US2341632A - Electric wave generator - Google Patents

Description

Feb. 15, 1944. T. M. LIBBY ELECTRIC WAVE GENERATOR 6 Sheets-Sheet 1 Filed Dec. 17, 1938 OUTPUT 2022/8 FIG. 5

INVENTOR T N M. LIBBY MM ATTORNEY Feb. 15, 1944. T, M, UBBY 2,341,632

ELECTRIC WAVE GENERATOR Filed Dec. 17, 1938 6 Sheets-Sheet 2 Flak INVENTOR Twmq M Ll BBY Mm ATTORNEY Feb. 15, 1944. T. M. LIBBY ELECTRIC WAVE GENERATOR Filed Dec. 17, 1938 6 Sheets-Sheet 3 JQNFFZQU J0 202200 m m ZOEEOU LID QFDO 8 5 W .3

INVENTOR TYNq M. LlBBY BY M ATTORN EY Feb. 15, 1944. T. M. LIBBY 2,341,632

ELECTRIC WAVE GENERATOR Filed Dec. 17. 1938 s Sheets -Sheet 4 INVENTOR TYNG M.L:agnr

BY W

A TORNEY ELECTRIC WAVE GENERATOR Filed Dec. 1'7, 1938 6 Sheets-Sheet 5 ATTEN UATION db 5 E 5 5 1.1 1.2 1.3 1.4 1.5 La, 1.7 (lg-RATIO OF CAPACITANCE FOR ATT- SHOWN TO CAPACITANCE FOR 2520 MT.

FI 1 Z INVENTOR TY M; M. LIBBY BY 0 p a ATTORNEY Feb. 15, 1944. UBBY 2,341,632

ELECTRIC WAVE GENERATOR Filed Dec. 1'7, 1938 6 Sheets-Sheet 6 TO 0 AT I40 MMFD.

CAPAClTANCE. C

, INVENTOR TYNG M .LnaaY ATTORNEY whose reactance quency.

Patented Feb. is, '1944 ELECTRIC WAVE cam naron Tyng M. Libby. Seattle, wash. Application Dece ber 17, 1933, Serial No. 246,358

14 Claims.

. .The present invention relates to electric wave generators. In particular, the present'invention relates to generators for the generation of electric waves having frequencies in the audible range. Such generators are of use in a voice-frequency carrier telegraph system, for measuring impedances of transmission networks and systems, for modulating the carrier of a radio telegraph transmitter, for loud-speaker testing, in an electric music instrument, and whe ever tone frequencies are required.

It is an object of the present invention to con-' Anotherobject is the provision in an electric tone generator, of an electric generator having a variable capacitance, the wave form of the output of the generator being a linear function of the wave form of capacitance variation.

- Another object is the provision of a variable inductance in an-electric tone generator, the wave form of the output of the generator being a linear function of the inductance variation.

It is an object of the present invention to effect a modulation of electric waves in both or either I amplitude and frequency, and to cont.ol the charstruct an electric generator for obtaining various wave-frequencies such as are desirable in the above-mentioned uses.

A further object is the generation of sub-audible-frequencies at comparatively large output power.

Another object isthe devising of a simple means for efficiently modulating a radio-frequency at audible and sub-audible rates.

Another object of the present invention is the production of electric waves of desired frequencies by the modulation of a carrier-frequency and the demodulation of the modulated carrier.

A further object of the present invention is the provision of means for keying electric waves in an electrical circuit by keying the biasing circuit of a demodulator.

Another object is the modulation of a supersonic-frequency by means of a resonant circuit is varied at the desired ire- It is an object. of the present invention to em- 'ciently transform radio-frequency power into audio-frequency power.

Another object is the production of a large gle radio-frequency source.

Another objectof the present invention is the provision of a tone generator wherein the instantaneous amplitudeof the output is a linear function of-a reactive variation in a one-hundred percent modulator;

An object of the present invention is to provide number of tone frequencies with the use of a sin- J a device for originating a prescribed electric waveform inwhich the wave form of the output more closely approaches a linear function of the variable reactor vices. a It is an object of the present invention to provide a carrier-wave modulator having a variable reactor in which the envelope of the modulated carrier'is alinear' function of thevariation of the reactance of acid reactor.

than is possible with previous deso is the production of a device which will produce a complex tone and allow the selection of various rates of onset and decay for variouscomponents of the tone.

It is an object of the present invention to effect a frequency modulation of electric waves of audible frequency by varying a carrier-frequency.

These objects are attained in a device such as an audible frequency, or tone, generator by amplitude modulating a high-frequency carrier, the modulator consisting of a network of reactive elements, one of which elements is periodically varied at a desired output frequency. The modulated carrier is then demodulated and the principal component of the output of the demodulator is the desired frequency.

The power output of the demodulator is controlled by means of biasing potentials, and provisions are made for varying the biasing potential and its associated control means is provided for each of the notes ofthemusical scale and each of the desired harmonics. The outputs of, the demodulators for the fundamental 'tones are connected to a common bus in the output circuit. Likewise all of the demodulators supplying harmonics of the same order are connected to a common bus in the'output circuit. A'volume control is provided in each bus enabling the relative amplitudes of the fundamentals and harmonics to be controlled as desired;

The simplest form of a modulating network comprises a series inductor and shunt capacitor, or a series capacitor and shunt inductor. In such a network thev loss introduced by the net-' work is a function of the variable reactance. There are many alternative forms of networkswhich will accomplish the modulation of a carrier when at least one of the reactances is peri-' odically varied.

One of the desirable effects'in music known as tremolo, comprises periodically varying thev amplitude of the component frequencies at the rate of about six cycles per second. In the present device, tremolo is produced byvarying the frequency of the carrier supply. The frequency of the audio-frequency output of the demodulator is independent of the frequency and amplitude of the supplied carrier, provided said carrier-frequency and amplitude are constant. Cyclicly varying the frequency of the carrier results in corresponding cyclic variations in the amplitude of the audio-frequency output; A periodic variation in the frequency of the carrier at the rate of six periods per second will result in a periodic variation of the audio-frequency amplitude at six periods per second. In this manner tremolo eifects are produced.

Another desirable effect in music, known as vibrato, comprises the periodic frequency variation of the component frequencies. present disclosure, vibrato is produced by first modulating the audio-frequencies appearing in the common output circuit of the demodulator by means of a fixed carrier-frequency and a balanced modulator. This balanced modulator suppresses the carrier-frequency and transmits the two side-bands. One of the said side-bands is then suppressed by means of a filter, the other of said side-bands being impressed upon a demodulator together with a second carrier-fre- 'quency which is varied in frequency at the desired vibrato rate. The output of this second demodulator will then, comprise the original tone-frequency varied at the vibrato rate. When several diiferent vibrato frequencies are desired, by the use of a plurality of band filters in the common output circuit of a. plurality of demodulators and byassociating with each band filter a vibrato producing system as above described, different vibrato rates may be impressed upon quantities inherent in a physical unit such as an inductance coil, condenser, or resistance unit.

An arm denotes a distinct set of elements electrically" isolated from all other conductors in a network except at two points. A series arm is one which conducts the main current in any part of a system in the direction of propaga-- tion. A shunt arm is one which diverts a part of the main current from one end of a series of,elements, forming a closed path.

arm (or from one end of a live wire) to a symmetrically located point'in a balanced system,

or to a ground wire in an unbalanced system.

A branch is one of several parallel paths in a System. w 7

A mesh is a combination of elements, or parts -A network is a. group of elements inserted in a system for the purpose of satisfying certain transmission requirements, the character, constants, and arrangements of whose elements therefore follow from the necessity of meeting these particular requirements. V

,A circuit consists of an entire system of interconnected apparatus and conductors designed to transfer electrical energy from one point to another. The term will also, be used to describe a part of a complete circuit where proper designation is given, or where the limitations involved are evident. Thus," the input circuit of a network is that part of the entirecircuit from which the network derives power.

Linearity in a mathematical concept, may be defined as the condition in which the derivative of the function with respect to the variable 'is a constant and is independent of the 'value of vchanical construction of the variable element.

In the upon the application of the device; for instance; in the application of this device as a tone gener ator for laboratory measurements not more than two percent deviation could be tolerated, while on the other hand in its'application to a musical and embodying the before mentioned features,

electric wave generator,

instrumentdeviations from linearity as great as ten percent may be tolerated. The method of designing a linear modulator in this disclosure is not based upon mathematical analyses but rather upon a trial and error method. It has been found that by this method the attenuation of a filter may be made a linear function of a variable reactor in the filter witha deviation not exceeding .one percent, over the range of 3 db. to 24 db. attenuation. For the purposes of this disclosure the term linearity shall be construed to mean that the device is linear within the permissibledeviations of the particular application of the device.

A wave filter is a selective circuitnetwork designed to pas currents within a continuous band or hands of frequencies, or direct current, and substantially reduce the amplitude of cur rent of undesired frequencies.

Devices for attaining the before mentioned objects and others herein mentioned and apparent,

are represented in the accompanying drawings,

in which: Figure 1 is a schematic diagramQoi a single Figures '2, 3, and 4 are modifications of the deviceshown in Figure 1.

Figure 5 is a schematic diagram of a variable inductor suitable for use as a periodically varying inductance in a modulating network shown in Figure 1.

Figure 6 is a view of the rotor of a variable capacitor suitable for use in a modulating network such as illustrated in Pigure 1.

, such as Figure 7 is a view ofa stator to be associated with the rotor shown in Figure 6.

Figure 8 is a sectional view of the stator and rotor shown in Figures 6 and 7, in their assem-. I

by the variablecapacitor I. -quencies,' the frequencies of the same order as the attenuation of the modulating network. By

periodically varying the capacitance of the capacitor I, the amplitude of the carrier impressed upon the demodulator is correspondingly periodically varied in amplitude. Such a modulated car rier is known to comprise the carrier-frequency plus side-frequencies. The output frequencies of the demodulator comprise the original input frequency plus the modulating frequency determined those of the carrier are freely transmitted by coupling capacitor Hi while the modulating frequency appears as a voltage across the capacitor l0; hence terminals/'22 and 23. may be viewed as the output terminals of an electric w'ave generator having a frequency-the same as that of the Figure 12 is a family of curves showing variation in attenuation as a function of capacitance similar to that shown in a modulating network in Figure 1. 1

Figure 13 is a graphic representation of the variations in attenuation asa function of capacitance in a modulating network "similar to that shown in Figure 1 Figure 1 is a schematic diagram of an electric wave generator. The rectangle l includes a modulating network, a demodulator, a demodulator biasing system, an input circuit, and an output circuit. The input circuit, comprising a transformer 2, couples a carrier-frequency source to the modulating network. The modulating network consists of series elements, 3 and i and shunt elements 5, 6, and 'l. A capacitor 52 allows the free transmission of carrier and audio-frequencies in the modulating network, and blocks. the transmission of direct current.

A demodulator Q'is connected between the drop ,pacitors l5 and 8 control the envelope of the initial and decay transients.

For the purpose of reference, connections with the rectangle i are called terminals and numbered. Terminalsifl and 2i constitute the carrier frequency input terminals for the modulate ing network. Terminals-22 and 23 are terminals for an output circuit. Terminals 2i and 25 are provided for connecting to sources of biasing potential, Terminals 25 and 26 are supplied for connecting a control key in the demodulator biasing system. A control key 27 is connected between the terminals 25 and 26. A source of potential 28 is connected to the terminals 23 and 24 to furnish a blocking bias to the demodulator 9. A' further source of potential 29 is connected to terminals 23 and 25 to furnish an operative bias to the demodulator 9. The rectangle 30 represents a source of carrier-frequency which may comprise any well known type of highfrequency oscillator.

In the operationof the device shown in Figure 1: the carrier-frequency from source 30 is transmlttod to the demodulator '9 by the modulating variation of capacitance of the capacitor '1. When the key 21 is open, the source of biasing potential 28 renders the demodulator inoperative. Under this condition zero voltage appears at the output terminals.

A demodulator suitable for'use in this disclosure is one such that a negative biasing voltage of proper value will make it inoperative. Such a biasing voltage will be called a blocking bias. A biasing voltage of proper value less than the blocking bias will permit the demodulator to function. Theoutput amplitude will be a tunetion of the bias voltage within the operating limits of the demodulator.

When the key 2'! is open, the voltage of source 28 is impressed across the demodulator s via resistors H and t2, the modulator network, and the output circuit. The polarity of'source 28 is such that no current from this source will flow through the demodulator. The voltage of source as is equal to the blocking bias of the demodulater 9. Since the D. C. resistance oi the modulating network and the output circuit is negligible compared to the resistance of the demodulator 9, the voltage of capacitor 8 is the same as the demodulator biasing voltage, under all conditions.

Upon closing the key ii, the D. C. potential sources 28 and 29 produce a. current in a clock wise direction in the circuit consisting of these sources in series with key 2i and the resistors ii I and. i3. This current produces, in resistor ii, a voltage which is in opposition to the voltage of the source 23, thereby reducing the voltage impressed upon the demodulator 5 and the capacitor 8. This reduction in the voltage impressed upon the capacitor 8 causes it to discharge through resistor E l. The rectifier lie is poled so that it is non-conducting to the discharge current from the capacitor 8. This discharge of capacitor 8 continues until its voltage is reduced to the difference between the voltages appearing across the resistor l2 and the source 28. The time required for the equalization of the voltage of the capacitor 8 with the difiference between the voltages across the resistor l2 and source 28, is a junction of the total voltage chang of the capacitor 8, they capacitance of the capacitor 8, and the resistance of the resistors H, l2, and I3. By making the parallel resistance of the resistors l2 and I 3 small compared to the resistance of the resistor H, the rate of change in the bias voltage may be controlled by adjusting the resistance of the resistor H.

When the final reduced voltage of the capacitor 8 is equal to the bias voltage for efficient operation of the demodulator 9, the closing or the key 21 causes the demodulator bias voltage to dey pressed upon the demodulator is a function of Of these output freulator operating value, and thus cause the amplitude of the electric wave in the demodulator output circuit to increase from zero-to its maximum value; Th rate of this amplitude increase, or

1 the initial transient, maybe controlled by addusting the resistance of the resistor EL As described above, when the key 21 has been closed for the duration of the initial transient, the biasin voltage, hence, the voltage of the capacitor 8 is equal to the difference between the voltages across the resistor l2 and the'source 28. The voltage of the capacitor 35 is zero. I

Upon openin the key 21, a transient-charging current flows through the capacitor it, resistors M, 43, and I2, and the sources 28 and 29 in a clockwise direction. This charging current decreases with time, hence the voltage across the resistor It decreases with time, and the difierence between the voltage across the resistor 52 and the voltage of the source 28 increases with time, until the capacitor i5 is charged to.the sum of the voltages of the sources 28 and 2e, whereupon the charging current becomes zero. .As the charg fng current of the capacitor l5 decreases, the voltage across the resistor-l2 decreases and the voltag impressed upon the capacitor 8 via the resistor H increases, thereby producing a charg current through the resistor i2, and since the current through this resistor varies with the capacitance of the capacitor i5 and the resistance of the resistor Hi .when the values of theeother associated elements are fixed, it follows'that the rate of change of the biasing voltage upon-openihg the keymay be controlled by adjusting the values of either the capacitor or the resistor in, or both. When the voltage of the source 25 ts equal to the blocking bias, opening the key 2? decreases the output of the demodulator s to zero, and the rate of this decrease, or the decay transient, may be controlled by adjusting the valus of capacitor l5 or the value of the resistor M. or both.

If the rectifier i la is not employed, the resistor 09 Will retard the decay transient. Under this condition, it may be made large for obtainin a very gradual initial transient, and then if a sharp decay transient is desired the resistance of resistor it should preferably be relatively small. It appears therefore that a. large value of resistance in resistor it will preclude obtaining a very short decay transient. This dificulty may be eliminated by connecting rectifier ii a in parallel with resistor i I, with the polarity shown in the figure. The function of resistor i i is unimpaired for the purpose of producing long initial transients. but by virtue of the one-way conductivity of the rectifier ml, the resistor H is effectively short circuited for the .decay transient currents produced upon opening the key 27. From the foregoing description it will be-seen that the initial transient and the decay transient of the electric wave may be independently con trolled.

By selection of. appropriate values for the restators H, i2, i3, and 14,1 have been able-to obtaln the sharp initial and the long'dec'ay trandent similar to that produced by striking a bell. By other adjustments of the resistors, a gradual I asaue aa crease from the blocking bias value to the demodinitial transient simulating um produced by pipe organs has been obtained.

.In Figure 1 the variable reactor l produced the periodic variation of attenuation in the modulatingnetwork. It is to be understood, however, that the reactor i may be fixed in value and modulation accomplished by periodically varying the inductance of the inductors 3, or d, or 5, singly or in combination. The operation of the modulating network and its component elements 1 will be described in more detail hereinafter.

' In Figure 1 the inductor 5,while shown as a separatecoil, in practice will not have a physical reality but will be obtained by the mutual inductance of inductors 3 and 3. As will be hereinafter described, the terminations of the modulating networks may require the inclusion of inductance. The inductance for these terminations may be incorporated in the inductors 3 and 6. 3

Figure 2 is a modification of a portion of the system shown in Figure 1. In Figure 2 the modulating network differs from that shown in Figure 1, in that in Figure 2 the series arm is a periodically daried inductor id and the shunt arm is a fixed capacitor 1?. The attenuation of the modulating network is a function of the ratio of eries and shunt reactances, hence, as it is periodically varied in value, the attenuation of the modulating network is likewise varied periodically which modulates the carrier impressed upon the input terminals 20 and ii. In a manner similar to that described for Figure 1, the

modulated carrier is demodulatedby demodulator 9, a tone-frequency appearing at the output terminals 22 and 23.

Figure 3 is another modification of Figure 1 and difders irom'Figure 1 in that one series arm of the modulating network consists of the periodically varied inductor it in parallel with a fixed capacitor is, while the shunt arm of the modulating network is the fixed capacitor ill. The attenuation of the modulating network is periodically varied by variable inductor it which thereby modulates the carrier impressed upon input terminals 2t and 21!. The'demodulator and associated elements are the same as and function as those described in relation to Figure 1.

Figure 4 is still another modification of Figure 1. The modulating network in Figure 4 difiers from the modulating network in'Figure 1 in that the series arm consists of a resistor i9, and the shunt arm consists of the periodically varied inductor It in parallel with the fixed capacitor id,

The attenuation of the network is periodically varied by inductor it, hence, the carrier im pressed upon input terminals 2@ and 2| is modulated in accordance with the variations in inductance in the inductor IS. The operation of the demodulator and its associated elements is 'essentially the same as that described for the demodulator of Figure 1.

While in Figures 1 to 4, inclusive, the demodulator biasing potential is supplied through the circuits connected to the demodulator, it is to be understood that the biasing potential may be impressed upon the demodulator through a suitable choke coil as is commonly known as the parallel feed system.

While there are innumerable networks which will accomplish thedesired demodulation in the manner above described, only four have been shown in this disclosure. A general treatment of modulating networks is given hereinafter.

ing material. Inductance coils 204 to 2| l, inclusive, are mounted about the periphery of rotor 200, the axes of the coils lie on radii of the rotor and are equally spaced in angular displacement. These coils when utilized with high-frequencies are'supported preferably by non-conducting material. As shown in Fig. 5, opposite coils are connected in series, as coils 2M and 205 are in 'ing rotor and stator tracks, exceeds the required capacitance for the, duty to be performed, the

stator electrodes may be connected in groups sov as to provide a number of variable capacitances havinggthe same frequency and a capacitance proportional to the number ,of electrode pairs series and connected to terminals 214 and 2i5 The eight coils constitute four generators of the samefrequency, any one or all of whlchmaybe used as the inductor I6, the terminals of which, 2 to 22I, inclusive, are arranged in pairs. The

inductance of the coils 204 to 2, inclusive, is

periodically varied, by rotating the shaft 2M, thereby alternately moving conducting pole tips in and out, of the magnetic field of the coils. The inductance is a minimum when a pole tip is coaxial with the coils, and the inductance is a maximum when the rotor slot is coaxial with the connected in a group. In order to obtain a continuously periodic variation in capacitance an integral number of electrodes must be used in each track, hence. the frequencies developed by the concentric tracks on a given rotor must be harmonically related. -When stator electrodes are connected in groups it. may be desirable to inter-connect diametrically opposite stator electrodes in order to eliminate undesired variations in capacitance due to wobble of the rotor or lack of parallelism between rotor and stator. It will be noted that all of the rotor electrodes are connected electrically, hence all of the capacitances have one common terminal. In practice it has been found desirable to ground this com-- mon terminal. When, using carrier-frequencies in the order of one-half megacycle or higher, the

capacitance between the shaft and its bearings forms a suitable circuit for connecting to the rator. This eliminates the necessity for rubbing contacts and the resultant noise-frequencies i which are ordinarily generated when they are coil. The frequency 01 the "inductance variation V is equal to the product of the number of poles on n the rotor and the rate of rotation of the rotor.

Figures 6, 7, and 8 are views of a variable capacitor suitable for use in modulating networks such as illustrated in Figure 1.

Figure 6 is a view of the rotor of the capacitor.

V The disc 300 which is carried upon a rotatable shaft 30! is preferably made of electrical conducting material such as .Woods metal or other metal suitable for die casting. Tooth electrodes 302 to 3115, inclusive", are set in conccntrictracks having an integral number of electrodes in each.

track. The electrodes in a track are equally spaced and have the same dimensions. While the circumferential dimensions of each electrode is here shown to be equal to the circumferential dimensions of the slot between the electrodes,

' many applications may require a different ratio of electrode to slot dimensions. For maximum capacity variation the depth of the slots should be greater than the slot width.

Figure 3 is a view of the stator of the variable capacitor. The plate did is made of a dielectric material suitable for mechanically supporting the groups of conducting electrodes 3M to lit, inclusive. These electrodes are positioned in concentrio circles of the same diameters as the corresponding rotor tracks illustrated in Figure-8. The angular displacement of the electrodes cor responds to that of 'the complementary rotor electrodes. As shown in Figure 'l the peripherial dimensions of the stator electrodes is two-thirds of the peripherial dimensions of the corresponding rotor electrodes. lthas been found that these proportions result in a capacitance variation closely approaching a sine wave. For other forms of capacitance variation different dimensions and shapes of electrodes may be emused.

Figure 8 is a sectional view of an assembly of the parts shown in Figures 6 and 7, showing on the left the rotor 30d, and shaft 3M, and on the right stator mounting plate 356 and state: electrodes 3H to Mt, inclusive. For economy in manufacture the rotor may be die cast of metal and the stator may be a moulded phenolic condensation product. hi Figures 6, 7, and 8 only a few large electrodes have been shown. By utilizing a disc having a diameter of six inches and one hundred and twenty-eight electrodes in the outer track driven at aspeed 01741.2 revolutions per second, I have obtained a frequency "variation in capacitance of 5273.6 per second which corresponds, to the highest note normally required in a musical instrument. This capacitor had a capacitance of lGG-l-sin (Qt-at) micromicroiarads, where n represents the frequency in cycles per second and t represents time in seconds. While in Figures 6, 7, and 8 a single variable capacitor is shown, when a multiplicity of such capacitors are located adjacent to each other they should preferably be electrostatically shielded from one another.

Figure 9 is a schematic diagram of a portion of an electric wave generating system embodying a plurality of individual generators, such as were hereinbefore described, and providing methods and means for combining and controlling different electric wave frequencies for the production of periodic electric waves of any desired shape. Any single valued periodic l waveform consists of integrally related frequencies having various amplitudes, the waveform being by the numbers and amplitudes of the integrally related frequencies. It is obvious,v therefore, by providing generators for fundamental and harmonic frequencies, together with means for controlling the amplitude of the individual frequencies, any desired waveform may be obtained.

In Figure 9 the rectangles Ail to Aid, inclusive, and Bil to Bid, inclusive, represent the modulator network demodulator, demodulator biasing system, and associated input and output circuits, representedin the rectangle I of Figures 1 to a, inclusive. Theinput circuits are energized by a common carrier-frequency source 30. The rectangle Afl generates a fundamental frequency. the rectangles Af2, M3, and M4 generate frequencies which are harmonics of AH, the order of the harmonic being designated by the numeral following the letters Af. The output from the generator Afl is connected to a common bus 59 and a fundamental bus 5|. generator AfZ, which is a second harmonic, is connected to the common bus 59 and to a second harmonic bus 52. The output ofthe generator Aft, which is a third harmonic, is connected to a third harmonic bus 53. The output of the generator Aft, being a fourth harmonic, is connected to a fourth harmonic bus 54. The output circuits of the group of tone generators Bf l to B'ft, inclusive, are connected to the fundamental and harmonic buses in a manner similar to the connections thereto of the group of tone generators Aft to Aft, inclusive. The amplitude of the output currents from the buses 51, 52, 53, and 54 are individually controlled by amplitude controls 55, 56, 51, and 58, respectively, while the amplitude of currents from all four buses is regulated by a common amplitude control 59. The output circuit of thecommon amplitude control 59 is connected to output terminals 90 and 6|. The output terminals 90 and 6! may be connected to an amplifier and loud speaker system or to a frequency modulating system, as hereinafter described. Other generators are connected to the fundamental bus 50 and the harmonic buses 52, 53, and 5 5 in a manner similar to that described for the groups Af and Bf. Likewise, if desired,

additional harmonic generators or sub-harmonic modulators operative and producing electric waves in the output circuits of each of the associated generators, in the manner described in connection with Figure 1. Similar operationof key B21, changesthe biasing potential on the demodulators in the generators Bf l to Bft, inclusive, making the demodulators operative and causing electric waves to appear in the output circuits of the & associated generators. Other groups may be similarly keyed. Since all of the fundamental sources'are connected to the common fundamental bus 5!, the amplitude of all fundamental frequenciesiwill be controlled by the resistance 55. Likewise, since all the second harmonic generators are connected to the second The output of the harmonic bus 52, the amplitude of all the second harmonics may be regulated by the resistance es. The third and fourth harmonic amplitudes may likewise be regulated by the resistances 5'! and 59,

respectively. By means of these independent ad-= switches may perfo the equivalent functions and give the same effects obtainable by the organ stops found in the Hammond patent, Number 1,956,350, granted April 24, 1934.

In Figures 1 to 4, inclusive, have been shown individual electric wave generators each supplied by a'separate carrier-frequency sourceiwhile in Figure 9 a plurality of electric wave generators have been shown as supplied from a single source of carrier-frequency. It is to be understood that while the carrier source has a low impedance, the

carrier source each generator should be connected to the common carrier source by a resistance pad which will fix the impedance of the carrier supply circuit and avoid undesired modulation com ponents from appearing in the carrier supply circuit. However, when a large number of generaters are supplied by a common carrier source the attenuation afforded by the division of power between the generator input circuits will reduce the amplitudes of spurious modulation products which may be produced by the individual modulating networks, and impedance fixing pads are not required.

Figure 10 is a schematic showing of an'electric wave generator capable of producing amplitude modulationeffects, known in music as tremolo. The rectangle 30 represents a carrier-frequency oscillator of conventional design employing a pentode vacuum tube 3|, a tank circuit inductor 32, and an output coupling indicator 93. The frequency generated by this oscillator system is determined by the reactance of inductor 32 together with the associated elements.

An electrical conductor 4| is placed in the field of the inductor 32, which conductor is rotated by a. shaft 42, gears t3 and 44, and a prime mover $5. The conductor M is rotated at a speed of approximately three revolutions per second. As is well known, the revolution of the conductor M will thenproduce a iodic variation in the inductance of the inductor 32, and thereby produce a periodic variation in the frequency of the oscillator. The frequency of this variation will be twice that of the frequency of rotation of the conductor M. The efiect of this periodic variation in the frequency of the carrier, as noted in the output of the demodulator 9, will be that eftect known musically as tremolo. The modulating network and the demodulator, to ether with the associated keying and control circuits between the input terminals 20 and 2! and the output' terminals 22 and 23, are like the elements of the assembly shown in Figure 1 or Figure 2, 3, o 4. The demodulator output terminals 22 and 23 are connected to the input terminals of an amplifier 92 by means of a volume control 59. The output terminals of the amplifier 92 are connected to a loud speaker 93. -Whi1e in Figure 10 only one modulator, demodulator, and associated keying and control circuits are shown, it is to be understood that a multiplicity of such modulator demodulator systems may be connected in multiple with the output of the oscillator 39 and the volume control 59, and amplitude moduletion produced in all the frequencies generated by the multiplicity of systems.

As described in connection with Figure 1, a constant frequency carrier impressed upon input ytermin'als 20 and 2| is'amplitude modulated by tion of the modulator is a function of the irequency of the carrier, the amplitude hi the car- 'rier in the output of the modulator may be periodlcally varied by varying the frequency of the carrier. I This may be done as above described by rotating the conductor H in the field oi the "inductor 32. Also, the amplitude of the electric wave in the output of demodulator 9 is proportional tothe product of the amplitude of the carrier and the amplitude of side frequencies, and

since the amplitude of the carrier is'periodically varied it follows that the amplitude of the electrio wave in the output circuit of the demodulator will vary at a frequency oftwlce that of the rotation of the element Bl. Hence, rotation of the conductor II at aspeed of three R. P. S.

'eflects a periodic variation at the rate of six C. PIS. in the amplitude of the electric wave in the output circuit of the demodulator. In order that the percentage of amplitude modulation may be controlled, means for varying the coupling between the inductor 32 and conductor ll oil via resistance pad lid. The purpose of the in combination with the transiormer is to fir. the impedance presented to balanced modulator. The output of transformer 52 is com nected to the input of a bandpass filter comprisinc elements l2, 58, i l, l5, l6, Tl, and i8, together with interconnecting" conductors. interposed be tween the transformer t2 and the hanrl pass ill ter is a balanced modulator, comprising elements (til, or, audit) which are copper oxide recti here such as are ordinarily used in teie phone systen'i modulators. A fixed carrier ir quehcy source is connected to intermediate terminals and it of the balanced modulator.

A resistance pad "it is provided for properly terurinating the hand-pass filter and for intercom-= nesting the filter and terminals and 8:! of balanced demodulator. interposed between the pad "l9 and an output transformer is a hal-. anced demodulator comprising elements 82, 83, Ed, and 35 which are copper oxide recti'flers such as are ordinarily used as demodulators in. carrier telephone systems.

@Jonnected to intermediate terminals and of the balanced demodulator is a carrier ire quency source l3!) similar to the one shown in Figure 10 in rectangle 3c. The rectangle 53E) represents a carrier-frequency oscillator of conventional design employing a pentode vacuum tube-HI, a tank circuit inductor H2, and an output coupiing' inductor I33. The frequency zenerated by this oscillatory system is determined by the reactance of the inductor I32 together with the reactance of the associated elements. An electrical conductor I4! is placed in the field of 6v the inductor I32, which conductor is rotated by a shaft 842, gears I43 and IN, and a prime mover H5. The conductor I is rotated at a speed of approximately three revolutions per second. As is well known, the revolutions of the 10 conductor I will then produce a periodic variation in the inductance of the inductor 532, and

'thereby produce a periodic variation in the frequency of the oscillator. The frequency of this variation will be twice that of the frequency of rotation of the conductor Ml, may alsobe aifected by periodically varying the capacitance associated with the inductor l32.

When electric waves are impressed upon terminals 63 and of the'bala'nced modulator via terminals 60 and 6!, pad 96, and the trans= former 62, and a carrier from the source ll is impressed upon intermediate terminals. 68 and Ill of the balanced modulator, the carrier is modulated by said electric waves and for each wave primary terminals 63 and 6t, while on the other hand the carrier-frequency voltage is suppressed.

The elimination of either one or the other of v these side-bands is accomplished by the band pass filter which is designed to freely transmit -only one of the side-bands. The transmitted side-hand after undergoing attenuation by the network pad i9 is impressed upon theprimary terminals Si! and 8E 01'' the balanced demodulator. The periodically varied carrier-frequency which is impressed upon the intermediate termirials and or of the demodulator has a mean frequency equal to the frequency of the source ll. The action of the demodulator, together with r the impressed side-bands and the periodically varied carrier, produces electric waves which are periodically varied; in frequency at a rate equal to two times the rotational frequency of element lll. The mean frequenc of each of the wavefrequencies, at the output terminals and ill. is identically the same as the wave-frequency, in the input circuit, which produced the side irequency from which any given output wave-frequency was derived. The rate of cyclic variation the frequency or the output wave is the same the cyclic variation in the frequency of the carrier, and these cyclic variations have a ire quency which is equal to twice the rotational frequency of element ldl. The extent of the variations in the carrier-frequency and in the output wave-frequency may be controlled by varying the coupling between rotating element Hi and inductor Q32, or by varying theresistance of element Ml. While in Figure 11 a source of electric waves and an output amplifier and speaker system have n'ot been shown, it is to be understood that the system of Figure 11 may be introduced between any electric wave source. and an associated amplifier for which the system is de- This variation frequency two side frequencies are produced.

Voltages of both side-bands appear between the &

ure 10 between terminals 60), El and 9d, 9!.

While in Figure 11 a simple form of balanced modulator and balanced demodulator has been shown, any one of a large number of such devices maybe utilized in the present application. The. particular type of balanced-modulator and demodulator shown herein has been completely disclosed and its operation described in the "Bell Laboratories Record of March 1937, Vol. XV.

.No. 7, published by Bell Telephone Laboratories,-

Incorporated, 463 West Street, NewYork city,

N. Y. In Figure ll the band pass filter shown is for a simple crystal filter but any one of a large number of types may be used in this application. For a description of filters suitable forgthis application reference is made to the "Bell System Technical Journal of October 1937, Vol. XVI, No. 4, published by the American Telephone and Telegraph Company, 195 Broadway, New York city, N. Y. s .1

Figure 12 is a family of curves showingvariation in attenuation as alfunction of capacitance in a modulating network similar to that shown in Figure 1. These curves are for m derived types of modulating networks, wherein the prototype network is a constant k filter. Attennation versus capacitance for the prototype filter is shown by the curve labeled "m equals 1.0.

Figure 13 is a graphic representation of the variations in attenuation as a function of capacltance in a modulating network similar to that shown in Figure 1. The curve A shows the variations in attenuation as a function of'the.ca

pacitance when the shunt resistance 6 in Figure 1 is azero. Curve B shows the variation in at tenuation'as a function of capacitance when theresistance 6, in ,Figure 1 is 200 ohms. Curve 0 shows the variation in attenuation as a function of the capacitance when the resistance 6, in Figure 1 is 200 ohms and th terminatingv impedances each have a magnitude of 937 ohms and a positive angle of 33 degrees. The curve C is for the total insertion loss of the modulating network, and illustrates the degree and range of linearity which may be obtained in a modulatin network such as shown in Figure 1. 1

In the application of modulating networks, such as shown in the drawings, to the production of electric waves, the total insertion loss of the network should be a linear function of the peri-.

tor, a pure sine wave of current having the same frequency as the sinusoidally varied capacitance will be obtained in the demodulator output circult. I

A modulating network in which the total in- 'sertion loss of the network is a linear function of "a"-periodically varied inductance,capacitance, or

resistance will be referred to as a linear modulating network.

, A modulating network should not; dissipate more than a few percent of the power delivered to it; thepercent modulation should be controllable such a filter is known 2,841,682 v e signed, such for instanceits introduction in Figbetween zero and,100%', it should consist of a minimum number of physically realizable ele- 'mentswhich are simple andinexpensive; and the characteristics of the network should be stable.

linear modulating-network having a periodically varying capacitance, will be presented.

-The simplest /form of a single section wave fil -ter consists of a series impedance Z1 and a shunt impedance Z2. The freely transmitting band of to occur for the values of Z1 and Z2 such that I For all other values of 21/421 the filter will ate tenuate. In Equation 1.it is evident that the transition from free transmission to attenuation occurs for the valuesz v -Z1/4Z2=0 (l?) Z Z2=1 (2) The attenuation of the filter may be varied by varying the value of Z1 or Z2 or the ratio Z1/Zs in the regions shown in Equation'Z.

The condition a is satisfied when: 7 (1) Z1 is zero'when Z2 is finite. j (2) Z1 is finite when Z2 is infinite.

(3 Z1'is zero and Z2 is infinite simultaneously. a

The condition'b in Equation 2 is satisfied when Z1 and Z2 are both finite but opposite in sign. This condition can be satisfied with a m ch simpler network than that required to satisfy condition'a above.

The requirements of b are satisfied by a simple symmetrical low-pass or high-pass wave file ter. The attenuation of such a filter is knownto A+ B=2 slnlk vzl ezz (a) where A is the attenuation constant andB is the phase constant.

The total'insertion loss of such a network, in addition to the attenuation above defined,'in-

' cludes the reflection losses between the iterative impedances of the network and the terminating impedances, plus the interaction loss.

The total insertidn loss for a symmetrical filter having equal sending end and receiving end ter= minating impedances may be expressed in the form LT=A +401og %l+20 log ll r R K cal analytical solution exists for determining thewhere A and B are as defined in Equation '3, Zn is the sending and receiving end impedance, and Z1; is the image impedance of the network.

-It is apparent from Equation 4 that no practivalues of Z1 and Z2 for a desired loss characteristic. It is therefore necessary to plot a family of curves for loss as a function of the parameters,

and from such curves, the necessary types and values of impedance elements may be' selected to sign formulae could be derived and were used, there would be no assurance that the analytically derivedvalues of Z1 and Z2, to meet a specified set of conditions, would be physically realizable, hence, empirical methods would be required to supplement any analytical design formulae.

I A L This condenser may consist of As an illustration a a method which 1 have used for designing allnear modulating network,

the following example is given.

nac os:

and the reactance Lo of the coil be designated by 7 @L then ioLo= d+iDwLo and the attenuation, from Equation 7 becomes with the methods of this disclosure; utilizing a variable condenser in a linear modulating network, the condenser must have'a frequency of capacitance variation of 5273.6 cycles per second. a stator and a rotor each having 128 teeth or electrodes, the rotor beingdriven at a speed 01' 41.2 revolutions per second.

when the rotor and the stator are parallel disks having electrode tracks or mean diameter d, gap clearance t, radialdimension r, with the circumrerential dimensions of each electrode equal to the circumferential dimensions or the slots between adjacent electrodes, air dielectric, and all dimensions in inches, the capacitance may be expressed in micromicrofarads as l.l13dr St tained by multiplying the revolutions per second by th number of rotor teeth, and where t is time in seconds.

Using suchacondenser as the shunt element in a l wp s constant fk type wave filter, at the wherein u is 21' times the impressed frequency, Lt is the inductance of the series element, and Co is the minimum capacitance of the shunt element. Let the value or Co=l50Xl0'- farads and let Lo=2 10 henries, then at the cutofl point from which the frequency of the impressed carrier is found to be 581,098 cycles per second and u=3,651,480. Introducing this value or together with the value or L0, Equation 6, gives Substituting this value of 2 /42 in Equation 3 results in By plotting values of attenuation versus Co, as

calculated from Equation 7, the attenuation is {311mg be far from linear (see curve m='1.0 e 12 However by inserting a small fixed resistance in series wvith the element Lo, the attenuation characteristic may be made linear over a portion or its length. This may be shown as follows:

.- lietthequotientoitheinsertedresistane lt (1 jd)2Cn l A Zsmh 300 (8) Wherein i= -:1 In (8) using the values of 200/300 which were used in Equation 7, the points for new curves maybe calculated for several values of d. While such curves are not herein shown, they have been plotted, and for the pres= ent example it was found that for d=0.2, the attenuation characteristic does not deviate more than 4.5% from absolute linearity over the range of 3 db. to 12 db. attenuation.

It can be similarly shown that the same results can be obtained by inserting resistance in the shunt element.

For many applications, the attenuation characteristic of the type k filter used in the foregoing example would .be satisfactory. The networks shown in Figure 2 and 3 may be designed in this manner. However, when larger variations of attenuation with lower minimum values are required, the "m derived types of filters are more satisfactory than the prototype.

Using the foregoing k type filter as the pro totype, the mid series m derived types have been determined and their attenuation characteristics versus capacitance has been computed. These characteristics are shown in Figure 12.

The method of designing m derived types of filters is fully disclosed in such publications as Transmission Circuits For Telephonic Communications," a book by K. S. Johnson published 1925 by D. Van Nostrand Company, Eight Warren Street, New York, N. Y., and Transmission Networks and Filters," a book by T. E. Shea published 1929 by D.-Van Nostrand Company, Incorporated, 250 'Fourth Avenue, New York, N. Y. 1

In Figure 12, the attenuation characteristics for the "m" derived types have double fiexures, these flexures occurring at higher values of atten nation as m approaches unity. The portion of the characteristic lying below the double flexure may be made to closely approach linearity by inserting resistance in either or both the series or shunt arms of the filter.

In Figure 13, the attenuation characteristic for the m derived filter wherein m=0.6 is shown by a curve A for the condition of zero resistance and by the curve B for 200 ohms inserted, in the shunt arm. A different value of resistance which would produce a much closer approach to linearity could be inserted, but 200 ohms was selected in order to obtain a linear characteristic for the .total insertion loss, as explained later.

In linear modulators of the type considered. the image impedance Z1: is a function ofthe variable reactor and therefore varies with the attenuation. The terminating impedances Za remain fixed. In the present example, both Zn and (A-HB) vary with the capacitance, and it is evident from Equation 4 that both the reflection and the interaction losses must likewise vary with the capacitance.

Note, in Equation 4, that when Zn and Z: are closely alike in both magnitude and angle, the reflection and interaction losses become very ire ' er 'as A increases.

small. Also, the interaction loss becomes small= That is, the interaction loss is of importance only when A is small. However, when A is small, the interaction loss is a func-- tion of B, and in the linear modulator B varies over a range of about two radians.

, Several values of terminating impedances, having values approximating Z'x for the low attenuation region, should be inserted in Equation 4, and the magnitudes of the reflection and interaction losses thus determined. A value of Zn must be found for which these losses are small over the operating range of the capacitance, or for which the sum of these losses is a linear function of the variable capacitance.

In the present example, a satisfactory value for the magnitude ofZa was found to beequal to the magnitude of Z1; when C=90 mmf. This is the value of C at which i=0 for the la prototype filter.

The optimum value of the angle for Zn is the mean between the angles of Zn for 0:90 mmf,

and C =85 mmfr The compromisevalueselected for Zn: is 937 433". This value of terminating impedance results in a total insertion loss characteristic which does not deviate from linearity by more thanill db. between 3 db. and 24 db.

The positive reactance required for the terminating impedances may be included in the inductance of the series arms of the'filter, thus avoiding the necessity of providing additional coils for this purpose. Likewise, the inductance in the shunt arm of the modulator may be derived from the mutual inductanceof; thecoils in the series arm. In this manners. single centertapped coil and a suitable variable condenser constitute the necessary reactance elements for the linear modulator and its terminations. In. a manner similar to the foregoing, it can be shown that; the periodically varying reactor could be an inductance rather than a capacitance.

The design problem wherein a variable inductance is used in the linear modulator is simplified by the fact that the reactance of a coil is a linear function of its inductance, whereas the reactance of a condenser is not a linear function of its capacitance.

Since the reactance of an inductor is a linear function of its inductance, the ratio of Z1/Zz, which includes a variable inductance, I

is likewise a linear function of the variable inductance. Families of curves showing the-attenuation characteristics of a large number of wave filters have been published in the references cited. Attenuation characteristics approximat-.

ing linearity may be selected from these curves,

is a linear function of its variable reactor. Such 5 a network is shown in Figure 1. Having thus described my invention, 1 claim 1. In an electric frequency modulator: a bal-' anced modulator having on input circuit and an output circult,-means for impressing upon the input of said modulator a constant frequency carrierwave, a wavefilter connected to. said output circuit and adapted-to transmitlbut one side-band from said modulator, a balanced demodulator having an output circuitand an insource will vary linearly with respect tions of the reactance of said reactor,- and means for cyclically varying the reactance of said reput circuit adapted to receive the output of said wave filter, and means for impressing upon said demodulator a second carrier wave having a periodically varied frequency; whereby electric wave frequencies impressed upon said input circuit will appear in said output circuit modulated in frequency at a frequency equal to the frequencyvariation of said periodically varied carrier. v r 4 2. An'electric wave generator, comprising: a source of carrier-frequency, a demodulator hav-;

ing an output circuit, ainodulating filter connecting said source to said demodulator, the

3 characteristics of theelements of said filter being such that it operates on its cutofi characteristic with respect to the carrier-frequency from u said source, said filter having therein a variable reactor, and means for cyclically varying the reactanceof said reactor for the purpose of modulating said carrier-frequency and oi producing a modulating frequency in the output of said demodulator. 1

3. An electric wave generator, comprising: a source of carrier-frequency, ademodulator' having an output circuit, a modulating filter connecting said source to said demodulator, the characteristics of the elements of said generator being suchthat the filter operates on its cutoff characteristic with respect to the carrierfrequency fromsaid source, saidfilter having therein a variable reactor, and means for cyclically varying the reactance of said reactor for the purpose of modulating said carrier and of producing a modulating frequency in the output of said demodulator. I

4. An electric wave generator, comprising: a

source of carrier-frequency, ademodulator having an output clrcuit,'a modulating filter cone,

nectlng said source to said demodulator, the characteristics of the elements of said generator being such that the filter operates on its cutofl-characteristic withrespect to the carrierfrequency from said source, said filter havingtherein a variable reactorjand a resistor, said 4 resistor being of a value that the attenuation of said filter to the carrier-frequency f om said variaactor for the purpose of modulatlnggsaid carrier and of producing ln the output of said demoduglator a wave form linear with respect to the variations in reactance of said reactor.

5. An electric wave generator, comprising: a

5 source of carrier-frequency, a demodulator hav- 'ing an output circuit, a modulating filter connecting said source to said demodulator, the characteristics of the elements of said-generator being such that said filter operates on its cutofl characteristic with respect to the carrier- ,frequency from said source, said filter having therein a variable reactor and a resistor. said resistor being of a value that the attenuation of said filter to the carrier-frequency from said source will vary linearly with respect to variations of the reactance of said reactor, and

' means for varying the reactance of said reactor linearly with respect to a desired wave form for the purpose of modulating said carrier and of producing in'the output. of said demodulator the desired wave form. Y

' 6. An electric wave generator, comprising: a

4 source of carrier-frequency, a demodulator having an output circuit and a biasing circuit, a

76 modulating filter connecting said sourceto said demodulator, the characteristics of the elements of said filter being such that itoperates on its cutoff characteristic with respect to the carrier-frequency from said source, said filter having therein a variable reactor, means for cyclically varying the reactance of said reactor for the purpose of modulating said carrier frequency and of'producing a modulating frequency in the output of said demodulator, and means for keying said biasing circuit.

7. An electric wave generator having a source of modulated carrier-frequency; and a demodulator having an input side connected to said source, an output circuit in which is produced through the action of said demodulator the modulating frequency of said carrier-frequency, a biasing circuit, and means for keying said biasing circuit to control in said output circuit the output of said demodulator. g

8. Anelectric wave generator having means for controlling the output of said generator, comprising a source of modulated carrier-frequency; and connected to said source, a demodulator having an input side connected to said source, an output circuit in which is produced through the action of said demodulator the modulating frequency of said carrier-frequency, and a biasing circuit, means in said biasing circuitior supplying a bias for biasing said demodulator, and means for varying said bias whereby in said output circuit the amplitude of the output of said demodulator is varied.

9. An electric wave generator, comprising: a source of carrier-frequency, a demodulator having an output circuit, a modulating filter connecting said source to said demodulator, thecharacteristic of the elements of said filter being such that it operates on its cutofl characteristic with respect to the carrier-frequency from said source, said filter having therein a variable reactor, and means for cyclically varying the reactance of said reactor for the purpose of modulating the carrier-frequency and of producing a modulating frequency in the output of said demodulator, means for biasing said demodulator and means for varying said bias to control the amplitude in the output of the demodulator.

10. An electric wave generator comprising: a source of modulated carrier-frequency, a demodulator connected to said source and having a biasing circuit, means for keying said circuit, and in said circuit a yariable impedance for controlling the amplitude of the output of said demodulator.

11. An electric wave generator, comprising:

a source of carrier-frequency, a demodulator having an output circuit, a modulating filter connecting said source to said demodulator, the characteristics of the elements of said filter being such that it operates on its cutofl characteristic with respect to the carrier-frequency from said source, said filter having therein a variable reactor, and means for cyclically varying the reactance of said reactor for the purpose of modulating said carrier-frequency and of producing a modulating-frequency in the output of said demodulator, and means for cyclically varying the frequency of the carrier produced by said source whereby the output of said demodulator will cyclically vary in amplitude.

12. In an electric wave generator employing a modulated carrier-frequency: a demodulator having an output and in input adapted to have impressed thereon a modulated carrier-frequency, a source of biasing voltage for said demodulator, and means connecting said demodulator and said source for impressing said biasing voltage upon said demodulator and for increasing and decreasing said impressed voltage.

13. In an electric wave generator employing a modulated carrier-frequency: a demodulator having an output and an input adapted to have impressed thereon a modulated carrier-frequency, a source of biasing voltage for said demodulator, and means connecting said demodulator and said source for impressing said biasing voltage upon said demodulator and for increasing and decreasing said impressed voltage, said means including a rectifier in parallel with an impedance for differentially controlling the rate of increase and decrease of said impressed biasing voltage whereby the output amplitude of the demodulator may be increased and decreased at a differential rate.

14. In an electric frequency-modulator: a balanced modulator having an input and an output circuit, means for impressing upon the input of said modulator a constant frequency carrier-wave, a wave filter connected to said output and adapted to transmit but one side-band from said modulator, a balanced demodulator having an output circuit and an input circuit adapted to receive the output of said wave filter,

- and meansfor impressing upon said demodulator a second carrier-wave having a varied frequency; whereby electric wave-frequencies impressed upon said input circuit will appear in said output circuit modulated in frequency I at a frequency equal to the'frequency variation of said varied carrier.

TYNG M. LIIBBY

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