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Explain the term modulation index for F.M. and F.M. band width. Give the advantages of frequency modulation. |
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Answer» Solution :Modulation index`(m_(f))`. The modulation index for FM, `m_(f)` is defined as `m_(f)=("maximum Frequency DEVIATION")/("modulating frequency")radians` `(delta)/(f_(m))=(k_(f)E_(m))/(2pif_(m))=(k_(f)E_(m))/(w_(m))` FM has an infinite number of side bands, the side frequencies are `(f_(c)+-f_(m)),(f_(c)+-2f_(m)),(f_(c)+-3f_(m))` etc. The modulation index determines the amplitudes of the side band components. The side- bands at equal distances from `f_(c)` have equal amplitudes, so that the side- band distribution is symmetrical about the carrier frequency. The amplitudes are negligible when the carrier and side frequencies are largely separated from each other. In practice, therefore, the number of significant side frequency pairs is limited. Band Width In frequency modulated signal, the information is contained in the side frequency components. Hence the band width required for transmission or reception through FM is given by `(BW)_(FM)=2nf_(m.)` where `n` is the number of side frequency pairs of significant amplitude. It must be noted that in AM, increased depth of modulation INCREASES the side band power and therefore the total transmitted power. In FM, however, the total transmitted power. In FM, however, the total transmitted power always remains constant, but with increase in `m_(f)` the required bandwidth increases. Here, the power in side frequency components is achieved at the reduction of power of the carrier component. Advantages of FM Following are the advantages of FM transmission : `1`. It gives noiseless reception, because noise is a form of amplitude variation. As in FM, amplitude remains constant, so noise factor does not creep up. `2`. Operating range is quite LARGE. `3`. It gives high fidelity reception. `4`. EFFICIENCY is high. |
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