US2179872A - Apparatus for and method of reducing distortion in the reception of electromagnetic waves - Google Patents

Description

Nov. 14, 1939. A. \INILIBUR 2.179.872 APPARATUS FOR AND METHOD OF REDUCING DISTORTION IN THE RECEPTION OF ELECTROMAGNETIC WAVES v Filed Se t. 10, 1937 Osu/la/or /Nl/.'/V 70/? .D EINALD .MLBUR ,4 7- TOR/V5 Y Patented Nov. 14, 1939 APPARATUS FOR/AND METHOD OF REDUC- ING DISTORTION IN THE RECEPTION OF ELECTROMAGNETIC WAVES Donald AfWilbur, Troy, N. Y.

Application September 10, 1937, Serial No. 163,283

12 Claims.

My invention relatesto a radio receiving device and more particularly to means and methods, of reducing or eliminating distortion in the recepf tion of radio signals or electromagnetic waves such, for. example, as those employed for transmission purposes in radio broadcasting.

Distortion of detected radio signals results from various causes, and the principal object of my 7 invention resides in the provision of a radio signal-receiving device, including a demodulator, in which the frequencies of the carrier of a received signal, comprising a carrier component and side band components, and a local oscillator are synchronized, or, correspondingly, the frequency of a local oscillatormay be synchronized with the transformed'frequency of the carrier as derived from an oscillator, with the result that in both cases energy, from the local oscillator,

may be added to the received signal at its carrier frequency and in an amount sufficient that, upon detection, the difference frequencies resulting between components of the signal, other than those due to reactions between any component and the carrier, are effectively reduced or eliminated.

The principal object of my invention, broadly, is attained by dividing the radio signal, as received, or,'as obtained in a different part of the frequency spectrum by a reaction between the received signal and the output or energy from a local oscillator, into'two separate signals, separating the carrier from one of these separated signals, modulating this carrier by energy from a second local oscillator, separating one of the side components produced by this modulation, modulating the other of the original separated signals, by this separated side component, separating the resulting modulation components which correspond-in form to the original signal but have as their carrier a component corresponding in frequency to that of said second local oscillator, adding energy from said second local oscillator'to these components in an amount such that upon detection the difference frequencies due to reactions between side components of the signal are effectively reduced or eliminated.

With the foregoing and other objects in View, my invention includes the novel elements and the combinations and arrangements thereof described below and illustrated in the accompanying drawing in which- Fig. 1 is a wiring diagram of a circuit embodying the principles of my invention;

Fig. 2 illustrates a modification of the circuit of Fig. 1; 7 i fl QW a fur e o i qa i i Of the circuit of Fig. 1 illustrating its adaptation for use with multi-electrode tubes; and

Fig. 4 illustrates a still further modified circuit for use with a balanced detector.

For the purpose of describing my method of procedure and the operation of my circuit, it will be assumed that the carrier of the received radio wave is of the form E0 sin'wt, where fw, the carrier frequency, equals ill/21%, and that the carrier is modulated by a signal E sin pt, where f the modulating frequency, equals p/21r. The resulting modulated signal e1 may be expressed,

The term E1 sin wt will be referred to as the carrier component, and any term involving a frequency other than that of the desired carrier, regardless of its source, will be referred to as a. side band component or a side component. In this case, then, the terms E2 cos (w-p)t and E2 cos (w+p)t are side components. In practice, the signalei is the signal applie to the input, whether it be the signal as received from the station or'as applied tothe input after intermediary processes such as occur in a superheterodyne in which the entire frequency range is shifted.

Referring first to Fig. 1, a received signal, represented by the above noted modulated signal e1, is separated into two like signals by means of coils I, 2 and 3 and the respective signals are induced into the two circuits indicated generally at 4 and 5. Circuit 4 includes a selective filter 6 which is designed to pass only the carrier component to the circuit indicated generally at 1. Circuit 1 supplies the carrier component to the grid of a modulator 8. Furthermore, energy in the form of a signal of any suitable frequency is induced in coil 9 in the grid circuit of modulator 8, which energy is induced from coil ID in the circuit of a local oscillator H. The oscillator ll creates an electrical oscillation which may be substantially of any frequency though preferably one which when combined with the carrier of a received signal orthe transformed carrier of a received signal produces components differing appreciably in frequency from that-of the carrier of the received signal or, its transformed carrier frequency. The plate circuit of modulator 8 preferably comprises a resonant circuit l2 which includes a coil l3 and it will be understood that circuit I2 is designed to be tuned to separate out a predetermined component of the signal from modulator 8 which separated component, by means of coils l3 and I4, is imparted through circuit l and coils l6 and I! to circuit 5. A signal, therefore, of the form of the received signal and the separated component from circuit I2 is supplied to the grid of modulator l8 of circuit 5. The plate circuit 19 of modulator l8 comprises a coil 20 which induces the signal in coil 2| of the resonant circuit 22. The local oscillator II is also connected to a coil 23 which serves to induce energy from the oscillator to coil 24 in the grid circuit of detector 25. The resultant signal is applied to the grid of detector 25 Where the signal is demodulated and supplied to the output, illustrated for example as a transformer In the foregoing, I have described the circuit embodied in one form of my invention for eliminating from the detected signal diiference frequencies resulting between components ,of the signal other than those due to reactions between any component and the carrier and in the following I will mathematically illustrate the operation of said circuit. 7 1

- For purposes of illustration, we-shall use square law detection and modulation and consider the distortion or detection terms only, although it is to be understood that my invention is in no sense limited to detectors or modulators, which operate according to this principle.

Referring again'to Fig. 1,'1et the received or i modulated signal 61 be supplied by means of coil I to circuits 4. and 5. The signal in circuit i is applied to the selective filter t, which is designed to pass only the carrier frequency to circuit I which carrier is then applied to the grid of modulator 8. Also energy from. the local oscillator at a frequency j where f =q/21r, is applied through coil ill to coil 9 and thence to the grid of modulator 8.

Considering the action of this modulator We have for the modulation'components in terms of currenti1=co(E1 sin wt-i-Ea sin qt) and omitting other than radio frequency terms we have 2'1=c1{E1 E3 cos (wq) t-E1Es cos (w+q-)t+1st and 2nd harmonic terms of both frequencies} By making f differ appreciably from fw these terms may be separated by the usual resonant circuit. Hence, resonant circuit 12 may be used. to separate out individually any of these 'com ponents and for purposes of illustration let it be resonant to and separate out the term I CIEIES cos (wq) t Considering the action'of this modulator we have for the modulation components in terms of current iz=c3{E2 cos (ui-p)t+Ersin wt- 1 7 E2 cos (w-i-p) t+E4 cos (w-q) t} and q may be made to differ from w by any desired amount. Let us consider now, the modulation components due to reactions between E4 cos (wq)t and the remainder of the signal. These will be The terms containing q and (210-11) respectively, as carriers, may be separated as separate groups by means of resonant circuits since (1 and w and hence q and (2 wq) may be made to differby any desired amount in frequency.

Therefore, we have induced in circuit 22, resonantto the terms with q as a carrier, by means and it is evident that this signal is of the same form as the original signal but that the frequency of the carrier is now J where f equals q/21r, or, is the same as that of the local oscillator and that this will always be true even though the local oscillator tends to drift in frequency since this would only mean that the signal would drift exactly in synchronism with it. Thus, since the carrier component of the signal we now wish to demodulate is of the same frequency as that of the local oscillator, we may induce in coil 24 by means of coil 23 energy from the local oscillator at the carrier component frequency and in any amount desired.

The signal applied to the grid of detector 25 in circuit 22, through the medium of coils 2| and 23, is then of the form I where E9 sin qt is the component added from. the

. local. oscillator and mayibeiof any desired magnitude and phase... This signal is then detected.

by detector 25 in the usual manner and the demodulation components appear in the output of transformer 26.

Now since in the usual detector the distortion due to reactions between the side components depends upon the amplitude of the side components and is independent of the amplitude of the carrier and since'the desired signal depends upon the amplitude of the side components and the amplitude of the carrier we may, therefore, by this addition of carrier reduce the distortion due ,to reactions between side components.

In Fig. 2, I have illustrated. a modified circuit wherein the received signalis combined with a component from a local oscillator, then applied to the grid of a demodulator, and thereafter appliedto a selective filter. In their construction, the circuits of Figs. 1 and 2 are essentially alike except for the rearrangement of the modulator and filter circuits to accomplish the above. The

circuit 21 of Fig. 2 comprises coil 28, in which is induced the signal from input coil'l, modulator 29 and coil 30, in which is induced a component from the local oscillator H through coil IU of the oscillator circuit. From coil 31 of the plate circuit 32 of modulator 29, the modulated signal E7 cos (q-i 'p) t+E9 sin q is induced'in coil 33 of a resonant circuit 34. Circuit 34 also includes a selective filter 6 whereby to circuit 35 is applied only the carrier component.

above described; hence, further description thereof is deemed unnecessary.

A mathematical illustration of the operation of Fig. 2 is as follows. Let the signal applied to the input be of the form where the terms have the same significance as before. This modulated signal 61 is then applied by means of coil to circuits 2'! and 5 and thence to the grids of modulators 29 and I8. There is also applied to the grid of modulator 29 a component from the local oscillator by means of coils I0 and 39. We thus have as the signal applied to the grid of modulator 29 By the process of modulation we have in coil 3|, in terms of current, the following components Since these two groups correspond to the original modulated signal but with carriers of frequency (w-q) and (w-I-q) respectively they may be separated since q may be made to differ from w by any desired amount. For purposes of illustration let the circuit 34 be resonant to those with (wq) as a carrier. Thus we have applied to selective filter 6 only those terms with the component I1 cos (wq)t as a carrier. Let selective filter 6 be resonant to and pass only the carrier component. We have thus in circuit 35 only the component I1 cos (w'q)t. This is then applied by means of coil I4, circuit l5 and coils l6 and H to circuit 3 and thence to the grid of modulator I8. Thus we have applied to the grid of modulator i8 the original signal plus a component E4 cos (w-q) t. The voltage applied to the grid of modulator I8 may then be represented by a signal of the form Considering the action of this modulator we have for the modulation components in terms of current E2 cos (w i-p) t-l-E4 cos (w-q) t} and it is well to note again that q may be made to differ from w by any desired amount.

Let us consider now the modulation components due to reactions between E4 cos (wq)t and the remainder of the signal. These will be The terms containing q and (Zw-q) respectively, as carriers, may be separated as distinct groups by means of resonant circuits since q and w and hence q and 2w-q may be made to differ by any desired amount in frequency.

Therefore, we have induced in circuit 22, resonant to the terms with q as a carrier, by means of coils 2t and 2| only the terms C4{E2E'4 cos (qp) t+E1E4 sin qt E2E4 cos (q-l-p) t} and it is evident that this signal is of the same form as the original signal but that the frequency of the carrier is noW fq, where f equals (1/211',

or, is the same as that of the'local oscillator. This signal relationship will always be true.

Thus since the carrier component of the signal we wish to demodulate is of the same frequency as .that of the local oscillator we may induce in coil 24 by means of coil 23 energy from the localoscillator and at the carrier component frequency and in any amount desired.

The signal applied to the grid of detector 25 by means of circuit 22 andcoil 24 is then of the form Fig. 3 discloses a circuit which functions in substantially like manner to the circuit shown in l but in the embodiment of my invention .disclosed in Fig, 3 the various signals are applied to separate electrodes in multi-eiectrode tubes. The action however is the same and the results are the same as for the circuit shown in Fig. 1

as should be evident 'to those skilled in the art.

For example, the carrier component of the separated, received signal in circuit 5 is supplied from filter 6 to circuit 7 and thence to one grid 36 of modulator 31. A component from the local oscillator H is induced through coils l5 and 38 and applied to the other grid 39 of modulator 31. Components of the output of modulator 31 are separated through a resonant circuit associated with the plate of said modulator and desired components supplied to the grid 30 of modulator 4| by means of coils 42 and 43. Circuit 5, of course, supplies theother of the separated, received signals to the other grid i lof modulator 4!. The output of modulator 4| is supplied to grid 45 of detector 46 and a component from the local oscillator is supplied through coils 23 and ll to the other grid 48 of detector 46, the output of detector 46 being fed to transformer 26. The similarity of the circuits of Figs. 1 and 3 should be apparent from a comparison thereof and, as above stated, their operations are identicaL' Fig, 4 illustrates a circuit which is substantially the same as the circuit shown in Fig. 1 except for the final detector. In this case a balanced detector is used. The signal corresponding in form to the original signal but having as its car, rier a component having the same frequency as that of the local oscillator is induced by means of coil 20 into circuit 49 and thence applied to the detectors-59 and 5|. Energy from the local oscillator is induced by means of coil 23 into coil 52 and thence is applied to detectors 58 and 5|. The detectors operate in the usual manner and the demodulation components thus appear in transformer 53. However, due to the arrangement of the plate circuits of detectors 5?! and 5|, only those demodulation components corresponding to reactions between the side components of the signal in circuit 49 and the carrier components in coil 52 appear in the output of transformer 53.

By my method of reception, distortion, due to a lowering of the level of the carrier anda consequent relative'incre'ase in amplitude of the 'difference frequency'terms due to reactions between ,side band components, as compared with reactions between. carrier and side band components, cannot occur due to the fact that the carrier may be maintained at any desired level by means of its introduction from the local oscillator. Also that distortion resulting from a change in the envelope of the signal produced by a decrease in the amplitude of the carrier regardless of its cause cannot occur since the original shape of the envelope may be recreated by the addition of carrier from the local oscillator. Distortion of the type described above is ordinarily noticeable when interference occurs between the ground and sky waves of, a station or when interference occurs between the signals from synchronized stations. It should be noted that distortion due to the above noted causes occurs regardless of the type of detector used and, hence, by employing the method herein described, such distortion will be eliminated or, in any event, considerably reduced regardless of the type of detector used.

Another desirable feature of my invention lies in the fact that synchronization between the signal applied to .the final detector and the local oscillator is independent of any reaction of the incoming signal upon the local oscillator and that it will always take place independent of the amplitude of the incoming signal or of frequency changes of the local oscillator.

. Still another desirable feature of my invention resides in the fact that extremely sharp tuning may be obtained since there is no modulated signal applied to the final detector and,

therefore, no output from the detector except when the carrier istuned to the resonant frequency of the filter. Hence, my invention will provide automatic, inter-station noise suppression with no loss of sensitivity.

It is also well to note that the resonant and selective circuits may be fixed in their tuning since in the superheterodyne type of receiver the incoming signal may be shifted to any desired frequency and hence may have its carrier tuned to the selective filter mentioned in the above descriptions.

While the condensers illustrated in the draw-' ings are shown as fixed condensers, it is to be understood that,,in the first instance, in order to secure proper tuning, these condensers may be adjustable. But once the circuits are properly tuned the condensers may be permanently set and it will not be necessary thereafter to adjust them. I

Furthermore, it is to be understood that the term oscillator as used in the foregoing description and appended claims is intended to designate any meansfor producing electrical oscillations.

While I have described my invention in its preferred embodiment, it is to be understood that the Words which I have used are words of description rather than of limitation. Hence, changes within the purview of the appended claims may be made Without departing from the true scope and spirit of my invention in its broader aspects.

. What I claim is: I

1. In a device for receiving modulated radio signals comprising carrier and side band components, the combination with an oscillator for creating electrical energy, of means for creating a signal component by a reaction between the carrier component .of a received signal and said created electrical energy, means for modulating the signal, as received, by said created "signal component, means for separating'fr'om the received signal, so modulated, components thereof corresponding inform to the received signal but having a carrier component of a frequency substantially equal to that of said created energy andrneans for adding to said last mentioned signal components energy from said oscillator.

2. In a device for receiving modulated radiosignals' comprising carrier and side band components, means for producing from the received signal first and second separate signals having corresponding carrier and side band components, an oscillator for creating electrical energy, means for transforming the first of said separated sigrials into a signal having a component produced by'areaction between the carrier component of said separated. signal and said created electrical modulated radio signals comprising carrier and side band components, means for producing from the received signal two separate signals having corresponding carrier and side components, means for modulating the carrier component only of the first of saidwtwo signals,'means for separating the side band components of themodulated carrier of said first'signal, means for modulating the sec ondro-f said two signals by one of said separated side band components, means for separating from said second signal, so modulated, components thereof corresponding in .form to the received signal, means for adding to said last mentioned components a signal component derived from said first mentioned modulating 'means and means for demodulating the resulting signal.

4. In a device for receiving modulated radio signals comprising carrierl andside band com.- ponents,'means' for producing from the received signal two separate'signals having corresponding carrier and side components, means for separating the carrier component from one of said two separate signals, an oscillator for creating electrical energy, means for modulating said separated carrier component by said created electrical energy, means for separating individually the side band components produced in said modulation, means for modulating the other of said separated signals by one of said separated side band components whereby a signal is produced having side band components corresponding in form to the original, received signal but having as their carrier a component having a frequency substantially equal to that of said created electrical energy', means for separating, from said last mentioned signal, components corresponding in form to the original, received signal but having at their carrier component a component having a frequen cy substantially equal to that of said created electrical energy, means for adding to said last mentioned, separated side components a component from said created electrical energy, and means for demodulating the signal so produced.

5. In a device for receiving modulated radio signals comprising carrier and side band components, means for producing from the received signal two separate signals having corresponding carrier an dside components, means for separating the carrier'componen't from one of said two separate signals, an oscillator for creating electrical energy, means for modulating said separated carrier component by said created electrical energy, means for separating the difference frequency component of said separated and mod ulated carrier, means for modulating the other of said separated signals by said difference frequency component, means for separating the side band components from said last modulated signal that correspond in form to the original, received signal but have as their carrier component a component having a frequency substantially. equal to that of said created electrical energy, means for adding to said separated side band components a component from said created band components a component from said created electrical energy, and means for demodulating the signal so produced.

6. In a device for receiving modulated radio signals comprising carrier and side band components, means for producing from the received signal two separate signals having corresponding carrier and side components, means for separating the carrier component from one of said two separate signals, an oscillator for creating electrical energy, means for modulating said separated carrier component by said created electrical energy, means for separating the summation frequency component of said separated and modulated carrier, means for modulating the other of said separated signals by said separated summation frequency component, means for separating the side band components from saidlast modulated signal that correspond in form tothe original signal but have as their carrier component a component having a frequency substantially equal to that of said created electrical energy, means for adding to said separated side band components a component from said created electrical energy, and means for demodulating the signal so produced.

7. In a device for receiving modulated radio signals comprising carrier and side band components, means for producing from the received signal two separate signals having corresponding carrier and side components, an oscillator for creating electrical energy, means for modulating one of said separated signals by said created electrical energy, means for separating a component produced by a reaction between the carrier component of said separated signal and said created electrical energy, means for modulating the other of said separated signals by said separated component, means for separating from said last modulated signal components corresponding in form to the original, received signal but having as their carrier component a component having a frequency substantially equal to that of said created electrical energy, means for adding to said last mentioned components a component from said created electrical energy, and means for demodulating the signal so produced.

8. In a device for receiving modulated radio signals comprising carrier and side band components, means for producing from the received signal two separate signals having corresponding carrier and side components, an oscillator for creating electrical energy, means for modulating one of said separated signals by said created electrical energy, means for separating the difference frequency component of said modulated, separated signal whereby a side band component is separated havinga frequency substantially equal to the difference between the frequency of the carrier of said separated signal and the frequency of the said created electrical energy, meansfor modulating the other of said separated signals by said separated side band component whereby side components are produced corresponding inv form to the original, received signal but having as their carrier a component having a frequency substantially equal to that of said created electrical energy, means for separating from said last modulate-d signal said last mentioned side components, means for adding to said separated, last mentioned side components a component from said created'electrical energy, and means for demodulating the signal so produced.

9. In a device for receiving modulated radio signals comprising carrier and side band components, means for producing from the received signal two separate signals having corresponding carrier and side components, an oscillator for creating electrical energy, means for modulating one of said separated signals by said created electrical energy, means for separating the summation frequency component of said modulated, separated signal whereby a side band component is separated having a frequency substantially equal to the sum of the frequency of the carrier of said separated, received signal and the frequency of said created electrical energy, means for modulating the other of said separated, received signals by said separated side band component, means for separating from said last modulated signal components corresponding in form to the original, received signal but having as their carrier component a component having a frequency substantially equal to that of said created electrical energy, means for adding to said separated, last mentioned components a component from said created electrical energy and means for demodulating the signal so produced.

10. In a device for receiving modulated radio signals comprising carrier and side band components, an electric circuit including means for producing from the received signal two separate signals of like characteristics, an oscillator, means for creating a signal having a side band component having a frequency dependent upon the frequencies of the carrier of said received signal and said oscillator, means for modulating one of said two separatedsignals with said created side band component whereby a signal component is produced corresponding in form to said received signal but having a carrier of the frequency of said oscillator, means for adding a component derived from said oscillator to said last mentioned signal component, and means for demodulating the resultant signal.

11. Those steps in the method of reducing distortion in the reception of modulated radio signals comprising carrier and side band components which comprise dividing the received signal into two separate signals, creating oscillating electrical energy, combining one of said separated signals and said electrical energy and separating a side band component from the signal so formed, modulating the other of said separated, received signals by said separated, side band component, separating from said last modutrical energy, producing a signal component by efiecting a reaction between the carrier compo nent of a received signal and said created electrical energy, modulating the signal, as received, by said signal component wherebysignal components are produced corresponding in form to the received signal but having a carrier component of a frequency substantially equal to that of said created electrical energy, separating said last mentioned signal components and adding 10 thereto energy from said created electrical energy. I a I DONALD A. WILBUR.

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