US2347098A - Electrical wave modulation - Google Patents

Description

April 1 8, 1944.' E. l. GREENy 2,347,098

ELEQTRICAL WAVE MODULATION Filed Jan. s. 1942 F/a/ 2 3 ,[5 A Af.

CHANNELS l -IZ RE C.

/NVENTOR E. GREEN TTORNE Y Patented Apr. 18, 1944 vinurl-:o STATES PATENT oFFics aLEcrmo'An WAVE MoDULATIoN Estill I. Green, Millburn, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 9, 1942, Serial No. 426,146

5 Claims. ('01. 179-1715) -high quality speech systems. measuring systems The present invention relates to the production of modulated electrical waves for carrier wave signaling or other purposes.

Many ways of producing electrical wave mociulation have been devised since the advent of wireless telephony and wire carrier telephony, the most commonly used types of modulators helng those using some form of vacuum tube circuit and those using solid element varistors such as copper-oxide rectiiiers or blocks of composite material having a non-linear volt-ampere characteristic. To a lesser extent, magnetic mcdulators have been used.

While these types have proved generally satisfactory for most uses, they have the drawback oi causing inteiniodulation products between all impressed wave components and where the sianal itself has 'a number of diiierent frequency components oi comparable amplitude the intermodulation products between such components will in some uses prove troublesome. These have commonly been kept to ncgligibly low value by and similar situations.

For convenience oi* expression the constant frequency wave that is to be modulated will be termed herein the carrier wave whether it he aca tually used as the transmission carrier wave or merely for frequency shifting purposes in a local circuit, as in a superheteiodyne system or heat wave system or certain types oi measuring circuits using a frequency shift.

A general object ofthe present invention is a modulating system or method oi such characterthat the intermodulation products between dii ferent components of the signal or other mcd ulating wave are inherentlyj kept at a low substantially zero value.

This object is attained in accordance with the use of a large carrier amplitude relative to signal amplitude. 4

One use in which the intermodulaticn products between input waves in the same input circuit are particularly troublesome is instanced in group modulators and demodulators, which are used for shitting anentire group of modulated high relative to the other input waves since the.

modulated wave channels incoming to' the group modulator or demodulator 'are often at fairly high level because' of transmission requirements' use of a modulator of the contact resistance type and varying its resistance oy mechanically vary ing the contact pressure at the carrier frequency.

The nature ofthe invention and its various features and objects will be more fully understood from the following detailed description taken in conjunction with the accompanying drawing, in which:

Fig. l is, a circuit diagram in block schematic oi' one terminal of a two-way carrier telegraph system incorporating group modulators in accorglance with the present invention;

Fig. 2 is a simplified circuit diagram showing impedance relations to he referred to in 'the description: and

Figs. 3, 4 and 5 are apparatus diagrams showing threedierent forms which the group mod 'ulator in accordance with the invention may placed on the system.4 In certain prior art systems the incoming channels have been provided with loss pads to lower their level to help bring about the high amplitude relation of the shifting wave with respect to the channel amplitudes.

Y This procedure requires ampliilcation after modulation to restore the channel levels to desired value for transmission purposes. i

Other cases in which it is desired to keep the intermodulation products at low level include take, these forms being applicable also to other types of modulation.

Referring to Fig. 1, the general plan of this circuit is old in the art..the feature of novelty inhering in the usein such circuit of the group modulators in accordance with the present invention,the nature o! these modulators to he referred to subsequently in the description. As

shown in the drawing, one group oi carrier telegraph channels comprising channels i to 12,. inclusive, transmit directly to the eastward line Il. Two of these channels are indicated in some suitable frequency such as 425 cycles per second,

followed by a transmitting circuit 2 and a 425- cycle lter '3, the pass-band of which is sumciently wide to accomodate the essential frequencies of whichl the signal in this channel is composed. The transmitter 2 is indicated as under the control of a key d. The next higher vchannel is indicated as using a carrier frequency of 595 cycles. Thisv same frequency separation of 170 cycles between channels is used throughout the group so that channel No. 12 of this group uses a frequency of 2,295 cycles as indicated on the gure. These twelve channels transmit out through the transmitting amplifier 5 and low-pass filter t to the outgoing line l il.

The second group of channels comprising channels-13 to 24, inclusive, may be entirely similar to channels Nos. l to 12 up to the point where the group modulator l2 is connected into the circuit. The purpose of the group modulator I2 is to raise the frequencies of these channels 13 to 24,

-transmitted through the group modulator filter I5 while the opposite side-band is suppressed. The selected side-band is amplied at I6 and transmitted through high-pass lter il to the line i0.

'Ihe manner in which these channels terminate at the opposite terminal may be seen from considering the west terminal for westward line 2li at the lower part of the gure. incoming over line 2li are selectively transmitted through direction'alloW-pass lter 2i and receiving amplifier 22 'to the input side of the twelve channel filters, one of which is shown at 23. These channel lters separate the message currents belonging to the individual channels and direct them into the various receivers, such as 2li, the outputs of which are connected to relays, such as 25, for receiving the message.

I'he upper group comprising channels 13 to 24 is selectively transmitted through the high group directional lter 2t and receiving amplier 21 and impressed on group modulator 2B. This group modulator is also supplied with some of the group shifting frequency 4,936 cycles from the output of lter it, by virtue of which these channels as a group are stepped downward in vfrequency to occupy the same frequency position l as they originally had before reaching the group modulator of the distant transmitting station. From this point on the manner in which these channels are terminated is entirely similar to that described for receiving channels 1 to 12.

Each of the transmitting wave sources l and the group shifting source I3 may in practice be a separate source or all of these frequencies may be obtained from a multifrequency generator or by means of a harmonic generator from a single .base frequency of 85v cycles per second frequency.

Y of the type disclosed in a paper entitled Voice frequency carrier telegraph system for cables by Hamilton, NyquistfLong, and Phelps, published in the Journal of the American Institute of Electrical Engineers, Volume XLIV, No. 3, March and described above, the twelve transmittingl 'channels comprising channels 1 to 12, inclusive,

communicate directly over the line l0 with the corresponding receiving channels at the distant station or stations by means of their individual carrier frequencies and without the necessity of frequency translation. Channels 13 to 24 have their frequencies stepped up by the group modulator I2 at the transmitting point for transmission over the line I0 to the distant terminal station or stations 'Where their frequencies are stepped downward as a group by a modulator corresponding to group modulator 28 for reception by the channel terminal apparatus. sired, other blocks or groups of channels may be similarly superposed i-n the frequency spectrum and transmitted over the same line by use of similar group modulators with appropriate shifting Channels 1 to 12 frequencies. It is important in this process of shifting the frequencies of the twelve channels as a group that intermodulation between individual channels be kept downto a minimum to avoid interference or crosstalk and reference will now be made to a type of modulator in accordance with the present invention which may be advantageously used as the group modulator in a system such as that shown in Fig. 1. It will be understood, of course, that the modulator of the invention is also capable of general application and that when used as a group modulator the actual system may diier widely from that shown in Fig. 1 which is to be taken merely as illustrative of the use of the modulator of the invention for group modulating purposes. v While the multiplex system chosen for illustration is a carrier telegraph system, this is not to be construed as limiting the invention, which is applicable to multiplex carrier telephony or other kinds of signal transmission systems. i

Referring to the elemental circuit diagram of Fig. 2, the modulators to be described in accordance with the present invention comprise in each case a variable resistance `R. connected in series .with the source of signal' voltage E and with the load impedance Z. The instantaneous value of the modulator resistance R is mechanically varied at the carrier frequency of q radians-per second. Further reference to this Fig. 2 will be made later on in the description in connection with the explanation of the theory of operatic involved.

One form which the group modulator of the invention may take is indicated in Fig. 3. It comprises a variable resistance composed of granular carbon 30 contained between an upper electrode 3| and a lower electrode 32. This variable resistance is s erially connected into the circuit by means of terminaison the conductive membersl and 32. The lower member 32 is mounted to be vibrated at the carrier frequency/,

by means of the piezo crystal 33 having upper and lower plates connected to the source of carrier waves. The upper plate is shown mechanically connectedby insulating layer 31 to the bottom conductive plate 32 of the carbon,

chamber. By this means mechanical pressure is exerted on the carbon granules, varying at the carrier frequency and the instantaneous value of the resistance offered, by triegranular carbon mass is therefore varied at thecarrier frequency. This resistance varying in carrier 2,347,098l frequency is serially included between the source.

of signals and the load circuit.

Fig. 4 is similar to Fig. 3 except that two such granular carbon reslstances 30 and 30 are con nected in push-pull relation and are mechanically driven by a common crystal 33.` Input transformer 34 and output transformer 35 with a mid-tap connection 36 leading from the midpoint of secondary windingv of transformer 36 to the mid-point of the primary winding oftransformer 35 are provided for coupling the modulator proper to the external circuit.

Referring to Fig. 5, another method of vibrating the granular carbon resistance 30 is shown comprising a magnetostriction member 40 provided with a bias winding 4| and bias source 42 and a driving Winding 43 supplied with current of the carrier frequency. Elongations and contractions of the magnetic core 4i) at the carrier frequency impart vibrations cf corresponding frequency to the mass of carbon 3U. For further information as to' how magnetos'triction may be employed for such purpose reference may be made to United States Patent 1,092,453 to P. M. Rainey, April 7, 1914.

Reference is made in connection with Fig.,4

to input and output terminals.4 It will be clear,

however, that the type of modulator shown in each of Figs. 3, 4 and 5 is bilateral so that either par of signal terminals shown may be input or output terminals.

A consideration of the theory of operation will serve to demonstrate that theoretically no crossmodulation products between components of the signal are generated in the contact resistance modulator of the invention. The well-known vacuum tube modulator is operated on a curved part of its characteristic, which means that if the carrier voltage is removed and4 only the signal (such as speech) is applied, an output consisting of the original speech, reproduced and possiblyamplifled. is obtained along with lntermodulation products between the different speechcompcnents since the input-output characteristic is assumed to be non-linear, the tube being operated as a modulator. If this simple :test is applied to magnetic, capacity or recti-v fier type modulators, it will be found that they product at least some degree of .intermodulation between signal components for the same reason thatl they are able to `modulate a carrier wave bythe signal, namely they operate with a non-linear input-output characteristic. In the case of the contact resistance modulator of the invention, if the carrier wave (in this lcase the source of mechanical vibration) be kept zero and the signal alone applied, no intermodulation between signal 'components appears since the modulator acts simply as an ohmic resistance of fixed value.

This may also be seen to be `the case by inspection of the formulae applicable to this type of circuit. Referring to Fig. 2, we can write,

R=To+r sin qt `beingq (inradianspersecond).

CII

The signal input'comprising several different frequencies may be designated as Ezar sin pli-l-az sin pzt ai and az being coefficients and p1 and pg being frequencies (in radians per second).

The current flowing in the circuit is E la' V:al sin pit-tag sin p2! Z- l-lt Z-i-rO-l-r sin qt Calling the denominator constants Zi this expands to It can be seen from inspection that when this expression is multiplied out,l it yields no term containing a product of sin pit times sin pet, which indicates that no intermodulation products between the separate signal components occur.

- What' is claimedgis:

1. A modulating system comprising a source of signal frequency electrical waves, a load cir-1 cuit and in series therewith a resistance contact whose resistance value is variable with variation of contact pressure, and means to vary the contact pressure at the frequency of the wave to be modulated.

2. Ir'i a modulating system, a source of modulating electrical waves comprising a band of frequencies, means for modulating a carrier frequency by said waves simultaneously and individually lcomprising a contact resistance, and means for mechanically varying the contact pressure at the carrier frequency.

3. In combination, a source of electrical waves covering a band of frequencies, a contact resistance and a load in series with said source, means to vary the contact pressure at a constant frequency outside said band, and means comprised in said load for responding to the modulated waves resulting from such variations in4 contact pressure.

of different frequencies comprised within a given frequency band, a. contact resistance and load in series with each other and said circuit, and

means to shift the frequencies of said pluralityv of signal waves to a different frequency level comprising means to vary the oontact'pressure of ce where ro is the constant component of the re said contact resistance at a frequency outside said band.'

5. In a modulator, a source of mechanical vibrations of carrier frequency, a contact resistance mounted to have its contact pressure and therefore its electrical resistance varied at carrier frequency by said mechanical vibrations, a`

source vof modulating electrical waves differing infrequency from said carrier frequency, a uti.- lization circuit, and means connecting said utilisation circuit, said contact resistance and said source of electrical waves in series with eachl other.

EBTILL I. GREEN.

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