US1654090A - Modulation indicator - Google Patents

Description

Dec. 27, 1927. 1,654,090

. A. A. OSWALD MODULATION INDICATOR Filed March 17. 1924 fiyfi 2 i J i 25x :8 I F 0 0 IE iF v men/0r:

Ari/2am. wWa/d by MA Patented Dec. 27, 1927.

UNITED STATES PATENT. OFFICE.

ARTHUR A. OSWALD, OF EAST ORANGE, NEW JERSEY, ABSIGNOB 'IO WESTERN ELEC- TRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

MODULATION INDICATOR.

Application filed March 17, 1924. Serial No. 898,716.

This invention relates to the measurement and indication of the degree of modulation effected in a system for the transmission of signals and more particularly to a'method of and means for indicating whether amphfying devices, used to increase the power of the oscillations representing the signals, are functioning properly.

In the transmission of speech by electric waves, the power of the Waves varies through a wide range in accordance with the modulations of the voice and the energies of the several tones. Certain of the tones, for example the lower pitched vowel tones of speech, produce oscillations of large amplitude and power, while other tonesa're conveyed with a very small amount of power. The quality of the transmission and the faithfulness with which it can be reproduced at the receiving points of the system depend upon the ability of the system to transmlt with equal degrees of purity both the strong and the weak oscillations.

in systems employing amplifiers to mcrease the average level of the transmission, it is well known that overloading the amphfying devices produces distortion of the wave form of the impressed oscillations and in the case of speech serious overloading may render the received speech almost unintelliible. g @n account of the irregular modulation of speech waves, modulation indicators that indicate the mean value of the current are generally insufficient to show what is happening to oscillations of large amplitude. The speech of particular indlvlduals or the music of certain types of musical instruments may be satisfactorily transmitted at an energy level corresponding to a particular mean current indication, while the speech of others and other forms of music in which the range of modulation is greater may still produce about the same mean current indication and yet, on account ot tl1e1r greater range of intensities, causesenous overloading with resultant distortlon in the reproduction.

in accordance with this invention, a modu lation indicator is provided, by means of which the maximum oscillation amplitudes are measured rather than the mean or the efiective values.

In one form of'the invention, a space discharge tube is used in accordance with the s stem of Patent No. 1,232,919, patented to eising July 10, 1917, to constitute a peak voltmeter. An indicator of this type gives most precise results when operated 1n accordance with a zero method, an adjustment of the device being made until the impressed voltage is neutralized. It is apparent that in using a device for the purpose of indicating the modulation effected in a transmis- S1011 system, it is not feasible to employ a zero method which requires continuous adjustment by an operator to secure indica tion. Accordingly, to make the device of the l' leising patent suitable for use as a transmission indicator, a plurality of such voltmeters s used each ad uSted to be responsive to impressed voltages of particular amplitudes, the response to these amplitudes being arranged in accordance with a series of increasing values. 7

In accordance with the foregoing, it is an object of the invention to provide a modulation indicator responsive without continuous adjustment to the maximum modulations of the signaling current.

A further object is to provide an indicating system whereby overloading of a trans mission system may be avoided.

A collateral object is to provide a transmission system having an improved. quality of reproduction.

The use of a plurality of peak voltmeters in accordance with this invention provides a visual indication of the modulation in a transmission system. They are readily responsive to the transmitted signal currents and are able to follow the comparatively rapid fluctuations of energy of low frequency currents corres onding to speech or music.

The amplitude at which distortion due to overloading an amplifying device becomes objectionable, may be determined for any particular type of amplifier by means of articulation tests. Such tests are capable of giving a close correlation between the amplitude and the percentage of spoken words that are correctly heard. When data of this kind have been obtained, the application of this invention insures that transmission may be accuratel controlled to deliver the greatest amount 0 energy conpredetermined control of the transmission stem in such cases requires a knowledge 0 the robable maximum amplitudes that may 0 associated with a. given mean current in the particular type of intelligence that is being ".r-tm smi t d- "The nature of the present invention and the o eration of its various parts will be more ully understood by reading the following detailed description in conjunction with the drawings, of which Fig. 1 shows the invention applied to a .high power wireless telephone transmission system; x

Fig. 2 shows an alternative method of connecting the modulation indicator to the system under control;.

Patent No. 1,434,555, patented to W. H.

Martin November 17, 1922, by means of which a high degree of attenuation is socured at a frequency close to the edge of the frequency band transmitted by the filter. The filter is proportioned to pass only one side band of frequencies and by means of its sharp attenuation close to the cutoil frequency the suppression of any residual waves of the carrier frequency Fig. 3 illustrates an alternative method may be completed. The energy transmitted of connecting the space discharge tubes of the modulation indicator;

Fig. 4 shows another embodiment of the modulation indicator in which two element discharge tubes are used as polarized recti-' tures of the operation of the indicator.

Referring to Fig. 1, 1 and 2 are the wires of a two-way transmission circuit which may be connected to 'a distant telephone station. By means of the bridge coil 3, directional separation of the currents in the two-way line is obtained, outgoing currents from the remote station being transmitted through the conductors 6 and 7 to the input of the amplifying and translating apparatus. The two-way line is balanced by a network which is indicated conventionally by the rectangle 4. The winding 5 of the bridge coil 3 receives the incoming circuits from a separate branch which is not illustrated as it plays no part in the o eration of the invention. The branch 6-- conducts the outgoing currents to a potentiometer 8, out of which they pass through the switch 50 to the speech input circuit of the balanced modulator 9. This modulator may be of the type described 'in Patent No. 1,343,307, June 15, 1920, to

J. R. Carson. v

A carrier wave'is supplied to the modulator from a high frequency oscillator 10 which may be of any convenient type. This carrier wave is impressed upon the input circuits of the modulator in parallel and the amplified carrier component is suppressed by the balanced arrangement of the output circuits. The purpose of this first stage of modulation is to secure complete suppression of the carrier and in order that this may be accomplished, the frequency of through the filter 11, consisting of a single side band without any attendant unmodulated carrier, is delivered to a second modulator 12 which may be of the same type as the modulator 9 and is indicated iagrammatically. A second carrier wave having a frequency suitable for radio transmission is supplied to modulator 12 by the oscillator 13. Modulation in modulator 12 takes place between the second carrier wave and the waves of approximately 30,000 C. P. S. of the single side band produced by the preceding apparatus. The products therefore include a pair of side bands separated by approximately 30,000 cycles from the frequency of the waves supplied by the oscillator 13. Output energy from the modulator 12 is delivered to a band filter 14 which is designed to pass only one of the side bands and hence serves to completely surpress the other side band and the carrier frequency components. The single side band transmitted through the filter 14 is supplied to the input circuit of amplifier 15,to which space current is supplied by generator 16 through a choke coil 17 and space current meter 18. The amplified side band is delivered to amplifier 19, which may be of the same type as the amplifier 15. The output energy from this amplifier is transmitted through condensers 41 to a tuned circuit 20 which is coupled to antenna 21 by means of coupling coil 22.

A system of this type is typical of wireless telephone systems used for long distance communication. In view of the large number of stages of modulation and amplification used in this system it is necessary that a precise and reliable means be provided for indicating the degree of modulation so that the operation of the system may be contill trolled to secure themost efiicient transmission.

In order to obtain an indication of the modulations efiected in a system of this type,

a part of the energy delivered to the antenna is supplied to the circuit 25, 26 by means of the coupling transformer 23, 24. The energy delivered to the circuit25, 26 1s 1mpressed u on the input electrodes of three space disc arge tubes 27 28 and 29, each of which is provided with a filament H an anode I and a control element J.

The filamentsare connected in series and? are supplied with current from the source 30 through a filter 31', the purpose of which is to suppress any fluctuations of the current from the source 30.

the excess voltage and determine the fila ment current strength, but serve also to establish the filaments at certain fixed potentials relatively to eachother and to the negative terminal of the supply;

The anodes I of the vacuum tubes are connected through ammeters 40 to a common lead 39, which in turn is connected to the ositive terminal of the source 30. T e impedance control element of tube .27 is conductively connected through a high lrequenc choke coil 37 to the negative terminal o the filament current source and is thus given a negative polarity with respect to the filament H by an amount determined b the fall of potential in resistance 34. ontrol elements of vacuum tubes 28 and 29 are similarly connected through high frequency choke coils 37 to the common points of resistances 35 and 36 which are connected between the respective filaments and the negative. terminal of the source.

By properly proportioning the resistances 35 and 36 and the resistance 34.- to establish the filament potentials, the normal negative polariaing potentials applied to the control elements or grids of the respective tubes may be maintained at any predeterminedvalues. It is necessary that the resistances 35, 36 be so large that a minimum amount oi current supplied by the source 30 is diverted from the filaments of the tubes 28 and 27.

Conductor 25 is connected to the grids through separate condensers 38 which constitute low impedance paths for the high frequency energy supplied through the cir- V cuit 25, 26 and also serve to prevent the polarizing potentials bung short circuited through the coil 24. Adjustment of the filament current within a lim ted range is indicator which is slightl "polarized also in like manner.

provided for by the resistance 32 which is variable;- 1

When the transmitter is idle, the polarizw tion of the grids J is sufiicient to prevent any space current flowing through the discharge devices and hence it is necessary, when the transmltter is operating, for the wave E. M. F. impressed u on the grids to overcome this polarization efore any indication is produced.

Fig. 2 shows an arrangement for cou pling the output of the amplifier 19 tothe transmission circuit and to the modulation different from thatshown in Fig. 1. is unit may be substituted for that portion of Fig. 1 to the right of vertical line Y-Y and above the horizontal line X--X. A potentiometer 42 is connected across the output circuit of the amplifier 19. The conductor 26 is connected to one terminal of the potentiometer, and the conductor 25 to the adjustable contact point. The tuned circuit 20 and the antenna 21 are replaced, in this figure by an impedance 55, which serves to indicate that t e energy may be delivered to any -t e of load circuit.

ig. 3 illustrates an alternative arrangement which may be used in place of. that part of Fig. 1 below the line XX. This arrangement differs from that of Fig. 1 in thatthe lead 25 through which high freuency energy is supplied to the space the 0 large tubes is connected to their anodes. The filamentsand the grids of the tubes are connected to the supply source 30 in the same manner as in Fig. 1, and the grids are Condenser 43 prevents the circuit 25., 26 from acting as a s ort circuit for the source supplying plate current to the tubes, and high frequencychoke coil 44 revents the high frequency oscillations rom being short-circuited through the direct current source 30.

The arrangement shown in Fig. 4 may also be substituted for that part of the apparatus below the line X-X of Fig. 1. Thethree' element tubes 27 28 and 29 are replaced by three two-element tubes 425, 46

and 4:7,the filaments of which are supplied with current from source 30 through inde-' pendent parallel circuits. Resistances 48 and 49 included in each filament circuit control the current supplied to the filaments and also the potentials of the filaments with lid respect to the negative terminal of the sup of Fig. 1 is as follows: By means of the coupling coil 23, 24 part of the energy delivered to the antenna is impressed across the terminals of the. hi h frequency choke coils 37, and consequent y between the grid and filament of the discharge tubes. Normally, no plate current flows in the discharge tubes on account of the polarizing potentials impressed upon their grids, and none can flow until one or another of the grids has a sufliciently large positive potential superimposed upon it to offset the blocking efiect of its stead negative potential. If the amplitude o the impressed oscillations is sufficiently great, the potential of the positive art of the cycle will ofi'set the blockin e ect of the negatively polarized grid an cause a momentary flow of current in the plate circuit.

The normal potentials impressed upon the grids of the dischar e tubes are graded so that, for example, tu e 27 will be rendered conductive for space current by the superppsition of a relatively small positive E.

F., space current will flow through tube 28 when a larger positive E. M. F. is impressed upon its grid and space current will flow through tube 29 when a still larger positive E. M. F. is sup lied to its grid. The relationship of these M. F.s is predetermined by the adjustment of the resistances associated with the filament circuits.

If the amplitude of the current delivered from the transmitting apparatus is small, the effect will be such as to produce only an occasional impulse of space current through the discharge tube 27 thereby causing an occasional sharp deflection of the pointer of the meter 40 associated with the tube 27.

As the amplitude of the current is increased,

the tube is more or less continuously conductive and the meter indication will be continuous and will vary approximately in accordance with the degree of modulation effected. Further increase in the level of the transmitted power produces occasional agitations of the meter pointer associated with the second tube 28 due to the amplitudes being sufliciently great to occasionally overcome the polarization of the grid. Still further increase in the power level results in continued agitation of the meter associated with the tube 28 and an occasional agitation of the meter associated with the tube 29, and so on, each device of the group be.-

coming operative in response to successively increasing amplitudes.

A group of three tubes is shown, but it is evident that as many may be used as desired and the voltage increments at which the succeeding tubes become operative may be proportioned in any desired manner.

As the power level of the transmission 7 is increased, the tubes responsive to the low power values will tend to be overloaded,

between the grid and filament of a thermionic vacuum tube is increased positivel the space current increases ro ressive y until a point is reached at w ic all the electrons emitted from the filament are continuously drawn off to the plate. After this occurs, further increases in the voltage applied to the space path produce no change in the space current. The discharge tube is then said to be in a condition of voltage saturation. The point at which voltage saturation occurs differs for each value of filament temperature. High temperatures permit the flow of large space currents, while at lower temperatures the value of the space current is smaller. Since the filament temperature is controlled by the current flowing through it, it follows that for each particular current value there is a limiting space current which cannot be exceeded no matter how great an electromotive force is impressed upon the electrodes. Hence, by using filament currents of the proper values the point at which voltage saturation occurs in the tubes 27, 28, 29 or 45, 46 and 47, may be predetermined so that the amplitude of the space current flowing through each tube cannot exceed a definite safe value.

In Fig. 5, the operating characteristics are shown in gra hlc form. The abscissa: represent the amp itudes of the alternating electromotive forces applied to the grids of the vacuum tubes from the transmission circuit. The ordinates represent the intensities of the space currents as indicated by the meters 40. The curve ODD represents the characteristic of the tube 27; curve OEE that of tube 28; and OFF that of tube 29. The minimum potentials required to neutralize the blocking effect of the steady grid polarizing potentials are represented by .the abscissae OD, OE and OF res ectively; the saturation current correspon ing to the particular filament current is re resented by the ordinate CG and the line (i By adjustment of the resistance 32 the filament current may be set at a value tor which the corresponding saturation current is sufficient to produce a full scale deflection of the plate current meters, but is insufiicient to cause any damage thereto because of overloading.

When a group of three tubes is used, as illustrated, it is convenient to adjust the values of the grid polarizing E. M. F.s so

dill! means of switch an that the minimum operating voltage of one of the tubes corresponds to the maximum power output from the transmitting system consistent with the requirements of the quality of reproduction. When the system is o crating at its full power, the meter associated with the tube selected should show no indication or only occasional small deflections, this tube corresponds to the tube 29 of Fig. 1 in accordance with the fore possible power output of the transmitting apparatus is being supplied to the transmission circuit.

For the purpose of calibration, the irregular modulations of speech in the transmission system may be replaced by a steady tone resultin in a steady output, the amplitude of which can be measured by means of an ammeter of any ordina'r type.

In the system shown in Fig. 1, the speech input circuit of modulator 9 may be disconnected from the tele hone line 1, 2 by connected instead to a low frequency oscillator 51. This oscillator may beset to generate a practically pure tone of some frequency, for example 1500 cycles, within the range of speech transmission. The voltage output is measured by voltmeter 52 and may be adjusted by means of potentiometer 54.

The amplitude of the oscillations flowing through t e output circuit of the power amplifier 19 may be measured by means of an ammeter53 included in its output circuit.

In carrying out the calibration, oscillators 10 and 13 are set to give their proper frequency and current strengths. The output of the low frequency oscillator 51 is ad usted by means of the potentiometer 54 until an out at current'is indicated by ammeter 53 in t e output circuit of the power amplifier which corresponds to the known permissible maximum space current of the particular amplifiers used. The coupling of transformer 23, 24 is then adjusted unt l the meter associated with the tube 29 just fails to indicate. The meters associated with the other tubes will then be operated since, as

stated above, they are adapted to operate at successively lower voltages the value of which in each case is predetermined by the value of the resistances in the respective filament circuits.

The operation of the modulation indicator of Fig. 3 is similar to that of the arrangehis ment shown in Fig. 1. The discharge tubes 27, 28 and 29 are similarly connected to the direct current source and the space current is normally suppressed by p0 arizing the grids negatively by different amounts, in the same manner as in the system of Fig. 1.

The voltage derived from the transmission system is applied through the blocking condenser iti to the plate circuits of the vacuum tubes. As is well known, the control effect upon the space current of a three element space discharge device due to an E. M. F. im ressed upon its grids, may be produced by a proportionally related E. M. F. impressed upon its anode. In general, this related E. M. F. is larger, its value depending upon the amplificationconstant of the tube. The arrangement shown in Fig. 3 therefore operates in a closely analogous manner to that of Fi 1, the larger electromotive force require for its operation being secured by increasin the coupling of the transformer 23, 24. natively the degree of polarization of the grids may be correspondingly reduced, making the increased cou ling between the indicating apparatus an the transmitting sys tem unnecessary.

The two element vacuum discharge tubes 45, 46 and 47 of the modulation indicator shown in Fig. 4 operate as polarized rectifiers. The degree of olarization of each tube is varied, as in the evices of Figs. 1 and 3, but in this arrangement the olarizationis obtained by making the ano es K negative with respect to the cathodes L. The amount of the polarization is determined b the resistances 48 and 49, by means of which the cathodes are connected to the current source 30. These resistances also serve to limit the current supplied to the filaments of the respective tubes, whereby each filament is heated to the proper temperature. The E. M. F. derived from the transmission circuit is applied through the conductors 25 and 26 to the cathodes and anodes of the tubes, the indicators 40 bein included in the individual anodes leads. [his arrange ment operates in a manner similar to that described above for the apparatus shown in Figs. 1 to 3, to indicate the de ree of modulation efiected by the transmitting system. Each device becomes operative when the positive half of the impressed oscillations 1s sufficient to overcome the negative olarizing potential applied to its anode. hen this condition exists, space current is caused to flow and an indication is produced in the meter 40 associated with the tube.

The term modulation is employed in the foregoing description to identify the variations in the intensity of electric currents corresponding to the variations of intensity that give the signal or the intelligence to be transmitted its recognizable characteristics.

lter- ,tudes of the signal waves byr tem described facilitates to some extent the measurement of the side-band amplitudes, but it is not essential that the transmitter be of the carrier suppression type. When the modulation indicator is used in connec tion with a transmitted carrier system, the initial adjustment and calibration carried out in the manner already described results in an increase of the polarizing potentials of the indicator discharge tubes by which the response to the unmodulated carrier is neutralized.

The invention may also be used in connection with low frequency amplifiers to indi cate the rapidly varying intensities of speech or other signal waves. In such case, it may be regarded as an ener y level indicator. In all its applications itis to be noted that the separate indicating devices are responsive either directly or indirectly to the amplitudes of the signal waves.

Various arrangements embodying the invention and their respective methods of operation have been described and the combinations and features of the invention which are believed to be novel will be set forth in the ap ended claims.

at is claimed is: v

1. The'method of indicating the energy level of a signal transmitted by electric waves which comprises detecting from the signal bearing waves a plurality of separate uni-directional currents, the number of said currents increasing in accordance with the amplitude of said waves.

2.- The method of indicating the energy level {of a; signal transmitted by electric waves which comprises separately combining the electromotive force of the signal bearing waves with a plurality of graded uni-directional potentials and detecting from each combination a current which is determined in intensity by the difi'erence of the two electromotive forces combined.

3. The method of indicating the energy level of a signal transmitted by electric waves which comprises separately combining the electromotive force of the signal bearing waveswith a plurality of graded uni-directional potentials and detecting from each combination a current which is determined in intensity by the excess of the wave electromotive force over the steady potential with which .it is combined.

4:. A modulation indicator comprising a plurality of space discharge devices, a circuit common to all of said devices and adapted tov receive and impress thereon signal modulated waves, and a plurality of graded uni-directional potentials respectively associated with said devices to oppose the space discharge therein whereby each device is made normally non-conductive and responsive only to signal waves of amplitude greater than a predetermined value.

5. An apparatus for lndicating the degree to which a carrier wave is modulated, comprising a plurality of indicating devices, a space discharge device associated with each of said indicating devices, each space discharge device responsive to currents having amplitudes of predetermined values, each of said predetermined values being different from the others, means for connecting said indicating devices to a transmission circuit to which the modulated wave is supplied whereby said Wave is impressed upon said indicating devices to produce an indication corresponding to the amplitude of said modulated wave.

6. In a wave transmission system including a transmission circuit, a plurality of space discharge devices having dischar e electrodes including an anode and acatho e, a source of space current connected to said anodes, an indicating device amociated with each anode and responsive to the space current flowing through the device, a polarizing potential applied to each of said discharge devices whereby the flow of space current is normally prevented, each'of said polarizing potentials being different from the others, an input circuit adapted to receive the ener of a wave to be supplied to said transmission circuit and to impress said energy upon the electrodes of said discharge devices, whereby energy in the transmission circuit in excess of predetermined amplitudes is caused to produce sensible indications.

7. A modulation indicator comprising a plurality of space discharge devices havlng electrodes including a cathode and an anode, an input circuitadapted to receive electrical oscillations and to lmpress them upon said electrodes, a source of energy for heating said cathodes, a source of space current connected to said anodes and cathodes of said discharge devices, a space current indicator connected inseries with each an ode, a source of steady potential, means for applying predetermined fractional amounts of sai steady potential to said dischar e devices whereby said indicators are made responsive to oscillations in said input circuit having amplitudes inexcess of predetermined values.

8. A modulation indicator comprising a plurality of space discharge devices each of said devices including a cathode, an anode and a control electrode, an input circuit adapted to receive electrical oscillations and connected to said cathodes and control electrodes, a source of space current connected to said anodes and said cathodes, means for indicating said space current associated with each anode, a source of energy for heating said cathodes, and a source of potential connected to said control electrodes for negatively polarizing them with respect to their cathodes to prevent the flow of space current, the polarizing potential of each of said control electrodes being different from the others.

9. In combination a modulation indicator in accordance with claim 7, and means to limit the energy supplied to the cathodes to produce a tion.

10. A modulation indicator comprising a plurality of space discharge devices having an anode, a control element and a filamentary cathode, a common source of supply for the space current of said devices and for energizing said cathodes, a circuit connectpredetermined voltage saturaing said cathodes to said source of supply containing resistances whereby the energizing currents are limited to predetermined values and whereby potentials of said cathodes with respect to each other are established at values also predetermined, space current indicators associated with said discharge devices, circuits connecting said anodes through said space current indicators to said source of supply, an input circuit adapted to receive electrical oscillations and to impress them between said control electrodes and said cathodes, means for polarizing said control electrodes by predetermined amounts so that said discharge devices are normally inoperative and become operative when the impressed oscillations exceed predetermined amplitudes.

11. In combination a modulation indicator in accordance with claim 10 and means for limiting the current supplied to the filamentary cathodes so that the maximum space current of voltage saturation corresponds to the maximum current capacity of the space current indicating devices.

In Witness whereof, I hereunto subscribe my name this 7th day of March A. D., 1924.

ARTHUR A. OSWALD.

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