US2048080A - Signaling with high frequency waves - Google Patents

Description

July 21, 1936. R K POTTER l Y 2,048,080

SIGNALING WITH HIGH FREQUENCY WAVES Filed Feb. l0, 1934 2 Sheets-Sheet l l\ e 4--E Q I/:Ly

ATTORNEY July 21 1935- R. K. POTTER SIGNALING' WITH HIGH FREQUENCY WAVES' 2 Sheets-Sheet 2 Filed Feb. lO, 1954 @WI/PML ATTORNEY Patented July 21, 1936 UNITED STATES PATENT Aorifice SIGNALING WITH HIGH FREQUENCY WAVES Application-February 10, 1934, :Serial No. .710,707

14 Claims.

This invention relates'to methods of andmeans for signaling vWhich involve the transmission of the'signalby themodulation of a'high-frequency carrier wave. More particularly the invention relates-to carrier transmission ,in which the carrier and a single side-band are transmitted, the other side-band ibeing suppressed at the transmitting endof `thesignaling system.

'-While the Vinvention is generally applicable to lO signaling with high-frequency waves in the manner rindicated hereinabove, whether the transmitting'medium be the ether ora circuit formed by one or more metallic conductors or a combination of one ormore radio linksand Wirelinks,

15 itis :particularly applicable to radio broadcast transmission and will bespecically described and discussed Vhereinafter with reference to such transmission.

Y .It is well-known in the art ofradio transmission that .it is desirable that the range of frequencies necessary to transmission from agiven transmitting station be reduced to limits as narroW1as due regardior the quality of the signal transmission vWill permit. Accordingly, in the case .of radio broadcasting, in which it is 4usually necessary to transmit the carrierfrequency along withthe'side frequencies produced by modulation of @the carrier in accordance with the signal, it has been proposed that one ofthe sidebands be 10 removed-'by suppression at the transmitting station. vOne objection to this proposal has been that the removal oi one of the side-.bands leads to 'substantialndistortiom as will be discussed at greater length hereinafter. UU The :ultimateobject of the present invention is toreduce'the range of .frequencies which it is necessary totransmit With'unsuppressed carrier transmission in order to produce high quality, and thus to permit a more eicient channel spacing :in such vtransmission as radio broadcasting; the proximate objectl ofthe invention is to eliminate for practical purposes, or, at the least, very greatly reduce the distortion normally resulting from the suppression of one side-bandin unsuppressed carrier transmission.

lngeneral, theabove-stated proximate object is accomplished byproducing at the transmitting stationan. auxiliary Wave and modulating the carrier lWaveby the auxiliary Wave, this auxiliary Wavehavingfa form such that there Will be efl fecteda substantial compensation for the distortion resultingfrom the suppression of the sideband. More specically, the proximate object mayibeaccomplished, after the suppression of 55 one-ofthe side-bands, by derivingat thetransmitting station 'from .a portion of the transmitted energy a ,demodulated signal wave, which willrbe a distorted Vrepresentation of the signal input =wave, -eiecting-adjustment of the amplitude and phase :of :this 'demodulated signal wave, and 5 combining the demodulated signal wave with the signal input wave for modulation with the carrier wave, ftheadjustment of the demodulated signal -wavebeingsuch as substantially to compensate for thefdistortion introduced as a result 10 of the suppression lof the one -side-band.

,The rfollowing `description of the invention is to be :readwith reference to the accompanying drawings, of .which- Figure "1, 4by means of `vector diagrams and 15 curves, illustrateslthe problemintroduced by the suppression of one side-band in unsuppressed carrier transmission;

-Fig.v2, -by `means-of curves, indicates graphically the :nature of the correction provided by 20 the applicant; :and

Fig. 3 shows .diagrammatically, and in part schematically, -a lsuitable arrangement of circuits for effecting .the applicants elimination or material ireduction of the distortion introduced 25 by the singleside-.band suppression.

With reference rst to Fig. 1 of the drawings, it 'to be understood that in the case of transmission-With the carriersuppressed at the transmitting end of the systemand reapplied for demodulation at the receiving end, the ratio of carrieramplitude to side-,band amplitude is readily adjustable and may be madehigh so that any distortion .introduced yas a `rresult of the suppression oi oneside-band at'the transmitting end 35 is negligible, v.Whereas in the `caseof the transmission .of VVthe .carrier With one side-band, -with whichthe applicantisdealing, the carrier amplitude `Will `exceed-the side-band amplitude by a verytmuchvsmaller amountif the same effective level of signal isto be maintained atthe receiver, andthe result.of.;the suppression of the one sideband :will be :distortion which is quite considerable, asshowninthis ligure. The-vector diagrams and curves .arendrawn to scale and represent a particular 4case cof '15% modulation of a highfrequency zwave by'a single tone, it-being understood that the single tonecase does not diier in general principle from the case of modulation by azvoice wave, -.for instance, `but permits of a 50 more graphic illustration.

`The upperzrow of diagrams of Fig. 1 represents inpterms ,of vectors 1 azhigh-frequency wave modulated bythe :tonein `.the ordinary manner and comprising .the :carrier (C) andthe two side- Y of carrier and two side-bands, the amplitude vav angles with the carrier vector at any time, the.V

direction of the resultantvector-is always that of the carrier vector. Accordingly, in ,thisY case riation represented by the resultant vectors (R1)` gives a modulated wave envelope corresponding to the modulating wave (tone). spondence isY graphically shown by thediagram appearing at the bottom` ofv Fig. 1, in which' the unbroken curve W1 is determined by plotting the amplitudes 'represented by the vectors R1, theenvelope amplitude varying'with time in a simple sinusoidal fashion." Thus,in Vthe case of carrier and double side-band transmission, the

, shape of the low-frequency signal wave' resulting from properdemodulation `at'the receiving end corresponds to the shape yof the low-frequency signal wave applied at the transmitting'end. Y

If, however, after the modulation of the Vhighfrequency wave, one Vof the side-bands issuppressed, the resultant-amplitude variation (or wave envelope) no longer corresponds to the shape of the modulating Wave. `In the lower row of vectordiagramsV of Fig. 1,` the unbroken,sin gle-headed vectors C and S-represent the carrier and the unsuppressed side-band (or` sidefrequency), respectivelyi It will be noted that in order to maintain the normal percentage amplitude modulation (75%) ortV thesame signal level at a receiving pointftheamplitude of the unsuppresse-d-side-band S `is made tWice'as-fgreat asv the amplitude of theside-band S1 or the sideband S2 in the case of carrier and double sideband transmissiondiscussed hereinabove.V Since the-side-band vector-S is now rotating with respect to the carrier Vector `C lwithout the directionally compensating effect of a side-band vector rotating in the opposite direction, the resultant vectors no longer coincide directionally With the carrier vector, except at the beginning and end of each half-cycle, and the resultant amplitudes represented atjintervals-of 40 by the lengths of the broke-n; double-headed vectors R2, are equal only at'those points to the resultant amplitudes at the corresponding points in the case of the carrierand double side-band transmission. Accordingly, with thelsuppression of the one side-band, Athe amplitude variation represented by the vectors R2 will give a modulated-'wave envelope which does notv` correspond to the modulatingxwave` (tone). vThe distortion is graphically shown inthe diagram at `the bottomV of Fig. l, in Vwhich theYA broken curve W2 is determined-by plotting the amplitudes represented by the vectors R2. It'will be noted that the envelope amplitudeY Varies with time in a non-sinusoidal fashion, the ldegree of distortion being indicated bythedisplacement ofi curve W2 with respect tocurve W1. It'will'nowreadily be seen than-inthe case of carrier and single sideband transmission, Vone side-band'- having been suppressed, the shape of thelow-freq'uency` signal-wave resulting fromdemodulation at.the

This corre- Y receiving end ofthe system Vis the resultof a receiving end does not, without the introduction of some correction, correspond to orvery closely approximate the shape of the low-frequency signal wave applied at the transmitting end. In other words the signal is seriously distorted. V

It can now be seen that, in the case of Vcarrier and single side-band transmission, if theV distortion resulting from the'suppression of the one side-band is to be eliminated or very'greatly reduced, there must be applied'torthe modul lator at the transmitting end, for modulation ofA the carrier wave, a wave of such shape that after themodulation and the suppression of the l side-band there can be derived at the receivingV end a. low-frequencyvwavesubstantially corre- 1 spondingv to the envelope shape of the modulated altered Yin shape to correspond substantially to the Wave W1. l Y 2 In V'order to present this pointy graphically',v

there Vare shown in Fig.. 2 the unbrokenwaveV side-band case (these two waves being the same as the waves of like designation in Figjl)V and-V in addition a' wave W3 (shown in mixedr curve),

which is the auxiliary or corrected wave whichV must be used at the transmitting end'toimodulate the carrier wave. of course, that Vthe wave Wsof Fig. Z'represents the requirement for perfect correction; lin prac-` tice, 'this wave will closely! approximate .the'form shown, with the object of substantial elimination or very great reduction of the distortionwhich would' otherwise resultfrom the side-band suppression. It can be seen from an examination of Fig. 2 that if the envelope shape of the. wavezfrom which the signal. output is to be derived'at the Y combination of a wave lhaving the shape of .curve W2 and a wave substantially corresponding to curve W3, there can be produced at, the receiving end a low-frequency wavehaving substantially the shape Vof the curve W1. It will be understood, however, that the wave representedv by W2 is .formed only as a result of the suppression of the one side-band.' Therefore, the problem must be solved by applying at the transmitting end for'modulation with the carrier a wave substantially in the'form of W3; in other words, the final modulating energy acting on the carrier in the modulator at the. transmitting station must be substantially'. represented by the curve W3. represent a distortion which .will be'substantially. eliminated or olset by thedistortion subsequently introduced as Va result of the side-band suppression. Viewed somewhat differently, the case is one in which there is introducedat theV transmitting end an auxiliary distortion which kwill in Vthe ultimate effect substantiallycompensate for the Vdistortion Vintroduced by theside-,band suppression. Y Y

An arrangement of apparatus suitable for the practice of the invention is shown in the circuit diagram of Fig. 3. At the transmitting station the signal energy from the audio-frequency input is passed through the transformer T1, the audiofrequency amplier AF and the audio-frequency lter F1 to the vacuum'tube circuits of the modulator, in which this low-frequency Wave modu- It will be understood,

The product of the-modulation will then lates the high frequency wave produced' inthev carrier frequency oscillator, associated Withthe product of the modulation comprises. the carrier and the double side-band components. The output of the modulator is then passed through a high-frequency amplifier and through a lter F2, which suppresses one of the two side-bands and passes the carrier and the other side-band. The carrier and single side-band energy is amplied in another. high-frequency amplifier and is radiated from the transmitting antenna A1. .Without correction in accordance with the invention, this energy, when picked up by the antenna A2 at the receiving station and demodulated, would give the distortion indicated in Fig. l, which distortion has been shown hereinabove to be the result of the suppression of the side-band, as in filter F2.

For the purpose of effecting the desired correction in accordance with the particular arrangement of Fig. 3, a portion of the high-frequency energy radiated from the antenna A1 is picked up by the auxiliary receiving antenna A3 at the transmitting station. In this auxiliary circuit a demodulator D, which is of the type preferably used at the receiving station, serves to demodulate the receivedportion of the signal energy, and the output of the demodulator is then passed through an audio-frequency amplifier. By means of a phase-correcting network N1 and an attenuation network Nz, elements which in themselves are well understood in the art, the demodulated signal is adjusted in phase and amplitude and is then impressed on the input or grid circuit of the audio-frequency amplier AF in the main transmission circuit, through the transformer T3. It has been seen that the signal inputis normally Vimpressed on this grid circuit of tube AF through transformer T1; it is now seen that with the circuit arrangement of Fig. 3, a combination of the normal input signal wave and the auxiliary demodulated signal wave, the latter having been subjected to suitable adjustment, can be impressed on the amplier AF and passed through the audio-frequency lter F1 for modulation of the carrier wave. The fundamental component from the auxiliary circuit should be adjusted in networks N1 and N2 so that it will be out of phase with the normal signal input and of amplitude approaching that of the signal input. With reference to Fig. 2, it will be understood that this adjustment should be such that the combination of the signal input and the adjusted auxiliary wave will produce a wave which takes substantially the form of the curve Ws (in the'single tone case presented). The modulation of the carrier by this wave (W3) introduces the eiect which substantially compensates for the distorting effect of the suppression of the sideband in filter F2, and the effective envelope shape 0f the carrier and single`side-band energy radi-A ated from the antenna A1, is changed to a degree such that the output of the receiver at the distant receiving station will substantially correspond to the original audio-frequency input at the transmitting station. I

In practice the degree of correspondence depends upon the extent to which the input from transformer T3 approaches that from transformer T1; to retain normal modulation it is required only that the output of transformer T1 be increased. VThe practical limit to the reduction of the distortion is determined by that distortion which arises from other unrelated sources.

=It will therefore be understood by those skilled in the art that in the applicants case of carrier andsingle side-band transmission with distortion correction introduced through -a feedback circuit, there should be a step-up of amplitude in the audio-frequency input, in comparison with the normal carrier modulation case.

The correction introduced as described hereinabove will necessarily result in additional sideband components that are not present in the case of the carrier and double side-band transmission. In other words, in order to eliminate the distortion produced at the output of the receiver demodulator, it is necessary to transmit components which are out of phase with and will neutralize those produced in the demodulation process at the receiver. However, .these additional sidebandcomponents will not linut the usefulness of the frequency band, and they need not extend beyond the band in such a way as to cause interference with adjacent channels. If, for example, the transmitted audio-frequency signal band has a range from 0 to 10,000 cycles, the regeneration can be limited to this range by means of the audio-frequency lter F1 of Fig. 3, connected in the circuit between the rst audiofrequency amplier (AF) and the modulator, this lter performing the dual function of limiting the f normal and the feed-back inputs to the modulator. Since, because of the presence of lter F1 in the circuit, no distortion component above 10,000 cycles is put back into the modulator, none will be transmitted. At the output of the receiver these will normally be suppressed in the circuits of the receiver, or, if not, will be tolerable because of their low amplitude and the natural limitations of the ear.

It will be understood that, while the simple case of modulation of the carrier by a single tone, used for illustration, would permit of correcting the envelope shape of the modulated wave through the simple adjustment of the signal input, in the practical case of the transmission of the voice or music this would not be so, since modulation components result for which no cornpensation would be eiected without some such arrangement as the applicants for applying the adjusted demodulated low-frequency wave.

While the invention has been disclosed as embodied in a specic form, for the purpose of illustration, it is to be understood that with respect to such embodiment many changes and modications may be made within the true scope of the invention as defined in the appended claims.

What is claimed is:

l. In high-frequency transmission involving the modulation of a carrier wave by a signal wave and the suppression of one of the side-bands resulting from such modulation, the method which consists in producing an auxiliary wave and modulating the carrier wave by said auxiliary wave, said auxiliary wave having a form such as to effect a substantial compensation for the modulated wave envelope distortion resulting from the suppression of the side-band. Y

2. In high-frequency transmission involving the normal modulation of a carrier wave by an audio-frequency signal wave and the suppression of one of the side-bands resulting from such modulation, the method which consists in producing an auxiliary audio-frequency wave and modulating the carrier wave by said auxiliary wave, said auxiliary wave having a form such as to effect a substantial compensation for the modulated wave of the side-band. l,

' 3.4In high-frequency transmission involving Y the modulation of a carrier Wave by a low-frelli) of the side-bands resulting, from such modulation, the carrier and the other side-band remaining'for transmission, the method Which consists in producing an auxiliary low-frequency Wave and modulating the carrier Wave by said auxiliary Wave, said auxiliary Wave having a form such as to eiect a substantial compensation for the modulated Wave envelope distortion resultingV from the suppression of the one'side-band.

4. The method of carrier wave signaling which consists in producing a high-frequency Wave, modulating said Wave in accordance With the signal, suppressing one of the side-bands resulting from the modulation, producing an auxiliary lowfrequencyY Wave, and combining said auxiliary Wave with the signal for the modulation of said high-frequency Wave, said auxiliary Wave being of such form that the effect of the modulation of the high-frequency Wave by the combinatio-n lowfrequency Wave Will substantially compensate for the envelope distorting effect of the suppression of the one side-band.

5.' In high-frequency transmission involving the modulation of a high-frequency carrier Wave by a low-frequency signal input Wave, the suppression ofV one of the side-bands resulting from such modulation and the transmission of the carrier and Vthe single remaining side-band, the method which consists in deriving from the transmitted Wave a demodulated 10W-frequency Wave, effecting an adjustment of said demodulated lowfreq'uency (Wave, and applying the signal input Wave and the adjusted demodulated low-frequency wavefin, combination for the modulation ofthe carrier Wave, said adjustment being such as'substantially to compensate for the wave en- Velope distortion introduced by the suppression of the one side-band. Y l

6. The method of high-frequency transmission which consists in modulating a carrier Wave with the signal input Wave to produce a compositewave of carrier andptWo side-bands, supressing Aone side-band, transmitting Vthe resultant energy composed of thecarrier and one side-band, deriving from a portioncf the transmitted energy a demodulated Ylow-frequency Wave which is a distorted representation of the signal input wave, effecting adjustment of the phase and amplitude of said demodulated low-frequency wave, and

applying the signal Vinput wave and said demodulated loW-frequency wave in combination for modulation with the carrier Wave, said adjustment of the demodulated low-frequency Wave being such as substantially to compensate for the modulated wave envelope distortion resulting from the suppression of the one side-band.

'7.-The method of 'signaling over a transmission system by meansof high-frequency Waves which consists in vproducing at the transmitting end of the system a high-frequency carrier Wave, modulating the Vcarrier wavevvith the signal input wave to produce a composite Wave of carrier andtwo side-bands, suppressing one side-band,

transmitting theV resultant energy composed of the carrier and the remaining side-band, deriving at the transmitting end of the system Afrom a portion of the transmitted energy a demodulated audio-frequency Wave which is a distorted :representation of the signal input wave, effecting adjustmentfof thevphase and amplitude oi saiddeting end. l

modulated audio-frequency Wave, applying thej signal input wave and the adjustedrdemodulated audio-frequency Wave in combination for modulation lwith the carrier wave at Ythe transmittingv end of' the system, transmitting the resultantV modulated Wave, after the suppression ofoneY side-band, to the receiving end of the system, and

demodulating the received Wave to produce theY iinal signal output wave, the adjustment ofthe mitting. end being such that the signal output Wave produced at the receiving end Will represent without substantial distortion thesignal input Wave applied tothe system at the transmit'- 8. In aV transmitting circuit forming a part of a high-frequency transmission system, means for producing. a high-frequency Vcarrier Wave, means for modulating saidcarrier Wave by=a low-frequency wave, means for suppressing one of the side-bands resulting from the Vaction, of said modulating means, means for producing an auxiliary low-frequency Wave of suchrform las? substantially to compensate forv the envelope distortion resulting from Vthe action of said suppressing means, and means for applying said auxiliary wave to said modulating means for modulation-of said carrier wave.

9. In a signaling system, means` for producing a high-frequency carrier Wave, means for modun side-bands resulting from the modulation, meansl at the transmitting endof the system for Tprothe v ducing an auxiliary audio-frequency wave ofV Y such form as substantially to compensate for the envelope distortion resulting from the suppres-V sion of the one side-band, Yand means for impressing said auxiliary Wave on. the 'low-frefluency-input circuit of said modulating means.V Y

one of the side-bands produced by the modula-i tion, means for transmitting the Wave composed of the carrier and the,V remaining side-band,

`means at the transmitting station for derivingV from a portion of the transmitted energy an auxiliary low-frequency Wave, means forV effecting adjustment 'of said auxiliary Wave, and means for combining the signal input vvave and the adjusted auxiliary Wave in theloW-frequen- Y cy application circuit of said modulator,V theV ad- Y `Vjustment of said'auxiliary Wave being such'that the combined 'low-frequency, energy. applied to the modulator Will result in substantial 'compensation for the envelope distortion introduced by the suppression of the one side-band.r

11. In a system for-theV transmission of signalsv by the modulation V.of a carrier Wave in accordance With the signal Wave, a modulator 'at theY transmitting end of the system, means'forimpressing ahigh-frequency carrier wave on said modulator, a circuit adapted to apply'loW-frequency energy to said modulator to modulate the carrier wave, means for suppressing one of thelside-bands produced by the action of 'said' modulator, means for transmitting the energy` composed of the carrier and the remainingsideband, means-at the transmitting end ofV the system for deriving from a portion of the transmitted energy an auxiliary low-frequency wave, means for effecting adjustment of the phase and amplitude of said auxiliary wave, and means for applying the adjusted.` auxiliary wave in combination With the signal Wave to the low-frequency input circuit of the modulator, said adjustment of the auxiliary wave being such as to produce substantial compensation for the wave distortion resulting from the suppression of the one side-band.

12. In a system for the transmission of signals by the modulation of a carrier wave in accordance With the signal wave, a modulator at the transmitting end of the system, means for impressing a high-frequency carrier wave on said modulator, a circuit adapted to apply lowfrequency energy to said modulator to modulate the carrier Wave, means for suppressing one of the side-bands produced by the action of said modulator, means for transmitting the energy composed of the carrier and the remaining sideband, means at the transmitting end of the system for deriving from a portion of the transmitted energy an auxiliary low-frequency Wave, means for effecting adjustment of the phase and amplitude of said auxiliary Wave, means for applying the adjusted auxiliary Wave in combination with the signal Wave to the low-frequency input circuit of the modulator, said adjustment of the auxiliary wave being such as to produce substantial compensation for the Wave distortion resulting from the suppression of the one side-band, and means at the receiving end of the system for demodulating the received Wave to produce the output signal Wave.

13. In carrier Wave signal transmission involving the suppression of one side-band, the method of preventing the distortion normally introduced by the side-band suppression which consists in employing at the transmitting end a carrier Wave and a modulating Wave and predistorting the modulating Wave to a shape such as to compensate for the envelope distortion introduced by the suppression of the one sideband.

14. In a carrier Wave signaling system, a modulator, means associated therewith for impressing thereon a high-frequency wave, a modui lating wave source, a circuit connecting said source and said modulator, means for suppressing one of the side-bands produced by the action of said modulator, and means associated with said connecting circuit for distorting the modulating Wave to a shape which will substantially compensate for the distortion of the modulated wave envelope introduced by the suppression of the side-band.

RALPH K. POTTER.

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