US 2159020 A
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y 1939- E. B. FERRELL 2,159,020
' AMODULATING SYSTEM Filed June 9, 1934 s Sheets-Sheet 1 F/ g a E g V a Lon) 2 msauzncr AMP.
man T FREQUENCY AMP.
lNVENTOR E B. F E RRE L L erg/aw ATTORNEY y 9 E. B. FERRELL 2,159,020
MODULATING SYSTEM Filed June 9, 1954 s Sheets-Sheet 2 FIG. 4
INVENTOR E. 8 FERRELL BYZW A TTORNE) y 1939- E. s. FERRELL 2,159,020
MODULATING SYSTEM Filed June 9, 1934 2' Sheets-Sheet a FIG. 7
lNl EN m E. 8. FERRELL ATTORNEY Patented May 23, 1939 PATENT OFFICE MODULATING SYSTEM Enoch B. Ferrell, Oakhurst,
Bell Telephone Laboratories,
N. J., assignor to Incorporated,
New York, N. Y., a corporation of New York Application June 9, 1934, Serial No. 729,735 2 Claims. (01. 179-111) This invention relates to modulating systems and more particularly to means for regulating the transmission characteristics for such systems.
I The invention has as its main object to reduce wave distortion in a modulator, particularly envelope distortion of a modulated wave.
Another object is to remove from an alternating current or carrier wave any undesired modu- 10 lation such as noise effects and slow amplitude variations.
' Noise and modulation effects have been successfully controlled in amplifiers by means of negative feedback or degeneration. The underlying principles involved in this method of control are disclosed by H. S. Black in an article entitled Stabilized feedback amplifiers, published in the January 1934 issue of the Bell System Technical J ournal, on pages 1-18 and in a copending appli- 20- cation, Serial No. 606,871, filed April 22, 1932, and
assigned to the same assignee as the present application. In a modulator, however, the problem of noise and distortion reduction is somewhat complicated by the necessary frequency changes 5 which occur.
In accordance with the invention, a detector is employed to derive a detected signal from a modulated carrier wave. In the absence of feedback in the system the detected signal varies somewhat 30- from the original signal in its wave form according to the amount of noise and distortion introduced by the modulator. To reduce this distortion the detected signal is fed back degeneratively upon the signal input of the modulator.
35' By making the fed-back component large compared with the original signal input and controlling its amplitude, the envelope distortion of the modulated wave may be reduced as much as may be desired.
40 Preferably a rectifier having a linear characteristic is used as the means for detecting the signal from the modulated wave since such a rectifier producesa faithful copy of the envelope of the modulated wave and introduces no addi- 45 tional signal distortion by itself.
In a modified form of the invention the principle of degenerative feedback is used to remove noise components and other undesired modulations from a carrier wave, thus providing a pure '50 sinusoidal wave of constant amplitude which may be supplied -to a modulating amplifier for'signal modulation. V
The invention will be more fully understood from the following detailed description and from 55 the accompanying drawings of which:
Fig. 1 shows a radio transmitting system embodying this invention;
Fig. 2 shows an application to the Heising system of modulation;
Fig. 3 shows the application to a grid circuit 5' modulator;
Fig. 4 shows the elimination of a separate detector at the transmitting station;
' Fig. 5 shows a reversed feedback arrangement I adapted to a screen grid amplifier;
Fig. 6 shows a modification of the system of Fig. 2 in order to utilize a signal input amplifier of the push-pull type; and
Fig. 7 shows a transmitting system incorporating initial noise reduction in a carrier supply amplifier.
In Fig. 1, a balanced high frequency generating system I is shown connected to a transmitting antenna system 2 through a high frequency am- I plifier 3. A signal source 4 is associated with the output circuit of the generating system I by means of a transformer 5, a bridge network 6 and a low frequency amplifier 1. A receiving antenna 8 is provided with a rectifying detector 9, H the output of the detector being connected through a filter I0 and transformer II to the bridge 6. Transformers 5 and I l are placed in the opposite diagonals of the bridge, with the amplifier 1 across one of the ratio arms. Dete'cv tor 9 may be a diode vacuum tube or a copper oxide rectifier or other rectifying device exhibiting a linear relationship between its input voltage and output current.
In the operation of the system shown in Fig. 1, the high frequency output of the generator I is modulated by amplified signals from source 4 transmitted through the transformer 5, the bridge 6 and the amplifier 1. The modulated high frequency wave is amplified in the amplifier 3 and radiated from the antenna 2. The antenna 8 picks up energy from the radiated wave for detector 9, which feeds the detected wave back in reversed phase upon the input circuit of the low frequency amplifier 1. The feedback operates to diminish noise effects and distortion 4 in the output wave from the system in accordance with the principles described in the hereinbefore mentioned paper by H. S. Black. The system diminishes noise and distortion originating in any portion of the loop comprising amplifier 1, generator I, amplifier 3, detector 9, and their coupling arrangements. The greatest reduction of disturbing effects Will occur when the feedback is as strong as possible and has a phase angle close to degrees. Strong reversed 5 feedback results, of course, in reduced output, but the modulation can be restored to its former strength by increasing the signal input. The phase requirement can generally be met by r..- versing one winding of the transformer ll, because with high grade amplifiers the phase shift in any stage will be either zero or degrees. If the total phase shift around the loop is 180 degrees or an odd multiple thereof, the transformer may not be needed.
The general principles of reversed feedback operation are set forth in Blacks article above cited. In the application of the principles to specific circuits it is well to note two definite limitations. One is that the system will not compensate for distortion which comes from interruption of the space current of a vacuum tube. Neither will it compensate for distortion due to exceeding the saturation voltage of a tube. The other limitation is imposed by the tendency to sing or set up self-sustaining oscillations. Nevertheless I have found by experiment that a substantial reduction of noise and distortion can be secured in radio transmitters by the use of my invention.
The bridge network 6 serves to isolate the feedback circuit from the signal supply circuit, making these circuits conjugate to each other and capable of independent adjustment. The filter I0 is employed to prevent high frequency feedback.
Fig. 2 shows a transmitter using the system for modulation disclosed in U. S. Patent 1,442,147 issued January 16, 1923 to R. A. Heising. A carrier generator I! is coupled to the transmitting antenna 2 by means of an amplifier I3. A low frequency amplifier I4 has its plate circuit coupled to the plate circuit of the modulating amplifier I3 through a choke coil I5 as is conventional in the Heising system. The detector 9 is coupled to the output circuit of amplifier I3 by high frequency coupling I6 and to the input of amplifier I4 by the low frequency transformer I I. The signal source 4 is associated with the input of amplifier I4 by means of the transformer 5.
In the operation of the system of Fig. 2, the signal wave from the source 4 is amplified in the vacuum tube I4. the amplified wave being used to effect modulation in the tube I3. The modulated wave is impressed upon the antenna 2 and also through coupling I6 upon the detector 9. The detected wave is fed back upon tube MI in reversed phase by means of the transformer II. The feedback in this case operates to reduce noise and distortion originating in the audio system and in the plate circuit of tube I3.
Fig. 3 shows, the application of the invention to a grid circuit modulator. The carrier generator I2 is connected to the antenna 2 through the modulating amplifier I3 and a power amplifier IT. The signal source 4 is connected into the grid circuit of amplifier I3 by means of the transformer 5. The detector Sis coupled by high frequency coupling I6 to the output circuit of amplifier I1 and by low frequency transformer II to the input of amplifier I3.
In the operation of the system of Fig.. 3, the
signal. wave is impressed upon the grid of tube I3. along with the carrier wave. The. modulated output of tube I3 is amplified by tube I1 and then radiated by antenna 2. The detector 9 picks up. energy from the output of amplifier I1 by means of the coupling I6. The detected wave is, fed back upon the grid of tube I3 in reversed phase relation to the signal wave by means of the transformer II.
Fig. 4 shows the application of the invention to a modulating amplifier of class B or class C, that is, to an amplifier operated with suifioient negative grid bias to cut off or nearly cut off the plate current in the absence of impressed alter nating current excitation. In the figure the carrier generator I2 is connected 'to 'theantenna 2 through the modulating amplifier I3 as in Fig. 2.
The signal source 4 is coupled to the input circuit of the amplifier I3 by means of the bridge 6 and the transformer 5. In the opposite bridge diagonal to the transformer 5 is inserted a low frequency coil I8 which is part of the plate circuit of the tube I3. The grid circuit of the tube I3 includes one of the ratio arms of the bridge. The plate supply battery is shown at I9.
In the operation of the system of Fig. 4, the carrier wave is impressed upon the grid circuit of the tube I3 along with the signal wave from; the bridge 6.
tential drop in the ratio arm included in the grid circuit. Due to the strong bias the plate current flows in pulses when the carrier and signal waves are impressed. In the presence of 1 the carrier without signals the pulses are uniform, but when the signals are applied the pulses vary in strength according to the signals. Consequently the plate' current contains a component of low frequency, which issubstantially a faithful copy of the envelope'of the modulated wave'in a properly adjusted modulator. The low frequency wave traverses coil I8, thus feeding back upon the-grid of tube I3. This feedback is inherently a reversed one as it will be evident from the figure that an increase. of plate current through coil I8 makes the grid more negative, while a decrease of plate current makes the grid less negative, these variations in the grid voltage being opposite in phase a large biasing battery 2| to effect class B or 0 operation. The signal source 4 is associated with the screen electrode of tube Zllby means of the transformer 5. Coupling between the plate and screen circuits at low frequencies is provided by the transformer II.
In the operation of the system of Fig. 5, the carrier is impressed upon the-grid and the signal.
upon the screen of the tube 20. --The modulated wave generated in the platecircuit is radiated by the antenna 2. The low frequency component of the plate current generated by class B or Coperation is fed backupon the screen by means of the transformer II which is-so poled that an increase of plate current results in a decrease in the posi-.
tive bias upon the screen and a decrease of plate current results in an increase of .screen; bias,
which conditions evidently constitute :reversedfeedback.v
v Fig. 6 shows the use of the invention in connection with a class Bsignal; amplifier in a, Heising The plate current from the battery !9 passes through the bridge 6 providing a strong negative grid bias for the tube, fixed by the po- Cit contant current modulating system. The carrier portion of the system is the same as shown in Fig. 2. The signal source 4 is connected through a pair of bridge networks 25 and 26 to push-pull connected tubes 23 and 24, the grids of which are normally biased to about the plate current cut-off point. The plate circuits of the tubes are coupled to the plate of tube I3 by means of a balanced transformer 21. Two detectors 9 and 29 are connected in series aiding relation to each other. The detectors are coupled to the antenna circuit by a high frequency coupling I6 and to the grids of the tubes 23 and 24 through the bridges 25 and 26, the latter having a common point at ground 80.
In the operation of the system of Fig. 6, the carrier is impressed upon the grid of the tube I3 and the signal upon the plate as is usual in the Heising system. The modulated wave is radiated by the antenna 2 and also a portion of its energy is transmitted to the detectors 9 and 29 by the coupling I6. The detected wave is fed back upon the grids of the tubes 23 and 24 through bridges 25 and 26 respectively, the detector circuit being balanced with respect to the ground because of the symmetry of the feedback path. While the symmetry is promoted by the use of two detectors as shown, in some cases sufficient balance may be obtained if one detector is omitted.
In this arrangement, as in that of Fig. 2, the principal effect of the feedback is to remove the distortion and noise originating in the low frequency signal amplifier.
Fig. 7 illustrates a reversed feedback system in which the wave fed back is a component of the plate current in a class B power amplifier. The carrier source I2 is connected to the antenna 2 through a carrier supply amplifier 30, modulating amplifier l3 and power amplifier I1. The carrier amplifier 30 is provided with a choke coil 3I which is common to the plate and grid circuits of the amplifier. The signal source 4 is coupled to the grid of the tube I4 through the transformer 5 and the bridge 6. The plate circuit of the tube I4 is coupled to the grid circuit of the tube I3 by means of a resistance 32. A choke coil I8 in the plate current return path of tube I! is coupled to the grid of tube I4 through the bridge 6. All the amplifiers may be class B operated.
In the operation of the system of Fig. '7, the carrier input to the modulator is substantially freed from noise effects by negative feedback in the supply amplifier 30. The vacuum tube being operated as a class B amplifier, any variation in the carrier amplitude results in a fluctuation of the average plate current. The fluctuation is transmitted in reversed phase to the grid circuit by coil 3|, producing a compensatory variation in the gain of the amplifier. That is, an increase of plate current causes the grid potential to become more negative, decreasing the gain, and a decrease of plate current makes the grid become less negative, increasing the gain. The controlled carrier wave is impressed upon the grid of tube [3 together with the signal wave from tube I4, the resulting modulated wave appearing in the plate circuit of tube I3. The modulated wave is amplified in tube H, the amplified wave being radiated by the antenna 2. The low frequency component of the plate current of tube I1 is fed back upon the grid of tube I4 by means of choke coil I 8. The system diminishes noise and distortion arising in amplifiers I3, I4 and I1 and also noise arising in generator I2 and tube 30.
What is claimed is:
l. A modulating system comprising a modulator tube and an amplifier tube, grid and plate circuits for each of said tubes, a carrier source coupled to the grid circuit of the modulator, a signal source, means responsive to the difference between the instantaneous values of any two waves impressed thereon, said means being connected between said signal source and said amplifier tube, and said signal source being isolated from said modulating device except through said differentially responsive means, an impedance element common to the plate circuits of both tubes, and a detector associated by high frequency coupling with the modulator plate circuit and by low frequency coupling with said differentially responsive means.
2. A modulating system comprising a modulator tube and an amplifier tube, grid and plate circuits for each of said tubes, means for maintaining carrier waves in the plate circuit of the modulator tube, a signal source, means responsive to the difference between the instantaneous values of any two waves impressed thereon, said means being connected between said signal source and said amplifier tube, and said signal source being isolated from said modulating device except through said differentially responsive means,
an impedance element common to the plate cira cuits of both tubes, and a detector associated by high frequency coupling with the modulator plate circuit and by low frequency coupling with said differentially responsive means.
' ENOCH B. FERRELL.