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Inventor, 00A/Ald K Dec. 21, 1954 D. K. WEAVER, JR 2,697,816 SINGLE SIDEBAND GENERATOR MODULATING NEGATIVE FEEDBACK Filed Feb. 3, 1954 INVENTOR, 00A/ALD K. #Vf/lvm, JR. pice 2,697,816 ­United States Patent O yPatented Dec. 21, 1954 1 2 feedback signal era to combining circuit 32 through low pass‘iilter 36. _Each of combining circuits20 and 22 2,697,816 provides an output which is proportional to the dilference ' SINGLE SIDEBAND GENERATOR MODULATING `between, the modulation signal input and a given propor tion F ofthe negative feedback signal input applied there NEGATIVE FEEDBACK A’ . to. As will be demonstrated below, this given proportion Donald K. Weaver, Jr., Palo Alto, Calif., assignor to the of negative feedback signal F is that which makes the United States of America as represented by the Secre amplitude of the single sideband output one-half‘as much tary of the Army . as it would be if no negative feedback were employed. 10 The equations for the carrier waves er1 and etz and Application February s, 1954, serial No. 408,052 modulation signals emi and~em2 are as follows: 1 claim. (cl. 332-45) ‘ en_-:sin wrt ’ j (l) e12=Er COS (wrt-Hor) (2) This invention-relates to a single `sideband generating 15 system, and more particularly‘to a single sideband gen emi`=cos wmt (3) erating system employing negative feedback. €m2=Ea Sill (Mittel-gom). One scheme, well known in the‘ art, for producing a where the carrier frequency is wr' and the modulationfre single sideband utilizes two carrier. waves` having 4the quency is` wm radians per second, the RF amplitude un same frequency and amplitude, but which are in phase 20 balance, that is, the ratio of the amplitude of erz relative quadrature, and two modulation signals having the same to that of er1 is Er, and the phase error in the phase frequency spectrum, the frequency components of which quadrature relationship between the two carrier waves have the same amplitude, but are in phase quadrature. 1s <pr, and similarly, the audio modulated unbalance is One of the carrier waves and one of the modulation Ea and the phase error in the modulation signal is tpm. signals are applied to one balanced modulator and the 25 With the four inputs given in Equations 1 to 4, it can other carrier wave and the other modulation signal are be shown by mathematical analysis that where the phase applied to another balanced modulator. By summing the errors are less than 10° and the amplitude unbalance is outputs of the two balanced modulators one sideband small, the output from the summing circuit with and with of the carrier wave frequency and the modulation fre quency spectrum is completely suppressed so that only 30 out negative feedback willÍ be to a very close approxima a single sideband is obtained. tion as follows: ` In practice, however, perfect equality in the amplitudes of the carrier waves and frequency components of the 1 modulation signals is never achieved. Furthermore, exact phase quadrature between the two carrier waves 35 B=rrr (6) and frequency components of the two modulation sig nals is never obtained. Because of these amplitude and phase errors an unwanted residual second sideband is AFiÑ-na-w <7) produced by practical single sideband generators of the type described above. 40 It is an object of this invention to reduce the amplitude of the residual second sideband for given errors in the phase quadrature relationship and the amplitudes of the where Bo is the relative amplitude of the desired single two carrier waves. sideband without negative feedback, B is the relative It is a further object of this invention to permit larger 45 amplitude of the wanted single sideband with negative errors in the amplitude and phase quadrature relation feedback, Ao is the relative amplitude of the unwanted ship of the two carrier waves to be tolerated for a given second sideband without negative feedback, and A is amplitude of the residual second sideband. The fea the relative amplitude of the unwanted second sideband tures of this invention which are believed to be novel are with negative feedback,-F is the given proportion of nega set forth with particularity in the appended claim. The 50 tive feedback applied to the balance modulator, and j invention itself, however, both as to its organization and is the square root of minus one. method of operation, together with further advantages For the special case where F=l and Ea=l Equations thereof, may best be understood by reference to the fol 6 and 8 respectively reduce to: lowing description taken in conjunction with the follow ing drawing, wherein the single ñgure is a block diagram 55 B: (9) of the preferred embodiment of the invention. Referring to the drawing, carrier wave er1 from source 10 is applied both to balanced modulator 24 and carrier 2 (l0) wave phase-shifting network 12. Network 12 shifts the phase of carrier wave er1 by approximately 90° with 60 by comparing Equations l0 and 8 it will be seen that little effect on its amplitude. The carrier wave output for the special case where F=1, the term containing era of network 12 is applied to balanced modulator 26. :pr is completely cancelled, and by comparing the real Carrier wave er1 is also applied to coherent detector 30 terms of Equations l0 and 9, and 7 and 5, it will be seen and carrier wave erz is also applied to coherent detector that the ratio of the amplitude of the unwanted second 32. A modulation signal em is provided by modulation 65 sideband to the desired single sideband with negative source 14. Modulation signal em is applied to both feedback, where Ea=l and F :1, is one-half the ratio of modulation phase shifting network 16 andmodulation the amplitude of the unwanted second sideband to the phase shifting network-18. The outputs of networks 16 wanted single sideband without negative feedback. and 18, emi and emz, which are approximately equal in While there has been described what is at present con amplitude and approximately in phase quadrature, are 70 sidered to be a preferred embodiment of the invention, respectively applied to combining circuits 20 and 22. it will be obvious to those skilled in the art that various The outputs of combining circuits 20 and 22 are respec changes and modification may be made therein without tively applied to balanced modulators 24 and 26. The departing from the invention, and it is, therefore, aimed outputs e1 and e2 of balanced modulators 24 and 26 in the appended claim to cover all such changes and are applied to summing circuit 28, which provides the 75 modifications as fall within the true spirit and scope of single sideband output eo. Negative feedback is achieved the invention. by applying the single sideband output e0 both to co What is claimed is: herent detectors 30 and 32. The output of coherent A single sideband modulation system comprising iirst detector 30 is applied as a negative feedback signal en and second combining circuits, each of which provides to combining circuit 20 through low-pass filter 34 and 80 an output proportional to the difference between a first the output of coherent detector is applied as a negative input signal and a given proportion of a second input 2,697,816 4 signal applied thereto, means producing iirst and second as first and second inputs respectively to said first coa modulation signals. having the same frequency spectrum herent detector,­ means for applying the output of said and wherein corresponding frequency components of summing circuit and said second carrier Wave as first and said iirst and second modulation signals have approxi~ second inputs respectively to said second coherent de , mately equal amplitudes and> are approximately in phase Dl tector, a first low-pass filter having the output of said _ quadrature, means for applying said iirst modulation s1g first coherent detector applied thereto for providing a nal as said ñrst input signal to said ñrst combining cir first negative feedback signal', a second low-pass filter cult,> means for applying said second modulation signal having theA output of said second coherent detector ap , assaid first input signal to said second combining circuit, plied thereto for providing a second negative feedback first and second balanced modulators, means producing signal, and means for applying said first and second nega iirst and second carrier waves having the same frequency -tive feedback signalsk respectively as said second input and approximately equal amplitudes and which are ap signals to said ñrst and second combining circuits, the proximately in phase quadrature, means for applying said given proportion of the second input signal appearing in íirst and second carrier. waves respectively as iirst inputs the output of each combining circuit being that which to said' first and second balanced modulators, means for makes the amplitude of the single sideband one-half as applying the outputs of said iirst and second combining much as it would be if no negative feedbacks were em ` circuits respectively as second inputs to said first and ployed. l second balanced modulators, a summing circuit for summing the output of said first balanced modu References Cited in the ñle of this patent lator and the output of said second balanced modu- ~ lator, whereby the output of said summing circuit is a UNITED STATES PATENTSl single sideband of the carrier wave frequency and the Number Name Date frequency spectrum of said modulation signals, first~ and 2,248,250 Peterson _____________ __ July 8, 1941 second coherent detectors, means for applying the out 2,557,194 Mitsom ______________ __ June 19, 1951 put of said summing circuit and said first carrier Wave 25 .
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