Modulasi frekuensi pertemuan 4
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Matakuliah: H0122 / Dasar Telekomunikasi Tahun: 2008. Modulasi Frekuensi Pertemuan 4. Learning Outcomes. Mahasiswa dapat menjelaskan teknik modulasi frekuensi dan karakteristiknya. Outline Materi. Prinsip Modulasi Frekuensi Bandwidth Hubungan dengan Modulasi Fasa. Angle Modulation.

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Modulasi Frekuensi Pertemuan 4

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Matakuliah: H0122 / Dasar Telekomunikasi

Tahun: 2008

Modulasi FrekuensiPertemuan 4

Learning Outcomes

Mahasiswa dapat menjelaskan teknik modulasi frekuensi dan karakteristiknya.

Outline Materi

  • Prinsip Modulasi Frekuensi

  • Bandwidth

  • Hubungan dengan Modulasi Fasa

Angle Modulation

  • The angle modulation can be expressed mathematically as:

    m(t) = Vccos [ωct + (t)]

    • m(t)= angle modulated wave

    • Vc= peak carrier amplitude (Volt)

    • c= carrier radian frequency (rad/sec)

    • (t )= instantaneous phase deviation (radians)

  • The magnitude of the frequency (f) and phase deviation ()is proportional to the amplitude of the modulating signal, Vm and the rate at which the changes are occurring is equal to the modulating signal frequency, fm.

Angle Modulation

  • Frequency & Phase Modulation (FM & PM) are both forms of Angle Modulation.

  • Because of its superior performance than AM, it is used extensively for commercial broadcasting radio broadcasting, television sound transmission, 2-way mobile radio, cellular radio, microwave and satellite communications systems.

Frequency Modulation

  • Frequency Modulation is the process of changing carrier frequency by the modulating signal while the carrier amplitude remains constant.

  • As the modulating signal amplitude increases, the carrier frequency increases and vice versa.

  • The amount of change in carrier frequency produced by the modulating signal is called Frequency Deviation (f). Meanwhile, the change in phase is called Phase Deviation ()

  • The deviation is proportional to the amplitude of the modulating signal.

Frequency Modulation

  • FM produces pairs of sidebands spaced from the carrier in multiples of the modulating frequency.

  • The modulation index m of FM signal is the ratio of the frequency deviation fd to the modulating frequency, fm (m = fd / fm)

  • The modulation index determines the number of significant pairs of sidebands in FM signals.

Kfis the frequency deviation sensitivity

Frequency Modulation

The frequency of a harmonic carrier signal is varied in such a way that the instantaneous frequency deviation i.e. the difference between the instantaneous frequency and the carrier frequency is linearly related to the size of the modulating signal at a given instant of time.

Modulation Index

  • Frequency modulation index is defined as

    m = Kf Vm/ωm

  • Frequency deviation which is the change in carrier when acted on by a modulating signal frequency is given by:

    • Peak frequency shift in hertz

    • Peak-to-peak frequency deviation of carrier swing

  • Therefore m can be rewritten as

    m=Δf /fm

PM & FM Waveform








Jn (m) is the Bessel function of the first kind



  • m=modulation index

  • Vc = peak amplitude of the unmodulated carrier

  • J0(m)= carrier component

  • J1(m)= first set of side frequencies displaced from

    the carrier by ωm

  • J2(m)=second set of side frequencies displaced

    from the carrier by 2ωm

  • Jn(m)=nth set of side frequencies displaced from

    the carrier by nωm


FM modulator: f = 10 kHz, fm = 10 kHz, Vc = 10 V, fc = 500 kHz, m=1


  • Low-index modulation (narrowband FM)

    • m <1 (fm>>f), B = 2fm

    • High-index modulation (wideband FM)

    • m >10 (f >>> fm), B = 2fm

  • Actual bandwidth

  • B = 2nfm

  • (use Bessel table, n = number of significant sidebands)

  • Carson’s rule (approx 98% of power)

    • B = 2 (f + fm)

    • Δf = peak frequency deviation

    • fm = modulating frequency

FM & PM Modulator

  • FM modulator = integrator followed by a PM Modulator

  • FM Demodulator = PM demodulator followed by a differentiator

  • PM Modulator = Differentiator followed by an FM Modulator

  • PM Demodulator = FM demodulator followed by an integrator

Modulating signal source

Frequency modulator


FM wave


Kf is the deviation sensitivity

Frequency Modulator


  • Telah dipelajari karakteristik modulasi sudut

  • Telah dipelajari modulasi frekuensi

  • Telah dipelajari peran sidebands.

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