US1794847A

US1794847A - Frequency eliminator

Info

Publication number: US1794847A
Application number: US176545A
Authority: US
Inventors: Estill I Green
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1927-03-18
Filing date: 1927-03-18
Publication date: 1931-03-03
Anticipated expiration: 1948-03-03

Description

March 3, 1931. GREEN 1,794,847

FREQUENCY ELIMINATOR Filed March 18, 1927 5% iger yr 3 Freyuemcy J v INVENTOR. 1:1 1:1 :3

El Gneen/ v ME 14/. Know/E Patented Mar. 3,1931

venom: AND TELEGRAPH COMPANY, ACORPORATIQN OFNEW' YORK Ap ication"iiieafiuareh'is, 1 927.l"se1 i'ai lief 176,545. :1 I

' This invention relates to signaling systems, 7 andparticularly to arrangements in such sy stems for elim nating currents of defin te frequencies.

carrier frequency corresponding to the voice frequency signals, it is frequently thepractice to employ'an electrical wave filter to sup-g press one ,of the side bands A and to freely the frequency spectrum, has been extremelydifiicult of accomplishment with ,theordin'ary designs of electrical wave filters; I any of the component frequencies of the undesired 7 side band are transmitted through theeleetrical wave filter, such transmitted frequencies will become demodulated at the receiv ing terminal of thesystem, andthereby constitute a source ofinterference, the seriousness of whichwill .depend largely upon the magnitudes of such transmitted frequencies and upon the phase relationship existent be-v tween the current of carrier frequency at the receiving terminal and the current'of car.-

rier frequency at the transmitting terminal.

Accordingly, it is an object of thisinvention to employ a p1ezo-electr1c structure to act as a means'of suppressing undes rable frequencies at or near the cut-0E pointofan 7 electrical wave filter.

It is'another object of this invention to as sociate a piezo-electric structurewith, a ea frequency spectrum. a

vice exhibiting attenuation which is variable with frequency, the pieZo-electric structure being operated so as to provide very high attenuation for a number of frequencies or for all frequencies within a small region of the While this invention will be pointed out with particularity in the appended claims, the invention itself, both as to its further objects and features, will be better understood from the detailed description hereinafter following, when read in connection with the In systems in which current of a carrier frequency is modulated by voiceffrequencyf signals so as to produce sidebands of the 'nsrrtii I. ennnn,, or EAST ORANGE, newuneau, assienoia ro-AMEnIci-in "rain-- accompanying drawing, which Figure l the inventionlv V Referringto Fig l of the drawing, there is shown a source o'f signals S connected to' the primaryfwinding of the transformer T The secondary windings of the transformer i represents a signaling'system embodying the a princ1plesj; of this invention, Fig; 31a repre- 1 sents aportlon of the signaling system shown sents curves characterist c of the elementsof T are connected to a duplex modulating system having two three-element thermionic tubes V and V the filaments of which are heated to electron-emitting temperatures by a commonbattery. Thesethermionic tubes are q preferably similar and equal in their fstrucfturealnd characteristics, and'maybe'replaced by "a single d uplex tube having a'single 'filament, two. grids and twoplates] The input circuits of these thermionic tubes are arrangedin parallel with respect-to the secondary winding oflthe transformerT which "winding suuseries with abatteryB n the: conductor" common to the u-mput circuits of these" tubes; The output I circuits of these tubes" are "connected through the primary windings ofa transformer T in parallel with respect to aconimon plate battery B The r i I J connected to an electrical wave filter which. -may be of any well-known type, preferably secondary winding of the transformer T is ofthe type disclosed inthe patent me. A.

' Campbell, No. 1,227,113, dated May22,1917.

The output terminals 'of the filter F are connected tothe primary winding of the The secondary winding of the transformer T is connectedto the input terminals of, a translating device, such as a three-element thermionic tubeV,jthe input transformerfT circuit of which includes the grid and filament of the thermionic tube V, a battery B and the secondary winding of the transformer T4. The filament of the thermionic tube -V also heated to an electron emitting temperature by alocal battery, whichmay, in this illus tration, be the same as the battery heats I the filaments of the thermionic tubes V and I V to electron-emitting, temperatures -The output circuit of the thermionic tube'V in-' T and a battery B The secondary winding of the transformer T is connected to an out- A piezo-electric device PE is type, is connected to the primary windingof J n V l V 'irequencies at or near the cut-off point of the put circuit S eludes the filament and plate of the thermionic tube V, the primaryjwinding of transformer connected inshuntwiththe input circuit of the thermionic tube V, the piezoelectric de-' 'vice being shown brldglng the secondary w nding of the transformer T known type, preferably of the vacuum tube the transformer T This oscillator generr ates current of the carrier frequency to be employed in the process of modulation in the system. By 'virtueof the connection of the up "oscillator O and the transformer T inthe duleX translatin s stem the rids ofthe tubes 7 V andV are both in phase with, respect to current oftheca'rrier frequency transmitted by this-oscillator. Oscillations of the'carr e'r frequency are thus impressed in the same direction on the input circuits of these tubes, the grids usually becoming equally biased'negatively by the battery B Under ordinary conditions, equal currents of the carrier frequency willfiow intheoutput circuits of the circuits ofthe tubes V and V ,modulate curtubes V and V which'may be oppositely directed by the primary windings of the transformer T so that these currents may elfectively balance and neutralize each other.

source S when impressed upon the input theoscillator O, sothat side bands of the carrier frequency, known as the upper and lower side bands, may appear in the out ut circuits of these tubes and may be transmitted through transformer T to the filter 'F, current of the carrier frequency being effectively suppressed by virtue'of the balanced arrangement of the I duplex system. This electrical wave filter F rmay, of course, be either a high-pass filter, a low-passfilter or a band-pass filter. I Yet, for illustrative purposes, it may be assumed that this filter is of the band-pass type, freely transmitting one ofthe side hands resulting from modulation, and substantially suppressing the other side hand. For illustrative purposes, it may also be assumed that the electrical Wave filter F substantially suppresses the lowerside band, while freely transmitting the upper side band. .In systems similar to the one illustrated, it has been discovered that if any, of the frequencies of the undesired side band, i. e., the lower side band, are transmitted through the filter F, the frequencies of the undesired side band, which are thus transmitted, will become demodulated at the receiving terminal of the system when beaten 1 with current of the-carrier frequency which is supplied at that terminal. The presence of the frequencies within the'undesired side band will constitute a source of interference An'oscillatorO, which may be wellslgnaling "currents; orig nating 1n the tending to distort thesignals. The eflect of these transmitted frequencies in causing. in-

terference will depend largely upon the amplitudes ofthese transmitted frequencies and upon the degree of synchronism betweenthe system.

c c r 10 carr er. currents supplied at the transm tting terminal and at the receiving'terminal of the v The piezoee-lectric device PE is designed tq have a resonant perio'd such that itmay 7 :present in. the input circuit ofthe thermionic tube Vaipath of very low impedance for V electrical wave filter Fr lVhen viewed from.

a different angle, the piezo-electric device may be cons dered one wh chexhlb ts very ly cqual to the period of the current of carrier frequency. In other Words, the vibratory period of the piezoelectric device may be the same, oralmost the same, as the period of the current ofjcarrier frequency. At frequencies very close to the natural frequency of the piezoelectric deviceiPE, there will be high attenuations, these' attenuations additively increasing the attenuations of the electrical wave filter F narrow range. I y

In Fig. 2 of the drawing, qbcrepresents a curve characteristic of an'electrical wave filter of the band-pass type, in which the ordinates represent loss orflattenuation and the abscissa represent frequency. This curveindicates that the ele'ctricalwave filter exhibits a low. and substantially uniform attenuation within a range of frequencies somewhat well-defined in the frequency spec trum. The curve def represents a curve characteristic of a pieZo-electric device, from. whlch 1t w ll be apparent that there 1s:very

high attenuation for each frequency within a small band also somewhat well-defined in the freql'iency spectrum. It will beapparent from the drawing that the band-pass filter for all frequencies within a will transmit freely and uniformly all frequencies within a desired side band and effectively suppress frequencies within the unde sired side'band very remote from the frequency of the carr er current. The niagnitudes of the loss or attenuation presented by the filter 1n the range of the undesired side band which is immediately adjacent to the frequency ofthe carrier current, may, however, be insuflicient to prevent interference and distortion of the signals if some of thefrequencies in the :undesired side band do become transmitted to the receiving terminal. The-employment of a piezo-electrical crystal designed to have a suitable vibratory period 2O I tory periods areshown connected in shunt will, however, provide a material improve ment in the suppression of frequencies of the undesired side band closely adjacent to the frequency of the carrier current. The curve clef shows the amount of'attenuation1ntro *duced by thepiezo-electric device, and'shows also how important such a device may be for providing additional attenuation in the range of frequencies at which the filter provides insufficient attenuation. a

It will be obvious that if the band of fire quencies intended for suppression in accord once with the principles of this inventiomis too wide for elimination with a single piezoelectric device, two or more ofsuch piezoelectric devices may be suitably combined to provide suflicient attenuation for all fre V quencies within that band. In the illustrated system of Fig. 1a,. additional piezo-electric devices PE a-nd'PE having different vibracially one which may exhibit attenuation which is variable with frequency.

While this lnvention has been shown and described 1n one particular embodiment,

; merely for the purpose of illustration, the

general principles of this invention maybe applied to other and w dely varied organizations without departing from the spirit of the invention and the scope of the appended claims, r J

What is claimed is: 1. The combination of an electrical wave filter having a flat transmission characteris tie for frequencies above a predetermined frequency and a variable attenuation characteristic for frequencies below said predetermined frequency, and a piezo-electric device coupled to said electrical wave filterrhaving a natural frequency of vibration adjacent to said predetermined frequency.

2. The combination of an electrical wave filter having an attenuation characteristic which varies with frequency immediately adjacent to the cut-off point of said filter, and a piezo-electric structure having a vibrato frequency close to the cut-off point of sai filter, and means whereby said PTGZO'. electric structure may exhibit a high attenuation for all frequencies within a narrow region of the frequency spectrum adjacent to its vibratory frequency.

3. The combination of an electrical wave filter having a cut-off point well defined in the frequency spectrum, and means coupled to said electrical wave filter including a piezo-electric structure having a vibratory period such that it provides high. attenuation for a range of frequencies immediately adjacent to the cut-ofi' point of said electri -l' cal wavefilter.

4. In a signaling system thes combination of a source of high frequency current, is sourceof signals, means for beating the current of said high. frequencysource with the current of said' s'ignaling- 'source'so a's to produce two side ban'ds ofv the high frequency V current reach characteristic of said signals, an electrical-wave filter for freely transmit tingnone ofithe side bands and for substan suppressing the other of the-side? of a source of signals, a source of'carrier current, a duplex translating circuit for beating the carrier current with said signals in order to provide a pair of side bands of the carrier current each characteristic of the signals, an electrical wave filter for suppressing one of the side bands and'for trans mitting the otherof the side bands, and-"a piezoelectric structure connected to said electrical wave filter which may be vibrated at a frequency substantially equal to the cut-off frequency of said wave filter adja- 7 cent toth'e1 frequency of the carrierv current, said piezo-electric structure sharpening the cut-off of said electrical wave filter at the frequency or frequencies at which'it is vibrated.

6. The combination of an electrical wave filter having a substantially. uniform transmission characteristic over a band of frequencies and havinga non-uniform attenuation characteristic for a range. of frequencies immediately adjacent to said band, and piezoelectric meansjconnected to said electrical wave filter to substantially increase the attenuation characteristic of said filter for all frequencies within a narrow band im- {)nediiately adjacentfto the first-mentioned- 7. The combination of a band-pass filter and a piezo-electric device coupled to said filter having a frequency of vibration. substantially equal to oneof the cut-off frequencies of said filter, 7

"8. A band filter comprising a plurality of piezo-electric devices of different vibratory periods connected in parallel relationship, the vibratory period of each device being within the limits'of the band, each piezo-electric device suppressing currents of frequencies within a narrow portion of said band, the 1 ious piezo-electric devices being adjacent to each other. I i

. 9. A band filter comprising a plurality'of portions. of said band suppressed'by the var-.

piezo-electric devices connected in parallel relationship and having closely spaced nat-' ural vibratory frequencies so thatthe Various piezot-electric devices may suppress cur-- rents having frequencies Within adjacent portions of the band to be eliminated.

10. The combination, of a band-pass filter and a pluralityofpiezo-electric devices cou-" pled to said filter having difi'ierent vibratory frequencies, the vibratory frequency ogf, each devicev being ar a selected cut-off frequency' of said filter.

In testimony whereof, I have signed my name to this specification this 17th day of 15 March,

ESTI-LL I. GREEN.