US2362898A

US2362898A - Signal transmission system

Info

Publication number: US2362898A
Application number: US435235A
Authority: US
Inventors: George W Gilman
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1942-03-18
Filing date: 1942-03-18
Publication date: 1944-11-14
Anticipated expiration: 1961-11-14

Description

sumo s'r 'rEs PATENT OFFICE assasss v r I SIGNAL raamsmssron srs'rm George W. Gilman, Short'llills,.N. 1., aaeignor to Bell Telephone Laboratories, Incorporated, New York, N. 1., a corporation of New York Application March re, 1942, Serial No. 435,235 3 Claims. (01. 178-44) This invention relates to electrical transmission systems and more particularly to electrical filter systems. Still more particularly this in- I vention relates to transmission systems employin: filters'or similar circuits or networks set up to transmit currents having a band of frequencies, the width of which may be changed in accordance with signal currents or voltages or other causes.

This invention involves a band selective network having limits which are variable under control of a signal current or voltage or other effect. The network may be supplied, for example, with a band of frequencies comprising a substantially continuous spectrum of randomly related currents extending over a wide band, such as the band that may be obtained from a noise generator or other source, and this band may be widened v or narrowed according to signals or other causes.

The network of this invention may include, for example, an oscillator for su plying current having a frequency which directly depends on the signal voltage or current, and a modulator for superimposing the received current on the oscillator current for raising the received band of frequencies in the frequency spectrum. The currents of the elevated frequencies may then be transmitted through a band filter of fixed limits which may transmit only a portion of the elevated frequencies. The frequencies transmitted by the filter may then be demodulated with the same oscillator current to return theelevated frequencies transmitted by the filter to their original position in the frequency spectrum. The demodulated band of frequencies will be of a width which will correspond to the magnitude of the impressed signal current or voltages or other cause. The filters employed may be of any suitable type,

' such as the crystal filters disclosed in Patent 2,045,991 granted June 30, 1936. to W. P. Mason. This invention will be better understood from the more detailed description hereinafter following when readin connection with the accompanyingzdrawing in which the circuit arrangement shown illustrates one form of apparatus for producing a band of frequencies of a width which depends on a signal current or voltage or other factor.

. Referring to thedrawing. there is shown an input circuit L; which may receive from a source 50 cycles according to the magnitude of the con-- electrons in a resistive impedance or the random fluctuations of the electron current in the space path of a thermionic vacuum tube. The input circuit Ll is connected to a band filter F1 which may transmit only a predetermined band of frequencies as, for instance. 1000 to 1100 cycles; all other received frequencies being suppressed by filter F1. The transmitted currents together with the current of an oscillator O are fed to a balanced modulator MD which .is of well-known type and serves to produce two side bands of the current obtainedrfrom oscillator 0, the current of oscillator 0 being suppressed in the balanced circuits of the modulator. The oscillator O is of the variable-frequency type, the frequency of which is varied in accordance with variations in the amplitude or phase of the signal current or voltage or other effect received over a signal control circuit In. The oscillator frequency may have a mean value of, for example, 148,950 cycles and many swing over a. range of plus or minus trol current or voltage. I

Assuming that the signalcurrent or voltage in circuit La is of zero magnitude or is entirely abmean value of 148,950 cycles. and the lower side band resulting from modulation will extend from 147,850-147,950 cycles while the upper side band will extend from 149,950-150,050 cycles. These side bands are then transmitted to a second band filter B: which has fixed limits of, for example, -156 kilocycles. This filter will suppress all of the lower side band obtained from modulator *MD as well as part of the upper side band. i. e. the part below the lower cutofl frequency of the filter. Hence those frequencies of the upper side band which are above the lower cutoil' point of the filter F: will alone be transmitted, or i50,000150,050 cycles. A band of this width traversing filter F2 and evelated in the frequency spectrum will correspond to an oscillator frequency which is at its mean value due to the absence of a control signal.

Currents of the frequencies passed by fllter Faare then fed along with the oscillator current to a balanced demodulator DM of well-known type, the demodulator acting to return the elevated band of frequencies to its original position in the frequency spectrum. The band of frequencies obtained from the demodulator DM in the illustration given will extend from 1050-1100'cycles and currents of these frequencies will appear in the output circuit La leading to a load circuit The reduced band of frequencies returned to its the band. An example of this lng action will now be given.

original position in the frequency spectrum will correspond to the condition of zero control voltage. The demodulator DM also creates an upper side band which may be eliminated by any simple selective means (not shown) such as an output transformer of suitable characteristic or a lowpass filter.

Instead of a control voltage. in line Ls of zero value, the control voltage may be positive or negative within certain limits. when the control voltage increases and reaches its maximum positive value, the frequency of the oscillator current will rise to. for example, 149 kilocycles. when this happens, the upper side bandresulting from modulation in modulator MD will extend from 150,000 to 150,100 cycles, all or the components oi which will lie within the frequency limits of filter F2. All of the-upper side band will therefore be transmitted by filter F: and the entire lower side band will be' suppressed. This upper side band will be returned by the action of demodulator DM to its original position in the frequency. spectrum, the demodulated band includme filter Fr. There will then. be substantially an absence oicurrent in line Ls.

Thus it will be observed that with an oscillator supplrins a current which varies in frequency in accordalice' with signals or'other effects, the band of frequencies fedto the output circuit Lswill be changed in widthto correspond to the values oi the signals, or other control eiiects. The band may be increased in width to limits which are determined by the frequency limits of filters F1 and 1'5, whichever is the narrower.

It has been shown that when the oscillator current frequen'cyis below the lower frequency limit of filter'l 'z, only the upper side band resulting from modulation contains components within the range-oi filter 1": which are transmitted and traversing filter 1'1.

thereafter demodulated. It is the lower regionot the upper side band that may be trimmed" or cut off due to the presence or absence of a control voltage as already explained. In a case where the oscillator current frequency is above theup-' per limit of filter 1'5 the upper region of the lower side band resulting from modulation .will

likewise be "trimmed" or removed due to the presence or absence of a control voltage.

filter'l 'i while not ,afiectingthe'lower-edge of type of modulat- Assume, for example, that the irequency' limits of filter I"; are 10,000-l0,100 cycles, that thsmean frequency of oscillator O'is145,950 cyeies and that the oscillator within limits of plus or minus cycles in acascaacs side band will extend from 155,950 to 156,050 cycles. If the filter I": has the frequency limits of 150-156 kilocycles, as was also assumed in prior illustrations, then the lower side band will be entirely suppressed while only part of the upper Fr, 1. e. 10,050-10,100 cycles, will be suppressed or trimmed. The demodulator DM- also creates an upper side band which may be eliminated by any simple selective means (-not shown) such as an output transformer of suitable characteristic or a lowpass filter.

As the frequency of oscillator O rises to, for

example, itsmaximum value of 146 kilocycles, then the upper side band resulting from modulation will extend from 156,000 to 156,100 cycles. All of this side band (and of course the lower side band) will be outside the limits of filter I": and will therefore be suppressed. The line 14 will therefore receive no current during this status. v

However, when the oscillator fmquency falls to. for example, its minimum value, or 145,900 cycles, the upper side band will comprise the frequencies between155,900 and 156,000 cycles, all of the components .of-which come within the range filter 1'5. Upon demodulation the band will comprise irequenciee between 10,000 and 10,100 cycles whichhasthesamewidthastheoriginalband Itwill be noted that the upper e'dge'oi the band traversing filter 1"1 may be "trimmed" by having the frequency spacing between the side bands resulting from modulation greater than thegband width of filter 1h. The filter 15 must have a pass band which iswide enough to permit oneoi'thesidebandstobesweptaemssitseue ataratetotheirequencyofthe modulating sisnal voltage or current incircuitlisl Whether-the upper or loweredge oi the band is trimmed is determined by the side band whichcomeswithinthersngeoifilterhand bythe' edge of the latter filter which the side band is swept across. This may be seneralissd as follows: When the lower side band from modulatorlmsweepsacrosstheloweredsoofthe filterhtheupperportionotthebandtraveru ing filter l'i will'have been "trimm Whenhave been trimmed, it this same side'band.

frequency is'variabl'e- I corda'nce with any ana] orcontrol voltage-in oircuitla as assumed in previous exsmplis. At (I is may beampllfisdif desired, and modulated casesthe"trimmingozthebandoffilterl ris ,of a frequency width which depends on and corresponds-to the signal current or voltage in circuith. w

Itwillbeunderstoodthatthecurrentsinline distant point the received band of frequencies may be amplified, if desired, demodulated and thereafter impressed upon a square law detector of well-known type to reproduce the original speech or other signals corresponding to the speech or other signals in circuit La. The currents of line Ls may be transmitted by systems of amplitude modulation or frequency modulation as will be clear to those skilled in the art.

It will be apparent that by arranging two networks, such as that shown in the'drawing, both connected in tandem, the trimming action may be caused to take place at both ends of theband simultaneously and, if desired, equal changes in width at the two edges of the band may be obtained.

The structure used to vary the frequency of the current or oscillator in accordance with the signal waves or other effects received over line La maybe any suitable well-known type, such as the structure" disclosed in Patent 2,250,104 granted be of any desired width.

The frequencies used in the examples and the swings in the oscillator frequency are given merely for illustrative purposes and are not to be construed as limitations on the invention.

'- While this invention has been shown and described. in certain particular embodiments merely.

for the purpose of illustration, it will be understood that the general principles of this invention may be applied to other and widely varied organizations without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. A signal transmission system ting a frequency band having its width varied in accordance with signals, comprising a source of alternating current, a source of signals, means for varying the frequency of said alternating curfor .transmitrent in accordance with said signals, a source supplying a substantially continuous band of frequencies having a predetermined constant width, and means responsive to changes in the frequency of said alternating current to vary proportionally the instantaneous width of the band of frequencies.

2. The combination of a source of currents having energy substantially continuously distributed over a band of frequencies of predetermined limits, said limits being fixed and constant, an, oscillator, an independent source of variable voltage, means for varying the frequency of said oscillator in accordance with changes in the voltage of the latter source, and means for changing the instantaneous width of the band of frequencies transmitted from the first source in proportion to the varying voltage, said means including a modulator and a filter and a demodulator all connected in tandem, the oscillator being connected in common to the modulator and the demodulator.

3. A' band selective transmission system, said system includin a source of signal currents havvary. in accordance with variations in the instantaneo'us amplitude of the signal currents, a second band pass filter having a pass band of substantially greater width than said band of frequencies and adapted to eliminate one of said side-bands and to trim part of the other sideband, the amount trimmed being proportional to the instantaneous amplitude of said signal currents, and a second modulator for restoring to their original positions in the frequency. spectrum those frequencies from said generator that were passed by the second band pass filter.

GEORGE w. GIIMAN.