US1745415A

US1745415A - Carrier amplitude control system

Info

Publication number: US1745415A
Application number: US145050A
Authority: US
Inventors: Estill I Green
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1926-10-29
Filing date: 1926-10-29
Publication date: 1930-02-04
Anticipated expiration: 1947-02-04

Description

Feb; 4, 1930. E. GREEN CARRIER AMPLITUDE CONTROL SYSTEM Filed Oct. 29, 1926 INVENTOR.

E Gre /0 ATTORNEYS.

ES TILL I. GREEN, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO AMERICAN TELE- Patented Feb. 4, 1930 UNITED STATES PATENT OFFICE PHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK CARRIER AMPLITUDE CONTROL SYSTEM Application filed October 29, 1926. Serial n 145,050.

I This invention relates to a carrier or radio system characterlzed by the simultaneous transmlssion of the carrier frequency and one or both of the side bands produced by the modulation of the carrier by a signal frequency, and particularly to a method and means for maintaining constant the ratio of the amplitude of the side band to the amplitude of the transmitted carrier.

In the method heretofore employed in radio transmission or broadcasting, and also in carrier systems in which the carrier frequency is simultaneously transmitted with one or both of the side bands resulting from modulation of the carrier by a frequency or band of frequencies representing the load to .be transmitted by the carrier, the amplitude of the carrier frequency per' se remains substantially constant, whereas the amplitude of the side bands increases and decreases in accordance with the instantaneous valuesof the frequency or frequencies employed in modulating the carrier. For example, during the transmission of speech, music, or othersound over a carrier system, there will be times when the side bands will cease to exist, which times correspond to a pause in the signal being. impressed upon the transmitter of the system. In such intervals, the carrier frequency continues to flow over the system to the detector at the receiving station. Any extraneous interference or noise lying within the frequencies of the side band will be.

demodulated with-this carrier by the detector at the receiving station. The objection to this situation is that during these pauses the receiving system is as responsive to noise conditions on the circuit as it is when 1 the signal band is being received.

' In-the Patent No. 1,681,972, that issued to Blackwell and Afiel, on August 28, 1928, is

disclosed, a method for controlling the amplitude of the carrier in accordance with the changes in the amplitude of the signal by which they carrier is modulated, the system being characterized-by a side band of variable amplitude and a carrier of constant amplitude.

My invention is'a modification of that disclosedin saidBlackwell and Afiel patent,

the modification consisting in a method and means for maintaining constant the ratio of the amplitude of the envelope of the transmitted side band to the amplitude of the envelope of the transmitted carrier.

In the arrangement of Messrs. Blackwell and Afi'el, it is proposed to reduce the noise during the periods when there is little or no signal input by reducing the amount of carrier transmitted at such times. Thus, the ar-' rangement provided for the variation of the envelope of the transmitted carrier in accordance with the variation in the envelope of the signal input wave. In such a system, the reduction of the noise during periods of light load or no load is accomplished at the expense of an increase in the transmitted power and received noise during the louder portions of the signal. In the present invention, the variations in the envelope of the signal wave are equally divided between the transmitted carrier and the transmitted side band, with the result that the noise during periods of light signal load or no load is kept a minimum, while the transmitted power and received noise during periods of heavy load are likewise kept a minimum.

This invention will be clearly understood from the following description when read in connection with the attached drawing of which Figure 1 shows schematically a form of the invention, and Fig. 2 shows a variation of a portion of the circuit shown in Fig. 1.

In the drawing a source of signal 1 is connected with the input circuit 2. This source is capable of producing a signal which, for purposes of discussion, may be represented by A sin pt. The circuit 2 is connected with the primary windingeof the transformer 3, the connection including a delay circuit 4, the function of which is to interpose a time lag in the application of the voltage produced by the source 1 upon the modulator M The circuit 2 is also connected with the primary winding of the transformer 5. The secondary of this transformer is connected with the input circuit of the vacuum tube 6. the output side of which is connected with the primary of the transformer 7. The circuit of the vacflowing through its out-put winding is proportional to the square root of the current flowing through the primary winding of the transformer 5. The secondary of the transformer 7 is connected with the input circuit of the rectifier 8, the output circuit of which is connected with the modulator M and the carrier supply regulator R The output circuit of tube 8 contains a low pass filter 9, the function of which is to suppress all currents except those of very low frequency, the cut-off value of the filter being slightly above the frequency corresponding to the changes in the envelope of the input signal wave.

The modulator M is of the well known balanced type comprising the

tubes

10 and 11, which are symmetrically connected with the secondary winding of'the transformer 3 and the primary winding of the transformer 12. The conductor that is common to the grids of both tubes contains a negative biasing battery 13, an inductance 14: across which the carrier voltage from the source 15 is impressed, and a resistance 16 across which the rectified voltage from the tube 8 is impressed. The

secondary winding of the transformer 12 is connected with the transmission circuit 17, the connection including the band filter 18 which is adapted to suppress the unwanted products of modulation. It may be arranged to transmit both of the side bands resulting from the modulation, although in this case, it is assumed to suppress one of the side bands of modulation and to transmit the other, as, for example, that represented by /AB cos (q p) t, where A is the "ariable coefficient of the signal input, and B is constant.

The carrier. frequency generator may be of any well known type, such, for example, as the vacuum tube oscillator. This generator produces a carrier wave which may be represented by B sin gt. The regulator R consist-s of a balanced tube structure, comprising

tubes

19 and 20 which are symmetrically connected with the

equal resistances

21 and 22 and also with the primary winding of the transformer 23. The carrier oscillator 15 is connected across the inductance 24 in the com mon conductor of the grid circuits of the

tubes

19 and 20,'which conductor also contains the biasing battery 25. The function of this regulator is to control the amplitude of the carrier that is sent over the line 17 simultaneously with the side band transmitted by the filter 18. Since the regulator is a balanced tube structure in which the carrier frequency is applied to the common conductor, the carrier will not be transmitted solong as the circuit is electrically balanced. \Vhenever a rectified voltage is impressed across the

resistances

21 and 22 in series as the result of the rectification of the signal, the balance will be disturbed and a carrier voltage will be applied across the line 17, the amplitude of which is dependent upon the rectified voltage applied to the

resistances

21 and 22. The choke coils 26 and 27 serve to prevent a transfer of high frequency currents over the circuits in which the choke coils have been placed and the condensers 28 and 29 serve to prevent direct currents flowing over the circuits in which they are placed.

The manner in which this circuit functions in order to attain the desired result, namely, a constant ratio between the amplitude of the side band and that of the carrier, is as follows: The signal voltage A sin pt, produced by the source 1, will be impressed across the primary windings of the

transformers

3 and 5, but due to the presence of the delay circuit 4 the time of application of the voltage to the winding of 3 will be somewhat later than its application to the winding of 5. This delay is interposed in order that the signal voltage may be set up across the winding'of the transformer 3 at approximately the same instant that the rectified voltage resulting from the transmission of the signal around the path comprising 5, 6, 7, 8, 9, and 26, is applied to the resistance 16.

The voltage A sin pt impressed across the input side of the device 6 will produce a voltage across the input side of the rectifier 8 that is proportional to the square root ofthe amplitude of the wave A sin pt. The rectified voltage will be proportional to the envelope of this wave, that is, the rectified voltage is proportional to the square root of A. This rectified voltage will of course vary from time to time, depending upon changes in the amplitude of the envelope of the signal wave. The rectified voltage will be impressed across the resistance 16 in the input circuit of the modulator M and will also be impressed across the

resistances

21 and 22 in the input circuit of the regulator R The amplitude of the carrier supplied by the source 15 to the modulator M will be constantr This carrier will be modulated by the signal that is impressed across the secondary of the transformer 3. The amplitude of the side bands produced by the modulator M would normally be proportional to the signal input, but in this case it will be controlled in an inverse manner by the voltage impressed across the reslstance 16, and this amplitude will therefore be directly proportional to the square root of the envelope of the signal, that is to say, the side band amplitude will be proportional to thesquare root of A. The modulating carrier will be balanced out, while one of the side bands, for example, that represented by fl 1B cos (qp) t, will be selected by the filter 18, and transmitted over the line 17. At the same time the rectified voltage impressed across the

resistances

21 and 22 will unbalance the regulator and permit the transfer to the line 17 of a carrier represented by 1/KB1 sin gt,whereAhas the same significance as before, and B is constant. The side band namely, and that the variations in the envelope of the original signal wave are divided uniformly between the transmitted carrier and the transmitted side band.

- For satisfactory transmission over a carrier system it is necessary that the variations in the output of the receiving demodulator be the same as the variations in the signal input.

There are numerous ways in which the transmitted carrier and side band may be related while still satisfying this condition, but three relations are of special interest. for purposes r of analysis. v

First, there is the ordinary type of transmission in which the carrier is constant while the side band envelope varies in accordance with envelope of the signal input. Using the same general notation as before, this system may be considered as transmitting to the line a carrier B sin gt and a side band AB cos 9-1?) t, where A varies according to the volume of the signal input, and B and B are constants.

Secondly, there' is a ty e of transmission precisely opposite to the a bove, namely, that in which there is transmitted to the line a constant side band accompanied by a variable carrier supply. For this method the side band may be designated B cos (9-12) t, and the.

carrier as AB sin qt.

Thirdly, there is the method of the present invention, which transmits to the line a side band equal to /AB cos I car'rier equal to JKB sin gt.

It is clear that for each of these three methods the demodulator output will contain the first power of A, and satisfactory transmission will be obtained. There are, however, very important differences between the three with respect to the amount of noise obtained at the receiver and the amount of transmitting power required. The reason for these differences lies in the well known fact that for satisfactory quality the magnitudeof the received carrier 'must always be greater than that of the received side band. v

If, now, in the ordinary system having a constant carrier and a variable side band, the ratio of carrier to side band is made correct for the maximum signal input there will be an excess of carrier, and consequently, an ex cess of noise, whenever the magnitude ofthe signal input is below the maximum value.

Furthermore, if in the system having a variable carrier and a constant side band, the

(9*?) and magnitude of the carrier is made correct for the minimum signal input there will be a great excess of carrier for the larger magnitudes of signal input, and this excess involves both an increase of noise during the loud periods and. an "increase in the transmitted power required at such times. The latter is, of course, objectionable in that it requires a greatly increased carrying capacity in the transmitting apparatus.

In the arrangement of the present invention, however, the ratio of carrier to side band is keptalways at the minimum value permissible for good quality, and as a result, the minimum amount of noise is obtained under all conditons of signal load, as well as the minimum amount carrying capacity in the transmitting apparatus. i

It is obvious that manyvariations may be introduced in the circuit arrangement of Fig. 1 without departing from the spirit of this invention. Thus, the modifying characteristic of the tube 6 may be ,combined with the rectifying action of the tube 8 so that both.

functions are performed in a single tube. Such a modification of that portion of the circuit is shown in Fig. 2, in which the transformer 5 is shown connected directly to a' rectifier tube marked 6 and 8, this tube having the proper characteristic to perform both functions.

Likewise, although the invention has been described as applied to a system in which only one of the side bands of modulation is transmitted, it may evidently be used in a system in which both side bands are transmitted, in which case, the envelopes of both side bands will be controlled in accordance with the square root of the envelope of the signal input.

Other methods of carrying out this invention which differ in their particular form and arrangement of parts from the arrangements -which have been described, will be readily apparent, and it is to be understood that these modifications still come within the spirit and scope of the appended claims. 7 1

What is claimed is:

1. In a signaling system in'which a carrier frequency is modulated by a signal current, the method which consists in varying the envelope of the transmitted carrier in' accordance with the square root of the envelope of the signal current, and simultaneousl varying the envelope of the transmitted sidi; band current in accordance with the square root of the envelope of the signal current.

2. In a signaling system in which a carrier frequency is modulated by a signal current, the method which consists in distributing the variations in the envelope of the signal current equally between the transmitted carrier and the transmitted side band fre-.

I r phone current wave of varying volume, a

source of carrier frequency, means for produc ng from saidtelephone current wave and said carrier frequency a side band wave whose envelope varies in accordance with the :squareroot of said telephone current wave,

'means for producing a carrier wave whose envelope varies in accordance with the square root of said telephone current wave,- and means for transmitting saidvarlable carrier wave in conjunction with said side band wave.- J p 4:. In a signaling system, a source of tele-' phone current wave of varying volume, a source of carrier frequency, means for modu+ lating said," carrier frequency with said telephone current wave, means for transmitting a side band wave producedyby said modulating means, means for transmitting a carrier Wave of varying magnitude, and means for .making constant the ratio of the envelope of the transmitted carrier to the envelope of the transmitted side band. I

5. In a carrier or radio signaling system a in which a carrier frequency and a side band are transmitted, the method which consists in maintaining constant the ratio of the envelope of the side band to the envelope of the carrier throughout the system.

6. In a signaling system, the combination with a source of a telephone current wave of v varying volume, of a source of a carrier frequency wave, means for modulating the sald carrier frequency Wave by the said telephone that is inversely proportional to the square current wave, means for securing in the said modulatmg means a degree of amphfication root of the amplitude of the envelope of the said telephone current wave, means for transmitting a part of the output of the said modulating means, means for varying the magnitude of the said carrier frequency in accordance with the square root oftlie amplitude of the-envelope of the said telephone current wave, and means for transmit-ting the carrier frequency wave as thus varied in magnitude.

In testimony whereof, I have si ed my .name to this specificaton this 27th day of I October, 1926.

- .ESTILL I. GREEN.