US2444534A

US2444534A - Single side-band modulation system

Info

Publication number: US2444534A
Application number: US644871A
Authority: US
Inventors: Hans W G Salinger
Original assignee: Farnsworth Research Corp
Current assignee: Farnsworth Research Corp
Priority date: 1946-02-01
Filing date: 1946-02-01
Publication date: 1948-07-06
Anticipated expiration: 1965-07-06

Description

July 6, 1948.

CARRIER WAVE SALINGER Filed Feb 1, 1946 SIGNAL SOURCE GENERATOR monuunon 1 lo l2 2: 3

FREQ. DOUBLER I PHASE GORREOTOR 2fc :e I?

\ V 9 4, fc+fm MODULATOR :5

,J FREQ.DOUBLER afcfm PHASE OORREOTOR W Z fc+fm MODULATOR E 'ffc-fm 2a 26 28 30 INVENTOR HANS W. G. SALINGER ATTORNEY Patented July 6, 1948 SINGLE SIDE-BAND MODULATION SYSTEM Hans W. G. Salinger, Fort Wayne, Ind., assignor, by mesne assignments, to Farnsworth Research Corporation, a corporation of Indiana Application February 1, 1946, Serial No. 644,871

12 Claims. (01. 179-1715) This invention relates to modulation systems, and more particularly relates to a system for developing a single-sideband modulated-carrier wave.

When the amplitude of a carrier wave is modulated in accordance with a signal of variable frequency, a modulated carrier wave is developed which comprises an upper and a lower sideband. It is well known that all the information contained in the signal is present if the carrier frequency and only one of the sideband-s is transmitted. Accordingly, one of the sidebands of a modulated carrier wave represents surplusage, and for this reason single sideband transmission where only the carrier frequency and one of the sidebands is transmitted has been proposed in the past. Present television standards, for example, demand transmission of the video signal by vestigial sideband transmission thereby to reduce the width of the channel required for the transmission of an image with given detail.

Modulation of a carrier wave in accordance with a video signal, which may include frequencies from zero to approximately megacycles, presents considerable difficulties. For this reason, modulation of the carrier wave at low levels is preferred because it is practically impossible to amplify the video signal to a high power level. On the other hand, amplification of a carrier wave modulated in accordance with a video signal also presents appreciable difliculties in view of the extremely wide sidebands of the carrier. Thus, for example, amplification of a modulated carrier wave by means of a class C amplifier, which is the most efficient amplification method, is not possible. Therefore, it has been proposed to remove one of the sidebands of the modulated carrier wave before further amplification takes place. The undesired sideband of a modulated carrier wave is conventionally suppressed by means of a filter network. However, unless the power amplifier stages used for amplifying the single-sideband modulated-carrier wave have exactly linear amplification, the undesired sideband is reinserted. Modulation systems have also been suggested where a modulated carrier Wave is developed having a single sideband only. This is effected by destroying the undesired sideband through interference, that is, by introducing a phase shift of 180 degrees between two components of the undesired sideband.

It is an object of the present invention, therefore, to provide a modulation method and system for developing a single-sideband modulated-car- 2 rier wave of appreciable power without requiring a high modulation level of the signal to be transmitted.

A further object of the invention is to utilize one of the two sidebands developed by modulating a carrier wave for feeding a utilization device, such as an antenna, while the other normally undesired sideband is utilized for developing a further sideband or further sidebands identical in frequency with the desired sideband, thereby to increase the amplitude or power of the desired sideband.

In accordance with the present invention, there is provided a single 'sideband modulation system comprising means for developing a signal and for developing a carrier wave of frequency ,1. Means are provided for modulating the carrier wave in accordance with the signal to develop a modulated carrier wave of frequency 1, having an upper and lower sideband. One of the two sidebands is the desired sideband and the other one, the undesired sideband. Means are further provided for doubling the frequency of the carrier wave to develop a carrier wave of frequency 2]. There is also provided a means for modulating the carrier wave of frequency 2] in accordance with the undesired sideband. Thus there is developed a modulated carrier wave of fre quency 2f having two sidebands, the lower one of which is substantially identical in frequency with the desired sideband. Accordingly, a further sideband is developed which has substantially the same frequencyas the desired sideband. Finally means are provided for adding these identical sidebands. In accordance with a, preferred embodiment of the invention the upper sideband is the desired sideband.

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

In the accompanying drawing, the single figure is a circuit diagram, partly in block form, of a single sideband modulation system embodying the invention.

Referring now to the drawing, there is illustrated a single sideband modulation system comprising carrier wave generator I. Generator l is arranged to develop a carrier wave at the frequency fc. A signal of variable frequency fin is developed by signal source 2. The signal developed by source 2 may, for example, be a video signal or an audio signal. In modulator 3, conheated to the outputs of generator I and signal Source 2, the amplitude of the carrier wave of frequency fc is modulated in accordance with the signal developed by source 2. Modulator 3 thus develops a modulated carrier wave having an upper sideband and a lower sideband. The frequencies of the modulated carrier wave are given as follows: fc+fm and fcfm, where thefirst expression represents the upper sideband and the second expression the lower sideband.

The two sidebands are segregated by filter network 4, having a high pass section 5 and a low pass section 6. High pass-filter section 5 comprises series condenser I and further comprises inductance element 8 and condenser I arranged in series and shunted between seriescondenser I and ground. High pass filtersection is ar ranged to pass the upper sideband of'the modulated carrier wave developed by modulator 3, that is, frequencies within the range fc+fm.

Low'passfilter section 6 comprises series inductance element I! and-further comprises-inductance element I2 and condenser I3 arranged in series and shuutedbetweeninductance element II and'ground. Low pass filter section 6 is des'igned to passthe lower sidebandof the modulated carrier wave developed by modulator 3; that'is frequencies within the range fc-Jm.

Filter network- 4- has been illustrated with lumped'elements but it is-to be understood that filter-network- 4 may also consist of suitable sections of'transmission lines having distributed-capacitance and inductance, as is well known in the art. Other types of filters including multisection structures may also be used.

The upper sideband including the carrier frequency, thatis, frequencies in the range fe+fm are passed by high pass filter section 5 which may becon'n'ected to antenna I 4 for the purpose'of radiating the single-sideband modulatedcarrier waveinto space. For single sideband transmission it is conventicnal practice to dissipate the undesired sideband, which is usually the lower sideband, by means of a load'resistor connected in parallel across inductance element t2 andcon'denser. I3 of low'pass filter section 6'. In accordance with the'present invention, the undesired sideband, that is the-lower sideband, is utilized for developing afurther sideband of the frequency fc-l-fini Tothisend thefrequency of the carrier' waveueveloped by generator I is doubled by"frequency doubler I51 The output of frequency-doubler" I5; therefore, is a carrier wave having the frequency of 2ft. The'amplitude of thecarrier-wave of frequency 2]0 is modulated in niodulator- I6'=-in accordance with the lower sideband fc3m obtained from low pass'filte'r sectlon 6-.

Theoutput'of'm'odulator I Baccordingly is a modulated carrier wave having an upper sideban'd of 'frequency 3fcfm and'a lower sideband of frequency-f+fm. Thus it-will be seen that the lower sideband of the modulated carrier wave developed by'modulatori6 is identical in frequency-wi'th'theupper sideband of the modulated carrier" wave developed by modulator 3.

For the purpose of segregating the 'two sidebands developed by i'rnodulator I6; thereis'provided filter: network" IT having a low pass'filter section- I8 and a high pass filter section 20. Low pass filter section Iais similar to-low'pass filter section while high" pass filter section 20 is sirriilar'to' high-pass filtersection 5: However, low pass filter section I8 is'designed to transmit 4 frequencies in the range of fc+fm, that is, the lower sideband of the modulated carrier wave developed by modulator I6. High pass filter section 20, on the other hand, is arranged to transmit frequencies within the range 3fc-f1n, that is, the upper sideband of the modulated carrier wave developed by modulator I6.

The sideband of iihflfleQllEHCK-fi-i-fxu which is developedby modulator: l6 and-passed by low pass filter section i8 is added to the sideband passed by high pass filter section 5. However, it is necessary to add the two identical sidebands of'frequency fc-I-fm in phase. If the two sidebands are out of phase, a phase corrector indicated at 2I may be arranged, for example, in the lead: connecting low pass filter section I8 to antenna Id.

The upper sideband developed by modulator I5 and passed by high pass filter section may be utilized again to develop a further sideband of the frequency fed-fin To this end. theirequency of the carrier wave developedby frequency doubler I5 is doubled again by frequency doubler 22. The carrier wave developed byv frequency doubler 22 has the frequency 4h. The amplitude of this carrier wave may be modulated in modulator'23 in accordance'with the upper sideband'developedby modulator I6 and having the frequency 3Jc-fm. The two sidebands of the modulated carrier wave developed by modulator 23'have the frequencies 7fcfm andfc-l-fm, respectively. Thus a further sideband of the desired frequency is developed.

For the purpose of segregating the lowersideband from the upper sideband of the modulated carrier Wave developed by modulator 23, there is provided filter network 25 comprising low pass filter section 25 and high pass filter section 25. Low passfil'ter section 25'is arranged to pass frequen'cies'within' the range 'fc+fm, while high pass filter section 26 is arranged to pass frequencies within the range 7f;fm.

The desired sideband of-frequency fe+fm which is-passed by low pass filter section 25' may also be 'fd"to antenna I4 through phase corrector 21 toinsure that the sideband is added to the other sideban'ds in phase. I

Theupperrsideband of frequency -'7fcfm which is passedby high pass filter section 26 may be dissipated by load resistor 28 shunted across inductance'elernent 30 and condenser 3| of filter section 25. Alternatively; it is feasible to utilize the uppersidebandzof frequency- 7f-fm for developing another desired sideband by modulating-therewith a'carrier wave of frequency 8ft Thisprocedure maybe repeated again to develop as many desired s'idebands as required.

The:modulation system of the invention has certain-advantages over those-of the prior art. The modulation level of signal source 2- need -not be very high for modulating the carrier wave developed'by generator I. The upper as'well asthe lower: sideband of the modulated. carrier wave developedby-rnodulator. 3 will have a higher power level than that of the" signal developed-by source. 2 r The lower sideband .of frequency fc-,fm is utilized for modulating Ia; carrier. Wave of frequency 2f. Thus-the-desired lower-sideband developedzbymcdulator I6 will'again have a higher power. level than that of the desired? sideband developed by modulator 3.

The difiiculties presentedby 'raising thezpowerlevelof a carrierwave are, of course, considerably less thanithose of amplifying a-modulated carrier wave-in view of.- the large frequencyrange of a modulated carrier wave. Since the method of obtaining a furtherdesired sideband may be repeated several times, the power levelgofj the single-sideband modulated-carrier wave maybe raised to an appreciable extentwithout requiring a high modulation level of the signal source.

Instead of developing a modulated carrier wave having an upper sideband only, it is also feasible in accordance with the invention to develop a modulated carrier wave having a lower sideband only. To this end the output of low pass filter section 6 should be connected to antenna l4, while the output of high pass filter section 5 should be connected to modulator l6. Thus. the

lower sideband of a frequency ,fefm is fed to antenna I4, while the upper sideband of a frequency fo+fm is passed to modulator IS. The modulated carrier wave d=eveloped-by modulator l6 has two sidebands of the frequencies 3fc+fm and ,fc-,fm, the latter being the sideband. Filter sections l8 and should, in this case, be arranged to pass the lower and the upper sideband developed by modulator l6 having, respectively, the frequencies fc--fm and 3fc+fm.

With the connections shown in the drawing, modulator 23 will then develop a modulated carrier wave having a lower sideband of the desired frequency fefm and an upper sideband of a frequency 7fc-fm.

Filter sections

25 and 26 should, in this case, be arranged to pass the lower and the upper sideband of the modulated carrier wave developed by modulator 23.

It will be observed that regardless of whether the upper or the lower sideband is the desired sideband, the lower sideband of the modulated carrier waves developed by modulators I6 and 23, respectively, is always the desired sideband.

In case the lower sideband is the desired sideband, the upper sideband developed by modulator 23, which has the frequency 7fe+fm, may be utilized again for developing a further desired sideband. To this end the output of high pass filter section 26 may be connected to a further modulator where a carrier wave of'frequency Sin is modulated therewith. In that case load resistor 28 may be omitted.

The modulation system of the invention will also operate when the amplitude of the desired or transmitted sideband is not'uniform throughout the frequency range. In some vestigial sideband transmission systems, the amplitude of the transmitted sideband decreases in the neighborhood of the carrier frequency and reaches zero amplitude at a point in the frequency spectrum located in the frequency range of the undesired sideband. As long as the points where-the desired sideband reaches zero amplitude and where the desired sideband reached maximum amplitude are symmetrically disposed with respect to the carrier frequency, the undesired sideband is a mirror image of the desired sideband and will, after its transformation to a further desired sideband, be identical in frequency with the desired sideband. The term single sideband modulation system as used in the claims is meant to include such a vestigial sideband transmission system.

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

What is claimedisz, v

,l. A single sideband modulation system comprising means for developing a signal, means for developing a carrier Wave of frequency 1, means for modulating said carrier wave of frequency f in accordance with said signal to'develop a modulated carrier wave of frequencyf-having an upper and alower sideband, one of said sidebands being the desired sideband the other one the undesired sideband, means for developing: a carrier wave of frequency 2], means for modulating said carrier wave of frequency 2 in accordance with said undesired sideband, thereby to develop a modulatedcarrier wave of frequency 2f having two sidebands, the lower oneof said sidebands being substantially identical in frequency with said desired sideband, and means for adding said identical sidebands. i a

2. A single side-band modulation system com.- prising means for developing a signal, means for developing-a carrier wave of frequency f, means for modulating the amplitude of said carrier wave of frequency fin accordance with said signal to develop a modulated carrier wav-eof frequency 1 having an upper and a lower sideband, one of said sidebands being the desired sideband and the other one the undesired sideband, meansfor doubling the frequency of said. carrier wave to develop a carrier wave of frequency 2f, means for modulating the amplitude of said carrier Wave of frequency 2 in accordance with said undesired sideband, thereby to developa modulated carrier wave of frequency 2 having two sidebands, the lower one of said sidebands being substantially identical in frequency with said desired sideband, and means including a phase correcting device for adding said identical sidebands in phase. I I l 3. A single sideband modulation system comprising m-eans for developing a signal, means for developing a-carrier wave of frequency means for modulating said carrier wave of frequency f in accordance with said signal to develop a modulated carrier wave of frequency 7 having an upper and a lower sideband, one of said sidebands being the desired sideband and the other one the undesired sideband, means for segregating said upper and said lower sidebands developed by modulating said wave of frequency ,f, means for developing a carrier wave of frequency 2 means for mod-' ula'ting said carrier wave of frequency 2 in accordance with said undesired sideband, thereby to develop a modulated carrier wave of frequency 2; having two sidebands, the lower one of said sidebands being substantially identical in frequency with said desired sideband, means for segregating said two sidebands developed by modulating said wave of frequency 2 and means for adding said identical sidebands in phase.

4. A single sideband modulation system comprising means for developing a signal, means for developing a carrier wave of frequency f;'means for modulating said carrier wave of frequency ,f in accordance with said signal to develop a mod-' ulated carrier Wave of frequency 1 having an upthe true spirit and per and a lower sideband, said upper sideband being the desired sideband, means for developing a carrier wave of frequency 2f, means for modulating said carrier wave of frequency 2 in accordance with said lower sideband, thereby to develop a modulated carrier wave of frequency 21 having two sidebands, the lower one of said sidebands being substantially identical in frequency with c eanser 7 said upper desired sideband, and means for adding said identical sidebands in phase.

5. A single sideband modulation system comprising means for developing a signal, means for developing. a: carrier wave of 1 frequency 1, means :for modulating the amplitude of said'carr-ier wave of frequency f in accordance with said signal to develop a modulated carrier wave offrequency-f having an upper and a lower sideband, means for doubling the frequency of said carrier wave to develops. carrier wave of frequency 2f, means for modulating the amplitude'of said carrier wave of frequency 2 in accordance with-said lower sidehand, thereby to develop a modulated carrier wave of frequency 21 having two sidebands, the lower one of said sidebands being substantiallyidentical in frequency with said upper sideband, and means including a phase correotor for adding said identical sid'ebands in phase.

6. A single sideband modulation system comprising means for developingasignal, means for developing axcarrierwave of frequencyv 1, means for modulating saidv carrier wave of frequency.

in accordance with said signal. to develop a modulatedlcarrier wave of frequency f'having an upper and-a lower sideband, means for segregating said upper and said lower sideband developed by modulating said wave of frequency 1, means for doubling the frequency ofsaid carrier wave to develop: av carrier wave offrequency 2f, means for modulating said carrier wave of frequency 27 in accordance with said lower sideband, thereby to developamodulatedcarrier wave of frequency 2 having two sidebands, the lower one of said sidebandsbeing substantially identical in frequency with said upper sideband, A means for segregating said. two. sidebands developed by modulating said wave of frequency 2 and means for-adding said identical sidebands i-nphase.

'I. A single sideband modulation system comprising means for developing aisignal -means for developing a-carrier wave of frequency 1, means for modulating said-carrier wave of frequency! in-accordance with said signal todevelop a modulated carrier wave of frequency I having an upper and alower sideband,..one of saidsidebands being the wanted sideband and theE-other one the unwanted sideband, means for successively doubling the frequency of said carrier wave to derive carrier waves each having a. frequency be.- ing :a multiple of 1, means for modulating "eachof said carrier waves having a frequency which is a multiple of f in accordance with the unwanted sideband developed by -themodulation of-the carrier wave of half the frequency thereof, thereby to develop modulated carrier waves each having twosidebands, the lower one-ofsaidsidebands being substantially identical in frequency with said wanted sideband, and means for adding-said identical sidebands in phase.

8. A single sideband .modulationsystemcome prising. means vfor developing ausignal means for developing a-carrier wave of frequency f, means forv modulating said carrier wavesof frequency. f in accordance with said signal tel-develop amodulatedcarrier wave of. frequency. having anupp er and a. lower sideband, one :of said sidehandsrbeing the wantedsideband: and the other one the unwan-ted sideband, means for segregating said. two sidebands developed'. by modulatingsaid wave of frequency f, means for successivelydoubling the frequency) of said carrierz'wa-ve. to derive carrier waves. each having a frequency being; .aemultiple of; gflxmiianszlfor modulating each ofs'said carrier waves having. a frequency which is 1 a multiple cf- 8 :f-i-n'aocordance with the unwanted sidebanddeveloped by the modulation of the carrier wave of" hailf'the frequency thereof, thereby to develop modulated carrier waves each having-two sidebands, the lower one of said sidebands being substantially identical in frequency with said wanted-sideband, means for segregating said two sidebands developed by modulating one of said waves having afrequency which is a multiple of 7, and means for adding said identical sidebands in phase.

.9. The method of developing a modulated carrier wave having av single sideband which comprises developing a signal, developing a carrier wave offrequency j, modulating said carrier wave offrequency f in accordance with said signal to develop .a modulated :carrier wave of. frequency j havingan upper and a lower sideband, one of said sidebands being the desired sideband and the other-one the undesired sideband, developing a carrier wave of frequency 2f, modulating said: carrier wave of frequency 21 in accordance with said undesired'sideband, thereby to develop a. modulated carrier wave of frequency '2! having two sidebands, the lower one of said sidebands being substantially identical in frequency with said desired sideband, and adding said identical sidebands in phase.

10. The method of developing a modulated carrier wave having a single sideband which comprises developing a signal, developing a carrier wave of frequency ,f, modulating said carrier wave of frequency f in accordance with said signal to develop a modulated carrier wave of frequency I having an upper and a-lowersideband, one of said sidebands being the desired sideband and the other one the undesired sideband, segregating said upper and said lower sideband developed by modulating said wave of frequency 1, doubling the frequency of said carrier wave to develop a carrier wave of frequency 2), modulatingsaid carrier wave of frequency 2 in accordance with said undesired sideband, thereby to develop a modulated carrier wave of frequency 2 having two sidebands, the lower one of said sidebands being substantially identical in frequency with said desired sideband, segregating said two sidebands developed by modulating said wave offrequency 21', and adding said identical sidebands in phase.

11'. The method of developing a modulated carrier wave having a single sideband which comprises developing a signal, developing a carrier wave of frequency f, modulating said carrier wave of frequency f in accordance with said signal to develop a modulated carrier wave of frequency f having an upper and a lower sideband, doubling the frequency of said carrier wave to develop a carrier wave of frequency 2 modulating said carrier wave of frequency 2 in accordance with said lower sideband; thereby to develop a modul'ated carrier wave of frequency 21 having two sidebands, the lower one of said sidebands being substantially identical in frequency with said upper sideband developed by modulating said wave of frequency f, and adding said identical sidebands in phase.

12. The method of'developing a modulated carrier wave having a single sideband which comprisesdeveloping a signal, developing a carrier wave offrequencyf, modulating saidcarrier wave of frequencyj in accordance with said signal to develop a modulated carrier wave of frequency I;

having .an upper and a lower sideband, one of said sidebands being the desired sideband and the other one the undesired sideband, successively doubling the frequency of said carrier wave to REFERENCES GITED develop carrier waves each having a frequency being a multiple of f, modulating each of said carrier waves having a frequency which is a The following references are of record in the file of this patent:

multiple of f in accordance with the undesired 5 UNITED STAT S PATENTS sideband developed by the modulation of the Number Name Date carrier wave of half the frequency thereof, there- 1 989 770 Reeves Feb 5 1935 by to develop modulated carrier waves each hav- 2:270:385 skiuman Jan 1942 ing two sidebands, the lower one of said sidebands being substantially identical in frequency 10 with said desired sideband, and adding said identical sidebands in phase.

HANS W. G. SALINGER.