Measuring Am Modulation Index

MEASURING AM MODULATING INDEX I. OBJE OBJECT CTIV IVES ES 1. To observ observe e an AM wave wave signal signal on oscill oscillosc oscope ope generat generated ed by a functi function on generator using internal or external modulation. 2. To measure measure the modulatio modulation n index of of an AM wave. wave. II.BASIC CONCEPTS Amplitude modulation is a process where the carrier frequency is varied in amplitude in the same rate as the intelligence (or message frequency. As the intell intellige igence nce amplit amplitude ude reach reaches es a maximu maximum m amplit amplitude ude posit positive ive value! value! the AM waveform has a maximum amplitude. "ame thing happen when the intelligence amplit amplitude ude reac reaches hes a maximum maximum negati negative ve value! value! the AM wavefo waveform rm reach reaches es a minimum value. "ee #gure 1 for the illustration.  The method used to produce a usable AM signal is to combine the carrier and intelligence frequency through a non$linear device. The output signal consists of  (1 %& level and (2 components at each of the two original frequencies.

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')*+, 1- AM aveform *nder /arying ntelligence "ignals  The extent to which a carrier voltage is varied by the message or intelligence voltage is called the modulation index or the percentage of modulation. ,i m 3 m 3 4 ,cgiven! the modulation index becomes f pea0$to$pea0 values are m 3 ,i

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m 36 $ A 67A

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III. PROCEDURE 1. "et up the instruments as shown in 'igure 1. The oscilloscope probe is connected to the 68& clip of the function generator (ground to ground. C"&DDC"&C?, '*8&TC8 ),8 &9 1 &9 2 68& ?+C6 2. "et a 15509: sine wave from the function , generator (The function generator

must be capable of producing an Am internal modulation. This could be a carrier frequency which must be modulated. ;. "elect AM mode ("9'T AM from the panel of the function generator from which to generate the modulating frequency internally. (The default internal modulating frequency of 8"T,< sfg215= is >559:. >. ?ill out the ",,? "?A8 0nob to set into AM modulation. +otate the 0nob enough to produce an AM wave. @ou can stabili:e the signal by adusting the variable 0nob of your oscilloscope. . ?erform and #ll up Table 1. The modulating frequency is adusted by rotating the 0nob of AM modulation. %o upto run > of Table 1. B. &ompute the percent of modulation and record it in Table 1. TABLE 1. RU N 1 2 ; >

Modulating Signal  ! f m1 f m2 f m; f m>

Ca""i#" "#$u#n%&  % 15509: 15509: 15509: 15509:

E!a'(V)

E!in(V)

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=. %raw E "0etch one run of AM wave. ndicate , max and ,min.

IV.

+UESTIONS TO PONDER

1. The antenna current of an AM transmitter is 12A. hen unmodulated but increases to 1;A when modulated. &alculate m.

2. hat are some of the possible results of overmodulationF  GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG   GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG   GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG  ;. A 155$/ carrier is modulated by a 1$09H sine wave. %etermine the side$ frequency amplitudes when m35.=.