oscillation and that when the cut-off bias is removed, regardless of the instant'voltage of an oscillatory swing, the oscillationsare terminated withinone-half of a cycle.

A carrier pulse of one Should the negative bias of a signal pulse be removed from grid 45 during the negative swing 51, the oscillatory swing will be completed, but as it passes through zero to positive, the tube commences to conduct thereby absorbing the charging voltage of the LC circuit. The trailing edge, like the leading edge, of each carrier pulse envelope is consistently-defined as substantially vertical. The leading edge of the carrier pulse envelope corresponds accurately in time to the leading edge of the corresponding signal pulse while the trailing edge is accurate within the timing of one-half cycle of the carrier wave.

It will be observed that the carrier wave modulator of my invention may even produce more truly rectangular pulse envelopes than that of the modulating pulses. This is so because the modulating tube 42 either permits the initiation of a positive undulation or absorbs substantially completely the energy attempting to form same, as the case may be.

The frequency of pulse modulation by my system is only limited by the capacity of the plateto-cathode electrodes of the tube 42. If this capacity is high, the tube will not cut-off at a high pulsing rate. this regard to select a tribe of low capacity.

To measure the peak voltage of the modulated carrier wave, especially for use in testing ofreceiver equipment, the load resistor or attenuator 3B is calibrated, preferably in mi-crovolts, and bridged by a voltmeter Bil By adjusting the source of positive bias at31, a given R. F. carrier voltage across attenuator 38 may be obtained for calibration of the attenuator 38 by observing the meter 60. By adjusting the movable contact 62, a carrier wave of any desired peak voltage may be obtained and read directly from the attenuator 38. Since the carrier pulses 26a, 21a, etc., are of the same amplitude as the unmodulated carrier wave 40, independent of the modulating pulse width, amplitude, frequency, etc., a measurement of the carrier wave 4!] by closing contacts 48, 50, determines the peak amplitude of the carrier pulse envelopes.

This accurate measurement of the carrier wave is very useful for testing receivers, not only for different peak voltages for input signals, but also for different carrier frequencies since the peak voltage of the carrier is not changed by change in frequency.

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.

I claim:

1. A modulating system comprising an oscillator having an oscillatory tank circuit for producing an oscillatory wave, a vacuum tube having plate, cathode and grid electrodes, means connecting said plate and cathode electrodes across a part of said oscillator tank circuit, means for connecting the grid electrode to a source of pulses of negative polarity, .a grid leak, and means ccnnecting said grid leak to the cathode electrode to normally give the grid electrode zero bias, such zero bias condition functioning to permit said tube to conduct upon occurrence of positive halves of oscillations thereby preventing occurrence of oscillations during such zero bias con-'- dition, and said pulses of negative'polarity opcrating to bias the tubeto'cut-ofi-thereby render.

It is therefore important in ing said tube non-conductive so as to permit said oscillator to produce oscillations during the occurrence of said pulses.

2. The system defined in claim 1 wherein said oscillator includes a load resistor calibrated according to a desired graduated output for different points along said resistor, and output means selectively connectable to said resistor at said points, whereby the peak power of the output oscillations is determinable.

3. The system defined in claim 1 wherein said oscillator includes a load resistor calibrated according to a desired graduated output for difierent points along said resistor and output means selectively connectable to said resistor at said points, and wherein the grid electrode of said tube is provided with means for biasing the tube to cut-ofi thereby enabling the oscillator to operate continuously so that the average voltage drop across said resistor corresponding to the peak voltage output thereof is obtainable by measuring said voltage drop.

4. In an oscillator having an oscillatory circuit and an output circuit, means to control the oscillating operation of said circuit including a vacuum tube having its plate and cathode electrodes connected across said oscillatory circuit and its grid electrode arranged for connection with a source of potential, said output circuit being provided with a calibrated load resistor and a voltmeter connected across said resistor, means to apply a given potential to said grid electrode to render said tube non-conductive to oscillations in said oscillatory circuit thereby enabling said voltmeter to indicate a steady voltage drop across said resistor, whereby the calibrations of said resistor may be valuated, and output means connectable at selectable points along the length of said resistor, whereby oscillations of a desired peak power are obtainable.

5. A modulating system comprising an oscillator having an oscillatory tank circuit for producing an oscillatory wave, a vacuum tube having plate, cathode and grid electrodes, means connecting said plate and cathode electrodes across a part of said oscillator tank circuit, means for connecting the grid electrode to a source of pulses of a given polarity, grid bias means, and means connecting said grid bias means to normally give the grid electrode a given bias, such bias condition functioning to permit said tube to conduct upon occurrence of given halves of oscillations thereby preventing occurrence of oscillations during such bias condition, and said pulses of given polarity operating to bias the tube to cut-of! thereby rendering said tube non-conductive so as to permit said oscillator to produce oscillations during the occurrence of said pulses.

6. A modulating system comprising an oscillator having an oscillatory tank circuit for producing an oscillatory wave, a vacuum tube having plate, cathode and grid electrodes, means connecting said plate and cathode electrodes across a part of said oscillator tank circuit, means for connecting the grid electrode to a source of pulses of negative polarity. grid bias means, and means connecting said grid bias means to normally give the grid electrode a given bias, such bias condition functioning to permit said tube to conduct upon occurrence of positive halves of oscillations thereby preventing occurrence of oscillations during such bias condition, and said pulses of negative polarity operating to bias the tube to cut-off thereby rendering said tube non-conductive so as to permit said oscillator to produce oscillations during the occurrence of said pulses.

DONALD D. GRIEG.

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

UNITED STATES PATENTS Germany July 18, 1928

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