US2187978A - Modulated-carrier signal-translating system - Google Patents

Description

Patented Jan. 23, 1940 PATENT. OFFICE MODULAT'ED-CARRIER SIGNAL-TRANSLAT- ING SYSTEM Harold M. Lewis, Great Neck, Y., assignor to- Hazeltine Corporation, a corporation of Delaware Application January 10, 1939, Serial No. 250,130

11 Claims.

This invention relates to modulated-carrier signal-translating systems and is particularly concerned with systems of the single-sideband type utilizing a carrier wave and only one sideband of modulation frequencies or a carrier wave and a band of modulation frequencies excluding at least a major part of one of the sidebands of modulation frequencies.

In accordance with conventional practice in modulated-carrier signal-translating systems, at the transmitter there is developed a high-frequency carrier wave and a lower-frequency signal wave representing the information to be communicated. The signal wave is impressed as modulation on the carrier wave and there is thereby developed a modulated-carrier wave comprising the carrier-frequency component and upper and lower sideband modulation-frequency components having frequencies equal, respectively, to the sums of the carrier frequency and each signal frequency, and to the difierences between the carrier frequency and each signal frequency. While the carrier-frequency component and each modulation sideband alone include components representative of all the information being transmitted, at the receiver, the carrier wave and both sidebands are generally detected to derive the originalmodulation signal for reproduction.

For various reasons it is frequently desirable to transmit the carrier wave and only one of the modulation sidebands. In television systems, for

example, where the modulation-signal frequencies extend over an exceedingly wide range,'of the order of four megacycles, the modulated carrier requires a relatively large portion of the limited part of the frequency spectrum 'which is available. For the purpose of economy with respect to the available part of the frequency spectrum, therefore, it has been proposed to develop and transmit single-sideband signals.

While in some systems single-sideband signals have been transmitted, received, and. reproduced with satisfactory results, it is well known that such systems are both theoretically and practically imperfect. A perfect replica of the modulation signal cannot be derived from a singlesideband and carrier-wave under all conditions. This may be explained as follows: In detecting the usual modulated-carrier wave with its two modulation sidebands, the detector derives from the signal input thereto beat notes between the carrier wave and each of the sideband-frequency components. Corresponding upper and lower sideband-frequency components produce with the carrier wave the same beat notes which represent the modulation signal and they are cumulative in the output circuit ofthe detector. The detector also produces beat notes between each sideband-frequency component and all others which do not-represent the modulation signal but constitute undesirable distortion components. Where-both sidebands are present, however, the undesired beat notes due-toone sideband cancel out those due to the other sideband. Where, however, the signal input to the detector comprises the carrier wave and only one modulation sideband, these undesired beat notes donot cancel but are present in the signal output and tend to destroy the fidelity of reproduction of the desired modulation components.

One manner of minimizing such undesirable results in single-sideband'systems has been to develop and transmit a signal having a carrier wave of a substantially greater amplitude than any of the sideband-frequency components. Consequently, in such systems, the beat notes between the sideband-frequency components themselves are of substantially lesser amplitudes than the beat notes between the sideband-frequency components and the carrier wave, and the undesirable effects are reduced.

In certain single-sideband systems, however, it is desirable that certain sideband-frequency components be of such amplitude relative, to that of the carrier wave that the beat notes therebetween are of appreciable Values relative to the useful beat notes, rendering detection thereof highly unsatisfactory. For example, in television systems, where there is a sharp contrast between adjacent incremental" areas of the image being transmitted, the boundary between these areas is represented in the signal by high-frequency modulation components comprising a-relatively great percentage of modulation so that the beat notes between such sideband-frequency components are sufficiently great to impair the reproduction of a single-sideband signal under certain conditions.

-It may be stated generally, therefore, that only where the signal impressed upon the detector comprises the carrier wave and both its modulation sidebands can the exact replica of the original modulation signal be derived under all conditions. For thisreason, it has been conventional practice to utilizedouble sideband; signals except where special considerations render expedient the compromise of utilizing single-sideband signals.

It is an object of the present invention to provide a modulated-carrier signal-translating system whereby there may be derived and utilized, from a signal comprising a carrier wave and a modulation-frequency band excluding at least a major portion of one of the sidebands of modulation, the excluded modulation-frequency components. I

It is another object of the invention to provide a modulated-carrier signal-translating system whereby there may be derived from an available single-sideband signal the modulation-frequency components corresponding to the sideband components which are missing from the available signal.

It is a further object of the invention to provide a modulated-carrier signal-translating system whereby benefits incident to both conventional single-sideband transmission and doublesideband reception may be'obtained.

In accordance with the present invention, there is provided a modulated-carrier signaltranslating system comprising an input circuit adapted to have a signal impressed thereon comprising a carrier wave and a modulation-frequency band excluding at least a major portion of one of the modulation-frequency sidebands. Non-linear signal translating means are coupled to the input circuit for distorting the impressed signal to derive therefrom signal-frequency components corresponding to the excluded portion of said sideband. An output circuit is coupled to the signal-translating means for utilizing the derived modulation-frequency components.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing, Fig. 1 is a schematic diagram of a complete superheterodyne radio receiver including a signaltranslating system embodying the present invention, while Fig. 2 is a circuit diagram of a modified form of the signal-translating system.

Referring now more particularly to Fig. 1, the system there illustrated comprises a receiver of the superheterodyne type including an antenna I0, I I connected to a radio-frequency amplifier I2 to which there is connected in cascade, in the order named, an oscillator-modulator I3, an intermediate-frequency amplifier I4, a signaltranslating system indicated generally at I5 and embodying the present invention, a detector I6, a modulation-signal amplifier I1 and a signalreproducing device I8. Where the receiver of Fig. 1 comprises a television receiver, the signalreproducing device may include scanning-wave generators, and a cathode-ray reproducing tube. Referring briefly to the general operation of the system just described, a signal intercepted by the antenna I0, I l is selected and amplified in the radio-frequency amplifier l2 and supplied to the oscillator-modulator I3, wherein it is converted into an intermediate-frequency signal which, in turn, is selectively amplified in intermediate-frequency amplifier I4 and delivered to the intermediate-frequency signal-translating system I5. The system I5 operates in accordance with the present invention, as will be presently fully explained, to deliver an intermediatefrequency signal to the detector [6 wherein the modulation signal is derived and from which it is supplied to the modulation-signal amplifier I I which amplifies this signal and supplies it to the signal-reproducing device l8 for reproduction. It is to be noted that the receiver of Fig. 1 is adapted for the reception of a single-sideband signal, comprising a carrier wave and a modulation-frequency band excluding at least a major portion of one of the modulation-frequency sidebands. The intermediate-frequency amplifier I4, therefore, isdesigned to pass the intermediatefrequency components corresponding to the received signal and developed therefrom by the oscillator-modulator l3.

Referring now more particularly to the por tion of the receiver of Fig. 1 which embodies the present invention, the intermediate-frequency signal-translating system I5 comprises an input v circuit I9, a non-linear signal-translating device 25 and an output circuit 2i. The input circuit is preferably a band-pass selector comprising a double-tuned transformer 22 having its primary circuit coupled to the output circuit of the intermediate-frequency amplifier I4 as shown. The response characteristic of the input circuit or selector I9 is indicated by curve R1 shown above the selector in Fig. 1, the mean frequency of the selector being indicated at o and the location of the carrier being indicated at 0. While any suitable elements may be utilized for the nonlinear signal-translating device 20, such as properly biased multi-grid tubes, in the arrangement shown a pair of diodes 23 and 24 are connected in parallel in opposite senses with suitable biasing batteries 25 and 26, respectively, included in their cathode circuits. The device 20 interconnects the high-potential terminals of the input circuit I9 and the output circuit 2| which have a common low potential terminal. The response or repeating ratio characteristic of the device 20 is illustrated by curve R2 shown above the de: vice in Fig. 1. More particularly, the upper and lower halves 1'1 and T2 of curve R2 represent the characteristics of the biased diodes 23 and 24, respectively, taken individually. For the purpose of clarity the curves 1'1 and r2 have been shown Withthe same zero base line but extending in opposite senses, since the diodes are connected in opposite senses and pass half-cycles of the signal of opposite polarities. It will be apparent that the device 20 has a non-linear response characteristic such that the signal translated thereby is distorted in a predetermined manner, as presently will be more fully explained.

The output circuit of the signal-translating system is preferably a band-pass selector of the dead-end filter type designed to pass a Wide band of frequencies. This circuit may comprise two double-tuned transformers 21 and 28 and a terminating resistor 29 interposed therebetween. The detector I6 is connected across the secondary circuit of transformer 21. The response characteristic of the output circuit or selector 2| is illustrated by curve R3 shown above the selector in Fig. 1 from which it is seen that the mean frequency of the pass band of the selector is the same as the frequency of intermediate-carrier wave and the selector is uniformly responsive over both the modulation sidebands.

In the operation of the signal-translating system l5, a single-sideband signal is impressed upon the input circuit [9 and is passed thereby with substantially uniform response over the single modulation-frequency sideband, as illustrated by curve R1. ,by the device 20 with a non-linear signal-repeating ratio, as illustrated by curve R2, so as to effect a predetermined distortion of the wave form of the signal. By virtue of the non-linear characteristic of the device I5 it not only passes the impressed orinput-signal frequency components but also it further functions to derive fromthe input signal the excluded or missing sidebandfrequency components. The complete signal including the carrier wave and both the modulation-frequency sidebands are thus translated to the output circuit 2| and this circuit as just explained having a band-pass characteristic illustrated by curve R3 passes all the frequency come The signal is then translated v E1 cos mt -(1) where w1=21rf1 and the modulation-frequency wave is represented .by

W .Ez cos wzt (2) where r w2=21rf2 and the modulated-carrier wave is represented by the formula:

em= E1E1+Ez cos was] cos at which when expanded gives:

If the lower-frequency sideband is suppressed,

the equation of the resultant carrier with only the upper-frequency sideband is:

v e=E COS w t+E /2 COS (w +w2)f I (4) Now the non-linear device 20 has an input voltage-output current characteristic which is represented by the general equation:

rt- 11 a e a e a e a e etc. (5)

It can be shown that only the odd power terms contribute to reproducing the sideband-frequency component missing from Equation 4. The characteristic of the non-linear device 28 of the present invention is of a form such that the even power terms cancel and its general equation (which also defines curve R2 of Fig. 1 of the drawings) is:

' The term the represents the linearly repeated component of the applied signal while each of the other terms contribute to the production of the missing sideband-frequency components. For

' simplicity it will be shown that this is true for the termase by rewriting Equation 4 as follows:

. a a e =a [E cos t+%*cos (w +wg)t] (7) This is of the form v +.V) Y+ Y +.V

and it can be shown that only the second term is-of interest as representing frequencies lower than thecarrier. Hence A Since E1 is constant, the last term may be written in the form:

KE .cos (w w )t (10) which is the third term of Equation 3 representing the suppressed sideband component.

nected in cascade, as shown. I may comprise a double-tuned transformer 33 preferably having its secondary circuit shunted shownthat, incase the upper or opposite sideband is suppressed and only the lower sideband is transmitted, the-upper sideband may be derived by the apparatus-of the present invention. Thus,

in accordance with the system of the present in-' vention, when a signal isimpressed thereon comprising a carrier wave and a modulation- In a manner similar to the above, it can be frequency sideband excluding one, or atleast a" .major portion of one, of the modulationfrequency sidebands, the excluded sidebandfreqenc'y components are derived from thissignali and reinserted in the output signal.

It can be shown "that the device 29 develops from the input signal, in addition to the desired 1 suppressed modulation sideband, modulation.

products corresponding to the higher order terms of its characteristic equation which constitute carrier waves of higher frequencies together with both their respective modulation-frequency sidebands.

their modulation sidebands. This may be readily accomplished by simply adjusting the output circuit 2| to be uniformly responsive over the desired. higher band of frequencies. Moreover, while the particular device 20 is so arranged that its characteristic equation includes only odd power terms, it will be appreciated that it is within the scope of the present invention to so design the devicev 2.0 that its characteristic equation includes only the even power terms or includes both the evenand odd powerterms, and

to design the output circuit so that it is responsive to the modulated-carrier waves derived under these conditions, which waves include the carrier wave and its sideband frequency components at various harmonics of the original signal-input frequencies.

In Fig. 2 thereis. illustrated a signal-translat-.

ing system a which is a modified form of the system I5 of Fig. 1 and maybe substitutedthere for in the receiver of Fig. 1. The'system I511, comprises an input circuit lga, a' signal-translating section Mia, and an output circuit 2Ia..' The input circuit comprises adouble-tuned'transformer '38 having its primary circuit adapted to be coupled to the outputcircuit of the intermediate- .'frequency amplifier l4. 1

The section a comprisestwo separate signal,- translating channels. One channel includes a In certain cases it may be desired to utilize the higher-frequency carrier waves and vacuum-tube amplifier 3!, of the triode or other suitable type, and a band-pass selector 3[2 conby a damping resistor "33b and so adjustedas to have a band-pass characteristic as illustrated by,

curve R4, its mean frequency'being located at substantially the centerfrequency cf thesingle sideband of frequencies of the input signal as indicated at o, and the carrier frequency being located near one edge of the pass band.

The other channel of the section Zita comprises a vacuum-tube amplifier 31a and selector -3-2a which may be substantially similar to the amplifier 3| and selector 32, respectively. 'Here, however, an intermediate tap on the secondary circuit of .the double-tuned transformer 33a is grounded and its opposite terminals are 'connected, by way of non-linear signal-translating devices, for example, diodes 36 and 35 with suitable biasing batteries 35 and'3lin series therewith, to the opposite terminalsof the primary circuit of a double-tuned transformer 38: The

The selector 32 latter transformer constitutes a band-pass selector 39 similar to the selectors 32 and 32a. An intermediate tap of the primary Winding of the transformer 38 is grounded. The diodes 34 and i 35 function similarly to the diodes 23 and 24 of Fig. ,1, their general repeating ,ratio characteristic taken together being the same as that illustrated by curve R2 of Fig. 1. Being connected to opposite terminals of the selectors 32a and 33, the diodes 33, 33 are connected in the same sense.. Suitable adjustment of the batteries 36 and 3'! serves effectively to shift the curves T1 and r2 laterally in opposite senses so that the desired over-all characteristic most suitable for developing the missing sideband-frequency components from the input signal may be obtained. The selector 39 has a band-pass characteristic illustrated by ,curveRs similar to that of the selector 32, but having a mean frequency substantially corresponding to the mean frequency of the missing sideband. derived by the nonlinear devices 34 and 35 so that it favors these derived frequencies. Also, the carrier frequency is located near the opposite edge of the pass band from that of selector 32.

The output circuit, Zia may comprise a pair of combining amplifiers 39 and 33 having their input electrodes coupled across the secondary circuits of the transformers 33 and 38 by way of biasing batteries 4| andv 42, respectively. A

common output circuit comprising a resistor 33 is provided for the tubes 33 and 43, across which the input circuit of the detector .15 may be connected. This output circuit is adapted to translate the signal and both its modulation-frequency sidebands. The biasing batteries 4!, 42 are adjusted so that the relative gains of the amplifiers 39 and All compensate for any difference in-the amplitudes of the signals as trans- .lated by the two channels.

The operation of the system IE4: is believed obvious from the above description. Briefly, the

single-sideband signal is supplied to the inputcircuit 19a and the channel including the tube ,3I andselector 32 amplifies and translates the carrier .wave and its sideband-frequency components to the output circuit 21a. In the other channel, however, while the input signal is also amplified and translated by the tube 3m and selector 32a, the diodes 34 and 35 operate with a repeating ratio characteristic equivalent to that illustrated by curve R2 of Fig. 1, that is, with a non-linear repeating ratio such as to distort the input signal and derive therefrom the missing side-band-frequency components. These derived frequency components are selectively translated by the selector 39 to the output circuit and the latter operates to combine them with the carrier wave and other sideband-frequency components translated by the channel 3|, 32. There is thus developed in the output circuit the complete signal including the carrier wave and both modulation-frequency sidebands, which signal may thereafter be delivered to the detector for demodulation.

While there have been described what are at present considered the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from this invention, and, therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What is claimed is:

1. A signal-translating system comprising, an input circuit adapted to have a signal impressed thereon comprising a carrier wave and a modula tion-frequency band excluding at least a major portion of one of the modulation sidebands, non-" linear signal-translating means coupled to said input circuit for distorting said impressed signal to derive therefrom signal components corresponding to said excluded portion of said sidee band, and an output circuit coupled to said sig-.

nal-translating means for utilizing said derived signal components.

2. A signal-translating system comprising, an input circuit adapted to have a signal impressed thereon comprising a carrier wave and a modulation-frequency band excluding at least a major portion of one of the modulation sidebands, nonlinear signal-translating means coupled to said input circuit for distorting said impressed signal to derive therefrom said excluded portion of said sideband, and an output circuit coupled to said signal translating means for translating said carrier wave and both its sidebands of modulation frequencies including said derived excluded portion of said other sideband.

3. A signal-translating system comprising, an input circuit adapted to have a signal impressed thereon comprising a carrier wave and a modulation-frequency band excluding at least a major portion of one of the modulation sidebands, nonlinear signal-translating means coupled to said input circuit and having a non-linear signalrepeating ratio for translating said impressed signal with a predetermined distortion to derive therefrom said excluded portion of said sideband, and an output circuit coupled to said signaltranslating means for utilizing said derived signal components. 7

4. A signal translating system comprising, an input circuit adapted to have a signal impressed thereon comprising a carrier Wave and a modulation-frequency band excluding at least a major portion of one of the modulation sidebands, nonlinear signal-translating means coupled to said input circuit and having a non-linear signalrepeating ratio for translating said impressed signal with a predetermined distortion to derive therefrom said excluded portion of said sideband, and an output circuit coupled to said signaltranslating means for translating said signalcarrier wave and both its modulation sidebands including said derived excluded portion of said other sideband.

5. A signal-translating system comprising, an input circuit adapted to have a signal impressed thereon comprising a carrier wave and a modulation-frequency band excluding at least a major portion of one of the modulation sidebands, said input circuit including a band-pass selector adapted to pass said signal with substantially uniform attenuation over said modulation band, non-linear signal-translating means coupled to said input circuit for distorting said impressed signal to derive therefrom said excluded portion of said sideband, and an output circuit coupled to said signal-translating means for translating said carrier Wave and both its modulation sidebands including said derived excluded portion of said other sideband and including a band-pass selector having a substantially uniform response characteristic over said sidebands.

6. A signal-translating system comprising, an input circuit adapted to have a signal impressed thereon comprising a carrier wave and a modulation-frequency band excluding at least a major portion of one of the modulation sidebands, said input circuit including a band-pass selector having a mean frequency at substantially the mean frequency of said band and adapted to pass said signal with uniform attenuation over said modulation band, non-linear signal-translating means coupled to said input circuit for distorting said impressed signal to derive therefrom said excluded portion of said sideband, and an output circuit coupled to said signal-translating means for translating said carrier wave and both its modulation sidebands and including a band-pass selector having a mean frequency equal to the carrier-frequency and a substantially uniformly.

responsive characteristic over said sidebands.

7. A signal-translating system comprising, an input circuit adapted to have a signal impressed thereon comprising a carrier wave and a modulation-frequency band excluding at least a major portion of one of the modulation sidebands, nonlinear signal-translating means including a pair of diodes coupled in opposite senses to said input circuit for non-linearly translating said impressed signal to effect a predetermined distortion thereof to derive therefrom said excluded portion of said sideband, and an output circuit coupled to said signal-translating means for utilizing said derived signal components.

8. A signal-translating system comprising, an input circuit adapted to have a signal impressed thereon comprising a carrier wave and a modulation-frequency band excluding at least a major portion of one of the modulation sidebands, signal-translating means, including a pair of diodes coupled in opposite senses to said input circuit, and means providing predetermined biases for said diodes, for non-linearly translating said impressed signal to effect a predetermined distortion thereof to derive therefrom said excluded portion of said sideband, and an output circuit coupled to said signal-translating means for utilizing said derived signal components. I

9. A signal-translating system comprising, an input circuit adapted to have a signal impressed thereon comprising a carrier wave and a modulation-frequency band excluding at least a major portion of one of the modulation sidebands, a first channel for translating said signal, a second channel including signal-translating means coupled to said input circuit for distorting said impressed signal to derive therefrom said excluded portion of said sideband, and a common output circuit for said channels for translating said carrier wave and both its modulation sidebands including said derived excluded portion of said other sideband. I

10. A signal-translating system comprising, an input circuit adapted to have a signal impressed thereon comprising a carrier wave and a modulation-frequency band excluding at least a major portion of one of the modulation sidebands, a

first channel for translating said signal, a second channel including signal-translating means coupled to said input circuit for distorting said impressed signal to derive therefrom said excluded portion of said sideband, a common output circuit for said channels for'translating said. carrierv Wave and both its modulation-frequency sidebands including said derived excluded portion of said other sideband, and means for adjusting the gain of one of said channels to adjust the relative amplitudes of said directly translated and said derived signals.

11. A signal-translating system comprising, an input circuit adapted to have a signal impressed thereon comprising a carrier Wave and a modulation-frequency band excluding at least a major portion of one of the modulation sidebands, a first channel for translating said signal including a band-pass selector adapted to pass said signal with uniform response over said modulation-frequency band with said carrier Wave at one edge of its pass band, a second channel including signal-translating means coupled to said input circuit for distorting said impressed signal to derive therefrom said excluded portion of said sideband, and a band-pass selector adapted to pass said excluded portion of said sideband with uniform response thereover and with said carrier at anopposite edge of its pass band, a common output circuit for said channels for translating said carrier wave and both its modulation-frequency sidebands including said derived excluded portion of said other sideband.

HAROLD M. LEWIS,

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