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[       }       ]        2'П                                                           (     -2
                                           .  rri .r
            The  mоdulаtiпg sigпаl m(t) is  limited  iп  гапgе gепегаllу iп the  fоllоwiпg mаппег to

            prevent distortion when demodulating the АМ signal:





            Given  this limitation  in  range  for  m(t), the value of [1  + m(lj] is  nonnegative,  and
            the  phase  of the  carrier  signal  [соs(2пfcаГГiег)  t]  does  not  change  as  its  carrier
            amplitude is  being varied. The constant 1 in the term  [1  + щl}] ensures that there

            is  а carrier  signal  should  the  modulating  signal  drop  to  О.  This  makes  sense
            because when the music ог voice signals drop in  а bгoadcast АМ radio  progгam, the
            radio-frequency (RF)  carгieг signal is still  pгesent.
                     Introduction to Suppressed-Carrier Amplitude

                                                   Modulation

            Foг а  suppressed-caгrier amplitude-modulated  signal,  theгe is  по caггier signal.
            That is,  the sinusoidal signal  соs(2пfcaггiег)t is not present in  а suppressed-carrier АМ
            signal.  Theгe аге generally  two  types  of  suppressed-carrieг АМ signal:s,  а

            doubIe-sidеЬапd suppгessed-caгrier (DSBSC)  АМ signal  and  а  signal-sideband
            suppressed-carrier  (SSBSC)  АМ signal.  Ву using  а combination  of DSBSC  АМ
            signa!ls,  опе сап also gепегаtе phase-modulated signals.
            The  basic  signal  to  provide  1 and  Q signals  and  single-sideband  signals  is  the
            doubIe-sidеЬапd signal S(t)DSBSC.  It is characterized  Ьу the following equation:



                                      t          2             t
                  О.              si                   .   i
            where  V sig ( l}  is  the  modulating  signal  and  gепегаllу is  ап alternating-current  (АС)
            signal,  and  the  гange of V   sig ( l}  is  not  necessarily  restricted  to  -1  and  + 1.  Figure

            16-3  provides  ал illustration  of  а  doubIe-sidеЬапd suppressed-carrier  signal
            S(l}DSBSC.  The  figuгe looks  гatheг strange  in  that  опе тау expect  to  see  that  а
            sinusoidal  modulating signal should  result in  а sinusoid envelope.
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