I. OBJECTIVES 1. To generate FM signals using VCO 2. To understand the modulator sensitivity and linear range o operation !. To examine the a"tors a#e"ting the shape o the spe"trum and the $and%idth o FM
II. BACKGROUND INFORMATION & simple and dire"t method o generating an FM is $y the use o a voltage "ontrolled os"illator ' VCO. The re(uen"y o su"h an os"illator "an $e varied $y the magnitude o an input )"ontrol* voltage. The $lo"+ diagram o VCO,FM generator is sho%n in -gure 1)a*. Figure 1)$* sho%s a snap shot o an FM signal together %ith the message rom %hi"h it %as derived. Note parti"ularly that there are no amplitude variations ' the envelope o an FM %aveorm is a "onstant. For VCO to %or+ as a re(uen"y modulator it has to maniest a linear operation $et%een the magnitude o the input signal and the output os"illation. /arge signal amplitude may ta+e the system out o its linear range o operation. Thereore Thereore a "areul design o the the deviation sensitivity sensitivity o the VCO is re(uired re(uired to ensure linear operation over the ull range o input signal amplitudes. 0nli+e amplitude modulation modulation the $and%idth o FM signals is not determined $y the message $and%idth only $ut also $y message )maximum* amplitude and deviation sensitivity. The produ"t o the last t%o a"tors yields re(uen"y deviation. The $and%idth $and%idth o FM signal signal "an $e approximated approximated $y )Carsons rule* rule*
III. MATERIALS REQUIRED E(uipment T3M4 5oard T3M4 Modules igital Os"illos"ope Mis"ellaneous pat"hing %ires and extension "ord
IV. E78ERIMENTAL 8ROCEDURES This linear has our parts. The -rst part studies the sensitivity and the range o linear operation o the voltage "ontrolled os"illator )VCO* in preparation or FM generation in the third part part 33 addresses designing the re(uen"y deviation ratio or the modulator. 4pe"trum analysis and $and%idth estimation %ill $e the su$7e"t o the last part.
S)n!iti1it0 and Lin)a,it0 o VCO The output re(uen"y o the VCO varies %ith the input voltage V in. The amount o variation )89:volt* "an $e "ontrolled $y the deviation sensitivity );&3N*. 5eore generating an FM %aveorm it is re(uired to set the deviation sensitivity to a value that ensures linearity o the VCO over the %hole range o message amplitudes. 1. VCO. 2. 0se the ront panel > o "ontrol to set the output re(uen"y )sin?t* "lose to 1@@ +8A. !. Conne"t the varia$le C voltage to the input )V in* o the VCO. B. The deviation sensitivity "an $e set %ith the ront panel ;&3N "ontrol. 4et this to a$out 2D o its ull "lo"+%ise rotation. 4ee -gure or approximate setting. . Vary the V&3&5/E C rom ,2V to 2V in steps o @.V and measure the output re(uen"y. Fill up the ta$le. 6. =ith the varia$le C on its minimum value set the ;&3N "ontrol )sensitivity* o the VCO to a$out 6@D )ma+e sure you do not overload the VCO the /E should $e light up*. edo step and ta$ulate the result also.
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=hi"h o the a$ove settings results in a more linear perorman"e in the given range o V inH etermine the linear range or the se"ond "ase.
0sing the ta$le only estimate the re(uen"y o the VCO %hen C input is 1.GV or $oth settings. =hi"h settings results in easier interpolationH =hyH
S)ttin" th) F,)-)n/0 D)1iation The re(uen"y deviation is e(ual to the produ"t o V in max and ;ain. Our o$7e"tive is to design the ;&3N that yields re(uen"y deviation o 1@ +8A or an input signal o B volts pea+,to,pea+. This "an $e done as ollo%s I. 4et a C voltage o ,2V as input to VCO. J. 4et the ;&3N "ontrol ully anti,"lo"+%ise and the re(uen"y to 1@@ +8A. 1@. &dvan"e the ;&3N "ontrol until the re(uen"y "hanges $y 1@ +8A. 11. Change the V&3&5/E C to 2V and "on-rm that the deviation is a$out 1@ +8A in the other dire"tion. e"ord the measured re(uen"y.
FM G)n),ation 12. epla"e the C voltage sour"e %ith the output rom an &03O O4C3//&TO. The re(uen"y deviation %ill no% $e a$out 1@ +8A sin"e the &03O O4C3//&TO output is a$out 2 volts pea+. =hy re(uen"y "ounter is still at 1@@ +8AH
1!. O$serve the generated FM on the os"illos"ope. &d7ust the range and Aoom,in to optimiAe the vie%. Try 2@Ks:div. ;raph the output.
1B. Vary the re(uen"y o the &03O O4C3//&TO. Explain the "hange in the modulated signal.
1. Vary the ;&3N o VCO. Explain the "hange in the modulated signal.
S6)/t,( Ana#0!i! and Bandwidth E!ti(ation Many interesting o$servation "an $e made regarding the FM spe"trum. 16. Fix the message re(uen"y rom the &03O O4C3//&TO to 2 +8A and the VCO gain to a$out 2D.
1G. Vary the message re(uen"y and des"ri$e the impa"t on the spe"trum o the FM signal.
